Solvent- and pH-Stable Eu(III)-Based Metal–Organic Framework with Phosphate-Ratio Fluorescence Sensing and Significant Proton Conduction

Lanthanide-based metal–organic frameworks show good potential for applications due to their unique structures and functional properties. A highly thermally and acid–base stable Eu-MOF was synthesized by a solvothermal method with the molecular formula {[(CH₃)₂NH₂]₂[Eu₂(NDDP)₂(H₂O)₂]·H₂O}_n (Eu-MOF, H₄NDDP = 5,5′-(naphthalene-2,6-diyl)diisophthalic acid). Eu-MOF takes a three-dimensional (4,4,8)-connected topology. The water molecules involved in the coordination, free water molecules, and [(CH₃)₂NH₂]⁺ cations in the pore can be used as proton carriers. The proton conductivity attains 1.25 × 10^–4 S cm^–1 at room temperature and 2.42 × 10^–3 S cm^–1 at 70 °C and 98% relative humidity. Combined with the dual-emission properties from the ligands and Eu(III) ions enables Eu-MOF to be used as a ratiometric fluorescent sensor for phosphate efficiently and rapidly, with a limit of detection of 0.12 μM in the Tris–HCl buffer solution. These results provide a new approach for the construction of MOFs with high proton conductivity and a ratiometric fluorescence response.