Surface functionalized fluorescent CdS QDs: selective fluorescence switching and quenching by Cu(2+) and Hg(2+) at wide pH range.

Fluorescent CdS QDs surface functionalized with organic functional unit, N-(2-hydroxybenzyl)-cysteine (L), has been synthesized using simple wet chemical approach in a single step. The as-prepared L-CdS QDs showed good fluorescence at 542 nm. Fluorescence sensor studies of L-CdS QDs showed selective fluorescence switching (542-600 nm) for Cu(2+) ions and quenching for Hg(2+) ions in aqueous solution. High selectivity of L-CdS QDs for Cu(2+) ions have been confirmed by interference studies. Interestingly, L-CdS QDs showed enhancement in the fluorescence intensity (4-fold) upon reducing pH from 2.0 to 10.0 without changing λmax. Importantly, selective fluorescent switching and quenching for Cu(2+) and Hg(2+) was observed from 2.0 to 10.0 pH range. The practical application of L-CdS QDs fluorescence sensing for Cu(2+) and Hg(2+) have also been demonstrated in real samples such as river, pond, tap and ground water.