Impedance spectroscopy of the potential response of MUO and AUT self-assembled monolayers on polycrystalline thin gold films

We investigate the dielectric properties of self-assembled monolayers (SAMs) of OH and NH₂ terminated alkyl-thiols on polycrystalline gold films by electrochemical impedance spectroscopy (EIS) in a non-electroactive electrolyte. We show that impedance spectroscopy, coupled to surface potential monitoring allows a fine inspection of the stability and homogeneity of these layers. In particular, we compare two preparation modes for these layers, respectively, pure ethanol and mixed ethanol-phosphate buffer. Using a simple electric model combining a constant phase element CPE with a highly resistive element r_S, both in series with a bulk resistor R, to fit the experimental EIS curves, we propose a direct survey of the SAM homogeneity during the gold film potential scan. The topological imperfections of polycrystalline gold films and self-assembled monolayers are interpreted by a constant phase element, i.e. an interfacial capacity behaving as a power law of frequency. We show that the type of medium chosen for the SAM preparation is not so critical, when the alkyl-thiol is not charged (OH), and these SAMs are very stable, independently of the gold surface potential. However, the preparation of charged SAMs, such as those obtained from NH₂ terminated alkyl-thiols requires the preparation medium to be similar to the type of solution in which it will be used afterwards, for instance if the SAM has to be used for aqueous medium applications, its preparation should be made in a mixed ethanol (organic solvent)-buffer solution. We also show that for both charged and uncharged SAMs, the periodic cycling of the gold potential promotes the homogenization of the SAM.