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.

Alkannin 是 Alkanet 的主要着色物质，Alkanet 是一种从 Alkanna tinctoria L. Shikonin 的根中提取的天然红色染料。 alkannin 的光学异构体 Shikonin 是从 Lithospermum erythrorhizon 中提取的。由于这两种红色染料仅微溶于水，普通拉曼光谱的应用受到限制。因此，表面增强拉曼光谱 (SERS) 可以成功地应用于红色染料溶液的研究。固体烷氨酸和紫草素通过普通拉曼光谱表征。密度泛函理论 (DFT) 方法用于计算染料的拉曼光谱并将实验拉曼谱带分配给它们的振动正常模式。测试了不同的 pH 条件，以确定 SERS 检测 alkannin 和紫草素的最佳条件。根据之前的结果，推断出红色染料在 Ag 基板上的垂直取向。最后，通过 SERS 光谱法在来自日本的 8 世纪手卷的染色纸样本中鉴定了紫草素。