The puzzle of the changing nanogold

14-06-2016

Sometimes scientists become detectives in their research, like in this story. Take an evasive nanogold cluster’s structure, a newly-developed investigative approach and three synchrotrons in different continents and you have a plot. And four years later, an outcome, even if an unexpected and puzzling one.

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Gold has been used in coins and jewelry for thousands of years for its durability, but shrink it to a size 10,000 times smaller than a human hair, and it becomes wildly unstable and unpredictable. At the nanoscale, gold likes to split apart other particles and molecules, making it a useful material for purifying water, imaging and killing tumors, and making solar panels more efficient, among other applications.

Though a variety of nanogold particles and molecules have been made in the lab, very few have had their secret atomic arrangement revealed. 

A team from the Columbia University, the Colorado State University and the European Synchrotron (ESRF), Argonne and Brookhaven National Laboratories have undergone some investigative work through the last four years to discover the structure of nanogold particles.

The leader of the team, Simon Billinge, who spent a year at the ESRF, used X-ray diffraction at ID11 beamline to unveil the structure of Gold-144.This molecule-sized nanogold cluster was first isolated in 1995, its structure had been theoretically predicted in 2009 but never confirmed. Gold-144 has found numerous applications, including in tissue-imaging.

The team combined the experiments with the method of atomic Pair Distribution Function (PDF) to interpret x-ray data, hoping they would confirm Gold-144’s structure. At the ESRF, Billinge worked with Gavin Vaughan, beamline scientist on ID11 at the time: "Although the PDF method has been around for more than 80 years, recent advances, both technical and analytical, mean that more and more quantitative information can be retrieved from data analyzed in this way, leading to the current explosion in the field. Due to the need for high energy x-rays and optimized detectors, the ESRF is one of the world's leading sites for the acquisition of PDF data". To their surprise, they found an angular core, and not the sphere-like icosahedral core predicted.  “The ESRF played a key role in the whole story, that eventually took us more than 4 years (and 3 synchrotrons) to figure out completely”, explains Billinge. When the team made a new sample and tried the experiment again, this time using synchrotrons at Brookhaven and Argonne national laboratories, the structure came back spherical.

polymorphs cropped.jpg

Setting out to confirm the predicted structure of Gold-144, researchers discovered an entirely unexpected atomic arrangement (right).  The two structures, described in detail for the first time, are chemically identical but uniquely shaped, suggesting they also behave differently. (Courtesy of Kirsten Ørnsbjerg Jensen)

 

“We didn’t understand what was going on, but digging deeper, we realized we had a polymorph,” said study coauthor Kirsten Jensen, formerly a postdoctoral researcher at Columbia, now a chemistry professor at the University of Copenhagen.

Further experiments confirmed the cluster had two versions, sometimes found together, each with a unique structure indicating they behave differently. The researchers are still unsure if Gold-144 can switch from one version to the other or, what exactly, differentiates the two forms.

To make their discovery, the researchers solved what physicists call the nanostructure inverse problem. How can the structure of a tiny nanoparticle in a sample be inferred from an x-ray signal that has been averaged over millions of particles, each with different orientations?

“The signal is noisy and highly degraded,” said Billinge. “It’s the equivalent of trying to recognize if the bird in the tree is a robin or a cardinal, but the image in your binoculars is too blurry and distorted to tell.”

“Our results demonstrate the power of PDF analysis to reveal the structure of very tiny particles,” added study coauthor Christopher Ackerson, a chemistry professor at Colorado State. "I’ve been trying, off and on, for more than 10 years to get the single-crystal x-ray structure of Gold-144. The presence of polymorphs helps to explain why this molecule has been so resistant to traditional methods."

The PDF approach is one of several methods being developed to bring nanoparticle structure into focus. Now that it has proven itself, it could help speed up the work of describing other nanostructures.

“This took four years to unravel,” said Billinge. “We weren’t expecting the clusters to take on more than one atomic arrangement. But this discovery gives us more handles to turn when trying to design clusters with new and useful properties.”

Indeed, the eventual goal is to design nanoparticles by their desired properties, rather than through trial and error, by understanding how form and function relate. Databases of known and predicted structures could make it possible to design new materials with a few clicks of a mouse. The detective work continues.

 

Reference
Polymorphism in magic-sized Au144(SR)60 clusters, K.M.Ø. Jensen, P. Juhas, M.A. Tofanelli, C.L. Heinecke, G. Vaughan, C.J. Ackerson & S.J.L. Billinge, Nature Communications 7,  11859 (2016); doi: 10.1038/ncomms11859.