Issue Date: October 22, 2007
A Golden Close-Up
BY SOLVING the structure of a gold-thiolate nanocrystal, scientists at Stanford University have gleaned new insights into how gold interacts with thiol ligands, as well as how gold clusters form (Science 2007, 318, 430).
Roger D. Kornberg and coworkers used X-ray crystallography to determine the structure of a nanoparticle composed of exactly 102 gold atoms surrounded by 44 p-mercaptobenzoic acid groups. Scientists have solved the X-ray structures of other metal clusters and nanoparticles in the past. Few of those materials, however, have as many potential applications as thiol-protected gold nanoparticles, which could find their way into molecular electronics, sensors, and biomedical diagnostics.
"There has thus been a real need to finally elucidate the chemical structure of these particles," says Mathias Brust, a nanochemistry professor at England's University of Liverpool. Thanks to this work, "we have now, for the first time, reliable structural information on such materials," he says.
The crystal structure reveals that 79 of the gold atoms form the core of the particle. The remaining 23 gold atoms make up an outer shell that interacts with the sulfur atoms on the thiolate ligands. The arrangement of these ligands renders the gold cluster chiral, suggesting it may be useful in enantioselective catalysis and separation science.
According to the conventional wisdom of how thiols behave on a gold surface, the thiolate ligands should simply sit above the smooth gold surface. "What we found contradicts this model," says Pablo D. Jadzinsky, a member of the Stanford team. Rather, the thiolates disrupt the smooth gold surface, nestling into the cluster's topmost layer to form covalent Au(I) thiolate moieties.
"This work is a great advance in our detailed understanding of the chemistry of gold and sulfur," says Rob Whetten, a metal cluster expert at Georgia Tech.
Scientists have been using thiol-coated nanocrystalline gold clusters since the mid-1990s, but until now no one had ever determined the absolute structure of these materials, Whetten explains. "This left room for doubt: Are they really distinct molecular species with definite compositions and structures? Or are they rather colloidal in nature, with continuously varying or ill-defined composition and structure?" he asks. "This report lays all that to rest, the implication being that they may all be well-defined."
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