Electric Potential Stability and Ionic Permeability of SAMs on Gold Derived from Bidentate and Tridentate Chelating Alkanethiols

Abstract

The interfacial electrochemical properties of self-assembled monolayers (SAMs) on gold derived from a structurally tailored series of monodentate, bidentate, and tridentate chelating alkanethiols were investigated. Specific adsorbates included 1-hexadecanethiol (C16), 2-tetradecylpropane-1,3-dithiol (C16C2), 2-tetradecyl-2-methylpropane-1,3-dithiol (C16C3), 2,2-ditetradecylpropane-1,3-dithiol (C16C16), and 1,1,1-tris(mercaptomethyl)pentadecane (t-C16). Reductive desorption of the SAMs as a function of potential was probed by voltammetric measurements, which indicated the following relative order of electric potential stability: t-C16 > C16C2 ≈ C16C3 ≈ C16C16 > C16. The ionic permeability was investigated under various applied cathodic potentials by electrochemical impedance spectroscopy (EIS). An examination of SAMs prepared at room temperature and accessed by EIS at open-circuit potential showed that the ionic permeability increased in the order C16C2 < C16 < C16C3 < C16C16 < t-C16. The ionic permeability of films was further influenced by the electric potential of the metal substrate and the temperature at which the monolayers were assembled. The potential dependence of the ionic permeability was qualitatively rationalized by considering both the initial ionic permeability and the electric potential stability of the SAMs. Similarly, the ionic permeability of the SAMs prepared at elevated temperature showed contributions from both their thermal stability and their insulating properties at room temperature.