A new approach to determine nanoparticle shape and size distributions of SERS-active gold-silver mixed colloids

This paper presents new results on surface enhanced Raman spectroscopy (SERS) of various probes (pyridine, acridine) deposited onto mixed Au100-xAgx colloids constituted of gold particles overlaid by less than a single layer of silver atoms. Since both electromagnetic and chemical theories predict that the Raman enhancement depends strongly on the morphology of colloidal particles, we have used two methods to approach the particle size and shape distributions of these mixed colloids. The first one compares the profiles of the experimental and calculated extinction bands of surface plasmon resonances. The second one deals with low frequency Raman spectra from particle mechanical vibrations (acoustic modes). Using a recent model that we have developed to simulate the band profiles of these spectra, we can obtain the size and shape of the resonant particles underlying the SERS effect. These distributions, deduced from both foregoing methods, are compared with those obtained by transmission electronic microscopy (TEM) and allow us to suggest that several scales of particle sizes lead to the SERS effect. This latter result is related to the fractal nature of partially aggregated colloids displaying scale invariance. We have also analyzed the physicochemical properties of the two probes (pyridine and acridine) when they are adsorbed onto these mixed colloids. The results show that the acridine species bound to the surface depends strongly on the addition of a very small amount of silver at the gold surface. These mixed colloids allow SERS spectra of acridine to be obtained when the laser excitation takes place in the green (514.5 nm) while in purr: gold colloids, acridine does not display any SERS spectrum at this excitation wavelength.