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nanotimes News in Brief

12-03 :: March/April 2012

Nanorods and Nanowires // Nanostarfruits are pure Gold for Research

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tarfruit-shaped gold nanorods synthesized by chemist Eugene Zubarev and Leonid Vigder-

man, a graduate student in his lab at Rice’s Bio- Science Research Collaborative, could nourish applications that rely on surface-enhanced Raman spectroscopy (SERS).

The researchers found their particles returned signals 25 times stronger than similar nanorods with smooth surfaces. That may ultimately make it possible to detect very small amounts of such organic molecules as DNA and biomarkers, found in bodily fluids, for particular diseases.

“There’s a great deal of interest in sensing applica- tions,” said Zubarev, an associate professor of che- mistry. “SERS takes advantage of the ability of gold to enhance electromagnetic fields locally. Fields will concentrate at specific defects, like the sharp edges of our nanostarfruits, and that could help detect the presence of organic molecules at very low concentra- tion.”

SERS can detect organic molecules by themselves, but the presence of a gold surface greatly enhances the effect, Zubarev said. “If we take the spectrum of organic molecules in solution and compare it to when they are adsorbed on a gold particle, the diffe- rence can be millions of times,” he said. The poten- tial to further boost that stronger signal by a factor of 25 is significant, he said.

Zubarev and Vigderman grew batches of the star- shaped rods in a chemical bath. They started with seed particles of highly purified gold nanorods with pentagonal cross-sections developed by Zubarev’s lab in 2008 and added them to a mixture of silver nitrate, ascorbic acid and gold chloride.

Over 24 hours, the particles plumped up to 550nm long and 55nm wide, many with pointy ends. The particles take on ridges along their lengths; photo- graphed tip-down with an electron microscope, they look like stacks of star-shaped pillows.

Why the pentagons turn into stars is still a bit of a mystery, Zubarev said, but he was willing to specu- late. “For a long time, our group has been interested in size amplification of particles,” he said. “Just add gold chloride and a reducing agent to gold nanopar- ticles, and they become large enough to be seen with an optical microscope. But in the presence of silver nitrate and bromide ions, things happen differently.”

When Zubarev and Vigderman added a common surfactant, cetyltrimethylammonium bromide (aka CTAB), to the mix, the bromide combined with the silver ions to produce an insoluble salt. “We believe a thin film of silver bromide forms on the side faces of rods and partially blocks them,” Zubarev said.

This in turn slowed down the deposition of gold on those flat surfaces and allowed the nanorods to ga- ther more gold at the pentagon’s points, where they