Dry-state SERS for the “in-situ” identification of natural textile dyes (June, 2021)

Natural dyes pose problems concerning their non-invasive identification in artefacts when most of the techniques usually employed for “in-situ” analysis of coloring materials are used. Nowadays, surface-enhanced Raman spectroscopy (SERS) is currently employed to recognise dyes extracted from textiles, as well as applied to extractionless analysis directly on fibers. Nevertheless, there is still a demand for a method based on SERS suitable for the “in-situ” identification of the dyes on intact fabrics in museums. The enhancement of the Raman scattering in SERS is due to two different mechanism, an electromagnetic and a chemical one. The former contributes to the SERS intensification in an order of magnitude of 1010, while the latter of 102. Therefore, in principle, the SERS enhancement can be observed also in absence of a chemisorption and thus, a dry-state analysis leading to a completely non-invasive approach should be possible. In this context, we are studying the possibility of using thin films obtained by deposition of silver colloids on an optically transparent support, i.e. a glass slide, to prepare in an easy way SERS probes suitable for “in situ” analysis by means of portable Raman instrumentation. Silver nanospheres obtained by different methods [2,3] or, alternatively, silver nanostars [4] were deposited on glass slides functionalised with (3-aminopropyl)trimethoxysilane, to promote the adherence and prevent the so-called “coffee ring” effect. The films were tested for the identification of anthraquinonic dyes by a portable Raman micro-probe in mock-up samples of dyed textile fibres. The possibility of embedding the nanoparticles into a polymeric matrix will be also considered to ensure higher stability and a lower impact on the examined object. Finally, we exploited for the first time, at least to the best of our knowledge, commercial electrochemically-deposited substrates (SERSitive) to observe dry-state SERS, obtaining positive results that encourage in perspective to experiment the electrochemical path to produce suitable substrates for our purposes.