Damien W. M. Arrigan

Nanoelectrodes, nanoelectrode arrays and their applications

This review deals with the topic of ultrasmall electrodes, namely nanoelectrodes, arrays of these and discusses possible applications, including to analytical science. It deals exclusively with the use of nanoelectrodes in an electrochemical context. Benefits that accrue from use of very small working electrodes within electrochemical cells are discussed, followed by a review of methods for the preparation of such electrodes. Individual nanoelectrodes and arrays or ensembles of these are addressed, as are nanopore systems which seek to emulate biological transmembrane ion transport processes. Applications within physical electrochemistry, imaging science and analytical science are summarised.

Tutorial review. Voltammetric determination of trace metals and organics after accumulation at modified electrodes

The applications of chemically modified electrodes (CMEs) to the determination of trace amounts of metals and organic analytes are discussed. The common feature of the CMEs reviewed is that prior to the voltammetric determination step the analyte is collected or accumulated onto the electrode. The accumulation step serves to preconcentrate the analytes from dilute solution, making their determination easier. It is analogous to the preconcentration step employed in stripping voltammetry at conventional electrodes. Trace analytes can be accumulated in several ways: ion-exchange, complexation, bioaccumulation, covalent attachment, and hydrophobic interaction accumulation schemes are discussed. Uses of CMEs in speciation analysis are outlined, and modified mercury-film electrodes and mercury films based on CMEs are discussed. Examples of the use of CMEs in the determination of trace metals [e.g., copper(II), lead(II), mercury(II), and silver(I) ions] and organic compounds are given.

DNA arrays, electronic noses and tongues, biosensors and receptors for rapid detection of toxigenic fungi and mycotoxins: A review

This paper presents an overview of how microsystem technology tools can be applied to the development of rapid, out–of–laboratory measurement capabilities for the determinations of toxigenic fungi and mycotoxins in foodstuffs. Most of the topics discussed are all under investigation within the European Commission–sponsored project Good–Food (FP6–IST). These are DNA arrays, electronic noses and electronic tongues for the detection of fungal contaminants in feed, and biosensors and chemical sensors based on microfabricated electrode systems, antibodies and novel synthetic receptors for the detection of specific mycotoxins. The approach to resolution of these difficult measurement problems in real matrices requires a multidisciplinary approach. The technology tools discussed can provide a route to the rapid, on–site generation of data that can aid the safe production of high–quality foodstuffs.

Voltammetric characterisation of silicon-based microelectrode arrays and their application to mercury-free stripping voltammetry of copper ions.

This paper describes the electrochemical characterisation of a range of gold and platinum microelectrode arrays (MEAs) fabricated by standard photolithographic methods. The inter-electrode spacing, geometry, numbers and dimensions of the electrodes in the arrays were found to influence the voltammetric behaviours obtained. Excellent correlation was found between experimental data and theoretical predictions employing published models of microelectrode behaviour. Gold MEAs were evaluated for their applicability to copper determination in a soil extract sample, where agreement was found between the standard analytical method and a method based on underpotential deposition-anodic stripping voltammetry (UPD-ASV) at the MEAs, offering a mercury-free alternative for copper sensing.

A study of L-cysteine adsorption on gold via electrochemical desorption and copper(II) ion complexation

The adsorption of L-cysteine on gold electrodes was studied by the methods of electrochemical oxidative desorption and Cu2+ complexation. Comparisons with L-cystine adsorption, via the oxidative method, revealed lower saturation surface coverages for L-cystine (1.7 ± 0.17 nmol cm–2) than for L-cysteine (2.7 ± 0.11 nmol cm–2), perhaps due to an incomplete cleavage of the S–S bond on adsorption and consequent effects on molecular orientation. Electrochemistry of solution-phase redox probes at L-cysteine monolayer electrodes have shown a porous open monolayer structure amenable to solvent, electrolyte and probe molecule permeation. Both the L-cysteine- and the L-cystine-derived electrodes were capable of complexation of Cu2+ ions from solution which were then detectable by cyclic voltammetry. The binding constant of Cu2+ ions to the L-cysteine monolayer was estimated to be 2 × 105 L mol–1 while the molar ratios of L-cysteine to Cu2+ on L-cysteine- and L-cystine-derived films were 2∶1 and 2.5∶1, respectively. Detection of Cu2+ down to 10–7 M was possible with cyclic voltammetry, indicating the possible use of such simple films for trace metal analysis.

Surface immobilisation of antibody on cyclic olefin copolymer for sandwich immunoassay

In this work, the surface functionalisation of the commercially available cyclic olefin copolymer (COC) materials, Zeonor and Zeonex, has been studied. The methodology employed involved oxidation in oxygen plasma, functionalisation of the oxidized surface with aminopropyl triethoxy silane and, finally, attachment of antibody using covalent linker molecules. 1,4-Phenylene diisothiocyanate was selected as the most suitable cross-linker for the attachment of protein, as assessed by fluorescent intensity measurements on immobilised FITC-labelled IgG antibody. The modification method was characterised by contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy (XPS) and fluorescence microscopy. The data are consistent with the deposition of a polymeric film of the silane chemisorbed to the oxidised plastic surface. The functionalised surfaces were employed in a sandwich immunoassay format using the reagents goat anti-human IgG (G alphaHIgG) and fluorescently labelled G alphaHIgG (Cy5-G alphaHIgG) as capture and detection antibodies, respectively, and with human IgG (HIgG) as the model analyte. The lowest concentration of HIgG detected was 0.1 ng ml(-1), with a relative standard deviation of 15%. Non-specific binding effects were also assessed. The method and supporting data demonstrate that simple approaches to surface functionalisation can be adapted to plastic-based devices.

A scanning force microscopy study of poly(phenol) films containing immobilized glucose oxidase

Scanning force microscopy has been used to characterize poly(phenol) films containing immobilized glucose oxidase (GOx). The films were grown by electrochemical polymerization of phenol from neutral aqueous solution at highly ordered pyrolytic graphite (HOPG) electrodes. Scanning force microscopy, in contact mode, shows the surface topography of the enzyme/polymer composite films. Films containing immobilized enzyme were found to be significantly rougher than either the bare substrate or enzyme free poly(phenol) films grown under the same conditions. The thickness of the electropolymerized films was determined by using the ‘nanodozing’ technique in which the scanning tip is used to scrape away an area of the polymer film to reveal the HOPG surface beneath. Using this method we find that the enzyme-containing films are 7·4 ± 0·8 nm thick whereas the enzyme free poly(phenol) films are 3 ± 1·2 nm thick. Amperometric measurements using the tetrahiafulvalene/tetrahiafulvalenium (TTF/TTF+) couple as a redox mediator for oxidation of the immobilized GOx confirm that the immobilized enzyme is catalytically active. Analysis of the amperometric response of the electrodes at different glucose concentrations allows us to determine the kinetic parameters for the immobilized enzyme. The amperometric responses are consistent with the presence of a thin active layer of GOx one or two molecules thick immobilized at the electrode surface and agree with the conclusions from the scanning force microscopy study.

An environmental monitoring system for trace metals using stripping voltammetry

Abstract There is increasing pressure on industry and regulatory bodies to monitor the discharge of trace metals into the aquatic environment. The determination of trace metals can be achieved to parts per billion (ppb) levels using anodic stripping voltammetry (ASV) at screen printed electrodes, controlled using an NMRC potentiostat coupled with software control. The disposable testheads consist of inexpensive materials and allow for low-cost production in batch processes. Voltammetric (the measurement of current as a function of potential) methods of analysis are attractive for the determination of copper, cadmium, lead and zinc. A three electrode set-up is used both in the preparation of the mercury film on a carbon electrode and in the subsequent anodic stripping voltammetric detection step. The performances of the reference electrodes, the screen-printed carbon and the FPGA based unit have been investigated in this paper.

Electrochemical Study of Insulin at the Polarized Liquid-Liquid Interface

This paper reports on the electrochemical behavior of bovine insulin at the interface between two immiscible electrolyte solutions (ITIES). The voltammetric ion-transfer response obtained in the presence of insulin was dependent on the aqueous phase pH conditions and on the nature of the organic phase electrolyte employed in experiments. Optimal detection was obtained at acidic pH below the isoelectric point of insulin where it was positively charged. A shift in transfer potentials to lower potential values was observed with decreasing hydrophobicity of the anion of the organic phase electrolyte. No ion-transfer response was observed at pH values of the aqueous phase above the isoelectric point, where insulin was negatively charged. These results suggest that the voltammetric response is due to ion-pairing interactions at the ITIES between positively charged insulin and the hydrophobic anion of the organic phase electrolyte, together with adsorption of the ion-pair at the interface. The voltammetric response was obtained for insulin at concentrations down to 1 muM. These results show that electrochemistry is useful in studying the behavior of this important protein molecule at the polarized water-1,2-DCE interface and provides an alternative detection mode for bioanalytical applications.

Bioanalytical Detection Based on Electrochemistry at Interfaces between Immiscible Liquids

Abstract The electrochemical properties of the interface between two immiscible electrolyte solutions (ITIES) have been studied intensely for more than 30 years now. Although there has always been an interest in the analytical applications of this electrochemical behavior, the opportunities it affords for the detection of biomolecular species, in particular, have become of increased interest in recent years. This mini-review discusses recent advances in this bioanalytical arena, highlighting the detection of molecules ranging from low-molecular-mass bioorganic substances (e.g., neurotransmitters, amino acids, drugs) through to proteins and nucleic acids.