Renewable Perfluorosulfonated Ionomer Carbon Paste Electrode for Competitive Homogeneous Electrochemical Immunoassays Using a Redox Cationic Labeled Hapten

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Abstract
A Nafion-loaded carbon paste electrode (CPE) was prepared and its square wave voltammetric response at a fresh surface was optimized using (ferrocenylmethyl)trimethylammonium iodide (1) as a model of a cation that accumulates irreversibly. Low current responses could be obtained reproducibly without preconditioning at a CPE loaded with 1-octanol and neutralized Nafion. Use of the former made the method more sensitive, and use of the latter gave lower background currents and stable analytical response over a long period of Nafion-loaded CPE use. Under optimized 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 μM 1, a detection limit of 5 nM 1 and a relative standard deviation of 5% for 0.5 μM 1. Using these optimized conditions, the competitive homogeneous immunoassay of phenytoin as a model hapten was carried out with cobaltocenium-labeled phenytoin as tracer. Clinical serum samples (5 μL) of phenytoin at therapeutic concentrations (20−80 μM) could be analyzed using this method.