The surface-enhanced Raman scattering (SERS) spectrum of L-tryptophan has been studied in the concentration range 1.4 × 10(-8) to 5 × 10(-4) M. A borohydride-reduced silver colloid was employed as the nanoparticle enhancing agent and different electrolytes have been tested for activation of the colloid. The optimum conditions have been determined for achieving high sensitivity of detection. The experimental procedure developed, which includes the use of a composite electrolyte (a mixture of NaHCO(3) and NaCl) for colloid activation, results in very high enhancement of the Raman signal (up to 10(8)). This gives the possibility of studying SERS spectra of L-tryptophan at concentrations as low as 10(-8) M, which is several orders of magnitude lower than previously reported in the literature. The observed SERS spectra were very reproducible and were detectable 2 minutes after mixing, reaching maximum strength approximately 15 minutes after mixing. The spectral characteristics were stable over the entire period of observation. We have found that SERS spectra of tryptophan in silver colloid differ in several features at low (below ∼10(-5) M) and at high (above ∼10(-4) M) concentrations. The most important difference is the absence of the peak near 1000 cm(-1) at low concentrations, which is usually assigned to the indole ring breathing mode. The observed spectra allow us to suggest that at low concentrations Trp molecules bind to the surface through the indole ring, which remains flat on the surface. This is in contrast to the previously reported observation of SERS spectra from Trp performed at concentration levels above 10(-5) M.

Surface Enhanced Raman Scattering (SERS) has been used to observe spectra of several amino acids and peptides. SERS signals from amino acids (tryptophan, phenylalanine and glycine) and peptides (Trp‐Trp and Gly‐Gly‐Gly) in silver colloids have been studied in three experimental configurations 1) free volume 2) liquid core waveguide (LCW) and 3) microfluidic channel. Even though all amino acids contain carboxyl and amine functional groups, the conditions for observing the SERS spectra are different for different amino acids. The impact of the type of electrolyte used for colloid aggregation and the experimental procedures for mixing of the components on the amplitude and the stability of the SERS signal have been studied. The optimum conditions for observing SERS signals for the above mentioned amino acids and peptides have been found. For Trp amino acid a SERS enhancement of Raman signal up to 107 was observed. Additional enhancement of 10 to 50 times can be obtained with the help of a liquid core waveguide technique. The results obtained will be useful in the development and optimization of microfluidic chip devices utilizing SERS. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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