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