Gold nanoparticles created by the Rice University laboratory of Eugene Zubarev take on the shape of starfruit in a chemical bath with silver nitrate, ascorbic acid, and gold chloride. Image: Zubarev Laboratory/Rice University |
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 laboratory at Rice’s
BioScience Research Collaborative, could nourish applications that rely on surface-enhanced
Raman spectroscopy (SERS).
The research appeared online in 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 laboratory in
2008 and added them to a mixture of silver nitrate, ascorbic acid, and gold
chloride.
Over 24 hrs, the particles plumped up to 550 nm long and 55
nm 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 laboratory 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.”
