Intrinsic Triple-Enzyme Mimetic Activity of V6O13 Nanotextiles: Mechanism Investigation and Colorimetric and Fluorescent Detections

Biomimetic synergistic effect of redox site and Lewis acid for construction of efficient artificial enzyme

In enzymatic catalysis, the redox site and Lewis acid are the two main roles played by metal to assist amino acids. However, the reported enzyme mimics only focus on the redox-active metal as redox site, while the redox-inert metal as Lewis acid has, to the best of our knowledge, not been studied, presenting a bottleneck of enzyme mimics construction. Based on this, a series of highly efficient MxV2O5·nH2O peroxidase mimics with vanadium as redox site and alkaline-earth metal ion (M2+) as Lewis acid are reported. Experimental results and theoretical calculations indicate the peroxidase-mimicking activity of MxV2O5·nH2O show a periodic change with the Lewis acidity (ion potential) of M2+, revealing the mechanism of redox-inert M2+ regulating electron transfer of V-O through non-covalent polarization and thus promoting H2O2 adsorbate dissociation. The biomimetic synergetic effect of redox site and Lewis acid is expected to provide an inspiration for design of enzyme mimics.

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.

A Nitro Functionalized MOF with Multi-Enzyme Mimetic Activities for the Colorimetric Sensing of Glucose at Neutral pH

Benefiting from the advantages like large surface area, flexible constitution, and diverse structure, metal-organic frameworks (MOFs) have been one of the most ideal candidates for nanozymes. In this study, a nitro-functionalized MOF, namely NO₂-MIL-53(Cu), was synthesized. Multi-enzyme mimetic activities were discovered on this MOF, including peroxidase-like, oxidase-like, and laccase-like activity. Compared to the non-functional counterpart (MIL-53(Cu)), NO₂-MIL-53(Cu) displayed superior enzyme mimetic activities, indicating a positive role of the nitro group in the MOF. Subsequently, the effects of reaction conditions on enzyme mimetic activities were investigated. Remarkably, NO₂-MIL-53(Cu) exhibited excellent peroxidase-like activity even at neutral pH. Based on this finding, a simple colorimetric sensing platform was developed for the detection of H₂O₂ and glucose, respectively. The detection liner range for H₂O₂ is 1-800 μM with a detection limit of 0.69 μM. The detection liner range for glucose is linear range 0.5-300 μM with a detection limit of 2.6 μM. Therefore, this work not only provides an applicable colorimetric platform for glucose detection in a physiological environment, but also offers guidance for the rational design of efficient nanozymes with multi-enzyme mimetic activities.

Catalase-Like Nanozymes: Classification, Catalytic Mechanisms, and Their Applications

The field of nanozymes has developed rapidly over the past decade. Among various oxidoreductases mimics, catalase (CAT)-like nanozyme, acting as an essential part of the regulation of reactive oxygen species (ROS), has attracted extensive research interest in recent years. However, CAT-like nanozymes are not as well discussed as other nanozymes such as peroxidase (POD)-like nanozymes, etc. Compared with natural catalase or artificial CAT enzymes, CAT-like nanozymes have unique properties of low cost, size-dependent properties, high catalytic activity and stability, and easy surface modification, etc., which make them widely used in various fields, especially in tumor therapy and disease treatment. Consequently, there is a great requirement to make a systematic discussion on CAT-like nanozymes. In this review, some key aspects of CAT-like nanozymes are deeply summarized as: 1) Typical CAT-like nanozymes classified by different nanomaterials; 2) The catalytic mechanisms proposed by experimental and theoretical studies; 3) Extensive applications in regard to tumor therapy, cytoprotection and sensing. Therefore, it is prospected that this review will contribute to the further design of CAT-like nanozymes and optimize their applications with much higher efficiency than before.

Self-template synthesis of mesoporous vanadium oxide nanospheres with intrinsic peroxidase-like activity and high antibacterial performance

Mesoporous vanadium oxide nanospheres are a very promising nanozyme for antibacterial and chemical sensing. However, controllable synthesis of mesoporous vanadium oxide nanospheres with uniform structure and small diameter (<200 nm) remains challenging. Herein, mesoporous vanadium oxide nanospheres (MVONs) with a small, uniform and adjustable particle size (52-105 nm), large mesopore size (5.1-5.8 nm), and high specific surface area (up to 63.7 m² g^-1) are constructed via a self-template strategy using tannic acid, formaldehyde and vanadium compounds as a polymerizable ligand, cross-linking agent and metal source, respectively. The relationships between synthesis conditions and material nanostructure are systematically investigated. The particle size and peroxidase-like activity of MVONs can be easily changed by adding different amounts of Pluronic block copolymer F127. Owing to the mesoporous structure, high specific surface area and small particle size, MVONs can effectively convert H₂O₂ into extremely toxic reactive oxygen species, and further kill Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). This research establishes a universal, reliable method for synthesizing mesoporous vanadium oxide nanospheres, which might be used in catalysis, biosensors, and antibacterial treatment.

Vanadium nitride@carbon nanofiber composite: Synthesis, cascade enzyme mimics and its sensitive and selective colorimetric sensing of superoxide anion

Nanozymes featuring with favorable activity, good stability and easy scale-up production, are promising to replace natural enzymes for various applications. However, it remains a challenge to explore the cascade reactions of multi-enzyme mimics, aiming at synergistic catalysis for various applications. Herein, vanadium nitride nanoparticles deposited on carbon nanofibers (VN@CNFs) composite was facilely prepared by typical electrospinning route with subsequently ammonia reduction process. The nanocomposite showed excellent peroxidase (POD)-like and superoxide dismutase (SOD)-like activities. Additionally, their catalytic mechanisms were systematically researched. Coupling of SOD-like with POD-like as cascade enzyme, a selective and sensitive colorimetric detection of superoxide anion (O₂^•-) was explored, which has two linear parts, 0.05-30 μM and 30-250 μM O₂^•- with the LOD of 0.0167 μM (S/N = 3). The as-proposed method was applicable to practical samples detection with satisfactory accuracy and recovery. Therefore, the VN@CNFs composite shows great prospect in biosensing, superoxide anion removal and biocatalysis.

Triple-enzyme mimetic activity of Fe3O4@C@MnO2 composites derived from metal-organic frameworks and their application to colorimetric biosensing of dopamine

Novel Fe₃O₄@C@MnO₂ composites were successfully synthesized for the first time via an interfacial reaction between magnetic porous carbon and KMnO₄, in which the magnetic porous carbon was derived from the pyrolysis of Fe-MIL-88A under N₂ atmosphere. Interestingly, the obtained Fe₃O₄@C@MnO₂ composites were found to have triple-enzyme mimetic activity including peroxidase-like, catalase-like, and oxidase-like activity. As a peroxidase mimic, Fe₃O₄@C@MnO₂ composites could catalyze the oxidation of TMB into a blue oxidized product by H₂O₂. As a catalase mimic, Fe₃O₄@C@MnO₂ could catalyze the decomposition of H₂O₂ to generate O₂ and H₂O. As an oxidase mimic, Fe₃O₄@C@MnO₂ could catalyze the direct oxidation of TMB to produce a blue oxidized product without H₂O₂. Reactive oxygen species measurements revealed that the oxidase-like activity originated from ¹O₂ and O₂^-∙and little∙OH generated by the dissolved oxygen, which was catalyzed by the Fe₃O₄@C@MnO₂ in the TMB oxidation reaction. The oxidase-like activity of Fe₃O₄@C@MnO₂ was investigated in detail. Under the optimized conditions, a rapid, sensitive, visual colorimetric method for dopamine detection was developed based on the inhibitory effect of dopamine on the oxidase-like activity. The proposed method allows for dopamine detection with a limit of detection of 0.034 μM and a linear range of 0.125-10 μM. This new colorimetric method was successfully used for the determination of dopamine in human blood samples.

Glucose oxidase decorated fluorescent metal-organic frameworks as biomimetic cascade nanozymes for glucose detection through the inner filter effect

Metal-organic frameworks (MOFs) as a peroxidase mimic have been integrated with glucose oxidase (GOx) to achieve one-step glucose detection. However, limited by the loading amount of GOx, the performances of the developed glucose sensing assays still remain to be further improved to meet sensing requirements in diverse biological samples. Herein, with Fe3+ as the metal ion and 2-amino-benzenedicarboxylic acid as a ligand, a fluorescent Fe-based organic framework (NH2-MIL-101) with peroxidase-like activity was synthesized. Due to the large specific surface area (791.75 m2 g-1), 68 μg mg-1 GOx could be immobilized through the amidation coupling reaction, and the product was designated GOx@NH2-MIL-101. With OPD as the substrate, Gox@NH2-MIL-101 achieved highly efficient biomimetic cascade catalysis for one-step glucose detection through an inner filter effect: upon reacting with glucose, GOx@NH2-MIL-101 catalytically oxidized glucose using dissolved O2, and the produced H2O2 concurrently oxidized o-phenylenediamine (OPD) to oxidized OPD (oxOPD), accompanied by the fluorescence of GOx@NH2-MIL-101 at 456 nm being quenched and that of oxOPD at 565 nm being enhanced. With the fluorescent ratio F565/F456 used as a readout signal, a wide linear range of 0.1-600 μM was obtained, and the detection limit was 0.0428 μM. Based on the excellent selectivity and high stability of GOx@NH2-MIL-101, the developed assay was successfully applied to glucose detection in human serum and saliva, presenting potential applications in diverse biological samples and even medical diagnosis.

NiCo2S4 microflowers as peroxidase mimic: A multi-functional platform for colorimetric detection of glucose and evaluation of antioxidant behavior

The regular design of atomic composition of materials endows a diverse range of enzyme mimics types and increases a broader application prospect. In this study, we designed a nickel-cobalt mixed metal sulfide and demonstrated that the as-prepared NiCo₂S₄ microflowers possessed intrinsic peroxidase-like activity. Nickel-cobalt sulfide (NiCo₂S₄) possessed high electron transfer capacity, which lead to good peroxidase-like activity. Compared with the reported enzyme-like materials, NiCo₂S₄ exhibited a smaller Km and a stronger affinity with substrate. A colorimetric assay was developed for the direct detection of hydrogen peroxide and indirect detection of glucose over a wide linear range (H₂O₂ was 20-200 μM, and glucose was 20-1000 μM) with a low detection limit (H₂O₂ was 5.19 μM, and glucose was 8.24 μM). Furthermore, a NiCo₂S₄ based platform was established to study the antioxidant behavior of three antioxidants. The antioxidant capacities of the antioxidants were found to rank in the order: tannic acid (TA) > ascorbic acid (AA)> gallic acid (GA). Moreover, the three antioxidants showed different inhibition mechanisms. This study indicated a new and important application field for NiCo₂S₄ and provides a basis for the rational design of enzyme-like mimics in future research.

Natural Polyphenol-Vanadium Oxide Nanozymes for Synergistic Chemodynamic/Photothermal Therapy

The selection of suitable nanozymes with easy synthesis, tumor specificity, multifunction, and high therapeutics is meaningful for tumor therapy. Herein, a facile one-step assembly approach was employed to successfully prepare a novel kind of natural polyphenol tannic acid (TA) hybrid with mixed valence vanadium oxide nanosheets (TA@VO_x NSs). In this system, VO_x is assembled with TA through metal-phenolic coordination interaction to both introduce superior peroxidase-like activity and high near infrared (NIR) absorption owing to partial reduction of vanadium from V⁵⁺ to V⁴⁺ . The presence of mixed valence vanadium oxide in TA@VO_x NSs is proved to be the key for the catalytic reaction of hydrogen peroxide (H₂ O₂ ) to ^. OH, and the corresponding catalytic mechanism of H₂ O₂ by TA@VO_x NSs is proposed. Benefitting from such peroxidase-like activity of TA@VO_x NSs, the overproduced H₂ O₂ of the tumor microenvironment allows the realization of tumor-specific chemodynamic therapy (CDT). As a valid supplement to CDT, the NIR absorption enables TA@VO_x NSs to have NIR light-mediated conversion ability for photothermal therapy (PTT) of cancers. Furthermore, in vitro and in vivo experiments confirmed that TA@VO_x NSs can effectively inhibit the growth of tumors by synergistic CDT/PTT. These results offer a promising way to develop novel vanadium oxide-based nanozymes for enhanced synergistic tumor-specific treatment.

Intrinsic Enzyme-like Activities of Cerium Oxide Nanocomposite and Its Application for Extracellular H2O2 Detection Using an Electrochemical Microfluidic Device

Artificial enzyme mimics have gained considerable attention for use in sensing applications due to their high stability and outstanding catalytic activity. We show that cerium oxide nanosheets (NSs) exhibit triple-enzyme mimetic activity. The oxidase-, peroxidase-, and catalase-like activities of the proposed nanoparticles are demonstrated using both colorimetric and electron paramagnetic resonance (EPR) spectroscopy. On the basis of the excellent catalytic activity of cerium oxide NSs toward hydrogen peroxide, an electrochemical approach for the high-throughput detection of H₂O₂ in living cells was established. This report presents an analytical microfluidic chip integrated with a cerium oxide NS mimic enzyme for the fabrication of a simple, sensitive, and low-cost electrochemical sensor. Three Au microelectrodes were fabricated on a glass substrate using photolithography, and the working electrode was functionalized using cerium oxide NSs. The operation of this biosensor is based on cerium oxide NSs and presents a high sensitivity over a wide detection range, between 100 nM and 20 mM, with a low detection limit of 20 nM and a high sensitivity threshold of 226.4 μA·cm^-2·μM^-1. This microfluidic sensor shows a strong response to H₂O₂, suggesting potential applications in monitoring H₂O₂ directly secreted from living cells. This sensor chip provides a promising platform for applications in the field of diagnostics and sensing.

Enzyme Mimic Nanomaterials and Their Biomedical Applications

Nanomaterials with enzyme-mimicking behavior (nanozymes) have attracted a lot of research interest recently. In comparison to natural enzymes, nanozymes hold many advantages, such as good stability, ease of production and surface functionalization. As the catalytic mechanism of nanozymes is gradually revealed, the application fields of nanozymes are also broadly explored. Beyond traditional colorimetric detection assays, nanozymes have been found to hold great potential in a variety of biomedical fields, such as tumor theranostics, antibacterial, antioxidation and bioorthogonal reactions. In this review, we summarize nanozymes consisting of different nanomaterials. In addition, we focus on the catalytic performance of nanozymes in biomedical applications. The prospects and challenges in the practical use of nanozymes are discussed at the end of this Minireview.

A differential photoelectrochemical method for glucose determination based on alkali-soaked zeolite imidazole framework-67 as both glucose oxidase and peroxidase mimics

A differential photoelectrochemical (PEC) method for glucose determination is reported using a nanocomposite with double mimic enzymes of glucose oxidase (GOx) and peroxidase. The nanocomposite was prepared by soaking zeolite imidazole framework-67 (ZIF-67) in 0.1 M NaOH solution at room temperature for 30 min, abbreviated as CoxOyHz@ZIF-67. The Michaelis-Menten constant of CoxOyHz@ZIF-67 to H2O2 and glucose is 121 μM and 3.95 mM, respectively. Using the photoelectrode of CoxOyHz@ZIF-67/TiO2 nanotubes (NTs), glucose was oxidized firstly by dissolved oxygen to generate H2O2 under the catalysis of CoxOyHz film as the mimics of GOx. The product of H2O2 enhanced the photocurrent of TiO2 NTs under the catalysis of ZIF-67 as the mimics of peroxidase. The molecular sieve effect of ZIF-67 frameworks reduces the interferences from molecules with size larger than the apertures in ZIF-67. Under the excitation of a 150 W xenon lamp with full spectrum, the photocurrent was measured in a two-electrode system without external additional potential. By using the photocurrent difference between two photocells, i.e CoxOyHz@ZIF-67/TiO2 NTs and Pt electrode, ZIF-67/TiO2 NTs and Pt electrode, as the signal, the selectivity for glucose determination is improved further. The differential PEC method was applied to the determination of glucose with a linear range 0.1 μM~1 mM and a detection limit of 0.03 μM. Graphical abstract.

Photoactivated oxidase mimetics derived from dicyandiamide and barbituric acid for colorimetric detection of glutathione

In this work, photoactivated oxidase mimetics was prepared by copolymerizing dicyandiamide with barbituric acid (BA) and characterized by X-ray diffraction pattern, Fourier transformed infrared spectrum, X-ray photoelectron spectroscopy, transmission electron microscopy, photoluminescence spectrum, diffuse reflectance spectrum. Experimental results and density functional theory calculation indicated that the substitution of nitrogen atoms by carbon atoms in tri-s-triazine structure due to the copolymerization of BA enhanced visible light absorption and weakened the barrier of photocarrier transfer. In the presence of visible light and oxygen, 3, 3', 5, 5'-tetramethylbenzidine was oxidized under the catalysis of photoactivated oxidase mimetics to produce a green colored product, which could be reduced by glutathione (GSH). Therefore, a facile method based on the photoactivated oxidase mimetic has been developed for colorimetric detection of GSH. The linear range for GSH was ranged from 2.0 to 50.0 μmol L^-1 (R² = 0.998) with the detection limit of 1.4 μmol L^-1. The proposed method was applied to detect the cellular GSH with satisfactory results.

Catalytic Mechanisms of Nanozymes and Their Applications in Biomedicine

The research on nanozymes has increased dramatically in recent years and a new interdiscipline, nanozymology, has emerged. A variety of nanomaterials have been designed to mimic the characteristics of natural enzymes, which connects an important bridge between nanotechnology and biological science. Unlike natural enzymes, the nanoscale properties of nanozymes endow them with the potential to regulate their enzymatic-like activity from different perspectives. The mechanisms behind those methods are intriguing. In this Review, we introduce these mechanisms from the aspects of surface chemistry, surface modification, molecular imprinting, and hybridization and then focus attention on some specific catalytic mechanisms of several representative nanozymes. The applications of nanozymes ranging from bioassay, imaging, to disease therapy are also discussed in detail to prove the fact that the inherent physicochemical properties of nanomaterials not only make nanozymes the analogues of biological enzymes, but also endow them with incomparable advantages and broad prospects in biomedical fields. Finally, four characteristics and some challenges of nanozymes are summarized.

VO x Quantum Dots with Multienzyme-Mimic Activities and the Application in Constructing a Three-Dimensional (3D) Coordinate System for Accurate Discrimination of the Hydrogen Peroxide over a Broad Concentration Range

The construction of efficient nanozyme with multienzyme activities in a simple way is vital for the wide biological and chemical applications. Generally, the mimic enzyme activities depend on their sizes, surface states, and materials types. Quantum dots (QDs), one type of zero-dimensional nanomaterials, are much appealing due to their abundant catalytically active surface deficiency. The vanadium oxide (VO _x) is one special transition metal oxides possessing different valence states. Inspired by these views, we synthesized VO _xQDs herein via a one-pot top-down ethanol-thermal method using bulk VO₂ as the precursor. The VO _xQDs showed not only oxidase- and peroxidase-like activities in ethanol as the main background solution (ethanol-BGS), but also exhibited additional superoxide dismutase mimetic activity in phosphate buffer solution. Furthermore, the TMB-VO _xQDs system in the ethanol-BGS produced three distinct colors in the presence of hydrogen peroxide (H₂O₂) at three different concentration gradients (10-90 μM, 0.1-10 mM, and 20-100 mM). Accordingly, we constructed a three-dimensional (3D) coordinate system (3D-CS) by using the three variables: the initial velocities, the maximum absorption values and the visual colors of the enzymatic reaction system. As a result, the rapid detection of H₂O₂ can be achieved while effectively avoiding the faked appearance due to the inhibition effects to the enzymatic system at too high H₂O₂ concentration. The applicability of the VO _xQDs based 3D-CS was further proved via the facile and accurate H₂O₂ assays in three different practical samples.

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.

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

Fluoride capped V6O13–reduced graphene oxide nanocomposites: high activity oxidase mimetics and mechanism investigation

A novel high activity of the oxidase-like nanozyme of V₆O₁₃–rGO NCs, whose activity can be significantly enhanced by fluoride capping through surface chemistry.

Heterogeneous Nanostructure Design Based on the Epitaxial Growth of Spongy MoS x on 2D Co(OH)2 Nanoflakes for Triple-Enzyme Mimetic Activity: Experimental and Density Functional Theory Studies on the Dramatic Activation Mechanism

In this study, we present a three-in-one catalytic platform for intrinsic oxidase-, peroxidase-, and catalase-like activity, which is enabled by epitaxial growth of the MoS _x nanosponge on 2D Co(OH)₂ nanoflakes [2D Co(OH)₂ NFs] (CoMo hybrids). First, the 2D Co(OH)₂ NFs are stripped from hierarchical three-dimensional Co(OH)₂ nanoflowers which are synthesized in an eco-friendly way via one-step surfactant-free chemical route. Next, the porous MoS _x nanosponge is decorated on the 2D Co(OH)₂ NFs' surface using a solvothermal process forming heterogeneous nanostructured CoMo hybrids. Finally, because of the host-guest interaction, that is, after the epitaxial growth of spongy MoS _x on 2D Co(OH)₂ NFs, the heterogeneous nanostructure of CoMo hybrids exhibits unpredictable triple-enzyme mimetic activity simultaneously. The mechanisms of the oxidase-like properties are investigated by density functional theory (DFT) calculations, and it is discovered that a simple reaction/dissociation of O₂ absorbed on Co-Mo thin films can explain the enhanced oxidase-like activity of the CoMo hybrids. In addition, the CoMo hybrids are also reproducible, stable, and reusable, that is, after 10 cycle uses, >90% mimic enzyme activity of the CoMo hybrids is still maintained. The oxidase-like activity of the CoMo hybrids enables it to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) producing the blue oxTMB, which can selectively oxidize ascorbic acid (AA) and pave a new avenue for colorimetric sensing of AA. The proposed colorimetric strategy has been successfully utilized to measure AA in rat brain during the cerebral calm/ischemia process. Our findings provide in-depth insight into the future research of enzyme-like activities and might help to elucidate the mechanism and understand the role of epitaxial growth on the properties and application of hybrid nanostructures.

Oxidase-mimicking activity of perovskite LaMnO3+δ nanofibers and their application for colorimetric sensing

Excellent oxidase-like mimics based on the ABX₃-type perovskite structure and the corresponding sensitive colorimetric detection of l-cysteine have been developed.