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

A novel sensor approach based on ac impedance measurement of capacitan ce changes produced during enzyme-catalyzed dissolution of polymer coa tings on electrodes, leading to a 4 orders of magnitude change in capa citance, is described, Electrodes were coated with an enteric polymer material, Eudragit S 100, which is based on methyl methacrylate, and d issolution was exemplified by utilizing the catalytic action of the en zyme urease, The resulting alkaline pH change caused dissolution of th e polymer film with a consequent large increase in capacitance, A mech anism for polymer breakdown is proposed which has been validated exper imentally using both ac impedance measurements and electron microscopy , The large changes in capacitance that are apparent using this techni que allow much greater sensitivity of measurement than, for example, p otentiometric electrodes, The potential broad clinical analytical appl ication of this technique is demonstrated in this report by applicatio n to urea measurement and to enzyme immunoassay, Urea measurement betw een 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 accou nt of both the effect of protein adsorption on the surface of the poly mer-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 ad dition, both competitive and noncompetitive enzyme immunoassays for hu man IgG based on the use of urease-antibody conjugates are reported, H uman IgG, or goat anti-human IgG (Fab specific), were immobilized cova lently onto cellulosic membranes via a diamine spacer group and the me mbranes placed over enteric polymer-coated electrodes, Specific measur ement of IgG in both formats was achieved over the concentration range 0.0001-100 mu g mL(-1). The performances of the impedance-based enzym e immunoassays were compared directly with identical assays employing spectrophotometric detection.