Jonathan M. Slater

Gas and vapour detection with poly(pyrrole) gas sensors

The response mechanism of the conducting polymer poly(pyrrole) to a selection of gases and vapours was investigated using two techniques: measurement of resistance change and mass changes using a piezoelectric quartz crystal microbalance with the objective of characterizing responses for incorporation in sensor arrays. Bromide-doped films were exposed to methanol, hexane, 2-2-dimethylbutane, ammonia and hydrogen sulfide. Polymers of different thicknesses were also exposed to methanol vapour and the response profiles were studied. The responses were all of a Fickian type except the piezoelectric signal, which exhibited an anomalous non-Fickian response to methanol. This suggests that the poly(pyrrole) resistance changes frequently observed are partly due to one stage in the two-stage sorption perhaps involving the swelling of the polymer. It was concluded that the response mechanism of poly(pyrrole) sensing of different gases and vapours is due to a mixed response involving electronic effects and physical effects.

Multi-layer conducting polymer gas sensor arrays for olfactory sensing

Conductivity sensors using the conducting polymer poly(pyrrole) and its derivatives have been prepared with a novel electrode design which allows the probing of resistance changes within zones of a single sensor. It was found that the application of principal component analysis to the sensor responses allowed methanol, ethanol and propanol to be distinguished. The use of layered conducting polymers improved the discrimination of the sensor. A sensor consisting of four electrode pairs and two polymer layers is capable of separating the response of certain alcoholic beverages.

Membrane design and photocuring encapsulation of flatpack based ion-sensitive field effect transistors

A flatpack mounted ion-sensitive field effect transistor (ISFET) device fixed in a Perspex flow cell provides a compact flow injection analysis (FIA) manifold. This allows for the injection of small sample volumes and rapid substitution of one ISFET sensor with another. Encapsulation of the chip was facilitated by the use of a “dry film” DuPont Riston photoresist material, which reproducibly patterned the encapsulant around the gate regions.Analytical conditions have been optimised using conventional solvent-cast PVC matrix membranes. As alternatives to these, an examination of a series of photopolymerisable methacrylate membranes showed butyl methacrylate to be a viable system. This has the advantage of ease of application to the FET gate and possible elimination of uneven membranes arising from evaporation of the solvent normally used to apply PVC. The butyl methacrylate-dioctylphenyl phosphonate-calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate} sensor system yields good flatpack mounted ISFET devices which perform well in the above FIA manifold. It has the expected response characteristics for the calcium ion-sensing system with lifetimes >21 d.

Examination of ammonia–poly(pyrrole) interactions by piezoelectric and conductivity measurements

The conducting polymer poly(pyrrole), electrochemically prepared and doped with anions, has been found to be a responsive coating for a piezoelectric gas detection system. Polymers doped with bromide, nitrate and sulphate ions were tested. It was found that samples of ammonia gas cause a measurable frequency decrease, interpreted as adsorption by the polymer coating of the quartz crystal; the linear range was 0.05–1% for mixtures of the gas in nitrogen. These signals were found to correspond to simultaneous conductivity changes of a similarly prepared poly(pyrrole) sample, showing analogies in the two sensing mechanisms. The duality of the poly(pyrrole) response increases the possibilities of using it as a gas sensor.

Modified poly(vinyl chloride) matrix membranes for ion-selective field effect transistor sensors

Various membrane compositions, in which a poly(vinyl chloride)(PVC)-poly(vinyl alcohol) copolymer (copolymer VAGH) and silicon tetrachloride were incorporated as additional components in the classical PVC matrix membrane composition for ion-selective electrodes, were studied for their adhesion characteristics to the gates of field effect transistor devices used for the construction of ion-selective field effect transistors (ISFETs). These studies were carried out on a potassium ion-sensing system based on valinomycin with dioctyl adipate as plasticising solvent mediator.The inclusion of 10–25%m/m copolymer VAGH and 1%m/m silicon tetrachloride in the membrane casting solution was found to increase the adhesion of the plastic membrane to the gate, reflected by an increase in ISFET functional lifetimes of 4–5 d over those which did not contain VAGH and by ca. 10 d over those which contained neither VAGH nor silicon tetrachloride. Leaching of membrane components is suggested as a factor limiting the ISFET lifetimes in adhesion studies.

Coated-wire and composite ion-selective electrodes based on doped poly(pyrrole)

Chemically and electrochemically prepared doped poly(pyrrole) has been evaluated as a membrane component for ion-selective electrodes (ISEs). Two types of ISE were prepared: coated-wire electrodes (CWEs) and classical ISEs with heterogeneous membranes. The classical ISE membranes investigated consisted of poly(pyrrole) particles dispersed in a polymer binder; the CWEs were doped poly(pyrrole) films produced by electropolymerization onto platinum wire or glassy carbon rods. Polymerization was carried out in aqueous electrolytes containing chloride, fluoride or nitrate counter anions. The characteristics of the electrodes varied with the electrode type and the conditioning procedure. The CWEs generally exhibited a sub-Nernstian response to the corresponding dopant anion, the actual response depending on polymerization conditions and subsequent conditioning of the electrode. In comparison, the classical ISEs with a heterogeneous membrane of electrochemically prepared poly(pyrrole) showed either reduced sub-Nernstian responses or cationic responses. The classical ISEs with membranes containing chemically polymerized pyrrole exhibited a sub-Nernstian response to the corresponding dopant anion. Electrode failure and poor electrode response appears to be related to the incorporation of cationic sites into the membrane.

Simple solid wire microdisc electrodes for the determination of vitamin C in fruit juices

Five micrometre gold microwire encapsulated in a thin sheath of borosilicate glass produced by the Taylor wire process can be used in the fabrication of gold microdisc electrodes. By combining glass-coated wire with fluorinated ethylene propylene lined poly(tetrafluoroethylene) tubing, disposable, high quality electrodes suitable for a range of applications, can be fabricated easily. Chemical etching techniques were investigated as a means of finishing the electrode surface, and the resulting surfaces were studied using scanning electron microscopy and evaluated electrochemically using model redox compounds. The electrodes obtained by this fabrication method were used to determine L-ascorbic acid and the results were compared with values obtained using high-performance liquid chromatography and iodimetry. Measurements were taken in buffered solutions, fruit juices, and directly in fruit. Previous studies have shown that ascorbic acid fouls the electrode surface during voltammetric analysis; these problems were overcome by using potential cycling techniques.

The optimisation of sorption sensor arrays for use in ambient conditions

Abstract Solvation models have been used to define the essential measurement parameters for a general purpose array and candidate sensor materials evaluated in ambient conditions. Where suitable existing coating materials were not available novel materials were synthesised and evaluated. Three different arrays each consisting of eight sensors were studied to derive a final array. The resulting array of eight sensors exhibited an excellent ability to distinguish different chemical functionalities and was capable of detecting a slight change of molecular size within the same chemical group. The discrimination of two component mixtures of various concentrations was also demonstrated. Principal component analysis (PCA) and discriminant function analysis (DFA) were employed to display the array responses.

Monitoring of engine oil degradation by voltammetric methods utilizing disposable solid wire microelectrodes

Gold wire microelectrode assemblies were used to perform cyclic and pulsed voltammetry directly in oil. The unmodified microelectrode behaviour was characterized by measuring ferrocene in mineral oil and a linear calibration was obtained over the concentration range 1–5 mM. The voltammograms remain sigmoidal with scan rates up to 200 mV s - 1 . An antioxidant, phenothiazine, was studied as a typical marker of engine oil degradation and a linear calibration was obtained over the normal additive concentration range for phenothiazine (0–10 mM). For the phenothiazine system, the voltammetric response remained sigmoidal although hysteresis was observed in the scan, which is thought to be due to the difficulty in deprotonation of the system for phenothiazine oxidation. The observations demonstrated that the sensor assembly allows voltammetry in previously inaccessible systems, the resistivity of mineral oil being five orders of magnitude greater than that of water.

On-chip microband array electrochemical detector for use in capillary electrophoresis

A new electrochemical detector, designed for use with conventional capillary electrophoresis systems, is described. The sensor incorporates a platinum microband array electrode and three larger electrodes on a silicon chip. The fluid flow is facilitated over the electrodes by an etched FI glass cap which allows simple connection to separation capillaries. The device was evaluated using a series of catecholamines with the lowest detectable amount of hydroquinone being 1.4 fmol (based on three times the standard deviation (3σ) of the background noise). Comparison with ultraviolet (UV) detection, using the same separation system, indicated that electrochemical detection was more sensitive for this range of analytes. The UV detection limit of hydroquinone was 58 fmol. Despite this increased sensitivity, the separation efficiency of the electrochemical method was reduced, as expected for the off-column detection mode.