Quantitative amplification of surface enhanced Raman scattering through plasmonic coupling in controlled nanoparticle assemblies

Metal nanoparticle assemblies of well-defined structure are investigated as substrates for quantitative surface enhanced Raman scattering (SERS). The ~100 nm structures are formed from oligonucleotide-functionalized gold core and satellite particles. Raman scattering from Cy5 incorporated on the core particles is detected before and after formation of the coupled plasmonic structures. The amplification of Raman scattering observed upon formation of the coupled structures matches quantitatively the increase in the fourth power of the surface E-field associated with coupling between particles. Raman scattering per core-satellite structure is determined by calibrating measured intensities using methanol as an intensity standard. The number of molecules that contribute significantly to the Raman signal and the mean cross section per adsorbed molecule is determined by analysis of the spatial non-uniformity of the core surface field distribution. Comparison of the wavelength dependence of the near field and the scattering spectrum using simulation reveals that the wavelengths of the maxima in near and far fields are more closely aligned for the coupled structures than for isolated cores.