Direct Observation of Two Types of Monolayer-Bound Ferricinium Ions on Gold: Implications for Electrocatalysis

Abstract

The direct surface-enhanced Raman spectroscopic (SERS) observation of two thermodynamically and electrocatalytically different types of ferricinium ions covalently bound to the redox-active monolayer of 9-mercaptononyl-5‘-ferrocenylpentanoate (MNFcP) on gold is reported. The presence of different surface redox centers generates an unusual electrocatalytic effect that might be called ion-gated mediated electron transfer. The ion gating is triggered by the substitution of the dominant anion in the supporting electrolyte. In the solution dominated by poorly hydrated (“hydrophobic”) perchlorate anions, the mediated electron transfer is sluggish, whereas the highly hydrated (“hydrophilic”) fluoride anions sharply accelerate the electrocatalytic electron transfer rate from the substrate (ascorbic acid) to the mediator. SERS data indicate that the inhibition of the mediated electron transfer is strongly related to the presence of the ion pairing between the ferricinium and hydrophobic perchlorate ions. In the fluoride-dominated solutions, at the initial stage of MNFcP monolayer oxidation, the ion pairing is suppressed. Thus, the hydrophilic fluoride ions act as accelerators, but hydrophobic perchlorate ions act as inhibitors of the mediated electron transfer.