Charge-transfer patterns for [Ru(NH3)6]3+/2+ at SAM modified gold electrodes : impact of the permeability of a redox probe

Electrochem. performance of a [Ru(NH3)6]3+/2+ redox couple at gold electrodes modified by alkanethiol self assembled monolayer (SAM) films of the type [-SH -(CH2)n - CH3] with different no. of methylene units (n = 2 to 10) in the presence and absence of glucose additives has been studied using fast scan cyclic and steady-state voltammetry. Specific scatter of measured rate consts. caused by enhanced sensitivity of this probe to minor defects of SAMs has been obsd. in a general agreement with the published data for thicker SAMs (n = 9 to 18). In addn., we have disclosed the anomalous viscosity-imposed drop of the heterogeneous rate const. for the case of Au electrodes modified by thinner n-alkanethiol SAMs (n = 2, 4). Taking into the account the fact of [Ru(NH3)6]3+/2+ couple's capability to penetrate into the SAM interior, we ascribe the obtained results to the manifestation of the solvent-friction mechanism under the condition where the redox species presumably together with a few of solvating water mols. reside in a SAM's peripheral interior marked by much higher local viscosity (slower dielec. relaxation) compared to the electrolyte soln.