Surface-enhanced Raman spectroscopy spectra of Mexican dyestuffs

Dyes were used in Mexico since early pre-Hispanic periods for coloring of fibers, codex writing, and mural paintings, among other purposes. From the wide variety of Mexican dyes, only a handful has been studied at length. Moreover, few studies had been devoted to the identification of these substances in artistic or historical objects, in part, because of the lack of a suitable technique that complies with the nondestructive or microdestructive requirements that involves working with such objects. Surface-enhanced Raman spectroscopy (SERS) may provide the tool needed to identify the more than 20 dyes that can be found in Mexico and the first step would be acquiring their SERS spectra in laboratory conditions. To fulfill that goal, carminic acid, cochineal (Dactylopius coccus), achiote (Bixa orellana), muitle (Justicia spicigera), zacatlaxcalli (Cuscuta sp.), brazilwood (Caesalpinia echinata), and cempazuchitl (Tagetes erecta) SERS spectra were recorded in aqueous solution and directly on dyed wool fibers, using silver colloids as SERS substrate. Ag nanoparticles were prepared by chemical reduction with sodium citrate and characterized by UVVis spectroscopy and high resolution transmission electronic microscopy. Pure carminic acid was used for control SERS spectra and then samples of cochineal, achiote, muitle, zacatlaxcalli, brazilwood, and cempazuchitl were tested. Samples were prepared either by extraction from their natural sources or by dyeing the fibers; in both cases traditional recipes were followed. The acquired spectra may serve in future research as comparative references for the identification of the dyes present in cultural artifacts, and in the case of cempazuchitl, muitle, and zacatlaxcalli, these are probably the first reported Raman spectra of such plants. Copyright (C) 2012 John Wiley & Sons, Ltd.