SERS spectroscopy is successfully employed in this work to reveal different components integrating the cochineal colorant employed for dying archaeological textile samples from the Arica Region in North Chile. This analysis was done by in-situ experiments that does not imply the material (colorant and biomolecules) extraction. The spectroscopic analysis of the archaeological textiles by SERS reveals the presence of bands attributed to carminic acid and nucleobases, mainly adenine and guanine. The identification of these biomolecules was also verified in raw cochineal extract and in cochineal dyed replica wool fibers fabricated by us following ancient receipts. The effect of Al on the complexation of carminic acid and other biomolecules was also tested in order to understand the changes induced by the metal interaction on the colorant structure. This study revealed that Al can also complex biomolecules existing in the cochineal extract. In particular, guanine residue seems to interact strongly with the metal, since SERS bands of this residue are enhanced.
Fig: Structure of carminic acid (a) and scheme of the interaction of carminic acid with an Ag surface (b). Raman spectra of carminic acid on glass (c) and on a Au surface (d). Raman spectrum of glucose on Au surface (e).
Furthermore, a theoretical analysis on the interaction of carminic acid and a silver surface was also performed in order to better understand the interaction mechanism between carminic acid and a metal surface that leads to the final SERS spectrum. The results of the present work will be very useful in the identification of different molecules and metal complexes that may be forming part of the cochineal colorant found in archaeological materials.
Celis, F., Segura, C., Gómez-Jeria, J.S. et al. Analysis of biomolecules in cochineal dyed archaeological textiles by surface-enhanced Raman spectroscopy. Sci Rep 11, 6560 (2021). https://doi.org/10.1038/s41598-021-86074-9