pH-Induced Conformational Change of Amine-Terminated Self-Assembled Monolayers

The present work describes a methodology to characterize the defects in self-assembled monolayers (SAMs) deposited on polycrystalline gold. This methodology combines numerical simulations with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of ferrocenemethanol (FcMeOH), an uncharged redox species. This approach is applied to amine-terminated SAMs for which we observe a pH-dependent electrochemical response, from almost perfectly blocking at low pH to somewhat leaky at high pH. This change with pH is reversible. Based on our numerical model extending the analytical model of Amatore et al. published for partially blocked electrodes (J. Electroanal. Chem. and Interfacial Electrochemistry 147, 39 (1983)), we determine the thickness of the SAM as well as the defect coverage. The pH dependence of the electrochemical behavior of an 11-amino-1-undecanethiol hydrochloride (AUT)-SAM is attributed to a conformal change of molecular chains within the SAM, while the defect coverage remains quasi-constant and neglectable.