Micro-Raman spectroscopy has been widely employed in the last few years for the study of artworks, allowing for the characterization of a high class of pictorial materials. However, the detection of organic dyes by conventional Raman spectroscopy is quite difficult, due to the high fluorescence provided by these compounds. Recently, remarkable improvements have been achieved by the introduction of the surface enhanced Raman spectroscopy (SERS) technique for the analysis of organic dyes.In the present work, a new method is presented, based on the use of a SERS probe made of agar-agar coupled with silver nanoparticles, for a non-destructive and minimally invasive micro-extraction of dyes from textiles. Ag-agar gel has been tested first on textile mock-ups dyed with alizarin, purpurin and carminic acid. SERS measurements have been performed adopting laser light excitations at 514.5 and 785 nm of a micro-Raman setup. Highly structured SERS band intensities have been obtained. After having verified the safety of the method by colorimetric, X-ray fluorescence and attenuated total reflectance Fourier transform infrared techniques, a real case, a pre-Columbian piece of textile, have been investigated by Ag-agar gel. This cutting-edge method offers new possibilities for a sensitive and non-destructive analysis of fluorescent materials. Figure 8. Comparison among SERS spectra of alizarin (8a-d), purpurin (8b-e) and carminic acid (8c-f) extracted from textiles by means of Ag-agar gel procedure, collected with diode laser excitation at 785 nm (8a-b-c) and argon ion laser excitation at 514.5 nm (8d-e-f).Detection of red organic dyes in Ag-agar gel

Indigoid dyes are well known as vat dyes. In their oxidized dichetonic form they are stable and insoluble in water, whereas in their reduced form, commonly known as leuco, they are soluble in water and able to be attached to fabric for dyeing purposes. These blue dyes are usually easily detectable in art objects by means of Raman spectroscopy by adopting for analyses a laser line at a high wavelength, such as a 785 nm diode laser. Unfortunately, in ancient artworks, that are often highly degraded, it is not always possible to collect high quality Raman spectra, which makes the analysis and identification of these compounds particularly challenging. In this work, we present a tailor-made methodology for the extraction and the recognition of indigoid dyes in works of art, which exploits the solubility of these compounds in their reduced form. Excellent Raman and surface enhanced Raman spectroscopy (SERS) spectra of indigo were acquired after micro-extraction on ancient and reference textiles, confirming the reliability of the presented procedure. Moreover, the methodology has been applied also for the extraction of the indigoid dye Tyrian purple on a reference textile, showing excellent results. This analytical method has been found to be extremely safe both for the reference textiles and the investigated ancient textiles, thus being a promising procedure for the selective analysis and detection of indigoid compounds in objects of artistic relevance.

A surface‐enhanced Raman spectroscopy (SERS) substrate made of agar gel coupled with silver nanoparticles has been applied to the micro‐extraction and the ultra‐sensitive detection of dyes on pieces of textile of different nature (silk, wool, and printed cotton) and on a mock‐up panel painting. In particular, the Ag‐agar gel substrate previously developed has been improved by the addition of the chelating agent ethylenediaminetetraacetic acid (EDTA), which has been found to play an important role as a stabilizer of the nanocomposite matrix and for the improvement of the micro‐extractive performances of the gel. The system has been confirmed to be non‐destructive and minimally invasive, showing a better capability of trapping the dye molecules inside its structure. Ag‐agar gel‐EDTA has been an invaluable and powerful tool for the characterization of the dye present in a printed cotton sample of unknown chemical composition. SERS analyses, performed on the dried cube of gel following extraction on the sample, have allowed for the identification of the dye. SERS results have been corroborated by high‐performance liquid chromatography analyses. The possibility of easily detecting compounds at very low concentration coupled with the high sensitivity of SERS makes this technique a valid and versatile method for the non‐destructive recognition of chemicals in works of art. Copyright © 2014 John Wiley & Sons, Ltd.

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In this work, a micro-Raman spectroscopic investigation was carried out in order to identify the pigments used in the canvas oil painting 'Rebecca at the well', which is preserved in MAON museum in Rende, southern Italy. The artwork's history is unknown, and no scientific investigation was performed on it before. Art historians believe that the painting was created in the XVIII century by an artist of the Neapolitan school. Raman spectra were collected by a Jobin Yvon micro-Raman LABRAM apparatus, with a He-Ne laser (632.8 nm) as excitation source. The main original pigments were identified and modern pigments like yellow-orange chrome and phthalocyanine were also found, providing important information about the restoration works and the painting's history.

Using density functional theory (DFT) and various cluster models for the simulation of the interaction between alizarin and silver nanoparticles, we calculated the SERS spectra of the AZ anion-Ag n (n=2, 4, 14) complexes and compared the results with experiment. The analysis of the calculated SERS spectra helped distinguishing the contribution of the chemical and electromagnetic mechanisms to the spectral enhancement, under the assumption that the excitation energies of the clusters are comparable with the local plasmon energies of nanoparticles. The results show a certain dependence of the relative Raman intensities and peak positions on the silver cluster size. Calculation of UV-VIS transition energies and Raman spectra of the complexes have been performed under the assumption that the AZ anion is bounded to the silver clusters through the oxygen atoms of the C=O groups in 1,9 positions, in a edge-on perpendicular orientation. The calculated SERS spectra show an acceptable similarity with the experimental SERS spectra carried out with excitation at 632 nm. The results of the calculation under pre-resonance condition with respect to chromophore located and cluster located excitations are compatible with mechanisms of enhancement acting on different parts of the AZ anion molecule. The calculations were performed using the B3LYP hybrid density functional. A 6-31g(d) basis set for H, C, O and LANL2DZ basis set for Ag were used. Vertical excitation energies and the corresponding oscillator strengths were calculated by means of time dependent density functional theory (TDDFT).

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