The art of SERS

Amanda D. Francoeur, amanda.francoeur@laurin.com

Surface-enhanced Raman spectroscopy (SERS) could help identify forged artworks. Researchers are using it to analyze paintings by 19th-century artist Mary Cassatt and the pastels she used to identify whether the materials were natural, synthetic or organic colorants. Determining the pastels’ chemical classes will help the scientists to distinguish between specific dyes, revealing the era in which a work of art was created and perhaps whether it is authentic.

Richard Van Duyne, a chemistry professor at Northwestern University, and colleagues have accomplished the first direct, extractionless and nonhydrolysis SERS study of delicate pieces of art.

Photomicrographs of pastel sticks used in Mary Cassatt’s artwork are pictured. Using SERS, investigators analyzed the grains of each stick to identify their chemical class and to determine whether the colorants were natural, organic or synthetic.

Raman spectroscopy has been one method of analyzing pigments in artworks. However, SERS is one to 100 million times more sensitive, Van Duyne said. “Normal Raman spectroscopy and resonance Raman spectroscopy were shown to be very useful for studying inorganic but not organic pigments in artworks,” he said. SERS and surface-enhanced resonance Raman spectroscopy both work with organic because the fluorescence of the pigments doesn’t interfere with the Raman. He added, “About half of all the pigments used are organic.”

For the first experiment, the researchers applied SERS to powder samples of pastel sticks taken from a pastel box owned by Cassatt. They painted grains from the sticks with sodium citrate-reduced silver colloids and used a 632.8-nm HeNe laser to excite the molecules. The 632.8-nm excitation wavelength gave the most intense signal with a lesser fluorescence interference.

They discovered that the shade called “carmine lake” was in the lilac, bright-red and light-pink pastel sticks, creating a purple hue. The pink pastel stick had a fuchsia pigment containing the chemical classes of rhodamine B and rhodamine 6G, with a presence of the mineral binder magnesite. Van Duyne identified that the pastels were made of synthetic organic pigment belonging to either or both of the dye classes β-naphthol and monoazo.

In a second experiment, performed on Cassatt’s “Sketch of Margaret Sloane, Looking Right,” two samples were taken with a tungsten needle, one from the flesh tone color of the face and the second from the mauve coloring within the white ruff. The strongest SERS spectra confirmed that the colors for the face contained the same dye as the purple pastel sticks, while the ruff revealed pink colorants and an unknown component. The researchers reported that many of the colors were produced by mixing pastels instead of layering them on the canvas; therefore, it is common to find spectra with more than one colorant.

A sample from the flesh tone color of the face in Cassatt’s “Sketch of Margaret Sloane, Looking Right” revealed purple pastel colorants, including lead-white and chrome yellow-orange. The mauve ruff of the subject’s blouse revealed a pink colorant and an unknown second component.

Other methods of identifying colorants include high-performance liquid chromatography and UV-VIS spectroscopy, but chromatography requires a large sample, which may not be available when studying artworks, and UV has poor specificity and difficulty with matrix interferences.

SERS can cause minor photodegradation from the laser, but it demonstrates photostability and can determine chromophores in an array of complex matrices, even in the presence of binders such as animal glue found on glass. Most importantly, SERS has provided a more sensitive examination of complex matrices and reduced sample sizes down to a single molecule. The team’s findings were published Sept. 1 in the journal Analytical Chemistry.