Optimising FRET-efficiency of Nd3+-sensitised upconversion nanocomposites by shortening the emitter-photosensitizer distance

Nd3+-Sensitised luminescent upconversion nanoparticles (UCNPs) have gained interest recently as theranostics due to their near-infrared (NIR) light excitation with a better tissue penetration depth. One example is the core/shell design NaYF4:Yb,Er@Nd,Yb. When harvesting the upconversion energy in such architectures, the long emitter-photosensitizer (i.e.Er3+-PS) distances lead to inefficient Forster resonance energy transfer (FRET). Herein, we report a new nanocomposite NaYF4:Nd,Yb@Yb@Yb,Er@Y with Nd(3+)ions in the core and Er(3+)ions in the shell to shorten the Er-PS distance to achieve better FRET. Furthermore, an outer non-emitting protective Y(3+)shell and a conducting Yb(3+)shell reduced surface quenching and Er3+-to-Nd(3+)energy back transfer effects, respectively. The upconversion FRET and downshifting emission efficiencies were simultaneously optimised by adjusting the thickness of the Y(3+)shell, and the FRET efficiency was at least 3.7 times that of the reference NaYF4:Yb,Er@Yb@Nd,Yb@Y in a photodynamic therapy (PDT) model.