Regulating the N Coordination Environment of Co Single-Atom Nanozymes for Highly Efficient Oxidase Mimics.

Single-atom catalysts with well-defined atomic structures and precisely regulated coordination environments have been recognized as potential substitutes for natural metalloenzymes. Inspired by the metal coordination structure of natural enzymes, we show here that the oxidase-like activity of single-atom Co catalysts greatly depends on their local N coordination around the Co catalytic sites. We synthesized a series of Co single-atom catalysts with different nitrogen coordination numbers (Co-Nx(C), x = 2, 3, and 4) and demonstrated that the oxidase-like activity of single-atom Co catalysts could be effectively tailored by fine-tuning the N coordination. Among the studied single-atom Co catalysts, the Co-N3(C) with three-coordinate N atoms shows the optimum oxygen adsorption structure and robust reactive oxygen species (ROS) generation, thus presenting the preferable oxidase-like catalytic activity. This work facilitates the future development of rational nanozyme designs for targeting reactions at the atomic level.

BSA-stabilized silver nanoclusters for efficient photoresponsive colorimetric detection of chromium(VI).

Hexavalent chromium is a highly toxic substance, which will pose a serious threat to human life and health and the entire ecosystem. Therefore, it is crucial to establish a simple and rapid detection method for hexavalent chromium. In this work, we fabricated bovine serum albumin-stabilized silver nanocluster (BSA-Ag13 NC) which exhibited photoresponsive oxidase-like activity, catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to the blue oxidized state TMB (oxTMB) in a short time. Interestingly, 8-hydroxyquinoline (8-HQ) can significantly inhibit the color reaction of TMB oxidation while Cr(VI) can interact specifically with 8-HQ to restore this chromogenic reaction. Based on the above facts, a colorimetric sensing system for detecting Cr(VI) was developed. The sensing system shows a wide linear range, and good selectivity, with a low detection limit of 2.32 nM. Moreover, this sensing system could be successfully applied to the detection of Cr(VI) in lake water, tap water, and sewage with satisfactory results.

Rational design and fabrication of MoSx nanoclusters decorated Mn0.3Cd0.7S nanorods with promoted interfacial charge transfer toward robust photocatalytic H2 generation.

In this work, we report a novel MoSx/Mn0.3Cd0.7S composite catalyst that has been designed and fabricated by in situ coupling MoSx nanoclusters with 1D Mn0.3Cd0.7S nanorods for photocatalytic H2 production. The catalyst features a 1D nanostructure with MoSx nanoclusters uniformly dispersed along the Mn0.3Cd0.7S nanorod. It was found that an intimate interface is built between MoSx nanoclusters and Mn0.3Cd0.7S nanorods thanks to the facile in situ photoreduction route, which contributes to a high-efficiency interfacial charge separation. The resulting MoSx/Mn0.3Cd0.7S photocatalyst shows a dramatically enhanced visible-light-driven photocatalytic H2 production activity compared with the control samples, owing to more efficient spatial charge separation as well as enriched active sites. This work is expected to provide an optimized structure model for rational design and constructing novel, inexpensive, efficient and stable cocatalyst/metal sulfide photocatalyst systems for H2 production.

Emerging interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic elements for electrocatalytic applications.

Palladium (Pd)-based nanomaterials have been identified as potential candidates for various types of electrocatalytic reaction, but most of them typically exhibit unsatisfactory performances. Recently, extensive theoretical and experimental studies have demonstrated that the interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic atoms (H, B, C, N, P, S) has a significant impact on their electronic structure and thus leads to the rapid development of one kind of promising catalyst for various electrochemical reactions. Considering the remarkable progress in this area, we highlight the most recent progress regarding the innovative synthesis and advanced characterization methods of nonmetallic atom-doped Pd-based nanomaterials and provide insights into their electrochemical applications. What's more, the unique structure- and component-dependent electrochemical performance and the underlying mechanisms are also discussed. Furthermore, a brief conclusion about the recent progress achieved in this field as well as future perspectives and challenges are provided.

Manganese-doped iron coordination polymer nanoparticles with enhanced peroxidase-like activity for colorimetric detection of antioxidants.

A convenient and sensitive antioxidant assay with high performance is essential for assessing food quality and monitoring the oxidative stress level of biological matrices. Although coordination polymer nanoparticles (CPNs)-based nanozymes have emerged as candidates in the analytical field, strategies to improve the catalytic activity of CPNs have been scarcely revealed and studied. Herein, we demonstrate a manganese (Mn) doping strategy to enhance the peroxidase-mimetic activity of Fe-based CPNs. By tuning the Mn doping amounts and selecting 2,5-dihydroxyterephthalic acid (H4DHTP) as ligands, the produced nanozymes in amorphous state followed the catalytic activity order of Fe5Mn-DHTP > Fe8Mn-DHTP > Fe2Mn-DHTP > Fe-DHTP > Mn-DHTP. Ulteriorly, benefitting from the best catalytic performance and definite catalytic mechanism of Fe5Mn-DHTP, versatile colorimetric assays for ultrasensitive detection of one exogenous antioxidant (ascorbic acid, AA) and two endogenous antioxidants (glutathione, GSH; cysteine, Cys) have been deftly devised based on the inhibition of the 3,3',5,5'-tetramethylbenzidine chromogenic reaction in presence of H2O2. It was found that mercaptan (GSH and Cys) and AA exhibited different inhibition mechanisms. Practically, such a colorimetric assay was viable to determine the total antioxidant capacity of drugs and foods with desirable results. This work proposes a feasible strategy for embellishing CPN nanozymes used for designing sensitive and convenient assays for various antioxidants based on an explicit detection mechanism.

Colorimetric detection of acetylcholinesterase and its inhibitor based on thiol-regulated oxidase-like activity of 2D palladium square nanoplates on reduced graphene oxide.

A convenient and sensitive colorimetric assay for acetylcholinesterase (AChE) and its inhibitor has been designed based on the oxidase-like activity of {100}-faceted Pd square nanoplates which are grown in situ on reduced graphene oxide (PdSP@rGO). PdSP@rGO can effectively catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) without the assistance of H2O2 to generate blue oxidized TMB (oxTMB) with a characteristic absorption peak at 652 nm. In the presence of AChE, acetylthiocholine (ATCh), a typical AChE substrate, is hydrolyzed to thiocholine (TCh). The generated TCh can effectively inhibit the PdSP@rGO-triggered chromogenic reaction of TMB via cheating with Pd, resulting in color fading and decrease in absorbance. Thus, a sensitive probe for AChE activity is constructed with a working range of 0.25-5 mU mL-1 and  a limit of detection (LOD) of 0.0625 mU mL-1. Furthermore, because of the inhibition effect of tacrine on AChE, tacrine is also detected through the colorimetric AChE assay system within the concentrations range 0.025-0.4 µM with a LOD of 0.00229 µM. Hence, a rapid and facile colorimetric procedure to sensitively detect AChE and its inhibitor can be anticipated through modulating the oxidase-like activity of PdSP@rGO. Colorimetric method for detection of AChE and its inhibitor is established by modulating the oxidase mimetic activity of {100}-faceted Pd square nanoplates on reduced graphene oxide (PdSP@rGO).

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

Enzyme-mimetic properties of nanomaterials can be efficiently tuned by controlling their size, composition, and structure. Here, ultrathin PdCu alloy nanosheet-assembled three-dimensional (3D) nanoflowers (Pd1Cux NAFs) with tunable surface composition are obtained via a generalized strategy. In presence of H2O2, the as-synthesized Pd1Cux NAFs can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the oxidized form of TMB (oxTMB) with a characteristic absorption peak at 652 nm. Interestingly, Pd1Cux NAFs show obviously composition-dependent peroxidase-like catalytic activities because of the synergistic interaction of nanoalloy. Additionally, different from 2D Pd nanosheets, the distinctive 3D superstructures are featured with rich approachable sites and proper layer spacing, which are in favor of fast mass transport and electron transfers during the catalytic process. Among the studied Pd1Cux NAFs, the Pd1Cu1.7 NAFs show the highest enzyme-like activities and can be successfully applied for the colorimetric detection of glucose with a low detection limit of 2.93 ± 0.53 µM. This work provides an efficient avenue to fabricate PdCu NAF nanozymes in biosensing toward glucose detection. Two-dimensional (2D) PdCu ultrathin nanosheet-assembled 3D nanoflowers (Pd1Cux NAFs) with tunable surface composition exhibit substantially enhanced intrinsic peroxidase-like catalytic activities. The Pd1Cu1.7 NAFs are successfully used as peroxidase mimic catalyst for the colorimetric detection of glucose with low detection limit of 2.93 µM.

Light-responsive Au nanoclusters with oxidase-like activity for fluorescent detection of total antioxidant capacity.

A fluorescent assay for total antioxidant capacity (TAC) detection based on the light-responsive oxidase-like activity of bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs) has been developed. Thiamine (TH) as the peroxidase substrate usually works at alkaline conditions and thus limits its practical applications. Here, by utilization the light-responsive oxidase-like activity of BSA-AuNCs, TH is oxidized to fluorescent thiochrome under neutral condition in two minutes due to the single oxygen generated by BSA-AuNCs upon light irradiation. After the introduction of antioxidants into the BSA-AuNCs-TH system, the formation of thiochrome is inhibited resulting in the fluorescence decrease. On the basis of the above facts, BSA-AuNCs-TH-based assay has been fabricated and applied successfully to detect antioxidants and the TAC of vitamin C tablets as well as some commercial fruit juice with satisfied results. This work may provide novel insights into developing light-responsive nanozymes-based fluorescent assays.

Alkaline Phosphatase-Triggered in Situ Formation of Silicon-Containing Nanoparticles for a Fluorometric and Colorimetric Dual-Channel Immunoassay.

Enzyme-triggered in situ chromogenic and/or fluorogenic reactions under accessible conditions are significant for developing enzyme activity and related spectroscopic assays. Here, we describe a facile one-pot synthetic strategy to prepare silicon-containing nanoparticles with yellow-green fluorescence and orange-red color by mixing N-[3-(trimethoxysilyl)propyl]ethylenediamine and p-aminophenol (AP) in aqueous solution at a mild temperature. Encouraged by the AP-regulated simple synthetic procedure and the generation of AP from alkaline phosphatase (ALP)-catalyzed hydrolysis of 4-aminophenol phosphate (APP), a fluorometric and colorimetric dual-readout ALP activity assay can be rationally envisioned and developed by employing APP as the substrate. In the wake of the good analytical performance of such ALP activity assay and its successful combination with enzyme-linked immunosorbent assay (ELISA), corresponding fluorometric and colorimetric dual-readout ALP-based ELISA has been constructed for highly sensitive and quantitative determination of human prostate-specific antigen (PSA), the key biomarker of prostate cancer in human serum. The convincing performance in evaluating the PSA level in serologic tests unambiguously reveals the great potential of our proposed fluorometric and colorimetric dual-channel immunoassay in early clinical diagnosis by monitoring disease biomarkers.

Colorimetric determination of the activity of alkaline phosphatase by exploiting the oxidase-like activity of palladium cube@CeO2 core-shell nanoparticles.

Core-shell palladium cube@CeO2 (Pd cube@CeO2) nanoparticles are shown to display oxidase-like activity. This is exploited in a method for determination of the activity of alkaline phosphatase (ALP). The Pd cube@CeO2 nanoparticles were thermally synthesized from Ce(NO3)3, L-arginine and preformed Pd cube seeds in water. The Pd cube@CeO2 nanoparticles catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by oxygen. This results in the formation of oxidized TMB (oxTMB) with an absorption peak at 652 nm. Ascorbic acid (AA) is generated from the hydrolysis of L-ascorbic acid 2-phosphate (AAP) catalyzed by ALP. It can reduce oxTMB to TMB, and this results in a decrease of the absorbance. The method allows for quantitative determination of the activity of ALP in the range from 0.1 to 4.0 U·L-1 and with a detection limit down to 0.07 U·L-1. Endowed with high sensitivity and selectivity, the assay can quantify ALP activity in biological system with satisfactory results. Graphical abstractSchematic illustration of Pd cube@CeO2 core-shell nanoparticles for colorimetric determination of alkaline phosphatase.

In Situ Formation of 2,3-Diaminophenazine for Evaluation of Alkaline Phosphatase Activity via the Inner Filter Effect.

We report a fluorescence assay for alkaline phosphatase (ALP) detection using in situ generation of 2,3-diaminophenazine (OPDox) through an inner filter effect (IFE). AgNO3 can oxidize o-phenylenediamine in a short time to obtain fluorescent OPDox. p-nitrophenol is obtained from ALP-catalyzed hydrolysis of p-nitrophenylphosphate, which can result in the fluorescence quenching of OPDox via the IFE. Consequently, in situ generation of OPDox can be utilized for evaluating the activity of ALP. The fluorescence of OPDox decreases linearly with increasing concentration of ALP in the range from 0.1 to 8.0 mU mL-1, and the detection limit is calculated to be 0.05 mU mL-1. More importantly, this assay has been used to construct a logic gate. The IFE-based fluorescence assay exhibits several distinctive advantages, including high sensitivity, good selectivity, and simple operation. Consequently, it may pave the way for the detection of ALP by utilization of the in situ generation of fluorophores.

Logically Regulating Peroxidase-Like Activity of Gold Nanoclusters for Sensing Phosphate-Containing Metabolites and Alkaline Phosphatase Activity.

Phosphate-containing metabolites and alkaline phosphatase (ALP) activity are useful biomarkers for many types of diseases. However, there are few straightforward, sensitive, and efficient colorimetric methods for the quantification of them only when resorting to unstable transition metal ions or specially designed organic substrates. Herein, we have demonstrated that histidine-protected gold nanoclusters (His-AuNCs) possess intrinsic peroxidase-like activity with Au atom facilitated formation of superoxide anions (O2•-) and their electron transfer ability. More interestingly, phosphate-containing metabolites can severely inhibit the peroxidase-like activity of His-AuNCs by blocking the generation of O2•- and electron transfer, and then ALP is able to restore the inhibition process through hydrolyzing the phosphate-containing metabolites. Therefore, using peroxidase-triggered chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) as an amplifier, a colorimetric on-off-on switch has been developed for sensing phosphate-containing metabolites and ALP based on the logical regulation of such deactivation and reactivation processes for the first time. According to the intrinsic mimic enzyme-catalyzed amplification and clear response mechanism, our colorimetric assay exhibits excellent sensitivity, selectivity, and sensing performance. Furthermore, on the basis of the proposed colorimetric sensors, a combinatorial "NOR+IMPLICATION" logic gate is further rationally constructed.

Engineering Two-Dimensional Pd Nanoplates with Exposed Highly Active {100} Facets Toward Colorimetric Acid Phosphatase Detection.

Enzyme-like activity and efficiency of nanomaterials are strongly controlled by their size, composition, and structure, and hence the structural parameters need to be optimized. Here, we report that two-dimensional Pd nanoplates enclosed by {100}-facets [{100}PdSP@rGO] exhibit substantially enhanced intrinsic oxidase-like activities relative to the {111}-facets ones and Pd nanocubes in catalyzing the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine. By taking ascorbic acid 2-phosphate as the substrate, which transforms to ascorbic acid in the presence of acid phosphatase (ACP), the {100}PdSP@rGO could be used as an efficient nanozyme for colorimetric ACP detection without resorting to destructive H2O2. A good linear relationship from 0.01 to 6.0 mU/mL with a detection limit of 8.3 µU/mL is obtained, which is better than most previously reported ACP assays. Importantly, the excellent assay performance could be successfully applied to ACP determination in serum samples with high accuracy. This study demonstrates that the enzyme-like activity of nanomaterials could be finely tuned simultaneously by controlling their exposed crystal facets and high specific surface area.

Spectrophotometric determination of the activity of alkaline phosphatase and detection of its inhibitors by exploiting the pyrophosphate-accelerated oxidase-like activity of nanoceria.

The oxidase-like activity of nanoceria is low. This limits its practical applications. It is demonstrated here that pyrophosphate ion (PPi) can improve the oxidase-like activity of nanoceria. Specifically, nanoceria catalyzes the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to give a blue product (oxTMB) with an absorption peak at 645 nm in the presence of PPi. If, however, alkaline phosphatase (ALP) is present, it will hydrolyze PPi, and this results in a decreased oxidase-like activity of nanoceria. Hence, less blue oxTMB willl be formed. On the other hand, if the ALP inhibitor Na3VO4 is added to the system, the oxidase-like activity of nanoceria is gradually restored. On the basis of the above results, a spectrophotometric method was developed for determination of the activity of ALP. It works in the 0.5 to 10 mU.mL-1 activity range and has a 0.32 mU.mL-1 detection limit. Na3VO4 causes a 50% ALP inhibition if present in 71 µM concentration. The assay was successfully applied to the determination of ALP in spiked human serum and gave good recoveries. Graphical abstract Schematic presentation of pyrophosphate (PPi)-induced acceleration of the oxidase-like activity of nanoceria (CeO2) for determination of alkaline phosphatase enzyme (ALP) activity and its inhibitor NaVO3.

Fluorometric and colorimetric dual-readout alkaline phosphatase activity assay based on enzymatically induced formation of colored Au@Ag nanoparticles and an inner filter effect.

An ultrasensitive fluorometric and colorimetric dual-mode assay is described for the determination of the activity of alkaline phosphatase (ALP). ALP catalyzes the decomposition of 2-phospho-L-ascorbic acid, and the ascorbic acid thus generated reduces silver ions. In the presence of gold nanoparticles, gold-silver nanoparticles (Au@Ag NPs) are formed. This is accompanied by a color change form pink to deep yellow. The Au@Ag NPs reduce the fluorescence of blue fluorescent graphene quantum dots due to spectral overlap. The changes of absorbance (measured at 410 and 520 nm) and fluorescence (measured at excitation/emission wavelengths of 346/415 nm) correlate well with the ALP activity in the 0.01-6 mU·mL-1 (absorption) and 0.01-2 mU·mL-1 (fluorescence) ranges, and the detection limits are 9 and 5 µU·mL-1 individually. Graphical abstract Schematic presentation of colorimetric and fluorometric dual-readout assay for alkaline phosphatase (ALP) activity. It is based on enzymatically induced formation of gold-silver nanoparticles (Au@Ag NPs), and the fluorescence quenching of graphene quantum dots due to inner filter effect.

Fluorometric determination of the activity of alkaline phosphatase and its inhibitors based on ascorbic acid-induced aggregation of carbon dots.

The authors describe a fluorometric method for determination of the activity of alkaline phosphatase (ALP) and its inhibitors. Nitrogen and boron co-doped carbon dots (C-dots) with excitation/emission peaks at 490/540 nm act as the fluorescent probe. The C-dots were prepared by hydrothermal carbonization starting from 3-aminophenylboronic acid as the sole precursor. On the basis of the boronic acid-triggered specific reaction with cis-diols, the boronic acid modified C-dots can bind to ascorbic acid that is generated by ALP-catalyzed hydrolysis of ascorbic acid 2-phosphate. This results in particle aggregation and quenching of fluorescence. If the ALP inhibitor Na3VO4 is introduced into the system, the activity of ALP is reduced and the fluorescence of C-dots recovers. This fluorometric method allows for the determination of ALP activity in the range from 0.2 to 6.0 mU mL-1 with a detection limit of 0.16 mU mL-1. The IC50 value for the inhibitor Na3VO4 is 3.6 µM. The method is convenient and cost-effective. It does not require complicated operations and in our perception widens the scope of applications of C-dots in bioanalytical sciences. Graphical abstract Schematic presentation of the nitrogen and boron co-doped carbon dot-based fluorometric method for determination of alkaline phosphatase (ALP) activity.

Highly sensitive fluorescent detection of glutathione and histidine based on the Cu(ii)-thiamine system.

In this work, we propose a fluorescence method for the simultaneous detection of glutathione (GSH) and histidine (His) based on the Cu(ii)-thiamine (Cu(ii)-TH) system. It is well established that non-fluorescent thiamine (TH) can be oxidized by Cu(ii) to generate fluorescent thiochrome (TC) under alkaline conditions. The introduction of GSH and His can inhibit the oxidation of TH by Cu(ii) due to the strong affinity between Cu(ii) and GSH or His. With this strategy, the detection limit for GSH and His is 10.5 nM and 26.4 nM, respectively. The developed method shows several obvious advantages: (1) simplicity in design and operation without the utilization of fluorescent nanomaterials or probes; (2) time-saving detection process since all the detection process is completed within 15 min; (3) cost-effectiveness by using TH as the fluorescent substrate; and (4) higher sensitivity and good selectivity. Therefore, it shows great potential in biosensing fields.

Fluorescence assay for alkaline phosphatase based on ATP hydrolysis-triggered dissociation of cerium coordination polymer nanoparticles.

Alkaline phosphatase (ALP) is a significant biomarker for diagnostics. Simple, selective and sensitive detection of ALP activity is thus of critical importance. In this study, an artful fluorescence assay for ALP is proposed based on adenosine triphosphate (ATP) hydrolysis-triggered disassociation and fluorescence quenching of cerium coordination polymer nanoparticles (CPNs). ATP, a recognized natural substrate of phosphatase, can serve as a superb "antenna" to sensitize the luminescence of Ce3+ with the aid of tris(hydroxymethyl) aminomethane (Tris), forming Ce3+-ATP-Tris CPNs. In the presence of ALP, ATP will be catalytically converted into adenosine and inorganic orthophosphate, however neither of them can sensitize Ce3+ in alkaline media. As a result, the obtained CPNs are disassociated, inducing the quenching of the fluorescence. On this basis, a straightforward fluorescence assay for ALP activity is rationally developed. The fluorescence quenching efficiency shows a linear relationship for ALP within the activity range from 0.1 to 10 mU mL-1 with a detection limit of 0.09 mU mL-1 under the optimal experimental conditions. Moreover, this facile yet effective fluorescence method featured simplicity, cost-effectiveness, high sensitivity and high selectivity and can be successfully utilized for the quantitative detection of ALP in human serum samples.

Fluorometric determination of sulfide ions via its inhibitory effect on the oxidation of thiamine by Cu(II) ions.

A fluorometric assay is described for sulfide ions determination. It is based on the finding that the oxidation of the non-fluorescent substrate thiamine (TH) by Cu(II) in basic solution to form fluorescent thiochrome is inhibited by sulfide ions. This results in a decrease in fluorescence intensity which is proportional to the concentration of sulfide ions. Under the optimized conditions, the decrease in fluorescence, best measured at excitation/emission wavelengths of 370/440 nm, decreases linearly in the 0.03 to 2.5 µM sulfide ions concentration range. The detection limit is 20 nM. The method shows excellent selectivity over many potentially interfering ions and has been successfully applied to the determination of sulfide ions in spiked tap water, lake water and the synthetic wastewater samples. The method is time-saving and environmentally friendly, and in our perception shows a great potential in water quality inspection and environmental monitoring. Graphical abstract A fluorescent assay for sulfide ions detection is proposed based on its inhibitory effect on the oxidation of thiamine by Cu(II) ions.

Carbon dots based fluorescent sensor for sensitive determination of hydroquinone.

In this paper, a novel biosensor based on Carbon dots (C-dots) for sensitive detection of hydroquinone (H2Q) is reported. It is interesting to find that the fluorescence of the C-dots could be quenched by H2Q directly. The possible quenching mechanism is proposed, which shows that the quenching effect may be caused by the electron transfer from C-dots to oxidized H2Q-quinone. Based on the above principle, a novel C-dots based fluorescent probe has been successfully applied to detect H2Q. Under the optimal condition, detection limit down to 0.1 µM is obtained, which is far below U.S. Environmental Protection Agency estimated wastewater discharge limit of 0.5 mg/L. Moreover, the proposed method shows high selectivity for H2Q over a number of potential interfering species. Finally, several water samples spiked with H2Q are analyzed utilizing the sensing method with satisfactory recovery. The proposed method is simple with high sensitivity and excellent selectivity, which provides a new approach for the detection of various analytes that can be transformed into quinone.