Imparting Biodegradability to Highly‐Efficient Upconversion Nanoparticles via Facet‐Selective Zirconium Doping

Alkali rare-earth fluorides have defined the golden era of upconversion nanoparticles (UCNPs) in biophotonics. Yet, their exceptional structural and chemical stability has raised concerns over long-term bioretention and potential chronic toxicity. Here, we present a strategy that bridges the gap between high-performance photonics and biological safety by simply substituting rare-earth ions with Zr⁴⁺ on the highly active [001] facet of LiREF₄ (RE = Yb³⁺), which induces the formation of a biodegradable domain while preserving the structural integrity and luminescent efficiency of the native lattice. Density functional theory calculations reveal that this domain can stably host Zr⁴⁺ in the form of hydrolytically active zirconium fluoride complexes with coordination numbers ranging from six to eight. The configurational diversity of these complexes contributes to a significantly extended degradation lifetime, with four days in the bare form and over one month when coated with silica via the Stöber process. Our strategy establishes a new design rule for imparting biodegradability to highly efficient upconversion materials with day-scale degradation, thereby paving the way for clinically translatable optical imaging and therapeutic applications.