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Gold nanoparticles revealed

Crystal study unveils chemistry of tiny gold tracers.

Go for gold: tiny particles of gold have been imaged for the first time. Credit: Punchstock

Nanoparticles are set to lose their air of mystery: the structure of a widely used class of gold nanoparticle has been unambiguously determined, and the structures of other nanoparticles could soon follow. The structure reveals that the molecules grown on the surface of the gold nanoparticles don?t behave quite as they were thought to.

Gold metallic nanoparticles are commonly used in the lab as a tracer, to detect the presence of specific proteins or DNA in a sample, for example. Such nanoparticles have proved to be adaptable, and so very useful.

Surprisingly, no one had a clear idea what these particles looked like ? perhaps they were colloidal, with a lumpy, messy range of shapes and sizes; or maybe they were discrete molecules of uniform size and structure. "Exactly what the structure was, remained kind of murky," says nanoparticle expert Ralph Nuzzo from the University of Illinois at Urbana-Champaign, who was not involved with this work.

Researchers have now succeeded in making a crystal structure of gold nanoparticles, and imaging this crystal to see inside the particles themselves and unpick their structure. The result settles some mysteries, and should also make the nanoparticles even easier to adapt and use.

Stuck together

Gold is used for nanoparticle applications because it is unreactive and isn?t sensitive to air or light. But gold does like to form bonds with itself. So to make sure the particles don?t clump together, their surfaces have to be covered with a layer of protective molecules. Sulphur is one of a few elements that gold happily bonds with, so sulphur-containing groups are often used for this protective coating.

These sulphur groups can also be functionalized ? given extra bits such as binding sites or fluorescent markers, for example, that can be picked up by a microscope. This turns the nanoparticles into tracers.

Roger Kornberg at Stanford University, California, and his team investigated the chemistry of such particles by creating a crystal of them ? a huge achievement in itself ? that could be investigated using X-ray crystallography. The particles they investigated consist of 102 gold atoms arranged in a ball and covered on the surface by a one-molecule-thick layer of 44 sulphur-containing molecules.

A clear, three-dimensional picture of the structure of this monster molecule, reported in Science, reveals that atoms in the core of this nanoparticle are arranged similarly to those in bulk gold, as expected. But this 'grand core' is then surrounded by two caps, each with 15 slightly twisted gold atoms.

And the sulphur groups, rather than binding directly to the gold surface as predicted in some models, forms an alliance with the outermost shell of gold, which then interacts weakly with the grand core. In addition to that, the nanoparticle is chiral ? it has a handedness ? introduced by the arrangement of the gold atoms with the sulphur groups.

Off the shelf

"Chemists and nanoscientists have worked for years with metal-rich gold cluster thiolates, without any unambiguous evidence of their structure," says Robert Whetten of the Georgia Institute of Technology in Atlanta. This new structure changes everything, he says. "Chemists treat something very differently when it is a substance that has a determined composition and structure. It can be handled, and bought and sold just like any other speciality chemical off the shelf."

This knowledge might eventually help to remove the suspicion that surrounds nanoparticles and their toxicity, researchers add. Knowing the structure of a molecule means researchers can better understand how it reacts.

"It means we have the potential to understand nanoparticles, to understand the chemistry on their surfaces," says Simon Billinge, part of the National Science Foundation-funded Nanoscale Interdisiplinary Research Team, based at Michigan State University, East Lansing.

"This is precisely the kind of discovery that finds its way in to textbooks," says Whetten.

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Sanderson, K. Gold nanoparticles revealed. Nature (2007). https://doi.org/10.1038/news.2007.178

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