Magic blue: Researchers develop dyes that glow through the skull

The Egyptians invented the color blue. This is suggested by ancient blue glass beads. They were found in a tomb dating to around 3300 to 3500 BC. Later, the pigment was also found in hieroglyphs: These were carved into the Pyramid of Unas in Saqqara, for example, and filled with the pigment. The pharaohs apparently had an insatiable demand for so-called "Egyptian blue." According to estimates, they needed around 1.4 tons to decorate a single temple.

The ancient Romans called the pigment "caeruleum," which means "color of the sky" in Latin. The oldest recorded recipe for its production comes from an architect named Vitruvius. He recorded it around 30 years before the beginning of our era: "Sand and salt are ground so finely that a flour-like product is formed. Copper is rubbed over the mixture with coarse files like sawdust."

Then, the balls had to be formed by hand and placed in earthen vessels in a hot furnace. "As soon as the copper and sand [. . .] unite under the intensity of the fire, they absorb each other's perspiration," Vitruvius continued. "Thus, they lose their previous properties and acquire a blue color."

Without this enormous effort, blue was unobtainable. The sky-colored pigment gave frescoes and mosaics brilliant blue hues, which one would look for in vain in the early Stone Age paintings on the cave walls of Lascaux or Altamira. Back then, people had to make do with what was available to them: ochre and charcoal. Blue dyes were not among them, as they are rare.

This is because the pigment must reflect blue light and simultaneously absorb red light. However, red rays are the longest-wavelength rays in the visible spectrum – and therefore have the lowest energy. "Absorbing such rays requires molecules with special electron transitions," says Robert Nissler of ETH Zurich. In Egyptian blue and other blue or violet mineral pigments, copper ions perform this task. "They are the luminous centers in the crystal lattice," says Nissler.

The nanoscientist, along with colleagues from the University of Zurich, Empa, and Germany, has developed a sophisticated manufacturing process to create fundamentally new color pigments. The new blue has the potential to play an important role in stroke research and in monitoring the durability of replacement joints. The researchers report. in the specialist magazine «Advanced Materials » .

However, it was only marginally about the color blue. Their interest focuses on another aspect that archaeologists first noticed almost 30 years ago: Egyptian blue not only absorbs red light, but also re-radiates a significant portion of the captured energy in the invisible infrared range. The chemically closely related "Han Blue" from ancient China also has this property. This means that the ancient dyes glow.

This makes them easy to detect with instruments that make infrared radiation visible – without damaging the precious ancient finds. In their article, the researchers describe them as "optically active" molecules. "The infrared radiation is what makes these dyes so special and unique," says Nissler.

At the interface between light and matter, even the slightest variations in the environment of the copper ions affect the blue hue and the infrared glow. For example, the slightly lighter Egyptian Blue contains additional calcium ions, while the slightly more violet Han Blue contains barium ions that are slightly larger, thus slightly distorting the crystal lattice.

To produce the new color pigments, the experts used to a process that has only been around for twenty years. Although the method is highly modern, it seems like a true alchemical play of colors: In the so-called flame synthesis, green juices are transformed into blue or violet powder.

In a first step, the researchers dissolve alkaline earth metals such as calcium, barium, or strontium in organic solvents. They then inject these green liquids into a flame heated to several thousand degrees. In this heat, the solvents evaporate instantly.

What remains are the alkaline earth metals, which clump together on the filter over the flame to form tiny, blue-green grains. These clumps, approximately 30 nanometers in size, are then placed in an oven heated to around 1,000 degrees Celsius for ten minutes, where the minerals ultimately form—and the crystalline, blue or violet pigments emerge.

Unlike conventional production processes, flame synthesis allows the various alkaline earth metals in the pigment to be combined almost arbitrarily, explains Nissler. The researchers have produced dozens of new dyes this way. His favorite shade—a "particularly intense dark blue"—is created by combining equal parts of barium and strontium, says the nanoscientist.

In their research, the experts further tweaked the manufacturing process – and discovered, for example, that the oven temperature and the time spent in the oven determine the color of the resulting pigment crystals. The frequencies at which the pigments glow in the infrared range also depend on these parameters.

The brighter and longer the wavelength of the infrared light, the better. This is because, starting at a wavelength of around 1000 nanometers, a so-called transparency window opens: Radiation of this wavelength penetrates biological tissue without being absorbed by red hemoglobin or water. This allows it to be intercepted at the other end of the body and used for imaging.

In fact, the experts discovered a mixture of the three metals with which they produce "ultra-bright" pigments that glow at exactly the desired wavelength. "Using the new method, we have produced a material that glows ten times brighter than currently available pigments," says Nissler.

The researchers are convinced that the new material could be used as a type of contrast agent. They dissolved the pigments in water and injected them into the blood of mice. The dye glowed through the intact skulls of the mice. Using infrared cameras, the researchers were able to observe not only exactly where the blood vessels in the brain run, but also how quickly the blood flows through these vessels.

Such information is important for stroke research, explains Nissler. It could provide information about how quickly blood flow returns to normal once a clot is dissolved, thus removing the blocking plug. Nissler also has another possible application in mind. The particles could, for example, be added to ceramic hip or knee prostheses. If small particles become detached from the prosthesis due to wear, surgeons could easily detect and remove them using infrared cameras.

These options are still a long way off. For now, the experts—with a group at Empa working on nanotoxicity—want to test how toxic the bright blue pigments are. It will likely be some time before further development of Egyptian blue can help people in health distress. But after more than 5,000 years, this wait isn't particularly important.

An article from the « NZZ am Sonntag »