Imagine a 20-carat gold nugget so light it could sit atop the foam on your cappuccino. That's what materials scientists in Switzerland have created. The nugget is actually an aerogel, a three-dimensional gold mesh consisting mostly of pores and is the lightest form of gold created.

The motivation behind the work, which took 18 months, was not only to produce a protein-based aerogel that could compare with cellulose or carbon nanotubes or even graphene aerogels, but also to exploit the possibility of using nanometer-fine protein fibers to synthesize in-situ gold nanoparticles and microcrystals. That’s according to Raffaele Mezzenga, a physicist and materials scientist at ETH Zurich and professor of food and soft materials.

Raffaele Mezzenga, a physicist and materials scientist at ETH ZurichRaffaele Mezzenga, a physicist and materials scientist at ETH Zurich“The result was a new form of gold aerogel with visual properties practically identical to real bulk gold, but with densities thousands of times lighter and unprecedented physical properties,” he says.

The gold aerogel even has a metallic shine. But in contrast to solid gold, the aerogel form is soft and malleable and lightweight. That’s because it consists of 98 parts air and two parts solid material, of which about four-fifths are gold and less than one-fifth is super-fine milk protein fibers, called amyloid fibrils. This corresponds to around 20 carat gold.

Developing this new gold proved to be about as time consuming as panning for it. First, the researchers heated milk proteins to produce the protein fibers from which they then produced a solution of gold salt. The protein fibers commingled into a basic structure as the gold crystallized into tiny particles, resulting in a gel-like gold fiber network.

Drying Challenge

A major challenge was figuring out how to dry this structure of protein fibers and gold particles without destroying it. Air drying wasn’t considered a sufficiently robust technique.

Instead, researchers opted for a method using carbon dioxide. “We used supercritical CO2 in a 100bar high-pressure environment, since the CO2 is then soluble with the alcohol used yet highly volatile when pressure is released,” Mezzenga says.

Using a process in which gold particles are crystallized directly during manufacture of the aerogel protein structure, rather than added to an existing scaffold, is new and comes with an advantage. It makes it easy to obtain a homogeneous gold aerogel that perfectly mimics gold alloys.

Manifold Applications

The new material could prove useful in many applications where gold is currently being used. One such application is in foods. “Because the material is formed by air, protein and gold, and gold is an approved food ingredient, one could think of several applications in the culinary industry; for example, chocolates, desserts and other dishes,” he says.

But applications extend well beyond the food industry. “An obvious application has to be in catalysis where the highly porous nature of this new type of gold perfectly suits the technological needs,” he says. “Another possibility is in optics, since gold nanoparticles and nanocrystals have plasmonic properties that bulk gold does not possess.”

As far as jewelry goes, however, this version isn’t likely to show up in watches, rings or gold necklaces anytime soon. But the possibilities may be intriguing. “This new material can be varied in content to fully meet jewelry standards of 18 carats or more while controlling the color of the gold and its density,” says Mezzenga. “Because of its light density, one could fill up huge volumes of solids with 18-carat gold; previously the cost of doing this would have been unaffordable.”