Gyromagnetic nanostars give new spin to imaging

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Researchers have created magnetically responsive gold nanostars that may offer a new approach to biomedical imaging, according to research in the July 22 issue of the Journal of the American Chemical Society.

The nanostars gyrate when exposed to a rotating magnetic field and can scatter light to produce a pulsating or "twinkling" effect, which allows them to stand out more clearly from noisy backgrounds like those found in biological tissue, the authors wrote.

Alexander Wei, PhD, a professor of chemistry, and Kenneth J. Ritchie, PhD, an associate professor of physics, led the Purdue University team that created the new gyromagnetic imaging method.

"This is a very different approach to enhancing contrast in optical imaging," said Wei, who also is a member of Purdue's Center for Cancer Research and the Oncological Sciences Center in West Lafayette, Ind. "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 researchers reported that 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 uniqueness of the twinkling nanostars enables them to be picked out from a field of stationary particles, some of which can be brighter than the nanostars.

Any signal that does not have a frequency corresponding to the rotating magnetic field can be suppressed in the images, eliminating background noise, according to Ritchie.

"It was surprising how well this method enhanced the imaging," Ritchie said. "It can improve the contrast of the particles to the background noise by more than 20 decibels and can clearly reveal a gyrating nanostar, where with existing direct imaging methods in many cases you wouldn't be able to definitively find a particle."

Gold nanostars and other gold nanoparticles have recently been examined as contrast agents for biomedical imaging because of their brightness at near-infrared wavelengths, which can penetrate through tissue better than visible light. However, giving them the ability to twinkle was key to developing a novel dynamic imaging method, Wei said.

"Gyromagnetic nanostars combine strong optical signaling with a unique mechanism for reducing noise, allowing one to pick out the proverbial needle from the haystack," Wei said. "Our analysis picks out signals at that frequency and translates that information into images of remarkable clarity."

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

"We have external control over the speed of rotation, so we will always know what frequency to focus on when looking for nanostars," Wei said.

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, Wei said. The team is continuing to investigate the biological effects of nanostars inside cells.

The National Institutes of Health (NIH) funded the research.