Application of surface-enhanced Raman scattering (SERS) for the identification of anthraquinone dyes used in works of art

Surface‐enhanced Raman scattering (SERS) was investigated for applications in the analysis of anthraquinone dyes used in works of art. Two SERS procedures were developed and evaluated with three frequently used anthraquinone dyes, alizarin, carminic acid and lac dye. The first procedure involves coating a layer of silver nanoparticles directly on pieces of filter paper stained with the dyes of interest by thermal evaporation to induce SERS effect. In the second procedure, a SERS‐active Ag‐Al2O3 substrate was prepared by spin‐coating an alumina‐nanoparticle layer onto a glass slide to provide the nanostructure of the substrate, followed by thermally evaporating a layer of silver nanoparticles on top of the alumina layer. Aliquots of dye solutions were delivered onto this substrate to be analyzed. Intense SERS spectra characteristic of alizarin, carminic acid and lac dye were obtained using both SERS procedures. The effects of two parameters, the concentration of the alumina suspension and the thickness of the silver nanoparticle layer on the performance of the Ag‐Al2O3 substrate were examined with alizarin as the model compound. Comparative studies were conducted between the Ag‐Al2O3 substrate and the SERS substrate prepared using Tollens reaction. The Ag‐Al2O3 substrate was shown to offer larger enhancement and improved reproducibility than the Tollens substrates. Finally, the potential applicability of the Ag‐Al2O3 substrate for the analysis of real artifact objects was illustrated by the identification of alizarin extracted from a small piece of textile dyed using traditional methods and materials. The limit of detection for alizarin was estimated to be 7 × 10−15 g from tests performed on solutions of known concentration. Copyright © 2005 John Wiley & Sons, Ltd.