Rare-Earth Nanoparticles with Enhanced Upconversion Emission and Suppressed Rare-Earth-Ion Leakage

Upconversion emissions from rare-earth nanoparticles have attracted much interest as potential biolabels, for which small particle size and high emission intensity are both desired. Herein we report a facile way to achieve NaYF4:Yb,Er@CaF2 nanoparticles (NPs) with a small size (1013 nm) and highly enhanced (ca. 300 times) upconversion emission compared with the pristine NPs. The CaF2 shell protects the rare-earth ions from leaking, when the nanoparticles are exposed to buffer solution, and ensures biological safety for the potential bioprobe applications. With the upconversion emission from NaYF4:Yb,Er@CaF2 NPs, HeLa cells were imaged with low background interference.