Surface-enhanced resonance Raman spectroscopy signals from single myoglobin molecules

The extremely large cross-section available from metallic surface enhancement has been exploited to investigate the Raman spectrum of heme myoglobin adsorbed on silver colloidal nanoparticles at very low concentrations. The study has been performed on particles both in solution and immobilized onto a polymer-coated glass surface. In both the cases, we have observed striking temporal fluctuations in the surface-enhanced resonance Raman spectroscopy (SERRS) spectra collected at short times. A statistical analysis of the temporal intensity fluctuations and of the associated correlations of the Raman signals has allowed us to verify that the single molecule limit is approached. The possible connections of these fluctuations with the entanglement of the biomolecule within the local minima of its rough energy landscape is discussed.