Direct analysis of anthraquinone dyed textiles by Surface Enhanced Raman Spectroscopy and Ag nanoparticles obtained by pulsed laser ablation

Detection of natural organic dyestuffs in art objects is a challenge for scientists and conservators, as it is limited by severe sampling restrictions. This work is aimed at developing a sampling-free method for the analysis of colorants traditionally used in historical textiles, with a focus on the preservation of the sample integrity to enable its further characterization with other techniques. Thus, in this study, Surface Enhanced Raman Spectroscopy (SERS) was performed directly on laboratory-dyed textiles for the identification of anthraquinone dyes, generally characterized by strong fluorescence signals that hinder conventional Raman spectra. For this purpose, SERS-active silver nanoparticles were produced by Pulsed Laser Ablation in Liquid (PLAL). PLAL presents some advantages when compared to conventional chemical preparation methods, namely the absence of reagents in the suspension and a stronger control over the nanoparticles morphology that can be obtained by changing the ablation conditions. For the analysis, a small piece of fiber was gently rubbed on a microscope glass slide where few drops of Ag nanoparticles were previously deposited and preconcentrated; micro-Raman measurement followed. To ensure the suitability of the analytical strategy for the characterization of historical artworks, the reversibility of the treatment was demonstrated by Laser Induced Breakdown Spectroscopy analysis of a wool fiber treated with nanoparticles and gently cleaned with diluted sodium citrate (p H 6-7).