Poly(phenol red)-based voltammetric sensor for the simultaneous quantification of hydroxycinnamic acids

Hydroxycinnamic acids are one of the most widely distributed class of natural phenolics in plants. Their coexistence requires selective methods for quantification. Voltammetry on chemically modified electrodes is one of the approaches to solve this problem. Electrodes based on the electropolymerized triphenylmethane dyes have shown sensitive and selective response to natural phenolic antioxidants of different classes. In this work, combination of functionalized single-walled carbon nanotubes and poly(phenol red) has been used as electrode surface modifier. The polymeric coverage has been obtained by potentiodynamic electropolymerization which conditions have been optimized on the basis of voltammetric response of hydroxycinnamic acids mixture. Poly(phenol red)-based electrode provides well-resolved peaks of caffeic, ferulic, and p-coumaric acids and statistically significant increase of the oxidation currents in comparison to bare glassy carbon and polyaminobenzene sulfonic acid functionalized single-walled carbon nanotubes modified electrodes. Electrooxidation of hydroxycinnamic acids is diffusion controlled process and involves proton transfer. Simultaneous voltammetric quantification of caffeic, ferulic, and p-coumaric acids has been performed for the first time. Two linear dynamic ranges of 0.10-2.5 µM for all acids and 2.5-100 µM for caffeic acid and 2.5-50 µM for ferulic and p-coumaric acids have been achieved using differential pulse voltammetry in acidic medium (Britton-Robinson buffer pH 2.0). The limits of detection are 47.6, 22.4, and 38.0 nM for the caffeic, ferulic, and p-coumaric acids, respectively. Then method developed has been successfully applied for the quantification of hydroxycinnamic acids in coffee.