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 (H2OBA) with Eu3+ and Tb3+ produced [M(OBA)(H2O)Cl] (M = Eu3+ (1) and Tb3+ (2)) and heteronuclear [Eu0.05Tb0.95(OBA)(H2O)Cl] (3). 3 emits the combined characteristic transitions of Eu3+ and Tb3+. 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 OBA2- turns into H2OBA based on the acid-base equilibrium. When pH = 3-11, the emission colors change from green to orange and ITb/IEu exhibits a linearity of ITb/IEu = 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 ITb/IEu = 0.30353 + 0.15042·VH2O% within 0-0.8% VH2O%. 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·pH2- 0.00551·pH3 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.