Application of surface-enhanced Raman scattering techniques to the ultrasensitive identification of natural dyes in works of art

Surface‐enhanced Raman scattering (SERS) is a powerful technique for the detection of natural dyes found in archeological and historical textiles, in paintings, and in other works of art. Natural organic products historically used as textile dyes or lake pigments are often fluorescent under normal dispersive Raman measurement conditions. To add to the fluorescence problem, the amount of dye actually present on works of art is minimal, requiring extremely sensitive analytical techniques. The enhancement of the Raman signal and the quenching of the background fluorescence resulting from the adsorption of dye molecules on metal nanoparticles in SERS concur to solve the problems encountered when studying dyes by Raman spectroscopy. Reproducible spectra of several reference dyes were obtained, and extraction and adsorption protocols to optimize the analysis of samples from actual work of art were developed. SERS supports evaluated include citrate‐reduced and hydroxylamine‐reduced Ag colloids, as well as Tollens mirrors and silver nanoisland films. Dyes for which SERS spectra were observed include: alizarin, purpurin, laccaic acid, carminic acid, kermesic acid, shikonin, juglone, lawsone, brazilin and brazilein, haematoxylin and haematein, fisetin, quercitrin, quercetin, rutin, and morin. The incompatibility with SERS of techniques traditionally used to extract dyes from artwork samples was demonstrated, and a nonextractive hydrolysis technique specially suited to prepare SERS microscopic samples was developed. Finally, SERS was successfully used to identify alizarin in a 1mm by 50 µm (diameter) single fiber sample form a sixteenth‐century tapestry. Copyright © 2006 John Wiley & Sons, Ltd.