Scientists Synthesize Star-Fruit-Shaped Gold Nanoparticles

Researchers continually seek to craft nanoparticles with complex, exotic structures to yield unique properties. Now chemists have made gold nanoparticles that look like star fruit (Langmuir, DOI: 10.1021/la300218z).

Scientists already use gold nanorods in medical imaging. And others have demonstrated that star-shaped nanoparticles have unique optical and electronic properties useful for detecting chemicals. Eugene R. Zubarev and Leonid Vigderman of Rice University thought that combining those two morphologies into star-fruit-shaped particles could lead to materials ideal for surface-enhanced Raman spectroscopy (SERS), a detection technique that relies on signals produced by molecules adsorbed onto gold surfaces and nanoparticles. Electromagnetic fields should concentrate at the tips of the star-fruit spikes, Zubarev says, and amplify the Raman signals of adsorbed molecules.

To make the gold star fruits, the researchers started by making gold nanorods with a pentagonal cross section using a technique Zubarev’s team reported in 2008 (J. Am. Chem. Soc., DOI: 10.1021/ja806043p). Next they stirred the rods into a solution of gold chloride, silver nitrate, and cetyltrimethylammonium bromide, to deposit more gold onto the nanorods. A few hours later, they filtered out the particles and observed the particles’ star-fruit shape using scanning electron microscopy.

The silver nitrate was the key to making the star-fruit shape, Zubarev says. Without it, the gold would have deposited evenly on the nanorods’ surface. But adding silver nitrate created a layer of silver bromide on the particles’ faces, hindering gold deposition. As a result, the highest rate of deposition happens at the rods’ five long edges, he says, “and the cross section becomes a star.”

The researchers also ran SERS experiments with the gold particles by placing them on a gold surface coated with 1,4-benzenedithiol. They found that Raman signals from 1,4-benzenedithiol were 25 times greater with the star-fruit particles than with the flat pentagonal nanorods.