Monitoring adsorption equilibria and kinetics at modified solid/liquid interfaces using surface-enhanced spectroscopy

Understanding the molecular interactions and adsorption/desorption chemistry at the liquid/solid interface is important in many areas of fundamental and applied science. To perform spectroscopic measurements on monolayer concentrations at interfaces, a surface selective or surface enhancing technique must be used. Surface-enhanced Raman spectroscopy (SERS) is a technique which enhances the inelastic scattering signals from molecules near a roughened metal surface. SERS is advantageous when monitoring adsorption/desorption chemistry because of its high surface specificity, the time scale for SERS measurements is within many adsorption/desorption kinetic processes, and SERS can provide information on the surface concentration, organization, and orientation.; We are interested in interactions that occur at hydrophobic alkyl and hydrophilic oxide surfaces. Thus, our goal is the development and characterization of a stable, reproducible surface-enhancing substrate that can be modified with an organic layer for the investigation of interfacial equilibria and kinetics. Immobilized gold and silver colloid substrates with alkyl silane or thiol siloxane overcoatings have been developed as SERS substrates that provide either a hydrophobic or hydrophilic interface at which adsorption can be observed. Adsorption equilibria and kinetics of the cationic surfactant cetylpyridinium chloride are measured on an immobilized gold colloid SERS substrate modified with octadecyltrimethoxysilane. Band shifts were observed in the SERS spectra as the cationic surfactant adsorbed to the C18 surface.; The adsorption of polycyclic aromatic hydrocarbons (PAH) at the C18 modified gold colloid interface was also studied. Detection of PAHs from very low concentration aqueous solutions was demonstrated. A C18 modified immobilized silver colloid substrate was characterized as a SERS substrate for PAH adsorption, and the silver was found to facilitate oxidation of the adsorbate; a gold colloid substrate was not observed to interact with the PAH.; The immobilized colloid substrate was evaluated for use in obtaining adsorption data with surface enhanced optical absorbance (SEA). Enhancement of optical absorbance at a hydrophilically modified metal colloid film occurs by a similar electromagnetic mechanism as SERS. The phenomenon was used to observe the adsorption of dyes at a hydrophilically modified silver colloid surface.