Erin L May | PubMed Health

PubMed2006

Scanning electrochemical mapping of spatially localized electrochemical reactions induced by surface potential gradients.

The influence of a surface potential gradient on the location and extent of electrochemical reactions was examined using a scanning electrochemical microscope. A linear potential gradient was imposed on the surface of a platinum-coated indium tin oxide electrode by applying two different potential values at the edges of the electrode. The applied potentials were used to control the location and extent of several electrochemical reactions, including the oxidation of Ru(NH3)6(2+), the oxidation of H2, and the oxidation of H2 in the presence of adsorbed CO. Scanning electrochemical mapping of these reactions was achieved by probing the feedback current associated with the oxidation products. The oxidation of Ru(NH3)6(2+) occurred at locations where the applied potential was positive of the formal potential of the Ru(NH3)6(2+/3+) redox couple. The position of this reaction on the surface could be spatially translated by manipulating the terminal potentials. The rate of hydrogen oxidation on the platinum-coated electrode varied spatially in the presence of a potential gradient and correlated with the nature of the electrode surface. High oxidation rates occurred at low potentials, with decreasing rates observed as the potential increased to values where platinum oxides formed. The extent of oxide formation versus position was confirmed with in-situ ellipsometry mapping. In the presence of adsorbed carbon monoxide, a potential gradient created a localized region of high activity for hydrogen oxidation at potentials between where carbon monoxide was adsorbed and platinum oxides formed. The position of this localized region of activity could be readily translated along the surface by changing the terminal potential values. The ability to manipulate electrochemical reactions spatially on a surface has potential application in microscale analytical devices as well as in the discovery and analysis of electrocatalytic systems.

Journal Article↗

PubMed2005

Rapid and reversible generation of a microscale pH gradient using surface electric fields.

We report a method for the rapid and reversible generation of microscale pH gradients using a spatially varied electric field. A linear gradient in electrochemical potential is produced on an electrode surface consisting of a platinum catalyst layer on indium-tin oxide-coated glass by the application of two different potential values at spatially distinct surface locations. The resulting potential gradient drives the oxidation and reduction of water at different rates along the surface, as dictated by the local applied potential. A nonuniform distribution of pH in the neighboring solution results due to the variation in surface reaction rates. The extent and magnitude of the pH gradient can be controlled by the appropriate selection of applied potential values. In addition, the gradient can be rapidly turned on or off and reversibly switched between various profiles. The size of the pH gradient can be readily modified by changing the dimensions of the electrode and contact pads to allow integration into chip-scale devices. Characteristics of the pH gradient are described, including experimental and theoretical evidence of significant improvement in time response over competing methods for the generation of microscale pH gradients.

Electricity↗