Redox-Active Europium-Based MOFs Showing Ratiometric Fluorescence Sensing for the Anthrax Biomarker and Oxytetracycline, and Proton Conduction

Redox-active metal–organic frameworks have received increasing attention for potential applications in conductivity, sensing, catalysis, and magnetism. In this work, a novel redox-active europium-based metal–organic framework, {[(CH3)2NH2][Eu(BTA-2OH)(H2O)2]·2H2O}n (Eu-MOF, H4BTA-2OH = 2,3,5,6-tetrakis(p-carboxyphenyl)hydroquinone), is prepared and structurally characterized. The Eu-MOF is characterized by a well-defined structure featuring a two-dimensional channel along the c-axis. Eu-MOF can be converted into Eu-MOF-Ox through the oxidation of hydroquinone to benzoquinone, which can maintain structural stability and enhance fluorescence emission. Compared with Eu-MOF, Eu-MOF-Ox displays a more sensitive visual identification of 2,6-pyridine dicarboxylic acid (DPA) or oxytetracycline (OTC) as a ratiometric fluorescent sensor with lower detection limits of 0.013 and 0.071 μM, respectively. Fluorescent composite films and smartphone-based readout systems based on Eu-MOFs have been constructed to achieve rapid on-site quantification and testing. Moreover, Eu-MOF displays a more outstanding proton conductivity value of 1.16 × 10–2 S cm–1 than Eu-MOF-Ox (2.01 × 10–5 S cm–1) at 50 °C and 100% RH, indicating its high performance and potential application in the field of proton conduction. Therefore, the Eu-MOF is a potential bifunctional material for fluorescence sensing and proton conduction.