Molecular Structure and Solvent Factors Influencing SERS on Planar Gold Substrates

Adsorption of analytes on surface enhanced Raman scattering (SERS) active nanometallic substrates is influenced by a variety of factors, including analyte–metal interactions, analyte–solvent interactions, and competition for the limited number of available sites on the surface. In this article, we present a study of the effects of solubility on the binding of analytes to metallic substrates as well as an equation to describe the effect of these interactions on the observed SERS signal. A variety of solvents are used to demonstrate the effect on binding of 1,2-bis(4-pyridyl)-ethylene and thiophenol to nanostructured gold SERS substrates. These solvents include mixtures of ethanol/water and acetonitrile/water with different ratios of organic/water fractions. In our previous work, two types of sites were observed on nanostructured gold substrates—nucleophilic and electrophilic—which tend to bind with analytes that are either electron poor or electron rich, respectively. The present study shows that the composition of the solvent has an important influence on the occupation of the different types of sites. Nonpolar organic solvents lead to occupation of only one of these types of sites, but evidence is found for rearrangement of the analytes on the surface as a function of concentration. Through observation of all of these effects, it is shown that to obtain the maximum signal intensity for potential SERS applications, careful consideration of the solvent composition is necessary.