Raman and surface-enhanced Raman spectra of chrysin, apigenin and luteolin

The FT-Raman and surface-enhanced Raman (SER) spectra of three flavonoids, namely chrysin, apigenin and luteolin, have been obtained. The SERS spectra were obtained on citrate reduced Ag colloids. Assignments of the experimentally obtained normal vibrational modes were aided by density functional theory (DFT) calculations using the B3LYP functional and the 6-31+G* basis set. Excellent fits were obtained for the observed spectra with little or no scaling. The most intense lines in the three flavonoids SERS spectra are those in the CO stretching region and around 1250 cm−1. The first ones are often weakened by proximity of the metal surface, whereas the latter are not affected by the Ag. On the other hand, the lines at lower wavenumbers, assigned to in-plane ring deformation, are strongly enhanced by the surface, indicating a perpendicular orientation of the flavonoids on the Ag surface. The spectra of the flavonoids are compared, and a case study of application to detect weld, a mixture of apigenin and luteolin, in a textile is presented.