Bright Infrared-to-Ultraviolet/Visible Upconversion in Small Alkaline Earth-Based Nanoparticles with Biocompatible CaF2Shells

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 CaF(2)shell. We synthesize 8 alkaline-earth host materials doped with Yb(3+)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@CaF(2)particles, with 7.3 % of total emission in the UV. In 10.9 nm SrYbF:1 %Tm3+@CaF(2)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.