Visual enantioselective recognition of aminoacids based on unmodified CdTe quantum dots

Chirality is a determinative feature of biological phenomena. Enantiomeric recognition of amino acids is very important in both process development and quality control because amino acids are essential bioactive substances and building blocks of proteins, polypeptides and various drugs [1]. Due to their unique optical properties, quantum dots (e.g., CdTe QDs), possess a tremendous potential of applications in optical sensing. Reported colorimetric chiral recognition probes for amino acids are based on the inherent chirality of metal surfaces [2] or chiral molecules adsorbed on the nanoparticle surface [3]. Metal nanoparticles with intrinsic chiral structure are numerable and adsorption of chiral molecules onto the surface of nanoparticles lead to complex chiral detection.Herein, we demonstrate a simple and novel luminescence sensing for visual chiral discrimination of Cysteine. TGA-capped QDs with green emission are directly synthesized in aqueous solution. The interaction between thiol containing Cysteine molecules and CdTe QDs lead to the aggregation of the QDs via hydrogen bonding. As a result of electronic coupling within these aggregates, a red-shift in the absorption and emission spectra of QDs are observed. The difference in kinetic of interaction between L- and D-Cysteine with CdTe QDs gives rise to chiral recognition of enantiomers. On addition of D-Cysteine to CdTe QDs’ solution in basic media, a green-to-yellow color change can be observed, whereas no emission color change is found in the presence of L-Cysteine after 2 hours. Compared with other proposed fluorimetric methods, the present strategy is more attractive because CdTe QDs do not need any labeling or modifying with chiral molecules. The chiral assay determine the enantiometric excess of D-Cysteine in the range from -100 % to 100 %. The response of the unmodified CdTe QDs in presence of other α-amino acids including Arginine, Tryptophan, Tyrosine, Proline and Histidine is small and the same for D- and L- form of each kind of amino acids. However, the response of D-Cysteine is the highest and distinct from L-Cysteine.