Metal Ion Reactivity with 1,4-Benzenedimethanethiol Monolayers on Gold

Exposure of Cu2+ ions to 1,4-benzenedimethanethiol (BDMT) monolayers on Au in solution results in the formation of copper functionalized monolayers which have been investigated in detail by surfaceenhanced Raman spectroscopy (SERS), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Upon exposure to copper ions, the free thiol groups at the surface of the monolayer disappear indicating the replacement of protons with copper ions. The reaction leads to a red shift in the C-S stretching frequency, but most of the other features are unaffected. Relative intensities of the peaks are largely the same; however, some new features are observed suggesting minor changes in the adsorbate structure that makes additional modes observable. Thermal stability of the monolayers has been reduced substantially as a result of reaction with metal ions, indicating that the chemical binding at the Au-monolayer interface is affected. XPS shows that copper is present at the surface. Electrochemical behavior before and after Cu2+ ion adsorption is significantly different, and the adsorbed metal ions undergo reversible redox transformations. The cyclic voltammograms of the adsorbed copper ions are discussed in relation to the oxidation state changes in aqueous electrolyte solutions.