Recent trends in nanozymes design: from materials and structures to environmental applications

Camilo A. S. Ballesteros† ^a, Luiza A. Mercante† ^b, Augusto D. Alvarenga ^cd, Murilo H. M. Facure ^ce, Rodrigo Schneider ^ce and Daniel S. Correa *^cde
aBachelor of Natural Sciences and Environmental Education, Pedagogical and Technological University of Colombia (UPTC), 150003, Tunja, Colombia
^bInstitute of Chemistry, Federal University of Bahia (UFBA), 40170-280, Salvador, BA, Brazil
^cNanotechnology National Laboratory for Agriculture (LNNA), Embrapa Instrumentação, 13560-970, Sao Carlos, SP, Brazil. E-mail: daniel.correa@embrapa.br
^dPPG-Biotec, Center for Exact Sciences and Technology, Federal University of São Carlos (UFSCar), 13565-905, São Carlos, SP, Brazil
^ePPGQ, Department of Chemistry, Center for Exact Sciences and Technology, Federal University of São Carlos (UFSCar), 13565-905, São Carlos, SP, Brazil

First published on 17th August 2021

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1. Introduction

The presence of emerging contaminants in water resources, including heavy metal ions, organic dyes, antibiotics and pesticides, can cause harmful effects on human health and the environment due to the high resistance of these contaminants to degradation.^1,2 Therefore, to protect human health and the environment, the development of adequate technologies for the detection and removal of these persistent pollutants is imperative. As highly efficient catalysts, natural enzymes have been widely applied as biorecognition elements in biosensors for the sensitive monitoring of contaminants,^3–6 and in bioremediation for wastewater treatment.^7,8 However, although they are promising to mitigate environmental problems, natural enzymes often suffer from intrinsic shortcomings, including their high cost of preparation and purification, harsh reaction conditions, poor stability and difficulties in recycling.^9,10

In this scenario, emerging applications of nanotechnology have contributed to the development of alternatives for replacing natural enzymes in environmental applications.^7,11,12 In particular, advances in nanomaterial design have opened up the possibility of developing nano-scaled materials with intrinsic enzyme-like features for applications ranging from water-quality screening to the degradation and removal of pollutants.^13–16 These biomimetic materials, known as nanozymes, present several advantages compared with natural enzymes, including higher stability, having wider chemical (e.g., pH) and physical (e.g., temperature) operational windows, easy synthetic protocols, tunability of catalytic activity, and reduced costs.^17–20 Furthermore, the unique physicochemical properties of nanosized materials endow nanozymes with many possibilities for structural modification and chemical functionalization, which provide a superior strategy for adjusting their catalytic activity.^21,22 Given these advantages, along with significant advances in biotechnology, and nanomaterials science, the past few years have witnessed remarkable advances in the rational design and controllable preparation of nanozymes for environmental applications.

The aim of this review is to highlight recent advances and prospects in the application of nanozymes for environmental applications, from a materials point of view, as illustrated in Scheme 1. First, we introduce some basic concepts regarding enzyme-like activities and the use of nanozymes for environmental applications, and then we illustrate the recent progress in the application of different types of enzyme-mimicking nanomaterials, including (but not limited to) carbon-, metal-, metal oxide-, and MOF-based nanomaterials in chemical (bio)sensing platforms for the analysis of contaminants that are deleterious to human health and the environment. We also discuss the use of nanozymes for advanced pollutant remediation. The influence of nanoscale factors, such as size, shape/morphology, surface modification, doping and composition on the nanozyme performance are also presented. These recent studies, presented in the next sections, exemplify the wide range of potential environmental applications of nanozymes towards remediation and sensing activities.

2. Key basic concepts

2.1. A brief overview of catalytic functions

Nanozymes are generally categorized into several subtypes based on their reactions and the types of natural enzyme they mimic. Most of the nanozymes employed for environmental applications exhibit oxidoreductase-like mimicking activity, e.g., peroxidase, oxidase, catalase and superoxide dismutase,^23,24 as shown in Scheme 1. Peroxidases encompass a large group of enzymes that catalyze the oxidation of substrates in the presence of peroxides, during which hydrogen peroxide (H₂O₂) or organic hydroperoxides (R–OOH) act as the electron acceptor and the substrate acts as the electron donor.^25,26 Oxidases catalyze the oxidation of diverse substrates (electron donors) using molecular oxygen (O₂) as the electron acceptor.²⁷ Subsequently, O₂ is reduced to water or H₂O₂, as depicted in Scheme 1. Generally, the specific name of the oxidase is given according to the substrate, i.e., glucose oxidase (GOx), alcohol oxidase (AOx) and uric acid oxidase (UOx) are the specific oxidases that catalyze the oxidation of glucose, ethanol, and uric acid, respectively.²⁸ Catalase is a natural enzyme that accelerates the decomposition of H₂O₂ into water and molecular oxygen.²⁶ The reactions catalyzed by superoxide dismutase enzymes involve the disproportionation of superoxide radicals (O₂˙⁻) into hydrogen peroxide and molecular oxygen.^25,26

The apparent steady-state kinetic parameters determined by the Michaelis–Menten model, such as the Michaelis constant (K_m) and the maximal reaction rate (V_max), are commonly employed to evaluate the catalytic performance of nanozymes.²⁹V_max describes the reactivity of the nanozyme (at a fixed concentration) when saturated with the substrate. K_m is defined as the concentration of a substrate when the enzymatic reaction reaches half of the maximum velocity (V_max).^14,30 According to this model, the K_m value is inversely proportional to the affinity of the nanozyme for the substrate, i.e., a small K_m indicates a high affinity. Therefore, K_m is used to determine the best substrate for a nanozyme.^14,16,30

2.2. Nanozymes in environmental applications

As an alternative to natural enzymes, nanozymes have been broadly exploited for sensing various pollutants, including toxic ions, pesticides, phenolic pollutants, antibiotic residues, and pathogens.^15,24 Depending on the type of nanozyme and the pollutant, different assays based on optical (e.g., colorimetric, fluorometric, chemiluminescent, and surface plasmon resonance) or electrochemical detection methods can be designed. As will be shown in the next section, optical biosensors are the most commonly reported class of nanozyme-based biosensors. In this regard, the use of nanozymes as optical probes involves accurately controlling the catalytic colored reactions and can be roughly categorized via two main strategies: (i) nanozymes catalyze the oxidation of different organic substrates (chromogenic, fluorogenic or chemiluminescent), where the optical change that accompanies these reactions provides an easy way to qualitatively identify and quantify the concentration of the target pollutant; or (ii) the catalytic efficiency of the nanozymes can be enhanced or inhibited by the target pollutant.^22,31 In electrochemical biosensors, nanozymes can be used as an electrode material for pollutant detection or as a tracing tag for signal amplification. The high surface area and the high density of capture sites of nanozymes could allow an increased loading of the electroactive species at their surfaces, resulting in improved electrochemical responses.³²

The potential applications of nanozymes for degrading environmental pollutants have been also investigated.^13–16 In general, the properties are strongly related to their ability to generate reactive oxygen species (ROS), which can quickly and non-selectively degrade most organic pollutants to CO₂ and H₂O.^33,34 The means of generating ROS may include chemical, electrochemical, sonochemical or photochemical processes.³⁵

3. Enzyme-mimicking nanomaterials for environmental applications

Considering that nanomaterials can be constructed rationally, the approach of grafting the structural properties of natural enzymes into the rational design of nanozymes has been proven to be one of the most promising strategies for optimizing their catalytic performance.³⁶ Consequently, the intrinsic enzyme-like activities of nanozymes are closely related to their composition, although other factors also have an impact, as will be discussed in Section 4. To date, according to their composition, nanozymes have been roughly divided as follows: carbon-, metal-, metal oxide-, and MOF-based nanozymes. The excellent properties of all these nanomaterials make them promising candidates as enzyme mimetics in environmental applications, as will be discussed in the next subsections. We will also cover the recent advances regarding the use of new classes of nanomaterials as nanozymes.

3.1. Carbon-based nanozymes

Carbon-based nanomaterials have been widely explored in many different applications due to their unique characteristics.^30,37 These nanomaterials have also aroused much interest to be used as nanozymes since they present specific electronic and geometric features that enable mimicking the catalytic center of natural enzymes.^24,30,38 In addition, carbon-based nanozymes exhibit high operational stability and excellent robustness against harsh conditions, they can be easily synthesized and tailored, and present a lower cost than natural enzymes.^39,40 Furthermore, they are a very interesting alternative for use as a metal-free catalyst for some (bio)chemical applications, since, depending on the medium employed, some Lewis bases can bind to the metal atoms or ions and hamper the catalytic activity of metal-based nanozymes.^30,40

Fullerenes, carbon nanotubes (CNTs), graphene, graphene oxide (GO), carbon dots (CDs), graphene quantum dots (GQDs), and carbon nitride have been reported as carbon-based nanozymes mimicking the catalytic activities of superoxide dismutase, peroxidase, oxidase, hydrolase, and catalase enzymes.^37,38 For instance, the superoxide dismutase-like activity of fullerene-based materials was first reported in the 1990s and 2000s.^41–43 After that, other forms of carbon nanomaterials began to be used as artificial enzymes. After the catalytic properties of CNTs were demonstrated,^44–46 in 2010 Song et al. reported the intrinsic peroxidase-like activity of single-walled carbon nanotubes (SWCNTs).⁴⁷ The authors showed that the activity was independent of the metal residues present in the SWCNTs. Next, GO modified with carboxyl groups (GO-COOH)⁴⁸ and CDs^49,50 were also demonstrated to present peroxidase catalytic activity. In 2013, GQDs were used in hydrogen peroxide (H₂O₂) detection, showing better catalytic activities than GO and a comparable performance to some enzyme-immobilized electrodes.^51,52 More recently, Ma et al. used graphdiyne oxide (GDYO), a new allotrope of the carbon family, to oxidize the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂.⁵³

The catalytic mechanism of carbon-based nanozymes is not yet fully understood, and currently there are just a few reports available in the literature that are devoted to understanding their catalytic mechanism.^30,39 In this regard, Qu's group studied the catalytic mechanism of GO–COOH⁴⁸ and GQDs⁵⁴ with peroxidase-like activity. The authors found that in the case of GO–COOH, the catalytic reaction followed a ping-pong mechanism. This indicates that, similar to HRP, GO–COOH binds and reacts with the first substrate (H₂O₂) to produce hydroxyl radicals and then reacts with the second substrate (TMB). Electron transfer was found to occur from the top of the valence band of the nanozyme to the lowest unoccupied molecular orbital (LUMO) of H₂O₂.^39,48 In the case of GQDs, the authors investigated the role of different oxygen-containing functional groups present on the GQD structure in the peroxidase-mimicking reaction.⁵⁴ By performing fluorescence experiments, they found that the –CO groups acted as catalytically active sites for the conversion of H₂O₂ to ˙OH, while the OC–O groups acted as substrate-binding sites for H₂O₂, and the presence of C–OH groups led to a decrease in the catalytic activity of the GQDs. In addition, theoretical calculations were conducted to gain further information on the mechanism. By determining the energy barriers and reaction energies, they could confirm that the –CO moieties presented a higher catalytic activity for H₂O₂ decomposition than the OC–O groups. They also found that, owing to the stronger hydrogen-bonding interaction, the binding energy of H₂O₂ on the OC–O groups (−0.67 eV) was lower than that for –CO (−0.30 eV) and C–OH (−0.17 eV), which supported the experimental evidence that the carboxylic groups acted as binding sites for the substrate.

Recently, Das et al. explored a systematic investigation on the mechanism of the peroxidase-like activity of CDs.⁵⁵ By combining different characterization techniques and density functional theory (DFT) calculations, they found that the carboxylic groups acted as the substrate-binding and the catalytic site for peroxidase-like activity in CDs. In addition, their studies revealed that the presence of an amine group in the vicinity of the carboxyl group increased the peroxidase-mimicking activity. The understanding of the catalytic activity of carbon-based nanozymes is of great importance, and further work should be carried out with the aim of improving the performance of these nanozymes in practical applications.

Although carbon-based nanozyme investigations that focus on catalytic mechanism are still scarce, their catalytic activity has been consistently improved, and some designed carbon-based nanozymes show comparable or even better results than natural enzymes. For instance, Fig. 1 shows the K_m and V_max of some carbon nanomaterials with intrinsic peroxidase-like activity, using TMB as the substrate, compared with horseradish peroxidase (HRP). The nanozymes located to the left of the HRP enzyme (vertical dashed line) presented a smaller value of K_m, i.e., a stronger affinity between the enzyme and the TMB substrate. Those above the horizontal dashed line of HRP presented a higher value of V_max, indicating faster catalysis towards TMB in the presence of H₂O₂. Such behavior, combined with the above-mentioned excellent physical and chemical properties, identifies the carbon nanozymes as promising materials to be used as artificial enzymes in environmental applications.^24,31,49,54

Devi et al. synthesized graphitic carbon nitride quantum dots (g-CNQDs) and graphene oxide quantum dots (GOQDs) for use in the colorimetric detection of fluoride ions (F⁻) in aqueous media.⁵⁸ The g-CNQDs were obtained using urea and trisodium citrate as the precursors via hydrothermal synthesis, while the GOQDs were produced by the pyrolysis of citric acid. The peroxidase-mimicking activity of the carbon materials was confirmed by the catalysis of the reduction of H₂O₂ and the oxidation of the chromogenic TMB, yielding a blue-colored product (oxTMB). The authors stated that the π-electron-rich surface of the quantum dots increased the electron density around the H₂O₂ molecule, favoring the generation of ˙OH radicals, which helped in the TMB oxidation, producing the oxTMB. Taking advantage of such a mechanism, fluoride ions were detected using the g-CNQDs. The p–π conjugation of the lone pairs in the fluorine atoms and the delocalized π-conjugated system of the g-CNQDs yielded an enhancement in the electric density of the highest occupied molecular orbital (HOMO). These free electrons participate in the formation of the ˙OH radicals, enhancing the blue color due to the production of the oxTMB product, as illustrated in Fig. 2A(i). Thus, an increase in the intensity of the blue color occurs with the addition of fluoride ions. The detection was monitored by recording the absorbance of the solution at 652 nm, as shown in Fig. 2A(ii). The use of g-CNQDs as the colorimetric probe allowed the detection of fluoride ions in the linear range of 10–120 μM with a limit of detection (LOD) of 4.06 μM, which is lower than the WHO permitted level (79 μM). The selectivity of the material and detection in real environmental samples were also tested. Graphitic carbon nitride (GCN) has also been employed as a nanozyme by other groups.^59,64 For example, Zhang et al. showed that the modified GCN with KOH and KCl presented bifunctional enzyme-mimicking behavior, combining the roles of oxidase and peroxidase.⁶⁵

Shamsipur and co-authors took advantage of the peroxidase-mimetic activity of CDs to detect mercury ions.⁶⁶ For that purpose, cysteine (Cys) was used to prevent the formation of the blue-colored oxTMB radical cation by impeding the redox reaction between H₂O₂ and TMB. Since Cys has a strong affinity for Hg²⁺ ions due to the presence of the thiol group, the detection of mercury ions could be performed via formation of the Hg–Cys complex, which allowed the redox reaction to occur recovering the blue color, as schematized in Fig. 2B(i). The optical sensor presented a LOD of 23 nM for Hg²⁺ and high selectivity towards mercury ions, as can be seen in Fig. 2B(ii). Furthermore, a tunable dynamic range was achieved by adjusting the Cys concentration, and the analysis of river water samples showed good recovery results from 91% to 113%.

Savas and Altintas used GQDs as an artificial enzyme to electrochemically detect the pathogen Yersinia enterocolitica.⁶⁸ Besides the peroxidase-like catalytic activity, the GQDs were chosen because they are 0D materials possessing a graphene structure that provides fast electrochemical responses allied to biocompatibility and low toxicity, which makes them suitable for the label-free detection of microorganisms.⁶⁹ The GQDs were laminated onto the electrode and the immobilization of the antibody was performed followed by inactivation of the unreacted carboxyl groups. The detection of the analyte was achieved with the reduction of H₂O₂ by the GQDs and with a decrease in the electron transfer hampered by the formation of the antigen–antibody complex. The GQD immunosensor presented high selectivity and sensitivity towards Y. enterocolitica with a LOD of 5 cfu mL⁻¹ under the investigated range of 1 to 6.23 × 10⁸ cfu mL⁻¹, indicating its potential for the detection of pathogens.

Zhu and co-authors used three carbon-based nanozymes to construct a nanozyme colorimetric sensor array for the detection of five pesticides.⁶⁷ The carbon nanozymes, i.e., nitrogen-doped graphene (NG), nitrogen and sulfur co-doped graphene (NSG), and GO presented peroxidase-like activities and were able to catalyze the oxidation of TMB. As schematized in Fig. 2C(i), the sensing mechanism was based on the reduction of the peroxidase-mimicking activity of the nanozymes in the presence of the aromatic pesticides. The sensor array was tested upon exposure to five pesticides, namely lactofen, fluroxypyr-meptyl, bensulfuron-methyl, fomesafen, and diafenthiuron. As shown in Fig. 2C(ii), the nanozymes responded to all the pesticides in a specific manner. To facilitate visualization of the pesticide detection, the colorimetric response patterns were converted into a 2D canonical score plot using linear discriminant analysis (LDA). As can be observed in Fig. 2C(iii), the pesticides were well separated into five different groups, showing the feasibility of the sensor array to detect aromatic pesticides in real environmental samples. In addition, the authors proved that the pesticides could be discriminated in terms of their concentration, in the presence of interferents, and in real soil samples.

Besides the intrinsic enzyme-like behavior of the carbon nanomaterials, they can also be combined with other materials or modified to attain enhanced enzymatic activities. In this regard, some groups have reported the doping of carbon nanomaterials with N and S atoms to specifically enhance their catalytic activities,^70–73 as will be discussed in detail in the next section. In addition, functionalization with poly(styrene sulfonate) to detect ascorbic acid,⁷⁴ with chitin for peroxide and glucose detection,⁷⁵ and with hemin for the degradation of dyes,⁷⁶ the detection of an insecticide,⁷⁷ and phenol pollutants⁷⁸ have also been reported.

Carbon-based nanomaterials have also been used for the degradation of pollutants.^34,76 For example, Safavi et al. used CDs with peroxidase activity for the degradation of azo dyes.⁷⁹ The CDs with a narrow size distribution and blue photoluminescence were synthesized using a microwave-assisted ionic liquid method. The azo dyes methyl red (MR) and methyl orange (MO) were degraded in the presence of H₂O₂. For MR the degradation rate constant was found to be 9.5 × 10⁻³ min⁻¹ with an efficiency of 83%. However, it is important to mention that there are only a few reports of pollutant degradation using carbon nanomaterials without any functionalization, and, as mentioned before, in general, such nanomaterials are used in combination with other materials aimed at improving the degradation efficiency. For instance, graphene oxide quantum dots (GOQD) were covalently immobilized in hydrogels for the degradation of the organic dye rhodamine B (RhB),⁸⁰ and graphene sheets were functionalized with polydopamine decorated with Fe₃O₄ magnetic nanoparticles for the detection and degradation of pesticides.⁸¹

3.2. Metal-based nanozymes

Since some metals present intrinsic catalytic behavior for different reactions, it is expected that metallic nanomaterials also present such catalytic activities. In fact, these nanomaterials have been extensively explored as artificial enzymes due to their electronic and catalytic properties.^14,29,82 Metalllic nanomaterials based on Au,^83,84 Ag,^85,86 Pt,^87,88 Pd,^89,90 Rh,^91,92 Ru,⁹³ Ir^19,20 and their multimetallic assemblies,^19,94,95 have been found to display different enzyme-like activities, including oxidase-, peroxidase-, catalase-, glucose oxidase- and/or superoxide dismutase-like activities. For instance, Fig. 3 shows some examples of metal-based nanomaterials with peroxidase-like activity and their performance in comparison with HRP. The availability of more active sites on the metal-based nanomaterial surfaces may be the reason for the lower K_m value than for natural HRP, which has only one active site per molecule. Such enzyme-mimicking behavior depends on many aspects, including the particle morphology and the size, the interparticle distance, the intrinsic nature of the catalytically active metals, the surface properties, and the specific experimental conditions, as discussed later on in this review.^92,96,97

For metal-based nanozymes, the catalytic mechanism arises from the adsorption, activation, and electron transfer of the substrate onto the metal surface, in contrast to mechanisms occurring by changes in the metal valence of the nanomaterial, as in the case of other metal compound-based nanozymes.¹⁰³ Therefore, the surface state of the metal-based nanomaterial is extremely important for its enzymatic performance. In this way, the high conductivity provided by the metal surface is a fundamental characteristic of such nanozymes.^14,24 Furthermore, metal nanoparticles are generally used with some capping or coating material, in order to tailor their surface properties and enhance the catalytic activity.¹⁰⁴ Additionally, changes in pH have been proven to be a crucial factor in the enzymatic activity of metal-based nanozymes.¹⁰⁵ Different studies have reported the peroxidase-like activities of Au, Ag, Pt, and Pd nanomaterials at acidic pH, and catalase-like activities at basic pH.¹⁸ Gao's group investigated this interesting behavior in detail.¹⁰⁵ By performing experimental and theoretical studies, they showed that the peroxidase-like activity at low pH is ascribed to the base-like decomposition of H₂O₂ on the metal surface, and the catalase-like activity manifested under high pH conditions is ascribed to the acid-like decomposition of H₂O₂. The pre-adsorbed ˙OH radicals on the surfaces, which are only favorably formed under basic conditions, trigger the switch between both activities and provide the pH-switchability.^13,14,24

These numerous interesting features allow metal-based nanozymes to be broadly applied in the detection and remediation of environmental pollutants.^22,97,106 Regarding detection applications, metal-based nanozymes have been used for the design of colorimetric,¹⁰⁷ spectrometric,²² fluorometric,¹³ and electrochemical⁸⁸ sensors, allowing the detection of different environmental pollutants, including heavy metals²² and organic compounds.¹⁰⁶ Several of these assays are based on Au nanostructures due to their ease of synthesis, surface tunability and high catalytic activity and stability.^18,108 For instance, Han and co-workers developed a gold nanozyme-based paper chip (AuNZ-PAD) for Hg²⁺ detection.¹⁰⁹ The colorimetric mercury assay was established based on the peroxidase-like catalytic activity of Au nanoparticles (AuNPs) promoted by the formation of the Au–Hg amalgam, which was correlated to the intensity of the colorimetric response resulting from the catalytic reaction between TMB and H₂O₂. More specifically, the presence of Hg²⁺ ions around the AuNPs caused changes in their surface properties, which accelerated the decomposition of H₂O₂, leading to a considerable improvement in the peroxidase-like catalytic activity of the nanoparticles. Therefore, an intense blue color was observed on the paper chip. By contrast, in the absence of Hg²⁺ ions, the authors did not observe the catalytic oxidation of TMB with H₂O₂, and no color change was observed on the paper chip. The AuNZ-PAD showed a high sensitivity with a LOD of 0.06 ng (30 μg L⁻¹). The paper chip also exhibited enhanced selectivity without interference from other metal/metalloid species (Ag⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺, Cd²⁺, As³⁺, Pb²⁺, Bi³⁺, Zn²⁺, Sb³⁺, Cu²⁺, Mn²⁺, Ca²⁺, and Na⁺). The inherent peroxidase-like activity of the Au nanomaterials was also explored for the detection of the pesticide acetamiprid.¹¹⁰

Weerathunge et al. reported the use of tyrosine-capped Ag nanoparticles for the detection of chlorpyrifos, an organophosphorus pesticide (OP).⁸⁶ In their strategy, an aptasensor was developed by exploring the dynamic non-covalent interaction of the chlorpyrifos specific aptamer (Chl) with the nanoparticle surface (sensor probe) vs. the analyte, which allowed the colorimetric detection of chlorpyrifos, as illustrated in Fig. 4A(i). The colorimetric property was due to the high catalytic activity of the Ag nanozyme for oxidizing the peroxidase substrate (TMB) with excellent kinetic parameters. In addition, the intensity of the blue color was directly proportional to the concentration of the pesticide and no color was observed when the sensor was exposed to other OPs (aldicarb, clothianidin, thiamethoxam, mancozeb, captan, diazinon, dichlorvos, phorate, monocrotophos, methamidophos, and azamethiphos), as shown in Fig. 4A(ii). The ability of the nanozyme to detect chlorpyrifos in river water demonstrated the potential practical applicability of the aptasensing platform.

Recently, He et al. reported the synthesis of osmium nanoparticles with peroxidase-like performance.¹⁰² The authors demonstrated that the nanoparticles possess a higher peroxidase-like activity and negligible oxidase-like activity compared with those of common noble metal-based nanozymes (Pt and Au). This behavior allowed the use of the Os nanozymes in colorimetric assays without the interference of O₂. In another study, copper nanoclusters exhibited excellent tetraenzyme-like activity, including peroxidase-, catalase-, superoxide dismutase-, and ascorbic acid oxidase-mimicking activities, and are a promising candidate for biosensing and biocatalytic applications.¹¹²

Besides single metal nanozymes, bimetallic and multimetallic nanomaterials have also displayed high enzymatic activities. Combining metals with different electronic characteristics allows the development of alloy nanozymes with unique electronic structures and enhanced catalytic properties due to the synergetic effect of the metals used.²⁴ For instance, several studies have reported the catalytic activity of Au@Pt.^94,113–115 With this in mind, Tian et al. designed a rod-shaped Au@Pt nanozyme as an integrated nanosensor for the detection of silver ions.⁹⁴ Their approach relied on two reactions: (i) the inhibitory effect of Ag⁺ on the catalytic decomposition of H₂O₂ in an acidic environment; and (ii) the reduction of Ag⁺ on the Au@Pt nanozyme surface in a basic environment, leading to a blue shift in the localized surface plasmonic resonance wavelength (LSPR λ_max) of the nanozyme. This method showed selectivity for Ag⁺ in the presence of other metal ions and a LOD of 500 nM, which is below the permitted level of Ag⁺ in drinking water by the US Environmental Protection Agency (EPA). Furthermore, this nanosensor was used to determine Ag⁺ in tap- and spring-water with recoveries in the range of 90–96% and 98–107%, respectively.⁹⁴ In another study, Song et al. reported the synthesis of peroxidase-like Au@AgPt nanoparticles via the decoration of hexoctahedral Au nanoparticles with Ag and Pt atoms.¹¹¹ The Au@AgPt nanoparticles showed good surface-enhanced Raman scattering (SERS) activity of Au and enhanced catalytic activity of Pt. As a strategy for the detection of Hg²⁺, 6-mercaptohexanol (MCH) was assembled onto Au@AgPt via thiol–metal bonds, to obtain a colorimetric/SERS dual-mode probe. Basically, the working principle of the colorimetric/SERS probe was based on the changes in the catalytic and SERS properties of the nanozyme in the presence of Hg²⁺, resulting in the color change from colorless TMB to blue oxTMB and the evolution of the SERS fingerprint spectrum of oxTMB, as illustrated in Fig. 4B. The sensing mechanism could be described as follows: in the presence of Hg²⁺, the Au@AgPt@MCH may selectively capture the mercury ions followed by the reaction between Hg²⁺ and Pt⁰ to generate Pt²⁺ and Hg⁰. The consumption of Pt atoms reduced the catalytic activity of Au@AgPt@MCH and the oxidation process was inhibited, which affected the catalytic reaction rate and the yield of oxTMB.¹¹¹ Therefore, with the increase in Hg²⁺ concentration, the consumption of TMB was reduced and the generation of oxTMB decreased, which was reflected by the color change of the reaction solution from blue to colorless and the decreased SERS signal of oxTMB. The selectivity of the nanozyme for Hg²⁺ was demonstrated over other ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Co²⁺, Cd²⁺, Li⁺, Mn²⁺ and Cr³⁺), indicating that it is a suitable platform for the on-site detection of Hg²⁺ over a wide concentration range.

There have also been significant advances in using metal-based nanozymes for the oxidation of environmental pollutants.²⁴ In this direction, Xu and Wang investigated the heterogeneous oxidation of 4-chloro-3-methyl phenol (CMP) using Fe⁰ nanoparticles.¹¹⁶ The authors investigated the influence of pH on the catalytic activity of the nanozyme and a complete CMP degradation could be achieved within 15 min at pH 6.1, while 63% of the total organic carbon was removed after 60 min. The catalytic activity decreased with time and the recyclability of the material was not demonstrated. In another study, Wang et al. demonstrated the potential of the Au/Pt/Co tri-metal nanozyme confined in a silica scaffold (DMSN@AuPtCo) as a water purifier for decontaminating organic wastewater and H₂O₂ sewage to turn them into safe resources.¹¹⁷ The nanozyme was capable of degrading 90% of the phenol within 60 min. Moreover, when the wastewater was treated over 24 h, the removal efficiency of phenol and H₂O₂ are both over 99%. This dual-function nanozyme not only showed outstanding peroxidase- and catalase-mimicking activities but also demonstrated high stability and well-conditioned tolerance. Besides, the nanozyme has the advantage of treating different sewage samples in one system, which avoids the problem of secondary pollution and greatly increases the efficiency and safety.

3.3. Metal oxide-based nanozymes

Since the discovery of the peroxidase-mimicking activity of Fe₃O₄ in 2007,⁶³ several studies on metal oxide nanozymes have been reported.²⁹ Among the metal oxide nanozymes, Fe₃O₄,^118,119 CeO₂,^120,121 and MnO₂^122,123 nanostructures stand out due to their unique ability to interchange between valence states. In addition, over the years, other metal oxide-based nanomaterials, such as VO_x,¹²⁴ CuO,¹²⁵ CeZrO₂,¹²⁶ GeO₂,¹²⁷ MoO₃,¹²⁸ and MnCo₂O₄,¹²⁹ have been explored to mimic different kinds of enzymes, including peroxidases, oxidases, hydrolases, and catalases. In order to give an overview to the reader, Fig. 5 shows a comparison of the enzymatic properties (K_m and V_max) of some metal oxide nanostructures with intrinsic peroxidase-like activity.

In the pioneering work of Yan's group in 2007, it was suggested that Fe₃O₄ nanoparticles can mimic the peroxidase activity towards H₂O₂ and TMB with improved efficiency compared with HRP.⁶³ Soon after this report, the peroxidase-like activity of magnetic iron oxide nanomaterials started to be explored for environmental applications.^23,24 For example, Wang et al. demonstrated the ability of Fe₃O₄ nanoparticles to act as a peroxidase mimetic to catalyze the breakage of H₂O₂ for the oxidative removal of organic pollutants.¹³³ The authors found that the nanoparticles prepared using the ultrasonic-assisted reverse co-precipitation method were able to remove almost 90% of RhB in 60 min, whereas only less than ca. 20% of RhB was degraded over nanoparticles prepared using the conventional mechanical stirring method. The significantly improved catalytic activity was attributed to the smaller particle size, the larger BET surface area and the higher dispersibility of the nanoparticles. In addition, the authors evaluated the influence of the chemical composition of the Fe₃O₄ nanoparticles on their peroxidase-like activity by varying the initial molar ratio of Fe²⁺/Fe³⁺. When the molar ratio was adjusted from 1:3 up to 3:1, the catalytic activity was significantly increased, and the removal of RhB within 3 min was increased from 21.1% up to 63.4%, implying that the Fe²⁺ sites on the nanoparticles’ surface are more important to their catalytic activity than the Fe³⁺ sites. This was attributed to the surface Fenton reactions between the Fe²⁺ sites and the adsorbed H₂O₂. Briefly, the Fenton reaction involves the initial formation of an Fe²⁺–H₂O₂ complex, which produces ˙OH radicals and other reactive oxygen species (like HO₂˙ and O₂˙⁻), and a radical chain reaction is promoted.¹³⁴ Iron oxide nanostructures have also been used for the Fenton-like degradation of other persistent organic pollutants, including dichlorophenol,¹³⁵ sulfathiazole¹³⁶ and bisphenol A.¹³⁷

In another study, Fe₃O₄ nanoparticles synthesized via the co-precipitation method exhibited enhanced bactericidal activity over a wide pH range.¹³⁸ In order to overcome the pH-dependent peroxidase activity of Fe₃O₄, which is restricted to an acidic environment, the authors used adenosine triphosphate disodium salt (ATP) as a synergistic agent to accelerate the ˙OH radical production, allowing an effective broad-spectrum antibacterial activity over a wide pH range. This catalytic activity was successfully confirmed via the bactericidal effect of Fe₃O₄ nanoparticles against both Gram-positive and Gram-negative bacterial strains under neutral pH conditions. Similar behavior was also observed by Xiao and co-workers.¹¹⁸

In addition to applications for the oxidation of pollutants and microorganisms, Fe₃O₄ nanomaterials have also been used for sensing applications. Wu et al. explored the peroxidase-like activity of Fe₃O₄ nanomaterials for the colorimetric quantification and discrimination of phenolic compounds.¹³⁹ Thanks to the favorable catalytic activity of the small nanoparticles, the nanozymes were able to promote the oxidative coupling of phenolic species and 4-aminoantipyrine (4-AAP) in the presence of H₂O₂, giving a remarkable color change related to the level of the target pollutants. The underlying molecular mechanism was proposed by the authors as follows: the Fe₃O₄ nanoparticles induce the decomposition of H₂O₂ to generate hydroxyl radicals, and these radicals with their strong oxidizing capacity can further remove a single electron from the hydroxyl group of phenol, generating quinoid radicals; after that, the redundant hydroxyl radicals trigger the oxidative coupling of colorless 4-AAP with the quinoid radicals, finally forming a pink quinone imine. As a result, this approach provided a linear response for phenol in the concentration range of 1.67 μM to 1.2 mM with a LOD down to 3.79 μM. Moreover, exploring the different reaction kinetics of different phenolic compounds (o-chlorophenol, m-chlorophenol, p-chlorophenol, m-aminophenol and o-nitrophenol) on the Fe₃O₄ surface, a colorimetric array was able to discriminate the six common phenolic pollutants even at a very low concentration. In another report, Liu et al. employed Fe₃O₄ nanoparticles as a peroxidase-mimicking nanozyme to catalyze the synthesis of fluorescent polydopamine (FDP) under mild conditions.¹⁴⁰ The mechanism of this reaction was not entirely clear, but it might be related to the preferential binding of the dopamine catechol groups to the Fe³⁺ sites of the Fe₃O₄ NPs. The resulting FDP was further applied for the selective detection of Zn²⁺ based on the fluorescence enhancement of FDP at 360 nm.

Cerium oxide (ceria) nanoparticles are also well known for their high catalytic performance in several applications owing to the presence of the Ce³⁺ and Ce⁴⁺ valence states, as well as the presence of oxygen vacancies.^121,141 Using a hydrothermal method, He et al. synthesized cerium oxide nanorods with haloperoxidase activity,¹⁴² which were found to be dependent on the aspect ratio due to the different concentrations of Ce³⁺ defect sites and exposed (110) facets. The CeO_2−x nanorods showed cytotoxicity to Escherichia coli and anti-fouling action in the presence of H₂O₂ and HOBr.¹⁴² Hu and co-workers demonstrated that the haloperoxidase activity and stability of the CeO_2−x nanorods were preserved by embedding them into PVA electrospun nanofibers.¹⁴³ Compared with nanozyme dispersions, polymer-embedded nanozymes are more convenient for handling and continuous processing. Moreover, the hydrophilic character of the PVA mats allowed the fibers to swell in the presence of water, further improving the access of the molecular substrates to the embedded CeO_2−x nanorods in the PVA nanofibers. In another report, Khulbe and co-workers observed that the introduction of oxygen vacancies into 5 nm CeO₂ nanoparticles transformed them into phosphotriesterase-like mimetic nanozymes, which allowed their application for the degradation of organophosphate pollutants.¹⁴⁴ The catalytically active vacancies on nanoceria led to degradation efficiencies of 90% for methyl paraoxon, 30–40% for ethyl paraoxon, and 0% for phenyl paraoxon, showing an action analogous to “lock–key” enzymatic selectivity.

Li et al.¹⁴⁵ and Zhao et al.¹⁴⁶ observed that fluoride ions can improve the oxidase activity of CeO₂ nanostructures. In the work reported by Zhao et al., the best catalytic performance was achieved with the addition of 5 mM of fluoride ions to the solution.¹⁴⁶ The activity enhancement was attributed to the interactions between F⁻ and cerium ions on the surface of CeO₂, which increased the concentration of Ce³⁺ and oxygen vacancies, facilitating the desorption of the ABTS oxidation product, and further enhancing the electron transfer. Electrochemical measurements also confirmed that the rate of electron transfer for the Ce⁴⁺/Ce³⁺ redox couple was enhanced by the fluoride ions. Studies have also shown that the catalytic activity of ceria nanostructures is also pH-dependent.¹⁴¹

Several authors have reported that MnO₂ nanostructures could also show enzyme-like activity.^122,147,148 Recently, the laccase enzyme-like activity of MnO₂ nanostructures was explored for the degradation of 17β-estradiol (E2).¹⁴⁷ The authors found that MnO₂ nanozymes were effective for removing E2 at pH 6 with 97.3% efficiency and reusability. More importantly, the authors investigated the effect of humic acid (HA), a major component of natural organic matter, in the removal and transformation of E2. The presence of HA hindered the E2 removal rate, due to the competition between HA and E2 for the catalytic sites on the MnO₂ nanozyme surface. However, HA could also be oxidized by the MnO₂ nanozyme to generate radical intermediates via a single-electron-transfer process, which coupled with the E2 radical intermediates and thus formed the heterocoupling products of E2–HA. The formation of homo-/hetero-coupling products between E2 and HA resulted in ablation of the phenolic group of E2, which is the essential functional group responsible for the strong estrogenicity of E2. Consequently, nano-MnO₂ as a laccase mimic was a useful tool for the removal and conversion of E2 but also for eliminating its estrogenic activity in the presence of HA.

Manganese oxide nanostructures can also mimic oxidases at acidic pH.¹⁴⁹ However, it was observed that oligonucleotides can adsorb on the surface of manganese oxide and therefore inhibit its oxidase activity.¹⁵⁰ Exploring this behavior, a colorimetric assay for the determination of Cd²⁺ and Hg²⁺ was developed by Wang et al. based on regulation of the oxidase-mimicking activity of Mn₃O₄ nanoparticles functionalized with oligonucleotides.¹⁵¹ The complexation of heavy metals with the oligonucleotides restored the catalytic activity of the nanozyme and caused a significant change in the color of the solution, allowing the detection of Cd²⁺ and Hg²⁺ over a wide linear range and with a low LOD (2.4 μg L⁻¹ for Cd²⁺ and 3.8 μg L⁻¹ for Hg²⁺). This sensor platform showed selectivity and efficiency to detect both heavy metals in real samples. In another study, MnO₂ and SiO₂@FeO₄ were applied to dye degradation and H₂O₂ detection to evaluate the multienzyme effect.

Recently, Luo et al. investigated the use of copper ions to regulate the enzyme-like activity of flower-like MMoO₄ (M = Co, Ni) nanostructures.¹⁵² The participation of Cu²⁺ resulted in a noticeable enhancement of the peroxidase-like activity of the MMoO₄ nanostructure; however, they showed a negligible effect on the oxidase-, catalase-, and superoxide dismutase-like activity. The enhancement mechanism was found to be related to the oxidation of M²⁺ ions in the nanomaterial structure by the Cu²⁺ ions, leading to the formation of multivalent M ions (M²⁺/M³⁺) on the surface of the nanomaterials and improving the overall efficiency of the peroxidase-like reaction. Based on this phenomenon, the authors developed a colorimetric strategy for detecting Cu²⁺ in the concentration range of 0.1–24 μM with a low LOD of 0.024 μM. Moreover, the method was applied for the analysis of Cu²⁺ in real water samples (drinking-, tap- and river-water), with recoveries of 99.4–102.1%.

It is important to stress that the synthetic strategy, and, consequently, the morphology, conformation, and composition of the metal oxide nanomaterials are determinants in their enzymatic activities and selectivity, and require more in-depth studies concerning the control of these aspects.

3.4. MOF-based nanozymes

Metal–organic frameworks (MOFs) are constituted of coordinate bonds between organic ligands and metal ions organized in a periodic structure, with well-determined lengths and highly ordered bonds, leading to a porous/channel-like morphology.¹⁵³ Due to the high surface area originating from these cavities and the possibility of pore size control through the synthesis and physical–chemical modifications, MOFs have gained increased popularity for several applications.¹⁵³ Notably, MOFs have been explored as nanozymes,^154,155 whose catalytic behavior originates from two aspects: from the metal ions that can act as biomimetic catalyst centers; and from the organic ligands that can act as electron mediators to accept electrons from a substrate and donate them to another one, thus catalyzing the reactions in a similar way to natural enzymes.¹⁵⁴ Furthermore, the high specific surface area, the tunable pore sizes, and the highly exposed active sites of the MOFs contribute to the achievement of high catalytic efficiency performances, and the organic ligands can provide attractive optical and electrical characteristics as well as abundant functional groups for chemical modifications.²⁴ MOFs can mimic the function of various enzymes, including superoxide dismutase,¹⁰⁸ catalase,¹⁰⁸ methane monooxygenase,¹⁵⁶ oxidase,¹⁵⁷ peroxidase,¹⁵⁸ and lipase.¹⁵⁹

Fig. 6 illustrates examples of MOFs with peroxidase-like activity, where some of them show superior catalytic activity over the natural HRP enzyme, demonstrating the possibility of improving the enzymatic performance in terms of the K_m and/or V_max. In this scenario, Li and co-workers synthesized 2D Ni/Fe MOF nanosheets with a high peroxidase-mimicking activity.¹⁵⁸ Based on the experimental results, the authors suggested that the high surface area and the mesoporous structure enabled the MOF nanozyme to adsorb H₂O₂ and TMB on its surface. The adsorbed H₂O₂ broke down into ˙OH and O₂˙⁻ at the Fe³⁺ active sites through a Fenton-like reaction. Subsequently, TMB was oxidized by the ˙OH radicals, producing the typical blue colored reaction. Kinetic studies confirmed the high affinity of the 2D Ni/Fe MOF nanosheet for H₂O₂ with a K_m of 0.037 mM, which was 100 times lower than that of HRP. This striking behavior was attributed to the good dispersion of the in situ-formed Fe MOF in the 2D Ni MOF nanosheet structure with coordinatively unsaturated metal sites. This allowed the 2D Ni/Fe MOF nanosheets to expose more active metal sites and to enhance the intrinsic catalytic activity of each site due to the synergistic interaction between the two metals. Owing to its high peroxidase-like activity, the 2D Ni/Fe MOF nanozyme was applied in the colorimetric assay for the detection of Hg²⁺ ions in real water samples, ranging from 100 nM to 200 μM and with a LOD of 100 nM. Other MOF-based nanozymes have also been employed for environmental-monitoring applications, including the sensing of other ionic species, such as F⁻,¹⁶⁰ Cu²⁺,^161,162 Cr⁶⁺,¹⁶³ Hg²⁺,^164,165 and Fe³⁺ ions.¹⁶⁶

Recently, Wang and Chen developed a MOF-based peroxidase-like nanozyme with a dual function, namely a catalyst and luminescent sensor, for the determination and degradation of 17β-estradiol (E2) and its derivatives (E1, E3, and EE2).¹⁷³ The nanozyme was prepared through the assembly of luminescent Tb³⁺ ions, catalytic hemin, and the light-harvesting ligand 4,4-oxybisbenzoic acid (OBBA). The Tb-OBBA-Hemin nanozyme was able to degrade E2 in the presence of H₂O₂. In addition, the product of E2 degradation improved the fluorescence of Tb-OBBA-Hemin, which allowed the monitoring of the degradation reaction. MOF-based nanozymes were also found to be useful for the remediation of other persistent organic pollutants such as dyes,¹⁷⁵ phenols,¹⁷⁶ pesticides,¹⁷⁷ and antibiotics.¹⁷⁸ Ren et al. evaluated the performance of three types of Fe-based MOFs, namely MIL-100, MIL-53, and MIL-68, in terms of aflatoxin B₁ (AFB₁) removal.¹⁷⁹ Their studies revealed that the three MOF structures had different catalytic and adsorption capacities, with MIL-68 presenting the best adsorption performance due to the presence of two different pore sizes (∼16 Å and ∼7.7 Å), allowing suitable porous interconnectivity for AFB₁ adsorption (Fig. 7A). Due to the presence of Fe³⁺ ions in the MOF structures, all the three compounds showed peroxidase-like activity allowing the catalytic degradation of AFB₁. In addition, X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectrometry (EDS) measurements revealed a high Fe content and the presence of Fe²⁺ ions in the MIL-53 structure, endowing it with the strongest catalytic capacity among the MOFs.

As previously stated, the abundant functional groups of the MOF structure allow chemical functionalization, which can modulate their properties and catalytic performance. For instance, Xu et al. synthesized two different peroxidase-like MIL-101(Fe) MOFs, one containing –NH₂ (electron-donating) and another containing –NO₂ (electron-withdrawing) functional groups.¹⁸⁰ According to the authors, the introduction of nitro groups enhanced the affinity of MIL-101 toward the substrate, increasing the MOF catalytic activity. Furthermore, the NO₂-MIL-101-based nanozyme was used as a biosensor for the determination of acetylcholinesterase (AChE) activity and OPs. The NO₂-MIL-101 showed a highly sensitive colorimetric response to AChE activity and OPs with a low LOD of 0.14 mU mL⁻¹ and 1 ng mL⁻¹, respectively.

Li et al. synthesized a very interesting MOF-818 that mimicked catechol oxidase.¹⁸¹ The MOF-818 nanozyme showed an F-centered cubic crystal structure containing a trinuclear copper center, with Cu in two oxidation states, Cu²⁺ and Cu⁺, as determined via XPS. This trinuclear structure was quite similar to the native Cu²⁺–Cu²⁺ and reduced Cu⁺–Cu⁺ states of catechol oxidase. Consequently, the MOF-818 nanozyme was capable of converting 3,5-di-tert-butylcatechol (3,5-DTBC) into 3,5-di-tert-butyl-o-benzoquinone (3,5-DTBQ) within 6 min. Moreover, the MOF-818 was found to be selectivity for DTBC oxidation, since no catalytic reaction occurred in the presence of other substrates, such as TMB, 2,2′-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), TMB/H₂O₂, and ABTS/H₂O₂. The oxidase-like activity of MOFs was also explored for detecting the AChE activity and for rhodamine B (RhB) degradation.¹⁸²

Fan and co-workers reported a comparative study of different MOF structures (ZIF-8, Ag/AgCl@ZIF-8, MOF-235, Bi₂WO₆/MIL-100(Fe), and BiOBr/MOF-5) for Microcystis aeruginosa inactivation aimed at the removal of algae from water sources.¹⁸³ The photocatalysis results showed that the Ag/AgCl@ZIF-8 displayed the best performance, reaching an algae removal of over 90% under sunlight, which was attributed to the oxidative stress caused by ROS originated from the ZIF-8 and Ag/AgCl heterojunction.

Furthermore, some MOF structures may exhibit two or more types of enzyme activity under the same conditions or in different environments.¹⁵⁴ For instance, cerium-based MOFs (Ce-BPyDC) containing Ce³⁺ and Ce⁴⁺ ions in their structure showed oxidase-like and peroxidase-like activities (Fig. 7B(i)).¹⁵⁷ The cube-like Ce-BPyDC particles of 200–250 nm size (Fig. 7B(ii)) showed a surface area of 1950 m² g⁻¹ and displayed oxidase-like properties to catalyze different substrates, including TMB, o-phenylenediamine (OPD), and ABTS, acting as an oxidase-like enzyme, as illustrated in Fig. 7B(iii). In addition, they observed that the TMB degradation activity was enhanced at pH 4. In the presence of H₂O₂, the authors found that Ce-BPyDC also showed peroxidase-like activity, with a K_m of 0.12 mM, suggesting a high affinity of Ce-BPyDC with TMB.

3.5. Other types of nanozymes

Nanotechnology has enabled the development of a plethora of materials at the nanoscale with different structures and specific properties, which have provided a new source of artificial enzymes. Over the past few years, perovskites,^184,185 metal sulfides,¹⁸⁶ metal dichalcogenides,^187,188 metal hydroxides,^189,190 metal phosphides,¹⁹¹ polymeric nanostructures^192,193 and other classes of nanomaterials have been explored in the context of artificial enzyme applications. Among them, transition metal dichalcogenides, which are semiconductors of the type MX₂, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), have emerged as a promising alternative to natural enzymes.¹⁹⁴ Recently, the intrinsic peroxidase-mimicking activity of the two-dimensional layered WS₂ nanosheets with rich active edges has been demonstrated by Tang and co-workers.¹⁹⁵ Kinetic studies revealed that the process of catalyzing the substrates TMB–H₂O₂ followed a ping-pong mechanism, indicating that WS₂ nanosheets might react with H₂O₂ to produce hydroxyl radicals, which further oxidized the second substrate to generate chromogenic products (oxTMB). The layered WS₂ nanosheets displayed higher V_max values compared with other peroxidase-mimic layered nanozymes such as GO-COOH, MoS₂ and MoO₃, indicating that the WS₂ nanosheets have a high catalytic activity. In addition, the authors employed the nanozyme to construct a colorimetric sensor for the detection of Pb²⁺ in environmental samples. Once the target heavy metal had been added, the electron transfer between the layered WS₂ nanosheets and H₂O₂ was blocked by Pb²⁺ and, consequently, the peroxidase-mimicking activity was inhibited and the substrate was no longer oxidized to produce the chromogenic product. The absorbance variation of the colorimetric sensor was proportionally related to the concentration of heavy metals, yielding a LOD of 4 mg L⁻¹, and excellent selectivity against other competitive metal ions (As³⁺, Cd²⁺, Hg²⁺, Ag⁺, K⁺, Zn²⁺, Ca²⁺, Co²⁺, Cu²⁺, Mg²⁺, Fe³⁺, and Al³⁺). Different authors have demonstrated that MoS₂ nanosheets also exhibit greatly enhanced peroxidase-like catalysis activity.^196,197

Recently, several layered double hydroxides (LDHs) have been found to exhibit enzyme-mimicking activity.¹⁹⁸ For instance, Xu et al. exploited the oxidase-like property of Fe–Co-layered double hydroxides (Fe/Co-LDH) for the detection of As³⁺ ions in water samples.¹⁹⁹ The catalytic mechanism indicated that the Fe/Co-LDH catalyzed the activation of dissolved O₂ to superoxide radicals, and the generated radicals further oxidized the colorless TMB substrate to oxTMB. Similar to other nanozymes, the oxidase-mimicking property of the Fe–Co-LDH was pH-dependent, showing a maximal catalytic activity at pH 4.

MXenes, a new family of 2D transition metal carbides, nitrides and carbonitrides with a large surface area, metallic conductivity, and antimicrobial activity, as well as acceptable biocompatibility, have aroused considerable attention for various applications.^200,201 For example, Zhu et al. reported 2D Ti₃C₂ as an oxidase mimic for the detection of α-naphthalene acetic acid (NAA) residues in farmland environments.²⁰² NAA was electrochemically detected over a wide linear range of 0.02–40 μM with a low LOD of 1.6 nM using a portable mini-workstation. In another study, the peroxidase-like activity of Ti₃C₂ nanosheets was demonstrated by Li and co-workers.²⁰³

Huang et al. demonstrated the oxidase-like activity of degradable manganite nanowires (γ-MnOOH NWs) for the catalytic oxidation of three common chromogenic substrates, i.e., TMB, ABTS and OPD.¹⁹⁰ The easy disintegration of the γ-MnOOH NWs into Mn²⁺ ions via the selective reaction with thiocholine (TCh) produced by AChE and acetylthiocholine iodide (ATCh) triggered a remarkable activity loss of the nanozymes, thus serving as a recognizer for AChE activity inhibitors. Taking advantage of this catalytic mechanism, the authors developed a colorimetric assay for the detection of OPs, as an AChE inhibitor, by measuring the change in absorbance at 652 nm or the blue color of the oxTMB products. To meet the simplicity, portability, reliability, and low-cost requirements of point-of-care testing, the nanozymes were further assembled onto a test paper and the final detection platform was proven to show high selectivity and sensitivity for the analysis of pesticide residues in vegetable samples.

Copper sulfide (CuS)¹⁸⁶ and iron phosphide (FeP)¹⁹¹ nanostructures were found to demonstrate peroxidase- and oxidase-mimicking activity, respectively. In both cases, the enzyme-mimicking activity was explored for the fluorescence detection of heavy metal ions. In the first case, silver ions could be successfully detected in the range of 0.05–75 μM with a detection limit of 5 nM.¹⁸⁶ In the second case, benefiting from the oxidase-like enzymatic activity of the FeP nanosheets, the authors developed a sensing platform for the detection of Cu²⁺.¹⁹¹ Recently, phase-engineered RuTe₂ nanorods with peroxidase-like activity were developed by Yan and collaborators.²⁰⁴ They observed that the amorphous RuTe₂ (a-RuTe₂) nanorods exhibited a superior catalytic activity over their crystalline counterpart (c-RuTe₂ nanorods). The improved performance was attributed to the large number of disordered atoms on the amorphous nanostructure.

Wei's group carried out a comprehensive study on the influence of the occupancy of e_g orbitals on the peroxidase-like activity of perovskite oxide nanozymes.¹⁸⁴ The study was conducted using ABO₃-type perovskite transition metal oxides with BO₆ octahedral sub-units (where A is a rare-earth or alkaline-earth metal and B is a transition metal). Using experimental measurements and theoretical studies, the authors identified that the catalytic activity of peroxidase-like nanozymes showed a dependence on the e_g occupancy. Namely, nanozymes with an e_g occupancy of ∼1.2 had the highest specific activity, whereas e_g occupancies of 0 or 2 corresponded to the lowest activities. They defined the specific activity as the V_max for a H₂O₂ substrate divided by the normalized BET surface area of nanozymes (i.e., BET area of nanozyme/BET area of LaNiO_3−δ). The systematic comparison of more than 20 representative peroxidase-like nanozymes revealed that LaNiO_3−δ showed the highest specific activity, which was one to two orders of magnitude higher than that of other representative peroxidase-like nanozymes, e.g., 91.4 and 49.0 times higher than that of Fe₃O₄ nanoparticles and Cu(OH)₂ supercages, respectively.

Enzyme-mimicking molecular imprinting polymers (MIPs) nanostructures, as a class of polymeric nanostructures created by polymerizing monomers around a template molecule, showed a higher catalytic hydrolysis activity and ability to eliminate organophosphate pollutants.¹⁹² The MIPs were successfully synthesized using two different macrocyclic amine functional monomers, (4-vinylbenzyl)-1,4,7-triazacyclononane (VBTN) and 1,4-bishydroxyethyl-7-(4-vinylbenzyl)-1,4,7-triazacyclononane (VBTNOH), and showed a hollowed nanocapsule morphology. The catalytic activity of MIP-VBTN and MIP-VBTNOH was found to be 377-fold and 416-fold greater compared with the self-hydrolysis of substrate methyl parathion, respectively. In addition, the hollowed nanocapsule morphology allowed the MIPs to effectively absorb p-nitrophenol from the hydrolysis product of OPs at the same time as catalytic degradation. In another study, the peroxidase-mimicking catalytic properties of chitosan were investigated through the use of a paper-based colorimetric sensor for the detection of H₂O₂.¹⁹³

Fig. 8 summarizes the experimental data (K_m and V_max values with TMB as the substrate) for other types of nanomaterials that have been applied as nanozymes for environmental applications. Regarding the binding affinity between the nanozyme and TMB, it is possible to observe that most of the nanostructures have K_m values close to that of HRP. On the other hand, with the exception of MoS₂ quantum dots, the V_max toward TMB for all the nanozymes was higher than that of HRP, which suggests that these nanozymes possess a high peroxidase-like activity. These data indicate that the remarkable peroxidase-mimicking activity originates mainly from the large surface area of the nanomaterials, which could offer a great number of active sites and facilitate the diffusion of reactants.

4. Tuning nanozyme properties for environmental applications

In order to improve the catalytic activity of nanozymes and make them superior in performance to natural enzymes, the research and engineering of nanomaterials have been greatly intensified in recent years. Typical nanoscale factors, such as size, morphology, surface modification and composition have been proved to affect the performance of nanozymes markedly, providing an efficient strategy for adjusting the nanozyme activity, as discussed in the next section.

4.1. Size and shape/morphology

The size of the particles and the arrangement of atoms at the micro/nanoscale can lead to changes in the physical, chemical, and biological properties of the materials,²⁰⁶ including the catalytic performance.³³ For example, Chen et al. demonstrated that ZIF-8 MOFs with a smaller particle size showed better catalytic activity.²⁰⁷ However, this enhancement was restricted to the range from 100 nm to 500 nm. Below 30 nm, the ZIF-8 particles tended to agglomerate, which decreased their enzyme-like performance. Similar behavior was observed for the iron–nitrogen carbon (Fe–N/C) nanozyme.¹⁷¹ With the particle size decreasing from 2.2 μm to 120 nm, the surface area increased from 216.88 to 996.02 m² g⁻¹, and the more abundant active sites of the latter contributed to a superior oxidase-like catalytic performance. However, when the particle was further reduced to 35 nm, the enzymatic activity was slightly decreased because of the agglomeration of nanoparticles.

The size of Pt nanoparticles (Pt NPs) loaded on the surface of dendritic porous silica nanospheres modified with the aminopropyl functionality (DPSNs-NH₂) also significantly affected the peroxidase-like activity of the DPSNs-NH₂@Pt nanozyme.²⁰⁸ With the increase in Pt NPs diameter, from 1.6 nm to 2.5 nm, the DPSNs-NH₂@Pt showed an increase in the TMB degradation rate. Despite the presence of more active sites (per volume) of the smaller 1.6 nm Pt NPs, the 2.5 nm Pt NPs showed a higher reaction rate. This behavior was attributed to a combination of the size and the valence state of the Pt NPs, where the 2.5 nm Pt NPs showed a higher crystallinity and lower percentage of Pt²⁺, favoring the catalytic activity.

In another study, it was demonstrated that size played a vital role on the peroxidase activity of β-casein functionalized with Au NPs (CM-AuNPs).²⁰⁹ The CM-AuNPs with a diameter of 4.2 nm presented the best peroxidase performance, followed by those with 2.8 and 8.7 nm diameters. The authors suggested that very small AuNPs (2.8 nm) have most of their surface covered with β-casein, which makes the access of the atoms to the AuNPs surface more difficult, hampering the occurrence of the catalytic reaction. In this context, Zhang et al. demonstrated the effect of the AuNP size on the oxidation of 17β-estradiol (E2).²¹⁰ The results showed that the diameter of the AuNP affected the oxidation capacity of the E2 substrate. By normalizing the catalytic activity with the catalytic area of the nanoparticles, it was shown that AuNPs with a diameter of ∼5 nm presented a ∼4.8-fold higher activity than AuNPs with a diameter of ∼13 nm.²¹⁰

The influence of size on the catalytic performance of metallic particles was also studied by Xi et al., who compared the catalytic efficiency of Pd–Ir nanoparticles with diameters of 3.3, 5.9, 9.8, and 13 nm.²⁰⁶ All the Pd–Ir NPs showed identical shapes and surface structures (e.g., the same capping agents and thickness of Ir shells), serving as an ideal system to investigate the size effect in nanozyme catalysis. Based on the k_cat values, which defined the maximum number of substrate conversions per second per catalyst, the authors found that with the increase of the Pd–Ir NP size from 3.3 nm to 13.0 nm, the k_cat increased continuously. This behavior was mainly ascribed to the enlarged catalytic surface area that enabled individual NPs to interact with more substrates.

Fig. 9A summarizes the effect of size on the K_m value for different types of nanozymes, employing TMB as substrate, in comparison with HRP. As can be noticed, the size of the nanomaterial can affect the peroxidase-like performance in different ways. For example, when evaluating the results concerning metal-based nanozymes, e.g., Au NPs, a decrease in the particle size suggests an increase in TMB affinity (K_m values close to or lower than HRP). It is important to observe that even larger particles (≥100 nm) of different materials can also show lower K_m values than HRP depending on the surface area and the active sites, as observed for Fe₃O₄ NPs (590 nm, K_m = 0.03 mM)²¹¹ in comparison with Fe₃O₄ NPs (9 nm, K_m = 0.285 mM).¹³⁰ Additionally, GQDs (2 nm) and Fe₂O₃ (5 nm) nanozymes showed V_max values higher than the other nanozymes of their respective class of materials, indicating that size control can drastically influence the reaction velocity. However, in general, peroxidase-like nanozymes with small sizes and higher affinities show V_max values smaller than or similar to the V_max of HRP, possibly as a result of the specificity and selectivity.

The catalytic performance of the nanozymes can also be modulated by adjusting the shape/morphology of the nanostructures, as highlighted in Fig. 9B. The different shapes and morphologies are associated with different amounts of dangling bonds and various arrangements of the surface atoms, which essentially determine the selectivity and reactivity of the nanozymes.^61,76 In addition, it has been recognized that the geometric and crystallographic structures affect the dissociation, coupling, and interaction of adsorbates during the catalytic events.^105,221 For instance, Biswas et al. compared the catalytic efficiency of gold nanorods (GNRs) with gold nanoparticles (GNPs).²¹⁷ GNRs with an aspect ratio of 2.8 were shown to have a 2.5 times higher peroxidase activity than GNPs. Besides, the catalytic behavior of these nanostructures was investigated by varying the pH and the temperature, showing that the intrinsic catalytic potential of GNRs is stable over a wide range of pH and temperatures, in contrast to the GNPs. As a proof of concept, the authors developed a simple and cheaper colorimetric inhibition assay for the malathion pesticide, where the presence of malathion produced an inhibitory effect on the peroxidase properties of the GNRs. The catalase-mimicking property of Co₃O₄ was also modulated as a function of their morphology.²²² The morphologies and compositions of the nanomaterials were characterized by different techniques, and the transmission electron microscopy (TEM) images are depicted in Fig. 10A. The different electron-transfer abilities of the nanomaterials explained why their catalase-like activities exhibited the following order: nanoplates > nanorods > nanocubes. On the other hand, the K_m values suggested that nanorods had the highest affinity towards the substrate (H₂O₂), which is related to the ability of H₂O₂ to coordinate most easily to the metal centers of the nanorods. This interaction might be so strong that the exchange rates were not high enough; therefore, the catalase-mimicking activity of the Co₃O₄ nanorods was lower than that of the nanoplates.

In another study, Liu et al. evaluated the catalytic activity of three distinct Fe₃O₄ nanostructures, namely cluster spheres, octahedra, and triangular plates (Fig. 10B), prepared by a hydrothermal procedure.²¹⁸ The peroxidase-like activities of the Fe₃O₄ nanomaterials were found to be structure dependent and followed the following order: cluster spheres > triangular plates > octahedra. The K_m value of the Fe₃O₄ cluster spheres towards TMB as the substrate was about two times lower than that of the triangular plates and 2.5 times lower than that of the octahedral nanostructures. Although the three Fe₃O₄ nanostructures were similar in size, according to BET analysis, the cluster spheres displayed a larger specific surface area than the triangular plates and the octahedral nanostructures. Therefore, for the cluster spheres, more catalytically active iron atoms were available at the surface to interact with the substrate, which explained their highest catalytic activity. In addition, the authors demonstrated that the triangular plates with (220) planes exhibited a superior catalytic activity to the octahedra with (111) planes. The diversity in shape is also related to the facet exposure on the surface of the nanomaterial. For instance, the oxidase-like activities of Pd nanotubes enclosed by the (100) surface and Pd octahedrons enclosed by the (111) surface showed higher oxidase-like activity due to the stronger ability of activating O₂ to generate singlet oxygen.²²³ Therefore, the uneven facet structure can lead to poor selectivity and thus limit the application of nanozymes. In this regard, several studies have reported the design of single-atom nanozymes as an efficient alternative to overcome this issue, since their catalytic activity and mechanism depend mainly on the steric configuration of the active centers, rather than the size, structure, or facet of the supported nanomaterials.^224,225

The strain effect is another important parameter to be examined in the catalytic performance of nanomaterials, as demonstrated by Xi and co-workers.⁸⁹ Their results demonstrated that strained Pd(111) icosahedra presented a higher peroxidase-like catalytic efficiency when compared with unstrained Pd(111) octahedra. DFT studies revealed that the tensile strain played an important role in the production of ˙OH and gave a larger tensile area and a greater compressive surface area, resulting in a significant improvement in the overall catalytic activity.⁸⁹

It is important to note that the change in shape/morphology may also lead to a change of size in different directions (volumetric strain). For instance, by tuning the amount of iron and cobalt precursors, CoFe₂O₄ with different shapes and sizes were synthesized by Zhang and co-workers.²²⁶ The nanostructures exhibited size- and shape-dependent peroxidase-like activity, which increased in the following order: spherical (4.1 nm), near corner-grown cubic (24.5 nm), star-like (32.1 nm), near cubic (45.2 nm) and nanopolyhedra (13.8 nm), with K_m values of 0.007, 0.017, 0.024, 0.035 and 0.055 mM, respectively.

4.2. Surface coating

The surface modification of nanozymes not only acts as a stabilizer in the nanomaterial synthesis, but is also of paramount importance for tuning their catalytic activity. Since many of the catalytic reactions take place on the surface, the surface coating of the nanomaterial may represent an efficient approach to adjust its surface charge, microenvironment, and active-site exposure.²²⁷Fig. 11 shows some examples of how the K_m values of nanozymes can be affected by the surface coating. Different strategies have been used for surface modification and, therefore, to modulate the nanozyme activity and selectivity, such as changing the electronic structure of their surfaces, tuning of surface acidity, blocking surface access, and/or promoting product desorption.²²⁸

Yu and co-workers evaluated the peroxidase-like activity of MoS₂ nanoflakes coated with polyethyleneimine (PEI) (positive charge), polyvinylpyrrolidone (PVP) (neutral charge), polyacrylic acid (PAA) (negative charge), and cysteine (Cys) (negative/positive charge).²¹⁵ The authors reported that PVP, PAA, and PEI modification inhibited the catalytic activity of the MoS₂ compared with that of raw MoS₂, while the catalytic activity of Cys-MoS₂ was promoted. As shown in Fig. 11, the K_m of MoS₂ decreased from 0.22 to 0.17 mM after coating with Cys, indicating a better affinity between the nanozyme and the chromogenic substrate (TMB). This behavior was explained in terms of the interaction of the sulfur-containing Cys group with both dangling Mo bonds at edge sites and the d_z² orbitals of Mo that protrude through the van der Waals surface of MoS₂, providing a pure electron-transfer bridge for the redox couple. Likewise, iron oxide NPs coated with citrate (ζ = −22.7 mV), carboxymethyl (ζ = −23.8 mV), heparin (ζ = −51.2 mV), glycine (ζ = +25 mV), polylysine (ζ = +28.9 mV) and poly(ethyleneimine) (ζ = +47.1 mV) showed different catalytic performances as a function of the surface coating.²²⁹ The authors observed that the NPs coated with positively charged ligands showed a better catalytic efficiency for ABTS. For instance, the NPs coated with poly(ethyleneimine) displayed an 11.5-fold higher activity than NPs functionalized with heparin. On the other hand, with TMB as the substrate, anionic nanoparticles had a high affinity and exhibited a high catalytic activity, and, therefore, the functionalization with heparin led to a 5.9-fold higher catalytic activity compared with modification with poly(ethyleneimine).

In another study, the influence of the surface coating was also evaluated on the peroxidase-mimicking activity of road-shaped Fe₃O₄ microcrystals.²³¹ Negatively charged Fe₃O₄ microrods coated with a thin graphite layer showed a superior peroxidase activity confirmed via the oxidation of TMB and pyrogallol. The high peroxidase activity of the microrods was correlated with the presence of a large number of Fe²⁺ and Fe³⁺ ions on their surface, compared with only one iron in HRP. This property was taken as advantageous for the degradation of organic pollutants, such as cationic and anionic dyes: rhodamine B (RhB) (degradation efficiency (DE) = 98% and K = 0.038 min⁻¹), methylene blue (MB) (DE = 77% and K = 0.011 min⁻¹) and methyl orange (MO) (DE = 60% and K = 0.007 min⁻¹). The lower degradation efficiency of MO was attributed to its anionic nature, which generated an electrostatic repulsion that prevented its adsorption onto the nanorods’ surface. On the other hand, the greater degradation efficiency towards RhB in comparison with MB, both cationic dyes, was attributed to the presence of carboxyl groups in RhB which might influence the formation of hydroxyl radicals and facilitate its attack on the dye molecules through hydrogen bonding.

In another report, three polysaccharides (dextran, hyaluronic acid, and chitosan) were employed for the surface modification of iron oxide nanoparticles (IONPs).²³² The affinity of the substrates for the catalytic site of the nanoparticles was found to be determined by the surface functional groups and the hydration layer of the polysaccharide coating on their surface. The low affinity of TMB for the hyaluronic acid@IONPs and chitosan@IONPs in comparison with that for the dextran@IONPs resulted from the thick hydration layer and/or high water-holding capacity of hyaluronic acid and chitosan, which decreased the diffusion rate of TMB to the surface of the IONPs. This hypothesis was also supported by the fact that the peroxidase-like activity of hyaluronic acid@IONPs and chitosan@IONPs was enhanced by the addition of ammonium chloride, known to reduce the thickness of the hydration layer, increasing the diffusion rate of TMB to the surface of the IONPs.

The enzyme-like activity of Au-core CeO₂-shell nanoparticles was also investigated upon changing the surface capping ligand.²³³ Due to the strong affinity of the thiol groups with the gold surface, it was observed that the shell layer of CeO₂ was also replaced during the replacement of cetyltrimethylammonium bromide (CTAB) by mercaptoundecanoic (11-MUA), which compromised the superoxide and catalase enzyme-like activities of the nanozyme, while the peroxidase activity was unaffected.

Liu and co-workers evaluated the peroxidase-like activities of FePt nanoparticles coated with dopamine and 3,4-dihydroxyhydrocinnamic (3,4-DHCA).²³⁰ The authors found that the K_m of FePt functionalized with dopamine was lower (0.079 mM) than that observed for FePt-3,4-DHCA (0.121 mM), indicating that the negative charge contributed to the interaction with the TMB substrate. Moreover, the V_max and catalytic efficiency of FePt-dopamine were also higher than FePt-3,4-DHCA. They observed the same behavior for Fe₃O₄ nanoparticles, as illustrated in Fig. 11.

4.3. Doping

Another interesting strategy to enhance, regulate, or even generate the enzyme-like activity of a nanozyme is through doping, i.e., the intentional introduction of species, generally heteroatoms, into the structure of the nanozyme. Doping can be used to provide the catalytic center, improve the electron transfer between the nanozyme and the substrate, and/or generate additional active sites.^9,27,82,234Fig. 12 shows some examples of nanozymes that had their peroxidase-like activity affected after doping, according to the decrease in the K_m value. As can be noticed, in some cases the affinity for the TMB substrate of the doped nanozyme becomes even higher than that of natural HRP enzyme, as the K_m value of the doped nanomaterial can be greatly lowered compared with the undoped nanozyme.

Doping nanozymes with heteroatoms has been extensively reported in the literature,^{71,95,239–241} and choosing the doping material depends on the application intended for the nanozyme. In this sense, the investigation of the catalytic mechanism can be important to discover the best strategy for doping.^9,82 It is well known that the catalytic center of the natural HRP enzyme is the iron porphyrin, while the heme edges act as the substrate-binding sites.⁹ Inspired by this, Qu et al. designed Fe³⁺-doped mesoporous carbon nanospheres to mimic the HRP enzyme.²⁴² In the system, Fe³⁺ acted as the catalytic center, while the carboxyl-modified carbon nanospheres were used to bind with the substrates. The catalytic mechanism was associated with the decomposition of the H₂O₂ through the breaking of the O–O bonds, generating ˙OH radicals that are stabilized in the nanozyme favoring the reaction with TMB.^239,240 For instance, the N doping boosted the peroxidase-mimicking activity of reduced graphene oxide (rGO) by stabilizing the radical oxygen species adjacent to the N sites. DFT calculations revealed the selective activation of H₂O₂ rather than O₂ and O₂˙⁻ in the catalytic mechanism due to the N doping.²³⁷

Li et al. fabricated Co-based homobimetallic hollow nanocages (HNCs) as nanozymes.¹⁸² Besides the Co metal, Ni, Mn, Cu, and Zn were also used to obtain doped-nanozymes (C–CoM–HNC, with M being Ni, Mn, Cu, or Zn). The oxidase-like activities of the nanozymes were calculated and C–CoCu–HNC exhibited the lowest K_m value (0.0741 mM, one-fifth of the nanozyme doped only with Co) and the highest V_max (2.56 × 10⁻⁸ M s⁻¹). The excellent catalytic activity enabled C–CoCu–HNC to be used for the degradation of RhB. Again, the C–CoCu–HNC nanozyme presented the highest degradation efficiency towards RhB. Using the peroxymonosulfate (PMS) activator, SO₄˙⁻ was produced, which enabled RhB degradation. Under the optimized conditions, the RhB concentration decreased 93.4% after 60 min using C–CoCu–HNC and PMS.

To enhance the catalytic activity of Fe–N–C single-atom catalysts, Jiao and co-authors used boron as a dopant.²³⁹ The doping induced charge-transfer effects and reduced the energy barrier for the formation of the hydroxyl radical by modulating the positive charge of the Fe atom, boosting the peroxidase-like activity of the nanozyme. A comparison between the undoped (FeNC) and the B-doped (FeBNC) nanozymes was performed using the absorbance value of the quantification of the oxidation state of TMB at 652 nm. The specific activity of the doped nanozyme (15.41 U mg⁻¹) was found to be much higher than the undoped SA (4.09 U mg⁻¹). The FeBNC nanozymes were also used to detect OPs. Activation of the peroxidase-like activity was performed via a decrease in the pH value using AChE. Since the OPs inhibited the activity of AChE, the increment in their concentration led to a hampering of the peroxidase-like activity of the nanozyme. Sensing experiments demonstrated that the OPs could be detected over a linear range from 8 to 1000 ng mL⁻¹ with a low LOD of 2.19 ng mL⁻¹.

Doped nanozymes have also been used as antimicrobial materials. As mentioned before, during the kinetic reaction, a nanomaterial with peroxidase-like activity can generate ROS that contribute to the antibacterial mechanism.^243–245 In this context, Liu et al. used degradable copper-doped phosphate-based glass (Cu-PBG) nanozymes with outstanding antibacterial efficacy against Gram-positive and Gram-negative bacteria.²⁴⁵ In this case, besides the generation of the hydroxyl radicals via the decomposition of H₂O₂, the antibacterial mechanism was also based on the release of copper. In acidic environments, the nanozymes caused lethal oxidative stress to the bacteria via catalyzing the decomposition of H₂O₂.

4.4. Synergistic effect of hybrid nanozymes

As the catalytic function of single nanozymes may be limited, the rational design and creation of new dual or multi-functional nanozymes have been pursued lately. Hybrid nanomaterials have attracted widespread interest since they harmonically combine the properties of distinct materials (in physical–chemical nature), and therefore promote synergistic effects between them. Inspired by this, nanozymes with improved catalytic capacities for environmental applications have been designed by the formation of hybrid nanostructures. As can be seen in Fig. 13, the synergistic effect led to substantial improvements in the performance of nanozymes reflected by the lower K_m values and, consequently, the higher affinity for the TMB substrate.

Although 2D nanomaterials can serve as promising enzyme mimics, their catalytic activity may be affected by agglomeration. In order to overcome this problem, Huang et al. reported the synthesis of chitosan-functionalized molybdenum(IV) selenide nanosheets (CS-MoSe₂ NS).²⁴⁸ The chitosan molecules bound to the nanomaterial, which facilitated the homogeneous dispersion of MoSe₂ sheets and protected them from aggregation caused by van der Waals or hydrophobic interactions. Moreover, chitosan molecules could act as an affinity agent to “capture” mercury ions, allowing monitoring of the Hg²⁺ concentration under optimized conditions. In addition, the successfully integrated CS-MoSe₂ NS/smartphone sensor provided sensitive, and rapid, reliable detection of Hg²⁺ on-site with a LOD of 8.4 nM, which performed well in real water samples. In another report, Wang et al. noticed that ZIF-8 MOFs protected carbon dots from the aggregation and quenching process, which increased the participation of carbon dots during catalytic reactions. The combination of carbon dots and ZIF-8 allowed rapid electron transfer, and their synergetic effect endowed the composite materials with a superior peroxidase-like activity.²⁴⁹ The synergy between conductive polymers and MOFs has also been explored in the context of potential catalytic applications.²⁵⁰

The catalytic activity of some nanomaterials may also be limited by the surrounding pH. In order to achieve a high catalytic performance over a broad pH range, Li and co-workers synthesized a hybrid nanomaterial with synergistically enhanced peroxidase-like activity based on Pd nanoparticles dispersed on CeO₂ nanotubes (Pd NPs/CeO₂ NTs).²⁴⁶ In their system, the interplay between the Pd NPs and the CeO₂ NTs entities resulted in a significant increase in the Ce³⁺/Ce⁴⁺ ratio, which further promoted the enzymatic activity and efficiency of the Pd NPs/CeO₂ NTs hybrid. The proposed Pd NPs/CeO₂ NTs exhibited no significant decline in activity when incubated in buffers with a wide range of pH or at different temperatures, confirming the excellent robustness of the peroxidase mimic.

Graphene-based nanomaterials have been widely explored in combination with other nanomaterials to optimize their enzymatic mimic activity.^81,95,130 On this basis, Boruah et al. prepared a Fe₃O₄/TiO₂/reduced graphene oxide (Fe₃O₄/TiO₂/rGO) nanocomposite with intrinsic peroxidase-mimicking activity and photocatalytic efficiency.¹³⁰ The synthesized nanocomposite showed a dual response including the highly sensitive colorimetric detection of harmful atrazine pesticide in an aqueous medium as well as the photocatalytic degradation of the pesticide. The authors found that the nanocomposite exhibited superior detection and degradation activity compared with Fe₃O₄/rGO, TiO₂/rGO, Fe₃O₄, TiO₂ and rGO. This occurred due to the synergistic effect of Fe₃O₄, TiO₂ and rGO sheets, which enhanced the ˙OH generation capability of the nanocomposite in the presence of H₂O₂. The rGO sheets also improved the interaction of the composite materials with TMB molecules by π–π interactions. Xiang et al. demonstrated the superior catalytic performance of a bimetallic-MOF assembled with GO (CuCo-BDC/GO).²⁵¹ The electrocatalytic property of CuCo-BDC/GO toward H₂O₂ was investigated using cyclic voltammetry and the superior performance of the nanocomposite was attributed to the following factors: (i) the MOF structure provided plenty of metal active sites (Cu²⁺ and Co²⁺) as the catalytic center with peroxidase-like activity; (ii) the synergetic catalysis of the two metal ions enhanced the catalytic capacity and acted as a current amplifier; (iii) CuCo-BDC with a hierarchical structure had a large specific surface area and porosity, which further increased the active sites of the composites; and (iv) GO also had a large specific surface area, an excellent catalytic performance and high conductivity. To improve the selectivity and sensitivity for the naked-eye detection of Hg²⁺, Zhao and co-workers synthesized Ag₂S nanoparticles functionalized with graphene oxide (Ag₂S@GO). The hybrid material was identified by virtue of the high oxidase-like activity of nano Ag₂S and the excellent physicochemical properties of GO.²⁵²

In another report, the peroxidase-like activity of CuS nanoparticles was tuned via functionalization with 2D graphitic carbon nitride (g-C₃N₄) and hexagonal boron nitride nanosheets (h-BN).²⁵³ The synthesized CuS/h-BN and CuS/g-C₃N₄ nanocomposites enhanced the decomposition of H₂O₂ to generate hydroxyl radicals catalyzed by Cu²⁺ ions on the catalyst surface through a Fenton-type reaction. Under the optimum experimental conditions, the absorbance value of oxTMB was found to be the highest in the presence of CuS/h-BN as compared with CuS/g-C₃N₄. The superior catalytic property of CuS/h-BN was ascribed to the large surface area and thus greater deposition of H₂O₂ molecules on the catalyst surface. In addition, the surface charge (zeta potential) of the catalyst surface influenced the electrostatic interaction between TMB molecules and the catalyst surface. More importantly, the synthesized hybrid nanomaterials were employed for the colorimetric detection of ibuprofen, a pharmaceutical pollutant, in an aqueous medium.

5. Summary and future perspectives

The unique physical and chemical features of nanozymes have opened new prospects in the environmental field, ranging from the detection to the degradation of various pollutants. In this review, we have illustrated how different classes of nanomaterials can be used as nanozymes for environmental applications. According to the Web of Science, since the unexpected discovery of the peroxidase enzyme-like activity of Fe₃O₄ nanoparticles in 2007, more than 1380 works have been published on nanozymes. The attractive enzyme-mimetic feature of different nanostructures, together with their versatile nature, has rapidly qualified them for varied applications, especially in environmental analysis and remediation, as highlighted through this review.

The importance of this emerging field can be visualized in Fig. 14A by the number of publications/citations in the field over the past decade, where the majority of the nanozymes have been developed using metal oxide-based nanomaterials (Fig. 14B), possibly due to their low cost, low toxicity, ease of synthesis and chemical stability. However, by comparing the graphs prepared for Section 3, it is possible to conclude that there is a higher number of enzymes based on carbon nanomaterials with a stronger affinity and faster catalysis towards TMB. Moreover, as demonstrated in Section 4.3, heteroatomic doping is one of the most effective strategies to improve the catalytic performance of carbon-based nanozymes, to which much effort has been devoted in the past few years. Therefore, we believe that the number of works related to the use of carbon-based nanozymes will significantly increase in the coming few years.

As highlighted in Section 4, several studies have demonstrated that a better catalytic activity for the nanozyme was achieved through a smaller size, larger surface area, higher surface-to-volume ratio, preferential exposure of catalytically active atoms, more open crystal planes and dangling bonds, which could facilitate interactions with large amounts of the substrate. In addition, recently the development of new hybrid nanostructures has gained importance and has witnessed great progress due to advances in nanoscience and nanotechnology. The successful development of hybrid nanozymes is intrinsically linked to the availability of nanomaterials, as highlighted in this review, and processing techniques. Indeed, hybrid nanozymes can be processed using many methodologies, including film-deposition methods,^32,254 electrospinning,^10,255 electrochemical deposition,^28,256 hydrothermal/solvothermal synthesis,^28,257 among others.

Despite these advantages, important challenges still remain for efficiently directing the development of nanozymes for real environmental applications. For instance, although numerous nanozymes have been proposed, the detailed systemic mechanisms of some of them are not yet fully understood. Therefore, a deeper investigation into the catalytic mechanisms and understanding of the structure–activity relationships of nanozymes, and discovery of the key factors that determine their activities are essential for the rational design of ideal nanozymes. Besides, most of them only exhibit oxidoreductase-like activities. In light of the huge number of chemical reactions catalyzed by enzymes, further investigations are indeed essential for constructing novel nanozymes with a wider range of enzymatic activities. The lack of substrate specificity is another obstacle for the application of nanozymes. Moreover, future efforts should also be aimed at translating such exciting advances into practical environmental applications, which will require close collaboration with environmental scientists and engineers, along with new technological breakthroughs and greater sophistication in nanozymes. Despite the unresolved issues and challenges, the unique properties and functions of these enzyme mimics and the promising results suggest that this field will continue to flourish in terms of research and applications for the detection of threats and for remediation processes, mainly via adsorptive removal and enhanced pollutant degradation. We believe that multifunctional nanozymes, capable of performing multiple tasks such as the “sensing, isolation and destruction” of environmental pollutants and chemical threats, will be the target pursued by scientists and engineers in the coming years.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (Grants 2017/10582-8, 2018/18468-2, and 2018/22214-6), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (CNPq/445982/2020-9), MCTI-SisNano (CNPq/402287/2013-4), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Código de Financiamento 001 and Rede Agronano-EMBRAPA from Brazil.

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Footnote

† These authors contributed equally to this work.

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