New gold foam almost as light as air

A nugget of 20 carat gold so light that it doesn’t sink in a cappuccino may sound unbelievable, but this is what researchers from ETH Zurich have now accomplished. Led by Raffaele Mezzenga, professor of food and soft materials, they have produced a new kind of foam out of gold.

"The so-called aerogel is a thousand times lighter than conventional gold alloys," says Mezzenga. "It is lighter than water and almost as light as air."

With the naked eye, the new gold foam can hardly be distinguished from conventional gold – the aerogel even has a metallic shine. In contrast to its conventional form, however, the gold foam is soft and malleable by hand. It comprises 98% air and only 2% solid material. This solid material is more than four-fifths gold, corresponding to around 20 carat gold, with milk protein making up the remainder.

The scientists created the porous material by first heating milk proteins to produce nanometer-fine protein fibers, so-called amyloid fibrils, which they then placed in a solution of gold salt. The protein fibers interlaced themselves into a basic structure onto which the gold simultaneously crystallized as small particles, producing a gel-like gold fiber network. The great advantage with this fabrication method is that it produces a homogeneous gold aerogel, perfectly mimicking gold alloys.

"One of the big challenges was how to dry this fine network without destroying it," explains Gustav Nyström, postdoc in Mezzenga's group and first author of the corresponding study in Advanced Materials. As air drying could damage the fine gold structure, the scientists opted for a gentle and laborious drying process using carbon dioxide.

The fabrication method also offers the researchers numerous possibilities for deliberately influencing the properties of the gold foam. "The optical properties of gold depend strongly on the size and shape of the gold particles," says Nyström. "Therefore we can even change the color of the material. When we change the reaction conditions in order that the gold doesn't crystallize into microparticles but rather smaller nanoparticles, it results in a dark-red gold." By the same means, the scientists can also influence other optical properties such as absorption and reflection.

The new material could find use in many of the applications where gold is currently used, says Mezzenga, but its lighter weight, smaller material requirements and porous structure would confer additional advantages. Applications in watches and jewellery are just one possibility. Another application demonstrated by the scientists is chemical catalysis: since the highly porous material has a huge surface area, it can efficiently catalyst chemical reactions that depend on the presence of gold. The material could also be used in applications where light is absorbed or reflected.

In addition, the scientists have shown how the gold foam could be used to manufacture pressure sensors. "At normal atmospheric pressure the individual gold particles in the material do not touch, and the gold aerogel does not conduct electricity," explains Mezzenga. "But when the pressure is increased, the material gets compressed and the particles begin to touch, making the material conductive."

This story is adapted from material from ETH Zurich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.