Anomalous upconversion amplification induced by surface reconstruction in lanthanide sublattices

Upconversion nanocrystals have been extensively investigated for optical imaging and biomedical applications1,2. However, their photoluminescence is strongly attenuated by surface quenching as the nanocrystal size diminishes3. Despite considerable efforts4,5, the quenching mechanism remains poorly understood. Here we report that surface coordination of bidentate picolinic acid molecules to NaGdF4:Yb/Tm nanoparticles enhances four-photon upconversion by 11,000-fold. Mechanistic studies indicate that surface ligand coordination reconstructs orbital hybridization and crystal-field splitting, minimizing the energy difference between the 4f orbitals of surface and inner lanthanide sensitizers. The 4f-orbital energy resonance facilitates energy migration within the ytterbium sublattice, impeding energy diffusion to surface defects and ultimately enhancing energy transfer to the emitters. Moreover, ligand coordination can exert energy-level reconstruction with a ligand–sensitizer separation of over 2 nm. These findings offer insights into the development of highly emissive nanohybrids and provide a platform for constructing optical interrogation systems at single-particle levels. Strong lanthanide-doped upconversion luminescence enhancement is achieved by the use of surface molecules which enhance four-photon upconversion emission. The results may lead to new, highly emissive, nanohybrid systems.