IDENTIFICATION OF COEXISTING INDIGO SPECIES IN AN ANCIENT GREEN THREAD USING DIRECT PLASMON-ENHANCED RAMAN SPECTROSCOPY

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  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.

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