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}<i>_n</i> (<b>Eu-MOF</b>, H₄BTA-2OH = 2,3,5,6-tetrakis(<i>p</i>-carboxyphenyl)hydroquinone), is prepared and structurally characterized. The <b>Eu-MOF</b> is characterized by a well-defined structure featuring a two-dimensional channel along the <i>c</i>-axis. <b>Eu-MOF</b> can be converted into <b>Eu-MOF-Ox</b> through the oxidation of hydroquinone to benzoquinone, which can maintain structural stability and enhance fluorescence emission. Compared with <b>Eu-MOF</b>, <b>Eu-MOF-Ox</b> 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 <b>Eu-MOF</b>s have been constructed to achieve rapid on-site quantification and testing. Moreover, <b>Eu-MOF</b> displays a more outstanding proton conductivity value of 1.16 × 10^-2 S cm^-1 than <b>Eu-MOF-Ox</b> (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 <b>Eu-MOF</b> is a potential bifunctional material for fluorescence sensing and proton conduction.