Underlying Electrode Surface Research Articles

Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly (EASA)

Highly ordered and vertically oriented mesoporous silica films can be generated by electro-assisted self-assembly (EASA). The method involves the electrogeneration of hydroxide ions at an electrode surface immersed in an hydrolyzed sol solution (containing typically tetraethoxysilane, TEOS, and cetyltrimethylammonium bromide, CTAB) in order to catalyze polycondensation of the precursors and self-assembly of hexagonally packed one-dimensional channels that grow perpendicularly to the support. Vertically aligned mesostructures have been demonstrated by TEM imaging and by grazing incidence X-ray diffraction (GIXD), this latter technique enabling characterization of thin films directly on their underlying electrode surface. The influence of the electrosynthesis medium composition (precursor and surfactant concentrations, surfactant chain length) on the mesostructural order and film thickness has been thoroughly examined. It was shown that the highly ordered and oriented mesoporous silica films can be obtained...

The influence of substrate effects when investigating new nanoparticle modified electrodes exemplified by the electroanalytical determination of aspirin on NiO nanoparticles supported on graphite

The apparent electrocatalytic detection of aspirin and salicylic acid is compared using NiO nanoparticles and microparticles supported on graphitic electrodes using abrasive and non-abrasive (drop-dry) immobilisation. However control experiments revealed that, the observed voltammetry is not due to the immobilised NiO materials, but is instead due to the underlying graphitic substrates. Abrasive immobilisation of NiO microparticles on a graphite electrode abrades the underlying electrode surface, introducing more electroactive edge-plane defects. Even when drop-dry immobilisation is used (i.e. non-abrasive), appropriate control experiments are still required as other experimental methods employed may change the nature of the underlying substrate.

In situ electropolymerised silica–polyaniline core–shell structures: Electrode modification and enzyme biosensor enhancement

Micro- and nanostructured electrode interfaces have the potential to exhibit improved performance characteristics. Here, we illustrate the interaction of an electropolymerised polyaniline film with silica microparticles on glassy carbon electrodes. It was found that the polyaniline grew preferentially around the silica particles rather than on the underlying electrode surface. This was found to be due to the adsorption of aniline monomers onto the silica surface, followed by in situ electropolymerisation. The thickness of the PANI layer could be controlled through the adjustment of polymerisation time from several to many tens of nanometres. In conjunction with immobilisation of the enzyme, horseradish peroxidase (HRP), the electrocatalytic reduction of hydrogen peroxide was found to be enhanced under certain conditions over polymer films grown in the absence of silica particles. It is believed that this may be due to the special structure of the silica–PANI core–shell particles. Silica particles coated with a nanofilm of PANI appear to adsorb HRP more effectively, while also increasing the active electrode surface area.

Using Scanning Electrochemical Microscopy (SECM) to Measure the Electron-Transfer Kinetics of Cytochrome c Immobilized on a COOH-Terminated Alkanethiol Monolayer on a Gold Electrode

Cytochrome c was electrostatically immobilized onto a COOH-terminated alkanethiol self-assembled monolayer (SAM) on a gold electrode at ionic strengths of less than 40 mM. Scanning electrochemical microscopy (SECM) was used to simultaneously measure the electron transfer (ET) kinetics of the bimolecular ET between a solution-based redox mediator and the immobilized protein and the tunneling ET between the protein and the underlying gold electrode. Approach curves were recorded with ferrocyanide as a mediator at different coverages of cytochrome c and at different substrate potentials, allowing the measurement of k(BI) = 2 x 10(8) mol(-1) cm3 s(-1) for the bimolecular ET and k degrees = 15 s(-1) for the tunneling ET. The kinetics of ET was also found to depend on the immobilization conditions of cytochrome c: covalent attachment gave slightly slower tunneling ET values, and a mixed CH3/COOH-terminated ML gave faster tunneling ET rates. This is consistent with previous studies and is believed to be related to the degree of mobility of cyt c in its binding configuration and its orientation with respect to the underlying electrode surface.

Template-Directed Formation of Hemispherical Cavities of Varying Depth and Diameter in a Silicate Matrix Prepared by the Sol−Gel Process

2-D ordered arrays of hemispherical cavities were formed in a sol−gel-derived silicate film on a glassy carbon electrode using polystyrene latex spheres as templates. The depth and the diameter of the cavities were varied over 1 order of magnitude by changing the diameter of the template and diluting the sol. Sulfate-stabilized polystyrene latex spheres ranging in size from 100 to 1000 nm were doped into a silica sol prepared from the hydrolysis and condensation of tetramethoxysilane, and the resultant solution was spin cast on a glassy carbon electrode. The 2-D ordered array of spheres in a thin silicate film was removed via soaking the film in chloroform. Scanning electron microscopy and atomic force microscopy (AFM) were used to image the films, and AFM with high-resolution TM tips was used measure the depth and diameter of the cavities. The cavities are open at the top and bottom and provide direct access to the underlying electrode surface as verified by cyclic voltammetry. Nanosized copper and polyaniline were electrodeposited in the cavities after pinholes were “capped” to create an ordered array of conducting nanostructures in a templated silica host.

Glucose oxidase entrapped in polypyrrole on high-surface-area Pt electrodes: a model platform for sensitive electroenzymatic biosensors

Enzyme entrapped in an electrosynthesized polymer film on a high-surface-area electrode is shown to be an attractive platform for the important class of amperometric biosensors based on oxidases and electrooxidation of the H 2O 2 generated by enzyme-catalyzed oxidation of analyte. Two Pt electrode surface morphologies, Pt black (Pt-BLK) and Pt nanowire brush (Pt-NW) were electrodeposited from chloroplatinic acid. Glucose oxidase (GOx) served as the model enzyme, which was entrapped in an electropolymerized polypyrrole (PPY) film on the high-surface-area Pt electrodes. The glucose sensitivity of the GOx electrodes can be controlled simply through Pt deposition conditions and varies roughly linearly with electrode microscopic roughness factor. A sensitivity of 103 μA mM −1 cm −2 was achieved with the Pt-BLK/PPY-immobilized GOx system at an underlying electrode surface roughness of 318, which is 150-fold higher than the sensitivity of a bare evaporated Pt/PPY-immobilized GOx electrode with a surface roughness of ∼2–3. The apparent Michaelis constant for PPY-immobilized GOx (∼30 mM) is similar to that for free enzyme (25 mM) suggesting that the overall electrode kinetics are not limited by glucose mass transfer. Scanning electron micrographs of enzyme electrode surfaces and a fluorescence assay to determine immobilized GOx concentration revealed that the PPY-GOx layer is deposited as a thin film that follows the contours of the electrode surface and that up to 3-fold greater enzyme is entrapped on the roughest electrodes examined. The orders of magnitude greater sensitivity of enzyme electrodes based on high-surface-area Pt appears likely to be due to increased H 2O 2 electrooxidation efficiency.