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Andrew C Hillier

Publications and source records attributed to Andrew C Hillier.

6 recordsLinked to original sources
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↗
PubMed2006

Grating-coupler assisted infrared reflection absorption spectroscopy for the characterization of organic thin films.

We demonstrate how grating-coupler assisted infrared reflection absorption spectroscopy can be used to simultaneously determine the chemical identity and relative thickness of organic thin films. With a grating substrate, a threshold anomaly associated with passing off of the -1 diffracted order occurs at grazing angles of incidence, resulting in a sharp absorbance in the infrared. The position of this peak is sensitive to the grating geometry as well as the dielectric environment near its surface. Thus, shifts in the peak position can be used to determine the relative thickness of adsorbed films or quantify molecular adsorption events. To illustrate the characteristics and sensitivity of this phenomenon, several samples were prepared and tested, including self-assembled alkanethiolate monolayers with 11-mercaptoundecanoic acid, 11-mercapto-1-undecanol, decanethiol, and a covalently linked layer of bovine serum albumin on a commercial, gold-coated grating. For these samples, the position of the threshold absorbance peak shifted to lower wavenumbers as film thickness increased, which is consistent with calculated shifts based upon an increasing refractive index at the interface. The sensitivity of this shift was measured to be 3.7 cm(-1) nm(-1). These results illustrate how a grating substrate can be exploited in a standard infrared reflectance measurement to provide additional information about the relative thickness of adsorbed surface films.

Journal Article↗
PubMed2006

Surface plasmon resonance imaging of biomolecular interactions on a grating-based sensor array.

A surface plasmon resonance sensor array based upon a grating substrate was developed for the detection of biomolecular interactions. The substrate consisted of a gold grating prepared by wet chemical treatment of a commercial recordable compact disk. A custom-built floating pin microspotter was constructed to deliver solutions containing omega-functionalized linear alkanethiols to the grating surface and produce an array of sensor elements with different exposed functional end groups. This array platform can be used to study biomolecular interactions in a label-free, sensitive, and high-throughput format. To illustrate the performance of this device, a test protein (bovine serum albumin) was exposed to sensor elements containing an array of functionalized alkanethiols possessing either activated carboxylic acid-, amine-, or hydroxyl-terminated regions. Local changes in plasmon resonance were monitored in a fixed-angle imaging configuration. Plasmon images clearly distinguish the degree of protein attachment at the various surfaces. The molecular binding events on the grating were also confirmed by ellipsometry. This grating-based SPR imaging platform represents a simple and robust method for performing label-free, high-sensitivity, and high-throughput detection of biomolecular interactions.

Electrodes↗
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↗