Surface‐enhanced Raman spectroscopy (SERS) on silver colloids for the identification of ancient textile dyes: Tyrian purple and madder

Abstract

Surface-enhanced Raman spectroscopy (SERS) was used for the identification of natural organic dyes belonging to indigoid and anthraquinone classes in archeological samples, and good agreement with the corresponding reference commercial materials was found. Special attention was paid to the well-known problem of anomalous bands that arise sometimes in the SERS spectra on colloids: as suggested in the literature, this problem could be reduced by the use of poly-L-lysine and ascorbic acid as aggregating agents, but we observed that also the addition first of the analyte and subsequently of suitable electrolytes to the colloid in an inverted order compared to the most widely used method can be of help in limiting the intensity of such spurious bands. This procedure allowed us to obtain, for the first time, the SERS spectra of both modern and ancient Tyrian purple and to solve a specific problem observed in the analysis of archeological wool samples dyed with madder lake, i.e. the competition in the SERS response between the dye and other compounds possibly deriving from the degradation of the peptide chain during the hydrolysis treatment during the extraction of the dye from the wool fiber.

Surface‐enhanced Raman spectroscopy (SERS) on silver colloids for the identification of ancient textile dyes. Part II: pomegranate and sumac

“…The resulting colloid could be kept under refrigeration in the dark (by wrapping the flask in an aluminium foil) and was characterized by determining the wavelength of the absorption maximum in the visible region; all the Lee-Meisel colloids prepared had an absorption maximum between 425 and 435 nm. [6] …”

“…In a previous paper, [6] we employed SERS on Ag colloids, with NaCl or poly-L-lysine and ascorbic acid as aggregating agents, to identify Tyrian purple on a purplish bone fragment and madder on a red wool thread, the latter found at the same excavation site in the Libyan Sahara to which the samples here investigated belong. In the present paper, the use of a different electrolyte, namely NaClO 4 , was evaluated, giving rise to significantly improved spectral quality compared to the previously used aggregating agents.…”

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

“…These methods are often proposed to overcome the spurious signal of the citrate that is used in the LeeMeisel synthesis to avoid the coalescence of nanoparticles. In fact, the spectrum of citrate can appear in the SERS spectra recorded alongside, or sometimes obscuring, the analyte signal due to competing adsorption processes on the surface of the silver nanoparticles [36,41,184]. The main drawbacks of in situ photoreduction though are, in turn, overheating of the sample and the appearance of carbonaceous bands.…”

Chemometric Analysis of Raman and IR Spectra of Natural Dyes

“…however, most of them are related to the determination of dye/pigment on the thread in context of the conservation of art [31,34,[107][108][109], but not in the biological systems.…”