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 [Eu_0.05Tb_0.95(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 OBA^2- turns into H₂OBA based on the acid-base equilibrium. When pH = 3-11, the emission colors change from green to orange and I_Tb/I_Eu exhibits a linearity of I_Tb/I_Eu = 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_Tb/I_Eu = 0.30353 + 0.15042·V_H% within 0-0.8% V_H%. 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.