Nanostars Seen as Superior for SERS

Synthesized starfruit-shaped gold nanorods could strengthen applications that rely on surface-enhanced Raman spectroscopy (SERS).

Rice University scientists Eugene Zubarev, an associate professor of chemistry, and graduate student Leonid Vigerman developed the starfruit-shaped particles using a chemical bath. In 2008, seed particles containing pure gold nanorods with pentagonal cross sections were developed in Zubarev’s lab. The nanorods were mixed in a chemical solution of silver nitrate, ascorbic acid and gold chloride. Over a period of 24 hours, the particles plumped in size to 55 nm in width and 550 nm in length. When observed under an electron microscope, they looked like star-shaped pillow stacks.

Seen from the side, the nanostarfruit produced at Rice University take on the appearance of carambola, or starfruit. The particles are about 55 nm wide and 550 nm long. (Images: Zubarev Lab/Rice University)

Why the pentagons turned into stars was unclear, but Zubarev 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 the scientists added a common surfactant, cetyltrimethylammonium bromide to the mixture, silver ions in the silver nitrate combined with bromide and resulted in the formation of an insoluble salt: silver bromide.

“We believe a thin film of silver bromide forms on the side faces of rods and partially blocks them,” Zubarev said. This slows down the process of gold deposition and enables the nanorods to collect more gold at the pentagon’s point, where they grew into ridges that gave the rods their starlike cross section.

When metal ions such as mercury, nickel, copper and iron were used instead of silver, they formed smooth nanorods. The metals did not form insoluble bromide salts and may explain the formation of smooth surfaces, Zubarev said.

The researchers observed that their starfruit-shaped particles gave back 25 times stronger signals when compared with smooth-surfaced nanorods. This characteristic could make it possible to detect minute amounts of organic molecules, including biomarkers and DNA, for particular diseases.

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 pentagon’s flat surfaces.

“There’s a great deal of interest in sensing applications,” Zubarev said. “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.”

The scientsist also grew longer nanowires that, along with their optical advantages, have unique electronic properties. Ongoing experiments will help characterize the nanowires’ ability to transmit a plasmonic signal, which could be useful for waveguides and other optoelectronic devices.

Zubarev’s primary interest for the technology, however, remains biological. “If we can modify the surface roughness such that biological molecules of interest will absorb 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.”

The study was published online in Langmuir.

For more information, visit: www.rice.edu