CuS and NiS Nanoparticle-Decorated Porous-Reduced Graphene Oxide Sheets as Efficient Peroxidase Nanozymes for Easy Colorimetric Detection of Hg(II) Ions in a Water Medium and Using a Paper Strip

Transition-metal sulfide (TMS)/ porous-reduced graphene oxide (p-rGO)-based nanocomposite materials exhibiting intrinsic peroxidase enzyme-like activity have been synthesized adopting simple solvothermal techniques and characterized with the help of different analytical techniques. The synthesized CuS/prGO and NiS/p-rGO artificial nanozymes can catalyze the oxidation process of chromogenic substrates like 3,3',5,5' tetramethyl benzidine (TMB) converted to a blue-colored oxidized TMB product (oxTMB) on addition of H2O2. It is observed that the presence of Hg(II) ions can inhibit the catalytic activity of nanozymes for the oxidation process of TMB because Hg(II) can easily bind with -S-containing sites of the nanozymes and thus block the catalytic active sites and reduce the catalytic efficiency. This mechanism is utilized for development of a simple colorimetric sensor for visual naked eye detection of Hg(II) ions in a water medium. The synthesized CuS/p-rGO and NiS/p-rGO nanozymes have excellent selectivity and sensitivity toward Hg(II) ions up to the lower concentrations of 13.05 and 48.73 nM, respectively. Moreover, we have reported here a portable, simple, cost-effective, and easy detection process of Hg(II) ions up to the lower limit of 50 nM using a CuS/p-rGO-based paper strip sensor.