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 °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/cm²).