Shaobo Wang,Yunchao Zhao,Zeyu Zhang,Yalong Zhang,Linlin Li,3,

1 School of Chemistry and Chemical Engineering,Center on Nanoenergy Research,Guangxi University,Nanning 530004,China

2 Beijing Institute of Nanoenergy and Nanosystems,Chinese Academy of Sciences,Beijing 100083,China

3 School of Nanoscience and Technology,University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:Amino acids Metal Coordination chemistry Cancer therapy Antibacterial applications Imaging

ABSTRACT Metal and amino acid(AA),as two kinds of entities,have been widely involved in biomaterials and nanomedicines.Recently,the marriage of them has developed new nanoformulations,amino acid-metal coordinated nanomaterials (AMCNs),which show great biomedical application potential in cancer therapy,antibacterial applications,biomedical imaging,etc.With the respective characteristics of metal and AA with rich biological and chemical properties,AMCNs can not only act as drug carriers with specific tumor targeting ability,but also realize synergistic therapy and imaging-guided therapy.Although the design and synthesis of amino acid-metal coordinated nanomaterials have been in-depth investigated,there are few systematic reviews on their biomedical application.In this review,we give a comprehensive summary of recent progresses in the design,fabrication,and biomedical applications of AMCNs.We also propose the future outlooks and challenges in aforementioned field.We expect that this review would contribute some inspiration for future research and development for amino acid metal coordinated nanomaterials.

1.Introduction

In human bodies,amino acid(AA)as organic ligands containing both carboxyl (–COOH) and amino (–NH2) groups are the fundamental constituent units of proteins and enzymes,and also precursors for the synthesis of other important biomolecules,such as hormones and neurotransmitters.For a long history,amino acids and derivatives are highly concerned molecules in pharmaceutics.For instance,potassium/magnesium aspartate are used to restore fatigue and treat a series of hypokalemia diseases,because aspartic acid has a strong affinity with cell membrane and helps potassium ions enter into the cells [1].Arginine can be used for therapy of hepatic encephalopathy caused by increased blood ammonia [2].As early as 1980,Gosalvez et al.[3] have developed an amino acid derivative thiazolidine-4-carboxylic acid (thioproline),which has been used as an antitumor agent for transforming tumor cells into normal cells.Amino acids has been widely utilized as modification ligands to enhance the solubility of poorly water-soluble drugs[4,5].Totally,amino acids have low toxicity,low immunogenicity,and high affinity to specific receptors in the bodies.Their structural diversity,chirality,and multiple functional groups enable them to act as ideal building blocks to fabricate nanomaterials for biomedical applications.

Metal ions play vital roles in living organisms,and most of them exist in bodies as metal complexes with proteins,lipids,and carbohydrates,such as ferrous ions in hemoglobin and myoglobin,and copper ions in ceruloplasmin,etc..Metal coordination chemistry offers attractive features for the design of biologically active molecules and nanomaterials.Through judicious selection of metal center,modulation of coordination number,labile groups,and bioactive or ancillary ligands,nanomaterials with distinct biological mechanisms of action could be fabricated and applied for disease diagnosis,sensing,and therapy [6].Because the transition metal centers may have higher coordination numbers than that of carbon with four bonds,increased “chemical space” is thereby available compared to pure organics [7–9].

In natural proteases,the coordination sites of metals and amino acids are usually catalytically active sites,in which metal ions stabilize the protease structure,act as an exciter or control agent in excitation and selectivity,and act as a Lewis acid to take part in the redox-reaction process [10],and circumambient amino acid species with specific side-chain groups determine the tertiary structure of the proteins and substrate-binding specificity.Recently,a number of metal amino acid nanomaterials have been fabricated through metal-AA coordination and have been applied in drug delivery,cancer imaging and therapy,and bacterial killing(Table 1).For these biomedical applications,they have showed unique advantages including good biocompatibility,facile and mild fabrication,versatile structure,and multifunction.In this review,we first outline the coordination chemistry of amino acids(AAs)with metal ions to fabricate nanomaterials(Fig.1).Then,different applications of the nanomaterials in cancer therapy,bacterial killing,and biomedical imaging are summarized.Finally,we conclude the open challenges and future perspectives of the amino acid metal coordinated nanomaterials.

Table 1Nanomaterials through coordination of amino acids and metals

Fig.1.Schematic illustration of AMCNs for biomedical applications.

2.Coordination between AA and metal for nanomaterials

An amino acid molecule contains aimmunot least two coordination sites with metal ions,amino and carboxyl.The side chain of amino acids includes many other functional groups,such as guanidine,thiol,and indole,which have many unique characteristics,including zwitterionic characteristics,low softening point and different hydrophilic-lipophilic properties.

As shown in Fig.2,metal ions can coordinate with N in amino,O in carboxyl and O/N/S in side chain groups.Generally,the strength of metal coordination bond is relatively weaker than covalent bonds,but stronger than non-covalent interactions (e.g.,hydrogen bonds and hydrophobic interactions).These several kinds of bonds could usually cooperate within a nanoparticle to drive the formation,self-assembly,and collapsion of the nanoparticle [43,44].The metal coordination bond is often dynamic and reversible.Using the coordinated nanomaterials as drug carrier,stimulusresponsive on-demand drug release can be achieved under the stimuli of pH,chemical substances,and enzymes [26,45–48],by which the nanoparticle can be disassembled to release the drugs.

In most cases,the coordination of metal ions with amino acids is tended to form one-dimensional (1D) nanostructures[23,29,31,49],which may be attributed to that the coordination number determines the growth along the 1D direction.Nanomaterials with other structures can be prepared by manipulation of the coordination and non-covalent interactions between metal and AAs/AA derivatives and side chain R groups bound to the metal,such as β-carboxylate group of glutamic and aspartic,imidazole group of histidine,thiol of cysteine,thioether group of methionine,or phenyl ring of tyrosine.These functional groups can bridge metal ions and determine spatial dimension and structure of nanoparticles[50,51].Amino acids provide multiple metal binding sites and can form metal organic frameworks (MOFs) or metal organic nanoparticles.With porous structure,MOFs with coordination bonds can prevent water molecules from attacking the open metal sites (OMSs),enhancing the stability of MOFs,and expose maximum metal active sites [52,53].In addition,because amino acids are rich in–NH2and–COOH groups,amino acid constituted or modified MOFs with large surface area and meso-/micro-pores can selectively adsorb different functional species,thereby applying for drug delivery,imaging,and different therapeutic purposes[35,54,55].

Fig.2.The potential coordination models of amino acids with metal ions.Before the ligand name,μ is used to mark the ligand as a bridging ligand,and the subscript number x after μ represents that the ligand is coordinated with x central atoms (μx).

Moreover,all left-handed(L)nature AAs except for glycine have right-handed(D)chiral enantiomers.Thus,coordinated nanomaterials with chirality can be obtained by selection of chiral AAs as building blocks [56–58],which have potential applications in imaging,diagnosis,and therapy.For example,through aqueous/organic interfacial coordinative polymerization,Imaz et al.[23]coordinated Cu2+with both carboxylate groups of D,L-aspartic acids(Asp) to form 1D nanowire with average diameters of 100–200 nm and maximum lengths of around one centimeter.Ma et al.fabricated Tb-Asp coordinated nanocrystals by the coordination between amino and carboxyl groups of Asp and Tb3+,which formed Tb-Asp tetrahedron with a planar hexagonal network structure [37].Aspartic acid with different chirality changed the stability of the coordination center.Compared with L-Asp,the nanocrystals with D-Asp formed a more stable coordination center,resulting in a 3-folds higher fluorescence emission intensity thanthat of Tb-L-Asp.Xin et al.[29]found nanofibers coordinated from zinc ions and D,L-aspartic acid can specifically bind to extracellular heat shock proteins in cancer cells,resulting in downregulation of nuclear factor-kappa B (NF-κB) and inhibiting the proliferation of cancer cells,but not damaging normal cells.The D-and L-Asp constituted nanofibers had different degrees of biochemical effects.

3.Biomedical Applications

3.1.Cancer therapy

In the past few decades,different anti-cancer drugs have been translated into clinic,including small molecule inhibitors,antibodies,chemodrugs,gene and nucleic acid drugs [59–64].However,their therapeutic effect are limited by the non-specific biodistribution of drugs and unfavorable pharmacokinetics after systemic administration,leading to undesirable side effects[65,66].For cancer therapy,some metal ions,such as Pt(II),can interact with DNA to inhibit cell growth,which makes metal-based complexes promising drug candidates [67].Several kinds of platinum drugs have been approved by the U.S.Food and Drug Administration(FDA),including cisplatin,carboplatin,and oxaliplatin,which are all metal complexes with high anti-tumor activity [68].AAs and AA derivatives themselves can play roles in cancer therapy through acting as antitumoral drugs or modification ligands.For example,Mustard gas has high toxicity and generally it cannot be used directly as an anti-cancer agent.However,when combined with L-phenylalanine to form phenylalanine mustard (L-PAM),it has reduced toxicity and can penetrate deeply into tumor tissue to realize an antitumoral effect [69].

With the combination of metals and AAs,it is expected that the coordinated nanomaterials can achieve a therapeutic effect of“1+1 >2”.As drug carriers,AMCNs can stabilize therapeutic agents,and deliver them to tumor tissues through AA-mediated specific targeting,thereby boosting therapeutic effect [70].In addition,other functional agents such as imaging agents,drug sensitizers,and photosensitizers can be simultaneously loaded into the nanomaterials for multifunctional and multimodal therapy.With these properties,they have been used for cancer therapy mediated by drug delivery[36],targeting[71],photothermal and photodynamic therapy [72],and chemodynamic therapy [26].

3.1.1.AA-mediated tumor targeting

Compared with normal cells,malignant tumor cells have higher requirements for proteins and amino acids,leading to overexpression of transmembrane amino acid transporters (i.e.,LAT1,ASCT2,ATB0,+).For instance,L-type amino acid transporter 1 (LAT1;also known as SLC7A5)mediates the transmembrane flux of large neutral amino acids in a Na+and pH-independent manner [73].Thus,these transporters can be utilized as potential targets for cancer therapy [74–77] with AAs as targeting ligands.For instance,Li et al.[75] conjugated glutamate with polyethylene stearate onto poly(lactic-co-glycolic acid) (PLGA) nanoparticles to target LAT1.Compared with unmodified PLGA nanoparticles,a significant increase in cellular uptake and cytotoxicity was observed.The targeted nanoparticles can circulate to tumor tissues within a short time and promote the release of drugs at a lowered pH (Fig.3a).In another work,Wang et al.[78] tethered glutamic acid,lysine,and tyrosine onto polyethylene glycol stearate that was further embedded into the phospholipid bilayer of liposomes,which can target deliver both irinotecan (CPT-11) through LAT1 and amino acid transporter B0,+(ATB0,+).Compared with the commercially available liposomes (Onivyde?),the tumor inhibition rate of the AA-modified liposomes loaded with CPT-11 was greatly increased from 39% to 87%.In this study,amino acids were modified on the surface of the liposomes as a small-molecule ligand,which can not only stabilize the circulation of nanoparticles in body fluids,but also help to target tumor cells.

Utilizing the tumor targeting ability and coordination of amino acids with metal,nanomaterials can be designed to achieve the purpose of simultaneous diagnosis and targeted therapy of cancer.For example,Chen et al.[80] prepared fluorescent nanoprobes for methionine-dependent malignant cells,in which methionine and NIR fluorescent dye (MPA) were covalently bound to Au NCs (Au-Met-MPA),and methionine conferred tumor targeting ability with low cytotoxicity.

Arginine is one of the essential substances for tumor growth.For auxotrophic tumors such as breast cancer,renal cell carcinoma,melanoma and liver cell carcinoma[81,82],arginine cannot be synthesized from citrulline due to argininesuccinate synthase (AS)deficiency,but only can be available externally.Therefore,arginine can be used to modify the nanomaterials to increase their specific uptake by tumor cells.For instance,Wang et al.[79]used arginine to modify manganese silicate nanobubbles(AMSNs)(Fig.3b).With the specific targeting,the nanobubbles showed selective uptake by Huh7 liver cancer cells (flow cytometry showed 92.4% positive cells at 4 h),while had low cytotoxicity to normal liver L02 cells(10.6%) (Fig.3c,d).Compared with MnO nanoparticles (MnOPEG) without arginine modification,AMSNs showed a faster release rate of Mn2+,which can be used as an effective glutathione(GSH) depletion reagent and lead to tumor-killing caused by ferroptosis (Fig.3e-g).

Gas therapy is considered as an alternative chemotherapy due to its low toxicity,side effects and drug resistance compared to the traditional methods.Nitric oxide(NO)as an endogenous gasotransmitter is involved in many physiological and pathological activities [83].L-Arginine (L-Arg) is a natural NO donor that can keep releasing NO in presence of irreducible NO synthase (iNOS)enzyme.In addition,L-Arg can also be oxidized by H2O2to generate NO,as depicted in Eq.(1).

Thus,the high-level H2O2in tumor microenvironment (TME)can assist to generate NO in situ for tumor gas therapy.Fan et al.[84] loaded glucose oxidase (GOD) and L-Arg onto hollow mesoporous organosilicon nanoparticles (HMONs),which can be released in the slightly acid TME for glucose starvation and NO gas therapy synergistically.Hu et al.[85]synthesized 1,5-bis[(L-pro line-1-yl)diazen-1-ium-1,2-diol-O2-yl]-2,4-dinitrobenzene (BPDB)and coordinated it with Fe2+to obtain Fe(II)-BNCP,which had a bidentate structure and a GSH-sensitive NO donor to release NO and produce ROS under H2O2.It improved the treatment specificity of liver cancer in mice through the synergy of NO gas therapy and chemodynamic therapy.

Fig.3.AA-mediated tumor targeting.(a) Probable mechanism of LAT1-mediated drug delivery.Reproduced with permission [75].Copyright 2016,Elsevier;(b) Schematic illustration of AMSNs;(c)Fluorescence intensity of AMSNs/DOX to Huh7 cells and L02 cells for different time periods;(d)Cytotoxicity of AMSNs against Huh7 cells and L02 cells after 24 h of incubation,(e,f)Mn ion release profiles(e)and cytotoxicity(f)of AMSNs and MnO-PEG nanoparticles;(g)Tumor volume changes with different treatments.Reproduced with permission [79].Copyright 2018,American Chemical Society.

3.1.2.Drug delivery

AMCNs as drug carriers can directly load drugs through physical loading in the pores,or through formation of coordination bonds with AA residues and metals.Thus,they often have high drug loading mount,controlled drug release,and can be targeted to tumor tissues using AA as the targeting ligand.In addition,amino acids contain both acidic carboxyl groups and basic amino groups,which endow them with amphoteric properties.Under pH value lower or higher than the isoelectric point (pI) of AA,AA will be protonated with positive charge or deprotonated with negative charge,respectively.For drug delivery,negatively charged nanocarriers tend to have longer circulation time in blood circulation,and positively charged nanocarriers have high affinity to cell membranes and higher endocytosis.Therefore,the drug delivery system modified by amphoteric amino acids can be trigged by the low pH value of TME,which can reverse their surface charge from negative to positive at the tumor site to improve the targeting efficiency,tumor penetration,and tumor cell endocytosis [86,87].It is a potential strategy to construct pH-triggered charge-reversible drug delivery system.

Cisplatin as a kind of metallodrugs can be modified with AA ligands to change the size and charge distribution,reducing its cytotoxicity to normal cells,but retaining its antitumor activity[88].Rijal et al.[89] found amino acid-linked platinum(II) complexes had increased tumor specificity.Huang et al.[36] prepared a pH-sensitive cisplatin delivery system,poly(L-glutamic acid-co-L-lysine)[P(Glu-co-Lys)],which was formed through the coordination of amino acids with platinum (Fig.4a).When the negatively charged CDDP/P(Glu-co-Lys)reached the weakly acidic tumor tissue,the surface charge of the nanoparticles was reversed from negative to positive,which enhanced cellular uptake and improved the anti-cancer efficiency of cisplatin.

For delivering chemotherapeutic drug doxorubicin(DOX),Chen et al.[90] designed a silica-based mesoporous nanosphere (MSN)as drug carrier,which used Cu2+as a bridging ion to tether Lcysteine functionalized gold nanoparticles (AuNPs).Cu2+can form coordination bond with the carboxyl and amino groups of AuNPs with MSN.Thereby,the functionalized AuNPs acted as a removable pore cover of MSN and hindered the release of DOX from the mesopores of MSN(Fig.4b).When pH was low than 5,ζ-potential of the L-cysteine-coated AuNPs changed from negative to positive and the coordination bond with copper ions was destroyed by adenosine triphosphate (ATP) simultaneously to trigger the release of Au NPs and loaded DOX.Similarly,Wang et al.[91] coordinated copper ions with recombinant ferritin to form nanocarrier of DOX and changed their feed ratio and reaction time to tune the morphology and size of nanoparticles [92].

Other chemotherapeutics are also delivered with AMCNs.Studies have shown that the poor pharmacokinetics of curcumin can be improved by combining diketo/enol moiety form with metal ions[93].Li et al.[94]fabricated a super-molecular curcumin nanoparticle through amino acid coordination-driven self-assembly.Curcumin was protected by metal coordination and molecular stacking through coordination and multiple non-covalent interactions to prevent hydrolysis.Compared with pure curcumin and curcumin-Zn2+complex,the coordination of amino acid with Zn2+nanoparticles can improve the stability of curcumin.The nanoparticles had a high drug loading and responsive drug release in tumor environment,and increased tumor accumulation of drugs.

Recently,nano drug delivery systems have been designed by using extracellular enzymes that are highly expressed in TME as triggers,thereby realizing high selectivity and sensitivity to tumor tissues.However,most of the reported enzymes as triggers are not uniquely expressed in tumor tissues,so it is particularly important to design a drug delivery system that can target the inherent enzyme triggers in tumor cells.Qiao et al.[12] prepared hybrid raspberry-like nanoparticles (HRNs) by coating mesoporous silica nanoparticles with uril complex of melon [8] and Fe3O4mediated by tryptophan,which acted as enzyme-triggered drug delivery system that can target specific tumor cells (Fig.4c).The indoleamine 2,3-dioxygenase 1(IDO1)enzyme that is only overexpressed in tumor tissue can trigger the release of DOX at tumor site.In the presence of IDO1,HRNs assembled with tryptophan reached 70%DOX release after 24 h of incubation.In comparison,DOX release from HRNs modified with phenylalanine or tryptophan remained at the minimum value.It confirmed the key role of tryptophan as an enzyme stimulator,which can trigger the controlled release of nanocarriers.

Fig.4.TME triggers anticancer drugs delivery of AMCNs.(a) pH-triggered cisplatin delivery [36].Reproduced with permission [36].Copyright 2013,American Chemical Society;(b)pH or ATP stimulation induced Au NPs removal and controlled release of DOX.Reproduced with permission[90].Copyright 2014,The Royal Society of Chemistry;(c) IDO1 enzyme triggered disintegration of the nanocarrier and drug release.Reproduced with permission.[12].Copyright 2019,Wiley-VCH Verlag GmbH &Co.KGaA.

3.1.3.Phototherapy

In addition to function as targeting ligands and building block of nanocarriers,AA can also coordinate with metal to form AMCNs that can deliver photoactive agents,thereby applying for caner phototherapy,including photothermal therapy(PTT)and photodynamic therapy (PDT).

Phototherapy,including PTT and PDT utilize photothemal effect and ROS generation ability of nanomaterials to kill cancer under light irradiation.However,photosensitive metal-based drugs have similar shortcomings of common metal drugs,such as poor water solubility,toxicity and non-specific tumor tissue delivery,thus discounting their therapeutic response.AMCNs can specifically deliver photothermal conversion agents or photosensitizers to tumor site for increasing their tumor accumulation and therapeutic response,and decreasing side effect.Considering the biocompatibility and targeting ability of AMCNs,it is possible to improve the shortcomings of metal-based drugs for phototherapy.

Liu et al.[22] used ferrous ion (Fe2+) as a bridging ion and designed a pH-switchable zwitterionic amino acid and gold coordinated nanoparticles(AuNPs-Fe-Glu-Lys),which was electrical neutrality at pH 7.4 to prevent the endocytosis by normal cells.They were changed to be positively charged at pH 6.8 in the TME,thereby promoting the endocytosis of these nanoparticles by tumor cells (Fig.5a).After cell uptake,transglutaminase (TGase)in tumor cells catalyzed the polymerization of glutamine and lysine,causing the intracellular assembly of these gold nanoparticles with enhanced near-infrared light absorption and photothermal conversion by about 3-folds (Fig.5b).Moreover,the coordinated Fe2+can decompose excess H2O2through Fenton reaction to produce hydroxyl radicals (?OH) and kill tumor cells.The nanoformulation showed over 8-times cytotoxicity to tumor cells than to normal cells (Fig.5c),and had long blood circulation time and high tumor selectivity for in vivo tumor therapy (Fig.5d).

Fig.5.Metal-AA coordinated nanomaterials improve phototherapy.(a) Schematic illustration of the fabrication of AuNPs-Fe-Glu-Lys,(b) intracellular enzyme-triggered thermal heating curves of AuNPs-Fe-Glu-Lys,(c) cell viability of HEM-l cells and A375 cells after incubation with different groups,(d) tumor volume curves by different formulations and conditions.Reproduced with permission[22].Copyright 2019,American Chemical Society;(e)Schematic illustration of AuNPs-Fe-Glu-Lys supramolecular metallo-nanodrugs for antitumor PDT,(f,g) response of Fmoc-H/Zn2+ to pH (f) and GSH (g) changes,(h) cell viability in vitro PDT,(d) tumor growth curves with different formulations.Reproduced with permission [30].Copyright 2018,American Chemical Society.

In human bodies,imidazolyl groups of histidine can form coordination compounds with Fe2+to promote iron absorption.By simulating this process,Yao et al.[48] prepared metallosupramolecular nanogels(SNG)though pH-sensitive metal coordination interaction between histidine and tetraphenylporphyrin zinc (Zn-Por).Histidine as a hydrophobic group was protonated in the acid TME,and SNG was broken down to release the codelivered DOX and photosensitizer for synergistic chemotherapy and PDT (Fig.4b).In another study,Li et al.[30] demonstrated self-assembled nanomedicine by coordinating Zn2+with two histidine derivatives,fluorenylmethoxycarbonyl-L-histidine (Fmoc-H)through imidazole and carboxyl group to encapsulate photosensitizer Chlorin e6 (Ce6).The nanomedicine was stable under normal physiological conditions,but at low pH or high GSH level,the coordination bond between metal and amino acid was broken to release Ce6 (Fig.5f,g).Fitting the profiles with a pharmacokinetic model showed that the in vivo half-lives of Ce6 for Fmoc-H/Zn2+/Ce6(8.71 h)and Z-HF/Zn2+/Ce6(6.33 h) were much longer than that of unencapsulated Ce6 (3.69 h).The cytotoxicity of the PDT nanoparticle was confirmed by the combination of irradiation and the metallo-nanodrugs.Significantly,the IC50values of Fmoc-H/Zn2+/Ce6(1.15 μg.ml-1)and Z-HF/Zn2+/Ce6(1.02 μg.ml-1) were much lower than the unencapsulated Ce6(3.83 μg.ml-1)(Fig.5h).And it can excellently inhibit the tumor growth in vivo (Fig.5i).

3.1.4.Chemodynamic therapy (CDT)

In recent several years,a new treatment method named chemodynamic therapy (CDT) has attracted more and more attention,which is based on transition metal-containing nanocatalysts to catalyze Fenton or Fenton-like reactions [95],as depicted in Eqs.(2)and (3).

In this method,transition metal ions (e.g.,Fe [95,96],Mn [97],Cu [98],Co [99],Au [100],and Pt [101]) catalyze overproduced H2O2in TME to produce highly toxic hydroxyl radicals(?OH),which induces cellular oxidative stress,irreversible damages to DNA,lipids and proteins,and finally leads to cell death [95,102].Compared with traditional PDT,CDT does not rely on external stimulation,which can be used to treat deep tumors.For possible applications,CDT still faces great challenges:1) most Fenton/Fenton-like reactions are relatively ineffective in the weakly acidic pH of TME;2) although higher than that in normal cells,the concentration of endogenous H2O2is still low to guarantee continuous ROS generation;3) the high-level GSH and other reducing substances in tumor tissues often offset the production of?OH.These limitations make it difficult to completely eliminate malignant tumors with mere PDT.Therefore,it is of great significance to develop CDT agents with high catalytic efficiency.

Selection of appropriate ligands of transition metals to construct CDT nanomedicines is an efficient strategy for improving CDT performance.CDT nanomedicines with high electron transfer rate under a wide pH range can significantly improve the therapeutic effect.For instance,it is reported cysteine as an “electronic shuttle”can speed up the Fe3+/Fe2+redox cycle in the Fenton reaction to generate?OH [103].The possible reaction mechanism includes:(i) formation of Fe3+-cysteine complex as Eq.(5);(ii)decomposition of the Fe3+-cysteine complex as Eq.(6);(iii)formation of cystine as Eq.(7) with a fast reaction rate.Fan et al.[15]introduced histidine onto the surface of Fe3O4nanoparticles to mimic natural peroxidase,which showed increased apparent affinity (KM) to the substrate H2O2with increased catalytic efficiency(kcat/KM) up to 20-folds.Thus,the modification of amino acid ligands on the surface of nanomaterials can improve not only catalytic activity but also chemical selectivity to substrate.

Fig.6.Metal-AA coordinated Cu-Cys NPs for CDT.(a) Scheme showing the mechanism of Cu-Cys NPs induced ROS augmentation and GSH depletion,(b) TEM of the nanoparticle,(c)terephthalic acid fluorescence for ? OH detection under different pH conditions,(d)GSH/GSSG ratio before and after treatment,(e)tumor-size variation during different treatments,and (f) average tumor mass excised.Reproduced with permission [26].Copyright 2018,American Chemical Society.

Fig.7.Metal-AA coordinated nanomaterials for antibacterial application.(a)Schematic illustration of amino acids-mediated synthesis of Ag NPs,(b,c)antimicrobial activity of the AgNPs against(b)L.monocytogenes and(c)E.coli.Reproduced with permission[106].Copyright 2015,Elsevier.(d)Antibacterial mechanism of Fmoc-F,(e)antibacterial effect of Fmoc-F against Gram-positive and Gram-negative bacteria,and (f) antibacterial effect of phenylalanine derivatives against S.aureus.Reproduced with permission [115].Copyright 2018,The Royal Society of Chemistry

Bu et al.[19] synthesized chelation nanocomplex ferrous-cyste ine-phosphotungstate nanoparticles (FcPWNPs)for pHindependent CDT,which can inhibit the formation of inert Fe(OH)xand accelerate the Fe2+/Fe3+redox cycle.The coordinated assembly of amino acids with ferrous irons accelerated electron transfer,which realized a pH-independent CDT.It can realize efficient cancer cell killing from pH neutral tumor surface to acidic tumor interior.

As above discussed,the high-level GSH in tumor cells that can eliminate ROS might discount ROS-involved dynamic therapy.Therefore,GSH depletion can enhance the efficacy of CDT.Our group recently synthesized copper-cysteine nanoparticles (Cu-Cys NPs) through the coordination of Cu2+with–SH of cysteine,which can be used for in-situ glutathione-activated and H2O2-reinforced CDT with high efficiency and selectivity (Fig.6a,b)[26].The Cu-Cys NPs endocytosed by tumor cells first reacted with local GSH and deplete GSH to produce oxidized GSH (GSSG),and Cu2+was converted to Cu+.Subsequently,the generated Cu+reacted with endogenous H2O2via Fenton-like reaction to generate toxic?OH and kill cancer cells(Fig.6c,d).The higher GSH and H2O2concentration in tumor ensured the significantly higher?OH generation in cancer cells but without damage to normal cells(Fig.6e,f).

3.2.Bacterial killing

Nanomaterials for antibacterial applications as substitute of antibiotics has drawn great attention.Metal nanoparticles as broad-spectrum antibacterial agents,especially silver nanoparticles (Ag NPs),have stronger antibacterial activity compared with antibiotics and small molecule biocides.However,most synthetic metal nanoparticles are encapsulated by surfactants or organic solvents,which might bring cytotoxicity to normal cells and systems.In severe cases,them may even cause thrombosis or cause allergic reactions [104,105].The development of environmentally friendly pathways to synthesize metal nanoparticles has aroused great demands.

Amino acids with reducing capacity,such as cysteine,tyrosine,tryptophan,can be used as environmentally friendly reducing agents and end-capping ligands to mediate theof metal nanoparticles with AA-metal coordination.Shankar et al.[106]synthesized Ag NPs using tyrosine and tryptophan as capping agents,and incorporated them into agar to form antibacterial agar/Ag NPs composite membranes (Fig.7a).The zwitterionic nature of the amino acids and their pH-responsive charge reversibility made them as not only reducing agents and capping agents of Ag NPs,but can efficiently interact with agar to stably disperse nanoparticles in aqueous solutions.The agar/Ag NPs composite membrane had strong antibacterial activity against Listeria monocytogenes (L.monocytogenes) and Escherichia coli (E.coli) (Fig.7b,c).

Bacterial biofilms are an important origin and contributor of bacterial drug resistance,which may evade the antibacterial challenge through multiple mechanisms [107].Therefore,it is necessary to design reagents to inhibit the formation of biofilms or decompose biofilms.Recent studies have shown that the incorporation of specific D-amino acids into the peptidoglycan of the cell wall can cause the bacteria release from the matrix of biofilms[108–110].Due to the biological benignity of D-amino acid,it can also prevent the formation of biofilms [111].To this end,continuous and on-demand release of D-amino acids is needed.Wei et al.[112] designed a nanodevices composed of upconverting nanoparticles (UCNPs) core and D-Tyrosine modified TiO2shell.Under NIR light irradiation,the nanodevices can release D-Tyr and produce ROS for biofilm dispersion and bacterial killing.In addition,aromatic amino acid derivatives can damage cell membranes,which led to the release of cytoplasmic components and cell death [113,114].

Gahane et al.[115] proved that fluorenylmethoxycarbonyl(Fmoc)-conjugated amino acid derivatives showed an antibacterial effect due to their surfactant-like properties (Fig.7d).Unlike known cationic antimicrobial peptides,Fmoc-phenylalanine(Fmoc-F)solution(1.5 mmol.L-1)had antibacterial activity against different Gram-positive bacteria (Fig.7e).Various phenylalanine derivatives had been tested,and loss of antibacterial activity was observed for all except for Fmoc-F (Fig.7f).In the presence of Fmoc-D-phenylalanine(Fmoc-D-Phe),the content of glutamic acid and betaine (maintaining osmotic balance in S.aureus) in the cell decreased sharply.The results showed that Fmoc-F can change the permeability and integrity of bacterial membranes and inhibit bacterial growth by inducing oxidation and osmotic pressure.Irwansyah et al.[116] utilize the intermolecular π-π stacking and hydrogen bonding to form supramolecular hydrogel.The combined assembly of Fmoc-F and Fmoc-leucine (Fmoc-L) antibacterial building blocks can produce supramolecular hydrogels with antibacterial activity.The co-assembled hydrogel exhibited selective Gram-positive antibacterial activity through destroying cell walls and membranes,and had no toxicity to normal cells.Similarly,Song et al.[32]reported an antibacterial biometallohydrogels through self-assembly and local mineralization of silver ion coordinated with Fmoc-amino acids (Fmoc-AAs).The Fmoc-AAs broke down the rigid structure of the bacterial cell wall and membrane,and then AgNPs and Ag+penetrated more efficiently into the cells.Compared with free Ag+and Fmoc-AAs,the Fmoc-AAs metallohydrogels had sustained Ag+release and significantly inhibited bacteria growth against both Gram-negative (E.coli) and Gram-positive(S.aureus) bacteria in vivo and in vitro.Moreover,the Fmoc-amino acid metallohydrogels also had a reduced cytotoxicity to normal cells than free Ag+.

In addition,transition metal-based nanomaterials are expected to reduce drug resistance of bacteria through ROS-based treatment strategies including PDT and CDT.The ROS-involved therapy has multi-target effect,which can destroy bacterial cell walls,nucleic acids,and proteins that are related to bacterial drug resistance.However,nanomaterials larger than 10 nm cannot effectively enter into bacteria,which will reduce the bactericidal effect,due to the inherent short lifespan (less than 200 ns) and diffusion distance(～20 nm) of ROS [118–120].Moreover,the accumulation of dead bacteria at the infected site might induce undesirable tissue inflammation,which is not conducive to wound healing [121].In order to overcome these problems,Sang et al.[122] designed Lcysteine modified MoS2-hydrogel to capture bacteria by electrostatic interaction and limit the ROS destruction region,thus effectively improving the antibacterial efficiency of the catalysisgenerated?OH.L-cysteine,providing carboxyl and amino groups,acted as an effective linking agent between MoS2nanoenzyme and the hydrogel.Compared with traditional nanoenzymes,the MoS2-hydrogel achieved better ROS utilization efficiency and antibacterial effect.

Fig.8.Metal-AA coordinated nanomaterials for multimodal imaging.(a)Synergistic construction of multimodal imaging for FEAD1 and accurate diagnosis and treatment of peritoneal metastatic tumors,(b) NIR-II fluorescence images with FEAD1,and (c) fluorescence images of tumor slice.Reproduced with permission [126].Copyright 2020,Wiley-VCH Verlag GmbH &Co.KGaA;(d) Scheme of the preparation procedure for PFDR-Au,(e) confocal images with PFDK-Au and PFDR-Au at different times,and (f)hemolytic activity of PFDR-Au and PFDK-Au under different pH values.Reproduced with permission [127].Copyright 2018,American Chemical Society.

Totally,amino acids as natural precursors and building blocks can be used as reducing agents,capping agents,and linkers to fabricate and modify metal ions containing nanomaterials.They can provide more binding sites for building antibacterial hydrogel,and enrich the generated ROS for enhancing antibacterial efficiency.

3.3.Biomedical imaging

Nanoscale imaging probes with high specificity have been extensively studied in the biomedical field.Tryptophan can absorb and emit electromagnetic radiation in the ultraviolet range due to the conversion between the energy states of the indole group[123].Recent studies have used amino acid coordination and self-assembly to fabricate nanomaterials with diagnostic capabilities [33,117].The strategies can be divided into two categories:1) by imitating natural fluorescent proteins,coordination interactions can be used to prepare AMCNs with inherent luminescent properties;2) external imaging agents can be introduced into the nanoparticle by coordination with amino acids for biological imaging.AMCNs functions in construction of biological imaging nanosystems by assisting self-assembly,adjusting photoluminescence properties of dyes and enhancing the contrast parameters of imaging contrast agents.

An ideal amino acid-based fluorophore should have a high fluorescence quantum yield and a long fluorescence lifetime.For example,tryptophan as the main source of ultraviolet absorption and fluorescence in proteins will be strongly affected by the surrounding environment.The coordination of tryptophan with metal containing nanoparticles can reduce the emission and enhance the lifetime of excited electronic states[124].Kim et al.[125]functionalized AuNPs with tryptophan for labeling human neuronal (SHSY5Y) cells,which had a high fluorescence in a simple,safe and sustainable way.Ling et al.[126] designed a TME-activatable NIRII nanotheranostics system (FEAD1) (Fig.8a),which was prepared by self-assembly of Fmoc-His,functionalized Ag2S quantum dots(MPA-Ag2S QDs),DOX and A1094 (a NIR absorber) into the nanoparticles.Once the nanoparticles FEAD1 entered the slightly acidic tumor tissue,the imidazole group of Fmoc-His and DOX were protonated and the metal coordination and hydrophobic force were weakened to trigger DOX release,and the rapid disassembly of FEAD1 induced fluorescence from “off” to “on”(Fig.8b).After the FEAD1 was injected into the abdomen of mice,the released DOX can treat peritoneal metastases,and simultaneously the NIR-II fluorescence imaging signals and bioluminescence signals can be detected for accurate cancer diagnosis (Fig.8c).

Ma et al.[127] fabricated a multifunctional poly(amino acid)-gold-magnetic nanoclusters (γ-PGA-Fe3O4-DOX-PA-Au) (PFDRAu) with self-degradation properties (Fig.8d).Through the dualresponse to near-infrared light and local acidic pH in tumor cells,multi-modal imaging-guided collaborative chemo-photothermal therapy was realized,which induced effective tumor ablation with a tumor inhibition rate of 94%.Compared with cationic poly(εlysine) modified nanoclusters (PFDK-Au),the PFDR-Au had a higher cytomembrane affinity due to the guanidine group in arginine,thereby effectively enhancing endocytosis and lysosome escape ability (Fig.8e,f).Using silica nanoparticles as core,Qi et al.[20] coordinated Fe2+ions with L-lysine to form brush and stable di-amino-acid coordinated iron nanogels (SiO2@MPGs).In the nanogel network,Fe2+ions served as active centers for the coordination of chain-like lysines of ferrase mimics.The SiO2@MPGs exhibited efficient multi-enzyme-like activities of superoxide dismutase (SOD) and peroxidase (POD).Furthermore,amplex red (AR) as the substrate of POD was oxidized to generate fluorescent molecules,realizing ROS-responsive fluorescence imaging.

Totally,nanomaterials formed by the coordination of metals and amino acids can solve the problem of imaging instability of amino acids themselves,and metals can act as active sites of biomimetic metal enzymes.The presence of metal coordination bonds allows nanomaterials to have an improved pH and enzyme triggering performance for selective imaging.

4.Conclusionsand Outlook

As stated in this review,amino acids and their derivatives with high biocompatibility and biodegradability,and low immunogenicity have been widely used in biomedicine.Metal ions have multiple oxidation states and play an important role in biological and biomedical processes.Many kinds of amino acid-metal coordinated nanomaterials have been designed and fabricated for cancer treatment,antibacterial application,biomedical imaging,and other applications with the advantages of convenient synthesis,good controllability,and versatile functions.It has become a promising platform for the development of new-generation nanomedicine.To improve their application performance,future researches still needs to solve the following problems:1) the thermal and chemical stability of AMCNs has not yet reached the requirements of traditional biological materials,which might hinder their further applications.2) the exact mechanism of the synergistic effect of amino acids with metals is still far from been clearly recognized,which raises questions about the multi-dose design and effectiveness.3) more treatment methods based on the coordination of amino acids with metals require deep and systematic in vivo toxicology and pathology research.We should bear in mind that nanomaterial preparation is only the first step for possible clinical practice.Prior to this,more laboratory and preclinical studies are needed,and further studies on feasibility verification are required.The clinical translation of AMCNs is still in its infancy.For the preclinical studies,the assessment of AMCNs should cover the impact of more factors.It is almost impossible to reproduce patients’heterogeneity by evaluating the diagnostic and therapeutic performance of nanomaterials only on small animal models.With the rapid development of nanotechnology and nanomedicine,these obstacles in the clinical translation of amino acid-metal coordinated nanomaterials will be overcome in the near future.We hope the collaboration from scientists in different fields will develop more effective,stable and comprehensive nanosystems based on the coordination of amino acids with metals in near future.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (82072065,81471784),the National Key Research and Development Program of the Minister of Science and Technology,China(2016YFA0202703),and the National Youth Talent Support Program.