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 NaYF 4 :Yb,Er@CaF 2 nanoparticles (NPs) with a small size (10–13 nm) and highly enhanced (ca. 300 times) upconversion emission compared with the pristine NPs. The CaF 2 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 NaYF 4 :Yb,Er@CaF 2 NPs, HeLa cells were imaged with low background interference.