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 {[(CH 3 ) 2 NH 2 ] 2 [Eu 2 (NDDP) 2 (H 2 O) 2 ]·H 2 O} n ( Eu-MOF , H 4 NDDP = 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 [(CH 3 ) 2 NH 2 ] + 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.