Gold electrodes with polyion multilayers and electrostatically bound redox couples

Self-assembled monolayers of alkane thiols with terminal ionized moieties readily adsorb polyions of the opposite charge to form a bilayer structure. A second polyion layer can be adsorbed electrostatically onto the first polyion layer to yield a trilayer. Ellipsometry suggests that the polyion layers have thicknesses comparable to that of the self-assembled monolayer. Each polyion layer can also electrostatically bind a redox couple, although only certain combinations of polyion and redox couple yield a stable redox wave. The reaction of ferroin and other iron complexes onto an anionic surface (monolayer or trilayer) leads to a pH-sensitive surface redox couple believed to be an oligomer of iron hydroxide. A two-terminal pH sensor of the type described by I. Rubinstein [1], and Hickman et al. Anals Chem 56, 1135 (1984). [2] is demonstrated using electrostatically bound redox couples. (C) 1997 Elsevier Science Ltd.