literature review on zinc

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

• View all journals
• Explore content
• About the journal
• Publish with us
• Sign up for alerts
• Review Article
• Open access
• Published: 03 January 2024

Cellular zinc metabolism and zinc signaling: from biological functions to diseases and therapeutic targets

• Bonan Chen   ORCID: orcid.org/0000-0002-2430-7934 1 , 2 , 3   na1 ,
• Peiyao Yu 4   na1 ,
• Wai Nok Chan 1 , 2 , 3 ,
• Fuda Xie 1 , 2 , 3 ,
• Yigan Zhang 5 ,
• Li Liang 4 ,
• Kam Tong Leung   ORCID: orcid.org/0000-0002-7695-2513 6 ,
• Kwok Wai Lo   ORCID: orcid.org/0000-0002-3488-6124 1 ,
• Jun Yu   ORCID: orcid.org/0000-0001-5008-2153 2 , 7 ,
• Gary M. K. Tse 1 ,
• Wei Kang   ORCID: orcid.org/0000-0002-4651-677X 1 , 2 , 3 &
• Ka Fai To   ORCID: orcid.org/0000-0003-4919-3707 1 , 2  

Signal Transduction and Targeted Therapy volume  9 , Article number:  6 ( 2024 ) Cite this article

8224 Accesses

8 Citations

12 Altmetric

Metrics details

• Cancer therapy
• Cell biology
• Tumour angiogenesis

Zinc metabolism at the cellular level is critical for many biological processes in the body. A key observation is the disruption of cellular homeostasis, often coinciding with disease progression. As an essential factor in maintaining cellular equilibrium, cellular zinc has been increasingly spotlighted in the context of disease development. Extensive research suggests zinc’s involvement in promoting malignancy and invasion in cancer cells, despite its low tissue concentration. This has led to a growing body of literature investigating zinc’s cellular metabolism, particularly the functions of zinc transporters and storage mechanisms during cancer progression. Zinc transportation is under the control of two major transporter families: SLC30 (ZnT) for the excretion of zinc and SLC39 (ZIP) for the zinc intake. Additionally, the storage of this essential element is predominantly mediated by metallothioneins (MTs). This review consolidates knowledge on the critical functions of cellular zinc signaling and underscores potential molecular pathways linking zinc metabolism to disease progression, with a special focus on cancer. We also compile a summary of clinical trials involving zinc ions. Given the main localization of zinc transporters at the cell membrane, the potential for targeted therapies, including small molecules and monoclonal antibodies, offers promising avenues for future exploration.

Similar content being viewed by others

Connecting copper and cancer: from transition metal signalling to metalloplasia

Alterations in ZnT1 expression and function lead to impaired intracellular zinc homeostasis in cancer

Targeting cuproplasia and cuproptosis in cancer

Introduction.

As an crucial trace element, zinc is critical for numerous biological functions, and its imbalance has been linked to a variety of pathologies, including cancer. 1 , 2 Understanding the intricacies of zinc metabolism at the cellular level, including encompassing the absorption, intracellular trafficking, utilization, storage, and expulsion of zinc, can shed light on the various effects of zinc in cell physiology and pathology. 3 Zinc, an essential component in the regulation of cellular homeostasis, is receiving increasing attention for its role in cancer. 4 , 5

Significantly, an extensive body underscores the crucial role of zinc homeostasis across various biological systems. Zinc is estimated to bind to around 3000 proteins in vivo, representing about 10% of the human proteome, 6 with over 3% of genes in human bodies encoding proteins containing zinc finger domains. Consequently, zinc assumes a pivotal position during numerous physiological processes, including cell cycle progression, 7 , 8 , 9 immune functions, 10 meiosis, 11 and many other physiological procedures. Intracellular zinc metabolism and zinc signaling are exceptionally precise. Cytoplasmic free zinc concentration remains within the picomolar range, while the overall zinc level is estimated to be about 200–300 μM. 12

Cellular zinc homeostasis is delicately regulated by a network of proteins, which includes the solute carrier (SLC) families SLC30 (ZnT) and SLC39 (Zrt- and Irt-like proteins/ZIP), as well as the zinc-binding (MTs). 2 , 13 These proteins are crucial in the maintenance of cellular zinc homeostasis. Traditionally, two transporter family members operate opposite directions to achieve this equilibrium. The SLC30 family, encoding ZnT proteins, facilitates zinc efflux through translocating zinc from the cytoplasm to the lumen of organelles or the extracellular space. 1 Conversely, the SLC39 family, also known as the ZIP family, functions in zinc influx, transporting zinc into the cytoplasm from the extracellular space of the cell or the intracellular storage compartment, effectively elevating zinc levels. 14 Meanwhile, MTs majorly handle zinc storage within the cell, safeguarding against potential toxicity while ensuring availability when required. 13 Increasingly, cellular zinc metabolism has been linked to disease progression. This review will explore the potential role of cellular zinc metabolism in biology, tumorigenesis, and drug applications.

Regulation of cellular zinc signaling

Zinc distribution.

Zinc is prevalent in various human tissues. Adults typically possess a zinc content ranging from 1.4 to 2.3 g. 15 Approximately 85% of zinc resides in the muscles as well as the bones. Besides, about 11% of zinc is in the skin and liver. The remaining 4% of zinc was scattered in other tissues. 16 Notably, the maximum zinc concentration has been found in the retina and choroid of the eye. 17 Additionally, zinc is found in considerable amounts in the prostate, bones, liver, and kidneys. 18

Notably, most of the zinc is intracellular. Approximately 30–40% of the content resides in nuclei of cells, with approximately half distributed across the cytosol, organelles, and specific vesicles, while the remaining zinc is associated with cell membranes. 19 Based on current research, the total pool of zinc, encompassing both intracellular and extracellular compartments, can be distinguished into three distinct categories. 20 , 21 Firstly, the term “Immobile zinc” refers to zinc that is firmly bound to metalloproteins or metalloenzymes, serving as either a structural component or a cofactor. This form of zinc is stable and non-reactive. Secondly, “Mobile reactive zinc” or “labile zinc” is loosely associated with low molecular weight ligands and MTs. This form is exchangeable and reactive. Notably, this mobile form constitutes about 5% of all intracellular zinc, playing a pivotal role in zinc transfer reactions and signaling processes. 22 , 23 Lastly, the “free zinc” pool is another reactive form of the element. In mammalian cells and in extracellular fluids, however, the concentration of this zinc is quite low, with values oscillating between roughly 5 pM and 1 nM. 24

MTs, colloquially referred to as “zinc storage,” maintain intracellular free zinc levels through their interaction with cysteine. 25 , 26 In addition to MTs, members of the zinc transporter family, including ZIPs and ZnTs, play a critical role in managing zinc homeostasis. Remarkably, the cellular zinc transport activity of ZnT7 is crucial for regulating the localization of ERp44 within the Golgi apparatus, a specific subcellular organelle. 27 Notably, many secretory enzymes obtain essential cellular zinc in the Golgi complex. Moreover, as a molecular chaperone acting in the early secretory pathway, ERp44 can bind to zinc to control the protein binding and release, thereby managing protein transport and stability.

In recent times, the essential and multifaceted function of zinc as a signaling molecular has attracted significant attention. The generation of zinc signals arises from three main sources: vesicular exocytosis, zinc transport facilitated by zinc transporters for entry or exit from the cell or organelle, and the binding or dissociation of MTs with zinc. These aspects will be expounded upon in the subsequent sections.

Intracellular zinc signaling

The total concentrations of zinc in cells range from 200–300 μM, 12 whereas the eukaryotic labile (“free”) zinc concentration is in the picomolar range, as mentioned earlier for each specific cell type. 24 Notably, the cytoplasm contains minimal free zinc since intracellular zinc is mainly sequestered in organelles like the ER, Golgi apparatus, and mitochondria, the so-called zinc store. 28 Growing evidence suggests that zinc functions not only as a neurotransmitter for cell-to-cell communication but also as an intracellular signaling molecule, facilitating the transduction of various signaling cascades in response to extracellular stimuli. This has led to the concept of zinc as the “calcium of the 21 st century”. 29

As previously mentioned, there are two pathways for intracellular zinc ion release, namely from intracellular zinc stores or zinc/sulfate sites in proteins, such as in MTs. Transient zinc increases may arise from various mechanisms, including efflux from vesicles known as zincosomes, 30 or changes in cellular redox potential facilitated by cytosolic proteins. 31 It is important to know that, in most cases, zinc signaling arises from the disturbance of intracellular zinc homeostasis, transiently and rapidly. The functioning of zinc ion transporters and MTs in the cell plays a role in maintaining cytoplasmic zinc homeostasis, which is referred to as “buffering” and “muffling”, two essential parameters that determine the availability and signaling processes of zinc. 32 Specifically, “buffering” involves zinc binding by proteins like MT, which helps maintain zinc concentration at the pM range in the cytosol. 33 The biology of MTs is characterized by zinc binding, movement within the cell, and transportation of zinc to various cellular compartments, including extracellular, endosomal, nuclear, and mitochondria. 34 The chelating agent will accelerate this process, but if gene expression such as MT is involved, “buffering” would be slow. 35 “Muffling”, on the other hand, is responsible for modulating transient changes in zinc concentrations under unsteady state conditions of cells, eventually restoring the cytosolic concentrations to their resting levels. 12 , 36 In the “muffling” process, zinc transporters regulate cellular zinc by importing, distributing, exporting, and providing zinc for zinc-dependent proteins. 36 For example, ZnT5,6 loads zinc for the enzymes of the secretory pathway, 37 , 38 while ZnT2,3,8 provides zinc for the exocytotic vesicles. 39 , 40 , 41 Moreover, MT is also responsible for zinc muffling by moving and sequestering zinc to cellular compartments, thus controlling kinetically ion concentrations. 42

In terms of time series, intracellular zinc serves as a second messenger, and its concentration transients are divided into two main types: early (fast) zinc signaling (EZS) and late zinc signaling (LZS) 43 (Fig. 1 ). The study further confirmed that EZS is transcription-independent, occurring over a timescale ranging from seconds to minutes, known as the “zinc wave”. 36 This phenomenon was first observed in mast cells and results from Fcε epsilon receptor I (FcεRI) stimulation, causing a transient, transcription-independent increase in intracellular zinc. 44 The “zinc wave” originates in the perinuclear region, including the ER, and depends on calcium influx and MEK activation. However, the precise mechanism of the “zinc wave” in cells remains poorly understood. In contrast, LZS requires the transcription of zinc transport proteins and has longer-lasting effects lasting for hours. In this case, diverse extracellular stimuli, including cytokines and growth factors, indulge the transcriptional modulation of zinc-associated proteins like ZIPs and ZnTs. Consequently, intracellular zinc homeostasis alterations regulate downstream molecular objectives, in addition to protein kinase C (PKC), ERK1/2 activation leading to neuronal cell death, cAMP-dependent protein kinase (PKA), Ca/calmodulin-dependent protein kinase II (CaMKII), phosphodiesterases (PDEs), protein tyrosine phosphatases (PTPs), and transcription factors, such as NF-κB.

Zinc signaling in the intracellular and extracellular regions. Zinc extracellular signaling is mainly involved in the physiological functions of neurosynapses and germ cells. In contrast, intracellular zinc signaling is primarily divided into two parts, EZS and LZS, which exert biological functions by activating downstream pathways, such as inflammatory signaling. Interestingly, the endoplasmic reticulum releases zinc to generate a specific zinc wave, observed within several minutes after FcεRI stimulation in mast cells. EZS early zinc signaling, LZS late zinc signaling. Green dots represent zinc

Notably, the elevated intracellular zinc has a bidirectional effect. On the one hand, zinc participates in various cellular signaling pathways, contributing to processes such as cell proliferation and differentiation. 45 , 46 , 47 , 48 For example, zinc promotes embryonic central nervous system (CNS) development by affecting STAT1 and STAT3 signaling pathways. 49 Interestingly, it has been shown that zinc has a more significant role in hematopoiesis than iron, at least in early hematopoietic stem cells. 50 In immune function-related signaling, zinc enhances the development of regulatory T cells, as induced by the transcription factor Foxp3. 51 , 52 On the other hand, excessive intracellular zinc accumulation can lead to apoptosis. Mitochondrial-derived zinc accumulation can impair mitochondrial structure and function, negatively impacting animal development and longevity in Caenorhabditis elegans . 53 Studies have also demonstrated that intracellular zinc release might occur as a response to oxidative or nitrosative stress, which could lead to the release of zinc from MT, a zinc buffer protein, thereby promoting apoptotic processes. 54 , 55 Furthermore, In a specific cell death pathway, the release of zinc and calcium within neurons leads to the subsequent phosphorylation of the potassium channel Kv2.1. 56 , 57 In conclusion, despite low intracellular free zinc concentrations, intracellular zinc signaling plays a broad and vital role in physiological functions.

Extracellular zinc signaling

Extracellular zinc is a significant signaling mediator in endocrine, paracrine, and autocrine systems. 58 , 59 It serves as a ligand for various receptor channels on the plasma membrane, including the zinc sensing receptor (ZnR/GPR39) that regulates neuronal excitation, 60 N-methyl-D-aspartate (NMDA) receptors, 61 α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, 62 voltage-dependent calcium channels (VDCC), 63 and γ-aminobutyric acid A (GABAA) receptors. 64 , 65 The progress within cell biology and chemistry has emphasized the presence and function of free or labile zinc in cellular responses, especially its neurotransmitter role in synaptic vesicles. 29 , 66 , 67 Fluctuations within brain zinc concentrations, corresponding to physiological experiences and long-term memories, indicate that free zinc is strongly associated with neurotransmitter performance. 68 Moreover, zinc released from the synapse directly activates a G-protein coupled receptor (mZnR/GPR39), sensing changes in extracellular zinc concentration and consequently regulating neuronal excitation. 69

In addition, fertilized mammalian embryos release zinc sparks. 8 , 11 The exocytotically released zinc ions coordinate with cellular calcium transients, modifying the structure of the zona pellucida to prevent polyspermy (Fig. 1 ).

Zinc signaling and tumorigenesis

Under normal circumstances, zinc concentration meets the demands of bioenergetic, synthetic, and catabolic, essential for manifesting the cells’ current activities, e.g., function, growth, and proliferation. Several mechanisms explain the antitumor function of zinc, encompassing DNA damage, DNA repair, immune function, oxidative stress, and inflammation. 70 , 71 , 72 As cell activity changes, its metabolism must be adjusted to accommodate any newly established biological energy/synthetic/catabolic requirements. Changes in zinc concentrations beyond the cell’s ability to coordinate can lead to tumorigenesis, as zinc provides the bioenergetic/ synthetic requirements of malignancy, such as the aberrant expression of zinc transporters and dysregulation of MTs binding proteins. 73 , 74 , 75

Indeed, zinc activation of two mitogen-activated protein kinase (MAPK) pathways linked to tumorigenesis, namely extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase (JNK), 44 plays a significant role. These MAPKs, including ERK and JNK, are serine/threonine protein kinases that regulate cell proliferation, differentiation, and apoptosis in tumorigenesis. 76 Regarding the late zinc signaling, STAT3 stimulates the transcriptional activity of ZIP6 in zebrafish. 70 As a result, STAT3-dependent ZIP6 expression leads to downstream activation of the transcriptional repressor Snail, which contributes to the epithelial-mesenchymal transition (EMT) during embryonic development and is associated with tumor metastasis mechanisms (Fig. 1 ). Similarly, ZIP4 induces EMT-promoting migration and invasion through the PI3K/Akt signaling pathway in nasopharyngeal carcinoma (NPC). 77 Additionally, elevated expression of ZIP13 activates the Src/FAK pathway, leading to increased expression of pro-tumor metastatic genes but decreased expression of tumor suppressor genes in ovarian cancer. 73 Overall, cancer cells appear to require stimulation of oncogenic pathways by zinc to maintain their aggressiveness.

Obviously, cellular zinc signaling benefits from the storage and release of organelles and subcellular structures, which are precisely regulated by the zinc transporters and MTs. Thus, maintaining zinc homeostasis requires a complex intracellular collaboration of these functional proteins. Hypothetically, would normal cells transform into cancer if zinc homeostasis were disrupted? A plethora of studies have substantiated that dysregulation of zinc transporter proteins not only affects cell proliferation and apoptosis but also induces alterations in various signaling pathways, thus promoting cancer progression. 73 , 74 , 75 Remarkably, the lysosomal cation channel MCOLN1 has been identified as a crucial mediator of zinc influx into the cytoplasm, thereby finely controlling oncogenic autophagy in cancerous cells. 78 Additionally, alterations in zinc homeostasis have been shown to modulate the tumor immune microenvironment, exerting a significant influence on cancer progression. 79 Furthermore, the involvement of zinc in heavy metal detoxification implies that its disruption could adversely affect detoxification pathways, thereby leading to cellular stress and subsequent cancer development. 26 In conclusion, the intricate link between zinc homeostasis and cancer is an emergent field that warrants further exploration to fully elucidate the underlying mechanisms that govern the transition from disrupted zinc homeostasis to tumorigenesis.

Regulation of cellular zinc metabolism

The basic knowledge of zinc transporters.

The SLC39 family comprises four distinct groups based on amino acid sequence similarities: subfamily I (ZIP9); subfamily II (ZIP1, 2, and 3); the LIV-1 subfamily (ZIP4, 5, 6, 7, 8, 10, 12, 13, and 14); and the gufA subfamily containing ZIP11. 80 All ZIP proteins have eight transmembrane (TM) domains with conserved histidine residues within TM 4 and 5, believed to be involved in zinc transportation. The C-terminal and N-terminal ends of ZIP are located either on the cell surface or within the lumen of the organelle. 81 , 82 Members of the LIV-I family, with the exception of ZIP13, are anticipated to possess one significant, extracellular N-terminal domain, suggested to function as extracellular zinc sensors. Recently, research has provided insights into the detailed structure of ZIP transporters, including a high-resolution 3.05 Å cryo-electron microscopy structure of a ZIP-family transporter from Bordetella bronchiseptica acquired in an inward-facing, inhibited conformation. 83 Each protomer of this homodimeric transporter comprises nine transmembrane helices and three metal ions. In this architecture, two metal ions create a binuclear pore structure, and the third ion is located at an egress site facing the cytoplasm. Notably, this egress site is covered by a loop, with two histidine residues on this loop interacting with the egress-site ion, crucially regulating its release. Understanding the structure and function of ZIP transporters may offer valuable insights for developing new therapeutic strategies targeting zinc transporters to treat various human diseases.

The ZIPs are typically synthesized on ribosomes attached to the endoplasmic reticulum (ER) and later transported to various intracellular compartments. 84 Similar to other protein expressions, unstable ZIP mutant proteins are often identified in the ER. Subsequently, they undergo retro translocation and degradation by cytosolic proteasomes in a ubiquitin-independent manner, as seen in the case of ZIP13 mutant. 85 Apart from the intracellular localization of certain ZIP members, the majority of ZIP transporters are positioned on the plasma membrane, facilitating metal ion uptake into cells. ZIP7 is situated in the Golgi apparatus and ER, while ZIP13, evolutionarily closest to ZIP7, is localized in the Golgi apparatus and cytoplasmic vesicles. 85 , 86 ZIP13 is responsible for mobilizing zinc from the lumen of these compartments and plays crucial roles in cellular signaling, including the BMP/TGF-β signaling pathway, by regulating the nuclear translocation of Smad proteins and maintaining ER homeostasis.

The expression levels of numerous ZIPs, such as ZIP1, 3, 4, 8, and 12, at the cell surface, are modulated by the available concentrations of zinc. 80 ZIP10 serves as a cell surface zinc importer. 87 The transcription of ZIP10 is upregulated in zinc-depleted cells 88 and downregulated in zinc-excess conditions. The regulation of zinc transcription is mediated by pausing Pol II transcription through the action of metal response element-binding transcription factor-1 (MTF-1). Furthermore, the positioning of certain ZIP proteins varies with zinc supply and specific physiological states. During adequate zinc intake, Zip5 aligns at the basolateral plasma membrane in polarized cells. 89 In a parallel manner, ZIP14 moves to the mouse hepatocyte’s sinusoidal membrane during sharp inflammatory events. 90 As a result, this boosts zinc absorption as part of the immediate response to inflammation.

While ZIP members are known for their primary role in transporting zinc, they can also mobilize other metals such as manganese and cadmium. 91 , 92 , 93 , 94 Biochemical studies have shown that ZIP8, in particular, can transport cadmium and manganese. 95 , 96 , 97 The expression of ZIP8 mRNA is upregulated by cadmium in an NF-κB-dependent manner, contributing to the risk of cadmium-mediated lung toxicity exposed to cigarette smoke. 98 ZIP14 is evolutionarily closely related to ZIP8. 99 Similar to ZIP8, ZIP14 has the ability to mobilize various divalent cations, including cadmium and manganese. 100 Moreover, ZIP14 and ZIP8 are capable of transporting iron. 101 , 102 ZIP14 plays a crucial role as an iron transporter in vivo, especially under iron overload conditions. 103 ZIP14’s capability to transport non-transferrin-bound iron (NTBI) is considered a vital contribution to iron homeostasis. 100 Interestingly, ZIP14 possesses two spliced variants: ZIP14A and ZIP14B. These variants are present on the plasma membrane and are involved in zinc uptake. In polarized cells, ZIP14A and ZIP14B are exclusively located on the apical surface. 99

The ZnT family belongs to the cation diffusion facilitator (CDF) family of proteins. Most ZnTs are located within organellar membranes, serving various functions, such as filling vesicular zinc stores, supplying organelles with zinc, and loading exocytotic vesicles with zinc for essential biological processes. The structure of ZnT proteins is inferred from the Escherichia coli homologs of YiiP, 104 which have six TM helices (TM helices I-VI) and their N- and C-termini situated on the cytoplasmic side. 104 , 105 ZnT5, on the other hand, possesses an unusually long N-terminal region with nine putative TM domains. 106 ZnT transporters are also expected to contain a conserved zinc-binding site on TM helices II and V, with critical residues determining their metal specificity. 14 , 105 Remarkably, ZnT10 demonstrates the molecular features of a manganese transporter, likely attributed to its possession of an Asn residue rather than His in the TM helix II. 104 Furthermore, the length and amino acid sequence of the initial TM structural domain of ZnT proteins, known for containing subcellular targeting signals, display substantial variations among different ZnT proteins. Based on their protein sequence similarities, the ZnT family members can be categorized into four groups: (1) ZnT6 and ZnT9, (2) ZnT1 and ZnT10, (3) ZnT2-4 and ZnT8, and (4) ZnT5 and ZnT7. Intriguingly, members belonging to the same subfamily exhibit similar cellular locations and functional characteristics 1 (Fig. 2 ).

The protein structure and gene family evolution of zinc transporters. a Cartoon of predicted structures of ZnT and ZIP transporter proteins. The picture on the left shows an atomic model of ZnT, which is the helical reconstruction of YiiP based on X-ray structure (PDB ID code: 7y5g). In detail, the schematic topology of ZnT transporters is proposed based on the three-dimensional structure of Escherichia coli homolog YiiP. ZnTs most likely have six TM domains divided into two bundles. Specifically, one of the ZnT’s bundles contains four TM domains (MI, MII, MIV, and MV), and the other one comprises two TM domains (MIII and MVI). Each of the former bundle’s domains can independently bind zinc, tetrahedrally coordinated by two D (aspartate) and two H (histidine) in the mammalian homologs. Similarly, the figure on the right presents putative TM domains of the ZIP family (PDB ID code: 7z6m). Moreover, the topology structure of ZIP is displayed, composed of 8 TM domains with a large N-terminal domain and a small C-terminal. The spatial distribution shows that it consists of three parts, the left and right parts each contain three TM domains (red), and in the middle are two TM domains (blue). Zinc could bind to the active site of TM domain IV and V, containing conserved HND (histidine, asparagine, aspartate) and HEH (two histidines and one glutamic acid) motifs, respectively. b Gene family evolutionary tree and isoform of the ZnTs and ZIPs. The lengths of the different isoforms are labeled in the front of the isoforms, and the color lines indicate the functional domain locations of each isoform. ZnTs belong to the Zn-cation diffusion facilitator (CDF) family, responsible for transporting zinc from intracellular to extracellular. ZIPs are divided into four subfamilies, namely ZIP subfamily I (ZIP9), GufA subfamily (ZIP11), ZIP subfamily II (ZIP1-3), and LIV-1 subfamily (ZIP4-8, ZIP10, ZIP12-14)

Functionally, in the SLC30 family, ZnT1 functions primarily as a zinc exporter on the cell membrane, transporting cytoplasmic zinc ions across the membrane to the extracellular space, while other ZnT proteins are situated on the membranes of intracellular organelles. 107 Besides, ZIP10 and ZnT1 are involved in renal zinc reabsorption. 108 , 109 Members of the subfamily II of the SLC30 proteins (ZnT2, ZnT3, ZnT4, and ZnT8) play a major role in secretory tissues, with ZnT3 involved in neurotransmission, ZnT8 in insulin storage, ZnT4 in prostate secretion, and ZnT2 in lactation. 40 , 108 , 110 , 111 Besides, Additionally, TMEM163 is a recently discovered zinc transporter with a predicted transmembrane domain structure and function similar to the CDF protein superfamily. 112 Some posit that TMEM163 could be a novel member of the mammalian ZnT transporter proteins. 113 Recent discoveries indicate its significant role in maintaining zinc balance in both nerves and blood. 114 , 115 , 116

The basic knowledge of MTs

Mammalian MTs are a superfamily of nonenzymatic polypeptides that typically consist of 61–68 amino acids. 25 They are characterized by a high cysteine content, accounting for approximately 30% of their amino acids, while aromatic amino acids are absent, and histidine residues are sparsely present. However, they contain abundant thiol groups that enable them to bind to heavy metals. MTs, with their abundant thiol groups, have the capacity to bind up to 7 zinc atoms: 3 zinc atoms in the β domain and 4 zinc atoms in the α domain. 117 , 118 This unique capability enables MTs to function as a cellular zinc reserve. It is crucial to highlight that while MTs can bind other essential metals such as copper and nonessential metals like cadmium, the predominant form in human tissue is zinc-bound MT.

Human MTs can be classified into four classes, namely MT1 to MT4, comprising a total of eleven functional isoforms, with eight of them belonging to class 1. 3 MT1 and MT2 are the predominant isoforms distributed throughout the human body and expressed in various organs. Conversely, MT3 is predominantly present in the CNS, while MT4 is primarily found in the skin and other stratified epithelium, representing the minor isoforms. 119 All isoforms have an approximate molecular weight of 7 kDa and lack aromatic amino acids. Moreover, they consist of twenty cysteine residues, endowing MTs with distinctive characteristics due to the properties of thiol groups. 120 Additionally, the transcription of MT1/2 genes is governed by MTF-1, a zinc finger transcription factor that regulates the expression of metal-responsive genes. Zinc is notably the sole known metal to activate MTF-1; however, studies propose that oxidative stress might also contribute to MTF-1 activation. 121 MTF-1 is involved in regulating the zinc-responsive transcription of ZnT1 and ZnT2 and inhibiting the expression of ZIP10, 87 , 122 , 123 emphasizing its vital role in zinc homeostasis.

In humans, MTs are structurally encoded by a family of genes located on chromosome 16q13, comprising at least 11 functional members: the MT1 genes consist of 18 isoforms, including 10 functional genes ( MT1A, MT1B, MT1E, MT1F, MT1G, MT1H, MT1M, and MT1X ) and 8 pseudogenes ( MT1CP, MT1DP, MT1JP, MT1L, MT1LP, MT1XP1, MT1P3, and MT1P1 ), in addition to MT2 (also known as MT2A), MT3 , and MT4 55 , 119 (Fig. 3 ). Remarkably, as the zinc store, MT can act as both zinc receptor and zinc donor, like two sides of the same coin. 118 , 124

The protein structure and gene family evolution of MTs. a Diagram of the predicted structure of the MT2 protein, which is modeled from the reconstructed X-ray structure (PDB ID code: 4mt2). The crystallographic structure of rat liver metallothionein has been accurately determined at a resolution of 2.0 Å, achieving a low R-value of 0.176 for all observed data. b Schematic representation of zinc binding in MTs. MTs contain abundant thiol groups capable of binding with heavy metals. Due to the high thiol content, MTs can bind up to 7 zinc atoms, with 3 zinc atoms located in the β domain and 4 zinc atoms in the α domain. c Gene family evolutionary tree and isoforms of MTs are depicted in Figure X. The isoform lengths are labeled in front of each isoform, and color lines indicate the position of the functional domain of each isoform. MTs are categorized into four subfamilies: MT1 (including MT1A , MT1B , MT1E , MT1F , MT1G , MT1H , MT1M , and MT1X ), MT2 (including MT2A ), MT3, and MT4. While MT1 and MT2 are universally expressed, MT3 is primarily expressed in the central nervous system, and MT4 is predominantly expressed in the skin and other stratified epithelium tissues

Role of cellular zinc metabolism under physiological conditions

The physiological role of zinc transporters, supporting immune function.

T cells are a critical component of the immune system. 125 Among the 14 ZIP family members, ZIP6, 8, and 13 are highly expressed in human CD4 + T cells, with ZIP6 predominantly localized to lipid rafts involved in the immune synapse (IS) formation following T cell receptor (TCR) stimulation. 126 Notably, the tyrosine phosphorylation of ZIP6 was observed to increase after five minutes of TCR stimulation due to its interaction with Zap70, a crucial kinase involved in early TCR signaling. In addition, the transcriptional activity of ZIP6 leads to zinc influx, promoting the expression of MTs, which plays a crucial role in supporting T cell proliferation and is essential for T cell survival and expansion in the elderly. 127 , 128 ZIP8 and ZIP13 are primarily expressed on the lysosome and ER/Golgi membrane of T cells, respectively. 86 , 129 During T cell activation, ZIP8 facilitates zinc transport from the lysosome to the cytoplasm, resulting in increased production of IFN-γ. Notably, ZIP8 expression can be induced in response to lipopolysaccharide (LPS) stimulation, 130 , 131 leading to enhanced IL-1β production downstream of the mTORC1S6K pathway. 132 Moreover, ZIP8 is a downstream target gene of NF-κB, which negatively regulates pro-inflammatory responses through zinc-mediated downregulation of Iκκ activity. 130 Comparatively, the deficiency of ZIP8 has a substantial impact on zinc influx in effector T cells and results in reduced TCR-mediated signaling, including NF-κB and MAPK signaling, which are involved in the differentiation of T helper (Th)17 cells. 133 Similarly, mice lacking ZIP3 exhibit decreased CD4 + CD8 + double-positive (DP) thymocytes but increased CD4 + and CD8 + single-positive thymocytes, indicating its role in regulating T cell development. 134 These findings open up new possibilities for immunotherapy to improve the prognosis by modulating the zinc transporter family genes on tumors or immune cells.

Undoubtedly, the adaptive branch of the immune system relies on both B cells and T cells. 135 ZIP9 and ZIP10 play essential roles in B cell receptor signaling pathways, influencing B cell activation 136 , 137 (Fig. 4 ). The release of zinc in B cells originates from the Golgi apparatus, with ZIP9 playing a crucial role as the zinc transport participant. 136 ZIP10, on the other hand, plays different roles in the early and late stages of B cell development, regulating distinct signaling cascades. The expression of ZIP10 is mechanistically regulated in a STAT3/STAT5-dependent manner, promoting early B cell survival by inhibiting caspase activation. 137 Additionally, ZIP10 deficiency in mature B cells has been shown to attenuate both T cell-dependent and -independent immune responses in vivo. 138 ZIP10 functions as a positive regulator of CD45R in B cell antigen receptor signaling transduction, playing a crucial role in setting a threshold for human immune responses. In hepatocellular carcinoma (HCC) cell lines, ZIP10 expression was found to be positively correlated with tumor-infiltrating lymphocytes and certain immune checkpoints, including CTLA4, TIM3, and TGFB1. 139 Moreover, ZIP10 is essential for zinc homeostasis within macrophages, where zinc is involved in antimicrobial responses. 140 activated macrophages, while crucial for immune responses, can also release large quantities of inflammatory cytokines, which may have the potential to harm the host. 141 ZIP10 was identified as a significant zinc importer in macrophages that activates macrophages and promotes cytokine expression. 142 Zinc deficiency (ZD) caused by knocking down ZIP10 leads to cytoplasmic p53 accumulation and nuclear translocation of AIF, ultimately triggering apoptosis. 142 Thus, targeting ZIP10 could be a promising approach to protect the liver from inflammation damage.

The main physiological functions of zinc transporters. The zinc transporter functions are basically classified into six parts: immunity, reproduction, muscle, intestinal function, glycolipid metabolism, and neuron function. Further, the function represented by each sector is mainly divided into three parts. Each part corresponds to specific zinc transporters. The outermost circle represents diseases and cancers caused by malfunctioning zinc transporters. SCD-EDS spondylocheirodysplastic ehlers-danlos syndrome, AE acrodermatitis enteropathica, IBD inflammatory bowel disease

Notably, sepsis is an acute systemic infection triggered by the invasion of pathogenic bacteria into the blood circulation and the production of toxins. 143 Circulating zinc levels lower than expected have been linked to high mortality in sepsis patients, with MT and ZIP8 identified as two of the most highly upregulated genes in non-survivors. 144 ZIP8, in particular, has been found to be the most significantly upregulated transporter in response to cytokines, bacteria, and sepsis, indicating its unique role in innate immune function. 130 , 145 , 146 As the closest homolog of ZIP8, ZIP14 also participates in response to sepsis and is implicated in the beneficial anti-inflammatory effects of supplemental dietary zinc during sepsis, indicating its potential as a therapeutic target. 147 Additionally, the existence of “zinc waves” in mast cells provides further evidence of the involvement of zinc transporters in immune functions. 44 The release of zinc from the ER is likely mediated by ZIP7, as ZIP7 predominantly resides in the ER, and silencing ZIP using siRNA prevented the occurrence of the zinc wave. 148 , 149 Besides, ZnT1/L-type voltage-gated calcium channels (LTCCs) also contribute to the zinc wave, which interacts with ZnT1 and modulates the zinc influx from extracellular space into the cytoplasm. 150 , 151 , 152

Assistance of reproduction

During meiotic maturation, total intracellular zinc increased by ~50%. After fertilization, zinc-rich oocytes induced zinc sparks, which decreased zinc concentration by approximately 20%. The role of zinc sparks requires further investigation, but some evidence suggests that these changes in zinc levels are crucial for subsequent developmental steps and may play a role in zinc-dependent processes regulating oocyte exit from meiosis I. 11 , 153 The ZIP transporter family is believed to regulate zinc influx, and ZIP6 and ZIP10, which share 43.5% sequence identity and are on the same clade of the ZIP family phylogenetic tree, 154 are highly expressed in the oocyte during the window of meiotic maturation 155 (Fig. 4 ). The ZIP6/ZIP10 heteromer is also critical for triggering zinc-mediated mitosis, 156 forming a zinc-dependent mitotic complex consisting of ZIP6, ZIP10, pS 727 STAT3, and pS 38 Stathmin, which play roles in proven mitotic pathways. As an illustration, they are involved in processes like stathmin- reliant microtubule reorganization or HistoneH3-mediated chromatin condensation. In order to stabilize pS 38 Stathmin throughout mitosis, STAT3 serves as an effector of ZIP6/ZIP10 heteromer regulating the expression of both genes. 137 , 157 Zinc levels are often higher in cancer tissues than normal tissues, possibly due to the increased demand for tumor growth. 158 In addition to using zinc chelators to inhibit the proliferative growth of cancer tissues, 78 , 159 , 160 , 161 another potential approach is to use ZIP6 or ZIP10-blocking antibodies to hinder mitosis in cancer progression.

Maintenance of muscle function

Research indicates that approximately 90% of zinc in the body is found in tissues with slow zinc metabolism, such as skeletal muscle and bone. 162 Zinc plays a vital role in stabilizing insulin, resulting in a synergistic effect on insulin stimulation of muscle cells. 163 , 164 On the other hand, nutritional ZD can hinder skeletal muscle growth, repair, and myoblast differentiation. 165 , 166 , 167 ZIP7, known as the zinc “gatekeeper”, is localized on the ER and Golgi membrane. It has been extensively studied for its role in skeletal muscle differentiation and the regulation of glucose metabolism 168 (Fig. 4 ). The localization of zinc in myoblasts and differentiated myotubes was found to correlate with the changing localization of ZIP7. 169 Silencing ZIP7 significantly reduces intracellular zinc levels and inhibits Akt phosphorylation, resulting in a decreased number of differentiated cells, even in the presence of extracellular zinc. 170

Similarly, in myoblasts, knocking down ZIP8 also hampers myotube formation by causing a significant reduction in cellular manganese, iron, zinc, and calcium levels, leading to decreased differentiation and proliferation of myoblasts 171 (Fig. 4 ). In comparison, ZIP13 plays crucial roles in the development of bone, tooth, and connective tissue. Mutations in ZIP13 have been linked to the spondylocheiro dysplastic form of Ehlers-Danlos syndrome (SCD-EDS), 172 , 173 characterized by abnormalities in hard and connective tissues. ZIP13 knockout mice exhibit delayed growth and skeletal and connective tissue abnormalities, mirroring the phenotypes observed in SCD-EDS patients. 173

Furthermore, zinc transporters play a direct role in regulating calcium channels, modulating calcium signaling, and subsequently influencing muscle contraction (Fig. 4 ). For instance, the interaction of ZnT1 with LTCCs enables zinc entry from the extracellular space into the cell membrane, thereby contributing to calcium signaling involved in excitation-contraction coupling in skeletal muscle. Additionally, ZnT1 directly inhibits the activity of L-type calcium channels by binding directly to the β-subunit, Ca v β. 151 ZIP7 and ZnT7 are involved in regulating the release of zinc into the sarcoplasmic reticulum (SR) in skeletal muscle. Intracellular zinc can then modulate ryanodine receptor (RyR)-mediated calcium release from the SR. Notably, the cytoplasmic C-terminal tail of ZnT1 alone can inhibit the channel, suggesting that the inhibition of L-type calcium channels by ZnT1 is independent of zinc channel function. 174

Regulating gastrointestinal (GI) function

The dietary complex releases zinc, which is primarily absorbed by enterocytes in the upper part of the small intestine. The luminal surface cells of the intestinal epithelium originate from intestinal stem cells (iSCs) and comprise various cell types, including enterocytes, goblet cells, enteroendocrine cells, tuft cells, and Paneth cells. These cells express members of both the ZIP and ZnT families involved in zinc transport. 175 ZIP4 is particularly important for zinc uptake and is closely related to the process. Loss of ZIP4 during embryonic development leads to lethality 176 (Fig. 4 ). Previous research has established that ZIP4 is predominantly localized to the apical brush border of enterocytes, facilitating zinc uptake from the intestinal lumen. Furthermore, the expression of ZIP4 is regulated through proteolytic processes that respond to changes in the zinc concentration within enterocytes. 177 , 178 Mutations in ZIP4 can lead to acrodermatitis enteropathica, a rare autosomal recessive metabolic disorder characterized by ZD, commonly observed in infants. 179 , 180 In the case of ZD, ZIP4 is translocated to the apical surface of the small intestinal epithelial cells. However, when zinc levels are adequate, the mRNA of ZIP4 becomes unstable, and the protein is internalized and quickly degraded. 181 Intestinal ZnT1 plays a crucial role in zinc acquisition and processing. It is highly expressed in the epithelium of the esophagus, duodenum of the small intestine, and cecum of the large intestine, suggesting its involvement in zinc efflux and absorption into the systemic circulation. 182 Remarkably, the expression of ZnT1 is influenced by dietary zinc supplementation. Upon zinc supplementation, there is an increase in ZnT1 mRNA expression. 183 As a result, both ZIP4 and ZnT1 play vital roles in regulating zinc intake.

Zinc plays a vital role in maintaining the homeostasis of intestinal epithelial cells, and its deficiency can lead to alterations in their integrity and function. 184 Zinc transporters play a significant role in regulating cellular function to support intestinal epithelial homeostasis. Among them, ZnT2 has been proven to be mainly expressed in Paneth cells, which are located within Lieberkühn crypts. 185 In these specialized secretory cells, ZnT2-mediated zinc absorption into intracellular vesicles is crucial for controlling cytoplasmic zinc levels and cellular function. 185 , 186 ZIP4, as mentioned earlier, is important for zinc uptake in the intestine and is essential for the differentiation and maintenance of Paneth cells. 176 Additionally, ZIP4 also contributes to the proliferation of intestinal epithelial cells. 176 Mice lacking ZIP4 exhibit disrupted villus integrity, highlighting the significance of ZIP4 in preserving the architecture of the intestinal epithelium. ZIP7, localized to the ER, is also highly expressed in the intestinal crypts. 108 , 187 Furthermore, the deletion of ZIP7 greatly enhances the ER stress response of proliferating progenitor cells, leading to apoptosis and disrupting intestinal epithelial cell proliferation and dryness. Indeed, the findings indicate that ZIP7 plays a vital role in promoting both the proliferation and maintenance of stemness in intestinal epithelial cells. 187

Recent studies have suggested that zinc plays a crucial role in preserving the integrity of mucosal barriers, which is linked to the immunological responses of gastrointestinal diseases in the mucosa 188 , 189 , 190 (Fig. 4 ). ZIP14, found at the basolateral membrane of enterocytes along the villus, is particularly abundant in the proximal region of the small intestine. 191 Deletion of ZIP14 in the intestine has been shown to result in compromised barrier function. 101 The reason is that ZIP14 maintains the intestinal barrier by stabilizing occludin’s phosphorylation, known as a tight junction protein. Studies have revealed that mice lacking ZIP14 display a disruption in the tight junction complex and increased permeability, potentially due to impaired zinc-dependent activation of ZnR/GPR39. The absence of ZIP14 in mice results in reduced zinc transport into enterocytes, which in turn results in a range of pathologies. These include reduced intestinal barrier function, adiposity, muscle wasting, impaired glucose processing, and skeletal defects that manifest with aging. 191 , 192 , 193

In the small intestine, ZnT2 assumes a vital function in cytoplasmic zinc buffering, which is essential to Toll-like receptor 4 (TLR4) expression, initiation of pathogen-activated NF-κβ translocation, in addition to the release of cytokine in response to infectious challenges. 185 , 194 Furthermore, ZnT2 is indispensable for the development of lysosome biogenesis and bacterial-stimulated autophagy, 195 facilitating a powerful host defense and resolution machinery against enteric pathogens. In conclusion, this evidence suggests ZnT2 serves as an innovative modulator for mucosal inflammation in colonic cells and plays a crucial role in coping with infectious colitis, opening up possibilities on manipulating ZnT2 as a novel treatment strategy to particular intestinal infections. 194 ZIP8 is crucial for T cell activation, and recent studies have highlighted its significance in T cell function and innate immunity, which may have important implications in the context of inflammatory bowel disease (IBD). 145 , 196 In a study by Li et al., a novel association between Crohn’s disease (CD) and ZIP8 was identified. 197 Healthy carriers of the ZIP8 variant exhibited changes in intestinal microbiota that partially overlapped with those observed in CD patients. This suggests that disturbances in zinc homeostasis could be linked to ecological imbalances in the gut, potentially contributing to the pathophysiology of CD.

Maintaining neuron functions

As a neuromodulator, zinc is crucial in managing diverse synaptic transmissions, such as glutamatergic, GABAergic, and glycinergic. 61 , 198 , 199 In addition, it modulates both short-term and long-term synaptic plasticity, enhances auditory processing, and refines sensory stimulus discrimination. 200 , 201 , 202 , 203 , 204 Following physiological activity, vesicular zinc is released and modulates neurotransmission by interfacing with postsynaptic neurotransmitter receptors and activating mZnR/GPR39 signaling. 199 , 205

So far, the specific functions of zinc transporters have been described in the brain (Fig. 4 ). ZnT3, a membrane zinc transporter responsible for concentrating zinc into neuronal presynaptic vesicles and co-released with glutamate upon depolarization, is pivotal in maintaining neuron functions. 206 , 207 , 208 ZnT3 exhibits predominant expression in the brain, particularly in key regions such as the hippocampus, amygdala, and cerebral cortex. 206 In various brain areas, including the cerebral cortex, hippocampus, amygdala, and dorsal cochlear nucleus (DCN), the transporter is abundantly present in excitatory neurons, playing a crucial role in channeling zinc into presynaptic vesicles. 209 Upon synaptic activity, vesicular zinc is released from terminals enriched with ZnT3 and diffuses across the synaptic cleft 61 to modulate multiple postsynaptic receptors, 199 , 210 l including the zinc-sensitive N-methyl-d-aspartate receptor (NMDAR). 61 The deletion of ZnT3 leads to the suppression of Erk1/2 signaling in MF terminals, resulting in the release of MAPK phosphatase and impairing hippocampus-dependent memory processes. 211

ZnT1, another zinc transporter, has been suggested to interact with NMDA receptors at synapses. 212 ZnT1 specifically associates with the C-tail of the NMDAR GluN2A subunit. This ZnT1/GluN2A complex may be influenced by synaptic plasticity, and disruptions in ZnT1 expression led to significant changes in dendritic spine morphology. 61 The primary targets of the released zinc are NMDARs containing GluN2A, which are responsive to nanomolar levels of extracellular zinc, thereby inhibiting receptor function. 213 Moreover, ZIP12, exclusively expressed in the CNS, plays a vital role in neuronal differentiation, including tubulin polymerization and neurite extension, by facilitating zinc uptake into the cytosol. 214 , 215 Excessive expression of ZIP12 has been observed in schizophrenia. 216

Additionally, different neuronal populations within the hippocampus express the plasma membrane zinc transporters ZIP1 and ZIP3. While ZIP1 controls the influx of zinc into postsynaptic cells, ZIP3 manages the re-uptake of zinc into dentate granule cells. 217 SHANK3, a critical scaffold protein in the PSD of excitatory glutamatergic synapses, is sensitive to changes in zinc concentrations. ZIP4 is found in the postsynaptic region and interacts with HOMER1 and SHANK3. 218

Furthermore, mutations in ZIP8 have been frequently reported in relation to the development of schizophrenia. Genome-wide association studies (GWAS) have indicated that a specific variant of the zinc transporter ZIP8 is significantly linked to the risk of schizophrenia and Parkinson’s disease (PD). 219 Severe homozygous loss-of-function mutations in ZIP8 lead to a type-II congenital disorder of glycosylation, increasing the risk of schizophrenia. 93 , 220 Furthermore, ZIP8 hypofunction may contribute to psychiatric risk by causing glutamate receptor hypofunction and heightened inflammation. As a result, selectively enhancing glutamate function and targeting anti-inflammatory mechanisms could be beneficial for schizophrenia patients with ZIP8 hypofunction. 221 , 222 In conclusion, zinc transporters are essential in neuronal cells to maintain neurological function primarily by keeping intracellular zinc ion homeostasis.

Involving in glucolipid metabolism

Zinc’s role in insulin crystal formation is widely recognized, with insulin crystallizing in hexamers when two or more zinc atoms are present. 223 Notably, systemic zinc dysregulation has been demonstrated in both type 1 and type 2 diabetes 224 (Fig. 4 ). Pancreatic β-cells, in particular, have elevated zinc concentrations compared to other cell types. 105 Therefore, if pancreatic β-cells maintain adequate zinc concentrations, the activation of zinc transporters is required.

ZnT8, found in β-islet cells, stands as the most extensively studied zinc transporter involved in insulin formation and secretion. 225 , 226 , 227 Particularly, the C variant of ZnT8 at single nucleotide polymorphism (SNP) rs13266634 has shown enrichment in individuals with type 2 diabetes, implying its potential influence on diabetes risk. 228 Notably, polymorphisms in ZnT8 are associated with both type 1 and type 2 diabetes mellitus. 229 , 230 , 231 Furthermore, ZnT8 autoantibodies are detected in approximately 60–80% of new cases that are clinically confirmed as being affected by type 1 diabetes within the patient population. 232 When combined with the preexisting detection markers such as protein tyrosine phosphatase IA2, the detection of type 1 diabetes-associated autoimmune responses increases to 98% at the onset. 233 Interestingly, a distinctive connection between ZnT3 and ZnT8 gene expression in insulin-secreting INS-1E cells has been observed. Conditions that cause an up-regulation of ZnT3 expression, such as high glucose concentration or DEDTC treatment, lead to a down-regulation of ZnT8 expression. 234 Conversely, knock-down of ZnT3 results in an up-regulation of ZnT8 expression, and vice versa. 235 Additionally, β-cells express ZIP4, ZIP6, and ZIP7, which play a role in zinc uptake into β-cells, essential for proper insulin packaging, 236 , 237 , 238 which is required for the proper insulin packaging (Fig. 4 ).

Currently, the majority of studies have focused on β cells, with only a limited number of studies involving α cells. α cells are responsible for secreting the hormone glucagon, which is essential for the regulation and control of hypoglycemia in the body’s metabolic system, and zinc plays a crucial role as a signal molecule in glucagon secretion. Interestingly, overexpression of ZnT8 in α cells leads to the inhibition of glucagon secretion, which may hold potential benefits for T2D. 239 Researchers have examined the expression of zinc transporters using fluorescent measurements. 238 ZIP1 and ZIP14 were found to be the most abundant influx transporters in pancreatic α cells, while ZnT4, ZnT5, and ZnT8 were the dominant efflux transporters.

Besides, zinc has been demonstrated to exert an insulin-mimetic effect on target organs, including adipocytes. 240 Specifically, it stimulates lipogenesis in fat cells, even in the absence of insulin. Among all zinc transporter functions in lipid metabolism, the role of ZIP13 serving in adipocyte browning has attracted much attention in recent years. 241 The browning of adipocytes means converting white adipocytes that store energy into beige adipocytes, the energy-consuming brown adipocytes. Fat atrophy is reported in patients with Ehers-Danlos syndrome with mutations in ZIP13 function loss. 173 Furthermore, ZIP13 has been established as a significant regulator of beige adipocyte differentiation, and it negatively regulates C/EBP-β protein levels. This suggests the physiological importance of the ZIP13-C/EBP-β axis in beige adipocyte biogenesis and thermogenesis, and also highlights its potential in obesity treatment. 242 Above all, abnormal glucolipid metabolism is not only contributing to the process of diabetes and obesity, but also involved in carcinogenesis, 243 , 244 suggesting the unique function of zinc transporters both in clinical and preclinical investigations.

The physiological role of MTs

Involvement in cell proliferation, differentiation, and apoptosis.

Numerous studies have demonstrated that MTs regulate zinc, notably in relation to cell cycle regulation and cell proliferation. 245 MT predominantly resides in the cytoplasm. 246 Its peak concentration appears during the late G1 and G1/S cell cycle stages. 247 The nucleus uptake of MTs may be linked to safeguarding cells from DNA damage, apoptosis, and gene transcription through various cell cycle phases. 248 , 249

Additionally, MT serves as a donor of zinc to an array of metalloproteins and transcription factors. 250 DNA-binding proteins featuring zinc finger domains are pivotal in orchestrating DNA transcription processes. The central domain of p53 contains a zinc finger motif, which relies on zinc for structural stability. Apothionein, also known as zinc-free MT, has the ability to remove zinc from p53, leading to a reduction in its transcriptional activity and subsequently suppressing its DNA binding capabilities. 251 Analogous interactions are observed with the p50 subunit of NF-κB, where MT plays a role in stabilizing the p50-DNA complex. Such interplays have been widely reported for other transcription factors, including Sp1 and TFIIIA. 252 , 253 Evidence suggests that MT can modulate cellular activity through the regulation of Zinc. For instance, the protein Bmi1, a member of the Polycomb group (PcG), serves as a crucial epigenetic modulator of stem cell behavior, including aspects like differentiation and self-renewal, throughout both typical maturation and in advanced organ systems. 254 MT1 plays a facilitating role in this modulation by enhancing resistance, particularly by improving the cellular capacity to combat oxidative stress encountered in their microenvironment, within the satellite cell clusters. 255 It is worth noting that in DCs treated with zinc chloride (ZnCl 2 ), MT1 insufficiency fails to promote a regulatory phenotype specifically aimed at modulating T cell behavior or stimulate the proliferation, such as active growth, of FoxP3 + T cells. 256 , 257 Besides, MT3’s important contribution to osteoblast differentiation is by counteracting oxidative stress. 258 Its inhibition of 3T3-L1 adipocyte differentiation is an indirect function, involving the suppression of PPARγ transcriptional activity and a decrease in reactive oxygen species (ROS) levels during early adipogenesis. This indicates that MT3 could be a new target for obesity prevention and treatment.

Furthermore, MTs have also been found to be involved in apoptosis. Recent research has identified XAF1 as a suppressor of MT2A, promoting apoptosis in cellular responses to heavy metals. 259 XAF1, an exclusive transcriptional target of MTF-1 involved in apoptotic signaling, opposes the survival effects of MT2A, which is also regulated by MTF-1. 259 Therefore, the induction of XAF1 by heavy metals leads to an apoptotic shift in the stress response by destabilizing MT2A. Additionally, MT mitigates nitrosative damage and cell death caused by angiotensin II (Ang II)-induced NOX. 260 More specifically, MT2A functions as an anti-apoptotic protein by reducing the expression of caspase-3, caspase-9, caspase-12, and BAX. 261 In addition, MT2A shields against cardiac failure induced by ER stress by reducing myocardial apoptosis.

Maintaining the redox balance

Oxidative stress is defined by an imbalance between oxidants and antioxidants, which arises from the excessive generation of ROS and a decrease in the rate of their elimination by the antioxidant defense system. 262 The excessive production of ROS, including superoxide, hydrogen peroxide (H 2 O 2 ), hydroxyl radicals (·OH), and NADPH-oxidase (NOX), combined with reduced antioxidant capacity, contributes to a pathological imbalance that leads to oxidative stress and inflammation. 121 Further, this condition would cause cellular and tissue damage, eventually leading to chronic illnesses such as obesity, diabetes, and cancer. 263 , 264

Apart from intracellular antioxidants like glutathione (GSH), heme oxygenase-1 (HO-1), superoxide dismutase-1, and nicotinamide adenine dinucleotide phosphate (NAPDH), MTs also serve as a redox buffer by interacting with and scavenging reactive species. 265 , 266 Additionally, as a key source of intracellular zinc, MTs play a vital role in the catalytic activation and structural stability of metalloenzymes. 19 , 267 Notably, it aids in the structural stability of nitric oxide synthase (NOS), 268 MMP-9, 269 and superoxide dismutase (Cu/Zn SOD). 270 Moreover, MTs become particularly active when the presence of the reduced GSH form is blocked. 271 , 272 In this condition, MTs effectively neutralize free radicals using the Zn-MT redox mechanism. MTs contribute to a new pool of thiol in the cell cytosol, mitigating the detrimental effects induced by GSH depletors. 273 They scavenge ROS through thiol groups present in cysteine residues, displaying stronger antioxidative activity than the majority of well-known antioxidants. 255 , 274 Remarkably, MT2A exhibits a 100-fold greater capacity to scavenge free •OH and peroxyl radicals when compared to GSH. In response to oxidative stress, the expressions of MT2A and HO-1 are heightened due to ROS. 275 MTs also modulate the phosphorylation of ERK and regulate ROS through HO-1. 276 The potency of MT3 in eliminating ROS has been notably linked to its metal-binding affinity. 277

MTs’ expression is subject to dynamic regulation by both oxidative stress and cellular zinc levels. 270 , 278 , 279 Under oxidative stress, disulfide bonds are formed, leading to the release of bound metals, particularly zinc, from MTs. While zinc lacks inherent redox capacity, it is regarded as a powerful and crucial antioxidant agent. 279 , 280 Several studies have linked cellular zinc depletion to elevated oxidant levels and oxidation parameters. Zinc’s antioxidant properties arise from its direct and indirect interference with target structures. 281 These functions comprise the induction of MT expression and GSH synthesis, regulation of oxidant production, association with cysteines (alongside release by other oxidants), and modulation of redox signaling. Typically, MT is found in the cytoplasm, but it can also translocate into the nucleus to safeguard DNA from damage and interact with transcription factors, which will be further elaborated on later. 34

In addition, MT1 and MT2 have differential effects on ROS levels in various organs and tissues. Transcriptionally induced MT1/2 strengthens the liver’s defense system against alcoholic toxicity by reducing ROS and inflammation. 282 Moreover, IL-22Fc induces MTs in the liver, resulting in decreased hepatic ROS production, stress kinase activation, and inflammatory functions, leading to the amelioration of nonalcoholic steatohepatitis. 283 MTs play a crucial part in the antioxidative effects of D609, a compound that safeguards RPE cells from oxidative cell death induced by sodium iodate (SI). 284 Dysregulated MT expression in ascending aortic smooth muscle cells from patients with bicuspid aortic valve (BAV) might lead to an insufficient response to oxidative stress, potentially triggering aneurysm formation. 285 Recently, MT3 has shown promise for future translational medicine research in osteogenesis due to its effective ROS elimination capabilities. 258

Besides, the transcription factor MTF-1 enhances cellular protection against oxidative stress, as it responds to alterations in the cell’s redox status. 286 Specifically, MTF-1 triggers the expression of the Selenoprotein 1 ( Sepw1 ) gene, responsible for encoding an antioxidant GSH-binding protein that effectively scavenges free radicals. 45 Furthermore, MTF1 can be activated by Sirt6, providing liver protection against alcohol-related liver disease. 282

Orchestrating inflammatory reactions

Extensive research has explored the implications of MTs in inflammation. As mentioned previously, oxidative stress acts as a potent catalyst for releasing inflammatory cytokines, 287 whereas MT1/2 effectively inhibits the activation of pro-inflammatory cytokines like IL-6, IL-12, and TNF-α. 288 Studies have demonstrated that bacterial endotoxin LPS acutely induces MT1 expression in various organs, such as liver, heart, kidney, and brain tissues involved in systemic response. 289 , 290 , 291 In the cellular environment of Histoplasma capsulatum-infected macrophages, the concentrations of MT1 and MT2 expression are regulated by the activation of STAT3 and STAT5 signaling pathways, which are also involved in zinc import, thereby regulating ZIP2. 292 Liu et al.‘s research revealed that MT2 knockdown increases LPS-induced IL-6 production in endothelial cells, 293 indicating a protective role against inflammatory responses. Similarly, the absence of MT1/2 significantly exacerbates renal oxidative damage and inflammation induced by intermittent hypoxia, with the Nrf2 signaling pathway implicated. 294

NF-κB, a crucial inflammation-associated transcription factor, mediates MT1 gene expression. 295 , 296 Restoring MT1 expression in cells lacking MT results in the recovery of NF-κB p65 subunit levels, along with a subsequent increase in NF-κB activity related to cellular signaling, and improved protection against apoptosis. These findings indicate that MT1 plays a significant role as a positive regulator of NF-κB activity. 297 In contrast, MT2A regulates the cell’s inflammatory response by inhibiting NF-κB and endothelial-overexpressed LPS-associated factor-1 (EOLA1). 293 The increased MT2 expression has demonstrated the ability to reduce NF-κB activity in tumor cells, keloid fibroblasts, and cardiomyocytes. 298 , 299 , 300 Furthermore, zinc functions as a robust and selective suppressor of IFN-λ3 signaling, resulting in elevated MT levels. 301

To summarize, MTs possess a wide-ranging and complex ability to regulate inflammatory responses. They serve crucial functions in maintaining a balance by restraining the release of pro-inflammatory cytokines and managing oxidative stress. MTs also influence inflammatory reactions through their impact on essential signal transduction pathways and the expression of diverse transcription factors. The intricate interplay between MTs and crucial elements like zinc forms a complex network of protective mechanisms.

Facilitating detoxification of metals

MTs are not only involved in the regulation of zinc homeostasis but also play significant roles in heavy metal detoxification, particularly for cadmium and arsenic. 302 , 303 Cadmium, listed as one of the most hazardous substances for human health, accumulates in various organs causing severe oxidative stress and other adverse effects. The protective role of MTs against cadmium toxicity becomes particularly notable here. Exposure to cadmium can displace zinc from MTs and other proteins, leading to an elevation in cytoplasmic zinc levels. This in turn activates MTF-1, inducing MT overexpression. 304 Interestingly, the cadmium/zinc quotient in MTs determines the level of protection offered to cells against cadmium toxicity. With a lower cadmium/zinc quotient, cells are more protected, while an increased quotient reduces this protection due to the decreased availability of zinc sites for cadmium interaction. 305 The effectiveness of this protection mechanism was vividly demonstrated in a study conducted among individuals living in a cadmium-contaminated area in China. The study found that individuals with a good zinc status had a notably lower prevalence of renal tubular dysfunction when compared to those who had lower levels of serum and hair zinc. 306

Exposure to arsenic can result in toxicity, primarily caused by the generation of reactive oxygen intermediates during its redox cycling and metabolic activation. 307 Zinc acts as a vital safeguard against acute arsenic toxicity through two distinct protective mechanisms: restoration of antioxidant activity and increased expression of MTs. 303 The enhancement of metal response element (MRE) and antioxidant response element (ARE) activation, facilitated by essential nutrients like zinc, holds the potential to be beneficial in reducing arsenic toxicity. These elements are crucial as they can transcribe the expression of MTs, particularly by minimizing ROS-mediated cytotoxicity, thus adding another layer of protection against arsenic’s harmful effects. 308 Thus, the multifaceted relationship between MTs and zinc contributes to both heavy metal detoxification and zinc metabolism. Their cooperative function safeguards cellular integrity against the toxicity of heavy metals.

Cellular zinc metabolism in tumorigenesis

As previously mentioned, there exists a correlation between changes in zinc levels and cancer progression. However, it is essential to acknowledge that the nature of this correlation may vary among various kinds of cancer. Multifaceted effects of zinc in promoting or inhibiting tumor growth underscores this complexity, with distinct mechanisms operating in various cancer types. Recent evidence has been accumulating, suggesting a link between ZD and the development of cancers. Numerous processes are involved in zinc’s anti-tumor activity, encompassing DNA damage and repair, oxygenation, immunity, and the inflammatory process. 45 , 51 , 309 , 310 , 311 Yet it is important to note an increased level of zinc concentration has also allowed for an improved rate of cancer. 312 , 313 Since zinc is always characterized by playing a crucial role in growth arrest after the first meiotic division, 153 , 314 it also contributes to the proliferation of cancer cells. Furthermore, zinc regulation towards cancer heavily relies on the involvement of zinc transporters. Abnormal expression of these two families is primarily a result of gene dysregulation and translocation from organelles, which result in tumorigenesis mainly through two ways, the regulation of downstream molecular targets and the unsteady state of zinc homeostasis. 315 Based on this point, we summarized several cancer types whose development is strongly associated with zinc transporters.

Breast cancer (BC)

Studies have reported that BCs, along with malignant cell lines, exhibit a higher accumulation of zinc in contrast to normal mammary epithelium. 316 , 317 Moreover, the degree of zinc accumulation has been linked to cancer progression and malignancy. 318 , 319 ZIP6 (also known as LIV-1), was initially recognized as an estrogen-mediated gene since 1988. 134 , 320 , 321 It is observed to be upregulated in estrogen receptor-positive breast cancers and shows a positive correlation with estrogen receptor status. During gastrulation in zebrafish, zip6 is transactivated by STAT3. Elevated expression of zip6 results in nuclear retention of Snail, which is also known to be a zinc-finger transcription factor, which subsequently represses the expression of E-cadherin, resulting in cell migration 322 (Fig. 5 ). Indeed, E-cadherin performs its function as a calcium-induced TM glycoprotein, with its decreased expression linked to BC metastasis. 323 , 324 Taylor’ research observed a positive association between STAT3 and ZIP6 in breast cancer samples. 320 Furthermore, the induction of ZIP6 expression by STAT3 induces the translocation of ZIP6 to the plasma membrane and facilitates zinc influx, which is triggered by N-terminal cleavage. 157 Consequently, the zinc influx activates the zinc influx/GSK-3β inhibition/Snail activation/E-cadherin loss pathway, resulting in cell rounding and detachment (Fig. 5 ).

The molecular mechanism of zinc transporters and MTs in BC and prostate cancers. The left figure represents the mechanism of ZIP-mediated proliferation and EMT procession in BC. ZIP7 locates on the endoplasmic reticulum and is highly expressed in tamoxifen-resistant BC cells. After CK2 phosphorylation, ZIP7 was stimulated to transport zinc from intracellular stores, for example, the Golgi apparatus. Subsequently, the increasing zinc concentration can promote proliferation by activating the downstream PTPs, AKT, and ERK1/2 signaling. ZIP6 and ZIP10 locate on the cytomembrane. In addition, ZIP6 is induced by STAT3 and then translocated to the plasma membrane, promoting the accumulation of cellular zinc. The zinc influx caused by ZIP6 and ZIP6/ZIP10 heteromer triggers the AKT pathway and inhibits GSK-3β, finally boosting the EMT process by reducing the nuclear translocation of Snail. MT2A play a dual role in zinc homeostasis and BC cell proliferation. They can chelate zinc ions to reduce zinc cytotoxicity-induced apoptosis, while also releasing zinc ions to promote cancer cell proliferation through cdc25A activation. The figure on the right elucidates the mechanism of zinc transporter involved in prostate cancer. RREB-1 downregulates ZIP1 expression, leading to zinc homeostasis imbalance in prostate cells. ZIP1 downregulation reduced zinc influx, thus degrading the Bax pore expression level, which is the channel for cyto-C releasing into the cytoplasm. Consequently, the apoptosis induced by cyto-C is inhibited. Moreover, decreasing zinc concentration attenuates the inhibition of m-aconitase, which drives citrate oxidization in the TCA cycle. Meanwhile, the inhibitory effect of zinc on the NF-κB signaling pathway was diminished, as well as the inhibitory effect on the expression of HIF-1α, PSA, AP-N, and VEGF, which contributes to the invasion and proliferation. Besides, HOXB13 upregulates the expression of ZnT4 in prostate cancer through transcriptional regulation. EMT epithelial-mesenchymal transition, CK2 casein kinase 2, PTPs protein tyrosine phosphatases, RREB-1 Ras-responsive element binding protein 1, m- aconitase mitochondrial aconitase, cyro-C cytochrome C, PSA prostate-specific antigen, AP-N activity of urokinase-type plasminogen activator and aminopeptidase N, VEGF vascular endothelial growth factor, TCA tricarboxylic acid

However, despite the above discoveries, a solid link of ZIP6 to lymph node metastasis has not yet been entirely determined. There is evidence that ZIP6 is negatively correlated with EMT. 325 E-cadherin is downregulated in the condition of ZIP6 silencing. 326 In BC cells, exposure to high glucose results in a notable elevation of intracellular zinc levels, and it also leads to decreased mRNA expression of ZIP6 in the context of hypoxia. This downregulation of ZIP6 is associated with increased cell viability and reduced E-cadherin expression. 327 Hypoxia, which arises due to the aggressive proliferation of tumor cells, has previously been shown to trigger BC cells to undergo EMT, thereby promoting cell survival and malignant progression. 328 , 329 Similarly, the knockdown of ZIP6 blocks the balance of intracellular zinc levels, resulting in more tolerant cells in hypoxic environments. 321 Furthermore, some evidence suggests ZIP6 is associated with a more favorable prognosis. An illustration of this is that ZIP6 serves as a biological marker for estrogen receptor-positive luminal-type-A breast cancer, which is a molecular subtype associated with a more favorable prognosis. 330 , 331 , 332

Among the ZIP zinc transporter family, ZIP10 shows the highest similarity to ZIP6, sharing 43.5% sequence identity, which implies that they likely possess comparable roles in the regulation of cell migration. 154 , 333 As an indicator of metastasis and aggressiveness in cancer progression, ZIP10’s clinical relevance extends to its correlation of estrogen receptor ERBB3 and STAT3 among BC cases, 320 , 334 , 335 like the previously mentioned ZIP6. In mitosis, ZIP6/ZIP10 heteromer-induced zinc influx into cells leads to the formation of pS 727 STAT3 from pY 705 STAT3. PY 705 STAT3 serves as a transcriptionally promoted form of the protein, 336 , 337 impelling numerous malignant cancer features, such as EMT in HER2-positive BCs. 338 Chandler et al. discovered that the elevated presence of ZIP10 as well as the reduction in ZIP4, ZIP7, and ZIP11 were consistent mechanisms linked to zinc overaccumulation in the cells of malignant mammary glands. 39

Furthermore, the expression of ZIP7 has been demonstrated to be remarkably upregulated in BC cells. 339 , 340 ZIP7 functions as a zinc importer, moving zinc from intracellular stores (i.e., ER, Golgi) to the cytoplasm upon stimulation by the phosphorylation of CK2 168 (Fig. 5 ). The upregulated expression of ZIP7 facilitates the proliferation and aggression of tamoxifen-resistant MCF-7 cells by activating epithelial growth factor receptor (EGFR), insulin-like growth factor receptor 1 (IGF1R), and tyrosine kinase Src. 339 Activated ZIP7 is essential to the proliferation of drug-resistant estrogen receptor-positive BC. 340 Additionally, it is of great importance to note that ZIP7 plays a vital role in ferroptosis, which may establish a connection between ferroptosis susceptibility and treatment-resistant cells, as described in reference. 159 Mechanistically, ZIP7 overexpression induces zinc mobilization from the ER and Golgi, 341 triggering tyrosine kinase signaling as well as enhancing the aggressiveness of MCF7 cells 148 , 339 (Fig. 5 ). Besides, ZIP13 expression and subsequent mobilization of zinc from the ER/Golgi are essential for stimulating BMP/TGF-β signaling in connective tissue. 173 Overexpression of ZnT2 has resulted in cell cycle shifts, increased apoptosis, and decreased proliferation and invasion capabilities within MDA-MB-231 cells. 39 To summarize, being a risk factor for BC, zinc ions are regulated by ZIPs and ZnTs. Unlike ZnTs, the transporter proteins responsible for zinc inward flow, ZIPs, appear to be oncogenes in BC.

Indeed, there is evidence of mechanistic heterogeneity in the function of zinc transporters across different subtypes of BC. A notable association has been found between ZIP6 mRNA expression and improved overall survival (OS) among the whole cohort, the same as patients with luminal A and HER2-positive tumors. 342 Conversely, in luminal B and triple-negative BC (TNBC) subtypes, patients with high levels of ZIP6 expression showed worse OS. Besides, within the context of this heterogeneity, ZIP4 transporter plays a distinct role, particularly in TNBC. The upregulated ZIP4 expression results in enhanced zinc influx and promotes tumorigenicity in TNBC. 343 Interestingly, the intracellular zinc concentration in the BrM2 cell line, which metastasized to brain tissue, was found to be twice as high as that in the TNBC cell line MDA-MB231. Additionally, ZIP8, ZIP9, and ZIP13 have been demonstrated to be upregulated in BrM2 cells. The correlation between intracellular zinc concentration and BC cell metastatic potential is implied.

However, excess zinc accumulation typically triggers apoptosis, necessitating mechanisms in malignant breast cells to protect themselves from zinc-induced cell death. MTs serve as buffers for cellular zinc and shield cells from zinc toxicity. Breast tumors are known to hyper-accumulate zinc, with tissue biopsies of invasive ductal carcinoma overexpressing MTs in up to 88% of cases, 344 reflecting aberrant zinc accumulation and associated with poor prognosis. Furthermore, MT expression inversely correlates with estrogen receptor expression, indicating an important protective role for MT overexpression in highly invasive and poorly differentiated breast carcinoma. Specifically, TCGA data showed that patients with estrogen receptor α-positive BC had reduced concentrations of MT1 genes. 345 Nevertheless, it should be noted that not all malignant breast cells express MTs, implying the presence of alternative mechanisms to prevent zinc cytotoxicity. ZnT2, similar to MTs, exhibits zinc-responsive expression due to MREs in its promoter, as previously mentioned. 122 The overexpression of ZnT2 has been observed in MT-null BC cells (T47D). It is positively correlated with zinc accumulation, thereby conferring a protective effect against excess zinc-induced cytotoxicity. 346

Additionally, MT overexpression is primarily observed in the invasive ductal carcinoma subtype of BC and is associated with p53 inhibition and resistance to apoptosis. 122 , 344 As previously mentioned, apo-MT was able to eliminate zinc from p53, and reduced the subsequent transcriptional activity, yet it was incapable of binding to DNA. 251 Moreover, MTs can influence BC growth through cell cycle effects. In BC cells, suppression of MT2A results in an upregulation of ataxia telangiectasia-mutated (ATM) expression and a concurrent decrease in cell division cycle 25 A (Cdc25A) levels., 347 which is known as playing a pivotal character in facilitating the cell cycle transition from G1 to S phase. Interestingly, cdc25c, which originated from the cdc25 protein family as well, has been characterized as a zinc-binding metalloprotein. Its role involves dephosphorylating and activating the Cyclin B/cdk1 complex, which subsequently governs the initiation and advancement of mitosis. 348 On the other hand, p53 is identified as the substrate related to ATM coping with DNA damage. 349 The subsequent induction of CDK inhibitor p21 CIP1/WAF1 transcriptional activity results in a G1-growth arrest. 350 Thus, MT2A may serve as a zinc donor and plausibly promote cell cycle progression through the ATM-cdc25A-dependent pathway in BC.

To sum up, zinc metabolism is critical to the pathogenesis of BC, with zinc transporters, particularly ZIP6, ZIP7, and ZIP10, along with MTs and ZnT2, having profound effects on cellular processes like cell migration, cell viability, and apoptosis. These molecules not only impact zinc homeostasis within the cancer cells but also modulate important signaling pathways and cellular responses to hypoxic environments, thereby influencing the progression and outcome of the disease.

Prostate cancer

Of all the soft tissues in human bodies, normal and hyperplastic prostate tissues have the highest concentrations of zinc accumulation. 351 On the other hand, zinc concentrations detected in prostate cancer were greatly reduced. 352 The peripheral zone, which is found to serve as the origin of prostate cancer, is responsible for secreting prostatic fluid. An essential and distinctive component of this fluid is the remarkably high concentration of citrate. 353 , 354 , 355 Traditionally, citrate is oxidized in the tricarboxylic acid (TCA) cycle, while high cellular zinc levels in normal prostate cells prevent this process by inhibiting the activity of mitochondrial aconitase (m-aconitase) 356 (Fig. 5 ). Furthermore, to preserve normal prostate function, physiological zinc levels induce apoptosis through various mechanisms in prostate cells. These include upregulating the Bax/Bcl-2 ratio in the mitochondria, 357 inducing HIF-1α degradation, 358 and involving with NF-κB pathway 359 (Fig. 6 ). Besides, zinc is also involved in the inhibition of invasion and adhesion in malignant prostate cancer cell through several ways: strongly prevents the enzymatic activity of prostate-specific antigen (PSA) and suppresses the invasion of LNCaP cells, 360 reduces the expression of vascular endothelial growth factor (VEGF), 361 interleukin (IL)-6, IL-8, matrix metalloproteinase-9 (MMP9), intercellular adhesion molecule-1 (ICAM1), diminished the activity of urokinase-type plasminogen activator and aminopeptidase N (AP-N) 362 (Fig. 5 ). Unfortunately, prostate cancer cells have significantly lower zinc levels, and hence they are unable to inhibit m-aconitase activity, ultimately resulting in the inability to obtain normal prostate fluid with citrate in tissue. 363 Also, m-aconitase activity can contribute to the proliferation and migration of prostate cancer cells. 364 Indeed, the low zinc concentration in malignant cells possesses mechanisms such as ZIP downregulation and ZnT upregulation.

The molecular mechanism of zinc transporters and MTs in PC. ZIP4 promotes PC carcinogenesis mainly through two transcription factors, CREB and ZEB1. ZEB1 promotes the procession of EMT by suppressing the expression of ZO-1 and CLDN1 and inducing the transcription of ITGA3. Moreover, the ZEB1 induces integrin α3β1 to phosphorylate JNK and ultimately blocks ENT1, a gemcitabine transporter, which results in chemoresistance. Besides, cellular zinc released by MT1G inhibits NF-κB, suppressing PC chemoresistance. CREB transcripts miR-373 to increase metastasis, invasion, and proliferation by activating the Hippo pathway yet inhibiting the expression of TP53INP1 and CD44 . Besides, PHLPP2, inhibited by miR-373, forms a malignant cycle through the suppression of CREB. However, the small molecule, circ ANAPC7, can block miR-373. As a target for PHLPP2 dephosphorylation, AKT increases the proliferation by upregulating cyclin D1 and promotes muscle wasting by phosphorylating STAT5. Another CREB-mediated downstream promoting muscle wasting is RAB27B. Mechanically, RAB27B promotes the release of HSP70 and HSP90 from MVB. Additionally, the CREB-mediated IL-6/STAT3/cyclin D1 pathway leads to proliferation in PC. ZIP4 could restrain apoptosis by inhibiting the activity of caspase9 and caspase7. The expression of ZIP3 is reduced by RREB-1. ZO-1, zonula occludens-1; ITGA3, integrin subunit alpha 3; JNK, c-Jun N-terminal kinase; MVB, multivesicular body; EMT, epithelial-mesenchymal transition

ZIP1 predominantly localizes at the basolateral membrane. Both normal and hyperplastic prostate glandular epithelial cells have in situ expression of ZIP1, where it transports zinc from the plasma into the cell. 365 , 366 In most cases, it plays a predominant role in zinc accumulation in benign prostatic hyperplastic epithelial cells. In contrast, ZIP1 is downregulated in malignant cells, resulting in the inability to accumulate zinc. 367 , 368 , 369 Therefore, prostate cancer can be characterized as a ZIP1-deficient tumor. 370 The expression of ZIP1 and ZIP2 detected by RT-in situ-PCR was lower in African Americans’ prostate epithelial cells than in Caucasian men, which could be involved in the higher susceptibility of African-Americans to prostate cancer. 367 Interestingly, overexpression of ZIP1 can sensitize the tumorigenic prostate epithelial cells (RWPE2) to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. 371 It was shown that the core promoter regions, contributing to the regulation of ZIP1 expression, are modulated by SP1 as well as CREB. 372 RREB-1, the downstream of ERK in the Ras/Raf/MAPK pathway, was upregulated in prostate cancer progression. 373 , 374 , 375 The inhibition of ZIP1 expression in prostate cancer implicates the mobilization of RREB-1, which could become one of the possibilities for the downregulated expression of the zinc transporter in malignant prostate disease 376 (Fig. 5 ). Besides, ZIP1-mediated rapid increase of zinc levels seems to be androgen-dependent. 377 Furthermore, by acting as an androgen cell membrane receptor, ZIP9 facilitates the mechanism of testosterone-dependent apoptosis in prostate carcinoma. 378 , 379

Unlike ZIP1, ZIP2 and ZIP3 are hardly localized to the basolateral membrane, both of which are mainly constrained to the apical membrane of the prostate tissue. 380 Studies on cell lines suggest that the functional role of ZIP2 and ZIP3 is to transport or reabsorb zinc from prostatic fluid back to the epithelium, 14 , 381 rather than accumulating cellular zinc from the blood circulation, which is the primary function of ZIP1. 382 Human prostate tissue sections examined by immunohistochemistry examination show significantly reduced regulation of ZIP2 and ZIP3 in adenocarcinoma glands, leading to dysfunction in accumulating zinc. 371 , 380 , 383 Thus, it is reasonable to propose that ZIP1, ZIP2, and ZIP3, all of which belong to the ZIP family, function as tumor suppressor genes in prostate carcinogenesis.

Regarding the ZnT transporter family, ZnT4 is five times higher in prostate cancer as measured in normal tissues. 111 Furthermore, ZnT4, as well as ZnT10, is highly induced by the HOXB13. 384 The introduction of exogenous HOXB13 decreases intracellular zinc levels in prostate cancer cells and activates NF-κB signaling, which promotes prostate cancer invasion. In addition, ZnT4 mRNA was found to be overexpressed in tumor samples acquired through radical prostatectomy versus normal tissues. 385 Interestingly, ZnT5 was also expressed at high levels in human prostate tissue. 386 Further study of the mechanistic impact of altered zinc transporter expression levels on prostate carcinogenesis has important implications for clinical treatment.

Additionally, studies investigating the relevance between MT expression and pathological/malignant conditions are severely limited in the prostate, and the regulatory mechanisms of zinc on MTs expression in prostate cells remain unclear. MT1/2 downregulation has been observed in benign prostatic hyperplasia (BPH), PC-3 cells, and malignant tissues of the human prostate. MT1/2 expression is notably enhanced by zinc therapy in both PC-3 and BPH cells, coincident with the restoration of intracellular zinc concentrations. Specifically, in BPH cells, MT3, acting as a growth inhibitory agent, was identified, and its levels were elevated by zinc. Furthermore, the expression of MT3 serves as a distinctive feature exclusively found in BPH cells. 387 MT1h, one of the components of the MT1 family, is commonly decreased in prostate cancer. The heavy methylation of its promoter has been observed. MT1h exerts its role as a tumor suppressor by activating euchromatin histone methyltransferase 1 (EHMT1), which leads to histone methylation and potentially suppresses gene expression. 388

Pancreatic cancer (PC)

Despite tremendous research efforts in the past few years, PC remains one of the most devastating diseases and has the highest fatality rate among all cancers. 389 Accumulating evidence indicates a strong correlation between zinc transporters and PC growth and progression. 75 , 312 , 390 , 391 , 392 However, the zinc levels and the molecular mechanisms through which zinc transporters regulate cancer growth in PC are not yet fully understood. Therefore, it is essential to study the effects of zinc transporters in PC carcinogenesis.

Overexpression of ZIP4 is widely described in human PC tissues and cell lines, contributing to tumor growth. 75 , 393 , 394 , 395 , 396 , 397 , 398 , 399 Obviously, the potential role by which ZIP4 is involved in PC growth and migration may be multifaceted. Knocking out ZIP4 is able to suppress the proliferation of PC through reducing cyclin D1 expression, 393 which serves as the downstream target of CREB/miR-373/PHLPP2 and CREB/IL-6/STAT3 pathway. Both pathways are activated by the overexpression of ZIP4, leading to PC cell proliferation 400 (Fig. 6 ). ZIP4 contributes to the mediation of metastasis in addition to the proliferation of PC cells. ZEB1 is the most critical EMT-associated transcription factor in PC, promoting stemness, invasion, and metastasis of PC. 401 Significantly, ZIP4 induces the expression of ZEB1, which mechanically is through phosphorylated STAT3. 395 Another report suggested that ZIP4 activates PC migration and invasion by mediating ZEB1 inhibition of ZO-1 and Claudin-1 expression 394 (Fig. 6 ). Additionally, ZIP4 is able to induce the expression of YAP1 by stimulating a miR-373-LATS2 pathway in PC, promoting organ formation and cell adhesion through the increasing expression of ITGA3. 74 Notably, the upregulation of ZEB1 inhibited expression of the gemcitabine transporter via ITGA3/ITGB1/α3β1 signaling and c- JNK pathway, which leads to chemoresistance both in vitro and in vivo. 395 Moreover, ZIP4 has a notable role in PC-related cachexia, where it facilitates the release of HSP70 and HSP90 via extracellular vesicles, thereby stimulating muscle atrophy. 75 Whereas the CircANAPC7 inhibited ZIP4/miR-373 mediated muscle wasting partially through STAT5/TGFβ signaling in PC. 400 These findings suggest that ZIP4 might serve as a potential PC diagnosis and therapy target (Fig. 6 ).

It could infer that aberrant overexpression of ZIP4 elevates zinc concentrations in PC cells. Using the nude mice model with subcutaneous xenograft, a study found that 80% more zinc was detected in the tumors implanted with ZIP4 stably overexpressed MIA-ZIP4 cells compared with the normal group. 393 However, clinical and preclinical indications disclose that zinc is persistently and significantly reduced in the early stage of PC compared with the normal or benign pancreas tissues, which is an essential malignant event. 402 Indeed, the reduction in zinc levels in pancreatic intraepithelial neoplasia (PanIN) lesions and malignancy is attributed to the downregulation of Ras responsive element binding protein 1 (RREB-1) and the silencing of ZIP3. 380 , 390 , 402 Another study has proved that PC cells are vulnerable to high zinc concentrations. The exposure of PC cells to physiological concentrations of zinc (0.01–0.5 mM) can lead to cytotoxic cell death, which is characterized by up-regulation of the zinc transporter ZnT1 gene expression. 312 Another study revealed that higher levels of zinc chloride (>50 μM) significantly reduced the proliferation of MIA-ZIP4 cells, suggesting that zinc activated the proliferation of PC cells only at comparatively low concentrations. 393 Besides, zinc provided by MT may be working with transcription factors. Research has shown that MT1G plays a crucial role as a tumor suppressor in pancreatic cancer stem cells. The downregulation of MT1G, caused by hypermethylation of its promoter, is associated with the maintenance of pancreatic cancer stemness. Mechanistically, MT1G exerts a negative regulatory effect on NF-κB signaling and facilitates the degradation of the NF-κB p65 subunit by upregulating the expression of E3 ligase TRAF7, consequently suppressing PDAC stemness. 403

Apparently, zinc is essential for cellular function, growth, reproduction, and metabolism. Thus, normal cells have evolved homeostatic mechanisms to maintain their normal required zinc levels and prevent the potential adverse effects of excessive zinc concentrations. However, the malignant cell has lost these normal protective conditions. PC cells require excess zinc to support proliferation and, on the other hand, avoid the adverse effects of zinc through other regulatory mechanisms.

Colorectal cancer (CRC)

Notably, a meta-analysis of human studies indicated that higher zinc intake was inversely associated with the overall risk of digestive tract cancers, especially for CRC. 404 It has been reported that zinc can inhibit the proliferation of colon cancer cells by arresting the cell cycle in the G2/M phase and disrupting the microtubule stability of cell-cell communication. 405 Hence, zinc transporters could be involved in GI disorders.

By bioinformatic analysis of microarray data in the GEO database, it has been identified that ZnT10 is one of the ten recommended candidate genes associated with CRC. 406 Consistently, a recent study reported ZnT10 as a methylation marker in the CRC, and the methylation epigenotype significantly correlated with KRAS and BRAF mutation in CRC. 407 In contrast, reduced expression of ZnT10 is associated with aggressive tumor phenotypes and poor patient outcomes in CRC. 408 ZnT10 acts as a competitive endogenous RNA for miR-21c to upregulate tumor suppressor gene APC expression, thus inhibiting CRC progression and metastasis. 408

Additionally, ZnT9 is the coactivator of β-catenin-mediated gene transcription, 409 , 410 which serves as the critical event in the Wnt signaling pathway and the development and progression of colon cancer. 411 Notably, the binding of ZnT9 and β-catenin can be competitively replaced by KCTD9, a tumor suppress gene which is negatively correlated with the clinical CRC stage, thus substantially inhibiting the transcription of downstream oncogenes, including MYC , CCND1 , and MMP7 409 (Fig. 7 ). In fact, ZIP7 also plays a crucial role in intestinal epithelial self-renewal. 187 Colorectal tumors have higher expression levels of ZIP7 than normal colon tissues. 412 It was demonstrated that the knockdown of ZIP7 induced G2/M cell cycle arrest and promoted apoptosis in colorectal cancer cells. 413 Furthermore, the downregulation of ZIP7 promoted the cleavage of PARP, enhanced the expression of Bad, Caspase-9, and cleaved-Caspase-3, and suppressed Bcl-2 expression in CRC. 413

The molecular mechanism of zinc transporters and metallothioneins in CRC, GC, and ESCC. In CRC, the binding of ZnT9 and β-catenin triggers the transcription of CCND1 , MYC , and MMP7 , resulting in proliferation and migration. KCTD9 can replace the binding of ZnT9 and β-catenin. Moreover, ZIP14 contains two alternative splicings, ZIP14-4A and ZIP14-4B. ZIP14-4B is upregulated by SRPK1 and SRSF1, two downstream targets of Wnt signaling, leading to increased Cd 2+ uptake. Concerning the GC microenvironment, ZIP7 is the upstream target of the AKT/mTOR signaling pathway. In GC, autophagic degradation of MT1E, MT1M, and MT1X initiated by USP2-E2F4 interaction leads to increased intracellular zinc storage vesicles, promoting GC cell growth. In contrast, MT2A inhibits NF-κB by releasing cellular zinc and thus ultimately suppresses GC cell proliferation. As for ESCC, ZIP6 activates PI3K/AKT and MAPK/ERK signaling pathways, which leads to the overexpression of downstream oncogenes such as MMP1 , MMP3 , MYC , and SLUG . Meanwhile, the cellular zinc released by MT2A promotes the oncogenic function of IGFBP2. NF-κBIA, NF-κB inhibitor alpha; IGFBP2, insulin-like growth factor binding protein 2

Alternative splicing is a critical step in generating protein diversity, and its misregulation has been observed in carcinogenesis. 414 , 415 , 416 Notably, alternative splicing of ZIP14 was found to be regulated by the Wnt pathway in CRC, most likely through the regulation of SRPK1 and SRSF1 417 (Fig. 7 ). ZIP14 contains two mutually exclusive exons, 4 A and 4B, and the ratio of exon 4 A/4B was significantly reduced in adenomas and cancers, which may be used as a tumor marker for identifying CRC and precancerous lesions. Specifically, the exon 4B isoform of ZIP14 is found to have an eightfold higher affinity for Cd 2+ than the exon 4 A isoform, which is known as a potent carcinogen. 99 Moreover, Cd 2+ has been found to influence several cellular processes, including apoptosis, differentiation, and cell growth, especially the inhibition of DNA mismatch repair, 418 , 419 thus setting off CRC carcinogenesis.

Beyond the roles of ZIP and ZnT transporters in CRC, our review extends to proteins such as MTs that regulate cellular zinc metabolism. Intriguingly, these MTs often act as tumor suppressor genes in CRC. A notable correlation between low MT1B, MT1H, or MT1L expression and an increased risk of adverse outcomes was identified. 420 Additionally, a distinct four-gene model, consisting of MT1F, MT1G, MT1L, and MT1X, effectively predicted survival and CRC prognosis. It has been reported that zinc potently enhances MT expression and is cytotoxic to cancer cells. 421 MT2A expression decreased in colorectal cancer and was linked to the patient’s tumor M stage. 422 , 423 The present research has mechanistically illustrated that MT2A upregulation promoted the expression of phosphorylated MST1, LATS2, and YAP1, which consequently inhibited the Hippo signaling pathway and controlled CRC cell proliferation and liver metastasis. 422 However, it is unclear whether the role of MT in controlling the MST1/LATS2/YAP1 signaling pathway depends on its regulation of zinc. Thus, the role of zinc and its regulatory mechanism in CRC requires further in-depth investigation.

Gastric cancer (GC)

In GC studies, the relationship between zinc intake and GC is contradictory. On the one hand, a large number of studies point out that lower zinc intake may increase the risk of GC. 424 , 425 , 426 For example, Cixian and Linxian are one of the higher-risk areas for upper GI cancer both in China and worldwide, where individuals have a zinc intake below the recommended daily allowance and higher incidence and mortality rates of GC than that of other regions. 427 , 428 , 429 However, a meta-analysis revealed that zinc intake was significantly associated with GC risk in Asia but not in America and Europe. 404 The heterogeneity in the results of zinc intake associated with GC risk may be due to the differences in the expression background of zinc transporters.

Multiple bioinformatic approaches revealed that high expression of five genes ( ZnT1 , 5-7 , and 9 ) was significantly correlated with better overall survival (OS), first progression survival (FPS), and post-progression survival (PPS), while upregulated ZnT2-4 , 8 , and 10 expressions was markedly associated with poor OS, FP, and PPS. 430 In addition, ssGSEA analysis indicated that SLC30 family genes were closely associated with the infiltration of immune cells, indicating that the ZnTs induced tumorigenesis partly because of immune infiltration. 430

In the GTEx and TCGA datasets, ZIP10 was highly expressed at the mRNA level in malignant GC cells compared to normal and adjacent non-tumor samples. 431 A previous study has demonstrated that ZIP10 expression was correlated with STAT activation in B cell lymphoma samples. 137 In GC, the novel natural product inhibitor of STAT3 termed XYA-2 might exert its anticancer activity by synergistically inhibiting the expression of MYC and ZIP10, two downstream genes of STAT3 in vitro and in vivo. 431 Meanwhile, ZIP6, another downstream target of STAT3, is involved in cancer development by forming a heterodimer with ZIP10. 157 Besides, the ZIP7 mRNA level was increased in both GC tissues and cell lines, which boosted cell proliferation and migration, while inhibiting apoptosis in GC. 432 Specifically, ZIP7 was negatively regulated by miR-139-5p and positively regulated GC development through Akt/mTOR signaling pathway, suggesting that ZIP7 may be a candidate target gene for GC treatment (Fig. 7 ).

Alarmingly, reduced expression of MT1 or MT2 has been observed in GC, a pattern correlated with worse prognoses. 433 There has been an observed decrease in MT2A and myeloid zinc-finger 1 (MZF1) expression in clinical specimens that are undergoing malignant transformation of the stomach. 434 Intriguingly, an important role played by zinc accumulation in controlling cancer through autophagy flux has been reported. 435 Autophagic degradation of MT1E, MT1M, and MT1X, initiated by E2F4 in GC, leads to an increase in zinc-stored vesicles within autophagosomes. This, in turn, lowers the levels of free intracellular zinc and facilitates the growth and invasion of GC cells. These findings offer a novel insight into how autophagy modulates zinc homeostasis in cancer cells. 48 In line with this, recent evidence has indicated that MT1M has the ability to dampen the malignancy and stem cell-like characteristics of GC by inhibiting GLI1, a component of the Hedgehog signaling pathway, known for its numerous zinc finger domains. 436 Besides, the MT1 gene cluster has been found to be hypermethylated in EBVaGC, suggesting redundant anti-EBV roles among various MT1 genes. 437 MT1 proteins provide cellular protection against OS via their antioxidant properties, 438 which account for their anti-EBV functions.

Furthermore, in human GC cell lines and primary tumors, the transcription factor MZF1 has been found to be epigenetically silenced, a finding associated with MT2A. MZF1 serves to deter gastric carcinogenesis by associating with MT2A to bind to the NFKBIA promoter (Fig. 7 ). Notably, this tumor-suppressive effect can be stimulated by diallyl trisulfide (DATS), a compound derived from garlic known to thwart the progression of GC. 439 In keeping with the ability of zinc to inhibit NF-kB activation in cancer cells, 440 , 441 , 442 zinc chelation likely plays a part in the anti-GC activity of the MT2A/MZF1–NF-kB pathway mediated by DATS. MT2A simultaneously controls zinc-binding proteins by adding or removing zinc and is transcriptionally inducible by these proteins to target its promoter region, which contains numerous regulatory elements, such as the MRE. 434 Therefore, the diminished expression of MZF1/MT2A significantly associates with the malignancy of GC and poor patient outcomes. Additionally, MT2A hinders cell growth via apoptosis and G2/M arrest, negatively influencing the NF-κB pathway through upregulation of IκB-α and downregulation of p-IκB-α and cyclin D1 expression. 298 ApoMT (metal-free MT) has been identified as a potential agent for extracting zinc from NF-κB, thereby rendering the NF-κB-mediated transcriptional activity inactive due to zinc chelation. 443 In conclusion, targeting GC by interfering with zinc metabolism appears to be a viable approach (Fig. 7 ).

Esophageal squamous cell carcinoma (ESCC)

Another essential type of digestive tract tumor is ESCC. Actually, ZD in dietary potentiates the effects of specific nitrosamines that act as esophageal carcinogens in rodents. 444 A study using x-ray fluorescence to measure zinc concentrations in tissues demonstrated that zinc concentration is inversely associated with the risk of incident ESCC. 445 Zinc replenishment rapidly induced apoptosis in esophageal epithelial cells and thereby substantially reduced the development of ESCC. 446

However, ZIPs, the proteins that translocate Zinc into cells, are associated with ESCC. Immunohistochemical staining of ESCC tissues showed that higher expression of ZIP6 predicted unfavorable prognosis in individuals with advanced ESCC. 447 ZIP6 overexpression is an “early” or “intermediate” event in the ESCC malignant progression, indicating that ZIP6 could serve as an early detector of high-risk subjects and prognostic biomarker. 448 Cheng et al. revealed that overexpression of ZIP6 or elevated intracellular zinc levels in cancer cells substantially activated the PI3K/AKT and MAPK/ERK signaling, which upregulated downstream oncogenes such as MMP1 , MMP3 , MYC , and SLUG . 449 This up-regulation of these molecules may be the underlying mechanism for the aggressive phenotypes of ESCC with ZIP6 overexpression (Fig. 7 ).

Similarly, studies suggested that ZIP5 protein and mRNA expression was highest in ESCC, intermediate in paraneoplastic tumors, and lowest in normal tissue. 450 Kumar et al. found that the dysregulation of zinc homeostasis in esophageal tumorigenesis is mainly reflected in the upregulation of ZIP5 and the downregulation of the zinc metabolism protein MT1G using cDNA microarray. 451 Besides, the downregulation of ZIP5 decreased the expression of COX2 and increased the expression of E-cadherin in the KYSE170K xenografts. 452 COX2 is an essential molecular basis for cancer progression, which promotes the proliferation and invasive ability of tumors and inhibits cancer cell apoptosis. 453 Collectively, knocking down ZIP5 by small interfering RNA might be a novel therapeutic strategy for ESCC with ZIP5 overexpression. Although some studies have shown that zinc ion intake might suppress tumor growth, overwhelming reports focus on the promoting role of zinc in tumor initiation and development, or even driving metas44tasis. MT2A, acting as a zinc donor, induces IGFBP2 and inhibits the expression of E-cadherin through a zinc finger protein. 454 , 455 Recombinant IGFBP2 promoted migration and invasiveness of ESCC cells via NF-κB, Akt, and Erk signaling pathways.

In pan-cancer copy number variation (CNV) and mutation analyses from the TCGA database, 456 most of the SLC30 and SLC39 family genes demonstrated gene amplification, especially SLC30A8 , SLC30A1 , SLC30A10 , SLC39A1 , and SLC39A4 . Notably, the gene for SLC30A8 and SLC39A4 amplification was co-occurring in almost all cancer patients. Interestingly, the cases with SLC39A14 deletion appear to be more than those with amplification (Fig. 8 ). Although ZIPs are more commonly regarded as oncogenes in cancer, prostate cancer is an exception. Studies also suggested that the function of the zinc transporters may be contradictory among different cancer types. As we delve into the gene alterations in MTs, our attention is captured by the astonishingly consistent variations observed among all MTs members (Fig. 8 ). Notably, the compelling set of data from representative tumor patients showcases the remarkably homogeneous trends in gene alterations among all MTs members. Such changes predominantly encompass amplifications and deep deletions, implying pivotal roles for MTs in the context of cancers. Despite the similar gene alteration trends, disparate mRNA expression profiles are observed for different MTs members. This intriguing observation suggests the involvement of intricate transcriptional regulatory mechanisms governing MTs genes. The diversity in mRNA expression levels might arise due to a myriad of factors, potentially linked to cellular context, tissue specificity, and even cancer types. Thus, research on zinc transporters and MTs in tumorigenesis is still a long way to go.

Genetic and mRNA alterations of zinc transporter and MT family genes in pan-cancer patients. The upper figure illustrates the gene alterations of zinc transporters. Out of the queried pan-cancer samples, 1526 (59%) showed copy number aberrations, mutations, and mRNA expression changes. The lower figure displays the gene alterations of MTs, where 324 (13%) of the queried pan-cancer samples demonstrated copy number aberrations, mutations, and mRNA expression changes. This diagram includes a total of 30 cancer types, marked with different colors. The data source is from the pan-cancer analysis of whole genomes dataset in cbioportal ( https://www.cbioportal.org/study/summary?id=pancan_pcawg_2020 )

Other cancers

Zinc homeostasis disruption has been observed in patients with various types of cancers. Studies have highlighted the significance of zinc-containing enzymes called matrix metalloproteinases (MMPs), which can be activated by zinc. 457 , 458 ZIP4, in particular, has been shown to regulate the expression of MMP2 and MMP9, influencing zinc concentration and promoting invasiveness and migration of hepatoma cells. 397 Notably, ZIP4 expression is linked to post-liver transplantation outcomes in HCC patients, making it a potential treatment target and prognostic marker for liver transplantation in HCC cases.

Besides, ovarian cancer, the most lethal gynecologic malignancy, exhibits rapid progression and widespread metastases. 459 Of note, ZIP13 was found to promote the proliferation, invasion, adhesion, and metastasis of ovarian cancer cells in vitro and in vivo. 73 The underlying mechanisms involve intracellular zinc distribution disruption and activation of the Src/FAK pathway, ultimately leading to ovarian cancer metastasis.

Drosophila melanogaster serves as a powerful model for cancer biology studies. Drosophila ZnT7 (dZnT7) acts as a tumor suppressor, negatively regulating JNK signaling. 460 dZnT7 knockdown induces JNK activation, promoting both cell-autonomous and nonautonomous autophagy, ultimately resulting in tumor overgrowth and migration.

Additionally, ZIP9 activation, through testosterone binding, induces an increase in cytosolic zinc in melanoma cells, thereby promoting cancer proliferation. 461 In gliomas, MT3 plays a key role in autophagy flux regulation via zinc-dependent lysosomal acidification, 435 contributing to glioma cell resistance to irradiation treatment. Targeting MT3 may thus enhance the efficacy of irradiation treatment. By elucidating the disruption of zinc homeostasis and its implications in cancer progression, these findings provide valuable insights into potential therapeutic strategies for diverse cancer types. Further research in this field may pave the way for improved cancer treatment and management.

Cellular zinc metabolism in cardiovascular disease

Noncommunicable diseases, such as cardiovascular disease (CVD) and cancer, are the leading causes of death worldwide. 462 The correlation between zinc and CVDs is a complex and multifaceted topic. Evidence suggests that zinc may be protective against certain CVDs, although the exact mechanisms are not fully understood. 270 , 463 , 464 Here, we focus on elucidating the crucial involvement of zinc in the progression of CVDs, specifically with regard to atherosclerosis (AS), diabetic cardiomyopathy, myocardial ischemia/reperfusion (I/R) injury, and heart events.

Atherosclerosis (AS)

Hyperlipidemic environments and inflammatory factors are known to significantly contribute to the development of AS. 465 Recent research highlights the critical role of ZD in the progression of this condition. 270 Zinc exerts influence on various characteristic aspects of AS, including increased apoptosis and disrupted NO levels. NO, synthesized in endothelial cells (ECs), acts as an essential endothelium-derived vasodilator. Reduced availability of NO occurs when there is a decrease in the expression or activity of endothelial NO synthase (eNOS), actively participating in the atherogenic process. 466 Additionally, it has been suggested that reduced NO generation in atheroprone regions, combined with increased ZnT1 and MT expression, may lead to decreased intracellular free zinc. 467 Studies using Zip13-KO mice have shown elevated levels of the cardiac fibrosis marker Col1a1 and the vascular inflammation-related gene eNOS, indicating the physiological importance of ZIP13 in maintaining cardiovascular homeostasis by resolving inflammation and stress response. 468

Moreover, the induction of EC apoptosis in response to oxidative stress is a characteristic atherogenic trait. Zinc is also associated with apoptosis and proliferation in vascular smooth muscle cells (VSMCs), 469 , 470 the primary contributors to the composition of atherosclerotic plaques. The regulators ZnT3 and ZnT10 play crucial roles in VSMC senescence and are susceptible to downregulation by Ang II and zinc. 471 Ang II signaling pathways become activated with age and contribute to developing AS and vascular senescence. 472 Interestingly, decreased catalase expression is observed, leading to ROS accumulation and induction of senescence. 471 ZnT3 and ZnT10 work to prevent increases in ROS levels by modulating the expression of catalase.

Myocardial ischemia/reperfusion (I/R) injury

Myocardial ischemia/reperfusion (I/R) injury is a prevalent cardiovascular condition associated with a high mortality rate. 473 Recent studies have revealed the importance of zinc homeostasis in cardiomyocytes during reperfusion, as zinc loss upon reperfusion contributes to I/R injury. 474 The crucial role of ZIP transporters in maintaining zinc homeostasis has been demonstrated, with ZIP2 playing a significant role in this process. 475 Deletion of the ZIP2 gene notably intensified myocardial I/R injury, whereas upregulation of ZIP2 demonstrated the potential to mitigate I/R injury. These findings suggest that ZIP2 exerts a cardioprotective effect against I/R injury by restoring zinc homeostasis. 475 Additionally, ZD has been shown to activate STAT3 through ER stress-induced Ca 2+ release and subsequent CaMKII activation, enhancing the transcriptional activity of ZIP9 and protecting against cellular ZD. 476

ZIP7 upregulation, on the other hand, hinders the accumulation of PINK1 and Parkin in mitochondria by increasing zinc outflow to the cytosol, contributing to the genesis of myocardial reperfusion injury by inhibiting mitophagy during reperfusion. 477 Consequently, the upregulation of ZIP7 is considered a significant feature of myocardial reperfusion injury and may present a novel therapeutic target for myocardial reperfusion injury and other cardiac diseases caused by oxidative stress or mitochondrial dysfunction.

Endogenous ZnT-1 has been shown to have a substantial protective effect against I/R injury, which is mediated by the C-terminal domain of the protein through the activation of Ras-ERK signaling. 478 Additionally, ZnT-1 serves various functions, such as binding to Raf1 and triggering the ERK cascade. 479 , 480 Additionally, it hinders LTCC activity by interacting with the β-subunit of the voltage-dependent calcium channel. The significance of ERK cascade activation in promoting cell survival after I/R injury has been extensively recognized. 481 Recent evidence suggests that nuclear factor (erythroid-derived 2)-like 2 (NRF2) activation/overexpression increases total zinc content in HCAEC with minimal changes in HCASMC, consistent with observed changes in ZnT1 and MT protein expression. 482 This finding further highlights the complex interplay between zinc, ROS, and endogenous antioxidant defenses regulated by NRF2.

Diabetic cardiomyopathy (DCM)

DCM is a prevalent and severe complication of diabetes. A link between systemic ZD and the increased incidence of diabetes and diabetic cardiovascular complications has been established. Notably, in diabetic mice, zinc supplementation has been shown to significantly protect against the development of DCM through the induction of cardiac MT. 483 , 484 , 485 , 486 MT has proven effective in countering cardiac fibrosis under stress conditions like diabetes and nicotine exposure. 484 , 487

MTs offer cardiomyocytes protection primarily through zinc-dependent antioxidant effects. During the early stages of diabetes, cardiac mitochondria experience cytochrome c release-dependent apoptosis. However, MT substantially inhibits this early cardiac apoptosis caused by diabetes by suppressing mitochondrial oxidative stress, particularly the depletion of GSH, which significantly prevents the development of DCM. 483 Moreover, MT suppresses Ang II-induced NOX-dependent nitrosative damage and cell death in both nondiabetic and diabetic hearts early in the injury process, effectively preventing the later development of Ang II-induced cardiomyopathy. 488 Furthermore, MT ameliorates ROS generation and cardiac fibrosis despite persistent cardiomyocyte contractile and intracellular Ca2þ derangement. 489 Both MT overexpression and direct MT administration can reduce DCM by suppressing peroxynitrite-derived nitrosative damage and ROS production in diabetic hearts. 490

Recently, it has been demonstrated that zinc-induced cardiac endogenous antioxidant MT blocks TRB3 induction, thereby preserving Akt2 signaling and preventing DCM. The development of pharmaceutical inducers of cardiovascular MT holds promise as a preventive measure against cardiomyopathy in diabetic patients. 491 In conclusion, the induction of MTs presents a potential therapeutic approach for preventing diabetic DCM.

Heart event

The zinc level in heart tissues is approximately 1 g or less, and it has been shown to have a positive correlation with ejection fraction in humans. 492 At a concentration of 1 nM, zinc can directly activate RyR2, which has a much higher affinity for zinc than Ca 2+ (about three-fold), providing an essential mechanistic explanation for the association between zinc dyshomeostasis and certain cardiomyopathies. 493 , 494 ZnT-1 is an endogenous negative regulator of the LTCC, particularly in the heart, where it appears to participate in cardiac electrical remodeling following atrial fibrillation. Increased ZnT1 expression is observed in patients with atrial fibrillation. 495 Mechanically, ZnT-1 was demonstrated to regulate the LTCC by interacting with its regulatory α1-subunit, thus limiting the plasma membrane expression of the LTCC. 151

Furthermore, serum zinc levels could serve as a valid diagnostic indicator for acute myocardial infarction (MI). 496 Meta-analysis data indicates that a lower dietary zinc intake is associated with an increased prevalence of coronary artery disease (CAD), and there is a direct relationship between zinc status and MI. 496 ZIP13 is ultimately in charge of CaMKII mobilization, while the suppression of ZIP13 aggravates myocardial infarction through destabilizing mitochondrial signalings. 497 Moreover, with respect to calcific aortic valve diseases, which is one of the most widespread heart valve disorders, the expression of ZIP13 is markedly enhanced. Correspondingly, ZIP13 knocking down resulted in the inhibition of human valve interstitial cells in an in vitro calcification model. 498 Thus, alterations in ZIP13 expression may occur due to cardiac stress, which may induce CVDs or promote their pathogenesis. Additionally, it has been demonstrated that ZnT5 is associated with heart function, and its deficiency causes osteopenia and sudden cardiac death. 386

During cardiac hypertrophy, the expression of ZIP2 was downregulated. 499 Inhibiting ZIP2 leads to the induction of interferon regulatory factor (IRF) 7 expression, which, in turns, triggers the activation of ZIP2 development. As a result, IRF7 functions the role of a feedback regulator to modulate ZIP2 expression according to its activity. Based on serial transgenic mouse models, it has been confirmed that IRF7, IRF8, and IRF9 were anti-hypertrophy factors that are consistently down-regulated in cardiac hypertrophy and heart failure. 500 , 501 , 502 To conclude, leveraging ZIP2 to modulate cellular zinc metabolism could offer an innovative approach for treating these two diseases. 499

Besides, zinc emerges as a novel inhibitor of Calcific aortic valve disease (CAVD). 498 The ZnR/GPR39 is reduced in calcified aortic valves from patients with CAVD. The anti-calcific effect of zinc on human valve interstitial cells (hVIC) calcification is, at least in part, mediated through the inhibition of apoptosis and osteogenic differentiation via the GPR39-dependent ERK1/2 signaling pathway. Additionally, ZIP13 and ZIP14 play important roles in hVIC in vitro calcification and osteogenic differentiation. 498

Additionally, left ventricular noncompaction (LVNC) is a cardiomyopathy caused by arrested compaction, characterized by excessive trabeculation with deep intertrabecular recesses and thin compact myocardium. 503 ZIP8 has been identified as a crucial factor in ventricular trabeculation and compaction, revealing a potentially novel regulator of ventricular myocardial development. As such, it may be included in the list of genes worth screening in patients with ventricular noncompaction or other diseases involving dysregulation of ECM degradation. 503

In conclusion, the effects of zinc on cardiovascular disease are multifaceted. Understanding the mechanisms by which cellular zinc metabolism and regulatory mechanisms influence these processes has the potential to develop new strategies for the treatment of cardiovascular disease.

Cellular zinc metabolism in autoimmune diseases

Zinc plays various roles in autoimmune diseases, including its function as an effector of the immune system, inflammation, and metabolism. As mentioned previously, the ZIP family, ZnT family, and MTs act as crucial regulators of zinc levels and are involved in developing different autoimmune diseases, such as the production of autoantibodies and inflammatory responses.

One specific autoimmune disease is type 1 diabetes, characterized by the destruction of pancreatic β cells mediated by T cells. Additionally, individuals with type 1 diabetes exhibit circulating autoantibodies targeting several β cell autoantigens. 504 In 2007, researchers identified zinc transporter 8 autoantibodies (ZnT8A), 233 which have since been recognized as one of the four major islet autoantibodies along with GAD65 autoantibodies (GADA), 505 islet antigen-2 autoantibodies (IA-2A), 506 and insulin autoantibodies (IAA). 507 In prospective studies involving hereditary relatives at first-degree risk for individuals of type 1 diabetes, ZnT8A typically emerges around the age of 3-4 years and persists until the onset of clinical disease. 233 , 508 ZnT8A serves as valuable markers for childhood-onset type 1 diabetes. 509 It is noteworthy that ZnT8A usually develops later in young individuals compared to IAA and GADA. The presence of ZnT8A, as well as IA-2A and ZnT8A positivity, can identify individuals with prediabetes who are at a high risk of rapidly progressing to clinical type 1 diabetes. 510 , 511 Moreover, the HLA class I A*24 allele, which is implicated in increased predisposition to type 1 diabetes, negatively correlates with the presence of ZnT8A at and before diagnosis, taking into account the age at onset. 512 , 513

Studies have proved that CD8 + T cells in individuals with diabetes recognize a range of ZnT8 peptides in different regions of the protein, including the transmembrane/loop and C-terminal regions. 514 , 515 Furthermore, isolated CD8 + T cells from individuals with diabetes show greater secretion of IFN-γ when stimulated by ZnT8. 516 Most of the mature ZnT8A responses target the C-terminal region of the protein, while only 10% recognize the N-terminal region. 517 Within the C-terminal region, ZnT8A can specifically target amino acid 325 of ZnT8, and this specificity is determined by the SLC30A8 polymorphism rs13266634. 518 Interestingly, the higher frequency of ZnT8A in childhood-onset patients is primarily due to an increased number of patients with aa325-nonrestricted ZnT8A. Additionally, the amino acid encoded by the polymorphic codon 325 (Arg, Trp, Gln) plays a significant role in the humoral autoreactivity of this protein. 518 , 519

In addition to T cells, a clinical trial discovered novel cryptic B cell epitopes in the ZnT8 autoantigen, which showed reduced levels of naturally occurring autoantibodies in diabetes. 520 ZnT8A titers decreased rapidly following the initiation of diabetes, reflecting the continuous loss of β-cell mass. 511 , 521 Although type 1 diabetes is commonly linked to other organic-specific autoimmune endocrine diseases, little evidence exists for a linkage between ZnT8A and markers of Addison’s disease (21OHA), autoimmune thyroiditis (TPOA), pernicious anemia (ATP4A-A), or celiac disease (TGA). 522 These findings suggest that islet autoantibodies are not pathogenic in type 1 diabetes but rather a consequence of the immune-mediated destruction of β-cells. From a clinical perspective, reducing ZnT8 transport activity or down-regulating its cellular expression is proposed as an anti-diabetogenic strategy, mimicking the protective effect of SLC30A8 haploinsufficiency in humans. 523

As previously mentioned, ZnT3 is crucial for transporting synaptic vesicular zinc, which can impact various signaling pathways downstream. Previous studies have suggested that zinc release/influx may be an initial event in the production of ROS induced by NADPH oxidase activation in experimental autoimmune encephalomyelitis (EAE). In mice, gene deletion of ZnT3 reduces the clinical symptoms of MOG35–55-induced EAE. This improvement is accompanied by reduced demyelination and the infiltration of encephalitogenic immune cells in the spinal cord. Furthermore, ZnT3 gene deletion inhibits the formation of EAE-associated aberrant synaptic zinc patches, MMP-9 activation, and disruption of the blood-brain barrier. 524 Additionally, Penkowa and Hidalgo demonstrated MT2 could become a prospective treatment candidate in multiple sclerosis, since it reduced cytokine expression in the CNS and prevent apoptotic neuronal death in an EAE model. 525

Genome-wide association studies have revealed an association between the SNP rs13107325 in SLC39A8/ZIP8 and Crohn’s disease. 197 Furthermore, microarray data from rheumatoid arthritis (RA) patients have shown a significant increase in the expression of ZIP8 in peripheral monocytes compared to healthy controls. 526 Monocytes and macrophages play crucial roles in the pathophysiology of RA by delivering enhanced costimulatory signaling and producing proinflammatory cytokines. 527 Since ZIP8 is constitutively expressed in resting monocytes and macrophages, it suggests that ZIP8-mediated zinc influx promotes inflammatory conditions in RA. Therefore, ZIP8 may represent a potential therapeutic target for various inflammatory disorders.

In conclusion, the regulation of cellular zinc metabolism and the involvement of zinc transporters and MTs play crucial roles in autoimmune diseases. This provides valuable insights into potential therapeutic targets and strategies for managing these complex conditions.

Cellular zinc metabolism in infectious diseases

Zinc, a divalent metal, holds a critical role in host-pathogen interactions by influencing microbial growth, pathogenicity, and the host’s immune defenses. Within innate and adaptive immune cells, two distinct and contrasting zinc-dependent mechanisms exist to combat pathogen invasion: nutritional immunity and zinc toxicity. Notably, nutritional immunity is a mechanism employed by immune cells to reduce the availability of zinc in the host, thereby hindering pathogen growth. In parallel, an excessive increase in zinc content within monocytes can induce zinc toxicity in pathogens, leading to their apoptosis. This intriguing interplay of zinc-related pathways highlights its multifaceted impact on the host-pathogen dynamic.

On one hand, nutritional immunity serves as a mechanism employed by immune cells to reduce the availability of zinc in the phagosome or cytoplasm, limiting its access and creating a phenomenon that restricts essential transition metal ions, including iron, zinc, selenium, and manganese, at the host-pathogen interface. This nutrient limitation strategy starves the invading pathogens. 292 , 528 Notably, in vitro studies have demonstrated the potential of zinc limitation strategies to combat carbapenem resistance caused by zinc metallo-β-lactamases, as evidenced by the restoration of carbapenem susceptibility in Acinetobacter baumannii and improved survival in mice infected with Aspergillus fumigatus when pathogens were starved with zinc chelators. 529 , 530 , 531 , 532 This approach may serve as an adjunctive therapy for difficult-to-treat pathogens like Aspergillus fumigatus. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has revealed that tissue abscesses caused by Staphylococcus aureus exhibit significantly lower levels of detectable zinc compared to the high zinc levels in surrounding healthy tissue. 533 While the specific factors responsible for sequestering zinc within abscesses remain unknown, the absence of nutrient zinc within the abscess appears to represent an immune strategy to control infection. Interestingly, in response to zinc sequestration, bacteria have developed mechanisms to overcome this limitation by expressing high-affinity zinc transporters. These zinc uptake systems can be categorized into two groups. The first category includes zinc transporter families with homology to the highly affinity ZnuABC transport system of Escherichia coli . 534 Additionally, both N. gonorrhoeae and N. meningitides express a specific zinc-import system called ZnuC, ZnuB, and ZnuA to improve intracellular zinc status. 535 The second category of zinc transporters is analogous with the eukaryotic ZIP family transporters, but ZIP homologs are exclusively discovered in Escherichia coli . 534

On the other hand, in certain infections like Mycobacterium tuberculosis , the zinc content in the phagosome is excessively increased, leading to zinc intoxication of the pathogen. 536 When monocytes are stimulated with Mycobacterium bovis BCG cell wall, they induce ZIP8 expression, suggesting that extracellular zinc can be drawn in to fuel the host’s zinc poisoning strategy. 131 Nutritional immunity and metal intoxication are feasible immune strategies to limit pathogen growth and control infection. Nutritional immunity primarily affects enzymatic and metabolic functions, while metal overload contributes to the generation of ROS, reactive nitrogen species, protein mismetallization, and subsequent respiratory arrest. 533 , 537 , 538 , 539

In particularly, within macrophages, two lines of host defense are observed: zinc sequestration and zinc intoxication. Sequestration of zinc by MTs deprives pathogens of this essential nutrient, making them susceptible to killing by superoxide. 540 Infection of macrophages with M. tuberculosis triggers zinc intoxication in both the host and the intracellular bacteria, indicating that the host-pathogen interaction disrupts zinc homeostasis in both organisms. The cytokines TNFα and IFNγ promote the accumulation of zinc in the phagosome of Mycobacterium avium-infected mouse macrophages, and phagosomal zinc levels increase over time in response to infection with Mycobacterium tuberculosis. 541 M. tuberculosis infection also up-regulates ZnT1 expression in human macrophages, 542 which probably facilitates the increase of zinc levels in macrophage phagosomes in conjunction with M. tuberculosis . 543 , 544 Additionally, ZIP8 has been identified as a feedback controller of macrophage inflammatory responses. 196 Its expression is upregulated by LPS and TNF, and the mechanism involves direct regulation by the transcription factor NF-κB. LPS also up-regulates ZIP14 mRNA from primary human macrophages, which acts as a limiting inflammatory response. 545 Furthermore, M. tuberculosis possesses a counter-defense strategy that involves extruding incoming zinc via the P1B-type ATPase efflux pump, CtpC, to resist zinc toxicity. 542 Mutant bacilli lacking CtpC are highly sensitive to zinc, rapidly accumulate the metal, and are killed by human macrophages. Macrophages adopt a similar zinc intoxication mechanism to challenge non-pathogenic Escherichia coli , indicating that zinc poisoning is a general defense strategy against intracellular bacteria. 546 Mycobacterial infection causes a “burst of free zinc” within macrophages and increases the levels of zinc-binding proteins, MT1 and MT2, and ZnT1. 547 Although macrophages are not yet proven to be capable of metallotoxicity against pathogenic Neisseria species, it has been shown that these immune cells can enhance zinc accumulation in cytoplasm and phagocytic vesicles through ZIPs. 131 , 140 , 548 This suggests that host-induced zinc toxicity may be relevant to pathogenic Neisseria infection. Therefore, high levels of zinc within macrophages can directly exert bactericidal effects.

Above all, cellular zinc metabolism influences host-pathogen interactions through nutritional immunity and zinc toxicity, affecting pathogen growth and host defense mechanisms. Zinc modulation offers potential therapeutic targets in infectious diseases.

Cellular zinc metabolism in neurodegenerative diseases

Zinc homeostasis alterations have been suggested to be closely associated with the development of certain neurodegenerative diseases. 66 , 549 , 550 In patients with PD, AD, and amyotrophic lateral sclerosis (ALS), there is a significant increase in the zinc content within the cerebrospinal fluid. ZD, on the other hand, was demonstrated to impact neurogenesis as well as augment neuronal apoptosis, resulting in impaired learning and memory, highlighting the importance of elucidating the involvement of cellular zinc metabolism in the pathogenesis of these diseases.

Altered neuronal zinc handling plays a pivotal role in AD pathogenesis. Zinc released during neurotransmission was found to bind to amyloid-β peptides, accelerating the assembly of amyloid-β into oligomers that impair synaptic function. 551 Multiple studies indicate that ZnT3 is crucial for reducing the risk of AD by facilitating the excretion of neuronal zinc. 40 , 552 , 553 , 554 , 555 The expression level of ZnT3 in the cortex has been observed to decline with age in individuals with AD and in healthy individuals. 40 , 556 Additionally, a rare copy number variant of the ZnT3 gene may be involved in the monogenic determination of autosomal dominant early-onset AD. Metal chaperones such as CQ and PBT2, which maintain metal ion homeostasis, have been shown to restore cognition, elevate zinc levels in the hippocampus, and restore levels of key proteins involved in learning/memory and synaptic plasticity in ZnT3 knockout mice. 552 This raises the interesting question of whether metal chaperones could serve as an alternative zinc transporter. It has been found that other transporters, such as vGlut1, may compensate for the deficiency of ZnT3 by loading zinc into synaptic vesicles. 557 In turn, Lang et al. 558 demonstrated that overexpression of the Drosophila homolog of human ZIP1 leads to zinc accumulation in Aβ42-expressing fly brains, and inhibition of ZIP1 expression reduces Aβ42 fibril deposits and improves cognition 16. Zinc binding to amyloid-β is also influenced by MT3 released by astrocytes. Furthermore, the decreased extracellular levels of MT3 observed in AD may facilitate hypermetallation of amyloid-β by zinc. 559 A study utilizing microarray data from the human frontal cortex has shown that the expression of ZNT3 and ZNT4 significantly decreases with age, while the expression of ZIP1, ZIP9, and ZIP13 significantly increases. 67

In vitro observations have confirmed the high enrichment of zinc within senile plaques. AD patients exhibit changes in ZnT proteins (ZnT-1, ZnT-4, and ZnT-6) 5. ZnT1 and ZnT4 are expressed throughout the senile plaque, whereas ZnT3, ZnT5, and ZnT6 are localized to the periphery of the plaque. 560 ZnT10 mRNA expression is significantly decreased in the frontal cortex of patients with AD, 561 similar to the case in APP/PS1 mice. Dysfunction of ZnT10 may contribute to Aβ deposition and the formation of senile plaques. Recently, research has shown that ZIP9 plays a key role in the effects of DHT in APP/PS1 mice. 562 Specifically, ZIP9 influences the expression levels of synaptic proteins, including PSD95, drebrin, and SYP. It also affects dendritic spine density in the hippocampus. These changes are mediated through the ERK1/2-eIF4E signaling pathway, which in turn has an impact on learning and memory processes. Therefore, new experimental evidence suggests that androgen supplementation improves learning and memory in AD.

In addition to AD, alterations in intracellular zinc homeostasis are considered a critical factor in the development of PD. Overwhelming evidence supports the notion that excessive intracellular zinc levels are implicated in the development of the disease. 34 , 563 Zinc directly interacts with α-synuclein, a causative agent of PD and other neurodegenerative diseases, promoting its aggregation. 564 Furthermore, zinc released from corticostriatal terminals may predominantly contribute to the deleterious effects associated with motor and cognitive symptoms in PD, as it acts synergistically with glutamate. 207 Excessive glutamatergic corticostriatal transmission has long been recognized for its contribution to the development of PD symptoms and neurotoxicity, leading to neuronal degeneration.

The relationship between zinc levels and Huntington’s disease (HD) presents contradictory findings. Synaptic dysfunction significantly contributes to the pathogenesis of HD, 565 with vesicular zinc playing a significant role in synaptic function. 566 , 567 Specifically, increased levels of zinc have been measured in HD patients, suggesting that mutant Htt (mHtt) may disturb zinc metabolism. 568 mHtt decreased ZnT3 expression by suppressing the conjugation of Sp1 with ZnT3 promoter. 569 As a result, it downregulates vesicular zinc levels in the brains of N171-82Q HD transgenic mice. However, ZD was observed in the hippocampus and cortex of the R6/1 mouse model of HD. 570 Previous studies have demonstrated significantly higher zinc levels in the cerebrospinal fluid of patients with ALS. Likewise, the protein levels of ZnT3 and ZnT6 are markedly and significantly reduced in the spinal cords of ALS patients, while ZnT5 levels show a tendency to decrease, although not significantly. 571 Importantly, dysregulation of zinc has recently been identified to be a possible procedure causing the disequilibrium in the nucleocytoplasmic distribution of SFPQ in neurodegenerative disorders, consisting of both AD and ALS. 572 SFPQ, an omnipresent nuclear RNA-binding protein intricately involved in diverse facets of RNA genesis, has been closely associated with neuropathological disorders, including AD and ALS. 573 , 574

In conclusion, cellular zinc metabolism appears to play a crucial role in the pathogenesis of neurodegenerative diseases. Altered zinc homeostasis can lead to the formation of senile plaques in AD and contribute to α-synuclein aggregation in PD. ZD and dysregulation have been implicated in synaptic dysfunction and impaired learning and memory. Understanding the intricate relationship between zinc and neurodegenerative diseases may offer potential therapeutic strategies for managing these conditions.

Therapeutic targets for cellular zinc metabolism

In the realm of medical research, identifying and understanding therapeutic targets for cellular zinc metabolism has become an intriguing area of study. The delicate balance of zinc within cells is critical for maintaining various cellular processes and overall physiological well-being. In this essay, we delve into the significance of therapeutic targets related to cellular zinc metabolism, shedding light on their potential implications for human health and developing novel therapeutic interventions.

Zinc transporters

Therapeutic potential of zinc transporters in carcinogenesis.

Zinc transporters not only contribute significantly to the onset and progression of cancer, but they are also implicated in the development of both chemoresistance and radiotherapy resistance. This positions zinc transporters as potential targets for breakthroughs in cancer therapy. Current therapeutic strategies primarily focus on the ZIP family of transporters, employing a variety of approaches, including antibody-drug conjugates (ADCs), siRNAs, and natural inhibitors (Table 1 ). These therapies have demonstrated promising efficacy, and as a result, we posit that the targeting of zinc transporters may emerge as a focal point in the development of future anticancer drugs.

The development of chemoresistance often limits the success of anti-cancer treatments. The acquired resistance is driven to some extent by intra-tumor heterogeneity, mainly directed by cancer stem cells (CSCs). 575 Moreover, the difference between CSCs and non-CSCs within the tumor microenvironment may be primarily attributable to a cell biological procedure called EMT. 576 , 577 Activation of the EMT program enables tumor cells to resist the therapeutic agents, which is consistent with the attribute of CSCs. 578 , 579 As previously mentioned, zinc transporters are pivotal in cell stemness and EMT programs, reflecting their function in chemoresistance. For example, ZIP4 increases gemcitabine resistance primarily due to the activation of ZEB1, via p-STAT3 in PC cells. 395 In other words, ZIP4 upregulated the expression of ZEB1 in PC, which in turn induced a substantial downregulation of gemcitabine uptake protein ENT1 by integrin α3β1, ultimately limiting drug internalization through activation of the MAP kinase JNK. Besides, ZEB1 has also been proven to confer PC drug resistance by suppressing miR-20331. 401 Nabhan et al. found that gemcitabine activity requires caspase activation in multiple myeloma. 580 Interestingly, ZIP4 regulates PC cell apoptosis through the cleavage of caspase. 581 So far, gemcitabine-based therapies have remained the standard of practice for treating advanced PC. 582 Obviously, ZIP4 knockdown combined with gemcitabine may be another promising novel approach for the treatment of PC metastasis and drug resistance.

Moreover, another study substantiated that ZIP4 facilitates EMT of NSCLC. The mechanism is through activation of the Snail-N-cadherin pathway. 583 Similarly, siZIP4 evoked an epithelioid phenotype in NSCLC, reduced the expression of CSC markers, and elevated cisplatin sensitivity. 584 In contrast, within high-grade serous ovarian cancer (HGSOC), the overexpression of ZIP4 increased chemoresistance to cisplatin and doxorubicin. 585 Mechanistically, ZIP4 is an upstream regulator of NOTCH3, a storable signature of CSC in HGSOC. NOTCH3 can regulate proliferation, acid resistance, and drug resistance in carcinomas. 586 , 587 Currently, developing more efficient siRNA delivery techniques is an active segment of ovarian cancer research, 588 and targeting ZIP4 holds excellent promise. In a study on osteosarcoma, the authors found that ZIP10 expression is induced by chemotherapy and that subsequent increased intracellular zinc content activated CREB and promoted ITGA10 expression. 589 Notably, ITGA10 predicted poor osteosarcoma survival because it could promote chemoresistance through PI3K/AKT signaling. Strikingly, the CREB inhibitor 666-15 as well as another small molecule, the PI3K/AKT inhibitor GSK690693, attenuated chemoresistance in the cancer cells with ZIP10 overexpression.

In addition to mediating chemoresistance in tumor cells, zinc transporters also contributed to chemoresistance mediated in stroma cells. It has been reported that interstitial space connection between cancer cells and matrix cells might underpin tumor proliferation and chemoresistance. 590 , 591 In the tumor microenvironment of the lung cancer model, the ZIP1 + CAF subgroup is enrichment after chemotherapy and developed potent gapped junctions with tumor cells via up-regulation of the CX43 protein. 592 This study described a fascinating zinc recycling procedure. Chemotherapy induces necrosis in dying cancer cells and releases unstable zinc to the extracellular compartment. In chemotherapy, tumor cells are inhibited from taking up zinc from the extracellular space, which may lead to ZD in tumor cells. However, ZIP1 + fibroblasts have the ability to serve as zinc reservoirs, allowing the transfer of zinc from fibroblasts into tumor cells, and leading to the induction of ABCB1-mediated drug efflux and chemoresistance. In summary, zinc transporters exert an imperative effect in the tumor microenvironment, helping cancer cells to generate chemoresistance by regulating zinc concentration.

It is well-documented that radiotherapy induces cancer cell apoptosis by DNA damaging. Zinc is essential for the protection of cells against DNA damage, and its role appears to be enhanced in cancer cells. 593 , 594 ZD significantly influences cell cycle. 595 For example, in ESCC, miR-193b modulates the expression of ZIP5 and Cyclin D1. 596 In ZD, miR-193b was observed to be silenced by methylation, which increases ZIP5 expression. Subsequently, ZIP5 overexpression enhanced cellular zinc content, thereby diminishing the DNA damage from radiotherapy. 596 Additionally, radiotherapy resistance is a major barrier limiting the favorable prognosis in NPC as it may lead to tumor recurrence. 597 Zeng et al. found that raised ZIP4 expression activated the PI3K/AKT pathway to induce EMT in NPC cell line C666-1. 77 Accordingly, ZIP4 inhibition augmented radiation-induced apoptosis of C666-1 cells ex vivo and in vivo. Crucially, targeting ZIP4 in conjunction with radiotherapy may be an effective new therapy for treating NPC. 77

ADC is a novel anti-cancer drug consisting of a monoclonal antibody coupled with a cytotoxic drug via chemical linker. 598 ZIP6 is the cell surfacing target that is critical in cancer progression, which is undoubtedly the best candidate for ADC therapy. 599 , 600 As a result, inhibitors of ZIP6, a promising target, are being developed. For example, Seattle Genetics (SGN)-LIV1A or ladiratuzumab vedotin (LV), is currently in clinical trials for metastatic BC. 156 , 601 LIV-1, also called ZIP6, is a transmembrane protein overexpressing in BC. As an ADC, (SGN)-LIV1A is composed of an antibody that specifically binds to ZIP6 on BC cells and a potent cytotoxic drug payload. Upon binding to ZIP6-positive BC cells, (SGN)-LIV1A delivers the cytotoxic drug directly to the cancer cells, inducing cell death.

In addition to ADCs targeting ZIP6, a few small molecules have been reported. For instance, M9S1 extracted from Moringa oleifera significantly downregulated the expression of ZIP6 in MDA-MB-231 tumor, 602 treatment with STAT3 inhibitor peptide, cell-permeable (#573096, Sigma). Besides, the antiestrogens (Faslodex and 4‐hydroxytamoxifen, each 100 nM) also indicated that the expression of LIV-1 was decreased in MCF7 cells. 157 Through phenotypic screening of compounds, a ZIP7 inhibitor, NVS-ZP7-4, was identified that dominates the Notch signaling pathway in T-cell acute lymphoblastic leukemia (T-ALL) cell lines and initiates apoptosis by inducing ER stress. 603 Another research group found that the administration of CK2 inhibitors, such as DMAT (dimehtylamino-4,5,6,7-tetrabromo-1H-benzamidazole) or TBB (4,5,6,7-tetrabromobenzotriazole), inhibited the activity of ZIP7 and was well tolerated by cancer patients. 168 Another advantage of targeting ZIP7 in cancer is that it inhibits the mobilization of a large amount of tyrosine kinases, preventing cancer cells from shifting into another signaling pathway for regeneration. 148

Additionally, testosterone promotes melanoma proliferation through the activation of ZIP9. 461 The classic FDA-approved androgen receptor inhibitor bicalutamide also inhibits ZIP9, thus the antagonist of the tumor-promoting role of testosterone in melanoma, 461 suggesting that ZIP9 may be an effective target for melanoma and other cancers. Correspondingly, novel evidence shows that another androgen, dihydrotestosterone, can increase migration and invasion via ZIP9-mediated intracellular Gαi/MAPK/MMP9 signaling in bladder cancer. 604 Furthermore, bladder cancer progression dependent on ZIP9 could be inhibited by dutasteride, a 5α-reductase inhibitor. 605

Notably, the transcription factor STAT3 was strongly activated and related to a worse outcome in GC. 606 XYA-2, a novel STAT3 naturally occurring product inhibitor, has recently been identified. It synergistically suppresses the expression of MYC and ZIP10 (two downstream genes of STAT3), which exerts an anti-carcinogenic activity. 431 Furthermore, ZIP6/ZIP10 heteromer plays an essential role in zinc-induced mitosis, involving breast cancer proliferation. 156 , 334 Therefore, targeting the ZIP6/ZIP10 heteromer could be a significant approach to inhibit breast cancer invasion. Nimmanon et al. 156 utilized ZIP6 residues 240–253 (ZIP6-Y) and ZIP10 residues 46–59 (ZIP10B) to target ZIP6 and ZIP10, preventing their heteromer formation and thereby impeding the progression of mitosis.

Most tumor-targeted therapeutic studies on zinc transporters have primarily focused on ZIPs, while fewer investigations have been conducted on ZnTs. However, several tumor types, such as pancreatic cancer 607 and GC, 430 exhibit low expression levels of ZnTs. Targeting low-expressed genes is a viable strategy. Gene therapy techniques, 608 such as viral vectors or nanoparticle-based delivery systems, could be employed to deliver ZnTs specifically to tumor cells, enhancing the expression of these low-expressed transporters and providing a targeted therapeutic effect. Alternatively, nanoparticle-based delivery 609 of ZnT’s activators may offer a targeted therapeutic approach. By targeting low-expressed zinc transporter proteins, especially members of the ZnT family, a novel perspective emerges to dysregulate zinc homeostasis in cancer cells.

Thoughtfully, zinc plays an essential physiological function in cells, presenting a dual impact in tumor therapy. Targeting zinc or zinc transporters for tumor therapy shows promise, but potential toxic effects must be considered. Inhibiting zinc transporters or chelating zinc can disrupt vital processes for cancer cell survival and proliferation, displaying potential as an anticancer strategy. Obviously, the potential of targeting zinc transporters in cancer therapy has been identified, and the development of targeted small molecule drugs for clinical cancer patients is imminent. The small molecules potentially targeting the aberrantly activated zinc transporters have been summarized in Table 1 . However, zinc’s significance in normal cellular functions, including DNA repair 610 and immune responses, 611 warrants caution to minimize off-target toxic effects. Precise optimization of zinc-targeted therapies is necessary to achieve tumor-selective cytotoxicity without harming healthy tissues. Understanding zinc’s specific molecular mechanisms in tumorigenesis is pivotal for developing low-toxicity targeted therapies.

Targeting zinc transporters in other diseases

While much of the current research on zinc transporter targeting has been concentrated on tumoral diseases, these essential proteins are not limited to oncological applications. Emerging evidence reveals their significant potential as therapeutic targets for a spectrum of disorders, including anemia, diabetes, malignant muscular dystrophy, and liver fibrosis. Table 2 summarizes the clinical value of targeting zinc transporters beyond cancer therapy.

Emerging research involving two distinctive models, including zip10 mutant zebrafish as well as the hematopoietic Zip10-deficient mice, has made significant strides in our understanding of hematopoiesis. 50 Intriguingly, both models demonstrated more pronounced hematopoietic impairment than their counterparts lacking transferrin receptor 1, an established iron-gatekeeper. Research outcomes suggest a larger effect of zinc than iron in early hematopoietic stem cells (HSCs), underlining the significance of ZIP10 and zinc homeostasis in promoting proliferation and differentiation of fetal HSCs. Thus, a new vista opens for developing of therapeutic strategies against early fetal anemia by targeting ZIP10.

As mentioned in the previous section, zinc and its transporter proteins are implicated in insulin synthesis, secretion, and utilization. A particular study shed light on Zip5, which was found to be down-regulated in pancreatic β-cells of a diabetic mouse model. 612 Intriguingly, the study revealed that zinc influx via Zip5 induced Glut2 expression through the activation of Sirt1-mediated Pgc-1α, proposing Zip5 as potential therapeutic target for diabetes-related diseases. Additionally, zinc transporters, specifically ZIP14, seem to be potential game-changers in the treatment of malignant muscular dystrophy. A conspicuous upregulation of ZIP14 was observed in dystrophic muscles from metastatic cancer. Further investigation revealed that ZIP14-mediated zinc accumulations in differentiating muscle cells cause deletion of myosin heavy chain. 613 This finding underscores the importance of zinc homeostasis regulation in metastatic carcinoma-induced muscular dystrophy and suggests new avenues for treatment by targeting ZIP14.

In the context of liver health, zinc and its transport proteins carry immense importance, particularly in cases of liver fibrosis or cirrhosis. A model of iron metabolism disorders, the Trf-LKO mouse model, was subjected to hepatocyte-specific Trf knockout. 614 The absence of hepatic Zip14 expression reduced hepatic iron build-up, thereby alleviating iron-death-mediated hepatic fibrosis triggered by a high-iron diet or CCl4 injection. Notably, Zip14 can transport iron ions in addition to zinc ions, providing another potential therapeutic avenue for preventing iron-death-induced liver fibrosis. Above all, the diverse roles of zinc transporters underscore their potential as therapeutic targets. The continued exploration of these transporter proteins will likely yield more significant insights and open the door to a broader range of therapeutic applications.

Therapeutic potential of MTs

MTs, by virtue of their metal-binding capabilities, are central to many physiological and pathophysiological processes. They notably regulate zinc and copper homeostasis, shield against oxidative stress, and detoxify heavy metals. 615 , 616 , 617 The exploratory frontier of MTs as potential therapeutic agents has been pushed substantially in recent times.

Neurodegenerative disorders such as AD and PD often exhibit aberrant metal homeostasis and pronounced oxidative stress, paving the way for the potential therapeutic application of MTs. 618 , 619 , 620 Despite the seemingly promising outlook, some investigations have paradoxically led to contrary outcomes. For instance, the Tg2576 mouse model for AD, when subjected to an MT1/2-deficiency, demonstrated a partial rescuing of mortality and body weight changes that were induced by the human amyloid precursor protein. 621 In addition, a reduction in amyloid plaque burden has been observed across both the cerebral cortex and hippocampus, although the overall effects on amyloid cascade, neuroinflammation, and behavior are complicated because of the deletion of MT1/2. 622 In another study focusing on ocular neovascularization, a contributory factor to blindness, MT1/2 was found to play significant roles in retinal and choroidal neovascularization. The authors proposed the potential of MT1/2 as novel therapeutic targets for diseases involving ocular angiogenesis. 623

Furthermore, MTs have demonstrated potential applicability in cancer therapy. Abnormal MT expressions have been detected in numerous cancer types, often exhibiting a correlation between the level of MTs in tumor tissue and disease prognosis. In the context of CRC, MTs are commonly viewed as oncogenes. There is experimental evidence indicating SPINK1’s role in promoting tumor survival in CRC via the suppression of MTs. 624 However, contrary studies have emerged, showing DC-SIGNR’s ability to encourage cancer cell metastasis in CRC through the promotion of MTs. 625 These opposing findings underscore the intricate interplay between MTs and cellular mechanisms during cancer progression.

In conclusion, despite the clearly apparent therapeutic potential of MTs, their role is convoluted and context-dependent. To grasp fully the biological functions of MTs and to harness them effectively for therapeutic strategies, we require a profound understanding which can only come from further dedicated research.

Zinc-based therapeutics and measurement

Beyond targeting cellular zinc metabolism components, the development of zinc-based therapeutics itself is a burgeoning field. Utilizing zinc ions or zinc complexes as therapeutic agents holds potential in various medical applications, including wound healing, antimicrobial treatments, and zinc supplementation for zinc-deficiency-related conditions. The clinical applications of zinc supplements, zinc chelators have been summarized in Table 3 . Meanwhile, Table 4 summarizes the measurements of cellular free zinc.

Zinc supplements

Zinc’s significance in maintaining overall health is extensively discussed in our review. Correspondingly, ZD results in developmental retardation of children, delayed genital development and hypogonadism, skin disorders, hair loss, teratogenic effects, as well as weakened immune function, leading to an increased susceptibility to infections. 626 , 627 Given the wide range of essential biological functions zinc performs, addressing ZD through proper nutrition could make a huge contribution to various facets of human health.

The European Food Safety Authority has delineated different reference daily intakes of zinc for different population groups. 628 , 629 , 630 Specifically, these intake guidelines prescribe a range of 9.4–16.3 mg for men, 7.5–12.7 mg for women, 9.1–14.3 mg for pregnant women, and a lower limit of 5.5–7.4 mg for children aged between 4 and 10 years. Furthermore, they propose an upper threshold for zinc intake, at 25 mg/day for adults, and 7–10 mg/day for children aged between 4 and 13 years, to prevent potential zinc toxicity. Regarding supplements or food fortification, the European Union has authorized several zinc compounds. Among these, zinc sulfate and zinc oxide stand out as popular choices due to their cost-effectiveness. 631 , 632 Zinc sulfate, being water-soluble and comprising 23% zinc, and zinc oxide, though water-insoluble but containing a substantial 80% zinc, are extensively used. 632 Concurrently, zinc citrate has emerged as a promising alternative due to its sensory attributes. This compound contains up to 31% zinc, is minimally insoluble in water, has no odor, and is relatively cost-effective, making it an ideal choice for supplementation. 633 However, data regarding the absorption efficacy of these compounds in humans remains somewhat limited. Research in rats have shown that supplementation with zinc gluconate or zinc citrate resulted in a significant increase in zinc concentrations in the prostate, while zinc sulfate had no effect. 634 Thus, understanding zinc intake recommendations and the efficiency of different zinc compounds for supplementation is crucial to fully optimize the benefits of zinc for various demographic groups. As further research unfolds, it will be important to monitor these developments, to refine and update guidelines accordingly.

Diabetics lose zinc due to increased urinary excretion, leading to diabetic complications. Zinc was described as having insulin-mimetic effects, so zinc supplements may be appropriate for people with diabetes. 635 The ameliorative benefit of zinc supplements in diabetics can be summarized as the potential hypoglycemic effect of zinc, beneficial modulation of concomitant metabolic aberrations and impaired anti-oxidant status, and attenuation of renal lesions. 636 , 637 A meta-analysis showed that zinc supplements dramatically reduced glycemic indices, including two-hour postprandial glucose, fast blood sugar (FBS), and hemoglobin A1c, in all randomized controlled trials. 638 Zinc also has a favorable effect on blood lipids. 639 In addition, low-dose (<25 mg/day), and prolonged (≥12 weeks) intake of zinc from supplements with potential biofortification may be beneficial in reducing risk factors for T2D and cardiovascular disease. 640

In addition, under physiological conditions, zinc binds preferentially to MT, further activating MT to exert its anti-oxidative stress function. Studies have shown that zinc supplementation alleviates MT and oxidative stress in renal tissues of streptozotocin-induced diabetic rats, thereby preventing the development of diabetic nephropathy. 641 Another animal study has shown that zinc supplementation, in particular, reduces the probability of hyperglycemia-mediated renal injury, which also involves the process of oxidative stress. 642 Similarly, an animal study involving streptozotocin-induced diabetic rats has shown that zinc supplementation may protect against diabetes-induced peripheral nerve damage by stimulating MT synthesis and decreasing oxidative stress. 643

Beyond MTs, zinc supplementation also significantly affects the expression of zinc transporters in diabetic patients. 644 Interestingly, the mRNA expression of ZnT8, a transporter closely tied to insulin secretion and hence diabetic conditions, displayed considerable variability. Notably, higher levels of HbA1c, an indicator of long-term glucose control, were found in those participants who exhibited ZnT8 expression compared to their counterparts with no detectable ZnT8 expression. 644 Besides, a positive correlation between the mRNA of ZnT5 and ZIP3 was observed exclusively among participants receiving zinc supplementation. However, the same supplementation seemed to nullify the correlation between ZnT5 and ZIP10. In addition to basic supplementation, recent research has made strides in applying zinc-based therapies for diabetes management. For instance, novel zinc coordination compounds 645 and zinc oxide nanoparticles 646 have been explored for their potential to improve clinical outcomes in diabetes.

Diarrhea leads to significant zinc loss, and zinc supplements have proven effective in their treatment. 647 However, the exact mechanism underlying zinc’s therapeutic effects and its role in preventing subsequent morbidity remains unclear. This may be because zinc is indispensable in maintaining normal immune function. 648 The WHO recommends zinc supplementation alongside oral rehydration salts for diarrhea management. Despite its benefits, zinc supplementation may lead to some side effects. In studies, infants and children receiving zinc gluconate (10 mg or 20 mg of elemental zinc, respectively) experienced more days with vomiting compared to the control group. 649 Besides, one systematic review reported a higher risk of vomiting with zinc gluconate compared to zinc sulfate or zinc acetate. 650 It has been suggested that the unpleasant taste of zinc contributes to vomiting, but this is more probably because of zinc’s gastric irritant properties. 651

In fact, higher concentrations of zinc have been found to disrupt the absorption of other essential trace elements, especially copper. 652 Consequently, patients with copper overload, such as those with Wilson’s disease, may gain from treatment with 50 mg of zinc acetate three or more times a day, which remains highly effective for up to 10 years. 653 However, it is crucial to be cautious about potential adverse effects. One concern is that zinc supplementation could result in copper deficiency, in turn causing severe anemia and neutropenia. 654 Moreover, supplementation with 80 mg of zinc per day for a week resulted in the suppression of mixed lymphocyte cultures in the body, demonstrating that high levels of zinc can impede immune function. 655 Thus, to ensure the safe and effective use of zinc supplementation, it is recommended to limit the daily dose to no more than 25 mg. 640 Higher dosages, especially extreme dosages of more than 75 mg/day, may increase the risk of developing aggressive prostate cancer. 640 , 656 These findings are in line with the tolerable upper intake levels (ULs) of zinc set in both the Americans (40 mg/day) and Europeans (25 mg/day). 657

Zinc chelators

In laboratory settings, researchers utilize specific zinc chelators to investigate processes that rely on zinc. One of the most used selective and membrane-permeable chelators for zinc ions is N, N, N’, N’-tetrakis (2-pyridinylmethyl)-1,2-ethanediamine (TPEN). TPEN exhibits the highest affinity for zinc compared to other chelators (Ka = 1015.58 M −1 ). 44 Numerous reports have shown that depletion of zinc from cells through chelation is considered a potential cancer treatment strategy. 160 , 658 , 659 However, it is essential to interpret zinc effects cautiously and assess their physiological relevance in such studies. TPEN’s strong zinc-binding affinity enables it to virtually eliminate the entire zinc response pool, a condition not attainable under normal or pathological circumstances, leading to predictable cell death.

In contrast, 2,3-dimercapto-1-propanesulfonic acid (DMPS), a heavy metal chelator, has the highest affinity for copper. 660 Interestingly, DMPS has also been identified as a zinc chelator and has been found to effectively antagonize Zn 2+ -dependent snake venom metalloproteinases in vitro. 661 Another widely used chelating agent is EDTA (Ethylenediaminetetraacetic acid), which forms stable complexes with various metal ions, including zinc. 662 For example, in the context of therapeutic modulation in traumatic brain injury (TBI), zinc has emerged as a target. 663 EDTA significantly increased the expression of neuroprotective genes and proteins after TBI.

Clioquinol, recently used as a topical agent for treating some skin infections, has drawn interest from researchers due to its zinc and copper chelating properties, making it a potential candidate for AD. 664 , 665 The chelating activity of zinc appears to play a direct role in heme production. 666 Both zinc and copper contribute to the deposition and stabilization of amyloid plaques, and chelators were shown to solubilize amyloid deposits. 667 Notably, as zinc is essential for heme synthesis, which is recognized as increased in the brain of AD sufferers leading to oxidative stress, clioquinol’s binding to zinc reduces heme synthesis and oxidative stress.

Zinc measurement

The complexity of distinguishing protein-bound zinc from unbound zinc in experimental setups has led to the development and employment of various methods for specific investigations. The techniques used can be broadly divided into two categories: analytical methods and fluorescence techniques.

Analytical methods such as atomic absorption/emission spectroscopy and inductively coupled plasma mass spectrometry offer a relatively straightforward means of measuring total zinc, including both bound and unbound forms. 668 These methods are particularly useful in obtaining a holistic view of zinc content within a given sample.

Moreover, fluorescence microscopy/spectroscopy is primarily employed to study the zinc pool without binding to protein. Two main fluorescence techniques are key in this aspect: low molecular weight (LMW) fluorescent/fluorogenic chelating agents (probes) and genetically encoded fluorescent proteins. 12 Typically bifunctional and comprising both chelating agent and fluorophore, LMW probes function mainly on the principle of photo-induced electron transfer (PET). 669 PET occurs among fluorophore and the chelating component, leading to fluorescence quenching, and this process is disrupted by zinc binding, leading to enhanced emission. 670

Further advancements in fluorescence techniques have led to the common utilization of Förster Resonance Energy Transfer (FRET) and Bioluminescence Resonance Energy Transfer (BRET) sensors, both genetically encoded specifically for zinc. 671 FRET sensors, with their inherently ratiometric nature, utilize interconnected donor as well as acceptor molecules, linked by a peptide sequence containing a zinc-binding domain. 669 Changes in zinc concentration lead to conformational changes that alter energy transmission and affect the strength of the emission fluorescence. 669 , 672 BRET, conversely, focuses on the transmission of energy across the fluorescent structural domains of the donor luciferase and the acceptor. Major advantages offered by BRET sensors are their resistance to photobleaching, absence of phototoxicity, and lack of background autofluorescence during measurement. 671 , 673 These characteristics make BRET an invaluable tool for examining dynamic interactions and enzymatic activity in living cells.

Besides, specific genetically encoded sensors like CALWY, Zap/ZifCY, and those based on carbonic anhydrase are increasingly being used to gain enhanced control over intracellular zinc concentration and location. 674 , 675 , 676 , 677 , 678 These sensors provide tailored advantages in managing intracellular variables, including concentration, localization, and calibration. Recently, a set of innovative organelle-targetable zinc fluorescent probes has been developed, comprising ZnDA-1H, ZnDA2H, and ZnDA-3H. 27 These cutting-edge probes feature HaloTag ligand (HTL) molecules, which facilitate precise localization within specific organelles, and provide an excellent means of studying the physiological functions of the ZIP members residing in the ER and Golgi apparatus.

In conclusion, from comprehensive analytical methods to fine-tuned fluorescence techniques like FRET and BRET, researchers are now equipped with diverse tools that provide multidimensional perspectives on zinc’s behavior and interactions. The synthesis of these tools within a clinical context could revolutionize patient care, fostering a new era of precision medicine where zinc measurement and manipulation become critical components in disease prevention, diagnosis, and treatment.

Conclusion and future direction

Undoubtedly, cellular zinc metabolism and zinc signaling are critical in a variety of biological functions, spanning from essential cellular processes to the development and progression of various diseases. Zinc acts as an essential modulator of cell homeostasis as well as is engaged in key signaling pathways that impact cell growth, proliferation, immune responses, and DNA repair. Dysregulation of zinc metabolism and signaling has been linked to numerous diseases, including cancer, neurodegenerative disorders, and infectious diseases.

Evidence suggests that a safe range of zinc intake is negatively associated with cancer risk. However, cancer cells inevitably require more zinc to maintain the oncogenic properties and metastasis, which functionally relies on the zinc transporter. Previous studies reported that the zinc transporter is aberrantly elevated and activated among multiple tumor types, particularly GI cancers. The significant upregulation of zinc transporters in GI cancers might be because that zinc absorption depends on the epithelial cells of the GI tract, which is the most vulnerable region for zinc homeostasis disorders. In BC and ESCC, zinc transporter ZIP6 is regarded as a diagnostic and prognostic biomarker. Similarly, ZIP10 is regarded as a cancer marker based on its methylation in CRC. Aberrant expression or hyperactivation of zinc transporters would also contribute to tumor resistance, which could be a malprognostic factor for cancer patients. Therefore, aiming at zinc transporters is expected to improve the efficacy of tumor therapies. Meanwhile, since zinc transporter proteins are predominantly distributed on cell membranes, developing small molecules or monoclonal antibodies for specific targeting is feasible.

Obviously, targeting zinc transporters offers potential strategies for treating various diseases, including cancer, neurodegenerative disorders, and infections. However, the study of zinc transporters is still at an infant stage. There are still several issues to be addressed, especially in cancer research. Firstly, the molecular mechanism for the expression of zinc transporters should be further elucidated. Nearly all the upstream regulatory mechanisms of the zinc transporter are still lacking. Thus, it is imperative to elucidate the critical transcriptional factors in regulating zinc transporter expression. Meanwhile, post-transcriptional and post-transcriptional regulation mechanisms need to be addressed. Next, several intellectual gaps still exist concerning the clinical relevance of zinc transporters and their downstream effectors in tumorigenesis. As the mechanisms of ZIPs and ZnTs are totally different in different cancer types, the detailed functional roles and underlying mechanisms are required to be comprehensively revealed. A comprehensive study of zinc transporter-related signaling might accelerate the development of combination therapeutic approaches specifically geared toward zinc transporters. Furthermore, apart from the cancer cell itself, the gut microbiota, including bacteria and viruses, has been implicated in playing a vital role in tumorigenesis and impacting the therapeutic efficacies of cancer patients, especially GI patients. We speculated that the gut microbiome might manipulate the zinc transporter expression and is involved in zinc-related signaling transduction. It will be a research focus on how the microbiome changes reshape the zinc transporters in tumor initiation and development. Finally, targeting zinc transporter is promising for eliminating cancer by developing small-molecule drugs and monoclonal antibodies. Notably, taking advantage of the fact that most zinc transporters are found to be localized on the membrane surface of cancer cells, targeting cancer cells with ADCs is also a potential therapeutic strategy. Meanwhile, it is required to carefully appraise the benefits and side effects of drugs targeting zinc transporters and develop novel delivery strategies. In conclusion, zinc transporters play multifaceted roles in solid tumors, and serve as diagnostic/prognostic tools and therapeutic targets.

Undeniably, the understanding of cellular zinc metabolism and zinc signaling is still evolving, and future investigations in this field are promising. The potential of zinc-based therapies, such as zinc supplements and zinc chelators, warrants exploration in the context of specific diseases. Understanding the optimal dosage, timing, and potential side effects of zinc supplementation or chelation will be crucial for the successful translation of these approaches into clinical practice. Besides, the detection of zinc levels and zinc-related molecular alterations in biological samples may serve as diagnostic biomarkers for various diseases, aiding early detection and guiding treatment decisions. In conclusion, research efforts in cellular zinc metabolism and zinc signaling will deepen the scope of our comprehension of fundamental biological processes and pioneer the way for emerging therapies to combat disease.

Huang, L. & Tepaamorndech, S. The SLC30 family of zinc transporters - a review of current understanding of their biological and pathophysiological roles. Mol. Asp. Med. 34 , 548–560 (2013).

Article   CAS   Google Scholar  

Kambe, T., Tsuji, T., Hashimoto, A. & Itsumura, N. The Physiological, Biochemical, and Molecular Roles of Zinc Transporters in Zinc Homeostasis and Metabolism. Physiol. Rev. 95 , 749–784 (2015).

Article   CAS   PubMed   Google Scholar  

Kimura, T. & Kambe, T. The Functions of Metallothionein and ZIP and ZnT Transporters: An Overview and Perspective. Int J. Mol. Sci. 17 , 336 (2016).

Article   PubMed   PubMed Central   Google Scholar  

Hu, H. et al. New anti-cancer explorations based on metal ions. J. Nanobiotechnol. 20 , 457 (2022).

Article   Google Scholar  

Stockwell, B. R., Jiang, X. & Gu, W. Emerging mechanisms and disease relevance of ferroptosis. Trends Cell Biol. 30 , 478–490 (2020).

Article   CAS   PubMed   PubMed Central   Google Scholar  

Andreini, C., Bertini, I. & Rosato, A. Metalloproteomes: a bioinformatic approach. Acc. Chem. Res. 42 , 1471–1479 (2009).

Angus-Hill, M. L. et al. A Rsc3/Rsc30 zinc cluster dimer reveals novel roles for the chromatin remodeler RSC in gene expression and cell cycle control. Mol. Cell. 7 , 741–751 (2001).

Kim, A. M. et al. Zinc sparks are triggered by fertilization and facilitate cell cycle resumption in mammalian eggs. ACS Chem. Biol. 6 , 716–723 (2011).

Lo, M. N. et al. Single cell analysis reveals multiple requirements for zinc in the mammalian cell cycle. Elife 9 , e51107 (2020).

Haase, H. & Rink, L. Multiple impacts of zinc on immune function. Metallomics 6 , 1175–1180 (2014).

Que, E. L. et al. Quantitative mapping of zinc fluxes in the mammalian egg reveals the origin of fertilization-induced zinc sparks. Nat. Chem. 7 , 130–139 (2015).

Maret, W. Analyzing free zinc(II) ion concentrations in cell biology with fluorescent chelating molecules. Metallomics 7 , 202–211 (2015).

Hennigar, S. R., Kelley, A. M. & McClung, J. P. Metallothionein and zinc transporter expression in circulating human blood cells as biomarkers of zinc status: a systematic review. Adv. Nutr. 7 , 735–746 (2016).

Bafaro, E., Liu, Y., Xu, Y. & Dempski, R. E. The emerging role of zinc transporters in cellular homeostasis and cancer. Signal Transduct. Target Ther. 2 , 17029- (2017).

Calesnick, B. & Dinan, A. M. Zinc deficiency and zinc toxicity. Am. Fam. Physician 37 , 267–270 (1988).

CAS   PubMed   Google Scholar  

Stefanidou, M., Maravelias, C., Dona, A. & Spiliopoulou, C. Zinc: a multipurpose trace element. Arch. Toxicol. 80 , 1–9 (2006).

Gilbert, R., Peto, T., Lengyel, I. & Emri, E. Zinc nutrition and inflammation in the aging retina. Mol. Nutr. Food Res. 63 , e1801049 (2019).

Article   PubMed   Google Scholar  

Pfeiffer, C. C. & Braverman, E. R. Zinc, the brain and behavior. Biol. Psychiatry 17 , 513–532 (1982).

Tapiero, H. & Tew, K. D. Trace elements in human physiology and pathology: zinc and metallothioneins. Biomed. Pharmacother. 57 , 399–411 (2003).

Costello, L. C., Fenselau, C. C. & Franklin, R. B. Evidence for operation of the direct zinc ligand exchange mechanism for trafficking, transport, and reactivity of zinc in mammalian cells. J. Inorg. Biochem. 105 , 589–599 (2011).

Maret, W. Zinc coordination environments in proteins as redox sensors and signal transducers. Antioxid. Redox Signal. 8 , 1419–1441 (2006).

Turan, B. & Tuncay, E. Impact of labile zinc on heart function: from physiology to pathophysiology. Int J. Mol. Sci. 18 , 2395 (2017).

Coyle, P., Philcox, J. C., Carey, L. C. & Rofe, A. M. Metallothionein: the multipurpose protein. Cell Mol. Life Sci. 59 , 627–647 (2002).

Outten, C. E. & O’Halloran, T. V. Femtomolar sensitivity of metalloregulatory proteins controlling zinc homeostasis. Science 292 , 2488–2492 (2001).

Blindauer, C. A. & Leszczyszyn, O. I. Metallothioneins: unparalleled diversity in structures and functions for metal ion homeostasis and more. Nat. Prod. Rep. 27 , 720–741 (2010).

Wang, X. L., Schnoor, M. & Yin, L. M. Metallothionein-2: an emerging target in inflammatory diseases and cancers. Pharm. Ther. 244 , 108374 (2023).

Amagai, Y. et al. Zinc homeostasis governed by Golgi-resident ZnT family members regulates ERp44-mediated proteostasis at the ER-Golgi interface. Nat. Commun. 14 , 2683 (2023).

Fang, H. et al. Simultaneous Zn(2+) tracking in multiple organelles using super-resolution morphology-correlated organelle identification in living cells. Nat. Commun. 12 , 109 (2021).

Frederickson, C. J., Koh, J. Y. & Bush, A. I. The neurobiology of zinc in health and disease. Nat. Rev. Neurosci. 6 , 449–462 (2005).

Eide, D. J. The SLC39 family of metal ion transporters. Pflug. Arch. 447 , 796–800 (2004).

Bin, B. H. et al. Molecular pathogenesis of spondylocheirodysplastic Ehlers-Danlos syndrome caused by mutant ZIP13 proteins. EMBO Mol. Med. 6 , 1028–1042 (2014).

Wang, Z., Tymianski, M., Jones, O. T. & Nedergaard, M. Impact of cytoplasmic calcium buffering on the spatial and temporal characteristics of intercellular calcium signals in astrocytes. J. Neurosci. 17 , 7359–7371 (1997).

Krezel, A. & Maret, W. Zinc-buffering capacity of a eukaryotic cell at physiological pZn. J. Biol. Inorg. Chem. 11 , 1049–1062 (2006).

Atrián-Blasco, E. et al. Chemistry of mammalian metallothioneins and their interaction with amyloidogenic peptides and proteins. Chem. Soc. Rev. 46 , 7683–7693 (2017).

Krezel, A. & Maret, W. Dual nanomolar and picomolar Zn(II) binding properties of metallothionein. J. Am. Chem. Soc. 129 , 10911–10921 (2007).

Colvin, R. A., Holmes, W. R., Fontaine, C. P. & Maret, W. Cytosolic zinc buffering and muffling: their role in intracellular zinc homeostasis. Metallomics 2 , 306–317 (2010).

Ueda, S. et al. Early secretory pathway-resident Zn transporter proteins contribute to cellular sphingolipid metabolism through activation of sphingomyelin phosphodiesterase 1. Am. J. Physiol. Cell Physiol. 322 , C948–c959 (2022).

Wagatsuma, T. et al. Pigmentation and TYRP1 expression are mediated by zinc through the early secretory pathway-resident ZNT proteins. Commun. Biol. 6 , 403 (2023).

Chandler, P. et al. Subtype-specific accumulation of intracellular zinc pools is associated with the malignant phenotype in breast cancer. Mol. Cancer 15 , 2 (2016).

Beyer, N. et al. ZnT3 mRNA levels are reduced in Alzheimer’s disease post-mortem brain. Mol. Neurodegener. 4 , 53 (2009).

Chimienti, F., Devergnas, S., Favier, A. & Seve, M. Identification and cloning of a beta-cell-specific zinc transporter, ZnT-8, localized into insulin secretory granules. Diabetes 53 , 2330–2337 (2004).

Maret, W. Redox biochemistry of mammalian metallothioneins. J. Biol. Inorg. Chem. 16 , 1079–1086 (2011).

Hirano, T. et al. Roles of zinc and zinc signaling in immunity: zinc as an intracellular signaling molecule. Adv. Immunol. 97 , 149–176 (2008).

Yamasaki, S. et al. Zinc is a novel intracellular second messenger. J. Cell Biol. 177 , 637–645 (2007).

Bonaventura, P., Benedetti, G., Albarède, F. & Miossec, P. Zinc and its role in immunity and inflammation. Autoimmun. Rev. 14 , 277–285 (2015).

Liu, W. et al. Lactate regulates cell cycle by remodelling the anaphase promoting complex. Nature 616 , 790–797 (2023).

Wang, L. et al. Co-implantation of magnesium and zinc ions into titanium regulates the behaviors of human gingival fibroblasts. Bioact. Mater. 6 , 64–74 (2021).

Xiao, W. et al. Therapeutic targeting of the USP2-E2F4 axis inhibits autophagic machinery essential for zinc homeostasis in cancer progression. Autophagy 18 , 2615–2635 (2022).

Supasai, S. et al. Zinc deficiency affects the STAT1/3 signaling pathways in part through redox-mediated mechanisms. Redox Biol. 11 , 469–481 (2017).

He, X. et al. The zinc transporter SLC39A10 plays an essential role in embryonic hematopoiesis. Adv. Sci. 10 , e2205345 (2023).

Feske, S., Wulff, H. & Skolnik, E. Y. Ion channels in innate and adaptive immunity. Annu Rev. Immunol. 33 , 291–353 (2015).

Chaigne-Delalande, B. & Lenardo, M. J. Divalent cation signaling in immune cells. Trends Immunol. 35 , 332–344 (2014).

Ma, T. et al. A pair of transporters controls mitochondrial Zn(2+) levels to maintain mitochondrial homeostasis. Protein Cell. 13 , 180–202 (2022).

Chen, H. C. et al. Sub-acute restraint stress progressively increases oxidative/nitrosative stress and inflammatory markers while transiently upregulating antioxidant gene expression in the rat hippocampus. Free Radic. Biol. Med. 130 , 446–457 (2019).

Si, M. & Lang, J. The roles of metallothioneins in carcinogenesis. J. Hematol. Oncol. 11 , 107 (2018).

Aras, M. A. & Aizenman, E. Redox regulation of intracellular zinc: molecular signaling in the life and death of neurons. Antioxid. Redox Signal. 15 , 2249–2263 (2011).

McCord, M. C. & Aizenman, E. Convergent Ca2+ and Zn2+ signaling regulates apoptotic Kv2.1 K+ currents. Proc. Natl Acad. Sci. USA. 110 , 13988–13993 (2013).

Millward, D. J. Nutrition, infection and stunting: the roles of deficiencies of individual nutrients and foods, and of inflammation, as determinants of reduced linear growth of children. Nutr. Res Rev. 30 , 50–72 (2017).

Ren, M. et al. Associations between hair levels of trace elements and the risk of preterm birth among pregnant Wwomen: a prospective nested case-control study in Beijing Birth Cohort (BBC), China. Environ. Int. 158 , 106965 (2022).

Chorin, E. et al. Upregulation of KCC2 activity by zinc-mediated neurotransmission via the mZnR/GPR39 receptor. J. Neurosci. 31 , 12916–12926 (2011).

Anderson, C. T. et al. Modulation of extrasynaptic NMDA receptors by synaptic and tonic zinc. Proc. Natl Acad. Sci. USA. 112 , E2705–E2714 (2015).

Medvedeva, Y. V., Ji, S. G., Yin, H. Z. & Weiss, J. H. Differential vulnerability of CA1 versus CA3 pyramidal neurons after ischemia: possible relationship to sources of Zn2+ accumulation and its entry into and prolonged effects on mitochondria. J. Neurosci. 37 , 726–737 (2017).

CAS   PubMed   PubMed Central   Google Scholar  

Michelotti, F. C. et al. PET/MRI enables simultaneous in vivo quantification of β-cell mass and function. Theranostics 10 , 398–410 (2020).

Carver, C. M., Chuang, S. H. & Reddy, D. S. Zinc selectively blocks neurosteroid-sensitive extrasynaptic δGABAA receptors in the hippocampus. J. Neurosci. 36 , 8070–8077 (2016).

Dostalova, Z. et al. Human α1β3γ2L gamma-aminobutyric acid type A receptors: high-level production and purification in a functional state. Protein Sci. 23 , 157–166 (2014).

Sensi, S. L., Paoletti, P., Bush, A. I. & Sekler, I. Zinc in the physiology and pathology of the CNS. Nat. Rev. Neurosci. 10 , 780–791 (2009).

Olesen, R. H. et al. Obesity and age-related alterations in the gene expression of zinc-transporter proteins in the human brain. Transl. Psychiatry 6 , e838 (2016).

Ren, L. et al. Amperometric measurements and dynamic models reveal a mechanism for how zinc alters neurotransmitter release. Angew. Chem. Int Ed. Engl. 59 , 3083–3087 (2020).

Hershfinkel, M. The zinc sensing receptor, ZnR/GPR39, in health and disease. Int J. Mol. Sci. 19 , 439 (2018).

Ho, E. & Ames, B. N. Low intracellular zinc induces oxidative DNA damage, disrupts p53, NFkappa B, and AP1 DNA binding, and affects DNA repair in a rat glioma cell line. Proc. Natl Acad. Sci. USA. 99 , 16770–16775 (2002).

Nuñez, N. N. et al. The zinc linchpin motif in the DNA repair glycosylase MUTYH: identifying the Zn(2+) ligands and roles in damage recognition and repair. J. Am. Chem. Soc. 140 , 13260–13271 (2018).

Lecane, P. S. et al. Motexafin gadolinium and zinc induce oxidative stress responses and apoptosis in B-cell lymphoma lines. Cancer Res. 65 , 11676–11688 (2005).

Cheng, X. et al. Zinc transporter SLC39A13/ZIP13 facilitates the metastasis of human ovarian cancer cells via activating Src/FAK signaling pathway. J. Exp. Clin. Cancer Res. 40 , 199 (2021).

Liu, M. et al. Zinc-dependent regulation of ZEB1 and YAP1 coactivation promotes epithelial-mesenchymal transition plasticity and metastasis in pancreatic cancer. Gastroenterology 160 , 1771–1783.e1771 (2021).

Yang, J. et al. ZIP4 promotes muscle wasting and cachexia in mice with orthotopic pancreatic tumors by stimulating RAB27B-regulated release of extracellular vesicles from cancer cells. Gastroenterology 156 , 722–734.e726 (2019).

Wagner, E. F. & Nebreda, A. R. Signal integration by JNK and p38 MAPK pathways in cancer development. Nat. Rev. Cancer 9 , 537–549 (2009).

Zeng, Q. et al. Inhibition of ZIP4 reverses epithelial-to-mesenchymal transition and enhances the radiosensitivity in human nasopharyngeal carcinoma cells. Cell Death Dis. 10 , 588 (2019).

Qi, J. et al. MCOLN1/TRPML1 finely controls oncogenic autophagy in cancer by mediating zinc influx. Autophagy 17 , 4401–4422 (2021).

Su, X. et al. Disruption of zinc homeostasis by a novel platinum(IV)-terthiophene complex for antitumor immunity. Angew. Chem. Int Ed. Engl. 62 , e202216917 (2023).

Jeong, J. & Eide, D. J. The SLC39 family of zinc transporters. Mol. Asp. Med. 34 , 612–619 (2013).

Zhang, T., Sui, D. & Hu, J. Structural insights of ZIP4 extracellular domain critical for optimal zinc transport. Nat. Commun. 7 , 11979 (2016).

Zhang, T. et al. Crystal structures of a ZIP zinc transporter reveal a binuclear metal center in the transport pathway. Sci. Adv. 3 , e1700344 (2017).

Pang, C. et al. Structural mechanism of intracellular autoregulation of zinc uptake in ZIP transporters. Nat. Commun. 14 , 3404 (2023).

Bogdan, A. R., Miyazawa, M., Hashimoto, K. & Tsuji, Y. Regulators of iron homeostasis: new players in metabolism, cell death, and disease. Trends Biochem Sci. 41 , 274–286 (2016).

Jeong, J. et al. Promotion of vesicular zinc efflux by ZIP13 and its implications for spondylocheiro dysplastic Ehlers-Danlos syndrome. Proc. Natl Acad. Sci. USA. 109 , E3530–E3538 (2012).

Bin, B. H. et al. Biochemical characterization of human ZIP13 protein: a homo-dimerized zinc transporter involved in the spondylocheiro dysplastic Ehlers-Danlos syndrome. J. Biol. Chem. 286 , 40255–40265 (2011).

Lichten, L. A. et al. MTF-1-mediated repression of the zinc transporter Zip10 is alleviated by zinc restriction. PLoS One 6 , e21526 (2011).

Ryu, M. S., Lichten, L. A., Liuzzi, J. P. & Cousins, R. J. Zinc transporters ZnT1 (Slc30a1), Zip8 (Slc39a8), and Zip10 (Slc39a10) in mouse red blood cells are differentially regulated during erythroid development and by dietary zinc deficiency. J. Nutr. 138 , 2076–2083 (2008).

Liuzzi, J. P. et al. Responsive transporter genes within the murine intestinal-pancreatic axis form a basis of zinc homeostasis. Proc. Natl Acad. Sci. USA. 101 , 14355–14360 (2004).

Taylor, K. M. & Nicholson, R. I. The LZT proteins; the LIV-1 subfamily of zinc transporters. Biochim. Biophys. Acta 1611 , 16–30 (2003).

Xin, Y. et al. Manganese transporter Slc39a14 deficiency revealed its key role in maintaining manganese homeostasis in mice. Cell Discov. 3 , 17025 (2017).

Polesel, M. et al. Functional characterization of SLC39 family members ZIP5 and ZIP10 in overexpressing HEK293 cells reveals selective copper transport activity. Biometals 36 , 227–237 (2023).

Boycott, K. M. et al. Autosomal-recessive intellectual disability with cerebellar atrophy syndrome caused by mutation of the manganese and zinc transporter gene SLC39A8. Am. J. Hum. Genet. 97 , 886–893 (2015).

Jorge-Nebert, L. F. et al. Comparing gene expression during cadmium uptake and distribution: untreated versus oral Cd-treated wild-type and ZIP14 knockout mice. Toxicol. Sci. 143 , 26–35 (2015).

Himeno, S., Yanagiya, T. & Fujishiro, H. The role of zinc transporters in cadmium and manganese transport in mammalian cells. Biochimie 91 , 1218–1222 (2009).

Nebert, D. W. & Liu, Z. SLC39A8 gene encoding a metal ion transporter: discovery and bench to bedside. Hum. Genomics. 13 , 51 (2019).

Liu, Z. et al. Cd2+ versus Zn2+ uptake by the ZIP8 HCO3–dependent symporter: kinetics, electrogenicity and trafficking. Biochem. Biophys. Res Commun. 365 , 814–820 (2008).

Napolitano, J. R. et al. Cadmium-mediated toxicity of lung epithelia is enhanced through NF-κB-mediated transcriptional activation of the human zinc transporter ZIP8. Am. J. Physiol. Lung Cell Mol. Physiol. 302 , L909–L918 (2012).

Girijashanker, K. et al. Slc39a14 gene encodes ZIP14, a metal/bicarbonate symporter: similarities to the ZIP8 transporter. Mol. Pharmacol. 73 , 1413–1423 (2008).

Pinilla-Tenas, J. J. et al. Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron. Am. J. Physiol. Cell Physiol. 301 , C862–C871 (2011).

Liuzzi, J. P. et al. Zip14 (Slc39a14) mediates non-transferrin-bound iron uptake into cells. Proc. Natl Acad. Sci. USA. 103 , 13612–13617 (2006).

Wang, C. Y. et al. ZIP8 is an iron and zinc transporter whose cell-surface expression is up-regulated by cellular iron loading. J. Biol. Chem. 287 , 34032–34043 (2012).

Jenkitkasemwong, S. et al. SLC39A14 is required for the development of hepatocellular iron overload in murine models of hereditary hemochromatosis. Cell Metab. 22 , 138–150 (2015).

Kambe, T., Matsunaga, M. & Takeda, T. A. Understanding the contribution of zinc transporters in the function of the early secretory pathway. Int J. Mol. Sci. 18 , 2179 (2017).

Davidson, H. W., Wenzlau, J. M. & O’Brien, R. M. Zinc transporter 8 (ZnT8) and beta cell function. Trends Endocrinol. Metab. 25 , 415–424 (2014).

Suzuki, T. et al. Zinc transporters, ZnT5 and ZnT7, are required for the activation of alkaline phosphatases, zinc-requiring enzymes that are glycosylphosphatidylinositol-anchored to the cytoplasmic membrane. J. Biol. Chem. 280 , 637–643 (2005).

Nishito, Y. & Kambe, T. Zinc transporter 1 (ZNT1) expression on the cell surface is elaborately controlled by cellular zinc levels. J. Biol. Chem. 294 , 15686–15697 (2019).

Lichten, L. A. & Cousins, R. J. Mammalian zinc transporters: nutritional and physiologic regulation. Annu Rev. Nutr. 29 , 153–176 (2009).

Wang, Y. et al. Zinc application alleviates the adverse renal effects of arsenic stress in a protein quality control way in common carp. Environ. Res. 191 , 110063 (2020).

Dwivedi, O. P. et al. Loss of ZnT8 function protects against diabetes by enhanced insulin secretion. Nat. Genet. 51 , 1596–1606 (2019).

Henshall, S. M. et al. Expression of the zinc transporter ZnT4 is decreased in the progression from early prostate disease to invasive prostate cancer. Oncogene 22 , 6005–6012 (2003).

Sanchez, V. B., Ali, S., Escobar, A. & Cuajungco, M. P. Transmembrane 163 (TMEM163) protein effluxes zinc. Arch. Biochem. Biophys. 677 , 108166 (2019).

Styrpejko, D. J. & Cuajungco, M. P. Transmembrane 163 (TMEM163) protein: a new member of the zinc efflux transporter family. Biomedicines 9 , 220 (2021).

do Rosario, M. C. et al. Variants in the zinc transporter TMEM163 cause a hypomyelinating leukodystrophy. Brain 145 , 4202–4209 (2022).

Kia, D. A. et al. Identification of candidate Parkinson disease genes by integrating genome-wide association study, expression, and epigenetic data sets. JAMA Neurol. 78 , 464–472 (2021).

Yuan, Y. et al. A zinc transporter, transmembrane protein 163, is critical for the biogenesis of platelet dense granules. Blood 137 , 1804–1817 (2021).

Braun, W. et al. Comparison of the NMR solution structure and the x-ray crystal structure of rat metallothionein-2. Proc. Natl Acad. Sci. USA. 89 , 10124–10128 (1992).

Krężel, A. & Maret, W. The bioinorganic chemistry of mammalian metallothioneins. Chem. Rev. 121 , 14594–14648 (2021).

Merlos Rodrigo, M. A. et al. Metallothionein isoforms as double agents - their roles in carcinogenesis, cancer progression and chemoresistance. Drug Resist. Updat. 52 , 100691 (2020).

Go, Y. M., Chandler, J. D. & Jones, D. P. The cysteine proteome. Free Radic. Biol. Med. 84 , 227–245 (2015).

Marreiro, D. D. et al. Zinc and oxidative stress: current mechanisms. Antioxidants. 6 , 24 (2017).

Guo, L. et al. STAT5-glucocorticoid receptor interaction and MTF-1 regulate the expression of ZnT2 (Slc30a2) in pancreatic acinar cells. Proc. Natl Acad. Sci. USA. 107 , 2818–2823 (2010).

Lu, Y. J. et al. Coordinative modulation of human zinc transporter 2 gene expression through active and suppressive regulators. J. Nutr. Biochem. 26 , 351–359 (2015).

Mocchegiani, E., Giacconi, R. & Malavolta, M. Zinc signalling and subcellular distribution: emerging targets in type 2 diabetes. Trends Mol. Med. 14 , 419–428 (2008).

O’Donnell, J. S., Teng, M. W. L. & Smyth, M. J. Cancer immunoediting and resistance to T cell-based immunotherapy. Nat. Rev. Clin. Oncol. 16 , 151–167 (2019).

Kim, B., Kim, H. Y. & Lee, W. W. Zap70 regulates TCR-mediated Zip6 activation at the immunological synapse. Front. Immunol. 12 , 687367 (2021).

Lee, W. W. et al. Age-dependent signature of metallothionein expression in primary CD4 T cell responses is due to sustained zinc signaling. Rejuvenation Res. 11 , 1001–1011 (2008).

Pommier, A. et al. Inflammatory monocytes are potent antitumor effectors controlled by regulatory CD4+ T cells. Proc. Natl Acad. Sci. USA. 110 , 13085–13090 (2013).

Aydemir, T. B., Liuzzi, J. P., McClellan, S. & Cousins, R. J. Zinc transporter ZIP8 (SLC39A8) and zinc influence IFN-gamma expression in activated human T cells. J. Leukoc. Biol. 86 , 337–348 (2009).

Liu, M. J. et al. ZIP8 regulates host defense through zinc-mediated inhibition of NF-kappaB. Cell Rep. 3 , 386–400 (2013).

Begum, N. A. et al. Mycobacterium bovis BCG cell wall and lipopolysaccharide induce a novel gene, BIGM103, encoding a 7-TM protein: identification of a new protein family having Zn-transporter and Zn-metalloprotease signatures. Genomics 80 , 630–645 (2002).

Kim, B. et al. Cytoplasmic zinc promotes IL-1beta production by monocytes and macrophages through mTORC1-induced glycolysis in rheumatoid arthritis. Sci. Signal. 15 , eabi7400 (2022).

Kang, J. A. et al. ZIP8 exacerbates collagen-induced arthritis by increasing pathogenic T cell responses. Exp. Mol. Med. 53 , 560–571 (2021).

Abd El-Rehim, D. M. et al. High-throughput protein expression analysis using tissue microarray technology of a large well-characterised series identifies biologically distinct classes of breast cancer confirming recent cDNA expression analyses. Int J. Cancer 116 , 340–350 (2005).

Lee, D. S. W., Rojas, O. L. & Gommerman, J. L. B cell depletion therapies in autoimmune disease: advances and mechanistic insights. Nat. Rev. Drug Discov. 20 , 179–199 (2021).

Taniguchi, M. et al. Essential role of the zinc transporter ZIP9/SLC39A9 in regulating the activations of Akt and Erk in B-cell receptor signaling pathway in DT40 cells. PLoS One 8 , e58022 (2013).

Miyai, T. et al. Zinc transporter SLC39A10/ZIP10 facilitates antiapoptotic signaling during early B-cell development. Proc. Natl Acad. Sci. USA. 111 , 11780–11785 (2014).

Hojyo, S. et al. Zinc transporter SLC39A10/ZIP10 controls humoral immunity by modulating B-cell receptor signal strength. Proc. Natl Acad. Sci. USA. 111 , 11786–11791 (2014).

Ma, Z. et al. SLC39A10 upregulation predicts poor prognosis, promotes proliferation and migration, and correlates with immune infiltration in hepatocellular carcinoma. J. Hepatocell. Carcinoma 8 , 899–912 (2021).

Stafford, S. L. et al. Metal ions in macrophage antimicrobial pathways: emerging roles for zinc and copper. Biosci. Rep. 33 , e00049 (2013).

Locati, M., Curtale, G. & Mantovani, A. Diversity, mechanisms, and significance of macrophage plasticity. Annu. Rev. Pathol. 15 , 123–147 (2020).

Gao, H. et al. Metal transporter Slc39a10 regulates susceptibility to inflammatory stimuli by controlling macrophage survival. Proc. Natl Acad. Sci. USA. 114 , 12940–12945 (2017).

Sriskandan, S. & Altmann, D. M. The immunology of sepsis. J. Pathol. 214 , 211–223 (2008).

Wong, H. R. et al. Genome-level expression profiles in pediatric septic shock indicate a role for altered zinc homeostasis in poor outcome. Physiol. Genomics. 30 , 146–155 (2007).

Besecker, B. et al. The human zinc transporter SLC39A8 (Zip8) is critical in zinc-mediated cytoprotection in lung epithelia. Am. J. Physiol. Lung Cell Mol. Physiol. 294 , L1127–L1136 (2008).

Besecker, B. Y. et al. A comparison of zinc metabolism, inflammation, and disease severity in critically ill infected and noninfected adults early after intensive care unit admission. Am. J. Clin. Nutr. 93 , 1356–1364 (2011).

Wessels, I. & Cousins, R. J. Zinc dyshomeostasis during polymicrobial sepsis in mice involves zinc transporter Zip14 and can be overcome by zinc supplementation. Am. J. Physiol. Gastrointest. Liver Physiol. 309 , G768–G778 (2015).

Hogstrand, C., Kille, P., Nicholson, R. I. & Taylor, K. M. Zinc transporters and cancer: a potential role for ZIP7 as a hub for tyrosine kinase activation. Trends Mol. Med. 15 , 101–111 (2009).

Adulcikas, J. et al. The zinc transporter SLC39A7 (ZIP7) harbours a highly-conserved histidine-rich N-terminal region that potentially contributes to zinc homeostasis in the endoplasmic reticulum. Comput Biol. Med. 100 , 196–202 (2018).

Uchida, R. et al. L-type calcium channel-mediated zinc wave is involved in the regulation of IL-6 by stimulating non-IgE with LPS and IL-33 in mast cells and dendritic cells. Biol. Pharm. Bull. 42 , 87–93 (2019).

Levy, S. et al. Molecular basis for zinc transporter 1 action as an endogenous inhibitor of L-type calcium channels. J. Biol. Chem. 284 , 32434–32443 (2009).

Maret, W. Zinc in cellular regulation: the nature and significance of “zinc signals”. Int J. Mol. Sci. 18 , 2285 (2017).

Kim, A. M., Vogt, S., O’Halloran, T. V. & Woodruff, T. K. Zinc availability regulates exit from meiosis in maturing mammalian oocytes. Nat. Chem. Biol. 6 , 674–681 (2010).

Taylor, K. M. et al. Zinc transporter ZIP10 forms a heteromer with ZIP6 which regulates embryonic development and cell migration. Biochem J. 473 , 2531–2544 (2016).

Kong, B. Y. et al. Maternally-derived zinc transporters ZIP6 and ZIP10 drive the mammalian oocyte-to-egg transition. Mol. Hum. Reprod. 20 , 1077–1089 (2014).

Nimmanon, T. et al. The ZIP6/ZIP10 heteromer is essential for the zinc-mediated trigger of mitosis. Cell Mol. Life Sci. 78 , 1781–1798 (2021).

Hogstrand, C. et al. A mechanism for epithelial-mesenchymal transition and anoikis resistance in breast cancer triggered by zinc channel ZIP6 and STAT3 (signal transducer and activator of transcription 3). Biochem. J. 455 , 229–237 (2013).

Mulay, I. L. et al. Trace-metal analysis of cancerous and noncancerous human tissues. J. Natl Cancer Inst. 47 , 1–13 (1971).

Chen, P. H. et al. Zinc transporter ZIP7 is a novel determinant of ferroptosis. Cell Death Dis. 12 , 198 (2021).

Makhov, P. et al. Zinc chelation induces rapid depletion of the X-linked inhibitor of apoptosis and sensitizes prostate cancer cells to TRAIL-mediated apoptosis. Cell Death Differ. 15 , 1745–1751 (2008).

Zhang, R. et al. Zinc regulates primary ovarian tumor growth and metastasis through the epithelial to mesenchymal transition. Free Radic. Biol. Med. 160 , 775–783 (2020).

Hernandez-Camacho, J. D., Vicente-Garcia, C., Parsons, D. S. & Navas-Enamorado, I. Zinc at the crossroads of exercise and proteostasis. Redox Biol. 35 , 101529 (2020).

Ohashi, K. et al. Zinc promotes proliferation and activation of myogenic cells via the PI3K/Akt and ERK signaling cascade. Exp. Cell Res. 333 , 228–237 (2015).

Lee, H. Y. et al. Deletion of Jazf1 gene causes early growth retardation and insulin resistance in mice. Proc. Natl Acad. Sci. USA. 119 , e2213628119 (2022).

Jinno, N., Nagata, M. & Takahashi, T. Marginal zinc deficiency negatively affects recovery from muscle injury in mice. Biol. Trace Elem. Res. 158 , 65–72 (2014).

Lin, P. H. et al. Zinc in wound healing modulation. Nutrients 10 , 16 (2017).

Postigo, A. A. & Dean, D. C. Differential expression and function of members of the zfh-1 family of zinc finger/homeodomain repressors. Proc. Natl Acad. Sci. USA. 97 , 6391–6396 (2000).

Taylor, K. M. et al. Protein kinase CK2 triggers cytosolic zinc signaling pathways by phosphorylation of zinc channel ZIP7. Sci. Signal. 5 , ra11 (2012).

Mnatsakanyan, H., Serra, R. S. I., Rico, P. & Salmeron-Sanchez, M. Zinc uptake promotes myoblast differentiation via Zip7 transporter and activation of Akt signalling transduction pathway. Sci. Rep. 8 , 13642 (2018).

Nimmanon, T. et al. Phosphorylation of zinc channel ZIP7 drives MAPK, PI3K and mTOR growth and proliferation signalling. Metallomics 9 , 471–481 (2017).

Mapley, J. I., Wagner, P., Officer, D. L. & Gordon, K. C. Computational and spectroscopic analysis of beta-indandione modified zinc porphyrins. J. Phys. Chem. A. 122 , 4448–4456 (2018).

Giunta, C. et al. Spondylocheiro dysplastic form of the Ehlers-Danlos syndrome–an autosomal-recessive entity caused by mutations in the zinc transporter gene SLC39A13. Am. J. Hum. Genet. 82 , 1290–1305 (2008).

Fukada, T. et al. The zinc transporter SLC39A13/ZIP13 is required for connective tissue development; its involvement in BMP/TGF-beta signaling pathways. PLoS One 3 , e3642 (2008).

Shusterman, E. et al. Zinc transport and the inhibition of the L-type calcium channel are two separable functions of ZnT-1. Metallomics 9 , 228–238 (2017).

Hennigar, S. R. & McClung, J. P. Zinc transport in the mammalian intestine. Compr. Physiol. 9 , 59–74 (2018).

Geiser, J., Venken, K. J., De Lisle, R. C. & Andrews, G. K. A mouse model of acrodermatitis enteropathica: loss of intestine zinc transporter ZIP4 (Slc39a4) disrupts the stem cell niche and intestine integrity. PLoS Genet. 8 , e1002766 (2012).

Dufner-Beattie, J., Kuo, Y. M., Gitschier, J. & Andrews, G. K. The adaptive response to dietary zinc in mice involves the differential cellular localization and zinc regulation of the zinc transporters ZIP4 and ZIP5. J. Biol. Chem. 279 , 49082–49090 (2004).

Dufner-Beattie, J. et al. The acrodermatitis enteropathica gene ZIP4 encodes a tissue-specific, zinc-regulated zinc transporter in mice. J. Biol. Chem. 278 , 33474–33481 (2003).

Kury, S. et al. Identification of SLC39A4, a gene involved in acrodermatitis enteropathica. Nat. Genet. 31 , 239–240 (2002).

Wang, K. et al. A novel member of a zinc transporter family is defective in acrodermatitis enteropathica. Am. J. Hum. Genet. 71 , 66–73 (2002).

Weaver, B. P., Dufner-Beattie, J., Kambe, T. & Andrews, G. K. Novel zinc-responsive post-transcriptional mechanisms reciprocally regulate expression of the mouse Slc39a4 and Slc39a5 zinc transporters (Zip4 and Zip5). Biol. Chem. 388 , 1301–1312 (2007).

Yu, Y. Y., Kirschke, C. P. & Huang, L. Immunohistochemical analysis of ZnT1, 4, 5, 6, and 7 in the mouse gastrointestinal tract. J. Histochem Cytochem. 55 , 223–234 (2007).

McMahon, R. J. & Cousins, R. J. Regulation of the zinc transporter ZnT-1 by dietary zinc. Proc. Natl Acad. Sci. USA. 95 , 4841–4846 (1998).

Wu, J., Ma, N., Johnston, L. J. & Ma, X. Dietary nutrients mediate intestinal host defense peptide expression. Adv. Nutr. 11 , 92–102 (2020).

Podany, A. B. et al. ZnT2-mediated zinc import into paneth cell granules is necessary for coordinated secretion and paneth cell function in mice. Cell Mol. Gastroenterol. Hepatol. 2 , 369–383 (2016).

Hennigar, S. R. & Kelleher, S. L. TNFalpha post-translationally targets ZnT2 to accumulate zinc in lysosomes. J. Cell Physiol. 230 , 2345–2350 (2015).

Ohashi, W. et al. Zinc transporter SLC39A7/ZIP7 promotes intestinal epithelial self-renewal by resolving ER stress. PLoS Genet. 12 , e1006349 (2016).

Turner, J. R. Intestinal mucosal barrier function in health and disease. Nat. Rev. Immunol. 9 , 799–809 (2009).

Higashimura, Y. et al. Zinc deficiency activates the IL-23/Th17 axis to aggravate experimental colitis in mice. J. Crohns Colitis 14 , 856–866 (2020).

Hering, N. A., Fromm, M. & Schulzke, J. D. Determinants of colonic barrier function in inflammatory bowel disease and potential therapeutics. J. Physiol. 590 , 1035–1044 (2012).

Guthrie, G. J. et al. Influence of ZIP14 (slc39A14) on intestinal zinc processing and barrier function. Am. J. Physiol. Gastrointest. Liver Physiol. 308 , G171–G178 (2015).

Kim, J. et al. Deletion of metal transporter Zip14 (Slc39a14) produces skeletal muscle wasting, endotoxemia, Mef2c activation and induction of miR-675 and Hspb7. Sci. Rep. 10 , 4050 (2020).

Aydemir, T. B. & Cousins, R. J. The multiple faces of the metal transporter ZIP14 (SLC39A14). J. Nutr. 148 , 174–184 (2018).

McGourty, K. et al. ZnT2 is critical for TLR4-mediated cytokine expression in colonocytes and modulates mucosal inflammation in mice. Int J. Mol. Sci. 23 , 11467 (2022).

Hennigar, S. R. et al. ZnT2 is a critical mediator of lysosomal-mediated cell death during early mammary gland involution. Sci. Rep. 5 , 8033 (2015).

Liu, M. J. et al. ZIP8 regulates host defense through zinc-mediated inhibition of NF-κB. Cell Rep. 3 , 386–400 (2013).

Li, D. et al. A pleiotropic missense variant in SLC39A8 is associated with Crohn’s disease and human gut microbiome composition. Gastroenterology 151 , 724–732 (2016).

Vergnano, A. M. et al. Zinc dynamics and action at excitatory synapses. Neuron 82 , 1101–1114 (2014).

Kalappa, B. I. et al. AMPA receptor inhibition by synaptically released zinc. Proc. Natl Acad. Sci. USA. 112 , 15749–15754 (2015).

Huang, Y. Z., Pan, E., Xiong, Z. Q. & McNamara, J. O. Zinc-mediated transactivation of TrkB potentiates the hippocampal mossy fiber-CA3 pyramid synapse. Neuron 57 , 546–558 (2008).

Pan, E. et al. Vesicular zinc promotes presynaptic and inhibits postsynaptic long-term potentiation of mossy fiber-CA3 synapse. Neuron 71 , 1116–1126 (2011).

Eom, K. et al. Intracellular Zn(2+) signaling facilitates mossy fiber input-induced heterosynaptic potentiation of direct cortical inputs in hippocampal CA3 pyramidal cells. J. Neurosci. 39 , 3812–3831 (2019).

Anderson, C. T., Kumar, M., Xiong, S. & Tzounopoulos, T. Cell-specific gain modulation by synaptically released zinc in cortical circuits of audition. Elife 6 , e29893 (2017).

Kumar, M., Xiong, S., Tzounopoulos, T. & Anderson, C. T. Fine control of sound frequency tuning and frequency discrimination acuity by synaptic zinc signaling in mouse auditory cortex. J. Neurosci. 39 , 854–865 (2019).

Besser, L. et al. Synaptically released zinc triggers metabotropic signaling via a zinc-sensing receptor in the hippocampus. J. Neurosci. 29 , 2890–2901 (2009).

Palmiter, R. D., Cole, T. B., Quaife, C. J. & Findley, S. D. ZnT-3, a putative transporter of zinc into synaptic vesicles. Proc. Natl Acad. Sci. USA. 93 , 14934–14939 (1996).

Sikora, J., Kieffer, B. L., Paoletti, P. & Ouagazzal, A. M. Synaptic zinc contributes to motor and cognitive deficits in 6-hydroxydopamine mouse models of Parkinson’s disease. Neurobiol. Dis. 134 , 104681 (2020).

Upmanyu, N. et al. Colocalization of different neurotransmitter transporters on synaptic vesicles is sparse except for VGLUT1 and ZnT3. Neuron 110 , 1483–1497.e1487 (2022).

McAllister, B. B. & Dyck, R. H. Zinc transporter 3 (ZnT3) and vesicular zinc in central nervous system function. Neurosci. Biobehav. Rev. 80 , 329–350 (2017).

Perez-Rosello, T. et al. Tonic zinc inhibits spontaneous firing in dorsal cochlear nucleus principal neurons by enhancing glycinergic neurotransmission. Neurobiol. Dis. 81 , 14–19 (2015).

Sindreu, C., Palmiter, R. D. & Storm, D. R. Zinc transporter ZnT-3 regulates presynaptic Erk1/2 signaling and hippocampus-dependent memory. Proc. Natl Acad. Sci. USA. 108 , 3366–3370 (2011).

Mellone, M. et al. Zinc transporter-1: a novel NMDA receptor-binding protein at the postsynaptic density. J. Neurochem. 132 , 159–168 (2015).

Krall, R. F. et al. Synaptic zinc inhibition of NMDA receptors depends on the association of GluN2A with the zinc transporter ZnT1. Sci. Adv. 6 , eabb1515 (2020).

Chowanadisai, W. et al. Neurulation and neurite extension require the zinc transporter ZIP12 (slc39a12). Proc. Natl Acad. Sci. USA. 110 , 9903–9908 (2013).

Kambe, T., Yamaguchi-Iwai, Y., Sasaki, R. & Nagao, M. Overview of mammalian zinc transporters. Cell Mol. Life Sci. 61 , 49–68 (2004).

Scarr, E. et al. Increased cortical expression of the zinc transporter SLC39A12 suggests a breakdown in zinc cellular homeostasis as part of the pathophysiology of schizophrenia. NPJ Schizophr. 2 , 16002 (2016).

Bogdanovic, M. et al. The ZIP3 zinc transporter is localized to mossy fiber terminals and is required for kainate-induced degeneration of CA3 neurons. J. Neurosci. 42 , 2824–2834 (2022).

De Benedictis, C. A. et al. Expression analysis of zinc transporters in nervous tissue cells reveals neuronal and synaptic localization of ZIP4. Int J. Mol. Sci. 22 , 4511 (2021).

Pickrell, J. K. et al. Detection and interpretation of shared genetic influences on 42 human traits. Nat. Genet. 48 , 709–717 (2016).

Park, J. H. et al. SLC39A8 deficiency: a disorder of manganese transport and glycosylation. Am. J. Hum. Genet. 97 , 894–903 (2015).

Müller, N. Inflammation and the glutamate system in schizophrenia: implications for therapeutic targets and drug development. Expert Opin. Ther. Targets 12 , 1497–1507 (2008).

Tseng, W. C. et al. Schizophrenia-associated SLC39A8 polymorphism is a loss-of-function allele altering glutamate receptor and innate immune signaling. Transl. Psychiatry 11 , 136 (2021).

Derewenda, U. et al. Phenol stabilizes more helix in a new symmetrical zinc insulin hexamer. Nature 338 , 594–596 (1989).

Barman, S. & Srinivasan, K. Diabetes and zinc dyshomeostasis: can zinc supplementation mitigate diabetic complications? Crit. Rev. Food Sci. Nutr. 62 , 1046–1061 (2022).

Davidson, H. W., Wenzlau, J. M. & O’Brien, R. M. Zinc transporter 8 (ZnT8) and β cell function. Trends Endocrinol. Metab. 25 , 415–424 (2014).

Rutter, G. A. & Chimienti, F. SLC30A8 mutations in type 2 diabetes. Diabetologia 58 , 31–36 (2015).

Tamaki, M. et al. The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance. J. Clin. Invest. 123 , 4513–4524 (2013).

Sladek, R. et al. A genome-wide association study identifies novel risk loci for type 2 diabetes. Nature 445 , 881–885 (2007).

Fukunaka, A. & Fujitani, Y. Role of zinc homeostasis in the pathogenesis of diabetes and obesity. Int J. Mol. Sci. 19 , 476 (2018).

Ma, Q. et al. ZnT8 loss-of-function accelerates functional maturation of hESC-derived β cells and resists metabolic stress in diabetes. Nat. Commun. 13 , 4142 (2022).

Regnell, S. E. & Lernmark, Å. Early prediction of autoimmune (type 1) diabetes. Diabetologia 60 , 1370–1381 (2017).

Lemaire, K. et al. Insulin crystallization depends on zinc transporter ZnT8 expression, but is not required for normal glucose homeostasis in mice. Proc. Natl Acad. Sci. USA. 106 , 14872–14877 (2009).

Wenzlau, J. M. et al. The cation efflux transporter ZnT8 (Slc30A8) is a major autoantigen in human type 1 diabetes. Proc. Natl Acad. Sci. USA. 104 , 17040–17045 (2007).

Smidt, K. et al. SLC30A3 responds to glucose- and zinc variations in beta-cells and is critical for insulin production and in vivo glucose-metabolism during beta-cell stress. PLoS One 4 , e5684 (2009).

Petersen, A. B. et al. siRNA-mediated knock-down of ZnT3 and ZnT8 affects production and secretion of insulin and apoptosis in INS-1E cells. Apmis 119 , 93–102 (2011).

Hardy, A. B. et al. Zip4 mediated zinc influx stimulates insulin secretion in pancreatic beta cells. PLoS One 10 , e0119136 (2015).

Liu, Y. et al. Characterization of zinc influx transporters (ZIPs) in pancreatic β cells: roles in regulating cytosolic zinc homeostasis and insulin secretion. J. Biol. Chem. 290 , 18757–18769 (2015).

Gyulkhandanyan, A. V. et al. Investigation of transport mechanisms and regulation of intracellular Zn2+ in pancreatic alpha-cells. J. Biol. Chem. 283 , 10184–10197 (2008).

Solomou, A. et al. Over-expression of Slc30a8/ZnT8 selectively in the mouse α cell impairs glucagon release and responses to hypoglycemia. Nutr. Metab. 13 , 46 (2016).

Balaz, M. et al. Subcutaneous adipose tissue zinc-α2-glycoprotein is associated with adipose tissue and whole-body insulin sensitivity. Obesity 22 , 1821–1829 (2014).

Wang, W. & Seale, P. Control of brown and beige fat development. Nat. Rev. Mol. Cell Biol. 17 , 691–702 (2016).

Fukunaka, A. et al. Zinc transporter ZIP13 suppresses beige adipocyte biogenesis and energy expenditure by regulating C/EBP-β expression. PLoS Genet. 13 , e1006950 (2017).

Hay, N. Reprogramming glucose metabolism in cancer: can it be exploited for cancer therapy? Nat. Rev. Cancer 16 , 635–649 (2016).

Luo, X. et al. Emerging roles of lipid metabolism in cancer metastasis. Mol. Cancer 16 , 76 (2017).

Gumulec, J. et al. Insight to physiology and pathology of zinc(II) ions and their actions in breast and prostate carcinoma. Curr. Med. Chem. 18 , 5041–5051 (2011).

Takahashi, Y., Ogra, Y. & Suzuki, K. T. Nuclear trafficking of metallothionein requires oxidation of a cytosolic partner. J. Cell Physiol. 202 , 563–569 (2005).

Nagel, W. W. & Vallee, B. L. Cell cycle regulation of metallothionein in human colonic cancer cells. Proc. Natl Acad. Sci. USA. 92 , 579–583 (1995).

Formigari, A., Santon, A. & Irato, P. Efficacy of zinc treatment against iron-induced toxicity in rat hepatoma cell line H4-II-E-C3. Liver Int. 27 , 120–127 (2007).

Chen, W. Y. et al. Expression of metallothionein gene during embryonic and early larval development in zebrafish. Aquat. Toxicol. 69 , 215–227 (2004).

Chen, W. Y., John, J. A., Lin, C. H. & Chang, C. Y. Expression pattern of metallothionein, MTF-1 nuclear translocation, and its dna-binding activity in zebrafish (Danio rerio) induced by zinc and cadmium. Environ. Toxicol. Chem. 26 , 110–117 (2007).

Xia, N., Liu, L., Yi, X. & Wang, J. Studies of interaction of tumor suppressor p53 with apo-MT using surface plasmon resonance. Anal. Bioanal. Chem. 395 , 2569–2575 (2009).

Rana, U. et al. Zinc binding ligands and cellular zinc trafficking: apo-metallothionein, glutathione, TPEN, proteomic zinc, and Zn-Sp1. J. Inorg. Biochem. 102 , 489–499 (2008).

Huang, M., Shaw, I. C. & Petering, D. H. Interprotein metal exchange between transcription factor IIIa and apo-metallothionein. J. Inorg. Biochem. 98 , 639–648 (2004).

Parreno, V., Martinez, A. M. & Cavalli, G. Mechanisms of Polycomb group protein function in cancer. Cell Res. 32 , 231–253 (2022).

Di Foggia, V. et al. Bmi1 enhances skeletal muscle regeneration through MT1-mediated oxidative stress protection in a mouse model of dystrophinopathy. J. Exp. Med. 211 , 2617–2633 (2014).

Dünkelberg, S. et al. The interaction of sodium and zinc in the priming of T cell subpopulations regarding Th17 and treg cells. Mol. Nutr. Food Res. 64 , e1900245 (2020).

Spiering, R. et al. Membrane-bound metallothionein 1 of murine dendritic cells promotes the expansion of regulatory T cells in vitro. Toxicol. Sci. 138 , 69–75 (2014).

Li, S. et al. Metallothionein 3 promotes osteoblast differentiation in C2C12 cells via reduction of oxidative stress. Int J. Mol. Sci. 22 , 4312 (2021).

Shin, C. H. et al. Identification of XAF1-MT2A mutual antagonism as a molecular switch in cell-fate decisions under stressful conditions. Proc. Natl Acad. Sci. USA. 114 , 5683–5688 (2017).

Korkola, N. C. & Stillman, M. J. Structural role of cadmium and zinc in metallothionein oxidation by hydrogen peroxide: the resilience of metal-thiolate clusters. J. Am. Chem. Soc. 145 , 6383–6397 (2023).

Ma, H. et al. HMBOX1 interacts with MT2A to regulate autophagy and apoptosis in vascular endothelial cells. Sci. Rep. 5 , 15121 (2015).

Murphy, M. P. et al. Guidelines for measuring reactive oxygen species and oxidative damage in cells and in vivo. Nat. Metab. 4 , 651–662 (2022).

Song, Q. X. et al. Potential role of oxidative stress in the pathogenesis of diabetic bladder dysfunction. Nat. Rev. Urol. 19 , 581–596 (2022).

Vatner, S. F. et al. Healthful aging mediated by inhibition of oxidative stress. Ageing Res. Rev. 64 , 101194 (2020).

Niu, B. et al. Application of glutathione depletion in cancer therapy: enhanced ROS-based therapy, ferroptosis, and chemotherapy. Biomaterials 277 , 121110 (2021).

Otterbein, L. E., Foresti, R. & Motterlini, R. Heme oxygenase-1 and carbon monoxide in the heart: the balancing act between danger signaling and pro-survival. Circ. Res. 118 , 1940–1959 (2016).

Maret, W. & Li, Y. Coordination dynamics of zinc in proteins. Chem. Rev. 109 , 4682–4707 (2009).

Pluth, M. D., Tomat, E. & Lippard, S. J. Biochemistry of mobile zinc and nitric oxide revealed by fluorescent sensors. Annu Rev. Biochem. 80 , 333–355 (2011).

Rowsell, S. et al. Crystal structure of human MMP9 in complex with a reverse hydroxamate inhibitor. J. Mol. Biol. 319 , 173–181 (2002).

Choi, S., Liu, X. & Pan, Z. Zinc deficiency and cellular oxidative stress: prognostic implications in cardiovascular diseases. Acta Pharm. Sin. 39 , 1120–1132 (2018).

D’Amico, E., Factor-Litvak, P., Santella, R. M. & Mitsumoto, H. Clinical perspective on oxidative stress in sporadic amyotrophic lateral sclerosis. Free Radic. Biol. Med. 65 , 509–527 (2013).

Wu, W., Bromberg, P. A. & Samet, J. M. Zinc ions as effectors of environmental oxidative lung injury. Free Radic. Biol. Med. 65 , 57–69 (2013).

Roel, M. et al. Crambescin C1 exerts a cytoprotective effect on HepG2 cells through metallothionein induction. Mar. Drugs 13 , 4633–4653 (2015).

Cavalca, E. et al. Metallothioneins are neuroprotective agents in lysosomal storage disorders. Ann. Neurol. 83 , 418–432 (2018).

Yang, M. & Chitambar, C. R. Role of oxidative stress in the induction of metallothionein-2A and heme oxygenase-1 gene expression by the antineoplastic agent gallium nitrate in human lymphoma cells. Free Radic. Biol. Med. 45 , 763–772 (2008).

Qu, W., Pi, J. & Waalkes, M. P. Metallothionein blocks oxidative DNA damage in vitro. Arch. Toxicol. 87 , 311–321 (2013).

Koh, J. Y. & Lee, S. J. Metallothionein-3 as a multifunctional player in the control of cellular processes and diseases. Mol. Brain. 13 , 116 (2020).

Álvarez-Barrios, A. et al. Antioxidant defenses in the human eye: a focus on metallothioneins. Antioxidants 10 , 89 (2021).

Maret, W. The redox biology of redox-inert zinc ions. Free Radic. Biol. Med. 134 , 311–326 (2019).

Oteiza, P. I. Zinc and the modulation of redox homeostasis. Free Radic. Biol. Med. 53 , 1748–1759 (2012).

Hübner, C. & Haase, H. Interactions of zinc- and redox-signaling pathways. Redox Biol. 41 , 101916 (2021).

Kim, H. G. et al. The epigenetic regulator SIRT6 protects the liver from alcohol-induced tissue injury by reducing oxidative stress in mice. J. Hepatol. 71 , 960–969 (2019).

Hwang, S. et al. Interleukin-22 ameliorates neutrophil-driven nonalcoholic steatohepatitis through multiple targets. Hepatology 72 , 412–429 (2020).

Wang, B. et al. D609 protects retinal pigmented epithelium as a potential therapy for age-related macular degeneration. Signal Transduct. Target Ther. 5 , 20 (2020).

Phillippi, J. A. et al. Basal and oxidative stress-induced expression of metallothionein is decreased in ascending aortic aneurysms of bicuspid aortic valve patients. Circulation 119 , 2498–2506 (2009).

Bahadorani, S., Mukai, S., Egli, D. & Hilliker, A. J. Overexpression of metal-responsive transcription factor (MTF-1) in Drosophila melanogaster ameliorates life-span reductions associated with oxidative stress and metal toxicity. Neurobiol. Aging 31 , 1215–1226 (2010).

Esposito, K. et al. Inflammatory cytokine concentrations are acutely increased by hyperglycemia in humans: role of oxidative stress. Circulation 106 , 2067–2072 (2002).

Stankovic, R. K., Chung, R. S. & Penkowa, M. Metallothioneins I and II: neuroprotective significance during CNS pathology. Int J. Biochem. Cell Biol. 39 , 484–489 (2007).

Inoue, K., Takano, H. & Satoh, M. Protective role of metallothionein in coagulatory disturbance accompanied by acute liver injury induced by LPS/D-GalN. Thromb. Haemost. 99 , 980–983 (2008).

Inoue, K. et al. Role of metallothionein in coagulatory disturbance and systemic inflammation induced by lipopolysaccharide in mice. Faseb J. 20 , 533–535 (2006).

Takano, H. et al. Protective role of metallothionein in acute lung injury induced by bacterial endotoxin. Thorax 59 , 1057–1062 (2004).

Subramanian Vignesh, K. et al. Granulocyte macrophage-colony stimulating factor induced Zn sequestration enhances macrophage superoxide and limits intracellular pathogen survival. Immunity 39 , 697–710 (2013).

Liu, Y. et al. EOLA1 protects lipopolysaccharide induced IL-6 production and apoptosis by regulation of MT2A in human umbilical vein endothelial cells. Mol. Cell Biochem. 395 , 45–51 (2014).

Wu, H. et al. Metallothionein deletion exacerbates intermittent hypoxia-induced renal injury in mice. Toxicol. Lett. 232 , 340–348 (2015).

Vasto, S. et al. Zinc and inflammatory/immune response in aging. Ann. N. Y. Acad. Sci. 1100 , 111–122 (2007).

Majumder, S. et al. Loss of metallothionein predisposes mice to diethylnitrosamine-induced hepatocarcinogenesis by activating NF-kappaB target genes. Cancer Res. 70 , 10265–10276 (2010).

Butcher, H. L. et al. Metallothionein mediates the level and activity of nuclear factor kappa B in murine fibroblasts. J. Pharm. Exp. Ther. 310 , 589–598 (2004).

Pan, Y. et al. Metallothionein 2A inhibits NF-κB pathway activation and predicts clinical outcome segregated with TNM stage in gastric cancer patients following radical resection. J. Transl. Med. 11 , 173 (2013).

Toh, P. P. et al. Modulation of metallothionein isoforms is associated with collagen deposition in proliferating keloid fibroblasts in vitro. Exp. Dermatol. 19 , 987–993 (2010).

Cong, W. et al. Metallothionein prevents age-associated cardiomyopathy via inhibiting NF-κB pathway activation and associated nitrative damage to 2-OGD. Antioxid. Redox Signal. 25 , 936–952 (2016).

Read, S. A. et al. Zinc is a potent and specific inhibitor of IFN-λ3 signalling. Nat. Commun. 8 , 15245 (2017).

Chen, Q. Y., DesMarais, T. & Costa, M. Metals and mechanisms of carcinogenesis. Annu. Rev. Pharm. Toxicol. 59 , 537–554 (2019).

Ganger, R. et al. Protective effects of zinc against acute arsenic toxicity by regulating antioxidant defense system and cumulative metallothionein expression. Biol. Trace Elem. Res. 169 , 218–229 (2016).

Polykretis, P. et al. Cadmium effects on superoxide dismutase 1 in human cells revealed by NMR. Redox Biol. 21 , 101102 (2019).

Petering, D. H., Loftsgaarden, J., Schneider, J. & Fowler, B. Metabolism of cadmium, zinc and copper in the rat kidney: the role of metallothionein and other binding sites. Environ. Health Perspect. 54 , 73–81 (1984).

Chen, X. et al. The association between renal tubular dysfunction and zinc level in a Chinese population environmentally exposed to cadmium. Biol. Trace Elem. Res. 186 , 114–121 (2018).

Hu, Y. et al. The role of reactive oxygen species in arsenic toxicity. Biomolecules 10 , 240 (2020).

Rahman, M. T. & De Ley, M. Arsenic induction of metallothionein and metallothionein induction against arsenic cytotoxicity. Rev. Environ. Contam Toxicol. 240 , 151–168 (2017).

Ho, E. Zinc deficiency, DNA damage and cancer risk. J. Nutr. Biochem. 15 , 572–578 (2004).

Song, Y. et al. Marginal zinc deficiency increases oxidative DNA damage in the prostate after chronic exercise. Free Radic. Biol. Med. 48 , 82–88 (2010).

Stepien, M. et al. Circulating copper and zinc levels and risk of hepatobiliary cancers in Europeans. Br. J. Cancer 116 , 688–696 (2017).

Jayaraman, A. K. & Jayaraman, S. Increased level of exogenous zinc induces cytotoxicity and up-regulates the expression of the ZnT-1 zinc transporter gene in pancreatic cancer cells. J. Nutr. Biochem. 22 , 79–88 (2011).

Wu, X., Tang, J. & Xie, M. Serum and hair zinc levels in breast cancer: a meta-analysis. Sci. Rep. 5 , 12249 (2015).

Seeler, J. F. et al. Metal ion fluxes controlling amphibian fertilization. Nat. Chem. 13 , 683–691 (2021).

Kambe, T., Hashimoto, A. & Fujimoto, S. Current understanding of ZIP and ZnT zinc transporters in human health and diseases. Cell Mol. Life Sci. 71 , 3281–3295 (2014).

Margalioth, E. J., Schenker, J. G. & Chevion, M. Copper and zinc levels in normal and malignant tissues. Cancer 52 , 868–872 (1983).

Gammoh, N. Z. & Rink, L. Zinc in infection and inflammation. Nutrients 9 , 624 (2017).

Cui, Y. et al. Levels of zinc, selenium, calcium, and iron in benign breast tissue and risk of subsequent breast cancer. Cancer Epidemiol. Biomark. Prev. 16 , 1682–1685 (2007).

Santoliquido, P. M., Southwick, H. W. & Olwin, J. H. Trace metal levels in cancer of the breast. Surg. Gynecol. Obstet. 142 , 65–70 (1976).

Taylor, K. M. et al. The emerging role of the LIV-1 subfamily of zinc transporters in breast cancer. Mol. Med. 13 , 396–406 (2007).

Kasper, G. et al. Expression levels of the putative zinc transporter LIV-1 are associated with a better outcome of breast cancer patients. Int J. Cancer 117 , 961–973 (2005).

Yamashita, S. et al. Zinc transporter LIVI controls epithelial-mesenchymal transition in zebrafish gastrula organizer. Nature 429 , 298–302 (2004).

Kowalski, P. J., Rubin, M. A. & Kleer, C. G. E-cadherin expression in primary carcinomas of the breast and its distant metastases. Breast Cancer Res. 5 , R217–R222 (2003).

Oka, H. et al. Expression of E-cadherin cell adhesion molecules in human breast cancer tissues and its relationship to metastasis. Cancer Res. 53 , 1696–1701 (1993).

Lopez, V. & Kelleher, S. L. Zip6-attenuation promotes epithelial-to-mesenchymal transition in ductal breast tumor (T47D) cells. Exp. Cell Res. 316 , 366–375 (2010).

Shen, H., Qin, H. & Guo, J. Concordant correlation of LIV-1 and E-cadherin expression in human breast cancer cell MCF-7. Mol. Biol. Rep. 36 , 653–659 (2009).

Matsui, C. et al. Zinc and its transporter ZIP6 are key mediators of breast cancer cell survival under high glucose conditions. FEBS Lett. 591 , 3348–3359 (2017).

Gao, T. et al. The mechanism between epithelial mesenchymal transition in breast cancer and hypoxia microenvironment. Biomed. Pharmacother. 80 , 393–405 (2016).

Dave, B., Mittal, V., Tan, N. M. & Chang, J. C. Epithelial-mesenchymal transition, cancer stem cells and treatment resistance. Breast Cancer Res. 14 , 202 (2012).

Chung, C. H., Bernard, P. S. & Perou, C. M. Molecular portraits and the family tree of cancer. Nat. Genet. 32 , 533–540 (2002).

Tozlu, S. et al. Identification of novel genes that co-cluster with estrogen receptor alpha in breast tumor biopsy specimens, using a large-scale real-time reverse transcription-PCR approach. Endocr. Relat. Cancer 13 , 1109–1120 (2006).

Althobiti, M. et al. Oestrogen-regulated protein SLC39A6: a biomarker of good prognosis in luminal breast cancer. Breast Cancer Res Treat. 189 , 621–630 (2021).

Kambe, T. [Overview of and update on the physiological functions of mammalian zinc transporters]. Nihon Eiseigaku Zasshi. 68 , 92–102 (2013).

Kagara, N., Tanaka, N., Noguchi, S. & Hirano, T. Zinc and its transporter ZIP10 are involved in invasive behavior of breast cancer cells. Cancer Sci. 98 , 692–697 (2007).

Pal, D., Sharma, U., Singh, S. K. & Prasad, R. Association between ZIP10 gene expression and tumor aggressiveness in renal cell carcinoma. Gene 552 , 195–198 (2014).

Pawlus, M. R., Wang, L. & Hu, C. J. STAT3 and HIF1alpha cooperatively activate HIF1 target genes in MDA-MB-231 and RCC4 cells. Oncogene 33 , 1670–1679 (2014).

Armanious, H. et al. STAT3 upregulates the protein expression and transcriptional activity of beta-catenin in breast cancer. Int J. Clin. Exp. Pathol. 3 , 654–664 (2010).

Chung, S. S., Giehl, N., Wu, Y. & Vadgama, J. V. STAT3 activation in HER2-overexpressing breast cancer promotes epithelial-mesenchymal transition and cancer stem cell traits. Int J. Oncol. 44 , 403–411 (2014).

Taylor, K. M. et al. ZIP7-mediated intracellular zinc transport contributes to aberrant growth factor signaling in antihormone-resistant breast cancer Cells. Endocrinology 149 , 4912–4920 (2008).

Ziliotto, S. et al. Activated zinc transporter ZIP7 as an indicator of anti-hormone resistance in breast cancer. Metallomics 11 , 1579–1592 (2019).

Huang, L., Kirschke, C. P., Zhang, Y. & Yu, Y. Y. The ZIP7 gene (Slc39a7) encodes a zinc transporter involved in zinc homeostasis of the Golgi apparatus. J. Biol. Chem. 280 , 15456–15463 (2005).

de Nonneville, A. et al. Prognostic and predictive value of LIV1 expression in early breast cancer and by molecular subtype. Pharmaceutics 15 , 938 (2023).

Vogel-Gonzalez, M., Musa-Afaneh, D., Rivera Gil, P. & Vicente, R. Zinc favors triple-negative breast cancer’s microenvironment modulation and cell plasticity. Int J. Mol. Sci. 22 , 9188 (2021).

Yap, X. et al. Over-expression of metallothionein predicts chemoresistance in breast cancer. J. Pathol. 217 , 563–570 (2009).

Jadhav, R. R. et al. Genome-wide DNA methylation analysis reveals estrogen-mediated epigenetic repression of metallothionein-1 gene cluster in breast cancer. Clin. Epigenetics. 7 , 13 (2015).

Lopez, V., Foolad, F. & Kelleher, S. L. ZnT2-overexpression represses the cytotoxic effects of zinc hyper-accumulation in malignant metallothionein-null T47D breast tumor cells. Cancer Lett. 304 , 41–51 (2011).

Lim, D., Jocelyn, K. M., Yip, G. W. & Bay, B. H. Silencing the Metallothionein-2A gene inhibits cell cycle progression from G1- to S-phase involving ATM and cdc25A signaling in breast cancer cells. Cancer Lett. 276 , 109–117 (2009).

Sun, L. et al. Zinc regulates the ability of Cdc25C to activate MPF/cdk1. J. Cell Physiol. 213 , 98–104 (2007).

Banin, S. et al. Enhanced phosphorylation of p53 by ATM in response to DNA damage. Science 281 , 1674–1677 (1998).

Deng, C. et al. Mice lacking p21CIP1/WAF1 undergo normal development, but are defective in G1 checkpoint control. Cell 82 , 675–684 (1995).

Li, D., Stovall, D. B., Wang, W. & Sui, G. Advances of zinc signaling studies in prostate cancer. Int J. Mol. Sci. 21 , 667 (2020).

Zhao, J. et al. Comparative study of serum zinc concentrations in benign and malignant prostate disease: a systematic review and meta-analysis. Sci. Rep. 6 , 25778 (2016).

McNeal, J. E. Normal histology of the prostate. Am. J. Surg. Pathol. 12 , 619–633 (1988).

Costello, L. C. & Franklin, R. B. A comprehensive review of the role of zinc in normal prostate function and metabolism; and its implications in prostate cancer. Arch. Biochem. Biophys. 611 , 100–112 (2016).

Vartsky, D. et al. Prostatic zinc and prostate specific antigen: an experimental evaluation of their combined diagnostic value. J. Urol. 170 , 2258–2262 (2003).

Dakubo, G. D. et al. Altered metabolism and mitochondrial genome in prostate cancer. J. Clin. Pathol. 59 , 10–16 (2006).

Feng, P. et al. The involvement of Bax in zinc-induced mitochondrial apoptogenesis in malignant prostate cells. Mol. Cancer 7 , 25 (2008).

Nardinocchi, L. et al. Zinc downregulates HIF-1alpha and inhibits its activity in tumor cells in vitro and in vivo. PLoS One 5 , e15048 (2010).

Uzzo, R. G. et al. Zinc inhibits nuclear factor-kappa B activation and sensitizes prostate cancer cells to cytotoxic agents. Clin. Cancer Res. 8 , 3579–3583 (2002).

Ishii, K. et al. Evidence that the prostate-specific antigen (PSA)/Zn2+ axis may play a role in human prostate cancer cell invasion. Cancer Lett. 207 , 79–87 (2004).

Uzzo, R. G. et al. Diverse effects of zinc on NF-kappaB and AP-1 transcription factors: implications for prostate cancer progression. Carcinogenesis 27 , 1980–1990 (2006).

Ishii, K. et al. Inhibition of aminopeptidase N (AP-N) and urokinase-type plasminogen activator (uPA) by zinc suppresses the invasion activity in human urological cancer cells. Biol. Pharm. Bull. 24 , 226–230 (2001).

Singh, K. K., Desouki, M. M., Franklin, R. B. & Costello, L. C. Mitochondrial aconitase and citrate metabolism in malignant and nonmalignant human prostate tissues. Mol. Cancer 5 , 14 (2006).

Fontana, F., Anselmi, M. & Limonta, P. Unraveling the peculiar features of mitochondrial metabolism and dynamics in prostate cancer. Cancers. 15 , 1192 (2023).

Costello, L. C. et al. Human prostate cancer ZIP1/zinc/citrate genetic/metabolic relationship in the TRAMP prostate cancer animal model. Cancer Biol. Ther. 12 , 1078–1084 (2011).

Costello, L. C. & Franklin, R. B. The clinical relevance of the metabolism of prostate cancer; zinc and tumor suppression: connecting the dots. Mol. Cancer 5 , 17 (2006).

Franklin, R. B. et al. hZIP1 zinc uptake transporter down regulation and zinc depletion in prostate cancer. Mol. Cancer 4 , 32 (2005).

An, Y. et al. A novel tetrapeptide fluorescence sensor for early diagnosis of prostate cancer based on imaging Zn(2+) in healthy versus cancerous cells. J. Adv. Res. 24 , 363–370 (2020).

Fong, L. Y. et al. Human-like hyperplastic prostate with low ZIP1 induced solely by Zn deficiency in rats. Proc. Natl Acad. Sci. USA. 115 , E11091–e11100 (2018).

Costello, L. C., Franklin, R. B., Zou, J. & Naslund, M. J. Evidence that human prostate cancer is a ZIP1-deficient malignancy that could be effectively treated with a zinc ionophore (Clioquinol) approach. Chemotherapy 4 , 152 (2015).

PubMed   Google Scholar  

Huang, L., Kirschke, C. P. & Zhang, Y. Decreased intracellular zinc in human tumorigenic prostate epithelial cells: a possible role in prostate cancer progression. Cancer Cell Int. 6 , 10 (2006).

Makhov, P. et al. Transcriptional regulation of the major zinc uptake protein hZip1 in prostate cancer cells. Gene 431 , 39–46 (2009).

Thiagalingam, A. et al. RREB-1, a novel zinc finger protein, is involved in the differentiation response to Ras in human medullary thyroid carcinomas. Mol. Cell Biol. 16 , 5335–5345 (1996).

Zhang, S. et al. p16 INK4a gene promoter variation and differential binding of a repressor, the ras-responsive zinc-finger transcription factor, RREB. Oncogene 22 , 2285–2295 (2003).

Gioeli, D. Signal transduction in prostate cancer progression. Clin. Sci. 108 , 293–308 (2005).

Milon, B. C. et al. Ras responsive element binding protein-1 (RREB-1) down-regulates hZIP1 expression in prostate cancer cells. Prostate 70 , 288–296 (2010).

Aguirre-Portoles, C. et al. ZIP9 is a druggable determinant of sex differences in melanoma. Cancer Res. 81 , 5991–6003 (2021).

Berg, A. H. et al. Identification and characterization of membrane androgen receptors in the ZIP9 zinc transporter subfamily: I. Discovery in female atlantic croaker and evidence ZIP9 mediates testosterone-induced apoptosis of ovarian follicle cells. Endocrinology 155 , 4237–4249 (2014).

Thomas, P., Pang, Y., Dong, J. & Berg, A. H. Identification and characterization of membrane androgen receptors in the ZIP9 zinc transporter subfamily: II. Role of human ZIP9 in testosterone-induced prostate and breast cancer cell apoptosis. Endocrinology 155 , 4250–4265 (2014).

Desouki, M. M. et al. hZip2 and hZip3 zinc transporters are down regulated in human prostate adenocarcinomatous glands. Mol. Cancer 6 , 37 (2007).

Kelleher, S. L., McCormick, N. H., Velasquez, V. & Lopez, V. Zinc in specialized secretory tissues: roles in the pancreas, prostate, and mammary gland. Adv. Nutr. 2 , 101–111 (2011).

Franklin, R. B. et al. Human ZIP1 is a major zinc uptake transporter for the accumulation of zinc in prostate cells. J. Inorg. Biochem. 96 , 435–442 (2003).

Prasad, R. R. et al. Stage-specific differential expression of zinc transporter SLC30A and SLC39A family proteins during prostate tumorigenesis. Mol. Carcinog. 61 , 454–471 (2022).

Kim, Y. R. et al. HOXB13 downregulates intracellular zinc and increases NF-kappaB signaling to promote prostate cancer metastasis. Oncogene 33 , 4558–4567 (2014).

Beck, F. W. et al. Differential expression of hZnT-4 in human prostate tissues. Prostate 58 , 374–381 (2004).

Inoue, K. et al. Osteopenia and male-specific sudden cardiac death in mice lacking a zinc transporter gene, Znt5. Hum. Mol. Genet. 11 , 1775–1784 (2002).

Wei, H. et al. Differential expression of metallothioneins (MTs) 1, 2, and 3 in response to zinc treatment in human prostate normal and malignant cells and tissues. Mol. Cancer 7 , 7 (2008).

Han, Y. C. et al. Metallothionein 1 h tumour suppressor activity in prostate cancer is mediated by euchromatin methyltransferase 1. J. Pathol. 230 , 184–193 (2013).

Siegel, R. L., Miller, K. D., Wagle, N. S. & Jemal, A. Cancer statistics, 2023. CA Cancer J. Clin. 73 , 17–48 (2023).

Costello, L. C. et al. Decreased zinc and downregulation of ZIP3 zinc uptake transporter in the development of pancreatic adenocarcinoma. Cancer Biol. Ther. 12 , 297–303 (2011).

Li, M. et al. Aberrant expression of zinc transporter ZIP4 (SLC39A4) significantly contributes to human pancreatic cancer pathogenesis and progression. Proc. Natl Acad. Sci. USA. 104 , 18636–18641 (2007).

Shakri, A. R. et al. Upregulation of ZIP14 and altered zinc homeostasis in muscles in pancreatic cancer cachexia. Cancers. 12 , 3 (2019).

Li, M. et al. Down-regulation of ZIP4 by RNA interference inhibits pancreatic cancer growth and increases the survival of nude mice with pancreatic cancer xenografts. Clin. Cancer Res. 15 , 5993–6001 (2009).

Liu, M. et al. ZIP4 promotes pancreatic cancer progression by repressing ZO-1 and Claudin-1 through a ZEB1-dependent transcriptional mechanism. Clin. Cancer Res. 24 , 3186–3196 (2018).

Liu, M. et al. ZIP4 increases expression of transcription factor ZEB1 to promote Integrin α3β1 signaling and inhibit expression of the gemcitabine transporter ENT1 in pancreatic cancer cells. Gastroenterology 158 , 679–692.e671 (2020).

Shi, X. et al. Circular RNA ANAPC7 inhibits tumor growth and muscle wasting via PHLPP2-AKT-TGF-β signaling axis in pancreatic cancer. Gastroenterology 162 , 2004–2017.e2002 (2022).

Xu, X. et al. ZIP4, a novel determinant of tumor invasion in hepatocellular carcinoma, contributes to tumor recurrence after liver transplantation. Int J. Biol. Sci. 10 , 245–256 (2014).

Zhang, Y. et al. ZIP4 regulates pancreatic cancer cell growth by activating IL-6/STAT3 pathway through zinc finger transcription factor CREB. Clin. Cancer Res. 16 , 1423–1430 (2010).

Zhang, Y. et al. A novel epigenetic CREB-miR-373 axis mediates ZIP4-induced pancreatic cancer growth. EMBO Mol. Med. 5 , 1322–1334 (2013).

Shi, X. et al. Circular RNA ANAPC7 inhibits tumor growth and muscle wasting via PHLPP2-AKT-TGF-beta signaling axis in pancreatic cancer. Gastroenterology 162 , 2004–2017.e2002 (2022).

Krebs, A. M. et al. The EMT-activator Zeb1 is a key factor for cell plasticity and promotes metastasis in pancreatic cancer. Nat. Cell Biol. 19 , 518–529 (2017).

Franklin, R. B., Zou, J. & Costello, L. C. The cytotoxic role of RREB1, ZIP3 zinc transporter, and zinc in human pancreatic adenocarcinoma. Cancer Biol. Ther. 15 , 1431–1437 (2014).

Li, K. et al. Metallothionein-1G suppresses pancreatic cancer cell stemness by limiting activin A secretion via NF-κB inhibition. Theranostics 11 , 3196–3212 (2021).

Li, P. et al. Association between zinc intake and risk of digestive tract cancers: a systematic review and meta-analysis. Clin. Nutr. 33 , 415–420 (2014).

Jaiswal, A. S. & Narayan, S. Zinc stabilizes adenomatous polyposis coli (APC) protein levels and induces cell cycle arrest in colon cancer cells. J. Cell Biochem. 93 , 345–357 (2004).

Shangkuan, W. C. et al. Risk analysis of colorectal cancer incidence by gene expression analysis. PeerJ 5 , e3003 (2017).

Yagi, K. et al. Three DNA methylation epigenotypes in human colorectal cancer. Clin. Cancer Res. 16 , 21–33 (2010).

Hou, L., Liu, P. & Zhu, T. Long noncoding RNA SLC30A10 promotes colorectal tumor proliferation and migration via miR-21c/APC axis. Eur. Rev. Med Pharm. Sci. 24 , 6682–6691 (2020).

CAS   Google Scholar  

Yao, H. et al. KCTD9 inhibits the Wnt/β-catenin pathway by decreasing the level of β-catenin in colorectal cancer. Cell Death Dis. 13 , 761 (2022).

Chen, Y. H. et al. Role of GAC63 in transcriptional activation mediated by beta-catenin. Nucleic Acids Res. 35 , 2084–2092 (2007).

Zhao, H. et al. Wnt signaling in colorectal cancer: pathogenic role and therapeutic target. Mol. Cancer 21 , 144 (2022).

Barresi, V. et al. Transcriptome analysis reveals an altered expression profile of zinc transporters in colorectal cancer. J. Cell Biochem. 119 , 9707–9719 (2018).

Sheng, N. et al. Knockdown of SLC39A7 inhibits cell growth and induces apoptosis in human colorectal cancer cells. Acta Biochim. Biophys. Sin. (Shanghai). 49 , 926–934 (2017).

Jbara, A. et al. RBFOX2 modulates a metastatic signature of alternative splicing in pancreatic cancer. Nature 617 , 147–153 (2023).

Marasco, L. E. & Kornblihtt, A. R. The physiology of alternative splicing. Nat. Rev. Mol. Cell Biol. 24 , 242–254 (2023).

Wan, L. et al. Splicing factor SRSF1 promotes pancreatitis and KRASG12D-mediated pancreatic cancer. Cancer Discov. 13 , 1678–1695 (2023).

Thorsen, K. et al. Alternative splicing of SLC39A14 in colorectal cancer is regulated by the Wnt pathway. Mol. Cell Proteom. 10 , M110 002998 (2011).

Cao, X. et al. Cadmium induced BEAS-2B cells apoptosis and mitochondria damage via MAPK signaling pathway. Chemosphere 263 , 128346 (2021).

Jin, Y. H. et al. Cadmium is a mutagen that acts by inhibiting mismatch repair. Nat. Genet. 34 , 326–329 (2003).

Hung, K. C. et al. The expression profile and prognostic significance of metallothionein genes in colorectal cancer. Int J. Mol. Sci. 20 , 3849 (2019).

Arriaga, J. M., Greco, A., Mordoh, J. & Bianchini, M. Metallothionein 1 G and zinc sensitize human colorectal cancer cells to chemotherapy. Mol. Cancer Ther. 13 , 1369–1381 (2014).

Liu, X. et al. Metallothionein 2 A (MT2A) controls cell proliferation and liver metastasis by controlling the MST1/LATS2/YAP1 signaling pathway in colorectal cancer. Cancer Cell Int. 22 , 205 (2022).

Arriaga, J. M. et al. Metallothionein expression in colorectal cancer: relevance of different isoforms for tumor progression and patient survival. Hum. Pathol. 43 , 197–208 (2012).

Chen, H. et al. Nutrient intakes and adenocarcinoma of the esophagus and distal stomach. Nutr. Cancer 42 , 33–40 (2002).

Rogers, M. A. et al. A case-control study of element levels and cancer of the upper aerodigestive tract. Cancer Epidemiol. Biomark. Prev. 2 , 305–312 (1993).

Pakseresht, M. et al. Dietary habits and gastric cancer risk in north-west Iran. Cancer Causes Control. 22 , 725–736 (2011).

He, Y. et al. Cancer incidence and mortality in Hebei province, 2013. Medicine 96 , e7293 (2017).

Li, D. et al. Cancer survival in Cixian of China, 2003-2013: a population-based study. Cancer Med. 7 , 1537–1545 (2018).

Liang, D. et al. Gastric cancer burden of last 40 years in North China (Hebei Province): a population-based study. Medicine 96 , e5887 (2017).

Guo, Y. & He, Y. Comprehensive analysis of the expression of SLC30A family genes and prognosis in human gastric cancer. Sci. Rep. 10 , 18352 (2020).

Guan, X. et al. Dual inhibition of MYC and SLC39A10 by a novel natural product STAT3 inhibitor derived from Chaetomium globosum suppresses tumor growth and metastasis in gastric cancer. Pharm. Res. 189 , 106703 (2023).

Zhang, Y. et al. SLC39A7, regulated by miR-139-5p, induces cell proliferation, migration and inhibits apoptosis in gastric cancer via Akt/mTOR signaling pathway. Biosci. Rep. 40 , BSR20200041 (2020).

Janssen, A. M. et al. Metallothionein in human gastrointestinal cancer. J. Pathol. 192 , 293–300 (2000).

Lin, S. et al. Transcription factor myeloid zinc-finger 1 suppresses human gastric carcinogenesis by interacting with metallothionein 2 A. Clin. Cancer Res. 25 , 1050–1062 (2019).

Cho, Y. H. et al. A role of metallothionein-3 in radiation-induced autophagy in glioma cells. Sci. Rep. 10 , 2015 (2020).

Li, K. et al. MT1M regulates gastric cancer progression and stemness by modulating the Hedgehog pathway protein GLI1. Biochem. Biophys. Res. Commun. 670 , 63–72 (2023).

Fiches, G. N. et al. Profiling of immune related genes silenced in EBV-positive gastric carcinoma identified novel restriction factors of human gammaherpesviruses. PLoS Pathog. 16 , e1008778 (2020).

Takahashi, S. Molecular functions of metallothionein and its role in hematological malignancies. J. Hematol. Oncol. 5 , 41 (2012).

Pan, Y. et al. Epigenetic upregulation of metallothionein 2 A by diallyl trisulfide enhances chemosensitivity of human gastric cancer cells to docetaxel through attenuating NF-κB activation. Antioxid. Redox Signal. 24 , 839–854 (2016).

Habel, N. et al. Zinc chelation: a metallothionein 2 A’s mechanism of action involved in osteosarcoma cell death and chemotherapy resistance. Cell Death Dis. 4 , e874 (2013).

Zalewska, M., Trefon, J. & Milnerowicz, H. The role of metallothionein interactions with other proteins. Proteomics 14 , 1343–1356 (2014).

Kolenko, V., Teper, E., Kutikov, A. & Uzzo, R. Zinc and zinc transporters in prostate carcinogenesis. Nat. Rev. Urol. 10 , 219–226 (2013).

Kim, C. H., Kim, J. H., Lee, J. & Ahn, Y. S. Zinc-induced NF-kappaB inhibition can be modulated by changes in the intracellular metallothionein level. Toxicol. Appl Pharmacol. 190 , 189–196 (2003).

Fong, L. Y. & Magee, P. N. Dietary zinc deficiency enhances esophageal cell proliferation and N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumor incidence in C57BL/6 mouse. Cancer Lett. 143 , 63–69 (1999).

Abnet, C. C. et al. Zinc concentration in esophageal biopsy specimens measured by x-ray fluorescence and esophageal cancer risk. J. Natl Cancer Inst. 97 , 301–306 (2005).

Fong, L. Y., Nguyen, V. T. & Farber, J. L. Esophageal cancer prevention in zinc-deficient rats: rapid induction of apoptosis by replenishing zinc. J. Natl Cancer Inst. 93 , 1525–1533 (2001).

Wu, C. et al. Genome-wide association study identifies common variants in SLC39A6 associated with length of survival in esophageal squamous-cell carcinoma. Nat. Genet. 45 , 632–638 (2013).

Cui, X. B. et al. SLC39A6: a potential target for diagnosis and therapy of esophageal carcinoma. J. Transl. Med. 13 , 321 (2015).

Cheng, X. et al. Solute carrier family 39 member 6 gene promotes aggressiveness of esophageal carcinoma cells by increasing intracellular levels of zinc, activating phosphatidylinositol 3-kinase signaling, and up-regulating genes that regulate metastasis. Gastroenterology 152 , 1985–1997.e1912 (2017).

Jin, J. et al. Knockdown of zinc transporter ZIP5 (SLC39A5) expression significantly inhibits human esophageal cancer progression. Oncol. Rep. 34 , 1431–1439 (2015).

Kumar, A., Chatopadhyay, T., Raziuddin, M. & Ralhan, R. Discovery of deregulation of zinc homeostasis and its associated genes in esophageal squamous cell carcinoma using cDNA microarray. Int J. Cancer 120 , 230–242 (2007).

Li, Q. et al. Knockdown of zinc transporter ZIP5 by RNA interference inhibits esophageal cancer growth in vivo. Oncol. Res. 24 , 205–214 (2016).

Huang, J. X. et al. Relationship between COX-2 and cell cycle-regulatory proteins in patients with esophageal squamous cell carcinoma. World J. Gastroenterol. 16 , 5975–5981 (2010).

PubMed   PubMed Central   Google Scholar  

Shimizu, M. et al. Metallothionein 2A expression in cancer-associated fibroblasts and cancer cells promotes esophageal squamous cell carcinoma progression. Cancers. 13 , 4552 (2021).

Wong, T. S., Gao, W. & Chan, J. Y. Transcription regulation of E-cadherin by zinc finger E-box binding homeobox proteins in solid tumors. Biomed. Res Int. 2014 , 921564 (2014).

ICGC/TCGA Pan-Cancer Analysis of Whole Genomes Consortium. Pan-cancer analysis of whole genomes. Nature 578 , 82–93 (2020).

Agrawal, A. et al. Zinc-binding groups modulate selective inhibition of MMPs. ChemMedChem 3 , 812–820 (2008).

Puerta, D. T. & Cohen, S. M. Examination of novel zinc-binding groups for use in matrix metalloproteinase inhibitors. Inorg. Chem. 42 , 3423–3430 (2003).

Lheureux, S., Braunstein, M. & Oza, A. M. Epithelial ovarian cancer: Evolution of management in the era of precision medicine. CA Cancer J. Clin. 69 , 280–304 (2019).

Wei, T. et al. ZnT7 RNAi favors Raf(GOF)scrib(-/-)-induced tumor growth and invasion in Drosophila through JNK signaling pathway. Oncogene 40 , 2217–2229 (2021).

Aguirre-Portolés, C. et al. ZIP9 is a druggable determinant of sex differences in melanoma. Cancer Res. 81 , 5991–6003 (2021).

Jaiswal, S. & Libby, P. Clonal haematopoiesis: connecting ageing and inflammation in cardiovascular disease. Nat. Rev. Cardiol. 17 , 137–144 (2020).

Bekele, T. H. et al. Dietary recommendations for ethiopians on the basis of priority diet-related diseases and causes of death in ethiopia: an umbrella review. Adv. Nutr. 14 , 895–913 (2023).

Mohammadifard, N. et al. Trace minerals intake: Risks and benefits for cardiovascular health. Crit. Rev. Food Sci. Nutr. 59 , 1334–1346 (2019).

Libby, P. The changing landscape of atherosclerosis. Nature 592 , 524–533 (2021).

Förstermann, U., Xia, N. & Li, H. Roles of vascular oxidative stress and nitric oxide in the pathogenesis of atherosclerosis. Circ. Res. 120 , 713–735 (2017).

Conway, D. E. et al. Endothelial metallothionein expression and intracellular free zinc levels are regulated by shear stress. Am. J. Physiol. Cell Physiol. 299 , C1461–C1467 (2010).

Hara, T. et al. Role of Scl39a13/ZIP13 in cardiovascular homeostasis. PLoS One 17 , e0276452 (2022).

Allen-Redpath, K. et al. Marginal dietary zinc deficiency in vivo induces vascular smooth muscle cell apoptosis in large arteries. Cardiovasc Res. 99 , 525–534 (2013).

Alcantara, E. H. et al. Long-term zinc deprivation accelerates rat vascular smooth muscle cell proliferation involving the down-regulation of JNK1/2 expression in MAPK signaling. Atherosclerosis 228 , 46–52 (2013).

Patrushev, N., Seidel-Rogol, B. & Salazar, G. Angiotensin II requires zinc and downregulation of the zinc transporters ZnT3 and ZnT10 to induce senescence of vascular smooth muscle cells. PLoS One 7 , e33211 (2012).

min, L. J., Mogi, M., Iwai, M. & Horiuchi, M. Signaling mechanisms of angiotensin II in regulating vascular senescence. Ageing Res Rev. 8 , 113–121 (2009).

Reed, G. W., Rossi, J. E. & Cannon, C. P. Acute myocardial infarction. Lancet 389 , 197–210 (2017).

McIntosh, R. et al. The critical role of intracellular zinc in adenosine A(2) receptor activation induced cardioprotection against reperfusion injury. J. Mol. Cell Cardiol. 49 , 41–47 (2010).

Du, L. et al. The critical role of the zinc transporter Zip2 (SLC39A2) in ischemia/reperfusion injury in mouse hearts. J. Mol. Cell Cardiol. 132 , 136–145 (2019).

Zhao, H. et al. Endoplasmic reticulum stress/Ca(2+)-calmodulin-dependent protein kinase/signal transducer and activator of transcription 3 pathway plays a role in the regulation of cellular zinc deficiency in myocardial ischemia/reperfusion injury. Front. Physiol. 12 , 736920 (2021).

Zhang, H. et al. The zinc transporter ZIP7 (Slc39a7) controls myocardial reperfusion injury by regulating mitophagy. Basic Res. Cardiol. 116 , 54 (2021).

Beharier, O. et al. ZnT-1 protects HL-1 cells from simulated ischemia-reperfusion through activation of Ras-ERK signaling. J. Mol. Med. 90 , 127–138 (2012).

Bruinsma, J. J., Jirakulaporn, T., Muslin, A. J. & Kornfeld, K. Zinc ions and cation diffusion facilitator proteins regulate Ras-mediated signaling. Dev. Cell. 2 , 567–578 (2002).

Lazarczyk, M. et al. Regulation of cellular zinc balance as a potential mechanism of EVER-mediated protection against pathogenesis by cutaneous oncogenic human papillomaviruses. J. Exp. Med. 205 , 35–42 (2008).

Murphy, E. & Steenbergen, C. Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury. Physiol. Rev. 88 , 581–609 (2008).

Smith, M. J. et al. Redox and metal profiles in human coronary endothelial and smooth muscle cells under hyperoxia, physiological normoxia and hypoxia: Effects of NRF2 signaling on intracellular zinc. Redox Biol. 62 , 102712 (2023).

Cai, L. et al. Attenuation by metallothionein of early cardiac cell death via suppression of mitochondrial oxidative stress results in a prevention of diabetic cardiomyopathy. J. Am. Coll. Cardiol. 48 , 1688–1697 (2006).

Wang, Y. et al. Inactivation of GSK-3beta by metallothionein prevents diabetes-related changes in cardiac energy metabolism, inflammation, nitrosative damage, and remodeling. Diabetes 58 , 1391–1402 (2009).

Dong, F. et al. Metallothionein prevents high-fat diet induced cardiac contractile dysfunction: role of peroxisome proliferator activated receptor gamma coactivator 1alpha and mitochondrial biogenesis. Diabetes 56 , 2201–2212 (2007).

Wang, J. et al. Cardiac metallothionein induction plays the major role in the prevention of diabetic cardiomyopathy by zinc supplementation. Circulation 113 , 544–554 (2006).

Hu, N. et al. Cardiac-specific overexpression of metallothionein rescues nicotine-induced cardiac contractile dysfunction and interstitial fibrosis. Toxicol. Lett. 202 , 8–14 (2011).

Zhou, G. et al. Metallothionein suppresses angiotensin II-induced nicotinamide adenine dinucleotide phosphate oxidase activation, nitrosative stress, apoptosis, and pathological remodeling in the diabetic heart. J. Am. Coll. Cardiol. 52 , 655–666 (2008).

Zhang, Y. et al. Cardiac overexpression of metallothionein rescues cold exposure-induced myocardial contractile dysfunction through attenuation of cardiac fibrosis despite cardiomyocyte mechanical anomalies. Free Radic. Biol. Med. 53 , 194–207 (2012).

Cai, L. et al. Inhibition of superoxide generation and associated nitrosative damage is involved in metallothionein prevention of diabetic cardiomyopathy. Diabetes 54 , 1829–1837 (2005).

Gu, J. et al. Metallothionein preserves Akt2 activity and cardiac function via inhibiting TRB3 in diabetic hearts. Diabetes 67 , 507–517 (2018).

Dabravolski, S. A. et al. Interplay between Zn(2+) homeostasis and mitochondrial functions in cardiovascular diseases and heart ageing. Int. J. Mol. Sci. 23 , 6890 (2022).

Woodier, J., Rainbow, R. D., Stewart, A. J. & Pitt, S. J. Intracellular zinc modulates cardiac ryanodine receptor-mediated calcium release. J. Biol. Chem. 290 , 17599–17610 (2015).

Gaburjakova, J. & Gaburjakova, M. The cardiac ryanodine receptor provides a suitable pathway for the rapid transport of zinc (Zn(2+)). Cells 11 , 868 (2022).

Mor, M. et al. ZnT-1 enhances the activity and surface expression of T-type calcium channels through activation of Ras-ERK signaling. Am. J. Physiol. Cell Physiol. 303 , C192–C203 (2012).

Liu, B., Cai, Z. Q. & Zhou, Y. M. Deficient zinc levels and myocardial infarction : association between deficient zinc levels and myocardial infarction: a meta-analysis. Biol. Trace Elem. Res. 165 , 41–50 (2015).

Wang, J. et al. Downregulation of the zinc transporter SLC39A13 (ZIP13) is responsible for the activation of CaMKII at reperfusion and leads to myocardial ischemia/reperfusion injury in mouse hearts. J. Mol. Cell Cardiol. 152 , 69–79 (2021).

Chen, Z. et al. Zinc ameliorates human aortic valve calcification through GPR39 mediated ERK1/2 signalling pathway. Cardiovasc. Res. 117 , 820–835 (2021).

Fang, Y. et al. Slc39a2-mediated zinc homeostasis modulates innate immune signaling in phenylephrine-induced cardiomyocyte hypertrophy. Front. Cardiovasc. Med. 8 , 736911 (2021).

Jiang, D. S. et al. IRF8 suppresses pathological cardiac remodelling by inhibiting calcineurin signalling. Nat. Commun. 5 , 3303 (2014).

Jiang, D. S. et al. Interferon regulatory factor 9 protects against cardiac hypertrophy by targeting myocardin. Hypertension 63 , 119–127 (2014).

Jiang, D. S. et al. Interferon regulatory factor 7 functions as a novel negative regulator of pathological cardiac hypertrophy. Hypertension 63 , 713–722 (2014).

Lin, W. et al. Zinc transporter Slc39a8 is essential for cardiac ventricular compaction. J. Clin. Invest. 128 , 826–833 (2018).

Lehuen, A., Diana, J., Zaccone, P. & Cooke, A. Immune cell crosstalk in type 1 diabetes. Nat. Rev. Immunol. 10 , 501–513 (2010).

Baekkeskov, S. et al. Identification of the 64 K autoantigen in insulin-dependent diabetes as the GABA-synthesizing enzyme glutamic acid decarboxylase. Nature 347 , 151–156 (1990).

Vehik, K. et al. Hierarchical order of distinct autoantibody spreading and progression to type 1 diabetes in the TEDDY study. Diabetes Care. 43 , 2066–2073 (2020).

Palmer, J. P. et al. Insulin antibodies in insulin-dependent diabetics before insulin treatment. Science 222 , 1337–1339 (1983).

Achenbach, P. et al. Autoantibodies to zinc transporter 8 and SLC30A8 genotype stratify type 1 diabetes risk. Diabetologia 52 , 1881–1888 (2009).

Kawasaki, E. et al. Differences in the humoral autoreactivity to zinc transporter 8 between childhood- and adult-onset type 1 diabetes in Japanese patients. Clin. Immunol. 138 , 146–153 (2011).

Vermeulen, I. et al. Contribution of antibodies against IA-2β and zinc transporter 8 to classification of diabetes diagnosed under 40 years of age. Diabetes Care. 34 , 1760–1765 (2011).

Wenzlau, J. M. et al. Kinetics of the post-onset decline in zinc transporter 8 autoantibodies in type 1 diabetic human subjects. J. Clin. Endocrinol. Metab. 95 , 4712–4719 (2010).

Long, A. E. et al. Humoral responses to islet antigen-2 and zinc transporter 8 are attenuated in patients carrying HLA-A*24 alleles at the onset of type 1 diabetes. Diabetes 62 , 2067–2071 (2013).

Ye, J. et al. Attenuated humoral responses in HLA-A*24-positive individuals at risk of type 1 diabetes. Diabetologia 58 , 2284–2287 (2015).

Énée, É. et al. ZnT8 is a major CD8+ T cell-recognized autoantigen in pediatric type 1 diabetes. Diabetes 61 , 1779–1784 (2012).

Scotto, M. et al. Zinc transporter (ZnT)8(186-194) is an immunodominant CD8+ T cell epitope in HLA-A2+ type 1 diabetic patients. Diabetologia 55 , 2026–2031 (2012).

Culina, S. et al. Islet-reactive CD8(+) T cell frequencies in the pancreas, but not in blood, distinguish type 1 diabetic patients from healthy donors. Sci. Immunol. 3 , eaao4013 (2018).

Lampasona, V. & Liberati, D. Islet autoantibodies. Curr. Diab. Rep. 16 , 53 (2016).

Wenzlau, J. M. et al. A common nonsynonymous single nucleotide polymorphism in the SLC30A8 gene determines ZnT8 autoantibody specificity in type 1 diabetes. Diabetes 57 , 2693–2697 (2008).

Kawasaki, E. et al. Association between anti-ZnT8 autoantibody specificities and SLC30A8 Arg325Trp variant in Japanese patients with type 1 diabetes. Diabetologia 51 , 2299–2302 (2008).

Shruthi, S., Mohan, V., Maradana, M. R. & Aravindhan, V. In silico identification and wet lab validation of novel cryptic B cell epitopes in ZnT8 zinc transporter autoantigen. Int J. Biol. Macromol. 127 , 657–664 (2019).

Hanna, S. J. et al. Slow progressors to type 1 diabetes lose islet autoantibodies over time, have few islet antigen-specific CD8(+) T cells and exhibit a distinct CD95(hi) B cell phenotype. Diabetologia 63 , 1174–1185 (2020).

Wenzlau, J. M. et al. Changes in zinc transporter 8 autoantibodies following type 1 diabetes onset: the type 1 diabetes genetics consortium autoantibody workshop. Diabetes Care. 38 , S14–S20 (2015).

Flannick, J. et al. Loss-of-function mutations in SLC30A8 protect against type 2 diabetes. Nat. Genet. 46 , 357–363 (2014).

Choi, B. Y. et al. Zinc transporter 3 (ZnT3) gene deletion reduces spinal cord white matter damage and motor deficits in a murine MOG-induced multiple sclerosis model. Neurobiol. Dis. 94 , 205–212 (2016).

Penkowa, M. & Hidalgo, J. Metallothionein I + II expression and their role in experimental autoimmune encephalomyelitis. Glia 32 , 247–263 (2000).

Kim, B. et al. Cytoplasmic zinc promotes IL-1β production by monocytes and macrophages through mTORC1-induced glycolysis in rheumatoid arthritis. Sci. Signal. 15 , eabi7400 (2022).

Yoon, B. R. et al. Preferential induction of the T cell auxiliary signaling molecule B7-H3 on synovial monocytes in rheumatoid arthritis. J. Biol. Chem. 291 , 4048–4057 (2016).

Cassat, J. E. & Skaar, E. P. Metal ion acquisition in Staphylococcus aureus: overcoming nutritional immunity. Semin. Immunopathol. 34 , 215–235 (2012).

Baum, M. K. et al. Randomized, controlled clinical trial of zinc supplementation to prevent immunological failure in HIV-infected adults. Clin. Infect. Dis. 50 , 1653–1660 (2010).

Kehl-Fie, T. E. & Skaar, E. P. Nutritional immunity beyond iron: a role for manganese and zinc. Curr. Opin. Chem. Biol. 14 , 218–224 (2010).

Bao, B. et al. Zinc supplementation decreases oxidative stress, incidence of infection, and generation of inflammatory cytokines in sickle cell disease patients. Transl. Res. 152 , 67–80 (2008).

Laskaris, P. et al. Administration of zinc chelators improves survival of mice infected with aspergillus fumigatus both in monotherapy and in combination with caspofungin. Antimicrob. Agents Chemother. 60 , 5631–5639 (2016).

Corbin, B. D. et al. Metal chelation and inhibition of bacterial growth in tissue abscesses. Science 319 , 962–965 (2008).

Hantke, K. Bacterial zinc uptake and regulators. Curr. Opin. Microbiol. 8 , 196–202 (2005).

Lappann, M. et al. In vitro resistance mechanisms of Neisseria meningitidis against neutrophil extracellular traps. Mol. Microbiol. 89 , 433–449 (2013).

Botella, H. et al. Metallobiology of host-pathogen interactions: an intoxicating new insight. Trends Microbiol. 20 , 106–112 (2012).

Branch, A. H., Stoudenmire, J. L., Seib, K. L. & Cornelissen, C. N. Acclimation to nutritional immunity and metal intoxication requires zinc, manganese, and copper homeostasis in the pathogenic neisseriae. Front Cell Infect. Microbiol. 12 , 909888 (2022).

Ishida, T. J. A. J. B. S. R. Review on the role of Zn2+ ions in viral pathogenesis and the effect of Zn2+ ions for host cell-virus growth inhibition. Am. J. Biomed. Sci. Res. 2 , 28–37, (2019).

Alamir, O. F., Oladele, R. O. & Ibe, C. Nutritional immunity: targeting fungal zinc homeostasis. Heliyon 7 , e07805 (2021).

Subramanian Vignesh, K. & Deepe, G. S. Jr. Immunological orchestration of zinc homeostasis: the battle between host mechanisms and pathogen defenses. Arch. Biochem. Biophys. 611 , 66–78 (2016).

Wagner, D. et al. Elemental analysis of Mycobacterium avium-, Mycobacterium tuberculosis-, and Mycobacterium smegmatis-containing phagosomes indicates pathogen-induced microenvironments within the host cell’s endosomal system. J. Immunol. 174 , 1491–1500 (2005).

Botella, H. et al. Mycobacterial p(1)-type ATPases mediate resistance to zinc poisoning in human macrophages. Cell Host Microbe 10 , 248–259 (2011).

Neyrolles, O., Wolschendorf, F., Mitra, A. & Niederweis, M. Mycobacteria, metals, and the macrophage. Immunol. Rev. 264 , 249–263 (2015).

Neyrolles, O., Mintz, E. & Catty, P. Zinc and copper toxicity in host defense against pathogens: mycobacterium tuberculosis as a model example of an emerging paradigm. Front. Cell Infect. Microbiol. 3 , 89 (2013).

Sayadi, A., Nguyen, A. T., Bard, F. A. & Bard-Chapeau, E. A. Zip14 expression induced by lipopolysaccharides in macrophages attenuates inflammatory response. Inflamm. Res. 62 , 133–143 (2013).

Stocks, C. J. et al. Uropathogenic Escherichia coli employs both evasion and resistance to subvert innate immune-mediated zinc toxicity for dissemination. Proc. Natl Acad. Sci. USA. 116 , 6341–6350 (2019).

Padilla-Benavides, T. et al. A novel P(1B)-type Mn2+-transporting ATPase is required for secreted protein metallation in mycobacteria. J. Biol. Chem. 288 , 11334–11347 (2013).

Chandrangsu, P., Rensing, C. & Helmann, J. D. Metal homeostasis and resistance in bacteria. Nat. Rev. Microbiol. 15 , 338–350 (2017).

Sensi, S. L. et al. The neurophysiology and pathology of brain zinc. J. Neurosci. 31 , 16076–16085 (2011).

Szewczyk, B. Zinc homeostasis and neurodegenerative disorders. Front Aging Neurosci. 5 , 33 (2013).

Walsh, D. M. et al. Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo. Nature 416 , 535–539 (2002).

Adlard, P. A. et al. Metal chaperones prevent zinc-mediated cognitive decline. Neurobiol. Dis. 81 , 196–202 (2015).

Bjorklund, N. L. et al. Absence of amyloid β oligomers at the postsynapse and regulated synaptic Zn2+ in cognitively intact aged individuals with Alzheimer’s disease neuropathology. Mol. Neurodegener. 7 , 23 (2012).

Bush, A. I. The metallobiology of Alzheimer’s disease. Trends Neurosci. 26 , 207–214 (2003).

Whitfield, D. R. et al. Depression and synaptic zinc regulation in Alzheimer disease, dementia with lewy bodies, and Parkinson disease dementia. Am. J. Geriatr. Psychiatry 23 , 141–148 (2015).

Adlard, P. A., Parncutt, J. M., Finkelstein, D. I. & Bush, A. I. Cognitive loss in zinc transporter-3 knock-out mice: a phenocopy for the synaptic and memory deficits of Alzheimer’s disease? J. Neurosci. 30 , 1631–1636 (2010).

Adlard, P. A. et al. A novel approach to rapidly prevent age-related cognitive decline. Aging Cell. 13 , 351–359 (2014).

Lang, M. et al. Genetic inhibition of solute-linked carrier 39 family transporter 1 ameliorates aβ pathology in a Drosophila model of Alzheimer’s disease. PLoS Genet. 8 , e1002683 (2012).

Meloni, G. et al. Metal swap between Zn7-metallothionein-3 and amyloid-beta-Cu protects against amyloid-beta toxicity. Nat. Chem. Biol. 4 , 366–372 (2008).

Lyubartseva, G., Smith, J. L., Markesbery, W. R. & Lovell, M. A. Alterations of zinc transporter proteins ZnT-1, ZnT-4 and ZnT-6 in preclinical Alzheimer’s disease brain. Brain Pathol. 20 , 343–350 (2010).

Bosomworth, H. J., Adlard, P. A., Ford, D. & Valentine, R. A. Altered expression of ZnT10 in Alzheimer’s disease brain. PLoS One 8 , e65475 (2013).

Song, L. et al. ZIP9 mediates the effects of DHT on learning, memory and hippocampal synaptic plasticity of male Tfm and APP/PS1 mice. Front Endocrinol. 14 , 1139874 (2023).

Sikora, J. & Ouagazzal, A. M. Synaptic zinc: an emerging player in Parkinson’s disease. Int J. Mol. Sci. 22 , 4724 (2021).

Valiente-Gabioud, A. A. et al. Structural basis behind the interaction of Zn 2+ with the protein α-synuclein and the Aβ peptide: a comparative analysis. J. Inorg. Biochem. 117 , 334–341 (2012).

Sepers, M. D. & Raymond, L. A. Mechanisms of synaptic dysfunction and excitotoxicity in Huntington’s disease. Drug Discov. Today 19 , 990–996 (2014).

Fourie, C. et al. Dietary zinc supplementation prevents autism related behaviors and striatal synaptic dysfunction in Shank3 Exon 13-16 mutant mice. Front. Cell Neurosci. 12 , 374 (2018).

Lee, K. et al. Dietary zinc supplementation rescues fear-based learning and synaptic function in the Tbr1(+/-) mouse model of autism spectrum disorders. Mol. Autism 13 , 13 (2022).

Squadrone, S., Brizio, P., Abete, M. C. & Brusco, A. Trace elements profile in the blood of Huntington’ disease patients. J. Trace Elem. Med. Biol. 57 , 18–20 (2020).

Niu, L. et al. Disruption of zinc transporter ZnT3 transcriptional activity and synaptic vesicular zinc in the brain of Huntington’s disease transgenic mouse. Cell Biosci. 10 , 106 (2020).

Ayton, S. et al. Brain zinc deficiency exacerbates cognitive decline in the r6/1 model of Huntington’s disease. Neurotherapeutics 17 , 243–251 (2020).

Kaneko, M. et al. Zinc transporters ZnT3 and ZnT6 are downregulated in the spinal cords of patients with sporadic amyotrophic lateral sclerosis. J. Neurosci. Res. 93 , 370–379 (2015).

Huang, J. et al. Structural basis of the zinc-induced cytoplasmic aggregation of the RNA-binding protein SFPQ. Nucleic Acids Res. 48 , 3356–3365 (2020).

Gordon, P. M., Hamid, F., Makeyev, E. V. & Houart, C. A conserved role for the ALS-linked splicing factor SFPQ in repression of pathogenic cryptic last exons. Nat. Commun. 12 , 1918 (2021).

Younas, N. et al. SFPQ and Tau: critical factors contributing to rapid progression of Alzheimer’s disease. Acta Neuropathol. 140 , 317–339 (2020).

Bayik, D. & Lathia, J. D. Cancer stem cell-immune cell crosstalk in tumour progression. Nat. Rev. Cancer 21 , 526–536 (2021).

Polyak, K. & Weinberg, R. A. Transitions between epithelial and mesenchymal states: acquisition of malignant and stem cell traits. Nat. Rev. Cancer 9 , 265–273 (2009).

Medema, J. P. Cancer stem cells: the challenges ahead. Nat. Cell Biol. 15 , 338–344 (2013).

Singh, A. & Settleman, J. EMT, cancer stem cells and drug resistance: an emerging axis of evil in the war on cancer. Oncogene 29 , 4741–4751 (2010).

Holohan, C. et al. Cancer drug resistance: an evolving paradigm. Nat. Rev. Cancer 13 , 714–726 (2013).

Nabhan, C. et al. Caspase activation is required for gemcitabine activity in multiple myeloma cell lines. Mol. Cancer Ther. 1 , 1221–1227 (2002).

Cui, X. et al. ZIP4 confers resistance to zinc deficiency-induced apoptosis in pancreatic cancer. Cell Cycle 13 , 1180–1186 (2014).

Hessmann, E., Johnsen, S. A., Siveke, J. T. & Ellenrieder, V. Epigenetic treatment of pancreatic cancer: is there a therapeutic perspective on the horizon? Gut 66 , 168–179 (2017).

Jiang, Y. et al. ZIP4 promotes non-small cell lung cancer metastasis by activating snail-N-cadherin signaling axis. Cancer Lett. 521 , 71–81 (2021).

Wu, D. M. et al. SLC39A4 expression is associated with enhanced cell migration, cisplatin resistance, and poor survival in non-small cell lung cancer. Sci. Rep. 7 , 7211 (2017).

Fan, Q., Zhang, W., Emerson, R. E. & Xu, Y. ZIP4 is a novel cancer stem cell marker in high-grade serous ovarian cancer. Cancers 12 , 3692 (2020).

Ivan, C. et al. Epigenetic analysis of the Notch superfamily in high-grade serous ovarian cancer. Gynecol. Oncol. 128 , 506–511 (2013).

Geles, K. G. et al. NOTCH3-targeted antibody drug conjugates regress tumors by inducing apoptosis in receptor cells and through transendocytosis into ligand cells. Cell Rep. Med. 2 , 100279 (2021).

Farra, R. et al. Strategies for delivery of siRNAs to ovarian cancer cells. Pharmaceutics 11 , 547 (2019).

Li, H. et al. ZIP10 drives osteosarcoma proliferation and chemoresistance through ITGA10-mediated activation of the PI3K/AKT pathway. J. Exp. Clin. Cancer Res. 40 , 340 (2021).

Chen, Q. et al. Carcinoma-astrocyte gap junctions promote brain metastasis by cGAMP transfer. Nature 533 , 493–498 (2016).

Maynard, A. et al. Therapy-induced evolution of human lung cancer revealed by single-cell RNA sequencing. Cell 182 , 1232–1251.e1222 (2020).

Ni, C. et al. ZIP1(+) fibroblasts protect lung cancer against chemotherapy via connexin-43 mediated intercellular Zn(2+) transfer. Nat. Commun. 13 , 5919 (2022).

Jia, C., Guo, Y. & Wu, F. G. Chemodynamic therapy via fenton and fenton-like nanomaterials: strategies and recent advances. Small 18 , e2103868 (2022).

Ho, E., Wong, C. P. & King, J. C. Impact of zinc on DNA integrity and age-related inflammation. Free Radic. Biol. Med. 178 , 391–397 (2022).

He, Y. et al. Evaluation of miR-21 and miR-375 as prognostic biomarkers in oesophageal cancer in high-risk areas in China. Clin. Exp. Metastasis. 34 , 73–84 (2017).

Jin, J. et al. Methylation-associated silencing of miR-193b improves the radiotherapy sensitivity of esophageal cancer cells by targeting cyclin D1 in areas with zinc deficiency. Radiother. Oncol. 150 , 104–113 (2020).

Kang, Y. et al. Advances in targeted therapy mainly based on signal pathways for nasopharyngeal carcinoma. Signal Transduct. Target Ther. 5 , 245 (2020).

Criscitiello, C., Morganti, S. & Curigliano, G. Antibody-drug conjugates in solid tumors: a look into novel targets. J. Hematol. Oncol. 14 , 20 (2021).

Nagayama, A., Vidula, N., Ellisen, L. & Bardia, A. Novel antibody-drug conjugates for triple negative breast cancer. Ther. Adv. Med. Oncol. 12 , 1758835920915980 (2020).

Trail, P. A., Dubowchik, G. M. & Lowinger, T. B. Antibody drug conjugates for treatment of breast cancer: novel targets and diverse approaches in ADC design. Pharm. Ther. 181 , 126–142 (2018).

Barroso-Sousa, R. & Tolaney, S. M. Clinical development of new antibody-drug conjugates in breast cancer: to infinity and beyond. BioDrugs 35 , 159–174 (2021).

Lim, W. F., Mohamad Yusof, M. I., Teh, L. K. & Salleh, M. Z. Significant decreased expressions of CaN, VEGF, SLC39A6 and SFRP1 in MDA-MB-231 xenograft breast tumor mice treated with moringa oleifera leaves and seed residue (MOLSr) extracts. Nutrients 12 , 2993 (2020).

Nolin, E. et al. Discovery of a ZIP7 inhibitor from a Notch pathway screen. Nat. Chem. Biol. 15 , 179–188 (2019).

Chen, J. et al. Androgen dihydrotestosterone (DHT) promotes the bladder cancer nuclear AR-negative cell invasion via a newly identified membrane androgen receptor (mAR-SLC39A9)-mediated Gαi protein/MAPK/MMP9 intracellular signaling. Oncogene 39 , 574–586 (2020).

Seok, J. et al. Anti-oncogenic effects of dutasteride, a dual 5-alpha reductase inhibitor and a drug for benign prostate hyperplasia, in bladder cancer. J. Transl. Med. 21 , 129 (2023).

Ashrafizadeh, M. et al. Noncoding RNAs as regulators of STAT3 pathway in gastrointestinal cancers: Roles in cancer progression and therapeutic response. Med. Res. Rev. , 43 , 1263–1321 (2023).

Yang, J. et al. Gene profile identifies zinc transporters differentially expressed in normal human organs and human pancreatic cancer. Curr. Mol. Med. 13 , 401–409 (2013).

Ferrari, G., Thrasher, A. J. & Aiuti, A. Gene therapy using haematopoietic stem and progenitor cells. Nat. Rev. Genet. 22 , 216–234 (2021).

Pramanik, S. K. et al. Nanoparticles for super-resolution microscopy: intracellular delivery and molecular targeting. Chem. Soc. Rev. 51 , 9882–9916 (2022).

Wandt, V. K. et al. Ageing-associated effects of a long-term dietary modulation of four trace elements in mice. Redox Biol. 46 , 102083 (2021).

Vrieling, F. & Stienstra, R. Obesity and dysregulated innate immune responses: impact of micronutrient deficiencies. Trends Immunol. 44 , 217–230 (2023).

Wang, X. et al. The zinc transporter Slc39a5 controls glucose sensing and insulin secretion in pancreatic β-cells via Sirt1- and Pgc-1α-mediated regulation of Glut2. Protein Cell. 10 , 436–449 (2019).

Wang, G. et al. Metastatic cancers promote cachexia through ZIP14 upregulation in skeletal muscle. Nat. Med. 24 , 770–781 (2018).

Yu, Y. et al. Hepatic transferrin plays a role in systemic iron homeostasis and liver ferroptosis. Blood 136 , 726–739 (2020).

Carvalho, C. S. et al. Blood cell responses and metallothionein in the liver, kidney and muscles of bullfrog tadpoles, Lithobates catesbeianus, following exposure to different metals. Environ. Pollut. 221 , 445–452 (2017).

Chen, G. H. et al. Functional analysis of MTF-1 and MT promoters and their transcriptional response to zinc (Zn) and copper (Cu) in yellow catfish Pelteobagrus fulvidraco. Chemosphere 246 , 125792 (2020).

Santoro, A. et al. The glutathione/metallothionein system challenges the design of efficient O(2) -activating copper complexes. Angew. Chem. Int Ed. Engl. 59 , 7830–7835 (2020).

Zaręba, N. & Kepinska, M. The function of transthyretin complexes with metallothionein in Alzheimer’s disease. Int J. Mol. Sci. 21 , 9003 (2020).

Manso, Y. et al. Characterization of the role of metallothionein-3 in an animal model of Alzheimer’s disease. Cell Mol. Life Sci. 69 , 3683–3700 (2012).

Kang, Y. C. et al. Cell-penetrating artificial mitochondria-targeting peptide-conjugated metallothionein 1 A alleviates mitochondrial damage in Parkinson’s disease models. Exp. Mol. Med. 50 , 1–13 (2018).

Carrasco, J. et al. Metallothionein-I and -III expression in animal models of Alzheimer disease. Neuroscience 143 , 911–922 (2006).

Manso, Y. et al. Characterization of the role of the antioxidant proteins metallothioneins 1 and 2 in an animal model of Alzheimer’s disease. Cell Mol. Life Sci. 69 , 3665–3681 (2012).

Nakamura, S. et al. Role of metallothioneins 1 and 2 in ocular neovascularization. Invest Ophthalmol. Vis. Sci. 55 , 6851–6860 (2014).

Tiwari, R. et al. SPINK1 promotes colorectal cancer progression by downregulating Metallothioneins expression. Oncogenesis 4 , e162 (2015).

Na, H. et al. Novel roles of DC-SIGNR in colon cancer cell adhesion, migration, invasion, and liver metastasis. J. Hematol. Oncol. 10 , 28 (2017).

Mendes Garrido Abregú, F., Caniffi, C., Arranz, C. T. & Tomat, A. L. Impact of zinc deficiency during prenatal and/or postnatal life on cardiovascular and metabolic diseases: experimental and clinical evidence. Adv. Nutr. 13 , 833–845 (2022).

Read, S. A., Obeid, S., Ahlenstiel, C. & Ahlenstiel, G. The role of zinc in antiviral immunity. Adv. Nutr. 10 , 696–710 (2019).

Gomes, M. J. C., Martino, H. S. D. & Tako, E. Zinc-biofortified staple food crops to improve zinc status in humans: a systematic review. Crit. Rev. Food Sci. Nutr. 63 , 4966–4978 (2023).

Gibson, R. S., King, J. C. & Lowe, N. A review of dietary zinc recommendations. Food Nutr. Bull. 37 , 443–460 (2016).

Fairweather-Tait, S. J. & de Sesmaisons, A. Approaches used to estimate bioavailability when deriving dietary reference values for iron and zinc in adults. Proc. Nutr. Soc. 78 , 1–7 (2018).

Google Scholar  

Duan, M. et al. Zinc nutrition and dietary zinc supplements. Crit. Rev. Food Sci. Nutr. 63 , 1277–1292 (2023).

Brown, K. H. et al. International Zinc Nutrition Consultative Group (IZiNCG) technical document #1. Assessment of the risk of zinc deficiency in populations and options for its control. Food Nutr. Bull. 25 , S99–S203 (2004).

Tran, C. D. et al. Zinc absorption as a function of the dose of zinc sulfate in aqueous solution. Am. J. Clin. Nutr. 80 , 1570–1573 (2004).

Sapota, A. et al. The bioavailability of different zinc compounds used as human dietary supplements in rat prostate: a comparative study. Biometals 27 , 495–505 (2014).

Chukwuma, C. I. et al. A comprehensive review on zinc(II) complexes as anti-diabetic agents: The advances, scientific gaps and prospects. Pharm. Res. 155 , 104744 (2020).

Jansen, J., Karges, W. & Rink, L. Zinc and diabetes–clinical links and molecular mechanisms. J. Nutr. Biochem. 20 , 399–417 (2009).

Tang, Y. et al. Zinc supplementation partially prevents renal pathological changes in diabetic rats. J. Nutr. Biochem. 21 , 237–246 (2010).

Jayawardena, R. et al. Effects of zinc supplementation on diabetes mellitus: a systematic review and meta-analysis. Diabetol. Metab. Syndr. 4 , 13 (2012).

Ranasinghe, P. et al. Effects of Zinc supplementation on serum lipids: a systematic review and meta-analysis. Nutr. Metab. 12 , 26 (2015).

Pompano, L. M. & Boy, E. Effects of dose and duration of zinc interventions on risk factors for type 2 diabetes and cardiovascular disease: a systematic review and meta-analysis. Adv. Nutr. 12 , 141–160 (2021).

Özcelik, D. et al. Zinc supplementation attenuates metallothionein and oxidative stress changes in kidney of streptozotocin-induced diabetic rats. Biol. Trace Elem. Res. 150 , 342–349 (2012).

Barman, S., Pradeep, S. R. & Srinivasan, K. Zinc supplementation alleviates the progression of diabetic nephropathy by inhibiting the overexpression of oxidative-stress-mediated molecular markers in streptozotocin-induced experimental rats. J. Nutr. Biochem. 54 , 113–129 (2018).

Liu, F. et al. Zinc supplementation alleviates diabetic peripheral neuropathy by inhibiting oxidative stress and upregulating metallothionein in peripheral nerves of diabetic rats. Biol. Trace Elem. Res. 158 , 211–218 (2014).

Foster, M., Chu, A., Petocz, P. & Samman, S. Zinc transporter gene expression and glycemic control in post-menopausal women with Type 2 diabetes mellitus. J. Trace Elem. Med Biol. 28 , 448–452 (2014).

Sakurai, H., Yoshikawa, Y. & Yasui, H. Current state for the development of metallopharmaceutics and anti-diabetic metal complexes. Chem. Soc. Rev. 37 , 2383–2392 (2008).

Tang, K. S. The current and future perspectives of zinc oxide nanoparticles in the treatment of diabetes mellitus. Life Sci. 239 , 117011 (2019).

Patel, A. et al. Therapeutic value of zinc supplementation in acute and persistent diarrhea: a systematic review. PLoS One 5 , e10386 (2010).

Chang, M. N. et al. Effects of different types of zinc supplement on the growth, incidence of diarrhea, immune function, and rectal microbiota of newborn dairy calves. J. Dairy Sci. 103 , 6100–6113 (2020).

Bhandari, N. et al. Substantial reduction in severe diarrheal morbidity by daily zinc supplementation in young north Indian children. Pediatrics 109 , e86 (2002).

Brooks, W. A. et al. Effect of weekly zinc supplements on incidence of pneumonia and diarrhoea in children younger than 2 years in an urban, low-income population in Bangladesh: randomised controlled trial. Lancet 366 , 999–1004 (2005).

Dong, J., Li, H. & Min, W. Preparation, characterization and bioactivities of Athelia rolfsii exopolysaccharide-zinc complex (AEPS-zinc). Int J. Biol. Macromol. 113 , 20–28 (2018).

Martinelli, D. et al. MEDNIK syndrome: a novel defect of copper metabolism treatable by zinc acetate therapy. Brain 136 , 872–881 (2013).

Camarata, M. A., Ala, A. & Schilsky, M. L. Zinc maintenance therapy for wilson disease: a comparison between zinc acetate and alternative zinc preparations. Hepatol. Commun. 3 , 1151–1158 (2019).

Duncan, A., Yacoubian, C., Watson, N. & Morrison, I. The risk of copper deficiency in patients prescribed zinc supplements. J. Clin. Pathol. 68 , 723–725 (2015).

Guo, C. H. & Wang, C. L. Effects of zinc supplementation on plasma copper/zinc ratios, oxidative stress, and immunological status in hemodialysis patients. Int J. Med. Sci. 10 , 79–89 (2013).

Hemilä, H. Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open. 8 , 2054270417694291 (2017).

Granum, B. Opinion of the Scientific Committee on Consumer safety (SCCS) - Final opinion on water-soluble zinc salts used in oral hygiene products. Regul. Toxicol. Pharmacol. 99 , 249–250 (2018).

Franklin, R. B. & Costello, L. C. The important role of the apoptotic effects of zinc in the development of cancers. J. Cell Biochem. 106 , 750–757 (2009).

Hashemi, M. et al. Cytotoxic effects of intra and extracellular zinc chelation on human breast cancer cells. Eur. J. Pharmacol. 557 , 9–19 (2007).

Richter, M. et al. Zinc chelators inhibit eotaxin, RANTES, and MCP-1 production in stimulated human airway epithelium and fibroblasts. Am. J. Physiol. Lung Cell Mol. Physiol. 285 , L719–L729 (2003).

Albulescu, L. O. et al. Preclinical validation of a repurposed metal chelator as an early-intervention therapeutic for hemotoxic snakebite. Sci. Transl. Med. 12 , eaay8314 (2020).

Nyborg, J. K. & Peersen, O. B. That zincing feeling: the effects of EDTA on the behaviour of zinc-binding transcriptional regulators. Biochem J. 381 , e3–e4 (2004).

Hellmich, H. L. et al. Protective effects of zinc chelation in traumatic brain injury correlate with upregulation of neuroprotective genes in rat brain. Neurosci. Lett. 355 , 221–225 (2004).

Bareggi, S. R. & Cornelli, U. Clioquinol: review of its mechanisms of action and clinical uses in neurodegenerative disorders. CNS Neurosci. Ther. 18 , 41–46 (2012).

Doraiswamy, P. M. & Finefrock, A. E. Metals in our minds: therapeutic implications for neurodegenerative disorders. Lancet Neurol. 3 , 431–434 (2004).

Labbé, R. F., Vreman, H. J. & Stevenson, D. K. Zinc protoporphyrin: a metabolite with a mission. Clin. Chem. 45 , 2060–2072 (1999).

Faller, P. & Hureau, C. Bioinorganic chemistry of copper and zinc ions coordinated to amyloid-beta peptide. Dalton Trans. 7 , 1080–1094 (2009).

Jackson, K. W. & Mahmood, T. M. Atomic absorption, atomic emission, and flame emission spectrometry. Anal. Chem. 66 , 252r–279r (1994).

Carter, K. P., Young, A. M. & Palmer, A. E. Fluorescent sensors for measuring metal ions in living systems. Chem. Rev. 114 , 4564–4601 (2014).

Denk, C. et al. Design, synthesis, and evaluation of a low-molecular-weight (11)C-labeled tetrazine for pretargeted PET imaging applying bioorthogonal in vivo click chemistry. Bioconjug. Chem. 27 , 1707–1712 (2016).

Aper, S. J., Dierickx, P. & Merkx, M. Dual Readout BRET/FRET Sensors for Measuring Intracellular Zinc. ACS Chem. Biol. 11 , 2854–2864 (2016).

Wei, T. et al. Directed evolution of the genetically encoded zinc(II) FRET sensor ZapCY1. Biochim Biophys. Acta Gen. Subj. 1866 , 130201 (2022).

Bacart, J. et al. The BRET technology and its application to screening assays. Biotechnol. J. 3 , 311–324 (2008).

Qin, Y. et al. Measuring steady-state and dynamic endoplasmic reticulum and Golgi Zn2+ with genetically encoded sensors. Proc. Natl Acad. Sci. Usa. 108 , 7351–7356 (2011).

Chabosseau, P. et al. Mitochondrial and ER-targeted eCALWY probes reveal high levels of free Zn2+. ACS Chem. Biol. 9 , 2111–2120 (2014).

Hessels, A. M. et al. eZinCh-2: a versatile, genetically encoded FRET sensor for cytosolic and intraorganelle Zn(2+) Imaging. ACS Chem. Biol. 10 , 2126–2134 (2015).

Hessels, A. M., Taylor, K. M. & Merkx, M. Monitoring cytosolic and ER Zn(2+) in stimulated breast cancer cells using genetically encoded FRET sensors. Metallomics 8 , 211–217 (2016).

Park, J. G., Qin, Y., Galati, D. F. & Palmer, A. E. New sensors for quantitative measurement of mitochondrial Zn(2+). ACS Chem. Biol. 7 , 1636–1640 (2012).

Lin, Y. et al. ZIP4 is a novel molecular marker for glioma. Neuro Oncol. 15 , 1008–1016 (2013).

Saravanan, R. et al. Zinc transporter LIV1: a promising cell surface target for triple negative breast cancer. J. Cell Physiol. 237 , 4132–4156 (2022).

Gou, Y. et al. The transcription of ZIP9 is associated with the macrophage polarization and the pathogenesis of hepatocellular carcinoma. Front Immunol. 13 , 725595 (2022).

Changizzadeh, P. N., Mukkamalla, S. K. R. & Armenio, V. A. Combined checkpoint inhibitor therapy causing diabetic ketoacidosis in metastatic melanoma. J. Immunother. Cancer 5 , 97 (2017).

Sveen, A. et al. The exon-level biomarker SLC39A14 has organ-confined cancer-specificity in colorectal cancer. Int J. Cancer 131 , 1479–1485 (2012).

Karandish, M. et al. The effect of curcumin and zinc co-supplementation on glycemic parameters in overweight or obese prediabetic subjects: a phase 2 randomized, placebo-controlled trial with a multi-arm, parallel-group design. Phytother. Res. 35 , 4377–4387 (2021).

Islam, M. R. et al. Zinc supplementation for improving glucose handling in pre-diabetes: a double blind randomized placebo controlled pilot study. Diabetes Res Clin. Pract. 115 , 39–46 (2016).

Foster, M., Petocz, P. & Samman, S. Inflammation markers predict zinc transporter gene expression in women with type 2 diabetes mellitus. J. Nutr. Biochem. 24 , 1655–1661 (2013).

Nazem, M. R. et al. Zinc supplementation ameliorates type 2 diabetes markers through the enhancement of total antioxidant capacity in overweight patients. Postgrad. Med. J. 99 , 862–867 (2023).

Fung, E. B. et al. Zinc supplementation improves markers of glucose homeostasis in thalassaemia. Br. J. Haematol. 190 , e162–e166 (2020).

Bao, B. et al. Zinc decreases C-reactive protein, lipid peroxidation, and inflammatory cytokines in elderly subjects: a potential implication of zinc as an atheroprotective agent. Am. J. Clin. Nutr. 91 , 1634–1641 (2010).

Ben Abdallah, S. et al. Twice-Daily Oral Zinc in the Treatment of Patients With Coronavirus Disease 2019: A Randomized Double-Blind Controlled Trial. Clin. Infect. Dis. 76 , 185–191 (2023).

Rodriguez, J. A. M. et al. Effect and tolerability of a nutritional supplement based on a synergistic combination of β-glucans and selenium- and zinc-enriched saccharomyces cerevisiae (ABB C1(®)) in volunteers receiving the influenza or the COVID-19 vaccine: a randomized, double-blind, placebo-controlled study. Nutrients 13 , 4347 (2021).

Faghfouri, A. H. et al. Regulation of NLRP3 inflammasome by zinc supplementation in Behçet’s disease patients: a double-blind, randomized placebo-controlled clinical trial. Int Immunopharmacol. 109 , 108825 (2022).

Faghfouri, A. H. et al. Immunomodulatory and clinical responses to zinc gluconate supplementation in patients with Behçet’s disease: a double-blind, randomized placebo-controlled clinical trial. Clin. Nutr. 41 , 1083–1092 (2022).

Bobat, R. et al. Safety and efficacy of zinc supplementation for children with HIV-1 infection in South Africa: a randomised double-blind placebo-controlled trial. Lancet 366 , 1862–1867 (2005).

Roy, S. K. et al. Zinc supplementation in children with cholera in Bangladesh: randomised controlled trial. BMJ 336 , 266–268 (2008).

Veenemans, J. et al. Effect of supplementation with zinc and other micronutrients on malaria in Tanzanian children: a randomised trial. PLoS Med. 8 , e1001125 (2011).

Fung, E. B. et al. Zinc supplementation improves bone density in patients with thalassemia: a double-blind, randomized, placebo-controlled trial. Am. J. Clin. Nutr. 98 , 960–971 (2013).

Guo, C. H., Chen, P. C., Hsu, G. S. & Wang, C. L. Zinc supplementation alters plasma aluminum and selenium status of patients undergoing dialysis: a pilot study. Nutrients 5 , 1456–1470 (2013).

Kobayashi, H. et al. Oral zinc supplementation reduces the erythropoietin responsiveness index in patients on hemodialysis. Nutrients 7 , 3783–3795 (2015).

Lin, L. C., Que, J., Lin, L. K. & Lin, F. C. Zinc supplementation to improve mucositis and dermatitis in patients after radiotherapy for head-and-neck cancers: a double-blind, randomized study. Int J. Radiat. Oncol. Biol. Phys. 65 , 745–750 (2006).

Ribeiro, S. M. et al. Effect of zinc supplementation on antioxidant defenses and oxidative stress markers in patients undergoing chemotherapy for colorectal cancer: a placebo-controlled, prospective randomized trial. Biol. Trace Elem. Res. 169 , 8–16 (2016).

Qiao, Y. L. et al. Total and cancer mortality after supplementation with vitamins and minerals: follow-up of the Linxian General Population Nutrition Intervention Trial. J. Natl Cancer Inst. 101 , 507–518 (2009).

Ye, W. et al. A sensitive FRET biosensor based on carbon dots-modified nanoporous membrane for 8-hydroxy-2’-Deoxyguanosine (8-OHdG) detection with Au@ZIF-8 nanoparticles as signal quenchers. Nanomaterials 10 , 2044 (2020).

Qin, Y. et al. Development of an optical Zn(2+) probe based on a single fluorescent protein. ACS Chem. Biol. 11 , 2744–2751 (2016).

Han, Y., Goldberg, J. M., Lippard, S. J. & Palmer, A. E. Superiority of SpiroZin2 Versus FluoZin-3 for monitoring vesicular Zn(2+) allows tracking of lysosomal Zn(2+) pools. Sci. Rep. 8 , 15034 (2018).

Nolan, E. M. & Lippard, S. J. Small-molecule fluorescent sensors for investigating zinc metalloneurochemistry. Acc. Chem. Res. 42 , 193–203 (2009).

Ueno, S. et al. Mossy fiber Zn2+ spillover modulates heterosynaptic N-methyl-D-aspartate receptor activity in hippocampal CA3 circuits. J. Cell Biol. 158 , 215–220 (2002).

Kao, Y. Y. et al. Zinc oxide nanoparticles interfere with zinc ion homeostasis to cause cytotoxicity. Toxicol. Sci. 125 , 462–472 (2012).

Sensi, S. L. et al. A new mitochondrial fluorescent zinc sensor. Cell Calcium 34 , 281–284 (2003).

You, Y. et al. Phosphorescent sensor for biological mobile zinc. J. Am. Chem. Soc. 133 , 18328–18342 (2011).

Meeusen, J. W., Tomasiewicz, H., Nowakowski, A. & Petering, D. H. TSQ (6-methoxy-8-p-toluenesulfonamido-quinoline), a common fluorescent sensor for cellular zinc, images zinc proteins. Inorg. Chem. 50 , 7563–7573 (2011).

Download references

Acknowledgements

This study was supported by National Natural Science Foundation of China (NSFC) (2022, No. 82272990), Health and Medical Research Fund (HMRF, 08190586), CUHK direct grant (2022.001 and 2020.004), and Cheng Yue Pui Charity Foundation. We acknowledge the TCGA Research Network ( http://cancergenome.nih.gov/ ) and the cBioPortal for Cancer Genomics ( https://www.cbioportal.org/ ) for providing the datasets and analysis. Part of the images was generated by BioRender ( https://biorender.com/ ) and GEPIA2 ( http://gepia2.cancer-pku.cn/#isoform ). We also appreciate the technical support from Core Utilities of Cancer Genomics and Pathobiology of the Department of Anatomical and Cellular Pathology, The Chinese University of Hong Kong.

Author information

These authors contributed equally: Bonan Chen, Peiyao Yu

Authors and Affiliations

Department of Anatomical and Cellular Pathology, State Key Laboratory of Translational Oncology, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong, China

Bonan Chen, Wai Nok Chan, Fuda Xie, Kwok Wai Lo, Gary M. K. Tse, Wei Kang & Ka Fai To

State Key Laboratory of Digestive Disease, Institute of Digestive Disease, The Chinese University of Hong Kong, Hong Kong, China

Bonan Chen, Wai Nok Chan, Fuda Xie, Jun Yu, Wei Kang & Ka Fai To

CUHK-Shenzhen Research Institute, The Chinese University of Hong Kong, Shenzhen, China

Bonan Chen, Wai Nok Chan, Fuda Xie & Wei Kang

Department of Pathology, Nanfang Hospital and Basic Medical College, Southern Medical University, Guangdong Province Key Laboratory of Molecular Tumor Pathology, Guangzhou, China

Peiyao Yu & Li Liang

Institute of Biomedical Research, Taihe Hospital, Hubei University of Medicine, Shiyan, China

Yigan Zhang

Department of Pediatrics, The Chinese University of Hong Kong, Hong Kong, China

Kam Tong Leung

Department of Medicine and Therapeutics, The Chinese University of Hong Kong, Hong Kong, China

You can also search for this author in PubMed   Google Scholar

Contributions

K.F.T. and W.K. offered directions on this manuscript. B.C., P.Y. drafted the manuscript together. B.C., W.N.C. and P.Y. made the figures and table. B.C., F.X. and Y.Z. reviewed the literature. L.L., K.T.L., K.W.L., J.Y. G.M.K.T. and W.K. reviewed the manuscript and gave comments. All authors have read and agreed to the published version of the manuscript.

Corresponding authors

Correspondence to Wei Kang or Ka Fai To .

Ethics declarations

Competing interests.

The authors declare no competing interests.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ .

Reprints and permissions

About this article

Cite this article.

Chen, B., Yu, P., Chan, W.N. et al. Cellular zinc metabolism and zinc signaling: from biological functions to diseases and therapeutic targets. Sig Transduct Target Ther 9 , 6 (2024). https://doi.org/10.1038/s41392-023-01679-y

Download citation

Received : 27 May 2023

Revised : 15 September 2023

Accepted : 10 October 2023

Published : 03 January 2024

DOI : https://doi.org/10.1038/s41392-023-01679-y

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

This article is cited by

Iron metabolism: backfire of cancer cell stemness and therapeutic modalities.

• Yinhui Hang

Cancer Cell International (2024)

Quick links

• Explore articles by subject
• Guide to authors
• Editorial policies

Zinc Deficiency as a General Feature of Cancer: a Review of the Literature

• Open access
• Published: 02 September 2023
• Volume 202 , pages 1937–1947, ( 2024 )

Cite this article

You have full access to this open access article

• Rie Sugimoto   ORCID: orcid.org/0000-0003-3906-9307 1 ,
• Lingaku Lee   ORCID: orcid.org/0000-0003-1005-5606 1 ,
• Yuki Tanaka   ORCID: orcid.org/0000-0002-1499-6764 1 ,
• Yusuke Morita 1 ,
• Masayuki Hijioka 1 ,
• Terumasa Hisano   ORCID: orcid.org/0000-0002-4965-086X 1 &
• Masayuki Furukawa   ORCID: orcid.org/0000-0002-1278-0775 1  

4285 Accesses

3 Citations

30 Altmetric

Explore all metrics

Trace elements are minerals that are present in very low concentrations in the human body and yet are crucial for a wide range of physiological functions. Zinc, the second most abundant trace element, is obtained primarily from the diet. After being taken up in the intestine, zinc is distributed to various target organs, where it plays key roles in processes such as immunity, protein folding, apoptosis, and antioxidant activity. Given the important role of zinc in a wide range of enzymatic reactions and physiological processes, zinc deficiency has been identified in a variety of diseases, notably cancer. In recent years, multiple meta-analyses and reviews looking at zinc levels in individual cancer types have been published, as have a plethora of primary studies demonstrating a link between low zinc levels and specific types of cancer. In this review, we summarize recent evidence implicating low zinc concentrations in serum or tissues as a characteristic in a wide range of cancers. We also discuss preliminary findings indicating that zinc level measurement could ultimately become a useful clinical tool for cancer diagnosis and predicting outcomes in patients with cancer. Finally, we suggest future directions for further elucidating the role of zinc deficiency in cancer development and progression.

Similar content being viewed by others

Cancer profiles in China and comparisons with the USA: a comprehensive analysis in the incidence, mortality, survival, staging, and attribution to risk factors

Vitamin D and human health: evidence from Mendelian randomization studies

Biological properties and clinical applications of berberine.

Avoid common mistakes on your manuscript.

Introduction

Trace elements are minerals that are present at very low concentrations in the human body and are crucial for proper physiological function. Some trace elements are considered to be nutritionally essential, meaning they must be obtained from the diet, while others are nonessential. Each trace element is typically present at a concentration of < 0.1% of the body volume and can be readily detected in the blood and other bodily tissues. Trace elements that are well known for their roles in human health and physiology include iron, zinc, iodine, fluoride, and copper. Zinc is the second most abundant trace element after iron [ 1 , 2 ]. Zinc is obtained primarily from the diet and absorbed in the intestine [ 3 , 4 ]. Its bioavailability is strongly affected by dietary composition, which affects its uptake by the digestive system [ 5 , 6 ]. After being taken up in the intestine, zinc is distributed to various target organs throughout the body, with concentrations tending to be highest in bone and muscle, followed by the skin and liver [ 7 ].

Zinc plays a variety of key roles in the human body (Fig.  1 ). For example, it is crucial to signaling processes involved in cell differentiation, proliferation, and apoptosis [ 8 ]. Zinc ions are required for the catalytic activity of a multitude of enzymes, notably the zinc finger proteins, which contain a zinc finger motif stabilized by a zinc ion. This large family of proteins has been implicated in processes as diverse as transcriptional regulation, ubiquitin-mediated protein degradation, signal transduction, actin targeting, and DNA repair [ 9 ]. In addition to these roles, zinc functions as an antioxidant and plays an essential role in immunity. As reviewed by Prasad, zinc inhibits inflammatory cytokine production and decreases plasma oxidative stress markers [ 10 , 11 ]. Other studies showed that copper-zinc superoxide dismutase protects tissues from reactive oxygen species, and alterations in the activity of this enzyme have been associated with a wide range of diseases [ 12 , 13 ].

The roles of zinc in human physiology. Some of the most important functions of zinc in the human body include supporting a healthy immune response by promoting differentiation of naïve T cells into activated Th17 cells; facilitating proper folding and activity of zinc-containing proteins; acting as a cofactor for superoxide dismutase (SOD) to exert antioxidant activity; and modulating the stability and activity of p53 to regulate apoptosis. SOD , superoxide dismutase ; Th17 , T helper 17. References: “Protein folding” representative image from [ 14 ] CC BY 4.0. “Apoptosis” representative image from [ 15 ] CC BY-NC 4.0. “Antioxidant activity” representative image from [ 16 ] CC BY 3.0

Unsurprisingly, given the crucial nature of zinc in many enzymatic reactions and physiological processes, low zinc levels are associated with a variety of disease states. Low levels of zinc have been reported in a range of cancers, with increasing evidence suggesting that low zinc levels in serum, plasma, and tissues may be linked to carcinogenesis [ 17 , 18 ]. In this review, we will present the recent evidence that low zinc levels in a variety of situations are a hallmark of various cancers. We will also speculate on the usefulness of zinc level as a biomarker and the potential prevention or treatment of cancer by zinc supplementation, as reported in preliminary studies. Finally, we will highlight the research priorities for increasing the understanding of the role that zinc deficiency plays in cancer and developing potential clinical applications on the basis of this association in the future.

Zinc Deficiency in Cancer

In this section, we provide a comprehensive review of the current state of knowledge regarding zinc levels in patients with various cancer types. Table 1 presents an overview of the key features of the studies discussed in this section. We used PubMed to identify 2470 articles on cancer and zinc published up to 2023. We excluded case reports, articles on animals, articles on side effects occurring during and after cancer treatment, articles in which the carcinoma was not specified, and basic research, resulting in a total of 151 articles. Clinical/epidemiological articles were further selected from these studies, and we excluded review articles; articles with the highest number of cases, highest level of evidence, and newest articles by cancer type were selected and are listed in Table 1 .

Zinc Deficiency in Esophageal Cancer

Esophageal cancer is one of the cancer types that is the most well studied in relation to zinc and cancer incidence. Lu et al. [ 19 ] reported in 2006 that the zinc intake of esophageal cancer patients in China was lower than that of healthy individuals, and Schaafsma et al. [ 20 ] conducted a database-based ecological analysis and reported that in Africa, zinc supply was lower in countries with a high risk for esophageal cancer incidence than in low-risk countries. In 2015, a significant negative correlation between zinc intake and the incidence of esophageal cancer was reported in the Golestan cohort study in a high-risk district for esophageal cancer [ 21 ]. Zinc levels in hair were measured in high- and low-risk areas for esophageal cancer, and low zinc levels were found to be associated with the development of esophageal cancer [ 22 ]. The molecular mechanisms linking esophageal cancer and zinc deficiency are also being elucidated, with a large body of evidence indicating that zinc deficiency promotes carcinogenesis by altering microRNA expression [ 52 , 53 , 54 ]. Li et al. [ 55 ] reported a significant correlation between zinc and risk of esophageal cancer in Asia, where zinc intake is naturally low, but no correlation was found in the USA or Europe. Dietary zinc intake also varies widely from region to region because of the strong regional specificity of dietary habits. When discussing the relationship between dietary zinc intake and disease, it may be necessary to examine each region separately.

Zinc Deficiency in Breast Cancer

There is considerable evidence for an association between zinc levels in tissue and serum and breast cancer, as demonstrated by many recent studies of zinc levels in a variety of patient tissues.

Early prospective studies provided clear indications that low zinc levels are closely linked to breast cancer development and progression. In their 2015 cross-sectional study conducted in Nigeria, Adeoti et al. [ 23 ] reported that the copper/zinc ratio was significantly higher in patients with breast cancer compared with healthy controls and that venous blood zinc levels were low in breast cancer patients. A retrospective in situ study by Costello et al., published in 2016, reported low zinc levels in breast invasive ductal carcinoma compared with normal ductal epithelium [ 24 ].

Several meta-analyses were published that collated evidence regarding zinc deficiency in breast cancer. A 2015 meta-analysis by Wu et al. [ 25 ] assessed zinc levels in various tissue samples from female breast cancer patients and healthy controls. The authors found that zinc levels in hair samples from breast cancer patients were lower than those in hair samples from a control group; however, they reported no significant difference in serum zinc levels between these two groups. More recently, a 2019 meta-analysis investigated zinc levels in breast tissue, plasma, serum, and hair samples from patients with breast cancer [ 26 ]. The authors reported that zinc levels were significantly lower in blood and hair samples from patients with breast cancer compared with controls, and significantly higher in tumor tissues. Furthermore, a 2020 meta-analysis by Feng et al. [ 27 ] evaluated data from studies assessing serum copper and zinc levels in patients with breast cancer. The results showed that elevated serum copper levels, elevated serum copper/zinc ratio, and decreased serum zinc levels were significantly associated with an increased risk of breast cancer. It should be noted, however, that in these meta-analyses, there was overlap in the cohorts used, as the hair sample cohort in the 2015 and 2019 meta-analyses was identical [ 25 , 26 ]. Furthermore, among the cohorts used in the 2019 and 2020 meta-analyses, half of the cohorts examined for serum zinc were the same (duplicates) [ 26 , 27 ].

Since the publication of these meta-analyses, additional clinical studies have shed further light on the close association between low zinc levels in tissues and serum and breast cancer. For example, in their 2022 study, Pala et al. [ 17 ] assessed the association between pre-diagnostic copper and zinc levels in the plasma and urine of women later diagnosed with breast cancer. The authors found that patients with a plasma copper/zinc ratio or a plasma and urine copper/zinc ratio in the upper tertile of the patient cohort had a significantly higher risk of developing breast cancer than those in the lower tertiles. A 2022 study by Mansouri et al. [ 28 ] reported that higher zinc levels were present in mammary tissue from healthy individuals than in breast cancer tissue from patients. In contrast, in a 2022 study of breast cancer patients, Barartabar et al. [ 29 ] reported lower zinc levels in adjacent non-cancerous areas compared with breast cancer tissue. This apparent discrepancy should be interpreted with caution, as Mansouri et al. assessed zinc levels in tissue from breast cancer patients compared with healthy controls, while Barartabar et al. examined zinc levels in healthy and diseased tissue from the same patient; thus, the studies may not be comparable.

Taken together, these meta-analyses and clinical studies build on the existing evidence for a correlation between breast cancer and zinc deficiency.

Zinc Deficiency in Liver Cancer

In addition to breast cancer, there is a substantial body of evidence demonstrating an association between low zinc levels and liver cancer.

Many studies have shown low zinc concentrations in liver cancer tissue. Kew et al. [ 30 ] reported that zinc concentrations in liver cancer tissue were significantly lower than those in non-cancerous areas, and further reports showed that zinc concentrations decrease as the degree of differentiation of liver cancer worsens [ 31 ]. In 2014, Costello et al. [ 56 ] highlighted the relationship between zinc levels and chronic liver disease/hepatocarcinogenesis, citing numerous studies that reported a marked decrease in zinc levels in hepatocellular carcinoma tissue compared with normal liver tissue. At present, no published papers have reported results indicating that zinc levels are not decreased in hepatocellular carcinoma tissue. Another study reported that serum zinc levels decrease as hepatocellular carcinoma progresses [ 32 ].

Low zinc levels also appear to correlate with poor outcomes in patients treated for liver cancer. For example, a retrospective analysis by Imai et al. [ 33 ] published in 2014 found that low serum zinc levels correlate with low disease-free and overall survival rates in patients undergoing hepatectomy for hepatocellular carcinoma. Interestingly, a 2019 study by Fang et al. [ 34 ] found that a higher serum copper/zinc ratio, but not a higher serum zinc level, may be associated with survival in patients with hepatocellular carcinoma. However, the actual risk of death in patients with low zinc levels in the study was not clear, as the participants were stratified into multiple groups with a range of zinc levels, and survival rates were compared among all of the groups. As copper and zinc absorption are antagonistic processes, more research is needed to determine whether copper or zinc plays a role in the prognosis of patients with liver cancer. Additionally, Hiraoka et al. reported that patients with low serum zinc levels had a worse prognosis than those with high levels, but there was no difference in recurrence-free survival. Furthermore, in the analysis of prognosis after treatment of early liver cancer, zinc levels correlated with hepatic reserve capacity [ 57 ]. Thus, the relationship between zinc and the prognosis of liver cancer is not only related to the prognosis of the cancer itself, but also to the prognosis related to hepatic reserve capacity.

Zinc deficiency appears to play a role not only in liver cancer outcomes but also in the risk of liver cancer development. Shigefuku et al. [ 35 ] prospectively observed 157 cirrhotic patients for 3 years and reported that hypozincemia was the only significant predictor of hepatic carcinogenesis. In 2020, Ozeki et al. [ 36 ] reported that low zinc levels were a risk factor for developing hepatocellular carcinoma after eradication of hepatitis C virus (HCV). Another report from Ozeki et al. [ 37 ] published in the same year found that low zinc levels are highly prevalent in patients with chronic liver disease, a strong risk factor for developing hepatocellular carcinoma. In considering the possible link between hepatocarcinogenesis and zinc, it is interesting to note that Franklin et al. [ 58 ] reported that tissue zinc deficiency is present in highly differentiated hepatomas and observed alongside downregulation of the ZIP14 zinc uptake transporters. This suggests that zinc depletion begins in the precancerous state. The connection between low zinc levels and hepatocarcinogenesis was explored in a 2022 review by Nishikawa et al.; the authors discussed a variety of studies demonstrating the relationship between hypozincemia and chronic liver disease, liver fibrosis, and the risk of liver carcinogenesis [ 59 ]. Whether hypozincemia is really the direct cause of hepatocarcinogenesis or whether hypozincemia correlates with liver fibrosis and only results in increased hepatocarcinogenesis remains unclear, and further research is needed.

Recent studies have explored the impact of zinc supplementation on the risk of developing liver cancer. A 2018 retrospective study by Hosui et al. [ 38 ] found that patients with chronic liver disease who received long-term zinc supplementation had a lower risk of developing liver cancer than patients who did not take the supplement. In 2021, the same group reported that supplementation with zinc after HCV eradication reduces the risk of developing hepatocellular carcinoma [ 18 ].

Together, these studies provide a strong argument for a key role for zinc in liver cancer development and progression.

Zinc Deficiency in Lung Cancer

Multiple recent studies support a link between zinc deficiency and lung cancer, and the association was extensively analyzed in two recent meta-analyses. Wang et al. found that serum zinc levels were significantly lower in patients with lung cancer than in controls in European and Asian populations [ 39 ]. Similarly, Zhang et al. found that lung cancer patients had a significantly higher serum copper/zinc ratio (implying low zinc levels) than healthy controls and patients with benign lung diseases; this ratio was significantly higher in patients with advanced lung cancer than in patients with early-stage lung cancer [ 40 ]. Notably, the two meta-analyses included 32 and 39 cohorts, but the results were not derived from completely separate populations because of the overlap of 23 cohorts. Additionally, Bai et al. reported that high zinc plasma reduces the risk of lung cancer, possibly by delaying telomere events and modulating the expression of some oncogenes; the authors suggest the possibility that zinc can be used to prevent lung cancer [ 41 ]. These studies clearly show that zinc deficiency is a factor in lung cancer development and progression.

Zinc Deficiency in Gynecological Cancer

Zinc deficiency has been well described in gynecological cancers.

The association between low zinc levels and cervical cancer was convincingly demonstrated in a 2018 meta-analysis by Xie et al. [ 42 ], which reported significantly lower serum zinc levels in cervical cancer cases than in controls in Asian women.

In endometrial cancer, reports on the relationship between cancer and zinc concentrations are mixed. Yaman et al. [ 60 ] reported that zinc concentrations in cancerous tissues were significantly lower than those in non-cancerous tissues, while Rzymski et al. [ 61 ] found no difference in zinc levels in cancerous areas despite an increased copper/zinc ratio. In contrast, Atakul et al. [ 62 ] reported lower serum copper and zinc levels but lower copper/zinc ratios in patients with endometrial cancer. Further study on the association between zinc concentration and endometrial cancer will be required.

Lin et al. reported a meta-analysis and Mendelian randomization study in ovarian cancer in 2021. The authors showed that ovarian cancer patients have significantly lower circulating zinc concentrations compared with healthy individuals [ 43 ].

Overall, the evidence strongly suggests that low zinc levels are associated with a range of gynecological cancers, and that an interaction with copper levels may also play a role. It should be noted that there is, as yet, no evidence that low zinc levels are a cause of carcinogenesis or cancer progression in gynecological cancers, and at this point, the results have only suggested a possible relationship between zinc levels and cancer patients.

Zinc Deficiency in Colon Cancer

Colon cancer has also been reported to be associated with zinc deficiency. A 2017 case–control study by Stepien et al. [ 44 ] performed using the large European Prospective Investigation into Cancer and Nutrition cohort reported an apparent link between copper/zinc ratios and colorectal cancer. More recently, a 2020 prospective observational study reported a correlation between low serum zinc levels and poor prognosis (including distant metastasis and survival) in patients with colon cancer [ 45 ]. While this association has not been explored as extensively as it has for other cancer types, the findings from these two studies suggest that zinc deficiency is also associated with colon cancer incidence and outcomes. However, while a Mendelian randomized study of 58221 European patients with colon cancer reported that a nominally significant association between colon cancer risk and zinc concentration was found, sensitivity analysis could not be performed, and ultimately a causal relationship with risk could not be proven [ 63 ]. Only a few mouse studies have examined whether zinc deficiency causes colon cancer [ 64 ]. Thus, more research is needed to determine whether zinc deficiency causes colon cancer.

Zinc Level in Oral Cancer

Notably, the association between zinc concentration and carcinogenesis in oral cancer is different from that of other carcinomas. Two recent case–control studies revealed a link between zinc levels and oral cancer. A 2019 study by Chen et al. [ 46 ] found that both excess and deficient serum levels of copper or zinc are correlated with oral cancer risk. A large 2022 case–control study by Wang et al. [ 65 ] assessed the association between oral cancer and a range of trace elements and reported that higher serum zinc levels correlated with a higher risk of oral cancer. Other studies reported that serum zinc levels [ 66 ] and salivary zinc levels [ 67 ] are higher in cancer patients than in healthy individuals. Whether the difference from other cancer types is because the oral mucosa is the site of direct contact with food and is susceptible to high concentrations of zinc, or whether it is from other causes, requires further investigation.

Zinc Deficiency in Other Cancers

In addition to the specific cancers covered in detail above, zinc deficiency has been reported in a range of other cancers. In 2016, a systematic review and meta-analysis by Zhao et al. [ 48 ] reported that serum zinc concentrations were significantly lower in patients with prostate cancer than in those with benign prostatic hyperplasia and in normal controls. Notably, however, high doses of zinc supplementation were reported to be a risk factor for prostate cancer [ 68 ]. More recently, a 2021 study from the UK by Murphy et al. [ 49 ] looked at micronutrient levels in patients with pancreatico-biliary cancer and identified low levels of serum zinc in 83% of the study cohort. A 2022 cohort study by Ganguly et al. [ 50 ] conducted in a cancer center in India in children aged ≤ 18 years found that zinc deficiency correlated with poor outcomes in patients with solid tumors. A 2023 prospective cohort study by Li et al. found that a high serum copper/zinc ratio was associated with poor outcomes and long-term survival in patients with newly diagnosed acute myeloid leukemia [ 51 ]. While these studies have not shown zinc deficiency to be a direct cause of cancer, they do provide clear preliminary evidence of an association between zinc deficiency and various types of cancer.

Potential Clinical Utility of Measuring Zinc Levels in Cancer

Given the substantial evidence for zinc deficiency as a universal feature of cancer that is common in a relatively large number of cancers, it seems likely that measuring zinc levels could become useful in the clinical setting in the near future. While this is still an emerging area of research, early indications suggest that zinc level measurement could potentially be used to diagnose cancer, predict cancer patient outcomes, and even treat patients with cancer. In this section, we discuss the preliminary evidence for these speculative applications.

Zinc as a Potential Diagnostic Biomarker for Cancer

Multiple studies have suggested that circulating zinc levels could be used as a diagnostic biomarker for cancer in patients suspected of having or being at risk of developing a malignancy. For example, a 2022 review by Venturelli et al. [ 69 ] explored the associations between cancer and minerals and trace elements. Among other factors, the authors noted a direct link between zinc levels and the development of different cancer types, suggesting that this micronutrient has potential for use as a biomarker for cancer, while highlighting the interdependence of micronutrient levels. Additionally, a 2017 review by Lo et al. [ 70 ] highlighted the low zinc levels found in prostate cancer tissues compared with healthy prostate tissue and recommended the use of zinc-responsive magnetic resonance imaging as a noninvasive means of diagnosing prostate cancer. Similarly, a prospective study by Maddalone et al. published in 2022 analyzed zinc levels in urine samples collected after prostatic massage from men referred for prostate biopsy; the results found that zinc levels were lower in the urine of cancer patients than in healthy subjects and decreased with disease progression [ 71 ]. The authors therefore suggested that urine zinc levels could be combined with other diagnostic modalities to identify patients with prostate cancer. The aforementioned 2020 study by Ozeki et al. [ 36 ] found that low serum zinc levels correlated with an increased risk of developing hepatocellular carcinoma following HCV eradication with direct-acting antivirals. The authors suggested that monitoring serum zinc levels in this specific patient population could enable early diagnosis of this malignancy. Thus, while the study of zinc as a potential biomarker for diagnosing a range of cancer types is still in its infancy, it is an idea that seems to be not only plausible but also highly applicable to specific patient populations. However, it is extremely important to note that serum zinc levels fluctuate under a variety of conditions. There are diurnal variations in zinc concentrations [ 72 ], and zinc levels also vary on the basis of sepsis [ 73 ], endotoxemia from alcohol consumption [ 74 ], steroid use [ 75 ], and mealtimes [ 76 ]. Michos et al. [ 77 ] also reported that zinc levels fluctuate on the basis of the menstrual cycle. These conditions need to be considered before zinc level is considered a biomarker for cancer.

Zinc as a Potential Prognostic Biomarker for Cancer

Zinc levels have also been proposed as a prognostic biomarker for predicting outcomes in patients with cancer. For example, a recent study by Iseki et al. [ 78 ] indicated an association between serum zinc status and postsurgical outcomes in patients with pancreatic ductal adenocarcinoma. Specifically, infectious complications were significantly associated with zinc deficiency in patients undergoing resection for this malignancy, suggesting that zinc deficiency could serve as a preoperative predictor of infectious complications after pancreatectomy in these patients. Gal et al. [ 79 ] also reported that, in patients undergoing exploratory laparotomy for suspected ovarian cancer, copper and zinc ratios, in addition to CA125, were sensitive in predicting ovarian cancer before laparotomy. Similarly, Harimoto et al. [ 80 ] reported in 2022 that serum zinc status may correlate with a range of outcomes in patients undergoing hepatic resection for hepatocellular carcinoma. Thus, zinc levels in these patients could be screened to predict which patients are at risk of suffering worse liver function, more severe liver fibrosis, a higher incidence of postoperative complications, and worse overall survival. Furthermore, a 2020 study by Tamai et al. [ 32 ] showed that a higher copper-to-zinc ratio in serum was associated with significantly improved survival rates in patients with hepatocellular carcinoma, indicating that the copper-to-zinc ratio in serum may be used to predict patient survival. A 2020 study by Hiraoka et al. [ 57 ] reported that in patients with early hepatocellular carcinoma from hepatitis virus, serum zinc levels were reduced in association with chronic liver disease grade progression, suggesting that zinc deficiency might be a significant prognostic factor for survival in this patient population. Thus, established links between zinc deficiency and patient outcomes could support the use of zinc level measurement as a useful tool for predicting cancer patient prognoses in the future. Notably, there are still many points to consider, such as the timing of measurement, zinc fluctuations because of background factors, and changes in zinc concentration due to complications.

Zinc as a Potential Preventive Method for Cancer

On the basis of the correlation between low zinc levels and poor cancer outcomes, many authors have speculated that zinc supplementation could be used to reduce the risk of developing cancer. A retrospective analysis published by Hosui et al. [ 38 ] in 2018 reported that zinc supplementation appears to maintain liver function and decrease the risk of developing hepatocellular carcinoma. A more recent study by the same group found that oral zinc supplementation decreased the risk of hepatocellular carcinoma development in patients who received direct-acting antivirals to eradicate HCV [ 18 ]. Valenzano et al. [ 81 ] also reported that in Barrett’s esophagus, administration of zinc gluconate resulted in the upregulation of several tumor-suppressive miRNAs and downregulation of inflammation-inducing proteins. Additionally, a 2022 mini-review by Iqbal et al. [ 82 ] noted limited evidence for a correlation between high dietary intake of zinc and a reduced risk of breast cancer, suggesting that dietary supplementation could decrease the chance of developing this malignancy. These preliminary studies suggest that there could be potential for using zinc supplementation in the clinical setting to help prevent cancer development.

Zinc supplementation has also been suggested by several studies as a potential adjunctive treatment for cancer. A series of papers by Costello et al. [ 56 ] directly addresses this possibility: a 2014 review by this group suggests that zinc supplementation could be a useful treatment for hepatocellular carcinoma, while also pointing out the limitations of existing in vitro models that have made it challenging to generate pre-clinical evidence for the utility of this therapeutic approach; a 2017 review presents compelling evidence that zinc treatment could help prevent the development and progression of prostate, liver, and pancreatic carcinomas [ 83 ]; and a 2020 review again points out the striking correlation between zinc deficiency and multiple cancer types and cites a single case report that demonstrated suppression of androgen-dependent prostate cancer progression in a patient treated with the zinc ionophore clioquinol [ 84 ]. Additionally, in a review published in 2020, Wang et al. [ 47 ] highlighted the prevalence of zinc dyshomeostasis in prostate cancer, breast cancer, and pancreatic cancer and provided an overview of the current evidence suggesting that zinc or zinc transporters could therefore be useful agents for cancer therapy. Furthermore, a 2021 systematic review by Hoppe et al. [ 85 ] that analyzed data from 19 publications found that zinc therapy helped reduce oral toxicities incurred during irradiation in patients mainly diagnosed with head and neck cancer. It should be noted, however, that high doses of zinc supplementation have been reported to be a risk factor for prostate cancer [ 68 ]. While a 2021 review by Singh et al. [ 86 ] mentions several studies that showed no beneficial effect of zinc supplementation on zinc levels in patients with prostate cancer, the authors proposed combined treatment with zinc and naturally occurring dietary phytochemicals that could lead to enhanced zinc bioaccumulation in the prostate. A 2022 review by Nishikawa et al. [ 59 ] describes the recent approval of zinc acetate hydrate for treating patients with hypozincemia associated with chronic liver disease in Japan and presents evidence from several studies showing a positive effect of zinc supplementation in patients with various forms of liver disease putting them at risk of developing liver cancer. Together, these studies have highlighted promising preliminary indications that zinc supplementation could be a useful addition to cancer treatment regimens and identified specific areas for future investigation.

The extensive evidence demonstrating the prevalence of zinc deficiency in a wide range of cancer types suggests that zinc deficiency should be considered a relatively widespread feature of multiple cancers. While research regarding the potential clinical utility of testing zinc levels in patients with or at risk of developing cancer is still preliminary, the data suggest that zinc deficiency may be a potential biomarker for identifying patients at risk of developing cancer, predicting outcomes in patients with cancer, and even as a preventive or adjunctive treatment for cancer.

There are several avenues for future research regarding the link between cancer and zinc deficiency that would be valuable to pursue. At a basic research level, future work should focus on elucidating the mechanisms by which zinc deficiency promotes carcinogenesis. Clinically, it would be beneficial to explore the relationship between serum zinc levels and local zinc concentrations in patients with cancer, as well as the reliability of serum zinc levels as a biomarker of carcinogenesis risk and cancer patient prognosis. Finally, prospective clinical studies should be carried out to determine the prognostic benefits of zinc supplementation. Given the universality of zinc deficiency in cancer, gaining a greater understanding of the molecular basis and clinical impact of this association is likely to yield substantial benefits for human health.

Data Availability

Not applicable.

Lim KHC, Riddell LJ, Nowson CA, Booth AO, Szymlek-Gay EA (2013) Iron and zinc nutrition in the economically-developed world: a review. Nutrients 5(8):3184–3211

Article   CAS   PubMed   PubMed Central   Google Scholar  

Maret W (2016) The metals in the biological periodic system of the elements: concepts and conjectures. Int J Mol Sci 17(1):66

Article   PubMed   PubMed Central   Google Scholar  

Krebs NF (2000) Overview of zinc absorption and excretion in the human gastrointestinal tract. J Nutr 130(5S Suppl):1374s–1377s

Article   CAS   PubMed   Google Scholar  

Lee HH, Prasad AS, Brewer GJ, Owyang C (1989) Zinc absorption in human small intestine. Am J Physiol 256(1 Pt 1):G87-91

CAS   PubMed   Google Scholar  

Trame S, Wessels I, Haase H, Rink L (2018) A short 18 items food frequency questionnaire biochemically validated to estimate zinc status in humans. J Trace Elem Med Biol 49:285–295

Hunt JR, Beiseigel JM, Johnson LK (2008) Adaptation in human zinc absorption as influenced by dietary zinc and bioavailability. Am J Clin Nutr 87(5):1336–1345

Jackson MJ (1989) Physiology of zinc: general aspects. In: Mills CF (ed) Zinc in Human Biology. Springer London, London, pp 1–14

Google Scholar  

Maret W (2017) Zinc in cellular regulation: the nature and significance of “zinc signals.” Int J Mol Sci 18(11):2285

Cassandri M, Smirnov A, Novelli F, Pitolli C, Agostini M, Malewicz M et al (2017) Zinc-finger proteins in health and disease. Cell Death Discov 3:17071

Prasad AS (2008) Clinical, immunological, anti-inflammatory and antioxidant roles of zinc. Exp Gerontol 43(5):370–377

Prasad AS (2008) Zinc in human health: effect of zinc on immune cells. Mol Med 14(5–6):353–357

Altobelli GG, Van Noorden S, Balato A, Cimini V (2020) Copper/zinc superoxide dismutase in human skin: current knowledge. Front Med (Lausanne) 7:183

Article   PubMed   Google Scholar  

Lewandowski Ł, Kepinska M, Milnerowicz H (2019) The copper-zinc superoxide dismutase activity in selected diseases. Eur J Clin Invest 49(1):e13036

Neuhaus D (2022) Zinc finger structure determination by NMR: why zinc fingers can be a handful. Prog Nucl Magn Reson Spectrosc 130–131:62–105

Yildiz A, Kaya Y, Tanriverdi O (2019) Effect of the interaction between selenium and zinc on DNA repair in association with cancer prevention. J Cancer Prev 24(3):146–154

María Clara F, Cassandra ND, Fabian HR, Alvaro GE (2013) Superoxide dismutase and oxidative stress in amyotrophic lateral sclerosis. In: Alvaro GE (ed) Current advances in amyotrophic lateral sclerosis. IntechOpen Limited, London. https://doi.org/10.5772/56488

Pala V, Agnoli C, Cavalleri A, Rinaldi S, Orlandi R, Segrado F et al (2022) Prediagnostic levels of copper and zinc and breast cancer risk in the ORDET cohort. Cancer Epidemiol Biomarkers Prev 31(6):1209–1215

Hosui A, Tanimoto T, Okahara T, Ashida M, Ohnishi K, Wakahara Y et al (2021) Oral zinc supplementation decreases the risk of HCC development in patients with HCV eradicated by DAA. Hepatology Communications 5(12):2001–2008

Lu H, Cai L, Mu LN, Lu QY, Zhao J, Cui Y et al (2006) Dietary mineral and trace element intake and squamous cell carcinoma of the esophagus in a Chinese population. Nutr Cancer 55(1):63–70

Schaafsma T, Wakefield J, Hanisch R, Bray F, Schüz J, Joy EJ et al (2015) Africa’s oesophageal cancer corridor: geographic variations in incidence correlate with certain micronutrient deficiencies. PLoS One 10(10):e0140107

Hashemian M, Poustchi H, Abnet CC, Boffetta P, Dawsey SM, Brennan PJ et al (2015) Dietary intake of minerals and risk of esophageal squamous cell carcinoma: results from the Golestan Cohort Study. Am J Clin Nutr 102(1):102–108

Ray SS, Das D, Ghosh T, Ghosh AK (2012) The levels of zinc and molybdenum in hair and food grain in areas of high and low incidence of esophageal cancer: a comparative study. Glob J Health Sci 4(4):168–175

Adeoti ML, Oguntola AS, Akanni EO, Agodirin OS, Oyeyemi GM (2015) Trace elements; copper, zinc and selenium, in breast cancer afflicted female patients in LAUTECH Osogbo, Nigeria. Indian J Cancer 52(1):106–109

Costello LC, Zou J, Franklin RB (2016) In situ clinical evidence that zinc levels are decreased in breast invasive ductal carcinoma. Cancer Causes Control 27(6):729–735

Wu X, Tang J, Xie M (2015) Serum and hair zinc levels in breast cancer: a meta-analysis. Sci Rep 5(1):12249

Jouybari L, Kiani F, Akbari A, Sanagoo A, Sayehmiri F, Aaseth J et al (2019) A meta-analysis of zinc levels in breast cancer. J Trace Elem Med Biol 56:90–99

Feng Y, Zeng J-W, Ma Q, Zhang S, Tang J, Feng J-F (2020) Serum copper and zinc levels in breast cancer: a meta-analysis. J Trace Elem Med Biol 62:126629

Mansouri B, Ramezani Z, Yousefinejad V, Nakhaee S, Azadi N, Khaledi P et al (2022) Association between trace elements in cancerous and non-cancerous tissues with the risk of breast cancers in western Iran. Environ Sci Pollut Res Int 29(8):11675–11684

Barartabar Z, Moini N, Abbasalipourkabir R, Mesbah-Namin SA, Ziamajidi N (2023) Assessment of tissue oxidative stress, antioxidant parameters, and zinc and copper levels in patients with breast cancer. Biol Trace Elem Res 201(7):3233–3244

Kew MC, Mallett RC (1974) Hepatic zinc concentrations in primary cancer of the liver. Br J Cancer 29(1):80–83

Tashiro-Itoh T, Ichida T, Matsuda Y, Satoh T, Sugiyama M, Tanaka Y et al (1997) Metallothionein expression and concentrations of copper and zinc are associated with tumor differentiation in hepatocellular carcinoma. Liver 17(6):300–306

Tamai Y, Iwasa M, Eguchi A, Shigefuku R, Sugimoto K, Hasegawa H et al (2020) Serum copper, zinc and metallothionein serve as potential biomarkers for hepatocellular carcinoma. PLoS ONE 15(8):e0237370

Imai K, Beppu T, Yamao T, Okabe H, Hayashi H, Nitta H et al (2014) Clinicopathological and prognostic significance of preoperative serum zinc status in patients with hepatocellular carcinoma after initial hepatectomy. Ann Surg Oncol 21(12):3817–3826

Fang A-P, Chen P-Y, Wang X-Y, Liu Z-Y, Zhang D-M, Luo Y et al (2019) Serum copper and zinc levels at diagnosis and hepatocellular carcinoma survival in the Guangdong Liver Cancer Cohort. Int J Cancer 144(11):2823–2832

Shigefuku R, Iwasa M, Katayama K, Eguchi A, Kawaguchi T, Shiraishi K et al (2019) Hypozincemia is associated with human hepatocarcinogenesis in hepatitis C virus-related liver cirrhosis. Hepatol Res 49(10):1127–1135

Ozeki I, Nakajima T, Suii H, Tatsumi R, Yamaguchi M, Arakawa T et al (2020) Predictors of hepatocellular carcinoma after hepatitis C virus eradication following direct-acting antiviral treatment: relationship with serum zinc. J Clin Biochem Nutr 66(3):245–252

Ozeki I, Arakawa T, Suii H, Tatsumi R, Yamaguchi M, Nakajima T et al (2020) Zinc deficiency in patients with chronic liver disease in Japan. Hepatol Res 50(3):396–401

Hosui A, Kimura E, Abe S, Tanimoto T, Onishi K, Kusumoto Y et al (2018) Long-term zinc supplementation improves liver function and decreases the risk of developing hepatocellular carcinoma. Nutrients 10(12):1955

Wang Y, Sun Z, Li A, Zhang Y (2019) Association between serum zinc levels and lung cancer: a meta-analysis of observational studies. World J Surg Oncol 17(1):78

Zhang L, Shao J, Tan S-W, Ye H-P, Shan X-Y (2022) Association between serum copper/zinc ratio and lung cancer: a systematic review with meta-analysis. J Trace Elem Med Biol 74:127061

Bai Y, Wang G, Fu W, Lu Y, Wei W, Chen W et al (2019) Circulating essential metals and lung cancer: risk assessment and potential molecular effects. Environ Int 127:685–693

Xie Y, Wang J, Zhao X, Zhou X, Nie X, Li C et al (2018) Higher serum zinc levels may reduce the risk of cervical cancer in Asian women: a meta-analysis. J Int Med Res 46(12):4898–4906

Lin S, Yang H (2021) Ovarian cancer risk according to circulating zinc and copper concentrations: a meta-analysis and Mendelian randomization study. Clin Nutr 40(4):2464–2468

Stepien M, Jenab M, Freisling H, Becker N-P, Czuban M, Tjønneland A et al (2017) Pre-diagnostic copper and zinc biomarkers and colorectal cancer risk in the European Prospective Investigation into Cancer and Nutrition cohort. Carcinogenesis 38(7):699–707

Wu X, Wu H, Liu L, Qiang G, Zhu J (2020) Serum zinc level and tissue ZIP4 expression are related to the prognosis of patients with stages I-III colon cancer. Transl Cancer Res 9(9):5585–5594

Chen F, Wang J, Chen J, Yan L, Hu Z, Wu J et al (2019) Serum copper and zinc levels and the risk of oral cancer: a new insight based on large-scale case–control study. Oral Dis 25(1):80–86

Wang J, Zhao H, Xu Z, Cheng X (2020) Zinc dysregulation in cancers and its potential as a therapeutic target. Cancer Biol Med 17(3):612–625

Zhao J, Wu Q, Hu X, Dong X, Wang L, Liu Q et al (2016) Comparative study of serum zinc concentrations in benign and malignant prostate disease: a systematic review and meta-analysis. Sci Rep 6(1):25778

Murphy DP, Kanwar MA, Stell MD, Briggs MC, Bowles MM, Aroori MS (2021) The prevalence of micronutrient deficiency in patients with suspected pancreatico-biliary malignancy: results from a specialist hepato-biliary and pancreatic unit. Eur J Surg Oncol 47(7):1750–1755

Ganguly S, Srivastava R, Agarwala S, Dwivedi S, Bansal PG, Gonmei Z et al (2022) Prevalence of micronutrient deficiency and its impact on the outcome of childhood cancer: a prospective cohort study. Clin Nutr 41(7):1501–1511

Li T, Shi L, Wei W, Xu J, Liu Q (2023) The trace that is valuable: serum copper and copper to zinc ratio for survival prediction in younger patients with newly diagnosed acute myeloid leukaemia. BMC Cancer 23(1):14

Fong LY, Jing R, Smalley KJ, Taccioli C, Fahrmann J, Barupal DK et al (2017) Integration of metabolomics, transcriptomics, and microRNA expression profiling reveals a miR-143-HK2-glucose network underlying zinc-deficiency-associated esophageal neoplasia. Oncotarget 8(47):81910–81925

Taccioli C, Chen H, Jiang Y, Liu XP, Huang K, Smalley KJ et al (2012) Dietary zinc deficiency fuels esophageal cancer development by inducing a distinct inflammatory signature. Oncogene 31(42):4550–4558

Taccioli C, Garofalo M, Chen H, Jiang Y, Tagliazucchi GM, Di Leva G et al (2015) Repression of esophageal neoplasia and inflammatory signaling by anti-miR-31 delivery in vivo. J Natl Cancer Inst 107(11):djv220

Li P, Xu J, Shi Y, Ye Y, Chen K, Yang J et al (2014) Association between zinc intake and risk of digestive tract cancers: a systematic review and meta-analysis. Clin Nutr 33(3):415–420

Costello LC, Franklin RB (2014) The status of zinc in the development of hepatocellular cancer. Cancer Biol Ther 15(4):353–360

Hiraoka A, Nagamatsu K, Izumoto H, Adachi T, Yoshino T, Tsuruta M et al (2020) Zinc deficiency as an independent prognostic factor for patients with early hepatocellular carcinoma due to hepatitis virus. Hepatol Res 50(1):92–100

Franklin RB, Levy BA, Zou J, Hanna N, Desouki MM, Bagasra O et al (2012) ZIP14 zinc transporter downregulation and zinc depletion in the development and progression of hepatocellular cancer. J Gastrointest Cancer 43(2):249–257

Nishikawa H, Asai A, Fukunishi S (2022) The significance of zinc in patients with chronic liver disease. Nutrients 14(22):4855

Yaman M, Kaya G, Simsek M (2007) Comparison of trace element concentrations in cancerous and noncancerous human endometrial and ovary tissues. Int J Gynecol Cancer 17(1):220–228

Rzymski P, Niedzielski P, Rzymski P, Tomczyk K, Kozak L, Poniedziałek B (2016) Metal accumulation in the human uterus varies by pathology and smoking status. Fertil Steril 105(6):1511-1518.e3

Atakul T, Altinkaya SO, Abas BI, Yenisey C (2020) Serum copper and zinc levels in patients with endometrial cancer. Biol Trace Elem Res 195(1):46–54

Tsilidis KK, Papadimitriou N, Dimou N, Gill D, Lewis SJ, Martin RM et al (2021) Genetically predicted circulating concentrations of micronutrients and risk of colorectal cancer among individuals of European descent: a Mendelian randomization study. Am J Clin Nutr 113(6):1490–1502

Yin X, Zhang Y, Wen Y, Yang Y, Chen H (2021) Celecoxib alleviates zinc deficiency-promoted colon tumorigenesis through suppressing inflammation. Aging (Albany NY) 13(6):8320–8334

Wang H, Wang J, Cao Y, Chen J, Deng Q, Chen Y et al (2022) Combined exposure to 33 trace elements and associations with the risk of oral cancer: a large-scale case-control study. Front Nutr 9:913357

Baharvand M, Manifar S, Akkafan R, Mortazavi H, Sabour S (2014) Serum levels of ferritin, copper, and zinc in patients with oral cancer. Biomed J 37(5):331–336

Ayinampudi BK, Narsimhan M (2012) Salivary copper and zinc levels in oral pre-malignant and malignant lesions. J Oral Maxillofac Pathol 16(2):178–182

Leitzmann MF, Stampfer MJ, Wu K, Colditz GA, Willett WC, Giovannucci EL (2013) Zinc supplement use and risk of prostate cancer. J Natl Cancer Inst 95(13):1004–1007

Article   Google Scholar  

Venturelli S, Leischner C, Helling T, Renner O, Burkard M, Marongiu L (2022) Minerals and cancer: overview of the possible diagnostic value. Cancers (Basel) 14(5):1256

Lo S-T, Martins AF, Jordan VC, Sherry AD (2017) Zinc as an imaging biomarker of prostate cancer. Isr J Chem 57(9):854–861

Maddalone MG, Oderda M, Mengozzi G, Gesmundo I, Novelli F, Giovarelli M et al (2022) Urinary zinc loss identifies prostate cancer patients. Cancers (Basel) 14(21):5316

McMillan EM, Rowe DJ, Halberg F (1987) Diurnal stage of circadian rhythm of plasma zinc in healthy and psoriatic volunteers. Prog Clin Biol Res 227b:295–303

Alker W, Haase H (2018) Zinc and Sepsis. Nutrients 10(8):976

Skalny AV, Skalnaya MG, Grabeklis AR, Skalnaya AA, Tinkov AA (2018) Zinc deficiency as a mediator of toxic effects of alcohol abuse. Eur J Nutr 57(7):2313–2322

Yunice AA, Czerwinski AW, Lindeman RD (1981) Influence of synthetic corticosteroids on plasma zinc and copper levels in humans. Am J Med Sci 282(2):68–74

Goode HF, Robertson DA, Kelleher J, Walker BE (1991) Effect of fasting, self-selected and isocaloric glucose and fat meals and intravenous feeding on plasma zinc concentrations. Ann Clin Biochem 28(Pt 5):442–445

Michos C, Kalfakakou V, Karkabounas S, Kiortsis D, Evangelou A (2010) Changes in copper and zinc plasma concentrations during the normal menstrual cycle in women. Gynecol Endocrinol 26(4):250–255

Iseki M, Mizuma M, Aoki S, Kawaguchi K, Masuda K, Ishida M et al (2022) What is the impact of zinc deficiency for pancreatectomies in patients with pancreatic ductal adenocarcinoma? Pancreatology 22(2):270–276

Gal D, Lischinsky S, Friedman M, Zinder O (1989) Prediction of the presence of ovarian cancer at surgery by an immunochemical panel: CA 125 and copper-to-zinc ratio. Gynecol Oncol 35(2):246–250

Harimoto N, Araki K, Muranushi R, Hoshino K, Yamanaka T, Hagiwara K et al (2022) Significance of zinc deficiency in patients with hepatocellular carcinoma undergoing hepatic resection. Hepatol Res 52(2):210–220

Valenzano MC, Rybakovsky E, Chen V, Leroy K, Lander J, Richardson E et al (2021) Zinc gluconate induces potentially cancer chemopreventive activity in Barrett’s esophagus: a Phase 1 pilot study. Dig Dis Sci 66(4):1195–1211

Iqbal S, Ali I (2022) Dietary trace element intake and risk of breast cancer: a mini review. Biol Trace Elem Res 200(12):4936–4948

Costello LC, Franklin RB (2017) Decreased zinc in the development and progression of malignancy: an important common relationship and potential for prevention and treatment of carcinomas. Expert Opin Ther Targets 21(1):51–66

Costello LC, Franklin RB (2020) Zinc: The wonder drug for the treatment of carcinomas. Acta Sci Cancer Biol 4(5):33–39

Hoppe C, Kutschan S, Dörfler J, Büntzel J, Büntzel J, Huebner J (2021) Zinc as a complementary treatment for cancer patients: a systematic review. Clin Exp Med 21(2):297–313

Singh CK, Chhabra G, Patel A, Chang H, Ahmad N (2021) Dietary phytochemicals in zinc homeostasis: a strategy for prostate cancer management. Nutrients 13(6):1867

Download references

Acknowledgements

We thank Edanz ( https://jp.edanz.com/ac ) for editing a draft of this manuscript.

We used official funds of Kyushu Cancer Center to fund the proofreading and submission of the paper.

Author information

Authors and affiliations.

Department of Hepato-Biliary-Pancreatology, National Hospital Organization Kyushu Cancer Center, 3-1-1 Notame, Minami-Ku, Fukuoka, 811-1395, Japan

Rie Sugimoto, Lingaku Lee, Yuki Tanaka, Yusuke Morita, Masayuki Hijioka, Terumasa Hisano & Masayuki Furukawa

You can also search for this author in PubMed   Google Scholar

Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Rie Sugimoto and Lingaku Lee. The first draft of the manuscript was written by Rie Sugimoto, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Rie Sugimoto .

Ethics declarations

Ethics approval, consent to participate, consent to publish, competing interests.

The authors declare no competing interests.

Additional information

Publisher's note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ .

Reprints and permissions

About this article

Sugimoto, R., Lee, L., Tanaka, Y. et al. Zinc Deficiency as a General Feature of Cancer: a Review of the Literature. Biol Trace Elem Res 202 , 1937–1947 (2024). https://doi.org/10.1007/s12011-023-03818-6

Download citation

Received : 18 May 2023

Accepted : 16 August 2023

Published : 02 September 2023

Issue Date : May 2024

DOI : https://doi.org/10.1007/s12011-023-03818-6

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Zinc deficiency
• Trace elements
• Cancer diagnosis
• Cancer prognosis
• Find a journal
• Publish with us
• Track your research

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings
• My Bibliography
• Collections
• Citation manager

Save citation to file

Email citation, add to collections.

• Create a new collection
• Add to an existing collection

Add to My Bibliography

Your saved search, create a file for external citation management software, your rss feed.

• Search in PubMed
• Search in NLM Catalog
• Add to Search

The clinical effects of zinc as a topical or oral agent on the clinical response and pathophysiologic mechanisms of acne: a systematic review of the literature

Affiliation.

• 1 Galderma Laboratories, LP, Fort Worth, TX, USA. [email protected]
• PMID: 23652948

This article reviews the published literature about the efficacy of oral and topical zinc as treatments for acne vulgaris. The medical literature was systematically reviewed to identify relevant articles. Each published study was assessed for pathophysiologic results and the quality of the clinical evidence the study provided based on Strength of Recommendation Taxonomy (SORT) criteria. Finally, the body of evidence for using oral or topical zinc in the treatment of acne was assessed, again using SORT criteria. A SORT strength of recommendation of B (inconsistent or limited-quality patient-oriented evidence) appears to be appropriate for both oral and topical zinc. The preponderance of evidence suggests zinc has antibacterial and anti-inflammatory effects and that it may decrease sebum production.

PubMed Disclaimer

Similar articles

• The role of nicotinamide in acne treatment. Walocko FM, Eber AE, Keri JE, Al-Harbi MA, Nouri K. Walocko FM, et al. Dermatol Ther. 2017 Sep;30(5). doi: 10.1111/dth.12481. Epub 2017 Feb 21. Dermatol Ther. 2017. PMID: 28220628 Review.
• Treatment of Acne in Pregnancy. Chien AL, Qi J, Rainer B, Sachs DL, Helfrich YR. Chien AL, et al. J Am Board Fam Med. 2016 Mar-Apr;29(2):254-62. doi: 10.3122/jabfm.2016.02.150165. J Am Board Fam Med. 2016. PMID: 26957383 Review.
• Antibiotic resistance: shifting the paradigm in topical acne treatment. Andriessen A, Lynde CW. Andriessen A, et al. J Drugs Dermatol. 2014 Nov;13(11):1358-64. J Drugs Dermatol. 2014. PMID: 25607703 Review.
• Newer approaches in topical combination therapy for acne. Fu LW, Vender RB. Fu LW, et al. Skin Therapy Lett. 2011 Oct;16(9):3-6. Skin Therapy Lett. 2011. PMID: 22089505
• A review of the anti-inflammatory properties of clindamycin in the treatment of acne vulgaris. Del Rosso JQ, Schmidt NF. Del Rosso JQ, et al. Cutis. 2010 Jan;85(1):15-24. Cutis. 2010. PMID: 20184207 Review.
• The role of skin care as an integral component in the management of acne vulgaris: part 1: the importance of cleanser and moisturizer ingredients, design, and product selection. Del Rosso JQ. Del Rosso JQ. J Clin Aesthet Dermatol. 2013 Dec;6(12):19-27. J Clin Aesthet Dermatol. 2013. PMID: 24765221 Free PMC article. Review.

Publication types

• Search in MeSH

Related information

• PubChem Compound (MeSH Keyword)

LinkOut - more resources

Full text sources.

• SanovaWorks

Other Literature Sources

• The Lens - Patent Citations
• MedlinePlus Health Information

• Citation Manager

NCBI Literature Resources

MeSH PMC Bookshelf Disclaimer

The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Unauthorized use of these marks is strictly prohibited.

• Open access
• Published: 26 June 2024

iPSCs chondrogenic differentiation for personalized regenerative medicine: a literature review

• Eltahir Abdelrazig Mohamed Ali 1 , 2   na1 ,
• Rana Smaida 3   na1 ,
• Morgane Meyer 2 , 3   na1 ,
• Wenxin Ou 2 , 6 , 7   na1 ,
• Zongjin Li 4 ,
• Zhongchao Han 5 ,
• Nadia Benkirane-Jessel 1 , 2 , 3 ,
• Jacques Eric Gottenberg 2 , 6 &
• Guoqiang Hua   ORCID: orcid.org/0000-0001-7639-5908 1 , 2  

Stem Cell Research & Therapy volume  15 , Article number:  185 ( 2024 ) Cite this article

190 Accesses

Metrics details

Cartilage, an important connective tissue, provides structural support to other body tissues, and serves as a cushion against impacts throughout the body. Found at the end of the bones, cartilage decreases friction and averts bone-on-bone contact during joint movement. Therefore, defects of cartilage can result from natural wear and tear, or from traumatic events, such as injuries or sudden changes in direction during sports activities. Overtime, these cartilage defects which do not always produce immediate symptoms, could lead to severe clinical pathologies. The emergence of induced pluripotent stem cells (iPSCs) has revolutionized the field of regenerative medicine, providing a promising platform for generating various cell types for therapeutic applications. Thus, chondrocytes differentiated from iPSCs become a promising avenue for non-invasive clinical interventions for cartilage injuries and diseases. In this review, we aim to highlight the current strategies used for in vitro chondrogenic differentiation of iPSCs and to explore their multifaceted applications in disease modeling, drug screening, and personalized regenerative medicine. Achieving abundant functional iPSC-derived chondrocytes requires optimization of culture conditions, incorporating specific growth factors, and precise temporal control. Continual improvements in differentiation methods and integration of emerging genome editing, organoids, and 3D bioprinting technologies will enhance the translational applications of iPSC-derived chondrocytes. Finally, to unlock the benefits for patients suffering from cartilage diseases through iPSCs-derived technologies in chondrogenesis, automatic cell therapy manufacturing systems will not only reduce human intervention and ensure sterile processes within isolator-like platforms to minimize contamination risks, but also provide customized production processes with enhanced scalability and efficiency.

Graphical abstract

Cartilage is a semi-rigid, load-bearing, avascular connective tissue, formed solely by cells known as chondrocytes. These cells are loosely embedded in an extracellular matrix (ECM) composed predominantly of collagens and, in some cases, elastic fibers, hyaluronan and proteoglycans [ 1 ]. Cartilage formation, also known as chondrogenesis, is a dynamic cellular process of a condensed mesenchyme tissue derived from the mesoderm germ layer during embryogenesis. Cartilage represents the fetal precursor tissue for skeletal development. In adults, it persists at almost all joints between bones and in structures that must be deformable as well as strong such as in the respiratory system. Based on the structure and composition of their ECMs, chondrocytes form three different types of cartilage; namely, hyaline cartilage, fibrocartilage and elastic cartilage [ 2 ].

Cartilage exhibits diverse clinical aspects and relevance to various medical disciplines, including orthopedics, rheumatology, and respiratory medicine. Cartilage defects are associated with various clinical conditions such as osteoarthritis (OA), rheumatoid arthritis, and cartilage dysplasias [ 1 ]. Understanding the clinical significance of cartilage is critical for the development of effective therapeutics and interventions in various healthcare settings. Orthopedic surgeries such as joint arthroplasty and cartilage transplantation are the most commonly used therapeutic interventions for cartilage repair or replacement [ 3 ]. However, these surgical interventions are invasive or minimally invasive, and their ability to restore normal joint function, alleviate pain, and improve the quality of life for individuals with cartilage-related issues is limited.

Therefore, it is crucial to develop other non-invasive therapeutic approaches with high safety and efficacy. Theoretically and due to their ability to repair injured tissues, adult stem cells can be a good source for developing therapies for a large number of diseases [ 4 ]. Mesenchymal stem cells (MSCs) which can be derived from various tissues such as bone marrow, adipose tissu, placenta, umbilical cord blood, and multiple dental tissues, are multipotent cells that have the potential to differentiate into the mesenchymal lineages including osteocytes, chondrocytes, and adipocytes, as well as other non-mesenchymal lineages, such as cardiomyocytes, astrocytes, neural cells, and endothelial cells [ 5 , 6 ]. Therefore, extensive efforts have been spent to develop MSCs-based cell therapies for a broad spectrum of diseases, encompassing cartilage and bone diseases, hematological diseases, inflammatory diseases, and graft-versus-host disease [ 7 ]. It is important to note that different transcription factors regulate the differentiation of MSCs to different lineages. Chondrogenic differentiation is determined by members the SOX (sex determining region Y (SRY)-related HMG-box) family of transcription factors SOX9, SOX5, and SOX6 while regulation of osteoblast differentiation involve the transcription factors runt-related transcription factor 2 (RUNX2), osterix, and β-catenin [ 8 , 9 ]. Among the different sources of MSCs, bone marrow-derived MSCs (BM-MSCs) are the most commonly used MSCs in regenerative medicine, particularly for cartilage and bone regeneration [ 10 ]. Although significant strides have been taken to improve the chondrogenic differentiation from BM-MSCs and other cell sources, several obstacles persist complicating the achievement of consistent and effective chondrocytes required for clinical application [ 11 ]. Several factors may lead to the failure of utilizing BM-MSCs for efficient treatment of cartilage diseases including but not limited to the restricted proliferation capabilities in cultures [ 12 ], donor variations, and immunogenicity triggered during culture and cryopreservation [ 13 ].

These challenges could be addressed by the induced pluripotent stem cell (iPSC) technology. iPSCs are pluripoent cells which have the capacity for self-renewal and differentiation into almost all cell types [ 14 ]. The concept of self-renewal is the ability of the cells to undergo infinite cell divisions without differentiation into other cell types, while pluripotency is the ability of the cells to produce specialized cells of the three embryonic layers: ectoderm, mesoderm, and endoderm [ 15 ]. iPSCs can be generated from any type of cells through non-integrating reprogramming method using specific transcription factors known as Yamanaka factors namely, Octamer binding transcription factor 3/4 (OCT3/4), SOX2, Krüppel-like factor 4 (KLF4), and Cellular-Myelocytomatosis c-MYC [ 15 ]. Simplicity and reproducibility are the attractive features of the iPSC technology and have attracted the biomedical scientists to generate and differentiate iPSCs from numerous normal and disease-specific cell types for disease modeling and drug screening applications [ 16 ]. Syngeneic non-integrated iPSCs and their derivatives have no or minimal immunogenic effect supporting the notion that these cells could be used for cellular therapy without causing harmful immune responses [ 17 ]. Therefore, generation of iPSC-derived chondrocytes has become indispensable to advance our understanding of the mechanisms of cartilage-related disorders and represents an important avenue in regenerative medicine. In the following section, we will summarize different strategies developed to differentiate iPSCs into chondrocytes aiming to recapitulate the in vivo microenvironment that support chondrogenesis, and to generate functional and stable iPSC-derived chondrocytes.

Generation of iPSC-derived chondrocytes

Chondrocytes can be differentiated from iPSCs though different intermediate stages, such as iPSC-derived MSCs (iPSC-MSCs), embryoid bodies (EBs) formation, induction of neural crest cells (NCCs), and primitive streak-mesendoderm and mesodermal lineage. iPSC-MSCs are morphologically highly similar to BM-MSCs and their gene expression profiling is also comparable to that of BM-MSCs [ 18 ], and exhibit traits that encompass features of both iPSCs and MSCs. iPSC-MSCs show reduced immunogenicity as compared to iPSCs [ 19 ], which renders them appropriate for allogeneic transplantation and enables development of off-the-shelf therapies. Moreover, patient-specific iPSC-MSCs open up the potential for developing personalized medicine for autologous transplantation, in vitro disease modeling, and drug screening [ 20 ]. These iPSC-MSCs were reported to differentiate into chondrocytes with growth factors, such as transforming growth factor-beta 3 (TGF-β3) (Fig.  1 A). Another commonly used approach to obtain chondrocytes from iPSCs in vitro is through formation of three-dimensional (3D) aggregates of pluripotent stem cells (PSCs) known as embryoid bodies (EBs) (Fig.  1 B). The EB has the capacity to generate ectodermal, mesodermal and endodermal cells due to its initiation of a process that resembles gastrulation-like events in embryonic development [ 21 ]. Several protocols have been developed under this category with slight variations in the number and concentration of growth factors used, the number of days required and whether an additional step such as differentiation of EBs to MSCs or paraxial mesoderm cells, is needed to differentiate iPSCs to chondrocytes [ 22 ]. NCCs are a multipotent group of transient embryonic cells in the vertebrate. They are derived from the ectoderm and differentiate to the peripheral nervous system cells and several non-neural cell types including pigment cells, and the cranio-facial cartilage and bones [ 23 ]. Taking the advantage of being multipotent, chondrogenic cells could be differentiated from the NCC-derived MSCs [ 24 ] (Fig.  1 C). Chondrocytes were also reported to be differentiated from human embryonic stem cells (hESCs) through primitive streak or mesendoderm to mesoderm [ 25 ]. Cheng et al. followed this method to differentiate iPSCs to chondrocyte in three short stages using different combination of growth factors in each stage [ 26 ] (Fig.  1 D). iPSCs can also be differentiated to chondrocytes by co-culture with primary chondrocytes (Fig.  1 D). This method is based on the fact that the primary chondrocytes secret paracrine factors which may induce chondrogenic differentiation of the stem cells by closely mimicking the in vivo tissue microenvironment for chondrogenesis [ 27 ]. Moreover, co-culture permits crosstalk between the stem cells and the primary chondrocytes influencing chondrocyte development. It facilitates physical contact between different cell types which stabilizes the cellular phenotype and allows for communication of molecular signals involved in chondrogenic differentiation [ 28 ].

Schematic representation of the current strategies for in vitro differentiation of iPSCs to chondrocytes. A Via iPSC-derived MSCs. B Via EBs formation. C Via induction of NCCs. D Via primitive streak-mesendoderm and mesodermal lineage. E Via co-culture with primary chondrocytes. BMP4: bone morphogenetic protein 4; BMP7: bone morphogenetic protein 7; CHIR99021: glycogen synthase kinase 3 (GSK-3) inhibitor; DM: dorsomorphin; EB: embryoid body; EGF: epidermal growth factor; FGF2: fibroblast growth factor 2; GDF5: growth/differentiation factor-5; hESC: human embryonic stem cell; iPSC: induced pluripotent stem cell; MSC: mesenchymal stem cell; NCC: neural crest cell; NT4: neurotrophin-4; PDGF: platelet-derived growth factor; PSC: pluripotent stem cell; SB431542: transforming growth factor-beta receptor inhibitor; TGF-β3: transforming growth factor-beta 3; Wnt3a: Wingless/Int1 family member 3A

The above-mentioned studies showed that cartilage cells differentiated from human iPSCs represent a promising tool for regenerative medicine to treat cartilage-related diseases, however some challenges remain. The variability in the quality and characteristics of different iPSC lines affects the efficiency and consistency of chondrogenic differentiation [ 29 ]. Since the suspension culture promotes the chondrogenic differentiation and enables removal of non-chondrocytic cells, Yamashita and colleagues reported that homogenous chondrogenic nodules derived from iPSCs cultivated in suspension culture has the potential to form scaffold-free hyaline cartilage in animal models [ 30 ]. How to generate homogenous cartilage cells without formation of hypertrophic chondrocytes which have the potential to trigger the process of initiating endochondral ossification in vivo remains the main challenge. Moreover, iPSCs have the potential to form teratomas, therefore it is crucial to ensure complete elimination of undifferentiated iPSCs from chondrogenic cultures to prevent teratoma formation upon transplantation [ 31 ]. Obtaining fully mature chondrocytes from iPSCs with a phenotype comparable to native chondrocytes, is challenging [ 32 ]. In addition, undesired development of chondrogenic hypertrophy and fibrocartilage in vitro may require modification of the growth factors cocktail used [ 33 ]. Due to bovine xenoproteins, use of fetal bovine serum (FBS) in cell culture may induce adverse response in transplant patient upon injection of MSCs [ 34 ]. Additionally, there is a risk of infection because of viral and prion contamination [ 35 ]. Interestingly, MSC induction in xeno-free conditions may tackle these problems and promote the safety and efficiency of iPSC-MSCs for clinical applications [ 36 ].

Genome-edited iPSC-derived chondrocytes

In the last decade, the clustered regularly interspaced short palindromic repeats (CRISPR-Cas9) approach has become an efficient and indispensable tool in biomedical research, and has been extensively explored in bone and cartilage research [ 37 , 38 ]. It has been used to edit genes associated with chondrogenic differentiation to enhance their expression [ 39 ] or to modify signaling pathways involved in chondrogenesis [ 40 ]. For example, chondrogenesis can be regulated by the expression of SOX9 and Stat3 [ 39 ]. Chondrogenic differentiation of MSCs can be promoted by knocking down the RUNX2 , a key transcription factor associated with osteoblast differentiation [ 41 ]. Genomic editing in iPSC-derived chondrocytes has been also reported in disease modeling. Efficient editing of cartilage related genes enables to investigate in depth the mechanisms underlying cartilage disorders and to identify potential therapeutic agents [ 42 ]. An interesting genome editing study showed simultaneous SOX9 activation and peroxisome proliferator-activated receptor gamma (PPAR-γ) repression in rat BM-MSCs, which promoted chondrocytes differentiation and regeneration of calvarial bone [ 43 ]. Various studies have investigated diverse targets for regeneration, paving the way for potential clinical trials in the near future. Genome editing has been employed to boost the regenerative potential of chondrocytes. This may involve editing genes related to ECM production, cell proliferation, or resistance to hypertrophy [ 41 , 44 , 45 ]. Although numerous studies have been reported on the application of genome-edited chondrocytes for in vivo cartilage repair, drug screening, and disease modeling [ 39 , 41 , 43 ], relatively few studies have been conducted specifically on iPSC-derived chondrocytes [ 40 , 46 , 47 ]. It was revealed that mutations in TRPV4 disrupted the bone morphogenetic protein (BMP) signaling pathway in iPSC-derived chondrocytes and blocked formation of hypertrophic chondrocytes providing potential targets for drug development for TRPV4-associated skeletal dysplasias [ 48 ]. The existing methods for chondrogenic differentiation from iPSCs may generate heterogeneous cell populations. To resolve this problem, a collagen, type II, alpha 1- green fluorescent protein (COL2A1-GFP) knock-in reporter allele generated by CRISPR-Cas9 system was used to purify the cells. The purified chondroprogenitors exhibited enhanced chondrogenic potential in comparison to unselected groups [ 40 ].

Transplantation of allogeneic human iPSC-derived cartilage have shown to be more effective than allogeneic BM-MSC-derived cartilage [ 49 ]. However, these cartilage cells can trigger immunological reactions [ 50 ]. To overcome this issue, it is necessary to reduce the immunological reactions. The β2 microglobulin, a component of MHC class I molecules, was knocked down in monkey iPSCs before their differentiation into chondrocytes. As expected, the allogeneic iPSC-derived cartilage transplanted in osteochondral defects in monkey knee joints showed increased proliferation of natural killer cells and leukocytes surrounding the knocked down PSC-derived cartilage. This indicates the intricate processes in the immune response of the transplanted allogeneic cartilage in osteochondral defects in vivo [ 47 ]. These studies highlight the tremendous advantages of the CRISPR-Cas9 system in understanding the pathogenesis, identification of promising drug targets, and development of feasible treatment interventions for cartilage diseases.

Cartilage organoids formed and differentiated from iPSCs

iPSC-derived cartilage organoids are 3D cell clusters that are created by differentiation of iPSCs in vitro. To support formation of cartilage organoids and their ability to self-renewal and self-organization, a number of biocompatible materials are used, such as Matrigel and synthetic hydrogels [ 51 ]. Cartilage organoid technology has been developed to facilitate drug screening through identification of important signaling pathways, recapitulate joint developmental events during embryogenesis and cartilage regeneration. Li and colleagues showed that long-term culturing of hiPSC-derived multi-tissue organoids (MTOs) in E8 medium results in a spontaneous emergence of hyaline cartilage tissues. Moreover, a transcriptome analysis indicated a strong association between the expression of chondrogenic markers in MTOs and fetal lower limb chondrocytes [ 52 ]. Another intriguing research demonstrated that subcutaneous implantation of iPSC-derived cartilage microtissues combined with pre-hypertrophic cartilage organoids in nude mice results in formation of both cartilaginous and bony regions [ 53 ]. Similarly, O’Connor and colleagues established osteochondral organoids using murine iPSCs through time-dependent sequential exposure of TGF-β3 and BMP2, to mimic natural bone development through the process of endochondral ossification. The generated organoids showed dual tissues consisting of cartilaginous and calcified bony regions [ 54 ]. A recent study showed a sequential differentiation process to produce matrix-rich cartilage spheroids from iPSC-MSCs by inducing NCCs in xeno-free environments. Efficient chondrogenic differentiation was induced by a thienoindazole derivative, TD-198946, a small molecule used to enhance differentiation of various human progenitor cells to chondrocytes. No hypertrophy, fibrotic cartilage formation, or dedifferentiation detected in vivo in the generated cartilage spheroids. These chondrogenic spheroids can serve as building blocks for biofabrication of engineered cartilage tissues, as they have the ability to fuse within a short timeframe of a few days [ 24 ]. It is worth mentioning that iPSC-derived cartilage organoids have also been reported to recruit osteogenic precursors for bone repair [ 55 ]. A recent study has revealed that allogeneic iPSC-derived cartilage organoids transplanted in the knee joints of a primate model of chondral defects integrated with articular cartilage of the host and prevented further degeneration of the surrounding cartilage [ 49 ]. These findings open new horizons for development of complex tissue engineered implants to promote zone-specific functionality by using pre-differentiated organoids as building blocks to establish articular cartilage grafts. Even though the research on iPSC-derived cartilage organoids is still in its infancy and creating fully functional cartilage organoids is still challenging, it is evident that they have demonstrated promising applications in drug screening, disease modeling, regeneration, and repair. It is of note that application of 3D bioprinting technology in development of iPSC-derived cartilage organoids can create more complex cartilage organoids and heighten their structural organization [ 56 ].

Therapeutic applications of iPSC-derived chondrocytes

Advanced disease modeling.

iPSC-derived chondrocytes have been utilized to recapitulate cartilage injuries and diseases in vitro (Table  1 ). The pluripotency and unlimited self-renewal capacity of the iPSCs make these cells vitally important for disease modeling, which permit us to investigate the mechanisms of various diseases, screen for potential treatment targets, and test therapeutic agents [ 57 ]. iPSC-derived disease models for both monogenic and complex cartilage diseases have been developed with more focus on single gene cartilage disorders [ 58 ]. Saitta et al. established an iPSC-based in vitro model of skeletal dysplasia to investigate the initial stages of abnormal cartilage formation. Mutations in the calcium channel gene TRPV4 lead to abnormal chondrogenesis during cartilage growth plate differentiation [ 59 ]. Isogenic iPSCs with wild-type or mutant NLRP3 have been generated from patients with neonatal-onset multisystem inflammatory disease. Both in vitro and in vivo chondrogenic differentiation were performed. Furthermore, immunodeficient mice that received mutant cartilaginous pellets in vivo experienced disordered endochondral ossification [ 60 ]. In vitro models of familial osteochondritis dissecans (FOCD) was developed using both patient BM-MSCs and iPSCs derived from patient fibroblasts to delineate the pathogenesis of this disease. The results showed that chondrogenic pellets with a high glycosaminoglycan (GAG) content but a poor structural integrity. Moreover, dysregulation of matrix production and assembly was evident. These findings show that how studying FOCD iPSC-derived chondrocytes can reveal insights into disease phenotype and pathogenesis offering a new in vitro model of OA and cartilage degeneration [ 61 ]. Esseltine et al. [ 62 ] converted fibroblasts from patient with oculodentodigital dysplasia (ODDD) into iPSCs, which provided a useful model for investigation of this disease. In this study, the iPSCs showed mutated Cx43 gene, decreased levels of Cx43 mRNA and protein, resulting in impaired channel function. Furthermore, the subcellular localization of Cx43 changed during the chondrogenic differentiation of ODDD-derived iPSCs. This altered localization may have contributed to the more compact cartilage pellet morphology observed in differentiated ODDD-derived iPSCs. Additionally, other research teams successfully developed iPSC-derived disease models for other genetic and complex multifactorial skeletal disorders including type II collagenopathy , fibrodysplasia ossificans progressive (FOP), OA, hand OA, and early-onset finger OA (efOA) [ 58 ]. Recently, a novel method was introduced to direct iPSC-derived sclerotome through a sequential transformation in a 3D pellet culture. The generated chondroprogenitors can further be differentiated into articular chondrocytes or, alternatively, transformed into hypertrophic chondrocytes capable of transitioning into osteoblasts. Moreover, distinctive gene expression signatures have been identified at critical developmental stages, highlighting the effectiveness of this system in modeling genetic disorders affecting cartilage and bone [ 63 ]. In general, these studies demonstrated that normal chondrogenesis can be recapitulated using an iPSC-derived model, and disease-specific iPSCs exhibit molecular evidence of aberrant chondrogenic developmental processes. These findings may be utilized to develop therapeutic strategies for cartilage-related disorders.

To overcome some limitations of scaffold-based 3D cell culture method, scaffold-free methods showed promising results as well. Nakumora et al. [ 64 ] reported efficient fabrication of unified, self-sufficient, and functional cartilaginous constructs by combining iPSCs and bio-3D printers using a Kenzan needle array technology. This approach may facilitate repairing of articular cartilage defects . Zhang et al. [ 65 ] established a rapid and efficient approach, employing a 3D rotary suspension culture system, to directly guide iPSC differentiation toward the chondrogenic mesoderm lineage. Subsequently, the research group introduced a tetracycline-controlled BMP4 gene regulation system for iPSCs, linking transcriptional activation of BMP4 with heightened chondrogenesis using the piggyBac (PB) transposon-based gene delivery system. Kotaka and associates used magnetically-labeled iPSCs and an external magnetic force to evaluate the safety and efficacy of magnetic field-mediated delivery of iPSCs for articular cartilage repair in nude rats. The results demonstrated the effectiveness and safety of this approach for in vivo cartilage repair [ 66 ] .

Drug screening

Surgical interventions are performed to prevent progressing of focal articular cartilage defects [ 29 ], however, no effective drugs are available for treatment of cartilage regeneration. Using human MSCs for screening of compounds that promote chondrogenesis has limitations due to limited expansion of MSC passages, variations between donors and the high cost [ 67 ]. The development of the iPSC technology and advancement in genome editing approaches provide crucial tools for drug screening by establishing iPSC-derived chondrocytes. Using human iPSCs, a 96-well screening platform was developed to identify chondrogenesis-inducing agents that can be used separately or combined with other techniques for cartilage regeneration and repair. Due to their ability to promote chondrogenesis in vitro and in vivo, AB235 and NB61, two chimeric ligands of Activin/BMP2, were used and tested separately at two different doses for validation of the 96-well chondrogenic screening format. Strikingly, elevated concentrations of each of these two agents resulted in improved chondrogenic differentiation [ 68 ]. Another OA drug screening study was conducted on iPSC-derived or native mouse cartilage samples. The inflammatory environment of OA was induced in these cells by interleukin-1α (IL-1α), and a 96-well plate format was used for screening of OA drug candidates. The high-throughput screening revealed that the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) inhibitor SC514 was the most effective drug candidate to reduce cartilage loss induced by IL-1α [ 69 ]. Increased mineralization in the FOP-derived iPSCs has been detected, a phenomenon that could be mitigated by the use of the BMP inhibitor DMH1 [ 70 ]. It has been demonstrated that statins could effectively rectify the degraded cartilage observed in both chondrogenically differentiated thanatophoric dysplasia type 1 (TD1)- and achondroplasia (ACH)-specific iPSCs [ 71 ]. These studies illustrate the potential of iPSCs to provide a suitable platform to identify novel therapeutic agents for cartilage-related disorders and facilitate development of personalized regenerative medicine.

Preclinical studies

Chondrocytes derived from iPSCs have demonstrated great promise in a variety of regenerative medicine applications, especially in relation to cartilage regeneration and repair [ 49 , 64 , 72 ]. These cells offer regenerative treatments for diseases such as OA and cartilage injuries (Table  1 ). They can be combined with biomaterial scaffolds or scaffold-free methods to create engineered cartilage grafts for transplantation [ 73 ]. Generation of cartilage tissues from patient-specific iPSCs reduces the risk of immunological rejection, thus this personalized strategy has a potential for treating diseases such as OA [ 19 ]. Before their clinical application, preclinical studies of the iPSC-derived chondrocytes are crucial to assess their viability, functionality, and safety [ 74 ]. iPSC-MSCs were used to repair cartilage defects in a rabbit model. Macroscopic and histological assessment revealed more cartilage repair in the experimental group as compared to both the control and scaffold implantation group. Furthermore, no teratoma formation detected in all the three groups indicating the safety and potential of iPSC-MSCs for cartilage regeneration [ 75 ]. Ko et al. [ 76 ] implanted iPSC-derived chondrocytes in osteochondral defects in immunosuppressed rats. The defects exhibited a significantly higher quality of cartilage repair than in the control. In another study, homogenous cartilaginous particles derived from chondrocyte-specific reporter hiPSC lines were transplanted into joint surface defects in immunodeficient rat and immunosuppressed mini-pig models. The neocartilage survived and integrated into native cartilage, and no tumor formation was observed in all the animal models following the transplantation [ 30 ]. The potential of MSC-based therapies is attributed to the release of trophic factors via paracrine signaling, with small extracellular vesicles (sEVs) potentially playing a significant role [ 77 ]. Zhu et al. [ 78 ] investigated the therapeutic efficacy of exosomes derived from synovial membrane MSCs (SM-MSC-Exos) and iPSC-MSCs (iPSC-MSC-Exos) in treatment of OA. The injected exosomes in an OA mouse model showed that iPSC-MSC-Exos exhibit a stronger therapeutic impact on OA compared to SM-MSC-Exos. Similarly, iPSC-MSC-derived sEVs injected in degenerative discs of intervertebral disc degeneration (IVDD) rat models revealed significant improvement in IVDD and senescence of nucleus pulposus cells of the IVD [ 79 ]. Given the poliferative capacity of autologous iPSC-MSCs, these cells ensure a consistent and abundant source of therapeutic sEVs, which could introduce a new therapeutic strategy for OA and IVDD treatment [ 78 , 79 ]. As previousely mentioned, Nejadnik et al. developed an effective method to directly differentiate human iPSCs (hiPSCs) into MSCs and chondrocytes without the need for EBs formation. Transplantation of these cells in OA rat models successfully repaired the osteochondral defects [ 33 ]. However, the traces of fibrocartilage and hypertrophic cartilage detected in the generated chondrocytes in vitro and use of FBS in the chondrogenic medium may prevent their clinical application. Use of Xeno-free media and thorough characterization of hiPSC-derived MSCs and chondrocytes will be essential prior to transplantation [ 33 ]. An intriguing study has demonestrated that chondrogenic spheroids derived from iPSC-MSCs retain cartilage phenotype in vivo comparable to the chondrogenic-like tissues generated from the same cell spheroids in vitro. In contrast to spheroids obtained from iPSC-MSCs, distinct bone-like tissue formation was evident in BM-MSC spheroids. This may prove the capacity of iPSC-MSC-derived chondrogenic spheroids to form cartilage-like tissues without endochondral ossification for treatment of cartilage defects in vivo [ 24 ]. Additionally, due to the ability of chondrogenic spheroids to fuse rapidly within a short timeframe, they can serve as as building blocks for constructing larger cartilage tissues using techniques like the Kenzan bioprinting method [ 56 ]. Current focus tends to shift towards investigating immune reactions in the context of allogeneic cartilage transplantation. Abe and colleagues were the first to conduct allogeneic cartilage transplantation into a primate model using major histocompatibility complex (MHC)-mismatched iPSC-derived cartilage organoids without the need for immunosuppressive drugs [ 49 ]. Remarkably, the transplanted organoids exhibited successful engraftment into chondral defects on the knee joint surface of the primate model, demonstrating survival, integration, and remodeling similar to native cartilage, without any observed immune reactions [ 49 ]. The findings of these preclinical studies demonstrate effective and clinically translatable approaches for regenerating cartilage tissue using hiPSC-derived MSCs and chondrocytes, offering potential enhancements in cartilage regeneration outcomes in cartilage diseases.

Clinical studies

Over the past decade, iPSCs have shown significant advancements, offering new prospects for personalized cell therapy. Patient-derived iPSCs exhibit a lower risk of rejection compared to allogeneic iPSCs. Therefore, some challenges such as tumorigenicity or immunogenicity must be addressed before the iPSCs can be extensively utilized in clinical therapy. To date, 89 clinical trials referenced under “induced pluripotent stem cells” have been registered on the World Health Organization (WHO)-managed main databases ( https://clinicaltrials.gov/ , International Clinical Trials Registry Platform (ICTRP), https://trialsearch.who.int/ ). Several studies from the Japan Primary Registries Network ( https://rctportal.niph.go.jp/en ) can be added to the list since most of their 21 iPSCs trials are not cross-referenced with the WHO’s platforms. Among the total 110 identified clinical trials, 51 trials were registered as interventional and the remaining as observational. Despite the low rejection risk, slow shifting from autologous to allogenic iPSC-derived therapy approach has been crucial due to the time and cost required for characterization and safety testing of each cell line. Furthermore, allogeneic iPSCs approach allow more time for the testing process, and once an approved cell line is established, it can be used to treat multiple patients. Opting for allogeneic cell therapy would result in a readily accessible therapeutic product for interventions [ 80 ].

Until recently, pluripotent cell-derived MSCs were not a popular focus in clinical research, with only a small number of studies exploring this area, despite the wide variety of potential tissues that could be produced. Currently, only three clinical trials involving ESC-derived MSCs [ 81 , 82 , 83 ], and six iPSC-MSCs clinical trials have been reported (Table  2 ) [ 84 , 85 ]. It is important to note that from the six clinical trials, cartilage regeneration through iPSC-MSCs was only addressed in two studies. In 2020, the University of Sydney and Cynata Therapeutics conducted phase 1 clinical trial to evaluate the safety, efficacy, and cost-effectiveness of an allogenic MSCs therapy (Cymerus MSCs) for tibiofemoral knee OA [ 86 ]. Lately, Cynata Therapeutics has reported that 321 subjects were recruited for the phase 3 SCUlpTOR clinical trial which will start in 2024 for 24 months (Trial ID: ACTRN12620000870954). In the foreseeable future, the phase 1 clinical trial sponsored by the Chinese Nuwacell Biotechnology company will investigate the safety and efficacy of the NCR100 allogenic iPSC-MSCs intra-articular injection for treatment of knee OA (Trial ID: NCT06049342). This is the first Chinese iPSC-derived cell product approved to be used in phase 1 clinical trial following six years of research and development, ( https://en.nuwacell.com/news ). It is to be noted that a study tried to directly differentiate allogenic iPSCs into chondrocytes without intermediate MSCs differentiation, to treat knee OA as well (Trial ID: jRCTa050190104). The 2020 Japanese interventional trial from Kyoto University was followed by a second observational trial in 2020 for post-treatment evaluation on the subject’s knees (Trial ID: jRCT1050220051).

As a concluding remark, there have been no results regarding cartilage regeneration through iPSC-derived cell therapy in these trials so far. The scarcity of iPSC-MSCs and cartilage-oriented clinical trials indicates significant potential for further advancement and enhancement. Hopefully with the extensively growing iPSCs research, cartilage regeneration for condition such as OA will receive greater attention.

Limitations of iPSC-derived chondrocyte in vitro models

Throughout this review, numerous studies have demonstrated the tremendous advantages offered by iPSC-derived chondrocytes for cartilage research. However, there are some limitations associated with iPSC-derived chondrocyte in vitro models. The first limitation is that the iPSC-derived chondrocytes may show an immature phenotype, and it is still challenging to obtain iPSC-derived chondrocytes with full maturation and stability [ 87 ]. The second limitation is the possibility to generate diverse cell populations with variation in maturation stages. This heterogeneity might complicate result interpretation and compromise the validity and reproducibility of experimental results [ 22 ]. Due to the potential of iPSCs to form teratomas, residual undifferentiated iPSCs in iPSC-derived cartilage grafts may pose a risk of tumor formation in transplantation studies [ 88 ]. Another main challenge is the variability in the efficiency of chondrogenic differentiation among different iPSC lines and even among clones of the same line [ 31 ]. Moreover, the culture conditions for differentiation of iPSCs to chondrocytes may not fully replicate the complex microenvironment of native cartilage tissue. The artificial culture conditions can influence cellular behavior and might not fully capture the in vivo physiological and mechanical complexity of chondrocytes [ 18 , 24 ]. Even though patient-derived iPSCs can potentially reduce the immunological rejection [ 89 ], the in vitro differentiation and manipulation processes may introduce foreign antigens, raising concerns about the immunogenicity of the generated chondrocytes [ 19 ]. In addition, the ability of iPSC-derived chondrocytes to produce a mature and robust ECM may be limited. The structure and organization of the ECM are essential for the functionality and integrity of cartilage tissue. Therefore, ECM defects may affect the utility of in vitro models [ 90 ]. Last, but not the least, the robustness of cartilage in vitro models may be affected by the technical aspects of iPSC maintenance, differentiation, and characterization, which may introduce variability [ 32 ]. These limitations illuminate the challenges associated with iPSC-derived chondrocyte in vitro models. Improvement and optimization of chondrogenic differentiation protocols may overcome these limitations and ensure reliable and comparable results across various studies.

Scaling-up of iPSC-derived cells

The potential of iPSC-derived technologies in chondrogenesis, offering significant benefits for OA and other medical conditions, is evident. However, unlocking these benefits encounters hurdles such as limited process understanding, outdated manufacturing techniques, and insufficient automation. Manual manufacturing and quality control processes prove labor-intensive and error prone. To address the anticipated demand for iPSC-derived cells, scalable production methods must be developed to uphold clinical-grade yields and immunomodulatory properties. Moreover, research indicates that human iPSCs might present an epigenetic edge compared to adult stem cells in producing chondrocytes on a large scale without a tendency towards hypertrophy. Ko and his team showcased heightened expression of key chondrogenic markers such as SOX9, COL2A1, and aggrecan (ACAN), alongside decreased levels of hypertrophic markers like COL10A1 and RUNX2 in iPSC-derived chondrocytes when compared to BM-MSC pellets [ 76 ].

It is crucial to establish robust protocols for large-scale iPSC production to support tasks like cell banking. Thorough evaluations of iPSC-derived chondrocytes in large-scale production settings are essential for consistent quality outcomes and to tackle the challenge of spontaneous differentiation. Closing the gap between research and clinical application necessitates the development of scaled production technologies spanning from initial seeding to final fill-and-finish stages. Embracing full automation in iPSCs cell therapy manufacturing and quality control is paramount for enhancing both product quality and production efficiency in this rapidly evolving field [ 91 ]. A recent study developed hiPSC-derived limb bud mesenchymal cells (ExpLBM cells) with strong chondrogenic potential and stable proliferation. Using a stirred bioreactor, this method outperformed conventional culture plate methods by yielding significant cartilage tissue with just 1 × 10 6 cells. This produced significant amounts of cartilaginous particles, suggesting a scalable method for cartilage regeneration without immune rejection. This efficient approach requires minimal cell quantities and offers potential scalability through adjustments in medium volume and cell numbers [ 92 ]. Another recent study has introduced GelMA microcarriers developed via step emulsification microfluidic devices as a degradable platform for amplifying iPSC-MSCs in scalable bioreactors, while maintaining typical MSC traits and immune-modulatory capabilities. These GelMA microcarriers, manufactured with efficiency and reproducibility in mind, facilitate substantial expansion of iPSC-MSCs (up to 16 times within 8 days) in vertical wheel bioreactors, with a post-digestion viability exceeding 95%. When compared to monolayer culture, iPSC-MSCs expanded on GelMA microcarriers exhibit at least similar, if not superior, immune-modulatory potential. This approach marks a notable progression in producing immune-modulatory iPSC-MSCs, providing scalability, cost-efficiency, and simplified cell retrieval through direct dissolution of microcarriers, thereby minimizing cell wastage [ 93 ].

A novel, good manufacturing practice (GMP)-compliant scalable manufacturing procedure is introduced for the fabrication of iPSC-MSCs, tackling the aforementioned hurdles. By employing xenogeneic-, serum-, and feeder-free conditions, alongside chemically defined maintenance for iPSCs, the process eliminates the necessity for murine feeders and accomplishes mesoderm induction, resulting in heightened performance of MSCs in immunopotency assessments. The manufacturing process comprises three phases: iPSC banking, iPSC expansion and differentiation into MSCs, and MSC expansion and formulation of the final clinical product. Impressively, one vial of iPSCs can yield an average of 3.2 × 10 10 MSCs, and the complete iPSC bank has the potential to generate 2.9 × 10 15 MSCs, equating to 29 million clinical doses, each containing 1 × 10 8 MSCs. This method presents a promising resolution to the challenges of supply, scalability, and consistency in iPSC-MSC production, paving the way for their utilization in clinical applications with heightened efficacy and safety. This optimized manufacturing process for iPSC-MSCs has been applied in treating steroid-resistant acute graft versus host disease (SR-aGvHD) in a phase 1 clinical trial but could be similarly employed in the iPSC-MSCs-Chondrocyte approach for chondrogenesis [ 84 ].

The aim of automating cell therapy manufacturing is to reduce human intervention, ensuring sterile processes within isolator-like platforms to minimize contamination risks. Despite notable advancements, challenges persist, including difficulties in executing specific biological procedures with robotic assistance, prompting the need for exploring new solutions and standardization. Establishing an automated manufacturing platform requires precise definition of process parameters and configurations through validated standard operating procedures (SOPs). To address these needs, an advanced automated cell manufacturing platform was employed to produce both equine and human iPSC-MSCs via EBs [ 94 ]. These iPSC-MSCs were further demonstrated their ability to differentiate into adipogenic, osteogenic, and chondrogenic lineages proficiently. The main goal of this study was to develop a simplified and uniform procedure for isolating MSCs from peripheral blood under GMP conditions, ensuring their viability and purity. Compared to existing protocols documented in the literature, this approach offers simplicity, scalability and consistently delivering robust cell purity [ 94 ]. Recently, another automatic system was reported to produce iPSC-derived therapies, covering a range of cell types including iPSC-MSCs, iPSC-derived chondrocytes, and extracellular vesicles [ 95 ]. iPSC expansion and differentiation into MSCs and chondrocytes take place in plates, while expansion of iPSC-derived MSCs and production of extracellular vesicles utilize microcarriers within stirred tank bioreactors. The system is designed to oversee iPSC expansion, differentiation, and the fill and finish of the products. Furthermore, this platform including a range of quality control assays such as microscopy, cell counting, viability assessment, qPCR, and endotoxin assays, aims to address these challenges by establishing an automated platform for producing cell therapies specifically targeting OA, and serves as an example of how existing automation technology can be customized and improved to enhance scalability and efficiency.

Conclusions

Genomic abnormalities detected during the reprogramming and subsequent expansion of iPSCs raised serious safety concerns [ 96 ]. Therefore, several factors including starting cell source, method of delivery, reprogramming factor and cell passage, should be taken into consideration for the generation of iPSCs in order to reduce not only genomic instability [ 97 ], but also immunogenicity [ 98 , 99 ].

The field of iPSC-derived cartilages is rapidly evolving, and several approaches and perspectives have been explored to tackle limitations and enhance the potential applications of these cells in regenerative medicine. Development of new or optimization of the current differentiation protocols to improve the maturation and stability of iPSC-derived chondrocytes is critical [ 25 ]. This can be achieved by further research on signaling pathways, culture conditions, and other factors that facilitate the maturation of iPSC-derived chondrocytes. It is significantly important to implement cutting-edge 3D culture systems combined with ink-free bioprinting technique to more closely mimic the in vivo microenvironment of cartilage tissue [ 56 ]. Using bioreactors, biomimetic scaffolds, 3D bioprinting and other advanced technologies can improve the functional characteristics of iPSC-derived chondrocytes for cartilage repair. Generation of heterogeneous cell populations remains one of the major challenges in development of efficient cartilage grafts [ 100 ]. To eliminate undesired cells and promote the homogeneity of iPSC-derived chondrocyte populations, sustained development of precise genome editing tools is quite essential. Moreover, it is necessary to identify the sources of heterogeneity in iPSC-derived chondrocyte populations to reduce variability and improve reproducibility [ 101 ]. Tumorigenicity associated with residual undifferentiated iPSCs can be addressed by advancements in purification methods and genetic modifications to increase the safety of iPSC-derived chondrocytes for clinical applications [ 102 ]. Moreover, scalability and cost-effectiveness of the methods used for generation of iPSC-derived chondrocytes should be improved by simplifying the differentiation protocols, optimizing culture conditions, and utilizing automation technologies [ 95 ]. Additionally, it is very crucial to enhance the development of in vivo models to investigate the safety and efficacy of iPSC-derived chondrocytes in preclinical studies [ 103 ]. Successful preclinical studies should be followed by well-designed clinical trials in patients with cartilage-related disorders. Furthermore, for personalized regenerative medicine, the design of preclinical and clinical trials should focus on the integration of patient-specific iPSCs with advanced gene editing technologies and highly efficient chondrogenic differentiation protocols. These future perspectives reflect the continuous endeavors to harness the full potential of iPSC-derived chondrocytes, opening the door for innovative approaches in cartilage regeneration and repair. Since this field is advancing rapidly, interdisciplinary collaborations and advancement in technologies will play a vital role in shaping the future of iPSC-based cartilage regeneration research.

Abbreviations

Two dimentional

Three dimentional

Achondroplasia

Bone marrow-derived Mesenchymal stem cells

Bone morphogenetic protein 2

Bone morphogenetic protein 4

Umbilical cord blood mononuclear cell

Cellular-Myelocytomatosis

Collagen, type II, alpha 1

Collagen, type II, alpha 1-green fluorescent protein

Clustered regularly interspaced short palindromic repeats

Dorsomorphin homolog 1

Embryoid bodies

Extracellular matrix

Early-onset finger osteoarthritis

Fibrodysplasia ossficans progressive

Good manufacturing practice

Human embryonic stem cells

Human induced pluripotent stem cells

Hand osteoarthritis

Identification number

• Induced pluripotent stem cells

Intervertebral disc degeneration

Krüppel-like factor 4

Knee osteoarthritis

Metaphyseal chondrodysplasia type Schmid

Multiple epiphyseal dysplasia

Mesenchymal stem cell

Not applicable

Neural crest cells

Kappa-light-chain-enhancer of activated B cells

Normal human epidermal keratinocytes

• Osteoarthritis

Octamer binding transcription factor 3/4

Peripheral blood mononuclear cells

Pluripotent stem cells

Runt-related transcription factor 2

Small extracellular vesicles

Standard operating procedures

SRY-related high mobility group box

Sex determining region Y

Thanatophoric dysplasia type 1

Transforming growth factor-beta 3

Krishnan Y, Grodzinsky AJ. Cartilage diseases. Matrix Biol. 2018;71–72:51–69.

Article   PubMed   PubMed Central   Google Scholar  

Sha’ban M. Histology and biomechanics of cartilage. Cartil Tissue Knee Jt Biomech [Internet]. Elsevier; 2024 [cited 2024 Feb 9]. pp. 25–35. Available from: https://linkinghub.elsevier.com/retrieve/pii/B9780323905978000013

Tokgoz E, Levitt S, Sosa D, Carola NA, Patel V. Robotics applications in total knee arthroplasty. Total knee arthroplasty. Cham: Springer Nature Switzerland; 2023. p. 155–74.

Google Scholar  

Trounson A, McDonald C. Stem cell therapies in clinical trials: progress and challenges. Cell Stem Cell. 2015;17:11–22.

Article   CAS   PubMed   Google Scholar  

Pittenger MF, Discher DE, Péault BM, Phinney DG, Hare JM, Caplan AI. Mesenchymal stem cell perspective: cell biology to clinical progress. NPJ Regen Med. 2019;4:22.

Uccelli A, Moretta L, Pistoia V. Mesenchymal stem cells in health and disease. Nat Rev Immunol. 2008;8:726–36.

Squillaro T, Peluso G, Galderisi U. Clinical trials with mesenchymal stem cells: an update. Cell Transpl. 2016;25:829–48.

Article   Google Scholar  

Nakashima K, Zhou X, Kunkel G, Zhang Z, Deng JM, Behringer RR, et al. The novel zinc finger-containing transcription factor osterix is required for osteoblast differentiation and bone formation. Cell. 2002;108:17–29.

Yoshida CA, Furuichi T, Fujita T, Fukuyama R, Kanatani N, Kobayashi S, et al. Core-binding factor β interacts with Runx2 and is required for skeletal development. Nat Genet. 2002;32:633–8.

Kangari P, Talaei-Khozani T, Razeghian-Jahromi I, Razmkhah M. Mesenchymal stem cells: amazing remedies for bone and cartilage defects. Stem Cell Res Ther. 2020;11:492.

Somoza RA, Welter JF, Correa D, Caplan AI. Chondrogenic differentiation of mesenchymal stem cells: challenges and unfulfilled expectations. Tissue Eng Part B Rev. 2014;20:596–608.

Kim HJ, Park J-S. Usage of human mesenchymal stem cells in cell-based therapy: advantages and disadvantages. Dev Reprod. 2017;21:1–10.

Galipeau J. The mesenchymal stromal cells dilemma—does a negative phase III trial of random donor mesenchymal stromal cells in steroid-resistant graft-versus-host disease represent a death knell or a bump in the road? Cytotherapy. 2013;15:2–8.

Article   PubMed   Google Scholar  

Takahashi K, Yamanaka S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell. 2006;126:663–76.

Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K, et al. Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell. 2007;131:861–72.

Karagiannis P, Takahashi K, Saito M, Yoshida Y, Okita K, Watanabe A, et al. Induced pluripotent stem cells and their use in human models of disease and development. Physiol Rev. 2019;99:79–114.

Guha P, Morgan JW, Mostoslavsky G, Rodrigues NP, Boyd AS. Lack of immune response to differentiated cells derived from syngeneic induced pluripotent stem cells. Cell Stem Cell. 2013;12:407–12.

Guzzo RM, Gibson J, Xu R, Lee FY, Drissi H. Efficient differentiation of human iPSC-derived mesenchymal stem cells to chondroprogenitor cells. J Cell Biochem. 2013;114:480–90.

Diekman BO, Christoforou N, Willard VP, Sun H, Sanchez-Adams J, Leong KW, et al. Cartilage tissue engineering using differentiated and purified induced pluripotent stem cells. Proc Natl Acad Sci. 2012;109:19172–7.

Article   CAS   PubMed   PubMed Central   Google Scholar  

Walters-Shumka JP, Sorrentino S, Nygaard HB, Willerth SM. Recent advances in personalized 3D bioprinted tissue models. MRS Bull. 2023;48:632–42.

Article   CAS   Google Scholar  

Schell JP. Spontaneous differentiation of human pluripotent stem cells via embryoid body formation. Hum stem cell man. Elsevier; 2012. p. 363–73.

De Kinderen P, Meester J, Loeys B, Peeters S, Gouze E, Woods S, et al. Differentiation of Induced pluripotent stem cells into chondrocytes: methods and applications for disease modeling and drug discovery. J Bone Miner Res. 2020;37:397–410.

Gammill LS, Bronner-Fraser M. Neural crest specification: migrating into genomics. Nat Rev Neurosci. 2003;4:795–805.

Zujur D, Al-Akashi Z, Nakamura A, Zhao C, Takahashi K, Aritomi S, et al. Enhanced chondrogenic differentiation of iPS cell-derived mesenchymal stem/stromal cells via neural crest cell induction for hyaline cartilage repair. Front Cell Dev Biol. 2023;11:1140717.

Oldershaw RA, Baxter MA, Lowe ET, Bates N, Grady LM, Soncin F, et al. Directed differentiation of human embryonic stem cells toward chondrocytes. Nat Biotechnol. 2010;28:1187–94.

Cheng A, Kapacee Z, Peng J, Lu S, Lucas RJ, Hardingham TE, et al. Cartilage repair using human embryonic stem cell-derived chondroprogenitors. Stem Cells Transl Med. 2014;3:1287–94.

Aung A, Gupta G, Majid G, Varghese S. Osteoarthritic chondrocyte–secreted morphogens induce chondrogenic differentiation of human mesenchymal stem cells. Arthritis Rheum. 2011;63:148–58.

Heng BC, Cao T, Lee EH. Directing stem cell differentiation into the chondrogenic lineage in vitro. Stem Cells. 2004;22:1152–67.

Tsumaki N, Okada M, Yamashita A. iPS cell technologies and cartilage regeneration. Bone. 2015;70:48–54.

Yamashita A, Morioka M, Yahara Y, Okada M, Kobayashi T, Kuriyama S, et al. Generation of scaffoldless hyaline cartilaginous tissue from human iPSCs. Stem Cell Rep. 2015;4:404–18.

Koyama N, Miura M, Nakao K, Kondo E, Fujii T, Taura D, et al. Human induced pluripotent stem cells differentiated into chondrogenic lineage via generation of mesenchymal progenitor cells. Stem Cells Dev. 2013;22:102–13.

Castro-Viñuelas R, Sanjurjo-Rodríguez C, Piñeiro-Ramil M, Hermida-Gómez T, Fuentes-Boquete IM, de Toro-Santos FJ, et al. Induced pluripotent stem cells for cartilage repair: current status and future perspectives. Eur Cell Mater. 2018;36:96–109.

Nejadnik H, Diecke S, Lenkov OD, Chapelin F, Donig J, Tong X, et al. Improved approach for chondrogenic differentiation of human induced pluripotent stem cells. Stem Cell Rev Rep. 2015;11:242–53.

Hemeda H, Giebel B, Wagner W. Evaluation of human platelet lysate versus fetal bovine serum for culture of mesenchymal stromal cells. Cytotherapy. 2014;16:170–80.

Marcus-Sekura C, Richardson JC, Harston RK, Sane N, Sheets RL. Evaluation of the human host range of bovine and porcine viruses that may contaminate bovine serum and porcine trypsin used in the manufacture of biological products. Biologicals. 2011;39:359–69.

Kamiya D, Takenaka-Ninagawa N, Motoike S, Kajiya M, Akaboshi T, Zhao C, et al. Induction of functional xeno-free MSCs from human iPSCs via a neural crest cell lineage. Npj Regen Med. 2022;7:47.

Fitzgerald J. Applications of CRISPR for musculoskeletal research. Bone Jt Res. 2020;9:351–9.

Vlashi R, Zhang X, Li H, Chen G. Potential therapeutic strategies for osteoarthritis via CRISPR/Cas9 mediated gene editing. Rev Endocr Metab Disord. 2023;25:339.

Mochizuki Y, Chiba T, Kataoka K, Yamashita S, Sato T, Kato T, et al. Combinatorial CRISPR/Cas9 approach to elucidate a far-upstream enhancer complex for tissue-specific Sox9 expression. Dev Cell. 2018;46:794-806.e6.

Adkar SS, Wu C-L, Willard VP, Dicks A, Ettyreddy A, Steward N, et al. Step-wise chondrogenesis of human induced pluripotent stem cells and purification via a reporter allele generated by CRISPR-Cas9 genome editing. Stem Cells. 2019;37:65–76.

Kim HJ, Park JM, Lee S, Cho HB, Park J-I, Kim J-H, et al. Efficient CRISPR-Cas9-based knockdown of RUNX2 to induce chondrogenic differentiation of stem cells. Biomater Sci. 2022;10:514–23.

Lilianty J, Bateman JF, Lamandé SR. Generation of a heterozygous COL2A1 (pG1113C) hypochondrogenesis mutation iPSC line, MCRIi019-A-7, using CRISPR/Cas9 gene editing. Stem Cell Res. 2021;56:102515.

Truong VA, Hsu M-N, Kieu Nguyen NT, Lin M-W, Shen C-C, Lin C-Y, et al. CRISPRai for simultaneous gene activation and inhibition to promote stem cell chondrogenesis and calvarial bone regeneration. Nucl Acids Res. 2019;47:e74–e74.

Bonato A, Fisch P, Ponta S, Fercher D, Manninen M, Weber D, et al. Engineering inflammation-resistant cartilage: bridging gene therapy and tissue engineering. Adv Healthc Mater. 2023;12:2202271.

Farhang N, Davis B, Weston J, Ginley-Hidinger M, Gertz J, Bowles RD. Synergistic CRISPRa-regulated chondrogenic extracellular matrix deposition without exogenous growth factors. Tissue Eng Part A. 2020;26:1169–79.

Dicks A, Wu C-L, Steward N, Adkar SS, Gersbach CA, Guilak F. Prospective isolation of chondroprogenitors from human iPSCs based on cell surface markers identified using a CRISPR-Cas9-generated reporter. Stem Cell Res Ther. 2020;11:66.

Okutani Y, Abe K, Yamashita A, Morioka M, Matsuda S, Tsumaki N. Generation of monkey induced pluripotent stem cell-derived cartilage lacking major histocompatibility complex class I molecules on the cell surface. Tissue Eng Part A. 2022;28:94–106.

Dicks AR, Maksaev GI, Harissa Z, Savadipour A, Tang R, Steward N, et al. Skeletal dysplasia-causing TRPV4 mutations suppress the hypertrophic differentiation of human iPSC-derived chondrocytes. Elife. 2023;12:e71154.

Abe K, Yamashita A, Morioka M, Horike N, Takei Y, Koyamatsu S, et al. Engraftment of allogeneic iPS cell-derived cartilage organoid in a primate model of articular cartilage defect. Nat Commun. 2023;14:804.

Kimura T, Yamashita A, Ozono K, Tsumaki N. Limited immunogenicity of human induced pluripotent stem cell-derived cartilages. Tissue Eng Part A. 2016;22:1367–75.

Crispim JF, Ito K. De novo neo-hyaline-cartilage from bovine organoids in viscoelastic hydrogels. Acta Biomater. 2021;128:236–49.

Li M, Abrahante JE, Vegoe A, Chai YW, Lindborg B, Toth F, et al. Self-organized emergence of hyaline cartilage in hiPSC-derived multi-tissue organoids. Cell Biol. 2021;2014:272481. https://doi.org/10.1101/2021.09.21.461213 .

Hall GN, Tam WL, Andrikopoulos KS, Casas-Fraile L, Voyiatzis GA, Geris L, et al. Patterned, organoid-based cartilaginous implants exhibit zone specific functionality forming osteochondral-like tissues in vivo. Biomaterials. 2021;273:120820.

O’Connor SK, Katz DB, Oswald SJ, Groneck L, Guilak F. Formation of osteochondral organoids from murine induced pluripotent stem cells. Tissue Eng Part A. 2021;27:1099–109.

Tam WL, Freitas Mendes L, Chen X, Lesage R, Van Hoven I, Leysen E, et al. Human pluripotent stem cell-derived cartilaginous organoids promote scaffold-free healing of critical size long bone defects. Stem Cell Res Ther. 2021;12:513.

Vignes H, Smaida R, Conzatti G, Hua G, Benkirane-Jessel N. Custom-made meniscus biofabrication. Trends Biotechnol. 2023;41:1467–70.

Singh VK, Kalsan M, Kumar N, Saini A, Chandra R. Induced pluripotent stem cells: applications in regenerative medicine, disease modeling, and drug discovery. Front Cell Dev Biol. 2015. https://doi.org/10.3389/fcell.2015.00002/abstract .

Lach MS, Rosochowicz MA, Richter M, Jagiełło I, Suchorska WM, Trzeciak T. The induced pluripotent stem cells in articular cartilage regeneration and disease modelling: are we ready for their clinical use? Cells. 2022;11:529.

Saitta B, Passarini J, Sareen D, Ornelas L, Sahabian A, Argade S, et al. Patient-derived skeletal dysplasia induced pluripotent stem cells display abnormal chondrogenic marker expression and regulation by BMP2 and TGFβ1 . Stem Cells Dev. 2014;23:1464–78.

Yokoyama K, Ikeya M, Umeda K, Oda H, Nodomi S, Nasu A, et al. Enhanced chondrogenesis of induced pluripotent stem cells from patients with neonatal-onset multisystem inflammatory disease occurs via the caspase 1–independent cAMP/protein kinase A/CREB pathway. Arthritis Rheumatol. 2015;67:302–14.

Xu M, Stattin E-L, Shaw G, Heinegård D, Sullivan G, Wilmut I, et al. Chondrocytes derived from mesenchymal stromal cells and induced pluripotent cells of patients with familial osteochondritis dissecans exhibit an endoplasmic reticulum stress response and defective matrix assembly. Stem Cells Transl Med. 2016;5:1171–81.

Esseltine JL, Shao Q, Brooks C, Sampson J, Betts DH, Séguin CA, et al. Connexin43 mutant patient-derived induced pluripotent stem cells exhibit altered differentiation potential. J Bone Miner Res Off J Am Soc Bone Miner Res. 2017;32:1368–85.

Lamandé SR, Ng ES, Cameron TL, Kung LHW, Sampurno L, Rowley L, et al. Modeling human skeletal development using human pluripotent stem cells. Proc Natl Acad Sci. 2023;120:e2211510120.

Nakamura A, Murata D, Fujimoto R, Tamaki S, Nagata S, Ikeya M, et al. Bio-3D printing iPSC-derived human chondrocytes for articular cartilage regeneration. Biofabrication. 2021;13:044103.

Zhang M, Niibe K, Kondo T, Limraksasin P, Okawa H, Miao X, et al. Rapid and efficient generation of cartilage pellets from mouse induced pluripotent stem cells by transcriptional activation of BMP-4 with shaking culture. J Tissue Eng. 2022;13:20417314221114616.

Kotaka S, Wakitani S, Shimamoto A, Kamei N, Sawa M, Adachi N, et al. Magnetic targeted delivery of induced pluripotent stem cells promotes articular cartilage repair. Stem Cells Int. 2017;2017:9514719.

Siddappa R, Licht R, Van Blitterswijk C, De Boer J. Donor variation and loss of multipotency during in vitro expansion of human mesenchymal stem cells for bone tissue engineering. J Orthop Res. 2007;25:1029–41.

Yang S-L, Harnish E, Leeuw T, Dietz U, Batchelder E, Wright PS, et al. Compound screening platform using human induced pluripotent stem cells to identify small molecules that promote chondrogenesis. Protein Cell. 2012;3:934–42.

Willard VP, Diekman BO, Sanchez-Adams J, Christoforou N, Leong KW, Guilak F. Use of cartilage derived from murine induced pluripotent stem cells for osteoarthritis drug screening. Arthritis Rheumatol. 2014;66:3062–72.

Matsumoto Y, Hayashi Y, Schlieve CR, Ikeya M, Kim H, Nguyen TD, et al. Induced pluripotent stem cells from patients with human fibrodysplasia ossificans progressiva show increased mineralization and cartilage formation. Orphanet J Rare Dis. 2013;8:190.

Yamashita A, Morioka M, Kishi H, Kimura T, Yahara Y, Okada M, et al. Statin treatment rescues FGFR3 skeletal dysplasia phenotypes. Nature. 2014;513:507–11.

Chang Y-H, Wu K-C, Ding D-C. Induced pluripotent stem cell-differentiated chondrocytes repair cartilage defect in a rabbit osteoarthritis model. Stem Cells Int. 2020;2020:1–16.

Lammi M, Piltti J, Prittinen J, Qu C. Challenges in fabrication of tissue-engineered cartilage with correct cellular colonization and extracellular matrix assembly. Int J Mol Sci. 2018;19:2700.

Yamashita A, Tsumaki N. Recent progress of animal transplantation studies for treating articular cartilage damage using pluripotent stem cells. Dev Growth Differ. 2021;63:72–81.

Xu X, Shi D, Liu Y, Yao Y, Dai J, Xu Z, et al. In vivo repair of full-thickness cartilage defect with human iPSC-derived mesenchymal progenitor cells in a rabbit model. Exp Ther Med. 2017;14:239–45.

Ko J-Y, Kim K-I, Park S, Im G-I. In vitro chondrogenesis and in vivo repair of osteochondral defect with human induced pluripotent stem cells. Biomaterials. 2014;35:3571–81.

Liang X, Ding Y, Zhang Y, Tse H-F, Lian Q. Paracrine mechanisms of mesenchymal stem cell-based therapy: current status and perspectives. Cell Transpl. 2014;23:1045–59.

Zhu Y, Wang Y, Zhao B, Niu X, Hu B, Li Q, et al. Comparison of exosomes secreted by induced pluripotent stem cell-derived mesenchymal stem cells and synovial membrane-derived mesenchymal stem cells for the treatment of osteoarthritis. Stem Cell Res Ther. 2017;8:64.

Sun Y, Zhang W, Li X. Induced pluripotent stem cell-derived mesenchymal stem cells deliver exogenous miR-105-5p via small extracellular vesicles to rejuvenate senescent nucleus pulposus cells and attenuate intervertebral disc degeneration. Stem Cell Res Ther. 2021;12:286.

Blair NF, Barker RA. Making it personal: the prospects for autologous pluripotent stem cell-derived therapies. Regen Med. 2016;11:423–5.

Yan L, Wu Y, Li L, Wu J, Zhao F, Gao Z, et al. Clinical analysis of human umbilical cord mesenchymal stem cell allotransplantation in patients with premature ovarian insufficiency. Cell Prolif. 2020;53:e12938.

Shin JH, Ryu C-M, Yu HY, Park J, Kang AR, Shin JM, et al. Safety of human embryonic stem cell-derived mesenchymal stem cells for treating interstitial cystitis: a phase I study. Stem Cells Transl Med. 2022;11:1010–20.

Guo B, Duan Y, Li Z, Tian Y, Cheng X, Liang C, et al. High-strength cell sheets and vigorous hydrogels from mesenchymal stem cells derived from human embryonic stem cells. ACS Appl Mater Interfaces. 2023;15:27586–99.

Bloor AJC, Patel A, Griffin JE, Gilleece MH, Radia R, Yeung DT, et al. Production, safety and efficacy of iPSC-derived mesenchymal stromal cells in acute steroid-resistant graft versus host disease: a phase I, multicenter, open-label, dose-escalation study. Nat Med. 2020;26:1720–5.

Rasko JEJ, Patel A, Griffin JE, Gilleece MH, Radia R, Yeung DT, et al. Results of the first completed clinical trial of an iPSC-derived product: CYP-001 in steroid-resistant acute GvHD. Biol Blood Marrow Transpl. 2019;25:S255–6.

Liu X, Robbins S, Wang X, Virk S, Schuck K, Deveza LA, et al. Efficacy and cost-effectiveness of stem cell injections for symptomatic relief and structural improvement in people with tibiofemoral knee OsteoaRthritis: protocol for a randomised placebo-controlled trial (the SCUlpTOR trial). BMJ Open. 2021;11:e056382.

Graceffa V, Vinatier C, Guicheux J, Stoddart M, Alini M, Zeugolis DI. Chasing chimeras–the elusive stable chondrogenic phenotype. Biomaterials. 2019;192:199–225.

Wang Z. Assessing tumorigenicity in stem cell-derived therapeutic products: a critical step in safeguarding regenerative medicine. Bioengineering. 2023;10:857.

Abe K, Tsumaki N. Regeneration of joint surface defects by transplantation of allogeneic cartilage: application of iPS cell-derived cartilage and immunogenicity. Inflamm Regen. 2023;43:56.

Saito T, Yano F, Mori D, Kawata M, Hoshi K, Takato T, et al. Hyaline cartilage formation and tumorigenesis of implanted tissues derived from human induced pluripotent stem cells. Biomed Res. 2015;36:179–86.

Dupuis V, Oltra E. Methods to produce induced pluripotent stem cell-derived mesenchymal stem cells: Mesenchymal stem cells from induced pluripotent stem cells. World J Stem Cells. 2021;13:1094–111.

Fujisawa Y, Takao T, Yamada D, Takarada T. Development of cartilage tissue using a stirred bioreactor and human iPSC-derived limb bud mesenchymal cells. Biochem Biophys Res Commun. 2023;687:149146.

Rogers RE, Haskell A, White BP, Dalal S, Lopez M, Tahan D, et al. A scalable system for generation of mesenchymal stem cells derived from induced pluripotent cells employing bioreactors and degradable microcarriers. Stem Cells Transl Med. 2021;10:1650–65.

Vieira CP, McCarrel TM, Grant MB. Novel methods to mobilize, isolate, and expand mesenchymal stem cells. Int J Mol Sci. 2021;22:5728.

Herbst L, Groten F, Murphy M, Shaw G, Nießing B, Schmitt RH. Automated production at scale of induced pluripotent stem cell-derived mesenchymal stromal cells, chondrocytes and extracellular vehicles: towards real-time release. Processes. 2023;11:2938.

Pera MF. The dark side of induced pluripotency. Nature. 2011;471:46–7.

Yoshihara M, Hayashizaki Y, Murakawa Y. Genomic Instability of iPSCs: challenges towards their clinical applications. Stem Cell Rev Rep. 2017;13:7–16.

Apostolou E, Hochedlinger K. iPS cells under attack. Nature. 2011;474:165–6.

Liu X, Li W, Fu X, Xu Y. The immunogenicity and immune tolerance of pluripotent stem cell derivatives. Front Immunol. 2017;8:645.

Murphy C, Mobasheri A, Táncos Z, Kobolák J, Dinnyés A. The potency of induced pluripotent stem cells in cartilage regeneration and osteoarthritis treatment. In: Turksen K, editor. Cell biology and translational medicine, vol. 1. Cham: Springer International Publishing; 2017. p. 55–68.

Chapter   Google Scholar  

Kim IG, Park SA, Lee S-H, Choi JS, Cho H, Lee SJ, et al. Transplantation of a 3D-printed tracheal graft combined with iPS cell-derived MSCs and chondrocytes. Sci Rep. 2020;10:4326.

Okita K, Matsumura Y, Sato Y, Okada A, Morizane A, Okamoto S, et al. A more efficient method to generate integration-free human iPS cells. Nat Methods. 2011;8:409–12.

Lo Monaco M, Merckx G, Ratajczak J, Gervois P, Hilkens P, Clegg P, et al. Stem cells for cartilage repair: preclinical studies and insights in translational animal models and outcome measures. Stem Cells Int. 2018;2018:1–22.

Castro-Viñuelas R, Sanjurjo-Rodríguez C, Piñeiro-Ramil M, Hermida-Gómez T, Rodríguez-Fernández S, Oreiro N, et al. Generation and characterization of human induced pluripotent stem cells (iPSCs) from hand osteoarthritis patient-derived fibroblasts. Sci Rep. 2020;10:4272.

Rim YA, Nam Y, Park N, Jung H, Lee K, Lee J, et al. Chondrogenic differentiation from induced pluripotent stem cells using non-viral minicircle vectors. Cells. 2020;9:582.

Ozaki T, Kawamoto T, Iimori Y, Takeshita N, Yamagishi Y, Nakamura H, et al. Evaluation of FGFR inhibitor ASP5878 as a drug candidate for achondroplasia. Sci Rep. 2020;10:20915.

Rim YA, Nam Y, Park N, Lee K, Jung H, Jung SM, et al. Characterization of early-onset finger osteoarthritis-like condition using patient-derived induced pluripotent stem cells. Cells. 2021;10:317.

Pretemer Y, Kawai S, Nagata S, Nishio M, Watanabe M, Tamaki S, et al. Differentiation of hypertrophic chondrocytes from human iPSCs for the in vitro modeling of chondrodysplasias. Stem Cell Rep. 2021;16:610–25.

Kimura T, Bosakova M, Nonaka Y, Hruba E, Yasuda K, Futakawa S, et al. An RNA aptamer restores defective bone growth in FGFR3-related skeletal dysplasia in mice. Sci Transl Med. 2021;13:eaba4226.

Lee SJ, Nam Y, Rim YA, Lee K, Ju JH, Kim DS. Perichondrium-inspired permeable nanofibrous tube well promoting differentiation of hiPSC-derived pellet toward hyaline-like cartilage pellet. Biofabrication. 2021;13:045015.

Download references

Acknowledgements

We thank the support of Institut national de la santé et de la recherche médicale (INSERM), Faculté de médecine et Faculté de chirurgie dentaire de Université de Strasbourg, and Lamina therapeutics. EAMA is financially supported by ANR ARTiTHERA, WO was supported by Chinese Scholarship Council (CSC N° 202309240005). We also thank Servier Medical ART for free medical images.

Not applicable.

Author information

Eltahir Abdelrazig Mohamed Ali, Rana Smaida, Morgane Meyer and Wenxin Ou have contributed equally to this work.

Authors and Affiliations

Institut National de la Santé et de la Recherche Médicale (INSERM), UMR 1260, Regenerative NanoMedicine (RNM), 1 Rue Eugène Boeckel, 67000, Strasbourg, France

Eltahir Abdelrazig Mohamed Ali, Nadia Benkirane-Jessel & Guoqiang Hua

Université de Strasbourg, 67000, Strasbourg, France

Eltahir Abdelrazig Mohamed Ali, Morgane Meyer, Wenxin Ou, Nadia Benkirane-Jessel, Jacques Eric Gottenberg & Guoqiang Hua

Lamina Therapeutics, 1 Rue Eugène Boeckel, 67000, Strasbourg, France

Rana Smaida, Morgane Meyer & Nadia Benkirane-Jessel

Nankai University School of Medicine, Tianjin, 300071, China

Beijing Engineering Laboratory of Perinatal Stem Cells, Beijing Institute of Health and Stem Cells, Health & Biotech Co, Beijing, 100176, China

Zhongchao Han

Centre National de Référence des Maladies Auto-Immunes et Systémiques Rares, Est/Sud-Ouest (RESO), Service de Rhumatologie, Centre Hospitalier Universitaire de Strasbourg, 67000, Strasbourg, France

Wenxin Ou & Jacques Eric Gottenberg

Chongqing Medical University, 1 Yixueyuan Road, Yuzhong District, Chongqing, 400016, China

You can also search for this author in PubMed   Google Scholar

Contributions

EAMA, RS, MM and WO wrote the draft of the manuscript. ZL, ZH, NBJ, JEG and GH revised the manuscript. All authors reviewed and approved the final manuscript.

Corresponding authors

Correspondence to Nadia Benkirane-Jessel , Jacques Eric Gottenberg or Guoqiang Hua .

Ethics declarations

Competing interests.

The authors declare that they have no competing interests.

Additional information

Publisher's note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/ ) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and permissions

About this article

Cite this article.

Ali, E.A.M., Smaida, R., Meyer, M. et al. iPSCs chondrogenic differentiation for personalized regenerative medicine: a literature review. Stem Cell Res Ther 15 , 185 (2024). https://doi.org/10.1186/s13287-024-03794-1

Download citation

Received : 28 March 2024

Accepted : 08 June 2024

Published : 26 June 2024

DOI : https://doi.org/10.1186/s13287-024-03794-1

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Chondrocytes
• Mesenchymal stem cells
• Cartilage regeneration
• Personalized regenerative medicine

Stem Cell Research & Therapy

ISSN: 1757-6512

• Submission enquiries: Access here and click Contact Us
• General enquiries: [email protected]

Information

• Author Services

Initiatives

You are accessing a machine-readable page. In order to be human-readable, please install an RSS reader.

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited. For more information, please refer to https://www.mdpi.com/openaccess .

Feature papers represent the most advanced research with significant potential for high impact in the field. A Feature Paper should be a substantial original Article that involves several techniques or approaches, provides an outlook for future research directions and describes possible research applications.

Feature papers are submitted upon individual invitation or recommendation by the scientific editors and must receive positive feedback from the reviewers.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

Original Submission Date Received: .

• Active Journals
• Find a Journal
• Proceedings Series
• For Authors
• For Reviewers
• For Editors
• For Librarians
• For Publishers
• For Societies
• For Conference Organizers
• Open Access Policy
• Institutional Open Access Program
• Special Issues Guidelines
• Editorial Process
• Research and Publication Ethics
• Article Processing Charges
• Testimonials
• Preprints.org
• SciProfiles
• Encyclopedia

Article Menu

• Subscribe SciFeed
• Recommended Articles
• PubMed/Medline
• Google Scholar
• on Google Scholar
• Table of Contents

Find support for a specific problem in the support section of our website.

Please let us know what you think of our products and services.

Visit our dedicated information section to learn more about MDPI.

JSmol Viewer

Zinc oxide—from synthesis to application: a review.

Graphical Abstract

1. Introduction

Zinc oxide, with its unique physical and chemical properties, such as high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption and high photostability, is a multifunctional material [ 1 , 2 ]. In materials science, zinc oxide is classified as a semiconductor in group II-VI, whose covalence is on the boundary between ionic and covalent semiconductors. A broad energy band (3.37 eV), high bond energy (60 meV) and high thermal and mechanical stability at room temperature make it attractive for potential use in electronics, optoelectronics and laser technology [ 3 , 4 ]. The piezo- and pyroelectric properties of ZnO mean that it can be used as a sensor, converter, energy generator and photocatalyst in hydrogen production [ 5 , 6 ]. Because of its hardness, rigidity and piezoelectric constant it is an important material in the ceramics industry, while its low toxicity, biocompatibility and biodegradability make it a material of interest for biomedicine and in pro-ecological systems [ 7 – 9 ].

The variety of structures of nanometric zinc oxide means that ZnO can be classified among new materials with potential applications in many fields of nanotechnology. Zinc oxide can occur in one- (1D), two- (2D), and three-dimensional (3D) structures. One-dimensional structures make up the largest group, including nanorods [ 10 – 12 ], -needles [ 13 ], -helixes, -springs and -rings [ 14 ], -ribbons [ 15 ], -tubes [ 16 – 18 ] -belts [ 19 ], -wires [ 20 – 22 ] and -combs [ 23 ]. Zinc oxide can be obtained in 2D structures, such as nanoplate/nanosheet and nanopellets [ 24 , 25 ]. Examples of 3D structures of zinc oxide include flower, dandelion, snowflakes, coniferous urchin-like, etc. [ 26 – 29 ]. ZnO provides one of the greatest assortments of varied particle structures among all known materials (see Figure 1 ).

In this review, the methods of synthesis, modification and application of zinc oxide will be discussed. The zinc oxide occurs in a very rich variety of structures and offers a wide range of properties. The variety of methods for ZnO production, such as vapour deposition, precipitation in water solution, hydrothermal synthesis, the sol-gel process, precipitation from microemulsions and mechanochemical processes, makes it possible to obtain products with particles differing in shape, size and spatial structure. These methods are described in detail in the following sections ( Table 1 ).

2. Methods of Synthesis of Nano- and Micrometric Zinc Oxide

2.1. metallurgical process.

Metallurgical processes for obtaining zinc oxide are based on the roasting of zinc ore. According to the ISO 9298 standard [ 68 ], zinc oxide is classified either as type A, obtained by a direct process (the American process); or type B, obtained by an indirect process (the French process).

The direct (American) process involves the reduction of zinc ore by heating with coal (such as anthracite), followed by the oxidation of zinc vapour in the same reactor, in a single production cycle. This process was developed by Samuel Wetherill, and takes place in a furnace in which the first layer consists of a coal bed, lit by the heat remaining from the previous charge. Above this bed is a second layer in the form of zinc ore mixed with coal. Blast air is fed in from below, so as to deliver heat to both layers and to carry carbon monoxide for zinc reduction. The resulting zinc oxide (of type A) contains impurities in the form of compounds of other metals from the zinc ore. The resulting ZnO particles are mainly needle-shaped, and sometimes spheroidal. To obtain a product with a permanent white color, the oxides of lead, iron and cadmium that are present are converted to sulfates. Increasing the permanence of the color is linked to increasing the content of water-soluble substances, and also increasing the acidity of the product. Acidity is desirable in the case of rubber processing technology, since it lengthens prevulcanization time and ensures the safe processing of the mixtures [ 69 ].

In the indirect (French) process, metallic zinc is melted in a furnace and vaporized at ca. 910 °C. The immediate reaction of the zinc vapour with oxygen from the air produces ZnO. The particles of zinc oxide are transported via a cooling duct and are collected at a bag filter station. The indirect process was popularized by LeClaire in 1844, and since then has been known as the French process. The product consists of agglomerates with an average particle size ranging from 0.1 to a few micrometres [ 70 ]. The ZnO particles are mainly of spheroidal shape. The French process is carried out in vertical furnaces, with an original vertical charge, vertical refining column, vaporizer with electric arc, and rotary combustion chamber [ 71 ]. Type B zinc oxide has a higher degree of purity than type A.

2.2. Chemical Processes

Because of its interesting properties, zinc oxide has been the subject of study by many researchers. This has led to the development of a great variety of techniques for synthesizing the compound. Unfortunately, methods that work in the laboratory cannot always be applied on an industrial scale, where it is important for the process to be economically effective, high yielding and simple to implement.

2.2.1. Mechanochemical Process

The mechanochemical process is a cheap and simple method of obtaining nanoparticles on a large scale. It involves high-energy dry milling, which initiates a reaction through ball–powder impacts in a ball mill, at low temperature. A “thinner” is added to the system in the form of a solid (usually NaCl), which acts as a reaction medium and separates the nanoparticles being formed. A fundamental difficulty in this method is the uniform grinding of the powder and reduction of grains to the required size, which decreases with increasing time and energy of milling. Unfortunately, a longer milling time leads to a greater quantity of impurities. The advantages of this method are the low production costs, small particle sizes and limited tendency for particles to agglomerate, as well as the high homogeneity of the crystalline structure and morphology.

The starting materials used in the mechanochemical method are mainly anhydrous ZnCl 2 and Na 2 CO 3 . NaCl is added to the system; this serves as a reaction medium and separates the nanoparticles. The zinc oxide precursor formed, ZnCO 3 , is calcined at a temperature of 400–800 °C. The process as a whole involves the following reactions (1) and (2):

The mechanochemical method was proposed by Ao et al. [ 30 ], they synthesized ZnO with an average crystallite size of 21 nm. The milling process was carried out for 6 h, producing ZnCO 3 as the zinc oxide precursor. Calcination of the precursor at 600 °C produced ZnO with a hexagonal structure. Tests showed that the size of the ZnO crystallites depends on the milling time and calcination temperature. Increasing the milling time (2–6 h) led to a reduction in the crystallite sizes (21.5–25 nm), which may indicate the existence of a “critical moment”. Meanwhile an increase in the calcination temperature from 400 to 800 °C caused an increase in crystallite size (18–35 nm).

The same system of reagents was used by Tsuzuki and McCormick [ 32 ]. They found that a milling time of 4 h was enough for a reaction to take place between the substrates, producing the precursor ZnCO 3 , which when calcined at 400 °C produced nanocrystallites of ZnO with an average size of 26 nm. Tsuzuki et al. showed that milling of the substrates without a thinner leads to the formation of aggregates measuring 100–1000 nm. This confirmed the important role played by zinc chloride in preventing agglomeration of the nanoparticles.

A milling process of ZnCl 2 and Na 2 CO 3 was also carried out by Moballegh et al. [ 33 ] and by Aghababazadeh et al. [ 34 ]. Moballegh et al. , investigated the effect of calcination temperature on particle size. An increase in the temperature of the process (300–450 °C) caused an increase in the size of the ZnO particles (27–56 nm). Aghababazadeh et al. obtained ZnO with an average particle size of approximately 51 nm and a surface area of 23 m 2 /g, carrying out the process at a temperature of 400 °C.

Stanković et al. [ 31 ] extended their previous study to investigate mechanical-thermal synthesis (MTS)—mechanical activation followed by thermal activation of ZnO from ZnCl 2 and oxalic acid (C 2 H 2 O 4 ·2H 2 O) as reactants with the intention of obtaining pure ZnO nanopowder. The study also aimed to examine the effects of oxalic acid as an organic PCA, and different milling times, on the crystal structure, average particle size and morphology of ZnO nanopowders. The mixture of initial reactants was milled from 30 min up to 4 h, and subsequently annealed at 450 °C for 1 h. Qualitative analysis of the prepared powders was performed using X-ray diffraction (XRD) and Raman spectroscopy. The XRD analysis showed perfect long-range order and the pure wurtzite structure of the synthesized ZnO powders, irrespective of the milling duration. By contrast, Raman spectroscopy indicates a different middle-range order of ZnO powders. From the SEM images, it is observed that the morphology of the particles strongly depends on the milling time of the reactant mixture, regardless of the further thermal treatment. A longer time of milling led to a smaller particle size.

2.2.2. Controlled Precipitation

Controlled precipitation is a widely used method of obtaining zinc oxide, since it makes it possible to obtain a product with repeatable properties. The method involves fast and spontaneous reduction of a solution of zinc salt using a reducing agent, to limit the growth of particles with specified dimensions, followed by precipitation of a precursor of ZnO from the solution. At the next stage this precursor undergoes thermal treatment, followed by milling to remove impurities. It is very difficult to break down the agglomerates that form, so the calcined powders have a high level of agglomeration of particles. The process of precipitation is controlled by parameters such as pH, temperature and time of precipitation.

Zinc oxide has also been precipitated from aqueous solutions of zinc chloride and zinc acetate [ 35 ]. Controlled parameters in this process included the concentration of the reagents, the rate of addition of substrates, and the reaction temperature. Zinc oxide was produced with a monomodal particle size distribution and high surface area.

A controlled precipitation method was also used by Hong et al. [ 36 ]. The process of precipitating zinc oxide was carried out using zinc acetate (Zn(CH 3 COO) 2 ·H 2 O) and ammonium carbonate (NH 4 ) 2 CO 3 . These solutions were dosed into a vigorously mixed aqueous solution of poly(ethylene glycol) with an average molecular mass of 10,000. The resulting precipitate was calcined by two different methods. In the first, calcination at 450 °C for 3 h produced ZnO labelled as “powder A”. In the second process, calcination took place following heterogeneous azeotropic distillation of the precursor; the resulting zinc oxide was labelled as “powder B”. Structural testing (XRD) and morphological analysis (TEM) showed that powder A contained particles with a diameter of 40 nm, while powder B contained particles with a diameter of 30 nm. Heterogeneous azeotropic distillation completely reduces the occurrence of agglomerates and decreases the ZnO particle size.

Lanje et al. [ 38 ] used the cost competitive and simple precipitation process for the synthesis of zinc oxide. The single step process with the large scale production without unwanted impurities is desirable for the cost-effective preparation of ZnO nanparticles. As a consequence, the low cost precursors such as zinc nitrate and sodium hydroxide to synthesize the ZnO nanoparticles ( ca. 40 nm) were used. In order to reduce the agglomeration among the smaller particles, the starch molecule which contains many O-H functional groups and could bind surface of nanoparticles in initial nucleation stage, was used.

Another process of controlled precipitation of zinc oxide was carried out by Wang et al. [ 39 ]. Nanometric zinc oxide was obtained by precipitation from aqueous solutions of NH 4 HCO 3 and ZnSO 4 ·7H 2 O by way of the following reactions (3) and (4): 5 ZnSO 4 ( aq ) + 10 NH 4 HCO 3 ( aq ) → Zn 5 ( CO 3 ) 2 ( OH ) 6 ( s ) + 5 ( NH 4 ) 2 SO 4 ( aq ) + 8 CO 2 ( g ) + 2 H 2 O ( l ) (3) Zn 5 ( CO 3 ) 2 ( OH ) 6 ( s ) → 5ZnO ( s ) + 2 CO 2 ( g ) + 3 H 2 O ( g ) (4)

This study was performed using a membrane reactor consisting of two plates of polytetrafluoroethylene (PTFE), with stainless steel as a dispersion medium. The ZnO obtained had a narrow range of particle sizes, from 9 to 20 nm. XRD analysis showed both the precursor and the ZnO itself to have a wurtzite structure exclusively. The particle size was affected by temperature, calcination time, flow rate and concentration of the supply phase.

In a report of Jia et al. [ 40 ], in situ crystallization transformation from Zn(OH) 2 to ZnO is demonstrated. Based on observations using X-ray diffraction (XRD) and scanning electron microscopy (SEM), two possible mechanisms from Zn(OH) 2 to ZnO are suggested. The formation mechanism of ZnO was studied in a time-resolved investigation by heating a water solution containing zinc salts (Zn(CH 3 COO) 2 ) and ammonium hydroxide (NH 4 OH) to 85 °C. Transformation of microcrystals of the stable intermediate ε-Z(OH) 2 to ZnO was observed to occur at various aging times. Transformation from ε-Z(OH) 2 to ZnO followed two mechanisms: dissolution−reprecipitation and in situ crystallization transformation involving dehydration and internal atomic rearrangements. From a fundamental point of view, these findings provide new insights into the growth of ZnO crystals and arm researchers with potential strategies for the controllable synthesis of ZnO in liquid media.

In processes of synthesis of nanopowders based on precipitation, it is increasingly common for surfactants to be used to control the growth of particles. The presence of these compounds affects not only nucleation and particle growth, but also coagulation and flocculation of the particles. The surfactant method involves chelation of the metal cations of the precursor by surfactants in an aqueous environment. Wang et al. [ 44 ] obtained nanometric zinc oxide from ZnCl 2 and NH 4 OH in the presence of the cationic surfactant CTAB (cetyltrimethylammonium bromide). The process was carried out at room temperature, and the resulting powder was calcined at 500 °C to remove residues of the surfactant. The product was highly crystalline ZnO with a wurtzite structure and with small, well-dispersed spherical nanoparticles in size of 50 nm. It was found that CTAB affects the process of nucleation and growth of crystallites during synthesis, and also prevents the formation of agglomerates.

Li et al. [ 45 ] synthesized microcrystals of zinc oxide with various shapes (including forms resembling rice grains, nuts and rods) from Zn(NO 3 ) 2 ·6H 2 O and NaOH in the presence of sodium dodecyl sulfate (SDS) and triethanolamine (TEA) as cationic surfactant. The presence of the surfactant was found to affect both the shape and size of the resulting ZnO particles. Li et al. suggested additionally that the transformation may take place via a mechanism of recrystallization. Figure 2 shows the effect of SDS on the structure of the ZnO crystal.

2.2.3. Sol-Gel Method

The obtaining of ZnO nanopowders by the sol-gel method is the subject of much interest, in view of the simplicity, low cost, reliability, repeatability and relatively mild conditions of synthesis, which are such as to enable the surface modification of zinc oxide with selected organic compounds. This changes in properties and extends its range of applications. The favourable optical properties of nanoparticles obtained by the sol-gel method have become a common topic of research, as reflected in numerous scientific publications [ 46 ]. Figure 3 shows two examples of synthesis by the sol-gel method: films from a colloidal sol ( Figure 3a ), and powder from a colloidal sol transformed into a gel ( Figure 3b ).

Benhebal et al. [ 47 ] prepared ZnO powder by sol-gel method from zinc acetate dihydrate, oxalic acid, using ethanol as solvent. The obtained product was characterized by using techniques such as nitrogen adsorption isotherms, X-ray difration (XRD), scanning electron microscopy (SEM), an UV-Vis spectroscopy. The prepared zinc oxide has a hexagonal wurtzite structure with the particles of a spherically shaped. A surface area obtained by the BET method of the calcined ZnO powder is equal to 10 m 2 /g, characteristic of a material with low prosity, or a crystallized material.

The sol-gel method was also used to obtain nanocrystalline zinc oxide by Ristić et al. [ 48 ]. A solution of tetramethylammonium hydroxide (TMAH) was added to a solution of zinc 2-ethylhexanoate (ZEH) in propan-2-ol. The resulting colloidal suspension was left for 30 min (alternatively for 24 h), and was then washed with ethanol and water. TMAH is a strong organic base, which comparably with an inorganic base (e.g., NaOH) is characterized by a pH of ~14. This high pH means that metal oxides are not contaminated with the cation from the base, which may have an effect on the ohmic conductance of the oxide material. A determination was made of the effect of the quantity of ZEH used and the maturing time of the colloidal solution. TEM images showed that the ZnO particles obtained have sizes of the order of 20–50 nm. The quantity of ZEH has a negligible effect on the particle size.

Yue et al. [ 49 ] also obtained ZnO by the sol-gel method. High-filling, unifrom, ordered ZnO nanotubes have been successully prepared by sol-gel method into ultrathin AAO membrane. Integrating the ultrathin AAO membranes with the sol-gel technique may help to fabricate high-quality 1D nanomaterials and to extend its application as a template for nanostructures growth.

2.2.4. Solvothermal and Hydrothermal Method

The hydrothermal method does not require the use of organic solvents or additional processing of the product (grinding and calcination), which makes it a simple and environmentally friendly technique. The synthesis takes place in an autoclave, where the mixture of substrates is heated gradually to a temperature of 100–300 °C and left for several days. As a result of heating followed by cooling, crystal nuclei are formed, which then grow. This process has many advantages, including the possibility of carrying out the synthesis at low temperatures, the diverse shapes and dimensions of the resulting crystals depending on the composition of the starting mixture and the process temperature and pressure, the high degree of crystallinity of the product, and the high purity of the material obtained [ 73 , 74 ].

An example of a hydrothermal reaction is the synthesis of zinc oxide as proposed by Chen et al. [ 50 ], using the reagents ZnCl 2 and NaOH in a ratio of 1:2, in an aqueous environment. The process took place by way of reaction (5):

The white Zn(OH) 2 precipitate underwent filtration and washing, and then the pH was corrected to a value of 5–8 using HCl. In the autoclave hydrothermal heating takes place at a programmed temperature for a set time, followed by cooling. The end product of the process is zinc oxide the following reaction (6):

The average size and the morphology of the resulting ZnO particles were analyzed using an X-ray diffractometer (XRD) and transmission electron microscope (TEM). The temperature and time of reaction were shown to have a significant effect on the structure and size of the ZnO particles. It was also found that as the pH of the solution increases, there is an increase in the crystallinity and size of the particles, which reduces the efficiency of the process.

A hydrothermal process was also used by Ismail et al. [ 51 ], who obtained zinc oxide by way of the following reactions (7) and (8):

The chemical reaction between Zn(CH 3 COO) 2 and NaOH was carried out in the presence of hexamethylenetetramine (HMTA), at room temperature. The resulting precipitate of Zn(OH) 2 was washed with water several times, and then underwent thermal treatment in a Teflon-lined autoclave. Based on SEM images, the authors concluded that the HTMA, as a surfactant, plays an important role in the modification of the ZnO particles. The shape of the particles is also affected by the time and temperature of the hydrothermal process. With an increase in time, temperature and surfactant concentration, the size of the particles increases. Hydrothermal processing of the precursor, followed by drying, produced spherical particles of ZnO with sizes in the range 55–110 nm depending on the conditions of synthesis.

Dem’Yanets et al. [ 52 ] used a hydrothermal method to synthesize nanocrystalline zinc oxide with different particle shapes and sizes. A reaction of zinc acetate or nitrate with a suitable hydroxide (LiOH, KOH, NH 4 OH) produced the precursor Zn(OH) 2 · n H 2 O. The process was carried out in an autoclave, in isothermal conditions or at variable temperature (120–250 °C). Dehydration of the precursor, followed by recrystallization, produced crystallites of ZnO with a hexagonal structure and sizes of 100 nm–20 μm. Increasing the time of the hydrothermal process caused an increase in the diameter of the ZnO particles. It was observed that an increase in temperature by 50–70 °C enabled a fourfold reduction in the time of the experiment, which is a very favourable phenomenon.

Musić et al. [ 53 ] determined the effect of chemical synthesis on the size and properties of ZnO particles. A suspension obtained from a solution of Zn(CH 3 COO) 2 ·2H 2 O and neutralized using different quantities of a solution of NH 4 OH underwent hydrothermal treatment in an autoclave at a temperature of 160 °C. It was found that the pH affected the size and shape of the ZnO particles. Maturing of the original aqueous suspension for 7 months (at a pH of 10, and at room temperature) led to the appearance of aggregates consisting of ZnO particles with sizes between ~20 and ~60 nm. Musić et al. , also synthesized zinc oxideu sing a sol-gel method, involving rapid hydrolysis of zinc 2-ethylhexanoate dissolved in propan-2-ol. The resulting nanoparticles cause distinct changes in the standard Raman spectrum of zinc oxide.

A number of studies [ 54 , 55 , 75 , 76 ] have shown that the use of microwave reactors in hydrothermal synthesis processes brings significant benefits. Microwaves make it possible to heat the solutions from which the synthesis products are obtained, while avoiding loss of energy on heating the entire vessel. Many chemical syntheses proceed with greater speed and yield when microwaves are used than in the case of traditional methods. Similar fast and voluminal heating of the reaction substrates can be achieved using electrical current flowing through the substrates. Strachowski et al. [ 77 , 78 ] carried out a systematic study comparing the ZnO obtained in reactors with different methods of reaction stimulation. The work was conducted in such a way that the reactions being compared took place at the same externally supplied power levels and with the same reaction vessel geometry. Reactors were used with reaction energy supplied using microwaves, electrical current, Joule heating, high-voltage pulses, and heating of the whole autoclave. Strachowski et al. found that nanopowders with phase composition closest to pure ZnO were obtained through microwave synthesis and in the traditional autoclave. The powders produced in the other reactors showed the presence of other phases (simonkolleite and hydroxyzincite) besides zinc oxide. The use of a microwave reactor made it possible to shorten the reaction time several fold, and also produced the purest product.

Microwaves were also used by Schneider et al. [ 56 ]. Zinc oxide was obtained by heating, using microwaves, zinc acetylacetonate and a zinc oxime complex in various alkoxyethanols (methoxy-, ethoxy- and butoxyethanol). Schneider et al. , showed that the morphology and aggregation of ZnO particles depends strongly on the precursor used. The zinc oxide obtained was analyzed using such methods as DLS (dynamic light scattering), BET surface area, SEM, TEM, XRD, TG, PL spectra and EPR (electronparamagnetic resonance). The size of the particles of the final product lay in the range 40–200 nm, depending on which precursor and alcohol were used. The smallest particles belonged to the zinc oxide obtained by heating a complex of zinc oxime in methoxyethanol. With an increase in the concentration and chain length of the alcohol, the particle size increased. The surface area of the ZnO lay in the range 10–70 m 2 /g. Thermal decomposition of both zinc acetylacetonate and zinc oxime enabled the obtaining of a product with the desired properties.

Zhang et al. [ 55 ] obtained ZnO particles in the shape of spheres and hollow spheres through a solvothermal reaction, in the presence of an ionic liquid (imidazolium tetrafluoroborate). The authors suggested that the solvothermal process may involve the following reactions (9) and (10):

The hollow spheres which were obtained had diameters of 2–5 μm and contained channels approximately 10 nm in diameter. The thickness of the wall of such a sphere was approximately 1 μm. The system proposed by Zhang et al. may combine the properties of both a solvothermal hybrid and an ionothermal system. It can be expected that a solvothermal hybrid and an ionothermal system may be successfully used to synthesize new materials with interesting properties and morphologies.

A solvothermal method was also used by Chen et al. [ 54 ], who prepared nanocrystalline ZnO, free of hydroxyl groups. It was obtained from a reaction of zinc powder with trimethylamine N -oxide (Me 3 N→O) and 4-picoline N -oxide (4-pic→NO), carried out in an environment of organic solvents (toluene, ethylenediamine (EDA) and N,N,N′,N′ -tetramethylenediamine (TMEDA)), in an autoclave at 180 °C. The process involved the following reactions (11) and (14):

The oxidizing agents used and the coordinating abilities of the solvents affected the morphology and size of the nanoparticles/nanowires of ZnO. The authors also determined the effect of the presence of water in the reaction system. It was found that the presence of trace quantities of water catalyzed the reaction between zinc powder and 4-picN→O and affected the size of the ZnO nanocrystallites. The zinc oxide obtained had diameters in the range 24–185 nm, depending on the reaction conditions.

2.2.5. Method Using an Emulsion or Microemulsion Environment

The classic definition of an emulsion as a continuous liquid phase in which is dispersed a second, discontinuous, immiscible liquid phase is far from complete. One very convenient way to classify emulsions is first to divide them into two large groups based on the nature of the external phase. The two groups are usually called oil-in-water (O/W) and water-in-oil (W/O) emulsions. The terms “oil” and “water” are very general; almost any highly polar, hydrophilic liquid falls into the “water” category in this definition, while hydrophobic, nonpolar liquids are considered “oils” [ 60 , 61 ].

Vorobyova et al. [ 58 ] used emulsion systems in their work. Zinc oxide was precipitated in an interphase reaction of zinc oleate (dissolved in decane) with sodium hydroxide (dissolved in ethanol or water). The process as a whole involved the reaction (15):

SEM and XRD analysis was performed on the ZnO powders obtained, following removal of the solvents and drying at room temperature. It was found that the reaction may take place in different phases, both in water and in the organic phase. The conditions of the process (temperature, substrates and ratio of two-phase components) affect the size of the particles and the location of their phases. Vorobyova et al. obtained zinc oxide with different particle shapes (irregular aggregates of particles, needle shapes, near-spherical and near-hexagonal shapes, and spherical aggregates) and with diameters in the range: 2–10 μm, 90–600 nm, 100–230 nm and 150 nm respectively, depending on the process conditions.

An emulsion method was also used in the work of Lu and Yeh [ 59 ]. The aqueous phase of the system was zinc acetate dissolved in de-ionized water, and the organic phase was heptane. To stabilize the water-in-oil emulsion, the surfactant Span 80 was added to the heptane. NH 4 OH was added to the emulsion in order to obtain the zinc cation. The precipitate was dried, and then calcined at 700–1000 °C. The resulting ZnO calcinates were analyzed by XRD, IR and SEM. Lu and Yeh concluded that ZnO precipitated in this emulsion system has a smaller range of particle sizes (0.05–0.15 μm) compared with ZnO obtained in a traditional system (0.10–0.45 μm). The product consisted of nearly spherical particles.

Zinc oxide was also obtained by precipitation in an emulsion system with zinc acetate used as a precursor of ZnO, and potassium hydroxide or sodium hydroxide as precipitating agent [ 60 ]. Cyclohexane, as an organic phase, and a non-ionic surfactant mixture were also used for preparation of the emulsion. By applying modifications of the ZnO precipitation process, such as changing the precipitating agent, composition of substrates and the rate of substrate dosing, some interesting structures of ZnO particles were obtained. The morphology of the modified samples was analyzed based on SEM (scanning electron microscope) and TEM (transmission electron microscope) images. Moreover the samples were analyzed by determination of their dispersive properties using the non-invasive back scattering method (NIBS), parameters of porous structure ( BET ) and crystalline structure (XRD). Thermogravimetric analysis (TG) as well as infrared spectrophotometry (FTIR) were also applied. For selected samples their electrical properties (dielectric permittivity and electric conductivity) were also measured. The zinc oxide obtained consisted of particles in the shapes of solids, ellipsoids, rods and flakes ( Figure 4 ), with sizes ranging from 164 to 2670 nm, and was found to have large surface area, with values as high as 20 m 2 /g.

Emulsions and microemulsions differ markedly from each other, which makes it relatively easy to identify the areas of their application. Microemulsions are stable, transparent, isotropic liquids consisting of an aqueous layer, and oil layer and a surfactant. The drop size in a microemulsion is significantly smaller than in an emulsion, and lies in the range 0.0015–0.15 μm [ 79 – 81 ]. In contrast to emulsions, microemulsions form spontaneously in appropriate conditions.

Li et al. [ 61 ] proposed a method of preparing nanometric zinc oxide using a microemulsion which is formed when alcohol is added to an emulsion system consisting of water, oil and emulsifier, until a transparent mixture is obtained. In this case the microemulsion consists of a solution of heptane and hexanol together with a non-ionic surfactant (such as Triton X-100). The growth of nanoparticles involves the exchange of the substrates Zn(NO 3 ) 2 and NaOH between the microemulsion drops and the medium (poly(ethylene glycol)—PEG 400), and aggregation of the formed nuclei. Drops of microemulsion act as a microreactor in which the desired reaction takes place. In the synthesis of ZnO, different concentrations of PEG 400 were used (0%–50%). Figure 5 illustrates the process of synthesis in microemulsion and the shape of ZnO nanoparticles as proposed by the aforementioned authors.

Another process of precipitation of zinc oxide in the environment of a microemulsion was proposed by Singhal et al. [ 62 ]. Zinc oxide was obtained from a microemulsion consisting of ZnO-AOT/ethanol/isooctane (AOT—sodium bis-(2-ethylhexyl)-sulfosuccinate). For converting Na(DEHSS) (sodium diethylhexylsulfosuccinate) into Zn(DEHSS) 2 , an appropriate solvent was used, which dissolves Zn(DEHSS) 2 but does not precipitate NaNO 3 . Appropriate quantities of Na(DEHSS) (dissolved in dry ether) and Zn(NO 3 ) 2 (dissolved in ethanol) were mixed for 4 h at room temperature. It is important that the solvents do not contain water, which might dissolve NaNO 3 during the reaction, causing contamination of the precursor. The resulting solution was filtered and dried. Residues of water in the Zn(DEHSS) 2 were removed by washing the precipitate with benzene. To ensure that the sample did not contain sodium, Zn(NO 3 ) 2 was also added to the solution. At the next stage an anhydrous microemulsion of alcohol in oil was prepared. For this purpose Zn(DEHSS) 2 was dissolved in isooctane, and then dry ethanol was added. To the microemulsion prepared in this way, zinc oxalate was added in excess, in the form of a fine powder, so as to precipitate Zn 2+ ions. The entire solution was mixed for 1 h. The precipitate was separated from the solution by centrifugation, and the precipitate obtained was then washed twice.

The first washing was performed using a methanol:chloroform mixture in a ratio of 1:1 by volume, to remove surfactant and oil. The second used an acetone:methanol mixture (1:1) to remove surfactant and excess oxalic acid. The dried precipitate was calcined at 300 °C for 3 h to produce nanoparticles of ZnO. Singhal et al. precipitated zinc oxide with particles in the range 11–13 μm and with a BET surface area of 82–91 m 2 /g, depending on the conditions applied.

The technique of obtaining ZnO using microemulsion was also used by Yildirim and Durucan [ 63 ]. They attempted to modify the microemulsion method so as to obtain monodisperse zinc oxide. They did not obtain zinc oxide directly from the microemulsion process, but used thermal decomposition of the zinc complex precipitated in the microemulsion process, followed by its calcination. The process was modified in that glycerol was used as the internal phase of a reverse microemulsion (Aeroloz OT:glycerol:heptane), similarly as Moleski et al. [ 64 ] did in preparing ZnO nanoparticles on amorphous silica. The basic aim of the work of Yildirim and Durucan was to determine how the concentration of surfactant and the temperature of calcination affect the size and morphology of the resulting ZnO particles. The final product was analyzed using such techniques as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetry (TGA), differential scanning calorimetry (DSC), and luminescence spectroscopy (PL). The zinc oxide obtained consisted of spherical and monodisperse particles measuring 15–24 nm.

In turn, Xu et al. [ 37 ] compared three methods for precipitating zinc oxide: emulsion, microemulsion, and chemical precipitation. In all of these methods the starting solution used was zinc nitrate (Zn(NO 3 ) 2 ). The emulsion consisted of Zn(NO 3 ) 2 and an appropriate surfactant (cationic, anionic or non-ionic). However the microemulsion was prepared from Zn(NO 3 ) 2 , cyclohexane, acrylonitrile-butadiene-styrene copolymer (ABS), butanol, and hydrogen peroxide (H 2 O 2 ). All processes were carried out in a reactor at a temperature of 25 °C (for the chemical and emulsion processes) or 60 °C (for the microemulsion process). The precipitate was dried at 80 °C for 24 h, and then calcined at 600 °C for 2 h. On comparing the methods, the authors concluded that the smallest ZnO particles were obtained from the process carried out in a microemulsion environment (≤20 nm), larger ones were obtained from the emulsion (20–50 nm), and the largest were obtained by chemical precipitation (>50 nm). The size of the ZnO particles precipitated from the emulsion system depended on the type of surfactant: cationic (40–50 nm), non-ionic (20–50 nm) or anionic (~20 nm). In summing up their work, Xu et al. noted that the size of the ZnO particles depends on the method of precipitation and the type of surfactant used.

2.2.6. Other Methods of Obtaining Zinc Oxide

There also exist many other methods of obtaining zinc oxide, including growing from a gas phase, a pyrolysis spray method, a sonochemical method, synthesis using microwaves, and many others.

Zinc oxide was obtained in the form of pure crystals by Grasza et al. [ 82 ]. Crystals of ZnO were grown from a gas phase (air, nitrogen, atmospheric oxygen, gaseous zinc and arsenic). A wide range of values of heating time and temperature were used. Particular emphasis was placed on analysing the surface during its interactions with the air, oxygen and gaseous zinc. The diversity of the morphology and the purity of the crystal surface were analyzed using AFM and XRD. It was found that thermal heating in the various gases led to similar changes in the crystal surface, although differences were observed in the rate of those changes. Grasza et al. , showed that heating in gaseous zinc leads to a surface roughness of less than 1 nm, while heating in gaseous arsenic causes degradation of the crystal surface. Tests showed that the porosity of the crystal surface increases with increasing temperature and heating time. The “milky” crystal surface obtained is a result of imperfections arising during the preparation of the surface for heating.

A thin layer of zinc oxide was obtained by Wei et al. [ 83 ] in an atmosphere of O 2 , under a pressure of 1.3 Pa, using the pulsed laser deposition method (PLD), with powdered and ceramic ZnO. They determined the effect of temperature on the structural and optical properties of the thin ZnO layer, using techniques such as XRD, SEM, FTIR and PL spectra. The results obtained by Wei et al. indicate that the best structural and optical properties of ZnO are obtained for a thin layer of zinc oxide produced at 700 °C using ZnO powder, and at 400 °C using ceramic ZnO. The PL spectra indicate that UV emission increases with increasing temperature. It was found that the quantity of UV emitted for a thin layer of ZnO made using powder was smaller than in the case of ceramic ZnO.

Using an aerosol pyrolysis method, Zhao et al. [ 65 ] obtained ultrapure particles of ZnO. As a zinc precursor they used Zn(CH 3 COO) 2 ·2H 2 O, in view of its high solubility and low temperature of decomposition. A determination was made of the mechanism and kinetics of the thermal decomposition of zinc acetate dihydrate, as well as a correlation of the mechanism with the results of the aerosol pyrolysis process. Analysis of the DTA and TG curves shows that the water of crystallization is lost below 200 °C, and anhydrous zinc acetate begins to form. At 210–250 °C the anhydrous zinc acetate decomposes into ZnO and organic compounds by way of endothermic and exothermic reactions. The process of decomposition of Zn(CH 3 COO) 2 ·2H 2 O is complete at 400 °C. Zinc oxide synthesized by the aerosol pyrolysis method consists of particles in the range 20–30 nm.

Hu et al. [ 57 ] produced connected rods of ZnO using a sonochemical process (exposure to ultrasound in ambient conditions), and by microwave heating. In the sonochemical method an aqueous solution of Zn(NO 3 ) 2 ·6H 2 O was added to (CH 2 ) 6 N 4 (hexamethylenetetramine, HMT). The resulting solution was exposed to ultrasound for 30 min, as a result of which the reaction temperature reached approximately 80 °C. In the second method, the solution was heated using microwaves at 90 °C for 2 min. The precipitates obtained were centrifuged, washed with water and ethanol, and dried at 60 °C for 2 h. The resulting ZnO was analyzed using XRD, TEM, SAED (selected area electron diffraction), EDS, and PL spectra. These methods of obtaining ZnO enable high yields, in excess of 90%. In summing up their work, Hu et al. stated that the sonochemical and microwave heating methods do not require surfactants, can be applied on a large scale with low production costs, and are simple and energy-efficient. The ZnO rods obtained can be successfully used in electronics and optoelectronics. The sonochemical method may be used in the future for the synthesis of single-dimensional structures of other metal oxides.

3. Methods of Modification of Zinc Oxide

The search for new possible applications of zinc oxide, the need to reduce its content in rubber mixtures, and the major problem of its tendency to form significant agglomerations have encouraged researchers in recent years to carry out numerous studies to find an optimum method of modifying the surface of the compound without impairing its physicochemical properties. Modification is also often carried out in order to improve its performance properties, such as high or low (depending on application) photocatalytic activity. In the following sections we will consider the methods of modification of zinc oxide proposed by various scientists. Figure 6 presents a schematic that summarizes all the method of modifiction of ZnO mentioned in the text.

Cao et al. [ 41 ] performed modification of zinc oxide using silica and trimethyl siloxane (TMS). The finest particles of ZnO were obtained by calcination of the precursor zinc carbonate hydroxide (ZCH). ZHC was obtained in a process of precipitation from substrates such as zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O), ammonium solution (NH 4 OH) and ammonium bicarbonate (NH 4 HCO 3 ). The surface of the ZCH was then successively modified by an in situ method using TEOS and hexamethyldisilazane (HMDS) in water. The ZHC functionalized in this way was calcined, to obtain ultrafine particles of ZnO. Modification of the ZnO particles made possible a solution to the problem of their agglomeration. Functionalization of the ZnO surface with an inorganic compound (silica) reduced the photocatalytic action of the oxide, while the organic compound (HMDS) increased the compatibility of the ZnO with an organic matrix. The highly transparent modified zinc oxide surface was found to provide excellent protection against UV radiation, which represents a significant advantage of the use of these modifying agents. A schematic representation of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method is shown in Figure 7 .

Modification with the use of silica was also performed by Xia and Tang [ 84 ]. By a method of controlled precipitation, clusters of zinc oxide were obtained on the surface of silica modified using triethanolamine N(CH 2 CH 2 OH) 3 (TEOH) and containing silanol (≡Si-OH) and siloxane (≡Si-O-Si≡) groups. Molecules of TEOH are adsorbed by the silica, and the siloxane and silanol networks are broken as a result of the changes occurring in the SiO 2 . The Zn 2+ ions, in reaction with triethanolamine, produce clusters of ZnO on the silica surface. In accordance with the theory of maturing and aggregation, the resulting clusters are susceptible to rapid collision with other clusters of zinc oxide, leading to an appropriate concentration of the compound. An important role in the proposed modification technique is played by TEOH, which enables complex structures to be obtained.

Hong et al. [ 36 ] also performed modification of zinc oxide using silica. They also performed an additional modification using oleic acid. Zinc oxide was obtained as a result of the reaction of zinc acetate with ammonium carbonate, followed by calcination of the resulting zinc precursor. To determine the compatibility between the inorganic nanoparticles and the organic matrix, the surface of the ZnO was covered with oleic acid. The FTIR spectra confirmed the presence on the surface of the modified ZnO of an organic layer and a chemical bond between the inorganic –OH groups and the organic chain macromolecules. The proposed mechanism for these processes was presented in terms of reaction (16):

When SiO 2 was used as a modifier, the FTIR spectrum indicates the presence of an interphase bond between ZnO and SiO 2 . Coverage of the zinc oxide surface with a thin film of amorphous silica improved the degree of dispersion, and thus reduced the agglomeration of nanoparticles. Moreover, based on photocatalytic degradation in aqueous solution using methyl orange, it was shown that silica-coated ZnO has lower catalytic activity than the original nanostructures. The work of Hong et al. showed that heterogeneous azeotropic distillation of the zinc precursor completely reduces the crystalline structure of ZnO, and thus makes it possible to avoid large aggregation and reduces the average particle size. Similar studies have been carried out and published by those authors in [ 85 ].

Hydrophobic ZnO nanoparticles were produced by Chen et al. [ 86 ] and a novel treatment process was developed by them to obtain highly dispersed and long-term stable ZnO nanoparticles, in an organic matrix. Aminopropyltriethoxysilane (APS) was grafted onto the surface of ZnO nanoparticles, and a long carbon chain of stearic acid (SA) was introduced through a condensation reaction between APS and the activated SA with N , N′ -carbonyldiimidazole (CDI). ZnO nanoparticles were analyzed by FTIR, TGA, SEM and a sedimentation test. The FTIR and TGA results showed that APS and SA were linked on the surface of ZnO nanoparticles through chemical bonds, and the CDI activator clearly promotes the condensation reaction and increases the grafting ratio of SA. Results from the SEM observations and sedimentation test indicate that the new surface treatment would considerably reduce aggregates of particles and enhance long-term stability in an organic matrix.

Modification of ZnO using an inorganic compound, namely Al 2 O 3 , was carried out by Yuan et al. [ 87 ]. Nanometric zinc oxide coated with Al 2 O 3 , with diameter 50–80 nm, was obtained by calcination of basic zinc carbonate (BZC) with simultaneous modification with a precipitate of Al(OH) 3 at 400–600 °C. The coating obtained was highly uniform, and had a thickness of 5 nm. The pH at the isoelectric point for ZnO nanoparticles with an Al 2 O 3 layer moved from around 10 to a value of 6, which may improve the dispersion of ZnO particles.

Wysokowski et al. [ 88 ] decided to develop a ZnO-containing composite material using β-chitin from Sepia officinalis cephalopod mollusk as the source of chitin. They suggest that application of morphologically defined β-chitin as a template for biomimetic ZnO deposition is very attractive from a technological point of view as it eliminates challenges associated with manufacturing chitin to chitosan and with processing to membranes or scaffolds.

Pyskło et al. [ 89 ] performed modification of zinc oxide using poly(ethylene glycol) and octadecyltrimethoxysilane, in order to improve its dispersion in rubber mixtures. The modification used zinc oxide synthesized by a hydrothermal method (microwave dehydration). The modification was carried out in the following way: in a solution containing 5% by mass of modifier (PEG or silane) relative to the mass of ZnO used, a ZnO nanopowder was dispersed. The resulting system was then mixed using an ultrasound disintegrator (in the case of silane the system was first mixed with a magnetic mixer, with simultaneous heating). The resulting precipitate was filtered and dried at a temperature of 80 °C for 48 h. Analysis of the FTIR spectra revealed the presence of -OH groups on the surface of the ZnO; for this reason the ZnO surface was modified with octadecyltrimethoxysilane. The modification was carried out as follows: ZnO was dispersed in an emulsion containing 5% by weight of silane. The whole was mixed with a magnetic mixer, with heating, for 5 min. The resulting precipitate was filtered and dried for 48 h at 80 °C. The addition to rubber mixtures of PEG-coated nano zinc oxide an increase in the degree of cross-linking of the vulcanizates, but there was also an increase in vulcanization reversion and a marked decrease in prevulcanization time. The samples of ZnO obtained were additionally analyzed using inverse gas chromatography (IGC), in order to determine the dispersive component of surface energy (γsD). Based on the results it was concluded that coating the surface of nanopowders with polyglycol or silane causes a decrease in the value of γsD (a better effect was obtained when PEG was used). The marked decrease in surface energy in the case of oxides modified with PEG and silane can be expected to facilitate their dispersion in nonpolar rubbers.

Modification of the surface of ZnO particles using silane was also performed by Kotecha et al. [ 90 ]. The modifier used was 3-methacryloxypropyltrimethoxysilane. Nanoparticles of zinc oxide were obtained using zinc acetate and potassium hydroxide as substrates. The precipitate was filtered and washed with methanol, and then dried at 130 °C. In this method the silane was introduced into the system during the precipitation. Concurrently with the formation of ZnO particles, a reaction takes place between silane and ZnO. In the course of this reaction H 2 O is generated and a side reaction takes place, during which the pH increases to 9. The silane-covered zinc oxide particles were introduced into an aqueous suspension and exposed to UV radiation. Based on interpretation of SEM images, the researchers concluded that unmodified zinc oxide contains particles around 100 nm in diameter, forming agglomerates. The introduction of silane into the ZnO structure caused a decrease in the particle size (40–100 nm) and an increase in the diameters of the aggregates, even to the order of micrometres. The irradiated ZnO particles had a fibrous structure “resembling wool”, and offered promising catalytic properties. UV radiation also changes the character of ZnO from hydrophobic to hydrophilic. Analysing the adsorption parameters, Kotecha et al. found that the surface area of silane-modified ZnO initially increases together with the concentration of silane, until that concentration reaches a value of approximately 1–2 mol—then the BET surface area starts to decrease. For the irradiated ZnO samples, the value of BET surface area continues to increase as the silane concentration increases, reaching a maximum of approximately 130 m 2 /g for the highest concentration. The results of Kotecha et al. imply that UV irradiation destroys organic domains. The resulting material has high porosity, large BET surface area, and hydrophilic properties.

Figure 8 shows example mechanisms taking place during the process of modification of zinc oxide using a selected silanol binding compound.

Chang et al. [ 91 ] modified the surface of ZnO using LiCoO 2 . Zinc oxide covered with a layer of LiCoO 2 was obtained by plasma-enhanced chemical vapour deposition (PE-CVD). In their work, Chang et al. confirmed the favourable effect of ZnO on the electrochemical yield and thermal stability of LiCoO 2 , which is used as a cathode material in Li-ion batteries. Covering ZnO with a layer of LiCoO 2 causes an increase in the surface area of the functionalized ZnO (from 0.4 to 1 m 2 /g).

Change of state of the surface of zinc oxide nanowires through plasma treatment is one of the most promising methods of ZnO modification. Experiments carried out by Ra et al. [ 92 ] aimed to determine how reactive chemical treatment using oxygen affects electrical transport, gas selectivity and the internal photoelectric effect of ZnO nanowires with a diameter of 80 nm, using a field-effect transistor (FET). A significant increase in the concentration of oxygen was observed, in the form of active oxygen centres (O 2− and OH − ) on the nanowire surface. After treatment the concentration of the carrier and mobility of the ZnO decreased. There was also an improvement in properties relating to the detection of hydrogen by the modified nanowires, and the time of photocurrent amplification in UV radiation. In summarizing their work, Ra et al. , stated that modification of the surface of ZnO using plasma treatment with oxygen opens up new possibilities for the production of electronic devices, catalysts and high-performance sensors.

In turn, Kang and Park [ 93 ] modified zinc oxide using silver ions. The ZnO was prepared using ultrasonic aerosol pyrolysis (FEAG) of a colloidal solution of zinc acetate. The size of the ZnO particles obtained by the FEAG method depended on the conditions of the operation and the type of solvent. Based on TEM images it was found that the ZnO obtained consisted of particles measuring approximately 12 nm. Next the ZnO was dispersed in a solution of silver nitrate in various ratios. As the ZnO:Ag mass ratio was increased, there was a change in the product’s surface area and particle size. Kang and Park obtained a ZnO-Ag composite with particles measuring approximately 120–250 nm and with a surface area of 3–6 m 2 /g.

Šćepanović et al. [ 94 ] modified ZnO using mechanical activation. A commercial ZnO powder was activated mechanically by grinding in a vibrating mill with steel rings, under continuous air circulation. The process was continued for 30 and 300 min. The product was subjected to comprehensive physicochemical analysis. Based on SEM images, Šćepanović et al. noted that the size of the crystallites of the modified ZnO was smaller, and the surface area was larger, than in the case of the unmodified product (for example, from 190 nm to 106 and 44 nm, and A BET from ca. 3 m 2 /g to 4 and 6 m 2 /g).

Wu et al. [ 95 ] produced ordered ZnO nanofibres by an electrospinning method, and modified the nanofibres using CdS with a nanocrystal layer deposition method. They then investigated the performance of hybrid solar cells based on the CdS/ZnO nanofibres and P3HT (poly(3-hexylthiophene)). The devices were optimized by changing the number of layers of cross-aligned ZnO nanofibres and the growth time of CdS on the ZnO. Wu et al. , found that the power conversion efficiency (PCE) of such a hybrid solar cell was improved by more than 100% after CdS modification. In addition, the lifetime of carriers at the bulk heterojunction was investigated using an impedance analyser and was found to be dramatically increased after CdS modification.

Over the past decade much work has been done on developing nanocomposites produced by the action of modified inorganic carriers with polymer matrices. Such procedures make it possible to produce new classes of polymeric materials which combine properties of both inorganic particles and organic polymer matrices (including process ability and elasticity). The MO/polymer composites produced in this way have unique electrical, thermal and optical properties, which enable their range of applications to be extended in many branches of industry [ 96 – 101 ].

Shim et al. [ 102 ] carried out modification of zinc oxide using poly(methyl methacrylate) (PMMA). A ZnO/PMMA composite was synthesized by means of polymerization in situ . The majority of microspheres of the MO/polymer composite are produced by coupling of existing polymer chains with the inorganic surface or by polymerization on the phase boundary of inorganic particles. Shim et al. , demonstrated that the stability of dispersion of ZnO in a monomer depends strongly on the nature of its surface, since this provides a precondition enabling dispersion of particles of the medium within drops of monomer and consequently their enclosure in PMMA. The most important condition in the production of the composite is the interphase compatibility between the inorganic compound and the polymer. For this purpose the surface of the inorganic system should be treated with a hydrophobic organic substance. The obtained inorganic-polymer composites form persistent microspheres and combine easily into highly processed polymers. Similar studies have been carried out and published by other researchers [ 103 , 104 ].

Tang et al. [ 105 ] modified zinc oxide using poly(methacrylic acid) (PMAA). The hydroxyl groups on the ZnO surface reacted with the carboxyl groups of the PMAA, producing a complex of poly(zinc methacrylate) on the surface of the zinc oxide. Interpreting the particle size distributions, it was found that the ZnO modified with PMAA contains particles with smaller diameter ( ca. 70 nm) compared with unmodified ZnO ( ca. 300 nm). Analysis of the dispersive stability of the ZnO showed that the modified particles of zinc oxide dispersed better in water than unmodified particles. Conventional inorganic nanoparticles have hydroxyl groups (-OH) on their surface, due to the effect of humidity and the environment and type of precipitation. These groups react with COO- groups to form small complexes of poly(zinc methacrylate) on the surface of the zinc oxide. Analysis using the FTIR, TGA, TEM and XRD techniques confirms the presence of polymer molecules on the zinc oxide surface.

Poly(methyl methacrylate) was also used as a ZnO surface modifier by Hong et al. [ 106 ]. Nanoparticles of zinc oxide with a diameter of approximately 30 nm were synthesized by means of homogeneous precipitation followed by calcination. In order to introduce reactive groups onto the ZnO surface, a reaction was carried out between the hydroxyl groups and a silane coupling agent (3-methacryloxypropyltrimethoxysilane). Graft polymerization was effected by means of a reaction between the ZnO, containing silanol groups, and the monomer. Tests showed that the polymerization does not alter the crystalline structure of the ZnO nanoparticles. Their dispersion in the organic solvent can greatly improve the graft polymerization of PMMA, and further improvement can be achieved by the addition of other surfactants. Modification of ZnO nanoparticles by grafted PMMA increases the degree of lyophilicity of the inorganic surface and reduces the formation of aggregates. The work of Hong et al. , showed that ZnO nanoparticles grafted with PMMA can increase the thermal stability of polystyrene.

Polystyrene (PS) is also the subject of interest as a surface modifier of zinc oxide particles. Chae and Kim [ 107 ] carried out a process of ZnO surface modification using that compound. In the process of obtaining a PS/ZnO composite, first an appropriate quantity of commercial ZnO (particle size 87 nm) was dispersed in a solvent with the help of ultrasound for 10 min. The solvent used was N,N -dimethylacetoacetamide (DMAc). Next, in the resulting DMAc/ZnO solution, polystyrene was dissolved, mixing vigorously for 2 h at 70 °C. To obtain a layer of nanocomposite the solutions were kept at a temperature of 90 °C for 4 days. Then the layer was dried (at 100 °C for 5 days) and hot pressed (at 200 °C), completely removing the remaining DMAc. For the structures obtained, the morphology, microstructure, thermal properties and mechanical properties were investigated. Spectral and X-ray identification were also performed, using techniques including TEM, FESEM, DSC, TGA, FTIR and WAXS. The tests confirmed that the solvent used is capable of breaking up the agglomerates that form, and prevents re-agglomeration during mixing of the solution.

An object of interest in recent years has been the resistance connections of ZnO particles embedded in MIM (metal-insulator-metal) structures. Work has focused on altering the layers of oxides, whose amorphous nature, porosity and lack of homogeneity constitute a problem. Researchers under the direction of Verbakel [ 108 ] investigated the resistance effects of the switching of diodes containing structures of nanometric ZnO covered with an active layer from a polystyrene matrix. These diodes consist of two PEDOT:PSS electrodes. Using an impedance spectroscope it was found that the electronic memory effect in nanostructured metal oxides can be affected by modification of the surface of the particles using coordinating ligands (e.g., amines and thiols), and this depends on the temperature of voltage measurements. This process provides new prospects for ecological modification of the surface of ZnO powder using inorganic hybrid materials.

Modification of ZnO nanoparticles using polystyrene was also performed by researchers under the direction of Tang [ 109 ]. Nanometric particles of zinc oxide (particle size ca. 40 nm) were “enclosed” in polystyrene, with a process of emulsion polymerization being carried out in situ in the presence of 3-mercaptopropyltrimethoxysilane (MPTMS) as a coupling agent and polyoxyethylene nonylphenyl ether (OP-10) as surfactant. The nano-ZnO surface had to have a hydrophobic character, in order to hermetically seal the ZnO nanoparticles perfectly in the monomer. This property was controlled by the creation of functional groups on the nano-ZnO surface with the use of a silane coupling agent (MPTMS). Consequently the MPTMS molecules were grafted on the surface of the nano-ZnO. MPTMS is an organic polymer chain which forms steric hindrances between inorganic particles, preventing their aggregation. However it was not simple to obtain perfect dispersion of the hydrophobic nano-ZnO particles in an aqueous polymerization system. To ensure stability of dispersion, a surfactant (OP-10) was added to the system, in a quantity smaller than that which properly saturates the surface, so as to avoid the formation of micelles of emulsifier. Tang et al. , proposed a mechanism for the polymerization ( Figure 9 ). Tests showed that the particles of the resulting polymer composite are monodisperse, with diameters in the range 150–250 nm.

Another modifier applied on the surface of ZnO is polyacrylnitryl (PAN). Studies with that compound were carried out by Chae and Kim [ 110 ]. ZnO nanopowder (particles of diameter 87 nm) was dissolved in DMAc to break up agglomerates. PAN was then added to the solution, and it was mixed vigorously at 70 °C. To obtain the nanocomposite, the PAN-ZnO solution was kept at 80 °C for 4 days, and at the next stage was dried at 100 °C for 5 days. The resulting precipitate underwent spectroscopic, thermal and mechanical analysis. The product exhibited better thermal stability than the starting material, due to the barrier role of ZnO. Moreover the ZnO nanoparticles caused a reduction in the crystallization temperature of the modifier (PAN) and an increase in the width of the crystallization peaks. This is linked to heterogeneous nucleation and the reduced mobility of the polymer chains. The introduction of ZnO nanoparticles into the polymer chain caused an increase in the modulus of elasticity on stretching and a reduction in the dynamic load resistance.

Xiong et al. [ 111 ] synthesized a new nanocomposite ZnO(PEGME), in which the ZnO nanoparticles and polymer groups (PEGME—poly(ethylene glycol) methyl ether) are linked by covalent bonds. The compound was analyzed in terms of composition, structure, fluorescence and specific conductance. The tests showed that the polymer nanocomposite synthesized by means of a chemical reaction has better properties than its equivalent obtained through physical mixing. The lasting stability of the properties of ZnO(PEGME) results from the strong chemical bond between the polymer and the nanoparticles. The hybrid ZnO(PEGME) has the capability of tuning luminescence spectra and has stable ionic conductance. These properties mean that the obtained compound can be used in luminescent devices and in electronic apparatus.

Modification of zinc oxide using carboxylic acids (such as stearic, tartaric, maleic, propanoic etc. ) makes it possible to introduce characteristic groups onto the surface of the ZnO and to alter its physicochemical properties. Studies of zinc oxide modified with carboxylic acids (wet modification) have shown that they do not significantly affect the morphological/dispersive or porous properties of zinc oxide. An apparently promising method is modification in situ , which causes a significant increase in the surface area of the zinc oxide (to as high as ca. 30 m 2 /g). Figure 10 shows an example mechanism taking place during modification of zinc oxide with maleic acid, and an FTIR spectrum confirming the effectiveness of the modification.

It has been experimentally demonstrated that alkanethiols may adsorb on ZnO surface [ 112 – 114 ]. For example, Singh et al. [ 113 ] have investigated adsorption, in ultrahigh vacuum, of methanethiol (MT), 1-dodecanethiol (DDT) and 3-mercaptopropyltrimetoxysilane (MPTMS) on sputter-cleaned ZnO(0001) via either the silane or thiol and of the molecule. They also presented the first ultraviolet photoelectron spectroscopy (UPS) investigation of thiol adsorption on zinc oxide. It was found that the MT frontier orbitals are strongly perturbed by adsorption on ZnO(0001), with the work function of the surface increasing by 0.7 eV. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopies confirmed adsorption, and in situ photoluminescence measurements showed the intensity of the visible emission peak is decreased by methanethiol adsorption. Their other work [ 114 ] demonstrates a previously unreported method of encapsulating zinc oxide nanoparticles and nanorods within an organic matrix consisting of a 1:2 Zn/thiol complex. The thickness and morphology of the encapsulating layer was controllable by the choice of thiol and preparation conditions. Singh et al. , concluded that this method may be useful in future photovoltaic applications in which one wishes to surround ZnO nanorods and whiskers with light-absorbing molecules, which could be achieved by using thiol-terminated dye molecules.

4. Applications of Zinc Oxide

Because of its diverse properties, both chemical and physical, zinc oxide is widely used in many areas. It plays an important role in a very wide range of applications, ranging from tyres to ceramics, from pharmaceuticals to agriculture, and from paints to chemicals. Figure 11 shows worldwide consumption of zinc oxide by region.

In the Figure 12 summarized application paths of ZnO are presented.

4.1. Rubber Industry

Global production of zinc oxide amounts to about 10 5 tons per year, and a major portion is consumed by the rubber industry to manufacture various different cross-linked rubber products [ 115 ]. The thermal conductivity of typical pure silicone rubber is relatively low; however, it can be improved by adding certain thermal conductivity fillers, including metal powders, metal oxides and inorganic particles. Some kinds of thermal conductivity powder, such as Al 2 O 3 , MgO, Al 2 N 3 , SiO 2 , ZnO, etc. , can improve the thermal conductivity of silicone rubber while retaining its high electrical resistance, and are thus promising candidates as high-performance engineering materials. The incorporation of nano-scale fillers can achieve high thermal conductivity even at a relatively low filling content. However, the ZnO nanoparticles tend to aggregate together to form particles of large size in the polymer matrix, due to the weak interaction between the surface of the nanoparticles and the polymer.

In order to solve this problem, surface modification techniques are applied to improve the interaction between the nanoparticles and the polymer. In the work of Yuan et al. [ 116 ], in order to prepare the silicone rubber with high thermal conductivity, pristine and surface-modified ZnO nanoparticles containing the vinyl silane group are incorporated into the silicone rubber via a hydrosilylation reaction during the curing process. The corresponding structure, morphology and properties of the silicone rubber/ZnO (SR/ZnO) and silicone rubber/SiVi@ZnO (SR/SiVi@ZnO) nanocomposites were investigated. Yuan et al. synthesized ZnO nanoparticles (with an average size below 10 nm) by a sol-gel procedure. Next the silicone coupling agent VTES was successfully incorporated onto the surface of the nanoparticles. The SR/SiVi@ZnO nanocomposites showed better mechanical properties and higher thermal conductivity due to the formation of a cross-linking structure with the silicone rubber matrix and better dispersion in that matrix.

Zinc oxide is a very effective and commonly used cross linking agent for carboxylated elastomers [ 117 , 118 ]. It can be used to produce vulcanizates with high tensile strength, tear resistance, hardness and hysteresis. The improved mechanical properties of ionic elastomers mainly result from their high capacity for stress relaxation, due to elastomer chain slippage on the ionic cluster surface and reformation of ionic bonds upon external deformation of the sample. Moreover, ionic elastomers have thermoplastic properties and can be processed in a molten state as a thermoplastic polymer [ 119 ]. However, there are some disadvantages to zinc-oxide-cross linked carboxylic elastomers. The most important are their scorchiness, poor flex properties and high compression set. In order to prevent scorchiness, carboxylated nitrile elastomers are cross linked with zinc peroxide or zinc peroxide/zinc oxide systems. The vulcanization of XNBR with zinc peroxide mainly leads to the formation of ionic crosslinks; covalent links are also formed between elastomer chains due to the peroxide action. However, higher vulcanization times are required to achieve vulcanizates with a tensile strength and crosslink density comparable to that of vulcanizates cross linked with zinc oxide. In the case of XNBR vulcanization with zinc peroxide/zinc oxide systems, curing is the sum of at least three processes: the very fast formation of ionic crosslinks due to the initial zinc oxide present, peroxide cross linking leading to the formation of covalent links (peroxide action), and ionic cross linking due to the production of zinc oxide from peroxide decomposition. The last process, which decays with vulcanization time, most likely involves the formation of ionic species. The achieved vulcanization times are considerably higher than in the case of XNBR cross linking with zinc oxide. Therefore, apart from the scorch problems, zinc oxide is still commonly used as a cross linking agent in carboxylated nitrile rubbers.

In view of the fact that, during the cross linking process, zinc oxide reacts with the carboxylic groups of the elastomer, which leads to the formation of carboxylic salts (ionic crosslinks), the most important parameters influencing the activity of zinc oxide are its surface area, particle size, and morphology. These parameters determine the size of the interphase between the cross linking agent and elastomer chains [ 120 ].

Przybyszewska et al. [ 121 ] used zinc oxides with different surface areas, particle sizes and morphologies (spheres, whiskers, snowflakes) as cross linking agents of carboxylated nitrile elastomer, in order to determine the relationship between the characteristics of zinc oxide and its activity in the cross linking process. They concluded that the use of zinc oxide nanoparticles produced vulcanizates with considerably better mechanical properties and higher crosslink density, as compared with vulcanizates cross linked with micro-sized zinc oxide, which is used commercially as a cross linking agent. Vulcanizates containing the same quantity of zinc oxide nanoparticles exhibited a tensile strength about four times greater than that of vulcanizates with micro-sized particles. Therefore the use of nano-sized zinc oxide enables the quantity of zinc oxide to be reduced by almost 40%. This is a very important ecological goal, since zinc oxide is classified as toxic to aquatic species, and the European Union requires that the amount of zinc oxide in rubber compounds be reduced. Moreover, it should be noted that vulcanizates of carboxylated nitrile elastomer cross linked with zinc oxide demonstrate heat shrinkability.

It is the morphology of zinc oxide particles which mainly affects the activity in the cross linking process. Particle size and surface area do not seem to have a significant influence on the efficiency of zinc oxide as a cross linking agent. The highest activity was observed for zinc oxide with a surface area of about 24 m 2 /g and three-dimensional snowflake particles. The specific shape and complex structure of ZnO aggregates, consisting of wires or plates growing from a single core, provide an increase in the size of the interphase between the elastomer carboxylic groups and the snowflake particle. As a result, Przybyszewska et al. , obtained vulcanizates exhibiting the best mechanical properties, mainly due to the high content of ionic clusters, which create multifunctional and labile crosslinks and can rearrange upon external stress, leading to stress relaxation. Moreover, the zinc oxide nanoparticles used by Przybyszewska et al. , have the lowest propensity for agglomeration in the elastomer matrix and create the smallest agglomerates, which concentrate the stresses during sample deformation to a smaller degree than the large agglomerates formed by other zinc oxides.

As mentioned above, the substantial usage of zinc oxide in rubber products has raised questions about the environmental impact of the rubber industry, particularly when this compound in finally released into the lithosphere during degradation of the rubber, after the end of a product’s life [ 122 ]. Environmental concerns are especially focussed on the effect of excess zinc on aquatic organisms [ 123 ], which has led to various efforts to reduce zinc levels in rubber compounds [ 124 ]. There are three basic methods of reducing the content of ZnO in rubber compounds:

replacing the commonly used zinc oxide of diameter 0.1–0.9 μm and surface area 4–10 m 2 /g with active zinc oxide of nanoscopic granularity and surface area of up to 40 m 2 /g;

modifying the surface of the zinc oxide with carboxylic acids (such as stearic acid, maleic acid and others);

using additional activators [ 69 ].

To find an alternative to conventional ZnO, which in higher dosage is toxic to aquatic systems, Thomas et al. [ 125 ] synthesized the novel accelerators N -benzylimine aminothioformamide(BIAT)-capped-stearic acid-coated nano-ZnO (ZOBS), BIAT-capped ZnO (ZOB), and stearic acid-coated nano zinc phosphate (ZPS), to investigate their effects in NR vulcanization. They studied the effect of these capped compounds on the curing and mechanical properties of natural rubber (NR) vulcanizates. The zinc oxide used in the research was prepared by a sol-gel method, and was then modified using accelerators such as BIAT and fatty acids such as stearic acid. This capping technique reduces agglomeration of nanoparticles of ZnO and is an effective method to improve the curing and physicochemical properties of NR. By capping ZnO with BIAT and stearic acid, it becomes possible to save the extra time and energy required for these particles to diffuse onto the surface of ZnO via the viscoelastic rubber matrix. This provides a further improvement in acceleration of vulcanization and improvement in the physicomechanical properties of the resulting vulcanizates. The mixture containing optimum concentration of BIAT-capped-stearic acid-coated zinc oxide (ZOBS) has superior curing and physicomechanical properties compared with other homologues and the reference mixture containing uncapped ZnO. The increased crosslink density caused by the ZPS particles could increase the stiffness of vulcanizates containing ZPS. The capping technique could improve the scorch safety of rubber compounds by the delayed release of BIAT from the capped ZnO into the rubber matrix for interaction with CBS (conventional accelerator).

Sabura et al. [ 126 ] prepared nano zinc oxide by a solid-state pyrolytic method. Microscopic images and surface area studies showed that the prepared zinc oxide had particle sizes in the range 15–30 nm and surface area in the range 12–30 m 2 /g. The researchers used this nano zinc oxide as a curing agent in neoprene rubber. The optimum dosage of ZnO was found to be low compared with commercial ZnO. The cure characteristic and mechanical properties of the rubber were compared with those containing conventional ZnO. It was found that a low dosage of zinc oxide was enough to give equivalent curing and mechanical properties compared to neoprene rubber containing a higher dosage of commercial zinc oxide.

4.2. The Pharmaceutical and Cosmetic Industries

Due to its antibacterial, disinfecting and drying properties [ 127 , 128 ], zinc oxide is widely used in the production of various kinds of medicines. It was formerly used as an orally administered medicine for epilepsy, and later for diarrhoea. At the present time it is applied locally, usually in the form of ointments and creams, and more rarely in the form of dusting powders and liquid powders. ZnO has properties which accelerate wound healing, and so it is used in dermatological substances against inflammation and itching. In higher concentrations it has a peeling effect. It is also used in suppositories. In addition it is used in dentistry, chiefly as a component of dental pastes, and also for temporary fillings. ZnO is also used in various types of nutritional products and diet supplements, where it serves to provide essential dietary zinc [ 129 ].

For many years, before sun creams began to contain nanoparticles of ZnO or TiO 2 , they contained thick preparations which did not rub easily into the skin and which were cosmetically unattractive. Due to their ability to absorb UVA and UVB radiation, these products began to be used in creams. A new cream formula, containing a combination of ZnO and TiO 2 , solved the problem of an insufficiently white layer and produced a new medium which is more transparent, less adhesive and much more easily rubbed into the skin [ 130 ]. A number of studies have shown that titanium and zinc oxides are extremely good media in sun creams, since they absorb UV radiation, do not irritate the skin, and are easily absorbed into the skin [ 131 – 133 ].

4.3. The Textile Industry

The textile industry offers a vast potential for the commercialization of nanotechnological products. In particular, water repellent and self-cleaning textiles are very promising for military applications, where there is a lack of time for laundering in severe conditions. Also in the world of business, self-cleaning and water repellent textiles are very helpful for preventing unwelcome stains on clothes. Protection of the body from the harmful UV portion of sunlight is another important area. Many scientists have been working on self-cleaning, water repellent and UV-blocking textiles [ 134 – 140 ].

For textile applications, not only is zinc oxide biologically compatible, but also nanostructured ZnO coatings are more air-permeable and efficient as UV-blockers compared with their bulk counterparts [ 141 ]. Therefore, ZnO nanostructures have become very attractive as UV-protective textile coatings. Different methods have been reported for the production of UV-protecting textiles utilizing ZnO nanostructures. For instance, hydrothermally grown ZnO nanoparticles in SiO 2 -coated cotton fabric showed excellent UV-blocking properties [ 142 ]. Synthesis of ZnO nanoparticles elsewhere through a homogeneous phase reaction at high temperatures followed by their deposition on cotton and wool fabrics resulted in significant improvement in UV-absorbing activity [ 143 ]. Similarly, ZnO nanorod arrays that were grown onto a fibrous substrate by a low-temperature growth technique provided excellent UV protection [ 144 ].

Zinc oxide nanowires were grown on cotton fabric by Ates et al. [ 145 ] to impart self-cleaning, superhydrophobicity and ultraviolet (UV) blocking properties. The ZnO nanowires were grown by a microwave-assisted hydrothermal method and subsequently functionalized with stearic acid to obtain a water contact angle of 150°, demonstrating their superhydrophobic nature, which is found to be stable for up to four washings. The UV protection offered by the resulting cotton fabric was also examined, and a significant decrease in transmission of the UV range was observed. The self-cleaning activity of the ZnO nanowire-coated cotton fabric was also studied, and this showed considerable degradation of methylene blue under UV irradiation. These results suggest that ZnO nanowires could serve as ideal multifunctional coatings for textiles.

Research on the use of zinc oxide in polyester fibres has also been carried out at Poznan University of Technology and the Textile Institute in Lodz [ 146 ]. Zinc oxide was obtained by an emulsion method, with particles measuring approximately 350 nm and with a surface area of 8.6 m 2 /g. These results indicate the product’s favourable dispersive/morphological and adsorption properties. Analysis of the microstructure and properties of unmodified textile products and those modified with zinc oxide showed that the modified product could be classed as providing protection against UV radiation and bacteria.

4.4. The Electronics and Electrotechnology Industries

Zinc oxide is a new and important semiconductor which has a range of applications in electronics and electrotechnology [ 147 – 149 ]. Its wide energy band (3.37 eV) and high bond energy (60 meV) [ 150 , 151 ] at room temperature mean that zinc oxide can be used in photoelectronic [ 152 ] and electronic equipment [ 153 ], in devices emitting a surface acoustic wave [ 154 ], in field emitters [ 155 ], in sensors [ 156 – 161 ], in UV lasers [ 162 ], and in solar cells [ 163 ].

ZnO also exhibits the phenomenon of luminescence (chiefly photoluminescence—emission of light under exposure to electromagnetic radiation). Because of this property it is used in FED (field emission display) equipment, such as televisions. It is superior to the conventional materials, sulfur and phosphorus (compounds exhibiting phosphorescence), because it is more resistant to UV rays, and also has higher electrical conductivity. The photoluminescent properties of zinc oxide depend on the size of crystals of the compound, defects in the crystalline structure, and also on temperature [ 164 – 170 ]. ZnO is a semiconductor, and thin films made of that material display high conductivity and excellent permeability by visible rays. These properties mean that it can be used for the production of light-permeable electrodes in solar batteries. It also has potential uses as a transparent electrode in photovoltaic and electroluminescent equipment, and is a promising material for UV-emitting devices [ 171 , 172 ].

Zinc oxide is also used in gas sensors. It is a stable material whose weak selectivity with respect to particular gases can be improved by adding other elements. The working temperature of ZnO is relatively high (400–500 °C), but when nanometric particles are used this can be reduced to around 300 °C. The sensitivity of such devices depends on the porosity and grain size of the material; sensitivity increases as the size of zinc oxide particles decreases. It is most commonly used to detect CO and CO 2 (in mines and in alarm equipment), but can also be used for the detection of other gases (H 2 , SF 6 , C 4 H 10 , C 2 H 5 OH). The zinc oxide used in the production of such equipment is obtained by a variety of methods (chemical vapour deposition, aerosol pyrolysis or oxidation of metallic zinc); it is important to control the process temperature, since this determines the properties of the product [ 173 – 175 ].

One of the most important applications of zinc oxide in electronics is in the production of varistors. These are resistors with a non-linear current-voltage characteristic, where current density increases rapidly when the electrical field reaches a particular defined value. They are used, among other things, as lightning protectors, to protect high-voltage lines, and in electrical equipment providing protection against atmospheric and network voltage surges. These applications require a material of high compactness, since only such a material can guarantee the stability and repeatability of the characteristics of elements made from it [ 176 , 177 ].

Certain unique electronic properties of ZnO are exploited in projection processes. The zinc oxide used for this purpose is produced from metallic zinc (from a suitable ore), so as to obtain a high-purity product. The photoconductor and semiconductor properties of ZnO are improved by thermal treatment, and also by the addition of other elements [ 178 , 179 ].

4.5. Photocatalysis

Intensive scientific work has taken place in recent years on photocatalysis. In this process, an electron-hole pair is produced below the intensity of light by means of oxidation or reduction reactions taking place on the surface of the catalyst. In the presence of a photocatalyst, an organic pollutant can be oxidized directly by means of a photogenerated hole or indirectly via a reaction with characteristic reactive groups (ROS), for example the hydroxyl radical OH·, produced in solution [ 180 – 182 ]. The most commonly used catalysts are TiO 2 and ZnO. TiO 2 exhibits photocatalytic activity below the intensity of UV light [ 183 , 184 ]. ZnO provides similar or superior activity to that of TiO 2 , but is less stable and less sensitive to photocorrosion [ 185 ]. Better stability, however, is provided by zinc oxide of nanometric dimensions, which offers better crystallinity and smaller defects [ 186 ]. The photocatalytic activity of ZnO can be further improved, and the range of the visible spectrum for zinc oxide can be extended, by adding other components [ 187 ].

The photocatalytic properties of zinc oxide, titanium dioxide and ZnO-TiO 2 composite were investigated by Guo et al. [ 188 ]. ZnO was obtained in solution, this being a low-temperature and low-cost method. The properties and photocatalytic applications of the ZnO obtained in this way were studied. A sample was placed on a Petri dish containing an aqueous solution of methyl orange (pH 6.7). While being exposed to UV radiation the solution was mixed and stimulated by sunlight with or without polycarbonate filters. Absorption was measured immediately before exposure to UV and at set time intervals, using a UV/Vis spectrometer. These tests showed that the ZnO nanorods have similar photocatalytic properties (with UV) or slightly better properties (with stimulated sunlight) compared with TiO 2 nanotubes. However, coating the surface of ZnO with a layer of TiO 2 causes deterioration of the photocatalytic properties, possibly due to an increase in the quantity of defects. Summarizing their work, Guo et al. stated that the photocatalytic properties of ZnO can be influenced by coating with various substances and by the thickness of such coating.

Li et al. [ 189 ] also studied the photocatalytic properties of ZnO. ZnO nanospheres were obtained using an electrochemical method, in the presence of POMs (polyoxometalates), at room temperature. The experiments showed that POMs play a very important role in the formation of ZnO nanospheres. The photocatalytic properties of ZnO were determined using the example of degradation of rhodamine (RhB). Based on this study Li et al. , concluded that ZnO displays high photocatalytic activity below the UV range. The proposed simple, single-stage method of synthesis makes it possible to obtain spherical ZnO particles and provides the possibility of controlling their shape.

Ma et al. [ 190 ] demonstrated superior photocatalytic performance on ZnO nanorods and nanoflowers compared with commercial ZnO particles on methyl orange (MO). Besides organic dyes, UV-induced photocatalytic degradation of stearic acid by ZnO nanowires was also reported [ 191 ]. By the incorporation of dopants or formation of a composite with other materials, the photocatalytic properties of ZnO could be enhanced. Xu et al. [ 192 ] demonstrated improved photodegradation of MO by doping with cobalt on hydrothermally grown ZnO powders. One-dimensional heterostructures of ZnO and carbon nanofibres were reported to have significantly enhanced the photodegradation of rhodamine B compared with a pure ZnO counterpart [ 193 ]. It has also been reported that ZnO nanorod films can disinfect E. coli contaminated water with UV illumination [ 194 ].

Other studies by numerous researchers prove that ZnO offers unique photocatalytic properties, making it possible for this oxide to be used as a photocatalyst in the process of degradation of various substances [ 195 – 197 ].

4.6. Miscellaneous Applications

Apart from the applications mentioned above, zinc oxide can also be used in other branches of industry, including for example concrete production. The addition of zinc oxide improves the process time and the resistance of concrete to the action of water. Also, the addition of ZnO to Portland cement slows down hardening and quenching (it reduces the gradual evolution of heat), and also improves the whiteness and final strength of the cement.

Zinc oxide reacts with silicates (e.g., sodium silicate) to produce zinc silicates, which are water- and fire-resistant materials used as binders in paints. These fire-resistant and adhesive substances are used in the binding of cements used in the construction industry.

Methanol, the third most-important chemical product of chemical industry, is produced using a Cu/ZnO/Al 2 O 3 catalyst, with small Cu particles promoted by their interaction with the ZnO substrate as the active component [ 198 ].

ZnO is also used for the production of typographical and offset inks. It imparts good printing properties (high fluidity). The addition of ZnO means that the inks have better covering power, pure shade and high durability, and prevents darkening. Zinc oxide is also used in pigments to produce shine.

It is added to many food products, including breakfast cereals. ZnO is used as a source of zinc, which is an essential nutrient. Thanks to their special chemical and antifungal properties, zinc oxide and its derivatives are also used in the process of producing and packing meat products (e.g., meat and fish) and vegetable products (e.g., sweetcorn and peas) [ 199 ].

As mentioned above, ZnO and its derivatives suppress the development and growth of fungi and moulds. Zinc oxide is added to fungicides to improve their effectiveness. Zinc oxide is also being used increasingly often as an animal feed additive, as it supports the correct growth of animals. It is also used as an artificial fertilizer [ 200 ].

Zinc oxide also has uses in criminology, in mechanical fingerprint analysis. It is also an ingredient in cigarette filters, as it selectively removes certain components from tobacco smoke. Filters are made of charcoal impregnated with ZnO and Fe 2 O 3 , which remove significant quantities of HCN and H 2 S from tobacco smoke without producing a smell. It also removes sulfur and its compounds from various liquids and gases, particularly industrial waste gases. Zinc also removes H 2 S from hydrocarbon gas, and desulfurizes H 2 S and other sulphur components.

ZnO and its derivatives are also used as an additive to car lubricating oils, reducing consumption and oxygen corrosion. Zinc oxide has also been used in various types of lubricants, such as those with EP additives, vibration-resistant lubricants and solid lubricants. In the future, advantage may also be taken of the adhesive properties of ZnO [ 201 ].

Because the compound is nontoxic, cheap, and chemically stable in the air, nanoparticles of zinc oxide can be used to make new eco-friendly substances for cell marking [ 202 ].

Recent advances in electrochemical biosensing based on a wide variety of nanostructures such as ZnO nanowires, nanotubes and nanoporous materials have attracted great interest in biosensor applications due to their remarkable properties such as non-toxicity, bio-safety, excellent biological compatibility, highelectron transfer rates, enhanced analytical performance, increased sensitivity, easy manufacture and low cost [ 203 – 205 ]. Moreover, ZnO has a highisoelectric point ( IEP ) of about 9.5, which can be expected to provide a positively charged substrate for immobilization of low- IEP proteins or enzymes such as uricase ( IEP ~ 4.6) at a physiological pH of 7.4 [ 206 , 207 ]. In addition, ZnO has high ionic bonding (60%), and it dissolves very slowly at biological pH values [ 208 ].

Many researchers have attempted to correlate the biological activity of inorganic antibacterial agents with the size of the constituent particles [ 209 , 210 ]. Inorganic nanocrystalline metal oxides are particularly interesting because they can be prepared with extremely high surface areas, and are more suitable for biological molecular applications [ 211 ]. ZnO semiconductors have been extensively studied as antimicrobial agents due to their photocatalytic activity under UV light [ 212 , 213 ]. These antimicrobial substances based on inorganic chemicals have been found to be effective for therapy [ 214 ]. Padmavathy et al. [ 215 ] showed that ZnO nanoparticles were more abrasive than bulk ZnO (particle sizes in the range 0.1–1 μm), and this contributes to the greater mechanical damage to the cell membrane and the enhanced bactericidal effect produced by ZnO nanoparticles.

5. Conclusions

Zinc oxide is a multifunctional material because of its many interesting properties (piezo- and pyroelectric), a wide range of UV absorption, and high photostability, biocompatibility and biodegradability. ZnO can also be obtained with a variety of particle structures, which determine its use in new materials and potential applications in a wide range of fields of technology. Therefore the development of a method of synthesizing crystalline zinc oxide which can be used on an industrial scale has become a subject of growing interest in science as well as industry.

As can be seen from the survey of recent literature presented here, particles of zinc oxide—both nano- and micrometric—can be produced by many different methods. These can be divided into metallurgical and chemical methods. In metallurgical processes, zinc oxide is obtained by the roasting of a suitable zinc ore, via a direct or indirect process. Chemical methods can be divided into two groups: dispersion methods and condensation methods. In dispersion (mechanochemical) processes, zinc oxide is obtained by the grinding of suitable precursors. The resulting product may contain particles measuring approximately 20 nm. The condensation methods (controlled precipitation, the sol-gel method, hydro- and solvothermal methods, formation in an emulsion or microemulsion environment, and many others) involve the use of a molecularly homogeneous solution subjected to a process of nucleation.

The need to reduce the content of zinc oxide in certain materials, and to limit the degree of agglomeration, has led to the development of various methods of modifying the ZnO surface. Numerous reports in the literature indicate that modification processes can be carried out using inorganic substances (oxides and hydroxides), organic substances (alkoxysilanes, carboxylic acids), and certain polymer matrices, depending on how the systems obtained are to be used. Crystalline oxide powders, combined with other materials, provide possibilities for obtaining improved chemical, mechanical, optical or electrical properties.

Technology and knowledge relating to oxide materials of nano- and micrometric dimensions are currently among the most rapidly developing scientific and technological disciplines. The use of such materials can provide, among other things, more durable ceramics, transparent solar filters blocking infrared and ultraviolet radiation, and catalysts. These materials are also useful in biomedical research and in the diagnosis and treatment of diseases. They can be used to deliver medicines directly to diseased cells, in a way that avoids adverse effects.

The survey of the literature that has been given here shows that zinc oxide can be classed as a multifunctional material. This is thanks to such properties as high chemical stability, low electrical constant, high electrochemical coupling index, wide range of radiation absorption, and high photostability. It can be expected that interest in zinc oxide will continue to grow, and that this will lead to the development of new possibilities for its application.

Acknowledgments

This work was supported by Poznan University of Technology research grant No. 32-443/2014-DS-PB.

Author Contributions

Agnieszka Kołodziejczak-Radzimska and Teofil Jesionowski write the manuscript together.

Conflicts of Interest

The authors declare no conflict of interest.

• Segets, D.; Gradl, J.; Taylor, R.K.; Vassilev, V.; Peukert, W. Analysis of optical absorbance spectra for the determination of ZnO nanoparticle size distribution, solubility, and surface energy. ACS Nano 2009 , 3 , 1703–1710. [ Google Scholar ]
• Lou, X. Development of ZnO series ceramic semiconductor gas sensors. J. Sens. Trans. Technol 1991 , 3 , 1–5. [ Google Scholar ]
• Bacaksiz, E.; Parlak, M.; Tomakin, M.; Özcelik, A.; Karakiz, M.; Altunbas, M. The effect of zinc nitrate, zinc acetate and zinc chloride precursors on investigation of structural and optical properties of ZnO thin films. J. Alloy. Compd 2008 , 466 , 447–450. [ Google Scholar ]
• Wang, J.; Cao, J.; Fang, B.; Lu, P.; Deng, S.; Wang, H. Synthesis and characterization of multipod, flower-like, and shuttle-like ZnO frameworks in ionic liquids. Mater. Lett 2005 , 59 , 1405–1408. [ Google Scholar ]
• Wang, Z.L. Splendid one-dimensional nanostructures of zinc oxide: A new nanomaterial family for nanotechnology. ACS Nano 2008 , 2 , 1987–1992. [ Google Scholar ]
• Chaari, M.; Matoussi, A. Electrical conduction and dielectric studies of ZnO pellets. Phys. B Condens. Matter 2012 , 407 , 3441–3447. [ Google Scholar ]
• Özgür, Ü.; Alivov, Y.I.; Liu, C.; Teke, A.; Reshchikov, M.A.; Dođan, S.; Avrutin, V.; Cho, S.J.; Morkoç, H. A comprehensive review of ZnO materials and devices. J. Appl. Phys 2005 , 98 . [ Google Scholar ] [ CrossRef ]
• Bhattacharyya, S.; Gedanken, A. A template-free, sonochemical route to porous ZnO nano-disks. Microporous Mesoporous Mater 2007 , 110 , 553–559. [ Google Scholar ]
• Ludi, B.; Niederberger, M. Zinc oxide nanoparticles: Chemical mechanism and classical and non-classical crystallization. Dalton Trans 2013 , 42 , 12554–12568. [ Google Scholar ]
• Banerjee, D.; Lao, J.Y.; Wang, D.Z.; Huang, J.Y.; Ren, Z.F.; Steeves, D.; Kimball, B.; Sennett, M. Large-quantity free-standing ZnO nanowires. Appl. Phys. Lett 2003 , 83 , 2061–2063. [ Google Scholar ]
• Hahn, Y.B. Zinc oxide nanostructures and their applications. Korean J. Chem. Eng 2011 , 28 , 1797–1813. [ Google Scholar ]
• Frade, T.; Melo Jorge, M.E.; Gomes, A. One-dimensional ZnO nanostructured films: Effect of oxide nanoparticles. Mater. Lett 2012 , 82 , 13–15. [ Google Scholar ]
• Wahab, R.; Ansari, S.G.; Kim, Y.S.; Seo, H.K.; Shin, H.S. Room temperature synthesis of needle-shaped ZnO nanorods via sonochemical method. Appl. Surf. Sci 2007 , 253 , 7622–7626. [ Google Scholar ]
• Kong, X.; Ding, Y.; Yang, R.; Wang, Z.L. Single-crystal nanorings formed by epitaxial self-coiling of polar-nanobelts. Science 2004 , 303 , 1348–1351. [ Google Scholar ]
• Pan, Z.W.; Dai, Z.R.; Wang, Z.L. Nanobelts of semiconducting oxides. Science 2001 , 291 , 1947–1949. [ Google Scholar ]
• Wu, J.J.; Liu, S.C.; Wu, C.T.; Chen, K.H.; Chenm, L.C. Heterostructures of ZnO–Zn coaxial nanocables and ZnO nanotubes. Appl. Phys. Lett 2002 , 81 , 1312–1314. [ Google Scholar ]
• Chen, W.J.; Liu, W.L.; Hsieh, S.H.; Tsai, T.K. Preparation of nanosized ZnO using α brass. Appl. Surf. Sci 2007 , 253 , 6749–6753. [ Google Scholar ]
• Liu, J.; Huang, X.; Duan, J.; Ai, H.; Tu, P. A low-temperature synthesis of multiwhisker-based zinc oxide micron crystals. Mater. Lett 2005 , 59 , 3710–3714. [ Google Scholar ]
• Huang, Y.; He, J.; Zhang, Y.; Dai, Y.; Gu, Y.; Wang, S.; Zhou, C. Morphology, structures and properties of ZnO nanobelts fabricated by Zn-powder evaporation without catalyst at lower temperature. J. Mater. Sci 2006 , 41 , 3057–3062. [ Google Scholar ]
• Nikoobakht, B.; Wang, X.; Herzing, A.; Shi, J. Scable synthesis and device integration of self-registered one-dimensional zinc oxide nanostructures and related materials. Chem. Soc. Rev 2013 , 42 , 342–365. [ Google Scholar ]
• Tien, L.C.; Pearton, S.J.; Norton, D.P.; Ren, F. Synthesis and microstructure of vertically aligned ZnO nanowires grown by high-pressure-assisted pulsed-laser deposition. J. Mater. Sci 2008 , 43 , 6925–6932. [ Google Scholar ]
• Cui, J. Zinc oxide naniwires. Mater. Charact 2012 , 64 , 43–52. [ Google Scholar ]
• Xu, T.; Ji, P.; He, M.; Li, J. Growth and structure of pure ZnO micro/nanocombs. J. Nanomater 2012 , 2012 . [ Google Scholar ] [ CrossRef ]
• Chiua, W.S.; Khiew, P.S.; Clokea, M.; Isaa, D.; Tana, T.K.; Radimanb, S.; Abd-Shukorb, R.; Abd–Hamid, M.A.; Huangc, N.M.; Limd, H.N.; et al . Photocatalytic study of two-dimensional ZnO nanopellets in the decomposition of methylene blue. Chem. Eng. J 2010 , 158 , 345–352. [ Google Scholar ]
• Jose-Yacaman, M.; Gutierrez-Wing, C.; Miki, M.; Yang, D.Q.; Piyakis, K.N.; Sacher, E. Surface diffusion and coalescence of mobile metal nanoparticles. J. Phys. Chem. B 2005 , 109 , 9703–9711. [ Google Scholar ]
• Polshettiwar, V.; Baruwati, B.; Varma, R.S. Self-asssembly of metal oxides into three-dimensional nanostructures: Synthesis and application in catalysis. ACS Nano 2009 , 3 , 728–736. [ Google Scholar ]
• Xie, Q.; Dai, Z.; Liang, J.; Xu, L.; Yu, W.; Qian, Y. Synthesis of ZnO three-dimensional architectures and their optical properties. Solid State Commun 2005 , 136 , 304–307. [ Google Scholar ]
• Liu, J.; Huang, X.; Li, Y.; Sulieman, K.M.; Sun, F.; He, X. Selective growth and properties of zinc oxide nanostructures. Scr. Mater 2006 , 55 , 795–798. [ Google Scholar ]
• Bitenc, M.; Orel, Z.C. Synthesis and characterization of crystalline hexagonal bipods of zinc oxide. Mater. Res. Bull 2009 , 44 , 381–387. [ Google Scholar ]
• Ao, W.; Li, J.; Yang, H.; Zeng, X.; Ma, X. Mechanochemical synthesis of zinc oxide nanocrystalline. Powder Technol 2006 , 168 , 128–151. [ Google Scholar ]
• Stanković, A.; Veselinović, L.J.; Skapin, S.D.; Marković, S.; Uskoković, D. Controlled mechanochemically assisted synthesis of ZnO nanopowders in the presence of oxalic acid. J. Mater. Sci 2011 , 46 , 3716–3724. [ Google Scholar ]
• Tsuzuki, T.; McCormick, P.G. ZnO nanoparticles synthesis by mechanochemical processing. Scr. Mater 2001 , 44 , 1731–1734. [ Google Scholar ]
• Moballegh, A.; Shahverdi, H.R.; Aghababazadeh, R.; Mirhabibi, A.R. ZnO nanoparticles obtained by mechanochemical technique and optical properties. Surf. Sci 2007 , 601 , 2850–2854. [ Google Scholar ]
• Aghababazadeh, R.; Mazinani, B.; Mirhabibi, A.; Tamizifar, M. ZnO nanoparticles by mechanochemical processing. J. Phys. Chem. Solids 2006 , 26 , 312–314. [ Google Scholar ]
• Kołodziejczak-Radzimska, A.; Jesionowski, T.; Krysztafkiewicz, A. Obtaining zinc oxide from aqueous solutions of KOH and Zn(CH 3 COO) 2 . Physicochem. Probl. Miner. Process 2010 , 44 , 93–102. [ Google Scholar ]
• Hong, R.; Pan, T.; Qian, J.; Li, H. Synthesis and surface modification of ZnO nanoparticles. Chem. Eng. J 2006 , 119 , 71–81. [ Google Scholar ]
• Xu, J.; Pan, Q.; Shun, Y.; Tian, Z. Grain size control and gas sensing properties of ZnO gas sensor. Sens. Actuators B Chem 2000 , 66 , 277–279. [ Google Scholar ]
• Lanje, A.S.; Sharma, S.J.; Ningthoujam, R.S.; Ahn, J.S.; Pode, R.B. Low temperature dielectric studies of zinc oxide (ZnO) nanoparticles prepared by precipitation method. Adv. Powder Technol 2013 , 24 , 331–335. [ Google Scholar ]
• Wang, Y.; Zhang, C.; Bi, S.; Luo, G. Preparation of ZnO nanoparticles using the direct precipitation method in a membrane dispersion micro-structured reactor. Powder Technol 2010 , 202 , 130–136. [ Google Scholar ]
• Jia, W.; Dang, S.; Liu, H.; Zhang, Z.; Yu, Ch.; Liu, X.; Xu, B. Evidence of the formation mechanism of ZnO in aqueous solution. Mater. Lett 2012 , 82 , 99–101. [ Google Scholar ]
• Cao, Z.; Zhang, Z.; Wang, F.; Wang, G. Synthesis and UV shielding properties of zinc oxide ultrafine particles modified with silica and trimethyl siloxane. Colloids Surf. A Physicochem. Eng. Asp 2009 , 340 , 161–167. [ Google Scholar ]
• Khoshhesab, Z.M.; Sarfaraz, M.; Houshyar, Z. Influence of urea on precipitation of zinc oxide nanostructures through chemical precipitation in ammonium hydrogencarbonate solution. Synth. React. Inorg. Met. Org. Nano Met. Chem 2012 , 42 , 1363–1368. [ Google Scholar ]
• Kumra, K.M.; Mandal, B.K.; Naidu, E.A.; Sinha, M.; Kumar, K.S.; Reddy, P.S. Synthesis and characterization of flower shaped zinc oxide nanostructures and its antimicrobial activity. Spectrochim. Acta Part A Mol. Biomol. Spectrosc 2013 , 104 , 171–174. [ Google Scholar ]
• Wang, Y.; Ma, C.; Sun, X.; Li, H. Preparation of nanocrystalline metal oxide powders with the surfactant-mediated method. Inorg. Chem. Commun 2002 , 5 , 751–755. [ Google Scholar ]
• Li, P.; Wei, Y.; Liu, H.; Wang, X.K. Growth of well-defined ZnO microparticles with additives from aqueous solution. J. Solid State Chem 2005 , 178 , 855–860. [ Google Scholar ]
• Mahato, T.H.; Prasad, G.K.; Acharya, B.S.J.; Srivastava, A.R.; Vijayaraghavan, R. Nanocrystalline zinc oxide for the decontamination of sarin. J. Hazard. Mater 2009 , 165 , 928–932. [ Google Scholar ]
• Benhebal, H.; Chaib, M.; Salomon, T.; Geens, J.; Leonard, A.; Lambert, S.D.; Crine, M.; Heinrichs, B. Photocatalytic degradation of phenol and benzoic acid using zinc oxide powders prepared by sol-gel process. Alex. Eng. J 2013 , 52 , 517–523. [ Google Scholar ]
• Ristić, M.; Musić, S.; Ivanda, M.; Popović, S. Sol–gel synthesis and characterization of nanocrystalline ZnO powders. J. Alloy. Compd 2005 , 39 , L1–L4. [ Google Scholar ]
• Yue, S.; Yan, Z.; Shi, Y.; Ran, G. Synthesis of zinc oxide nanotubes within ultrathin anodic aluminum oxide membrane by sol-gel method. Mater. Lett 2013 , 98 , 246–249. [ Google Scholar ]
• Chen, D.; Ciao, X.; Cheng, G. Hydrothermal synthesis of zinc oxide powders with different morphologies. Solid State Commun 2000 , 113 , 363–366. [ Google Scholar ]
• Ismail, A.A.; El-Midany, A.; Abdel-Aal, E.A.; El-Shall, H. Application of statistical design to optimize the preparation of ZnO nanoparticles via hydrothermal technique. Mater. Lett 2005 , 59 , 1924–1928. [ Google Scholar ]
• Dem’Yanets, L.N.; Li, L.E.; Uvarova, T.G. Zinc oxide: Hydrothermal growth of nano- and bulk crystals and their luminescent properties. J. Mater. Sci 2006 , 41 , 1439–1444. [ Google Scholar ]
• Musić, S.; Dragčević, D.; Popović, S.; Ivanda, M. Precipitation of ZnO particles and their properties. Mater. Lett 2005 , 59 , 2388–2393. [ Google Scholar ]
• Chen, S.J.; Li, L.H.; Chen, X.T.; Xue, Z.; Hong, J.M.; You, X.Z. Preparation and characterization of nanocrytalline zinc oxide by a novel solvothermal oxidation route. J. Cryst. Growth 2003 , 252 , 184–189. [ Google Scholar ]
• Zhang, J.; Wang, J.; Zhou, S.; Duan, K.; Feng, B.; Weng, J.; Tang, H.; Wu, P. Ionic liquid-controlled synthesis of ZnO microspheres. J. Mater. Chem 2010 , 20 , 9798–9804. [ Google Scholar ]
• Schneider, J.J.; Hoffmann, R.C.; Engstler, J.; Klyszcz, A.; Erdem, E.; Jakes, P.; Eichel, R.A.; Pitta-Bauermann, L.; Bill, J. Synthesis, characterization, defect chemistry, and FET properties of microwave-derived nanoscaled zinc oxide. Chem. Mater 2010 , 22 , 2203–2212. [ Google Scholar ]
• Hu, X.L.; Zhu, Y.J.; Wang, S.W. Sonochemical and microwave-assisted synthesis of linked single-crystalline ZnO rods. Mater. Chem. Phys 2004 , 88 , 421–426. [ Google Scholar ]
• Vorobyova, S.A.; Lesnikovich, A.I.; Mushinski, V.V. Interphase synthesis and characterization of zinc oxide. Mater. Lett 2004 , 58 , 863–866. [ Google Scholar ]
• Lu, C.H.; Yeh, C.H. Emulsion precipitation of submicron zinc oxide powder. Mater. Lett 1997 , 33 , 129–132. [ Google Scholar ]
• Kołodziejczak-Radzimska, A.; Markiewicz, E.; Jesionowski, T. Structural characterization of ZnO particles obtained by the emulsion precipitation method. J. Nanomater 2012 , 2012 . [ Google Scholar ] [ CrossRef ]
• Li, X.; He, G.; Xiao, G.; Liu, H.; Wang, M. Synthesis and morphology control of ZnO nanostructures in microemulsions. J. Colloid Interface Sci 2009 , 333 , 465–473. [ Google Scholar ]
• Singhal, M.; Chhabra, V.; Kang, P.; Shah, D.O. Synthesis of ZnO nanoparticles for varistor application using Zn-substituted Aerosol OT microemulsion. Mater. Res. Bull 1997 , 32 , 239–247. [ Google Scholar ]
• Yildirim, Ö.A.; Durucan, C. Synthesis of zinc oxide nanoparticles elaborated by microemulsion method. J. Alloy. Compd 2010 , 506 , 944–949. [ Google Scholar ]
• Moleski, R.; Leontidis, E.; Krumeich, F. Controlled production of ZnO nanoparticles from zinc glycerolate in a sol-gel silica matrix. J. Colloid Interface Sci 2006 , 302 , 246–253. [ Google Scholar ]
• Zhao, X.; Zheng, B.; Li, C.; Gu, H. Acetate-derived ZnO ultrafine particles synthesized by spray pyrolysis. Powder Technol 1998 , 100 , 20–23. [ Google Scholar ]
• Petzold, F.G.; Jasinski, J.; Clark, E.L.; Kim, J.H.; Absher, J.; Toufar, H.; Sunkara, M.K. Nickel supported on zinc oxide nanowires as advanced hydrodesulfurization catalyst. Catal. Today 2012 , 198 , 219–227. [ Google Scholar ]
• Akgul, F.A.; Attenkofer, K.; Winterer, M. Structural properties of zinc oxide and titanium dioxide nanoparticles prepared by chemical vapor synthesis. J. Alloy. Compd 2013 , 554 , 177–181. [ Google Scholar ]
• International Organization for Standardization (ISO), Rubber Compounding Ingredients—Zinc Oxide—Test Methods ; ISO 9298:1995; ISO: Geneva, Switzerland, 2010.
• Pyskło, L.; Parasiewicz, W.; Pawłowski, P.; Niciński, K. Zinc Oxide in Rubber Compounds ; Instytut Przemyslu Gumowego: Piastow, Poland, 2007. [ Google Scholar ]
• Mahmud, S.; Abdullah, M.J.; Putrus, G.A.; Chong, J.; Mohamad, A.K. Nanostructure of ZnO fabricated via French process and its correlation to electrical properties of semiconducting varistors. Synth. React. Inorg. Met. Org. Nano Met. Chem 2003 , 36 , 155–159. [ Google Scholar ]
• Tsuzuki, T.; McCormick, P.G. Mechanochemical synthesis of nanoparticles. J. Mater. Sci 2004 , 39 , 5143–5146. [ Google Scholar ]
• Znaidi, L. Sol-gel-deposited ZnO thin films: A review. Mater. Sci. Eng 2010 , 174 , 18–30. [ Google Scholar ]
• Djurišcić, A.B.; Chen, X.Y.; Lung, Y.H. Recent progress in hydrothermal synthesis of zinc oxide nanomaterials. Recent Pat. Nanotechnol 2012 , 6 , 124–134. [ Google Scholar ]
• Tsuzuki, T.; Dawkins, H.; Dunlop, J.; Trotter, G.; Nearn, M.; McCormick, P.G. Nanotechnology and cosmetic chemist. Cosmet. Aeorosol Toilet. Aust 2002 , 15 , 10–24. [ Google Scholar ]
• Ma, J.; Liu, J.; Bao, Y.; Zhu, Z.; Wang, X.; Zhang, J. Synthesis of large-scale uniform mulberry-like ZnO particles with microwave hydrothermal method and its antibacterial property. Ceram. Int 2013 , 39 , 2803–2810. [ Google Scholar ]
• Bondioli, F.; Ferrari, A.M.; Braccini, S.; Leonelli, C.; Pellacani, G.C.; Opalinska, A.; Chudoba, T.; Grzanka, E.; Palosz, B.; Łojkowski, W. Microwave hydrothermal synthesis of nanocrystalline pre-doped zirconia powders at pressures up to 8 MPa. Solid State Phenom 2003 , 94 , 193–196. [ Google Scholar ]
• Strachowski, T.; Grzanka, E.; Palosz, B.; Presz, A.; Œlusarski, L.; Łojkowski, W. Microwave driven hydrothermal synthesis of zinc oxide nanopowders. Solid State Phenom 2003 , 94 , 189–192. [ Google Scholar ]
• Choduba, T.; Łojkowski, W.; Reszke, E.; Strachowski, T. Way of Conducting of Synthesis and Chemical Electrode Reactor, US3450617 A, 28 May 2009.
• Lissan, K.J. Emulsion and Emulsion Technology ; Marecel Dekker Inc: New York, NY, USA, 1974. [ Google Scholar ]
• Kumar, R.; Kumar, M.S.; Mohadevan, N. Multiple emulsions: A review. Int. J. Recent Adv. Pharm. Res 2012 , 2 , 9–19. [ Google Scholar ]
• Paul, B.K.; Moulik, S.P. Uses and applications of microemulsions. Curr. Sci 2001 , 80 , 990–1001. [ Google Scholar ]
• Grasza, K.; Łusakowska, E.; Skupiński, P.; Kopałko, K.; Bąk-Misiuk, J.; Mycelski, A. Effect of annealing atmosphere on the quality of ZnO crystal surface. Phys. Status Solidi 2007 , 244 , 1468–1472. [ Google Scholar ]
• Wei, X.Q.; Zhang, Z.; Yu, Y.X.; Man, B.Y. Comparative study on structural and optical properties of ZnO thin films prepared by PLD using ZnO powder target and ceramic target. Opt. Laser Technol 2009 , 41 , 530–534. [ Google Scholar ]
• Xia, H.L.; Tang, F.Q. Surface synthesis of zinc oxide nanoparticles on silica spheres: Preparation and characterization. J. Phys. Chem. B 2003 , 107 , 9175–9178. [ Google Scholar ]
• Hong, R.; Qian, J.; Miao, C.; Li, H. Synthesis and surface modification of ZnO nanoparticles. Spec. Petrochem 2005 , 2 , 1–4. [ Google Scholar ]
• Chen, H.; Guo, Z.; Jia, L. Preparation and surface modification of highly dispersed nano-ZnO with stearic acid activated by N,N′-carbonyldiimidazole. Mater. Lett 2012 , 82 , 167–170. [ Google Scholar ]
• Yuan, F.; Peng, H.; Yin, Y.; Chunlei, Y.; Ryu, H. Preparation of zinc oxide nanoparticles coated with homogeneous Al 2 O 3 layer. Mater. Sci. Eng. B 2005 , 122 , 55–60. [ Google Scholar ]
• Wysokowski, M.; Motylenko, M.; Stöcker, H.; Bazhenov, V.V.; Langer, E.; Dobrowolska, A.; Czaczyk, K.; Galli, R.; Stelling, A.L.; Behm, T.; et al . An extreme biomimetic approach: Hydrothermal synthesis of β-chitin/ZnO nanostructured composites. J. Mater. Chem. B 2013 , 1 , 6469–6476. [ Google Scholar ]
• Pyskło, L.; Niciński, K.; Piaskiewicz, M.; Bereza, M.; Łojkowski, W. Synthesis of zinc oxide with nanometric particle size, its characteristics and influence on the properties of rubber compounds. Elastomery 2007 , 11 , 10–19. [ Google Scholar ]
• Kotecha, M.; Veeman, W.; Rohe, B.; Tausch, M. NMR investigations of silane-coated nano-sized ZnO particles. Microporous Mesoporous Mater 2006 , 95 , 66–75. [ Google Scholar ]
• Chang, W.; Choi, J.W.; Im, J.C.; Lee, J.K. Effects of ZnO coating on electrochemical performance and thermal stability of LiCoO 2 as cathode material for lithium-ion batteries. J. Power Sources 2010 , 195 , 320–326. [ Google Scholar ]
• Ra, H.W.; Khan, R.; Kim, J.T.; Kang, B.R.; Bai, K.H.; Im, Y.H. Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment. Mater. Lett 2009 , 63 , 2516–2519. [ Google Scholar ]
• Kang, Y.C.; Park, S.B. Preparation of zinc oxide-dispersed silver particles by spray pyrolysis of colloidal solution. Mater. Lett 1999 , 40 , 129–133. [ Google Scholar ]
• Šćepanović, M.; Srećković, T.; Vojisavljević, K.; Ristić, M.M. Modification of the structural and optical properties of commercial ZnO powder by mechanical activation. Sci. Sinter 2006 , 38 , 169–175. [ Google Scholar ]
• Wu, S.; Li, J.; Lo, S.C.; Tai, Q.; Yan, F. Enhanced performance of hybrid solar cells based on ordered electrospun ZnO nanofibers modified with CdS on the surface. Org. Electron 2012 , 13 , 1569–1575. [ Google Scholar ]
• Zou, H.; Wu, S.; Shen, J. Polymer/silica nanocomposites: Preparation, characterization, properties, and applications. Chem. Rev 2008 , 108 , 3893–3957. [ Google Scholar ]
• Amalvy, J.I.; Percy, M.J.; Armes, S.P. Characterization of the nanomorphology of polymer−silica colloidal nanocomposites using electron spectroscopy imaging. Langmuir 2005 , 21 , 1175–1179. [ Google Scholar ]
• Du, X.W.; Fu, Y.S.; Sun, J.; Han, X.; Liu, J. Complete UV emission of ZnO nanoparticles in a PMMA matrix. Semicond. Sci. Technol 2006 , 21 , 1202–1206. [ Google Scholar ]
• Ashraf, M.; Campagne, C.; Prewuelz, A.; Champagne, A.; Leriche, A.; Courtois, C. Development of superhydrophilic and superhydrophobic polyester fabric by growing zinc oxide nanorods. J. Colloid Interface Sci 2013 , 393 , 545–553. [ Google Scholar ]
• Sivakumar, K.; Senthil Kumar, V.; Haldorai, Y. Zinc oxide nanoparticles reinforced conducting poly(aniline-co-p-phenylenediamine) nanocomposite. Compos. Interfaces 2012 , 19 , 397–409. [ Google Scholar ]
• Zhang, J.; Gao, G.; Liu, F. Preparation of zinc oxide nanocrystals with high stability in the aqueous phase. J. Appl. Polym. Sci 2013 , 128 , 2162–2166. [ Google Scholar ]
• Shim, J.W.; Kim, J.W.; Han, S.H.; Chang, I.S.; Kim, H.K.; Kang, H.H.; Lee, O.S.; Suh, K.D. Zinc oxide/polymethylmethacrylate polymerization and their morphological study. Colloids Surf. A Physicochem. Eng. Asp 2002 , 207 , 105–111. [ Google Scholar ]
• Tang, E.; Cheng, G.; Pang, X.; Ma, X.; Xing, F. Synthesis of nano-ZnO/poly(methyl methacrylate) composite microsphere through emulsion polymerization and its UV-shielding property. Colloid Polym. Sci 2006 , 284 , 422–428. [ Google Scholar ]
• Tang, E.; Cheng, G.; Ma, X. Preparation of nano-ZnO/PMMA composite particles via grafting of the copolymer onto the surface of zinc oxide nanoparticles. Powder Technol 2006 , 161 , 209–214. [ Google Scholar ]
• Tang, E.; Cheng, G.; Ma, X.; Pang, X.; Zhao, Q. Surface modification of zinc oxide nanoparticle by PMAA and its dispersion in aqueous system. Appl. Surf. Sci 2006 , 252 , 5227–5232. [ Google Scholar ]
• Hong, R.Y.; Qian, J.Z.; Cao, J.X. Synthesis and characterization of PMMA grafted ZnO nanoparticles. Powder Technol 2006 , 163 , 160–168. [ Google Scholar ]
• Chae, D.W.; Kim, B.C. Characterization on polystyrene/zinc oxide nanocomposites prepared from solution mixing. Polym. Adv. Technol 2005 , 16 , 846–850. [ Google Scholar ]
• Verbakel, F.; Meskers, S.C.J.; Janssen, R.A.J. Surface modification of zinc oxide nanoparticles influences the electronic memory effects in ZnO-polystyrene diodes. J. Phys. Chem. C 2007 , 111 , 10150–10153. [ Google Scholar ]
• Tang, E.; Liu, H.; Sun, L.; Zheng, E.; Cheng, G. Fabrication of zinc oxide/poly(styrene) grafted nanocomposite latex and its dispersion. Eur. Polym. J 2007 , 43 , 4210–4218. [ Google Scholar ]
• Chae, D.W.; Kim, B.C. Effects of zinc oxide nanoparticles on the physical properties of polyacrylonitrile. J. Appl. Polym. Sci 2006 , 99 , 1854–1858. [ Google Scholar ]
• Xiong, H.M.; Wang, Z.D.; Liu, D.P.; Chen, J.S.; Wang, Y.G.; Xia, Y.Y. Bonding polyether onto ZnO nanoparticles: An effective method for preparing polymer nanocomposites with tunable luminescence and stable conductivity. Adv. Funct. Mater 2005 , 15 , 1751–1756. [ Google Scholar ]
• Im, J.; Singh, J.; Soares, J.W.; Steeves, D.M.; Whitten, J.E. Synthesis and optical properties of dithiol-linked ZnO/gold nanoaprticle composites. J. Phys. Chem 2011 , 115 , 10518–10523. [ Google Scholar ]
• Singh, J.; Im, J.; Watters, E.J.; Whitten, J.E.; Soares, J.W.; Steeves, D.M. Thiol dosing of ZnO single crystal and nanorods: Surface chemistry and photoluminescence. Surf. Sci 2013 , 609 , 183–189. [ Google Scholar ]
• Singh, J.; Im, J.; Whitten, J.E. Encapsulation of zinc oxide nanorods and nanoparticles. Langmuir 2009 , 25 , 9947–9953. [ Google Scholar ]
• Das, A.; Wang, D.Y.; Leuteritz, A.; Subramaniam, K.; Greenwell, H.C.; Wagenknecht, U.; Heinrich, G. Preparation of zinc oxide free, transparent rubber nanocomposites using a layered double hydroxide filler. J. Mater. Chem 2011 , 21 , 7194–7200. [ Google Scholar ]
• Yuan, Z.; Zhou, W.; Hu, T.; Chen, Y.; Li, F.; Xu, Z.; Wang, X. Fabrication and properties of silicone rubber/ZnO nanocomposites via in situ surface hydrosilylation. Surf. Rev. Lett 2011 , 18 , 33–38. [ Google Scholar ]
• Mandal, U.K.; Tripathy, D.K.; De, S.K. Dynamic mechanical spectroscopic studies on plasticization of an ionic elastomer based on carboxylated nitrile rubber by ammonia. Polymer 1996 , 37 , 5739–5742. [ Google Scholar ]
• Ibarra, L.; Marcos-Fernandez, A.; Alzorriz, M. Mechanistic approach to the curing of carboxylated nitrile rubber (XNBR) by zinc peroxide/zinc oxide. Polymer 2002 , 43 , 1649–1655. [ Google Scholar ]
• Chatterjee, K.; Naskar, K. Development of thermoplastic elastomers based on maleated ethylene propylene rubber (n-EPM) and propylene (PP) by dynamic vulcanization. Express Polym. Lett 2007 , 1 , 527–534. [ Google Scholar ]
• Hamed, G.R.; Hua, K.C. Effect of zinc oxide particle size on the curing of carboxylated NBR and carboxylated SBR. Rubber Chem. Technol 2004 , 77 , 214–226. [ Google Scholar ]
• Przybyszewska, M.; Zaborski, M. The effect of zinc oxide nanoparticle morphology on activity in crosslinking of carboxylated nitrile elastomer. Express Polym. Lett 2009 , 3 , 542–552. [ Google Scholar ]
• Chapman, A.V.; Porter, M.; Roberts, A.D. Natural Science and Technology ; Oxfrod University Press: New York, NY, USA, 1988. [ Google Scholar ]
• Fosmire, G.J. Zinc toxicity. Am. J. Clin. Nutr 1990 , 51 , 225–227. [ Google Scholar ]
• Heideman, G.; Datta, R.N.; Noordermeer, J.W.M.; van Baarle, B. Influence of zinc oxide during different stages of sulfur vulcanization. Elucidated by model compound studies. J. Appl. Polym. Sci 2005 , 95 , 1388–1404. [ Google Scholar ]
• Thomas, S.P.; Mathew, E.J.; Marykutty, C.V. Synthesis and effect of surface modified nano ZnO in natural rubber vulcanization. J. Appl. Polym. Sci 2012 , 124 , 3099–3107. [ Google Scholar ]
• Sabura Begum, P.M.; Mohammed Yusuff, K.K.; Joseph, R. Preparation and use of nano zinc oxide in neoprene rubber. Int. J. Polym. Mater 2008 , 57 , 1083–1094. [ Google Scholar ]
• Liu, H.; Yang, D.; Yang, H.; Zhang, H.; Zhang, W.; Fang, Y.; Liu, Z.; Tian, L.; Lin, B.; Yan, J.; et al . Comparative study of respiratory tract immune toxicity induced by three sterilization nanoparticles: Silver, zinc oxide and titanium oxide. J. Hazard. Mater 2013 , 248 , 478–486. [ Google Scholar ]
• Mirhosseini, M.; Firouzabadi, F. Antibacterial activity of zinc oxide nanoparticle suspensions on food-borne pathogens. Int. J. Dairy Technol 2012 , 65 , 1–5. [ Google Scholar ]
• Mason, P. Physiological and medicinal zinc. Pharm. J 2006 , 276 , 271–274. [ Google Scholar ]
• Newman, M.D.; Stotland, M.; Ellis, J.I. The safety of nanosized particles in titanium dioxide- and zinc oxide-based sunscreens. J. Am. Acad. Dermatol 2009 , 61 , 685–692. [ Google Scholar ]
• Pirot, F.; Millet, J.; Kalia, Y.N.; Humbert, P. In vitro study of percutaneous absorption, cutaneous bioavailability and bioequivalence of zinc and copper from five topical formulations. Skin Pharmacol 1996 , 9 , 10–20. [ Google Scholar ]
• Lansdown, A.B.; Taylor, A. Zinc and titanium oxides: Promising UV-absorbers, but what influence do they have on the intact skin? Int. J. Cosmet. Sci 1997 , 19 , 167–172. [ Google Scholar ]
• Cross, S.E.; Innes, B.; Roberts, M.S.; Tsuzuki, T.; Robertson, T.A.; McCormick, P. Human skin penetration of sunscreen nanoparticles: In vitro assessment of novel micronized zinc oxide formulation. Skin Pharmacol 2007 , 20 , 148–154. [ Google Scholar ]
• Uddin, M.J.; Cesano, F.; Scarano, D.; Bonino, F.; Agostini, G.; Spoto, G.; Bordiga, S.; Zecchina, A. Tailoring the activity of Ti-based photocatalysts by playing with surface morphology and silver doping. J. Photochem. Photobiol 2008 , 199 , 64–72. [ Google Scholar ]
• Gao, Q.; Zhu, Q.; Guo, Y. Formation of highly hydrophobic surfaces on cotton and polyester fabrics using silica sol nanoparticles and nonfluorinated alkylsilane. Ind. Eng. Chem. Res 2009 , 48 , 9797–9803. [ Google Scholar ]
• Atienzar, P.; Ishwara, T.; Illy, B.N.; Ryan, M.P.; O’Regan, B.C.; Durrant, J.R.; Nelson, J. Control of photocurrent generation in polymer/ZnO nanorod solar cells by using a solution-processed TiO 2 overlayer. Phys. Chem. Lett 2010 , 1 , 708–713. [ Google Scholar ]
• Lim, Z.H.; Chia, Z.X.; Kevin, M.; Wong, A.S.W.; Ho, G.W. A facile approach towards ZnO nanorods conductive textile for room temperature multifunctional sensors. Sens. Actuators B Chem 2010 , 151 , 121–126. [ Google Scholar ]
• Gomez, J.L.; Tigli, O. Zinc oxide nanostructures: From growth to application. J. Mater. Sci 2013 , 48 , 612–624. [ Google Scholar ]
• Tanasa, D.; Vrinceanu, N.; Nistor, A.; Aristodor, C.M.; Popovivi, E.; Bistricianu, I.L.; Brinza, F.; Chicet, D.L.; Coman, D.; Pui, A.; et al . Zinc oxide-linen fibrous composites: Morphological, structural, chemical and humidity adsorptive attributes. Text. Res. J 2012 , 82 , 832–844. [ Google Scholar ]
• Vigneshwaran, N.; Kumar, S.; Kathe, A.A.; Varadarajan, P.V.; Prasad, V. Functional finishing of cotton fabrics using zinc oxide-soluble starch nanocomposites. Nanotechnology 2006 , 17 , 5087–5095. [ Google Scholar ]
• Yadav, A.; Prasad, V.; Kathe, A.A.; Raj, S.; Yadav, D.; Sundaramoorthy, C.; Vigneshwaran, N. Functional finishing in cotton fabrics using zinc oxide nanoparticles. Bull. Mater. Sci 2006 , 29 , 641–645. [ Google Scholar ]
• Mao, Z.; Shi, Q.; Zhang, L.; Cao, H. The formation and UV-blocking property of needle-shaped ZnO nanorod on cotton fabric. Thin Solid Films 2009 , 517 , 2681–2686. [ Google Scholar ]
• Becheri, A.; Maximilian, D.; Lo Nostro, P.; Baglioni, P. Synthesis and characterization of zinc oxide nanoparticles: Application to textiles as UV-absorbers. J. Nanopart. Res 2008 , 10 , 679–689. [ Google Scholar ]
• Wang, R.; Xin, J.H.; Tao, X.M.; Daoud, W.A. ZnO nanorods grown on cotton fabrics at low temperature. Chem. Phys. Lett 2004 , 398 , 250–255. [ Google Scholar ]
• Ates, E.S.; Unalan, H.E. Zinc oxide nanowire enhanced multifunctional coatings for cotton fabrics. Thin Solid Films 2012 , 520 , 4658–4661. [ Google Scholar ]
• Jesionowski, T.; Kołodziejczak-Radzimska, A.; Ciesielczyk, F.; Sójka-Ledakowicz, J.; Olczyk, J.; Sielski, J. Synthesis of zinc oxide in an emulsion system and its deposition on PES nonwoven fabrics. Fibers Text. East. Eur 2011 , 19 , 70–75. [ Google Scholar ]
• Liu, Y.; Zhou, J.; Larbot, A.; Persin, M. Preparation and characterization of nano-zinc oxide. J. Mater. Process. Technol 2007 , 189 , 379–383. [ Google Scholar ]
• Mansouri, S.; Bourguiga, R.; Yakuphanoglu, F. Analytic model for ZnO-thin film transistor under dark and UV illumination. Curr. Appl. Phys 2012 , 12 , 1619–1623. [ Google Scholar ]
• Gunaratne, K.D.; Berkdemir, C.; Harmon, C.L.; Castelman, A.W., Jr. Investigating the relative stabilities and electronic properties of small zinc oxide clusters. J. Phys. Chem. A 2012 , 116 , 12429–12437. [ Google Scholar ]
• Kong, Y.C.; Yu, D.P.; Zhang, B.; Fang, W.; Feng, S.Q. Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach. Appl. Phys. Lett 2001 , 78 , 407–409. [ Google Scholar ]
• Sundara Venkatesh, P.; Jeganthan, K. Investigations on the growth and characterization of vertically aligned zinc oxide nanowires by radio frequency magnetron sputtering. J. Solid State Chem 2013 , 200 , 84–89. [ Google Scholar ]
• Purica, M.; Budianu, E.; Rusu, E. ZnO thin films on semiconductors substrate for large area photo-detector applications. Thin Solid Films 2001 , 383 , 284–286. [ Google Scholar ]
• Aoki, T.; Hatannaka, Y.; Look, D.C. ZnO diode fabricated by excimer-laser doping. Appl. Phys. Lett 2000 , 76 , 3257–3258. [ Google Scholar ]
• Gorla, C.R.; Emanetoglu, N.W.; Liang, S.; Mayo, W.E.; Lu, Y.; Wraback, M.; Shen, H. Structural, optical and surface acoustic wave properties of epitaxial ZnO films grown on (011 over-bar 2) sapphire by metalorganic chemical vapor deposition. J. Appl. Phys 1999 , 85 , 2595–2602. [ Google Scholar ]
• Jo, S.H.; Lao, J.Y.; Ren, Z.F.; Farrer, R.A.; Baldacchini, T.; Fourkas, J.T. Field-emission studies on thin films of zinc oxides nanowires. Appl. Phys. Lett 2003 , 83 , 4821–4823. [ Google Scholar ]
• Arnold, M.S.; Avouris, P.; Pan, Z.W.; Wang, Z.L. Field-effect transistors based on single semiconducting oxide nanobelts. J. Phys. Chem 2003 , 107 , 659–663. [ Google Scholar ]
• Lin, F.C.; Takao, Y.; Shimizu, Y.; Egashira, M. Hydrogen-sensing mechanism of zinc oxide varistor gas sensor. Sens. Actuators B Chem 1995 , 24 , 843–850. [ Google Scholar ]
• Weissenrieder, K.S.; Muller, J. Conductivity model for sputtered ZnO—Thin film gas sensors. Thin Solid Films 1997 , 300 , 30–41. [ Google Scholar ]
• Muller, J.; Weissenrieder, K.S. ZnO—Thin film chemical sensors. J. Anal. Chem 1994 , 349 , 380–384. [ Google Scholar ]
• Water, W.; Chen, S.E.; Meen, T.H.; Ji, L.W. ZnO thin film with nanorod arrays applied to fluid sensor. Ultrasonics 2012 , 52 , 747–752. [ Google Scholar ]
• Singh, J.; Im, J.; Whitten, J.E.; Soares, J.W.; Meehan, A.M.; Steeves, D.M. Adsorption of mercaptosilanes on nanocrystalline and single crystal zinc oxide surfaces, Proceedings of the SPIE 7030, Nanophotonic Materials V, 70300T, San Diego, CA, USA, 10 August 2008.
• Yan, H.Q.; He, R.R.; Johnson, J.; Law, M.; Saykally, R.J.; Yang, P. Dendritic nanowire ultraviolet laser array. J. Am. Chem. Soc 2003 , 125 , 4728–4729. [ Google Scholar ]
• Senoussaoui, N.; Krause, M.; Müller, J.; Bunte, E.; Brammer, T.; Stiebig, H. Thin-film solar cells with periodic grating coupler. Thin Solid Films 2004 , 397 , 451–452. [ Google Scholar ]
• Lima, S.A.M.; Sigoli, F.A.; Jafelicci, M., Jr.; Davolos, M.R. Luminescent properties and lattice correlation defects on zinc oxide. Int. J. Inorg. Mater 2001 , 3 , 749–754. [ Google Scholar ]
• Mikrajuddin, F.; Okuyama, K.; Shi, F.G. Stable photoluminescence of zinc oxide quantum dots in silica nanoparticles matrix prepared by the combined sol-gel and spray drying method. J. Appl. Phys 2001 , 89 , 6431–6434. [ Google Scholar ]
• Tang, Z.K.; Wong, G.K.L.; Yu, P.; Kawasaki, M.; Ohtomo, A.; Koinuma, H.; Segawa, Y. Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films. Appl. Phys. Lett 1998 , 72 , 3270–3272. [ Google Scholar ]
• Kim, M.S.; Nam, G.; Kim, S.; Kim, D.Y.; Lee, D.Y.; Kim, J.S.; Kim, S.O.; Kim, J.S.; Son, J.S.; Leem, J.Y. Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol-gel method. J. Lumin 2012 , 132 , 2581–2585. [ Google Scholar ]
• Khranovskyy, V.; Lazorenko, V.; Lashkarev, G.; Yakimova, R. Luminescence anisotropy of ZnO microrods. J. Lumin 2012 , 132 , 2643–2647. [ Google Scholar ]
• Zhong, K. Photoluminescence from zinc oxide quantum dots embedded in silicon dioxide matrices. Spectrosc. Lett 2013 , 46 , 160–164. [ Google Scholar ]
• Soares, J.W.; Whitten, J.E.; Oblas, D.W.; Steeves, D.M. Novel photoluminescence properties of surface-modified nanocrystalline zinc oxide: Toward a reactive scaffold. Langmuir 2008 , 24 , 371–374. [ Google Scholar ]
• Wang, M.; Wang, X. Electrodeposition zinc-oxide inverse opal and its application in hybrid photovoltaics. Sol. Energy Mater. Sol. Cells 2008 , 92 , 357–362. [ Google Scholar ]
• Wastermark, K.; Rensmo, H.; Lees, A.C.; Vos, J.G.; Siegbahn, H. Electron spectroscopic studies of bis-(2,2′-bipyridine)-(4,4′-dicarboxy-2,2′-bipyridine)-ruthenium(II) and bis-(2,2′-bipyridine)-(4,4′-dicarboxy-2,2′-bipyridine)-osmium(II) absorbed on nanostructured TiO 2 and ZnO. Surf. J. Phys. Chem 2002 , 106 , 10108–10113. [ Google Scholar ]
• Roy, S.; Basu, S. Improved zinc oxide films for gas sensor applications. Bull. Mater. Sci 2002 , 25 , 513–515. [ Google Scholar ]
• Samarasekara, P.; Yapa, N.; Kumara, N.; Perera, M. CO 2 gas sensitivity of sputtered zinc oxide thin films. Bull. Mater. Sci 2007 , 30 , 113–116. [ Google Scholar ]
• Larbi, T.; Ouni, B.; Boukachem, A.; Boubaker, K.; Amlouk, M. Electrical measurments of dielectric properties of molybdenum-doped zinc oxide thin films. Mat. Sci. Semicon. Proc 2014 , 22 , 50–58. [ Google Scholar ]
• Tsonos, C.; Kanapitsas, A.; Triantis, D.; Anastasiadis, C.; Stavrakas, I.; Pissis, P.; Neagu, E. Interface states and MWS polarization contributions to the dielectric response of low voltage ZnO varistor. Ceram. Int 2011 , 37 , 207–214. [ Google Scholar ]
• Tu, Y.; He, J.; Wang, Q.; Liu, M.; Xu, G.; Ding, L. Measurement of thermally stimulated current in ZnO varistor. Proc. CSEE 2010 , 30 , 116–121. [ Google Scholar ]
• Bhachu, D.S.; Ankar, G.; Parkin, I.P. Aerosol assisted chemical vapor deposition of transparent conductive zinc oxide films. Chem. Mater 2012 , 24 , 4704–4710. [ Google Scholar ]
• Long, W.; Hu, J.; Liu, J.; He, J.; Zong, R. The effect of aluminium on electrical properties of ZnO varistor. J. Am. Ceram. Soc 2010 , 93 , 2441–2444. [ Google Scholar ]
• Jain, N.; Bhargava, A.; Panwar, J. Enhanced photocatalytic degradation of methylene blue using biologically synthesized “protein-capped” ZnO nanoparticles. Chem. Eng. J 2014 , 243 , 549–555. [ Google Scholar ]
• Lam, S.M.; Sin, J.C.; Abdullah, A.Z.; Mohamed, A.R. Degradation of wastewaters containing organic dyes photocatalyzed by zinc oxide: A review. Desalin. Water Treat 2012 , 41 , 131–169. [ Google Scholar ]
• Lin, C.J.; Lu, Y.T.; Hsieh, C.H.; Chien, S.H. Surface modification of highly ordered TiO 2 nanotube arrays for efficient photoelectrocatalytic water splitting. Appl. Phys. Lett 2009 , 94 , 113102–113104. [ Google Scholar ]
• Kuo, T.J.; Lin, C.N.; Kuo, C.L.; Huang, M.H. Growth of ultralong ZnO nanowires on silicon substrates by vapor transport and their use as recyclable photocatalysts. Chem. Mater 2007 , 19 , 5143–5147. [ Google Scholar ]
• Janitabar Darzi, S.; Mahjoub, A.R. Investigation of phase transformations and photocatalytic properties of sol–gel prepared nanostructured ZnO/TiO 2 composites. J. Alloy. Compd 2009 , 486 , 805–808. [ Google Scholar ]
• Hariharan, C. Photocatalytic degradation of organic contaminants in water by ZnO nanoparticles: Revisited. Appl. Catal. A Gen 2006 , 304 , 55–61. [ Google Scholar ]
• Xiao, Q.; Ouyang, L.L. Photocatalytic photodegradation of xanthate over Zn 1− x Mn x O under visible light irradiation. J. Alloy. Compd 2009 , 479 , L4–L7. [ Google Scholar ]
• Bizarro, M. High photocatalytic activity of ZnO and ZnO:Al nanostructured films deposited by spray pyrolysis. Appl. Catal. B Environ 2010 , 97 , 198–203. [ Google Scholar ]
• Guo, M.Y.; Fung, M.K.; Fang, F.; Chen, X.Y.; Ng, A.M.C.; Djurišič, A.B.; Chan, W.K. ZnO and TiO 2 1D nanostructures for photocatalytic applications. J. Alloy. Compd 2011 , 509 , 1328–1332. [ Google Scholar ]
• Li, Q.; Wang, C.; Ju, M.; Chen, W.; Wang, E. Polyoxometalate-assisted electrochemical deposition of hollow ZnO nanospheres and their photocatalytic properties. Microporous Mesoporous Mater 2011 , 138 , 132–139. [ Google Scholar ]
• Ma, S.S.; Li, R.; Lv, C.P.; Xu, W.; Gou, X.L. Faciale synthesis of ZnO nanorod arrays and hierarchical nanostructures for photocatalysis and gas sensor applications. J. Hazard. Mater 2010 , 192 , 730–740. [ Google Scholar ]
• Kenanakis, G.; Katsarakis, N. Light-induced photocatalytic degradation of stearic acid by c-axis oriented ZnO nanowires. Appl. Catal. A Gen 2011 , 378 , 227–233. [ Google Scholar ]
• Xu, C.; Cao, L.X.; Su, G.; Liu, W.; Qu, X.F.; Yu, Y.Q. Preparation, characterization and photocatalytic activity of Co-doped ZnO powders. J. Alloy. Compd 2010 , 497 , 373–376. [ Google Scholar ]
• Mu, J.B.; Shao, C.L.; Guo, Z.C.; Zhang, Z.Y.; Zhang, M.Y.; Zhang, P.; Chen, B.; Liu, Y.C. High photocatalytic activity of ZnO-carbon nanofiber heteroarchitectures. ACS Appl. Mater. Interafces 2011 , 3 , 590–596. [ Google Scholar ]
• Rodriguez, J.; Paraguay-Delgado, F.; Lopez, A.; Alarcon, J.; Estrada, W. Synthesis and characterization of ZnO nanorod films for photocatalytic disinfection of contaminated water. Thin Solid Film 2010 , 519 , 729–735. [ Google Scholar ]
• Xie, J.; Li, Y.; Zhao, W.; Bian, L.; Wei, Y. Simple fabrication and photocatalytic activity of ZnO particles with different morphologies. Powder Technol 2011 , 207 , 140–144. [ Google Scholar ]
• Zhu, Q.; Chen, J.; Zhu, Q.; Cui, Y.; Liu, L.; Li, B.; Zhou, X. Monodispersed hollow microsphere of ZnO mesoporous nanopieces: Preparation, growth mechanism and photocatalytic performance. Mater. Res. Bull 2010 , 45 , 2024–2030. [ Google Scholar ]
• Pyne, S.; Sahoo, G.P.; Bhui, D.K.; Bar, H.; Sarkar, P.; Samanta, S.; Maity, A.; Misra, A. Enhanced photocatalytic activity of metal coated ZnO nanowires. Spectrochim. Acta Part A Mol. Biomol. Spectrosc 2012 , 93 , 100–105. [ Google Scholar ]
• Wöll, C. The chemistry and physics of zinc oxide surfaces. Prog. Surf. Sci 2007 , 82 , 55–120. [ Google Scholar ]
• Espitia, P.J.P.; Soares, N.F.F.; Coimbra, J.S.R.; de Andrade, N.J.; Cruz, R.S.; Medeiros, E.A.A. Zinc oxide nanoparticles: Synthesis, antimicrobial activity and food packaging applications. Food Bioprocess Technol 2012 , 5 , 1447–1464. [ Google Scholar ]
• Moezzi, A.; McDonagh, A.M.; Cortie, M.B. Zinc oxide particles: Synthesis, properties and applications. Chem. Eng. J 2012 , 185–186 , 1–22. [ Google Scholar ]
• Klingshirn, C. ZnO: From basics towards applications. Phys. Status Solidi 2007 , 244 , 3027–3073. [ Google Scholar ]
• Tang, X.; Choo, E.S.G.; Li, L.; Ding, J.; Xue, J. One-pot synthesis of water-stable ZnO nanoparticles via a polyol hydrolysis route and their cell labeling applications. Langmuir 2009 , 25 , 5271–5275. [ Google Scholar ]
• Usman Ali, S.M.; Ibupoto, Z.H.; Chey, C.O.; Nur, O.; Willander, M. Functionalized ZnO nanotubes arrays for the selective determination of uric acid with immobilized uricase. Chem. Sens 2011 , 19 , 1–8. [ Google Scholar ]
• Jianrong, C.; Yuqing, M.; Nogyue, H.; Xiaohua, W.; Sijiao, L. Nanotechnology and biosensors. Biotechnol. Adv 2004 , 22 , 505–518. [ Google Scholar ]
• Wang, D.H.; Kou, R.; Gil, M.P.; Jacobson, H.P.; Tang, J.; Yu, D.H.; Lu, Y.F. Templated synthesis, characterization, and sensing application of macroscopic platinum nanowire network electrodes. J. Nansci. Nanotechnol 2005 , 11 , 1904–1909. [ Google Scholar ]
• Topoglidis, E.; Palomares, E.; Astuti, Y.; Green, A.; Campbell, C.J.; Durrant, J.R. Immobilization and electrochemistry of negatively charged proteins on modified nanocrystalline metal oxide electrodes. Electroanalysis 2005 , 17 , 1035–1041. [ Google Scholar ]
• Wang, J.X.; Sun, X.W.; Wei, A.; Lei, Y.; Cai, X.P.; Li, C.M.; Dong, Z.L. Zinc oxide nanocomb biosensor for glucose detection. Appl. Phys. Lett 2006 , 88 , 233106–233108. [ Google Scholar ]
• Usman Ali, S.M.; Alvi, N.H.; Ibupoto, Z.; Nur, O.; Willander, M.; Danielsson, B. Selective potentiometric determination of uric acid with uricase immobilized on ZnO nanowires. Sens. Actuators B Chem 2011 , 152 , 241–247. [ Google Scholar ]
• Brayner, R.; Ferrari-Iliou, R.; Brivois, N.; Djediat, S.; Benedetti, M.F.; Fievet, F. Toxicological impact studies based on Escherichia coli bacteria in ultrafine ZnO nanoparticles colloidal medium. Nano Lett 2006 , 6 , 866–870. [ Google Scholar ]
• Stoimenov, P.K.; Klinger, R.L.; Marchin, G.L.; Klabunde, K.J. Metal oxide nanoparticles as bactericidal agents. Langmuir 2002 , 18 , 6679–6686. [ Google Scholar ]
• Ahmad, M.; Zhu, J. ZnO based advanced functional nanostructures: Synthesis, properties and applications. J. Mater. Chem 2011 , 21 , 599–614. [ Google Scholar ]
• Fortuny, A.; Bengoa, C.; Font, J.; Fabregat, A. Bimetallic catalysts for continuous catalytic wet air oxidation of phenol. J. Hazard. Mater 1999 , 64 , 181–193. [ Google Scholar ]
• Rana, S.; Rawat, J.; Sorensson, M.M.; Misra, R.D.K. Antimicrobial function of Nd 3+ -doped anatase titania-coated nickel ferrite composite nanoparticles: A biomaterial system. Acta Biomater 2006 , 2 , 421–432. [ Google Scholar ]
• Shanmugam, S.; Viswanathan, B.; Varadarajan, T.K. A novel single step chemical route for noble metal nanoparticles embedded organic–inorganic composite films. Mater. Chem. Phys 2006 , 95 , 51–55. [ Google Scholar ]
• Padmavathy, N.; Vijayaraghavan, R. Enhanced bioactivity of ZnO nanoparticles—An antimicrobial study. Sci. Technol. Adv. Mater 2008 , 9 . [ Google Scholar ] [ CrossRef ]

Click here to enlarge figure

Note: BET —surface area calculated based on BET equation; D —particles diameter; L —particles length.

© 2014 by the authors; licensee MDPI, Basel, Switzerland This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

Share and Cite

Kołodziejczak-Radzimska, A.; Jesionowski, T. Zinc Oxide—From Synthesis to Application: A Review. Materials 2014 , 7 , 2833-2881. https://doi.org/10.3390/ma7042833

Kołodziejczak-Radzimska A, Jesionowski T. Zinc Oxide—From Synthesis to Application: A Review. Materials . 2014; 7(4):2833-2881. https://doi.org/10.3390/ma7042833

Kołodziejczak-Radzimska, Agnieszka, and Teofil Jesionowski. 2014. "Zinc Oxide—From Synthesis to Application: A Review" Materials 7, no. 4: 2833-2881. https://doi.org/10.3390/ma7042833

Article Metrics

Article access statistics, further information, mdpi initiatives, follow mdpi.

Subscribe to receive issue release notifications and newsletters from MDPI journals

• Search the site GO Please fill out this field.
• Newsletters

The 9 Healthiest Cheese Types to Eat

Liudmila Chernetska / Getty Images

Cheese is often an important part of diets worldwide. Though cheese sometimes gets a bad rap in the nutrition world due to its high calorie and fat content, most cheeses are nutritious and may benefit your health in several ways. Research shows that people who eat cheese are less likely to develop heart disease and stroke and are less likely to die from heart-related illnesses compared to those who don't.

There are hundreds of cheeses, including those made with cow's milk, goat's milk, and sheep's milk. From soft cheeses like ricotta to hard cheeses like parmesan, there's a cheese for nearly every flavor, texture, and ingredient preference.

Here are nine of the healthiest types of cheese, plus tips for enjoying cheese as part of a healthy diet.

1. Feta Cheese

Candice Bell / Getty Images

Here's the nutrition breakdown for a 1-ounce serving of feta cheese:

• Calories: 75.1
• Protein: 4.03 grams (g)
• Carbohydrates: 0 g
• Calcium: 140 milligrams (mg), or 11% of the Daily Value (DV)
• B12: 0.479 micrograms (mcg), or 20% of the DV
• Sodium: 323 mg

Feta cheese is a tangy, soft cheese traditionally made from goat's or sheep's milk. This salty, slightly acidic cheese is a staple in Mediterranean diets and is featured in nutritious recipes like Greek salad and pasta dishes.

Feta is high in several nutrients, including calcium and B12 , a B vitamin essential for neurological (brain and nervous system) function, red blood cell production, metabolism, and DNA synthesis.

Because feta is so flavorful, a little goes a long way. Try adding a bit of feta to salads and grain bowls or sprinkling feta on top of soups and pasta to add a creamy texture and tangy flavor.

2. Parmesan

Image Source / Getty Images

A 1-ounce serving of hard parmesan cheese provides:

• Calories: 111
• Protein: 10.1 g
• Fat: 7.09 g
• Carbohydrates: 0.9g
• Calcium: 335 mg, or 26% of the Daily Value (DV)
• Selenium: 6.38 mcg, or 11% of the DV
• B12: 0.34 mcg, or 14% of the DV
• Sodium: 335 mg
• Zinc: 1.15 mg or 10% of the DV

Parmesan, or Parmigiano Reggiano, is a beloved hard Italian cheese commonly enjoyed in pasta dishes and salads. It has a sharp, salty, complex flavor and is naturally low in lactose, a type of sugar found in milk products.

Up to 75% of the world’s population is intolerant to lactose and may experience symptoms like bloating, diarrhea , and gas after consuming lactose-rich foods and drinks. Most people with lactose intolerance can tolerate parmesan, as it contains less than 10 mg of lactose per kilogram (kg). Foods containing less than 10 mg/kg of lactose can be labeled lactose-free and are unlikely to cause symptoms in people with lactose intolerance.

In addition to being low in lactose, parmesan is high in protein and vitamins and minerals like calcium, selenium , and B12. A 1-ounce serving covers over 25% of your daily needs for calcium, a mineral that provides structure to the bones and teeth and is necessary for critical processes such as nerve and muscle function and hormone secretion.

3. Cottage Cheese

elenaleonova / Getty Images

Here's the nutrition breakdown for a 1-cup serving of 2% cottage cheese:

• Calories: 180
• Protein: 24.2 g 
• Fat: 5.06 g
• Carbohydrates: 9.48 g
• Calcium: 227 mg, or 17% of the DV
• B12: 0.92 mcg, or 38% of the DV
• Vitamin A: 154 mcg, or 17% of the DV
• Selenium: 32.1 mcg, or 58% of the DV
• Sodium: 706 mg

Cottage cheese is a fresh, soft cheese that tastes much milder than other cheeses. Commonly enjoyed as a breakfast or snack option, cottage cheese pairs well with sweet and savory ingredients such as berries and herbs.

Cottage cheese is an excellent source of highly absorbable protein, providing over 24 g of protein per cup. Protein slows digestion and stimulates the release of satiety hormones, helping you feel full after eating. Plus, adding protein-rich foods to your meals and snacks can help support healthy blood sugar regulation by slowing glucose absorption into the bloodstream.

Cottage cheese is also high in several vitamins and minerals, such as selenium, a mineral essential for thyroid function and a powerful antioxidant. One cup of 2% cottage cheese covers over 50% of your daily needs for this critical nutrient.

Though cottage cheese is nutritious, it is relatively high in sodium . If you're following a low-sodium diet, aim for smaller portions or choose reduced-sodium cottage cheese products.

HandmadePictures / Getty Images

A 1-ounce serving of cheddar cheese provides:

• Calories: 115
• Protein: 6.78 g 
• Fat: 9.46 g
• Carbohydrates: 0.59 g
• Calcium: 199 mg, or 15% of the DV
• B12: 0.246 mcg, or 10% of the DV
• Vitamin A: 278 mcg, or 31% of the DV
• Selenium: 7.92 mcg, or 14% of the DV
• Zinc: 1.05 mg, or 10% of the DV
• Sodium: 180 mg

Cheddar is a popular type of cheese that comes in many flavors, such as mild, sharp, and extra-sharp. Cheddar, especially aged cheddar, is low in lactose and is usually well-tolerated by people with lactose intolerance.

In addition to being low in lactose, cheddar is packed with vitamins and minerals, like calcium, B12, selenium, zinc , and vitamin A. Vitamin A is a nutrient that's needed for proper growth and development, cellular communication, immune function, and other critical processes in the body.

Cheddar is also high in protein and provides essential amino acids like leucine, which plays an important role in protein synthesis and muscle repair. Studies show that eating cheddar can help increase blood levels of amino acids like leucine and promote muscle growth.

Try adding cheddar to salads and sandwiches or pairing cheddar with fresh fruit and nuts for a filling snack.

tashka2000 / Getty Images

A half-cup serving of whole milk ricotta cheese provides:

• Calories: 204
• Fat: 14.2 g
• Carbohydrates: 8.85 g
• Calcium: 289 mg, or 15% of the DV
• B12: 1.01 mcg, or 42% of the DV
• Vitamin A: 164 mcg, or 18% of the DV
• Selenium: 7.1 mcg, or 13% of the DV
• Sodium: 135 mg

Ricotta cheese is a creamy Italian dairy product popular in Mediterranean diets. It has a high moisture content and is very mild in taste, so it can be used in both sweet and savory recipes, such as cheesecake and lasagna.

Ricotta is available in several fat percentages, including whole milk and skim. It can suit a variety of dietary preferences. For example, whole milk ricotta can be used to make keto-friendly desserts, like cheesecakes, while low-fat ricotta can add a rich and creamy flavor to soups and pastas without adding additional fat.

It's a rich source of several nutrients but is particularly high in B12, vitamin A, and calcium. It's also low in sodium and can add flavor to dishes by those on salt-restricted diets .

6. Manchego

fcafotodigital / Getty Images

Here's the nutrition breakdown for a 1-ounce serving of manchego cheese:

• Calories: 120
• Protein: 5.99 g
• Carbohydrates: 0.4 g
• Calcium: 250 mg, or 19% of the DV
• Vitamin A: 300 mcg, or 33% of the DV
• Sodium: 170 mg

Manchego is an aged cheese made in the La Macha region of Spain from milk from a specific type of sheep called Manchega sheep. Because it's made with sheep's milk and is naturally low in lactose, it can be used as an alternative by people who experience digestive issues after eating cow's milk-based products.

Manchego cheese has a crumbly texture and a tangy, slightly sweet flavor that intensifies as the cheese ages. It's used in traditional Spanish dishes like pisto, a vegetable-based stew-like dish topped with shaved manchego.

Manchego is a good source of protein and is high in calcium. A diet high in calcium and protein-rich foods , like manchego, has been shown to reduce the the risk of bone-related conditions such as low bone mineral density and fractures.

It's also rich in vitamin A and relatively low in sodium, making it an all-around healthy choice.

7. Blue Cheese

grafvision / Getty Images

A 1-ounce serving of blue cheese provides:

• Calories: 100
• Protein: 6.07 g
• Fat: 8.14 g
• Carbohydrates: 0.6 g
• Calcium: 150 mg, or 12% of the DV
• B12: 0.346 mcg, or 14% of the DV
• Pantothenic acid (B5): 0.49 mg, or 10% of the DV
• Sodium: 326 mg

Blue cheese is a name for a group of cheeses made with cultures of a mold called Penicillium Roquefort . Blue cheeses, like Roquefort, Gorgonzola, and Stilton, have a characteristic sharp and salty taste and range in texture from soft and creamy to crumbly.

Blue cheeses are a good source of minerals like calcium and several B vitamins , including B12 and B5, a vitamin needed for the production of coenzymes, neurotransmitters, and cholesterol that also helps the body obtain energy from food.

Blue cheese is delicious crumbled on salads and can be added to dishes like pasta, sandwiches, and vegetable recipes. However, its powerful flavor can easily overwhelm a dish if too much is used.

8. Mozzarella

A 1-ounce serving of mozzarella provides:

• Calories: 84.8
• Protein: 6.29 g
• Fat: 6.26 g
• Calcium: 143 mg, or 11% of the DV
• B12: 0.646 mcg, or 27% of the DV
• Selenium: 4.82 mcg, or 9% of the DV
• Sodium: 138 mg

Mozzarella is an Italian cheese commonly found in dishes like lasagna, caprese salad, and pizza. It has a mild taste and a highly elastic texture when melted.

This popular cheese is a rich source of vitamin B12 and also provides smaller amounts of vitamin A and selenium. It's lower in sodium than other cheeses, making it a good choice for cheese lovers watching their salt intake.

9. Goat Cheese

Ramses02 / Getty Images

Here's the nutrition breakdown for a 1-ounce serving of soft goat cheese:

• Calories: 74.8
• Protein: 5.24 grams (g)
• Fat: 5.98 g
• Copper: 0.208 mg, or 23% of the DV
• Riboflavin (B2): 0.108 mg, or 8% of the DV
• Vitamin A: 81.6 mcg, or 9% of the DV
• Sodium: 130 mg

Goat cheese, also known as chèvre, is a mild-tasting cheese made from goat's milk . It's a good source of protein and contains zero carbs, making it a popular ingredient amongst those following low-carb diets . Since it's made with goat's milk, it's safe for people sensitive or intolerant to cow's milk products.

Goat cheese is lower in calcium compared to other cheeses, but it's high in other essential nutrients like copper, which is needed for red blood cell formation, growth and development, energy production, iron metabolism , and neurotransmitter synthesis. It's also high in riboflavin, a B vitamin that plays an essential role in energy production and functions as an antioxidant in the body.

Goat cheese is delicious in sweet and savory recipes and can add a creamy texture to soups, salads, and desserts like cakes and mousses.

How Much Cheese Should You Eat Per Day?

Your calorie and macronutrient needs vary depending on factors like activity levels, body size, and health goals. Cheeses are generally high in calories and fat, so it's usually recommended to consume cheese in moderation.

Though your body needs fat to function and fat isn't inherently bad for your health, some people, such as those with familial hypercholesterolemia , are more sensitive to high-fat, high-cholesterol foods like cheese. For people with high cholesterol, it's best to limit foods high in saturated fat and cholesterol, like cheese, to reach and maintain healthy blood lipid levels and prevent the development of heart disease.

The American Heart Association recommends that most people stick to about three servings of cheese per day, which equates to 3 ounces (oz) of cheese, and choose lower-fat cheeses over full-fat cheeses when possible. However, unless you're very sensitive to cholesterol-rich foods, you still enjoy small portions of full-fat cheeses, like cheddar and ricotta, in moderation as part of a healthy diet.

A Quick Review

Cheese is a nutrient-dense dairy product that's a staple in many diets. There are hundreds of types of cheese, most of which are packed with protein, vitamins, and minerals.

If you're looking for an easy way to hit your daily needs for essential nutrients like protein, calcium, and B12, consider adding some of the cheeses listed above to your diet.

Zhang M, Dong X, Huang Z, et al. Cheese consumption and multiple health outcomes: an umbrella review and updated meta-analysis of prospective studies .  Adv Nutr . 2023;14(5):1170-1186. doi:10.1016/j.advnut.2023.06.007

U.S Department of Agriculture. FoodData Central. Cheese, feta .

National Institutes of Health: Office of Dietary Supplements. Vitamin B12 .

U.S Department of Agriculture. FoodData Central. Cheese, parmesan, hard .

Dekker PJT, Koenders D, Bruins MJ. Lactose-free dairy products: market developments, production, nutrition and health benefits.   Nutrients . 2019;11(3). doi:10.3390/nu11030551

Li A, Zheng J, Han X, et al. Advances in low-lactose/lactose-free dairy products and their production .  Foods . 2023;12(13):2553. doi:10.3390/foods12132553

Fructuoso I, Romão B, Han H, et al. An overview on nutritional aspects of plant-based beverages used as substitutes for cow’s milk . Nutrients . 2021;13(8):2650. doi:10.3390/nu13082650

LibreTexts Medicine. Calcium . 

U.S Department of Agriculture. FoodData Central. Cheese, cottage, 2% milk fat .

Moon J, Koh G. Clinical evidence and mechanisms of high-protein diet-induced weight loss . J Obes Metab Syndr . 2020;29(3):166-173. doi:10.7570/jomes20028

National Institutes of Health: Office of Dietary Supplements. Selenium . 

U.S Department of Agriculture. FoodData Central. Cheese, cheddar, sharp, sliced .

National Institutes of Health: Office of Dietary Supplements. Vitamin A .

de Hart NMMP, Mahmassani ZS, Reidy PT, et al. Acute effects of cheddar cheese consumption on circulating amino acids and human skeletal muscle .  Nutrients . 2021;13(2):614. doi:10.3390/nu13020614

U.S Department of Agriculture. FoodData Central. Cheese, ricotta, whole milk .

Mangione G, Caccamo M, Natalello A, Licitra G. Graduate Student Literature Review: History, technologies of production, and characteristics of ricotta cheese .  J Dairy Sci . 2023;106(6):3807-3826. doi:10.3168/jds.2022-22460

U.S Department of Agriculture. FoodData Central. PRESIDENT, DON BERNARDO, MANCHEGO CHEESE .

Shrestha A, Samuelsson LM, Sharma P, Day L, Cameron-Smith D, Milan AM. Comparing response of sheep and cow milk on acute digestive comfort and lactose malabsorption: a randomized controlled trial in female dairy avoiders .  Front Nutr . 2021;8:603816. doi:10.3389/fnut.2021.603816

Cairoli E, Aresta C, Giovanelli L, et al. Dietary calcium intake in a cohort of individuals evaluated for low bone mineral density: a multicenter Italian study .  Aging Clin Exp Res . 2021;33(12):3223-3235. doi:10.1007/s40520-021-01856-5

U.S Department of Agriculture. FoodData Central. Cheese, blue .

Chávez R, Vaca I, García-Estrada C. Secondary metabolites produced by the blue-cheese ripening mold penicillium roqueforti; biosynthesis and regulation mechanisms .  J Fungi (Basel) . 2023;9(4):459. doi:10.3390/jof9040459

Hanna M, Jaqua E, Nguyen V, Clay J. B vitamins: functions and uses in medicine . Perm J . 26(2):89-97. doi:10.7812/TPP/21.204

U.S. Department of Agriculture. FoodData Central. Cheese, mozzarella, whole milk .

U.S. Department of Agriculture. FoodData Central. Cheese, goat, soft type .

National Institutes of Health. Copper .

Vaezi Z, Amini A. Familial hypercholesterolemia . In:  StatPearls . StatPearls Publishing; 2024.

Centers for Disease Control and Prevention. Familial hypercholesterolemia .

American Heart Association. Suggested servings from each food group .

Related Articles

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings

Preview improvements coming to the PMC website in October 2024. Learn More or Try it out now .

• Advanced Search
• Journal List
• Materials (Basel)

Zinc Oxide—From Synthesis to Application: A Review

Zinc oxide can be called a multifunctional material thanks to its unique physical and chemical properties. The first part of this paper presents the most important methods of preparation of ZnO divided into metallurgical and chemical methods. The mechanochemical process, controlled precipitation, sol-gel method, solvothermal and hydrothermal method, method using emulsion and microemulsion enviroment and other methods of obtaining zinc oxide were classified as chemical methods. In the next part of this review, the modification methods of ZnO were characterized. The modification with organic (carboxylic acid, silanes) and inroganic (metal oxides) compounds, and polymer matrices were mainly described. Finally, we present possible applications in various branches of industry: rubber, pharmaceutical, cosmetics, textile, electronic and electrotechnology, photocatalysis were introduced. This review provides useful information for specialist dealings with zinc oxide.

1. Introduction

Zinc oxide, with its unique physical and chemical properties, such as high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption and high photostability, is a multifunctional material [ 1 , 2 ]. In materials science, zinc oxide is classified as a semiconductor in group II-VI, whose covalence is on the boundary between ionic and covalent semiconductors. A broad energy band (3.37 eV), high bond energy (60 meV) and high thermal and mechanical stability at room temperature make it attractive for potential use in electronics, optoelectronics and laser technology [ 3 , 4 ]. The piezo- and pyroelectric properties of ZnO mean that it can be used as a sensor, converter, energy generator and photocatalyst in hydrogen production [ 5 , 6 ]. Because of its hardness, rigidity and piezoelectric constant it is an important material in the ceramics industry, while its low toxicity, biocompatibility and biodegradability make it a material of interest for biomedicine and in pro-ecological systems [ 7 – 9 ].

The variety of structures of nanometric zinc oxide means that ZnO can be classified among new materials with potential applications in many fields of nanotechnology. Zinc oxide can occur in one- (1D), two- (2D), and three-dimensional (3D) structures. One-dimensional structures make up the largest group, including nanorods [ 10 – 12 ], -needles [ 13 ], -helixes, -springs and -rings [ 14 ], -ribbons [ 15 ], -tubes [ 16 – 18 ] -belts [ 19 ], -wires [ 20 – 22 ] and -combs [ 23 ]. Zinc oxide can be obtained in 2D structures, such as nanoplate/nanosheet and nanopellets [ 24 , 25 ]. Examples of 3D structures of zinc oxide include flower, dandelion, snowflakes, coniferous urchin-like, etc. [ 26 – 29 ]. ZnO provides one of the greatest assortments of varied particle structures among all known materials (see Figure 1 ).

Examples of zinc oxide structure: flower ( a ); rods ( b ); wires ( c , d ) (created based on [ 17 , 27 , 29 ] with permission from Elsevier Publisher and [ 11 ] AIP Publishing LLC).

In this review, the methods of synthesis, modification and application of zinc oxide will be discussed. The zinc oxide occurs in a very rich variety of structures and offers a wide range of properties. The variety of methods for ZnO production, such as vapour deposition, precipitation in water solution, hydrothermal synthesis, the sol-gel process, precipitation from microemulsions and mechanochemical processes, makes it possible to obtain products with particles differing in shape, size and spatial structure. These methods are described in detail in the following sections ( Table 1 ).

Summary of methods of obtaining zinc oxide.

Note: BET —surface area calculated based on BET equation; D —particles diameter; L —particles length.

2. Methods of Synthesis of Nano- and Micrometric Zinc Oxide

2.1. metallurgical process.

Metallurgical processes for obtaining zinc oxide are based on the roasting of zinc ore. According to the ISO 9298 standard [ 68 ], zinc oxide is classified either as type A, obtained by a direct process (the American process); or type B, obtained by an indirect process (the French process).

The direct (American) process involves the reduction of zinc ore by heating with coal (such as anthracite), followed by the oxidation of zinc vapour in the same reactor, in a single production cycle. This process was developed by Samuel Wetherill, and takes place in a furnace in which the first layer consists of a coal bed, lit by the heat remaining from the previous charge. Above this bed is a second layer in the form of zinc ore mixed with coal. Blast air is fed in from below, so as to deliver heat to both layers and to carry carbon monoxide for zinc reduction. The resulting zinc oxide (of type A) contains impurities in the form of compounds of other metals from the zinc ore. The resulting ZnO particles are mainly needle-shaped, and sometimes spheroidal. To obtain a product with a permanent white color, the oxides of lead, iron and cadmium that are present are converted to sulfates. Increasing the permanence of the color is linked to increasing the content of water-soluble substances, and also increasing the acidity of the product. Acidity is desirable in the case of rubber processing technology, since it lengthens prevulcanization time and ensures the safe processing of the mixtures [ 69 ].

In the indirect (French) process, metallic zinc is melted in a furnace and vaporized at ca. 910 °C. The immediate reaction of the zinc vapour with oxygen from the air produces ZnO. The particles of zinc oxide are transported via a cooling duct and are collected at a bag filter station. The indirect process was popularized by LeClaire in 1844, and since then has been known as the French process. The product consists of agglomerates with an average particle size ranging from 0.1 to a few micrometres [ 70 ]. The ZnO particles are mainly of spheroidal shape. The French process is carried out in vertical furnaces, with an original vertical charge, vertical refining column, vaporizer with electric arc, and rotary combustion chamber [ 71 ]. Type B zinc oxide has a higher degree of purity than type A.

2.2. Chemical Processes

Because of its interesting properties, zinc oxide has been the subject of study by many researchers. This has led to the development of a great variety of techniques for synthesizing the compound. Unfortunately, methods that work in the laboratory cannot always be applied on an industrial scale, where it is important for the process to be economically effective, high yielding and simple to implement.

2.2.1. Mechanochemical Process

The mechanochemical process is a cheap and simple method of obtaining nanoparticles on a large scale. It involves high-energy dry milling, which initiates a reaction through ball–powder impacts in a ball mill, at low temperature. A “thinner” is added to the system in the form of a solid (usually NaCl), which acts as a reaction medium and separates the nanoparticles being formed. A fundamental difficulty in this method is the uniform grinding of the powder and reduction of grains to the required size, which decreases with increasing time and energy of milling. Unfortunately, a longer milling time leads to a greater quantity of impurities. The advantages of this method are the low production costs, small particle sizes and limited tendency for particles to agglomerate, as well as the high homogeneity of the crystalline structure and morphology.

The starting materials used in the mechanochemical method are mainly anhydrous ZnCl 2 and Na 2 CO 3 . NaCl is added to the system; this serves as a reaction medium and separates the nanoparticles. The zinc oxide precursor formed, ZnCO 3 , is calcined at a temperature of 400–800 °C. The process as a whole involves the following reactions (1) and (2):

The mechanochemical method was proposed by Ao et al. [ 30 ], they synthesized ZnO with an average crystallite size of 21 nm. The milling process was carried out for 6 h, producing ZnCO 3 as the zinc oxide precursor. Calcination of the precursor at 600 °C produced ZnO with a hexagonal structure. Tests showed that the size of the ZnO crystallites depends on the milling time and calcination temperature. Increasing the milling time (2–6 h) led to a reduction in the crystallite sizes (21.5–25 nm), which may indicate the existence of a “critical moment”. Meanwhile an increase in the calcination temperature from 400 to 800 °C caused an increase in crystallite size (18–35 nm).

The same system of reagents was used by Tsuzuki and McCormick [ 32 ]. They found that a milling time of 4 h was enough for a reaction to take place between the substrates, producing the precursor ZnCO 3 , which when calcined at 400 °C produced nanocrystallites of ZnO with an average size of 26 nm. Tsuzuki et al. showed that milling of the substrates without a thinner leads to the formation of aggregates measuring 100–1000 nm. This confirmed the important role played by zinc chloride in preventing agglomeration of the nanoparticles.

A milling process of ZnCl 2 and Na 2 CO 3 was also carried out by Moballegh et al. [ 33 ] and by Aghababazadeh et al. [ 34 ]. Moballegh et al. , investigated the effect of calcination temperature on particle size. An increase in the temperature of the process (300–450 °C) caused an increase in the size of the ZnO particles (27–56 nm). Aghababazadeh et al. obtained ZnO with an average particle size of approximately 51 nm and a surface area of 23 m 2 /g, carrying out the process at a temperature of 400 °C.

Stanković et al. [ 31 ] extended their previous study to investigate mechanical-thermal synthesis (MTS)—mechanical activation followed by thermal activation of ZnO from ZnCl 2 and oxalic acid (C 2 H 2 O 4 ·2H 2 O) as reactants with the intention of obtaining pure ZnO nanopowder. The study also aimed to examine the effects of oxalic acid as an organic PCA, and different milling times, on the crystal structure, average particle size and morphology of ZnO nanopowders. The mixture of initial reactants was milled from 30 min up to 4 h, and subsequently annealed at 450 °C for 1 h. Qualitative analysis of the prepared powders was performed using X-ray diffraction (XRD) and Raman spectroscopy. The XRD analysis showed perfect long-range order and the pure wurtzite structure of the synthesized ZnO powders, irrespective of the milling duration. By contrast, Raman spectroscopy indicates a different middle-range order of ZnO powders. From the SEM images, it is observed that the morphology of the particles strongly depends on the milling time of the reactant mixture, regardless of the further thermal treatment. A longer time of milling led to a smaller particle size.

2.2.2. Controlled Precipitation

Controlled precipitation is a widely used method of obtaining zinc oxide, since it makes it possible to obtain a product with repeatable properties. The method involves fast and spontaneous reduction of a solution of zinc salt using a reducing agent, to limit the growth of particles with specified dimensions, followed by precipitation of a precursor of ZnO from the solution. At the next stage this precursor undergoes thermal treatment, followed by milling to remove impurities. It is very difficult to break down the agglomerates that form, so the calcined powders have a high level of agglomeration of particles. The process of precipitation is controlled by parameters such as pH, temperature and time of precipitation.

Zinc oxide has also been precipitated from aqueous solutions of zinc chloride and zinc acetate [ 35 ]. Controlled parameters in this process included the concentration of the reagents, the rate of addition of substrates, and the reaction temperature. Zinc oxide was produced with a monomodal particle size distribution and high surface area.

A controlled precipitation method was also used by Hong et al. [ 36 ]. The process of precipitating zinc oxide was carried out using zinc acetate (Zn(CH 3 COO) 2 ·H 2 O) and ammonium carbonate (NH 4 ) 2 CO 3 . These solutions were dosed into a vigorously mixed aqueous solution of poly(ethylene glycol) with an average molecular mass of 10,000. The resulting precipitate was calcined by two different methods. In the first, calcination at 450 °C for 3 h produced ZnO labelled as “powder A”. In the second process, calcination took place following heterogeneous azeotropic distillation of the precursor; the resulting zinc oxide was labelled as “powder B”. Structural testing (XRD) and morphological analysis (TEM) showed that powder A contained particles with a diameter of 40 nm, while powder B contained particles with a diameter of 30 nm. Heterogeneous azeotropic distillation completely reduces the occurrence of agglomerates and decreases the ZnO particle size.

Lanje et al. [ 38 ] used the cost competitive and simple precipitation process for the synthesis of zinc oxide. The single step process with the large scale production without unwanted impurities is desirable for the cost-effective preparation of ZnO nanparticles. As a consequence, the low cost precursors such as zinc nitrate and sodium hydroxide to synthesize the ZnO nanoparticles ( ca. 40 nm) were used. In order to reduce the agglomeration among the smaller particles, the starch molecule which contains many O-H functional groups and could bind surface of nanoparticles in initial nucleation stage, was used.

Another process of controlled precipitation of zinc oxide was carried out by Wang et al. [ 39 ]. Nanometric zinc oxide was obtained by precipitation from aqueous solutions of NH 4 HCO 3 and ZnSO 4 ·7H 2 O by way of the following reactions (3) and (4):

This study was performed using a membrane reactor consisting of two plates of polytetrafluoroethylene (PTFE), with stainless steel as a dispersion medium. The ZnO obtained had a narrow range of particle sizes, from 9 to 20 nm. XRD analysis showed both the precursor and the ZnO itself to have a wurtzite structure exclusively. The particle size was affected by temperature, calcination time, flow rate and concentration of the supply phase.

In a report of Jia et al. [ 40 ], in situ crystallization transformation from Zn(OH) 2 to ZnO is demonstrated. Based on observations using X-ray diffraction (XRD) and scanning electron microscopy (SEM), two possible mechanisms from Zn(OH) 2 to ZnO are suggested. The formation mechanism of ZnO was studied in a time-resolved investigation by heating a water solution containing zinc salts (Zn(CH 3 COO) 2 ) and ammonium hydroxide (NH 4 OH) to 85 °C. Transformation of microcrystals of the stable intermediate ε-Z(OH) 2 to ZnO was observed to occur at various aging times. Transformation from ε-Z(OH) 2 to ZnO followed two mechanisms: dissolution−reprecipitation and in situ crystallization transformation involving dehydration and internal atomic rearrangements. From a fundamental point of view, these findings provide new insights into the growth of ZnO crystals and arm researchers with potential strategies for the controllable synthesis of ZnO in liquid media.

In processes of synthesis of nanopowders based on precipitation, it is increasingly common for surfactants to be used to control the growth of particles. The presence of these compounds affects not only nucleation and particle growth, but also coagulation and flocculation of the particles. The surfactant method involves chelation of the metal cations of the precursor by surfactants in an aqueous environment. Wang et al. [ 44 ] obtained nanometric zinc oxide from ZnCl 2 and NH 4 OH in the presence of the cationic surfactant CTAB (cetyltrimethylammonium bromide). The process was carried out at room temperature, and the resulting powder was calcined at 500 °C to remove residues of the surfactant. The product was highly crystalline ZnO with a wurtzite structure and with small, well-dispersed spherical nanoparticles in size of 50 nm. It was found that CTAB affects the process of nucleation and growth of crystallites during synthesis, and also prevents the formation of agglomerates.

Li et al. [ 45 ] synthesized microcrystals of zinc oxide with various shapes (including forms resembling rice grains, nuts and rods) from Zn(NO 3 ) 2 ·6H 2 O and NaOH in the presence of sodium dodecyl sulfate (SDS) and triethanolamine (TEA) as cationic surfactant. The presence of the surfactant was found to affect both the shape and size of the resulting ZnO particles. Li et al. suggested additionally that the transformation may take place via a mechanism of recrystallization. Figure 2 shows the effect of SDS on the structure of the ZnO crystal.

Effect of sodium dodecyl sulfate (SDS) surfactant on the structure of a ZnO crystal (created based on [ 45 ] with permission from Elsevier Publisher).

2.2.3. Sol-Gel Method

The obtaining of ZnO nanopowders by the sol-gel method is the subject of much interest, in view of the simplicity, low cost, reliability, repeatability and relatively mild conditions of synthesis, which are such as to enable the surface modification of zinc oxide with selected organic compounds. This changes in properties and extends its range of applications. The favourable optical properties of nanoparticles obtained by the sol-gel method have become a common topic of research, as reflected in numerous scientific publications [ 46 ]. Figure 3 shows two examples of synthesis by the sol-gel method: films from a colloidal sol ( Figure 3a ), and powder from a colloidal sol transformed into a gel ( Figure 3b ).

Overview showing two examples of synthesis by the sol-gel method: ( a ) films from a colloidal sol; ( b ) powder from a colloidal sol transformed into a gel (created based on [ 72 ] with permission from Elsevier Publisher).

Benhebal et al. [ 47 ] prepared ZnO powder by sol-gel method from zinc acetate dihydrate, oxalic acid, using ethanol as solvent. The obtained product was characterized by using techniques such as nitrogen adsorption isotherms, X-ray difration (XRD), scanning electron microscopy (SEM), an UV-Vis spectroscopy. The prepared zinc oxide has a hexagonal wurtzite structure with the particles of a spherically shaped. A surface area obtained by the BET method of the calcined ZnO powder is equal to 10 m 2 /g, characteristic of a material with low prosity, or a crystallized material.

The sol-gel method was also used to obtain nanocrystalline zinc oxide by Ristić et al. [ 48 ]. A solution of tetramethylammonium hydroxide (TMAH) was added to a solution of zinc 2-ethylhexanoate (ZEH) in propan-2-ol. The resulting colloidal suspension was left for 30 min (alternatively for 24 h), and was then washed with ethanol and water. TMAH is a strong organic base, which comparably with an inorganic base (e.g., NaOH) is characterized by a pH of ~14. This high pH means that metal oxides are not contaminated with the cation from the base, which may have an effect on the ohmic conductance of the oxide material. A determination was made of the effect of the quantity of ZEH used and the maturing time of the colloidal solution. TEM images showed that the ZnO particles obtained have sizes of the order of 20–50 nm. The quantity of ZEH has a negligible effect on the particle size.

Yue et al. [ 49 ] also obtained ZnO by the sol-gel method. High-filling, unifrom, ordered ZnO nanotubes have been successully prepared by sol-gel method into ultrathin AAO membrane. Integrating the ultrathin AAO membranes with the sol-gel technique may help to fabricate high-quality 1D nanomaterials and to extend its application as a template for nanostructures growth.

2.2.4. Solvothermal and Hydrothermal Method

The hydrothermal method does not require the use of organic solvents or additional processing of the product (grinding and calcination), which makes it a simple and environmentally friendly technique. The synthesis takes place in an autoclave, where the mixture of substrates is heated gradually to a temperature of 100–300 °C and left for several days. As a result of heating followed by cooling, crystal nuclei are formed, which then grow. This process has many advantages, including the possibility of carrying out the synthesis at low temperatures, the diverse shapes and dimensions of the resulting crystals depending on the composition of the starting mixture and the process temperature and pressure, the high degree of crystallinity of the product, and the high purity of the material obtained [ 73 , 74 ].

An example of a hydrothermal reaction is the synthesis of zinc oxide as proposed by Chen et al. [ 50 ], using the reagents ZnCl 2 and NaOH in a ratio of 1:2, in an aqueous environment. The process took place by way of reaction (5):

The white Zn(OH) 2 precipitate underwent filtration and washing, and then the pH was corrected to a value of 5–8 using HCl. In the autoclave hydrothermal heating takes place at a programmed temperature for a set time, followed by cooling. The end product of the process is zinc oxide the following reaction (6):

The average size and the morphology of the resulting ZnO particles were analyzed using an X-ray diffractometer (XRD) and transmission electron microscope (TEM). The temperature and time of reaction were shown to have a significant effect on the structure and size of the ZnO particles. It was also found that as the pH of the solution increases, there is an increase in the crystallinity and size of the particles, which reduces the efficiency of the process.

A hydrothermal process was also used by Ismail et al. [ 51 ], who obtained zinc oxide by way of the following reactions (7) and (8):

The chemical reaction between Zn(CH 3 COO) 2 and NaOH was carried out in the presence of hexamethylenetetramine (HMTA), at room temperature. The resulting precipitate of Zn(OH) 2 was washed with water several times, and then underwent thermal treatment in a Teflon-lined autoclave. Based on SEM images, the authors concluded that the HTMA, as a surfactant, plays an important role in the modification of the ZnO particles. The shape of the particles is also affected by the time and temperature of the hydrothermal process. With an increase in time, temperature and surfactant concentration, the size of the particles increases. Hydrothermal processing of the precursor, followed by drying, produced spherical particles of ZnO with sizes in the range 55–110 nm depending on the conditions of synthesis.

Dem’Yanets et al. [ 52 ] used a hydrothermal method to synthesize nanocrystalline zinc oxide with different particle shapes and sizes. A reaction of zinc acetate or nitrate with a suitable hydroxide (LiOH, KOH, NH 4 OH) produced the precursor Zn(OH) 2 · n H 2 O. The process was carried out in an autoclave, in isothermal conditions or at variable temperature (120–250 °C). Dehydration of the precursor, followed by recrystallization, produced crystallites of ZnO with a hexagonal structure and sizes of 100 nm–20 μm. Increasing the time of the hydrothermal process caused an increase in the diameter of the ZnO particles. It was observed that an increase in temperature by 50–70 °C enabled a fourfold reduction in the time of the experiment, which is a very favourable phenomenon.

Musić et al. [ 53 ] determined the effect of chemical synthesis on the size and properties of ZnO particles. A suspension obtained from a solution of Zn(CH 3 COO) 2 ·2H 2 O and neutralized using different quantities of a solution of NH 4 OH underwent hydrothermal treatment in an autoclave at a temperature of 160 °C. It was found that the pH affected the size and shape of the ZnO particles. Maturing of the original aqueous suspension for 7 months (at a pH of 10, and at room temperature) led to the appearance of aggregates consisting of ZnO particles with sizes between ~20 and ~60 nm. Musić et al. , also synthesized zinc oxideu sing a sol-gel method, involving rapid hydrolysis of zinc 2-ethylhexanoate dissolved in propan-2-ol. The resulting nanoparticles cause distinct changes in the standard Raman spectrum of zinc oxide.

A number of studies [ 54 , 55 , 75 , 76 ] have shown that the use of microwave reactors in hydrothermal synthesis processes brings significant benefits. Microwaves make it possible to heat the solutions from which the synthesis products are obtained, while avoiding loss of energy on heating the entire vessel. Many chemical syntheses proceed with greater speed and yield when microwaves are used than in the case of traditional methods. Similar fast and voluminal heating of the reaction substrates can be achieved using electrical current flowing through the substrates. Strachowski et al. [ 77 , 78 ] carried out a systematic study comparing the ZnO obtained in reactors with different methods of reaction stimulation. The work was conducted in such a way that the reactions being compared took place at the same externally supplied power levels and with the same reaction vessel geometry. Reactors were used with reaction energy supplied using microwaves, electrical current, Joule heating, high-voltage pulses, and heating of the whole autoclave. Strachowski et al. found that nanopowders with phase composition closest to pure ZnO were obtained through microwave synthesis and in the traditional autoclave. The powders produced in the other reactors showed the presence of other phases (simonkolleite and hydroxyzincite) besides zinc oxide. The use of a microwave reactor made it possible to shorten the reaction time several fold, and also produced the purest product.

Microwaves were also used by Schneider et al. [ 56 ]. Zinc oxide was obtained by heating, using microwaves, zinc acetylacetonate and a zinc oxime complex in various alkoxyethanols (methoxy-, ethoxy- and butoxyethanol). Schneider et al. , showed that the morphology and aggregation of ZnO particles depends strongly on the precursor used. The zinc oxide obtained was analyzed using such methods as DLS (dynamic light scattering), BET surface area, SEM, TEM, XRD, TG, PL spectra and EPR (electronparamagnetic resonance). The size of the particles of the final product lay in the range 40–200 nm, depending on which precursor and alcohol were used. The smallest particles belonged to the zinc oxide obtained by heating a complex of zinc oxime in methoxyethanol. With an increase in the concentration and chain length of the alcohol, the particle size increased. The surface area of the ZnO lay in the range 10–70 m 2 /g. Thermal decomposition of both zinc acetylacetonate and zinc oxime enabled the obtaining of a product with the desired properties.

Zhang et al. [ 55 ] obtained ZnO particles in the shape of spheres and hollow spheres through a solvothermal reaction, in the presence of an ionic liquid (imidazolium tetrafluoroborate). The authors suggested that the solvothermal process may involve the following reactions (9) and (10):

The hollow spheres which were obtained had diameters of 2–5 μm and contained channels approximately 10 nm in diameter. The thickness of the wall of such a sphere was approximately 1 μm. The system proposed by Zhang et al. may combine the properties of both a solvothermal hybrid and an ionothermal system. It can be expected that a solvothermal hybrid and an ionothermal system may be successfully used to synthesize new materials with interesting properties and morphologies.

A solvothermal method was also used by Chen et al. [ 54 ], who prepared nanocrystalline ZnO, free of hydroxyl groups. It was obtained from a reaction of zinc powder with trimethylamine N -oxide (Me 3 N→O) and 4-picoline N -oxide (4-pic→NO), carried out in an environment of organic solvents (toluene, ethylenediamine (EDA) and N,N,N′,N′ -tetramethylenediamine (TMEDA)), in an autoclave at 180 °C. The process involved the following reactions (11) and (14):

The oxidizing agents used and the coordinating abilities of the solvents affected the morphology and size of the nanoparticles/nanowires of ZnO. The authors also determined the effect of the presence of water in the reaction system. It was found that the presence of trace quantities of water catalyzed the reaction between zinc powder and 4-picN→O and affected the size of the ZnO nanocrystallites. The zinc oxide obtained had diameters in the range 24–185 nm, depending on the reaction conditions.

2.2.5. Method Using an Emulsion or Microemulsion Environment

The classic definition of an emulsion as a continuous liquid phase in which is dispersed a second, discontinuous, immiscible liquid phase is far from complete. One very convenient way to classify emulsions is first to divide them into two large groups based on the nature of the external phase. The two groups are usually called oil-in-water (O/W) and water-in-oil (W/O) emulsions. The terms “oil” and “water” are very general; almost any highly polar, hydrophilic liquid falls into the “water” category in this definition, while hydrophobic, nonpolar liquids are considered “oils” [ 60 , 61 ].

Vorobyova et al. [ 58 ] used emulsion systems in their work. Zinc oxide was precipitated in an interphase reaction of zinc oleate (dissolved in decane) with sodium hydroxide (dissolved in ethanol or water). The process as a whole involved the reaction (15):

SEM and XRD analysis was performed on the ZnO powders obtained, following removal of the solvents and drying at room temperature. It was found that the reaction may take place in different phases, both in water and in the organic phase. The conditions of the process (temperature, substrates and ratio of two-phase components) affect the size of the particles and the location of their phases. Vorobyova et al. obtained zinc oxide with different particle shapes (irregular aggregates of particles, needle shapes, near-spherical and near-hexagonal shapes, and spherical aggregates) and with diameters in the range: 2–10 μm, 90–600 nm, 100–230 nm and 150 nm respectively, depending on the process conditions.

An emulsion method was also used in the work of Lu and Yeh [ 59 ]. The aqueous phase of the system was zinc acetate dissolved in de-ionized water, and the organic phase was heptane. To stabilize the water-in-oil emulsion, the surfactant Span 80 was added to the heptane. NH 4 OH was added to the emulsion in order to obtain the zinc cation. The precipitate was dried, and then calcined at 700–1000 °C. The resulting ZnO calcinates were analyzed by XRD, IR and SEM. Lu and Yeh concluded that ZnO precipitated in this emulsion system has a smaller range of particle sizes (0.05–0.15 μm) compared with ZnO obtained in a traditional system (0.10–0.45 μm). The product consisted of nearly spherical particles.

Zinc oxide was also obtained by precipitation in an emulsion system with zinc acetate used as a precursor of ZnO, and potassium hydroxide or sodium hydroxide as precipitating agent [ 60 ]. Cyclohexane, as an organic phase, and a non-ionic surfactant mixture were also used for preparation of the emulsion. By applying modifications of the ZnO precipitation process, such as changing the precipitating agent, composition of substrates and the rate of substrate dosing, some interesting structures of ZnO particles were obtained. The morphology of the modified samples was analyzed based on SEM (scanning electron microscope) and TEM (transmission electron microscope) images. Moreover the samples were analyzed by determination of their dispersive properties using the non-invasive back scattering method (NIBS), parameters of porous structure ( BET ) and crystalline structure (XRD). Thermogravimetric analysis (TG) as well as infrared spectrophotometry (FTIR) were also applied. For selected samples their electrical properties (dielectric permittivity and electric conductivity) were also measured. The zinc oxide obtained consisted of particles in the shapes of solids, ellipsoids, rods and flakes ( Figure 4 ), with sizes ranging from 164 to 2670 nm, and was found to have large surface area, with values as high as 20 m 2 /g.

Zinc oxide structures: ( a ) solids; ( b ) ellipsoids; ( c ) rods; and ( d ) flakes [ 60 ].

Emulsions and microemulsions differ markedly from each other, which makes it relatively easy to identify the areas of their application. Microemulsions are stable, transparent, isotropic liquids consisting of an aqueous layer, and oil layer and a surfactant. The drop size in a microemulsion is significantly smaller than in an emulsion, and lies in the range 0.0015–0.15 μm [ 79 – 81 ]. In contrast to emulsions, microemulsions form spontaneously in appropriate conditions.

Li et al. [ 61 ] proposed a method of preparing nanometric zinc oxide using a microemulsion which is formed when alcohol is added to an emulsion system consisting of water, oil and emulsifier, until a transparent mixture is obtained. In this case the microemulsion consists of a solution of heptane and hexanol together with a non-ionic surfactant (such as Triton X-100). The growth of nanoparticles involves the exchange of the substrates Zn(NO 3 ) 2 and NaOH between the microemulsion drops and the medium (poly(ethylene glycol)—PEG 400), and aggregation of the formed nuclei. Drops of microemulsion act as a microreactor in which the desired reaction takes place. In the synthesis of ZnO, different concentrations of PEG 400 were used (0%–50%). Figure 5 illustrates the process of synthesis in microemulsion and the shape of ZnO nanoparticles as proposed by the aforementioned authors.

Synthesis and morphology of crystalline ZnO synthesized in a microemulsion system: ( a ) without PEG 400; and with the addition of: ( b ) 12.5%–25% PEG 400; ( c ) 50% PEG 400 (created based on [ 61 ] with permission from Elsevier Publisher).

Another process of precipitation of zinc oxide in the environment of a microemulsion was proposed by Singhal et al. [ 62 ]. Zinc oxide was obtained from a microemulsion consisting of ZnO-AOT/ethanol/isooctane (AOT—sodium bis-(2-ethylhexyl)-sulfosuccinate). For converting Na(DEHSS) (sodium diethylhexylsulfosuccinate) into Zn(DEHSS) 2 , an appropriate solvent was used, which dissolves Zn(DEHSS) 2 but does not precipitate NaNO 3 . Appropriate quantities of Na(DEHSS) (dissolved in dry ether) and Zn(NO 3 ) 2 (dissolved in ethanol) were mixed for 4 h at room temperature. It is important that the solvents do not contain water, which might dissolve NaNO 3 during the reaction, causing contamination of the precursor. The resulting solution was filtered and dried. Residues of water in the Zn(DEHSS) 2 were removed by washing the precipitate with benzene. To ensure that the sample did not contain sodium, Zn(NO 3 ) 2 was also added to the solution. At the next stage an anhydrous microemulsion of alcohol in oil was prepared. For this purpose Zn(DEHSS) 2 was dissolved in isooctane, and then dry ethanol was added. To the microemulsion prepared in this way, zinc oxalate was added in excess, in the form of a fine powder, so as to precipitate Zn 2+ ions. The entire solution was mixed for 1 h. The precipitate was separated from the solution by centrifugation, and the precipitate obtained was then washed twice.

The first washing was performed using a methanol:chloroform mixture in a ratio of 1:1 by volume, to remove surfactant and oil. The second used an acetone:methanol mixture (1:1) to remove surfactant and excess oxalic acid. The dried precipitate was calcined at 300 °C for 3 h to produce nanoparticles of ZnO. Singhal et al. precipitated zinc oxide with particles in the range 11–13 μm and with a BET surface area of 82–91 m 2 /g, depending on the conditions applied.

The technique of obtaining ZnO using microemulsion was also used by Yildirim and Durucan [ 63 ]. They attempted to modify the microemulsion method so as to obtain monodisperse zinc oxide. They did not obtain zinc oxide directly from the microemulsion process, but used thermal decomposition of the zinc complex precipitated in the microemulsion process, followed by its calcination. The process was modified in that glycerol was used as the internal phase of a reverse microemulsion (Aeroloz OT:glycerol:heptane), similarly as Moleski et al. [ 64 ] did in preparing ZnO nanoparticles on amorphous silica. The basic aim of the work of Yildirim and Durucan was to determine how the concentration of surfactant and the temperature of calcination affect the size and morphology of the resulting ZnO particles. The final product was analyzed using such techniques as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetry (TGA), differential scanning calorimetry (DSC), and luminescence spectroscopy (PL). The zinc oxide obtained consisted of spherical and monodisperse particles measuring 15–24 nm.

In turn, Xu et al. [ 37 ] compared three methods for precipitating zinc oxide: emulsion, microemulsion, and chemical precipitation. In all of these methods the starting solution used was zinc nitrate (Zn(NO 3 ) 2 ). The emulsion consisted of Zn(NO 3 ) 2 and an appropriate surfactant (cationic, anionic or non-ionic). However the microemulsion was prepared from Zn(NO 3 ) 2 , cyclohexane, acrylonitrile-butadiene-styrene copolymer (ABS), butanol, and hydrogen peroxide (H 2 O 2 ). All processes were carried out in a reactor at a temperature of 25 °C (for the chemical and emulsion processes) or 60 °C (for the microemulsion process). The precipitate was dried at 80 °C for 24 h, and then calcined at 600 °C for 2 h. On comparing the methods, the authors concluded that the smallest ZnO particles were obtained from the process carried out in a microemulsion environment (≤20 nm), larger ones were obtained from the emulsion (20–50 nm), and the largest were obtained by chemical precipitation (>50 nm). The size of the ZnO particles precipitated from the emulsion system depended on the type of surfactant: cationic (40–50 nm), non-ionic (20–50 nm) or anionic (~20 nm). In summing up their work, Xu et al. noted that the size of the ZnO particles depends on the method of precipitation and the type of surfactant used.

2.2.6. Other Methods of Obtaining Zinc Oxide

There also exist many other methods of obtaining zinc oxide, including growing from a gas phase, a pyrolysis spray method, a sonochemical method, synthesis using microwaves, and many others.

Zinc oxide was obtained in the form of pure crystals by Grasza et al. [ 82 ]. Crystals of ZnO were grown from a gas phase (air, nitrogen, atmospheric oxygen, gaseous zinc and arsenic). A wide range of values of heating time and temperature were used. Particular emphasis was placed on analysing the surface during its interactions with the air, oxygen and gaseous zinc. The diversity of the morphology and the purity of the crystal surface were analyzed using AFM and XRD. It was found that thermal heating in the various gases led to similar changes in the crystal surface, although differences were observed in the rate of those changes. Grasza et al. , showed that heating in gaseous zinc leads to a surface roughness of less than 1 nm, while heating in gaseous arsenic causes degradation of the crystal surface. Tests showed that the porosity of the crystal surface increases with increasing temperature and heating time. The “milky” crystal surface obtained is a result of imperfections arising during the preparation of the surface for heating.

A thin layer of zinc oxide was obtained by Wei et al. [ 83 ] in an atmosphere of O 2 , under a pressure of 1.3 Pa, using the pulsed laser deposition method (PLD), with powdered and ceramic ZnO. They determined the effect of temperature on the structural and optical properties of the thin ZnO layer, using techniques such as XRD, SEM, FTIR and PL spectra. The results obtained by Wei et al. indicate that the best structural and optical properties of ZnO are obtained for a thin layer of zinc oxide produced at 700 °C using ZnO powder, and at 400 °C using ceramic ZnO. The PL spectra indicate that UV emission increases with increasing temperature. It was found that the quantity of UV emitted for a thin layer of ZnO made using powder was smaller than in the case of ceramic ZnO.

Using an aerosol pyrolysis method, Zhao et al. [ 65 ] obtained ultrapure particles of ZnO. As a zinc precursor they used Zn(CH 3 COO) 2 ·2H 2 O, in view of its high solubility and low temperature of decomposition. A determination was made of the mechanism and kinetics of the thermal decomposition of zinc acetate dihydrate, as well as a correlation of the mechanism with the results of the aerosol pyrolysis process. Analysis of the DTA and TG curves shows that the water of crystallization is lost below 200 °C, and anhydrous zinc acetate begins to form. At 210–250 °C the anhydrous zinc acetate decomposes into ZnO and organic compounds by way of endothermic and exothermic reactions. The process of decomposition of Zn(CH 3 COO) 2 ·2H 2 O is complete at 400 °C. Zinc oxide synthesized by the aerosol pyrolysis method consists of particles in the range 20–30 nm.

Hu et al. [ 57 ] produced connected rods of ZnO using a sonochemical process (exposure to ultrasound in ambient conditions), and by microwave heating. In the sonochemical method an aqueous solution of Zn(NO 3 ) 2 ·6H 2 O was added to (CH 2 ) 6 N 4 (hexamethylenetetramine, HMT). The resulting solution was exposed to ultrasound for 30 min, as a result of which the reaction temperature reached approximately 80 °C. In the second method, the solution was heated using microwaves at 90 °C for 2 min. The precipitates obtained were centrifuged, washed with water and ethanol, and dried at 60 °C for 2 h. The resulting ZnO was analyzed using XRD, TEM, SAED (selected area electron diffraction), EDS, and PL spectra. These methods of obtaining ZnO enable high yields, in excess of 90%. In summing up their work, Hu et al. stated that the sonochemical and microwave heating methods do not require surfactants, can be applied on a large scale with low production costs, and are simple and energy-efficient. The ZnO rods obtained can be successfully used in electronics and optoelectronics. The sonochemical method may be used in the future for the synthesis of single-dimensional structures of other metal oxides.

3. Methods of Modification of Zinc Oxide

The search for new possible applications of zinc oxide, the need to reduce its content in rubber mixtures, and the major problem of its tendency to form significant agglomerations have encouraged researchers in recent years to carry out numerous studies to find an optimum method of modifying the surface of the compound without impairing its physicochemical properties. Modification is also often carried out in order to improve its performance properties, such as high or low (depending on application) photocatalytic activity. In the following sections we will consider the methods of modification of zinc oxide proposed by various scientists. Figure 6 presents a schematic that summarizes all the method of modifiction of ZnO mentioned in the text.

Schematic diagram of the most popular modifying methods of ZnO.

Cao et al. [ 41 ] performed modification of zinc oxide using silica and trimethyl siloxane (TMS). The finest particles of ZnO were obtained by calcination of the precursor zinc carbonate hydroxide (ZCH). ZHC was obtained in a process of precipitation from substrates such as zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O), ammonium solution (NH 4 OH) and ammonium bicarbonate (NH 4 HCO 3 ). The surface of the ZCH was then successively modified by an in situ method using TEOS and hexamethyldisilazane (HMDS) in water. The ZHC functionalized in this way was calcined, to obtain ultrafine particles of ZnO. Modification of the ZnO particles made possible a solution to the problem of their agglomeration. Functionalization of the ZnO surface with an inorganic compound (silica) reduced the photocatalytic action of the oxide, while the organic compound (HMDS) increased the compatibility of the ZnO with an organic matrix. The highly transparent modified zinc oxide surface was found to provide excellent protection against UV radiation, which represents a significant advantage of the use of these modifying agents. A schematic representation of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method is shown in Figure 7 .

Schematic representation of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method (created based on [ 41 ] with permission from Elsevier Publisher).

Modification with the use of silica was also performed by Xia and Tang [ 84 ]. By a method of controlled precipitation, clusters of zinc oxide were obtained on the surface of silica modified using triethanolamine N(CH 2 CH 2 OH) 3 (TEOH) and containing silanol (≡Si-OH) and siloxane (≡Si-O-Si≡) groups. Molecules of TEOH are adsorbed by the silica, and the siloxane and silanol networks are broken as a result of the changes occurring in the SiO 2 . The Zn 2+ ions, in reaction with triethanolamine, produce clusters of ZnO on the silica surface. In accordance with the theory of maturing and aggregation, the resulting clusters are susceptible to rapid collision with other clusters of zinc oxide, leading to an appropriate concentration of the compound. An important role in the proposed modification technique is played by TEOH, which enables complex structures to be obtained.

Hong et al. [ 36 ] also performed modification of zinc oxide using silica. They also performed an additional modification using oleic acid. Zinc oxide was obtained as a result of the reaction of zinc acetate with ammonium carbonate, followed by calcination of the resulting zinc precursor. To determine the compatibility between the inorganic nanoparticles and the organic matrix, the surface of the ZnO was covered with oleic acid. The FTIR spectra confirmed the presence on the surface of the modified ZnO of an organic layer and a chemical bond between the inorganic –OH groups and the organic chain macromolecules. The proposed mechanism for these processes was presented in terms of reaction (16):

When SiO 2 was used as a modifier, the FTIR spectrum indicates the presence of an interphase bond between ZnO and SiO 2 . Coverage of the zinc oxide surface with a thin film of amorphous silica improved the degree of dispersion, and thus reduced the agglomeration of nanoparticles. Moreover, based on photocatalytic degradation in aqueous solution using methyl orange, it was shown that silica-coated ZnO has lower catalytic activity than the original nanostructures. The work of Hong et al. showed that heterogeneous azeotropic distillation of the zinc precursor completely reduces the crystalline structure of ZnO, and thus makes it possible to avoid large aggregation and reduces the average particle size. Similar studies have been carried out and published by those authors in [ 85 ].

Hydrophobic ZnO nanoparticles were produced by Chen et al. [ 86 ] and a novel treatment process was developed by them to obtain highly dispersed and long-term stable ZnO nanoparticles, in an organic matrix. Aminopropyltriethoxysilane (APS) was grafted onto the surface of ZnO nanoparticles, and a long carbon chain of stearic acid (SA) was introduced through a condensation reaction between APS and the activated SA with N , N′ -carbonyldiimidazole (CDI). ZnO nanoparticles were analyzed by FTIR, TGA, SEM and a sedimentation test. The FTIR and TGA results showed that APS and SA were linked on the surface of ZnO nanoparticles through chemical bonds, and the CDI activator clearly promotes the condensation reaction and increases the grafting ratio of SA. Results from the SEM observations and sedimentation test indicate that the new surface treatment would considerably reduce aggregates of particles and enhance long-term stability in an organic matrix.

Modification of ZnO using an inorganic compound, namely Al 2 O 3 , was carried out by Yuan et al. [ 87 ]. Nanometric zinc oxide coated with Al 2 O 3 , with diameter 50–80 nm, was obtained by calcination of basic zinc carbonate (BZC) with simultaneous modification with a precipitate of Al(OH) 3 at 400–600 °C. The coating obtained was highly uniform, and had a thickness of 5 nm. The pH at the isoelectric point for ZnO nanoparticles with an Al 2 O 3 layer moved from around 10 to a value of 6, which may improve the dispersion of ZnO particles.

Wysokowski et al. [ 88 ] decided to develop a ZnO-containing composite material using β-chitin from Sepia officinalis cephalopod mollusk as the source of chitin. They suggest that application of morphologically defined β-chitin as a template for biomimetic ZnO deposition is very attractive from a technological point of view as it eliminates challenges associated with manufacturing chitin to chitosan and with processing to membranes or scaffolds.

Pyskło et al. [ 89 ] performed modification of zinc oxide using poly(ethylene glycol) and octadecyltrimethoxysilane, in order to improve its dispersion in rubber mixtures. The modification used zinc oxide synthesized by a hydrothermal method (microwave dehydration). The modification was carried out in the following way: in a solution containing 5% by mass of modifier (PEG or silane) relative to the mass of ZnO used, a ZnO nanopowder was dispersed. The resulting system was then mixed using an ultrasound disintegrator (in the case of silane the system was first mixed with a magnetic mixer, with simultaneous heating). The resulting precipitate was filtered and dried at a temperature of 80 °C for 48 h. Analysis of the FTIR spectra revealed the presence of -OH groups on the surface of the ZnO; for this reason the ZnO surface was modified with octadecyltrimethoxysilane. The modification was carried out as follows: ZnO was dispersed in an emulsion containing 5% by weight of silane. The whole was mixed with a magnetic mixer, with heating, for 5 min. The resulting precipitate was filtered and dried for 48 h at 80 °C. The addition to rubber mixtures of PEG-coated nano zinc oxide an increase in the degree of cross-linking of the vulcanizates, but there was also an increase in vulcanization reversion and a marked decrease in prevulcanization time. The samples of ZnO obtained were additionally analyzed using inverse gas chromatography (IGC), in order to determine the dispersive component of surface energy (γsD). Based on the results it was concluded that coating the surface of nanopowders with polyglycol or silane causes a decrease in the value of γsD (a better effect was obtained when PEG was used). The marked decrease in surface energy in the case of oxides modified with PEG and silane can be expected to facilitate their dispersion in nonpolar rubbers.

Modification of the surface of ZnO particles using silane was also performed by Kotecha et al. [ 90 ]. The modifier used was 3-methacryloxypropyltrimethoxysilane. Nanoparticles of zinc oxide were obtained using zinc acetate and potassium hydroxide as substrates. The precipitate was filtered and washed with methanol, and then dried at 130 °C. In this method the silane was introduced into the system during the precipitation. Concurrently with the formation of ZnO particles, a reaction takes place between silane and ZnO. In the course of this reaction H 2 O is generated and a side reaction takes place, during which the pH increases to 9. The silane-covered zinc oxide particles were introduced into an aqueous suspension and exposed to UV radiation. Based on interpretation of SEM images, the researchers concluded that unmodified zinc oxide contains particles around 100 nm in diameter, forming agglomerates. The introduction of silane into the ZnO structure caused a decrease in the particle size (40–100 nm) and an increase in the diameters of the aggregates, even to the order of micrometres. The irradiated ZnO particles had a fibrous structure “resembling wool”, and offered promising catalytic properties. UV radiation also changes the character of ZnO from hydrophobic to hydrophilic. Analysing the adsorption parameters, Kotecha et al. found that the surface area of silane-modified ZnO initially increases together with the concentration of silane, until that concentration reaches a value of approximately 1–2 mol—then the BET surface area starts to decrease. For the irradiated ZnO samples, the value of BET surface area continues to increase as the silane concentration increases, reaching a maximum of approximately 130 m 2 /g for the highest concentration. The results of Kotecha et al. imply that UV irradiation destroys organic domains. The resulting material has high porosity, large BET surface area, and hydrophilic properties.

Figure 8 shows example mechanisms taking place during the process of modification of zinc oxide using a selected silanol binding compound.

Probable mechanism occurring during modification of ZnO using vinyltrimethoxysilane.

Chang et al. [ 91 ] modified the surface of ZnO using LiCoO 2 . Zinc oxide covered with a layer of LiCoO 2 was obtained by plasma-enhanced chemical vapour deposition (PE-CVD). In their work, Chang et al. confirmed the favourable effect of ZnO on the electrochemical yield and thermal stability of LiCoO 2 , which is used as a cathode material in Li-ion batteries. Covering ZnO with a layer of LiCoO 2 causes an increase in the surface area of the functionalized ZnO (from 0.4 to 1 m 2 /g).

Change of state of the surface of zinc oxide nanowires through plasma treatment is one of the most promising methods of ZnO modification. Experiments carried out by Ra et al. [ 92 ] aimed to determine how reactive chemical treatment using oxygen affects electrical transport, gas selectivity and the internal photoelectric effect of ZnO nanowires with a diameter of 80 nm, using a field-effect transistor (FET). A significant increase in the concentration of oxygen was observed, in the form of active oxygen centres (O 2− and OH − ) on the nanowire surface. After treatment the concentration of the carrier and mobility of the ZnO decreased. There was also an improvement in properties relating to the detection of hydrogen by the modified nanowires, and the time of photocurrent amplification in UV radiation. In summarizing their work, Ra et al. , stated that modification of the surface of ZnO using plasma treatment with oxygen opens up new possibilities for the production of electronic devices, catalysts and high-performance sensors.

In turn, Kang and Park [ 93 ] modified zinc oxide using silver ions. The ZnO was prepared using ultrasonic aerosol pyrolysis (FEAG) of a colloidal solution of zinc acetate. The size of the ZnO particles obtained by the FEAG method depended on the conditions of the operation and the type of solvent. Based on TEM images it was found that the ZnO obtained consisted of particles measuring approximately 12 nm. Next the ZnO was dispersed in a solution of silver nitrate in various ratios. As the ZnO:Ag mass ratio was increased, there was a change in the product’s surface area and particle size. Kang and Park obtained a ZnO-Ag composite with particles measuring approximately 120–250 nm and with a surface area of 3–6 m 2 /g.

Šćepanović et al. [ 94 ] modified ZnO using mechanical activation. A commercial ZnO powder was activated mechanically by grinding in a vibrating mill with steel rings, under continuous air circulation. The process was continued for 30 and 300 min. The product was subjected to comprehensive physicochemical analysis. Based on SEM images, Šćepanović et al. noted that the size of the crystallites of the modified ZnO was smaller, and the surface area was larger, than in the case of the unmodified product (for example, from 190 nm to 106 and 44 nm, and A BET from ca. 3 m 2 /g to 4 and 6 m 2 /g).

Wu et al. [ 95 ] produced ordered ZnO nanofibres by an electrospinning method, and modified the nanofibres using CdS with a nanocrystal layer deposition method. They then investigated the performance of hybrid solar cells based on the CdS/ZnO nanofibres and P3HT (poly(3-hexylthiophene)). The devices were optimized by changing the number of layers of cross-aligned ZnO nanofibres and the growth time of CdS on the ZnO. Wu et al. , found that the power conversion efficiency (PCE) of such a hybrid solar cell was improved by more than 100% after CdS modification. In addition, the lifetime of carriers at the bulk heterojunction was investigated using an impedance analyser and was found to be dramatically increased after CdS modification.

Over the past decade much work has been done on developing nanocomposites produced by the action of modified inorganic carriers with polymer matrices. Such procedures make it possible to produce new classes of polymeric materials which combine properties of both inorganic particles and organic polymer matrices (including process ability and elasticity). The MO/polymer composites produced in this way have unique electrical, thermal and optical properties, which enable their range of applications to be extended in many branches of industry [ 96 – 101 ].

Shim et al. [ 102 ] carried out modification of zinc oxide using poly(methyl methacrylate) (PMMA). A ZnO/PMMA composite was synthesized by means of polymerization in situ . The majority of microspheres of the MO/polymer composite are produced by coupling of existing polymer chains with the inorganic surface or by polymerization on the phase boundary of inorganic particles. Shim et al. , demonstrated that the stability of dispersion of ZnO in a monomer depends strongly on the nature of its surface, since this provides a precondition enabling dispersion of particles of the medium within drops of monomer and consequently their enclosure in PMMA. The most important condition in the production of the composite is the interphase compatibility between the inorganic compound and the polymer. For this purpose the surface of the inorganic system should be treated with a hydrophobic organic substance. The obtained inorganic-polymer composites form persistent microspheres and combine easily into highly processed polymers. Similar studies have been carried out and published by other researchers [ 103 , 104 ].

Tang et al. [ 105 ] modified zinc oxide using poly(methacrylic acid) (PMAA). The hydroxyl groups on the ZnO surface reacted with the carboxyl groups of the PMAA, producing a complex of poly(zinc methacrylate) on the surface of the zinc oxide. Interpreting the particle size distributions, it was found that the ZnO modified with PMAA contains particles with smaller diameter ( ca. 70 nm) compared with unmodified ZnO ( ca. 300 nm). Analysis of the dispersive stability of the ZnO showed that the modified particles of zinc oxide dispersed better in water than unmodified particles. Conventional inorganic nanoparticles have hydroxyl groups (-OH) on their surface, due to the effect of humidity and the environment and type of precipitation. These groups react with COO- groups to form small complexes of poly(zinc methacrylate) on the surface of the zinc oxide. Analysis using the FTIR, TGA, TEM and XRD techniques confirms the presence of polymer molecules on the zinc oxide surface.

Poly(methyl methacrylate) was also used as a ZnO surface modifier by Hong et al. [ 106 ]. Nanoparticles of zinc oxide with a diameter of approximately 30 nm were synthesized by means of homogeneous precipitation followed by calcination. In order to introduce reactive groups onto the ZnO surface, a reaction was carried out between the hydroxyl groups and a silane coupling agent (3-methacryloxypropyltrimethoxysilane). Graft polymerization was effected by means of a reaction between the ZnO, containing silanol groups, and the monomer. Tests showed that the polymerization does not alter the crystalline structure of the ZnO nanoparticles. Their dispersion in the organic solvent can greatly improve the graft polymerization of PMMA, and further improvement can be achieved by the addition of other surfactants. Modification of ZnO nanoparticles by grafted PMMA increases the degree of lyophilicity of the inorganic surface and reduces the formation of aggregates. The work of Hong et al. , showed that ZnO nanoparticles grafted with PMMA can increase the thermal stability of polystyrene.

Polystyrene (PS) is also the subject of interest as a surface modifier of zinc oxide particles. Chae and Kim [ 107 ] carried out a process of ZnO surface modification using that compound. In the process of obtaining a PS/ZnO composite, first an appropriate quantity of commercial ZnO (particle size 87 nm) was dispersed in a solvent with the help of ultrasound for 10 min. The solvent used was N,N -dimethylacetoacetamide (DMAc). Next, in the resulting DMAc/ZnO solution, polystyrene was dissolved, mixing vigorously for 2 h at 70 °C. To obtain a layer of nanocomposite the solutions were kept at a temperature of 90 °C for 4 days. Then the layer was dried (at 100 °C for 5 days) and hot pressed (at 200 °C), completely removing the remaining DMAc. For the structures obtained, the morphology, microstructure, thermal properties and mechanical properties were investigated. Spectral and X-ray identification were also performed, using techniques including TEM, FESEM, DSC, TGA, FTIR and WAXS. The tests confirmed that the solvent used is capable of breaking up the agglomerates that form, and prevents re-agglomeration during mixing of the solution.

An object of interest in recent years has been the resistance connections of ZnO particles embedded in MIM (metal-insulator-metal) structures. Work has focused on altering the layers of oxides, whose amorphous nature, porosity and lack of homogeneity constitute a problem. Researchers under the direction of Verbakel [ 108 ] investigated the resistance effects of the switching of diodes containing structures of nanometric ZnO covered with an active layer from a polystyrene matrix. These diodes consist of two PEDOT:PSS electrodes. Using an impedance spectroscope it was found that the electronic memory effect in nanostructured metal oxides can be affected by modification of the surface of the particles using coordinating ligands (e.g., amines and thiols), and this depends on the temperature of voltage measurements. This process provides new prospects for ecological modification of the surface of ZnO powder using inorganic hybrid materials.

Modification of ZnO nanoparticles using polystyrene was also performed by researchers under the direction of Tang [ 109 ]. Nanometric particles of zinc oxide (particle size ca. 40 nm) were “enclosed” in polystyrene, with a process of emulsion polymerization being carried out in situ in the presence of 3-mercaptopropyltrimethoxysilane (MPTMS) as a coupling agent and polyoxyethylene nonylphenyl ether (OP-10) as surfactant. The nano-ZnO surface had to have a hydrophobic character, in order to hermetically seal the ZnO nanoparticles perfectly in the monomer. This property was controlled by the creation of functional groups on the nano-ZnO surface with the use of a silane coupling agent (MPTMS). Consequently the MPTMS molecules were grafted on the surface of the nano-ZnO. MPTMS is an organic polymer chain which forms steric hindrances between inorganic particles, preventing their aggregation. However it was not simple to obtain perfect dispersion of the hydrophobic nano-ZnO particles in an aqueous polymerization system. To ensure stability of dispersion, a surfactant (OP-10) was added to the system, in a quantity smaller than that which properly saturates the surface, so as to avoid the formation of micelles of emulsifier. Tang et al. , proposed a mechanism for the polymerization ( Figure 9 ). Tests showed that the particles of the resulting polymer composite are monodisperse, with diameters in the range 150–250 nm.

Mechanism of nano-ZnO/PS composite synthesis by in situ emulsion polymerization (created based on [ 109 ] with permission from Elsevier Publisher).

Another modifier applied on the surface of ZnO is polyacrylnitryl (PAN). Studies with that compound were carried out by Chae and Kim [ 110 ]. ZnO nanopowder (particles of diameter 87 nm) was dissolved in DMAc to break up agglomerates. PAN was then added to the solution, and it was mixed vigorously at 70 °C. To obtain the nanocomposite, the PAN-ZnO solution was kept at 80 °C for 4 days, and at the next stage was dried at 100 °C for 5 days. The resulting precipitate underwent spectroscopic, thermal and mechanical analysis. The product exhibited better thermal stability than the starting material, due to the barrier role of ZnO. Moreover the ZnO nanoparticles caused a reduction in the crystallization temperature of the modifier (PAN) and an increase in the width of the crystallization peaks. This is linked to heterogeneous nucleation and the reduced mobility of the polymer chains. The introduction of ZnO nanoparticles into the polymer chain caused an increase in the modulus of elasticity on stretching and a reduction in the dynamic load resistance.

Xiong et al. [ 111 ] synthesized a new nanocomposite ZnO(PEGME), in which the ZnO nanoparticles and polymer groups (PEGME—poly(ethylene glycol) methyl ether) are linked by covalent bonds. The compound was analyzed in terms of composition, structure, fluorescence and specific conductance. The tests showed that the polymer nanocomposite synthesized by means of a chemical reaction has better properties than its equivalent obtained through physical mixing. The lasting stability of the properties of ZnO(PEGME) results from the strong chemical bond between the polymer and the nanoparticles. The hybrid ZnO(PEGME) has the capability of tuning luminescence spectra and has stable ionic conductance. These properties mean that the obtained compound can be used in luminescent devices and in electronic apparatus.

Modification of zinc oxide using carboxylic acids (such as stearic, tartaric, maleic, propanoic etc. ) makes it possible to introduce characteristic groups onto the surface of the ZnO and to alter its physicochemical properties. Studies of zinc oxide modified with carboxylic acids (wet modification) have shown that they do not significantly affect the morphological/dispersive or porous properties of zinc oxide. An apparently promising method is modification in situ , which causes a significant increase in the surface area of the zinc oxide (to as high as ca. 30 m 2 /g). Figure 10 shows an example mechanism taking place during modification of zinc oxide with maleic acid, and an FTIR spectrum confirming the effectiveness of the modification.

( a ) Probable mechanism of ZnO surface modification with maleic acid; and ( b ) exemplary FTIR spectra of obtained products.

It has been experimentally demonstrated that alkanethiols may adsorb on ZnO surface [ 112 – 114 ]. For example, Singh et al. [ 113 ] have investigated adsorption, in ultrahigh vacuum, of methanethiol (MT), 1-dodecanethiol (DDT) and 3-mercaptopropyltrimetoxysilane (MPTMS) on sputter-cleaned ZnO(0001) via either the silane or thiol and of the molecule. They also presented the first ultraviolet photoelectron spectroscopy (UPS) investigation of thiol adsorption on zinc oxide. It was found that the MT frontier orbitals are strongly perturbed by adsorption on ZnO(0001), with the work function of the surface increasing by 0.7 eV. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopies confirmed adsorption, and in situ photoluminescence measurements showed the intensity of the visible emission peak is decreased by methanethiol adsorption. Their other work [ 114 ] demonstrates a previously unreported method of encapsulating zinc oxide nanoparticles and nanorods within an organic matrix consisting of a 1:2 Zn/thiol complex. The thickness and morphology of the encapsulating layer was controllable by the choice of thiol and preparation conditions. Singh et al. , concluded that this method may be useful in future photovoltaic applications in which one wishes to surround ZnO nanorods and whiskers with light-absorbing molecules, which could be achieved by using thiol-terminated dye molecules.

4. Applications of Zinc Oxide

Because of its diverse properties, both chemical and physical, zinc oxide is widely used in many areas. It plays an important role in a very wide range of applications, ranging from tyres to ceramics, from pharmaceuticals to agriculture, and from paints to chemicals. Figure 11 shows worldwide consumption of zinc oxide by region.

Worldwide consumption of zinc oxide.

In the Figure 12 summarized application paths of ZnO are presented.

Schematic representation all the application of ZnO mentioned in the text.

4.1. Rubber Industry

Global production of zinc oxide amounts to about 10 5 tons per year, and a major portion is consumed by the rubber industry to manufacture various different cross-linked rubber products [ 115 ]. The thermal conductivity of typical pure silicone rubber is relatively low; however, it can be improved by adding certain thermal conductivity fillers, including metal powders, metal oxides and inorganic particles. Some kinds of thermal conductivity powder, such as Al 2 O 3 , MgO, Al 2 N 3 , SiO 2 , ZnO, etc. , can improve the thermal conductivity of silicone rubber while retaining its high electrical resistance, and are thus promising candidates as high-performance engineering materials. The incorporation of nano-scale fillers can achieve high thermal conductivity even at a relatively low filling content. However, the ZnO nanoparticles tend to aggregate together to form particles of large size in the polymer matrix, due to the weak interaction between the surface of the nanoparticles and the polymer.

In order to solve this problem, surface modification techniques are applied to improve the interaction between the nanoparticles and the polymer. In the work of Yuan et al. [ 116 ], in order to prepare the silicone rubber with high thermal conductivity, pristine and surface-modified ZnO nanoparticles containing the vinyl silane group are incorporated into the silicone rubber via a hydrosilylation reaction during the curing process. The corresponding structure, morphology and properties of the silicone rubber/ZnO (SR/ZnO) and silicone rubber/SiVi@ZnO (SR/SiVi@ZnO) nanocomposites were investigated. Yuan et al. synthesized ZnO nanoparticles (with an average size below 10 nm) by a sol-gel procedure. Next the silicone coupling agent VTES was successfully incorporated onto the surface of the nanoparticles. The SR/SiVi@ZnO nanocomposites showed better mechanical properties and higher thermal conductivity due to the formation of a cross-linking structure with the silicone rubber matrix and better dispersion in that matrix.

Zinc oxide is a very effective and commonly used cross linking agent for carboxylated elastomers [ 117 , 118 ]. It can be used to produce vulcanizates with high tensile strength, tear resistance, hardness and hysteresis. The improved mechanical properties of ionic elastomers mainly result from their high capacity for stress relaxation, due to elastomer chain slippage on the ionic cluster surface and reformation of ionic bonds upon external deformation of the sample. Moreover, ionic elastomers have thermoplastic properties and can be processed in a molten state as a thermoplastic polymer [ 119 ]. However, there are some disadvantages to zinc-oxide-cross linked carboxylic elastomers. The most important are their scorchiness, poor flex properties and high compression set. In order to prevent scorchiness, carboxylated nitrile elastomers are cross linked with zinc peroxide or zinc peroxide/zinc oxide systems. The vulcanization of XNBR with zinc peroxide mainly leads to the formation of ionic crosslinks; covalent links are also formed between elastomer chains due to the peroxide action. However, higher vulcanization times are required to achieve vulcanizates with a tensile strength and crosslink density comparable to that of vulcanizates cross linked with zinc oxide. In the case of XNBR vulcanization with zinc peroxide/zinc oxide systems, curing is the sum of at least three processes: the very fast formation of ionic crosslinks due to the initial zinc oxide present, peroxide cross linking leading to the formation of covalent links (peroxide action), and ionic cross linking due to the production of zinc oxide from peroxide decomposition. The last process, which decays with vulcanization time, most likely involves the formation of ionic species. The achieved vulcanization times are considerably higher than in the case of XNBR cross linking with zinc oxide. Therefore, apart from the scorch problems, zinc oxide is still commonly used as a cross linking agent in carboxylated nitrile rubbers.

In view of the fact that, during the cross linking process, zinc oxide reacts with the carboxylic groups of the elastomer, which leads to the formation of carboxylic salts (ionic crosslinks), the most important parameters influencing the activity of zinc oxide are its surface area, particle size, and morphology. These parameters determine the size of the interphase between the cross linking agent and elastomer chains [ 120 ].

Przybyszewska et al. [ 121 ] used zinc oxides with different surface areas, particle sizes and morphologies (spheres, whiskers, snowflakes) as cross linking agents of carboxylated nitrile elastomer, in order to determine the relationship between the characteristics of zinc oxide and its activity in the cross linking process. They concluded that the use of zinc oxide nanoparticles produced vulcanizates with considerably better mechanical properties and higher crosslink density, as compared with vulcanizates cross linked with micro-sized zinc oxide, which is used commercially as a cross linking agent. Vulcanizates containing the same quantity of zinc oxide nanoparticles exhibited a tensile strength about four times greater than that of vulcanizates with micro-sized particles. Therefore the use of nano-sized zinc oxide enables the quantity of zinc oxide to be reduced by almost 40%. This is a very important ecological goal, since zinc oxide is classified as toxic to aquatic species, and the European Union requires that the amount of zinc oxide in rubber compounds be reduced. Moreover, it should be noted that vulcanizates of carboxylated nitrile elastomer cross linked with zinc oxide demonstrate heat shrinkability.

It is the morphology of zinc oxide particles which mainly affects the activity in the cross linking process. Particle size and surface area do not seem to have a significant influence on the efficiency of zinc oxide as a cross linking agent. The highest activity was observed for zinc oxide with a surface area of about 24 m 2 /g and three-dimensional snowflake particles. The specific shape and complex structure of ZnO aggregates, consisting of wires or plates growing from a single core, provide an increase in the size of the interphase between the elastomer carboxylic groups and the snowflake particle. As a result, Przybyszewska et al. , obtained vulcanizates exhibiting the best mechanical properties, mainly due to the high content of ionic clusters, which create multifunctional and labile crosslinks and can rearrange upon external stress, leading to stress relaxation. Moreover, the zinc oxide nanoparticles used by Przybyszewska et al. , have the lowest propensity for agglomeration in the elastomer matrix and create the smallest agglomerates, which concentrate the stresses during sample deformation to a smaller degree than the large agglomerates formed by other zinc oxides.

As mentioned above, the substantial usage of zinc oxide in rubber products has raised questions about the environmental impact of the rubber industry, particularly when this compound in finally released into the lithosphere during degradation of the rubber, after the end of a product’s life [ 122 ]. Environmental concerns are especially focussed on the effect of excess zinc on aquatic organisms [ 123 ], which has led to various efforts to reduce zinc levels in rubber compounds [ 124 ]. There are three basic methods of reducing the content of ZnO in rubber compounds:

• (i) replacing the commonly used zinc oxide of diameter 0.1–0.9 μm and surface area 4–10 m 2 /g with active zinc oxide of nanoscopic granularity and surface area of up to 40 m 2 /g;
• (ii) modifying the surface of the zinc oxide with carboxylic acids (such as stearic acid, maleic acid and others);
• (iii) using additional activators [ 69 ].

To find an alternative to conventional ZnO, which in higher dosage is toxic to aquatic systems, Thomas et al. [ 125 ] synthesized the novel accelerators N -benzylimine aminothioformamide(BIAT)-capped-stearic acid-coated nano-ZnO (ZOBS), BIAT-capped ZnO (ZOB), and stearic acid-coated nano zinc phosphate (ZPS), to investigate their effects in NR vulcanization. They studied the effect of these capped compounds on the curing and mechanical properties of natural rubber (NR) vulcanizates. The zinc oxide used in the research was prepared by a sol-gel method, and was then modified using accelerators such as BIAT and fatty acids such as stearic acid. This capping technique reduces agglomeration of nanoparticles of ZnO and is an effective method to improve the curing and physicochemical properties of NR. By capping ZnO with BIAT and stearic acid, it becomes possible to save the extra time and energy required for these particles to diffuse onto the surface of ZnO via the viscoelastic rubber matrix. This provides a further improvement in acceleration of vulcanization and improvement in the physicomechanical properties of the resulting vulcanizates. The mixture containing optimum concentration of BIAT-capped-stearic acid-coated zinc oxide (ZOBS) has superior curing and physicomechanical properties compared with other homologues and the reference mixture containing uncapped ZnO. The increased crosslink density caused by the ZPS particles could increase the stiffness of vulcanizates containing ZPS. The capping technique could improve the scorch safety of rubber compounds by the delayed release of BIAT from the capped ZnO into the rubber matrix for interaction with CBS (conventional accelerator).

Sabura et al. [ 126 ] prepared nano zinc oxide by a solid-state pyrolytic method. Microscopic images and surface area studies showed that the prepared zinc oxide had particle sizes in the range 15–30 nm and surface area in the range 12–30 m 2 /g. The researchers used this nano zinc oxide as a curing agent in neoprene rubber. The optimum dosage of ZnO was found to be low compared with commercial ZnO. The cure characteristic and mechanical properties of the rubber were compared with those containing conventional ZnO. It was found that a low dosage of zinc oxide was enough to give equivalent curing and mechanical properties compared to neoprene rubber containing a higher dosage of commercial zinc oxide.

4.2. The Pharmaceutical and Cosmetic Industries

Due to its antibacterial, disinfecting and drying properties [ 127 , 128 ], zinc oxide is widely used in the production of various kinds of medicines. It was formerly used as an orally administered medicine for epilepsy, and later for diarrhoea. At the present time it is applied locally, usually in the form of ointments and creams, and more rarely in the form of dusting powders and liquid powders. ZnO has properties which accelerate wound healing, and so it is used in dermatological substances against inflammation and itching. In higher concentrations it has a peeling effect. It is also used in suppositories. In addition it is used in dentistry, chiefly as a component of dental pastes, and also for temporary fillings. ZnO is also used in various types of nutritional products and diet supplements, where it serves to provide essential dietary zinc [ 129 ].

For many years, before sun creams began to contain nanoparticles of ZnO or TiO 2 , they contained thick preparations which did not rub easily into the skin and which were cosmetically unattractive. Due to their ability to absorb UVA and UVB radiation, these products began to be used in creams. A new cream formula, containing a combination of ZnO and TiO 2 , solved the problem of an insufficiently white layer and produced a new medium which is more transparent, less adhesive and much more easily rubbed into the skin [ 130 ]. A number of studies have shown that titanium and zinc oxides are extremely good media in sun creams, since they absorb UV radiation, do not irritate the skin, and are easily absorbed into the skin [ 131 – 133 ].

4.3. The Textile Industry

The textile industry offers a vast potential for the commercialization of nanotechnological products. In particular, water repellent and self-cleaning textiles are very promising for military applications, where there is a lack of time for laundering in severe conditions. Also in the world of business, self-cleaning and water repellent textiles are very helpful for preventing unwelcome stains on clothes. Protection of the body from the harmful UV portion of sunlight is another important area. Many scientists have been working on self-cleaning, water repellent and UV-blocking textiles [ 134 – 140 ].

For textile applications, not only is zinc oxide biologically compatible, but also nanostructured ZnO coatings are more air-permeable and efficient as UV-blockers compared with their bulk counterparts [ 141 ]. Therefore, ZnO nanostructures have become very attractive as UV-protective textile coatings. Different methods have been reported for the production of UV-protecting textiles utilizing ZnO nanostructures. For instance, hydrothermally grown ZnO nanoparticles in SiO 2 -coated cotton fabric showed excellent UV-blocking properties [ 142 ]. Synthesis of ZnO nanoparticles elsewhere through a homogeneous phase reaction at high temperatures followed by their deposition on cotton and wool fabrics resulted in significant improvement in UV-absorbing activity [ 143 ]. Similarly, ZnO nanorod arrays that were grown onto a fibrous substrate by a low-temperature growth technique provided excellent UV protection [ 144 ].

Zinc oxide nanowires were grown on cotton fabric by Ates et al. [ 145 ] to impart self-cleaning, superhydrophobicity and ultraviolet (UV) blocking properties. The ZnO nanowires were grown by a microwave-assisted hydrothermal method and subsequently functionalized with stearic acid to obtain a water contact angle of 150°, demonstrating their superhydrophobic nature, which is found to be stable for up to four washings. The UV protection offered by the resulting cotton fabric was also examined, and a significant decrease in transmission of the UV range was observed. The self-cleaning activity of the ZnO nanowire-coated cotton fabric was also studied, and this showed considerable degradation of methylene blue under UV irradiation. These results suggest that ZnO nanowires could serve as ideal multifunctional coatings for textiles.

Research on the use of zinc oxide in polyester fibres has also been carried out at Poznan University of Technology and the Textile Institute in Lodz [ 146 ]. Zinc oxide was obtained by an emulsion method, with particles measuring approximately 350 nm and with a surface area of 8.6 m 2 /g. These results indicate the product’s favourable dispersive/morphological and adsorption properties. Analysis of the microstructure and properties of unmodified textile products and those modified with zinc oxide showed that the modified product could be classed as providing protection against UV radiation and bacteria.

4.4. The Electronics and Electrotechnology Industries

Zinc oxide is a new and important semiconductor which has a range of applications in electronics and electrotechnology [ 147 – 149 ]. Its wide energy band (3.37 eV) and high bond energy (60 meV) [ 150 , 151 ] at room temperature mean that zinc oxide can be used in photoelectronic [ 152 ] and electronic equipment [ 153 ], in devices emitting a surface acoustic wave [ 154 ], in field emitters [ 155 ], in sensors [ 156 – 161 ], in UV lasers [ 162 ], and in solar cells [ 163 ].

ZnO also exhibits the phenomenon of luminescence (chiefly photoluminescence—emission of light under exposure to electromagnetic radiation). Because of this property it is used in FED (field emission display) equipment, such as televisions. It is superior to the conventional materials, sulfur and phosphorus (compounds exhibiting phosphorescence), because it is more resistant to UV rays, and also has higher electrical conductivity. The photoluminescent properties of zinc oxide depend on the size of crystals of the compound, defects in the crystalline structure, and also on temperature [ 164 – 170 ]. ZnO is a semiconductor, and thin films made of that material display high conductivity and excellent permeability by visible rays. These properties mean that it can be used for the production of light-permeable electrodes in solar batteries. It also has potential uses as a transparent electrode in photovoltaic and electroluminescent equipment, and is a promising material for UV-emitting devices [ 171 , 172 ].

Zinc oxide is also used in gas sensors. It is a stable material whose weak selectivity with respect to particular gases can be improved by adding other elements. The working temperature of ZnO is relatively high (400–500 °C), but when nanometric particles are used this can be reduced to around 300 °C. The sensitivity of such devices depends on the porosity and grain size of the material; sensitivity increases as the size of zinc oxide particles decreases. It is most commonly used to detect CO and CO 2 (in mines and in alarm equipment), but can also be used for the detection of other gases (H 2 , SF 6 , C 4 H 10 , C 2 H 5 OH). The zinc oxide used in the production of such equipment is obtained by a variety of methods (chemical vapour deposition, aerosol pyrolysis or oxidation of metallic zinc); it is important to control the process temperature, since this determines the properties of the product [ 173 – 175 ].

One of the most important applications of zinc oxide in electronics is in the production of varistors. These are resistors with a non-linear current-voltage characteristic, where current density increases rapidly when the electrical field reaches a particular defined value. They are used, among other things, as lightning protectors, to protect high-voltage lines, and in electrical equipment providing protection against atmospheric and network voltage surges. These applications require a material of high compactness, since only such a material can guarantee the stability and repeatability of the characteristics of elements made from it [ 176 , 177 ].

Certain unique electronic properties of ZnO are exploited in projection processes. The zinc oxide used for this purpose is produced from metallic zinc (from a suitable ore), so as to obtain a high-purity product. The photoconductor and semiconductor properties of ZnO are improved by thermal treatment, and also by the addition of other elements [ 178 , 179 ].

4.5. Photocatalysis

Intensive scientific work has taken place in recent years on photocatalysis. In this process, an electron-hole pair is produced below the intensity of light by means of oxidation or reduction reactions taking place on the surface of the catalyst. In the presence of a photocatalyst, an organic pollutant can be oxidized directly by means of a photogenerated hole or indirectly via a reaction with characteristic reactive groups (ROS), for example the hydroxyl radical OH·, produced in solution [ 180 – 182 ]. The most commonly used catalysts are TiO 2 and ZnO. TiO 2 exhibits photocatalytic activity below the intensity of UV light [ 183 , 184 ]. ZnO provides similar or superior activity to that of TiO 2 , but is less stable and less sensitive to photocorrosion [ 185 ]. Better stability, however, is provided by zinc oxide of nanometric dimensions, which offers better crystallinity and smaller defects [ 186 ]. The photocatalytic activity of ZnO can be further improved, and the range of the visible spectrum for zinc oxide can be extended, by adding other components [ 187 ].

The photocatalytic properties of zinc oxide, titanium dioxide and ZnO-TiO 2 composite were investigated by Guo et al. [ 188 ]. ZnO was obtained in solution, this being a low-temperature and low-cost method. The properties and photocatalytic applications of the ZnO obtained in this way were studied. A sample was placed on a Petri dish containing an aqueous solution of methyl orange (pH 6.7). While being exposed to UV radiation the solution was mixed and stimulated by sunlight with or without polycarbonate filters. Absorption was measured immediately before exposure to UV and at set time intervals, using a UV/Vis spectrometer. These tests showed that the ZnO nanorods have similar photocatalytic properties (with UV) or slightly better properties (with stimulated sunlight) compared with TiO 2 nanotubes. However, coating the surface of ZnO with a layer of TiO 2 causes deterioration of the photocatalytic properties, possibly due to an increase in the quantity of defects. Summarizing their work, Guo et al. stated that the photocatalytic properties of ZnO can be influenced by coating with various substances and by the thickness of such coating.

Li et al. [ 189 ] also studied the photocatalytic properties of ZnO. ZnO nanospheres were obtained using an electrochemical method, in the presence of POMs (polyoxometalates), at room temperature. The experiments showed that POMs play a very important role in the formation of ZnO nanospheres. The photocatalytic properties of ZnO were determined using the example of degradation of rhodamine (RhB). Based on this study Li et al. , concluded that ZnO displays high photocatalytic activity below the UV range. The proposed simple, single-stage method of synthesis makes it possible to obtain spherical ZnO particles and provides the possibility of controlling their shape.

Ma et al. [ 190 ] demonstrated superior photocatalytic performance on ZnO nanorods and nanoflowers compared with commercial ZnO particles on methyl orange (MO). Besides organic dyes, UV-induced photocatalytic degradation of stearic acid by ZnO nanowires was also reported [ 191 ]. By the incorporation of dopants or formation of a composite with other materials, the photocatalytic properties of ZnO could be enhanced. Xu et al. [ 192 ] demonstrated improved photodegradation of MO by doping with cobalt on hydrothermally grown ZnO powders. One-dimensional heterostructures of ZnO and carbon nanofibres were reported to have significantly enhanced the photodegradation of rhodamine B compared with a pure ZnO counterpart [ 193 ]. It has also been reported that ZnO nanorod films can disinfect E. coli contaminated water with UV illumination [ 194 ].

Other studies by numerous researchers prove that ZnO offers unique photocatalytic properties, making it possible for this oxide to be used as a photocatalyst in the process of degradation of various substances [ 195 – 197 ].

4.6. Miscellaneous Applications

Apart from the applications mentioned above, zinc oxide can also be used in other branches of industry, including for example concrete production. The addition of zinc oxide improves the process time and the resistance of concrete to the action of water. Also, the addition of ZnO to Portland cement slows down hardening and quenching (it reduces the gradual evolution of heat), and also improves the whiteness and final strength of the cement.

Zinc oxide reacts with silicates (e.g., sodium silicate) to produce zinc silicates, which are water- and fire-resistant materials used as binders in paints. These fire-resistant and adhesive substances are used in the binding of cements used in the construction industry.

Methanol, the third most-important chemical product of chemical industry, is produced using a Cu/ZnO/Al 2 O 3 catalyst, with small Cu particles promoted by their interaction with the ZnO substrate as the active component [ 198 ].

ZnO is also used for the production of typographical and offset inks. It imparts good printing properties (high fluidity). The addition of ZnO means that the inks have better covering power, pure shade and high durability, and prevents darkening. Zinc oxide is also used in pigments to produce shine.

It is added to many food products, including breakfast cereals. ZnO is used as a source of zinc, which is an essential nutrient. Thanks to their special chemical and antifungal properties, zinc oxide and its derivatives are also used in the process of producing and packing meat products (e.g., meat and fish) and vegetable products (e.g., sweetcorn and peas) [ 199 ].

As mentioned above, ZnO and its derivatives suppress the development and growth of fungi and moulds. Zinc oxide is added to fungicides to improve their effectiveness. Zinc oxide is also being used increasingly often as an animal feed additive, as it supports the correct growth of animals. It is also used as an artificial fertilizer [ 200 ].

Zinc oxide also has uses in criminology, in mechanical fingerprint analysis. It is also an ingredient in cigarette filters, as it selectively removes certain components from tobacco smoke. Filters are made of charcoal impregnated with ZnO and Fe 2 O 3 , which remove significant quantities of HCN and H 2 S from tobacco smoke without producing a smell. It also removes sulfur and its compounds from various liquids and gases, particularly industrial waste gases. Zinc also removes H 2 S from hydrocarbon gas, and desulfurizes H 2 S and other sulphur components.

ZnO and its derivatives are also used as an additive to car lubricating oils, reducing consumption and oxygen corrosion. Zinc oxide has also been used in various types of lubricants, such as those with EP additives, vibration-resistant lubricants and solid lubricants. In the future, advantage may also be taken of the adhesive properties of ZnO [ 201 ].

Because the compound is nontoxic, cheap, and chemically stable in the air, nanoparticles of zinc oxide can be used to make new eco-friendly substances for cell marking [ 202 ].

Recent advances in electrochemical biosensing based on a wide variety of nanostructures such as ZnO nanowires, nanotubes and nanoporous materials have attracted great interest in biosensor applications due to their remarkable properties such as non-toxicity, bio-safety, excellent biological compatibility, highelectron transfer rates, enhanced analytical performance, increased sensitivity, easy manufacture and low cost [ 203 – 205 ]. Moreover, ZnO has a highisoelectric point ( IEP ) of about 9.5, which can be expected to provide a positively charged substrate for immobilization of low- IEP proteins or enzymes such as uricase ( IEP ~ 4.6) at a physiological pH of 7.4 [ 206 , 207 ]. In addition, ZnO has high ionic bonding (60%), and it dissolves very slowly at biological pH values [ 208 ].

Many researchers have attempted to correlate the biological activity of inorganic antibacterial agents with the size of the constituent particles [ 209 , 210 ]. Inorganic nanocrystalline metal oxides are particularly interesting because they can be prepared with extremely high surface areas, and are more suitable for biological molecular applications [ 211 ]. ZnO semiconductors have been extensively studied as antimicrobial agents due to their photocatalytic activity under UV light [ 212 , 213 ]. These antimicrobial substances based on inorganic chemicals have been found to be effective for therapy [ 214 ]. Padmavathy et al. [ 215 ] showed that ZnO nanoparticles were more abrasive than bulk ZnO (particle sizes in the range 0.1–1 μm), and this contributes to the greater mechanical damage to the cell membrane and the enhanced bactericidal effect produced by ZnO nanoparticles.

5. Conclusions

Zinc oxide is a multifunctional material because of its many interesting properties (piezo- and pyroelectric), a wide range of UV absorption, and high photostability, biocompatibility and biodegradability. ZnO can also be obtained with a variety of particle structures, which determine its use in new materials and potential applications in a wide range of fields of technology. Therefore the development of a method of synthesizing crystalline zinc oxide which can be used on an industrial scale has become a subject of growing interest in science as well as industry.

As can be seen from the survey of recent literature presented here, particles of zinc oxide—both nano- and micrometric—can be produced by many different methods. These can be divided into metallurgical and chemical methods. In metallurgical processes, zinc oxide is obtained by the roasting of a suitable zinc ore, via a direct or indirect process. Chemical methods can be divided into two groups: dispersion methods and condensation methods. In dispersion (mechanochemical) processes, zinc oxide is obtained by the grinding of suitable precursors. The resulting product may contain particles measuring approximately 20 nm. The condensation methods (controlled precipitation, the sol-gel method, hydro- and solvothermal methods, formation in an emulsion or microemulsion environment, and many others) involve the use of a molecularly homogeneous solution subjected to a process of nucleation.

The need to reduce the content of zinc oxide in certain materials, and to limit the degree of agglomeration, has led to the development of various methods of modifying the ZnO surface. Numerous reports in the literature indicate that modification processes can be carried out using inorganic substances (oxides and hydroxides), organic substances (alkoxysilanes, carboxylic acids), and certain polymer matrices, depending on how the systems obtained are to be used. Crystalline oxide powders, combined with other materials, provide possibilities for obtaining improved chemical, mechanical, optical or electrical properties.

Technology and knowledge relating to oxide materials of nano- and micrometric dimensions are currently among the most rapidly developing scientific and technological disciplines. The use of such materials can provide, among other things, more durable ceramics, transparent solar filters blocking infrared and ultraviolet radiation, and catalysts. These materials are also useful in biomedical research and in the diagnosis and treatment of diseases. They can be used to deliver medicines directly to diseased cells, in a way that avoids adverse effects.

The survey of the literature that has been given here shows that zinc oxide can be classed as a multifunctional material. This is thanks to such properties as high chemical stability, low electrical constant, high electrochemical coupling index, wide range of radiation absorption, and high photostability. It can be expected that interest in zinc oxide will continue to grow, and that this will lead to the development of new possibilities for its application.

Acknowledgments

This work was supported by Poznan University of Technology research grant No. 32-443/2014-DS-PB.

Author Contributions

Agnieszka Kołodziejczak-Radzimska and Teofil Jesionowski write the manuscript together.

Conflicts of Interest

The authors declare no conflict of interest.