High Up-Conversion Efficiency of YVO₄:Yb,Er Nanoparticles in Water down to the Single-Particle Level

We report up-conversion emission from an aqueous solution of YVO 4:Yb 3+,Er 3+ nanocrystals synthesized by an original method that produces nanoparticles with excellent crystallinity and no porosity. We show that these YVO 4:Yb 3+,Er 3+ nanocrystals are not very sensitive to nonradiative relaxations, leading to a high green-to-red emission ratio of 6.3. Using a comparison with YVO 4:Eu 3+ particles, we determined the quantum yield of the up-conversion emission of the aqueous YVO 4:Yb 3+,Er 3+ dispersion to be 0.09 ± 0.04% for an excitation intensity of only 0.55 kW·cm −2 at 970 nm. Furthermore, single YVO 4:Yb,Er particles with an estimated size down to 10 nm can be detected using a wide-field microscope under a 970 nm, 8 kW·cm −2 excitation. Because of their unexpectedly high up-conversion emission without intermittency, their water dispersibility, and their photostability, YVO 4:Yb 3+,Er 3+ nanoparticles are highly appropriate both for single-biomolecule and for in vivo imaging.