Surface‐enhanced Raman spectroscopy of various madder species on wool fibers: the role of pseudopurpurin in the interpretation of the spectra

Abstract

Madder has been used as a textile dye and pigment in works of art since antiquity. Madder dyes from different botanical species are characterized by different series of anthraquinone derivatives. The occurrence of one or more of these compounds in various relative amounts may provide a useful indication of the plant species used to obtain the dye. In this work, surface‐enhanced Raman spectroscopy (SERS) was applied to the analysis of textile fibers dyed with madder from two different botanical species, Oldenlandia umbellata L. and Rubia tinctorum L. The resulting SERS spectra were interpreted in light of the actual composition of the madder dyes as determined by high‐performance liquid chromatography. Interestingly, the SERS spectra were found to exhibit very distinctive spectral features depending on the different anthraquinone derivatives present in the dyes. In particular, the SERS spectrum of O. umbellata L. was found to match the spectrum of alizarin, while the SERS spectrum of R. tinctorum L. was surprisingly dominated by signals due to the less studied anthraquinone compound pseudopurpurin. With this study, we demonstrate that SERS spectra may offer valuable information regarding the major coloring constituents present in different madder species. Moreover, our work shows that not only can SERS be used successfully to differentiate among closely related anthraquinone derivatives, but also that this technique is particularly suitable for the detection of pseudopurpurin, leading to the identification of this compound in a number of works of art. Copyright © 2015 John Wiley & Sons, Ltd.