Effect of hexacyanoferrate(II/III) on self-assembled monolayers of thioctic acid and 11-mercaptoundecanoic acid on gold

The change in characteristics of self-assembled monolayers (SAMs) of thioctic acid (TA) and 11-mercaptoundecanoic acid (MUA) with time in buffered hexacyanoferrate(II/III) solutions of pH 7.4 has been studied with electrochemical impedance spectroscopy. Frequency scans were recorded at regular time intervals at +0.2 V versus SCE and a superimposed sinusoidal potential with an amplitude of 10 mV, in the frequency range from 10 kHz to 50 mHz. The stability of the SAMs was studied in the dark and during subsequent exposure of the solution to light. A TA SAM, a MUA SAM, and clean gold can be modeled with the same equivalent circuit. From this model, it was found that, as expected, charge transfer and diffusion of the redox probe through the MUA SAM are more inhibited than through the TA SAM. Further, the MUA SAM has a larger mean monolayer thickness, as is revealed from the lower value of the capacitive component. After exposure of the solution to light, a relatively rapid decrease in the resistances and increase in the double-layer capacitance are observed. Despite the racemic mixture of TA used, the characteristics of the MUA SAM change much faster than those of the TA SAM, and after 6 h of exposure, the equivalent circuit has to be modified to give an appropriate fit. The manner in which the values of the elements describing the TA SAM change suggests that TA molecules are gradually removed. A possible mechanism is etching of the gold substrate by CN- ions through pinholes in the SAM. Support of this mechanism is obtained from the large difference in characteristics of the gold electrode before SATM formation and after long-term incubation in illuminated buffered hexacyanoferrate(II/III) solution, pointing to a change in the structure of the gold, and by the observation that thin gold layers are etched in this solution.