A novel spectroscopic approach, correlated surface-enhanced Raman scattering (SERS) and fluorescence microscopy, is used to identify organic materials in two 18th century oil paintings. The vibrational fingerprint of analyte molecules is revealed using SERS, and corresponding fluorescence measurements provide a probe of local environment as well as an inherent capability to verify material identification. Correlated SERS and fluorescence measurements are performed directly on single pigment particles obtained from historic oil paintings with Ag colloids as the enhancing substrate. We demonstrate the first extractionless nonhydrolysis SERS study of oil paint as well as the potential of correlated SERS and fluorescence microscopy studies for the simultaneous identification of organic colorants and binding media in historic oil paintings.

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.

Identifying natural, organic dyes and pigments is important for the conservation, preservation, and historical interpretation of works of art. Although previous SERS studies have demonstrated high sensitivity and selectivity for red lake pigments using various pretreatment conditions, corresponding investigations of yellow lake pigments and paints are relatively sparse. Here, surface-enhanced Raman scattering (SERS) spectroscopy is used to identify a variety of yellow organic dyestuffs and lake pigments in oil paint. High-quality SERS spectra of yellow dyestuffs (i.e., turmeric, old fustic, Buckthorn berries) and corresponding paints could be obtained with or without sample pretreatment using microliter quantities of HCl and methanol at room temperature. However, the SERS spectra of yellow lake pigments (i.e., Stil de Grain, Reseda lake) and their corresponding oil paints were only observed upon sample pretreatment. Ultimately, we demonstrate a reliable sample treatment protocol for SERS-based identification of turmeric, old fustic, Buckthorn berries, Stil de Grain, and Reseda lake as well as for microscopic samples of the corresponding oil paints.

Surface-enhanced Raman scattering (SERS) studies of art represent an attractive way to introduce undergraduate students to concepts in nanoscience, vibrational spectroscopy, and instrumental analysis. Here, we present an undergraduate analytical or physical chemistry laboratory wherein a combination of normal Raman and SERS spectroscopy is used to identify both inorganic and organic fluorescent colorants in an oil painting. On the basis of their experimental observations, students make procedural decisions to adjust acquisition settings and use SERS, thereby enabling the successful identification of unknowns. This laboratory engages undergraduate students by applying what they have learned about quantum mechanics, nanoscience, and spectroscopy to the real-world, problem-solving context of art conservation. S urface-enhanced Raman scattering (SERS) spectroscopy has become a powerful technique in analytical and physical chemistry. By integrating nanoscience with vibrational spectroscopy, SERS provides for the unambiguous and ultrasensitive detection of a wide variety of analytes. 1 For example, SERS is increasingly applied to the field of art conservation to identify colorants in minute samples from cultural heritage objects. 2−8 Indeed, SERS studies of artists' materials represent an excellent method to familiarize undergraduate students with modern applications of spectroscopy and nanoscience. Various experiments have been presented to introduce undergraduates to the field of art conservation using Raman spectroscopy, 9 UV/vis absorption, 10 NIR imaging, 11 and XRF. 12 Similarly, several laboratories have been developed to demonstrate the SERS effect and estimate enhancement factors, 13−16 but there are no existing experiments devoted to the real-world application of SERS to art conservation. The integration of SERS with art conservation enriches the learning experience by adding elements of nanoscience and advanced spectroscopy within the engaging context of art. Furthermore, this laboratory simulates a problem-solving scenario for students to detect unknown colorants in art. Students encounter problems throughout this experiment that require generating testable hypotheses and making new procedural decisions. In this problem-based learning (PBL) approach, 17 the instructor serves as a problem-solving guide by asking questions and facilitating group discussion. Ultimately, the combination of nanoparticle synthesis, normal Raman spectroscopy, and SERS in this PBL experiment enables students to learn how to identify both inorganic and natural, organic pigments in small samples from an actual oil painting.In this experiment, students are presented with an Andy Warhol-inspired oil painting of four red flowers on a gray background. The composition for the painting is adapted from the print "Flowers" by Warhol to take advantage of the basic fields of color and to connect the experiment to an iconic element of art history. Each flower contains one of the following colorants bound in linseed oil: madder lake, lac dye, ca...

A novel treatment flowchart approach for surface-enhanced Raman scattering (SERS) is used to identify both blue and yellow organic pigments in a single microscopic sample from a series of reference oil paints as well as an actual 18th century oil painting. In particular, several treatment strategies using acids and solvents are integrated into a specific flowchart designed to enable the minimally invasive identification of unknown blue (i.e., indigo, Prussian blue) and yellow organic (i.e., Reseda lake, Stil de Grain, gamboge) pigments in one sample. We demonstrate the first successful identification of a yellow lake pigment in a historic painting using SERS as well as the utility of our treatment flowchart approach for identifying pigments of varying resonance conditions, surface affinities, and treatment requirements in a single microscopic sample from a historic oil painting.