Surface-Enhanced Raman Spectroscopy (SERS) of historical dyes on textile fibers: evaluation of an extractionless treatment of samples

Most natural organic dyes are fixed on textiles by mordanting, i.e. by treating the fiber with metal salts (e.g. aluminium salts) so that the metal ion mediates the interaction between the dye molecule and the fiber itself. As a result, for SERS analysis of historical textiles it is usually necessary to extract the dye itself from the ancient artefact by more or less aggressive hydrolysis methods. Of course, the extraction procedure is time-consuming and leads to the destruction of the sample. For this reason, in recent times many researchers began to propose various extractionless procedures, to be applied directly on mordanted textile fibers [Leona et al., 2006] [Jurasekova et al., 2008][Brosseau et al., 2009]. On the other hand, these methods require silver colloids obtained by unconventional synthesis: microwave reduction of Ag2SO4, in situ photoreduction of Ag nanoparticles or SiO2 spheres covered with silver by thermal deposition. The present work studies the applicability of a procedure, based on the use of an Ag Lee-Meisel colloid [Lee and Meisel, 1982], to obtain SERS spectra directly from fibers. This sol synthesis is simple and doesn't require any specific instrumentation. The analyzed fibers are both wool threads dyed in our laboratory according to historical recipes and ancient textile samples. Reference wool threads had been washed, treated with alum, KAl(SO4)2 and with acid potassium tartrate and finally dyed [Bruni et al., 2011 (a)], while the ancient samples analyzed come from textile manufacts belonging to the gallery Moshe Tabibnia, Milan. Special attention was paid to the possibility to obtain a reliable SERS spectrum from amounts of sample as small as possible, so that the method can be considered, if not entirely non destructive, at least micro-destructive and to the possibility to use different excitation wavelengths besides the green one frequently employed, for example a near infrared radiation as in the FT-Raman technique. As regards the use of this excitation source, it is well known that different excitation wavelengths can enhance different vibrational modes in the resulting SERS spectrum: indeed, there are significant differences between the SERS spectra of our previous database [Bruni et al., 2011 (b)] recorded at 532 nm and the FT-SERS spectra collected with excitation at 1064 nm. Thus FT-SERS spectra of reference dyes in solution were collected in order to obtain more suitable references in the near-infrared (NIR) region. The aim to reduce, as much as possible, the size of the thread used for the analysis has been achieved: starting from 5 mm of thread initially subjected to analysis, we decreased the sample dimensions, in most cases, to a single fiber of wool, thus making the technique nearly non-destructive. As regards the second point, FT-SERS spectra on dyed wool were recorded on a wide range of colorants, leading to the construction of a new database. It collects spectra of some chromophores and of many natural organic dyes which belong to several molecular classes, namely anthraquinones, flavonoids, neoflavonoids, biflavonoids, carotenoids, curcuminoids, naphthoquinones, and gallotannins. Good results were also achieved on ancient textile samples with the use of an excitation wavelength of 532 nm, while studies about the development of a FT-SERS procedure for the identification of dyes in ancient samples are still in progress.