A hetero-MOF-based bifunctional ratiometric fluorescent sensor for pH and water detection.

The detection of pH and water is of significance in natural and production processes. The ionothermal reactions of 4,4'-oxybisbenzoic acid (HOBA) with Eu and Tb produced [M(OBA)(HO)Cl] (M = Eu (1) and Tb (2)) and heteronuclear [EuTb(OBA)(HO)Cl] (3). 3 emits the combined characteristic transitions of Eu and Tb. Its energy transition processes can be disturbed by pH and water. The emission colors of 3 follow pH changes. It emits blue light when pH = 1-2, in which OBA turns into HOBA based on the acid-base equilibrium. When pH = 3-11, the emission colors change from green to orange and I/I exhibits a linearity of I/I = 6.7482-0.5971·pH; the emissions are quenched at pH = 12-13, due to OH destroying the delocalized conjugated system of 3. 3 shows a fluorescence response to water with a linearity of I/I = 0.30353 + 0.15042·V% within 0-0.8% V%. The green (G) and red (R) color intensities of the paper-based MOF sensor of 3 reveal a trinomial fitting equation of G/R = 4.16334 - 1.23014·pH + 0.14036·pH- 0.00551·pH when pH = 3-11. 3 can be used as a ratiometric fluorescent sensor to detect pH and water and the paper-based MOF sensor can also be used in on-site pH detection.