Selective Detection of Vitamins A and C based on Covalent Organic Framework Modified Electrodes

The engineering and regulation of electrode interfaces are of great significance for accurate analysis of specific analyte in complex samples. Covalent organic frameworks (COF), as a class of crystalline polymers formed precisely and periodically by building blocks, have provoked exponential interest due to their pre-designable and adjustable pore sizes, hydrophobicity, geometry and so on. Therefore, varying the structures of monomers is able to modulate the molecular permeability of COF and achieve selective detection of different analytes. In this work, the polycondensation reaction between the amine monomer, namely 1,3,5-tris(4-aminophenyl) benzene (TAPB), and two different aldehyde monomers, namely terephthalaldehyde (PDA) and 2,5-dibromoterephthalaldehyde (BrPDA), at the liquid/liquid interface was conducted to prepare continuous and uniform imine-linked covalent organic framework (COF) membranes. The membranes were subsequently transferred onto the surface of indium tin oxide (ITO) glass electrodes and then the electrochemical responses of hydrophobic vitamin A (VA) and hydrophilic vitamin C (VC) at two COF modified electrodes were investigated. Due to the existing of Br groups on the pore walls of TAPB-BrPDA COF, it is hydrophobic with a contact angle of 122 degrees, while the TAPB-PDA COF membrane is hydrophilic with a contact angle of 73 degrees. The electrochemical responses of two COF modified electrodes towards hydrophobic VA and hydrophilic VC were studied in 1 mol/L NaCl solution. The results of cyclic voltammetry revealed that TAPB-BrPDA COF could prohibit effectively the permeability of VC whereas allow that of VA, eventually achieving the selective electrochemical detection of the latter with a detection linear range of 5 similar to 100 mu mol/L and a limit of detection at 1.32 mu mol/L. In contrast, TAPB-PDA COF/ITO electrode can reject the access of VA whereas permit that of VC, thus showing good electrochemical selectivity toward VC and yielding a linear detection range of 5 similar to 200 mu mol/L and a limit of detection at 1.35 mu mol/L. Finally, two electrodes were successfully used for the quantitative determination of VA and VC in multivitamin tablets with recoveries of 104% and 101%, respectively. This work constitutes a step in the surface engineering of electrodes by COF to realize pre-designed composition and functions and thus excellent selectivity towards specific molecules.