Paul H. Treloar

Plasticized poly(vinyl chloride) as a permselective barrier membrane for high-selectivity amperometric sensors and biosensors

Abstract A poly(vinyl chloride) (PVC) membrane system is described for use as a high-selectivity barrier in amperometric sensors and biosensors. Membrane casting and electrode fabrication techniques are presented. The membrane properties, both physical and with respect to permselectivity, are outlined. This new form of homogeneous membrane barrier shows very much greater selectivity for H 2 O 2 in oxidase-based enzyme electrodes than any previously reported barrier. Selectivity for the phenolic compounds catechol, hydrocaffeic acid, 4-aminophenol and paracetamol, as model electrochemically active compounds, is described and related to interference from ascorbate and urate; the responses of equimolar catechol: ascrobate and catechol: urate are both 61 500:1, and that of 1 mM catechol: undiluted serum is 60 000:1. Comparative results for a low molecular weight cut-off cellulose acetate layer, commonly employed in amperometric sensors and biosensors for clinical monitoring, demonstrates the superior selectivity of PVC, showing the PVC to have 7 and 180 times greater selectivity for H 2 O 2 and paracetamol, respectively, against ascorbate. Biocompatibility is also excellent, with no loss of signal after prolonged exposure to serum.

Engineering the right membranes for electrodes at the biological interface; solvent cast and electropolymerised

A generic approach to adapting many sensors to the rigours of biomedical measurement is to protect appropriate components with selective solute discriminating membrane layers. Our work has utilised external barrier membranes for mass transport control to enzyme electrodes in order to adapt their behaviour to requisite clinical ranges; these membranes have had the additional important effect of reducing surface deposition of colloid and other material on electrode surfaces. Internal membranes, especially those interposed between the enzyme layer and the working electrode of an amperometric sensor, have been found by us to be remarkably important in determining sensor operational stability. This paper will review such membrane materials, including coating technologies, and suggest appropriate directions for future work.

Whole blood assay of glucose-6-phosphate dehydrogenase: potential for simplified immunoassay

Enzyme labels combined with electrochemical product detection have considerable advantages in the development of non isotopic immunoassays. In particular, there is the possibility of signal detection in whole blood samples using cheap, robust and ultimately portable instrumentation. In this study the amperometric measurement, in whole blood, of glucose-6-phosphate dehydrogenase (G6PDH), an enzyme commonly employed in homogeneous immunoassays, has been investigated using a simple membrane covered electrode. On comparison of several compounds for electrochemical mediation of the enzyme cofactor NADH, in whole blood, napthoquinone sulphonic acid has been found to be optimal giving lower background responses (<1 nA vs. Ag/AgCl) whilst mediating effectively NADH detection at low overpotentials (≤0.15 V vs. Ag/AgCl). Employing this mediator it is possible to measure electrochemically NADH and subsequently G6PDH at concentrations down to 0.6 U ml−1 in the presence of whole blood. This simplified G6PDH detection system will facilitate greatly the development of dehydrogenase based amperometric immunoassays in whole blood.

The Use of Chemical Sensor Systems for Sampling, Selectivity and Speciation

Chemical sensors are ideally suited to miniaturisation and reformatting in order to conform to particular types of flowing, static and low volume samples. An important additional need is the complementary tailoring of a measurement cell in order to create a functional, integrated sensor-based measuring system. In this context the importance of sampling, membrane barriers, electrode modification and transduction strategy have been outlined and specific examples provided. A description of the sampling technique, open micoflow, is given which demonstrates how measurements can be performed in colloid containing samples without associated sensor fouling. The permselectivity and porosity of microporous and homogeneous barrier membranes can be tailored by addition of suitable modifiers (eg. surfactant) to alter hydrophilic/hydrophobic properties. Specifically, this has facilitated the control of solute flux and permselectivity towards neutral, charged, polar or non-polar species. Ultrathin non-conducting electropolymerised films are an alternative route in creating permselectivity barriers. It is shown that by judicious choice of monomer derivative both permselectivity and functionality can be achieved. Impedance spectroscopy and spectral reflectance as sensor transduction strategies have been explored in specific relation to ligand containing conducting polymer films. Such techniques have enabled enhanced sensitivity and selectivity to be achieved which can be extended to a wide range of chemical sensor applications.

Ex-Vivo Sensors; Microencapsulation of Enzyme Layer in Biosensors

Various solids, solid/liquid composites and quasi-liquid membranes have been developed to protect the enzyme layer and working electrode of enzyme electrodes. These include isopropyl myristate-containing porous membranes, liposomes and PVC with a high plasticiser content. The additional value of such membranes beyond their surface ‘biocompatibility’ has been their facility for controlling mass transport to the enzyme layer. In this way effective enzyme Km values have been manipulated, the dependence on co-substrate has been reduced, and where appropriate the co-substrate has been retained with the enzyme without a specific need for a co-substrate chemical immobilisation so avoiding the inherent problems of co-substrate mobility. Specific examples of these new encapsulated systems are provided.