Twinkling nanostars: a golden opportunity?

27 Jul 2009

New spin on bioimaging exploits the light-scattering properties of rotating nanoparticles.

Scientists at Purdue University (West Lafayette, IN) have published details of a new gyromagnetic imaging technique that exploits the light scatter from rotating gold nanoparticles to suppress the background noise associated with optical interrogations of biological tissue.

Alexander Wei, a professor of chemistry, and Kenneth Ritchie, an associate professor of physics, headed up the team behind the breakthrough, details of which are featured on the cover of the latest issue of the Journal of the American Chemical Society.

The gold "nanostars", which measure about 100 nm tip to tip, contain an iron-oxide core that causes them to spin when exposed to a rotating magnet. The arms of the nanostar reflect incident light to a camera – in effect, twinkling at rates that can be precisely controlled by the speed of the rotating magnetic field. It's the unique signature of the twinkling nanostars that enables them to be picked out from a field of stationary particles, some of which may be brighter than the nanostars.

"This is a very different approach to enhancing contrast in optical imaging," said Wei, who also is a member of the Purdue University Center for Cancer Research. "Brighter isn't necessarily better for imaging; the real issue is background noise, and you can't always overcome this simply by creating brighter particles. With gyromagnetic imaging we can zero in on the nanostars by increasing signal strength, while cutting down on background noise."

To perform gyromagnetic imaging, the team placed a sample of cells containing the nanostars under a standard microscope equipped with a white-light source and a rotating magnet. Light was sent through a polarizing beam splitter and into the sample, and then reflected back through the beam splitter. The camera collected images at 120 frames per second, capturing the signal from the nanostars as they spun at approximately five revolutions per second.

"To translate a new imaging technique into something practical for broad use, it needs to be done without specialized equipment," Ritchie added. "Many other imaging techniques require expensive equipment or lasers, but this method can be done with a halogen lamp and a $10,000 camera."

In testing whether nanostars might harm cells during the imaging process, the researchers found that the particles were not only biocompatible, but could actually promote cell growth. The team is continuing to investigate the biological effects of nanostars inside cells.