A Cu2(OH)3Cl-CeO2 nanocomposite with peroxidase-like activity, and its application to the determination of hydrogen peroxide, glucose and cholesterol

Scalable synthesis of Au@CeO2 nanozyme for development of colorimetric lateral flow immunochromatographic assay to sensitively detect heart-type fatty acid binding protein

Nanozymes with core@shell nanostructure are considered promising biolabeling materials for their multifunctional properties. In this work, a simple one-pot strategy has been proposed for scalable synthesis of gold@cerium dioxide core@shell nanoparticles (Au@CeO2 NPs) with strong localized surface plasmon resonance (LSPR) absorption and high peroxidase-like catalytic activity by redox reactions of Ce3+ ions and AuCl4- ions in diluted ammonia solution under room temperature. A colorimetric lateral flow immunochromatographic assay (LFIA) has been successfully fabricated for sensitive detection of heart-type fatty acid binding protein (H-FABP, an early cardiac biomarker) by using the Au@CeO2 NPs as reporters. The as-developed LFIA with Au@CeO2 NP reporter (termed as Au@CeO2-LFIA) exhibits a dynamic range of nearly two orders of magnitude, and a limit of detection (LOD) as low as 0.35 ng mL-1 H-FABP with nanozyme-triggered 3,3',5,5'-tetramethylbenzidine (TMB) colorimetric amplification. Furthermore, the practicality of Au@CeO2-LFIA has been demonstrated by profiling the concentrations of H-FABP in 156 blood samples of acute myocardial infarction (AMI) patients, and satisfactory results are obtained.

Graphene quantum dots on TiO2 nanotubes as a light-assisted peroxidase nanozyme

Hybrid nanozyme graphene quantum dots (GQDs) deposited TiO2 nanotubes (NTs) on titanium foil (Ti/TiO2 NTs-GQDs) were manufactured by bestowing the hybrid with the advantageous porous morphology, surface valence states, high surface area, and copious active sites. The peroxidase-like activity was investigated through the catalytic oxidation of chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2, which can be visualized by the eyes. TiO2 NTs and GQDs comprising oxygen-containing functional groups can oxidize TMB in the presence of H2O2 by mimicking peroxidase enzymes. The peroxidase-mimicking activity of hybrid nanozyme was significantly escalated by introducing light illumination due to the photosensitive features of the hybrid material. The peroxidase-like activity of Ti/TiO2 NTs-GQDs enabled H2O2 determination over the linear range of 7 to 250 μM, with a LOD of 2.1 µM. The satisfying peroxidase activity is possibly due to the unimpeded access of H2O2 to the catalyst's active sites. The porous morphology provides the easy channeling of reactants and products. The periodic structure of the material also gave rise to acceptable reproducibility. Without material functionalization, the Ti/TiO2 NTs-GQDs can be a promising substitute for peroxidases for H2O2 detection.

Peroxidase activity of a Cu-Fe bimetallic hydrogel and applications for colorimetric detection of ascorbic acid

A Cu-Fe bimetallic hydrogel (2-QF-CuFe-G) was constructed through a simple method. The 2-QF-CuFe-G metallohydrogel possesses excellent peroxidase-like activity to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. The catalytic mechanism was confirmed by the addition of •OH radical scavenger isopropyl alcohol (IPA), tert-butyl alcohol (TBA) and ˙OH trapping agent terephthalic acid (TA). Remarkably, the resultant blue ox-TMB system can be used to selectively and sensitively detect ascorbic acid (AA) with an LOD of 0.93 μM in the range of 4-36 μM through the colorimetric method. Moreover, the assay based on the 2-QF-CuFe-G metallohydrogel can be successfully applied to detect AA in fresh fruits.

Porphyrin-Based Covalent Organic Frameworks with Donor-Acceptor Structure for Enhanced Peroxidase-like Activity as a Colorimetric Biosensing Platform

Hydrogen peroxide (H₂O₂) and glucose play a key role in many cellular signaling pathways. The efficient and accurate in situ detection of H₂O₂ released from living cells has attracted extensive research interests. Herein, a new porphyrin-based porous covalent organic framework (TAP-COF) was fabricated via one-step condensation of 1,6,7,12-tetrachloroperylene tetracarboxylic acid dianhydride and 5,10,15,20-tetrakis (4-aminophenyl)porphyrin iron(III). The obtained TAP-COF has high surface areas, abundant surface catalytic active sites, and highly effective electron transport due to its precisely controllable donor-acceptor arrangement and 3D porous structure. Then, the new TAP-COF exhibited excellent peroxidase-like catalytic activity, which could effectively catalyze oxidation of the substrate 3,3',5,5'-tetramethylbenzidine by H₂O₂ to produce a typical blue-colored reaction. On this basis, simple, rapid and selective colorimetric methods for in situ H₂O₂ detection were developed with the detection limit of 2.6 nM in the wide range of 0.01 to 200 μM. The colorimetric approach also could be used for in situ detection of H₂O₂ released from living MCF-7 cells. This portable sensor based on a COF nanozyme not only opens a new path for point-of-care testing, but also has potential applications in the field of cell biology and clinical diagnosis.

Tailoring of the photocatalytic activity of CeO2 nanoparticles by the presence of plasmonic Ag nanoparticles

The present study investigates basic features of a photoelectrochemical system based on CeO₂ nanoparticles fixed on gold electrodes. Since photocurrent generation is limited to the absorption range of the CeO₂ in the UV range, the combination with metal nanoparticles has been studied. It can be shown that the combination of silver nanoparticles with the CeO₂ can shift the excitation range into the visible light wavelength range. Here a close contact between both components has been found to be essential and thus, hybrid CeO₂@Ag nanoparticles have been prepared and analyzed. We have collected arguments that electron transfer occurs between both compositional elements of the hybrid nanoparticles.The photocurrent generation can be rationalized on the basis of an energy diagram underlying the necessity of surface plasmon excitation in the metal nanoparticles, which is also supported by wavelength-dependent photocurrent measurements. However, electrochemical reactions seem to occur at the CeO₂ surface and consequently, the catalytic properties of this material can be exploited as exemplified with the photoelectrochemical reduction of hydrogen peroxide. It can be further demonstrated that the layer-by layer technique can be exploited to create a multilayer system on top of a gold electrode which allows the adjustment of the sensitivity of the photoelectrochemical system. Thus, with a 5-layer electrode with hybrid CeO₂@Ag nanoparticles submicromolar hydrogen peroxide concentrations can be detected.

Investigation on the Peroxidase-like Activity of Vitamin B6 and Its Applications in Colorimetric Detection of Hydrogen Peroxide and Total Antioxidant Capacity Evaluation

The peroxidase-like activity of vitamin B6 (VB6) was firstly demonstrated by catalyzing the peroxidase chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB) at the existence of H2O2. The influence of different factors on the catalytic property of VB6, including pH, temperature, VB6 concentration, and incubation time, were investigated. The steady-state kinetic study results indicate that VB6 possesses higher affinity to H2O2 than natural horseradish peroxidase and some other peroxidase mimics. Besides, the radical quenching experiment results confirm that hydroxyl radical (•OH) accounts for the catalytic process. Based on the excellent peroxidase-like catalytic activity of VB6, the colorimetric methods for H2O2 and gallic acid (GA) detection were developed by measuring the absorbance variance of the catalytic system. Under the optimal conditions, the linear ranges of the methods for H2O2 and GA determination with good selectivity are 50.0–600.0 μM and 10.0–50.0 μM, respectively. In addition, the developed method was applied in the detection of H2O2 in milk samples and evaluation of total antioxidant capacity of different tea infusions. This study may broaden the application prospect of VB6 in environmental and biomedical analysis fields, contribute to profound insight of the physiological functions of VB6, as well as lay foundation for further excavation of small-molecule peroxidase mimics.

N, S-co-doped carbon/Co1-xS nanocomposite with dual-enzyme activities for a smartphone-based colorimetric assay of total cholesterol in human serum

We fabricated a novel N,S-co-doped carbon/Co_1-xS nanocomposite (NSC/Co_1-xS) using a facile sol-gel approach, which featured a multiporous structure, abundant S vacancies and Co-S nanoparticles filling the carbon-layer pores. When the Co_1-xS nanoparticles were anchored onto the surface of N,S-co-doped carbon, a synergistic catalysis action occurred. The NSC/Co_1-xS nanocomposites possessed both peroxidase-like and oxidase-mimetic dual-enzyme activities, in which the oxidase-mimetic activity dominated. By scavenger capture tests, the nanozyme was demonstrated to catalyze H₂O₂ to produce h⁺, •OH and •O₂^-, among which the strongest and weakest signals were h⁺ and •OH, respectively. The multi-valence states of Co atoms in the NSC/Co_1-xS structure facilitated electronic transfer that enhanced redox reactions, thereby improving the resultant color reaction. Based on the NSC/Co_1-xS's enzyme-mimetic catalytic reaction, a visual colorimetric assay and Android "Thing Identify" application (app), installed on a smartphone, offered detection limits of 1.93 and 2.51 mg/dl, respectively, in human serum samples. The selectivity/interference experiments, using fortified macromolecules and metal ions, demonstrated that this sensor had high selectivity and low interference potential for cholesterol analysis. Compared to standard assay kits and previously reported visual detection, the Android smartphone-based assays provided higher accuracy (recoveries up to 93.6-104.1%), feasibility for trace-level detection, and more convenient on-site application for cholesterol assay due to the superior enzymatic activity of NSC/Co_1-xS. These compelling performance metrics lead us to posit that the NSC/Co_1-xS-based nanozymic sensor offers a promising methodology for several practical applications, such as point-of-care diagnosis and workplace health evaluations.

Ionic-Liquid-Stabilized TiO2 Nanostructures: A Platform for Detection of Hydrogen Peroxide

Hydrogen peroxide (H2O2) acts as a signaling molecule to direct different biological processes. However, its excess amount results in oxidative stress, which causes the onset of different types of cancers. TiO2 nanostructure was synthesized by a facile hydrothermal method. The prepared material was characterized by FTIR spectroscopy, XRD, SEM, EDX, TGA, and Raman spectroscopy, which confirmed the formation of nanostructured material. Subsequently, the prepared nanoparticles (NPs) were capped with 1-H-3-methylimidazolium acetate ionic liquid (IL) to achieve its deagglomeration and functionalization. A new colorimetric sensing probe was prepared for the detection of H2O2 based on ionic liquid-capped TiO2 nanoparticles (TiO2/IL) and 3,3',5,5'-tetramethylbenzidine (TMB) dye, which acts as an oxidative chromogenic substrate. H2O2 reacts with TMB, in the presence of ionic liquid-coated TiO2 NPs, to form a blue-green product. The color was visualized with the naked eye, and the colorimetric change was confirmed by a UV-vis spectrophotometer. To obtain the best response of the synthesized sensor, different parameters (time, pH, concentrations, loading of nanomaterials) were optimized. It showed a low limit of detection 8.61 × 10-8 M, a high sensitivity of 2.86 × 10-7 M, and a wide linear range of 1 × 10-9-3.6 × 10-7 M, with a regression coefficient (R 2) value of 0.999. The proposed sensor showed a short incubation time of 4 min. The sensing probe did not show any interference from the coexisting species. The TiO2/IL sensor was effectively used for finding H2O2 in the urine samples of cancer patients.

Ultrathin PdCu alloy nanosheet-assembled 3D nanoflowers with high peroxidase-like activity toward colorimetric glucose detection

Enzyme-mimetic properties of nanomaterials can be efficiently tuned by controlling their size, composition, and structure. Here, ultrathin PdCu alloy nanosheet-assembled three-dimensional (3D) nanoflowers (Pd₁Cu_x NAFs) with tunable surface composition are obtained via a generalized strategy. In presence of H₂O₂, the as-synthesized Pd₁Cu_x NAFs can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the oxidized form of TMB (oxTMB) with a characteristic absorption peak at 652 nm. Interestingly, Pd₁Cu_x NAFs show obviously composition-dependent peroxidase-like catalytic activities because of the synergistic interaction of nanoalloy. Additionally, different from 2D Pd nanosheets, the distinctive 3D superstructures are featured with rich approachable sites and proper layer spacing, which are in favor of fast mass transport and electron transfers during the catalytic process. Among the studied Pd₁Cu_x NAFs, the Pd₁Cu_1.7 NAFs show the highest enzyme-like activities and can be successfully applied for the colorimetric detection of glucose with a low detection limit of 2.93 ± 0.53 μM. This work provides an efficient avenue to fabricate PdCu NAF nanozymes in biosensing toward glucose detection. Two-dimensional (2D) PdCu ultrathin nanosheet-assembled 3D nanoflowers (Pd₁Cu_x NAFs) with tunable surface composition exhibit substantially enhanced intrinsic peroxidase-like catalytic activities. The Pd₁Cu_1.7 NAFs are successfully used as peroxidase mimic catalyst for the colorimetric detection of glucose with low detection limit of 2.93 μM.

The excellent peroxidase-like activity of uniform CuCo2O4 microspheres with oxygen vacancy for fast sensing of hydrogen peroxide and ascorbic acid

The uniform CuCo₂O₄ microspheres with oxygen vacancy were firstly found to possess excellent peroxidase-like activity which is essential for constructing a rapid and facile colorimetric sensor to determine H₂O₂ and interrelated biomolecules.

Cu2O nanocubes-grafted highly dense Au nanoparticles with modulated electronic structures for improving peroxidase catalytic performances

Based on the intermediate states of metal ions in metal oxide nanomaterials (NMs) that acted as the primary active species, the design of high-performance nanozymes has greatly stimulated current research in diverse biomedical applications. Herein, Cu₂O nanocubes-grafted highly dense Au nanoparticles (NPs) was developed as an appealing nanozyme for H₂O₂ colorimetric sensor and antioxidant detections. The obtained Au/Cu₂O heterostructures show efficient electron-transfer from metallic NPs to Cu₂O nanocubes owing to the difference of Fermi energy between two components. The modulated electronic structure of Au/Cu₂O hybrids enables them to possess enhanced peroxidase catalytic activity for the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) in the presence of H₂O₂, which is about 32% higher than that of pristine Cu₂O nanocubes. Then, an excellent H₂O₂ colorimetric sensor was established by using Au/Cu₂O heterostructures with a low limit of detection (LOD) of 0.054 μM, which is much lower than the H₂O₂ allowance level of US FDA regulations (ca.15 μM, 0.05 wt%). The obtained Au/Cu₂O nanoproducts exhibit pronounced long-time stability with 95% peroxidase activity maintained after keeping them for 30 days, while residual 64.5% via Cu₂O nanocubes. Furthermore, we assessed the anti-oxidative behavior of three natural antioxidants (tannic acid, gallic acid, tartaric acid) with the LODs as low as 0.039, 0.16 and 1.55 μM, respectively, and the antioxidant capacity in the following order: tannic acid > gallic acid > tartaric acid. Therefore, it is believed that the as-prepared Au/Cu₂O nanozymes have promising potential applications in fields of biomedicine and food safety.

Colorimetric detection of H2O2 based on the enhanced peroxidase mimetic activity of nanoparticles decorated Ce2(WO4)3 nanosheets

In this paper, nanoparticles decorated Ce₂(WO₄)₃ nanosheets (CWNSs) with negative potential and large specific surface area were synthesized and developed as highly efficient peroxidase mimics for colorimetric detection of H₂O₂. CWNSs can efficiently catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to induce an obvious color variation. Kinetic analysis indicated that the catalytic behaviors of CWNSs obey the typical Michaelis-Menten mechanism. The peroxidase-like catalytic mechanism of CWNSs was proposed according to the active species trapping experiments, verifying that ·O₂^- radicals played primary roles in the catalytic reaction. Based on the strong and stable peroxidase-like catalytic activity of CWNSs, a simple, rapid, selective, and ultrasensitive method was successfully established for colorimetric detection of H₂O₂. The method has a good linear response ranging from 0.5 μM to 100 μM for H₂O₂ concentration with a lower detection limit of 0.15 μM. Benefitting from the sensitive response and good stability, the method is applied in real sample detection and shows a favorable reproducibility and feasibility. This work not only provides a novel enzymatic mimics with remarkable catalytic activities for biomedical and environmental analysis, but also extends the application area of Ce₂(WO₄)₃ materials.

Ficin encapsulated in mesoporous metal-organic frameworks with enhanced peroxidase-like activity and colorimetric detection of glucose

Ficin has been reported to possess peroxidase activity, but its applications in some respects have been limited because of its relatively low activity. Herein, a mesoporous metal-organic framework, PCN-333(Fe), was synthesized, which was selected to encapsulate ficin to form ficin@PCN-333(Fe). Compared with ficin, the peroxidase-like activity of ficin@PCN-333(Fe) toward 3,3',5,5'-tetramethylbenzidine (TMB) oxidation was about 3 times increase in the presence of H₂O₂, and followed classical Michaelis-Menten model. The kinetic parameters showed that stronger affinity and higher catalytic constant (K_cat) of ficin@PCN-333(Fe) to both TMB and H₂O₂ compared with ficin, and K_cat of ficin@PCN-333(Fe) was increased by 3.65 folds and 3.59 folds for TMB and H₂O₂, respectively. Taking advantages of higher catalytic property of ficin@PCN-333(Fe), we developed a colorimetric method with high sensitivity and selectivity to detect glucose, which displayed a good linear response toward glucose in the range of 0.5-180 μM with a limit of detection of 97 nM. Furthermore, ficin@PCN-333(Fe) has been proven to successfully detect glucose in human serum, implying its great potentialities and wide applications as peroxidase mimics.

A double carbon dot system composed of N, Cl-doped carbon dots and N, Cu-doped carbon dots as peroxidase mimics and as fluorescent probes for the determination of hydroquinone by fluorescence

A fluorescence method is described for the determination of hydroquinone based on the double carbon dot system as peroxide mimic enzymes and fluorescent probes. Deep eutectic solvent (DES)-based fluorescent carbon dots (N/Cl-CDs) and copper-doped carbon dots (N/Cu-CDs) were prepared by the hydrothermal method. Both carbon dots were characterized with transmission electron microscopy (TEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectron spectrometry (XPS), Fourier transform infrared (FT-IR) spectroscopy, and fluorescence spectroscopy. N/Cl-CDs displayed intrinsic peroxidase-like activity and were able to catalyze the oxidation of hydroquinone (H₂Q) to p-benzoquinone (BQ) along with an intermediate. The intermediate (BQ) did quench the N/Cu-CD photoluminescence (PL) at 450 nm using an excitation wavelength of 347 nm. Based on the results, a fluorescent platform is proposed for the determination of hydroquinone with a promising determination limit of 0.04 μM (linear range, 1.0-75 μM). The recoveries of spiked water samples were in the range 89.5-105.1%, with relative standard deviations (RSDs) of 1.5-2.9%. This method was applied to determination of H₂Q in environmental water samples. Graphical abstract A fluorescence method was established for the determination of hydroquinone based on the double carbon dot system as peroxide-mimic enzymes and fluorescent probes. Chlorine-doped carbon dots (N/Cl-CDs) derived from deep eutectic solvent (DES) displayed intrinsic peroxidase-like activity, and were able to catalyze the oxidation of hydroquinone (H2Q) to p-benzoquinone (BQ) along with an intermediate. The intermediate (BQ) did quench the N/Cu-CD photoluminescence (PL). This method was applied to H2Q in environmental water samples.

Copper-tripeptides (cuzymes) with peroxidase-mimetic activity

Peroxidases are enzymes that use hydrogen peroxide to oxidize substrates such as 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ATBS). In this study, we showed that copper-tripeptide complexes ("cuzymes") also exhibited peroxidase-like activities. Different cuzymes could be formed by using various tripeptide ligands, such as GGG, GGH or HGG. However, the peroxidase-like activity of cuzymes depends on the sequence of the tripeptide (Cu-GGG > Cu-HGG > Cu-GGH). When ABTS was used as the substrate, the activity of Cu-GGG was 326 ± 1.5 U mg^-1 which was 2.5 times higher than that of horseradish peroxidase (HRP). Copper-tripeptide complexes were also used to degrade trypan blue dye. By using 0.2 mM Cu-GGG and 0.2% H₂O₂, 200 μM trypan blue could be degraded in 15 min at 50 °C. The degradation reaction followed second-order kinetics; the reaction rate was proportional to both H₂O₂ concentration and the copper-tripeptide concentration, but it was independent of the trypan blue concentration. Because copper-tripeptides catalyzed the oxidation reactions involving H₂O₂ effectively, they may have potential applications in biochemical assays and environmental remediation.

Organic-Inorganic Composite Nanorods as an Excellent Mimicking Peroxidases for Colorimetric Detection and Evaluation of Antioxidant

N,N'-Dicarboxy methyl perylene diimide-coated CeO₂ nanorods (PDI/CeO₂ NR) were synthesized via an ultrasonic-assisted method. PDI/CeO₂ exhibits the superb mimic peroxidase functions confirmed by the catalyzed oxidation of TMB by hydrogen peroxide in less than 60 s along with color transformation from colorless to blue. The catalytic mechanism was confirmed to be an electronic transfer mechanism by fluorescence experiment. Thus, a visual colorimetric sensor was constructed for hydrogen peroxide detection with a low detection limit (LOD = 2.23 μM). Comparing the inhibition effects of l-cysteine (Cys), ascorbic acid (AA) and glutathione (GSH) on the catalytic oxidation of TMB, it can be found that they possessed different types of inhibition on the oxidation of TMB by H₂O₂ using PDI/CeO₂, and AA has better antioxidant effect, followed by Cys and GSH. On the basis of the excellent antioxidant effect of AA, a low-cost colorimetric sensor was also used to detect AA, and a lower LOD value (0.68 μM) was obtained in the linear range of 5.0-30 μM.

Fe/C magnetic nanocubes with enhanced peroxidase mimetic activity for colorimetric determination of hydrogen peroxide and glucose

Iron-carbon (Fe/C) magnetic nanocubes were synthesized by direct pyrolysis of Prussian blue nanocubes under an inert gas atmosphere. They are shown to possess intrinsic peroxidase mimicking activity to catalyze the oxidation of peroxidase substrate N,N-diethyl-p-phenylenediamine sulfate salt to form a purple colored product in the presence of H₂O₂. The values for K_m and V_max are 74 μM and 46 nmol s^-1, respectively. Steady-state kinetic analysis also indicates that the catalysis reaction follows a ping-pong mechanism. Based on these findings, an ultrasensitive colorimetric H₂O₂ assay was worked. Absorbance (best value measured at 550 nm) increases linearly in the 10 nM to 0.2 mM H₂O₂ concentration range, and the limit of detection is 1.5 nM. The method was also applied to the quantification of glucose, which is oxidized by glucose oxidase in the coexistence of H₂O₂. The response covers the 0.1 to 500 μM glucose concentration range, and the limit of detection is 16 nM. The method was applied to the determination of H₂O₂ in rainwater samples. The glucose assay was used to analyze serum samples, and satisfactory results were obtained. Other attractive features include good chemical activity, low cost, easy storage, and high catalytic efficiency. Graphical abstract Schematic presentation of converting Prussian blue nanoparticles into Fe/C magnetic nanocubes by a pyrolysis technique and the use of glucose oxidase and Fe/C magnetic nanocubes to establish a one-step spectrophotometric method for the determination of glucose and hydrogen peroxide.

Colorimetric detection of hydrogen peroxide and glucose by exploiting the peroxidase-like activity of papain

Papain, a natural plant protease that exists in the latex of Carica papaya, catalyzes the hydrolysis of peptide, ester and amide bonds. In this work, we found that papain displayed peroxidase-like activity and catalyzed the oxidation of 3,3',5',5'-tetramethylbenzidine (TMB) in the presence of H2O2. This results in the formation of a blue colored product with an absorption maximum at 652 nm. The effects of experimental parameters including pH and reaction temperature on catalytic activity of papain were investigated. The increase of absorbance induced by the catalytic effect of papain offers accurate detection of H2O2 in the range of 5.00-90.0 μM, along with a detection limit of 2.10 μM. A facile colorimetric method for glucose detection was also proposed by combining the glucose oxidase (GOx)-catalyzed glucose oxidation and papain-catalyzed TMB oxidation, which exhibited a linear response in the range of 0.05-0.50 mM with a detection limit of 0.025 mM. The method proposed here displayed excellent selectivity, indicating that common coexisting substances (urea, uric acid, ascorbic acid, maltose, lactose and fructose) in urine did not interfere with detection of glucose. More importantly, the suggested method was successfully used to precisely detect the glucose concentration in human urine samples with recoveries over 96.0%.

Peroxidase-Like Activity of Smart Nanomaterials and Their Advanced Application in Colorimetric Glucose Biosensors

Diabetes is a dominating health issue with 425 million people suffering from the disease worldwide and 4 million deaths each year. To avoid further complications, the diabetic patient blood glucose level should be strictly monitored despite there being no cure for diabetes. Colorimetric biosensing has attracted significant attention because of its low cost, simplicity, and practicality. Recently, some nanomaterials have been found that possess unexpected peroxidase-like activity, and great advances have been made in fabricating colorimetric glucose biosensors based on the peroxidase-like activity of these nanomaterials using glucose oxidase. Compared with natural horseradish peroxidase, the nanomaterials exhibit flexibility in structure design and composition, and have easy separation and storage, high stability, simple preparation, and tunable catalytic activity. To highlight the significant progress in the field of nanomaterial-based peroxidase-like activity, this work discusses the various smart nanomaterials that mimic horseradish peroxidase and its mechanism and development history, and the applications in colorimetric glucose biosensors. Different approaches for tunable peroxidase-like activity of nanomaterials are summarized, such as size, morphology, and shape; surface modification and coating; and metal doping and alloy. Finally, the conclusion and challenges facing peroxidase-like activity of nanomaterials and future directions are discussed.

A facile preparation of FePt-loaded few-layer MoS2 nanosheets nanocomposites (F-MoS2-FePt NCs) and their application for colorimetric detection of H2O2 in living cells

BACKGROUND

Rapid and sensitive detection of H2O2 especially endogenous H2O2 is of great importance for series of industries including disease diagnosis and therapy. In this work, uniform FePt nanoparticles are successfully anchored onto Few-layer molybdenum disulfide nanosheets (F-MoS2 NSs). The powder X-ray diffraction, transmission electron microscopy, UV-Vis spectra and atomic force microscopy were employed to confirm the structure of the obtained nanocomposites (F-MoS2-FePt NCs). The prepared nanocomposites show efficient peroxidase-like catalytic activities verified by catalyzing the peroxidation substrate 4,4'-diamino-3,3',5,5'-tetramethylbiphenyl (TMB) with the existence of H2O2.

RESULTS

The optimal conditions of the constructed colorimetric sensing platform is proved as 35 °C and pH 4.2. Under optimal catalytic conditions, the detection limit for H2O2 detection reaches 2.24 μM and the linear ranger is 8 μM to 300 μM. Furthermore, the proposed colorimetric sensing platform was successfully utilized to detect the intracellular H2O2 of cancer cells (MCF-7).

CONCLUSIONS

These findings indicated that the F-MoS2-FePt-TMB-H2O2 system provides a potential sensing platform for hydrogen peroxide monitoring in living cells.

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.

A Colorimetric Sensor for Dopamine Detection Based on Peroxidase-like Activity of Ce2(MoO4)3 Nanoplates

Introduction:

Artificial enzyme mimics are materials with similar catalytic function of natural enzymes. Among several types of artificial enzymes, nanomaterial-based products or nanozymes have been of particular interest to researchers.

Materials and Methods:

In this work, Ce2(MoO4)3 nanoplates were synthesized via a one-pot hydrothermal approach. SEM and EDS characterizations show a plated-like architecture with high purity. These nanoplates are shown to have an intrinsic peroxidase-mimetic activity. In the presence of H2O2, Ce2(MoO4)3 nanoplates could catalyse the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with high performance to produce a blue dye (with an absorbance maximum at 652 nm). Dopamine (DA) has some reducibility due to the phenol hydroxyl group, which results in using H2O2 and causing the blue shallowing of the reaction solution by inhibiting the reaction between H2O2 and TMB. Based on that, a visual, sensitive and simple colorimetric method using Ce2(MoO4)3 nanoplates as peroxidase mimics was developed for detecting DA.

Results and Conclusions:

Suitable linear relationship for DA was obtained from 0.1 to 10 µM. The limit of detection (LOD) of the proposed method was calculated as 0.05 µM and the relative standard deviation (RSD) was less than 4.0%. The proposed method was successfully applied to DA detection in human serum sample.

Peroxidase-like activity of magnetic poly(glycidyl methacrylate-co-ethylene dimethacrylate) particles

Poly(glycidyl methacrylate) (PGMA) is prone to modifications with different functional groups, magnetic fluids or direct coupling with biological molecules. The purpose of this research was to synthesize new magnetically responsive particles with peroxidase-like activity. Poly(glycidyl methacrylate-co-ethylene dimethacrylate) [P(GMA-EDMA)] particles containing carboxyl groups were obtained by emulsifier-free emulsion polymerization and hydrolysis and oxidation of PGMA with KMnO4, resulting in poly(carboxymethyl methacrylate-co-ethylene dimethacrylate) [P(CMMA-EDMA)] particles. Thionine (Th) was also attached to the particles [(P(CMMA-EDMA)-Th] via EDC/NHS chemistry to observe its effect on electron transfer during the oxidation reaction. Finally, the particles were coated with a nitric acid-stabilized ferrofluid in methanol. The resulting magnetic particles were characterized by several methods, including scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. The effect of EDMA on the P(CMMA-EDMA) particle size and size distribution was investigated; the particle size changed from 300 to 340 nm, and the particles were monodispersed with a saturation magnetization of 11 Am2/kg. Finally, the effects of temperature and pH on the peroxidase-like activity of the magnetic P(CMMA-EDMA) and P(CMMA-EDMA)-Th particles were investigated. The particles, which exhibited a high activity at pH 4-6 and at ∼37 °C, represent a highly sensitive sensor component potentially useful in enzyme-based immunoassays.

Intrinsic peroxidase-like activity of Cu2ZnSn(SxSe1-x)4 nanocrystals, and their application to the colorimetric detection of H2O2

Nanocrystals (NCs) of type Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) were prepared via a solvothermal approach. They are shown to be highly efficient peroxidase (POx) mimics for colorimetric detection of H₂O₂. By varying the molar ratio of S and Se during preparation, the NCs showed different crystal structures, morphologies, surface properties, and POx-like activities. Among them, the type CZTSSe-0.25 NCs exhibit the strongest POx-like activities towards the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine in the presence of H₂O₂ to generate a blue product. The enhanced activity is attributed to its more negative potential and larger specific surface of the NCs. Based on these findings, a rapid and ultrasensitive method was developed for the visual and colorimetric determination of H₂O₂. The method is selective, and the NCs are reusable and long-term stable. The detection limit of H₂O₂ is 50 nM. Kinetic and active species trapping experiments were performed to elucidate the POx-like mechanism of the NCs. Graphical abstract Schematic presentation of the process of Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals catalyzing the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ to induce a typical blue color reaction, which can be applied in colorimetric detection of H₂O₂.

A functional FePt@MOFs (MIL-101(Fe)) nano-platform for high efficient colorimetric determination of H2O2

Surface-modified FePt nanoparticles were loaded into metal–organic frameworks in a facile in situ way and the resulting material (FePt@MOFs NCs) acted as a high efficient colorimetric detection agent for H₂O₂.

Colorimetric determination of mercury(II) via the inhibition by ssDNA of the oxidase-like activity of a mixed valence state cerium-based metal-organic framework

This work demonstrates the inhibition effects of single-stranded (ssDNA) on the oxidase-like activity of a mixed-valence state cerium-based metal-organic framework, denoted as MVC-MOF. The MVC-MOF was synthesized by partial oxidation of cerium(III) which leads to the presence of both Ce(III) and Ce(IV) ions. The latter endows the MVC-MOF with a typical oxidase-like activity. However, on addition of ssDNA, the catalytic activity of the MVC-MOF is inhibited because it binds the MVC-MOF and thereby shield its active sites. This prevents the access of substrates. The inhibition by ssDNA depends on its length but not its sequence. By contrast, negligible changes in the oxidase-mimicking activity are observed if double-stranded DNA (dsDNA) is added. By employing a thymine-rich ssDNA (T-ssDNA) as a model DNA, a colorimetric assay was developed for the determination of Hg(II). This ion binds to T-ssDNA and causes the formation of T-dsDNA. Hence, the oxidase-mimicking activity is compromised. By using the oxidase substrate 3,3',5,5'-tetramethylbenzidine that gives a colored product in the presence of oxygen, the assay has a linear response that covers 0.05 to 6 μM Hg(II) with a detection limit of 10.5 nM, and exhibits high selectivity over other metal ions. The assay was successfully applied to the determination of Hg(II) in environmental water samples. Graphical abstract Schematic of the inhibition effect of ssDNA on the oxidase-like activity of MVC-MOF that converts colorless TMB to oxTMB with blue color in the presence of oxygen, and its application in the construction of a colorimetric assay for Hg(II) determination.

Fabrication of Liquid-Crystal-Based Optical Sensing Platform for Detection of Hydrogen Peroxide and Blood Glucose

Rapid and accurate determination of H₂O₂ is of great importance in practical applications. In this study, we demonstrate construction of liquid-crystal (LC)-based sensing platforms for sensitive and real-time detection of H₂O₂ with high accuracy for the first time. Single-stranded DNA (ssDNA) adsorbed onto the surface of nanoceria is released to the aqueous solution in the presence of H₂O₂, which disrupts arrangement of the self-assembled cationic surfactant monolayer decorated at the aqueous/LC interface. Thus, the orientation of LCs changes from a homeotropic to planar state, leading to change in the optical response from dark-to-bright appearance. As H₂O₂ can be produced during oxidation of glucose by glucose oxidase (GOx), detection of glucose is also fulfilled by employing the H₂O₂ sensing platform. Our system can detect H₂O₂ and glucose with concentrations as low as 28.9 nM and 0.52 μM, respectively. It shows high promise of using LC-based sensors for the detection of H₂O₂ and its relevant biomarkers in practical applications.

Synthesis of Porous CoFe₂O₄ and Its Application as a Peroxidase Mimetic for Colorimetric Detection of H₂O₂ and Organic Pollutant Degradation

Porous CoFe₂O₄ was prepared via a simple and controllable method to develop a low-cost, high-efficiency, and good-stability nanozyme. The morphology and microstructure of the obtained CoFe₂O₄ was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), specific surface area and pore analysis, and Raman spectroscopy. The results show that the annealing temperature has an important effect on the crystallinity, grain size, and specific surface area of CoFe₂O₄. CoFe₂O₄ obtained at 300 °C (CF300) exhibits the largest surface area (up to 204.1 m² g^−1) and the smallest grain size. The peroxidase-like activity of CoFe₂O₄ was further verified based on the oxidation of peroxidase substrate 3,3’,5,5’-tetramethylbenzidine (TMB) in the presence of H₂O₂. The best peroxidase-like activity for CF300 should be ascribed to its largest surface area and smallest grain size. On this basis, an effective method of colorimetric detection H₂O₂ was established. In addition, the porous CoFe₂O₄ was also used for the catalytic oxidation of methylene blue (MB), indicating potential applications in pollutant removal and water treatment.

Enhanced peroxidase-like activity of porphyrin functionalized ZnFe2O4 hollow nanospheres for rapid detection of H2O2 and glucose

Using a simple one-pot solvothermal method, binary metal oxide, magnetic and hollow ZnFe₂O₄ nanospheres functionalized with 5,10,15,20-tetrakis(4-carboxylpheyl)-porphyrin (Por–ZnFe₂O₄ HSs) were prepared and subsequently applied as a substitute for natural peroxidase.

A gold electrode modified with a nanoparticulate film composed of a conducting copolymer for ultrasensitive voltammetric sensing of hydrogen peroxide

A film consisting of poly(γ-glutamic acid) modified with 3-aminothiophene (ATh-γ-PGA) was prepared by macromolecular self-assembly and electropolymerization. ATh-γ-PGA is amphiphilic and electrically conductive. The copolymers undergo self-assembly to form nanoparticles (NPs) on decreasing the pH value of an aqueous solution. A conducting film of NPs was formed on the surface of a gold electrode by casting the ATh-γ-PGA NPs and subsequently electropolymerizing the thiophene units. Next, horseradish peroxidase and Nafion were cast onto the film to obtain an enzymatic biosensor for H₂O₂. Due to the electropolymerization step, a cross-conjugated polymer network is created that improves electron transfer rates and thus enhances the response. This endows the biosensor with high sensitivity. Two linear ranges are present, the first ranging from 1 × 10^-11 to 1 × 10^-8 mol·L^-1, and the second from 1 × 10^-8 to 1 × 10^-5 mol·L^-1. The detection limit is as low as 3 × 10^-12 mol·L^-1. The sensor is stable, repeatable, and was successfully applied to the determination of H₂O₂ in a commercial disinfecting solution. Graphical abstract Preparation of a conducting nanoparticle (NP) film on the gold electrode (GE) by self-assembly of poly(γ-glutamic acid) that was modified with electroactive 3-aminothiophene (ATh-γ-PGA). It served as a platform for the fabricationof an ultrasensitive voltammetric enzyme-based biosensor for H₂O₂.

Intrinsic peroxidase-like activity of rhodium nanoparticles, and their application to the colorimetric determination of hydrogen peroxide and glucose

The intrinsic peroxidase-like activity of rhodium nanoparticles (RhNPs) and their use as catalytic labels for sensitive colorimetric assays is presented. RhNPs catalyze the oxidation of the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue reaction product with a maximum absorbance at 652 nm. Kinetic studies show catalysis to follow Michaelis-Menten kinetics and a "ping-pong" mechanism. The calculated kinetic parameters indicate high affinity of RhNPs for both the substrate TMB and H₂O₂. In fact, they are better than other peroxidase mimicking nanomaterials and even the natural enzyme horseradish peroxidase. On the other hand, RhNPs exhibit no reactivity towards saccharides, thiols, amino acids and ascorbic acid. Based on these findings, a sensitive and selective colorimetric method was worked out for the determination of H₂O₂ in real samples with a linear response in the 1-100 μM concentration range. By employing glucose oxidase, the glucose assay has a linear range that covers the 5 to 125 μM glucose concentration range. The detection limits are <0.75 μM for both species. The methods were applied to the determination of H₂O₂ in spiked pharmaceutical formulations, and of glucose in soft drinks and blood plasma. Figures of merit include (a) good accuracy (with errors of <6%), (b) high recoveries (96.5-103.7%), and (c) satisfactory reproducibility (<6.3%). Graphical abstract Rhodium nanoparticles catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue reaction product. The effect is exploited in photometric assays for hydrogen peroxide and glucose.

Layered vanadium(IV) disulfide nanosheets as a peroxidase-like nanozyme for colorimetric detection of glucose

The authors have discovered that vanadium disulfide (VS₂) nanosheets, synthesized by a hydrothermal method, exert stable peroxidase-like activity. The catalytic activity, with H₂O₂ as a cosubstrate, follows Michaelis-Menten kinetics and varies with temperature, pH value and H₂O₂ concentration. Two-dimensional VS₂ sheets acting as peroxidase (POx) mimics can replace horseradish peroxidase due to their availability, robustness, and reusability. The POx-like activity of VS₂ sheets was exploited to design a colorimetric glucose assay by using 3,3',5,5'-tetramethylbenzidine as a substrate and by working at an analytical wavelength of 652 nm. The assay covers the 5 to 250 μM glucose concentration range with a 1.5 μM detection limit. It was applied to the analysis of glucose in fruit juice. In our perception, the peroxidase-like nanozyme out of the family of transition metal dichalcogenides presented here has a wide scope in that it may stimulate promising biocatalytic applications in biotechnology and analytical chemistry. Graphical abstract Layered VS₂ nanosheets were prepared via hydrothermal synthesis and are shown to exert superior peroxidase-mimicking activity. Using these POx nano-mimics, a sensitive colorimetric assay for glucose was developed and applied to fruit juice analysis. This work unlocks the access of VS₂ to biocatalysis and bioassays.