A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications — TEI-tagged view

Qianwen LiupersName, Amin ZhangpersName, Ruhao WangpersName, Qian ZhangpersName, Daxiang CuipersName (2021date). “A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications.” Nano-Micro Letterstitle·j.

Object of study —

Research question —

Contribution — integrated synthesis of nanozymeterm·dom mechanisms, challenges, applications, and future directions

Full text — tagged

The reference list is not part of the text we hold for this paper. Our copy was extracted from the publisher's page and the bibliography did not survive that extraction, so the article below ends before its references. Nothing has been removed by us.

Abstract. Highlights The characteristics of metal- and metal oxide-based nanozymesterm·dom with diverse construction are dissertated. The intrinsic properties and catalytic mechanism of metal- and metal oxide-based nanozymesterm·dom are discussed. The recent applications of metal- and metal oxide-based nanozymesterm·dom in biological analysis, relieving inflammation, antibacterial, and cancer therapy are reviewed. Keywords: Metal- and metal oxide-based nanozymesterm·dom, Intrinsic properties, Catalytic mechanism, Applications Abstract Since the ferromagnetic (Fe 3 O 4 ) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymesterm·dom has been made with deep investigation of diverse nanozymesterm·dom and rapid development of related nanotechnologies. As promising alternatives for natural enzymesterm·def, nanozymesterm·dom have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymesterm·dom owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymesterm·dom. This reviewterm·mth digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymesterm·dom, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymesterm·dom constituted of metal and metal oxide nanomaterials. Introduction Enzymesterm·def are environmentally friendly biomaterials with remarkable catalytic efficiency and substrate specificity produced by living cells [ 1 , 2 ]. Most of the natural enzymesterm·def are proteins, while a small part are RNA. The past decades have witnessed the extensive progress of biological enzymesterm·def in biology, medicine, chemistry, and industrial science [ 3 ]. Nevertheless, the complicated preparation procedure, unstable catalytical activity and intrinsic environmental sensitivity have restricted the scalable utilization of natural enzymesterm·def [ 4 , 5 ]. Therefore, the exploration of alternative artificial enzymesterm·def to overcome shortcomings of natural catalysts has become an issue of increasing concern. The evolution of nanotechnology and biology provides a bridge toward novel artificial enzymesterm·def. After the pioneering work of Gao et al. [ 6 ] reporting ferromagnetic (Fe 3 O 4 ) nanoparticles (NPs) with enzyme-mimicking property in 2007, a bunch of nanozymesterm·dom have been demonstrated as natural catalysts mimics. For instance, Au@Co–Fe hybrid NPs [ 7 ], CuCo 2 S 4 NPs [ 8 ], MnO 2 nanowires (NWs) [ 9 ], Pt nanoclusters (NCs) [ 10 ], Au@Pt nanorods (NRs) [ 11 ], and carboxyl-modified graphene oxide (GO–COOH) [ 12 ] have been reported as peroxidase (POD) mimics. Nanozymesterm·dom with multi-enzyme-type activities (e.g., Co(OH) 2 /FeOOH/WO 3 ternary nanoflowers [ 13 ], AuNPs [ 14 , 15 ], Co 3 O 4 NPs [ 16 ], AgPt NPs [ 17 ], N-doped sponge-like carbon spheres [ 18 ], Mn 3 O 4 NPs [ 19 ]) have been exploited in diverse investigation. Up to date, more than 540 types of nanozymesterm·dom have been synthesized by over 350 research laboratories from 30 countries [ 20 ]. Generally, existing nanozymesterm·dom are affiliated with two categories, namely oxidoreductase family and hydrolase family. Carbon-based materials, metal, and transition metal compounds are the most common nanozymeterm·dom composition materials [ 21 ]. Wu et al. reviewed the history of nanozymeterm·dom and draw a brief timeline for the evolution of artificial enzymesterm·def and natural enzymesterm·def (Fig. 1 ) [ 22 ]. With extensive efforts devoted to the investigation of artificial enzymesterm·def and nanotechnology, creative breakthroughs have been made steadily on the catalytic mechanisms and intrinsic properties of nanozymesterm·dom, as well as the application field. In the past two years, the investigation on single-atom nanozymeterm·dom (SAN) has aroused numerous attention due to their outstanding activity and selectivity [ 23 , 24 ]. In the research of Kim et al. [ 25 ], the Fe–N–rGO SAN showed the best catalytic efficiency for different substrates among various classical POD mimics and natural HRP. Niu et al. [ 26 ] reported that the Fe–N–C SAN not only possessed excellent enzymatic activities, but also exerted splendid stability and robustness within a broad temperature and pH range. Fig. 1 A brief timeline for the evolution of artificial enzymesterm·def and natural enzymesterm·def. Reproduced from Ref. [ 22 ] with permission Since nanozymesterm·dom are recognized as a class of functional nanomaterials, they possess both the unique nature of nanomaterials and enzyme-like activity [ 27 ]. The surfaces of metal and metal oxide nanomaterials are covered with a large amount of charge, which was responsible for their superb electron properties [ 28 ]. Consequently, metal- and metal oxide-based nanozymesterm·dom stand out in the area of electrocatalysis, sensing and fuel cells [ 29 , 30 ]. Furthermore, as promising alternatives for natural biocatalysts, they commonly retained better stability and robustness under extreme conditions than natural enzymesterm·def [ 5 ]. The prominent physicochemical properties (e.g., high surface energy, superior optical, and photothermal conversion properties), as well as simplicity in preparation and storage also broaden their applications [ 31 ]. Interestingly, the catalytic performance and physicochemical properties of metal and metal oxide nanomaterials could be easily regulated according to the practical demand [ 32 , 33 ]. For instance, surface modification has been revealed as a promising strategy to increase the biocompatibility of these nanozymesterm·dom [ 34 – 36 ]. The structure design associated with the catalytic efficiency is flexible through suitable control of synthetic conditions [ 37 ]. Given the above ascendency, the research fields of metal- and metal oxide-related nanozymesterm·dom have gradually extended from environment to chemical industry, food, agriculture, biomedicine, medicine, and so forth [ 38 – 40 ]. Even though tremendous efforts were devoted, further promotion of this kind of nanozymesterm·dom is still facing difficulties. For example, the enhancement of catalytic activity and selectivity, closely associated with the sensitivity and specificity of nanozyme-based biosensors, remains a challenge [ 25 , 41 ]. In addition, the strengthened physiological stability and biological safety is vital for the spread application of nanozymesterm·dom in clinical medicine [ 42 ]. Therefore, novel nanozymesterm·dom and biotechnology are urgently needed to make up these defects. Dozens of excellent reviewsterm·mth concerned with nanozymesterm·dom have been published in recent years. Some of the reviewsterm·mth involved the research progress of nanozymesterm·dom in a particular field [ 5 , 21 , 27 , 43 , 44 ]. Some researchers organized and revealed the natural activities and working mechanisms of specific nanozymesterm·dom [ 45 – 49 ]. In 2019, Huang et al. [ 50 ] systematically discussed the classification, intrinsic nature, enzymatic mechanisms and potential applications of nanozymesterm·dom for the first time. However, a thorough overview for metal- and metal oxide-based nanozymesterm·dom is still lacking. In this reviewterm·mth, we firstly elucidate the characteristics and synthetic methods of metal- and metal oxide-based nanozymesterm·dom. Then, we will dig into the catalytic mechanisms and property regulation of these nanozymesterm·dom. After introducing their appliance in biological analysis, relieving inflammation, antibacterial and cancer therapy, we finally discuss the present challenges and give a future perspective for the research of nanozymesterm·dom constituted of metal and metal oxide. Preparing Diverse Nanozymesterm·dom with Constructive Feature Generally speaking, the existing metal- and metal oxide-based nanozymesterm·dom can be roughly assorted into monometal [ 51 ], metal alloy [ 52 – 54 ], metal oxide [ 6 , 55 , 56 ], metallic core/shell nanostructure [ 57 – 59 ], and hybrid [ 60 ] nanomaterials in terms of constructive feature. Monometal nanozymesterm·dom are usually noble metal nanomaterials possessing prominent chemical stability under natural conditions. They commonly possess facile conjugation sites to diverse biomolecule ligands and antibodies, remarkable surface plasmon resonance (SPR) properties, superior optical, and photothermal conversion properties [ 61 – 63 ]. However, bare monometal nanoparticles (e.g., Ag, Pt) tend to aggregate into nanoclusters, resulting in decreasing of catalytic activity [ 64 ]. What is worse, most bare noble metal nanozymesterm·dom (except Au) have biological toxicity, thus limiting their application in clinical medicine. The structure, size, and morphology have been proved to influence the catalytic properties of these nanozymesterm·dom [ 65 – 67 ]. Monometal nanozymesterm·dom could be prepared through preformed-seed-mediated growth [ 68 ], high-temperature reduction method [ 2 , 69 – 71 ], electrochemical synthesis, photochemical method, biosynthesis [ 72 , 73 ], and spatially confined medium/template approach [ 74 ]. With different methods, various forms of noble metal nanomaterials (e.g., nanoparticles [ 14 , 15 ], nanoclusters [ 10 ], nanorods [ 75 ], nanosheets [ 76 ], nanocubes) could be obtained. The preformed‐seed‐mediated growth is feasible for size control by changing the concentration and nature of seeds in the growth solution [ 77 ]. A variety of small molecules (e.g., tannic acid [ 71 ], citrate [ 78 ]) and macromolecular templates including DNA [ 79 ], dendrimers [ 80 ], and proteins (e.g., bovine serum albumin, human serum albumin, lactoferrin, pepsin, insulin) [ 2 , 70 ] have been employed for monometal nanozymesterm·dom synthesis via the high‐temperature reduction procedure. The electrochemical strategy could modulate the size and morphology of noble metal nanomaterials through controlling electrodeposition parameters during the deposition process. Metal alloy nanozymesterm·def, containing bimetal alloys and multimetallic alloys, could be obtained via common chemical synthesis such as the one-pot strategy [ 81 ], galvanic replacement reaction [ 82 , 83 ], co-reduction method [ 84 , 85 ], hydrothermal growth [ 86 ], and electrodeposition method [ 87 , 88 ]. Besides, biological strategy [ 89 ] and bimetallic nanomaterials printing [ 90 ] have been present as favorable synthesis method as well. The biological strategy is widely known as a green synthesis method with biological elements as the reducing agents or growth template (e.g., leaf extract, plant extract, DNA) [ 91 , 92 ]. Along with the preparation of diverse nanoalloys, researchers found that the composition as well as structure affected the enzymatic characteristic of metal alloy nanozymesterm·def [ 93 , 94 ]. Therefore, adjusting the proportion of various metals, enlarging porosity and specific surface area of alloy nanomaterials have been recognized as effective approaches to regulate activity. Generally, the cost of metal alloy nanozymesterm·def is much lower than that of monometal nanomaterials as the incorporation of non-precious metals. Owing to the synergistic effect of the two components, bimetal nanoalloys tend to exhibit superiorly optical and chemical properties, as well as better catalytic performance compared with noble metal nanomaterials [ 95 ]. Furthermore, the introduction of magnetic metal (e.g., Co, Fe, and Ni) could endow alloys with magnetism besides optimizing their enzymatic properties [ 83 , 84 ]. Possessing high surface energy and surface-to-volume ratio, metal oxide nanozymesterm·dom have been considered as promising artificial enzymesterm·def for decades [ 96 ]. The most common metal oxide nanozymesterm·dom like CeO 2 , Fe 2 O 3 , Fe 3 O 4 , Co 3 O 4 , Mn 2 O 3 , and Mn 3 O 4 nanomaterials have all been reported to possess multi-enzyme-like activities [ 97 ]. In addition, they exhibit plenty of unique properties such as magnetic, fluorescence quenching and dielectric properties [ 98 ]. Compared with precious metal nanomaterials, metal oxide nanozymesterm·dom commonly exert lower price and concise synthesis process [ 99 ]. Furthermore, the low biological toxicity and favorable accumulation in biological tissues have broadened their application toward biopharmaceutical [ 100 ]. Nevertheless, there are some disadvantages of unmodified metal oxide nanozymesterm·dom in terms of biology. For instance, they might show awful stability and accelerate the generation of harmful free radicals under physiological conditions [ 101 ]. Additionally, the improper surface ligands coating would lead to the failure control of drug release [ 102 ]. In recent, diverse methods have been employed for metal oxide nanozymesterm·dom preparation, including the hydrothermal [ 103 , 104 ], solvothermal [ 105 , 106 ], pulsed laser ablation [ 107 ], co-precipitation [ 108 , 109 ], sol–gel [ 110 ], and thermal decomposition method [ 111 ]. The metallic core/shell (inorganic/inorganic) nanostructure-based nanozymesterm·dom could be prepared through the hydrothermal reaction [ 112 ], solvothermal method [ 113 ], sol–gel approach [ 114 ], and atomic layer deposition [ 115 ]. By combining different materials and modifying structure, researchers could regulate the stability and functionality of core/shell structure-based nanozymesterm·dom conveniently [ 116 ]. For example, the introduction of SiO 2 as coating significantly realized good stability and reduced bulk conductivity of the core particles [ 117 ]. The dispersion and biological safety of magnetite NPs encapsulated by silica could also be improved when existed in physiological environment [ 118 ]. In addition, the Au-coated nanostructure-based nanozymesterm·dom have demonstrated to show enhanced chemical stability, biocompatibility, and optical properties [ 119 , 120 ]. However, the accessibility between substrate and the active phase of nanozymesterm·dom could be affected by coating materials [ 121 ]. Therefore, regulating coatings’ thickness, porosity, and synthesis procedure was demanded to modulate enzyme-like capacity and other chemical properties of nanozymesterm·dom. The metal- and metal oxide-based hybrid nanozymesterm·dom could be prepared with organic molecules or polymers modified on the surface of metal or metal oxide nanomaterials [ 122 , 123 ]. The modifications on the surface of hybrid nanozymesterm·dom are used to optimize the catalytic performance, instead of acting as stabilizer during the synthesis process [ 124 ]. Generally, the intrinsic properties of hybrid nanozymesterm·dom might be ascribed to size, content, and components structure [ 125 , 126 ]. For instance, polymer/metal nanozymesterm·dom have been revealed to show stable catalytic capacity in which metal nanoparticles are evenly dispersed in polymer [ 127 , 128 ]. In parallel with enhancing catalytic activity and selectivity, the incorporation of polymer or organic molecule endows hybrid nanozymesterm·dom with amazing physical, chemical, and mechanical properties (e.g., adsorption [ 129 ], water solubility [ 130 ], biodegradability [ 131 ]), thereby expanding their application in miscellaneous fields [ 124 ]. The catalytic activities and efficiency of metal- and metal oxide-based nanozymesterm·dom involved in the recent reports are listed in Table 1 . These nanozymesterm·dom mainly imitate four kinds of natural enzymesterm·def, namely POD, oxidase (OXD), catalase (CAT), and superoxide dismutaseterm·def (SOD). The Michaelis–Menten constant ( K m ) and maximal velocity ( V max ) reflects the enzymeterm·def affinity with its substrate and maximal reaction velocity respectively [ 132 ]. And the K cat is the maximum number of substrate molecules converted to product per enzymeterm·def molecule per second. The lower value of K m and the higher value of V max indicate the stronger catalytic activity of nanozymesterm·dom. Table 1 Intrinsic activity and catalytic efficiency of typical metal- and metal oxide-based nanozymesterm·dom Nanomaterial Surface modification Activity Catalyst efficiency: k cat (s −1 ), substrate, K m (mM), V max (μM s −1 ) References Monometal Au NPs GOx 18.52, glucose, 6.97, 0.63 [ 133 ] Au NCs Amine-terminated PAMAM dendrimer POD,CAT,SOD –, H 2 O 2 ,16.0,0.452 (CAT) [ 80 ] Pt NPs BSA POD –, TMB, 0.119, 0.21 [ 134 ] –, H 2 O 2 , 41.8, 0.167 Pt NCs POD –, TMB, 0.096, 0.1414 [ 135 ] –, H 2 O 2 , 3.07, 0.1817 Pd NPs Carboxylated chitosan POD –, TMB,0.09, 0.177 [ 136 ] –, H 2 O 2 , 537.71, 0.112 Ru NPs HRP, OXD –, TMB,0.234, 0.0825 (HRP) [ 137 ] –, H 2 O 2 , 2.206, 0.583 (HRP) Cu NCs POD –, TMB, 0.648, 0.0596 [ 138 ] –, H 2 O 2 , 29.16, 0.0422 Os NPs Citrate POD 1.72 × 10 3 ,TMB, 0.096, 0.412 [ 139 ] 2.35 × 10 3 , H 2 O 2 , 3.88, 0.565 Ir NPs Citrate POD,CAT,OXD 5 × 10 2 , TMB, 0.0906, 1.7 (POD) [ 140 ] 4.4 × 10 2 , H 2 O 2 , 0.27, 1.5 (POD) –, H 2 O 2 , 21.09, – (CAT) Rh NPs Citrate POD 3.87 × 10 2 , TMB, 0.198, 0.0678 [ 141 ] 1.38 × 10 3 , H 2 O 2 , 0.38, 0.241 Metal alloy Au 2 Pt CAT –, H 2 O 2 , 7.7066, 0.9018 [ 142 ] AgPt NPs BSA CAT,POD 0.751 × 10 3 ,OPD,0.129,89.71 (POD) [ 17 ] 1.075 × 10 3 , H 2 O 2 ,76.05, 128.49 (POD) 183.735 × 10 3 , H 2 O 2 ,54.30, 16.2 (CAT) Au–Pt NCs Guanosine monophosphate (GMP) OXD –, TMB, 6.805, 2.538 [ 143 ] –, ABTS, 0.1321,0.1798 Fe–Pt NPs OXD –, TMB, 0.030, 0.0142 [ 144 ] Pd/Pt NWs OXD –, TMB, 0.058, 0.114 [ 33 ] NiPd NPs CAT,POD,OXD –, TMB,0.11, 0.0152 (POD) [ 83 ] –, H 2 O 2 , 0.66, 0.2618 (POD) Metal oxide MnO 2 NSs HSA OXD –, TMB, 0.042,0.212 [ 145 ] Mn 3 O 4 NPs OXD –, TMB, 0.08, 0.4817 [ 146 ] Fe 3 O 4 histidine POD 1.8256 × 10 5 , TMB, 6.22, 0.157 [ 105 ] 1.6965 × 10 5 , H 2 O 2 , 10.58, 0.1459 CeO 2 NPs Phosphatase –, pNPP, 0.74, 7.33 × 10 –6 [ 147 ] CeO 2 NRs SO 4 2− OXD 16.55, TMB, 0.22, 0.48 [ 148 ] Co 3 O 4 NPs CAT 1.63 × 10 4 , H 2 O 2 , 34.3, 11.2 [ 103 ] Co 3 O 4 NPs OXD –, TMB, 0.051, 0.033 [ 104 ] –, ABTS, 0.037,0.032 Co 3 O 4 nanoflowers POD,CAT, OXD,SOD –, TMB, 0.2830, 0.1052 (POD) [ 149 ] –, H 2 O 2 , 5.9322, 0.0985 (POD) –, H 2 O 2 ,839.85, 1466.66 (CAT) –, TMB, 0.0469, 0.0459 (OXD) NiO nanoflowers SOD 2.6 × 10 10 , O 2 ˙ − , 0.043,35 [ 106 ] Core/shell nanostructure Au@Pt POD 1.475 × 10 3 ,TMB, 0.00243, 0.04425 [ 150 ] 2.004 × 10 3 , H 2 O 2 , 0.00407, 0.06013 Fe 3 O 4 @MoS 2 POD –, TMB,0.25, 0.111 [ 151 ] –, H 2 O 2 , 1.39, 1.63 Fe 3 O 4 @C NWs POD –, TMB,0.20, 0.0134 [ 152 ] –, H 2 O 2 , 0.23,0.0241 Co@Fe 3 O 4 POD –, TMB,1.17, 0.379 [ 153 ] –, H 2 O 2 , 0.19,0.715 Au/CeO 2 NPs POD,CAT,SOD –, TMB, 0.29, 0.039 (POD) [ 154 ] –, H 2 O 2 , 44.69, 0.0223 (POD) Cu 2 O@TiO 2 NWs OXD 15.25, glucose, –, 0.915 [ 155 ] Pd cube@CeO 2 NPs OXD –, TMB, 0.21, – [ 156 ] Hybrid nanozymesterm·dom PVP/IrPt NPs CAT,POD,OXD –, TMB, 0.16, 2.25 (POD) [ 157 ] –, H 2 O 2 , 0.75, 2.66 (POD) Fe 3 O 4 /CoFe-LDH POD –, TMB, 0.395, – [ 158 ] –, H 2 O 2 , 10.24, – Co 3 O 4 @β-cyclodextrin NPs POD –, TMB, 0.17, 0.0281 [ 36 ] –, H 2 O 2 , 1.42, 0.0285 HS-Pt NPs OXD –, TMB, 0.01012, – [ 159 ] His@AuNCs/RGO OXD –, TMB, 0.031, 0.0655 [ 160 ] BSA bovine serum albumin, PVP polyvinylpyrrolidone, PNPP p -nitrophenyl phosphate, LDH layered double hydroxides, HS heparin sodium, RGO reduced graphene oxide, His histidine, GOx glucose oxidase, HRP horseradish peroxidase, OPD o-phenylenediamine, NSs nanosheets, HSA human serum albumin Properties of Metal- and Metal Oxide-Based Nanozymesterm·dom Catalytic Mechanism Catalase-Like Activity CAT is a kind of binding enzymeterm·def with iron porphyrin as its prosthetic group [ 161 ]. CAT presents in the living tissues could catalyze hydrogen peroxide (H 2 O 2 ) into oxygen and water, hence protecting tissues from excessive H 2 O 2 [ 162 ]. Up to now, a series of metal-associated nanozymesterm·dom, such as platinum (Pt) [ 51 ], gold (Au) [ 163 ], CeO 2 [ 164 ], Mn 3 O 4 [ 19 ], have been demonstrated to show CAT-like activity. Although promising in anti-inflammatory, tumor treatment, biological detection and many other fields, considerable CAT mimics still constrained by the obscure mechanism [ 165 , 166 ]. Li et al. [ 167 ] verified that the pre-adsorbed OH group on the surface of noble metal served as the active site for CAT-like catalytic reaction. Although most reported nanomaterial-based CAT mimics showed favorable catalysis ability in neutral and alkaline environment, LiupersName et al. [ 80 ] firstly reported that amine-terminated PAMAM dendrimer encapsulated gold nanoclusters (AuNCs-NH 2 ) displayed CAT-mimicking property not only in acidic environment but also over physiological pH range (i.e., pH 4.8–7.4). They speculated that the protonation of tertiary amines from dendrimers in acidic solution could stimulate pre-adsorbing OH, thus providing active sites for H 2 O 2 decomposition to generate oxygen and water. In terms of metal oxide nanozymesterm·dom, Celardo et al. put forward a possible catalytic model of CeO 2 NPs with CAT-mimicking properties in 2011 [ 168 ]. In the system, H 2 O 2 was firstly bind to the 2Ce 4+ binding site presented by the oxygen vacancy site of CeO 2 NPs (Fig. 2 a➀, ➁). Then, the fully reduced oxygen vacancy site was formed as the protons released and two electrons transferred to the two Ce 4+ (Fig. 2 a➂). The oxygen was generated from the reduced oxygen vacancy site (Fig. 2 a➃). Afterwards, another H 2 O 2 molecule was bind to the 2Ce 3+ site (Fig. 2 a➄). The homolysis of O–O bond happened with the transfer of two electrons and a uptake of two protons (Fig. 2 a➅). After H 2 O molecules released, the initial Ce 4+ sites were regenerated on nanoceria surface. Interestingly, Mu et al. reported that a larger concentration of the perhydroxyl anion (OOH − ) contained in H 2 O 2 molecule were existed in the neutral and alkaline solution [ 103 ]. The OOH − then might interact with metal centres of Co 3 O 4 and form the · O 2 H due to its prominent nucleophilic ability compared with H 2 O 2 . With terephthalic acid as the fluorescent probe, it could be found that the production efficiency of the hydroxyl radical ( · OH) depended on the Co 3 O 4 concentration, indicating that the CAT-type property of Co 3 O 4 NPs would influence the decomposition of H 2 O 2 to · OH. Moreover, thermodynamic and kinetic analysis revealed that there might be more “active sites” on the surface of Co 3 O 4 NPs than natural CAT owing to the stronger affinity between H 2 O 2 and Co 3 O 4 compared with natural CAT. Fig. 2 a Electron transfer mechanism for the CAT-mimetic activity of CeO 2 NPs. b Top view (left) and side view (right) of the CeO 2 (111) structural model. c Atomistic-level catalytic mechanisms for the CAT-mimicking reactions of nanoceria. d , e Energy profiles for steps (1) and (2) for CeO 2 (111). Adapted from a Ref. [ 168 ], b – e [ 171 ] with permission The existing hypothetical mechanisms for the CAT-like property of CeO 2 NPs and Co 3 O 4 NPs mentioned above still show certain limitations due to the neglect of the real structural features discussion [ 169 ]. Therefore, Guo et al. [ 170 ] investigated the possible catalytic mechanism of CAT-type activity at atomic or molecular level, involving the base-like dissociative, acid-like dissociative, and bi-hydrogen peroxide associative mechanisms. Based on the calculation of thermochemical energies and associated activation barriers, they reported that the bi-hydrogen peroxide associative mechanism was most viable for the CAT-mimicking catalytic recycle for Co 3 O 4 . WangpersName et al. deeply investigated the structural and electronic properties of nanoceria to propose the atomistic-level mechanisms (Fig. 2 b, c) [ 171 ]. In their model, the CeO 2 (111) surface oxidized H 2 O 2 molecule to form O 2 and a reduced H 2 -CeO 2 (111) surface. Then, anotehr H 2 O 2 molecule would react with the H 2 -CeO 2 (111) surface to produce H 2 O. As shown in Fig. 2 d, the reaction between H 2 O 2 and CeO 2 (111) surface was exoenergetic (energy difference Δ E = − 1.40 eV) with a small energy barrier ( E a ) of 0.35 eV. Since Δ E = − 2.09 eV and E a = 0.82 eV, the interaction between H 2 -CeO 2 (111) surface and H 2 O 2 was also exoenergetic and kinetically favorable as well (Fig. 2 e). Peroxidase-Like Activity Peroxidase, produced by microorganisms or plants, is closely related to the growth of animals and plants [ 172 , 173 ]. The peroxidase family is very huge, and most peroxidases are heme enzymesterm·def with ferric protoporphyrin IX (protoheme) as the prosthetic group (e.g., horseradish peroxidase, lignin peroxidases, myeloperoxidase) [ 174 – 177 ]. Following the blooming exploration on enzymesterm·def, peroxidases with selenium (glutathione peroxidase, GPx), manganese (manganese peroxidase), and vanadium (bromoperoxidase) as active centers have been widely reported [ 178 – 180 ]. Peroxidase catalytically oxidizes organic substrates in which H 2 O 2 acted as an electron acceptor, thereby decomposing H 2 O 2 and effectively eliminating the toxicity of phenols and amines. In 2007, GAO et al. discovered that magnetite (Fe 3 O 4 ) nanoparticles had a special property that similar to HRP [ 6 ]. Since then, a series of nanomaterials have been unraveled to serve as POD mimics, including metal materials [ 181 ], metal oxides [ 182 ], conducting polymers [ 183 ], metal organic frameworks [ 184 ], carbon nanomaterials [ 185 ], single-atom catalysts [ 186 ] and so on. The catalytic mechanisms of various nanomaterial-based POD mimics could generally be concluded as Fenton or Fenton-like reaction or the electron transfer process [ 117 ]. WangpersName et al. [ 187 ] prepared Fe 3 O 4 magnetic nanoparticles (Fe 3 O 4 MNPs) via a reverse co-precipitation method under ultrasonic irradiation. The possible catalytic mechanism of Fe 3 O 4 MNPs with POD-type activity was displayed in Fig. 3 a. The bound Fe 2+ and Fe 3+ activated H 2 O 2 molecules that adsorbed on the surface of Fe 3 O 4 MNPs to produce · OH and oxygen superoxide anion (O 2 · − )/hydroperoxyl radicals (HO 2 · ). Then, the · OH and O 2 · − /HO 2 · radicals would induce the subsequent degradation and mineralization of Rhodamine B (RhB). However, Maxim et al. [ 188 ] put forward different opinions about the generation of · OH under conditions of the biologically relevant superoxide-driven Fenton reaction. Based on the spin-trapping electron paramagnetic resonance (EPR) experiments, they discovered that the reactions (Eqs. 1 – 3 ) at the nanoparticles’ surface rather than the metal ions released by the nanoparticles were responsible for the POD-mimicking property of γ-Fe 2 O 3 and Fe 3 O 4 NPs (Fig. 3 b). What is more, the production effect of the catalytic centers on the surface of γ-Fe 2 O 3 was demonstrated to be at least 50-fold higher than that of the dissolved metal ions. 1 Fe 3 + + O 2 · - → Fe 2 + + O 2 2 2 O 2 · - + 2 H + → H 2 O 2 + O 2 3 Fe 2 + + H 2 O 2 → Fe 3 + + OH - + · OH Fig. 3 a Mechanism for the POD-like activity of Fe 3 O 4 MNPs in the degradation of organic pollutants. b Mechanism mediated by γ-Fe 2 O 3 NPs. c View of Single layer from the V 2 O 5 structure. d Possible mechanism for the catalytic reaction of the V 2 O 5 NWs. e Catalytic mechanism of M13-E4@MnO 2 NWs with POD-type properties. f Corresponding reaction equation of TMB oxidized by H 2 O 2 with the Ir NPs as POD mimics. ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)). Adapted from a Ref. [ 187 ], b Ref. [ 188 ], c , d Ref. [ 189 ], e Ref. [ 9 ], f Ref. [ 71 ] with permission The nanocrystalline structure of nanozymesterm·dom was also considered to make contribution to the H 2 O 2 -activating ability. André et al. reported that the intrinsic POD-like activity of V 2 O 5 nanowires was attributed to surficial properties of the nanozymesterm·dom instead of free orthovanadate anions [ 189 ]. They proposed a likely reaction mechanism based on analyzing the layered V 2 O 5 orthorhombic structure (Fig. 3 c). The (001) surface and the (110) surface were predominantly connected to the selective oxidation of hydrocarbons and total oxidation, respectively. The surface sites on the exposed (010) lattice planes of V 2 O 5 NWs was assumed to be related to their enzyme-like property. The V atoms in the (010) plane and the electron lone pairs of the bridging oxygen atoms, respectively, acted as Lewis acid and base sites. Consequently, an intermediate peroxo species was produced after the reaction between V 2 O 5 NWs and H 2 O 2 (Fig. 3 d). Afterward, the ABTS was bind to the vanadium peroxo species via a nucleophilic attack and then oxidized into ABTS* + species. The regeneration of the V 2 O 5 NWs required another ABTS molecule since H 2 O 2 is a two‐electron oxidant. In recent years, the electron transfer-related mechanism was applied to a bunch of POD mimics such as IrO 2 /rGO nanocomposites [ 123 ], FePt-Au hybrid NPs [ 190 ], Co 3 O 4 NPs [ 191 ], and AuNPs@CDs nanocomposites [ 122 ]. Han et al. [ 9 ] obtained recyclable biotemplate-based MnO 2 nanowires with genetically engineered filamentous phages M13 as template. As illustrated in Fig. 3 e, an electron transfer model was proposed for the reaction mechanism. With an electron transferred to MnO 2 NWs, the first substrate ABTS was oxidized. Then, another electron would transfer from MnO 2 to H 2 O 2 and hence produced H 2 O molecules. According to the chromogenic reaction and a series of control experiments, the enhanced POD-mimetic capacity of 1D M13-E4@MnO 2 nanozymesterm·dom could be attributed to the surface effect, the small size effect and the homogeneous distribution of nanocrystals. When it comes to noble metal nanozymesterm·dom, CuipersName et al. [ 71 ] speculated that Ir NPs could serve as the electron transfer mediators between H 2 O 2 and 3,3′,5,5′-tetramethylbenzidine (TMB) (Fig. 3 f). TMB adsorbed on the Ir surface provided lone-pair electrons from amino group to the Ir NPs, whose electron density was consequently increased. The electrons that transferred from the Ir NPs to peroxides would accelerate the oxidation of TMB and the reduction of H 2 O 2 . Oxidase-Like Activity Oxidases catalytically oxidize substrate (electron donor) and produce H 2 O or H 2 O 2 in the presence of oxygen, which is served as the electron acceptor. The oxidase family is classified according to the acting group of donors, including amino groups, CH-OH group (GOx), Ph-OH group (polyphenol oxidase), sulfur group (sulfite oxidase, SuOx), and ferrous ions (ferroxidase and cytochrome c oxidase) [ 192 ]. Among them, the OXD-mimetic nanozymesterm·dom that acting on amino groups were widely investigated. Up to date, a large amount of metal-based and metal oxide-based oxidase mimics have been uncovered, such as CuO [ 193 ], MnFe 2 O 4 [ 194 ], and Pt@MnO 2 [ 58 ]. The formation of intermediates (e.g., singlet oxygen, oxygen superoxide anion) and electron transfer process have been demonstrated to have important impacts on the OXD-type properties of these nanozymesterm·dom [ 195 ]. The possible reaction mechanism of Mn 3 O 4 NPs proposed by ZhangpersName et al., which was illustrated in Fig. 4 a [ 196 ]. The electrons that transferring from manganese to O 2 caused the formation of O 2 · − , part of which was responsible for the generation of H 2 O 2 and O 2 via non-enzymatic or SOD-catalyzed dismutation. Then, some of produced H 2 O 2 would react with the dissolved Mn 2+ and decomposed into · OH. Afterward, the intermediate · OH/O 2 · − and Mn 3+ would oxidize the TMB, thus forming the TMB–Mn 3 O 4 NP system. As a concerned nanomaterial, the CeO 2 has been demonstrated to exhibit multi-enzyme-mimicking activities. Cheng et al. probed into the O 2 -dependent catalytic behavior of nanoceria and confirmed its OXD-type activity under the studied conditions [ 197 ]. In the reaction mechanism, the O 2 molecules were adsorbed onto defect sites of nanoceria and converted into O 2 · − under acidic conditions (Eq. 4 ). As the surface Ce 4+ reduced to Ce 3+ , the TMB was oxidized into TMB ox (Eq. 6 ). As the main intermediate, the in situ produced O 2 · − finally regenerated Ce 4+ via the oxidation of Ce 3+ , accompanied by the generation of water (Eq. 7 ). Alternatively, the oxidation of TMB could be directly initiated by O 2 · − as well (Eq. 5 ). 4 O 2 + Ce 3 + CeO 2 → O 2 · - + Ce 4 + CeO 2 5 O 2 · - + TMB red → H 2 O + TMB ox 6 CeO 2 + TMB red → Ce 2 O 3 + TMB ox 7 Ce 2 O 3 + O 2 · - + 2H + → CeO 2 + H 2 O Fig. 4 a Reaction of the TMB oxidized by Mn 3 O 4 NPs with OXD-like activity. b Possible reaction mechanism for the SuOx-type activity of P‐MoO 3 −x NPs in the presence of sulfite and K 3 [Fe (CN) 6 ]. c Catalytic mechanism of Au NPs as GOx mimics. Adapted from a Ref. [ 196 ], b Ref. [ 198 ], c Ref. [ 199 ] with permission Mechanism study on the nanozymesterm·dom mimicking the other members of the oxidase family has made great progress as well. Following the exploration on the MoO 3 NPs as SuOx mimics [ 198 ], Chen et al. synthesized PEGylated (polyethylene glycol)‐MoO 3 −x nanoparticles (P‐MoO 3 −x NPs) that could catalytically oxidize sulfite. As shown in Fig. 4 b, the sulfite was oxidized into sulfate with the two electron oxidative hydroxylation. Following the reduction of [Fe(CN) 6 ] 3− , one electron then transferred in succession to the Mo V intermediate for the stabilization of the inactive Mo IV state. In terms of nanozymesterm·dom with GOx-like acticity, Comotti et al. put forward a two-electron mechanism to explain the intrinsic catalytic activity of the Au NPs (Fig. 4 c) [ 199 ]. In their model, the hydrated glucose anions that formed in the presence of alkali were adsorbed on the surface of AuNPs. The gold surface atoms on the hydrated glucose then activated molecular oxygen and formed the dioxogold intermediate, which provided a bridge (Au + –O 2 − or Au 2+ –O 2 2− couples) for the electron transfer. After two electrons transferring from glucose to dioxygen, the gluconic acid and H 2 O 2 were finally generated. ZhangpersName et al. [ 200 ] prepared crown-jewel-structured Au/Pd nanoclusters with high reactivity. The anionic charge on the top Au atoms may directly contribute to the high GOx-like activity since a hydroperoxo-like species was formed during the electron transfer progress form the anionic top Au atoms to O 2 . In addition, the PtCu NPs were reported to possess ferroxidase-like activity isolated from the impact of other ions based on the Fenton-like reaction [ 201 ]. Despite the obscure mechanism, the Pt NPs (as catechol oxidase mimics) [ 202 ], Au nanorod/ Pt nanodot structures (as ferroxidase mimics) [ 203 ], Cu 2 O NPs (as cytochrome c oxidase mimics) [ 204 ] and many other metal- and metal oxide-based nanozymesterm·dom have broaden the way toward the prosperity of OXD mimics. Superoxide Dismutase-Like Activity Superoxide dismutaseterm·def is a kind of metalloenzyme that mainly distributed in microorganisms, plants and animals. Oxidative stress, involving the increasing concentration of reactive oxygen species (ROS), is considered to be an important factor in aging and disease [ 205 ]. ROS refers to the reduction products of oxygen in the body, including oxygen radicals (e.g., O 2 · − , · OH, HO 2 · ) and certain nonradical oxidizing agents (e.g., ozone, H 2 O 2 , hypochlorous acid) [ 206 ]. SOD is selected as a favorable tool to anti-oxidation and anti-aging since it could transform superoxide anion radicals into H 2 O 2 and O 2 [ 207 ]. Numerous nanomaterials have been proven as SOD mimics, such as Mn 3 O 4 [ 208 ], Au[ 63 ], MnO 2 [ 209 ], and CeO 2 [ 210 ]. The coupled electron-transfers model was once accepted as a rational mechanism to explain the SOD mimetic property of CeO 2 NPs as shown in Fig. 5 a [ 168 ]. Following the oxidative half-reaction (Fig. 5 a➀–➃, same as that in Fig. 2 a), a O 2 · − molecule would bind to the reduced oxygen vacancy site (Fig. 5 a➄). Then, H 2 O 2 was released with the absorption of two protons and the transfer of electron from one Ce 3+ (Fig. 5 a➅). The original nanoceria state would be regenerated by repeating this reaction with a second O 2 · − molecule (Fig. 5 a➆). However, this model was questioned since Cafun et al. demonstrated the absence of spin-unpaired Ce 3+ sites in colloidal nanoceria via means of high-energy resolution hard X-ray spectroscopy [ 211 ]. Given profound consideration about the true structure and electronic characteristics of cerium oxide, WangpersName et al. proposed a polished catalytic cycle mechanism for nanoceria as SOD mimics [ 171 ]. The surface defect states were critical to the enzyme-like activity in this model. After the coadsorption of HO 2 · onto the surface of CeO 2 , the intermediate was formed as shown in Fig. 5 b. Then, the reaction between the intermediate and another HO 2 · radicals could release H 2 O 2 and O 2 , with the nanoceria restored to the initial state. Fig. 5 a Electron transfer model for the oxidation of H 2 O 2 by nanoceria as SOD mimics. b Reaction mechanism for the SOD mimetic activity of nanoceria. c , d Calculated reaction energy profiles for the SOD-mimic activity of Co 3 O 4 and Fe 3 O 4 . Adapted from a Ref. [ 168 ], b Ref. [ 171 ], c Ref. [ 170 ], d [ 212 ] with permission With the assistance of rigorous density functional theory and microkinetic modeling, Guo et al. investigated the Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms to describe the SOD-like activity of Co 3 O 4 [ 170 ] and Fe 3 O 4 [ 212 ] respectively. As illustrated in Fig. 5 c, the ER mechanism is more viable for Co 3 O 4 as the barriers involved through ER mechanism was lower than those along LH mechanism [ 170 ]. The O 2 · − molecule would capture a proton from water to form OH − and HO 2 · . The ER mechanism began with the chemisorption of HO 2 · on the surface of Co 3 O 4 to generate the intermediate (INT1) and the adsorption energy was − 35.52 kcal mol −1 . Hereafter, INT1 would react with a second HO 2 · to release H 2 O 2 and O 2 , accompanied by the regeneration of Co 3 O 4 . The activation barrier of the elementary reaction passing through the transition state (TS1) was 19.02 kcal mol −1 . When it comes to Fe 3 O 4 , the LH mechanism is viable since the barrier along the LH mechanism is lower (Fig. 5 d) [ 212 ]. Two HO 2 · molecules were absorbed on the surface of Fe 3 O 4 to from the intermediate (INT2) with OOH* and HOO* species. Then, the O–H bond of OOH* species was split and the H atom was combined with the nearby O atom of HOO* (TS2). The H 2 O 2 and O 2 molecule were produced with the O 2 molecule binding to the Fe site. Finally, the H 2 O 2 and O 2 molecule were released. Others Compared with oxidoreductive family, the reports about metal- and metal oxide-based nanomaterials with hydrolase mimetic activities are relatively rare. The peptide-functionalized monolayer protected gold clusters (Au MPCs) have been demonstrated as mimics of nuclease, esterase and silicatein [ 213 – 216 ]. The functional groups present on the protecting shells of Au MPCs were fundamental to their catalytic activities [ 217 ]. In addition, the CeNPs have been uncovered to show phosphatase-like property since they could cleave the phosphate ester bond of ATP, pNPP, and o- phospho-l-tyrosine [ 218 – 220 ]. The key to their catalytic phosphate ester bond cleavage lied on the availability of cerium(III) sites. Dhall et al. prepared CeNPs with phosphatase and CAT-mimetic activities via the wet chemical method [ 147 ]. The kinetic studies using pNPP as the substrate indicated that their phosphatase-type catalytic mechanism followed the saturation-based kinetics with V max and K m values of 0.44 nmol min −1 and 0.74 mM, respectively. In their study, the tungstate and molybdate tend to inhibit the phosphatase mimetic activity of CeNPs owing to the interaction of anions with the CeNPs surface. Regulation of Catalytic Activity Morphology Previous studies have demonstrated that the morphology control would affect the catalytic activity of nanozymesterm·dom to a large extent [ 146 ]. Exploration on the relevance between morphology and catalytic activity mainly involved surface area, pore size and volume. Tian et al. prepared VO 2 NPs in three kinds of morphologies (fibers, sheets and rods) as POD mimics [ 221 ]. The VO 2 nanofibers performed best in the H 2 O 2 and glucose colorimetric assay due to their largest specific surface area. Singh et al. [ 222 ] compared Mn 3 O 4 NPs in cube-, polyhedron-, hexagonal plates-, lakes- and flower-like morphology (Mnf). The larger size and higher surface area seemed to create higher catalytic activity of Mnf. Moreover, the multi‐enzymeterm·def property of Mnf could be ascribed to the larger pore size, which would hold the substrates and cofactor for the catalytic reactions. The effect of surface facets has gradually become a focus in morphology control as it determines surface energy or surface reactivity [ 223 ]. Huang et al. [ 55 ] found the OXD-type activity of CeO 2 nanorods with unique {110} planes was more ingenious than that of nanopolyhedra and nanocubes. In the research of Mu et al. [ 224 ], the catalytic activities of Co 3 O 4 materials were in the order of nanoplates > nanorods > nanocubes. The difference in lowering energy barrier and electron transfer ability might be related to distinct POD-like properties of three kinds of Co 3 O 4 nanozymesterm·dom. Ge et al. [ 67 ] reported that the Pd octahedrons enclosed by {111} facet structure showed lower surface energy, which were more sensitive to CAT-type property and ROS-eliminating capacity than the Pd nanocubes enclosed by {100} facet structure. As shown in Fig. 6 a, the reaction energy on Pd {111} and Pd {100} was 2.81 and 2.64 eV respectively, indicating the more possible homolytic dissociation of H 2 O 2 molecule on the surface of Pd {111} facet. In contrast, Fang et al. found that OXD- and POD-type activities of Pd nanocubes {100} were higher than that of Pd octahedrons {111} [ 225 ]. The binding between O 2 and Pd {100} facet (an adsorption energy of − 1.40 eV) was much stronger than that between O 2 and Pd {111} facet due to the higher adsorption energy at Pd {100} facet (Fig. 6 b). Also, the activation energy of surficial O 2 dissociation for {100} facets (0.31 eV) was lower than that for the {111} facets (0.67 eV). Thus, the energetically more favorable dissociative adsorption of the O 2 molecule on the Pd {100} facet explained its higher OXD-like activity. In terms of POD capacity, the homolytic dissociation reaction on the Pd {100} facet was more feasible than on the Pd {111} facet considering the reaction energy (Fig. 6 c). Fig. 6 a Lowest-energy adsorption structures and reaction energies (in eV) for the reactions on the Pd {111} and {100} facets. b , c Relative energies (eV) of O 2 dissociative adsorption and O–O atomic distances (Å) on the Pd {111} and {100} facets. Adapted from a Ref. [ 67 ], b , c Ref. [ 225 ] with permission Size Generally speaking, size sheds significant influence on the properties of diverse nanomaterials [ 226 ]. In most cases, the nanozymesterm·dom with smaller size tend to be more active in catalytic reactions ascribed to the larger specific surface area. For example, Xi et al. [ 32 ] reported the size‐dependent POD-type properties of Pd–Ir NPs within the size range from 3.3 to 13.0 nm. With an enzymeterm·def‐linked immunosorbent assay (ELISA) as a model platform, they attributed the higher catalytic properties of the smaller nanoparticles to their diffusivities and reduced steric effect. Luo et al. considered that the amount of surficial Au atoms was the key point to control the catalytic reaction rate, thus explaining the size-related GOx mimics activities of AuNPs [ 133 ]. They prepared CeO 2 NPs with SOD- and CAT-mimetic capacities in four kinds of sizes (4.5, 7.8, 23, and 28 nm) [ 227 ]. The decreased particle sizes could increase the Ce 3+ fraction along with enhancing catalytic efficiency. Interestingly, LiupersName et al. [ 228 ] discovered that the catalytic activity of β-Casein–AuNPs (β-casein functionalized AuNPs) was increased in the order of 4.2, 2.8, and 8.7 nm. Obviously, the smallest β-Casein–AuNPs did not possess the best POD-like activity. They deduced that the coated protein might affect the proximity between substrates and the nanozymeterm·dom core, which also determined the enzyme-like property. Surface Valence State The controls of the surface valence state and oxygen vacancies are considered as essential factors to modulate catalytic properties. Researches have demonstrated that the surface oxidation state of nanoceria played a considerable role in tuning the enzyme-like properties of CeO 2 due to the association between Ce 3+ and oxygen vacancies. Pirmohamed et al. verified that the H 2 O 2 decomposition rate of nanoceria increased with the decreasing of Ce 3+ /Ce 4+ redox state ratios [ 229 ]. In contrast, the reduced Ce 3+ /Ce 4+ ratio was responsible for the decay of SOD mimetic capacity [ 230 ]. Besides CeO 2 nanozymesterm·dom, WangpersName et al. reported that the POD mimicking activity in Ni-based nanozymesterm·dom was associated with the oxidation state of Ni [ 231 ]. In their study, the catalytic performance of porous LaNiO 3 perovskite was about 58- and 22-fold higher than that of NiO and Ni NPs, indicating the Ni oxidation state-dependent POD-like properties of Ni-based nanomaterials. Moreover, they proved the significance of Ni 3+ in regulating catalytic activities via the comparison between LaNiO 3 -H 2 and LaNiO 3 nanocubes, in which the ratios of Ni 3+ were different. With tuning copper states from Cu 0 to Cu 2+ , Xi et al. found that the multi-enzyme-like activities (POD, CAT and SOD) of copper/carbon nanozymesterm·dom were closely related to the Cu state [ 232 ]. Fan et al. realized surface valence state control on Au-based nanozymesterm·dom for the first time [ 233 ]. In their system, the catalytic efficiency for substrate oxidation (TMB and H 2 O 2 ) decreased with the reduced ratio of Au(I) complex in Au Aerogels. Composition The composition control of nanozymesterm·dom provides possibility to tune their catalytic activity [ 33 ]. Some studies demonstrated that the catalytic performance and Raman scattering (SERS) activities of AgAu, AgPd, and AgPt NPs are more obvious than that of Ag NPs [ 234 – 236 ]. Similarly, alloying with other metals (e.g., Pd, Au, Cu, and Co) has also been regarded as feasible solutions to catalytic ability regulation of Pt NPs [ 237 ]. In fact, adjusting the proportion of components and designing metallic core/shell structure-based nanomaterials are both feasible solutions modulate the enzyme-like properties [ 154 , 238 ]. LiupersName et al. speculated that the Pt/Ru molar ratio would affect electronic variation and electronic charge transfer effects of PtRu nanoalloy, thereby tuning their POD- and OXD-like activity [ 239 ]. In their work, the enzyme-type property was enhanced in the order of Pt 40 Ru 60 , Pt, Pt 75 Ru 25 , and Pt 90 Ru 10 . He et al. reported that the change of Au/Pt molar ratio not only influenced structure of AuPt alloy NPs, but also improved the catalytic reaction rates when increasing Pt/Au ratio [ 85 ]. To investigate the metallic core/shell structure-based nanomaterials, Xia et al. adjusted the amount of Ir precursor to obtain Pd–Ir cubes with different Ir shells [ 240 ]. In this work, the Ir shells at certain thicknesses would effectively increase the surface reactivity of Pd and reduce the dissociation difficulty of H 2 O 2 molecules. Moreover, the thickness of Ir shells could enhance or weaken the ligand effect stemming from the interaction of Ir monolayer with Pd substrate, in which the Pd(100) surface with single Ir layer was more active than that with three Ir layers during the oxidation process of TMB. Owing to the synergetic effects between ceria and heteroatoms, doping CeO 2 with suitable foreign atoms is favorable to boost the catalytic activity [ 241 ]. By replacing Ce 4+ ion in the CeO 2 lattice, the incorporation of heteroatoms tends to strengthen surface defects in the CeO 2 lattice via generating more oxygen vacancies for oxygen migration and diffusion [ 242 , 243 ]. Among diverse heteroatoms, the introduction of one-dimensional nanowires achieved the best catalytic activity enhancement effect [ 244 ]. ZhangpersName et al. synthetized CeO 2 nanozymesterm·dom doped with different metal elements (such as Ag, Cr, Co, Rh, Pd, Mn, and Ni) and possessed multi-enzyme-like activities, herein the Cr/CeO 2 nanozymesterm·dom owned best catalytic performance.The Cr 3+ incorporation could improve surficial Ce 3+ /Ce 4+ ratio, thus reinforcing the catalytic capacity of CeO 2 NPs [ 245 ]. In addition to the types of doped atoms, the amounts are critical to regulate activity of nanozymesterm·dom as well. Jampaiah et al. revealed that the catalytic efficiency toward TMB oxidation of 6% Fe 3+ -doped CeO 2 NRs was the best among the CeO 2 NRs incorporated with 3, 6, 9, and 12% Fe respectively [ 246 ]. The Raman and X-ray photoelectron spectroscopy (XPS) results indicated the higher amount of surface defects including Ce 3+ ions and oxygen vacancies in the 6% Fe 3+ -doped CeO 2 nanozymesterm·dom. Surface Modification Surface modification ranging from functional group, inorganic ions and small molecules to macromolecules has been revealed as a promising strategy to regulate the mimetic enzymeterm·def properties of metal- and metal oxide-based nanozymesterm·dom by affecting their surface chemistry [ 247 – 249 ]. For instance, ligands such as glutathione (GSH), dendrimer, DNA, and protein tend to protect metal nanoclusters from aggregation, thence reinforcing the stability, biocompatibility and catalytic activity of nanozymesterm·dom [ 250 , 251 ]. LiupersName et al. reported that the catalytic efficiency of the DNA-capped iron oxide NPs as POD mimics was about tenfold higher than that of naked NPs [ 252 ]. The DNA coatings not only strengthened combining capacity with the amino groups of TMB via hydrogen bonding, but also provided the π – π stacking for nucleobase interacting with the benzene rings of TMB, which effectively enhanced the affinity of Fe 3 O 4 NPs toward TMB. Huo et al. modified Co 3 O 4 nanoplates with the amino group (NH 2 -Co 3 O 4 ), carboxyl group (COOH-Co 3 O 4 ), hydroxyl group (OH-Co 3 O 4 ), and sulfhydryl group (SH-Co 3 O 4 ) in respective, and then systematically studied their catalytic activities [ 253 ]. Except hydroxyl group, the other functional groups all possessed positive effect to enhance POD-like activities, and among which the NH 2 -Co 3 O 4 nanoplates ranked the first. Huo et al. considered the functional groups’ influence on the electron transfer ability of nanozymesterm·dom was critical to modulating their catalytic properties. Yue et al. [ 254 ] prepared functionalized ceria nanorods catalysts M/CeO 2 (M = Fe 3+ , Co 2+ , Mn 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) via chelating metal ions onto ceria nanorods CeO 2 surface. These metal-chelated nanocerias have all possessed enhanced POD-mimicking property and Mn(II)/CeO 2 showed best catalytic performance. The researchers found that the synergistic effect of metal ions and CeO 2 , along with the carboxyl groups served as substrate binding sites, was critical to the promotional effect on the enzymatic activity. The addition of F − into nanoceria obviously caused the generation of more oxygen vacancies, facilitating electron transfer between the Ce 4+ /Ce 3+ redox couple as well as the stimulating product desorption, thereby enhancing OXD-mimetic capacity of nanoceria by fluoride capping [ 255 ]. External Triggers pH and temperature Up to date, the enzyme-like activities of numerous metal- and metal oxide-based nanozymesterm·dom have been verified to be sensitive to pH and temperature [ 17 , 256 – 258 ]. The POD-type property of Fe@PCN-224 NPs was optimal in pH 3.5 with the temperature of 45 °C [ 259 ]. And the activity could remain 80% and 90% of the highest activity at 25 and 37 °C, respectively. Although an increasing number of novel nanomaterials have shown high enzyme-like property within a wide temperature range, the catalytic activity of nanozymesterm·dom would slightly decrease when the temperature was not at optimal [ 260 ]. LiupersName et al. [ 261 ] found that the ROS eliminating activity of Pt NPs was strengthened with the increment of environment pH by the assistance of electron spin resonance (ESR) spectroscopy and spin traps. It has been reported that Pt NPs [ 261 ], Ag NPs [ 262 ] functioned as POD mimics in acidic conditions while exhibited CAT-like activities in neutral and alkaline environment. What is more, Pt and Au NPs were demonstrated to show SOD mimetic capacity under neutral conditions [ 63 , 261 ]. Li et al. [ 167 ] dug into the pH-switchable enzyme-like properties of Au, Ag, Pt, and Pd nanozymesterm·dom. The adsorption of H + and OH − ions on the metal surface was feasible under acidic and basic conditions, respectively. The base-like decompositions of H 2 O 2 in low-pH conditions was fundamental to the POD-like activities of Au, Ag, Pt and Pd nanozymesterm·dom while their CAT-type activity was related to the acid-like decompositions of H 2 O 2 in high-pH conditions. (2) Hydronium The catalytic activity of nanozymesterm·dom could also be affected by metal ions (e.g., Fe 3+ , Hg 2+ , Ni 2+ , Cd 2+ , and Al 3+ ) and anions (e.g., S 2– , F – , Cl – , Br – , and I – ) [ 136 , 263 , 264 ]. For example, heavy metal ions might inhibit catalytic activities of metal- and metal oxide-based nanozymesterm·dom, which could be ascribed to the metallophilic interaction between nanozymesterm·dom and heavy metal ions, including the deposition of metal ions [ 265 ], the formation of alloy on the surface of nanomaterials [ 266 ], and the leaching of surface atoms [ 267 ]. The integration between heavy metal ions and the surface ligands also affected the catalytic performance of nanocomposites by deposing of ligands or decreasing affinity toward substrate [ 268 , 269 ]. Han et al. conjectured that the promotional or block effects of Ca 2+ , Fe 3+ , Hg 2+ , and Mn 2+ toward the CAT-type property of Co 3 O 4 NPs were related to their influence on the electron transfer rate in Co 3 O 4 [ 270 ]. In the report of LiupersName et al., the S 2– at low ion concentration tended to inhibit the POD-mimetic catalytic reactions of β-casein stabilized Pt NPs (CM–PtNPs) toward TMB while switch on their enzyme-like activity toward ABTS [ 264 ]. Besides, the sulfide-mediated activity switching efficiency decreased with the increment of S 2– concentration. Fluorescence spectra and X-ray photoelectron spectroscopy (XPS) data revealed that the key of S 2– -mediated activity switching mechanism lied in the structure change of protein molecule and ratio change of Pt 2+ /Pt 0 with the introduction of sulfide ions. (2) Light The photothermal effect and light-induced electron transfer have been demonstrated to be involved with the photo-enhanced enzyme-like activity of nanozymesterm·dom [ 271 – 273 ]. With AuNPs and α-FeOOH microcrystals grown on porous carbons, ZhangpersName et al. obtained Au/α-FeOOH–FPC catalysts with visible-light-driven enzymatic property [ 274 ]. Herein, the system temperature was raised to accelerate the process of glucose oxidation when the Au NPs converted the absorbed light energy into heat. And the generated gluconic acid could lower surrounding pH to stimulate the enzymatic reaction. Furthermore, hot electrons from plasmon-excited AuNPs promoted charge separation at the interface of Au/α-FeOOH, resulting in efficient cycling of Fe 3+ /Fe 2+ to produce Fenton reaction. The introduction of visible light has increased the POD-type activity of Fe 2 O 3 NPs by at least 1.2 times in the research of Zhu et al. [ 275 ]. They found that the light-related catalytic property of Fe 2 O 3 nanozymesterm·dom was concerned with the bandgap and light absorption range, which were responsible for the barrier density generation and the light energy absorption. In addition, the influence on the enzymeterm·def mimetic properties changed according to the type of light excitation. WangpersName et al. discovered that the catalytic activity of Au/Si/Azo (AuNPs encapsulated and dispersed by the azobenzene- modified expanded mesoporous silica) was activated under UV illumination while inhibited under visible light [ 276 ]. The control of the host–guest interaction between Azo and cyclodextrin (CD) via the isomerization between trans and cis conformations of Azo was significant to the activity regulation by UV or visible light. (4) Others Nucleoside triphosphates (NTPs) including adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP) and uridine triphosphate (UTP) have been considered as promoters for nanozymesterm·dom owing to the coupling of their hydrolysis with oxidative reaction [ 220 ]. Vallabani et al. discovered that the employment of ATP could reinforce the affinity between Fe 3 O 4 NPs and their substrate, thus maintaining the POD mimetic capacity of Fe 3 O 4 nanozymesterm·dom within a wide range of pH and temperature [ 277 ]. Interestingly, Cheng et al. [ 197 ] found that the introduction of ATP might restrain the enzymetic reaction of nanoceria in prolonged reactions despite its initial enhancing effect. They attributed the inhibition to Ce–PO 4 complexes formation in the presence of ATP, which could interact with nanoceria and shield active centers. Furthermore, Jia et al. [ 278 ] reported that the antioxidants possessed inhibitory effect on the POD-type property of Co 3 O 4 NPs. The addition of gallic acid (GA), tannic acid (TA) and ascorbic acid (AA) would slow the catalytic reaction toward the TMB or OPD, among which the influence of TA was the highest because of its numerous phenolic groups. Applications of Metal- and Metal Oxide-Based Nanozymesterm·dom Applications in Analytical Field As mentioned above, metal- and metal oxide-based nanozymesterm·dom normally come along with unique physicochemical properties including high surface-to-volume ratio, enzymatic activity and good biocompatibility. These capabilities endow them with promising applications in target substances detection following the extensive exploration of biosensing schemes [ 279 ]. The integration of nanozymesterm·dom and conventional determination technologies containing colorimetric, electrochemical, and fluorescence has gradually become optimal candidate for biological analysis. The past decade has witnessed the inclusive utilization of novel nanozyme-based sensors in detecting proteins, glucose, heavy metal ions, pathogen microorganisms and many other substances. Heavy Metal Ions Previous studies have illustrated that excessive heavy metal ions are one of the culprits of environmental pollution [ 280 ]. Furthermore, heavy metal ions could invade human body through water and food, resulting in permanent chronic poisoning [ 281 ]. Therefore, detecting heavy metal ions is of great significance to protect ecology and human health. Nevertheless, most analytical platforms (e.g., atomic absorption spectrometry, energy-dispersive X-ray, and inductively coupled plasma mass spectrometry) for heavy metal ion analysis relied on expensive instruments and professional technicians [ 282 ]. Nanozymesterm·dom provided a potential to simultaneously improve the performance of metal ion detection with low cost. For instance, Han et al. designed a portable paper chip based on AuNPs (AuNZ-PAD) to investigate Hg 2+ in distilled and tap water samples, in which Au–Hg 2+ integration could influence enzyme-like catalytic activity of AuNPs and caused paper discoloration (Fig. 7 a) [ 226 ]. This ultrasensitive AuNZ-PAD further cooperated with mobile phone camera, effectively reducing the cost of assay and simplifying the operation. Fig. 7 a Detection progress of the Hg 2+ in distilled and tap water samples with AuNZ-PAD based on the TMB–H 2 O 2 catalytic reaction. b Principle of quantitative detection of Hg 2+ ions in seawater (3.5% NaCl) using OEG-AuNPs compared with that using bare AuNPs. c Detection principle of TAC based on the reaction between antioxidants and H 2 O 2 in the presence of Pt nanozymesterm·dom as POD mimics. d Synthesis of cDNA@Pt@P-MOF(Fe) as the signal probe for the analysis of telomerase activity. e Preparation of Ag-Au/AgCl nanohybrid with OXD-like and POD-like activity and the working mechanism of Spm detection. GCE Glassy carbon electrode, TP telomerase primer. Adapted from a Ref. [ 226 ], b Ref. [ 293 ]. c Ref. [ 301 ], d Ref. [ 302 ], e Ref. [ 303 ] with permission Among the classical analytical assays basing nanozymesterm·dom, colorimetric stood out for the operation convenience. Some references concluded that heavy metal ions might enhance or inhibit the POD-like property of nanozymesterm·dom [ 283 – 285 ]. Hence, histidine(His)-Pd [ 268 ], MMoO 4 (M = Co, Ni) [ 286 ], DNA-Ag/Pt [ 287 ], MnO 2 [ 288 ] have been synthesized for Ag + [ 268 ], Cu 2+ [ 286 ], Hg 2+ [ 287 , 288 ] monitoring by colorimetric assay. In addition, Pb 2+ ions would accelerate the AuNPs leaching in presence of S 2 O 3 2− and lead to less oxidation of TMB, expanding the Pb 2+ determination with the assistance of nanozymesterm·dom [ 289 , 290 ]. Xie et al. [ 291 ] fabricated a colorimetric probe by using metallic nanozymeterm·dom to determine Pb 2+ . The Au@Pt NPs served as POD mimics were introduced, which could detect Pb 2+ ions in the lake water samples within a linear range from 20 to 800 nM. As high electrolyte has an adverse effect on the catalytic performance and stability of nanozymesterm·dom, analyzing heavy metal ions in seawater is much more difficult than other liquid samples such as lake water and drinking water [ 292 ]. Logan et al. quantitatively determined mercury ions in complicated water matrices using OEG-Au complex by functionalizing AuNPs with oligo-ethylene glycol (OEG) [ 293 ]. In this proposal, OEG-AuNPs exhibited enhanced stability and weakened catalytic properties in a wide pH range under high NaCl concentration, which effectively ameliorated the poor stability of bare-AuNPs (Fig. 7 b). The Hg 2+ detection limit of coastal seawater by this platform was 13 ppb in only 45 min. Biomarkers Biomarkers refer to biochemical indicators that mark the structure or functional changes of biosystems including organ, tissue and cell. The exploration of biomarkers is beneficial to clinical diagnosis, drug analysis and ecosystem protection. Enormous effort has been made in nanozyme-based biomarker detecting, including biological macromolecules (e.g., acid phosphatase (ACP) for prostate cancer [ 149 ]; human epidermal growth factor receptor-2 (HER2) for breast cancer [ 294 , 295 ]; carcinoembryonic antigen (CEA) for rectal cancer [ 296 , 297 ] and benzo[ a ]pyrene-7,8-diol 9,10-epoxide–DNA (BPDE–DNA) for woodsmoke exposure [ 298 ]) and small molecule biomarkers (e.g., sarcosine for prostate cancer [ 299 ] and uric acid [ 300 ]). Pedone et al. [ 301 ] developed a colorimetric approach to determine the total antioxidant capacity (TAC) in saliva on basis of the reaction between antioxidants and H 2 O 2 in the presence of Pt nanozymesterm·dom, which was acted as POD mimics. TAC acted as an important biological indicator closely associated with oxidative stress. It reflected the total effects of enzymesterm·def and non-enzymatic analytes in the body. The combination of Pt nanozymesterm·dom and · OH radical substrates allowed the detection scheme sensitive to both single electron transfer (SET) and hydrogen atom transfer (HAT) reactions (Fig. 7 c). The improvement in signal transduction rate is a breakthrough to raise the sensitivity of biomarker detection [ 298 ]. Thence, metal- and metal oxide-based nanozymesterm·dom functioned as signal amplification has boosted biomarkers analysis in sundry assays involving electrochemical, fluorescent and so on [ 300 ]. Ling et al. obtained Pt@ P-MOF (Fe) nanozymesterm·dom by growing ultra-small Pt nanoparticles on metalloporphyrin metal organic frameworks [ 302 ]. The novel artificial nanozymesterm·dom were employed as signal probe, allosteric switch of DNA and Exo III recycling amplification in their electrochemical template for telomerase detection (Fig. 7 d). The catalytic property of Pt NPs on P-MOF (Fe) could decompose H 2 O 2 , and hence strengthened the electrochemical signal. Kuo et al. [ 303 ] synthesized Ag-Au/AgCl nanohybrid with OXD- and POD-type capacities for spermine (Spm) analysis in urine, which could act as the diagnostic indicators for liver cancer and stroke. As is shown in Fig. 7 e, Spm inhibited fluorescent molecules generation of H 2 O 2 -Amplex Red (AR) system when in the presence of Ag-Au/AgCl, thereby realizing highly selective and ingenious determination of Spm. Pathogen Microorganisms The analysis of pathogenic microorganisms, ranging from viruses, bacteria, parasites to prions, is crucial to prevention and control of infectious diseases [ 304 ]. The nanozymesterm·dom have become powerful competitors for natural enzymesterm·def in field of pathogen detection due to their low-cost (especially for foodborne bacteria), timesaving operation and sensitivity [ 305 – 307 ]. For instance, Cheng et al. employed Pd@Pt NPs as a signal amplifier in the lateral flow immunoassay (LFIA) assays for Salmonella Enteritidis ( S. enteritidis ) and Escherichia coli ( E. coli ) O157:H7 [ 57 ]. The integration of Pd@Pt NPs and smartphone-based device offered a portable platform for fast detection of foodborne pathogens. The studies involving nanozyme-based pathogen analysis in the past 5 years are listed in Table 2 . All the metal- and metal oxide nanozymesterm·dom mentioned in this table were functioned as POD mimics. Table 2 Nanozymeterm·dom and analysis method for pathogen microorganism detection reported in recent years Pathogenic microorganisms Nanozymeterm·dom Method