Some considerations of the SERS effect of L-phenylalanine in the near-infrared region using silver colloid solution

The surface‐enhanced Raman scattering (SERS) spectra of L‐phenylalanine were examined in the near‐infrared region using silver colloid solutions. It was shown that near‐infrared SERS (NIR‐SERS) was not detected for L‐phenylalanine in a normal silver colloid solution, whereas in the visible region a distinct SERS intensity enhancement was realized for such sample systems. In the present study, this situation was considered from the viewpoint that the degree of aggregation of colloid particles necessary to cause the SERS effect is different between visible and near‐infrared radiation. In this work, silver colloid solution containing dissolved L‐phenylalanine was dropped on to a filter‐paper and this filter‐paper sample was then dried. It was confirmed by SEM (scanning electron microscopy) that as a result of this sample preparation, the degree of aggregation of colloid particles increased considerably. It was shown that the near‐infrared SERS was detected with high sensitivity for L‐phenylalanine adsorbed on silver particles condensed on filter‐paper. From consideration of the magnitudes of the SERS intensity enhancement in this case, it is suggested that the mechanism of NIR‐SERS for the present sample system is the electromagnetic effect, related to the excitation of surface plasmons (the surface plasmon polariton mechanism). © 1997 John Wiley & Sons, Ltd.