Andrew Doherty

Electrocatalytic oxidation of ascorbic acid at [osmium(2,2′-bipyridyl)2-(poly-4-vinylpyridine)10Cl]Cl modified electrodes; implications for the development of biosensors based on osmium-containing redox relays

The oxidation of ascorbic acid at electrodes modified with the redox polymer [Os(bipy)2(PVP)10Cl]Cl (where bipy = 2,2′-bipyridyl and PVP = poly-4-vinylpyridine) has been studied. The reaction proceeds electrocatalytically at 0.250 V versus SCE by mediation via the OsII/OsIII redox couple immobilized in the polymer film. The reaction is negative first order with respect to proton concentration and positive first order with respect to ascorbic acid concentration. At pH3.0, limiting currents are diffusionally controlled and the kinetic regime may be classified as an Sk″ mechanism. The second order rate constant for the surface electrocatalytic reaction k″ at pH 4.5 is 7.8 ± 0.4 × 10–4dm3mol–1 cm s–1, while at pH 7.0 k″ is 9.1 ± 0.6 × 10–4 dm3 mol–1 cm s–1. At pH ⩽2.0 ascorbic acid permeates the polymer film and reacts at the mediator sites within the polymer film giving an Lk mechanism, while at potentials >0.4 V versus SCE the reaction proceeds both at the underlying electrode and with the OsIII mediator sites within the polymer film. The modified electrode has been characterized as an electrochemical sensor for ascorbic acid in flow injection systems.

Electrochemical Determination of pKa of N-Bases in Ionic Liquid Media

Two electrochemical techniques have been used to measure the pK(a) of N-bases in several ionic liquids (ILs). The first method corresponds to a potentiometric titration of a strong acid with the N-base using a platinized Pt indicator electrode immersed in the IL solution and maintained under dihydrogen atmosphere via gas bubbling. The second approach involves performing cyclic voltammetry at a platinized Pt electrode in a solution containing both strong acid and the conjugate weak acid of the N-base. Values of pK(a) obtained by one or the other approach are in good agreement with each other. The experimental data clearly demonstrated that acid/base chemistry in ILs is similar to that observed in molecular nonaqueous solvents; i.e., the relative strengths of the bases were in the right order and spaced (ΔpK(a)). It was also observed that the strength of N-bases is highly dependent on the anion of the ionic liquid; this observation indicates that pH-dependent reactions could be controlled by the appropriate choice of anion for bulk ILs or as an added co-ion to bulk IL.

Electrocatalytic reduction of nitrite at an [Os(bipy)2(PVP)10Cl]Cl-modified electrode

The electrocatalytic reduction of nitrite at an electrode modified with the redox polymer [Os(bipy)2(PVP)10Cl]Cl, where bipy = 2,2′–bipyridyl, PVP = poly(4-vinylpyridine), is described. A possible mechanism for the mediated reaction is proposed. The kinetics of the mediated reaction are examined and compared to a theoretical rate equation describing the proposed mechanism. The reaction most likely proceeds through the species, NO+. Kinetic analysis reveals that the kinetic situation Lk best describes the electrocatalytic reaction, with mediation occurring throughout the polymer film and electrode currents being controlled by the rate constant for the cross-exchange reaction. An equation relating limiting currents to known and easily measurable parameters is also presented which may be used for the internal calibration of sensors constructed from these electrodes.

Flow injection amperometric determination of nitrite at a carbon fibre electrode modified with the polymer [Os(bipy)2(PVP)20Cl]Cl

The development of carbon fibre electrodes modified with the polymer [Os(bipy)2(PVP)20Cl]Cl for the flow amperometric determination of nitrite is described. This osmium polymer modifier greatly enhances the kinetics of nitrite reduction compared with the reaction at bare carbon electrodes. Various electrode characteristics were optimized using both cyclic voltammetry and flow injection. The calibration graph yielded a slope of 0.197 nA cm3µg–1 over the linear range 0–400 µg cm–3 with a limit of detection of 0.1 µg cm–3. The modified electrode was shown to exhibit good short-term reproducibility yielding a relative standard deviation of 2.15%(n= 20). After a 3 week period of monitoring, involving 240 standard injections and 30 meat extract injections, the electrode continued to function with no significant change in sensitivity. The electrode was used to analyse a processed meat sample for nitrite content and the results compared favourably with those obtained using a standard reference spectrophotometric method.

Electrochemical reduction of butyraldehyde in the presence of CO2

The reduction of butyraldehyde in nominally dry acetonitrile has been investigated using cyclic voltammetry and in situ FTIR spectroelectrochemistry. The major products (butanol, enolate anion equilibrium species and the aldol) have been identified, the formation of which may be attributed to the electrochemical formation of the highly basic radical anion that abstracts protons form free butyraldehyde ultimately resulting in the well-known aldol condensation reaction. However, in the presence of electrophilic CO2, the aldol and enolate formations are suppressed, and to account for this observation a reaction scheme is proposed involving the nucleophilic coupling of the enolate carbanion with CO2 to form the carboxylate.

Combined effect of temperature and electrolyte on triton X-100 micellar diffusion

Abstract The self-diffusion coefficients ( D s ) of Triton X-100 micelles have been determined voltammetrically as a function of KCl concentration and temperature. Normalised diffusion coefficients ( D s η o / T ) were found to decrease linearly with increasing [KCl] at fixed temperatures over the range 288–313 K due to electrolyte induced micellar growth. D s η o / T was also found to decrease with increasing temperature due to temperature-induced micellar growth although the temperature-dependent intermicellar interaction parameters were found to decrease with increasing temperature suggesting increasing attractive interactions at higher temperatures where as the interaction parameter increases with [KCl]. An electrolyte concentration dependent transition (suppression of D s η o / T ) was observed at temperatures below the onset of extensive micellar growth and appears to be unrelated to the usual phase separation behaviour frequently observed and may possibly be related to a significant increase in polydispersity.

Electrochemistry in true reverse micelles

In this communication we demonstrate for the first time the electrochemistry of a simple redox couple Fe(CN)4/3−6 in a true reverse micellar system (i.e. not a microemulsion). The reverse micelles were formed from aerosol-OT (AOT=sodium bis-2-ethylhexylsulfosuccinate) in isooctane with a [H2O]/[AOT] ratio (W) of 10 using the phase transfer technique in the absence of supporting electrolyte. Ferricyanide was introduced by dissolving in the aqueous phase prior to mixing with the AOT-containing isooctane phase. The reduction of Fe(CN)3−6 within the reverse micellar environment was carried out at a carbon fibre microelectrode using a two-electrode assembly with a porous-polymer-coated silver wire acting as the pseudo-reference electrode. Nernstian electrochemical responses were observed and the limiting currents were well defined from which micellar diffusion coefficients could be obtained.

The electrochemical reduction of dimethyl maleate and dimethyl fumarate

Abstract The electrochemical reduction of dimethyl maleate (DMM) and dimethyl fumarate (DMF) in acetonitrile and methanol solutions has been investigated using cyclic voltammetry (CV), rotating ring-disk electrode voltammetry (RRDE) and in situ FT-IR spectroelectrochemistry. In both solvents, the electrochemically generated radical anion of DMM under goes both a rapid cis–trans isomerisation process forming the dimethyl fumarate radical anion (DMF·−) and a rapid radical anion dimerisation reaction. The radical anion dimerisation of these species is rapid with second order rate constants of ≈1×107 mol dm−3 s−1. An autocatalytic electron cross-exchange reaction between the DMF·− species and DMM results in DMF and DMM·− consequently leading to a rapid build-up of the trans isomer in solution.

Interaction of cetyltrimethylammonium chloride micelles under static and hydrodynamic conditions

Self-diffusion coefficients (Dm) of cetyltrimethylammonium chloride (CTAC) micelles have been measured as a function of surfactant concentration (Cs) under static and hydrodynamic conditions using cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry with ferrocene acting as an electroactive probe. The diffusion coefficients obtained from both techniques were found to be an inverse function of surfactant concentration with the behaviour conforming to the relationship Dm=Dom/[1 +ƒ(Cs– c.m.c.)], having a common value, Dom, at the critical micelle concentration (c.m.c.). This behaviour has been interpreted in terms of an inter-micellar interaction parameter ƒ. At surfactant concentrations above the c.m.c., clear differences between the statically measured (CV) and the hydrodynamically measured (RDE) diffusion coefficients were observed, with significantly higher values being obtained with the RDE, giving ƒ= 13.5 ± 0.2 dm3 mol–1 under static conditions and ƒ= 7.9 ± 0.2 dm3 mol–1 under hydrodynamic conditions, i.e. inter-micellar interaction is a function of the experimental conditions. The difference between the hydrodynamically and statically measured diffusion coefficients is shown to be a function of inter-micellar distance.

Carbons, Ionic Liquids, and Quinones for Electrochemical Capacitors

Carbons are the main electrode materials used in electrochemical capacitors, which are electrochemical energy storage devices with high power densities and long cycling lifetimes. However, increasing their energy density will improve their potential for commercial implementation. In this regard, the use of high surface area carbons and high voltage electrolytes are well known strategies to increase the attainable energy density, and lately ionic liquids have been explored as promising alternatives to current state of the art acetonitrile-based electrolytes. Also, in terms of safety and sustainability ionic liquids are attractive electrolyte materials for electrochemical capacitors. In addition, it has been shown that the matching of the carbon pore size with the electrolyte ion size further increases the attainable electric double layer (EDL) capacitance and energy density. The use of pseudocapacitive reactions can significantly increase the attainable energy density, and quinonic-based materials offer a potentially sustainable and cost effective research avenue for both the electrode and the electrolyte. This perspective will provide an overview of the current state of the art research on electrochemical capacitors based on combinations of carbons, ionic liquids and quinonic compounds, highlighting performances and challenges and discussing possible future research avenues. In this regard, current interest is mainly focused on strategies which may ultimately lead to commercially competitive sustainable high performance electrochemical capacitors for different applications including those requiring mechanical flexibility and biocompatibility.