Synthesis, multicolour tuning, and emission enhancement of ultrasmall LaF3:Yb3+/Ln3+ (Ln = Er, Tm, and Ho) upconversion nanoparticles

Abstract

The development of efficient sub-10 nm ultrasmall upconversion nanoparticles will open the door to the exciting application in biological labelling and imaging. In this paper, we demonstrated a facile method for the synthesis of monodisperse sub-10 nm hexagonal-phased LaF3 nanoparticles doped with upconverting lanthanide ions (Yb3+/Ln3+, Ln = Er3+, Tm3+, and Ho3+). The particle size of the as-synthesized LaF3 nanoparticles can be tuned by varying the preparation temperature. Upon excitation at 980 nm, the LaF3:Yb3+/Ln3+ nanoparticles showed intense upconversion emissions, and the colour output can be precisely modulated by changing the species and concentration of the lanthanide activators. In order to further enhance the upconversion emission intensity of the ultrasmall LaF3:Yb3+/Ln3+ nanoparticles, we adopted the strategy of core–shell nanostructured design to minimize the surface quenching effect. After coating an inert LaF3 shell, a maximum ninefold enhancement in upconversion luminescence was achieved under 980 nm excitation. These as-prepared lanthanide-doped LaF3 upconversion nanoparticles may find promising applications in biomedicine fields as luminescent nanoprobes.