L-Cysteine-coated CdSe/CdS core-shell quantum dots as selective fluorescence probe for copper(II) determination

Quantum dots (QDs) or semiconductor nanocrystals have been receiving great interest in the last few years. In this paper, L-cysteine-coated CdSe/CdS core-shell QDs (λem = 585 nm) have been prepared, which have excellent water-solubility. The full width at half maximum (FWHM) of the photoluminescence of these nanocrystals is very narrow (about 30 nm), and the quantum yield (QY) is 15% relative to Rhodamine 6G in ethanol (QY = 95%). With excess free L-cysteine in the solution, the fluorescence intensity of L-cysteine-coated CdSe/CdS QDs showed improved stability. It was found that the fluorescence of L-cysteine-capped CdSe/CdS QDs could be quenched only by copper (II) ions and was insensitive to other physiologically important cations, such as Ca2+, Mg2+, Zn2+, Al3+, Fe3+, Mn2+ and Ni2+ etc. Based on this finding, the quantitative analysis of Cu2+ with L-cysteine-capped CdSe/CdS QDs has been established. The linear range was from 1.0 × 10− 8 to 2.0 × 10− 7 mol L− 1 and the limit of detection (LOD) was 3.0 × 10− 9 mol L− 1 (S/N = 3). The proposed method has first been applied to the determination of Cu2+ in vegetable samples with recoveries of 99.6–105.8%.