Microscopic morphology and SERS activity of Ag colloidal particles

We have experimentally and theoretically studied Raman spectra from single molecules on Ag particles. Extremely large enhancement was observed at the Ag aggregates not only for cationic dye (R6G) but also for DNA base molecules. This is in good agreement with a numerical simulation of the local electric field on the Ag surfaces using FDTD (Finite Difference Time Domain) method, as two touching Ag particles with various shapes and sizes show vast enhancement at the junction. In fact, single molecule sensitivity is predicted for ordinary molecules with a scattering cross section of 10(-30) cm(2)/molecules, which corresponds to an enhancement factor of ca. 10(10) (ca. 340 as an amplitude enhancement) in our micro-Raman equipment. Blinking (drastic intensity change with time) of the Raman signal is probably due to thermal diffusion of the adsorbed molecules on the Ag particles between different sites with distinct interaction and enhancement factors. (C) 2002 Elsevier Science B.V. All rights reserved.