Voltammetry of multi-electron electrode processes of organic species

Classically the analysis of the voltammetric response of multi-electron transfer processes is achieved through the use of the Randles- Ševčík equations based on analytical theory. In such it is assumed that the after the 'rate determining step' all electron transfers are highly driven. Conversely, it is commonly found experimentally that the formal potentials for different electrochemical steps (1, 2, 3 ...) are found to be at comparable potentials (i.e. Ef,1θ∼Ef,2θ∼Ef,3θ); this leads to significant deviations from the Randles-Ševčík analysis. This article highlights various 'simple' electrochemical mechanisms (EE, EC, EEC) and discusses how the voltammetric peak height resulting from linear sweep voltammetry is expected to vary with scan rate and other parameters, with the aim of providing a general theoretical basis upon which analysis of complex voltammetric systems may be approached and understood. © 2012 Elsevier B.V. All rights reserved

Thin-Film Modified Rotating Disk Electrodes: Models of Electron-Transfer Kinetics for Passive and Electroactive Films

This work explores the influence of both passive and electroactive thin films upon the steady-state voltammetric response of a rotating disk electrode, proposing simple physical and algebraic models. In both cases, it is clearly evidenced how the alteration of the mass-transport regime adjacent to the electrochemical interface leads to an apparent change in the electron-transfer kinetics. These results are of great significance because of the wide adoption of the rotating disk electrode technique for studying new electrocatalytic materials

Thin-Film Modified Rotating Disk Electrodes: Models of Electron-Transfer Kinetics for Passive and Electroactive Films

© 2014 American Chemical Society. This work explores the influence of both passive and electroactive thin films upon the steady-state voltammetric response of a rotating disk electrode, proposing simple physical and algebraic models. In both cases, it is clearly evidenced how the alteration of the mass-transport regime adjacent to the electrochemical interface leads to an apparent change in the electron-transfer kinetics. These results are of great significance because of the wide adoption of the rotating disk electrode technique for studying new electrocatalytic materials

Characterising and evidencing the effects of porosity in nano-electrochemistry

Porosity in nanoparticles and their ensembles can profoundly alter a system's electrochemical behaviour by both changing the number and identity of the available catalytic sites, and through influencing the mass transport in the vicinity of the electrochemical interface. This review focuses first on recent advancements in characterising porosity and heterogeneity at the nanoscale and second on developments in electrochemical simulation which provide insight into how a porous architecture can lead to apparent ‘catalytic’ effects by virtue of altering the masstransport in the vicinity of the electrode

Electrochemical oxidation of guanine: electrode reaction mechanism and tailoring carbon electrode surfaces to switch between adsorptive and diffusional responses.

The electrochemical oxidation of guanine is studied in aqueous media at various carbon electrodes. Specifically edge plane pyrolytic graphite (EPPG), basal plane pyrolytic graphite (BPPG), and highly ordered pyrolytic graphite (HOPG) were used, and the voltammetry was found to vary significantly. In all cases, signals characteristic of adsorbed guanine were seen and the total charge passed varied from surface to surface in the order roughened BPPG > EPPG > BPPG > HOPG. It is of note that the peak height for the EPPG electrode is less than that found for roughened BPPG; furthermore, across the series of electrodes, there is a significant decrease in peak potential with increasing density of edge plane sites present at the electrode surface. This leads us to conclude that there are two dominating and controlling factors present: (i) the density of basal plane sites on which guanine can adsorb and (ii) the density of edge plane sites necessary for the electro-oxidation of the analyte. This conclusion is corroborated through further experiments with multi- and single-walled carbon nanotubes. Adsorption was seen to be enhanced by modification of the EPPG surface with alumina particles, and as such, increased peak signals were observed in their presence. It is further reported that via the pre-adsorption of acetone onto the graphite surface that the adsorption of guanine may be blocked, resulting in a diffusional voltammetric signal. This diffusional response has been successfully modeled and gives insight into the complex -4e(-), -4H(+) oxidation mechanism; specifically, it enables explanation of the observed change in rate-determining step with scan rate. The oxidation of guanine first proceeds via a two-electron oxidation followed by a chemical step to form 8-oxoguanine, then 8-oxoguanine is then further oxidized to form nonelectroactive products. The change is mechanism is attributed to the variation in potential of the first and second electron transfer with scan rate

The Copper(II)-Catalyzed Oxidation of Glutathione.

The kinetics and mechanisms of the copper(II)-catalyzed GSH (glutathione) oxidation are examined in the light of its biological importance and in the use of blood and/or saliva samples for GSH monitoring. The rates of the free thiol consumption were measured spectrophotometrically by reaction with DTNB (5,5'-dithiobis-(2-nitrobenzoic acid)), showing that GSH is not auto-oxidized by oxygen in the absence of a catalyst. In the presence of Cu(2+) , reactions with two timescales were observed. The first step (short timescale) involves the fast formation of a copper-glutathione complex by the cysteine thiol. The second step (longer timescale) is the overall oxidation of GSH to GSSG (glutathione disulfide) catalyzed by copper(II). When the initial concentrations of GSH are at least threefold in excess of Cu(2+) , the rate law is deduced to be -d[thiol]/dt=k[copper-glutathione complex][O2 ](0.5) [H2 O2 ](-0.5) . The 0.5(th) reaction order with respect to O2 reveals a pre-equilibrium prior to the rate-determining step of the GSSG formation. In contrast to [Cu(2+) ] and [O2 ], the rate of the reactions decreases with increasing concentrations of GSH. This inverse relationship is proposed to be a result of the competing formation of an inactive form of the copper-glutathione complex (binding to glutamic and/or glycine moieties)

Optimising amperometric pH sensing in blood samples: an iridium oxide electrode for blood pH sensing

Amperometric pH sensing in blood samples has been studied using iridium oxide electrodes. The iridium oxide electrodes are made by electrodeposition of iridium oxide onto an iridium micro-disc electrode from an alkaline solution of iridium(iii) oxide. The response of the electrode is studied in aqueous solutions and authentic samples of sheep's blood employing both cyclic voltammetry and square wave voltammetry. Uncertainties of pH measurement in blood samples via cyclic voltammetry (±0.07 pH units) were improved by a factor of two using square wave voltammetry (±0.03 pH units). Limitations of amperometric pH sensing in blood samples are considered as caused by the uncertainty of the required reference measurements (via a conventional glass electrode) and also the use of matrix-free and low ionic strength buffers to calibrate a standard glass electrode for the measurement of blood pH

Electrooxidative decarboxylation of vanillylmandelic acid: voltammetric differentiation between the structurally related compounds homovanillic acid and vanillylmandelic acid.

Vanillylmandelic acid (VMA) and homovanillic acid (HVA) are the major end products of catecholamine metabolism. Abnoramally high levels in both plasma and urine may be indicative of a number of diseases including neuroblastoma and phaeochromocytoma. Commonly the VMA:HVA ratio is used as a disease marker, so that any measurement techniques need to be able to differentiate between these two structurally similar compounds. Electrochemistry is often limited in selectivity due to many organic molecules being oxidized or reduced at similar potentials. This work investigates the electrochemical oxidation mechanism of VMA at an edge-plane pyrolytic graphite electrode and highlights how, although structurally similar to HVA, their voltammetric responses may be differentiated through appropriate selection of the electrode material. The oxidation of VMA exhibits two clear peaks and the mechanism is shown to proceed through the decarboxylation of VMA to form vanillin, which is further oxidized resulting in the second peak. Modification of the electrode with a porous layer of multiwalled carbon nanotubes so as to change the mass transport to that of a thin layer system causes the voltammetric resolution between the two species to be enhanced. Differential pulse voltammetry is used to measure the limits of detection for VMA on an edge-plane pyrolytic graphite electrode and on commercially available multiwalled carbon nanotube screen printed electrode, with limits of detection of 1.7 and 1.0 microM, respectively. These limits of detection are well within the range of sensitivity required for clinical sample measurement

The physicochemical aspects of DNA sensing using electrochemical methods

As our understanding of the human genome increases there is an ever expanding demand for fast, sensitive and selective methods of DNA analysis. Due to the low associated production costs, and high sensitivity and selectivity of many electrochemical systems, development of these methods holds much promise. Production of a portable low-cost system suitable for DNA analysis has the potential to revolutionise modern health care. Single-nucleotide polymorphisms (SNPs) are a common form of genomic variation. These alterations to the genetic code can cause a change in a given genes’ function and as such may increase an individuals susceptibility to a disease. Consequently it is imperative that any system of DNA analysis is able to distinguish between single changes in the base pair sequence. This review aims to build an understanding of DNAs structure and physicochemical properties, focusing on the thermodynamics and kinetics of DNA hybridisation. From this a wide overview of the current methods of electrochemical DNA sensing is provided with the discussion of both labeled and non-labeled methods. Recent work in which DNA sensing has been taken beyond single-analyte detection is also discussed

Single nanoparticle detection in ionic liquids

Nanoimpacts are novelly observed in a room temperature ionic liquid with the oxidation of silver nanoparticles in 1-butyl-3-methylimidazolium tetrafluoroborate. The addition of chloride facilitates the oxidation of the silver nanoparticles to silver chloride, which is observed as spikes in the current that correspond to single nanoparticles occurring via nanoimpacts, whereby random diffusion (Brownian motion) brings particles to within electron tunnelling distance of an electrode. © 2016 American Chemical Society