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/p-rGO 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.