Identification of anthocyanins in plant sources and textiles by surface-enhanced Raman spectroscopy (SERS)

The aim of the present study was to provide experimental procedures for the identification of anthocyanin-based dyes used in antiquity. In particular, we assessed the possibility to identify anthocyanins, both in plant extracts and in dyed textiles, by means of surface-enhanced Raman spectroscopy (SERS), a very chemically specific technique that is moreover sensitive to the changes in structures of molecules, phenomena that occur extensively in the chemistry of anthocyanins. The choice of the plant sources (bilberry, elderberry, sumac, purple corn and hollyhock) was based on their attested use in history as dyeing matters. Suitable extraction and pre-treatment procedures were optimized both for plant sources (berries, cob glumes and flowers) and textiles dyed with such sources in the laboratory, followed by SERS analyses at different pH values. Finally, special attention was paid to the well-known instability of anthocyanins: dyed wool samples were exposed to artificial aging in order to verify the possibility to identify such molecules also in faded textiles. The achievement of reliable surface-enhanced Raman spectra from these samples encourages us to suggest the protocol for the analysis of historical objects.