Electrochemical impedance Spectroscopy for investigations on ion permeation in ω-functionalized self-assembled monolayers

Electrochemical impedance spectroscopy was employed to explore the possibility of relating the permeation of electrolyte ions in co-functionalized self-assembled monolayers to structural or polarity changes induced by interaction with metal ions. The monolayers were based on alkanethiols modified with a phosphorylated tyrosine analogue, which from previous work are known to drastically change their organization on gold surfaces upon interaction with aluminum and magnesium ions. The ion permeation was evaluated by using relatively low excitation frequencies, 1000 to 2 Hz, and quantified by an extra resistive component in the equivalent circuit (R-SAM). The extent of ion permeation influenced by the do potential, the electrolyte concentration, the functional group, and the thiol length were also investigated. It was, for example, found that RSAM decreased similar to 20% when the thiol organization collapsed and that RSAM increased similar to 4-5 times when the electrolyte concentration was decreased by 1 order of magnitude. Interesting observations were also made regarding the potential dependence of R-SAM and the double layer capacitance. The evaluation of the ion permeation can be used to indirectly detect whether the organization of a SAM is influenced by, for example, electric fields or chemical and biological interactions. This analysis can be performed without addition of redox species, but is on the other hand complicated by the fact that other factors also influence the presence of ions within the monolayer. In addition, a second parallel RC process was obtained in some of the impedance spectra when using even lower frequencies, and its resistive component revealed different results compared to RSAM. Such data maybe useful for the understanding of complex double layer phenomena at modified electrodes.