A single source-precursor route for the one-pot synthesis of highly luminescent CdS quantum dots as ultra-sensitive and selective photoluminescence sensor for Co2+ and Ni2+ ions

In this study, we have demonstrated a facile, simple one-pot and low cost method for the synthesis of 3-mercaptopropionic acid (MPA)-capped, water-soluble CdS quantum dots (QDs) with highly tunable optical properties. Initially Cd2+ coordinates with MPA at about pH 5, and the CdS QDs were then formed at a higher pH (7-12) under refluxing conditions through the disruption of coordination interaction with the release of sulfur. Here MPA played a dual role, as both, a source of sulfur and as a stabilizer. The particle size and the optical properties of the as-prepared CdS QDs were found to be dependent on the refluxing time for a given concentration ratio of the reactants and pH of the initial mixture. The broadness and large Stokes shift of emission of MPA-CdS QDs are due to the surface-trap state photoluminescence (PL). The PL peak around 510 nm-650 nm is due to the recombination of shallow trapped electrons in sulfur vacancy defect states with holes in the valence band, and a similar to 665 nm peak (shoulder) arises from deep-trap states. The origin of the longer lifetime is presumed to be due to the involvement of surface-trap states and their environment. Use of MPA as a capping agent eventually enhances the water solubility as well as the stability of CdS QDs, which makes them useful for the ultra-sensitive detection of Co2+ and Ni2+. The selective coordination interaction of Co2+ and Ni2+ with MPA-CdS QDs through the carboxyl group of MPA provides a turn-off photoluminescence-based assay for sensitive detection of these metal ions without any interference of other commonly coexisting metal ions. The limit of detection (LOD) is 10 nM for Co2+ ions and 50 nM for Ni2+ ions. Co2+-induced color (from colorless to yellow) and UV-vis spectral change of MPA-CdS QDs is the simple way to distinguish Co2+ from Ni2+ in a higher concentration range (more than 5 mu M). On the other hand the lower stability of the Co(II)-MPA complex than the Ni(II)-MPA complex provides a disodium salt of ethylenediaminetetraacetic acid (EDTA)-induced, time dependent turn-on photoluminescence-based technique to distinguish Co2+ from Ni2+ in the entire range of concentrations. EDTA-induced time dependent PL recovery of MPA-CdS QDs occurs via rapid dissociation of Co2+ ions from the surface of QDs than that of Ni2+. Thus our synthesized MPA-CdS QDs offer a very simple, rapid, cost-effective, turn-off-on photoluminescence-based technique for ultra-sensitive and selective detection of either Co2+ or Ni2+ in aqueous solution without interference of other common metal ions.