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Electrochemical study of chalcogen-containing species under aqueous conditions is often hindered by unwanted oxidation and surface adsorption. Disulfides, diselenides, thiols, and selenols react readily with metallic electrodes to form self-assembled monolayers (SAMs) and conductive films. We have shown that the intrinsic reactivity between selenium and gold yields a stable and reproducible electrode surface modification that makes the reduction of selenocystine to selenocysteine accessible at neutral pH. In this work, we expand our electrochemical characterization of the selenocystine / selenocysteine redox couple to examine the pH dependence of the electron-transfer process, evaluate preparative conditions for formation of the selenium-gold adsorbate, and compare surface reactivity and solution behavior on gold and platinum. We demonstrate that the electrochemically generated surface modification can be removed via anodic stripping voltammetry and restored via subsequent cycling with little to no loss in current. X-ray photoelectron spectroscopy confirms the presence of selenium on the surface of substrates that have been reductively cycled in the presence of selenocystine. These findings mark an important advance in the electroanalysis of chalcogen-containing biological species under physiological conditions.

The voltammetric response of seleno-l-cystine has been studied at gold substrates under physiological conditions. The reactivity of diselenides is utilized to generate a modified electrode surface, which in turn allows for electroanalysis of solution-based selenium species. Stable and reproducible voltammetry is observed for selenocystine reduction at pH 7 on selenium-modified gold substrates (Emid = −486 mV vs. Ag/AgCl) and is consistent with a diffusionally controlled process. Modification of the gold surface is readily achieved via electrochemical cycling in the presence of a diselenide source at conventional scan rates. These studies afford voltammetric characterization of the selenocystine/selenocysteine redox couple under physiologically relevant conditions and highlight the potential utility of selenium-modified substrates for electroanalysis of chalcogen-containing species. Keywords: Selenium, Selenocystine, Diselenide, Voltammetry

Psychrophilic organisms play a significant role in global nutrient cycling and bioremediation, and thus there is great interest in understanding the details of their cellular processes. Here, we report the first electrochemical measurements of cytochrome c552 isolated from the model psychrophile, Colwellia psychrerythraea. Quasi-reversible redox behavior is observed at thiol-modified and bare gold electrodes with midpoint potentials of 247 and 277 mV vs SHE, respectively. Comparison of the voltammetric response on the two substrates illustrates the need for a modified surface to direct protein orientation and facilitate electron transfer in a reproducible manner. Temperature-dependence measurements of the current profile illustrate the protein’s ability to participate in electron transfer reactions over a 50° range, which is supported by recent structural evidence of enhanced flexibility. These electrochemical findings allow the behavior of this psychrophilic cytochrome to be evaluated in the context of m...

The direct electrochemical oxidation of ammonia has been examined in both the organic solvent dimethylformamide (DMF) and the room temperature ionic liquid 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][N(Tf)2]. The corresponding voltammetric responses have been shown to be similar in each solvent with a broad oxidative wave occurring upon the introduction of ammonia to the solution and the appearance of a new reductive wave following the oxidation. The oxidative reaction process has been examined and a suitable reaction pathway has been deduced, corresponding to the formation of ammonium cations after oxidation of the ammonia. A linear response of limiting current against vol% ammonia was observed in both DMF and [EMIM][N(Tf)2], suggesting potential application for analytical methods.

The results detail a novel methodology for the electrochemical determination of ammonia based on its interaction with hydroquinone in DMF. It has been shown that ammonia reversibly removes protons from the hydroquinone molecules, thus facilitating the oxidative process with the emergence of a new wave at less positive potentials. The analytical utility of the proposed methodology has been examined with a linear range from 10 to 95 ppm and corresponding limit-of-detection of 4.2 ppm achievable. Finally, the response of hydroquinone in the presence of ammonia has been examined in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide, [EMIM][N(Tf) 2 ]. Analogous voltammetric waveshapes to that observed in DMF were obtained, thereby confirming the viability of the method in either DMF or [EMIM][N(Tf) 2 ] as solvent.

The base pair stack of DNA has been demonstrated as a medium for long-range charge transport chemistry both in solution and at DNA-modified surfaces. This chemistry is exquisitely sensitive to structural perturbations in the base pair stack as occur with lesions, single base mismatches, and protein binding. We have exploited this sensitivity for the development of reliable electrochemical assays based on DNA charge transport at self-assembled DNA monolayers. Here, we discuss the characteristic features, applications, and advantages of DNA-mediated electrochemistry.

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C 4 mim][NTf 2 ]), and the conventional aprotic solvent, acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim ® was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2 Br - → Br 2 + 2 e - Br 2 + Br - ⇌ k b k f Br 3 - K eq Athigher potentials, the tribromide anion dissociates to bromine, Br 2 , and bromide, Br − , which is immediately oxidised, leading to the emergence of a second anodic wave. Irreversible electrode kinetics were inferred from Tafel analysis and removal of the first electron is believed to be the rate-determining step in the formation of bromine. The equilibrium constant, K eq , where K eq = [ Br 3 - ] [ Br 2 ] [ Br - ] was found to be 3 × 10 3 and 9 × 10 6 M −1 in the ionic liquid and in acetonitrile, respectively. An equilibrium constant, K complex for the complexation of 1-ethyl-3-methylimidazolium bromide in acetonitrile was found to be 35.1 M −1 .

Redmond Red, a fluoropore containing a redox-active phenoxazine core, has been explored as a new electrochemical probe for the detection of abasic sites in double-stranded DNA. The electrochemical behavior of Redmond Red-modified DNA at gold surfaces exhibits stable, quasi-reversible voltammetry with a midpoint potential centered around −50 mV versus NHE. Importantly, with Redmond Red positioned opposite an abasic site within the DNA duplex, the electrochemical response is significantly enhanced compared to Redmond Red positioned across from a base. Redmond Red, reporting only if well-stacked in the duplex, represents a sensitive probe to detect abasic sites electrochemically in a DNA-mediated reaction.

The electrochemical oxidation of cobalt(II) at gold, boron-doped diamond, basal and edge plane pyrolytic graphite, and highly oriented pyrolytic graphite electrodes in aqueous solutions containing NH3 has been studied using cyclic voltammetry, with subsequent chemical and electrochemical processes explained in detail. Furthermore, the electroreduction of [Co(NH 3)6]3+ in the presence of different electrolytes has also been studied to obtain a better understanding of the oxidation pathway of the Co(II)-ammine complexes. In aqueous solution the mechanism can be described by the following scheme: (Chemical presented) © 2006 Wiley-VCH Verlag GmbH and Co. KGaA.

The electrochemical windows of acetonitrile solutions doped with 0.1 M concentrations of several ionic liquids were examined by cyclic voltammetry at gold and platinum microelectrodes. These results were compared with those observed in the commonly used 0.1 M tetrabutylammonium perchlorate/acetonitrile system as well as with neat ionic liquids. The use of a trifluorotris(pentafluoroethyl)phosphate-based ionic liquid, specifically, as supporting electrolyte in acetonitrile solutions affords a wider anodic window, which is attributed to the high stability of the anionic component of these intrinsically conductive and thermally robust compounds.

The electrochemical reduction of aqueous hexamminecobalt(III) is studied using cyclic voltammetry, observing the current-voltage response using a range of scan rates covering seven orders of magnitude. At lower scan rates, it is clear that the voltammetry is diffusion-controlled, but as the scan rate is increased, the system undergoes a transition to a regime that is governed by adsorption. These experimental results are compared with a previously developed theoretical treatment of adsorption (F.G. Chevallier, O.V. Klymenko, L. Jiang, T.G. Jones, R.G. Compton, J. Electroanal. Chem. 574 (2004) 217). © 2005 Elsevier B.V. All rights reserved.

Some twenty-five years after they first came to prominence as alternative electrochemical solvents, room temperature ionic liquids (RTILs) are currently being employed across an increasingly wide range of chemical fields. This review examines the current state of ionic liquid-based electrochemistry, with particular focus on the work of the last decade. Being composed entirely of ions and possessing wide electrochemical windows (often in excess of 5 volts), it is not difficult to see why these compounds are seen by electrochemists as attractive potential solvents. Accordingly, an examination of the pertinent properties of ionic liquids is presented, followed by an assessment of their application to date across the various electrochemical disciplines, concluding with an outlook viewing current problems and directions.