Strategies for photoluminescence enhancement of AgInS2 quantum dots and their application as bioimaging probes

We report the effect of the initial Ag : In stoichiometry, capping ligand concentration, and reaction temperature on the optical properties (such as photoluminescence [PL] quantum yield, PL full width at half-maximum, and wavelength of maximum PL emission) of AgInS2 quantum dots (QDs). The fabricated QDs exhibit excellent optical characteristics, including PL quantum yields of up to 22% and the appearance of an excitonic absorption peak. This is the first reported observation of such a well-defined exciton absorption feature in AgInS2 QDs. Its appearance could indicate a relatively narrow size distribution or minimization of nonradiative recombination at the surface sites of the QDs. Moreover, we report one-pot two-step synthesis of highly luminescent AgInS2–ZnS QDs by injection of ZnS precursors directly into the same reactor with existing AgInS2 cores without any purification. In addition, to demonstrate their potential biomedical application, the AgInS2–ZnS QDs were coated with poly(maleic anhydride-alt-1-octadecene) and further conjugated with folic acid for human liver carcinoma (HepG2) tumor cell labeling. The folate-receptor-mediated cellular uptake of the folic-acid-conjugated AgInS2–ZnS QDs was confirmed by confocal imaging characterization. Additionally, the small dimensions and high solubility of the folic-acid-conjugated AgInS2–ZnS QDs were also exploited in prefixed-cell staining; the QDs entered the complex cellular matrix and stained both the nucleoli and cytoplasm of HepG2 cells.