For thousands of years, people have been using the red dye carminated from scale insects. But only now has researchers succeed in deciphering his crystal structure. They came across a surprise: the molecules are stored into a complex structure with nanopores. So far, such materials have been considered innovations of modern synthetic chemistry and can help, among other things, to bind pollutants from the air. The findings could help develop new materials based on the animal substance.
Bright red candy, lemonades or sausages often contain the dye carmine, which usually appears on the list of ingredients as an additive E120. The substance is obtained from dried female Cochenille scale insects-and for thousands of years. Historical sources suggest that the animal extract was used to color textiles around 700 BC. Artists such as Rembrandt and Van Gogh also painted with carmine. The pigment is characterized by a bright red color, which hardly fades even over time.
Porous structure
“Due to its widespread historical and today’s use, a lot of speculation about the structure of carmine and a possible explanation for its special color brilliance and durability,” reports a team led by Erik Svensson Grape from the University of Oregon. “But even though karmin is a crystalline substance, an analysis with traditional methods such as single-crystal X-ray was not possible because you could not breed enough crystals for it.” Svensson Grape and his team therefore used modern techniques of electron microscopy, including the so-called 3D electron base, which can also analyze crystals with a size of less than a micrometer.
The researchers came across an astonishing structure: the carmine consists of tiny microcrystals with a diameter of 0.3 to 0.6 micrometers, composed of butterfly -shaped metal complexes. These contain two calcium and aluminum ions, surrounded by four carmine acid molecules. If several of these molecular butterflies are lagging together, a structure with tiny pores with a diameter of about 1.8 nanometers is created.
Manufacture according to historical recipe
“It was really surprising that a pigment that has been used for a long time, which is made from a naturally occurring molecule, has this type of structure,” says Svensson Grape. “Chemists have only recently started to develop and use materials with this type of porous architecture – for example in catalysis, pollutant separation and energy storage. The first commercially available nanoporous materials were only developed in the middle of the 20th century.”
For their analyzes, the researchers used various modern variants of carmine, which they acquired in the artist needs and from a manufacturer of chemicals. On the other hand, they produced the dye even according to a historical recipe from the 19th century. To do this, they mixed the dried and grinded scale insects with citric acid, milk and with the aluminum -containing substance alum. “Both the modern rehearsals and the meadows made according to the historical model open the same crystal structure,” reports the team.
The new findings can help to identify carmine more clearly in historical artifacts than before. They also open potential new applications for animal dye. Thanks to its porous structure, it could possibly form the basis for new adsorption agents that bind pollutants from the air.
Source: Erik Svensson Grape (University of Oregon, USA) et al., Crystal Growth & Design, DOI: 10.1021/acs.cgd.5c00185
