Application to historical samples of in situ, extractionless SERS for dye analysis

Silver colloidal pastes, obtained by concentrating through centrifugation chemically reduced silver colloids, have been proposed by Brosseau et al. [2009, (1)] as suitable substrates for in situ extractionless Surface Enhanced Raman Scattering (SERS) of art samples such as pigment grains and dyed fibers. The extractionless approach lowers to few nanograms the amount of sample required for the analysis and thus it is very suitable for investigating historical artworks. As far as sample preparation is concerned, the main issue consists in achieving a suitable silver coating of the complex surfaces under investigation. The morphology of the coating in fact plays the major role in promoting signal enhancement. Moreover, the interpretation of SERS spectra may be difficult due to the possible presence of degradation products, dust and restoration materials that can contribute to the overall SERS spectrum [Brosseau et al., 2009, (2)]. In this work, the potentiality of the application of silver colloidal pastes for the identification of natural dyes has been explored on various samples obtained from historical textiles and on cross sections obtained from painted art objects. Cross sections are usually prepared to investigate multilayered pictorial films, as they may provide large information with very small sampling. The investigation of such sections is generally carried out through optical microscopy with visible and UV light, microchemical tests and micro-Raman spectroscopy [Appolonia et al., 2009]. The latter technique generally provides a detailed molecular characterization of inorganic pigments, but generally fails in the identification of painting lakes [Bellot-Gurlet et al., 2006], which owe their color to the presence of organic dyes in small amounts. SERS represents therefore a promising tool overcoming the lack of information about the dyes used in polychromies [Casadio et al., 2010]. The textiles samples considered here are few wool bundles detached from the mantle of a cope (probably dating to XVIth century) conserved in Museo del Tesoro of Aosta’s cathedral (Italy) and a very small red silk fragment of a unique tapestry dating from the end of XVth century, representing the Deposition from the Cross and conserved in Milano’s cathedral (Italy). The latter sample was already detached from the artwork and was recovered during the handling for the ongoing conservation intervention. In addition, several cross sections obtained by mounting in epoxy resin the polychrome finishing of wooden statues from Aosta Valley were considered. The corresponding painted areas, analyzed with in situ X-ray fluorescence spectroscopy, did not evidenced key-elements that might suggest the presence of a specific pigment, thus indicating that the color was obtained by organic dyes, possibly employed as lakes. Historical fibers were mildly pre-treated with water and methanol, in order to reduce the interference of contaminants on SERS spectra; fibers were then treated with the silver colloidal paste for SERS investigation. On the other hand, the coating with silver nanoparticles of the polished cross sections was obtained by employing less concentrated colloids, that allowed to record SERS spectra from the painting lakes. This work demonstrated that silver colloidal pastes are suitable for investigating dyes in different historical art samples. The concentration of the pastes can be modified according to the specific sample, in order to achieve the optimal coating for SERS analysis. Surface Enhanced Raman Scattering permits to deepen the knowledge of the colouring matters used in works of art as it can be applied with an extractionless approach to very small samples or to cross sections prepared for morphological and compositional analysis of painted layers.