MOF@COFs with Strong Multiemission for Differentiation and Ratiometric Fluorescence Detection

Aggregation-caused quenching (ACQ) is often observed in covalent organic frameworks (COFs) for their low emission. Here, we propose that limited COF layers form on UiO-66 to eliminate the ACQ by the formation of UiO@COF composites. UiO-66 is selected because this metal-organic framework (MOF) is easily prepared in nanosize with Zr⁴⁺ ion and 2-aminoterephthalic acid (BDC-NH₂). The high affinity of the Zr⁴⁺ ion to phosphate species improves sensing selectivity. The surface -NH₂ reacts with 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. The hydrogen bond formed between the hydroxyl group in Tp and imine nitrogen realizes excited-state intramolecular proton transfer; therefore, multiemission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470, and 613 nm for UiO@COF1 and at 370, 470, and 572 nm for UiO@COF2. When phosphate ion is added in the composites, the emissions from the COFs keep stable, while that from UiO-66 is enhanced. However, adenosine-5'-triphosphate (ATP) improves the emissions from UiO-66 and COF's enol state, but that from the keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multiemission, the affinity of Zr⁴⁺ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multiemission, affinity, and structural selectivity to improve the sensing performance for differentiation and ratiometric detection.