Quantitative Amplification of Cy5 SERS in ‘Warm Spots’ Created by Plasmonic Coupling in Nanoparticle Assemblies of Controlled Structure

Abstract

Metal nanoparticle assemblies of well-defined structure are investigated as substrates for quantitative surface enhanced Raman scattering (SERS). The ∼100 nm structures are composed of oligonucleotide-functionalized gold nanoparticles of two sizes; a large particle serves as a template for assembly of multiple small particles. Satellite structure formation is driven by hybridization of linker oligonucleotides to thiolated single-stranded DNA bound to the particle surfaces and is verified by transmission electron microscopy and Rayleigh scattering spectroscopy. Raman scattering spectra are collected from dispersed particles and core−satellite assemblies (CSA) formed with core particles that incorporate Cy5 in the core particle surface strands. Quenching of Cy5 fluorescence allows Raman spectra to be clearly detected both from dispersed and assembled core−satellite structures. Raman scattering from Cy5 in the coupled assemblies is amplified by a factor of 8, relative to the Raman scattering collected from dispersed cores. The amplification of Raman scattering from Cy5 on the core particle surface is quantitatively consistent with the amplification expected on the basis of surface field intensities that increase when satellite particles assemble at controlled distance from the core. Raman scattering per core-satellite structure is determined by calibrating measured intensities using methanol as a Raman intensity standard. The number of Cy5 molecules per core−satellite structure contributing to the Raman scattering is determined from analysis of the spatial nonuniformity of the simulated core surface field distribution. The Raman cross section of the immobilized Cy5 species is estimated from the Raman scattering per CSA using the Cy5 coverage per core particle and the calculated mean electromagnetic enhancement on the surface of the CSA. While uncertainties in Cy5 coverage, properties of the Raman standard, core−satellite geometry, and concentration contribute modest errors in the Raman cross section estimates, weak electromagnetic enhancement allows the cross sections of the immobilized Cy5 molecules to be assessed without the usual orders of magnitude uncertanties associated with strongly enhanced near fields.