Ultrasound-assisted synthesis of chiral cysteine-capped CdSe quantum dots for fluorometric differentiation and quantitation of tryptophan enantiomers

A room temperature ultrasound-assisted method was applied to synthesize L- and D-cysteine-capped CdSe quantum dots (QDs). The QDs were characterized by XRD, FT-IR, and TEM. They have diameters of 5-7 nm and are shown to be viable probes for highly selective chiral recognition of tryptophan (Trp) enantiomers by fluorometry. The green fluorescence of the capped QDs (with excitation/emission maxima at 380/527 nm and 380/520 nm for L-Cys and D-Cys QDs, respectively) is differently quenched by D- and L-Trp in a high selective manner, with negligible interference by other species. The calibration plots and corresponding Stern-Volmer plots for both Trp enantiomers were investigated by two different approaches: In the first, each individual enantiomer was tested. In the second, each enantiomer was tested in the presence of a 100-folds excess of the other enantiomer. The detection limits for the recognition of L- and D-Trp are 4.2 and 4.7 nM, respectively, for the first approach. In the presence of the other enantiomer, the LODs are 4.4 and 4.8 nM. The linear range extends from 0.1 to 15 mu M for both enantiomers.