Sreenath Bolisetty

Aerogels seem to enchant even scientists who sometimes call it ‘solid smoke’ (my Aug. 20, 2012 posting). This latest aerogel is made of gold according to a Nov. 25, 2015 news item on Nanowerk,

A nugget of real 20 carats gold, so light that it does not sink in a cappuccino, floating instead on the milk foam – what sounds unbelievable has actually been accomplished by researchers from ETH Zurich. Scientists led by Raffaele Mezzenga, Professor of Food and Soft Materials, have produced a new kind of foam out of gold, a three-dimensional mesh of gold that consists mostly of pores. It is the lightest gold nugget ever created. “The so-called aerogel is a thousand times lighter than conventional gold alloys. It is lighter than water and almost as light as air,” says Mezzenga.

A Nov. 25, 2015 ETH Zurich press release (also on EurekAlert), which originated the news item, provides more information about the ‘gold smoke’,

The new gold form can hardly be differentiated 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. This corresponds to around 20 carat gold.

Here’s what it looks like,

Caption: Even when it seems unbelievable: these are genuine photographs, in which nothing has been faked. E.g. the 20 carats gold foam is lighter than milk foam.
Credit: Gustav Nyström and Raffaele Mezzenga / (copyright) ETH Zurich

The press release provides more technical details,

The scientists created the porous material by first heating milk proteins to produce nanometre-fine protein fibres, so-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, postdoc in Mezzenga’s group and first author of the corresponding study 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, Professor of Process Engineering.

Dark-red gold

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 manufacturing technique also offers scientists numerous possibilities to deliberately influence the properties of gold in a simple manner. ” 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 microparticles 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 their advantages. Applications in watches and jewellery are only one possibility. Another application demonstrated by the scientists is chemical catalysis: since the highly porous material has a huge surface, chemical reactions that depend on the presence of gold can be run in a very efficient manner. The material could also be used in applications where light is absorbed or reflected. Finally, the scientists have also shown how it becomes possible to manufacture pressure sensors with it. “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.”

Here’s a link to and a citation for the paper,

Amyloid Templated Gold Aerogels by Gustav Nyström, Maria P. Fernandez-Ronco, Sreenath Bolisetty, Marco Mazzotti, Raffaele Mezzenaga. Advanced Materials DOI: 10.1002/adma.201503465 First published: 23 November 2015

This paper is behind a paywall.

The researchers didn’t start out by trying to develop a ‘gold-milk hybrid’ material; that came later, according to the June 18, 2013 news item on Nanowerk (Note: Links have been removed),

Raffaele Mezzenga, professor of food and soft materials, came up with the idea of “gold paper” a year ago. At the time, his group was working on an unusual hybrid material, a wafer-thin, paper-like mixture of graphene and protein fibres (see ETH Life report). The recipe is universally applicable and relatively simple: you mix fibroid objects with plate-like entities in a watery solution and filter the mixture with the aid of vacuum. The plates and fibres congregate and remain on the filter as a thin film.

As a result, Mezzenga set two of his team members, Chaoxu Li and Sreenath Bolisetty, the task of producing a kind of gold leaf out of protein fibres and gold plates (“Hybrid Nanocomposites of Gold Single-Crystal Platelets and Amyloid Fibrils with Tunable Fluorescence, Conductivity, and Sensing Properties”). First of all, the researchers had to make the fibres by stretching them naturally from milk globular proteins, the so-called beta-lactoglobulin, with the aid of heat and acid. Like all proteins, milk proteins are also composed of a chain of numerous individual amino acids that form complex compact structures under native conditions. Heat or chemicals break open the compact configuration, causing the chains to unravel.

The June 18, 2013 ETH Life news article [Eidgenössische Technische Hochschule Zürich] by Peter Rüegg. which originated the news item, describes what happens after the milk protein’s amino acid chains unravel,

Several of these milk protein fibres then organise themselves into thicker, helical fibres. The researchers added gold in the form of a salt to the acidic solution of the fibres. The protein fibres allow the gold to reduce into small plates with a diameter of one micrometre and a thickness of 100 nanometres. The gold grows as a so-called monocrystal and the gold ions form a crystal lattice completely devoid of any defects.

Gold plates and fibres then accumulate in layers. The thin film that remains after filtration is formed in much the same way as paper from cellulose. The novel hybrid material is very stable, but remarkably changes its physical and optical properties when it comes into contact with water.

Here’s an image of the ‘golden milk’,

The hybrid film on a filter (r.a.) and on glass (ETH logo). REM reveals the micro (upper left) and nano (bottom left) structure of this particular material. (Images: from Li, C., Adv. Mater. 2013) [downloaded from https://www.ethlife.ethz.ch/archive_articles/130618_goldfolie_per/index_EN]

The researchers have some ideas for how this material could be commercialized,

Mezzenga sees an initial application in gastronomy. In culinary applications, pure gold has an approved E-number code (E-175) allowing his use as additive in foods and indeed gold leafs have long been used to decorate desserts, drinks and other specially prepared foods. Because the new hybrid material is made of gold and dietary proteins, the researchers do not anticipate any hurdles in using it for culinary purposes, thereby considerably reducing the cost of using pure gold.

Even more interesting, however, are the unusual optical properties of the “gold paper”, especially as the gold is available as monocrystals. These properties change according to the pH value, for instance, which means the hybrid material could be used for acidity measurements in sensors. The “paper” is also conductive to varying degrees depending on its composition and lends itself to applications in microelectronics.

…

Because, at face value, the gold paper is barely distinguishable from gold leaf – it has the lustre and colour of gold – it may also be interesting for the clock and jewellery industries, which could reduce their demand for the precious metal with protein gold leaf. In order to imitate gold leaf, the hybrid material only needs a ratio of one third weight percentage of gold. The new material would thus be just the ticket for gold-plating the numbers on the faces of wristwatches, for instance. “When you consider how much pure gold costs, this new material makes a massive difference,” says the ETH-Zurich professor. On the one hand, it could help to reduce the global demand for gold and thus relieve the pressure on natural resources; on the other hand, the hybrid material broadens the fields of application for the metal.

The researchers have filed a patent for their invention. Mezzenga hopes that industry will show an interest in the new material. “Gold is a delicate subject. Nonetheless, the potential for applications is vast.”

Here’s a link to and reference for the research paper,

Li C, Bolisetty S and Mezzenga R (2013). Hybrid Nanocomposites of Gold Single-Crystal Platelets and Amyloid Fibrils with Tunable Fluorescence, Conductivity, and Sensing Properties. Adv. Mater. doi: 10.1002/adma.201300904

This paper is behind a paywall.

In the category of idle thoughts, perhaps I should have called this ‘milky gold’, eh?

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Tag Archives: Sreenath Bolisetty

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