Sodium Alginate Modified Platinum Nanozymes With Highly Efficient and Robust Oxidase-Like Activity for Antioxidant Capacity and Analysis of Proanthocyanidins.

Platinum nanozymes exhibiting highly efficient and robust oxidase-like activity are successfully synthesized and modified using sodium alginate (SA-PtNPs). According to a steady-state dynamic assay, Michaelis-Menton constant (K m ) is calculated as 11.6 µM, indicating that the affinity of SA-PtNPs toward the substrate, 3, 3', 5, 5'-tetramethylbenzidine (TMB), is high. It shows in the paper that SA-PtNPs exhibit a significant oxidant effect on substrate-O2 to produce O 2 • - as an oxidase mimic. Moreover, the oxidase-like activity fluctuated slightly under changes in environmental pH and incubation time, implying that SA can increase the dispersibility and stability of PtNPs. A colorimetric assay for oligomeric proanthocyanidins (OPC) was realized given how few explorations of the former there are. We found that the significant inhibitory effect of OPC on the oxidase-like activity is due to the competitive effect between OPC and TMB for binding to the active site of SA-PtNPs, resulting in a color change. Under optimal conditions, the logarithmic value of the chromogenic difference (ΔA450nm) to OPC concentration was linear (4-32.5 µM, r = 0.999) with a limit of detection (LOD) of 2.0 µM. The antioxidant capacity of OPC obtained by the Soxhlet extraction method from grape seeds was 2.85 U/mg. The recovery from the experiment in which OPC was added to grape seeds ranged from 97.0 to 98.6% (RSDs of 0.5-3.4%), suggesting a high accuracy in OPC detection. These findings are important because OPC is an internationally recognized antioxidant that eliminates free radicals in the human body and, therefore, may prevent a variety of diseases. Thus, we envisage that this Pt nanozyme-based assay may be prevalent for antioxidant capacity evaluation and analytical applications.

Heparin-platinum nanozymes with enhanced oxidase-like activity for the colorimetric sensing of isoniazid.

In this study, a colorimetric sensing assay of isoniazid based on excellent oxidase-like activity of heparin sodium stabilized platinum nanoparticles (HS-PtNPs) has been demonstrated. The newly prepared HS-PtNPs exhibit a great dispersion with an average size distribution of 4.8 ± 0.6 nm, and maintain more than 90% catalytic activity under strong acid and alkali or long-term storage conditions, indicating a robust nanomaterial with attractive potential. The HS-PtNPs show distinct oxidase-like activity with an ultrahigh affinity (Km = 0.01012 mM) for 3, 3', 5, 5'-tetramethylbenzidine (TMB). More significantly, we found that the pyridine ring of isoniazid has a strong reductive hydrazyl substitution, which can compete with TMB for the catalytic site of HS-PtNPs resulting in a colorless solution. Accordingly, a colorimetric sensing of isoniazid was fabricated. A linear relationship for isoniazid was achieved in 2.5 × 10-6 to 2.5 × 10-4 M (R2 = 0.998) with a low limit of detection 1.7 × 10-6 M (S/N = 3). Recovery experiments in drug tablets show that the standard recovery rates were 95%-103%. The quantitative detection data for isoniazid in drug tablets calculated respectively from the standard method and this method exhibited a high correlation coefficient (a slope of 0.9995), suggesting that high accuracy in isoniazid detection.

Colorimetric tyrosinase assay based on catechol inhibition of the oxidase-mimicking activity of chitosan-stabilized platinum nanoparticles.

It is found that catechol inhibits the oxidase-mimicking activity of chitosan-protected platinum nanoparticles (Chit-PtNPs) by competing with the substrate for the active site of the Ch-PtNPs. The inhibition mechanism of catechol is different from that of ascorbic acid in that it neither reacts with O2•- nor reduces the oxidized 3,3',5,5'-tetramethylbenzidine (TMB). Tyrosinase (TYRase) catalyzes the oxidation of catechol, thus restoring the activity of oxidase-mimicking Chit-PtNPs. By combining the Chit-PtNP, catechol, and TYRase interactions with the oxidation of TMB to form a yellow diamine (maximal absorbance at 450 nm), a colorimetric analytical method was developed for TYRase determination and inhibitor screening. The assay works in the 0.5 to 2.5 U·mL-1 TYRase activity range, and the limit of detection is 0.5 U·mL-1. In our perception, this new assay represents a powerful approach for determination of TYRase activity in biological samples. Graphical abstract Schematic representation of a colorimetric method for tyrosinase (TYRase) detection and inhibitor screening. It is based on the fact that catechol can inhibit the oxidase-like activity of chitosan-stabilized platinum nanoparticles (Ch-PtNPs) by competing with the substrate for the active sites and TYRase can catalyze the oxidation of catechol.

Engineering a New Chloroplastic Photorespiratory Bypass to Increase Photosynthetic Efficiency and Productivity in Rice.

Over the past few years, three photorespiratory bypasses have been introduced into plants, two of which led to observable increases in photosynthesis and biomass yield. However, most of the experiments were carried out using Arabidopsis under controlled environmental conditions, and the increases were only observed under low-light and short-day conditions. In this study, we designed a new photorespiratory bypass (called GOC bypass), characterized by no reducing equivalents being produced during a complete oxidation of glycolate into CO2 catalyzed by three rice-self-originating enzymes, i.e., glycolate oxidase, oxalate oxidase, and catalase. We successfully established this bypass in rice chloroplasts using a multi-gene assembly and transformation system. Transgenic rice plants carrying GOC bypass (GOC plants) showed significant increases in photosynthesis efficiency, biomass yield, and nitrogen content, as well as several other CO2-enriched phenotypes under both greenhouse and field conditions. Grain yield of GOC plants varied depending on seeding season and was increased significantly in the spring. We further demonstrated that GOC plants had significant advantages under high-light conditions and that the improvements in GOC plants resulted primarily from a photosynthetic CO2-concentrating effect rather than from improved energy balance. Taken together, our results reveal that engineering a newly designed chloroplastic photorespiratory bypass could increase photosynthetic efficiency and yield of rice plants grown in field conditions, particularly under high light.

Alkaline peroxidase activity of cupric oxide nanoparticles and its modulation by ammonia.

We herein report the intrinsic alkaline peroxidase-like activity exhibited by CuO nanoparticles when 3-(4-hydroxyphenyl)propionic acid was employed as a substrate. Based on this observation, a fluorometric assay method with a low detection limit of 0.81 µM was established for H2O2 determination under alkaline conditions. Notably, ammonia was found to inhibit the alkaline peroxidase-like activity of the CuO nanoparticles. Thus, a sensing platform for the determination of urea and urease was successfully constructed, with the limits of detection for urea and urease being 27 µM and 2.6 U L-1, respectively. This platform was then applied for the detection of urea in human urine and urease in soil, which yielded satisfactory results. These results suggest that it is possible to extend the catalytic potential of peroxidase and its mimetics from acidic and neutral conditions to include activity in alkaline media as well. Furthermore, this strategy is a novel method for the analysis of urea and urease. The assay developed in this work is facile, inexpensive, convenient, and highly selective and sensitive. Therefore, it is expected that this system can serve as a template for the development of similar enzyme nano-mimics.

Chitosan-stabilized platinum nanoparticles as effective oxidase mimics for colorimetric detection of acid phosphatase.

Capping molecules on the surface of nanomaterials not only enhance the dispersion and stability of nanomaterials but also greatly facilitate their surface modification and biological applications. However, most capping molecules can severely block the active sites of the catalytic core, thereby decreasing the enzymatic activity of nanomaterial-based enzyme mimics. This work demonstrates the superiority of chitosan (Ch) as a capping molecule for synthesizing catalytic platinum nanoparticles (PtNPs). The experimental results show that Ch simultaneously exhibits an excellent stabilizing effect and enhances the oxidase-like activity of PtNPs. Kinetic studies indicate that Ch-PtNPs have a higher affinity for 3,3',5,5'-tetramethylbenzidine (TMB) than other kinds of oxidase mimics. Furthermore, the TMB chromogenic reaction catalyzed by Ch-PtNPs is found to be much faster in an acidic medium, thus adapting well to the optimal pH for acid phosphatase (ACP). Therefore, a novel colorimetric approach for ACP determination is developed for the first time, which is based on the Ch-PtNP-catalyzed oxidation of TMB, the inhibitory effect of ascorbic acid (AA) on the oxidase-like activity of Ch-PtNPs, and the ACP-catalyzed hydrolysis of AA 2-phosphate (AAP) into AA. The linear range for ACP is 0.25-2.5 U L-1 and the limit of detection is measured to be 0.016 U L-1. This new colorimetric method is utilized to detect ACP in real biological samples and to screen ACP inhibitors. We believe that these new PtNPs, which exhibit high colloidal stability, excellent catalytic performance, good biocompatibility, simple preparation, and easy modification, can be promising candidates for a broad range of applications in optical sensing, environmental monitoring, clinical diagnosis, and drug discovery.

Self-cascade reaction catalyzed by CuO nanoparticle-based dual-functional enzyme mimics.

It is desirable but challenging to assemble various biomimetic properties into a functional catalytic cascade system. In this work, cupric oxide nanoparticles were investigated as oxidase mimics for the aerobic oxidation of cysteine to cystine with the generation of hydrogen peroxide. Coupling this property with the peroxidase-like activity of CuO nanoparticles, we constructed a self-organized cascade reaction system based on a single-component nanozyme, which includes the oxidation of cysteine to yield cystine and hydrogen peroxide and the hydrogen peroxide-mediated oxidation of terephthalic acid to produce a fluorescence change. Based on this artificial enzymatic cascade reaction system, a fluorometric assay method with a low detection limit of 6.6nM was established for cysteine determination. This platform was then applied for the detection of cysteine in pharmaceutical products and human plasma, which yielded satisfactory results. Our investigations open up a new route and holds promise for the development and applications of multifunctional nanomaterials as enzyme mimics.