Identification of coexisting indigo species in an ancient green thread using direct plasmon-enhanced Raman spectroscopy

Celis, F. Tirapegui, C. García, M. Aracena, A. Aliaga, A. Campos-Vallette, M.

Keywords: : Ancient thread, Indigo, Leuco-Indigo, Raman, SERS

Abstract

A green ancient thread sample from a Chilean mummy turban was analyzed by plasmon-enhanced Raman scattering spectroscopy using a direct drop-colloidal method. The enhanced-Raman signals in the sample are associated with biomolecules from the thread and two coexisting dyes, indigo and leuco-indigo. The presence of indigo (blue colour) was identified from its most characteristic vibrational bands. Leuco-indigo (yellow colour) was identified for the first time in an ancient textile; its SERS signals are coincident with the SERS bands of a synthesized leuco-indigo. The interconversion leuco-indigo to indigo was followed by UV-visible spectroscopy. Based on theoretical calculations it is proposed that the interconversion involves a pi electron delocalization mainly around the NC-CN bridge. The mixture of both dyes (indigo and leuco-indigo) is the responsible for the green colour observed.

Más información

Título de la Revista: Journal of the Chilean Chemical Society
Volumen: 65
Editorial: 2013
Fecha de publicación: 2020