Gold is a genuinely fascinating material. It’s incredibly rare, created only during the most powerful stellar explosions in the universe. It’s relatively easy to shape, conducts electricity and heat pretty well, and looks rather beautiful, so we assign a high economic value to it. It’s also quite dense, however, so it may strike you as unusual that researchers at ETH Zurich have revealed that they’ve created a gold aerogel so light it can float on the frothy surface of a well-made cappuccino. Their study is published in the journal Advanced Materials.
This new form of gold is one thousand times lighter than its conventional form, despite the fact that it looks identical to a lump of gold. It’s lighter than water and even has a density approaching that of just air. It’s so soft that it can be molded into shapes by hand. Even its value compared to conventional gold has barely slipped – a piece the size of a dime is still 20 carats, meaning that this aerogel is made of 83 percent pure gold. The rest of it is composed of nothing but porous spaces filled with air, and a little milk.
To make it, milk proteins were heated until they formed nanometer-sized fibers called amyloid fibrils, before they were placed into a solution containing positively-charged gold ions. As the solution was allowed to evaporate, the fibers arranged themselves automatically into a 3D lattice-like network, along which the gold began to crystallize – thus producing a golden grid full of spaces. As air drying could damage the gold, the team chose a slow, delicate drying process using carbon dioxide to remove the liquid solution.
Image credit: The gold is so light that it can rest on the top of a small flower. Nystrom and Mezzenga/ETH Zurich
Often dubbed “frozen smoke,” aerogels such as this are synthetically manufactured, highly porous solids. But instead of a liquid component, there’s gas mixed in; as a result, the material has an incredibly low density. Somewhat misleadingly, they aren’t actually gels; they’re merely made from gels, which are more “jelly-like” solid-liquid mixtures.
Most aerogels are made by causing chemical constituents to crystallize on a pre-existing grid network; in this instance, the grid formed in synchronization with the precious metal. Not only does this quickly produce a pure gold aerogel with no traceable contaminants, but it means that its optical properties can be customized.
“The optical properties of gold depend strongly on the size and shape of the gold particles,” said Gustav Nyström, postdoc at ETH Zurich and first author of the study, in a statement. “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.”
Apart from its use in creating easy-to-shape, valuable jewelry, the aerogel has multiple engineering applications as well. Gold is a chemical catalyst, meaning that it can speed up the reactions of huge industrial processes, including during the combustion of hydrocarbons. The huge surface area of the gold aerogel would improve its catalytic abilities to a perhaps unprecedented degree.
In addition to this, it can also be used as a pressure-sensitive conductor of electrical currents. Normally, the gold particles within the aerogel do not touch, so it acts as an insulator. If put under significant pressure, however, they connect, and a circuit is formed.