Metal nanostructures with single molecule sensitivity in surface enhanced Raman scattering

Modified Raman microscope was used to correlate surface morphology, surface plasmon extinction spectra of Ag particles and Raman spectra from adsorbed molecules. All the hot particles providing large enhancement consist of Ag aggregates with a connecting axis parallel to the polarization of the excitation light. They give broad extinction peaks at ca. 500 nm from localized surface plasmon (LSP) to be in resonance at lambda(ex) = 488 nm. Moreover, huge enhancement is obtained at the junctions due to coupling of the LSP on neighboring Ag particles. Considerable number of blinking particles at room temperature showed the invariant signal intensity, i.e. bright and dark images, at 77 K. It indicates the blinking is thermally activated, most probably due to thermal diffusion of adsorbed molecules on the Ag particle with distinct enhancement as suggested by numerical simulations of the local electric field. In addition, DNA base molecules such as adenine, thymine, guanine or cytosine showed similar blinking features in SERS signal, which have no electronic transition in visible wavelength at a bulk state. Thus, SMS-SERS using a junction of touching Ag nanoparticles is quite promising to characterize various adsorbed species. (C) 2004 Elsevier B.V. All rights reserved.