With support from the Chemical Measurement and Imaging Program (via the SciArt solicitation), Drs. Marco Leona of the Metropolitan Museum of Art and John Lombardi of CUNY are devising means of identifying the materials present in works of art - complex assemblages of inorganic and organic materials, often with unknown histories. The work is essential to understanding the history and improving the conservation of artworks. The problem is challenging because many materials of interest (e.g., markers for provenance, attribution, and use history) are present in very low concentrations and/or are affected by degradation processes. Strict sampling limitations further complicate the undertaking. The team is adapting surface-enhanced Raman spectroscopy (SERS) for the identification of organic colorants, an important class of artists' materials found in textiles, paintings, and other polychrome works of art. Specifically, they are developing sensitive and reproducible SERS substrates and new sample preparation methods, and using a broadband tunable laser to enable resonant excitation for increased sensitivity. They are investigating computational modeling in parallel with experimental adsorption measurements to address important problems affecting SERS, such as variations in response among closely related dyes and the influence of interfering compounds.
The high-sensitivity non-destructive techniques being developed should have applicability beyond the confines of cultural heritage research, in areas such as biochemical, pharmaceutical, environmental, and forensic, and homeland security. The increased understanding of the SERS phenomenon will be helpful in elucidating the relative importance of electromagnetic and chemical contributions to the SERS mechanism. The work will thus have broad impact through improved understanding and conservation of works of art and cultural heritage; enhanced cross-disciplinary research training for undergraduate, graduate, and postdoctoral chemists with case studies from art conservation and art historical studies; and science outreach through inclusion of scientific results in museum publications and public programs.
With this project we developed new techniques for the analysis of works of art, based on surface-enhanced Raman scattering (SERS), a phenomenon that occurs when organic molecules are adsorbed on the surface of metal nanoparticles. We used SERS to detect organic colorants, molecules that in ancient times used to be extracted from plants and insects and were traded all over the world. In the 19th century, synthetic analogues begun to be created: these are the pigments that we encounter every day in colored inks and other products (food colorants, lipstick, etc.). The problem in identifying these materials is that because of their coloring power, organic colorants are used in very small amount, making it difficult to detect them, particularly when one can obtain only microscopic samples, as is the case with works of art. Using SERS, we were able to detect the first example of use of organic colorants in history, in a 4,000 years old Ancient Egyptian leather piece. We were also able to identify the first use of lac dye in European art, in the 1100s. This colorant was obtained from an insect native of India, and had to be traded all over the Islamic world to reach Europe. Finding it in a religious sculpture in Southern France shows the extent of global trade at the time of the Crusades. Closer to our time, we used SERS to study Japanese prints from the end of the 19th century. This period sees Japan opening its doors to commerce with Europe and the United States. Analysis of the prints from this period shows that the synthetic dyes that were being synthesized in Europe find quick diffusion in Japan. Comparing patent dates for the dyes with the dates of the prints we can build a chronology of trade, and at the same time, develop a new tool for dating unsigned works. Finally, we developed a new tool for chemical analysis by coupling laser ablation with SERS. By using a precisely aimed short laser pulse to vaporize a very small amount of a sample (which is then collected on a SERS active film) we can analyze areas of a complex sample only a few micrometers wide. This allows us to select single grains of pigment within a paint layer for instance. The technique might even have applications in other fields. One could imagine looking at a section of tissue being investigated for medical reasons: Laser Ablation SERS could target a specific region or group of cells, to track for instance where particular metabolites might accumulate. While we did not actually use Laser Ablation SERS exactly in this context, we did use SERS to track carbon nanotubes in tissue extract, in collaboration with biomedical researchers. The postdoctoral researcher who conducted most of the work in the project is now an Associate Scientist at Yale. A graduate student working with us went on to obtain a high school teaching position with the NYC public school system and used the research work he conducted with us as part of his credentials process. In addition, we hosted for the first time two high school students for summer internships. The students had primary responsibility for developing a computer controlled positioning stage for a laser ablation system, and a hyperspectral imaging camera. This shows the education potential for fundamental scientific research carried out in a highly interdisciplinary setting.
