Gold foam a thousand times lighter than conventional gold alloys

The new gold form is almost impossible to differentiate from conventional gold with the naked eye - the aerogel even has a metallic shine. But in contrast to its conventional form, it is soft and malleable by hand. It consists of 98 parts air and only two parts of solid material. Of this solid material, more than four-fifths are gold and less than one-fifth is milk protein fibrils.

The scientists created the porous material by first heating milk proteins to produce nanometre-fine protein fibres, called amyloid fibrils, which they then placed in a solution of gold salt. The protein fibres interlaced themselves into a basic structure along which the gold simultaneously crystallised into small particles. This resulted in a gel-like gold fibre network.

"One of the big challenges was how to dry this fine network without destroying it," explains Gustav Nyström, a post-doctoral researcher in Professor Raffaele Mezzenga's group at ETH Zurich, and first author of an article describing the work in the journal, Advanced Materials. As air drying could damage the fine gold structure, the scientists opted for a gentle and laborious drying process using carbon dioxide. They did so in an interdisciplinary effort assisted by researchers in the group of Marco Mazzotti, a professor of process engineering.

The method chosen, in which the gold particles are crystallised directly during manufacture of the aerogel protein structure (and not, for example, added to an existing scaffold) is new. The method's biggest advantage is that it makes it easy to obtain a homogeneous gold aerogel, perfectly mimicking gold alloys.

"The optical properties of gold depend strongly on the size and shape of the gold particles," says Nyström. "Therefore we can even change the colour of the material. When we change the reaction conditions in order that the gold doesn't crystallise into micro-particles but rather smaller nanoparticles, it results in a dark-red gold." By this means, the scientists can influence not only the colour, but also other optical properties such as absorption and reflection.

The new material could be used in many of the applications where gold is currently being used, says Mezzenga. The substance's properties, including its lighter weight, smaller material requirement and porous structure, have applications in watches and jewellery, chemical catalysis or in applications where light is absorbed or reflected.

The scientists have also their new material 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."