Scientists can detect the movements of single molecules by using fluorescent tags or by pulling them in delicate force measurements, but only for a few minutes. A new technique by Rice University researchers will allow them to track single molecules without modifying them -- and it works over longer timescales.
In the current issue of Nanotechnology, a team led by Jason Hafner, an associate professor of physics and astronomy and of chemistry, has shown that the plasmonic properties of nanoparticles can "light up" molecular interactions at the single-molecule limit in ways that will be useful to scientists.
Hafner's method takes advantage of the ability of metal nanoparticles to focus light down to biomolecular scales through an effect called localized surface plasmon resonance (LSPR). The gold nanoparticles ultimately used in the experiment scatter light in visible wavelengths, which can be detected and spectrally analyzed in a microscope.
"The exact peak wavelength of the resonance is highly sensitive to small perturbations in the nearby dielectric environment," said graduate student Kathryn Mayer, the lead student on the experiment. "By tracking the peak with a spectrometer, we can detect molecular interactions near the surface of the nanoparticles."
Hafner first discussed their progress at a 2006 conference after a presentation on gold nanostars his lab had developed. "We had extremely preliminary data, and I said, 'Maybe we've got it.' I thought we were close," he recalled.
What took time was finding the right particle. "We started with nanorods, which don't scatter light well, at least not the small nanorods we produce in my lab. Then we tried nanostars and found they were very bright and sensitive, but each was a different shape and had a different peak wavelength."
The team settled on bipyramids, 140-nanometer-long, 10-sided gold particles that focus light at their sharp tips, creating a halo-
|Contact: David Ruth|