Atomic-Level Passivation of Individual Upconversion Nanocrystal for Single Particle Microscopic Imaging

Lanthanide-doped upconversion nanoparticles (UCNPs) have significant applications for single-molecule probes and high-resolution display. However, one of their major hurdles is the weak luminescence, and this remains a grand challenge to achieve at the single-particle level. Here, 484-fold luminescence enhancement in LuF₃:Yb³⁺, Er³⁺ rhombic flake UCNPs is achieved, thanks to the Yb³⁺-mediated local photothermal effect, and their original morphology, size, and good dispersibility are well preserved. These data show that the surface atomic structure of UCNPs as well as transfer from amorphous to ordered crystal structure is modulated by making use of the local photothermal conversion that is generated by the directional absorption of 980 nm light by Yb³⁺ ions. The confocal luminescence images obtained by super-resolution stimulated emission depletion also show the great enhancement of individual LuF₃:Yb³⁺, Er³⁺ nanoparticles; the high signal-to-noise ratio images indicate that the laser treatment technology opens the door for single particle imaging and practical application.