Surface-coverage dependence of surface-enhanced raman scattering from gold nanocubes on self-asssembled monolayers of analyte

Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on gold substrates are presented for SAMs onto which gold nanocubes have been electrostaticaly immobilized. In the absence of nanocubes, no SERS signals from 4-MBA SAMs are observed. Upon addition of the gold nanocubes to the SAM, a sandwich architecture is formed, allowing for coupling between the localized surface plasmon of the nanocubes and the surface plasmon of the gold substrate. This creates a large electromagnetic field in the area where the 4-MBA molecules reside, causing the characteristic vibrational modes of 4-MBA to appear. SERS intensities increase linearly with increasing nanocube coverage up to a factor of 7 in the best case studied here, with enhancement factors of up to 1013.