Electroanalysis And Cyclic Voltabsorptometry Of Flavonoids On Carbon Paste Electrodes

Flavonoids are widely distributed in plants, fruits, and vegetables and are important constituents of the human diet. The oxidation of flavonoids is of great interest because of their action as antioxidants with the ability to scavenge superfluous superoxide free radicals in human body. This project began with the electroanalysis of the flavonoid quercetin and rutin using a carbon nanotubes-nujol paste electrode (CNTPE), followed by the major study in which the redox processes of quercetin and rutin were investigated by cyclic voltabsorptometry (CVA) on a graphite-wax paste electrode (WGE) using a self-designed UV-Vis long path-length thin-layer spectroelectrochemical cell.As compared with graphite-nujol paste electrode or glass carbon electrode, CNTPE displayed great power to adsorb quercetin and rutin, hence obviously increased their redox peak currents of differential pulse voltammetry (DPV). The order of magnitude of the detection limits (s/n = 3) could reach to 10^-8 mol L^-1 , indicating that carbon nanotube is a potential excellent electrode material for the adsorptive-stripping determination of quercetin and rutin. A reversing differential pulse voltammetry (RDPV) with preconcentration step was developed for simultaneous determination of quercetin and rutin at CNTPE, in which the anodic DPV peak of quercetin and the reversing cathodic peak of rutin were selected as the analytical singals for simultaneous determination of them in a mixture. The applicability of the method to real sample analysis was evaluated.For the purpose of investigating the redox mechanisms, cyclic voltabsorptiograms (CVAs) were recorded in different potential ranges for quercetin and rutin, respectively, at characteristic wavelengths of species. The derivative cyclic voltabsorptiograms (DCVAs) were analysed for reconstruction of cyclic voltammograms (CVs) of these species appeared in the solution during electrolysis using self-developed computer programs. The reconstructed 'CVs' correspond to the variation of their concentrations in the thin-layer chamber provided more detailed information on the redox system under investigation. Based on these, the redox mechanisms of rutin and quercetin in different potentials were discussed, and conclusions can be drawn as follows:(1) The first oxidation step of rutin at E_m of 0.58 V vs Ag/AgCl(sat. KCl) leads to the formation of o-quinonic structure at catechol moiety via an adsorption-controlled 2-electrons and 2-protons reaction, in which the total current is mostly contributed from the adsorbed species. The contributions of both the species in solution and those pre-adsorbed on the electrode surface to the total redox currents can be estimated based on the optical absorption signals for the first time.(2) The second oxidation of rutin occurs at much higher potentials (E_p = 1.1 V), accompanying an O₂-evolution reacton. However, no CV peak was obtained from the reconstruction, which indicates that no species in solution were involved in the second reaction of rutin. Thus, it should be a reaction of adsorbed species, and most probably correspondes to the transformation of 7-OH group at the A ring into 7-position oxygen free radical. This radical can possible be an excellent catalyst for the O₂-generation, and can be further oxidized at more positive potentials leading to the C-ring opening. The later may also undergo o-dicarbonyl reduction if the potential scan was reversed, especially at high scan rates.(3) The first oxidation step of quercetin (E_m = 0.48 V) follows an EC mechanism: quercetin-o-quinoid was electrogenerated via a 2-electrons and 2-protons step, followed by a homogeneous transition of o-quinoid to the corresponding p-quinoid. The total oxidation current is mostly contributed from the species in solution at slow scan rates, but it is dominated by the pre-adsorbed species under faster scan (≥50 mV s^-1). The reaction rate of the chemical step was estimated from the optical absorption singals of the final product (p-quinoid) for the first time, which was found to be in the order of magnitude of 10^-6 mol L^-1 s^-1.(4) The succedent oxidations of quercetin presented multi anodic CV peaks with no reconstructed CV peaks at corresponding potentials for quercetin and its products in solution. This indicates that the succedent oxidation of quercetin almostly involves only the species adsorbed on the electrode surface. In this case the CVA method in reflected absorbtion mode is expected to be more effective.The present work demonstrated that our developed long path-length thin-layer CVA method in combination with the DCVA reconstruction program is a very useful technique for the study of complex electrochemical reactions coupled with surface adsorption and chemical steps.