'Nanostars' Could be Powerful Chemical Sensors

New optics research from Rice University's Laboratory for Nanophotonics (LANP) suggests that tiny gold particles called nanostars combine some of the best properties of photonic particles like nanorods and quantum dots while also having unique spectral signatures that could be used as powerful chemical sensors.

New research from Rice University's Laboratory for Nanophotonics suggests that each spike on a tiny gold particle called a nanostar has a unique spectral signature that could be useful for 3-D molecular sensing. (Photo: Jason Hafner/Rice University)

Nanophotonics is a rapidly growing field of study that looks at ways to generate and manipulate light using ultrasmall, engineered structures. The virus-sized nanostars, so named because of their spiky surface, are one of a growing number of intricately shaped particles that are increasingly drawing the attention at LANP and other photonics labs, the researchers said.

"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 lead researcher Jason Hafner, associate director of LANP and assistant professor of chemistry, physics and astronomy. "Today, researchers are increasingly interested in intricate shapes and the specific ways that those shapes affect a particle's interaction with light."

Most nanophotonic research at LANP involves the study of plasmons, waves of electrons that flow like a fluid across metal surfaces. Light can be used to amplify plasmon waves on metal nanoparticles. Like a child in a bathtub, rhythmically building waves until they slosh out of the tub, the plasmons on the particles dramatically amplified with wavelengths of light that correspond to the rhythm of the electron waves. The study of plasmonics is one of the fastest growing fields in optics because it could prove useful for a wide range of applications in biological sensing, microelectronics, chemical detection, medical technology and others, the researchers said.

Nanostars incorporate some of the best properties of photonic particles like nanorods and quantum dots, Hafner said. For example, they deliver strong spectral peaks that are easy to distinguish with relatively low-cost detectors. But his team found unique properties too. A painstaking analysis revealed that each spike on a nanostar has a unique spectral signature, and preliminary tests show that these signatures can be used to discern the three-dimensional orientation of the nanostar, which could open up new possibilities for 3-D molecular sensing, he said.

"We are just getting started with our followup 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."

Co-authors of the study include physics and astronomy graduate student Colleen Nehl and chemistry graduate student Hongwei Liao. The research was supported by the Army Research Office, the National Science Foundation and the Welch Foundation. The findings are due to appear in an upcoming issue of the journal Nano Letters.

For more information, visit: www.rice.edu