CoO-supported ordered mesoporous carbon nanocomposite based nanozyme with peroxidase-like activity for colorimetric detection of glucose

Hexagonal Boron Nitride Quantum Dots Embedded on Layer-by-Layer Films for Peroxidase-Assisted Colorimetric Detection of β-Galactosidase Producing Pathogens

Pathogen detection has become a major research area all over the world for water quality surveillance and microbial risk assessment. Therefore, designing simple and sensitive detection kits plays a key role in envisaging and evaluating the risk of disease outbreaks and providing quality healthcare settings. Herein, we have designed a facile and low-cost colorimetric sensing strategy for the selective and sensitive determination of β-galactosidase producing pathogens. The hexagonal boron nitride quantum dots (h-BN QDs) were established as a nanozyme that showed prominent peroxidase-like activity, which catalyzes 3,3',5,5'-tetramethylbenzidine (TMB) oxidation by H2O2. The h-BN QDs were embedded on a layer-by-layer assembled agarose biopolymer. The β-galactosidase enzyme partially degrades β-1,4 glycosidic bonds of agarose polymer, resulting in accessibility of h-BN QDs on the solid surface. This assay can be conveniently conducted and analyzed by monitoring the blue color formation due to TMB oxidation within 30 min. The nanocomposite was stable for more than 90 days and was showing TMB oxidation after incubating it with Escherichia coli (E. coli). The limit of detection was calculated to be 1.8 × 106 and 1.5 × 106 CFU/mL for E. coli and Klebsiella pneumonia (K. pneumonia), respectively. Furthermore, this novel sensing approach is an attractive platform that was successfully applied to detect E. coli in spiked water samples and other food products with good accuracy, indicating its practical applicability for the detection of pathogens in real samples.

Construction of Mn-decorated zeolitic imidazolate framework-90 nanostructure as superior oxidase-like mimic for colorimetric detection of glucose and choline

A Mn decorated zeolitic imidazolate framework-90 (ZIF-90) nanozyme (Mn/ZIF-90) was constructed through an effective and rapid post-synthetic strategy for the first time. The Mn in Mn/ZIF-90 exists in mixed valence states, which is doped to the ZIF-90 through the formation of Mn-O bond. The Zn-N coordination structure of ZIF-90 may change the electronic arrangement of oxygen atoms in the free carbonyl groups (-CHO), allowing the coordination of Mn with O. The prepared Mn/ZIF-90 possesses outstanding oxidase-like activity and remarkable stability. Besides, the catalytic activity of Mn/ZIF-90 can be inhibited in the presence of H2O2. Therefore, using the Mn/ZIF-90-triggered chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) as an amplifier, a versatile enzyme cascade-based colorimetric method for the detection of glucose and choline with good sensitivity and selectivity was developed. The linear ranges for glucose and choline are 6.25-500 μM and 5-1000 μM, respectively. Furthermore, the developed method was applied in the detection of glucose and choline in rabbit plasma samples, and the recoveries are 89.5-107.3 % and 96.0-109.3 %, respectively. In short, the simple and efficient post-synthetic doping method may provide a new thought for the rational designs of enzyme mimics with improved catalytic performance. Moreover, the colorimetric method based on the excellent catalytic activity of Mn/ZIF-90 may be extended to detect other H2O2-generating or consuming molecules and evaluate the activity of bio-enzymes that can catalyze the generation of glucose or choline.

Development of a flow injection chemiluminescence immunoassay based on DES-mediated CuCo2O4 nanoenzyme for ultrasensitive detection of zearalenone in foods

Nanoenzymes have been widely used to construct biosensors because of their cost-effectiveness, high stability, and easy modification. At the same time, the discovery of deep eutectic solvents (DES) was a great breakthrough in green chemistry, and their combination with different materials can improve the sensing performance of biosensors. In this work, we report an immunosensor using CuCo2O4 nanoenzyme combined with flow injection chemiluminescence immunoassay for the automated detection of zearalenone (ZEN). The immunosensor exhibited excellent sensing performance. Under the optimal conditions, the detection range of ZEN was 0.0001-100 ng mL-1, and the limit of detection (LOD) was 0.076 pg mL-1 (S/N = 3). In addition, the immunosensor showed excellent stability with a relative standard deviation (RSD) of 2.65% for  15 repetitive  injections. The method has been successfully applied to the analysis of real samples with satisfactory recovery results, and can hence provide a reference for the detection of small molecules in food and feed.

Co-based Nanozymatic Profiling: Advances Spanning Chemistry, Biomedical, and Environmental Sciences

Nanozymes, next-generation enzyme-mimicking nanomaterials, have entered an era of rational design; among them, Co-based nanozymes have emerged as captivating players over times. Co-based nanozymes have been developed and have garnered significant attention over the past five years. Their extraordinary properties, including regulatable enzymatic activity, stability, and multifunctionality stemming from magnetic properties, photothermal conversion effects, cavitation effects, and relaxation efficiency, have made Co-based nanozymes a rising star. This review presents the first comprehensive profiling of the Co-based nanozymes in the chemistry, biology, and environmental sciences. The review begins by scrutinizing the various synthetic methods employed for Co-based nanozyme fabrication, such as template and sol-gel methods, highlighting their distinctive merits from a chemical standpoint. Furthermore, a detailed exploration of their wide-ranging applications in biosensing and biomedical therapeutics, as well as their contributions to environmental monitoring and remediation is provided. Notably, drawing inspiration from state-of-the-art techniques such as omics, a comprehensive analysis of Co-based nanozymes is undertaken, employing analogous statistical methodologies to provide valuable guidance. To conclude, a comprehensive outlook on the challenges and prospects for Co-based nanozymes is presented, spanning from microscopic physicochemical mechanisms to macroscopic clinical translational applications.

Recent advances in colorimetric sensors based on nanozymes with peroxidase-like activity

Nanozymes have been widely used to construct colorimetric sensors due to their advantages of cost-effectiveness, high stability, good biocompatibility, and ease of modification. The emergence of nanozymes greatly enhanced the detection sensitivity and stability of the colorimetric sensing platform. Recent significant research has focused on designing various sensors based on nanozymes with peroxidase-like activity for colorimetric analysis. However, with the deepening of research, nanozymes with peroxidase-like activity has also exposed some problems, such as weak affinity and low catalytic activity. In view of the above issues, existing investigations have shown that the catalytic properties of nanozymes can be improved by adding surface modification and changing the structure of nanomaterials. In this review, we summarize the recent trends and advances of colorimetric sensors based on several typical nanozymes with peroxidase-like activities, including noble metals, metal oxides, metal sulfides/metal selenides, and carbon and metal-organic frameworks (MOF). Finally, the current challenges and prospects of colorimetric sensors based on nanozymes with peroxidase-like activity are summarized and discussed to provide a reference for researchers in related fields.

Dual-Enzyme Cascade Composed of Chitosan Coated FeS2 Nanozyme and Glucose Oxidase for Sensitive Glucose Detection

Immobilizing enzymes with nanozymes to catalyze cascade reactions overcomes many of the shortcomings of biological enzymes in industrial manufacturing. In the study, glucose oxidases were covalently bound to FeS₂ nanozymes as immobilization carriers while chitosan encapsulation increased the activity and stability of the immobilized enzymes. The immobilized enzymes exhibited a 10% greater increase in catalytic efficiency than the free enzymes while also being more stable and catalytically active in environments with an alkaline pH of 9.0 and a high temperature of 100 °C. Additionally, the FeS₂ nanozyme-driven double-enzyme cascade reaction showed high glucose selectivity, even in the presence of lactose, dopamine, and uric acid, with a limit of detection (LOD) (S/N = 3) as low as 1.9 × 10^-6 M. This research demonstrates that nanozymes may be employed as ideal carriers for biological enzymes and that the nanozymes can catalyze cascade reactions together with natural enzymes, offering new insights into interactions between natural and synthetic biosystems.

One-Pot Preparation of Imidazole-Ring-Modified Graphitic Carbon Nitride Nanozymes for Colorimetric Glucose Detection

Nanozymes are highly desired to overcome the shortcomings of natural enzymes, such as low stability, high cost and difficult storage during biosensing applications. Herein, by imitating the structure of natural enzymes, we propose a one-pot annealing process to synthesis imidazole-ring-modified graphitic carbon nitride (g-C₃N₄-Im) with enhanced peroxidase-like activity. g-C₃N₄-Im shows enhanced peroxidase-like activity by 46.5 times compared to unmodified g-C₃N₄. Furthermore, imidazole rings of g-C₃N₄-Im make it possible to anchor Cu(II) active sites on it to produce g-C₃N₄-Im-Cu, which shows a further increase in peroxidase-like activity by three times. It should be noted that the as-prepared g-C₃N₄-Im-Cu could show obvious peroxidase-like activity over a broad range of pH values and at a low temperature (5 °C). The ultrahigh peroxidase-like activity is attributed to the electronic effect of imidazole rings and the active sites of Cu(II) for ·OH production. Based on the enhanced peroxidase-like activity, a H₂O₂ and glucose biosensor was developed with a high sensitivity (limit of detection, 10 nM) and selectivity. Therefore, the biosensor shows potential for applications in diabetic diagnoses in clinical practice.

The valine-based N-doped carbon dots with high peroxidase-like activity

Developing artificial nanoparticles with peroxidase-like activity are being attracted great interests because their catalytic activity are not easy to lose under harsh conditions compared with natural enzymes. In this work, the N-doped carbon dots (CDs) were prepared, and it was found that the N-doped CDs showed high peroxidase-like catalytic activity towards the reaction system of hydrogen peroxide - 3, 3', 5, 5'-tetramethylbenzidine (TMB). The catalytic mechanism of the N-doped CDs was explored by usingelectron spin resonance (ESR) spectra, cyclic voltammetry (CV) method and simulation of density functional theory calculation. This work can offer a new feasible method for synthesizing N-doped CQDs used as artificial peroxidases in biological and environment applications.

CeO2/Co3O4@N-doped hollow carbon microspheres with improved peroxidase-like activity for the determination of quercetin

Nanozymes, as substitutes for natural enzymes, have been used to construct some fast cheap sensing platforms by virtue of the high catalytic activity. Herein, we firstly study the peroxidase-like activity of CeO₂/Co₃O₄@N-doped hollow carbon microspheres as nanozymes. In the presence of H₂O₂, CeO₂/Co₃O₄@NCH exhibits high peroxidase-like activity evaluated by the catalytic oxidation of the chromogenic substrate, 3,3',5,5'-tetramethylbenzidine (TMB) into a blue oxTMB visually in 1 min. The larger surface area, pore-like structure, and oxygen vacancies contribute to the enhanced peroxidase-like activity of CeO₂/Co₃O₄@NCH. The active species •O^2- is detected during the catalytic process. Thus, based on the excellent peroxidase-like activity of CeO₂/Co₃O₄@NCH, a facile and effective biosensor is fabricated for sensitive and selective determination of H₂O₂ and quercetin, respectively. CeO₂/Co₃O₄@NCH shows high affinity towards H₂O₂ (K_m = 4.001 mM) and TMB (K_m = 0.086 mM). The detection limit of H₂O₂ and quercetin is as low as 0.086 mM and 1.19 μM (3σ/slope), respectively. This work remarkably extends the utilization of CeO₂/Co₃O₄@NCH in designing a colorimetric sensing platform related to biosensing, food, and environmental monitoring. A fast colorimetric sensing platform for quercetin based on the excellent peroxidase-like activity of CeO₂/Co₃O₄@NCH.

A Copper-Based Biosensor for Dual-Mode Glucose Detection

Glucose is a source of energy for daily activities of the human body and is regarded as a clinical biomarker, due to the abnormal glucose level in the blood leading to many endocrine metabolic diseases. Thus, it is indispensable to develop simple, accurate, and sensitive methods for glucose detection. However, the current methods mainly depend on natural enzymes, which are unstable, hard to prepare, and expensive, limiting the extensive applications in clinics. Herein, we propose a dual-mode Cu₂O nanoparticles (NPs) based biosensor for glucose analysis based on colorimetric assay and laser desorption/ionization mass spectrometry (LDI MS). Cu₂O NPs exhibited excellent peroxidase-like activity and served as a matrix for LDI MS analysis, achieving visual and accurate quantitative analysis of glucose in serum. Our proposed method possesses promising application values in clinical disease diagnostics and monitoring.

Colorimetric Aptasensor Based on Fe₃O₄-Cu2+ Nanozyme with Intrinsic Peroxidase-Like Activity in the Detection of Breast Cancer Exosomes

Because breast cancer cells such as MCF-7, exhibit vital and developmental signs by exosome secretion, diagnosing them in the blood can provide a good index of the presence of breast cancer. However, accurate and inexpensive detection of exosomes in clinical practice faces challenges. Therefore, in the presents study, an aptasensor based on CD63 aptameriron oxide-copper ion nanozymes (Fe₃O₄-Cu²⁺-NZs) was designed with the ability of the CD63 aptamer to interact with the exosome and the release of the Fe₃O₄-Cu²⁺-NZs for peroxidase-like activity on the tetramethylbenzidine (TMB). After fabrication of CD63 aptamer-Fe₃O₄-Cu²⁺-NZs based on hydrothermal method, their physicochemical properties were investigated with the SEM, TEM, DLS, Zeta, XRD and magnetization. To investigate the interaction of CD63 aptamer-Fe₃O₄-Cu²⁺-NZs with exosomes, the required exosomes were extracted from cultured MCF-7 cells. The performance of CD63 aptamer-Fe₃O₄-Cu²⁺-NZs on TMB degradation in the presence and absence of exosomes was investigated through UV-vis adsorption and ocular observations based on colour changes on laboratory and real samples. The results show that the absence of exosomes significantly inhibited the peroxidase-like activity of CD63 aptamer-Fe₃O₄-Cu²⁺-NZs due to the aptamer coating. Under optimal conditions, the designed CD63 aptamer-Fe₃O₄-Cu²⁺-NZs is capable of detecting exosomes in the range of 1.4 × 10⁴-5.6 × 10⁵ particles/μL with a detection limit of 5.91 × 10³ particles/μL. Also, this method showed a satisfactory outcome in detection of cancer cells in real samples. Overall, this colorimetric aptasensor can be used to diagnose breast cancer cells based on a simple and inexpensive approach.

Highly sensitive and low-cost colourimetric detection of glucose and ascorbic acid based on silver nanozyme biosynthesized by Gleditsia australis fruit

In this study, a simple, eco-friendly and low-cost approach was used to fabricate silver nanoparticles (AgNPs) from an aqueous extract of Gleditsia australis (GA) fruit. The nanoparticles synthesized in the optimal condition have an average size of 14 nm. The peroxidase-like activity of GA-AgNP in the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in combination with hydrogen peroxide (H₂O₂) was investigated. Further, optimal conditions for the use of peroxidase-like catalytic activity in sensing applications were identified. The colourimetric detection of H₂O₂ showed a linear range of 1-8 mM with a limit of detection (LOD) of 0.34 mM. The oxidation of TMB (red-TMB) enables the detection of glucose, which is converted into H₂O₂ and gluconic acid in the presence of the enzyme glucose oxidase. The observations showed linearity from 0.05 to 1.5 mM with a LOD of 0.038 mM. Moreover, the blue colour of oxidized TMB (ox-TMB) was reduced according to ascorbic acid (AA) concentration, with a linear range of 0.03-0.14 mM and a LOD of 3.0 μM. The practical use of the sensing system for the detection of AA was studied using real fruit juice and showed good sensitivity. Hence, the easy-to-use peroxidase-like sensor provides a new platform for the detection of bioactive compounds in biological systems.

CoO Nanozymes with Multiple Catalytic Activities Regulate Atopic Dermatitis

Herein, we prepared CoO nanozymes with three types of enzyme catalytic activities for the first time, which have SOD-like, CAT-like, and POD-like catalytic activities. This is the first study to report the preparation of CoO nanoparticles with three types of enzyme catalytic activities by the one-pot method. By modifying the surface of CoO nanozymes with a carboxyl group, its biocompatibility enhanced, so it can be used in the field of life sciences. In vitro cytotoxicity and anti-H₂O₂-induced ROS experiments proved that CoO nanozymes can protect HaCaT cells against ROS and cytotoxicity induced by H₂O₂. In addition, an atopic dermatitis (AD) mouse model was established by topical application of MC903, which verified the anti-inflammatory effect of CoO nanozymes on the AD mouse model. Traditional drugs for the treatment of AD, such as dexamethasone, have significant side-effects. The side-effects include skin burns, telangiectasias, and even serious drug dependence. CoO nano-enzymes have a low cytotoxicity and its multiple enzyme-like catalytic activities can effectively protect cells and tissues in ROS environments, which proves that CoO nano-enzymes have high application potential in the field of anti-inflammation.

β-Cyclodextrin-Stabilized Biosynthesis Nanozyme for Dual Enzyme Mimicking and Fenton Reaction with a High Potential Anticancer Agent

The myth of inactivity of inorganic materials in a biological system breaks down by the discovery of nanozymes. From this time, the nanozyme has attracted huge attention for its high durability, cost-effective production, and easy storage over the natural enzyme. Moreover, the multienzyme-mimicking activity of nanozymes can regulate the level of reactive oxygen species (ROS) in an intercellular system. ROS can be generated by peroxidase (POD), oxidase (OD), and Fenton-like catalytic reaction by a nanozyme which kills the cancer cells by oxidative stress; therefore, it is important in CDT (chemo dynamic therapy). Our current study designed to investigate the enzyme mimicking behavior and anticancer ability of cerium-based nanomaterials because the cerium-based materials offer a high redox ability while maintaining nontoxicity and high stability. Our group synthesized CeZrO₄ nanoparticles by a green method using β-cyclodextrin as a stabilizer and neem leaf extract as a reducing agent, exhibiting POD- and OD-like dual enzyme activities. The best enzyme catalytic activity is shown in pH = 4, indicating the high ROS generation in an acidic medium (tumor microenvironment) which is also supported by the Fenton-like behavior of CeZrO₄ nanoparticles. Inspired by the high ROS generation in vitro method, we investigated the disruption of human kidney cells by this nanoparticle, successfully verified by the MTT assay. The harmful effect of ROS in a normal cell is also investigated by the in vitro MTT assay. The results suggested that the appreciable anticancer activity with minimal side effects by this synthesized nanomaterial.

Transferrin guided quasi-nanocuboid as tetra-enzymic mimics and biosensing applications

Given the promising prospect of nanozymes system with multi-enzyme mimetic activities, it is also a challenge for designing a controllable nanostructure as multi-enzymes mimics by protein-guided strategy. Here, transferrin (Trf)-directed manganese oxide with 3D nanomorphology was developed. Trf-Mn₃O₄ quasi-nanocuboids (NCs) was obtained by improved conditions for biomineralization, which employing Trf as biotemplate and Mn²⁺ as metal source in alkaline solution. Fortunately, not only was the controllable structure discovered, but Trf-Mn₃O₄ NCs also showed tetra-enzyme mimic activities involving peroxidase-, ascorbic acid oxidase-, catalase- and superoxide dismutase-mimic activities. Further, the catalytic properties and steady-state kinetic of Trf-Mn₃O₄ NCs was explored, systematically. Moreover, we develop simple colorimetric sensor based on the peroxidase-mimic activity of Trf-Mn₃O₄ NCs for the detection of gallic acid (GA) with the linear range within 0.1-40 μΜ, and the limit of detection was 41.2 nM (S/N = 3). Besides, a novel fluorimetric sensor for the detection of AA based on the ascorbic acid oxidase-mimic activity was proposed with the linear range of 0.5 μM-10 μM (R² = 0.9906) and 10 μM-5 mM (R² = 0.9927), and LOD of 0.24 μM (S/N = 3) was obtained. Both the proposed sensors showed outstanding detection performance, accuracy and repeatability, which realized the detection of GA and AA in real sample, respectively. The as-synthesized tetra-enzymic mimics not only enriched the species of nanozymes, particularly for the nanozymes with multi-enzyme mimic activities, the proposed sensors showed promising potential applications for biosensor in complex samples.

Immobilization of horseradish peroxidase on lysine-functionalized gum Arabic-coated Fe3O4 nanoparticles for cholesterol determination

BACKGROUND

Horseradish Peroxidase (HRP) is ranked as one of the most important industrial enzymes that is extensively used in industry. Cholesterol is routinely detected indirectly by cholesterol oxidase in the presence of O₂, liberating H₂O₂ as a by-product. The H₂O₂ content is determined through the HRP activity in the presence of a redox dye, producing a red colored quinoneimine which can be measured quantitatively. Herein, we have designed a magnetic nanoparticle for reusing and easily separating HRP as the most expensive compartment for the low-cost cholesterol assay.

METHODS

The gum Arabic coated magnetic nanoparticles were functionalized with L-lysine linker for maintaining protein flexibility on nanoparticle. Enzyme-loaded nanoparticles were characterized by TEM, FTIR, DLS, VSM and XRD analysis.

RESULTS

The immobilization efficiency was ∼65% and the immobilized HRP retained 60% of its activity after 8 times reuse. The optimum pH and thermal stability shifted from 7.0 to 8.0 and 60 to 70 °C after immobilization, respectively. Storage stability of HRP was improved by 10%, at 4 °C for 60 days. Immobilized HRP showed more catalytic activity in presence of Fe²⁺, Ca²⁺ and Na⁺. The designed system has cholesterol detection linearity range from 0.2 to 5.0 mM and detection limit of 0.08 mM and acceptable correlation coefficient of 0.9973 and 0.9982 on sample serum using both chromogens.

CONCLUSION

The HRP-loaded magnetic nanoparticles are capable of being used as a cost-effective system for cholesterol determination in laboratory due to its reusability and stability benefits.

Amplified Peroxidase-like Activity of Co2+ Using 8-Hydroxyquinoline and Its Application for Ultrasensitive Colorimetric Detection of Clioquinol

8-Hydroxyquinoline (8HQ) and its derivatives display diverse bioactivities and therapeutic potentials. In this study, we unveiled that 8HQ can boost the peroxidase-like activity of Co²⁺ in the presence of bicarbonate (HCO₃ ^- ) in neutral pH at room temperature. With 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS) as the substrate, the formed Co²⁺ /8HQ/HCO₃ ^- complex shows robust catalytic activity with the turnover number (k_cat ) tens to hundreds of times higher than that of Co₃ O₄ and other Co²⁺ complexes in terms of per cobalt ion. This system was used to design colorimetric sensors for ultrasensitive detection of 8HQ-based drugs by activating the activity of Co²⁺ . Take detecting clioquinol as an example, a detection limit of 2.4 nM clioquinol with a linear range from 0.01 to 0.2 μM was obtained. This work not only revealed a new kind of ligand that activated the activity of Co²⁺ , but also provided a facile, low-cost, ultrasensitive, easy-to-use, and universal strategy for sensing various 8HQ-based drugs. Further development of this catalytic system might be beneficial to overcome drug resistance by combined medication.

CuxO nanorods with excellent regenerable NADH peroxidase mimics and its application for selective and sensitive fluorimetric ethanol sensing

Cu_xO nanorods with excellent NADH peroxidase mimics were synthesized by a simple hydrothermal method. The catalytic oxidation of NADH to NAD cofactor strictly follows the enzymatic kinetics with high catalytic rate and strong affinity. The catalytic mechanism of Cu_xO NRs was that in the presence of hydrogen peroxide, the catalytic oxidizing NADH to NAD ⁺ involving with O₂^.-.anion production, making it realistic to mutually convert between coenzymes. Considering that the mutual transformation of NADH/NAD cofactors plays an important role in biological function, combination of Cu_xO NRs with alcohol dehydrogenase, a highly selective method for fluorimetric detection of ethanol was established. The as-proposed sensing platform is capable of dectecting alcohol with the limit of detection of 26.7 μM (S/N = 3) and applied in practical sample with satisfied accuracy and recovery. The as-developed regenerable NADH peroxidase mimics would also cast lights in biocatalysis, synthetic biology and bioenergy.

Enzyme (Single and Multiple) and Nanozyme Biosensors: Recent Developments and Their Novel Applications in the Water-Food-Health Nexus

The use of sensors in critical areas for human development such as water, food, and health has increased in recent decades. When the sensor uses biological recognition, it is known as a biosensor. Nowadays, the development of biosensors has been increased due to the need for reliable, fast, and sensitive techniques for the detection of multiple analytes. In recent years, with the advancement in nanotechnology within biocatalysis, enzyme-based biosensors have been emerging as reliable, sensitive, and selectively tools. A wide variety of enzyme biosensors has been developed by detecting multiple analytes. In this way, together with technological advances in areas such as biotechnology and materials sciences, different modalities of biosensors have been developed, such as bi-enzymatic biosensors and nanozyme biosensors. Furthermore, the use of more than one enzyme within the same detection system leads to bi-enzymatic biosensors or multi-enzyme sensors. The development and synthesis of new materials with enzyme-like properties have been growing, giving rise to nanozymes, considered a promising tool in the biosensor field due to their multiple advantages. In this review, general views and a comparison describing the advantages and disadvantages of each enzyme-based biosensor modality, their possible trends and the principal reported applications will be presented.

Copper sulfide nanoclusters with multi-enzyme-like activities and its application in acid phosphatase sensing based on enzymatic cascade reaction

Nanozymes are artificial enzymes, which can substitute natural enzymes for wide range of catalysis-based applications. However, it is challenging to explore novel mimic enzymes or multi-enzyme mimics. Herein we report the facile preparation of uniform CuS nanoclusters (NCs), which possessed outstanding tetra-enzyme mimetic catalytic activities, including peroxidase (POD)-mimics, catalase (CAT)-mimics, ascorbic acid oxidase (AAO)-mimics and superoxide dismutase (SOD)-mimics. The catalytic mechanism of POD-like was coming from the oxygen vacancies of CuS. Furthermore, the steady-state kinetics of POD-, CAT- and AAO mimics of CuS NCs were systematically explored. On basis of the enzymatic cascade reaction that acid phosphatase (ACP) involved in a weak acidic environment, in the presence of O-phenylenediamine, quinoxaline fluorescent substance will be generated. Thus, a fluorescent biosensor platform was proposed for detection of ACP with the linear range of 0.05-25 U L^-1 and limit of detection of 0.01 U L^-1. The as-proposed method was applicable to real serum sample detection accurately and reproducibly. Considering the simple preparation, good stability, excellent multi-enzyme activities and controllable experimental operation, CuS NCs would provide a basis for expanding to other biocatalytic and biomedical applications.

Modification of ordered mesoporous carbon for removal of environmental contaminants from aqueous phase: A review

Contamination of water bodies by potentially toxic elements and organic pollutants has aroused extensive concerns worldwide. Thus it is significant to develop effective adsorbents for removing these contaminants. As a new member of carbonaceous material families (activated carbon, biochar, and graphene), ordered mesoporous carbon (OMC) with larger specific surface area, ordered pore structure, and higher pore volume are being evaluated for their use in contaminant removal. In this paper, modification techniques of OMC were systematically reviewed for the first time. These include nonmetallic doping modification (nitrogen, sulfur, and boron) and the impregnation of nano-metals and metal oxides (iron, copper, cobalt, nickel, magnesium, and rare earth element). Reaction conditions (solution pH, reaction temperature, sorbent dosage, and contact time) are of critical importance for the removal performance of contaminants onto OMC. In addition, the pristine and modified OMC have been investigated for the removal of a range of contaminants, including cationic/anionic toxic elements and organic contaminants (synthetic dye, phenol, and others), and involving different and specific mechanisms of interaction with contaminants. The future research directions of the application of pristine and modified OMC were proposed. Overall, this review can provide sights into the modification techniques of OMC for removal of environmental contaminants.

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.

2D Co3O4 stabilizing Rh nano composites developed for visual sensing bioactive urea and toxic p-aminophenol in practice by synergetic-reinforcing oxidase activity

To enlarge the perspective of nanozyme, 2-dimensional Co₃O₄ stabilizing Rh nano composite (2D Co₃O₄@Rh NC) was identified and developed first by one-pot surfactant-aided oxido-reduction. By virtue of the synergetic-reinforcing oxidase activity between 2D Co₃O₄ substrate and Rh nano particles, the obtained 2D Co₃O₄@Rh NC could catalyze the oxidation of chromogenic substrate 3,3',5,5,'-tetramethylbenzidine (TMB) to blue oxTMB with quite a low Michaelis-Menten constant (K_m) of 0.018 mM and a quick v_max of 6.45 × 10^-8 M s^-1, expressing superior oxidase-like catalysis with a wide temperature range from 20 to 60 °C. Importantly, either bioactive urea or toxic p-aminophenol (p-Ap) could exclusively alter the existed state of oxTMB with differentiable color changes. Under the optimized conditions, 2D Co₃O₄@Rh NC was successfully applied for ratiometric colorimetric sensing urea and p-Ap in environmental water, soil and urine samples with low detection limits (1.1 μM for urea and 0.68 μM for p-Ap) and satisfactory recoveries (96.0-105.8%). The synergetic enhanced oxidase-like activity of 2D Co₃O₄@Rh NC and the different reaction mechanisms of the 2D Co₃O₄@Rh NC-TMB system to urea and p-Ap were investigated. Not only does the work provide an efficient way for sensing organic pollution of p-Ap, it will offer an efficient potential for diagnosing urea-related diseases on clinical medical testing in future.

PVP-stabilized PtRu nanozymes with peroxidase-like activity and its application for colorimetric and fluorometric glucose detection

Nanozymes have significant advantages over natural enzymes. The intrinsic peroxidase-like activity of Pt-based nanomaterials can be enhanced by alloying with other transition metals, such as Ru, that have great catalytic activity. In this study, we used polyvinylpyrrolidone (PVP) to synthesize well-dispersed and homogeneous nanostructures. PVP-stabilized Pt-Ru nanozymes (PVP/PtRu NZs) were synthesized and characterized. The PVP/PtRu NZs had an average size of 3.54 ± 0.84 nm and exhibited an intense peroxidase-like activity. The PVP/PtRu NZs were used as peroxidase mimics for colorimetric and fluorometric glucose determination by the glucose oxidase and PVP/PtRu NZs cascade reaction. In the colorimetric assay, the linearly detectable range was 0.25-3.0 mM, with an R² and limit of detection (LOD) of 0.988 and 138 μM, respectively. In the fluorometric assay, a linear relationship was found when the glucose concentration was between 5.0 and 300 μM (R² = 0.997), with an LOD of 1.11 μM. Compared to the colorimetric assay, the fluorometric assay had greater sensitivity and a lower detection limit for the determination of glucose. Moreover, the PVP/PtRu NZs had high storage stability over a month and great recovery values in human serum and artificial urine, with a range of 94-106 %. From these results, PVP/PtRu NZs are expected to be used as promising peroxidase mimics in various fields such as biosensing, pharmaceutical processing, and the food industry.

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.

Green synthesis of platinum nanoclusters using lentinan for sensitively colorimetric detection of glucose

The sensitive colorimetric detection of glucose using nanomaterials has been attracting considerable attention. To improve the detection sensitivity, highly stable lentinan stabilized platinum nanoclusters (Pt-LNT NCs) were prepared, in which lentinan was employed as a mild reductant and stabilizer. The size of platinum nanoclusters (Pt NCs) was only 1.20 ± 0.29 nm. Pt-LNT NCs catalyzed the oxidation of substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂) to produce a blue oxidation product with absorption peak at 652 nm, indicating their peroxidase-like properties. Their enzymatic kinetics followed typical Michaelis-Menten theory. In addition, fluorescence experiments confirmed their ability to efficiently catalyze the decomposition of H₂O₂ to generate •OH, which resulted in the peroxidase-like mechanism of Pt-LNT NCs. Moreover, a colorimetric method for highly selective and sensitive detection of glucose was established by using Pt-LNT NCs and glucose oxidase. The linear range of glucose detection was 5-1000 μM and the detection limit was 1.79 μM. Finally, this method was further used for detection of glucose in human serum and human urine. The established colorimetric method may promote the development of biological detection and environmental chemistry in the future.

A review on metal nanozyme-based sensing of heavy metal ions: Challenges and future perspectives

Large scale mining, manufacturing industries, exploitation of underground water, depletion of groundwater level, and uncontrolled discharge of industrial wastes have caused severe heavy metal ion pollution to the environment throughout the world. Therefore, the rapid detection of such toxic metal ions is inevitable. However, conventional methods require sophisticated instruments and skilled manpower and are difficult to operate in on-field conditions. Recently, metal nanozyme-based assays have been found to have the potential as an alternative to conventional methods due to their portability, simplicity, and high sensitivity to detect metal ion concentration to as low as parts per trillion (ppt). Metal nanozyme-based systems for heavy metal ions enable rapid and cheap screening on the spot with a very simple instrument such as a UV-vis absorption spectrophotometer and therefore, are convenient for use in field operations, especially in remote parts of the world. The sensing mechanism of a nanozyme-based sensor is highly dependent on its surface properties and specific interactions with particular metal ion species. Such method often encounters selectivity issues, unlike natural enzyme-based assays. Therefore, in this review, we mainly focus our discussion on different types of target recognition and inhibition/enhancement mechanisms, and their responses toward the catalytic activity in the sensing of target metal ions, design strategies, challenges, and future perspectives.

Gold nanoplates with superb photothermal efficiency and peroxidase-like activity for rapid and synergistic antibacterial therapy

Gold nanoplates exhibit 68.5% photothermal conversion efficiency and peroxidase-like activity, and AuNPTs (50 μg mL⁻¹)/H₂O₂ (0.1 mM)/NIR (1 W cm⁻², 3 min) show excellent synergistic antibacterial ability and promote MRSA-infected wound healing in vivo.

Facile one-pot synthesis of Mn3O4 nanorods and their analytical application

One-pot synthesis of Mn3O4 nanorods in aqueous solution at room temperature without using templates and surfactants was achieved for the first time, opening a new route for preparing various metal nanorods for detecting H2O2-related targets.

Facile Preparation of Homogeneous Copper Nanoclusters Exhibiting Excellent Tetraenzyme Mimetic Activities for Colorimetric Glutathione Sensing and Fluorimetric Ascorbic Acid Sensing

Nanozymes are artificial enzymes, which can substitute traditional biological enzymes for multifield applications. However, to date, it remains challenging to search novel mimic enzymes or multienzyme mimics. Herein, a facile and green method for preparing monodisperse, homogeneous copper nanoclusters (Cu NCs) with smaller size was developed, which used cysteamine as a template and hydrazine hydrate as a reductant to reduce Cu²⁺. The as-prepared Cu NCs exhibited excellent tetraenzyme-like activities, including peroxidase (POD)-, catalase (CAT)-, superoxide dismutase (SOD)-, and ascorbic acid oxidase (AAO)-mimic activities. The mechanisms, kinetics, and catalytic performances of Cu NCs were systematically studied. Moreover, based on the POD-like activity of Cu NCs, sensitive and simple colorimetric sensing glutathione (GSH) was explored, with the low limit of detection of 0.89 μM GSH (S/N = 3). Additionally, a novel fluorimetric ascorbic acid (AA) sensor was developed with the linear range of 0.5-30 μM and limit of detection (LOD) of 0.144 μM, on the basis of the principle that AA is oxidized to dehydroascorbic acid (DHAA) specifically catalyzed by the AAO-like activity of Cu NCs, while DHAA can further react with o-phenylenediamine (OPDA) to generate a highly fluorescent quinoxaline (DFQ) derivative. The as-proposed colorimetric GSH sensor and the fluorimetric AA sensor were capable of detecting GSH and AA, respectively, in real samples accurately and reproducibly. Thus, the Cu NCs-based multienzyme mimic is a promising candidate for biocatalysis and biosensing.

Colorimetric Assay of Bacterial Pathogens Based on Co3O4 Magnetic Nanozymes Conjugated with Specific Fusion Phage Proteins and Magnetophoretic Chromatography

It is important to detect pathogens rapidly, sensitively, and selectively for clinical medicine, homeland security, food safety, and environmental control. We report here a specific and sensitive colorimetric assay that incorporated a bovine serum albumin-templated Co₃O₄ magnetic nanozyme (Co₃O₄ MNE) with a novel specific fusion phage protein and magnetophoretic chromatography to detect Staphylococcus aureus. The Co₃O₄ MNE was conjugated to S. aureus-specific fusion-pVIII (Co₃O₄ MNE@fusion-pVIII), screened from the S. aureus-specific phage AQTFLGEQD (the phage monoclone is denoted by the peptide sequence). The as-prepared triple-functional Co₃O₄ MNE@fusion-pVIII particles were capable of capturing S. aureus in sterile milk, which were then isolated from milk magnetically. Assisted by polyethylene glycol, the Co₃O₄ MNE@fusion-pVIII@S. aureus complex was separated from the free Co₃O₄ MNE@fusion-pVIII by magnetophoretic chromatography in an external magnetic field. After transferring the isolated Co₃O₄ MNE@fusion-pVIII@S. aureus complexes into a 96-well plate, diammonium salt of 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) and H₂O₂ were added to develop color because of the peroxidase mimetics activity of the Co₃O₄ MNE. A S. aureus concentration within 10-10,000 cfu/mL in milk can be detected (detection limit: 8 cfu/mL). The as-developed method is simple, cost-efficient, and sensitive, which is useful for rapidly diagnosing pathogenic bacteria and helpful to prevent disease outbreaks induced by pathogens in developing countries.

A label-free immunosensor for the sensitive detection of hepatitis B e antigen based on PdCu tripod functionalized porous graphene nanoenzymes

Nanomaterials with enzyme properties possess excellent catalytic activity and stability. We prepared new nanoenzymes to construct a label-free electrochemical immunosensor for the detection of hepatitis B e antigen (HBe Ag). In this study, PdCu tripod (PdCu TP) functionalized porous graphene (PG) nanoenzymes (PdCu TPs/PG) were prepared through the in situ reduction of PdCu tripods onto porous graphene. The catalytic Michaelis-Menten kinetic parameters of PdCu TPs/PG are better than horseradish peroxidase (HRP) and show enhanced peroxidase-like activity. Therefore, we used PdCu TPs/PG to catalyse the electrochemically active matrix of H₂O₂ and generate the synergistically amplified current signal for the subsequent sensitive detection of HBe Ag. Due to the good conductivity, large specific surface area and synergistic amplification of PdCu TPs/PG, the quantitative detection of HBe Ag shows a detection limit of 20 fg·mL^-1 and linear range from 60 fg·mL^-1 to 100 ng·mL^-1. During the detection of human serum samples, PdCu TPs/PG shows good accuracy based on the standard addition method and a comparison with an ELISA. The prepared immunosensors exhibiting good selectivity, stability and reproducibility provide an important basis for determining the prognosis of hepatitis B and show potential applications in medical applications.

Rapid colorimetric sensing of ascorbic acid based on the excellent peroxidase-like activity of Pt deposited on ZnCo2O4 spheres

Pt/ZnCo₂O₄ composites were firstly found to act as artificial peroxidases and used to construct colorimetric sensing platforms for detecting H₂O₂ and ascorbic acid.

Rock salt type NiO assembled on ordered mesoporous carbon as peroxidase mimetic for colorimetric assay of gallic acid

Mimetic enzymes are of scientific and practical significance as alternatives for natural enzymes because the latter ones have drawbacks such as instability and high cost. Here we report that ordered mesoporous carbon supported rock salt type NiO nanocomposite (NiO-OMC) exhibited interesting peroxidase-like activity in the typical reaction of H₂O₂ and 3,3',5,5'-tetramethylbenzidine. Further, based on gallic acid (GA) quenching the catalytic activity of NiO-OMC nanocomposite, selective colorimetric detection of GA was developed. The decreased absorbance @650 nm in the presence of GA is linear with GA concentration within 0.2-10 μM. The limit of detection is 0.112 μM GA. The proposed method is applicable to real samples measurements.