Mechanistic investigation of photon upconversion in Nd³⁺- sensitized core-shell nanoparticles

A new type of core-shell upconversion nanoparticles which can be effectively excited at 795 nm has been designed and synthesized through spatially confined doping of neodymium (Nd³⁺) ions. The use of Nd³⁺ ions as sensitizers facilitates the energy transfer and photon upconversion of a series of lanthanide activators (Er³⁺, Tm ³⁺, and Ho³⁺) at a biocompatible excitation wavelength (795 nm) and also significantly minimizes the overheating problem associated with conventional 980 nm excitation. Importantly, the core-shell design enabled high-concentration doping of Nd³⁺ (∼20 mol %) in the shell layer and thus markedly enhanced the upconversion emission from the activators, providing highly attractive luminescent biomarkers for bioimaging without autofluorescence and concern of overheating.