Sample Treatment Considerations in the Analysis of Organic Colorants by Surface-Enhanced Raman Scattering

Abstract

The introduction of surface-enhanced Raman spectroscopy (SERS) in the field of cultural heritage has significantly improved the analysis of the organic dyes and their complexes that have been used as textile dyes and pigments in paintings and other polychrome works of art since antiquity. Over the last five years, a number of different procedures have been developed by various research groups. In this Article, we evaluate the effect of pretreating samples by exposing them to hydrofluoric acid (HF) vapor prior to SERS analysis, a step designed to hydrolyze the dye-metal complexes and increase analyte adsorption on the nanosized metallic support, thus enhancing the SERS signal. Materials studied include pure colorants, commercial lake pigments, and fibers from dyed textiles, as well as actual aged samples, such as microscopic fragments of lakes on paper and ancient pigments and glazes from several works of art, covering a wide range of time, from the second century B.C. to the early 20th century. In each case, SERS spectra obtained with or without HF hydrolysis were critically evaluated. The pretreatment with HF vapor resulted in faster analysis and increased sensitivity in most cases, with the exception of dyed silk fibers, where silk protein hydrolyzates were found to interfere with SERS analysis. As a final point, a two-step procedure including SERS on untreated and treated samples is proposed as a standard approach: by analyzing a sample first without hydrolysis, and then, following removal of the colloid, upon HF treatment, the best and most reliable results for a great number of dyes and substrates are assured.

Van Gogh’s Irises and Roses: the contribution of chemical analyses and imaging to the assessment of color changes in the red lake pigments

“…This step is often fundamental to hydrolyze natural dyes-based lakes and make the free dyes available for adsorption on the nanoparticles, as previously demonstrated [23]. Once the sample holder was removed from the chamber, 2 μL of silver colloid were placed directly on the sample followed by 0.5 μL of a 0.5 M KNO 3 solution to induce the aggregation.…”

Organic dyes in illuminated manuscripts: a unique cultural and historic record

“…Microspectrofluorimetry showed an excitation maximum at 560 nm and emission maximum at 593-608 nm, which is in agreement with the brazilwood recipe 8 from Livro de Como se Fazem as Cores, which also contains gypsum (electronic supplementary material, figure S7) [5]. Raman spectroscopy showed a high fluorescence background signal and for this reason the identification of the dye was only achieved by SERS (following HF treatment), as presented in table 3 and figure 3, through its bands at 367, 417, 462, 485, 522 and 546 cm −1 owing to ring deformation, at 638, 671, 713, 724 and 808 cm −1 assigned to the out-of-plane δ(CH) and δ(CO), at 1105 cm −1 attributed to the in-plane δ(CH), at 1165 cm −1 owing to the δ(CCH) and ν(C-C), at 1201 cm −1 ascribed to δ(C-C), at 1350 cm −1 attributed to the ν(C-O) and, finally, at 1487 and 1556 cm −1 both assigned to ν(C=C) [50,51]. The SERS spectrum is in agreement with the spectra collected for the brazilein chromophore, but even more with the brazilwood reconstruction, which is also in agreement with the microspectrofluorimetry results.…”

Tailored micro-extraction method for Raman/SERS detection of indigoids in ancient textiles

“…In the last decade, the successful application of the high sensitive surface enhanced (resonance) Raman spectroscopy [SE(R)RS] for the detection of organic dyes in works of art has strongly improved the possibility of identifying fluorescent organic dyes according to minimal invasive [10][11][12][13][14] and microdestructive approaches [15][16][17]. Up to the present, the majority of the developed SERS procedures dealt mainly with the detection of red and yellow dyes [15][16][17][18][19][20][21][22][23][24][25][26], whereas only a few works have investigated the characterization of indigoids by means of SERS/SERRS. One of the main problems is that indigo is insoluble in the aqueous silver colloid dispersion and it must be treated by specific solvents prior to SERS analyses.…”