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

Abstract

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 Förster resonance energy transfer (FRET). Herein, we report a new nanocomposite NaYF₄:Nd,Yb@Yb@Yb,Er@Y with Nd³⁺ ions in the core and Er³⁺ ions in the shell to shorten the Er–PS distance to achieve better FRET. Furthermore, an outer non-emitting protective Y³⁺ shell and a conducting Yb³⁺ shell reduced surface quenching and Er³⁺-to-Nd³⁺ energy back transfer effects, respectively. The upconversion FRET and downshifting emission efficiencies were simultaneously optimised by adjusting the thickness of the Y³⁺ 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.