Role of ZnS shell on stability, cytotoxicity and photocytotoxicity of water-soluble CdSe semiconductor quantum dots surface modified with glutathione

Biomedical applications of quantum dots (QDs) have become a subject of a considerable concern in the past few decades. The present study examines the stability and cytotoxicity of two QDs systems in cell culture medium in the presence and absence of a thin layer of ZnS shell. The two systems were built from core, CdSe QDs, surface modified with glutathione (GSH), named CdSe similar to GSH and CdSe/ZnS similar to GSH. Our results demonstrated that 0.7 nm layer of ZnS shell played a significant role in the stability of CdSe/ZnS similar to GSH QDs in supplemented cell culture medium (RPMI). Also, a significant improvement in the physicochemical properties of the core CdSe QDs was shown by maintaining their spectroscopic characteristics in RPMI medium due to the wide band gap of ZnS shell. Both systems showed insignificant reduction in cell viability of HFB-4 or MCF-7 cell lines in the dark which was attributed to the effective GSH coating. Following photoirradiation with low laser power (irradiance 10 mW cm(-2)), CdSe similar to GSH QDs showed a significant decrease in cell viability after 60 min irradiation which may result from detachment of GSH molecules. Under the same irradiation condition, CdSe/ZnS similar to GSH QDs showed insignificant decrease in cell viability or after 2 h incubation from laser irradiation which was attributed to the strong binding between ZnS and GSH coatings. It can be concluded that the stability of CdSe core QDs was significantly improved in cell culture medium by encapsulation with a thin layer of ZnS shell whereas their cytotoxicity and photo-cytotoxicity are highly dependent on surface modification.