Biosynthesis of bifunctional silver nanoparticles for catalytic reduction of organic pollutants and optical monitoring of mercury (II) ions using their oxidase-mimic activity

Fluorescent/colorimetric probe for the detection of Cr(Ⅵ) based on MIL-101(Fe)-NH2 with peroxidase-like activity

In the present research, Fe-based metal-organic frameworks (MIL-101(Fe)-NH2) nanoparticles were synthesized by simple solvothermal methods and used to assay Cr(Ⅵ). The MIL-101(Fe)-NH2 performs dual functions: the 2-aminoterephthalic acid (NH2-BDC) ligand endows a strong fluorescence emission, and the Fe metal nodes are able to facilitate the oxidation of 3,3',5,5'- tetramethylbenzidine (TMB) directly, resulting in the generation of oxidized-TMB (ox-TMB). Our research results showed that reducing agents such as ascorbic acid (AA) can collapse the structures of MIL-101(Fe)-NH2 because of the reduction of Fe3+ by AA, resulting in release of NH2-BDC. In the presence of Cr(Ⅵ), the fluorescence intensity of the MIL-101(Fe)-NH2 + AA system will be decreased due to the competitive reduction of Fe3+ and Cr(Ⅵ). Nevertheless, Cr(Ⅵ) can significantly accelerate the oxidation of TMB by MIL-101(Fe)-NH2 as it boosts the electron transfer rate between Fe3+ and Fe2+. Therefore, a fluorescent/colorimetric dual-mode platform was developed for the detection of Cr(Ⅵ) with an extensive linear range (7.5-750 μg/L and 13.3-1000 μg/L) as well as a remarkably low detection limit (0.99 μg/L and 1.98 μg/L). This MOF with the ability to release ligands not only provides inspiration for the design of new luminescent materials, but also offers a novel and reliable solution for the detection of Cr(Ⅵ).

Colorimetric detection of chromium (VI) via its instigation of oxidase-mimic activity of CuO

Extensive use of chromium makes it one of the major pollutants of water resources. Chromium (VI) in particular is toxic and has detrimental health effects. Because of its high toxicity a tolerable concentration limit of chromium (VI) in drinking water has been recommended. Here we report a colorimetric method for determination of chromium (VI) in water based on the oxidase-like activity of solvothermal synthesized copper oxide. The particles have been characterized by X- ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). Using 3, 3', 5, 5'- tetramethylbenzidine (TMB) as a chromogenic substrate, CuO exhibited a pronounced oxidase-like activity in the presence of chromium (VI). This method enables successful colorimetric detection of chromium (VI). It demonstrated excellent selectivity for detection of chromium (VI) ions against potentially interfering ions. The method's feasibility to real sample analysis has been proven by testing tap water. Hence, we anticipate that the method can be successfully applied for analysis of chromium (VI) in environmental samples.

Peroxidase-like activity of biosynthesized silver nanoparticles for colorimetric detection of cysteine

Cysteine is one of the important amino acids that is involved in various physiological processes, food industries, pharmaceuticals, and personal care. It also serves as a biomarker for some diseases. The large use of cysteine necessitates rapid, cheap, and accurate determination of cysteine in a range of samples. Although many techniques have been employed for the detection of cysteine, they suffer from limitations that make them unsuitable for routine analysis. Here we report on a cheap colorimetric method using biosynthesized silver nanoparticles (AgNPs) as nanozymes. The AgNPs were characterized by UV/visible spectrophotometry, scanning electron microscopy (SEM), and surface-enhanced Raman spectroscopy (SERS). The AgNPs exhibit peroxidase-like activity using o-phenylenediamine (OPD) as a chromogenic reagent. The low K_m values observed for OPD and H₂O₂ (0.9133 and 61.56 mM respectively) show strong affinity of the substrates to AgNPs. The peroxidase-like activity of AgNPs, however, was inhibited on the addition of cysteine. The results show that the absorption intensity of the oxidized OPD decreased linearly with the concentration of cysteine in the range of 0.5-20 μM. The limit of detection (LOD) in this linear range was found to be as low as 90.4 nM. The recovery from urine sample (spiked with cysteine) analyses demonstrated the feasibility of the method in real sample application. From our findings, we anticipate that our method can be applied for the analysis of cysteine in various samples.

Effect of size on physicochemical, antibacterial, and catalytic properties of Neolamarckia cadamba (burflower-tree) synthesized silver/silver chloride nanoparticles

Aqueous extract of Neolamarchia cadamba leaves were used in the synthesis of silver/silver chloride nanoparticles (Ag/AgCl NPs). Further they were separated based on their using step-wise centrifugation approach at 09,000, 12,000, and 15,000 rpm. Thus obtained NPs were characterized for their physicochemical features. NPs showed maximum absorbance at 455 nm, 415 nm, and 402 nm. All the NPs were found to be crystalline in nature with average crystallite size (nm) of 58.31, 23.43, and 09.56. Particle size distribution (nm) of NPs was observed to 435.43, 276.75, and 105.49, Surface charge (-mV) of NPs was observed to be 14.59, 23.90, and 32.17. Ag/AgCl NPs-rpm@15,000 showed antibacterial activity against Escherichia coli, coagulase-negative Staphylococci, and Staphylococcus aureus with zone of inhibition (mm) of 16.65, 13.69, and 14.02 at 50 µg per well, respectively. Ag/AgCl NPs-rpm@15,000 showed excellent catalytic activity in degradation of methyl red, methylene blue, rhodamine-B, and methyl orange dyes in the presence of sodium borohydride under 4, 6, 5, and 4 min with pseudo-first order rate constant (min^-1) of 0.981 (96.4%), 0.666 (97.1%), 0.905 (98.1%), and 1.032 (96.6%), respectively. Furthermore, Ag/AgCl NPs-rpm@15,000 showed good catalytic efficiency even under different dye combinations. Total combination was degraded under 18 min.