Extremely large enhancement factors in surface-enhanced Raman scattering for molecules on colloidal gold clusters

In agreement with previous results reported for colloidal silver clusters, effective surface-enhanced Raman cross sections of about 10-(16) cm(2) per molecule, corresponding to enhancement factors on the order of 10(14), have also been obtained for molecules attached to colloidal gold clusters. Spat ially isolated nearly spherical colloidal gold particles of about 60 nm siz e show maximum enhancement factors on the order of 10(3) at 514 nm excitati on, close to the single plasmon resonance. The enhancement factor increases by eleven orders of magnitude when colloidal gold clusters are formed by a ggregation of the gold colloids and when near-infrared excitation is applie d. The large effective surface-enhanced Raman cross section has been estima ted by a straightforward method based on steady-state population redistribu tion due to the pumping of molecules to the first excited vibrational state via the strongly enhanced Raman process. Our experimental finding confirms the important role of colloidal clusters for extremely large surface-enhan ced Raman scattering (SERS) enhancement factors. Simultaneously, it suggest s colloidal gold clusters as a substrate for high-sensitivity surface-enhan ced Raman scattering, which can provide an enhancement level sufficient for Raman single molecule detection. Due to its chemical inactivity, gold migh t have some advantages compared to silver, particularly in biomedical spect roscopy.