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

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