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Alchemists at it again. Is there any terra firma in science?

Posted on July 19th, 2016 by in New Materials & Applications

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Alchemists have striven for centuries to convert base metals to gold, shrouded in mystery and secrecy.  One of the last famous alchemists was Sir Isaac Newton, according to a publication by The Royal Society in the United Kingdom.  Belief in alchemy continued into the 20th century strongly enough to raise the ire of the scientific community.  Berkeley Daily Gazette reported, on January 16, 1936, the formation of an alchemy society headed by a Professor J. R. Partington from the University of London.  The Society set out “to end all talk of alchemy and prove to the world that man can’t make gold out of base metals”.  That this debunking did not take place earlier in scientific development is certain to surprise most people.

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.  Though a variety of nanogold particles and molecules have been made in the lab, very few have had their secret atomic arrangement revealed.  But recently, new technologies are bringing these minuscule structures into focus.  Researchers describe two unique structures or polymorphs for the iconic gold nanocluster Au144(SR)60, better known as Gold-144, including a version never seen before.

Gold-144, a molecule-sized nanogold cluster was first isolated in 1995 by Professor Robert Whetten, at Georgia Institute of Technology, now a professor of chemical physics at the University of Texas, San Antonio.  He was the first unlikely alchemist who led the team that first isolated Gold-144.  Its structure was theoretically predicted in 2009, though never confirmed.

Simon Billinge is a Professor of Applied Physics and Applied Mathematics at Columbia University.  He might be considered another unlikely alchemist proving the existence of a new structure of gold.  Billinge asked chemists at the Colorado State University to make tiny samples of Gold-144,

“This took four years to unravel,” said Simon Billinge, a physics professor at Columbia Engineering and a member of the Data Science Institute. “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.”

High-energy x-ray beams are fired at a sample of nanoparticles. Advanced data analytics are 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. Samples made at Colorado State University were tested using the atomic Pair Distribution Function (PDF) analysis, for interpreting this scattering data, pioneered by Billinge and his lab.

Hoping the test would confirm Gold-144’s structure they analyzed the clusters at the European Synchrotron Radiation Source in Grenoble, and used the PDF method to infer their structure. To their surprise, they found an angular core, and not the sphere-like icosahedral core predicted (Figure 1).  When they made a new sample and tried the experiment again, this time using synchrotrons at Brookhaven and Argonne national laboratories, the structure came back spherical.

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Figure 1. Two Polymorphs of Gold – (a) Spherical-like icosahedral core and (b) angular core
Courtesy: Professor Kirsten Ørnsbjerg Jensen, University of Copenhagen

“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.

The study revealed structural polymorphism in these archetypal nanoclusters. In addition to confirming the theoretically predicted icosahedral-cored cluster, also samples were found with a truncated decahedral core structure, with some samples exhibiting a coexistence of both cluster structures. Although the clusters were monodisperse in size, structural diversity is apparent. The researchers are still unsure if Gold-144 can switch from one version to the other or, what exactly, differentiates the two forms.

Single nanoparticles of gold are being used as building blocks for sensing devices, and in biomedical, material, optical, and industrial applications. The good news is gold nanostructures are expected to continue finding applications for purifying water, imaging and killing tumors, and making solar panels more efficient, among other applications.

New analytic approaches have brought two unique atomic structures of gold into limelight. The discovery of gold nanocluster “double” hints at other shape changing particles yet to be discovered.

Einstein upended our perception of motion thus came the Theory of Relativity.  Now we are told there are new types of gold!  You would have thought gold is just gold and that is the end of the story. Not quite so….

Sources

  1. Kim Martineau, Discovery of Gold Nanocluster “Double” Hints at Other Shape Changing Particles, Columbia University, New York, 2016.
  2. Kirsten M.Ø. Jensen, Pavol Juhas, Marcus A. Tofanelli, Christine L. Heinecke, Gavin Vaughan, Christopher J. Ackerson & Simon J. L. Billinge, Polymorphism in magic-sized Au144(SR)60 clusters, Nature Communications, June 14, 2016.
  3. Whetten, J. T. Khoury, M. M. Alvarez, S. Murthy, I. Vezmar, Z. L. Wang, P. W. Stephens, C. L. Cleveland, W. D. Luedtke, and U. Landman, Nanocrystal gold molecules, Advanced Materials, May 1996.

All opinions shared in this post are the author’s own.

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