Highly Selective Ag-Ca@CuO Nanocomposite-Based Sensor for Trace-Level Cr (VI) Detection in Water

Abstract

Cr (VI) is a highly toxic metal ion and is considered as a carcinogen, teratogen, and mutagen. To address its sensing in water samples, Ag-Ca@CuO nanocomposites are synthesised using a simple, less expensive, effective method and are used for determining Cr (VI) by colorimetric approach. The nanocomposites have been characterized through several methods. Scanning electron microscopy revealed the formation of rice grain-shaped CuO nanoparticles. Upon incorporation of Ca and Ag, a change in the morphology was noticed. The oxidase-like activity is shown by Ag-Ca@CuO nanocomposites in the presence of chromium (VI) and a chromogenic substrate i.e., 3, 3′, 5, 5′- tetramethylbenzidine (TMB). Our established method showed superior sensitivity with a detection limit of 0.033 µM, a linear range of 10–600 µM and recoveries ranged from 100.16% to 110.32%. Reactive oxygen species include superoxide anion (O₂* −), hydroxyl radicals (OH*), singlet oxygen (¹O₂), etc. Superoxide radicals (O₂^•−) are generated from the prepared nanocomposites and these radicals in the presence of chromium (VI) caused oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), and are recovered as oxTMB in blue colour. The role of O₂^•− in the oxidation of TMB was confirmed from the reactive oxygen species (ROS) scavenging studies using ascorbic acid as an O₂^•− scavenger. In addition, singlet oxygen also caused oxidation of TMB and is confirmed from diphenylisobenzofuran (DPBF) assay. Selective detection of Cr (VI) in the Yamuna River water sample, by the obtained nanocomposites is confirmed from the interference studies carried out in the existence of several metal ions.