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By adjusting the coordination environment of single-atom catalysts, the enzyme-like activity can be finely tuned for highly sensitive biosensing. Herein, we demonstrated that coordinatively unsaturated cobalt-nitrogen sites doped within porous carbon (SA-CoN3) could serve as highly efficient oxidase mimic. Compared with the typical planar four-coordination structure (SA-CoN4), the as-obtained single-atom Co nanozymes anchored by three nitrogen atoms are found to display much higher oxidase-like catalytic efficiency. Combined theoretical and experimental analysis revealed that the coordinatively unsaturated Co sites could facilitate adsorption and activation of O2 molecule and thus improve their oxidase-like activity. Based on the enhanced oxidase-like activity of SA-CoN3, a paper/smartphone sensor for organophosphorus pesticides (OPs) was successfully constructed and used to quantify glyphosate in environmental and food samples with a low detection limit of 0.66 µM. This work not only highlights the important role of coordination unsaturation of SA nanozymes for promoting oxidase-like activity, but also provides an easy and cost-effective way to conduct effective quantification of OPs in the field. [ABSTRACT FROM AUTHOR]

Ultrathin porous and highly curved two-dimensional PdCu alloy metallene are shown to be highly efficient oxidase mimics. Serving as intrinsic oxidase mimic, the ultrathin porous structure of the PdCu metallenezymes could effectively utilize all the Pd atoms of the metallenezymes during catalytic reactions. By using the oxidation capability of 3,3′5,5′–tetramethylbenzidine as distinctive chromogenic substrate, the PdCu metallenezymes was used as oxidase-like mimics for determination of total antioxidant capacity (TAC) of vitamin C containing real products including fresh orange juice, commercial beverages, Vitamin C tablets and dermo-cosmetic products. AAP was hydrolyzed using ALP to generate AA and the corresponding ALP activity was successfully detected in the 0–100 U/L range with a lowest detection limit of 0.9 U/L. This study demonstrates the significant catalytic performance and oxidase-like activity of PdCu metallene nanozyme providing a strategy to develop a TAC assay for the assessment of antioxidant food quality as well as oxidative stress in skin and health care products. [ABSTRACT FROM AUTHOR]

Palladium (Pd)-based nanomaterials are the emerging class of catalysts with individual physiochemical properties. Unlike traditional catalysts, metallenes showed abundant active sites, large surface area, and high atomic utilization. Based on these benefits, we demonstrate a highly active 2D graphene-like Pd metallene with abundant accessible active sites serving as a highly efficient oxidase mimic. The structure and morphology of Pd metallenezymes were controlled to enhance the catalytic performance and to efficiently utilize all the Pd atoms. Pd metallenezymes with excellent oxidase-like activity were successfully applied for colorimetric-based chromium (VI) (Cr(VI)) detection in a real environment. 3,3',5,5'-Tetramethylbenzidine (TMB) was used as a typical chromogenic substrate catalyzed by Pd metallene to show that blue oxidized TMB (ox TMB) was significantly reduced to colorless TMB by the reducing agent 8-hydroxyquinoline (8-HQ). The reaction process was impressively reversed by the addition of Cr(VI), which interacted with 8-HQ to restore the blue color of TMB. Based on the above results, a facile and effective colorimetric sensing system for the detection of Cr(VI) with a low detection limit of 2.8 nM was developed and could be successfully applied to the detection of Cr(VI) in a real environment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

High levels of uric acid (UA) in humans can cause a range of diseases, and traditional assays that rely on uric acid enzymes to break down uric acid are limited by the inherent deficiencies of natural enzymes. Fortunately, the rapid development of nanozymes in recent years is expected to solve the above-mentioned problems. Hence, we used a host–guest strategy to synthesize a platinum nanoparticle confined in a metal–organic framework (Pt NPs@ZIF) that can sensitively detect UA levels in human serum. Unlike previously reported free radical-catalyzed oxidation systems, its unique electron transfer mechanism confers excellent peroxidase-like activity to Pt NPs@ZIF. In addition, UA can selectively inhibit the chromogenic reaction of TMB, thus reducing the absorbance of the system. Therefore, using the peroxidase-like activity of Pt NPs@ZIF and using TMB as a chromogenic substrate, UA can be detected directly without relying on natural enzymes. The results showed a relatively wide detection range (10–1000 μM) and a low detection limit (0.2 μM). Satisfactory results were also obtained for UA in human serum. This study with simple operation and rapid detection offers a promising method for efficiently detecting UA in serum. [ABSTRACT FROM AUTHOR]

Pd-based nanomaterials have shown great promise as potential mimic enzymes, but conventional catalysts use only a small fraction of the Pd content that located on the catalyst's surface. Herein, we demonstrated that maximum atom utilization could be achieved by using single-atom Pd catalysts as oxidase mimic. The single-atom Pd nanozymes exhibit significantly enhanced catalytic efficiency, with a catalytic rate constant (Kcat) and the catalytic efficiency (Kcat/Km) values more than 625 and 4,837 times higher than those of horseradish peroxidase, respectively. A combined experimental and theoretical calculation reveals reactive oxygen species involved catalytic mechanism which endows single-atom Pd catalysts with excellent colorimetric analysis performance. Benefiting from the maximum atom utilization efficiency and well-defined structural features, the single-atom Pd nanozymes could be successfully applied for the total antioxidant capacity of fruit, determining the serum acid phosphatase activity as well as constructing NAND logic gate. This finding not only provides an effective strategy to maximize the noble-metal atom utilization efficiency as enzyme mimics, but also provides a new idea for extending their possible applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The structure of the “black soil” in Northeast China has been greatly deteriorated by long-term intensive conventional mouldboard plow tillage (CT) practices. In this study, micromorphological observation and image analysis of soil thin sections were conducted to evaluate the impacts of 21 years (1986–2007) of no tillage (NT) on soil structure as compared to CT in an experiment near Gongzhuling City, Jilin Province. Soil organic matter (SOM), wet aggregate stability and saturated hydraulic conductivity (Ks) were also analyzed. Total SOM was not significantly affected by tillage systems, but fresher SOM was observed in the surface layer under NT. The aggregates under NT showed different hierarchies in the form of crumbs, and the mean weight diameter (MWD) of NT was significant higher than that of CT in the surface layer. Platy and blocky aggregates were frequently observed in the lower layers under CT practice. The compound pore structure with intertwined intra- and inter- aggregates pores under NT was well developed in a layer from 0–5 cm to 20–25 cm. While under CT system, more inter-aggregate pores and fewer intraaggregate pores were observed, and planes and channels were frequently found in the 20–25 cm layer, where macroporosity decreased significantly and a plow pan was evident. The Ks values of NT were significantly lower at 0–5 cm but significantly higher at 20–25 cm compared with CT, which showed the same trend with macroporosity. These results confirmed that long-term CT practice fragmented the tillage layer soil and compacted the lower layer soil and formed a plow pan. While long-term NT practice in the black soil region favored soil aggregation and a stable porous soil structure was formed, which are important to the water infiltration and prevent soil erosion. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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.Schematic presentation of the nitrogen and boron co-doped carbon dot-based fluorometric method for determination of alkaline phosphatase (ALP) activity. [ABSTRACT FROM AUTHOR]

Soil structure represents a basis for soil water retention and fertiliser availability. Here, we performed a micromorphological analysis of thin soil sections to evaluate the effects of 10 years of organic planting (OPP), pollution-free planting (PFP), and conventional planting (CPP) on greenhouse soil structure in the North China Plain. We also analysed soil bulk density, soil organic matter (SOM), and wet aggregate stability. The CPP soil microstructure included weakly separated angular block or plate forms and weak development of soil pores (fissured or simply accumulated pores) with the highest bulk density (1.33 g cm−3) and lowest SOM (26.76 g kg−1). Unlike CPP, the OPP soil microstructure was characterised by highly separated granular and aggregated structures and an abundance of plant and animal remains. OPP was associated with the highest total porosity (55.4%), lowest bulk density (1.17 g cm−3), and highest SOM (54.81 g kg−1) in the soil surface layer. OPP also improved the ventilation pore content (proportion of pores >0.1 mm, 44.09%). OPP aggregates showed different hierarchies of crumb microstructure and higher mean weight diameter and geometric mean diameter values than did CPP. These results confirm the benefits of long-term OPP for soil structure and quality in the greenhouse. [ABSTRACT FROM AUTHOR]

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.A fluorescent assay for sulfide ions detection is proposed based on its inhibitory effect on the oxidation of thiamine by Cu(II) ions. [ABSTRACT FROM AUTHOR]