Art conservationists and historians increasingly have used Raman spectroscopy to identify the molecules in the inks and paints used in documents and artworks. But the technique generally requires them to remove a sample from the object. Now, researchers have found a way to avoid this invasive step by adapting a Raman technique that uses a metal tip to scan across the surface of a document (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5027612).
Identifying the compounds in inks and paints is of great value to scientists, conservationists, and archaeologists. For example, it could help conservationists pick the right materials to restore a damaged painting or in some cases could help historians determine the authenticity of a lost work from a master artist.
Raman spectroscopy enables researchers to obtain this molecular information, in particular through a technique called surface-enhanced Raman spectroscopy (SERS). In SERS, researchers pluck a tiny piece of the paint or ink they want to study and adsorb it onto a gold or silver surface, often in the form of nanoparticles. They then shine laser light on the metal surfaces, which boost the often weak Raman signals produced by the excited molecules. The resulting Raman signature is then used to identify the dye molecules.
Richard P. Van Duyne of Northwestern University, an early pioneer of using SERS in art conservation, and his colleagues wanted a technique that had the power of SERS but wouldn’t damage objects of cultural and artistic significance.
They found the answer in tip-enhanced Raman spectroscopy (TERS). Like SERS, it involves putting pigment molecules in contact with a Raman-signal-boosting metal. But in TERS, the metal is coated onto a scanning-probe microscopy tip. This tip can move across an object, enhancing the Raman signals of molecules it comes in contact with. Also, the tip can probe areas on the nanometer scale, letting researchers achieve single-molecule resolution.
As a proof of concept, the researchers obtained Raman spectra from a 19th-century document written with a common ink, known as iron gall. The researchers used a silver-coated atomic force microscopy tip and zeroed in on a crack in the paper. The thinner paper at the crack allowed more light through, making it easier to obtain a signal. The resulting spectrum had most of the same peaks as a conventional Raman spectrum of gallic acid, a key component of iron gall ink.
Van Duyne says one limitation of TERS is that analyzing large areas of an object could take a long time, because the resolution is so fine and the tool can be cumbersome to operate. As is the case for other tools, TERS wouldn’t work in all situations and often would work best in conjunction with other techniques, the researchers say.
John R. Lombardi, an analytical chemist at the City College of New York who specializes in Raman spectroscopy in the context of art, lauds the work as “a very valuable application of TERS.” But he warns that oils, dirt, or varnishes could block the Raman signals from pigment molecules in paintings. Written documents, having fewer of these problems, may be easier to analyze, he says.
Barbara Berrie, a conservation scientist at the National Gallery of Art, in Washington, D.C., says that in part because of the method’s fine resolution, it can tease out the pigments’ Raman signals from those of the paper. Collecting molecular information about both the paper and ink could help researchers understand how the pigment molecules change over time or interact with the paper. That could help researchers devise better ways to preserve paper or canvases without compromising the pigments.