Photoinduced toxicity of CdSe/ZnS quantum dots with different surface coatings to Escherichia coli

Quantum dot (QD) nanoparticles are increasingly used for various biomedical applications. To increase their biocompatibility, QDs are frequently coated with different surface coatings. Since QDs are photoactive under UV irradiation and may lead to oxidative stress, in this study the photoinduced toxicities of CdSe/ZnS QDs with three different coatings at three different wavelengths (530, 580 and 620 nm), along with a reference sample, a non-cadmium based QD CuInS2/ZnS were tested with E. coli under solar irradiation and dark conditions. QEI QDs coated with polyethylenimine (PEI) were found to be highly phototoxic to E. coli, conversely, QSA QDs with polyethyleneglycol coating and QSH QDs with amphiphilic polymer coating did not show toxicity. LC50 values for QEI QDs are smaller in the light exposure groups than those in the dark exposure groups. CuInS2/ZnS was less toxic to E. coli both under the light and dark conditions. The release of cadmium ions from the QD core was found to be negligible. Instead, the primary mechanism of the phototoxicity of QDs is the oxidative stress via formation of hydroxyl radicals that leads to lipid peroxidation and a concomitant decrease in reduced glutathione. The results also indicated that CdSe/ZnS QDs with PEI coating are highly toxic to E. coli and the non-cadmium based QD CuInS2/ZnS have better biocompatibility.