Raman scattering of L-tryptophan enhanced by surface plasmon of silver nanoparticles: vibrational assignment and structural determination

Vibrational bands of L-tryptophan which was adsorbed on Ag nanoparticles (∼10 nm in diameter) have been investigated in the spectral range of 200–1700 cm−1 using surface-enhanced Raman scattering (SERS) spectroscopy. Compared with the normal Raman scattering (NRS) of L-tryptophan in either 0.5 M aqueous solution (NRS-AS) or solid powder (NRS-SP), the intensified signals by SERS have made the SERS investigation at a lower molecular concentration (5 × 10−4 M) possible. Ab initio calculations at the B3LYP/6-311G level have been carried out to predict the optimal structure and vibrational wavenumbers for the
zwitterionic form of L-tryptophan. Facilitated with the theoretical prediction, the observed vibrational modes of L-tryptophan in the NRS-AS, NRS-SP, and SERS spectra have been analyzed. In the spectroscopic observations, there are no significant changes for the vibrational bands of the indole ring in either NRS-AS, NRS-SP, or SERS. In contrast, spectral intensities involving the
vibrations of carboxylate and amino groups are weak in NRS-AS and NRS-SP, but strong in SERS. The intensity enhancement in the SERS spectrum can reach 103 –104-fold magnification. On the basis of spectroscopic analysis, the carboxylate and amino groups of L-tryptophan are determined to be the preferential terminal groups to attach onto the surfaces of Ag nanoparticles in the SERS measurement.