Reference Electrodes for Electrochemical Sensors Based on Redox Couples Immobilized within Nafion Films

The immobilization of both components of a redox couple within films of the sulfonated tetrafluoro­ethylene polymer, Nafion, on the surface of carbon electrodes is reported. In particular, cationic and/or water-insoluble species are used to develop stable and robust reference electrodes by adjustment and optimization of both the film thickness and the level of doping of the film, as an alternative to using standard commercial reference electrodes. Different redox couples were evaluated: methyl viologen dication/cation (MV²⁺/MV⁺), tris­(2,2′-bipyridyl)­ruthenium­(II)/ruthenium­(III) (Ru­(bpy)₃²⁺/Ru­(bpy)₃³⁺), ferrocene/ferrocenium (Fc/Fc⁺), and poly­(vinylferrocene)/poly­(vinylferrocenium) (PVFc/PVFc⁺). These reference electrodes demonstrate a good stability over 10 days and 1000 successive scans; in particular, Nafion-confined Fc/Fc⁺PF₆^– was shown to display the best performance. This type of reference electrode, as a solid-state electrolyte-free reference electrode, has several advantages, including full retention of the redox species within the films together with ease of both fabrication and miniaturization.