gold nugget
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Not gold enough?
New form of gold is much golder than normal gold

All that glitters is not gold - but sometimes it really, really is. Researchers have made a new kind of gold crystal that is even more gold-like than regular gold.

Gold is a precious metal, which means that as well as being attractively shiny it is almost entirely chemically inert. Unlike other metals, it does not rust when exposed to air, and retains its lustre indefinitely.

It's said this property is why wedding rings are traditionally made of gold: it represents an eternity of love. Silver is another such "noble metal", but even silver reacts slowly with oxygen in the air, so requires occasional polishing.

In 2015 Giridhar Kulkarni of the Centre for Nano and Soft Matter Sciences in Bangalore, India and his colleagues described a new form of gold: microcrystals measuring between 2 and 17 micrometres. They made them by heating gold chloride to 220 °C for 30 minutes in the presence of a second chemical called tetraoctylammonium bromide. They look like angular, knobbly sausages.

Microcrystal test

Now, Kulkarni has now found that the microcrystals are even less reactive than ordinary gold, by exposing them to two substances that normally dissolve the shiny stuff.

The first was mercury, a metal that is liquid at room temperature. The team dunked the microcrystals into pots of mercury and did the same to similar-sized chunks of ordinary gold. While the normal gold disappeared within 6 minutes, the microcrystals were unchanged after 27 hours.

Next, they used aqua regia, a mixture of nitric acid and hydrochloric acid once used by alchemists to test for fake gold - hence the phrase "acid test". Ordinary gold dissolved within 70 minutes, but the microcrystals survived - although they did succumb when the aqua regia was made more concentrated.

The team calls the microcrystals "nobler than the noblest". "It appears that these are the least reactive substances produced," says Kulkarni.

The microcrystals are so unreactive because of the way gold atoms in them are arranged. The atoms in ordinary gold are arranged in a cubic pattern, but in the microcrystals the cubes have been stretched or distorted. This means they have unusually-shaped surfaces, which the other chemicals can't latch onto.

This new gold might ultimately be useful for making other novel substances. In a bizarre twist, in 2017 the team showed that "they exhibit catalytic activity, which is unknown for ordinary bulk gold," says Kulkarni. That means despite not reacting themselves, they can drive other chemical reactions, which could help make new discoveries.

Journal reference: Angewandte Chemie , DOI: 10.1002/anie.201804541