Improving 800 nm Triggered Upconversion Emission for Lanthanide-Doped CaF2 Nanoparticles through Sodium Ion Doping

Lanthanide ions (Ln(3+))-doped CaF2 luminescent nanoparticles exhibit unique optical properties and represent promising candidates for bioapplications; however, their inherent weak luminescence intensity greatly limits their application, especially for Nd3+-sensitized ones. We have doped Ln(3+) ions along with Na+ ion into CaF2 lattice to maintain charge neutrality and investigated the influence of Na+ ion on the 800 nm triggered upconversion process. As compared with Na+-free (shell layer) nanoparticles, the crystallinity of Na+-ion-codoped ones has been improved greatly, and the average lifetimes of the excited Nd3+, Yb3+, and Er3+ states have been prolonged after Na+ ion codoping in the sensitizing shell. Thus the energy-transfer efficiency from sensitizers (Nd3+) to activators (Er3+) has been increased by 67%. As a result, the upconversion luminescence intensity of the nanoparticles with suitable Na+ doping reaches 67 times higher than Na+-free nanoparticles upon 800 nm irradiation, although fewer Ln(3+) ions were doped into CaF2 in this case. The as-synthesized CaF2:Yb/Er@CaF2:Nd/Yb core-shell structured (CS) UCNPs may be good candidates as excellent imaging nanoprobes because of their good biocompatibility.