Showing papers by "Ana Maria Oliveira Brett published in 2003"

Electrochemical Oxidation of Quercetin

Abstract: The mechanism of electrochemical oxidation of quercetin on a glassy carbon electrode has been studied using cyclic, differential pulse and square-wave voltammetry at different pH. It proceeds in a cascade mechanism, related with the two catechol hydroxyl groups and the other three hydroxyl groups which all present electroactivity, and the oxidation is pH dependent. Quercetin also adsorbs strongly on the electrode surface; and the final oxidation product is not electroactive and blocks the electrode surface. The oxidation of the catechol 3,4-dihydroxyl electron-donating groups, occurs first, at very low positive potentials, and is a two electron two proton reversible reaction. The hydroxyl group oxidized next was shown to undergo an irreversible oxidation reaction, and this hydroxyl group can form a intermolecular hydrogen bond with the neighboring oxygen. The other two hydroxyl groups also have an electron donating effect and their oxidation is reversible.

Atomic Force Microscopy of DNA Immobilized onto a Highly Oriented Pyrolytic Graphite Electrode Surface

Abstract: Magnetic AC mode atomic force microscopy (MAC Mode AFM) was used to characterize the process of adsorption of DNA on a highly oriented pyrolytic graphite (HOPG) electrode surface using different concentrations of DNA and adsorption procedures. AFM of DNA immobilized on the HOPG showed that both single-stranded DNA and double-stranded DNA molecules have the tendency to spontaneously self-assemble from solution onto the solid support and the process was very fast. DNA condensed on the substrate in a tight and well-spread two-dimensional lattice covering the entire surface uniformly. The interaction of DNA with the hydrophobic HOPG surface induced DNA superposition, overlapping, and intra- and intermolecular interactions. The application of a positive potential of 300 mV (vs Ag wire) to the HOPG electrode during adsorption was studied. The applied potential considerably enhanced the robustness and stability to mechanical stress of the DNA films, through multiple electrostatic interactions between the negatively charged hydrophilic sugar-phosphate backbone and the positively charged carbon surface. The characteristics of the DNA films and the apparent height of the network wires were dependent on the DNA concentration and the immobilization procedure. The DNA lattices were held together on the substrate surface only by noncovalent interactions such as hydrogen bonding, base stacking, electrostatic, van der Waals, and hydrophobic interactions.

Studies on Self-Assembled Alkanethiol Monolayers Formed at Applied Potential on Polycrystalline Gold Electrodes

Abstract: 1-Dodecanethiol assembly on polycrystalline gold electrodes at fixed positive potentials has been investigated by chronoamperometry and electrochemical quartz crystal microbalance and the films formed characterized by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that 1-dodecanethiol adsorption on gold is enhanced by application of positive potentials to the electrode surface and that adsorption proceeds faster than in the case of open circuit deposition. Compact defect-free monolayers of capacitance values of 1.1–1.6 μF cm−2 are produced in time intervals as short as 100 s, with no roughness, as demonstrated for the first time by electrochemical impedance analysis. Control of surface potential during alkanethiol assembly appears to improve monolayer quality and to allow for shorter assembly periods. Monolayers can be removed by cycling in alkaline solution or in dilute sulfuric acid. These results are important for the fast construction of defect-free bilayers.

Step and Pulse Techniques

Abstract: The sections in this article are Introduction Potential Step Single Potential Step Double Potential Step Potential Step Chronocoulometry Staircase Voltammetry Pulse Techniques Normal Pulse Voltammetry (NPV) Differential Pulse Voltammetry (DPV) Square Wave Voltammetry (SWV) Other Pulse Techniques Comparison between Potential Pulse Techniques Current Step and Chronopotentiometry Variation of Potential with Time Current Steps and Potentiometric Stripping Analysis Concluding Remarks