Core-Size-Controlled and Ligand-Modified CuInS 2 /ZnS Quantum Dots for Enhancing the Red Photoluminescence Quantum Yield and Metal Ion Sensing in an Aqueous Solution

Red-emitting quantum dots (QDs) are particularly valuable in fluorescence-based chemical sensing, as longer-wavelength emission reduces background fluorescence and improves the signal-to-noise ratio in complex environments. CuInS2 (CIS) QDs are a promising cadmium-free alternative with tunable emission. However, red-shifting their photoluminescence (PL) peak often decreases their PL quantum yield (PLQY), which limits their further application. In this work, we synthesize CIS/ZnS core–shell QDs using the precursor thermal decomposition method. We further enlarged the CIS core to decrease its surface-to-volume ratio, thereby reducing interfacial defect density between the core and shell and narrowing the bandgap of the core, which achieves red PL emission with a peak wavelength of 668 nm and a PLQY of 82.3% in toluene. Subsequent ligand exchange with 3-mercaptopropionic acid (MPA) endows CIS/ZnS QDs with aqueous dispersibility. The resulting CIS/ZnS QDs with MPA ligands exhibit a high PLQY of 40.3% in a phosphate-buffered saline (PBS) aqueous solution at pH 8 and retain good storage stability for one month. Furthermore, the QDs enable selective detection of Cu2+ among 12 metal ions in PBS solutions, with clearly observable fluorescence quenching.