DFT and surface-enhanced Raman scattering study of tryptophan–silver complex

Surface-enhanced Raman scattering (SERS) study of tryptophan was carried out in silver hydrosol. The surface adsorption properties of tryptophan were investigated due to its biological importance. Tryptophan is an essential amino acid needed for the normal growth in infants and for nitrogen balance in adults. DFT calculations using B3LYP functional with LANL2DZ basis set was carried out to support the experimental Raman and SERS data. The strong enhancement of 1343 cm−1 band, assigned to the CO2 sym. stretching vibration in the SERS spectrum along with a red shift of 63 cm−1, manifests that chemical mechanism contributes to the SERS activity. Moreover, the observed features in the SERS spectrum as well as theoretical calculations infer that tryptophan is chemisorbed to the silver surface directly through the oxygen and nitrogen atoms of the carboxylate and amino groups with an edge-on orientation with the indole ring lying nearly perpendicular to the silver surface. The SERS enhancement factors for various Raman vibrations of tryptophan were found to be of the order of 105–106.

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► Tryptophan is an essential amino acid needed for the normal growth in infants.
► Its surface adsorption properties were investigated due to its biological importance.
► Strong enhancement and red shift observed in CO2 sym. stretch in the SERS spectrum.
► Chemical mechanism contributes to the SERS activity.
► DFT calculations support the experimental Raman and SERS data.