Silver colloidal pastes for the analysis via Surface-Enhanced Raman Scattering of colored historical textile fibers: some morphological and spectroscopic considerations

In recent years Surface Enhanced Raman Spectroscopy (SERS) has become a valid tool to investigate organic colorants in samples of historical and artistic significance and offers now new analytical strategies for scientific investigation in the field of art history and conservation.[1] The noble metal substrate effectively overcomes problems affecting normal Raman spectroscopy, such as high fluorescence due to the complex chemical environment and lack of signal due to very low concentrations of the molecules under investigation. The main challenge in performing dye analysis with SERS is the feasibility of delivering suitable SERS substrates directly on the sample, in order to avoid further treatments of the sample itself and to minimize down to a micro-scale the fragments that have to be detached from the artwork. A variety of SERS substrates have been proposed for art analysis including metal films over nanospheres (FONs),[2,3] concentrated silver colloids[4] and nanoparticles obtained by in-situ photo-reduction[5] or laser ablation.[6] Among them, chemically reduced silver colloids are presently the most popular substrates for SERS in art analysis. The present work reports the results of a thorough investigation of silver colloids[7] that are concentrated according to Brosseau et al.[4] (henceforth SC pastes) for analysis of different types of natural fibers dyed red with brazilwood or cochineal. The morphological, physical and optical characteristics of the SC paste are investigated and discussed. Specifically, the SC paste is tested on modern wool and silk dyed with cochineal or brazilwood, as well as on cotton and flax tinted with brazilwood, at various concentrations. Moreover, historical textiles from an important collection of Mariano Fortuny (1871-1949) textiles at the Art Institute of Chicago are examined, in order to test the efficacy of the SC paste on aged samples, and to shed light on whether it is true that, at a time when the chemical industries were flooding the market with bright and attractive new industrial products, Fortuny did not use synthetic dyes. Scanning electron microscopy is employed to highlight the morphological characteristics of the paste itself and to image the coatings that develop when the paste is spread on the various fibers. Spherical nanoparticles (30 to 100 nanometers in diameter) with a minority of rod-shaped particles (50 to 200 nm in length) are observed. SEM images show that the hydrophobic nature of the wool fiber’s surface limits the coverage of the silver coating, whereas the highly hydrophilic vegetal fibers are almost completely covered. Despite the different extent of coverage of the silver coating, areas with a homogenous layer of nanoparticles that is SERS active were observed in all the considered reference samples. The SC paste was effective in enhancing the signals of the dyeing molecules of cochineal and brazilwood, although finding SERS activity was somewhat harder when analyzing wool fibers with respect to other fibers. Besides the presence of spurious signals from the SC paste itself, the spectra obtained from reference samples did not show additional peaks that can be attributed to the proteinaceous or glycosidic fibers. Regarding the historical samples, the use of natural colorants was confirmed in the Fortuny fiber samples that were analyzed. Cochineal and brazilwood were found in both the silk velvets and cotton fibers examined, testifying that a skillful combination of such dyes (here identified simultaneously on the same fiber with direct extractionless SERS with SC pastes) was put in place to obtain a variety of different hues contributing to the enduring allure of such beautiful textiles.