Researchers at Purdue University have created magnetically responsive gold nanostars that may offer a new approach to biomedical imaging. The nanostars gyrate when exposed to a rotating magnetic field and can scatter light to produce a pulsating or "twinkling" effect. This twinkling allows them to stand out more clearly from noisy backgrounds such as those found in biological tissue. Alexander Wei, Ph.D., and Kenneth Ritchie, Ph.D., M.Sc., led the team that created the new gyromagnetic imaging method. The work appears in a paper published in the Journal of the American Chemical Society.
"This is a very different approach to enhancing contrast in optical imaging," said Dr. Wei. "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."
The gold nanostars are about 100 nanometers from tip to tip and contain an iron oxide core that causes them to spin when exposed to a rotating magnet. The arms of the nanostar are designed to respond to a light source and reflect light to a camera when properly aligned. This gives nanostars the appearance of twinkling at rates that can be precisely controlled by the speed of the rotating magnetic field. The unique signature of the twinkling nanostars enables them to be picked out easily from a field of stationary particles, some of which can be brighter than the nanostars.
Any signal that does not have the frequency corresponding to the rotating magnetic field can be suppressed in the images, eliminating background noise, Dr. Ritchie said. "It was surprising how well this method enhanced the imaging. It can improve the contrast of the particles to the background noise by more than 20 decibels and can clearly reveal a gyrating nanostar, whereas with existing direct imaging methods, in many cases you wouldn't be able to definitively find a particle."