P.A. Payne

Reagentless biosensing using electrochemical impedance spectroscopy

The use of electrochemical impedance spectroscopy (EIS) and the conducting polymer, poly (pyrrole), as an integrated recognition and transduction system for reagentless biosensor systems was demonstrated with two different systems. The first system being an immunoassay for detection of luteinising hormone (LH) with the antibody being entrapped with in the poly (pyrrole) matrix and the second, a construct for DNA hybridisation discrimination able to differentiate single- and double-stranded DNA based on the interaction of the DNA with poly (pyrrole).

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.

High-frequency measurements of conducting polymers: development of a new technique for sensing volatile chemicals

Electrically conducting polymers have previously been used for the detection of vapours and gases by measuring the reversible change in their DC resistance. The sensors have very broad chemical specificity with higher sensitivity to polar chemicals. The change in the AC impedance characteristics of poly-N-(2-pyridyl)pyrrole in the presence of three different volatile chemicals was investigated, in order to determine whether measurement of multifrequency parameters could be used as the basis of chemical discrimination. The results demonstrate quantification of concentration and discrimination between the chemicals tested on the basis of capacitance, conductance, resistance and dissipation factor as a function of frequency. This technique may be generally applied for discrimination between many chemical species.

An intelligent gas sensing system

Electrically conducting organic polymers are widely used as a means of gas, odour or aroma analysis using multi-element array techniques coupled with direct current (d.c.) interrogation techniques. Recently it has been established that the use of alternating current (a.c.) interrogation gives rise to improved performance. In addition, the need to use multi-element arrays is much reduced since a single sensor can be interrogated at a wide range of frequencies. This gives rise to much increased information content for the measurements. This paper describes the use of alternating current (a.c.) interrogated conducting organic polymers coupled with neural network pattern recognition techniques for a system to determine the compositional fraction of volatile vapour mixtures. Experiments have been conducted on binary, tertiary and quaternary mixtures of vapours and compositional fractions have been determined to within 5%.

A new Golay code system for ultrasonic pulse echo measurements

Many measurements involving ultrasonic pulse echo techniques are limited by poor signal-to-noise ratios. It is well known that random or pseudorandom codes in conjunction with cross correlation can enhance signal-to-noise ratio and it has been shown that a form of pseudo-random sequence known as a Golay code enables the signal-to-noise ratio improvement to be obtained whilst requiring only two sequences of the code to overcome problems of self noise. A significant parameter associated with a system employing this technique is the amplitude of the coded signal applied to the transducer. The authors describe the development and application of a system having a transmitter capable of delivering peak-to-peak signals of 100 V. Measurements show that this instrument provides considerable signal-to-noise improvements when compared with conventional equipment.

A NEW LASER-ULTRASOUND TRANSDUCER FOR MEDICAL APPLICATIONS

A new type of probe is presented, which combines an ultrasound receiver constructed from piezoelectric polymer materials with a coaxial optical fibre for laser-induced ultrasound. The initial application of this probe, which may be further miniaturized, is in intra-arterial imaging and therapy. Overall diameters of the prototype probes presently range from 2.5 mm to 10.0 mm. Pulsed laser beams are introduced through the optical fibre to form a photoacoustic source within the material under investigation. Ultrasonic waves are induced in the material due to the photo-acoustic effect and received on return by a forward-looking piezoelectric polymer transducer. In principle, other high-power laser beams can also be introduced through the same optical fibre for therapeutic use. Initial development and characterization of this combined probe is reported in this paper, together with some results.

An integrated multi-element array transducer for ultrasound imaging

Abstract Much progress has been made towards integrating the electronic circuitry associated with either linear or phased ultrasonic array scanning into the hand-held case of the transducer. The number of wires required to connect the transducer back to the display system has been dramatically reduced and the path length between transducer elements and the driver circuits kept to a few millimetres. To achieve this new construction, polymer transducer arrays have been fabricated and the pulse-control circuitry has been integrated onto custom-designed silicon chips.

Pseudo-random binary sequence interrogation technique for gas sensors

Abstract Electrically conducting organic polymers are widely used as a means of gas, odour or aroma analysis using multi-element array techniques coupled with direct current (d.c.) interrogation techniques. Recently, it has been established that the use of alternating current (a.c.) interrogation gives rise to improved performance. In addition, the need to use multi-element arrays is much reduced since a single sensor can be interrogated at a wide range of frequencies. This paper describes the use of pseudo-random binary sequences (PRBS) as interrogation signals for semi-conducting organic polymer gas sensors. Preliminary experiments have been conducted upon volatile vapours and results are presented herein.

Characterisation of laser-ultrasound signals from an optical absorption layer within a transparent fluid

Characteristics of laser-ultrasound signals are presented from photoacoustic interaction with a layered, optically absorbing medium surrounded by a transparent fluid. A thermoelastic model is presented describing the interaction, with signal predictions in the fluid arising from polymer transducer detection. By taking the optical absorption coefficient and finite layer thickness into account, the amplitude and shape of transient elastic waves are calculated for both forward and backward travelling directions. Additionally, wave interaction with the PVDF transducer has been characterised using a discrete-time algorithm for the transducer response. With just three constants to define transducer response characteristics, the response function may be used to predict voltage signals. Good agreement with experimental waveforms is demonstrated, so that the response function may form the basis of system modelling when miniature laser-ultrasound probes are used in various applications.

Integrated ultrasound transducers

Piezoelectric polymer materials are of interest for high frequency transducer array applications primarily because the array geometry can be achieved using photolithography. The UMIST research team have fabricated up to 64-element polymer transducer arrays. They have adopted a unique approach whereby the pulse control electronics can be mounted in close physical proximity with the transducer array. In order to accomplish this both transmit and receive circuitry has been integrated onto in-house custom designed silicon chip sets. A notable achievement is the design of a monolithic 16-channel programmable pulse generator chip fabricated in 1.5 μm CMOS technology. By employing novel design techniques time delays with 1 ns time resolution and a dynamic range of 219 have been achieved. A description of the system is provided and preliminary results of beam plotting measurements are given