"Just a few years ago, everyone's attention was on the size of nanoparticles because altering size was a straightforward way to change the wavelength of light that the particle reacted with," said Jason Hafner of Rice University. "Today, researchers are increasingly interested in intricate shapes and the specific ways that those shapes affect a particle's interaction with light."

Nanoparticles of noble metals such as gold exhibit localized surface plasmon resonance (LSPR), a phenomenon which causes optical extinction at certain wavelengths and gives the materials applications in sensing, imaging and as nanoscale optical waveguides.

Hafner and colleagues made the star-shaped nanoparticles by adding 10 nm diameter gold colloid particles to a gold chloride growth solution containing surfactant. The same process is normally used with surfactant-stabilized seed nanoparticles in place of the colloid particles, which produces gold nanorods. In this case, though, the result was star-shaped gold nanoparticles around 100 nm in size, with around 14% of the particles having at least three tips.

The nanostars had extinction peaks in both the visible and near-infrared regions. Each nanostar tip appeared to have a distinct resonant wavelength: analysis showed that each peak was also polarized at a different angle.

The nanostars' resonances were also extremely sensitive to the dielectric environment, undergoing a shift depending on the conditions. The LSPR shift for other nanoparticles is typically 500–800 meV/RIU (change in refractive index unit), whereas the team measured shifts for the star-shaped nanoparticles of 649 and 1410 meV/RIU.

"We are just getting started with our follow-up work, but nanostars clearly offer some exciting possibilities," said Hafner. "Their extreme sensitivity to the local dielectric environment is a particularly attractive quality for molecular sensing."

The researchers reported their work in Nano Letters.