Alkaline Post-Treatment of Cd(II)–Glutathione Coordination Polymers: Toward Green Synthesis of Water-Soluble and Cytocompatible CdS Quantum Dots with Tunable Optical Properties

In this study, we demonstrate a facile and environmentally friendly method for the synthesis of glutathione (GSH)-capped water-soluble CdS quantum dots (QDs) with a high cytocompatibility and a tunable optical property based on alkaline post-treatment of Cd–GSH coordination polymers (CPs). Cd–GSH CPs are synthesized with the coordination reaction of Cd2+ with GSH at different pH values, and the CdS QDs are then formed by adding NaOH to the aqueous dispersion of the Cd–GSH CPs to break the coordination interaction between Cd2+ and GSH with the release of sulfur. The particle size and optical property of the as-formed CdS QDs are found to be easily tailored by simply adjusting the starting pH values of GSH solutions used for the formation of Cd–GSH CPs, in which the wavelengths of trap-state emission of the QDs red-shift with an increase in the sizes of the QDs that is caused by an increase in the starting pH values of GSH solutions. In addition, the use of GSH as the capping reagent eventually endows the as-formed CdS QDs with enhanced water solubility and good cytocompatibility, as demonstrated with HeLa cells. The method demonstrated here is advantageous in that the cadmium precursor and the sulfur source are nontoxic and easily available, and the size, optical properties, water solubility, and cytocompatibilty of the as-formed CdS QDs are simply achieved and experimentally regulated. This study offers a new and green synthetic route to water-soluble and cytocompatible CdS QDs with tunable optical properties.