Application of scanning electrochemical microscopy to biological samples

The scanning electrochemical microscope can be used in the feedback mode in two-dimensional scans over biological substrates to obtain topographic information at the micrometer level. In this mode, the effect of distance between a substrate (either conductive or insulating) and a scanning ultramicroelectrode tip on the electrolytic current flowing at the tip is recorded as a function of the tip x-y position. Scans of the upper surface of a grass leaf and the lower surface of a Ligustrum sinensis leaf (which show open stomata structures) immersed in aqueous solution are shown. Scans of the upper surface of an elodea leaf in the dark and under irradiation, where the tip reaction is the reduction of oxygen produced by photosynthesis, demonstrate the possibility of obtaining information about the distribution of reaction sites on the substrate surface

Use of Electrochemical Microscopy to Examine Counterion Ejection from Nafion Coatings on Electrodes

The ideal cation permselectivity exhibited by electrode coatings prepared from the polyelectrolyte Nafion leads to ejection of countercatlons from the coatings during electrochemical oxidation of Os(bpy)_3^(2+) counterions incorporated in the coatings. If the coatings are saturated with Os(bpy)_3^(2+) so that these are the only counterions present, one-third of the incorporated cations are ejected during the oxidation of Os(bpy)_3^(2+) to Os(bpy)_3^(3+). The cyclic voltammetry of the incorporated Os(bpy)_3^(3+/2+) couple is altered substantially in the absence of additional counterions. The changes are attributed to contributions to the voltammetric potentials from the free energy of transfer of these strongly bound counterions. Microtip electrodes positioned just above Nafion-coated electrodes were used to monitor the ejection of both Os(bpy)_3^(3+) and Os(bpy)_3^(2+) from Nafion coatings. Much more of the former complex lo ejected from saturated coatings which is believed to be the result of electric field-assisted ejection

Highly Catalytic Electrochemical Oxidation of Carbon Monoxide on Iridium Nanotubes: Amperometric Sensing of Carbon Monoxide

The nanotubular structures of IrO2 and Ir metal were successfully synthesized without any template. First, IrO2 nanotubes were prepared by electrospinning and post-calcination, where a fine control of synthetic conditions (e.g., precursor concentration and solvent composition in electrospinning solution, temperature increasing rate for calcination) was required. Then, a further thermal treatment of IrO2 nanotubes under hydrogen gas atmosphere produced Ir metal nanotubes. The electroactivity of the resultant Ir metal nanotubes was investigated toward carbon monoxide (CO) oxidation using linear sweep voltammetry (LSV) and amperometry. The anodic current response of Ir metal nanotubes was linearly proportional to CO concentration change, with a high sensitivity and a short response time. The amperometric sensitivity of Ir metal nanotubes for CO sensing was greater than a nanofibrous counterpart (i.e., Ir metal nanofibers) and commercial Pt (20 wt% Pt loading on carbon). Density functional theory calculations support stronger CO adsorption on Ir(111) than Pt(111). This study demonstrates that metallic Ir in a nanotubular structure is a good electrode material for the amperometric sensing of CO

Application of scanning electrochemical microscopy to generation/collection experiments with high collection efficiency

The technique of scanning electrochemical microscopy (SECM) was introduced by Bard and co-workers (1) who have utilized it in a variety of experiments (2). The use of a microelectrode placed close to a substrate electrode to detect electroactive products generated at the latter, as described by Engstrom and co-workers (3,4),is one attractive application of the SECM technique (1). In previous studies, this type of generation/detection experiment has usually been carried out with the microelectrode used as the detector electrode (1-5). In the present study, a larger (100 µm diameter) substrate electrode is used to detect and collect electrode reaction products generated at a microtip electrode positioned at various distances above the substrate electrode. The collection efficiency (in the sense employed for rotating ring-disk electrodes (6)) of such an arrangement is virtually 100% at easily achievable separation distances. The advantages of carrying out generation/collection experiments with the present apparatus are exemplified with some simple experimental systems. Comparisons with similar recent experiments in which arrays of microband electrodes are utilized (7,8) are also provided

Application of scanning electrochemical microscopy to generation/collection experiments with high collection efficiency

The technique of scanning electrochemical microscopy (SECM) was introduced by Bard and co-workers (1) who have utilized it in a variety of experiments (2). The use of a microelectrode placed close to a substrate electrode to detect electroactive products generated at the latter, as described by Engstrom and co-workers (3,4),is one attractive application of the SECM technique (1). In previous studies, this type of generation/detection experiment has usually been carried out with the microelectrode used as the detector electrode (1-5). In the present study, a larger (100 µm diameter) substrate electrode is used to detect and collect electrode reaction products generated at a microtip electrode positioned at various distances above the substrate electrode. The collection efficiency (in the sense employed for rotating ring-disk electrodes (6)) of such an arrangement is virtually 100% at easily achievable separation distances. The advantages of carrying out generation/collection experiments with the present apparatus are exemplified with some simple experimental systems. Comparisons with similar recent experiments in which arrays of microband electrodes are utilized (7,8) are also provided

Amperometric Sensing of Carbon Monoxide: Improved Sensitivity and Selectivity via Nanostructure-Controlled Electrodeposition of Gold

A series of gold (Au) nanostructures, having different morphologies, were fabricated for amperometric selective detection of carbon monoxide (CO), a biologically important signaling molecule. Au layers were electrodeposited from a precursor solution of 7 mM HAuCl4 with a constant deposition charge (0.04 C) at various deposition potentials. The obtained Au nanostructures became rougher and spikier as the deposition potential lowered from 0.45 V to 0.05 V (vs. Ag/AgCl). As prepared Au layers showed different hydrophobicity: The sharper morphology, the greater hydrophobicity. The Au deposit formed at 0.05 V had the sharpest shape and the greatest surface hydrophobicity. The sensitivity of an Au deposit for amperometric CO sensing was enhanced as the Au surface exhibits higher hydrophobicity. In fact, CO selectivity over common electroactive biological interferents (L-ascorbic acid, 4-acetamidophenol, 4-aminobutyric acid and nitrite) was improved eminently once the Au deposit became more hydrophobic. The most hydrophobic Au was also confirmed to sense CO exclusively without responding to nitric oxide, another similar gas signaling molecule, in contrast to a hydrophobic platinum (Pt) counterpart. This study presents a feasible strategy to enhance the sensitivity and selectivity for amperometric CO sensing via the fine control of Au electrode nanostructures