Nanostarfruit rods enhance applications SERS

Nanostarfruit rods enhance applications of SERS

6:40 AM, 5th April 2012
Nanostarfruit rods enhance applications of SERS
Nanostarfruits begin as gold nanowires with pentagonal cross-sections. Rice chemist Eugene Zubarev believes silver ions and bromide combine to form an insoluble salt that retards particle growth along the pentagons’ flat surfaces.

HOUSTON, US: Eugene Zubarev, Associate Professor, Rice Univeristy and Leonid Vigderman, Graduate Student, Rice Univeristy synthesized starfruit-shaped gold nanorods, which 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.

“There’s a great deal of interest in sensing applications. 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. 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. The potential to further boost that stronger signal by a factor of 25 is significant,” said Zubarev.

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 nanometre long and 55 nanometre 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.

“For a long time, our group has been interested in size amplification of particles. 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. We believe a thin film of silver bromide forms on the side faces of rods and partially blocks them,” said Zubarev. When Zubarev and Vigderman added a common surfactant, cetyltrimethylammonium bromide (CTAB), to the mix, the bromide combined with the silver ions to produce an insoluble salt.

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. 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,” explained Zubarev.

The researchers also grew longer nanowires that, along with their optical advantages, may have unique electronic properties. “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,” said Zubarev.

© Rice University News

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