This image shows the two possible atomic arrangements of Gold-144: the predicted sphere-like icosahedral core (left) and the angular core (right). Image: Kirsten Ørnsbjerg Jensen.Although chemically the same, graphite and diamonds are as physically distinct as two substances can be: one opaque and soft, the other translucent and hard. What makes them unique is their differing arrangement of carbon atoms.
Polymorphs, or materials with the same composition but different structures, are common in bulk materials, and now a new study in Nature Communications confirms they exist in nanomaterials too. Researchers describe two unique structures for the iconic gold nanocluster Au144(SR)60, better known as Gold-144, including a version never seen before.
"We discovered that the same number of gold atoms can arrange to form two different versions of the nanosized cluster," said co-first author Pavol Juhas, a physicist at the US Department of Energy's (DOE) Brookhaven National Laboratory. Their discovery gives engineers a new material to explore, along with the possibility of finding other polymorphic nanoparticles.
"This took four years to unravel," said co-author Simon Billinge, a physicist at Brookhaven Lab, a physics professor at Columbia Engineering and a member of the Data Science Institute at Columbia University. "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."
Bulk gold is fairly unreactive, but at the nanoscale it likes to split apart other particles and molecules. This makes it a useful material for purifying water, imaging and killing tumors, and making solar panels more efficient, among other applications.
Though various nanogold particles and molecules have been made in the lab, very few have had their atomic arrangements revealed. But recently, new technologies are bringing these miniscule structures into focus.
One of these technologies involves firing high-energy x-ray beams at a sample of nanoparticles. Advanced data analytics are then used to interpret the x-ray scattering data and infer the sample's structure, which is key to understanding how strong, reactive or durable the particles might be.
Billinge and his lab have pioneered a novel method for interpreting this scattering data, which they term atomic pair distribution function (PDF) analysis. To test their PDF method, Billinge asked chemists at Colorado State University to make tiny samples of Gold-144, a molecule-sized nanogold cluster first isolated in 1995 that has since found numerous applications, including in tissue imaging. The structure of Gold-144 was theoretically predicted in 2009, but has never been confirmed experimentally.
Hoping to provide this confirmation, the team analyzed the clusters at the European Synchrotron Radiation Source in Grenoble, France, and used the PDF method to infer their structure. To their surprise, they found an angular core, rather than the sphere-like icosahedral core that had been predicted. When they made a new sample and tried the experiment again, this time using Brookhaven Lab's National Synchrotron Light Source and Argonne National Laboratory's Advanced Photon Source (both DOE Office of Science User Facilities), they came up with the predicted sphere-like core.
"We didn't understand what was going on, but digging deeper, we realized we had a polymorph," said co-first author Kirsten Jensen, formerly a postdoctoral researcher at Columbia and now a chemistry professor at the University of Copenhagen in Denmark.
Further experiments confirmed that the cluster came in two versions, 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.
"While we still have much to learn about how the gold nanoparticles take on different shapes and what those shapes specifically are, we now know that polymorphism can exist and thus should be considered when preparing nanoparticles from other materials," said Juhas, who collaborated with Jensen on modeling and analyzing the PDF data.
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," explained 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 co-author 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 rival methods being developed to bring nanoparticle structures into focus. Now that it has proven itself, it could help speed up the work of describing other nanostructures.
The eventual goal is to design nanoparticles in terms of 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. This study is a first step.
"We've had a structure model for this iconic gold molecule for years and then this study comes along and says the structure is basically right but it's got a doppelgänger," said Robert Whetten, a professor of chemical physics at the University of Texas, San Antonio, who led the team that first isolated Gold-144. "It seemed preposterous to have two distinct structures that underlie its ubiquity, but this is a beautiful paper that will persuade a lot of people."
This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.