A hydrazone-linked covalent organic framework as a dual-mode colorimetric and fluorescence pH sensor

Covalent organic frameworks (COFs) have emerged as a class of crystalline porous materials with stable structures and tuneable functionalities, attracting great interest in recent years. Benefiting from regular pore structure, adjustable functional unit, and excellent photophysical properties, the fluorescent COFs exhibit promising prospects for sensing application. In this study, a hydrazone-linked COF (MOH-Tf12Tf21) was synthesized by Schiff-base reaction of 2-hydroxy-1,3,5-benzenetricarbaldehyde (Tf1), 2,4-dihydroxy-1,3,5-benzenetricarbaldehyde (Tf2) and 2,5-dimethoxybenzene-1,4-dicarbohydrazide (MOH) under solvothermal conditions. The resulting MOH-Tf12Tf21 exhibits high crystallinity, excellent porosity, and good thermal and chemical stability. N2 adsorption tests assessed the surface area, further determined by the BET with a value of 507 m2 g−1. The MOH-Tf12Tf21 could also be found to be responsive to the pH values by fluorometry and colorimetry. The results suggested that MOH-Tf12Tf21 should be suitable as a sensitive, visual, and reusable fluorescence pH sensor. Moreover, the fluorescence intensity of MOH-Tf12Tf21 showed an excellent linear relationship with pH values in the range of 1.0–6.0. The nonlinear relationship between the fluorescence intensity and high pH values (7.0–14.0) was well established. Theoretical calculations were further performed to illustrate the remarkable color change and fluorescence turn-on/off response upon MOH-Tf12Tf21 interaction with different pH values. This work should provide new opportunities for developing COFs-based sensing platforms.