The Metropolitan Museum of Art in New York (The Met) has teamed up with Columbia University scientists to bring techniques used in immunology to help identify the exact nature of materials used in art works. Museums generally utilize Fourier-transform infrared spectroscopy and gas chromatography/mass spectrometry to learn what ingredients were used to make a paint, but these techniques have their limitations. The new partnership will help develop better techniques to identify which proteins and polysaccharides are present in tiny samples of art.
From a statement by the National Science Foundation:
Immunological methods rely on the specificity of one antibody for one target molecule, called an antigen. In applying that kind of technique to art, the proteins or gums found in an artwork serve as the antigen.
Enzyme-linked Immunosorbent Assay (ELISA)–a technique commonly used in biological research and currently employed for art analysis at the MMA–harnesses antigen-antibody specificity for identification purposes. The antigen-antibody complex is detected because it attaches to a “reporting system”, in this case an enzyme-catalyzed reaction that yields a colored product when there is a positive result. The intensity of the colored response can be visible to the naked eye, and is recorded by a spectrophotometer.
Knowing which proteins or gums are in a sample is only half of the answer. The location of the materials in the stratigraphy of an artwork can determine if there are egg-based paints beneath oil paint layers, or if an egg-white coating was applied in between layers, for example.
At the MMA, a different reporting system is being applied to the localization of proteins in situ using indirect ELISA analysis on cross-sections of paint samples.
The reporting system is a Surface-enhanced Raman Spectroscopy (SERS) nanoparticle. It is composed of a Raman-active dye surrounding a gold colloid, encapsulated in a silica shell that is functionalized to bind a target molecule, in this case an antibody. The gold nanoparticle core acts as a substrate for SERS, and increases the Raman signal of the reporting dye so it gives the most intense spectrum in the cross-section.
The SERS-nanotag-antigen-antibody complex is allowing the unambiguous localization of proteins in a given multi-layered cross-section.
Images: Top: Scenes From the Life of Saint John the Baptist (Francesco Granacci; ca. 1506-1507), egg tempera, oil, and gold on wood and cross section of paint layers from the painting. Side: ELISA plate after the enzyme-catalyzed color reaction; positive reaction in one well.
Press release: What Lies Beneath? Understanding Art Using Science