L-tryptophan adsorption differentially changes the optical behaviour of pseudo-enantiomeric cysteine-functionalized quantum dots: Towards chiral fluorescent biosensors

Water-soluble chiral graphene quantum dots (GQDs) with a strong blue emission were synthesized by covalently immobilizing L-cysteine or D-cysteine onto the GQDs. Either the amine or the thiol group of cysteine was used to make the bond through amide coupling or thiol-ene click chemistry respectively. The functionalized chiral GQDs were the characterized by FT-IR and UV-vis. The enantiomeric pairs exhibit equal but opposite bands in circular dichroism spectra suggesting that there is no difference in the efficacy of conjugation. The fluorescent response of these chiral GQDs when exposed to L-tryptophan was then studied. The fluorescence of the amide-conjugated GQDs was quenched with the addition of L-Trp regardless of which enantiomer of cysteine was present on the surface. The thiol-linked D- Cys GQDs fluorescence was also quenched on exposure to L-Trp, but the fluorescence of the thiol-linked L-Cys GQDs was unaffected under the same conditions.