An efficient and sensitive fluorescent pH sensor based on amino functional metal–organic frameworks in aqueous environment

Abstract

A pH sensor is fabricated via a reaction between an Al^III salt and 2-aminoterephthalic acid in DMF which leads to a MOF (Al-MIL-101-NH₂) with free amino groups. The Al-MIL-101-NH₂ samples show good luminescence and an intact structure in aqueous solutions with pH ranging from 4.0 to 7.7. Given its exceptional stability and pH-dependent fluorescence intensity, Al-MIL-101-NH₂ has been applied to fluorescent pH sensing. Significantly, in the whole experimental pH range (4.0–7.7), the fluorescence intensity almost increases with increasing pH (R² = 0.99688) which can be rationalized using a linear equation: I = 2.33 pH + 26.04. In addition, error analysis and cycling experiments have demonstrated the accuracy and utilizability of the sensor. In practical applications (PBS and lake water), Al-MIL-101-NH₂ also manifests its analytical efficiency in pH sensing. And the samples can be easily isolated from an aqueous solution by incorporating Fe₃O₄ nanoparticles. Moreover, the possible sensing mechanism based on amino protonation is discussed in detail. This work is on of the few cases for integrated pH sensing systems in aqueous solution based on luminescent MOFs.