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In this paper a comparison between three commercially-available, screen-printable graphite inks for the construction of phenolic biosensors is made. The enzyme tyrosinase was immobilised within a polymer matrix and the substrate catechol was used to characterise the bio-electroanalytical response of each electrode. Biosensors fabricated from Gwent graphite inks exhibited the greatest sensitivity (5740 mA mol cm −2 ) compared to Dupont and Acheson graphite-based inks. This difference in sensitivity was attributed to a combination of a larger electroactive surface area, and thus a greater number of immobilised enzyme molecules. However, the dynamic range was considerably smaller (0.025–14 μM) indicating that the enzyme molecules were easily accessible to the substrate catechol. The surface properties of the biosensors were characterised using ac impedance, which indicated that the presence of the polymer on the electrode surface not only increased the charge-transfer kinetics of the three biosensors, but also increased the surface roughness of biosensors fabricated from Gwent inks. On the basis of these results Gwent graphite-based inks were used for analysis of phenolic compounds in lager beers by flow-injection analysis. The biosensor displayed favourable response characteristics, but cannot differentiate between the various phenolic compounds present in the samples. Nevertheless, the biosensor maybe suitable for indicating the phenolic status of beer or brew samples compared to time-consuming traditional methods, e.g. colorimetric or chromatographic methods.

TiO2 electrodes were prepared by the electrophoretic immobilisation of Degussa P25 TiO2 powder on titanium alloy (Ti–6Al–4V). These electrodes were used as photoanodes in a two compartment photoelectrochemical cell to investigate the photocatalytic oxidation of oxalate. The effects of electron acceptors, i.e. oxygen and Fe3+, on the rate of mineralisation of oxalate were determined. Significant increases in the short-circuit photocurrent densities were observed upon the addition of oxalate to the anode compartment when oxygen was present only in the cathode compartment. When oxygen was present in both the anode and cathode compartments, little photocurrent was observed. Oxygen scavenges both conduction band electrons from the TiO2 surface and primary radicals formed in the initial one electron oxidation of oxalate. An increase in the anodic photocurrent and rate of degradation of oxalate was observed when oxygen was replaced with Fe3+ as the electron acceptor in the cathode compartment. The TiO2 coated anodes behaved as microporous nanocrystalline film electrodes where the anodic photocurrent is dependent on the diffusion of majority charge carriers through the film. Oxalate acts as an efficient hole acceptor, inhibiting the photoanodic production of oxygen.

The effect of different hole acceptors on the photocurrent response of particulate TiO2 anodes was investigated. In all cases there was a significant increase in the photocurrent over that observed for the photooxidation of water. The photocurrent efficiency observed in the presence of hole acceptors followed the order: oxalate > formate > acetate > methanol. The order of efficiency of the hole acceptors does not correlate with reported free hydroxyl radical reactivity constants or with reported oxidation potentials for the substrates studied. Adsorption measurements made in the dark showed that oxalate was much more strongly adsorbed on the electrode surface than either acetate or formate. Impedance spectroscopy indicated an increase in the capacitance of the system due to the presence of oxalate, both in the dark and under illumination.

Apple tissue containing polyphenol oxidase was incorporated in carbon paste matrix to make a biosensor for the analysis of flavanols in beers. The electrochemical and structural properties of different carbon paste compositions were assessed using ac impedance, other electrochemical techniques and scanning electron microscopy. Results are discussed regarding electroactive surface, mechanical stability, impedance parameters and surface roughness of the carbon pastes. From these results, an optimum graphite/Nujol ratio of 4 was chosen and applied to the construction of apple-based biosensors. The electrochemical, morphological and analytical properties of these sensors were investigated with respect to biomolecule loading. The analytical properties (sensitivity, Imax) of these biosensors increased with plant tissue loading. This result was correlated with ac impedance parameters, electroactive surface and carbon paste morphology. A biosensor using a plant tissue/carbon paste ratio of 0.086 was used to assess sensitivity and selectivity to various flavanols and phenolic compounds.

Various plant tissues containing polyphenol oxidase were incorporated in carbon paste electrodes to make biosensors for the determination of catechol related components in beers, principally flavanols such as catechins and their dimers and trimers. Calibration graphs were produced for each plant tissue with both catechol and (−) epicatechin as standards. The response of the banana based sensor was rather erratic and showed a large zero error, probably due to flavanols in the banana. Potato, wet apple and dried apple were all satisfactory. The response with dried apple was 60% that of wet apple, but showed greater stability. The response of (−)epicatechin was 1.4 to 1.5 times that of catechol with wet or dried apple. Banana and potato based sensors were used for the determination of total flavanols in a range of commercial beers and lagers. Results with the banana were rather high due to the zero error. Good analytical data were obtained with potato, comparable to those obtained using colorimetric or liquid Chromatographic analyses.

Variable sweep rate voltammetry of carbon dioxide at a glassy carbon electrode in aqueous solutions containing tetraethylammonium perchlorate gave two successive one-electron kinetically controlled waves. The apparently low peak potentials (−0.74 V vs. SCE at pH 10) were rationalised by consideration of the ionic radius of the tetraethylammonium cation relative to other cations. The rate limiting process appeared to be release of carbon dioxide molecules from aqueous bicarbonate. Equilibrium constants for this process were measured by rotating disc electrode voltammetry and found to be 0.069 at pH 9.1 and 0.076 at pH 10. The rate constant for the formation of carbon dioxide molecules was calculated from Kl 1 2 and found to be kf = 1.6 s−1 at pH 10. However, these data conflicted with published data for this process from homogeneous kinetics. An adsorption model was applied to rationalise the data. The surface coverage was found to be 0.019 at pH 10, in line with published data.

There are three basic electrochemical processes which are useful in transducers for biosensors: (i) potentiometry, the measurement of a cell potential at zero current; (ii) voltammetry (amperometry), in which an oxidising (or reducing) potential is applied between the cell electrodes and the cell current is measured; and (iii) conductimetry, where the conductance (reciprocal of resistance) of the cell is measured by an alternating current bridge method.

Skin contact electrodes require electrolyte gels between the skin and the electrode in order to ensure good electrical contact. The effect of different types of electrolyte gel on skin impedance was studied. The main types of gels used were wet gels, karaya-gum based hydrogels and synthetic copolymer-based hydrogels [2-acrylamido-2-methylpropanesulfonic acid–N, N′-methylenebis(acrylamide) copolymers]. The effect of variation in gel composition on the impedance of the skin was investigated.

Cyclic voltammograms of quinones were recorded in acetonitrile in the presence of various substrates: carbonyl compounds, halobenzenes, Methyl Viologen and Neutral Red. When illuminated with light of λ >410 nm, catalytic waves were observed. From the ratio of the catalysed to uncatalysed peak current, electron transfer rate constants were calculated using the working curves of Saveant and coworkers. The values of these rate constants were compared with the values obtained by Shukla and Rusling for different systems using a similar method and with quenching rate constants calculated using Rehm–Weller–Marcus theory.

We have studied voltammetric reduction waves for carbon dioxide in water, dimethylsulphoxide, acetonitrile and propylene carbonate using electrodes of glassy carbon, mercury, platinum, gold and lead as applicable. The apparent number of electrons involved was determined from the current function and in some cases by chronoamperometric analysis of the diffusion controlled tail of the wave. Results were compared with those from other workers.

While cyclic voltammetry is a widely used electroanalytical technique,1 variable voltage sweep rates have not been extensively exploited; though the theory for a variety of mechanistic situations has been worked out,2, 3 and the feasibility of its use up to 50 000 V s–1 has been tested experimentally.4 The main advantage of the method is that a simple picture is obtained which may indicate the existence of an intermediate.In this paper a range of experimental situations is discussed, including that of stable intermediates, unstable electro-active intermediates, and unstable electro-inactive intermediates. The range of information about the nature of the intermediates obtainable by cyclic voltammetry is given and hence the ability to infer the nature of the intermediate from these data is shown. Future developments involving the digital acquisition of data are discussed; these will enable faster sweep rates to be used and hence shorter-lived intermediates to be detected.