Synthesis of silver nanoparticles and their attachment to zinc oxide-iron oxide hybrid structure: analyte-mediated enhanced peroxidase-like activity for colorimetric detection

Abstract

Natural and human activities such as mining of gold, coal fired power stations, coal burning, and different industrial processes have contributed to the emission of mercury to the environment. This necessitates regular monitoring of mercury in the environment. The requirement of skilled personnel, high cost, and time-consuming processes make the conventional techniques used in detection of mercury unsuitable for routine analysis. Colorimetric methods relieve these limitations. In this study, silver nanoparticles (AgNPs) loaded ZnO/Fe 3 O 4 (ZMN) composites (shortly AgZMN) with peroxidase-like activity are produced. Ferrous sulphate is used to reduce Ag + to Ag 0 that is ascribed to the difference in the reduction potentials of the two reacting species. Surface-enhanced Raman spectroscopy (SERS) was used indirectly to investigate the formation of AgNPs. Using 3,3'5,5'-tetramethylbenzidine (TMB) as chromogenic peroxidase substrate colorimetric method for the detection of Hg 2+ is developed. The sensing relies on enhancement of the peroxidase-like activity of AgZMN on addition of Hg 2+ to the catalytic oxidation reaction system. By relating the absorbance of the oxTMB and the concentration of Hg2+ a relation that fits linear equation in the range of 3 -65 μM was obtained. The limit of detection (LOD) was predicted from the calibration curve and found to be 0.943 μM. The assay exhibits a high selectivity towards Hg 2+ detection. The applicability of this method in determination of Hg2+ from tap and borehole ground water shows its practicality for real sample analysis. The green reduction method and deposition of the AgNPs could be generalized to other metal ions with different reduction potentials from ferrous ions that favors reduction.