Surface-Enhanced Raman Spectroscopy Detection of Ionic Solutes by Surfactant-Mediated Adsorption to a Hydrophobic Surface

Chemical modification of the surfaces of surface-enhanced Raman spectroscopy (SERS)-active metals with thin molecular layers expands the variety of molecular species that can be attracted to the SERS surface from solution. This approach can provide selective detection of new classes of molecules that would not otherwise be detectable through direct interaction with a SERS-active metal. For example, polycyclic aromatic compounds can be attracted from aqueous solution to gold or silver SERS substrates that are modified with alkylsilanes or alkanethiols. While n-alkane monolayers attract hydrophobic solutes to a SERS-active surface, they are not well suited to adsorbing more water-soluble, ionized species from solution. In this work, we address SERS detection of ionic solutes by applying the principles of ion-interaction chromatography, where a charged surfactant is added to solution and adsorbs to an n-alkane-modified, SERS-active surface. The adsorbed charged surfactant serves to attract an ionic solute of opposite charge to the surface, where it can be detected. This concept was tested with a model anionic solute, 3-nitrobenzenesulfonate (NBS⁻), with a cationic surfactant, cetylpyridinium (CP⁺), that adsorbs to a 1-dodecanethiol (C₁₂)-modified silver surface. The interfacial populations of both the surfactant and anionic solute can be determined simultaneously from SERS spectra. The adsorption equilibrium of CP⁺ to the C₁₂ surface was fit to a Langmuir model, and the effect of supporting electrolyte on its adsorption equilibria was also investigated by SERS. The retention of NBS⁻ at the C₁₂ surface depends on the concentration of CP⁺. The binding of NBS⁻ to adsorbed CP⁺ is described by an ion-interaction model that includes competition for the NBS⁻ population due to association with surfactant ions in solution. While the strength of this binding interaction is not as great as the hydrophobic interactions that drive aromatic hydrocarbons to hydrophobic SERS surfaces, SERS detection of analyte ions by this approach could be accomplished at concentrations two orders of magnitude lower compared with Raman detection in free solution.