Gold nanostructures focus light to a molecular length scale at their surface, creating the possibility to visualize molecular structure. The high optical intensity leads to surface enhanced Raman scattering (SERS) from nearby molecules. SERS spectra contain information on molecular position and orientation relative to the surface but are difficult to interpret quantitatively. Here we describe a ratiometric analysis method that combines SERS and unenhanced Raman spectra with theoretical calculations of the optical field and molecular polarizability. When applied to the surfactant layer on gold nanorods, the alkane chain is found to be tilted 25° to the surface normal, which matches previous reports of the layer thickness. The analysis was also applied to fluid phase phospholipid bilayers that contain tryptophan on the surface of gold nanorods. The lipid double bond was found to be oriented normal to the bilayer and 13 Å from the nitrogen atom. Tryptophan was found to sit near the glycerol headgroup region with its indole ring 43° from the bilayer normal. This new method can determine specific interfacial structure under ambient conditions, with microscopic quantities of material, and without molecular labels.
Keywords: Surface enhanced raman scattering (SERS); lipid membrane; molecular structure; near field; plasmonics; tryptophan.