Research Progress in Iron-Based Nanozymes: Catalytic Mechanisms, Classification, and Biomedical Applications

Natural enzymes are crucial in biological systems and widely used in biology and medicine, but their disadvantages, such as insufficient stability and high-cost, have limited their wide application. Since Fe₃O₄ nanoparticles were found to show peroxidase-like activity, researchers have designed and developed a growing number of nanozymes that mimic the activity of natural enzymes. Nanozymes can compensate for the defects of natural enzymes and show higher stability with lower cost. Iron, a nontoxic and low-cost transition metal, has been used to synthesize a variety of iron-based nanozymes with unique structural and physicochemical properties to obtain different enzymes mimicking catalytic properties. In this perspective, catalytic mechanisms, activity modulation, and their recent research progress in sensing, tumor therapy, and antibacterial and anti-inflammatory applications are systematically presented. The challenges and perspectives on the development of iron-based nanozymes are also analyzed and discussed.

Kanamycin triggered nanozyme for homogeneous and amplified colorimetric detection of T4 polynucleotide kinase

It is of significance to develop efficient methods for detecting the activity of T4 polynucleotide kinase (T4 PNK) due to its essential role in the modulation of different life activities. In this work, we constructed a novel nanozyme using Kanamycin (KANA) as a trigger for the [Fe(CN)₆]^3- coordinated Cu₂(OH)₃NO₃ (Cu₂(OH)₃NO₃/[Fe(CN)₆]^3-) nanorods, and designed an amplified colorimetric method to detect T4 PNK. That was, the free KANA efficiently triggered the peroxidase-like activity of Cu₂(OH)₃NO₃/[Fe(CN)₆]^3-, while the bound KANA by its aptamer lost the stimulative capability for the nanomaterials. On the basis of the bioreaction regulated generation of the KANA aptamer, a highly sensitive colorimetric assay aided by the rolling circle amplification (RCA) reaction for the detection of T4 PNK was realized. Results showed that this assay can detect T4 PNK from 1.0 × 10^-3 to 10.0 U/mL, with a limit of detection (LOD) of 1.42 × 10^-4 U/mL. The assay also showed acceptable performance in the detection of T4 PNK in serum samples. In addition to the satisfactory sensitivity and selectivity, the displayed T4 PNK assay also presented merits of operational convenience, without labeling or immobilization process and did not require costly instrument. It is expected that the KANA as a stimulator would have extended biosensing applications by coupling various bioreactions that can produce the KANA aptamer.

Ru(bpy)32+ as a photoinduced oxidase mimic for colorimetric detection of biothiols

We have found that tris (2,2'-bipyridyl) ruthenium (II) (Ru(bpy)₃²⁺) possesses a high photo-induced oxidase-like activity and is capable of catalyzing the color reaction of 3,3',5,5'-tetramethylbenzidine (TMB) with dissolved oxygen. Ru(bpy)₃²⁺ has a catalytic constant (K_cat) that is twice as high as that of fluorescein, 170 and 275-fold higher than that of 9-mesityl-10-methyl acridine and Eosin Y, respectively. Electron spin resonance spectroscopy (ESR) and radical scavenging experiments have verified the major active radicals involved in the color reaction are •OH. A colorimetric biothiol assay has been successfully developed for the oxidase-like activity of Ru(bpy)₃²⁺ can be suppressed by sulfhydryl compounds. A linear dependence between the decrease in absorbance and the logarithm of thiol concentrations can be found ranging from 5.0 to 50 μM, with a detection limit of 1.0 μM. This work reveals a new oxidase mimic with high catalytic activity and will facilitate the utilization of this oxidase mimic in biochemical analysis.

Self-coordinated nanozyme on Cu3BiS3 nanorods for high-performance aptasensing

A novel strategy is reported to access high-performance nanozymes via the self-coordination of ferrocyanides ([Fe(CN)₆]^4-) onto the surface of the Cu₃BiS₃ (CBS) nanorods. Notably, the in situ formed nanozymes had high catalytic activity, good stability, low cost, and easy mass production. The formed nanozyme catalyzed the oxidation of the typical chromogenic substrate of 3,3',5,5'-tetramethylbenzidine (TMB) with a distinctive absorption peak at 652 nm, accompanied by a blue color development. Moreover, the attachment of deoxyribonucleoside 5'-monophosphates (dNMP) beforehand onto the surface of CBS prevented coordination of ferrocyanides and resulted in the tunable formation of the nanozyme, thereby enabling the construction of an exquisite biosensing platform. Taking the aptasensing of chloramphenicol (CAP) as an example, the engineered nanozyme allowed the construction of a homogenous, label-free, and high-performance bioassay in terms of its convenience and high sensitivity. Under the optimal conditions, changes in the absorption intensity at 652 nm for the oxidized TMB provides a good linear correlation with the logarithm of CAP concentrations in the range 0.1 pM to 100 nM, and the limit of detection was 0.033 pM (calculated from 3σ/s). Considering a vast number of bioreactions can be connected to dNMP production, we expect the engineerable nanozyme as a universal signal transduction scaffold for versatile applications in bioassays. Through the attachment of deoxyribonucleoside 5'-monophosphate (dNMP) on the surface of CBS to regulate the generation of self-coordinated nanozyme CBS/BiHCF, a homogeneous, label-free, and high-performance universal aptasensing platform was constructed.

Tin Porphyrin-Based Nanozymes with Unprecedented Superoxide Dismutase-Mimicking Activities

As the oxidative stress is related to human aging and many diseases, a diversity of antioxidant biomimetic enzymes to eliminate reactive oxygen species in vivo and maintain the redox balance has attracted intensive attention. Of particular interest are superoxide dismutase (SOD)-mimicking artificial enzymes that bear inherent characteristics of natural counterparts but overcome their deficiencies in thermal and acidic stability. Inspired by the metallized active center of natural SODs, here, we engineered different groups of metalloporphyrins and found that Sn-metallized porphyrins can act as novel SOD mimics, in which Sn-metallized meso-tetra(4-carboxyphenyl) porphine (Sn-TCPP) can more effectively catalyze the disproportionation of superoxide radical anions (•O₂^-) into hydrogen peroxide and oxygen. Especially, Sn-TCPP-based metal-organic frame nanozyme (Sn-PCN222) displays an unusually high catalytic activity that remarkably exceeds those of commonly used counterparts. Such unprecedented catalytic behaviors are proposed to depend on the Sn(IV)/Sn(II) transition at the center of Sn-TCPP. In addition, the metal-organic framework (MOF) nanozymes also display higher thermal and acidic stability than natural SODs. Interestingly, we find that Sn-complexed methylated tetra-(4-aminophenyl) porphyrin shows an aggregation-induced SOD activity in an acidic environment, whereas conventional SOD mimics do not function well in this case. Given these unique features, our reported Sn-porphyrin-based nanozymes would be potent alternatives for natural SODs to be widely used in clinical treatments of oxidative stress-related diseases.

In-situ generation of highly active and four-in-one CoFe2O4/H2PPOP nanozyme: Mechanism and its application for fast colorimetric detection of Cr (VI)

Nanozymes have been widely utilized in colorimetric sensors and developing nanomaterials with multienzyme functions have more application prospects due to their cascaded catalytic efficiency. Here, a unique organic-inorganic nanocomposite CoFe₂O₄/H₂PPOP was synthesized by depositing CoFe₂O₄ nanocubes on a fully conjugated porphyrin-based porous organic polymer (H₂PPOP) in situ. CoFe₂O₄/H₂PPOP revealed outstanding tetra-enzyme-like activities, namely oxidase-like, peroxidase-like, catalase-like and superoxide dismutase-like activities. Compared with pure CoFe₂O₄ nanocubes, the catalytic activities of CoFe₂O₄/H₂PPOP were significantly boosted because of the large surface area and extended conjugated structure of H₂PPOP, abundant active substances (CoFe₂O₄) on the surface and the effective electronic transfer between CoFe₂O₄ and H₂PPOP. Based on the oxidase-like activity of CoFe₂O₄/H₂PPOP, a colorimetric platform was constructed for Cr (VI) with a wide linear range (0.6-100 μM) and a low detection limit (26 nΜ). Further utilizing the double oxidase-like and peroxidase-like activities, a more sensitive colorimetric platform with a faster detection speed for Cr (VI) was realized with the LOD as low as 2 nΜ. This work opens up a new way to prepare multi-enzyme active nanozyme and excavates its potential for detecting environmental pollutants.

A label-free and modification-free ratiometric electrochemical strategy for enhanced natural enzyme detection using a bare electrode and nanozymes system

Ratiometric electrochemical assays have been demonstrated to be more sensitive and selective in various sensing events, mainly due to their affordable built-in correction and good self-reference capability. But it is known that complicated modification and labeling operations usually are necessary for the construction of ratiometric electrochemical assays, therefore is a hot and important issue needing consideration carefully. We herein report a new yet simple bare electrode-based ratiometric electrochemical bioassay to achieve sensitive and selective analysis of alkaline phosphatase (ALP), using a liquid phase system that contains CoOOH nanozymes and commercially available indicator substrate. This proposed bioassay works based on the ratiometric change of dual electrochemical signals, arising from an exclusive target ALP-triggered hydrolysis of electrochemical substrate p-nitrophenyl phosphate (PNPP). In this design, the two hydrolyzed products of electrochemically active p-nitrophenol (PNP) and electrochemically inactive phosphate anion (PO₄^3-) are responsible together for the ratiometric electrochemical analysis of ALP. PNP exhibits a straightforward current response toward ALP content; however, PO₄^3- cannot show a direct electrochemical signal thus is rationally designed to offer an alternative response by linking it with the specific CoOOH nanozyme-catalyzed reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂, in which the nanozyme-catalyzed product oxTMB shows a direct reduction current at the GCE, and significantly decreases with increasing PO₄^3- species due to the good inhibition of PO₄^3- toward CoOOH nanozyme activity. As a result, a ratiometric electrochemical strategy for ALP analysis with a low limit of detection of 0.366 U/L (S/N = 3) was successfully achieved by integrating the above direct and indirect dual electrochemical responses. This developed bioassay can allow the quantitative diagnosis of ALP activity especially with a label-free and modification-free merit, therefore paving the way for simple, convenient, and portable electroanalytical tools in biosensing design and application.

Unveiling the interplay between homogeneous and heterogeneous catalytic mechanisms in copper–iron nanoparticles working under chemically relevant tumour conditions

This work studies in depth the interplay between glutathione (GSH) and a copper–iron oxide nanocatalyst. We describe the activation and interplay of homogeneous and heterogeneous processes that induce a cascade of reactions against cancer cells.

Water-soluble PANI:PSS designed for spontaneous non-disruptive membrane penetration and direct intracellular photothermal damage on bacteria

The major challenge in the field of antibacterial agents is to overcome the low-permeability of bacteria cell membranes that protects the cells against diverse drugs. In this work, water-soluble polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) is found to spontaneously penetrate bacteria cellular membranes in a non-disruptive way, leaving no evidence of membrane poration/disturbance or cell death, thus avoiding side effects caused by cationic ammonia groups in traditional ammonia-containing antibacterial agents. For aqueous synthesis, which is important for biocompatibility, the polymer is synthesized via an enzyme-mimetic route relying on the catalysis of a nanozyme. Owing to its fluorescent properties, the localization of as-prepared PANI:PSS is determined by the confocal microscope, and the results confirm its rapid entry into bacteria. Under 808 nm near-infrared (NIR) irradiation, the internalized PANI:PSS generates local hyperthermia and destroys bacteria highly efficiently from inside the cells due to its excellent photothermal effects. Staphylococcus aureus (S. aureus), M ethicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli (E. coli) could be effectively eliminated as well as the corresponding bacterial biofilms. Results of in vivo antibacterial experiments demonstrate excellent antibacterial activities of the water-soluble PANI:PSS without side effects. Therefore, the prepared water-soluble polymer in this study has great potential in the treatment of various bacterial infections.

Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

Nanomaterial-based artificial enzymes (or nanozymes) have attracted great attention in the past few years owing to their capability not only to mimic functionality but also to overcome the inherent drawbacks of the natural enzymes. Numerous advantages of nanozymes such as diverse enzyme-mimicking activities, low cost, high stability, robustness, unique surface chemistry, and ease of surface tunability and biocompatibility have allowed their integration in a wide range of biosensing applications. Several metal, metal oxide, metal-organic framework-based nanozymes have been exploited for the development of biosensing systems, which present the potential for point-of-care analysis. To highlight recent progress in the field, in this review, more than 260 research articles are discussed systematically with suitable recent examples, elucidating the role of nanozymes to reinforce, miniaturize, and improve the performance of point-of-care diagnostics addressing the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to the end user) criteria formulated by World Health Organization. The review reveals that many biosensing strategies such as electrochemical, colorimetric, fluorescent, and immunological sensors required to achieve the ASSURED standards can be implemented by using enzyme-mimicking activities of nanomaterials as signal producing components. However, basic system functionality is still lacking. Since the enzyme-mimicking properties of the nanomaterials are dictated by their size, shape, composition, surface charge, surface chemistry as well as external parameters such as pH or temperature, these factors play a crucial role in the design and function of nanozyme-based point-of-care diagnostics. Therefore, it requires a deliberate exertion to integrate various parameters for truly ASSURED solutions to be realized. This review also discusses possible limitations and research gaps to provide readers a brief scenario of the emerging role of nanozymes in state-of-the-art POC diagnosis system development for futuristic biosensing applications.

Superoxide dismutase nanozymes: an emerging star for anti-oxidation

Superoxide dismutases (SODs) are a group of metalloenzymes that catalyze the dismutation of superoxide radicals (O2˙-) into hydrogen peroxide (H2O2) and oxygen (O2). As the first line of defense against reactive oxygen species (ROS)-mediated damage, SODs are expected to play an important role in the treatment of oxidative stress-related diseases. However, the clinical applications of SODs have been severely limited by their structural instability and high cost. Compared with natural enzymes, nanozymes, nanomaterials with enzyme-like activity, are more stable, and economical, can be easily modified and their activities can be adjusted. Due to their excellent characteristics, nanozymes have attracted widespread attention in recent years and are expected to become effective substitutes for natural enzymes in many application fields. Importantly, some nanozymes with SOD-like activity have been developed and proved to have a mitigating effect on diseases caused by oxidative stress. These studies on SOD-like nanozymes provide a feasible strategy for breaking through the dilemma of SOD clinical applications. However, at present, the specific catalytic mechanism of SOD-like nanozymes is still unclear, and many important issues need to be resolved. Although there are many comprehensive reviews to introduce the overall situation of the nanozyme field, the research on SOD-like nanozymes still lacks a systematic review. From the structure and mechanism of natural SOD enzymes to the structure and regulation of SOD-like nanozymes, and then to the measurement and application of nanozymes, this review systematically summarizes the recent progress in SOD-like nanozymes. The existing shortcomings and possible future research hotspots in the development of SOD-like nanozymes are summarized and prospected. We hope that this review would provide ideas and inspirations for further research on the catalytic mechanism and rational design of SOD-like nanozymes.

Smart nanozyme of silver hexacyanoferrate with versatile bio-regulated activities for probing different targets

Smart nanozymes that can be facile and rapidly produced, while with efficiently bio-regulated activity, are attractive for biosensing applications. Herein, a smart nanozyme, silver hexacyanoferrate (Ag₄[Fe(CN)₆]), was constructed in situ via the rapid, direct reaction between silver(I) and K₄[Fe(CN)₆]. And the activity of the nanozyme can be rationally modulated by different enzymatic reactions including the glucose oxidase (GOx, taken as a model oxidoreductase), alkaline phosphatase (ALP), and acetylcholinesterase (AChE). On the basis of which, a multiple function platform for the highly sensitive detection of glucose, ALP and AChE were developed through colorimetry. Corresponding detection limits for the above three targets were found to be as low as 0.32 μM, 3.3 U/L and 0.083 U/L (S/N = 3), respectively. The present study provides a novel nanozyme that can be produced in situ, which rules out the harsh, cumbersome, and time-consuming synthesis/purification procedures. In addition, it establishes a multiple function platform for the amplified detection of versatile targets by the aid of the developed nanozyme, whose detection has the advantages of low cost, ease-of-use, high sensitivity, and good selectivity.

The chain-like Au/carbon dots nanocomposites with peroxidase-like activity and their application for glucose detection

The nanocomposites with highly synergistic effect show important potential application as nanozymes. Herein, the chain-like Au/carbon dots (CDs) (GCDs) nanocomposites were prepared by self-assembly method. The negatively charged Au nanoparticles (NPs) and positively charged CDs were connected by the electrostatic interaction. Then, the electron transfer between Au and CDs induces the strong catalytic effect of GCDs nanocomposites. The cross-linking reaction occurred between amino groups and carboxyl groups on the surface of CDs, which led to form the chain-like Au aggregation surrounded by carbon shells. By FDTD simulation, the aggregation of Au NPs may enhance the electromagnetic field so that the surface-enhanced Raman scattering (SERS) signal can be increased based on GCDs nanocomposites as substrate. Otherwise, the GCDs nanocomposites can also be used to catalyze the oxidation of colourless3,3^',5,5'-tetramethylbenzidine (TMB) to blue oxTMB in the presence of H₂O₂, which displays the enhanced peroxidase-like activity compared with alone Au NPs or CDs. The obvious oxidation process of TMB molecules may be monitored by the change of SERS signal during the catalytic reaction. On this basis, GCDs nanocomposites can be further used for detection of glucose. The detection level of glucose is obtained as low as 5 × 10^-7 M. Therefore, this provides a method to detect the glucose based on GCDs nanocomposites as an enzyme mimic.

Enhancing the peroxidase-like activity and stability of gold nanoparticles by coating a partial iron phosphate shell

Using citrate-capped gold nanoparticles (AuNPs) as peroxidase-mimicking enzymes to design biosensors is hindered by their low catalytic activity and poor colloidal stability, resulting in limited sensitivity and large variations. Herein, the growth of a partial iron phosphate (FeP) shell with Fe²⁺ ions on citrate-capped AuNPs boosted the activity of the AuNPs by up to 20-fold. The FeP-enhanced activity was demonstrated on AuNPs of different sizes, and gold nanostars. When the FeP layer is thick enough to block the access to the Au/FeP interface, the activity was inhibited. Capping the remaining Au surface by thiol also inhibited the activity, suggesting that faster reactions occurred at the interfaces of Au/FeP. Moreover, a FeP shell can stabilize AuNPs against freezing and a high NaCl concentration of 1 M. Sensitive detection of Fe²⁺ was achieved with a detection limit of 0.41 μM, while no other tested transition metal phosphates enhanced the peroxidase-like activity of AuNPs.

Molecule-gated surface chemistry of Pt nanoparticles for constructing activity-controllable nanozymes and a three-in-one sensor

Herein, a simple strategy for constructing activity-controllable nanozymes is proposed based on the glutathione (GSH)-gated surface chemistry of citrate-capped Pt nanoparticles (PtNPs). PtNPs have been shown to have oxidase-like activity that can effectively catalyze the oxidation of 3,3',5,5'-tertamethylbenzidine (TMB) by O2, resulting in a typical color reaction from colorless to blue. We found that GSH can inhibit the oxidase-like activity of PtNPs as a molecule-gated surface chemistry element, resulting in a dramatic decrease of the oxidation of TMB. The addition of copper ions (Cu2+) could oxidize GSH into glutathione disulfide (GSSG), resulting in the distinct suppression of GSH-modulated PtNP surface chemistry and oxidase-like activity inhibition, which further results in a significant acceleration of TMB oxidation and the obvious recovery of intense blue color. Furthermore, the color-based detection signal associated with the redox of TMB indicator here was found to show good fluorescence and a photothermal effect and exhibit sensitive and selective response toward the proposed molecule-gated surface chemistry and Cu2+ target. On the basis of this phenomenon, we successfully constructed a three-in-one sensor for Cu2+ with a triple signal readout, colorimetric, photothermal (temperature), and fluorescence, depending on the proposed in situ modulation method for PtNP catalysis. The applicability of the three-in-one sensor was also demonstrated by measuring Cu2+ in human serum with a standard addition method, and the results are of satisfactory accuracy as confirmed by ICP-MS measurements.

Temperature-responsive iron nanozymes based on poly(N-vinylcaprolactam) with multi-enzyme activity

Iron (Fe)-based nanozymes are widely applied in the biomedical field due to their enzyme-like catalytic activity. Herein, Fe(ii)-based coordination polymer nanohydrogels (FeCPNGs) have been conveniently prepared as a new type of nanozyme by the chelation reaction between ferrous iron and polymer nanohydrogels. The P(VCL-co-NMAM) nanohydrogels prepared by a reflux precipitation polymerization method using N-vinylcaprolactam (VCL) and N-methylol acrylamide (NMAM) as monomers and N,N-methylenebisacrylamide (MBA) as a crosslinker were esterified using P2O5 and then chelated with Fe(ii) ions to form nanozymes with peroxidase and superoxide dismutase (SOD) activity. It was found by dynamic light scattering (DLS) and transmission electron microscopy (TEM) that the nanohydrogels prepared with a monomer concentration of 4% and mass ratio of 1 : 1 (VCL : NMAM) had more uniform particle size, better dispersion and a distinct temperature response. The results of Fourier transform infrared (FTIR), DLS, TEM, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicated the successful preparation of the esterified nanohydrogel and FeCPNGs. Of particular importance is that such FeCPNGs can functionally mimic two antioxidant enzymes (peroxidase and superoxide dismutase) by UV analysis of catalytic oxidation between 3,3,5,5-tetramethylbenzidine (TMB) and H2O2 and the kit analysis of SOD-like activity.

Copper-based metal-organic xerogels on paper for chemiluminescence detection of dopamine

In this work, copper(ii)-containing metal-organic xerogels (Cu-MOXs), which were composed of copper as the central ion and 2,2'-bipyridine-6,6'-dicarboxylic acid as the ligand, were quickly synthesized by a mild facile strategy. The Cu-MOXs exhibited superior catalytic performance for the luminol-H₂O₂ chemiluminescence (CL) system. The possible mechanism was studied via CL spectra, UV-Vis absorption and electron paramagnetic resonance (ESR). Since dopamine (DA) can inhibit the reaction of this system, a sensitive paper-based CL device for the detection of DA was established. Under the optimal experimental conditions, the linear range of this method was 40-200 nM with a detection limit of 10 nM. The proposed method was used for the determination of DA in urine samples.

Iron-Based Nanozymes in Disease Diagnosis and Treatment

Iron-based nanozymes are currently one of the few clinical inorganic nanoparticles for disease diagnosis and treatment. Overcoming the shortcomings of natural enzymes, such as easy inactivation and low yield, combined with their special nanometer properties and magnetic functions, iron-based nanozymes have broad prospects in biomedicine. This minireview summarizes their preparation, biological activity, catalytic mechanism, and applications in diagnosis and treatment of diseases. Finally, challenges to their future development and the trends of iron-based nanozymes are discussed. The purpose of this minireview is to better understand and reasonably speculate on the rational design of iron-based nanozymes as an increasingly important new paradigm for diagnostics.

Nanozymes: A New Disease Imaging Strategy

Nanozymes are nanomaterials with intrinsic enzyme-like properties. They can specifically catalyze substrates of natural enzymes under physiological condition with similar catalytic mechanism and kinetics. Compared to natural enzymes, nanozymes exhibit the unique advantages including high catalytic activity, low cost, high stability, easy mass production, and tunable activity. In addition, as a new type of artificial enzymes, nanozymes not only have the enzyme-like catalytic activity, but also exhibit the unique physicochemical properties of nanomaterials, such as photothermal properties, superparamagnetism, and fluorescence, etc. By combining the unique physicochemical properties and enzyme-like catalytic activities, nanozymes have been widely developed for in vitro detection and in vivo disease monitoring and treatment. Here we mainly summarized the applications of nanozymes for disease imaging and detection to explore their potential application in disease diagnosis and precision medicine.

Counter Anion-Directed Growth of Iron Oxide Nanorods in a Polyol Medium with Efficient Peroxidase-Mimicking Activity for Degradation of Dyes in Contaminated Water

Development of nanozymes, which are nanomaterials with intrinsic enzymatic properties, has emerged as an appealing alternative to the natural enzymes with tremendous application potential from the chemical industry to biomedicine. The self-assembled growth of micrometer-sized oxide materials with controlled nonspherical shapes can be an important tool for enhancing activity as artificial enzymes, as the formation of these superstructures often results in high surface area with favorable impact on catalytic activity. Herein, the growth of rod-shaped Fe3O4 microstructures via a one-pot microwave-based method and using a water-poly(ethylene glycol) mixture as a solvent is reported, without the involvement of external shape-directing agents. The precursor metal salt played a key role in the size, shape, and phase selective evolution of iron oxide micro/nanomaterials. Whereas self-assembled microrod superstructures were obtained using Fe(NO3)3 as the metal salt precursor, use of FeCl3 or Fe-acetate as precursors afforded hollow Fe2O3 microparticles and Fe3O4 nanoparticles, respectively. A graphitic layer was deposited on the Fe3O4 surface, imparting a negative surface charge as a result of a high-temperature treatment of poly(ethylene glycol). The rod-shaped Fe3O4 microcrystals show efficient peroxidase-mimicking activity toward 3,3,5,5'-tetramethylbenzidine and pyrogallol as peroxidase substrates with a Michaelis-Menten rate constant (K m) value of 0.05 and 0.52 mM, respectively. The proficient enzyme mimicking behavior of these magnetic superstructures was further explored for the degradation of organic dyes that includes rhodamine B, methylene blue, and methyl orange with a rate constant (k) of 0.038, 0.011, and 0.007 min-1 respectively, using H2O2. This fast and simple method could help to develop a new pathway for differently shaped oxide nanoparticles in a sustainable and economical manner that can be harnessed as nanozymes for industrial as well as biological applications.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

Mustard seeds derived fluorescent carbon quantum dots and their peroxidase-like activity for colorimetric detection of H2O2 and ascorbic acid in a real sample

Herein, we were synthesized fluorescent carbon quantum dots via facile one-step hydrothermal treatment of mustard seeds (M-CQDs). It showed excellent optical property with fluorescent quantum yield 4.6%. The as-prepared M-CQDs exhibited peroxidase-like mimetic activity and catalyzed the oxidation of chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue color reaction mixture with the prominent peak at 652 nm. Furthermore, the peroxidase-like catalytic performance of M-CQDs followed the steady-state kinetics behavior and exhibited similar catalytic activity as that of natural enzyme Horseradish peroxidase (HRP). In addition to this, the double reciprocal plot showed a parallel line which suggested the occurrence of Ping-Pong type of mechanism. The H₂O₂ dependent oxidation of TMB was helpful for the colorimetric detection of H₂O₂ in the linear range of 0.02-0.20 mM with the limit of detection (LOD) of 0.015 mM. Interestingly, the oxidized TMB (ox-TMB) was further reduced to native TMB by the reducing agent ascorbic acid. Hence M-CQDs showed its potential towards the selective and sensitive detection of ascorbic acid in the linear range of 10-70 μM having a correlation coefficient of 0.998 with LOD of 3.26 μM. The practical feasibility of the proposed detection method of AA was also investigated in common fresh fruits.

Silver-gold-apoferritin nanozyme for suppressing oxidative stress during cryopreservation

Reactive oxygen species (ROS) cause oxidative stress, which involves in the pathogenesis of many serious diseases. Apoferittin containing gold-silver nanoparticles (Au-Ag-AFT) was designed and evaluated as a nanozyme for scavenging the ROS. The nanozyme consisting of silver-gold nanohybrid in apoferittin cage represents superoxide dismutase, catalase and peroxidase mimetic activities. The Au-Ag-AFT nanozyme was characterized by spectroscopy, FESEM, TEM and dynamic light scattering. The inhibition process for pyrogallol autoxidation was used for assaying the superoxide dismutase mimetic activity and measuring the kinetic parameters of Au-Ag-AFT nanozyme. Additionally, Aebi method and standard protocol was used for evaluating the catalase and peroxidase mimetic activity. The k_cat values for superoxide dismutase, catalase and peroxidase mimetics activity were 1.4 × 10⁶, 0.1 and 9 × 10³ s^-1 respectively. These values indicated that Au-Ag-AFT nanozyme could act as a suitable ROS scavenger. Additionally, Au-Ag-AFT nanozyme was examined as a protective agent for human sperm against oxidative stress induced during the cryopreservation process. Presence of the nanozyme in the sperm media significantly increased the motility and viability of the cells and also decreased the ROS, apoptosis and necrosis (P < 0.05) compare to the control group.

A novel photoswitchable enzyme cascade for powerful signal amplification in versatile bioassays

This report outlines the construction of an advanced, exquisite photoswitchable enzyme cascade on the basis that tyrosinase (TYR) catalyzes the generation of dihydroxyphenylalanine (DOPA) coordinated TiO₂ nanoparticles (NPs) to form a light responsive nano-trigger that subsequently photoactivates the enzymatic activity of horseradish peroxidase (HRP). This photoswitchable enzyme cascade has a powerful signal transduction/amplification ability in TYR-based bioassays, and holds great promise to be applied in versatile applications.

An iron hydroxyl phosphate microoctahedron catalyst as an efficient peroxidase mimic for sensitive and colorimetric quantification of H2O2 and glucose

An iron hydroxyl phosphate that possesses high peroxidase activity is synthesized via a one-step hydrothermal method and used for colorimetric glucose detection in human serum.

In Situ Enzymatically Generated Photoswitchable Oxidase Mimetics and Their Application for Colorimetric Detection of Glucose Oxidase

In this study, a simple and amplified colorimetric assay is developed for the detection of the enzymatic activity of glucose oxidase (GOx) based on in situ formation of a photoswitchable oxidase mimetic of PO₄³⁻-capped CdS quantum dots (QDs). GOx catalyzes the oxidation of 1-thio-β-d-glucose to give 1-thio-β-d-gluconic acid which spontaneously hydrolyzes to β-d-gluconic acid and H₂S; the generated H₂S instantly reacts with Cd²⁺ in the presence of Na₃PO₄ to give PO₄³⁻-stabilized CdS QDs in situ. Under visible-light (λ ≥ 400 nm) stimulation, the PO₄³⁻-capped CdS QDs are a new style of oxidase mimic derived by producing some active species, such as h⁺, ^•OH, O₂^•− and a little H₂O₂, which can oxidize the typical substrate (3,3,5,5-tetramethylbenzydine (TMB)) with a color change. Based on the GOx-triggered growth of the oxidase mimetics of PO₄³⁻-capped CdS QDs in situ, we developed a simple and amplified colorimetric assay to probe the enzymatic activity of GOx. The proposed method allowed the detection of the enzymatic activity of GOx over the range from 25 μg/L to 50 mg/L with a low detection limit of 6.6 μg/L. We believe the PO₄³⁻-capped CdS QDs generated in situ with photo-stimulated enzyme-mimicking activity may find wide potential applications in biosensors.

Morphological tuning of Eu2O2S nanoparticles, manifestation of peroxidase-like activity and glucose assay use

We present a single source precursor driven synthesis of Eu₂O₂S with variable shapes, which effectively mimic peroxidase activity, and were successfully employed to selectively detect and determine glucose.

Self-catalytic polymerization of a water-soluble selenium/polypyrrole nanocomposite and its nonlinear optical properties

A water-soluble Se/PPy nanocomposite was synthesized by a single step and eco-friendly method, and its nonlinear optical properties were studied by Z-scan tests.

Superior peroxidase mimetic activity of carbon dots–Pt nanocomposites relies on synergistic effects

CDs–Pt nanocomposites were prepared and proposed as robust enzyme mimetics for the first time.

Solvent assisted and solvent free orientation of growth of nanoscaled lanthanide sulfides: tuning of morphology and manifestation of photocatalytic behavior

We observed solvent-mediated and solvent-free growth of lanthanide sulfides where EuS emerged as a promising candidate for visible-light induced photocatalysis towards the degradation of toxic organic dyes.

A green route to water-soluble polyaniline for photothermal therapy catalyzed by iron phosphates peroxidase mimic

Water-soluble polyaniline showing the photothermal effect was synthesized via a green route using iron phosphates peroxidase mimic as a catalyst.

Colorimetric Detection of Sulfite in Foods by a TMB-O2-Co3O4 Nanoparticles Detection System

In this paper, we first discovered that Co3O4 nanoparticles (NPs) possess intrinsic oxidase-like activity and can catalytically oxidize peroxidase substrates, such as 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) and 3,3',5,5'-tetramethylbenzidine (TMB), to form colored products, in the absence of exogenously added H2O2. The presence of sulfite inhibited the TMB-O2-Co3O4 NPs reaction system and caused a change in color of the reaction system. On the basis of this phenomenon, a colormetric approach to detect sulfite was established with a good linear relationship ranging from 0.2×10(-6) to 1.6×10(-5) M and a detection limit of 5.3×10(-8) M. The method was used to detect sulfite in foods. Good recoveries ranging from 93.8% to 100.5% were obtained. Furthermore, the mechanism was studied and results showed that the oxidase-like activity of the Co3O4 NPs was not from •OH or O2•- radical generated. It may probably originate from their ability to transfer an electron between the peroxidase substrate and oxygen absorbed on the surface of the Co3O4 NPs.

Polyaniline:poly(sodium 4-styrenesulfonate)-stabilized gold nanoparticles as efficient, versatile catalysts

Gold nanoparticles were stabilized by a polyaniline:poly(sodium 4-styrenesulfonate) (PANI:PSS) matrix and readily dispersed in water over a wide range of pH. In contrast to nanoparticles passivated by alkanethiolates that formed a compact capping layer on the nanoparticle surface, the PANI:PSS-Au nanocomposites exhibited apparent catalytic activity in the reduction of 4-nitrophenol in the presence of excessive NaBH4, with reasonably good recyclability, which was likely due to the large accessible surface area. In addition, the PANI:PSS-Au nanocomposites also demonstrated peroxidase-like catalytic activity as evidenced by the colorimetric detection of H2O2 and glucose with PANI:PSS-Au as the enzymatic mimic. The present method may find potential applications in the design, preparation and functionalization of noble nanoparticles as efficient, versatile, and recyclable catalysts with high dispersibility and stability in aqueous media.

Dual responsive enzyme mimicking activity of AgX (X=Cl, Br, I) nanoparticles and its application for cancer cell detection

Chitosan (CS) modified silver halide (AgX, X=Cl, Br, I) (CS-AgX) nanoparticles (NPs) were found to possess dual responsive enzyme mimetic activities. In the presence of H2O2, they were able to oxidize various colorimertic dyes, namely, peroxidase-like activity. Upon photoactivation, CS-AgX NPs could also oxidize the typical substrates in the absence of H2O2. Taking CS-AgI as an example, it was found that the photostimulated enzyme mimetics of CS-AgI NPs showed several unprecedented advantages over natural peroxidase or other existing alternatives based on nanomaterials, such as excellent enzyme-like activity over a broad pH range (3.0-7.0), the independence of hydrogen peroxide on activity, the easily regulated activity by light irradiation, and the good reutilization without significant loss of catalytic activity. The mechanism of the dual responsive enzyme-like activity of CS-AgI was investigated. On the basis of these findings, the photoactivated CS-AgI was designed to develop a facile, cheap, rapid, and highly sensitive colorimetric assay to detect cancer cells. The detection limit of the method for MDA-MB-231 was estimated to be as low as 100 cells, which was much lower than that reported by the method using peroxidase mimetics based on nanomaterials. We believe that CS-AgX NPs with dual responsive enzyme-mimicking activity, especially the excellent photostimulated enzyme-like activity, may find widely potential applications in biosensors.

CoxFe3−xO4 hierarchical nanocubes as peroxidase mimetics and their applications in H2O2 and glucose detection

A facile approach was proposed for the synthesis of hierarchical Co_xFe_3−xO₄ (CF) nanocubes, using Prussian Blue (PB) as precursor. Then, an efficient and simple colorimetric biosensor for H₂O₂ and glucose was fabricated using CF nanocubes as peroxidase mimetic.

A non-aggregation colorimetric assay for thrombin based on catalytic properties of silver nanoparticles

In this paper, we developed a simple and rapid colorimetric assay for protein detection based on the reduction of dye molecules catalyzed by silver nanoparticles (AgNPs). Aptamer-modified magnetic particles and aptamer-functionalized AgNPs were employed as capture and detection probes, respectively. Introduction of thrombin as target protein could form a sandwich-type complex involving catalytically active AgNPs, whose catalytic activity was monitored on the catalytic reduction of rhodamine B (RhB) by sodium borohydride (NaBH4). The amount of immobilized AgNPs on the complex increased along with the increase of the thrombin concentration, thus the detection of thrombin was achieved via recording the decrease in absorbance corresponding to RhB. This method has adopted several advantages from the key factors involved, i.e., the sandwich binding of affinity aptamers contributed to the increased specificity; magnetic particles could result in rapid capture and separation processes; the conjugation of AgNPs would lead to a clear visual detection. It allows for the detection limit of thrombin down to picomolar level by the naked eye, with remarkable selectivity over other proteins. Moreover, it is possible to apply this method to the other targets with two binding sites as well.