ELECTROCHEMICAL SENSORS BASED ON IMPEDANCE MEASUREMENT OF ENZYME-CATALYZED POLYMER DISSOLUTION - THEORY AND APPLICATIONS

A novel sensor approach based on ac impedance measurement of capacitance changes produced during enzyme-catalyzed dissolution of polymer coatings on electrodes, leading to a 4 orders of magnitude change in capacitance, is described, Electrodes were coated with an enteric polymer material, Eudragit S 100, which is based on methyl methacrylate, and dissolution was exemplified by utilizing the catalytic action of the enzyme urease, The resulting alkaline pH change caused dissolution of the polymer film with a consequent large increase in capacitance, A mechanism for polymer breakdown is proposed which has been validated experimentally using both ac impedance measurements and electron microscopy, The large changes in capacitance that are apparent using this technique allow much greater sensitivity of measurement than, for example, potentiometric electrodes, The potential broad clinical analytical application of this technique is demonstrated in this report by application to urea measurement and to enzyme immunoassay, Urea measurement between 2 and 100 mM has been achieved with a change in response over this concentration rang by over 4 orders of magnitude, We have taken account of both the effect of protein adsorption on the surface of the polymer-coated and bare electrodes and the effect of buffer capacity when carrying out these measurements in buffered solutions containing 8% (w/v) protein and have demonstrated that the method should allow simple, interference-free measurement of urea in serum and whole blood, In addition, both competitive and noncompetitive enzyme immunoassays for human IgG based on the use of urease-antibody conjugates are reported, Human IgG, or goat anti-human IgG (Fab specific), were immobilized covalently onto cellulosic membranes via a diamine spacer group and the membranes placed over enteric polymer-coated electrodes, Specific measurement of IgG in both formats was achieved over the concentration range 0.0001-100 mu g mL(-1). The performances of the impedance-based enzyme immunoassays were compared directly with identical assays employing spectrophotometric detection.