The effect of supporting electrolyte on the electron transfer at mixed self-assembled monolayers containing ferrocene moieties

• The redox of ferrocene (Fc) moieties in mixed SAMs was studied.
• The apparent redox potential of Fc was affected by supporting-electrolyte anions.
• This would be explained by the anions' ability of charge neutralization.

The effect of supporting electrolyte on the apparent redox potential (Eapp∘) of ferrocene (Fc) moieties in mixed self-assembled monolayers (SAMs) has been studied by means of cyclic and ac voltammetry. When the Fc moieties are exposed on the surface of the SAM (i.e., for the mixed SAM of Fc(CH2)6SH and 1-hexylmercaptan), the Eapp∘ is little dependent on the supporting electrolytes used, i.e., sodium salts of 4-hydroxybenzenesulfonate (HBS), p-toluenesulfonate (TS), and 2-naphtalenesulfonate (NS). On the other hand, when the Fc moieties are buried in the SAM (i.e., for the mixed SAM of Fc(CH2)6SH and 1-octadecylmercaptan), the Eapp∘ is significantly shifted to more positive potentials and is further affected by the nature of the electrolyte anions. The more hydrophobic the anion becomes in the order HBS < TS < NS, the less positive the Eapp∘ value. This effect has been successfully explained by the theory proposed by Smith and White (Anal. Chem., 64 (1992) 2398–2405), with an additional consideration on the anions' ability to neutralize the positive charge of oxidized Fc moieties.

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