Controlled Tuning of the Ferri/Ferrocyanide Electron Transfer at Oligo(Ethylene Glycol)-Modified Electrodes

The modification of gold electrodes by a short thiolated hepta(ethylene glycol) self-assembled monolayer (SAM) significantly inhibits the electron transfer kinetics of the [Fe(CN)6]3−/4− couple. By contrast, other redox species such as Fc(CH2OH)2, Fc+, [Ru(NH3)6]3+ and methylene blue are barely affected by the presence of the SAM. Considering the properties of the hydrated monolayer and the documented peculiarities of the [Fe(CN)6]3−/4− electron transfer kinetics, hydration is hypothesized to be a key factor to explain the particular behaviour of the ferri/ferrocyanide system. Further support to the role of hydration is provided through the effect of “water structure breakers” ions, whose presence in solution promotes a gradual concentration-dependent recovery of the electron transfer. Taking advantage of the facile kinetics displayed by other redox species, these are used at micromolar concentrations as mediators to catalytically amplify the electron transfer of the ferri-ferrocyanide couple at the SAM. The voltammetric behaviours recorded in the presence of these redox species are in very good agreement with the theoretical framework of homogeneous redox catalysis of electrochemical processes. The library of available redox mediators enables one to tune the electrochemistry of [Fe(CN)6]3−/4− in a desired potential range.