Bright Infrared‐to‐Ultraviolet/Visible Upconversion in Small Alkaline Earth‐Based Nanoparticles with Biocompatible CaF₂ Shells

Abstract

Upconverting nanoparticles (UCNPs) are promising candidates for photon-driven reactions, including light-triggered drug delivery, photodynamic therapy, and photocatalysis. Herein, we investigate the NIR-to-UV/visible emission of sub-15 nm alkaline-earth rare-earth fluoride UCNPs (M_1-x Ln_x F_2+x, MLnF) with a CaF₂ shell. We synthesize 8 alkaline-earth host materials doped with Yb³⁺ and Tm³⁺ , with alkaline-earth (M) spanning Ca, Sr, and Ba, MgSr, CaSr, CaBa, SrBa, and CaSrBa. We explore UCNP composition, size, and lanthanide doping-dependent emission, focusing on upconversion quantum yield (UCQY) and UV emission. UCQY values of 2.46 % at 250 W cm^-2 are achieved with 14.5 nm SrLuF@CaF₂ particles, with 7.3 % of total emission in the UV. In 10.9 nm SrYbF:1 %Tm³⁺ @CaF₂ particles, UV emission increased to 9.9 % with UCQY at 1.14 %. We demonstrate dye degradation under NIR illumination using SrYbF:1 %Tm³⁺ @CaF₂ , highlighting the efficiency of these UCNPs and their ability to trigger photoprocesses.