Identification of Nucleophilic and Electrophilic Binding Sites on Gold Surface-Enhanced Raman Substrates

We have studied the binding of several nitrogen-containing aromatic chemicals to nanostructured gold surfaces from aqueous media in order to better understand the equilibrium thermodynamics and nature of the binding. These studies have revealed that a complex equilibrium state exists between the molecules and the gold surface that cannot be explained by a simple Langmuir isotherm. Instead, the concentration dependence of the surface-enhanced Raman spectroscopy response suggests two equilibrium states. We present evidence that these two equilibrium states correspond to nucleophilic and electrophilic adsorption sites, both of which can be filled depending on the state of the analyte molecule.