Surface-Enhanced Raman Scattering of Azobenzene-Containing Long-Chain Fatty Acids Adsorbed on Metal Colloids

We have investigated the surface geometry of azobenzene-containing long-chain fatty acids (nAmH; n = 8, 12, m = 3, 5) adsorbed on silver and gold colloids as well as the interaction between nAmH and the two kinds of metal colloidal surface by means of near-infrared and visible surface-enhanced Raman scattering (SERS). The 488.0 and 514.5 nm excited Raman spectra of nAmH adsorbed on silver colloid and the 1064 nm excited Fourier transform (FT)-Raman spectrum adsorbed on gold colloid show a marked SERS effect. The lowest concentration at which the SERS signals were observed was 10⁻¹⁰ and 10⁻⁸ M for the silver and gold colloids, respectively, in the present study. The 488.0 nm excited SERS spectrum of 12A3H adsorbed on silver colloid is very similar to a Raman spectrum of 12A3H in chloroform, while the 1064 nm excited SERS spectrum of 12A3H adsorbed on gold colloid is quite different; a number of new bands appear, and the relative intensity of bands changes markedly in the 1064 nm excited SERS spectrum. Probably, a chemical mechanism contributes significantly to the emergence of the SERS effect for 12A3H on gold colloid. Particularly striking in the SERS spectrum of 12A3H adsorbed on gold colloid is the observation that bands at 1586, 1166, and 834 cm⁻¹ due to the phenyl ring modes show remarkable enhancement. It seems likely that the COO⁻ group is adsorbed directly on the silver colloid surface, while one of the phenyl groups is adsorbed directly on the gold colloid surface. The effect of the particle size on the SERS intensity has been explored for 12A3H on gold colloid. It was found from the frequency of the -N=N- stretching band that 12A3H assumes a cis conformation on the gold colloid with the smaller particle size.