Study Of Electroanalysis And Electro-oxidation Mechanism Of Quercetin

Quercetin, one of flavanoids, is abundant in vegetables, fruits and other plants. Owing to its strong antioxidant properties, quercetin can scavenge superfluous superoxide free radicals in human body, preventing DNA and cells from oxidative damage. It shows pharmacological effects in anti-tumour, anti-inflammation and anti-senility, etc. Quercetin has five hytroxyl groups which all present electro-activity. So far little attention has been paid to the complex electro-oxidation mechanism, which therefore remains unsolved. The present work studied the electrochemistry behaviors of quercetin on glassy carbon electrode (GCE), paraffin-graphite powder electrode (PGPE) and carbon-nanotubes paste electrode (CNTPE), respectively, in Britton-Robinson buffer solutions involving 0.5 mol·L-1 KCl as supporting electrolyte, using cyclic voltammetry and differential pulse voltammetry and in situ thin layer spectroelectrochemistry.Results indicate that there are great differences in both absorption and electrocatalysis activity among the three electrodes. The most CV peaks with the highest currents were obtained on CNTPE, indicating that carbon nanotubes have extremely strong absorption and electrocatalysis activity to quercetin. The best stability of data was shown on PGPE. Differential pulse voltammetry indicates that there is a good linear relationship between the main oxidation peak current of quercetin on PGPE and the concentration over the range 2.0 × 10-8 - 4.0× 10-6 mol·L-1, therefore the electrode PGPE can be used for electroanalysis of quercetin.Cyclic voltammetry and in situ spectroelectrochemistry showed a very complex electro-oxidation mechanism of quercetin, involving all the five hydroxyl groups with electro-activity. All the redox reactions were proton participated. The oxidation of the 3'4'-dihydroxyl electron-donating groups occurred first at the lowest positive potentials, producing quercetin o-quinone via a two electron two proton reversible reaction. This o-quinone could isomerize to p-quinone methylene compound at low scan rates. The C3 hydroxyl group at ring C oxidized next was shown to undergo an irreversible oxidation reaction. This oxidation became the main oxidation peak during thefollowing cyclic potential scans, due to an especial product-absorption mechanism. The other two hydroxyl groups at ring A can be oxidized at more positive potentials with poor reversibility. Due to the strong influence of the electrode potential over a wide potential range, the adsorption and desorption of both quercetin and the oxidation products occurred periodically with the repeated cyclic potential scans. The final oxidation products which absorbed onto the electrode surface during positive scans blocked the electrode surface, hindering the continuing reaction of quercetin from the solution. Some interesting CV behaviors were observed, which can be attributed to the multi-groups with electro-activity in quercetin molecule. The electro-oxidation mechanism of quercetin has been discussed detailedly in this paper, based on the abundant kinetic information from the in situ spectro-electrochemistry measurements.