Multifunctional Covalent Organic Framework Coupled with Sc ^<b>3+</b> -Sensitized Emission for Ratiometric Detection of Enrofloxacin

Rare-earth functionalized covalent organic frameworks (COFs) have played an important role in antibiotic detection. However, the single optical change and fixed sensing mode limit its development. In this work, a COF-50% featuring strong fluorescence emission and abundant chelation sites was designed and synthesized. As an imine-linked COF, COF-50% overcomes fluorescence quenching caused by π-π stacking and bond rotation through aggregation-induced emission and intramolecular hydrogen bonding. With a quantum yield of 10.5%, it serves as a stable built-in fluorescent reference signal. In addition, partial replacement of linkers within the structure further enhanced the coordination ability toward metal ions. Subsequently, a novel ratiometric fluorescent probe (COF-50%-Sc) was constructed by exploiting the fluorescence sensitization of Sc³⁺ on enrofloxacin (ENR). The resulting system enhances sensing performance in water, exhibiting a linear fluorescence response to enrofloxacin with a detection limit of 5.96 nM. Notably, the fluorescence color changes from orange to purple during detection, and a modular color recognition device was further developed for portable, rapid, and real-time quantification of ENR. Overall, this study offers a novel strategy for the synergistic integration of rare-earth ions and COF materials in optical sensing applications.