Krishna C. Persaud

Towards an integrated electronic nose using conducting polymer sensors

Abstract This paper describes progress that has been made towards realizing an artificial nose based on arrays of conducting polymers. Electrically conducting organic polymers based on heterocyclic molecules display reversible changes in conductivity when exposed to polar volatile chemicals. In the sensor described, the polymers are interrogated for resistance changes by means of an application-specific integrated circuit (ASIC) realized in BiCMOS technology. The ASIC and the polymer array are housed on a single thick-film ceramic substrate.

On the study of feature extraction methods for an electronic nose

In this study, we analyzed the transient of microsensors based on tin oxide sol–gel thin film. A novel method to this research field for data analysis and discrimination among different volatile organic compounds is presented. Moreover; several feature extraction methods have been considered, both steady-state (fractional change, relative, difference and log) and transient (Fourier and wavelet descriptors, integral and derivatives) information. Feature extraction methods have been validated qualitatively (by using principal component analysis) and quantitatively on the classification rate (by using a radial basis function neural network). # 2002 Elsevier Science B.V. All rights reserved.

Polymers for chemical sensing

Chemical sensors play an increasingly important role in monitoring the environment we live in, providing information on industrial manufacturing processes and their emissions, quality control of foods and beverages, and a host of other applications. Electrically conductive plastics are being developed for many useful applications. Improvement in understanding of the physical and chemical mechanisms by which electrical conduction occurs in these materials is now leading to a new generation of chemical sensors, which are reviewed in this article.

Capacitance-modulated transistor detects odorant binding protein chiral interactions

Peripheral events in olfaction involve odorant binding proteins (OBPs) whose role in the recognition of different volatile chemicals is yet unclear. Here we report on the sensitive and quantitative measurement of the weak interactions associated with neutral enantiomers differentially binding to OBPs immobilized through a self-assembled monolayer to the gate of an organic bio-electronic transistor. The transduction is remarkably sensitive as the transistor output current is governed by the small capacitance of the protein layer undergoing minute changes as the ligand–protein complex is formed. Accurate determination of the free-energy balances and of the capacitance changes associated with the binding process allows derivation of the free-energy components as well as of the occurrence of conformational events associated with OBP ligand binding. Capacitance-modulated transistors open a new pathway for the study of ultra-weak molecular interactions in surface-bound protein–ligand complexes through an approach that combines bio-chemical and electronic thermodynamic parameters.

Evaluation of a radial basis function neural network for the determination of wheat quality from electronic nose data

Abstract Odorous contaminants in wheat have been detected using a conducting polymer array. A radial basis function artificial neural network (RBFann) was used to correlate sensor array responses with human grading of off-taints in wheat. Wheat samples moulded by artificial means in the laboratory were used to evaluate the network, operating in quantitative mode, and also to develop strategies for evaluating real samples. Commercial wheat samples were then evaluated using the RBFann as a classifier network with great success, achieving a predictive success of 92.3% with no bad samples misclassified as good in a 40-sample population (24 good, 17 bad) using a training set of 92 samples (72 good, 20 bad).

Screening for bacterial vaginosis : a novel application of artificial nose technology

The AromaScan system was used to analyse vaginal swabs from 68 women attending a genitourinary clinic. Using clinical criteria, subjects were assessed for bacterial vaginosis. After training the AromaScan system to recognise patterns generated from four patients with and four patients without bacterial vaginosis, 16 of the 17 (94%) remaining subjects were correctly identified as having the condition. The positive predictive value of the test was 61.5%. These results indicate that the AromaScan technology may be of value as a screening test for bacterial vaginosis.

Development of an enzyme-based biosensor for atrazine detection

An amperometric biosensor for detection of the photosynthetic inhibiting herbicide atrazine is described. The sensor, incorporating the enzyme tyrosinase, was sensitive to a wide range of di- and triphenols. It displayed a fast response, with 95% of the steady-state current being obtained within 25 s, and had a half-life of 8 d but retained some activity for up to 20 d. This biosensor displayed a notable decrease in response in the presence of atrazine. Atrazine inhibition of the biosensor response was found to be reversible. The electrode construction consisted of tyrosinase cross-linked on a poly(pyrrole) coated gold electrode surface. The inhibition of enzyme activity in the presence of atrazine was measured amperometrically. Atrazine concentrations of 5 × 10–6 mol dm–3 were measured in a hydrodynamic cell and of 1 × 10–5 mol dm–3 in a flow-through system.

Sensor array techniques for mimicking the mammalian olfactory system

Scales of human odour perception are subjective and there is much need for automated methods of odour measurement in a variety of industries. Organic conducting polymers have been developed as sensing devices, and many materials have been synthesised and characterised. The sensors show rapid adsorption and desorption characteristics and allow rapid measurements to be made. The responses are proportional to the concentration of the volatile chemical being sensed, and with calibration can be used to quantify single chemical species. Arrays of sensors produce patterns of responses that can be used as descriptors for discriminating complex odours. Examples of applications in food quality monitoring and agriculture malodours are given. The sensor array response may be correlated with olfactometric measurements in the case of pig slurry malodour.

The perception of visual images encoded in musical form: a study in cross-modality information transfer.

This study demonstrates the ability of blind (previously sighted) and blindfolded (sighted) subjects in reconstructing and identifying a number of visual targets transformed into equivalent musical representations. Visual images are deconstructed through a process which selectively segregates different features of the image into separate packages. These are then encoded in sound and presented as a polyphonic musical melody which resembles a Baroque fugue with many voices, allowing subjects to analyse the component voices selectively in combination, or separately in sequence, in a manner which allows a subject to patch together and bind the different features of the object mentally into a mental percept of a single recognizable entity. The visual targets used in this study included a variety of geometrical figures, simple high‐contrast line drawings of man‐made objects, natural and urban scenes, etc., translated into sound and presented to the subject in polyphonic musical form.

Multi-frequency measurements of organic conducting polymers for sensing of gases and vapours

Electrically conducting organic polymers display electrical conductivities that are dependent on the concentration of dopant ions incorporated in the material. The conductivity, usually measured using direct current (dc) techniques, may be modulated reversibly and rapidly at ambient temperature by adsorption and desorption of volatile chemicals. This phenomenon has immense practical use for gas and odour sensing. By using arrays of conducting polymer sensors having broadly overlapping specificity to a range of volatiles, we are able to measure and assign descriptors to the volatiles. In this paper we show that similar descriptors can be generated by using alternating current (ac) at suitable frequencies to follow the changes in sensor capacitance, conductance and dissipation factor. We also show that using a single sensor we can obtain discrimination between chemical species. The ac response characteristics of these sensors have been modelled using a simple electrical circuit equivalent and we show that the model is a good predictor of sensor performance.