Surface-enhanced Raman scattering of biological molecules on metal colloid II: Effects of aggregation of gold colloid and comparison of effects of pH of glycine solutions between gold and silver colloids

As the second paper of a series of our studies on surface-enhanced Raman scattering (SERS) of biological molecules on metal colloid, this paper reports effects of aggregation of gold colloid on the SERS intensity of glycine (Gly) in aqueous solutions and comparison of effects of pH of the Gly solutions between gold and silver colloids. In order to investigate the relation between the SERS effect and colloidal aggregation, we have developed a spectroscopic system by which we can measure simultaneously time-resolved SERS spectra and a time-dependent intensity change in a surface plasmon absorption of a metal colloid system. By use of this instrument we monitored time-dependent intensity changes in the SERS signals and in an absorption band at 802 nm of Gly on gold colloid at pH 3.9 and 6.0, For both pH values, the intensities of the SERS and the absorption band change in concert with each other at the emergence of the SERS effect. The intensity changes in the SERS signals are rather slow for the solution of pH 6.0 while rapid for that of pH 3.9, The coagulation proceeds slowly near the isoelectric point of Gly, whereas the rapid coagulation happens in the acidic region. SERS spectra of Gly adsorbed on silver colloid were measured at various pH values, and it was found that, in sharp contrast to the gold colloid system, which we reported in our previous paper, the SERS intensity of Gly on the silver colloid decreases markedly at pH 2.0 where most of Gly in the solution has a COOH group instead of a COO- group. It seems that the coagulation of the silver colloid particles does not occur very much below pK(1) (2.35) because the COOH groups do not interact strongly with the positively charged silver particles.