A facile cation exchange-based aqueous synthesis of highly stable and biocompatible Ag₂S quantum dots emitting in the second near-infrared biological window.

Second near-infrared (NIR-II) emitting Ag2S quantum dots (QDs) with high stability and biocompatibility were synthesized and developed toward an ideal nanoprobe. This study reports a facile synthesis of NIR-II Ag2S QDs on the basis of cation exchange between visible-emitting CdS QDs and Ag(+) ions in aqueous solution. Experimental data testified that the cation exchange was quick and complete and that the resultant products were single monoclinic Ag2S without CdS QDs. The prepared Ag2S QDs were systematically characterized, showed typical NIR-II emission and high PL stability, and had small diameters (~3.5 nm) and a quantum yield up to 2.3%. The results of cytotoxicity assay suggested that the Ag2S QDs produced negligible effects in altering the cell proliferation or in generating reactive oxygen species, indicating an ultralow cytotoxicity and an excellent biocompatibility. These properties have opened up the possibility of using Ag2S QDs for effective bioimaging applications.