Characterization of ferrocene-modified electrode using electrochemical surface forces apparatus

A electrochemical surface forces apparatus (EC-SFA) was employed to measure the interactions between gold electrodes modified with self-assembled monolayers of ferrocene alkyl thiol (Fc-SAM) and oxidized ferrocene (ferrocenium cation, Fc⁺-SAM) in a 1 mM aqueous electrolyte. The double-layer repulsion in both cases of the Fc-SAM and Fc⁺-SAM electrodes was observed. The surface charge density (σ) evaluated from the double-layer repulsions between the Fc⁺-SAM electrodes in 1 mM aqueous KClO ₄ was 0.0027 C/m², which was 2.5 times greater than that of the Fc-SAM, at 0.0011 C/m². The σ values of the Fc ⁺-SAM were evaluated for various counteranions using the same method, which were 0.0048, 0.0040, and 0.0104 C/m² for NO₃ ^-, SO₄^2-, and CF₃SO₃ ^-, respectively. The degrees of dissociation (α_d) between the ferrocenium cation and these counteranions were obtained from σ and the density of the ferrocenium on the electrode. The α_d value of CF₃SO₃^-, 4.1%, was the highest, followed in the order, SO₄^2- > NO ₃^- > ClO₄^-, indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counteranions.