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

Abstract

Lanthanide ions (Ln 3+ )-doped CaF 2 luminescent nanoparticles exhibit unique optical properties and represent promising candidates for bioapplications; however, their inherent weak luminescence intensity greatly limits their application, especially for Nd 3+ -sensitized ones. We have doped Ln 3+ ions along with Na + ion into CaF 2 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 Nd 3+, Yb 3+, and Er 3+ states have been prolonged after Na + ion codoping in the sensitizing shell. Thus the energy-transfer efficiency from sensitizers (Nd 3+ ) to activators (Er 3+ ) 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 CaF 2 in this case. The as-synthesized CaF 2:Yb/Er@CaF 2:Nd/Yb core–shell structured (CS) UCNPs may be good candidates as excellent imaging nanoprobes because of their good biocompatibility.