Web Date: December 17, 2012
Ancient Pigment Forms Nanoscale Sheets
Ancient Egyptians used a copper-based blue pigment to paint murals and to decorate their pottery. Scientists have found that this pigment easily forms nanosheets that emit near-infrared light (J. Am. Chem. Soc., DOI: 10.1021/ja310587c). Because of this property, the researchers say, the blue material could find use in medical imaging or in counterfeit-proof documents.
Tina Salguero, an inorganic chemist at the University of Georgia, Athens, and her colleagues study materials that consist of layered sheets. They split these bulk materials into nanosheets that have desirable properties, like storing charge or conducting electricity.
While watching a television show, Christopher Barrett, a postdoctoral scholar working with Salguero, learned about a different kind of layered material: pigments used by artists in the ancient world. Salguero encouraged him to investigate these pigments, sending the chemists into the worlds of archeology and cultural heritage.
“This project has been a lot of fun because it combines information from other areas that I’ve been generally interested in but had no reason to go into,” Salguero says.
One of the pigments is called Egyptian blue. It consists of a mixture of calcium copper tetrasilicate (CaCuSi4O10) and silicon oxide impurities, such as quartz and glass. Egyptian blue is the earliest complex synthetic pigment, says Mark Abbe, an archeologist at the University of Georgia, who was not involved with the work.. Starting around the early third millennium B.C., people across Egypt, Greece, and Mesopotamia used the pigment as a replacement for one based on the rock lapis lazuli.
In recent years, scientists have become aware that Egyptian blue pigment emits near-IR light when bathed in visible light (Chem. Commun., DOI: 10.1039/B902563D). This unusual property helped conservation scientists at the British Museum identify invisible traces of the pigment on artifacts from the Parthenon. “If there’s one pigment particle on a work of art, you’ll see it,” Abbe says. “No other ancient material has such strong luminescence.”
Few materials emit near-IR light, Salguero says. She wanted to see if she could separate Egyptian blue pigment into light-emitting nanosheets. If it formed nanosheets, Salguero figured the material could find interesting applications.
For a pigment that remains stable on ancient artifacts for millennia, it separated into nanosheets surprisingly easily, Salguero says. All her team had to do was stir the material in hot water for several days. Using atomic force microscopy, they found that single-layered sheets of the pigment were about 1.2 nm thick. The scientists then confirmed that the nanosheets of CaCuSi4O10 emit near-IR light when hit with visible light.
Using an inkjet printer, the researchers easily applied the material as nanosheets to surfaces. Salguero says that as a result, the pigment could find use in printing documents prone to counterfeit. Also, near-IR light easily penetrates biological tissue. Biologists could use the nanosheets to build dyes for biomedical imaging, she adds.
Abbe says the chemists’ work reveals how the pigment could degrade on a nanoscale, possibly helping archeologists understand the pigment deterioration they have observed at burial sites.
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