Experimental Study of the Interplay between Long-Range Electron Transfer and Redox Probe Permeation at Self-Assembled Monolayers:Evidence for Potential-Induced Ion Gating

摘要

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH_3)_6~(3+) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode,by using electrochemical techniques that operate at distinct time scales.Two limiting voltammetric behaviors are observed,consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH_2)_(10)-COOH and HS-(CH_2)_n-CH_2OH (n = 3,5,and 10) SAMs.The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH_3)_6~(3+) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times.However,long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied,which takes the form of a chronoamperometric spike.Redox probe permeation is shown to be a kinetically slow process,whose activation strongly depends on redox probe concentration,applied potential,and chemical composition of the intervening medium.The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM,whereas those of mercaptohexanol,mercaptoundecanol,and mercaptoundecanoic acid undergo an order/ disorder transition below a critical potential,which facilitates the approach of the redox probe toward the electrode surface.