Robust Oxidase-Mimetic Supramolecular Nanocatalyst for Lignin Biodegradation.

Enzymatic catalysis presents an eco-friendly, energy-efficient method for lignin degradation. However, challenges arise due to the inherent incompatibility between enzymes and native lignin. In this work, we introduce a supramolecular catalyst composed of fluorenyl-modified amino acids and Cu2+, designed based on the aromatic stacking of the fluorenyl group, which can operate in ionic liquid environments suitable for the dissolution of native lignin. Amino acids and halide anions of ionic liquids shape the copper site's coordination sphere, showcasing remarkable catechol oxidase-mimetic activity. The catalyst exhibits thermophilic property, and maintains oxidative activity up to 75 °C, which allows the catalyzed degradation of the as-dissolved native lignin with high efficiency even without assistance of the electron mediator. In contrast, at this condition, the native copper-dependent oxidase completely lost its activity. This catalyst with superior stability and activity offer promise for sustainable lignin valorization through biocatalytic routes compatible with ionic liquid pretreatment, addressing limitations in native enzymes for industrially relevant conditions.

Histidine-Based Supramolecular Nanoassembly Exhibiting Dual Enzyme-Mimetic Functions: Altering the Tautomeric Preference of Histidine to Tailor Oxidative/Hydrolytic Catalysis.

Challenges persist in replicating enzyme-like active sites with functional group arrangements in supramolecular catalysis. In this study, we present a supramolecular material comprising Fmoc-modified histidine and copper. We also investigated the impact of noncanonical amino acids (δmH and εmH), isomers of histidine, on the catalytic process. The Fmoc-δmH-based nanoassembly exhibits an approximately 15-fold increase in oxidative activity and an ∼50-fold increase in hydrolytic activity compared to Fmoc-εmH (kcat/Km). This distinction arises from differences in basicity and ligation properties between the ε- and δ-nitrogen of histidine. The addition of guanosine monophosphate further enhances the oxidative activity of the histidine- and methylated histidine-based catalysts. The Fmoc-δmH/Cu2+-based nanoassembly catalyzes the oxidation/hydrolysis cascade of 2',7'-dichlorofluorescein diacetate, benefiting from the synergistic effect between the copper center and the nonligating ε-nitrogen of histidine. These findings advance the biomimetic catalyst design and provide insights into the mechanistic role of essential residues in natural systems.

Modifying Proton Relay into Bioinspired Dye-Based Coordination Polymer for Photocatalytic Proton-Coupled Electron Transfer.

Proton-coupled electron transfer (PCET) imparts an energetic advantage over single electron transfer in activating inert substances. Natural PCET enzyme catalysis generally requires tripartite preorganization of proton relay, substrate-bound active center, and redox mediator, making the processes efficient and precluding side reactions. Inspired by this, a heterogeneous photocatalytic PCET system was established to achieve higher PCET driving forces by modifying proton relays into anthraquinone-based anionic coordination polymers. The proximally separated proton relays and photoredox-mediating anthraquinone moiety allowed pre-assembly of inert substrate between them, merging proton and electron into unsaturated bonds by photoreductive PCET, which enhanced reaction kinetics compared with the counter catalyst without proton relay. This photocatalytic PCET method was applied to a broad-scoped reduction of aryl ketones, unsaturated carbonyls, and aromatic compounds. The distinctive regioselectivities for the reduction of isoquinoline derivatives were found to occur on the carbon-ring sides. PCET-generated radical intermediate of quinoline could be trapped by alkene for proton relay-assisted Minisci addition, forming the pharmaceutical aza-acenaphthene scaffold within one step. When using heteroatom(X)-H/C-H compounds as proton-electron donors, this protocol could activate these inert bonds through photooxidative PCET to afford radicals and trap them by electron-deficient unsaturated compounds, furnishing the direct X-H/C-H functionalization.

Reduction-triggered Disassembly of Organic Cationic Nanorods Produces a Cysteine Protease Mimic (Miravet et†al.).

The emergence of catalytic activity associated with a disassembly process is reported, reminiscent of complex biological systems. A cystine derivative with pendant imidazole groups self-assembles into cationic nanorods in the presence of the cationic surfactants cetylpyridinium chloride (CPC) or cetyltrimethylammonium bromide (CTAB). Disulfide reduction triggers nanorod disassembly and the generation of a simple cysteine protease mimic, which shows a dramatically improved catalytic efficiency in the hydrolysis of p-nitrophenyl acetate (PNPA).

Spindle-shaped Cu-Ru mesoporous nanospheres with enhanced enzyme-like activity for visual differentiation of toxic o-/m-aminophenol and recognition mechanisms.

To effectively differentiate toxic aminophenol isomers, a kind of spindle-shaped Cu-Ru bimetal mesoporous nanozyme (Cu-Ru MPNZ) with high specific surface was developed by one-pot homogeneous reduction method, directed by hexadecyl trimethyl ammonium bromide (CTAB) in this work. By virtue of the distinctive microstructure, Cu-Ru MPNZ expressed superior bi-functional oxidase- and peroxidase-mimic activity to catalyze the oxidation of 3,3',5,5,'-tetramethylbenzidine (TMB) and 2,2'-azinobis (3-ethylbenzothiazoline-6- sulfonic acid) ammonium salt (ABTS) with low Michaelis-Menten constants and quick reaction rates. Especially, toxic aminophenol isomers could exclusively react with the oxydates of TMB or ABTS to express differentiable signals in color. Under the optimal conditions, Cu-Ru MPNZ was successfully applied for visual differentiation of toxic aminophenol isomers in real aqueous, juices and medicinal samples with low detection limits (1.60 × 10-8 mol/L for o-aminophenol and 3.25 × 10-8 mol/L for m-aminophenol) and satisfactory recoveries (96.6-103.5%). The different recognition mechanisms of Cu-Ru MPNZ to toxic o- and m-aminophenol isomers were proposed for the first time as far as we known. This work will provide a potential way to monitor different organic isomer pollution in future.

An Unprecedented FeMo6 @Ce-Uio-66 Nanocomposite with Cascade Enzyme-Mimic Activity as Colorimetric Sensing Platform.

Introducing the idea of integrated design and cascade activity into nanozyme, the novel integrated nanozymes (INAzymes), FeMo6 @Ce-Uio-66 (FC-66(n)), were designed and synthesized by encapsulating iron-based polyoxometalates (FeMo6 ) into the ceria-based metal-organic framework (Ce-Uio-66). Due to the oxygen-driven reversible Ce3+ /Ce4+ couple sites, the "Fenton-like" effect by iron centers, the "nanoscale proximity" effects by nanocages, and their synergistic effects, FC-66(n) as INAzymes exhibit elegant cascade enzyme-mimic activities (oxidase-, peroxidase-, and Fenton-like activity), which realizes INAzyme activities based on polyoxometalates based metal-organic framework (POMOFs). By employing dopamine (DA) detection as a model reaction, a high-efficient fluorescent "turning-on-enhanced" platform under near neutral conditions was established.

The synthesis of Pt doped WO3 nanosheets and application on colorimetric detection of cysteine by naked eye using response surface methodology for optimization.

We present a simple, sensitive, and specific colorimetric using the peroxidase properties method based on Pt doped WO3 nanosheets to detect the cysteine. Pt@WO3NSs were synthesized by hydrothermal method and characterized by Fourier transform infrared (FTIR), Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction patterns (XRD) methods. The response surface methodology (RSM) method based on the central composite design (CCD) was used to optimize test parameters such as pH, nanosheet concentration, and temperature. When cysteine is present in the environment due to its competition with 3,3', 5,5'-Tetramethylbenzidine (TMB) in the use of hydrogen peroxide, the blue discoloration is reduced compared to the absence of cysteine and leads to its detection. We have favorably created a peculiar approach for sensing cysteine based on the colorimetric method in solution and paper with linear range 0.01-15 µM, 0.005-14 µM and R2 = 0.9887 and R2 = 0.9871 respectively. The detection limit for solution-based is 1.2 nM and for paper-based is 1 nM.

Nitrogen-doped carbon dots/Ni-MnFe-layered double hydroxides (N-CDs/Ni-MnFe-LDHs) hybrid nanomaterials as immunoassay label for low-density lipoprotein detection.

Nitrogen-doped carbon dots/Ni-MnFe-layered double hydroxides (N-CDs/Ni-MnFe-LDHs) are demonstrated as superior peroxidase mimic antibody labels alternative to horseradish peroxidase (HRP) in an immunoassay, potentially overcoming some of the inherent disadvantages of HRP and other enzyme mimicking nanomaterials. They revealed efficient peroxidase-like activity and catalyzed the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to form the intense blue product (at 620 nm) in the presence of hydrogen peroxide (H2O2). Using low-density lipoprotein (LDL) as a model target, an ultra-low limit of detection (0.0051 mg/dL) and a linear range of 0.0625-0.750 mg/dL were achieved, exhibiting higher sensitivity than the HRP-based immunoassay. Thus, the proposed N-CDs/Ni-MnFe-LDHs can be used as HRP mimicking analogs for developing highly sensitive colorimetric immunosensors for detection of biomarkers, as well as trace chemical analysis.

Applications of Nanozymology in the Detection and Identification of Viral, Bacterial and Fungal Pathogens.

Nanozymes are synthetic nanoparticulate materials that mimic the biological activities of enzymes by virtue of their surface chemistry. Enzymes catalyze biological reactions with a very high degree of specificity. Examples include the horseradish peroxidase, lactate, glucose, and cholesterol oxidases. For this reason, many industrial uses of enzymes outside their natural environments have been developed. Similar to enzymes, many industrial applications of nanozymes have been developed and used. Unlike the enzymes, however, nanozymes are cost-effectively prepared, purified, stored, and reproducibly and repeatedly used for long periods of time. The detection and identification of pathogens is among some of the reported applications of nanozymes. Three of the methodologic milestones in the evolution of pathogen detection and identification include the incubation and growth, immunoassays and the polymerase chain reaction (PCR) strategies. Although advances in the history of pathogen detection and identification have given rise to novel methods and devices, these are still short of the response speed, accuracy and cost required for point-of-care use. Debuting recently, nanozymology offers significant improvements in the six methodological indicators that are proposed as being key in this review, including simplicity, sensitivity, speed of response, cost, reliability, and durability of the immunoassays and PCR strategies. This review will focus on the applications of nanozymes in the detection and identification of pathogens in samples obtained from foods, natural, and clinical sources. It will highlight the impact of nanozymes in the enzyme-linked immunosorbent and PCR strategies by discussing the mechanistic improvements and the role of the design and architecture of the nanozyme nanoconjugates. Because of their contribution to world health burden, the three most important pathogens that will be considered include viruses, bacteria and fungi. Although not quite seen as pathogens, the review will also consider the detection of cancer cells and helminth parasites. The review leaves very little doubt that nanozymology has introduced remarkable advances in enzyme-linked immunosorbent assays and PCR strategies for detecting these five classes of pathogens. However, a gap still exists in the application of nanozymes to detect and identify fungal pathogens directly, although indirect strategies in which nanozymes are used have been reported. From a mechanistic point of view, the nanozyme technology transfer to laboratory research methods in PCR and enzyme-linked immunosorbent assay studies, and the point-of-care devices such as electronic biosensors and lateral flow detection strips, that is currently taking place, is most likely to give rise to no small revolution in each of the six methodological indicators for pathogen detection and identification. While the evidence of widespread research reports, clinical trials and point-of-care device patents support this view, the gaps that still exist point to a need for more basic research studies to be conducted on the applications of nanozymology in pathogen detection and identification. The multidisciplinary nature of the research on the application of nanozymes in the detection and identification of pathogens requires chemists and physicists for the design, fabrication, and characterization of nanozymes; microbiologists for the design, testing and analysis of the methodologies, and clinicians or clinical researchers for the evaluation of the methodologies and devices in the clinic. Many reports have also implicated required skills in mathematical modelling, and electronic engineering. While the review will conclude with a synopsis of the impact of nanozymology on the detection and identification of viruses, bacteria, fungi, cancer cells, and helminths, it will also point out opportunities that exist in basic research as well as opportunities for innovation aimed at novel laboratory methodologies and devices. In this regard there is no doubt that there are numerous unexplored research areas in the application of nanozymes for the detection of pathogens. For example, most research on the applications of nanozymes for the detection and identification of fungi is so far limited only to the detection of mycotoxins and other chemical compounds associated with fungal infection. Therefore, there is scope for exploration of the application of nanozymes in the direct detection of fungi in foods, especially in the agricultural production thereof. Many fungal species found in seeds severely compromise their use by inactivating the germination thereof. Fungi also produce mycotoxins that can severely compromise the health of humans if consumed.

Transesterification of Non-Activated Esters Promoted by Small Molecules Mimicking the Active Site of Hydrolases.

The synthesis of small molecules able to mimic the active site of hydrolytic enzymes has been largely pursued in recent decades. The high reaction rates and specificity shown by natural hydrolases present an attractive target, and yet the preparation of suitable small-molecule mimics remains challenging, requiring activated substrates to achieve productive outcomes. Here we present small synthetic artificial enzymes which mimic the catalytic site and the oxyanion hole of chymotrypsin and N-terminal hydrolases and are able to perform, for the first time, the transesterification of a non-activated ester such as ethyl acetate with methanol under mild and neutral reaction conditions.

Platinum-Gold Alloy Catalyzes the Aerobic Oxidation of Formic Acid for Hydrogen Peroxide Synthesis.

On-site hydrogen peroxide production through electrocatalytic and photocatalytic oxygen reduction reactions has recently attracted broad research interest. However, practical applications have thus far been plagued by the low activity and the requirement of complex equipment. Here, inspired by the process of biological hydrogen peroxide synthesis catalyzed by enzymes, we report a Pt-Au alloy to mimic the catalytic function of natural formate oxidase for hydrogen peroxide synthesis through aerobic oxidation of formic acid. The mass activity of the Pt-Au alloy is three times higher than that of formate oxidase. Density functional theory calculations revealed that the efficient dehydrogenation of formic acid and the high selectivity of the subsequent reduction of oxygen to hydrogen peroxide account for the high hydrogen peroxide productivity. In addition, the formic acid aqueous solution provides an acidic environment, which is conducive to the utilization of the in situ generated hydrogen peroxide for oxidation reactions, including C-H bond oxidation and sterilization.

Strain Effect in Palladium Nanostructures as Nanozymes.

While various effects of physicochemical parameters (e.g., size, facet, composition, and internal structure) on the catalytic efficiency of nanozymes (i.e., nanoscale enzyme mimics) have been studied, the strain effect has never been reported and understood before. Herein, we demonstrate the strain effect in nanozymes by using Pd octahedra and icosahedra with peroxidase-like activities as a model system. Strained Pd icosahedra were found to display 2-fold higher peroxidase-like catalytic efficiency than unstrained Pd octahedra. Theoretical analysis suggests that tensile strain is more beneficial to OH radical (a key intermediate for the catalysis) generation than compressive strain. Pd icosahedra are more active than Pd octahedra because icosahedra amplify the surface strain field. As a proof-of-concept demonstration, the strained Pd icosahedra were applied to an immunoassay of biomarkers, outperforming both unstrained Pd octahedra and natural peroxidases. The findings in this research may serve as a strong foundation to guide the design of high-performance nanozymes.

Monodispersed gold nanoparticles entrapped in ordered mesoporous carbon/silica nanocomposites as xanthine oxidase mimic for electrochemical sensing of xanthine.

Monodispersed Au nanoparticles in ordered mesoporous carbon/silica (Au/OMCS) nanocomposites were prepared by the solvent evaporation induced self-assembly. Au/OMCS nanocomposites were characterized through XRD, BET, and TEM. The obtained nanocomposites exhibit uniform mesopores with the size of 18 ± 2 nm. And ultrafine Au nanoparticles with the size of 3~7 nm are well dispersed in the cavities. An ultrasensitive nanoenzyme sensor was fabricated based on a Au/OMCS-modified electrode. The Au/OMCS-modified electrode displays high xanthine oxidase-like catalytic activity evaluated through cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The DPV response currents are linearly dependent on concentrations of xanthine (Xa) in the range 0.10-20 µM, along with a high sensitivity of 6.84 µA µM-1 cm-2 and very low detection limit of 0.006 µM (S/N = 3) under the optimal working potential of 0.64 V vs. SCE. Interference experiments show that the nanoenzyme sensor has no obvious responses to most potentially interfering species at a potential of 0.64 V. The fabricated sensor has been applied to the determination of Xa in spiked urine samples with recoveries ranging from 98.26 to 101.4%. Graphical abstract.

Polyethylenimine-stabilized silver nanoclusters act as an oxidoreductase mimic for colorimetric determination of chromium(VI).

A new and efficient assay is proposed for the photometric determination of Cr6+ by employing polyethylenimine-stabilized Ag nanoclusters (PEI-AgNCs) as an oxidoreductase mimic. Cr6+ with certain oxidicability is able to specifically react with 3,3',5,5'-tetramethylbenzidine (TMB), giving a color change from colorless to blue indicating the presence of Cr6+. However, the redox kinetics is so slow that the sensitivity obtained for Cr6+ determination is very poor. It is interestingly found that PEI-AgNCs can act as an oxidoreductase-like nanozyme to significantly promote the sluggish reaction, making it possible to rapidly detect toxic Cr6+ with remarkably enhanced performance. With the use of PEI-AgNCs, fast and convenient determination of Cr6+ was realized, with a limit of detection as low as 1.1 µM. Additionally, the proposed assay exhibited excellent selectivity; other ions, including Cr3+, hardly affected the determination of Cr6+. Graphical abstract Polyethylenimine-stabilized silver nanoclusters (PEI-AgNCs) act as an oxidoreductase mimic to catalyze the redox reaction of Cr6+ and 3,3',5,5'-tetramethylbenzidine (TMB), enabling the high-performance colorimetric determination of toxic Cr6+.

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.

Recent developments in colorimetric immunoassays using nanozymes and plasmonic nanoparticles.

Enzyme-linked immunosorbent assay (ELISA) is a popular detection technique for the screening and diagnosis of diseases. The sensitivity of ELISA can be increased by the incorporation of nanoparticles. Through this article, we discuss the utilization of nanoparticles in ELISA. Nanoparticles possess an intrinsic biological peroxidase-like activity which allows it to act as an enzyme mimic for the development of an improved analysis method. Different nanoparticles (gold nanoparticles, silver nanoparticles, etc.) carry different peroxidase-mimic characteristics. Besides this, nanoparticles can also perform as a colorimetric substrate in ELISA where it gives a more prominent color change compared to the commonly used colorimetric substrate TMB. This article also focuses on the mechanisms behind this color change including aggregation, in situ nanoparticle growth, seeding, and etching.

Gold-platinum nanoflowers as a label and as an enzyme mimic for use in highly sensitive lateral flow immunoassays: application to detection of rabbit IgG.

The authors describe the preparation of gold-platinum nanoflower (AuPt NFs) and show that they can be simultaneously used as a label and as an enzyme mimic in lateral flow immunoassays (LFIs). The AuPt NFs were prepared by growing Pt nanowires on the surface of gold nanoparticle. The assay involves the capture of target proteins (here: rabbit IgG as a model analyte) by the immobilized capture antibody, and by using AuPt NF-labeled secondary antibody. The AuPt NFs are thus captured by the test zone and produce a characteristic black band for visual detection of the antigen (IgG). The coloration of the test line can be further enhanced by addition of the chromogenic substrate 3-amino-9-ethyl-carbazole which is catalytically oxidized by the captured Pt nanowires on the AuPt NF and produce a red coloration. Quantitative results were obtained by reading the test line intensities with a portable strip reader. The LFI has a 5 pg mL-1 detection limit for IgG under optimized experimental conditions. This is 100 times lower than that of the conventional AuNP-based LFI. Conceivably, this assay has a wide scope in that it may be applied to numerous other targets for which appropriate antibodies are available. Graphical abstract Gold-platinum nanoflowers are used as a label and as an enzyme mimic in a highly sensitive lateral flow immunoassay for IgG. The detection limit of gold-platinum nanoflower-based lateral flow assay is 100 times lower than that of the conventional gold nanopaticle-based lateral flow assay.

Etched PtCu nanowires as a peroxidase mimic for colorimetric determination of hydrogen peroxide.

Etched PtCu nanowires (NWs) were synthesized by a hydrothermal reaction and chemical etching process. The NWs are shown to be viable peroxidase (POx) mimics capable of catalyzing the oxidation of the substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2 to form a blue-green coloration. The mechanism of catalysis was investigated and the results demonstrated that H2O2 is decomposed to form hydroxyl radicals which oxidize TMB in the presence of the NWs. Under optimized conditions, a steady-state kinetic analysis revealed that the NWs possess a stronger affinity for H2O2 and TMB compared to the enzyme horseradish POx. Based on the high POx-like activity, a colorimetric assay for H2O2 was established. Absorbance at 652 nm increases linearly in the 0.1-300 µM H2O2 concentration range, and the detection limit is 0.06 µM (at S/N = 3). The assay was successfully applied to the determination of H2O2 in (spiked) milk and contact lens solution. Furthermore, a highly sensitive test strip was designed which represents a low cost and fast alternative for the visual determination of H2O2. Graphical Abstract Schematic presentation of the colorimetric detection of H2O2. PtCu nanowires (PtCu NWs) can catalyze 3,3',5,5'-tetramethylbenzidine (TMB) oxidation by H2O2 to produce blue-green oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB). Based on the color change, test strips were designed for H2O2 detection.

Rapid colorimetric determination of the pesticides carbofuran and dichlorvos by exploiting their inhibitory effect on the aggregation of peroxidase-mimicking platinum nanoparticles.

A novel and highly sensitive enzyme inhibition assay was developed for the rapid detection of the organophosphate pesticide dichlorvos and the carbamate pesticide carbofuran. It achieves signal amplification by the secondary catalysis of platinum nanoparticles. Acetylcholinesterase (AChE) is capable of catalyzing the hydrolysis of acetylthiocholine to form thiocholine. Thiocholine causes the aggregation of citrate-capped platinum nanoparticles which then lose their peroxidase-mimicking properties. After addition of pesticides, the activity of AChE is inhibited, less thiocholine is produced, less aggregation occurs, and the peroxidase-mimetic properties are increasingly retained. In the presence of tetramethylbenzidine and H2O2, a deep blue coloration with an absorption maximum at 650 nm will be formed. The assay was applied to the determination of dichlorvos and carbofuran, and detection limits of 2.3 µg·L-1 and 1.4 µg·L-1 were obtained, respectively. Recovery experiments with spiked tap water and pears gave satisfactory relative standard deviations. Graphical abstract The blue product formed by platinum nanoparticle-catalyzed oxidation of 3,3'5,5'-tetramethylbenzidine (TMB) by H2O2 is reduced if acetylthiocholine (ATCh) is hydrolyzed by acetylcholinesterase (AChE) to form thiocholine. However, if AChE is inhibited by pesticides, color formation will recover.

Copper(II) ions enhance the peroxidase-like activity and stability of keratin-capped gold nanoclusters for the colorimetric detection of glucose.

A method is described for the preparation of copper(II)-modified keratin-capped gold nanoclusters (AuNCs) with adjustable Au/Cu molar ratio through a two-step synthetic route. The introduction of Cu(II) is known to cause quenching of the fluorescence of such AuNCs. It is found, however, that the Cu(II) loaded AuNC (AuNC-Cu2+) display strongly enhanced peroxidase-like activity and improved chemical stability. This is assumed to be due to the synergistic effect of the gold and copper atoms and in contrast to the single components (pure AuNCs and copper ions). The kinetic parameters of the new peroxidase mimic show a higher Kcat value (12.1 × 10-4 s-1) and a lower Km value (53 µM) for H2O2 (compared to those of conventional AuNCs). The catalytic activity is stable and remains essentially unchanged after two months. The interactions of AuNCs with Cu(II) were characterized by fluorescence spectroscopy, UV-vis spectroscopy and X-ray photoelectron spectroscopy. Based on these findings, a glucose colorimetric assay at 452 nm was developed that has a detection range from 1.6 to 800 µM and a 0.26 µM detection limit. Graphical abstract Copper ion-modified keratin-capped gold nanoclusters (AuNC-Cu2+) exhibit enhanced peroxidase-like activity owing to the synergistic effect of the gold and copper atoms which is in contrast to pure AuNCs.