ZnO/ZnS core–shell quantum dots with enhanced ultraviolet fluorescence and low cytotoxicity for cell imaging

Zinc oxide quantum dots (ZnO QDs) have gained wide attention due to their wide excitation spectrum, large Stokes shift, adjustable photoluminescence (PL) spectrum, and excellent biocompatibility. However, low fluorescence intensity and poor stability restrict their further applications. In this work, zinc sulfide (ZnS) as a surface modifier, ZnO/ZnS core-shell QDs with type-I core-shell structure and particle size of 5 nm were prepared via sol-gel method. Transmission electron microscope characterization demonstrates the core-shell structure and spherical morphology of the as-synthesized ZnO/ZnS QDs. The PL spectra show that ultraviolet fluorescence has been greatly enhanced. The maximum fluorescence intensity of ZnO/ZnS core-shell QDs increases by 5288.6% compared with that of bare ZnO QDs. The PL quantum yield increases from 9.53% to 30.95%. After being stored for three weeks, the fluorescence performance can be well retained. Furthermore, the cytotoxicity tests confirm the excellent biocompatibility of ZnO/ZnS core-shell QDs, demonstrating they are good candidates for cell imaging.