Pretreatment Strategies for SERS Analysis of Indigo and Prussian Blue in Aged Painted Surfaces

Abstract

Surface-enhanced Raman scattering (SERS) spectroscopy is increasingly applied to the identification of organic colorants in cultural heritage objects because vibrational fingerprints can be measured from microscopic samples. However, the development of SERS into a reliable, broad-spectrum method for art analysis requires the study of a wide variety of organic and inorganic colorants as well as colorant mixtures in paint. Here, we demonstrate reliable protocols for SERS-based identification of insoluble indigo, Prussian blue (PB), and mixtures thereof in aged painted surfaces. The use of simple salts and acids for sample pretreatment is evaluated. High-quality SERS spectra of PB and indigo are elucidated upon sample pretreatment with H(2)SO(4). In several cases, SERS spectra of the colorants could not be obtained without sample pretreatment. We demonstrate the use of H(2)SO(4) to solubilize PB as well as perform an in situ conversion of insoluble indigo to soluble indigo carmine (IC) on indigo, indigo oil paint, and actual samples from historic painted surfaces. A microscopic H(2)SO(4)-treated sample from the Portrait of Evelyn Byrd produced a SERS spectrum that is consistent with a mixture of PB and IC. To our knowledge, this work represents the first SERS spectrum of indigo in oil paint and the first simultaneous detection of a mixture of blue organic and inorganic colorants in a single art sample using SERS.

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

“…Indigo dye is usually easily detectable in dispersive conditions by means of Raman spectroscopy by adopting for analyses laser lines at high wavelength [6], which are sufficient to quench the fluorescence background and to detect the molecule. Unfortunately, the recognition of this compound may be quite difficult when analyzing ancient artworks, as for instance textiles, where the dye is present at very low concentrations, or paint layers, where the colorant is generally incorporated within complex pigment mixtures [7][8][9]. Moreover, degradation processes or bad state of conservation of the art object may further affect the analyses of the blue dye.…”

“…The first semiquantitative study of indigo using SERS/SERRS was reported by Shadi et al [27], who investigated methanolic solutions of the blue dye combined with silver colloidal dispersions aggregated by means of HCl. In the specific case of cultural heritage applications, SERS-based detection of indigo has been reported for the analysis of paintings [28], textiles [8,29,30], and archaeological objects [31]. Recently, a pioneering application of tip enhanced Raman spectroscopy (TERS) has also been presented for the in situ identification of indigo in paper artworks [32].…”

Surface-Enhanced Raman Spectroscopy: Using Nanoparticles to Detect Trace Amounts of Colorants in Works of Art

“…Beside successfully detecting red lakes in combination in a few cases, SERS was used by Oakley et al (2012) upon treatment with sulfuric acid to probe mixtures of blue inorganic pigments and organic colorants, i.e. Prussian blue and indigo, in a single sample from the early 18th-century oil painting Portrait of Evelyn Byrd (1725-26).…”

“…Articles about the detection of flavonoids (Jurasekova et al 2006(Jurasekova et al , 2008(Jurasekova et al , 2012Wang et al 2007;Teslova et al 2007;Cañamares et al 2009;Corredor et al 2009;Mayhew et al 2013), indigoids (Bruni et al 2010;Oakley et al 2012), and dyes belonging to other molecular classes Cañamares et al 2008aCañamares et al , b, 2010Cañamares et al , 2014Fig. 1 Schematic of the surface-enhanced Raman scattering process.…”

Rapid and label-free Raman detection of azodicarbonamide with asthma risk

“…Holding the wish for SERS application in food and environmental monitoring [26][27][28][29] and SERS-based diagnose [30,31], it is of the essence for exploring the appropriate synthesis methods to develop the high performance nanostructures for improving the sensitivity and reproducibility of SERS [32]. In amongst, bimetallic silver-coated gold colloids (Au@Ag NPs) were prepared for identification of toxic food additives.…”