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 diminishes³. 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.

上转换纳米晶体已被广泛研究用于光学成像和生物医学应用^1,2。然而，随着纳米晶体尺寸的减小，它们的光致发光会因表面淬火而大大减弱³。尽管付出了相当大的努力^4,5，淬火机制仍然知之甚少。在这里，我们报告了二齿吡啶甲酸分子与 NaGdF ₄的表面配位：Yb/Tm 纳米颗粒将四光子上转换提高了 11,000 倍。机理研究表明，表面配体配位重建了轨道杂交和晶体场分裂，最大限度地减少了表面和内部镧系元素敏化剂的 4 f轨道之间的能量差异。4楼-轨道能量共振促进镱亚晶格内的能量迁移，阻止能量扩散到表面缺陷并最终增强能量到发射器的传输。此外，配体配位可以进行能级重建，配体-敏化剂分离超过 2 nm。这些发现为高发射纳米杂化物的发展提供了见解，并为在单粒子水平上构建光学询问系统提供了一个平台。