Surface-enhanced Raman scattering (SERS) spectra of many non-resonant biomolecules exhibit very broad vibrational bands. Using tyrosine (Tyr) as an example, we demonstrate that this broadening is inhomogeneous. We find that the single-molecule components behind the inhomogeneous distribution can be observed in experiments on immobilized 90-nm Ag-particles. Remarkably, single-molecule dynamics is even observed in measurements that average over a comparatively large number of particles (N ∼ 100) and molecules (0.1 mM), demonstrating that the number of “hot” adsorption sites is a limiting factor in ensemble averaged SERS. This conclusion is corroborated throughexperiments at small N and at single-molecule stoichiometry, and in agreement with previously reported results on fluorescent dyes. We estimate the surface enhancement at “hot sites” to the order 1012. Experiments with small Ag-particles (30 nm) or NaCl show that “hot sites” are not due to atomic-scale features, such as Tyr complexes with adatoms or Cl-ions, indicating a dominant enhancement mechanism of the “classical” electromagnetic type. We argue that the inhomogeneous broadening originates in a heterogeneous metal-molecule interaction and that the rich spectral dynamics observed are transitions between different long-lived chemisorption states. We attempt to characterize some of these states through quantum chemical calculations of various Tyr – Ag complexes.
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Bjerneld et al. (2000) studied this question.