The effects of electrostatic interaction between biological nano-metal colloid molecules and on near-infrared surface-enhanced Raman scattering

This paper reports effects of electrostatic interaction between amino acid and gold colloid on the intensity of surface-enhanced Raman scattering (SERS). Near-infrared (NIR) excited 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. Note that the SERS intensity changes dramatically with pH. 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 that determine the intensities of the SERS signals. A similar SERS study has been carried out for glycine (Gly) on gold colloid at various pH values. It also suggests that the positively charged amino groups interact with the negatively charged gold surface, yielding a SERS effect. The chemical interactions are much less important for the emergence of SERS of Gly on the gold colloid.