Heightened electromagnetic fields between metal nanoparticles: Surface enhanced raman scattering from metal - Cytochrome c-metal sandwiches

This manuscript reports the demonstration of strong interparticle electromagnetic coupling in nanoparticle: molecule:nanoparticle sandwiches. These assemblies were prepared by adsorption of cytochrome c (Cc)-coated colloidal Au nanoparticles onto aggregated colloidal Ag. Surface enhanced Raman scattering (SERS) spectra for the Cc:Au conjugates (Ag:Cc:Au) were compared to those obtained by direct adsorption of Cc (Ag:Cc) at several visible excitation wavelengths. The use of identically prepared samples with the same effective [Cc] and the same laser power allows the effects of sample geometry and interparticle coupling to be probed. As expected, at all excitation wavelengths, Ag:Cc:Au gave less intense spectra than Ag:Cc, because (i) only a small percentage of the Cc molecules were close to the more SERS-active metal (Ag), and (ii) the heme, from which all observed vibrations arise, was oriented toward the Au, not the Ag. Nonetheless, the ratio of Ag:Cc:Au to Ag:Cc SERS intensity varied significantly with excitation wavelength, a phenomenon that can be accounted for only by a wavelength-dependent electromagnetic field. This, in turn, results from wavelength-dependent electromagnetic coupling between closely spaced colloidal Ag and colloidal Au. In addition to wavelength-dependent SERS intensity ratios, three separate lines of evidence support the existence of strong electromagnetic coupling: (i) the presence of a wavelength-shifted single particle surface plasmon band for Cc:Au conjugates upon exposure to agggregated Ag, (ii) demonstration of excitation wavelength-dependent photodriven conformational changes in Ag:Cc:Au, and (iii) dramatic increases in SERS intensities from protein - colloid conjugates prepared with Ag in place of Au.