Raman, SERS and DFT analysis of the natural red dyes of Japanese origin alkannin and shikonin

Alkannin is the main coloring matter of Alkanet, a natural red dye extracted from the root of Alkanna tinctoria L. Shikonin, the optical isomer of alkannin, is extracted from Lithospermum erythrorhizon. As both red dyes are only slightly soluble in water, the application of ordinary Raman spectroscopy is limited. Thus, Surface-enhanced Raman spectroscopy (SERS) can be successfully applied to the study of the red dyes solutions. Solid alkannin and shikonin were characterized by ordinary Raman spectroscopy. Density Functional Theory (DFT) methods were used to calculate the Raman spectrum of the dyes and to assign the experimental Raman bands to their vibrational normal modes. Different pH conditions were tested in order to determine the optimal conditions for the SERS detection of alkannin and shikonin. Based on the previous results, a perpendicular orientation of the red dyes on the Ag substrate was deducted. Finally, shikonin was identify by SERS spectroscopy in a dyed paper sample from an 8th century handscroll from Japan.