Convenient and sensitive colorimetric detection of melamine in dairy products based on Cu(ii)-H2O2-3,3′,5,5′-tetramethylbenzidine system

The illegal adulteration of melamine in dairy products for false protein content increase is a strong hazard to human health. Herein, a simple and sensitive colorimetric method was developed for the quantification of melamine in dairy products based on a Cu²⁺-hydrogen peroxide (H₂O₂)-3,3′,5,5′-tetramethylbenzidine (TMB) system. In this strategy, Cu²⁺ exhibits peroxidase-like activity and can catalyze the oxidation of TMB to oxidized TMB (oxTMB) in the presence of H₂O₂ with a blue colour change of the solution. However, the presence of melamine quickly interacts with H₂O₂ leading to the consumption of H₂O₂ and thus strongly hinders the oxidation of TMB. Under the optimal conditions, the absorbance change of oxTMB has a linear response to the concentration of melamine from 1 to 100 μM with a detection limit of 0.5 μM for melamine. The proposed method has many merits including more simplicity, good selectivity, and more cost-effectiveness without using any nanomaterials. The method was further successfully applied to detect melamine in dairy products including milk and infant formula powder.

Convenient and sensitive colorimetric detection of melamine in dairy products based on a Cu(ii)-H₂O₂-3,3′,5,5′-tetramethylbenzidine system was reported.     

Colorimetric sensing of glucose and GSH using core–shell Cu/Au nanoparticles with peroxidase mimicking activity

The catalytic properties of bimetallic nanoparticles have been widely studied by researchers in many fields. In this paper, core–shell Cu/Au nanoparticles (Cu/Au NPs) were synthesized by a simple and mild one-pot method, and their peroxidase activity was proved by catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) with color change to blue. The change of solution color and absorbance strongly depends on the concentration of H₂O₂, so it can be used for direct detection of H₂O₂ and indirect detection of glucose. What''s more, GSH can efficiently react with the hydroxyl radicals from H₂O₂ catalyzed by core–shell Cu/Au NPs to inhibit the production of ox-TMB. Thus, the concentration of GSH can be determined by the decrease in the absorbance of the solution at 652 nm. The results showed that our proposed strategy had good detection range and detection limit for the detection of glucose and GSH. This method has been used in the detection of practical samples and has great application potential in environmental monitoring and clinical diagnosis.

Core–shell Cu/Au nanoparticles were synthesized by a one pot method, their peroxidase activity was proved by catalysing the oxidation of 3,3′,5,5′-tetramethylbenzidine with colour change to blue. Results showed a good range and limit for the detection of glucose and GSH.     

Molecular imprinting on PtPd nanoflowers for selective recognition and determination of hydrogen peroxide and glucose

PtPd nanoflowers (PtPd NFs) exhibit intrinsic peroxidase-like activity as nanozymes, but the nanozymes lack substrate specificity and have low catalytic activity. Herein, a molecularly imprinted nanogel on PtPd NFs was prepared by using 3,3′,5,5′-tetramethylbenzidine (TMB) as the template through the aqueous precipitation polymerization method. After the TMB was washed out, many substrate binding pockets were retained in the PtPd NFs. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) were employed to characterize the molecularly imprinted polymer (MIP) PtPd nanoflowers (T-MIP-PtPd NFs). The obtained T-MIP-PtPd NFs exhibited enhanced catalytic activity and specific recognition for TMB. Compared with PtPd NFs, T-MIP-PtPd NFs showed a linear range from 0.01–5000 μM and a detection limit of 0.005 μM toward the detection of H₂O₂. Glucose can also be sensitively detected through cascade reaction by the T-MIP-PtPd NFs and glucose oxidase. Therefore, molecular imprinting on nanozymes technology shows promising application in biocatalysis and sensing fields.

PtPd nanoflowers (PtPd NFs) exhibit intrinsic peroxidase-like activity as nanozymes, but the nanozymes lack substrate specificity and have low catalytic activity.     

Sweetsop-like α-Fe2O3@CoNi catalyst with superior peroxidase-like activity for sensitive and selective detection of hydroquinone

Hydroquinone (HQ) is poorly degradable in the ecological environment and is highly toxic to human health even at a low concentration. The colorimetric method has the advantages of low cost and fast analysis, which provides the possibility for simple and rapid detection of HQ. In this work, a new colorimetric method has been developed for HQ detection based on a peroxidase-like catalyst, α-Fe₂O₃@CoNi. This sweetsop-like α-Fe₂O₃@CoNi catalyst enables H₂O₂ to produce hydroxyl (˙OH), leading to the oxidization of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxTMB. In the presence of HQ, the blue oxTMB is reduced to colorless, which allows for colorimetric detection of HQ in water samples. This method has been validated by detecting HQ in water samples with high selectivity, rapid response, broad detection range (0.50 to 30 μM), and low detection limit (0.16 μM).

A sweetsop-like α-Fe₂O₃@CoNi catalyst with superior peroxidase-like activity was synthesized and successfully applied to the detection of hydroquinone (HQ) based on the colorimetric principle.     

Co nanoparticles decorated with N-doped carbon nanotubes as high-efficiency catalysts with intrinsic oxidase-like property for colorimetric sensing

Artificial nanozymes are designed for pursuing the functions of splendid catalytic efficiency and prominent selectivity of natural enzymes, meanwhile obtaining higher stability than that of natural enzymes. This emerging technology shows widespread application in the crossing field between nanotechnology and biomedicine. In this work, we employed a universal approach to fabricate a Co@N-CNTs hybrid nanocomposite as an oxidase mimic, in which fine Co nanoparticles were wrapped in N-doped carbon nanotubes, stacking on a hollow dodecahedron carbon skeleton. The synergistic effects of nanostructure engineering, N-doping and carbon coating, as well as the derived interfacial effect contribute to the glorious oxidase-like activity, stability and reusability. It can catalytically oxidize the colorless substrate 3,3′,5,5′-tetramethylbenzidine (TMB) to a blue oxidation product (ox-TMB). As a result, a colorimetric technique with excellent selectivity and sensitivity for detecting ascorbic acid (AA) with naked eyes was established, in view of specific inhibitory effects towards oxidation of TMB. Under optimal detection conditions, this method exhibits a good linearity ranging from 0.1 to 160 μM with a low limit of detection (LOD) of 0.076 μM. For practical applications, Co@N-CNTs hybrid catalyst as a mimic oxidase was used for the determination of AA in human serum, which yielded satisfactory results. This work may serve as a new research thought to guide the design of high-performance nanozymes and establish a sensing platform for the detection of AA.

In this work, we designed a Co@N-CNTs hybrid nanocomposite as an oxidase mimic for the colorimetric detection of ascorbic acid with the naked eye.     

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%.

We reported a simple colorimetric method for the detection of glucose based on GOx-catalyzed glucose oxidation and papain-catalyzed TMB oxidation.     

Synthesis of a new Ag+-decorated Prussian blue analog with high peroxidase-like activity and its application in measuring the content of the antioxidant substances in Lycium ruthenicum Murr.

A new Prussian blue analog (PBA) that contains three metal elements and has peroxidase-like activity was synthesized by a simple method. Then, AgNO₃ solution was added slowly to the PBA solution under continuous stirring. We found that this synthesis method could be used to prepare other PBAs, and that the anchoring of Ag⁺ on the surface of PBA could enhance the peroxidase-like activity of the material, suggesting potential applications for the Ag⁺-decorated Prussian blue analog (Ag-PBA) in traditional Chinese medicine. Ag-PBA is a new type of multi-metal cubic nano-enzyme that exhibits good stability and excellent peroxidase-like activity; as such, it could catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ and Ag-PBA. We then developed a new method to measure the content of antioxidant substances in Chinese herbs by using the excellent peroxidase-like activity of Ag-PBA. Using the Chinese herb Lycium ruthenicum Murr. as a model compound, we measured the content of the antioxidant substances in Lycium ruthenicum Murr. by this new method. After optimization of reaction temperature, concentrations of TMB and H₂O₂, and reaction time, the content of the antioxidant substances was measured and calculated in comparison with anthocyanidin standards. The results of the Ag-PBA method and the classical DPPH method were compared by a paired t-test, with no statistically significant difference found between the methods. Hence, these two methods can be used interchangeably, although the Ag-PBA method had the advantages of simplicity, rapidness, and good stability. Moreover, the Ag-PBA method has a low limit of quantification and a shorter reaction time, which are improvements on the DPPH method, and it is not necessary to avoid light. Therefore, we anticipate that the Ag-PBA method may be used widely for the measurement of the content of antioxidant substances in Chinese herbs.

An Ag⁺-decorated Prussian blue analog (Ag-PBA) was synthesized and used to measure the content of antioxidant substances in Lycium ruthenicum Murr.     

Mesoporous MnFe2O4 magnetic nanoparticles as a peroxidase mimic for the colorimetric detection of urine glucose

Mesoporous MnFe₂O₄ magnetic nanoparticles (mMnFe₂O₄ MNPs) were prepared with a one-step synthesis method and characterized to possess intrinsic peroxidase-like activity, and had obvious advantages over other peroxidase nanozymes in terms of high catalytic affinity, high stability, mono-dispersion, easy preparation, and quick separation. The mMnFe₂O₄ MNPs were used as a colorimetric sensor for indirect sensing of urine glucose based on the sensing principle that H₂O₂ can be produced from glucose oxidation catalyzed by glucose oxidase (GOx), and under the catalysis of the mMnFe₂O₄ MNPs nanozyme, H₂O₂ can oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to produce a blue color in a few minutes. This sensor is simple, cheap, sensitive, and specific to glucose detection with a detection limit of 0.7 μM, suggesting its potential for on-site glucose detection.

Schematic illustration of glucose detection with glucose oxidase (GOx) and mMnFe₂O₄ MNPs-catalyzed system.     

Cu@Au(Ag)/Pt nanocomposite as peroxidase mimic and application of Cu@Au/Pt in colorimetric detection of glucose and l-cysteine

Nanomaterial-based artificial peroxidase has attracted extensive interests due to their distinct advantages over natural counterpart. Cu@Au/Pt and Cu@Ag/Pt nanocomposite with rambutan-like structure were prepared and discovered to function like peroxidase, which was illustrated by catalyzing the oxidation reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) accompanied with a blue color change. Steady-state investigation indicates that the catalytic kinetics of Cu@Au/Pt and Cu@Ag/Pt all followed typical Michaelis–Menten behaviors and Cu@Au/Pt showed a strong affinity for H₂O₂, while Cu@Ag/Pt showed strong affinity for TMB. The color change and absorbance intensity strongly depend on the concentration of H₂O₂, thus the direct determination of H₂O₂ and indirect detection of glucose were demonstrated using Cu@Au/Pt with a detection limit of 1.5 μM and 6 μM, respectively. What is more important, the method was applied for detection of glucose in 50% fetal bovine serum with a detection limit of 80 μM, which is much lower than the lowest glucose content in blood for diabetes (7 mM). Moreover, the Cu@Au/Pt nanocomposite were also successfully applied for sensing l-cysteine because of the inhibition effect. Considering the good peroxidase-like activity and novel structure, Cu@Au(Ag)/Pt is expected to have a wide range of applications in bioassays and biocatalysis.

Cu@Au(Ag)/Pt nanocomposite possess good peroxidase-like activity and can be used for detection of glucose and l-cysteine.     

Protein-mediated sponge-like copper sulfide as an ingenious and efficient peroxidase mimic for colorimetric glucose sensing

Strenuous efforts have been made to develop nanozymes for achieving the performance of natural enzymes to broaden their application in practice, but the fabrication of high-performance and biocompatible nanozymes via facile and versatile approaches has always been a great challenge. Here, sponge-like casein-CuS hybrid has been facilely synthesized in the presence of amphiphilic protein-casein through a simple one-step approach. Casein-CuS hybrid exhibits substrates-dependent peroxidase-like activity. Casein-CuS hybrid exhibits well peroxidase-like activity with 3,3′,5,5′-tetramethylbenzidine (TMB) and 1,2-diaminobenzene (OPD) as substrates, and the affinity of OPD towards the hybrid nanozyme is much higher than that of TMB. More importantly, due to the high affinity of OPD and the well biocompatibility of the hybrid nanozyme, a superior enzyme cascade for glucose based on the well cooperative effect of casein-CuS hybrid and glucose oxidase is developed. The proposed glucose sensor exhibits a wide linear range of 0.083 to 75 μM and a detection limit of 5 nM. This suggests the promising utilization of protein–metal hybrid nanozymes as robust and potent peroxidase mimics in the medical, food and environmental detection fields.

Strenuous efforts have been made to develop nanozymes for achieving the performance of natural enzymes, but the fabrication of high-performance and biocompatible nanozymes via facile and versatile approaches has always been a great challenge.     

Peroxidase catalytic activity of carbon nanoparticles for glutathione detection

Peroxidases are present widely in microorganisms and plants, and catalyze many reactions. However, the activity of natural peroxidases is susceptible to external conditions. We prepared carbon nanoparticles (CNPs) using an environmentally friendly and simple method. These CNPs were demonstrated to possess intrinsic peroxidase-like activity. CNPs could catalyze the reaction of a peroxidase substrate, 3,3,5,5-tetramethylbenzidine (TMB), in the presence of H₂O₂ to produce a blue solution at 652 nm. CNPs exhibited higher peroxidase activity than that of other carbon-based nanomaterials. Moreover, CNPs retained their high peroxidase activity after being reused several times. Glutathione (GSH) can change the blue color of oxidized TMB into a colorless hue at 652 nm. Based on this fact, qualitative and quantitative approaches were employed to detect GSH using a colorimetric method. This method showed a broad detection range (2.5–50 μM) with a limit of detection of 0.26 μM. This method was shown to be accurate for GSH detection in a cell culture medium compared with that using a commercial assay kit. Our findings could facilitate application of CNPs in biomedical areas.

Peroxidases are present widely in microorganisms and plants, and catalyze many reactions.     

Highly selective colorimetric determination of catechol based on the aggregation-induced oxidase–mimic activity decrease of δ-MnO2

Multiple enzyme-like activities of manganese oxides (MnO₂) have been reported and applied in catalysis, biosensors, and cancer therapy. Here, we report that catechol can be determined colorimetrically based on the 3,3′,5,5′-tetramethylbenzidine (TMB) oxidase-like activity of δ-MnO₂. The detection was based on pre-incubation of catechol containing water samples with δ-MnO₂, and then the residual TMB oxidase-like activity of reacted δ-MnO₂ was linearly dependent on the catechol concentration in the range of 0.5 to 10 μM. This determination method was stable at pH 3.73–6.00 and was not affected by ion strength up to 200 μM. Common co-solutes in water bodies (50 μM) have negligible effects and excellent selectivity of catechol among various phenolic compounds (15 μM) was facilitated. Both reduction and aggregation of δ-MnO₂ were observed during the incubation process with catechol, and aggregation-induced TMB oxidase–mimic activity decrease was the main factor for this colorimetric determination.

A new determination mechanism for catechol: aggregation-induced oxidase-mimic activity decrease of δ-MnO₂.     

A nano-sized Cu-MOF with high peroxidase-like activity and its potential application in colorimetric detection of H2O2 and glucose

Peroxidase widely exists in nature and can be applied for the diagnosis and detection of H₂O₂, glucose, ascorbic acid and other aspects. However, the natural peroxidase has low stability and its catalytic efficiency is easily affected by external conditions. In this work, a copper-based metal–organic framework (Cu-MOF) was prepared by hydrothermal method, and characterized by means of XRD, SEM, FT-IR and EDS. The synthesized Cu-MOF material showed high peroxidase-like activity and could be utilized to catalyze the oxidation of o-phenylenediamine (OPDA) and 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The steady-state kinetics experiments of the oxidation of OPDA and TMB catalyzed by Cu-MOF were performed, and the kinetic parameters were obtained by linear least-squares fitting to Lineweaver–Burk plot. The results indicated that the affinity of Cu-MOF towards TMB and OPDA was close to that of the natural horseradish peroxidase (HRP). The as-prepared Cu-MOF can be applied for colorimetric detection of H₂O₂ and glucose with wide linear ranges of 5 to 300 μM and 50 to 500 μM for H₂O₂ and glucose, respectively. Furthermore, the specificity of detection of glucose was compared with other sugar species interference such as sucrose, lactose and maltose. In addition, the detection of ascorbic acid and sodium thiosulfate was also performed upon the inhibition of TMB oxidation. Based on the high catalytic activity, affinity and wide linear range, the as-prepared Cu-MOF may be used for artificial enzyme mimics in the fields of catalysis, biosensors, medicines and food industry.

A Cu-MOF with high peroxidase-like activity was prepared and could be used for colorimetric detection of H₂O₂ and glucose with high selectivity and good linear range (50–500 μM).     

AuNPs@PMo12 nanozyme: highly oxidase mimetic activity for sensitive and specific colorimetric detection of acetaminophen

The design of a highly specific and sensitive approach for the quantitative and qualitative determination of acetaminophen (AP) is crucial from a human health point of view. In this study, AuNPs@PMo₁₂, as a nanozyme, has been developed for the highly sensitive and selective detection of AP with 3,3′,5,5′-tetramethylbenzidine (TMB) within a few seconds without adding oxidizing reagents (e.g. H₂O₂). Synthesized nanosensors are able to oxidize TMB to yellow-brown oxidized TMB (oxTMB). The maximum peak wavelength of oxTMB was observed at 450 nm. The addition of AP and then increasing its concentration led to the production of different products in blue color. In experimental measurements, the limit of detection was obtained as 14.52 mg L⁻¹. The quantitative determination of AP concentrations can be carried out using UV-vis spectroscopy. The design of nanosensors is cost-effective and application of them in H₂O₂-free and enzyme-free conditions provides a rapid sensing approach for practical use in disease monitoring and diagnosis.

The design of a highly specific and sensitive approach for the quantitative and qualitative determination of acetaminophen (AP) is crucial from a human health point of view.     

Mn3O4 microspheres as an oxidase mimic for rapid detection of glutathione

Exploiting a rapid and sensitive method for biomarker detection has important implications in the early diagnosis of diseases. Here, we synthesized Mn₃O₄ microspheres which worked as a nanozyme to exhibit outstanding oxidase-like activity for rapid colorimetric determination of glutathione (GSH). The Mn₃O₄ microspheres of about 800 nm in size could be prepared through a hydrothermal method, and we found that the as-prepared Mn₃O₄ microspheres could quickly oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to its oxidized form (TMBox) in the absence of H₂O₂. After adding glutathione (GSH), TMBox was able to be changed into to its original form and resulted in the corresponding decrease in absorbance value at 652 nm. The Mn₃O₄-TMB system had good linearity with GSH concatenation in the range of 5–60 μM, and the limit of detection was 0.889 μM. Furthermore, this assay possessed high selectivity specificity, which made it possible to detect GSH in human serum samples. Thus, the obtained assay based on the oxidase mimic of Mn₃O₄ would enlarge and exploit the application fields of nanozymes in bio-analysis.

The oxidase-like activity of Mn₃O₄ was used to detect the GSH level directly and rapidly in the absence of H₂O₂.     

Highly efficient redox reaction between potassium permanganate and 3,3′,5,5′-tetramethylbenzidine for application in hydrogen peroxide based colorimetric assays

Potassium permanganate (KMnO₄) is one of the most important oxidants, which plays important roles in many fields. Here, we found that KMnO₄ could directly induce the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to generate an oxidized product with a color change. This redox reaction is highly efficient, and 1 μM KMnO₄ is enough to cause detectable changes in the absorbance signal. Meanwhile, this reaction is very fast and the generated blue product can stabilize for a relatively long period, which has great advantages in practical applications. Since hydrogen peroxide (H₂O₂) is able to react with KMnO₄ under acidic conditions, the KMnO₄-TMB system can be used for the detection of H₂O₂; the absorbance signal induced by 5 μM H₂O₂ can be easily detected in this method. Meanwhile, the KMnO₄-TMB system can also be used for the detection of glucose by monitoring the generation of H₂O₂, which is the main product of glucose oxidation; this method permits detection of concentrations as low as 10 μM glucose, and the sensitivity is comparable to or higher than most peroxidase mimetic based methods, but avoiding the preparation and storage of the nanomaterials. Furthermore, the KMnO₄-TMB system can even be used for analyzing glucose in serum samples, which can also be expected to be used in immunoassays.

The redox reaction between potassium permanganate and 3,3′,5,5′-tetramethylbenzidine is fast and highly efficient, which can be used for different biosensing.     

Ginkgo biloba leaf polysaccharide stabilized palladium nanoparticles with enhanced peroxidase-like property for the colorimetric detection of glucose

Sensitive glucose detection based on nanoparticles is good for the prevention of illness in our bodies. However, many nanoparticles lack stability and biocompatibility, which restrict their sensitivity to glucose detection. Herein, stable and biocompatible Ginkgo biloba leaf polysaccharide (GBLP) stabilized palladium nanoparticles (Pd_n-GBLP NPs) were prepared through a green method where GBLP was used as a reducing and stabilizing agent. The results of Pd_n-GBLP NPs characterized by UV-visible spectroscopy (UV-Vis), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS) confirmed the successful preparation of Pd_n-GBLP NPs. TEM results indicated that the sizes of Pd NPs inside of Pd_n-GBLP NPs (n = 41, 68, 91 and 137) were 7.61, 9.62, 11.10 and 13.13 nm, respectively. XPS confirmed the successful reduction of PdCl₄²⁻ into Pd (0). Dynamic light scattering (DLS) results demonstrated the long-term stability of Pd_n-GBLP NPs in different buffer solutions. Furthermore, Pd₉₁-GBLP NPs were highly biocompatible after incubation (500 μg mL⁻¹) with HeLa cells for 24 h. More importantly, Pd₉₁-GBLP NPs had peroxidase-like properties and followed a ping-pong mechanism. The catalytic oxidation of substrate 3,3′,5,5′-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB) by Pd₉₁-GBLP NPs was used to detect the glucose concentration. This colorimetric method had high selectivity, wide linear range from 2.5 to 700 μM and a low detection limit of 1 μM. This method also showed good accuracy for the detection of glucose concentrations in blood. The established method has great potential in biomedical detection in the future.

Ginkgo biloba leaf polysaccharide stabilized palladium nanoparticles had high stability, good biocompatibility and low detection limit for glucose.     

Ultrasensitive detection of uric acid in serum of patients with gout by a new assay based on Pt@Ag nanoflowers

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed. H₂O₂ was formed by the reaction of uricase and UA and produced the hydroxyl radical (˙OH). The system was catalyzed by Pt@Ag NFs to change the color of 3,3′,5,5′-tetramethylbenzidine (TMB) from colorless to blue, and the morphology and chemical properties of Pt@Ag NFs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Under the optimized conditions, a linear relationship between the absorbance and UA concentration was in the range of 0.5–150 μM (R² = 0.995) with a limit of detection of 0.3 μM (S/N = 3). The method can be applied to detection of UA in actual samples with satisfactory results. The proposed assay was successfully applied to the detection of UA in human serum with recoveries over 96.8%. Thus, these results imply that the UA assay provides an effective tool in fast clinical analysis of gout.

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed.     

MoS2 nanosheet mediated ZnO–g-C3N4 nanocomposite as a peroxidase mimic: catalytic activity and application in the colorimetric determination of Hg(ii)

A novel colorimetric sensing platform using the peroxidase mimicking activity of ternary MoS₂-loaded ZnO–g-C₃N₄ nanocomposites (ZnO–g-C₃N₄/MoS₂) has been developed for the determination of Hg(ii) ions over co-existing metal ions. The nanocomposite was prepared using an exfoliation process, and the product was further characterized using SEM, TEM, XRD and FTIR analysis. The ZnO–g-C₃N₄/MoS₂ possesses excellent intrinsic catalytic activity to induce the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in aqueous solution in the presence of H₂O₂ to generate deep blue coloured cation radicals (TMB⁺) which can be viewed with the naked eye and produce absorbance at a wavelength of 652 nm. The addition of a well known bioradical scavenger, glutathione (GSH), to the solution hinders the generation of cation radicals and turns the solution colourless. The introduction of Hg(ii) to this solution brings the blue colour back into it, due to the strong affinity of the thiol in the GSH. Based on this mechanism, we have developed a simple and rapid colorimetric sensor for the highly sensitive and selective detection of Hg(ii) ions in aqueous solution with a low detection limit of 1.9 nM. Furthermore, the prepared colorimetric sensor was effectively applied for the quantification analysis of real water samples.

A novel colorimetric sensing platform using the peroxidase mimicking activity of ternary MoS₂-loaded ZnO–g-C₃N₄ nanocomposites (ZnO–g-C₃N₄/MoS₂) has been developed for the determination of Hg(ii) ions over co-existing metal ions.     

Enzyme cascade-amplified immunoassay based on the nanobody–alkaline phosphatase fusion and MnO2 nanosheets for the detection of ochratoxin A in coffee

Ochratoxin A (OTA) is a common food contaminant with multiple toxicities and thus rapid and accurate detection of OTA is indispensable to minimize the threat of OTA to public health. Herein a novel enzyme cascade-amplified immunoassay (ECAIA) based on the mutated nanobody–alkaline phosphatase fusion (mNb–AP) and MnO₂ nanosheets was established for detecting OTA in coffee. The detection principle is that the dual functional mNb–AP could specifically recognize OTA and dephosphorylate the ascorbic acid-2-phosphate (AAP) into ascorbic acid (AA), and the MnO₂ nanosheets mimicking the oxidase could be reduced by AA into Mn²⁺ and catalyze the 3,3′,5,5′-tetramethyl benzidine into blue oxidized product for quantification. Using the optimal conditions, the ECAIA could be finished within 132.5 min and shows a limit of detection of 3.38 ng mL⁻¹ (IC₁₀) with an IC₅₀ of 7.65 ng mL⁻¹ and a linear range (IC₂₀–IC₈₀) of 4.55–12.85 ng mL⁻¹. The ECAIA is highly selective for OTA. Good recovery rates (84.3–113%) with a relative standard deviation of 1.3–3% were obtained and confirmed by high performance liquid chromatography with a fluorescence detector. The developed ECAIA was demonstrated to be a useful tool for the detection of OTA in coffee which provides a reference for the analysis of other toxic small molecules.

Enzyme cascade-amplified immunoassay using nanobody–alkaline phosphatase fusion and MnO₂ nanosheets for sensitive and selective detection of ochratoxin A in coffee.