Near-infrared excited surface-enhanced Raman scattering of biological molecules on gold colloid I: Effects of pH of the solutions of amino acids and of their polymerization

Near-infrared (NIR) excited surface-enhanced Raman scattering (SERS) has been measured for lysine (Lys) on gold colloid at pH 10.8 and 12.0 where its epsilon-amino group is ionized and both the alpha- and epsilon-amino groups are neutralized, respectively, The intensities of SERS signals from Lys are much stronger at pH 10.8 than at pH 12.0. This result suggests that the electrostatic interaction between the positive charge of the epsilon-NH3+ group and the negative charge of the gold surface is one of the most important factors which determine the intensities of the SERS signals. A similar SERS study has been carried out for glycine (Gly) on gold colloid at various pH. The SERS spectra of Gly on gold colloid are quite different in terms of both the frequencies and the band intensities from those of Gly on silver colloid, previously reported; the zwitter ion is dominant on the surface of the gold colloid, while the anion form is major on the silver colloid surface. The chemical interactions are much less important for the emergence of SERS of Gly on the gold colloid, NIR SERS has also been performed for oligomers and polymers of Lys and Gly. The intensities of the SERS signals decrease markedly with their polymerization. The effects of the steric hindrance due to the polymerization and of the distance between the gold surface and amino acid unit probably determine the SERS intensities.