This paper focuses on recent advances in the use of ionomer-coated electrodes and nanoelectrode ensembles for the determination of redox ions at trace concentration level.
In the first part, stress is put on recent developments concerning the deposition and electroanalytical uses of electrodes coated with ultrathin films of ionomers (Nafion, Tosflex, Eastman AQ) obtained by using Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques. Problems and achievements for obtaining LB- and LS-films of these polymers are described.
Morphological characteristics of these coatings are obtained by atomic force microscopy and quartz crystal microbalance. Voltammetry allows the ion-exchange and electrochemical characterization of these ultrathin films. Advantages and limits with respect to micrometer-thick coatings obtained by "conventional" techniques (dip-coating, spin-coating, microvolume evaporation) are critically discussed.
The second part of the talk deals with the development and electroanalytical use of a new kind of nanostructured electrodes named nanoelectrode ensembles (NEEs). They are prepared by template deposition of metal nanofibers within the pores of microporous tracked-etched polycarbonate membranes. NEEs can find application in a variety of fields, from sensors to electronics, from energy storage to magnetic materials.
From an analytical viewpoint, NEEs display improved electroanalytical detection limits relative to conventional electrodes because the Faradaic current (signal) is proportional to the total geometric area of the ensemble (nanodiscs and insulator), while the background current (double-layer charging current) is proportional only the nanodiscs (active) area.
NEEs show also peculiar kinetic properties since they allow the determination of very high heterogeneous charge transfer rate constants. They behave, in fact, like partially blocked electrode surfaces. Therefore, the heterogeneous electron transfer kinetics is ruled by an apparent rate constant which is diminished with respect to the true rate constant by a factor which depends on the ratio between the nanodiscs (active) area and the blocked area (membrane).
Prospects for applying the discussed electrode devices to sensing purposes are critically evaluated. In particular, advantages and limits of the use of LB- and LS-ionomer-coated electrodes and NEEs for shuttling electrons to redox mediators and (possibly) directly to redox proteins are discussed.
P. Ugo, L.M. Moretto, F. Vezza, ChemPhysChem, (2002), 3, 917-925 (and references cited therein.)