Enhancing Förster Resonance Energy Transfer (FRET) Efficiency of Titania–Lanthanide Hybrid Upconversion Nanomaterials by Shortening the Donor–Acceptor Distance

Several robust titania (TiO(2)) coated core/multishell trivalent lanthanide (Ln) upconversion nanoparticles (UCNPs) hybrid architecture designs have been reported for use in photodynamic therapy (PDT) against cancer, utilizing the near-infrared (NIR) excited energy down-shifting and up-conversion chain of Nd(3+) (λ(793-808 nm)) → Yb(3+) (λ(980 nm)) → Tm(3+)(λ(475 nm)) → TiO(2) to produce reactive oxygen species (ROS) for deep tissue-penetrating oxidative cytotoxicity, e.g., NaLnF(4):Yb,Tm (Ln = Y, Gd). Herein, we demonstrate that by doping the Tm(3+) emitter ions in the outer shell and the Nd(3+) sensitizer ions in the core, the newly designed NaYF(4):Nd,Yb@Yb@Yb,Tm@TiO(2) hybrid UCNPs exert more ROS production than the reference NaYF(4):Yb,Tm@Yb@Nd,Yb@ TiO(2) with the Tm(3+) ions in the core and the Nd(3+) ions in the outer shell, upon 793 nm laser irradiation, primarily due to the shortening of the Tm(3+)-TiO(2) distance of the former with greater Förster resonance energy transfer (FRET) efficiency. After coating with polyallylamine hydrochloride (PAH)/polyethylene glycol folate (PEG-FA), the resulting NaYF(4):Nd,Yb@Yb@Yb,Tm@TiO(2)-PAH-PEG-FA hybrid nanocomposites could be internalized in MDA-MB-231 cancer cells, which also show low dark cytotoxicity and effective photocytotoxicity upon 793 nm excitation. These nanocomposites could be further optimized and are potentially good candidates as nanotheranostics, as well as for other light-conversion applications.