Ag₂S-Glutathione quantum dots for NIR image guided photothermal therapy

Aqueous synthesis of glutathione (GSH) coated Ag₂S quantum dots (QD) with strong emission in the medical imaging window (700-900 nm) was performed from AgNO₃. Variations in temperature, time, pH and sulfur source (Na₂S and thioacetamide) were studied to tune the emission wavelength and intensity of the QDs. The optimum synthesis conditions to produce the smallest QDs (ca. 7 nm) with the highest quantum yield (70%) and excellent stability were determined as 50 °C and pH 10 where Na₂S was used in 2 h reactions. The cytotoxicity was evaluated with MTT assay on the HT29 and MCF7 cancer cell lines and indicated no significant toxicity of the QDs up to 200 μg mL^-1. The Photothermal Therapy (PTT) potential was first investigated using a 795 nm, continuous-wave fiber coupled diode laser using aqueous solutions of QDs and then on HT29 and MCF7 cells. QD (150 μg mL^-1) treated MCF7 cells almost quantitatively died after 10 min irradiation at 795 nm (700 mW, 1.82 W cm^-2) and the viability of HT29 cells dropped to 40%. Most cell death was late apoptotic/necrotic. This study indicates that such a laser irradiation procedure and the QDs are safe, yet their combination provides dramatic cell death upon short laser treatment. PTT coupled with strong intracellular optical signal promotes Ag₂S-GSH QDs as potential theranostic nanoparticles.