Gold
nanoparticles created by the Rice University lab of Eugene Zubarev take
on the shape of starfruit in a chemical bath with silver nitrate,
ascorbic acid and gold chloride.
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.
This in turn slowed down the deposition of gold on those flat
surfaces and allowed the nanorods to gather more gold at the pentagon's
points, where they grew into the ridges that gave the rods their
star-like cross-section. "Silver bromide is likely to block flat
surfaces more efficiently than sharp edges between them," he said.
The researchers tried replacing silver with other metal ions such as
copper, mercury, iron and nickel. All produced relatively smooth
nanorods. "Unlike silver, none of these four metals form insoluble
bromides, and that may explain why the amplification is highly uniform
and leads to particles with smooth surfaces," he said.
The researchers also grew longer nanowires that, along with their
optical advantages, may have unique electronic properties. Ongoing
experiments with Stephan Link, an assistant professor of chemistry and
chemical and biomolecular engineering, will help characterize the
starfruit nanowires' ability to transmit a plasmonic signal. That could
be useful for waveguides and other optoelectronic devices.
But the primary area of interest in Zubarev's lab is biological. "If
we can modify the surface roughness such that biological molecules of
interest will adsorb selectively on the surface of our rugged nanorods,
then we can start looking at very low concentrations of DNA or cancer
biomarkers. There are many cancers where the diagnostics depend on the
lowest concentration of the biomarker that can be detected."
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