The effects of composition and surface chemistry on the toxicity of quantum dots

Recently, researchers have paid much attention to the toxicity of QDs because of their rapidly increasing application in biomedicine. Recent investigations have indicated that QDs have influences on biological systems at the cellular, subcellular, and protein levels during the processes of imaging and therapeutic applications. The toxicity of QDs is related to their composition, surface functionality, size, shape, and charge, etc. among which composition and surface modification are two important elements. This feature article mainly concentrates on the latest developments in reducing QD toxicity by manipulating their composition and surface modification. Besides the cadmium-based QDs, the assessment of toxicity in vitro and in vivo for other QDs such as carbon dots, graphene QDs, silicon QDs, Ag₂Se QDs, CuInS₂@ZnS, InP QDs, hybrid QDs of carbon and CdSe@ZnS, etc., is generalized. Each kind of QD has its own advantages. Cadmium-based QDs have broad UV excitation, narrow emission and bright photoluminescence (PL), while cadmium-free ones present low toxicity. In fact, a lot of investigations show that the toxicity of QDs is dose dependent, whatever the composition. Consequently, surface modification becomes very important to reduce toxicity and simultaneously impart biocompatibility, stability, and specificity to QDs. Therefore, the functionalization of QDs with inorganic shells (e.g., CdSe@ZnS, CdSe@SiO₂), polymers, bio- or natural macromolecules is summarized. Future research work should concentrate on preparing novel QDs with appropriate surface modification and investigating the long-term influence of QDs on absorption, distribution, metabolism, and elimination in vivo, especially for cadmium-free QDs such as carbon-based QDs, Ag₂Se QDs, CuInS₂@ZnS QDs and InP QDs, etc.