This article reports on a new method, based on surface-enhanced Raman scattering (SERS), for the nondestructive identification of organic colorants in objects whose value or function precludes sampling, such as drawings, prints, historic and archeological textiles, handwritten or printed documents, and forensic evidence.
A bead of a polymer hydrogel loaded with a solution containing water, an organic solvent, and a chelating agent were used to extract minimal amounts of the colorants from the work of art for SERS analysis. Using a gel as a medium for the solvent mixture confined its action only to the areas of the work of art covered by the gel bead. The gel bead was then removed from the work of art, covered with a drop of Ag colloid, and examined with a Raman microscope. Transfer of the dye from the substrate to the gel did not require removing a sample from the work of art, thus preserving the physical integrity of the object. Spectrophotometric color measurements confirmed that color change was below the limit perceivable by a human observer. Finally, the size of the polymer bead can be reduced to a fraction of a millimeter to further minimize any impact on the work of art, without detriment to the effectiveness of the method. The technique has been successfully used for the analysis of a mordant dye on the 15th century Netherlandish tapestry, “The Hunt for the Unicorn”, and of a synthetic lake pigment on a Meiji period Japanese woodblock print. (publisher abstract modified)