To study the composition of fragile ancient artifacts, scientists often turn to surface-enhanced Raman spectroscopy (SERS), a technique that can identify low levels of small organic molecules, such as dyes. But current SERS methods don’t work with some oily or otherwise insoluble compounds. Now researchers report a a laser vaporization method that allows analysis of previously SERS-incompatible materials (Anal. Chem. 2013, DOI: 10.1021/ac400440c).
In a typical SERS experiment, scientists dissolve a very small sample in a solution containing gold or silver nanoparticles so the sample molecules adsorb onto the nanoparticles. When illuminated with laser light, the metal surfaces greatly amplify the characteristic signals produced by the excited molecules. These signals allow scientists to identify the compounds and measure their concentrations.
It’s hard to predict how each sample will react with the nanoparticles, says Pablo S. Londero, a cultural heritage scientist at the Metropolitan Museum of Art in New York City. “You never know what an artist put into a material,” he says.
What’s more, many materials are insoluble or don’t adsorb well to the metal surfaces. For example, Londero and his colleague Marco Leona were studying 3,000-year-old dyed leather from an ancient Egyptian chariot. The dyes sit within the leather’s collagen matrix, making it tricky to prepare a sample for a traditional SERS analysis.
The researchers thought that a laser pulse might vaporize a pinpoint of the material of interest and that the vaporized material would adsorb to a metal surface more readily. Londero, Leona, and collaborator John R. Lombardi of the City College of New York fine-tuned the idea to come up with a new SERS method that takes about 15 minutes to analyze a single sample.
They put a sample in a small vacuum chamber and use a microscope to choose a part of the sample to hit with an intense laser pulse. The laser vaporizes a few micrometers of the material, which then deposits onto a thin film of silver nanoparticles sitting just above the sample. The scientists then shine another laser on the silver to record the characteristic signals produced by the deposited material.
The scientists tried out their technique on the dyes embedded in the Egyptian leather. From the spectra, the team confirmed that the pigment is madder lake, a dye made from the madder plant (Rubia), combined with a metal that helps it bind to the leather.
The setup is clever, says Renato Zenobi, an analytical chemist at the Swiss Federal Institute of Technology, Zurich. He likes that the direct vaporization keeps the whole experiment within one sample holder. But the greatest advantage, he notes, is that the technique produces spectra of completely insoluble materials that are out of reach with typical SERS methods.