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.

Colors and dyes of archaeological textiles from Tarapacá in the Atacama Desert (South Central Andes)

“…For the pre-Columbian period, studies usually involve the analysis of a few samples, commonly reds or blues, with little information about other colors or shade variations [3, 48-55, 57-67, 70]. Further colors like yellow, green, white, or brown are less studied [49,50,53,56,64,66,67,69,70]. Only a few of them have widened the analyses of the dyes and focused on the mordants [50,51,54,67,68,70].…”

“…From an analytical perspective, they applied chromatographic or spectrometric techniques including gas chromatography-mass spectrometry (GC-MS), high-performance liquid chromatography with diode array detectors (HPLC-DAD) and/or mass spectrometry (HPLC-MS), direct analysis in real time mass spectrometry (DART-MS), which albeit sensitive are destructive methods [3, 48-50, 53, 54, 56-60, 62-64, 66, 70]. Analysis by surface-enhanced Raman spectroscopy (SERS) was applied in just five publications to analyze archaeological textiles from Peru and northern Chile [55,61,65,69,73]. Finally, the available literature shows a particular emphasis on the analysis of polychrome textiles from the Paracas and Nazca cultures (700 BC-800AD), on South-Central Perú [51-53, 59-61, 66, 67, 70].…”

Colors and dyes of archaeological textiles from Tarapacá in the Atacama Desert (South Central Andes)

“…In synthesis, for the pre-Columbian period, studies usually involve the analysis of a few samples, commonly reds or blues, with little information about other colors or shade variations [3,[48][49][50][51][52][53][54][55][57][58][59][60][61][62][63][64][65][66][67]70]. Other colors like yellow, green, white, or brown are less studied [49,50,53,56,64,66,67,69,70]. Only a few of them have widened the analyses of the dyes and focused on the mordants [50,51,54,67,68,70].…”

“…From an analytical point of view, they have been applied chromatographic or spectrometric techniques (CG-MS; HPLC; LC-DAD-MS; DART-MS, between others), which albeit sensitive are destructive methods [3,[48][49][50]53,54,[56][57][58][59][60][62][63][64]66,70]. SERS spectroscopy analysis was applied in just four cases to analyze in two studies the archaeological textiles from Peru and, in the other two, samples from northern Chile [55,61,65,69]. Finally, the available literature shows a particular emphasis on the analysis of polychrome textiles from the Paracas and Nazca cultures (700 BC-800AD), on South -Central Perú [51-53,59-61,66,67,70].…”

“…Finally, the available literature shows a particular emphasis on the analysis of polychrome textiles from the Paracas and Nazca cultures (700 BC-800AD), on South -Central Perú [51-53,59-61,66,67,70]. For northern Chile, only two studies have been published addressing the physicochemical analysis of textile dyes: one related to textiles from San Pedro de Atacama, almost 500 km to the south of our region of study, and one for Arica textiles, 270 km to the north [63,69]. Other results were just announced at a congress and are still unpublished [55].…”