Synthesis of Zn–Cu–In–S/ZnS Quantum Dots via Ligand Exchange for Bioimaging and Comprehensive Toxicity Evaluation

Abstract

Zn–Cu–In–S/ZnS (ZCIS/ZnS) quantum dots (QDs) stand out for their strong photoluminescence, red-to-near-infrared emission, and long fluorescence lifetimes. However, most ZCIS/ZnS QDs are synthesized with hydrophobic ligands, which limit their compatibility. Despite their promise as safer, cadmium-free alternatives, toxicological data on these QDs remain scarce. Ligand exchange allows a better understanding of how surface chemistry affects their optical and biological properties. In this study, ZCIS/ZnS QDs were functionalized with three hydrophilic ligands: 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA), and reduced glutathione (GSH). The resulting QDs exhibited bandgap energies of 2.6, 2.7, and 2.8 eV; quantum yields of 18%, 17%, and 14%; and fluorescence lifetimes of 115, 108, and 107 ns, respectively. Cytotoxicity tests in Chinese hamster ovary (CHO) cells showed minimal toxicity across 20–150 μg/mL. In vivo tests in mice indicated an LD50 of 500 mg/kg (GHS category 4), while zebrafish embryo assays confirmed safety with LC50 values above 300 μg/mL. Fluorescence imaging revealed efficient uptake of MPA- and TGA-capped QDs in CHO and SKOV-3 cells, whereas GSH-capped QDs showed weaker labeling. These findings highlight the influence of ligand structure on optical and biological behavior, guiding the development of safe, cadmium-free QDs for bioimaging.