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

Abstract

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). High affinity of Zr4+ ion to phosphate species improves sensing selectivity. The surface -NH2 reacts 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. Hydrogen bond formed between hydroxyl group in Tp and imine nitrogen realizes excited state intramolecular proton transfer, so multi-emission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470 and 613 nm for UiO@COF1 and 370, 470, and 572 nm for UiO@COF2. When added phosphate ion 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 keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multi-emission, the affinity of Zr4+ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multi-emission, affinity, and structural selectivity to improve sensing performance for differentiation and ratiometric detection.