news
SERS enters the art scene While the famous American painter Mary Cassatt was running around Paris with the likes of Edgar Degas and other Impressionists in the late 1800s, she could not have imagined that ⬎100 years later someone in a Chicago chemistry lab would take a laser to one of her sketches. And yet, that is what Richard Van Duyne and colleagues at Northwestern University (NU), the Art Institute of Chicago (AIC), and the Getty Conservation Institute have done. To demonstrate the power of surface-enhanced Raman spectroscopy (SERS) for art conservation and authentication, the researchers matched the organic colorants in Cassatt’s pastel sticks with colored grains carefully removed from Sketch of Margaret Sloane, looking right. The results of the analysis are reported in AC (DOI 10.1021/ac901219m). An ideal method for artwork analysis should be sensitive and nondestructive. Several such methods exist for the identification of inorganic pigments, but the detection of organic pigments has been a nagging problem in the artconservation field. Organic colorants are composed of carbon, of course, so an elemental signature doesn’t help. In addition, the tinting strength of organic pigments is very high, so only a tiny amount of the pigment typically is present in an already small sample provided by conservators. Over the years, HPLC, IR spectroscopy, resonance Raman spectroscopy, and other methods have been applied to the problem, but they either require too much sample or are too sensitive to interferences. As the art world was pondering how to best analyze organics, Van Duyne and colleagues were developing a highly sensitive technique called SERS, in which metal nanoparticles are added to samples to boost Raman signals, to solve problems in electrochemistry and materials science. (Typically, SERS amplifies Raman signals ⬃106⫺108⫻, says Van 7128
ANALYTICAL CHEMISTRY /
Leona of the Metropolitan Museum of Art was surprised when he read this part of the paper. “It’s a really big result that they can just take it without doing any treatments” beforehand, he says. Other researchers have used SERS to analyze reference materials for art, but to Van Duyne’s knowledge, no one had applied it to a real sketch. In preparation for this feat, Christa Brosseau, a postdoctoral fellow in Van Duyne’s lab, analyzed and identified organic pigments in several pastel sticks in a box that belonged to Cassatt. In some cases, Brosseau could make identifications with a single grain of pigment. However, the organic pigments in some sticks could not be identified, which Van Duyne says points to the need for more comprehensive reference databases. Finally, the NU⫺AIC team showed that one, or possibly two, of these sticks was used to sketch the face of the cherubic Margaret Sloane SERS on colored flecks removed from Sketch of on tan wove paper. Margaret Sloane, looking right revealed that the Of the analyses, Paul Whitmore of flesh color of the face was drawn with pastel the Art Conservation Research Censticks #1 and/or #7 from Mary Cassatt’s pastel box. ter at Carnegie Mellon University says, “It’s a big advance in trying to discussion about SERS evolved. “It was attack a long-standing, difficult proba very serendipitous and wonderful lem.” The characterization of single thing that happened,” says Van Duyne. grains is impressive, but he cautions that That meeting was the beginning of a many traditional pigments were concocfruitful collaboration that led to the tions made from the extracts of plants or publication of several papers on the apinsects, so several organic dye molecules plication of SERS to art analysis. In their could be present. Leona points out that latest manuscript, the researchers first pastels have almost no binders, so the tested whether SERS could detect curreal test will be whether the researchers cumin dye contained in silk yarn, mixed can identify pigments in the background with animal glue, painted on glass, and of linseed oil in paints. “But definitely, precipitated on alumina. In this proofthis is a very substantial step forward,” of-concept study, the team showed that says Leona. “I can see that they really the technique could distinguish the dye pushed it to the next level.” —Katie Cottingham from other materials that could be present in art samplesOand they did so without extraction or hydrolysis. “Believe me, very few techniques can pull out the [dye] of interest in a very complex mixture of this sort without preseparating it,” says Van Duyne. Marco Duyne.) The connection to art conservation was made when Francesca Casadio of the Art Institute of Chicago visited his office 4⫺5 years ago and a
SEPTEMBER 1, 2009
10.1021/AC901678F 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 08/10/2009
