Electrooxidation of quercetin at glassy carbon electrode studied by a.c. impedance spectroscopy

The present paper reports cyclic voltammetry and a.c. impedance spectroscopy studies on adsorption and electrooxidation of quercetin (3,3′,4′,5,7-pentahydroxyflavone) compound at glassy carbon electrode surface in 0.1 M sodium acetate–acetic acid buffer in 90% methanol solution. The resulted information provided support for a cascade electrooxidation mechanism, which process commences with oxidation of catechol hydroxyl groups and involves strongly adsorbed reaction intermediate. The significance of each oxidation step is explained through associated charge-transfer resistance (derived for all individual oxidation steps and electrosorption of quercetin) and capacitance parameters. This work also presents an original way to regenerate the surface of glassy carbon electrode (after being blocked by quercetin oxidation products) through voltammetric cycling over the potential range negative to the hydrogen reversible potential. The above is realized by means of in situ evolved hydrogen, which species is capable of electrochemically reducing products formed during the cascade electrooxidation reaction of quercetin.