Europium doping promoted intermolecular charge transfer in UiO-67-CDC for ultrasensitive turn-on colorimetric detection of an anthrax biomarker

Abstract

A post-synthetic modification was employed to incorporate lanthanide Eu³⁺ ions into carbazole-functionalized UiO-67 metal–organic frameworks, successfully fabricating an efficient dual-emission fluorescence probe. The Eu³⁺-doped composite manifests superior luminescent properties and remarkable fluorescence stability, attributable to the sensitization and reinforcement afforded by the parent framework. Notably, the ligand's capability to capture external light and sensitize Eu³⁺ emissions led to observing characteristic peaks at 435 nm for the intrinsic ligand and 591, 614, 651, and 701 nm for Eu³⁺. The introduction of dipicolinic acid (DPA) serves a dual role: it acts as an antenna to absorb photons and replaces the coordinated water molecules of Eu³⁺, thereby preventing fluorescence quenching by water's O–H vibrations. Additionally, establishing hydrogen bonding between DPA and the carbazole dicarboxylic acid (CDC) initiates an intermolecular charge transfer (ICT) process, enhancing the ligand's fluorescence. Besides, the addition of DPA concurrently enhances the fluorescence emissions of both the ligand and Eu³⁺, displaying an unusual dual-enhanced “turn-on” fluorescence mode with an impressively low detection limit of 0.538 μM, accompanied by discernible color changes visible to the naked eye. Utilizing Eu@UiO-67-CDC as a test paper facilitates rapid on-site detection of DPA, eliminating the need for complex instrumentation. Moreover, the Eu@UiO-67-CDC probe can quantify DPA in environmental matrices such as river and lake water, with better recovery rates, indicating its significant potential for practical applications.