Electroactive Covalent Organic Frameworks/Carbon Nanotubes Composites for Electrochemical Sensing

An electroactive covalent organic framework (COFThi–TFPB) was prepared by a dehydration condensation reaction between 1,3,5-tris(p-formylphenyl)benzene (TFPB) and thionine (Thi) and wound with carbon nanotubes (CNTs) to design COFThi–TFPB–CNT composites for electrochemical sensing of ascorbic acid (AA) and pH. COFThi–TFPB was a highly ordered two-dimensional (2D) crystalline nanosheet, and its thickness was 0.75 nm. The loading of CNTs onto COFThi–TFPB nanosheets could significantly improve the dispersibility, stability, and catalytic activity of the CNTs. Here CNTs could well catalyze the oxidation of AA, while COFThi–TFPB had a pair of redox peaks that were inert for AA. Accordingly, a ratiometric electrochemical AA sensor was constructed by using the peak of COFThi–TFPB as the reference signal. The ratiometric AA sensor showed a detection limit of 17.68 μM and linear ranges of 53.04 μM to 4.00 mM and 4.00–8.00 mM, revealing a good performance, together with satisfactory selectivity, reproducibility, and stability. The redox peak potential of COFThi–TFPB was linearly shifted with the pH, so a pH sensor was constructed. The linear range and sensitivity of the pH sensor were 1–12 and 54 mV pH–1, respectively. The electroactive COFThi–TFPB–CNT might also be applied to construct other electrochemical sensors.