L-cysteine adsorption on thiol-coated quantum dot (QD) in aqueous solution: An implication on the QD speciation in biological media

The adsorption of L-cysteine at the thiol-capped QD-water interface was investigated by monitoring its photoluminescence andin-situ ATR-FTIR spectral features. Upon exposure to L-cysteine in aqueous solution, quenching of the QD fluorescence during the first few hours, followed by slow and steady enhancement of the fluorescence during the next several days were observed. Moreover, it was also found that the temporal changes in fluorescence intensities were strongly dependent on the concentration of L-cysteine. Based on these results combined within-situ ATR-FTIR spectroscopic observation, a reaction pathway of L-cysteine adsorption on thempaQD colloidal particles was proposed, which is initially driven by the formation of kinetically favorable intermediate species, involving coordination of L-cysteine thiol group to the QD surface and then followed by the slow formation of thermodynamically favored QD species, probably involving bidentate coordination of both amine and thiol groups of L-cysteine to the surface of QD nanoparticles. Considering the photoluminescence and colloidal property changes ofmpaQDs exposed to widely used cell culture media (i.e., RPMI 1640 and DMEM), this spectroscoscopic results and proposed reaction pathway at the QDwater interface have important implications in understanding the potential biological fate and cytotoxicity mechanisms of engineered nanomaterials, which recently induced increasing concerns.