Philip N. Bartlett

Electrochemical immobilisation of enzymes: Part II. Glucose oxidase immobilised in poly-N-methylpyrrole

Electrochemical polymerisation of N-methylpyrrole in buffered aqueous solution containing the enzyme glucose oxidase produces adherent films at the electrode surface containing the active enzyme. Electrodes prepared in this manner can be used to detect glucose in solution in the range 0–0.22 mol dm−3. The observed response from such electrodes as a function of glucose concentration, enzyme loading, and film thickness are in excellent agreement with theory. From a comparison of the experimental results and the theory we are able to characterise the enzyme kinetics and diffusion within the conducting polymer film. The results are consistent with a model in which the hydrogen peroxide produced by the enzymatic reaction reacts at the electrode surface and not at the polymer itself.

Electrochemical immobilisation of enzymes: Part I. Theory

Abstract Electrochemical polymerisation is an attractive method for the immobilisation of redox enzymes at electrode surfaces since it allows precise control over the amount and spatial distribution of the immobilised enzyme. In this paper we present an approximate analytical treatment of the response of an amperometric enzyme electrode made in this way. We derive appropriate expressions for the response of such a sensor to the addition of substrate. On the basis of the model we are able to distinguish between direct electrochemical reaction of the product of the enzyme reaction at the electrode and reaction throughout the film on the polymer used for immobilisation. This is especially relevant when conducting polymers are used to immobilise the enzyme.

Sensors and sensory systems for an electronic nose

1. Odours -- The Stimulus for an Electronic Nose G.H. Dodd, P.N. Bartlett, J.W. Gardner. 2. Biophysical Properties of Olfactory Receptor Neurones H.A. Schultens, D. Schild. 3. Molecular Modeling and the Selective Sensor Response M. Thompson, D.C. Strone. 4. Odour Sensors for an Electronic Nose P.N. Bartlett, J.W. Gardner. 5. Fundamentals and Recent Developments of Homogeneous Semiconducting Sensors D. Kohl. 6. Fine-Tuning of Electron- and Ion-Conducting Materials for Sensor Arrays W. Gopel, K-D. Schierbaum, S. Vaihinger, U. Weimar. 7. Microsensors Based on Modulation of Work Function J. Janata. 8. Studies of Interactions Between Organic Vapours and Organic Semiconductors Aplications to Chemical Sensing M. Josowicz, P. Topart. 9. Silicon Based Surface Acoustic Wave Gas Sensors M.S. Nieuwenhuizen, A.J. Nederlof. 10. Miniaturisation of Gas Sensor Substrates. Problems and Benefits of Microelectronic Technology U. Dibbern. 11. Pattern Recognition in Odour Sensing J.W. Gardner, P.N. Bartlett. 12. Desired and Achieved Characteristics of Sensor Arrays G. Horner, R. Muller. 13. Use of Pattern Recognition Techniques Applied to Signals Generated by a Multielement Gas Sensor Array as a Means of Compensating for Poor Individual Element Response A.W.J. Cranny, J.K. Atkinson. 14. Pattern Recognition in Electronic Noses by Artificial Neural Network Models T. Moruzumi, T. Nakamoto, Y. Sakuraba. 15. Sensor Arrays Using Conducting Polymers for an Artificial Nose K.C. Persaud, P. Pelosi. 16. Monitoring ofFish Freshness Using Tin Oxide Sensors R. Olafsson, E. Martinsdottir, G. Olafsdottir, P.I. Sigfusson, J.W. Gardner. 17. Chemical Sensor Arrays: Practical Insights and Examples J.R. Stetter. 18. Electronic Noses Based on Field Effect Structures I. Lundstrom, E. Hedborg, A. Spetz, H. Sundgren, F. Winquist. Index.

Conducting polymer gas sensors Part III: Results for four different polymers and five different vapours

The application of four different conducting polymers (polypyrrole, poly-N-methylpyrrole, poly-5-carboxyindole and polyaniline) as sensors for organic vapours has been investigated. The sensors are formed by the electrochemical polymerization of the appropriate monomers across a 12 μm gap between two gold microband electrodes. Upon exposure to vapours the polymers show conductivity changes that are rapid and in general reversible at room temperature. Of the four polymers investigated, under the deposition conditions employed and for the vapours used (methanol, ethanol, acetone, ether and toluene), poly-5-caboxyindole is found to give the most stable, reproducible behaviour and to be the most promising material for sensor applications. The use of these materials in intelligent gas sensors is discussed.

Strategies for the development of amperometric enzyme electrodes

There are now a number of distinct strategies which can be employed to make amperometric enzyme electrodes. These include the use of homogeneous mediators, modified electrodes and organic conducting salts. In this paper we review these strategies and discuss their application to NAD(P)H dependent dehydrogenase and flavoprotein based biosensors. In addition we discuss recent work on the immobilisation of glucose oxidase in polypyrrole, poly-N-methylpyrrole, polyaniline and polyphenol films electrochemically grown at the electrode surface and on the covalent attachment of redox mediators to glucose oxidase in order to achieve direct electron transfer to the electrode.

Electronic nose for monitoring the flavour of beers

The flavour of a beer is determined mainly by its taste and smell, which is generated by about 700 key volatile and non-volatile compounds. Beer flavour is traditionally measured through the use of a combination of conventional analytical tools (e.g., gas chromatography) and organoleptic profiling panels. These methods are not only expensive and time-consuming but also inexact due to a lack of either sensitivity or quantitative information. In this paper an electronic instrument is described that has been designed to measure the odour of beers and supplement or even replace existing analytical methods. The instrument consists of an array of up to 12 conducting polymers, each of which has an electrical resistance that has partial sensitivity to the headspace of beer. The signals from the sensor array are then conditioned by suitable interface circuitry and processed using a chemometric or neural classifier. The results of the application of multivariate statistical techniques are given. The instrument, or electronic nose, is capable of discriminating between various commercial beers and, more significantly, between standard and artificially-tainted beers. An industrial version of this instrument is now undergoing trials in a brewery.

Conducting polymer gas sensors part I: fabrication and characterization

Abstract A simple but effective method for the fabrication of reusable dual-microband electrodes for use as gas sensors based on electrochemically polymerized conducting polymers is described. The electrodes are made by sputtering gold onto both sides of thin (12 μm) Mylar films and then encapsulating the resulting gold/Mylar/gold sandwich so that only the edge is exposed. The resulting electrodes are characterized using cyclic voltammetry, a.c. impedance and chronoamperometry. Following the electrochemical deposition of polypyrrole onto the dual-microband electrodes, they can be used as gas-sensitive chemiresistors. Preliminary results for the change of resistance of such a device on exposure to methanol vapour are presented.

Conducting polymer gas sensors part II: response of polypyrrole to methanol vapour

Abstract Conducting films of polypyrrole deposited across a narrow gap between two gold electrodes can be used to sense methanol vapour by following changes in the resistance of the polymer. The response is rapid and reversible at room temperature. The effects of the concentration of methanol, the operating temperature and the film thickness on the response have been investigated. The data are consistent with a model in which the methanol interacts with sites either on, or within, the polymer. Simple gas sensors of this type are promising candidates for use in an ‘intelligent’ gas sensor based on an array of gas-sensing elements.

Oxidation of β-nicotinamide adenine dinucleotide (NADH) at poly(aniline)-coated electrodes

Poly(aniline)–poly(vinylsulfonate) composite-coated glassy carbon electrodes are found to give stable and reproducible electrocatlytic responses to NAD(P)H in citrate–phosphate buffer at pH 7. Analysis of these amperometric responses as a function of NAD(P)H concentration, film thickness, electrode potential and rotation speed show that the reaction occurs within the film, rather than just at the outside, and that the reaction is reversibly inhibited by the oxidation product [NA(P)D + ]. Based on a full kinetic analysis, rate constants for the various processes are determined.

An Enzyme Switch Employing Direct Electrochemical Communication between Horseradish Peroxidase and a Poly(aniline) Film.

An enzyme switch, or microelectrochemical enzyme transistor, responsive to hydrogen peroxide was made by connecting two carbon band electrodes (∼10 μm wide, 4.5 mm long separated by a 20-μm gap) with an anodically grown film of poly(aniline). Horseradish peroxidase (EC 1.11.1.7) was either adsorbed onto the poly(aniline) film or immobilized in an insulating poly(1,2-diaminobenzene) polymer grown electrochemically on top of the poly(aniline) film to complete the device. In the completed device, the conductivity of the poly(aniline) film changes from conducting (between - 0.05 and + 0.3 V vs SCE at pH 5) to insulating (>+0.3 V vs SCE at pH 5) on addition of hydrogen peroxide. The change in conductivity is brought about by oxidation of the poly(aniline) film by direct electrochemical communication between the enzyme and the conducting polymer. This was confirmed by measuring the potential of the poly(aniline) film during switching of the conductivity in the presence of hydrogen peroxide. The devices can be reused by rereducing the poly(aniline) electrochemically to a potential below +0.3 V vs SCE. A blind test showed that the device can be used to determine unknown concentrations of H(2)O(2) in solution and that, when used with hydrogen peroxide concentrations below 0.5 mmol dm(-)(3), the same device maybe reused several times. The possible development of devices of this type for use in applications requiring the measurement of low levels of hydrogen peroxide or horseradish peroxidase is discussed.