A novel approach towards optical detection and detoxification of Cr(VI) to Cr(III) using L-Cys-VS2QDs

Highly stable and water-soluble L-cysteine vanadium disulfide quantum dots (L-Cys-VS(2)QDs) were synthesized using the hydrothermal method. L-Cys was used as a capping agent, sulfur source as well as a stabilizer, to restrain the hydrothermal growth of QDs. The L-Cys capped VS(2)QDs (L-Cys-VS(2)QDs) were interacted with different physiological and toxic environmental pollutant metal ions such as Zn (II), Cu (II), Pb (II), Fe (III), Cr (VI) and Cd (II) in aqueous solution and their effect was studied using photoluminescence (PL) spectroscopy. It was found that L-Cys-VS(2)QDs showed highly selective quenching towards the Cr (VI) ion in the concentration range of 1 mu M to 160 mM with linearity up to 20 mM. On the other hand, metal ions such as Zn (II), Cu (II), Pb (II) and Cd (II) did not show significant quenching. The quenching mechanism has been proposed by the complex formation between L-Cys-VS(2)QDs and Cr (VI) ions. The interaction of L-Cys-VS(2)QDs with Cr (VI) ions by PL emission exhibited to follow the Stern-Volmer positive type binding with Stern-Volmer constant of 0.129 mM(-1). The Cr (VI) ions form thioester complex with L-Cys ligands that detached from the L-Cys-VS(2)QDs. The detailed mechanism is explained in the following sections. Further, the decomposition of thioester leads to detoxification of Cr (VI) to Cr (III) ions. This L-Cys-VS(2)QDs exhibits a synergistic effect by following optical detection through PL response and reduction of Cr (VI) to Cr (III). The detoxification of Cr (VI) to Cr (III) ions was further confirmed by electron paramagnetic resonance (EPR) spectroscopy.