Michael Thompson

The Potential of the Bulk Acoustic Wave Device as a Liquid-Phase Immunosensor

A summary of previous experiments is presented, which is concerned with partial immersion of bulk and surface acoustic wave devices in liquid media in order to produce chemical sensors. The the- ory of kinetics of antibody-antigen complexation at a planar surface is described together with a qualitative treatment of the propagation of acoustic shear waves through the interface between the solid device and the bulk liquid. Surface treatment of a bulk wave crystal by Lang- muir-Blodgett film technology and by silanization is employed to dem- onstrate that the structure of the interface is important in determining the time-dependent frequency of the device. Immobilization of antisera to the crystal-electrode surface by two different methods is used to examine experimentally the response of the device to interfacial reac- tion with an antibody. The future potential of liquid-phase acoustic wave sensors is evaluated.

Adsorption on film-free and antibody-coated piezoelectric sensors

Frequency responses associated with adsorption of gas-phase toluene, o-nitrotoluene, valproic acid and the pesticides parathion, malathion and disulfoton on uncoated and protein-coated piezoelectric sensors are reported. Valproic acid antiserum and antibody against parathion were included among the set of protein films studied. Responses, which were reversible in all cases, are attributed to chemisorption or physisorption on electrode and protein surfaces. Under the conditions used, no signals corresponding to selective immunochemical binding were found. Highly sensitive responses of protein-coated sensors to the pesticide species were confirmed, with the lower limit of detection being about 0.01 μg l−1 in nitrogen carrier gas.

A chemoreceptive bilayer lipid membrane based on an auxin-receptor ATPase electrogenic pump

Auxin-binding proteins have been extracted from coleoptiles and primary leaves of maize and diffusion--reconstituted in phosphatidyl choline/partially-oxidized cholesterol membranes. Measurement of membrane ion flux at 25 mV external potential with buffered KCl electrolyte was performed for the receptor support matrix and various combinations of ATP, receptor and naphthalene-1-acetic acid. Addition of the three components in any order results in a substantial increase in current with a limit-of-detection for auxin of about 10(-7)M. The pH-dependence of the response is consistent with previous suggestions that an ATPase pump acts to translocate protons in the presence of K+ and Mg2+ and that the pump can be activated by auxin. This work provides the first direct link between the binding of a plant hormone to a putative receptor and the evocation of a biochemical response.

Biosensors and bioprobes

Abstract Dedicated microsensors for automation and process control are becoming pervasive in many aspects of high technology. New developments in chemical sensing mechanisms and the design, miniaturization and fabrication of biosensors and bioprobes is leading to exciting potential applications in invasive biological analysis, remote sensing of harsh environments and biotechnological synthesis.

Chromatographic analysis of phospholipid and cholesterol oxidation.

Capillary thin layer and gas chromatographic methods for analysis of the extent of oxidation in phosphatidyl choline/cholesterol samples are described. Examples of systems suitable for qualitative and quantitative analysis, based on use of unmodified samples or of their derivatives, are illustrated. A method for concurrent quantitative determination of phospholipid and sterol without preseparation is introduced and is based on extension of a previous lipid trans-methylation technique.

The structure and electrochemical properties of a polymer-supported lipid biosensor

Abstract Bilayer lipid membranes have great potential as electrochemical sensors for trace organic compounds. Their most serious disadvantage is the limitation imposed by the delicate nature of the membrane. A polyamide polymer substrate has been developed to support a lipid structure similar in function to the BLM. This new system is sufficiently rugged for laboratory use and does not compromise electrochemical response characteristics. The membrane has been studied by transmission electron microscopy and x-ray photoelectron spectroscopy.

The analytical potential of chemoreception at bilayer lipid membranes

Abstract The potential use of the bilayer lipid membrane as an electrochemical sensor is discussed through a study of model systems known to cause increased membrane conductance. The limit of detection for amphotericin B, a molecule capable of forming membrane pores, is in the region of 1O -9 M. The current—time profile is discussed in terms of a mechanism which involves micelle formation in the aqueous and lipid phases. Unlike previous experiments, two current maxima with time are observed for valinomycin response (limit of detection 1O -11 M). The first transient signal is attributed to increased membrane permeability caused by a conformational change in valinomycin in the “surface” volume of the bilayer. Selective interactions at membranes and the nature of membrane responses are discussed in terms of analytical parameters.

The electroanalytical response of the bilayer lipid membrane to valinomycin: membrane cholesterol content

Abstract Under certain conditions the current—time response of the phosphatidyl choline bilayer membrane to valinomycin is biphasic. Residual, final, and first and second maximum currents have been measured for a series of membranes of different overall concentrations of cholesterol and its derivatives generated by oxidation. The factors contributing to the variation of results for particular membrane compositions are discussed. The first short-time maximum is not related to local concentration effects, but is likely associated with a perturbation of the membrane surface structure where transport parameters are changing with time. Thermal properties of membrane electrochemistry are also discussed.

Lipid membrane technology for chemical and biosensor development

Abstract New developments and applications of selective biosensors specifically designed for clinical assay and invasive biological analysis are revolutionizing chemical sensing. One area which has remained largely unaddressed consists of the chemical sensing systems found in nature, which offer splendid examples of efficient and sensitive devices from which much can be learned. Efforts to devise lipid-based biosensors, which are very limited models of the highly refined natural system, have recently attracted much attention and financing. The superior sensing characteristics demonstrated by the prototype systems tested to date indicate that this technology may provide the basis for the ultimate artificial biosensor of the future.

The bilayer lipid membrane as a basis for a selective sensor for ammonia

The development of an electrochemical method for the selective sensing of ammonia gas, based on a modified bilayer lipid membrane, is described. Membrane selectivity for ammonium ion is achieved through incorporation of the antibiotic nonactin as ion-carrier. The detection limits compare favourably with those for conventional ammonia gas-sensing electrodes, but the selectivity is much superior. Theoretical evaluation of the potential sensitivity of the new gas-sensor with respect to design parameters is described.