Lanthanide-doped ultrasmall yttrium fluoride nanoparticles with enhanced multicolor upconversion photoluminescence

Ultrasmall upconversion (UC) nanoparticles with efficient multicolor photoluminescence (PL) are quite desirable for high contrast in vitro and in vivo bioimaging. Here, we present a simple and reproducible thermolysis approach to synthesize 3.7 nm sized lanthanide-doped YF₃ UC nanocrystals. The PL intensity of these ultrasmall nanoparticles was found to be about ten times brighter than that of 4.5 nm sized CaF₂ counterparts, which were reported to be one of the smallest efficient upconversion nanoparticles. We also showed that the color output of these nanocrystals can all be simply modulated by adjusting the sensitizer Yb³⁺ concentration. Moreover, to further enhance the PL output, a one nm thick epitaxial layer was grown onto the YF ₃ nanocrystals. These lanthanide-doped ultrasmall upconversion YF₃ nanocrystals with modulated and enhanced multicolor output are ideal for a variety of in vitro and in vivo biological imaging applications.