Dual-Ligand Surface Passivation Enables Monodisperse Ag₂S Colloidal Quantum Dots for Efficient Near-Infrared Photothermal Therapy

Silver sulfide (Ag2S) colloidal quantum dots (CQDs) have attracted attention as promising infrared materials owing to their broad bandgap tunability and nontoxic composition. However, synthesizing highly monodisperse Ag2S CQDs has been challenging, because they readily fuse with each other. Here, we introduce a dual-ligand passivation approach for the synthesis of highly monodisperse Ag2S CQDs. Leveraging both oleic acid and oleylamine as coligands for surface passivation, we achieve enhanced confinement of CQD morphology and effectively prevent CQD fusion. This contrasts with conventional Ag2S CQDs prepared by using solely oleylamine ligands, which show a wide size distribution due to inter-CQD fusion. This enables the exhibition of an efficient photothermal conversion capability upon illumination with an 808 nm laser, causing a rapid increase of temperature from 25 to 70 degrees C within 3 min. We demonstrate that incubation with 500 nM CQDs results in nearly 100% death of MCF-7 cells (human breast cancer cells) after just 5 min of 808 nm laser irradiation (1.5 W/cm(2)).