Fluorine Doping Mediated Epitaxial Growth of NaREF 4 on TiO 2 for Boosting NIR Light Utilization in Bioimaging and Photodynamic Therapy.

By integrating TiO 2 with rare earth upconversion nanocrystals (NaREF 4 ), efficient energy transfer can be achieved between the two subunits under near-infrared (NIR) excitation, which hold tremendous potential in the fields of photocatalysis, photodynamic therapy (PDT), etc. However, in the previous studies, the combination of TiO 2 with NaREF 4 is a non-epitaxial random blending mode, resulting in a diminished energy transfer efficiency between the NaREF 4 and TiO 2 . Herein, we present a fluorine doping-mediated epitaxial growth strategy for the synthesis of TiO 2 -NaREF 4 heteronanocrystals (HNCs). Due to the epitaxial growth connection, NaREF 4 can transfer energy through phonon-assisted pathway to TiO 2 , which is more efficient than the traditional indirect secondary photon excitation. Additionally, F doping brings oxygen vacancies in the TiO 2 subunit, which further introduces new impurity energy levels in the intrinsic band gap of TiO 2 subunit, and facilitates the energy transfer through phonon-assisted method from NaREF 4 to TiO 2 . As a proof of concept, TiO 2 -NaGdF 4  : Yb,Tm@NaYF 4 @NaGdF 4  : Nd@NaYF 4 HNCs were rationally constructed. Taking advantage of the dual-model up- and downconversion luminescence of the delicately designed multi-shell structured NaREF 4 subunit, highly efficient photo-response capability of the F-doped TiO 2 subunit and the efficient phonon-assisted energy transfer between them, the prepared HNCs provide a distinctive nanoplatform for bioimaging-guided NIR-triggered PDT.