Single Molecule Vibrational Fine-structure of Tyrosine Adsorbed on Ag Nano-Crystals

作者：Erik J. Bjerneld, Patrik Johansson, Mikael Käll · 发表于：Single Molecules · 年份：2000 · DOI：10.1002/1438-5171(200009)1:3<239::aid-simo239>3.0.co;2-4 · 被引用次数：79 · 研究领域：Gold and Silver Nanoparticles Synthesis and Applications、Nanocluster Synthesis and Applications、Protein Interaction Studies and Fluorescence Analysis

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.