RENEWABLE PERFLUOROSULFONATED IONOMER CARBON-PASTE ELECTRODE FOR COMPETITIVE HOMOGENEOUS ELECTROCHEMICAL IMMUNOASSAYS USING A REDOX CATIONIC LABELED HAPTEN

A Nafion-loaded carbon paste electrode (CPE) was prepared and its squa re wave voltammetric response at a fresh surface was optimized using ( ferrocenylmethyl)trimethylammonium iodide (1) as a model of a cation t hat accumulates irreversibly. Low current responses could be obtained reproducibly without preconditioning at a CPE loaded with 1-octanol an d neutralized Nafion. Use of the former made the method more sensitive , and use of the latter gave lower background currents and stable anal ytical response over a long period of Nafion-loaded CPE use. Under opt imized conditions and using a 5 min accumulation period, the modified electrode gave a linear current response over the range of 5 nM to 1.5 mu M 1, a detection limit of 5 nM 1 and a relative standard deviation of 5% for 0.5 mu M 1. Using these optimized conditions, the competiti ve homogeneous immunoassay of phenytoin as a model hapten was carried out with cobaltocenium-labeled phenytoin as tracer. Clinical serum sam ples (5 mu L) of phenytoin at therapeutic concentrations (20-80 mu M) could be analyzed using this method.