| Abstract
| - This paper experimentally and theoretically investigates the influence of an underlying metallic substrate(i.e., gold and silver) on the surface plasmon resonance (SPR) of labeled gold nanoparticles and the concomitantimpact on the surface-enhanced Raman scattering (SERS) signal from the labels. These experiments employnanoparticles of varied sizes (30−100 nm) that are coated with a bifunctional Raman scatterer composed of(1) a disulfide for chemisorption to the nanoparticle surface, (2) a succinimidyl ester for formation of a covalentlinkage to an amine-terminated self-assembled monolayer on the underlying substrate, and (3) an aryl nitrogroup with an intrinsically strong Raman active vibrational mode. This approach allows facile systematicassessments of how variations in nanoparticle size, substrate composition, and the gap between the nanoparticleand substrate affect the SPR of the bound particles. Both UV−vis transmission and reflection absorption(incident angle of 58°) spectroscopy are used to characterize the effect of each of these parameters on SPR.These results are then correlated with SERS enhancement factors (EFs) that were determined by accountingfor particle surface concentrations, which were measured by atomic force microscopy, and the absolute numberof labels, which were calculated on the basis of the surface area of each of the different-sized particles. AllSERS spectra were collected at an incident angle of 58° with respect to the surface normal. As expected, theSPR for particles in solution red-shifts with increasing particle size. More importantly, the SPR moves toeven longer wavelengths as the size of immobilized particles increases and as the gap between the immobilizedparticle and substrate decreases. The red shift is also greater for a gold nanoparticle tethered to a gold substratecompared to a silver substrate. A theoretical model for the extinction of a particle above a flat substrate,corrected for surface scattering, radiation damping, and dynamic depolarization, is also briefly detailed. SPRresults calculated with the model are consistent with the shifts observed in the SPR position for each of themanipulated experimental variables. The largest EFs are found for samples with an SPR maximum (λmax)between the wavelengths for laser excitation (633 nm) and the Raman band for the symmetric nitro stretchof the particle coating (690 nm). As an example, an order of magnitude in the SERS enhancement factor isgained for a 60-nm particle immobilized 1.2 nm above a gold substrate (SPR λmax = 657 nm) compared tothat for a 30-nm particle (SPR λmax = 596 nm).
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