A label-free colorimetric biosensor utilizing natural material for highly sensitive exosome detection

Exosomes, extracellular vesicles secreted by cells, play a crucial role in intercellular communication by transferring information from source cells to recipient cells. These vesicles carry important biomarkers, including nucleic acids and proteins, which provide valuable insights into the parent cells' status. As a result, exosomes have emerged as noninvasive indicators for the early diagnosis of cancer. Colorimetric biosensors have garnered significant attention due to their cost-effectiveness, simplicity, rapid response, and reproducibility. In this study, we employ sporopollenin microcapsules (SP), a natural biopolymer material derived from pollen, as a substrate for gold nanoparticles (AuNPs). By modifying the SP-Au complex with CD63 aptamers, we develop a label-free colorimetric biosensor for exosome detection. In the absence of exosomes, the SP-Au complex catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), resulting in a color change from colorless to blue. However, the addition of exosomes inhibits the catalytic activity of the SP-Au complex due to coverage of exosomes on AuNPs. This colorimetric biosensor exhibits high sensitivity and selectivity for exosome detection, with a detection limit of 10 particles/μL and a wide linear range of 10 - 108 particles/μL. Additionally, the SP-Au biosensor demonstrates remarkable resistance to serum protein adsorption and excellent catalytic stability even in harsh environments, making it highly suitable for clinical diagnostics.

Radix Pueraria Flavonoids Assisted Green Synthesis of Reduced Gold Nanoparticles: Application for Electrochemical Nonenzymatic Detection of Cholesterol in Food Samples

Using radix pueraria flavonoids (RPFs) as a reducing and stabilizing agent, we report a simple, cost-effective, and ecologically friendly green synthesis technique for gold nanoparticles (AuNPs) in the present study. Ultraviolet-visible (UV) spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared (FTIR), and X-ray diffraction (XRD) investigations were used to characterize the AuNPs. The results demonstrated that the produced AuNPs were nearly spherical and that their particle sizes had a mean diameter of 4.85 ± 0.75 nm. The "Green" AuNPs, exhibiting remarkable peroxidase-like activity and Michaelis-Menten kinetics with high affinity for H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB), were effectively applied to the fabrication of a sensitive nonenzymatic enhanced electrochemical sensor for the detection of cholesterol (Cho). Under optimum circumstances, it was possible to establish two linear ranges of 1-100 and 250-5000 μmol/L with a detection limit of 0.259 μmol/L (signal/noise ratio (S/N) = 3). The suggested sensor was utilized with satisfactory findings to determine the amount of Cho in food samples.

Supramolecular Enzymatic Labeling for Aptamer Switch-Based Electrochemical Biosensor

Here we report a novel labeling strategy for electrochemical aptasensors based on enzymatic marking via supramolecular host–guest interactions. This approach relies on the use of an adamantane-modified target-responsive hairpin DNA aptamer as an affinity bioreceptor, and a neoglycoconjugate of β-cyclodextin (CD) covalently attached to a redox enzyme as a labeling element. As a proof of concept, an amperometric aptasensor for a carcinoembryonic antigen was assembled on screen-printed carbon electrodes modified with electrodeposited fern-like gold nanoparticles/graphene oxide and, by using a horseradish peroxidase-CD neoglycoenzyme as a biocatalytic redox label. This aptasensor was able to detect the biomarker in the concentration range from 10 pg/mL to 1 ng/mL with a high selectivity and a low detection limit of 3.1 pg/mL in human serum samples.

Single-Stranded DNA-Encoded Gold Nanoparticle Clusters as Programmable Enzyme Equivalents

Nanozymes have emerged as a class of novel catalytic nanomaterials that show great potential to substitute natural enzymes in various applications. Nevertheless, spatial organization of multiple subunits in a nanozyme to rationally engineer its catalytic properties remains to be a grand challenge. Here, we report a DNA-based approach to encode the organization of gold nanoparticle clusters (GNCs) for the construction of programmable enzyme equivalents (PEEs). We find that single-stranded (ss-) DNA scaffolds can self-fold into nanostructures with prescribed poly-adenine (polyA) loops and double-stranded stems and that the polyA loops serve as specific sites for seed-free nucleation and growth of GNCs with well-defined particle numbers and interparticle spaces. A spectrum of GNCs, ranging from oligomers with discrete particle numbers (2-4) to polymer-like chains, are in situ synthesized in this manner. The polymeric GNCs with multiple spatially organized nanoparticles as subunits show programmable peroxidase-like catalytic activity that can be tuned by the scaffold size and the inter-polyA spacer length. This study thus opens new routes to the rational design of nanozymes for various biological and biomedical applications.

Metalloporphyrin and gold nanoparticles modified hollow zeolite imidazole Framework-8 with excellent peroxidase like activity for quick colorimetric determination of choline in infant formula milk powder

Metalloporphyrin and gold nanoparticles (AuNPs) were fixed on the surface of hollow zeolite imidazole framework-8 (HZIF-8) by cation exchange and cross-linking reaction. The obtained hybrid nanozyme, Au/HZIF-8@TCPP(Fe), was fully characterized by TEM, HRTEM, EDS element mapping and XPS. Then, its peroxidase-like activity was explored with Km of 1.74 mM and Vmax of 9.60 × 10^-8 M·S^-1 towards H₂O₂, indicating excellent catalytic activity. Based on cascade reaction between choline oxidase and Au/HZIF-8@TCPP(Fe), a quick colorimetric method was established for choline detection in infant formula milk powder. After comprehensive verification, this method presented the merits of simple operation, satisfied detection limit (0.05 mM), wide linear range (0.05-2.0 mM), high accuracy (recovery of 92.2%-105.2%) and nice selectivity. This colorimetric method was applied to the determination of choline in milk powders of five brands. Our study could offer valuable reference for finding highly efficient nanozyme and constructing novel optical biosensors.

A dual-readout sandwich immunoassay based on biocatalytic perovskite nanocrystals for detection of prostate specific antigen

Nanozymes have been regarded as an excellent alternative for natural enzymes because of their high stability, low cost, and high activity. However, their use in disease diagnosis is still challenging, since the complex biological samples foul the nanozymes' surface and generate interference signals, thereby compromising the performance of nanozyme-based assays. Here, we report a dual-readout, CsPbBr₃ NCs-based sandwich immunoassay for the detection of prostate specific antigen (PSA). Thanks to their excellent fluorescence and intrinsic peroxidase-like catalytic activity, the designed phospholipid-coated CsPbBr₃ NCs (PL-CsPbBr₃ NCs) served as an attractive dual signal generator (fluorescent and colorimetric), which is hardly achieved by other nanozymes. The Michaelis-Menten constant (K_M) values of PL-CsPbBr₃ NCs for H₂O₂ and tetramethylbenzidine are 2.85 mM and 1.42 mM, respectively. Meanwhile, the lipid shell around CsPbBr₃ NCs not only greatly improves their aqueous stability, but also helps them resist the unspecific adsorption of biological impurities. Thus, the proposed dual-readout immunoassay enables precise, cost-effective, and anti-jamming detection of PSA in real serum samples with a low detection limit of 0.29 ng mL^-1 (colorimetric) and 0.081 ng mL^-1 (fluorescence). This enhanced immunoassay opens new insights for the application of perovskites in bioanalysis, especially for protein assay, holding great potential for disease diagnosis.

Trimetallic AuPtCo Nanopolyhedrons with Peroxidase- and Catalase-Like Catalytic Activity for Glow-Type Chemiluminescence Bioanalysis

Chemiluminescence (CL) with stable and glowing light emission is vital for the accurate detection of biomarkers. Moreover, the catalyst plays an important role in CL systems. Herein, the trimetallic AuPtCo nanopolyhedrons with peroxidase- and catalase-like catalytic activities are readily synthesized via a one-step reduction method. After reaction with the substrate ABEI and oxidant H₂O₂, the AuPtCo nanozyme can catalyze the CL emission in a flash type. Interestingly, it has been found that the biofunctionalization of the AuPtCo surface can endow the catalytic interface with a slow-diffusion effect, thereby prolonging the emission of glow-type CL. On this basis, two biofunctionalized AuPtCo nanocomposites, named as AuPtCo@Cys and AuPtCo@Ab, are prepared, achieving sensitive and selective detection of H₂O₂ and lipoprotein-associated phospholipase A₂ (Lp-PLA₂), respectively. Further, the proposed glow-type CL assays are successfully applied for the determination of H₂O₂ and Lp-PLA₂ in female vaginal discharge and human serum samples, respectively, which exhibit good correlation with the clinical results. Overall, the trimetallic AuPtCo nanozyme-based glow-type CL analysis has demonstrated as a powerful and robust tool for biomarker analysis, which holds great promise in clinical applications.

Two-dimensional layered WS2 nanosheets as peroxidase mimetics in a colorimetric chemosensor for simple and rapid detection of acetone

Two-dimensional (2D) nanomaterials with catalytic activity have attracted considerable attention from researchers, but their application in the detection of hazardous substances needs to be further expanded. Herein, layered tungsten sulfide (WS₂) nanosheets with peroxidase-mimicking activity were used to construct a colorimetric chemosensor for rapid detection of acetone. WS₂ nanosheets can decompose H₂O₂ to generate hydroxyl radicals (·OH), which will further oxidize o-phenylenediamine (OPD) through hydrogen atom transfer (HAT) to form the yellow product 2,3-diaminophenazine. Acetone can block the HAT from OPD to ·OH, thus causing obvious inhibition of the peroxidase activity of WS₂ nanosheets, making the solution appear pale yellow or even colorless. The investigation of catalytic kinetics indicates that the catalytic reaction consists of the 'ping pong' mechanism, and the regulatory effect of acetone on WS₂ nanosheets is confirmed to be an irreversible inhibition. The chemosensor can easily distinguish a trace amount of acetone by the naked eye in less than 20 min, and has a limit of detection for acetone of as low as 3.08 mg  l^-1. The application in actual samples displays the accuracy and stability of the chemosensor, suggesting that such a method is promising for acetone detection.

Synthesis of Au–Cu Alloy Nanoparticles as Peroxidase Mimetics for H2O2 and Glucose Colorimetric Detection

The detection of hydrogen peroxide (H2O2) is essential in many research fields, including medical diagnosis, food safety, and environmental monitoring. In this context, Au-based bimetallic alloy nanomaterials have attracted increasing attention as an alternative to enzymes due to their superior catalytic activity. In this study, we report a coreduction synthesis of gold–copper (Au–Cu) alloy nanoparticles in aqueous phase. By controlling the amount of Au and Cu precursors, the Au/Cu molar ratio of the nanoparticles can be tuned from 1/0.1 to 1/2. The synthesized Au–Cu alloy nanoparticles show good peroxidase-like catalytic activity and high selectivity for the H2O2-mediated oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB, colorless) to TMB oxide (blue). The Au–Cu nanoparticles with an Au/Cu molar ratio of 1/2 exhibit high catalytic activity in the H2O2 colorimetric detection, with a limit of detection of 0.141 μM in the linear range of 1–10 μM and a correlation coefficient R2 = 0.991. Furthermore, the Au–Cu alloy nanoparticles can also efficiently detect glucose in the presence of glucose oxidase (GOx), and the detection limit is as low as 0.26 μM.

ZIF-67-derived Co3O4 hollow nanocage with efficient peroxidase mimicking characteristic for sensitive colorimetric biosensing of dopamine

Co₃O₄ hollow nanocages (Co₃O₄ HNCs) were prepared by simple calcination with ZIF-67 as the precursor. Compared with ordinary nano-sized Co₃O₄, skeletal Co₃O₄ HNCs have a larger specific surface area and porosity, lead to better dispersion, which can expose more catalytic active sites, and obtain higher catalytic activity. Experiments indicate that Co₃O₄ HNCs are used as a catalyst to make H₂O₂ generate O₂. At the same time, Co₃O₄ HNCs act as bridge to accelerate the electrons transfer from the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) to the dissolved oxygen and efficiently obtain blue oxidized TMB (oxTMB) at low concentration of H₂O₂. Steady-state kinetic analysis shows a lower K_m and a higher V_max value than other materials, indicating its excellent affinity and high catalytic efficiency. Based on the inhibitory effect of dopamine (DA) on TMB oxidation in the system, a sensitive, visual colorimetric biosensing method is developed. The calibration curve of DA has a good linear response at both high and low concentrations. Compared with other system, it has the unique advantage of very low detection limit, while retaining a wide detection range, and realizes the accurate detection of actual samples with different concentrations.

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

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

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.

Simultaneous photocatalytic degradation of ibuprofen and H2 evolution over Au/sheaf-like TiO2 mesocrystals

Considerable effort has been devoted to the efficient degradation of pharmaceuticals and personal care products (PPCPs), while the chemical energy in these processes has been widely overlooked. In this study, we demonstrated the simultaneous hydrogen production and ibuprofen degradation through heterogeneous photocatalysis. By anchoring Au nanoparticles (NPs) on the (101) surface of sheaf-like TiO₂ mesocrystals with [001] orientation, efficient charge separation is achieved, which is essential for the photocatalytic redox reactions. XPS analysis showed that the binding energies of Ti 2p and O 1s indicated no shift after Au addition. Peaks observed at 81.8 and 85.5 eV due to Au 4f_7/2 and Au 4f_5/2 of metallic gold on the surface of Au/meso-TiO₂, confirmed the formation of Au NPs. The as-synthesized anatase TiO₂ mesocrystals are composed of small nanocrystals with a size of 8 nm and exhibit the uniform sheaf-like morphology along [001] orientation. As expected, the 1 wt% Au/TiO₂ mesocrystals shows the largest photocurrent density, highest H₂-evolution rate, and fastest photodegradation rate of ibuprofen under simulated sunlight irradiation among all the studied catalyst. Furthermore, the effect of solution pH, common anions (Cl^-, NO₃^-, and SO₄^2-) and cations (Na⁺, K⁺, and Ca²⁺) on photocatalytic H₂ evolution and degradation of ibuprofen were individually investigated and discussed. A mechanism for the simultaneous photocatalytic hydrogen generation and degradation of ibuprofen has also been proposed. This work opens up new opportunities for the development of energy efficient techniques for PPCPs degradation.

Hydroquinone colorimetric sensing based on platinum deposited on CdS nanorods as peroxidase mimics

Pt deposited on CdS nanorods (Pt/CdS) have been prepared via the UV light photoreduction method. The Pt/CdS nanocomposites possess highly significant peroxidase-like activity with the assistance of the colorless substrate 3,3,5,5-tetramethylbenzidine (TMB). In the presence of peroxidase mimic Pt/CdS, TMB is quickly oxidized into a typical blue product (oxTMB, which has an obvious absorption at 652 nm) by H2O2 only in 3 min, which is easily detected visually. The catalytic activity of Pt/CdS originates from the accelerated electron transfer between the reactants. Combining the peroxidase-like activity of Pt/CdS with the blue change of TMB, a fast colorimetric sensing platform for detection of H2O2 has been constructed with a linear range 0.10-1.00 mM and a detection limit of 45.5 μM. The platform developed is further used to detect hydroquinone (HQ) in the range1.0-10 μM with a lower detection limit of 0.165 μM. The colorimetric platform has a potential to detect HQ residue in real water samples with recoveries ranging from 83.56 to 91.76%. Graphical abstract.

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.

Fe-Loaded MOF-545(Fe): Peroxidase-Like Activity for Dye Degradation Dyes and High Adsorption for the Removal of Dyes from Wastewater

Methods to remove dye pollutants with natural enzyme, like horseradish peroxidase (HRP), are still limited due to high costs and low stability levels. The development of such a method with similar enzymatic activity is important and could be helpful in wastewater disposal. A metal organic framework material, Fe-loaded MOF-545 (Fe), was synthesized in our study as a new way to remove dyes due to its peroxidase-like activity. The structural characterizations of Fe-loaded MOF-545(Fe) was investigated using scanning electron microscopy (SEM), UV-Vis absorption spectra, and X-ray diffraction (XRD). The peroxidase-like (POD-like) activity of Fe-loaded MOF-545(Fe) was investigated under different pH and temperature conditions. Because of the Fe added into the MOF-545 structure, the absorption of Fe-loaded MOF-545(Fe) for acid (anionic) dyes (methyl orange (MO)) was better than for basic (cationic) dyes (methylene blue (MB)). The Fe-loaded MOF-545(Fe) could give a significant color fading for MO and MB over a short time (about two hours) with peroxidase-like activity. The remarkable capacity of Fe-loaded MOF-545(Fe) to remove the MO or MB is due to not only physical adsorption, but also degradation of the MO and MB with POD-like activity. Therefore, Fe-loaded MOF-545(Fe) has significant potential regarding dye removal from wastewater.

Sulfur and phosphorus co-doped graphene quantum dots for fluorescent monitoring of nitrite in pickles

Doping graphene quantum dots (GQDs) with heteroatoms can change their band gap and electronic density, thus enhancing their fluorescence quantum yield (QY). In this work, we for the first time reported a nontoxic, rapid, and one-pot hydrothermal method to synthesize sulfur and phosphorus co-doped GQDs (S, P-GQDs). Citric acid was functioned as a carbon source, whereas sodium phytate and anhydrous sodium sulfate are used as the P and S sources, respectively, in this bottom-up synthesis. The resulting S, P-GQDs exhibit high heteroatomic doping ratios of 9.66 at.% for S and 3.34 at.% for P, and higher QY than those obtained from monoatomic doped GQDs. Additionally, the as-prepared S, P-GQDs exhibit excitation-dependent behavior, pH sensitivity between 8.0 and 13.0, high tolerance of ionic strength. More importantly, the as-synthesized S, P-GQDs show a sensitive and selective behavior for sensing nitrite (NO₂^-) in the concentration range of 0.7-9 μmol/L, and the detection limit was as low as 0.3 μmol/L. Additionally, the S, P-GQDs was successfully used in detecting NO₂^- in pickled foods, showing their promise for potential applications in realistic analysis.

Dynamic interactions between peroxidase-mimic silver NanoZymes and chlorpyrifos-specific aptamers enable highly-specific pesticide sensing in river water

With growing environmental and health concerns over persistent organic compounds such as organophosphates, regulatory bodies have imposed strict regulations for their use and monitoring in water bodies. Although conventional analytical tools exist for the detection of organophosphorus pesticides, new strategies need to be developed to fulfil the ASSURED (affordable, sensitive, specific, user-friendly, rapid, equipment-free and deliverable to end users) criteria of the World Health Organisation. One such strategy is to employ the ability of certain nanoparticles to mimic the enzymatic activity of natural enzymes to develop optical sensors. We show that the intrinsic peroxidase-mimic NanoZyme activity of tyrosine-capped silver nanoparticles (Ag-NanoZyme) can be exploited for highly specific and rapid detection of chlorpyrifos, an organophosphorus pesticide. The underlying working principle of the proposed aptasensor is based on the dynamic non-covalent interaction of the chlorpyrifos specific aptamer (Chl) with the NanoZyme (sensor probe) vs. the pesticide target (analyte). The incorporation of the Chl aptamer ensures high specificity leading to a colorimetric response specifically in the presence of chlorpyrifos, while the sensor remains unresponsive to other pesticides from organophosphate and non-organophosphate groups. The robustness of the sensor to work directly in environmental samples was established by evaluating its ability to detect chlorpyrifos in river water samples. The excellent recovery rates demonstrate the sensor robustness, while the simplicity, and rapid sensor response (2 min) to detect the presence of chlorpyrifos highlights the capabilities of the proposed colorimetric sensing system.

One-pot synthesized Cu/Au/Pt trimetallic nanoparticles as a novel enzyme mimic for biosensing applications

Multimetallic nanomaterials have aroused special attention owing to the unique characteristics of chemical, optical and enhanced enzyme mimetic capabilities resulting from the synergistic effect of different metal elements. In this work, we present a facile, gentle, fast and one-pot method for preparing Cu/Au/Pt trimetallic nanoparticles (TNPs), which possess intrinsic and enhanced peroxidase-like activity as well as excellent stability, sustainable catalytic activity, and robustness to harsh environments. Kinetic analysis indicated that Cu/Au/Pt TNPs exhibited strong affinities with H2O2 and 3,3,5,5-tetramethylbenzidine (TMB) as the substrates. To investigate the feasibility of Cu/Au/Pt TNPs-based strategy in biological analysis, H2O2 was chosen as a model analyte and a sensitive and specific detection for H2O2 was acquired with a detection limit of 17 nM. By coupling with glucose oxidase (GOD), this assay could also achieve a sensitive and selective detection of glucose with a detection limit of 33 μM, indicating the versatility of the method. In view of the potential combination with diverse enzyme-related reactions, the Cu/Au/Pt TNPs-based strategy is promising as a universal platform for biosensors.

Porphyrin functionalized Co(OH)2/GO nanocomposites as an excellent peroxidase mimic for colorimetric biosensing

A colorimetric sensor based on the enhanced peroxidase-like activity of Por/Co(OH)₂/GO for the sensitive detection of H₂O₂ and GSH.

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

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

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

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

Nanostructured silver fabric as a free-standing NanoZyme for colorimetric detection of glucose in urine

Enzyme-mimicking catalytic nanoparticles, more commonly known as NanoZymes, have been at the forefront for the development of new sensing platforms for the detection of a range of molecules. Although solution-based NanoZymes have shown promise in glucose detection, the ability to immobilize NanoZymes on highly absorbent surfaces, particularly on free-standing substrates that can be feasibly exposed and removed from the reaction medium, can offer significant benefits for a range of biosensing and catalysis applications. This work, for the first time, shows the ability of Ag nanoparticles embedded within the 3D matrix of a cotton fabric to act as a free-standing peroxidase-mimic NanoZyme for the rapid detection of glucose in complex biological fluids such as urine. The use of cotton fabric as a template not only allows high number of catalytically active sites to participate in the enzyme-mimic catalytic reaction, the absorbent property of the cotton fibres also helps in rapid absorption of biological molecules such as glucose during the sensing event. This, in turn, brings the target molecule of interest in close proximity of the NanoZyme catalyst enabling accurate detection of glucose in urine. Additionally, the ability to extract the free-standing cotton fabric-supported NanoZyme following the reaction overcomes the issue of potential interference from colloidal nanoparticles during the assay. Based on these unique characteristics, nanostructured silver fabrics offer remarkable promise for the detection of glucose and other biomolecules in complex biological and environmental fluids.

Titania-coated gold nanorods with expanded photocatalytic response. Enzyme-like glucose oxidation under near-infrared illumination

Gold nanorods coated with a uniform titanium dioxide nanoshell have been prepared and used as glucose-oxidase surrogates. Remarkably, this core-shell photocatalytic nanostructure has been able to induce complete oxidation of glucose at near room temperature (32-34 °C) in a wide range of pH values with the aid of a near-infrared (NIR) irradiation source. In contrast, the uncoated gold nanorods exhibit negligible photo-oxidation response under identical experimental conditions thereby proving the photoactivity of the titania shell towards glucose oxidation. The process takes place via in situ photo-generation of singlet oxygen or hydroxyl radicals as reactive oxidative species (ROS). This underlines the role played by the core nanorods as plasmonic light harvesters in the NIR range and constitutes the first example of a NIR-activated enzyme-like catalyst.

N,N′-di-caboxy methyl perylene diimide (PDI) functionalized CuO nanocomposites with enhanced peroxidase-like activity and their application in visual biosensing of H2O2 and glucose

Perylene diimide functionalized CuO nanobelts were demonstrated to possess higher intrinsic peroxidase-like activity than that of pure CuO nanobelts.

Oxidase-like mimic of Ag@Ag3PO4 microcubes as a smart probe for ultrasensitive and selective Hg(2+) detection

An oxidase-like mimic system based on facilely synthesized Ag@Ag3PO4 microcubes (Ag@Ag3PO4MCs) was designed and utilized to detect mercury ions with high selectivity and ultrasensitivity. Ag@Ag3PO4MCs with an average size of ca. 1.6 μm were synthesized by the reaction of [Ag(NH3)2](+) complex and Na2HPO4 and subsequent photoreduction under ultraviolet light. The as-prepared Ag@Ag3PO4MCs can effectively catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and o-phenylenediamine (OPD) in the presence of dissolved oxygen in slightly acidic solution, exhibiting oxidase-like activities rather than peroxidase-like activity. Interestingly, the introduction of Ag nanoparticles (AgNPs) on the surfaces of Ag3PO4MCs can dramatically enhance the oxidase-like activities due to a synergistic effect between AgNPs and Ag3PO4MCs, as evidenced by the faster oxidation speed of TMB and OPD than that of native Ag3PO4MCs in the presence of dissolved oxygen. The enzyme kinetics can be well-explained by the Michaelis-Menten equation. As "poisoning" inhibitor, Hg(2+) ions can inhibit the enzyme reaction catalyzed by Ag3PO4MCs or Ag@Ag3PO4MCs. On the basis of this effect, a colorimetric Hg(2+) sensor was developed by the enzyme inhibition reaction of Ag3PO4MCs or Ag@Ag3PO4MCs. The excellent specific interaction of Hg-Ag or Hg(2+)-Ag(+) provides high selectivity for Hg(2+) over interfering metal ions. Meanwhile, the sensitivity of this sensor to Hg(2+) is extremely excellent with a limit of detection as low as 0.253 nM for Ag@Ag3PO4MCs. Considering the advantages of low detection limit, low cost, facile preparation, and visualization, the colorimetric Ag@Ag3PO4MCs sensor shows high promise for the testing of Hg(2+) in water samples.

Intrinsic peroxidase-like activity and the catalytic mechanism of gold@carbon dots nanocomposites

The mechanism of AuNPs@CDs as nano-enzyme catalysing the oxidation of TMB in the presence of H₂O₂.

Peroxidase-like activity of gold nanoparticles stabilized by hyperbranched polyglycidol derivatives over a wide pH range

The aim of this work was to carry out comparative studies on the peroxidase-like activity of gold nanoparticles (AuNPs) stabilized with low molecular weight hyperbranched polyglycidol (HBPG-OH) and its derivative modified with maleic acid residues (HBPG-COOH). The influence of the stabilizer to gold precursor ratio on the size and morphology of nanoparticles obtained was checked, and prepared nanoparticles were characterized by means of transmission electron microscopy and UV-Vis spectroscopy. The results indicated the divergent effect of increasing the concentration of stabilizers (HBPG-OH or HBPG-COOH) on the size of the nanostructures obtained. The gold nanoparticles obtained were characterized as having intrinsic peroxidase-like activity and the mechanism of catalysis in acidic and alkaline mediums was consistent with the standard Michaelis-Menten kinetics, revealing a strong affinity of AuNPs with 2, 2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 3, 3', 5, 5'-tetramethylbenzidine (TMB), and significantly lower affinity towards phenol. By comparing the kinetic parameters, a negligible effect of polymeric ligand charge on activity against various types of substrates (anionic or cationic) was indicated. The superiority of steric stabilization via the application of tested low-weight hyperbranched polymers over typical stabilizers in preventing salt-induced aggregation and maintaining high catalytic activity in time was proved. The applied hyperbranched stabilizers provide a good tool for manufacturing gold-based nanozymes, which are highly stable and active over a wide pH range.