Resveratrol (3,5,4'-trihydroxystilbene) is an organic metabolite produced by plants in response to fungal infection. It is found in various plant fruits and is abundant in the skins of unripe grapes and related products. This photosensitive molecule exists in two isomeric forms, trans and cis-resveratrol. The antioxidant activity of resveratrol, both trans and cis forms, was evaluated by means of cyclic, differential pulse and square-wave voltammetry over a wide pH range, using a glassy carbon electrode. Voltammograms of resveratrol presented two oxidation peaks; the first oxidation peak corresponds to the oxidation of the phenol group and was shown to undergo an irreversible oxidation reaction. The second oxidation peak corresponds to the oxidation of the resorcinol moiety and is also irreversible. The influence of pH on the electrochemical oxidation process of resveratrol was investigated.

Palladium nanoparticles and nanowires electrochemically deposited onto a carbon surface were studied using cyclic voltammetry, impedance spectroscopy and atomic force microscopy. The ex situ and in situ atomic force microscopy (AFM) topographic images showed that nanoparticles and nanowires of palladium were preferentially electrodeposited to surface defects on the highly oriented pyrolytic graphite surface and enabled the determination of the Pd nanostructure dimensions on the order of 50-150 nm. The palladium nanoparticles and nanowires electrochemically deposited onto a glassy carbon surface behave differently with respect to the pH of the electrolyte buffer solution. In acid or mild acid solutions under applied negative potential, hydrogen can be adsorbed/absorbed onto/into the palladium lattice. By controlling the applied negative potential, different quantities of hydrogen can be incorporated, and this process was followed, analysing the oxidation peak of hydrogen. It is also shown that the growth of the Pd oxide layer begins at negative potentials with the formation of a pre-monolayer oxide film, at a potential well before the hydrogen evolution region. At positive potentials, Pd(0) nanoparticles undergo oxidation, and the formation of a mixed oxide layer was observed, which can act as nucleation points for Pd metal growth, increasing the metal electrode surface coverage. Depending on thickness and composition, this oxide layer can be reversibly reduced. AFM images confirmed that the PdO and PdO 2 oxides formed on the surface may act as nucleation points for Pd metal growth, increasing the metal electrode surface coverage.

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The electrochemical behaviour of the flavone chrysin and the dihydroflavonol (AE)-taxifolin was investigated by different electrochemical techniques at different pH values. Dihydroflavonol (AE)-taxifolin presented two oxidation peaks. The first electrooxidation reaction of (AE)-taxifolin is reversible over the whole pH range. The dependence of peak current on pH showed a maximum around neutral pH values with a decrease in acidic and alkaline media. The influence of the catechol group deprotonation on ring B is related to the electron/proton donating capacity in (AE)-taxifolin and to its radical scavenging antioxidant activity. In chrysin, where the ring B has an unsubstituted phenyl, the antioxidant active moiety is in ring A. Chrysin presents an irreversible oxidation peak at higher oxidation potentials than those usually obtained for the catechol moiety ion in ring B and a pH dependent behaviour. The ÀOH groups on ring A are affected by the 4-keto group as shown by the lower pK a value of the 7-OH, indicating that the 7-OH group is the more acidic. The 5-OH group is less acid and may be explained by the acidic weakening effect of an intramolecular H-bond between the 5-OH and 4-keto group of taxifolin and chrysin.

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