Topological Control of Lanthanide-Doped Upconversion Nanocrystals for Improved FRET Coupling Efficiency in Second Near-Infrared-Triggered Photodynamic Therapy

The precise control of the topological morphology and hierarchical composition is essential for the specific design of multifunctional lanthanide-doped nanocrystals (NCs). This control also enhances our understanding of the epitaxial growth mechanism in core–shell nanostructures, particularly in the case of NaErF4-based NCs, which exhibit minimal detectable upconversion emission without an epitaxial shell. Herein, we established a precise regulation strategy for the fabrication of disk- and rod-shaped core–shell NCs, catering to the growing interest in anisotropic core–shell NCs and aiming to improve Förster resonance energy transfer (FRET) efficiency for photodynamic therapy (PDT) applications. We synthesized NaErF4@NaREF4 NCs (RE = Y, Lu) with distinct topological morphologies by deliberately controlling the Ostwald ripening process. Furthermore, we observed that sacrifice NCs obtained through varying treatment times had diverse ion and oleate ligand release rates. These differences in the ripening reaction pathways were responsible for the ultimate variation in topological morphologies. Steady-state and dynamic luminescence studies revealed that the ligand-free rod-shaped NCs exhibited more robust upconversion emissions, and higher FRET coupling efficiency (54.0%) with photosensitizers, resulting in more generation efficiency of singlet oxygen. These findings suggest promising applications in second near-infrared-triggered PDT.