Greg M. Swain

Electrochemistry and the environment

Advances in electrochemical methods for pollutant remediation, recycling and sensing are reviewed. Additionally, applications of these methods in the drinking water industry, and for disinfection scenarios are discussed. Lastly, new electrode materials for environmental applications are described. In a companion review, photoelectrochemical methods will be discussed.

Conductive diamond thin-films in electrochemistry

Diamond electrodes offer superb properties for a variety of electrochemical technologies, properties that include: corrosion resistance, low background current, good responsiveness without pretreatment, resistance to fouling, and optical transparency. Electroanalysis, electrocatalysis, spectroelectrochemistry, and bioelectrochemistry are areas of electrochemistry in which diamond thin-films, both microcrystalline and nanocrystalline, are being successfully researched. A brief review is given herein of some of our R&D efforts in each of these areas.

Anthraquinonedisulfonate Electrochemistry: A Comparison of Glassy Carbon, Hydrogenated Glassy Carbon, Highly Oriented Pyrolytic Graphite, and Diamond Electrodes

The electrochemistry of anthraquinone-2,6-disulfonate (2,6-AQDS) at glassy carbon (GC), hydrogenated glassy carbon (HGC), the basal plane of highly oriented pyrolytic graphite (HOPG), and boron-doped diamond was investigated by cyclic voltammetry and chronocoulometry. Quantitative determination of the surface coverage and qualitative assessment of the physisorption strength of 2,6-AQDS adsorption on each of these electrodes were done. The diamond and HGC surfaces are nonpolar and relatively oxygen-free, with the surface carbon atoms terminated by hydrogen. The polar 2,6-AQDS does not adsorb on these surfaces, and the electrolysis proceeds by a diffusion-controlled reaction. Conversely, the GC and HOPG surfaces are polar, with the exposed defect sites terminated by carbon-oxygen functionalities. 2,6-AQDS strongly physisorbs on both of these surfaces at near monolayer or greater coverages, such that the electrolysis proceeds through a surface-confined state. Less than 40% of the initial surface coverage can be removed by rinsing and solution replacement, reflective of strong physisorption. The results show the important role of the surface carbon-oxygen functionalities in promoting strong dipole-dipole and ion-dipole interactions with polar and ionic molecules such as 2,6-AQDS. The results also support the theory that diamond electrodes may be less subject to fouling by polar adsorbates, as compared to GC, leading to improved response stability in electroanalytical measurements. The relationship between the 2,6-AQDS surface coverage, the double-layer capacitance, and the heterogeneous electron-transfer rate constant for Fe(CN)(6)(3)(-)(/4)(-) for these four carbon electrodes is presented.

The Susceptibility to Surface Corrosion in Acidic Fluoride Media: A Comparison of Diamond, HOPG, and Glassy Carbon Electrodes

The chemical inertness and corrosion resistance of a boron-doped diamond thin film electrode, grown by chemical vapor deposition (CVD), have been studied during potential cycling (PC) for 2 h in a solution of 1.0M HNO 3 +0.1M NaF at 50 o C. Similar experiments were performed on highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC), for comparison. The physicochemical properties of the electrode surface were characterized before and after PC by cyclic voltammetry, optical and scanning electron microscopy, Raman spectroscopy, and ac impedance spectroscopy. The results indicated that the diamond electrode possesses a superior degree of chemical inertness and corrosion resistance, has no microstructural damage nor was surface oxidation observed after PC

Applications of Diamond Thin Films in Electrochemistry

Electrochemical reactions typically involve electron transfer between an electrode and a dissolved chemical species at a solid-electrode/liquid-electrolyte interface. Three broad classes of electrochemical applications may be identified: (1) synthesis (or destruction), in which an applied potential is used to bring about a desired chemical oxidation or reduction reaction; (2) analysis, in which the current/potential characteristics of an electrode are used to determine the type and concentration of a species; and (3) power generation. These broad types of applications require stable, conductive, chemically robust, and economical electrodes. Diamond electrodes, fabricated by chemical vapor deposition, provide electrochemists with an entirely new type of carbon electrode that meets these requirements for a wide range of applications. The first reports of electrochemical studies using diamond were in the mid-1980s. During the past several years, the field has attracted increasing attention. This review summarizes the electrochemical properties of diamond that make it a unique electrode material and that distinguish it from conventional carbon electrodes.

Activation of colonic mucosal 5-HT(4) receptors accelerates propulsive motility and inhibits visceral hypersensitivity.

BACKGROUND & AIMS 5-hydroxytryptamine receptor (5-HT(4)R) agonists promote gastrointestinal motility and attenuate visceral pain, but concerns about adverse reactions have restricted their availability. We tested the hypotheses that 5-HT(4) receptors are expressed in the colonic epithelium and that 5-HT(4)R agonists can act intraluminally to increase motility and reduce visceral hypersensitivity. METHODS Mucosal expression of the 5-HT(4)R was evaluated by reverse-transcriptase polymerase chain reaction and immunohistochemical analysis of tissues from 5-HT(4)R(BAC)-enhanced green fluorescent protein mice. Amperometry, histology, and short-circuit current measurements were used to study 5-HT, mucus, and Cl(-) secretion, respectively. Propulsive motility was measured in guinea pig distal colon, and visceromotor responses were recorded in a rat model of colonic hypersensitivity. 5-HT(4)R compounds included cisapride, tegaserod, naronapride, SB204070, and GR113808. RESULTS Mucosal 5-HT(4) receptors were present in the small and large intestines. In the distal colon, 5-HT(4) receptors were expressed by most epithelial cells, including enterochromaffin and goblet cells. Stimulation of 5-HT(4)Rs evoked mucosal 5-HT release, goblet cell degranulation, and Cl(-) secretion. Luminal administration of 5-HT(4)R agonists accelerated propulsive motility; a 5-HT(4)R antagonist blocked this effect. Bath application of 5-HT(4)R agonists did not affect motility. Oral or intracolonic administration of 5-HT(4)R agonists attenuated visceral hypersensitivity. Intracolonic administration was more potent than oral administration, and was inhibited by a 5-HT(4)R antagonist. CONCLUSIONS Mucosal 5-HT(4) receptor activation can mediate the prokinetic and antinociceptive actions of 5-HT(4)R agonists. Colon-targeted, intraluminal delivery of 5-HT(4)R agonists might be used to promote motility and alleviate visceral pain, while restricting systemic bioavailability and resulting adverse side effects.

Enhanced Signal-to-Background Ratios in Voltammetric Measurements Made at Diamond Thin-Film Electrochemical Interfaces

Large signal-to-background (S/B) ratios for the Fe(CN)(6)(3)(-)(/4)(-) and IrCl(6)(2)(-)(/3)(-) redox couples in KCl have been observed in cyclic voltammetric measurements made at a conductive diamond thin-film electrode without any conventional surface pretreatment. The S/B ratios were a factor of ∼16 and 8 larger at diamond than at freshly polished glassy carbon (GC) for Fe(CN)(6)(3)(-)(/4)(-) and IrCl(6)(2)(-)(/3)(-), respectively. The polycrystalline diamond film, grown on a p-Si(100) substrate, possessed significant cubic {100} faceting, as evidenced by AFM images, and was of high quality, as indicated by Raman spectroscopy. The high degree of electrochemical activity without surface pretreatment, the enhanced S/B ratios, and the excellent response stability demonstrate that diamond might be an attractive new electrode material for electroanalysis.

Effect of sp2-Bonded Nondiamond Carbon Impurity on the Response of Boron-Doped Polycrystalline Diamond Thin-Film Electrodes

The physical and electrochemical properties of boron-doped polycrystalline diamond thin-film electrodes, prepared with varying levels of sp 2 -bonded nondiamond carbon impurity, were systematically investigated. This impurity was introduced through adjustment of the methane-to-hydrogen (C/H) source gas ratio used for the deposition. Volumetric gas ratios of 0.3, 0.5, 1, 2, 3, and 5% were employed. Proportional increases in the fraction of grain boundary, the extent of secondary nucleation, and the sp 2 -bonded carbon impurity content resulted in increasing C/H ratio. Variations in the morphology and microstructure were monitored using atomic force microscopy (AFM) and Raman spectroscopy, respectively. The electrode response was assessed using Fe(CN) 3-/4- 6 , Ru(NH 3 ) 3+/2+ 6 , Fe 3+/2+ . and 4-tert-butylcatechol (4-tBC). All were 1 mM in concentration and dissolved in either 1 M KCl or 0.1 M HClO 4 . While increased sp 2 -bonded carbon content had little effect on the cyclic voltammetric peak separation (ΔE p ) and peak current for the first two redox systems, the impurity had a significant impact on the latter two, as ΔE p decreased proportionally with increased sp 2 -bonded carbon content. The effect of the impurity on the reduction of oxygen in 0.1 M HClO 4 and 0.1 M NaOH was also investigated. A direct correlation was found between the relative amount of the impurity, as estimated from Raman spectroscopy, and the overpotential for oxygen reduction. The greater the nondiamond content, the lower the kinetic overpotential for the reduction reaction. Tafel plots yielded an apparent exchange current density that increased and a transfer coefficient that decreased with the increased nondiamond carbon content. The results demonstrate that the grain boundaries, and the sp 2 carbon impurity presumably residing there, can have a significant impact on the electrode reaction kinetics for certain redox systems and little influence for others.

Electrochemical Modification of Boron‐Doped Chemical Vapor Deposited Diamond Surfaces with Covalently Bonded Monolayers

Electrochemical reduction of phenyl diazonium salts in acetonitrile at boron-doped diamond electrodes yielded covalent bonding of aromatic groups to the sp 3 carbon surface. Diamond surfaces modified with nitrophenyl, trifluoromethylphenyl, and nitroazobenzene showed strong X-ray photoelectron spectroscopy (XPS) signals for surface nitrogen or fluorine, which were stable to exposure to air or solvents. Raman spectra of chemisorbed nitroazobenzene on boron-doped diamond were obtained, and were similar to those observed for derivatized glassy carbon. Estimated surface coverages of 50-70% of a compact monolayer were calculated from XPS spectra, indicating that the coverage is too high to be attributed solely to modification of sp 2 carbon impurities or boron dopant. The high coverages of covalently bonded molecules on diamond achievable by diazonium reduction imply that a variety of surface functionalities may be introduced on the normally unreactive diamond surface.

Fabrication and Evaluation of Platinum/Diamond Composite Electrodes for Electrocatalysis Preliminary Studies of the Oxygen-Reduction Reaction

A catalytic electrode was prepared using a new electrically conducting and corrosion resistant carbon support material, boron-doped diamond. Fabrication of the composite electrode involves a three-step process: (i) continuous diamond thin-film deposition on a substrate, (ii) electrodeposition of Pt catalyst particles on the diamond surface, and (iii) short-term diamond deposition to entrap the metal particles into the surface microstructure. The process results in a conductive, morphologically, and Microstructurally stable composite electrode containing metal particles of somewhat controlled composition, size, and catalytic activity. The metal catalyst particles were galvanostatically deposited from a K 2 PtCl 6 /HClO 4 solution, with the metal particle size (50-350 nm) and distribution (∼10 9 cm 2 ) being controlled by adjusting the galvanostatic deposition and secondary diamond-growth conditions. For a 300 s Pt deposition time, the estimated loading was 75.8 μg/cm 2 , assuming a 100% current efficiency. The composite electrode was extremely stable, both structurally and catalytically, during a 2 h polarization in 85% H 3 PO 4 at 170°C and 0.1 A/cm 2 . The electrodes catalytic activity was evaluated using the O 2 reduction reaction at room temperature in 0.I M solutions of H 3 PO 4 . H 2 SO 4 , and HClO 4 . The kinetic parameters (Tafel slope and exchange current density) were obtained by cyclic voltammetry and were found to be comparable to those for a polycrystalline Pt electrode in the same media. Tafel slopes of -63 to -80 mV/dec were observed at low overpotentials, with the lowest slope in HClO 4 and highest in H 3 PO 4 . The exchange current density ranged from 10 12 to 10 10 A/cm 2 , and increased in the order of H 3 PO 4 < H 2 SO 4 < HClO 4 . The potential advantages of the composite electrode, as compared with commercial sp 2 carbon electrodes, are (i) the corrosion resistance of the diamond support, resulting in highly stable relation centers at high potentials, current densities, and temperatures, and (ii) the fact that all of the catalyst particles are strongly anchored at the film surface and are not contained inside pores.