Fourier-transform surface-enhanced Raman spectroscopy (FT-SERS) applied to the identification of natural dyes in textile fibers: an extractionless approach to the analysis

In the present study, an application of a silver colloid substrate in order to obtain Fourier-transform surface-enhanced Raman (FT-SER) spectra of natural historical dyes is presented. In detail, we collected a spectral database from solutions of pure dyes and then we carried out extractionless both hydrolysis and non-hydrolysis FT-SERS analyses on wool fibers previously dyed in our laboratory and on ancient textiles. The term 'extractionless' refers to a method of SERS analysis applied directly on the fiber, thus avoiding the extraction of dyes from textile samples. The combination of a low-energy source of radiation, as in the FT-Raman technique, with SER spectroscopy can bring the important advantage of reducing the fluorescence typical of ancient samples and organic dyes. In some historical textile samples, for which SER spectra by use of visible excitation could not be obtained, the FT-SER spectrum of an iron-gall dye was recorded without hydrolysis, while, with an HF hydrolysis pre-treatment on ancient fibers, madder, lac dye and brazilwood were clearly recognized.