An aqueous approach to synthesize capped SnS quantum dots (QDs) followed by optional capping molecule extension by attaching one or more extending molecules to the capping molecule via peptide bond formation at elevated temperature. The capped SnS QDs may have a capping molecule:Sn:S molar ratio of 16:3:1 to 16:12:1. A suspension of SnS QDs was heat-treated at 200° C. for 0.5-4 hrs. The obtained SnS QDs showed an NIR emission peak at 820-835 nm with an excitation wavelength at 690 nm. The as synthesized SnS QDs were found to have high positive zeta potential of ˜30 mV and thus were toxic to cells. By neutralizing the SnS QDs the cytotoxicity was reduced to an accepted level. The heat-treatment step can be obviated by adding a glycerol solution containing S2− anions and capping molecule to a glycerol solution of Sn2+ ions.