Fluorine Doping Mediated Epitaxial Growth of NaREF₄ on TiO₂ for Boosting NIR Light Utilization in Bioimaging and Photodynamic Therapy

By integrating TiO₂ with rare earth upconversion nanocrystals (NaREF₄), 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 TiO₂ with NaREF₄ is a non-epitaxial random blending mode, resulting in a diminished energy transfer efficiency between the NaREF₄ and TiO₂. Herein, we present a fluorine doping-mediated epitaxial growth strategy for the synthesis of TiO₂-NaREF₄ heteronanocrystals (HNCs). Due to the epitaxial growth connection, NaREF₄ can transfer energy through phonon-assisted pathway to TiO₂, which is more efficient than the traditional indirect secondary photon excitation. Additionally, F doping brings oxygen vacancies in the TiO₂ subunit, which further introduces new impurity energy levels in the intrinsic band gap of TiO₂ subunit, and facilitates the energy transfer through phonon-assisted method from NaREF₄ to TiO₂. As a proof of concept, TiO₂-NaGdF₄ : Yb,Tm@NaYF₄@NaGdF₄ : Nd@NaYF₄ HNCs were rationally constructed. Taking advantage of the dual-model up- and downconversion luminescence of the delicately designed multi-shell structured NaREF₄ subunit, highly efficient photo-response capability of the F-doped TiO₂ subunit and the efficient phonon-assisted energy transfer between them, the prepared HNCs provide a distinctive nanoplatform for bioimaging-guided NIR-triggered PDT.