Anomalous upconversion amplification induced by surface reconstruction in lanthanide sublattices

Upconversion nanocrystals have been extensively investigated for optical imaging and biomedical applications(1,2). However, their photoluminescence is strongly attenuated by surface quenching as the nanocrystal size diminishes(3). Despite considerable efforts(4,5), 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.

Automated summary

The coordination of bidentate picolinic acid molecules on the surface of nanoparticles can significantly enhance four-photon upconversion, providing new insights into the development of highly emissive nanohybrids. By modulating surface coordination, the reconstruction of orbital energy levels and crystal-field splitting can enhance energy transfer efficiency, offering a new platform for optical interrogation systems at the single-particle level.