They look like fruit, and indeed the nanoscale stars of new research at Rice University have tasty implications for medical imaging and chemical sensing.
Starfruit-shaped gold nanorods synthesized by chemist Eugene Zubarev and Leonid Vigderman, a graduate student in his lab at Rice's BioScience Research Collaborative, could nourish applications that rely on surface-enhanced Raman spectroscopy (SERS).
The research appeared online this month in the American Chemical Society journal Langmuir.
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 applications," said Zubarev, an associate professor of chemistry. "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 concentration."
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 difference can be millions of times," he said. The potential 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 550 nanometers long and 55 nanometers wide, many with pointy ends. The particles take on ridges along their lengths; photographed 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 speculate. "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 nanoparticles, 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.