Green Synthesis of Carboxymethyl Chitosan-Based CuInS QDs with Luminescent Response toward Pb Ion and Its Application in Bioimaging.

Polysaccharide-based QDs have attracted great attention in the field of biological imaging and diagnostics. How to get rid of the high heavy metal toxicity resulting from conventional Cd- and Pb-based QDs is now the main challenge. Herein, we offer a simple and environmentally friendly approach for the "direct" interaction of thiol-ending carboxymethyl chitosan (CMC-SH) with metal salt precursors, resulting in CuInS QDs based on polysaccharides. A nucleation-growth mechanism based on the LaMer model can explain how CMC-CuInS QDs are formed. As-prepared water-soluble CMC-CuInS QDs exhibit monodisperse particles with sizes of 5.5-6.5 nm. CMC-CuInS QDs emit the bright-green fluorescence at 530 nm when excited at 466 nm with the highest quantum yield of ∼18.0%. Meanwhile, the fluorescence intensity of CMC-CuInS QD aqueous solution is quenched with the addition of Pb and the minimal limit of detection is as little as 0.4 nM. Furthermore, due to its noncytotoxicity, great biocompatibility, and strong biorecognition ability, CMC-CuInS QDs can be exploited as a possible cell membrane imaging reagent. The imaging studies also demonstrate that CMC-CuInS QDs are suitable for Pb detection in live cells and living organisms (zebrafish). Thus, this work offers such an efficient, green, and practical method for creating low-toxicity and water-soluble QD nanosensors for a sensitive and selective detection of toxic metal ion in live cells and organisms.