Fourier transform-Raman and surface-enhanced Raman spectroscopy analysis of safflower red-dyed washi paper: pH study and bands assignment

Safflower red is a natural dye extracted from the petals of Carthamus tinctorius L. Its main coloring matter is carthamin. Despite its poor lightfastness, safflower was greatly appreciated in Asia for its bright red color. It was particularly important in nishiki-e multicolor Japanese woodblock printing. In the present work, safflower red was analyzed by Fourier transform (FT)-Raman and surface-enhanced Raman scattering (SERS) spectroscopies and the band assignment was carried out with the support of density functional theory calculations. Different pH values were tested in order to determine the optimal conditions for SERS analysis. Changes in the relative intensity of the Raman bands at neutral and acid pH suggest a deprotonation of the acidic groups in the molecule. From the obtained results, the adsorption geometry of the dye molecule on Ag nanoparticles was deducted. Finally, SERS was successfully applied to the detection of safflower red in a paper fiber from a Japanese woodblock print.