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

ABSTRACT 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 Zr ⁴⁺ on the highly active [001] facet of LiREF ₄ (RE = Yb ³⁺ ), 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 Zr ⁴⁺ 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.