Electroanalytical determination of caffeic acid – Factors controlling the oxidation reaction in the case of PEDOT-modified electrodes

Abstract

The present studies focus on the intrinsic role of poly(3,4-ethylenedioxythiophene) (PEDOT) in electrooxidation of caffeic acid (CA) in view of the application of this electrode material in electroanalytical sensing. Electrosynthesized PEDOT layers with different thickness and different doping counterions are investigated. Based on the scan rate dependence of the voltammetric peak currents it is found that depending on the thickness of the polymer layer different factors, i.e. adsorption for thin layers and diffusion for thick layers, are rate determining. In terms of electroanalytical characteristics it is established that adsorption control provides the opportunity to obtain higher electroanalytical sensitivity in a narrow concentration range of linear response. Diffusion control results in markedly lower sensitivity values but extended range of linearity in the concentration dependence. In the case of adsorption control a hyperbolic equation of the Langmuir/Michaelis–Menten type is suggested to model the experimentally obtained non-linear concentration dependence of the peak currents. The use of a non-linear calibration curve provides an extended concentration range for electroanalytical work under the established adsorption controlled conditions. Doping counterions (polysterene sulfonate and dodecyl sulfate) are found to have no effect on the electroanalytical performance of PEDOT for CA oxidation.