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 (H(2)OBA) 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 OBA(2-) turns into H(2)OBA 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 center dot 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.pH(2) - 0.00551.pH(3) 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.

Automated summary

This study investigated the ionothermal reactions of Eu3+ and Tb3+ with 4,4'-oxybisbenzoic acid, and their luminescent properties under different pH and water content. The emission colors and intensities of the produced compounds were analyzed for pH and water detection purposes.