Post-modification amino-functionalized metal–organic framework for construction ratiometric fluorescent sensors with dual emissions

The most promising chemical sensing application on metal–organic frameworks (MOFs) is to construct the MOFs-based ratiometric fluorescent sensors due to their self-calibration against background signal interferences to afford the high accuracy and reliability of the sensing detection. In order to realize the precise linearity control over photophysical characteristics and sensing sensitivity at the molecular level, a novel amino-functionalized metal–organic framework (Zn-MOF-An) has been designed and synthesized using 5′-(4-aminophenyl)-[1,1′:3′,1′'-terphenyl]-3,3′'-dicarboxylic acid as the ligand, in which –NH2 groups arranged parallel within the pores can react with pyrene-1-carboxylic acid to form ammonium salts, and thus introduce organic luminescent molecules into the frameworks of MOFs (Zn-MOF-An-Py). The fluorescence intensity of Zn-MOF-An remains almost constant, while the emission of pyrene-1-carboxylic acid is selectively quenched by Cu2+ and Cr3+, which enables the ratio fluorescence sensing of Cr3+ and Cu2+ in water. Different blue shift effects of Zn-MOF-An towards Cu2+ and Cr3+ can also help to distinguish them. The linear concentration ranges and detection limits (LODs) of Cr3+ and Cu2+ are 2 ∼ 100 μM and 1.01 μM, 0.5 ∼ 80 μM and 0.32 μM, respectively. The results of PXRD, FT-IR and UV–vis confirm that the detection mechanism can be attributed to the interaction between organic luminescent molecules and metal ions, and the framework of Zn-MOF-An-Py does not collapse before and after detection. The high recovery rate and low standard deviation in real water confirms the reliability and practicability of Zn-MOF-An-Py.