Bacteria and viruses have long been an important threat to human life and health. For example, SARS- CoV-2, which has caused a global storm of disease transmission^[1], Staphylococcus aureus, which causes pneumonia, Helicobacter pylori, which causes gastritis, and Salmonella, which causes enteritis. To illustrate, pneumonia due to bacteria accounts for about one third of the causes of pneumonia^[2]. These hazardous pathogens are even more damaging to vulnerable people such as children and patients. Pneumonia is estimated to account for 21% of deaths among children, based on an analysis of epidemiological data from 42 countries. This is similar to the findings of 19% of the WHO in 2000^[3]. WHO indicates that pneumonia is in the top three factors contributing to death in children under five years of age. Acquired pneumonia has been a very common and threatening occurrence of nosocomial infections^[4-5], with methicillin-resistant Staphylococcus aureus (MRSA) evading antimicrobial drugs and causing people to death^[6]. The growth of resistant bacteria has also posed a significant challenge to the development of antimicrobial drugs. The dose of antibiotics increases with the increase of behavior of drug-resistant bacteria, and the overuse of antibiotics leading to the emergence of more resistant bacteria^[7-8]. Moreover, a huge public health crisis is brewing along with the stagnation of antibiotic research and development. It is necessary to develop new antibacterial and antiviral drugs that are effective and do not generate bacterial resistance.

The development of nanotechnology has brought new inspiration to human medicine, whether it is nanovaccines^[9-10] or nanodrug delivery agents^[11-12], giving hope that this new technology can be used to solve difficult problems^[13]. Importantly, since their special properties were discovered in 2007^[14], as a new role of nanomaterials, nanozymes have been given a new mission.

Nanozymes are a class of nanomaterials with intrinsic enzyme-like activity. Since the first discovery of the intrinsic peroxidase-like activity of Fe₃O₄ nanoparticles by Gao et al. in 2007, nanozymes with other enzyme-like activities, including catalase-like^[15], superoxide dismutase- like^[16], oxidase-like^[17] and glutathione peroxidase-like^[18] activities have been subsequently revealed. The catalytic efficiency of nanozymes is close to or even better than natural enzymes, allowing them to be utilized in various catalysis-related aspects. Researchers have used nanozymes for various fields such as disease diagnosis^[19], disease treatment^[20-21] and detection^[22⇓-24]. In addition, many types of nanozymes have been used for antibacterial and antiviral treatments: metals^[25], metal oxides^[26], carbon dots^[27], mesoporous silica^[28], etc.

These various effective nanozymes identified at present have some commonalities in the mechanisms of action. This review summarizes and analyzes the latest research advances in the field of antibacterial and antiviral nanozymes from a mechanistic point of view (Fig. 1). It is hoped that the introduction of antibacterial and antiviral strategies of nanozymes can provide inspiration for the design and development of novel antibacterial and antiviral drugs for challenging conditions.

Nanozymes are so called enzyme-mimics because the mechanism by which their function is primarily enzymatic catalysis. Nanozymes with peroxidase-like activity catalyze the generation of lethal hydroxyl radicals from H₂O₂, one of the most damaging reactive oxygen species (ROS) that destroys biological molecules and thus causes the death of the target organisms. Nanozymes with oxidase-like activity typically reduce O₂ to H₂O₂ or H₂O, and can also produce the toxic superoxide radical O₂^·-. Nanozymes with catalase-like activity catalyze the generation of O₂ from H₂O₂ can improve the oxygen-depleted environment or be used in combination with other catalytic activities to enhance the target effect. Superoxide dismutase-like nanozymes can convert superoxide radicals into H₂O₂ and O₂, which can be used as an important antioxidant to protect normal tissues from ROS attack^[29⇓-31]. In the treatment of anti-microbial infections, aggressive ROS generated by nanozymes generally play a key role.

Huang et al.^[32] have designed a bimetallic quasi-MOF nanozyme (Q-MOF_Ce0.5) with peroxidase-like activity. Such nanosheets featured by a layered heterogeneous junctional interface can continuously produce ROS to achieve effective killing of Escherichia coli (99.74%) and Staphylococcus aureus (99.35%) in vitro, and the morphology analysis clearly that the attacked bacteria lose their normal shapes (Fig. 2). Optimizing the catalytic activity of nanozymes by doping, modification, etc. can improve their antimicrobial properties. Pan et al.^[33] compared the peroxidase-like activity of vigorous chitosan grafted Fe-doped carbon dots (CS@Fe/CDs) and carbon dots without Fe-doping (CS@CDs), found that nanozyme based on CS@Fe/CDs performs better catalytic activity. Authors pointed out that the mechanism of the peroxidase-like activity is the increased electron density in the active center and the reduced resistance of the steric sites due to the abundant carboxyl groups on the surface of the nanozyme, leading to an increased Fenton- like reaction. In antimicrobial experiments, CS@Fe/CDs showed satisfying killing effect on Staphylococcus aureus and Pseudomonas aeruginosa in the presence of H₂O₂.

Through rational design, nanozymes with diverse enzymatic activities may achieve the protection of normal tissues while killing microorganisms. Cao et al.^[26] successfully constructed an iron phosphate nanozyme- hydrogel (FePO₄-HG) with multiple enzymatic activities. This nanozyme exhibited peroxidase-like activity under acidic conditions to kill bacteria, but showed superoxide dismutase-like and catalase-like activity under neutral to weak alkaline conditions to protect normal tissues from exogenous H₂O₂ damage. In addition, the nanozyme possessing positive charges and large pores is able to trap bacteria within a certain range, thereby enhancing its killing effect, which was confirmed in animal experiments.

In order to cope with SARS-CoV-2, researchers have also found inspiration from nanozymes. Wang et al.^[34] have designed a highly effective anti-viral TiO₂ supported Ag atoms single atom nanozyme (Ag-TiO₂ SAN) (99.65%) by loading atomically dispersed Ag atoms onto TiO₂ nanoparticles. This single atom nanozyme can be phagocytosed by macrophages and co-localized with lysosomes through strong interaction of Ag atoms to the cysteine and asparagine of viral spike proteins, and has strong adsorption to SARS-CoV-2 (Fig. 3). In acidic environment, the high peroxidase-like activity exhibited by Ag-TiO₂ SAN gives it the ability to generate ROS and to kill SARS-CoV-2.

Nanozymes also were applied to create anti- microbial fabrics based on their antibacterial and antiviral properties. Inspired by the structure of SARS-CoV-2, Ni et al.^[35] proposed a new idea to fight superbugs with super viruses by designing a triple-shell porous graphitic carbon nitride (g-C₃N₄) embedded with cobalt nanoparticles based on the morphology of coronaviruses. This artificial virus has oxidase-like activity, can penetrate the bacterial membrane barrier and cause ROS damage to bacteria. The artificial virus also has photodynamic and photothermal properties triggered by light due to the good photothermal conversion properties of the material, and can also be enriched and recycled by using the magnetic properties of the material. This kind artificial nanozyme-based virus has been used for river water circulation disinfection and in vivo wound healing.

Nanozymes are a special class of materials that possess both the enzymatic activity and the properties of inorganic nanomaterials inherent in them. Many studies therefore applied nanozymes in the direction of synergistic therapy.

When the size of nanoparticles is much smaller than the wavelength of incident light, and the frequency of incident light is the same as the frequency of free electron oscillation, it can cause the collective oscillation of free electrons on the surface, and this phenomenon is named local surface plasmon resonance (LSPR)^[36]. Metal nanoparticles such as Au, Ag, Cu, etc. have the property of absorbing light in a specific range. In a different interpretation of the mechanism, a special class of nanozymes, plasmonic nanoparticles, were found to have LSPR properties in addition to the good catalytic activity of nanoparticles^[37]. Liao et al.^[38] found that the prepared Au nanozymes and Cu metal-organic framework nanosheets (Cu-MOFNs) had faster reaction kinetics under LSPR excitation. The dark-field scattering spectra of this nanoparticle indicated that its LSPR promotes hot electron transfer, resulting in faster reaction kinetics, and it showed good results in antimicrobial therapy and wound healing. The LSPR method has also been used by researchers in studies for the treatment of tumors^[39].

Doping affects the LSPR property of nanozymes. The impurities or holes introduced by doping can change the LSPR properties of the nanozymes due to the changed carrier density. Doping with plasma metals such as Au enhances the LSPR field, which enhances the light absorption properties of the material and enhances the thermal transport rate so that the nanozymes can have higher photothermal conversion efficiency. Meanwhile, the hot electron driving of plasma provides higher generation rate of energetic electron-hole pairs for the catalytic properties of the nanozymes, which improves the catalytic efficiency of the nanozymes^[40-41]. Compared to MoO_3-x, the Au-doped MoO_3-x (Au/MoO_3-x) plasmonic- semiconductor hybrid was 3.6 times more effective in terms of peroxidase-like activity. In addition, the Au/MoO_3-x exhibited a better photothermal conversion efficiency (52.40%) than the undoped MoO_3-x (41.11%) due to the enhanced LSPR effect of the doped material. Through higher peroxidase-like activity and better photothermal property, the material achieved effective removal of MRSA bacteria (99.76%)^[26].

The photothermal effect refers to the interaction of photon energy with the lattice when the material is irradiated by incident light, and the absorbed light energy is converted into thermal motion energy of the lattice, causing the rise of temperature. Materials with high photothermal conversion efficiency usually have complex microstructure and high light absorption capacity^[42]. Taking advantage of the good photothermal properties of the nanomaterials, Chen et al.^[43] designed 4-mercaptophenylboronic acid (MPBA)-functionalized CuSe nanoprobes (CuSeNPs@MPBA) based on chemiluminescence (CL) for the construction of a multifunctional platform for the effective inactivation of bacteria. The nanozyme have peroxidase-like activity to catalyze the conversion of flash CL to luminescent CL, improving the stability and accuracy of the assay, as well as excellent and stable photothermal performance, with outstanding photothermal conversion efficiency (37%) that can significantly increase the temperature under NIR irradiation to cause inactivation of E. coli (99.2%) and S. aureus (99.8%).

However, photothermal therapy is often accompanied by concerns about whether the heat causes damage to normal tissues. Li et al.^[44] have synthesized hollow mesoporous Prussian Blue nanoparticles (HMPBNPs) with peroxidase-like activity for photothermal therapy, which requires not only a lower effective temperature (48 ℃) than the usual effective temperature for photothermal therapy (>53 ℃), but also a lower concentration of H₂O₂. These properties allow for the effective killing of both E. coli which represent Gram-negative bacteria and S. aureus which represent Gram-positive bacteria under mild conditions.

Similarly, Lin et al.^[45] constructed Bacitracin-modified dextran-MoSe₂ nanosheets (AMP/dex-MoSe₂ NSs) for low-temperature photothermal antimicrobial activity in the NIR irradiation. The lower degree of photothermal heating also increases the efficiency of the production of hydroxyl radicals by peroxidase-like catalysis which is one of the most active bactericidal components. The nanozyme can also bind to bacterial membranes by electrostatic adsorption to achieve a multimodal antimicrobial effect.

As a material with catalytic activity and good modifiable properties, the nanozymes coupled with photosensitizers can endow them with photodynamic properties that lead to photosensitization of oxygen to produce ROS, enhancing their catalytic activity and achieving the killing effect on microorganisms^[46]. The shortcomings of nanozymes for anti-infection therapy mainly stem from their inadequate production of lethal ROS, both in terms of their yield and the time they can be sustained release. To overcome this limit, Gao et al.^[47] optimized a new photoactivated nanozyme, Ag/Bi₂MoO₆ (Ag/BMO), by charge separation engineering, which not only exhibits catalytically enhanced photodynamic properties under NIR (II) irradiation, but also peroxidase-like properties on a sustained level. This is due to the fact that the appropriate light and acidity make it easier to separate the light-triggered electron-hole pairs to produce ROS (Fig. 4), while the electron transfer generated by laser irradiation at specific wavelengths makes it easier for the reversible Mo⁵⁺/Mo⁶⁺ change to take place, thus exhibiting enhanced catalytic activity. To improve the antibacterial activity of nanozymes, He et al.^[48] have designed a multifunctional nanozyme using polydopamine (PDA)- modified copper oxide (Cu_xO-PDA). The peroxidase-like activity of the nanozyme can be enhanced under near infrared irradiation (NIR). The negative charge on the surface of this nanozyme changes to a positive charge as the neutral or basic conditions change to acidic conditions. This property allows bacteria with a negative surface charge to be trapped by electrostatic interactions. Moreover, after the nanozyme adhering to the bacterial membrane, the bacteria aggregate under the driving of the nanozyme under NIR irradiation, and this ability to lock and capture the bacteria allows the nanozyme to trap bacteria within the killing range of ROS, thus enhancing the antibacterial effect.

Using different catalytic activities, Loukanov et al.^[49] found that the histidine-modified carbon nanodots exhibited ideal inhibition of bacterial proliferation under visible light irradiation using their oxidase-like catalytic properties.

The positively charged carbon nanodots tend to attract negatively charged bacteria, thus causing severe damage to bacterial membranes and achieving antibacterial effects.

Many nanozymes have multi-enzyme activities, such as peroxidase-like, catalase-like, and oxidase-like activities. These multi-enzyme activities can be combined to perform various functions, such as killing pathogens while protecting normal tissues from harsh environments. Secondly, nanozymes can be cascaded with natural enzymes to achieve a triggered reaction or amplify the signal. Cascade reactions can be used to solve complex problems when the signal is insufficient or the concentration of the reaction substrate is inadequate.

Du et al.^[50] used iron oxide nanoparticles coated with a glucose oxidase shell (Fe₃O₄-GOx) to implement a cascade reaction to treat difficult-to-heal wounds. Fe₃O₄-GOx behaves glucose oxidase (GOx), catalase (CAT) and peroxidase (POD)-like activities (Fig. 5). Under acidic conditions, the GOx/POD cascade produces hydroxyl radicals, which inhibit bacterial growth and aid wound healing. Under neutral conditions, the GOx/CAT cascade produces oxygen, which addresses hypoxia and oxidative stress in difficult-to-heal wounds and accelerates normal tissue proliferation. This approach allows regrowth of difficult-to-heal wounds with minimal toxicity to normal tissues.

In addition, the low substrate concentration requirement of the cascade reaction also gives it some advantages. For example, Zhang et al.^[51] developed a cascade nanozyme (Fe₂(MoO₄)₃@GOx) by loading glucose oxidase on the outside of a nanozyme with peroxidase-like activity. The nanozyme can solve the problem of poor therapeutic efficacy due to unstable H₂O₂ levels in the environment. GOx loaded on the surface digests glucose to produce gluconic acid and H₂O₂, which is then converted into hydroxyl radicals using the peroxidase-like activity of the nanozyme. This in vivo harmless cascade of nanozyme acts synergistically by causing starvation and generating ROS for antibacterial purposes against two typically drug-resistant bacteria, extended-spectrum beta- lactamases producing E. coli and methicillin- resistant S. aureus, both showed ideal antimicrobial efficacy (98.396% and 98.776%, respectively).

Bio-orthogonal catalysis refers to catalytic reactions that can be carried out in living cells or tissues without interfering with the organism's inherent biochemical reactions. Bio-orthogonal catalysis makes it possible for catalytic processes in situ in cells or tissues to produce effective drugs without affecting the surrounding environment^[52-53]. As a class of inorganic nanomaterials with catalytic activity, the good biosafety of nanozymes allows them to be used as bio-orthogonal catalytic processes. Microorganism intracellular infections a difficult problem to solve, due to their complex environment and the difficulty in adjusting the dose of antibiotics. Nanozymes can be used to solve this problem, as their stability and modifiability ensures that they are not degraded intracellularly and that the orthogonal catalysis does not have effective impact on normal cellular tissues due to high antibiotic doses.

Joseph Hardie et al.^[54] designed a nanozyme consist of mannose-functionalized gold nanoparticle and iron tetraphenyl porphyrin (FeTPP) that can be used as bio-orthogonal catalyst to treat intracellular bacterial infections of Lactobacillus sp. The nanozyme, which was used for bio-orthogonal catalysis, could be taken up by macrophages via the cell surface mannose receptor (CD206) and subsequently convert the inactive antibiotic precursor (pro-ciprofloxacin) to ciprofloxacin inside the cell, and showed better killing of Salmonella relative to non-pathogenic Lactobacillus. As a new strategy, it could also be taken as next step in typhoid and tuberculosis.

Similarly, Akash Gupta et al.^[55] used gold nanoparticles as a backbone and FeTPP as a catalyst to synthesize nanozymes (Man-NZ). The researchers modified gold nanoparticles on the surface of the RBCs, which have good biocompatibility and are susceptible to hemolyzed by bacterial toxins, through electrostatic interactions. The stability and the catalytic activity of the nanozyme was checked to be not influenced by the functionalized RBCs. Afterwards, the authors chose arylazide protected moxifloxacin (pro-Mox) as an antibiotic precursor, which was found to be well activated and acted upon by studying MRSA with E. coli, increasing the specificity of the treatment. The nanozyme can catalyze this antibiotic precursor in situ into effective antibiotic and then kill bacteria.

Jing et al.^[56] designed shape-based bio-orthogonal catalytic nanozyme that performs antibacterial activity. They fabricated a shell on the surface of the bacteria, following the shape of the bacteria, and then separated the bacteria and the shell by calcination to obtain the designed bio-orthogonal catalytic nanozyme. This nanozyme can selectively recognize the corresponding template pathogenic bacteria. As a bio-orthogonal catalyst, the nanozyme can catalyze the precursor into an active antimicrobial molecule 6 in situ (Fig. 6) and act as a bacterial killer. And this nanozyme have been used in treatment of E. coli and S. aureus infections in vivo. However, since the recognition is based on shape, it is still a great challenge to distinguish bacteria of the same shape or different subtypes of bacteria.

As newly discovered nanomaterials with special functions, nanozymes are showing great potential for various applications (Table 1). In the fight against microbial infections, nanozymes work by relying on their enzyme-like catalytic properties, cleverly exploiting the environment to inhibit microorganisms through the production of lethal ROS. However, some individual studies indicated that antibacterial active nanozymes have different killing effects on Gram-negative and Gram- positive bacteria. The designed nanozyme kills Pseudomonas aeruginosa more effectively than S. aureus^[33]. Therefore, the mechanism behind this needs to be carefully investigated.

In addition to the properties of enzyme-mimic, as functional inorganic nanomaterials, nanozymes can also perform anti-microbial infection functions by combining their intrinsic physiochemical properties and catalytic properties. As materials with surprisingly special properties at the nanoscale, nanozymes can possess good photothermal conversion efficiency inherently or after modification for photothermal combination therapy; nanozymes can be designed photo-triggered or composed of photosensitive materials with photodynamic properties to enhance catalytic effects. The LSPR properties of nanozymes can also be exploited to give nanozymes superior photothermal properties and enhanced catalytic activity. In addition, nanozymes can be carefully designed to achieve specific functions in complex environments. For example, cascade-based nanozymes can perform different functions at different pH or overcome the problem of unstable catalytic substrate H₂O₂ levels in the environment. Due to their good security, biocompatibility and catalytic properties, nanozymes can also be used as orthogonal catalysts to achieve precise in situ cell killing effects.

Unfortunately, the mechanism of ROS production of nanozymes against microbial infections is not well elaborated and classified by researchers. In addition, there are two issues that need to be addressed in the field of anti-microbial infection application of nanozymes as follows.

1) Antimicrobial specificity. The principle of nanozymes functioning as antibiotics in vivo is mainly to produce ROS to kill pathogens, but it needs to be discussed whether the recognition between pathogens and normal tissues by nanozymes is so good that ROS does not attack normal tissues. Moreover, whether the specificity of the nanozymes for the target pathogen is adequate to avoid the normal survival of non-target bacteria also needs to be further studied.

2) Drug resistance. Drug resistance caused by antibiotics is one of the major issues of public healthcare nowadays. Due to the different mechanisms of effect, nanozymes mainly contribute to the removal of pathogens by means of adsorption and contact killing, ROS production, etc. These methods with no interference to biochemical and metabolic processes of bacteria have a low risk of resistance development. However, further studies are necessary support whether this property also leads to the development of drug resistance in pathogens.

In the future, the wide range of properties of nanozymes as inorganic materials can be explored more deeply in order to make a better combination of various excellent properties including catalytic activity to make contribution in the field of anti-microbial infection.