Scientists have devised a technique for spotting beads with a radius as small as 12.5 nm in solution, and they have improved the signal-to-noise ratio for sensing the influenza A virus by a factor of 10.
Theoretical simulation of the mode field in a cross-section of the toroidal microresonator.
Over the past 15 years, scientists have been leveraging the properties of optical microresonators to detect smaller and smaller particles. One group of researchers has devised a technique for spotting beads with a radius as small as 12.5 nm in solution, and they have improved the signal-to-noise ratio for sensing the influenza A virus by a factor of 10 (Proc. Natl. Acad. Sci. USA 108, 5976).
The key, according to lead researchers Kerry Vahala (California Institute of Technology, U.S.A.) and Tao Lu (University of Victoria, Canada), was to compare the beam from the microresonator with a thermally controlled interferometer for noise reduction.
For a resonator, the group used a glass torus about 100 µm in diameter. They split the source emission from a 680-nm laser into two beams: one through the toroidal resonator and the other through a reference interferometer, immersed in an ice-water bath to compensate for the fluctuations in the laser signal. The resonator sat in a room-temperature tank containing water with dilute concentrations of polystyrene beads of 50 nm, 25 nm or 12.5 nm radius in various experimental runs.
The thermal control of the reference interferometer was a key improvement to the detection setup. Otherwise, the laser noise would have exceeded the sensitivity of the toroid.
Whenever the beads attach themselves to the equator of the toroid, they change the resonant frequency of the resonator by a tiny amount. The larger the radius of the particle, the larger the frequency shift, which scales with the volume of the bead. For the smallest beads, the frequency shift corresponds to a wavelength change of about 0.5 fm.
The influenza A virus, which is about 40 to 50 nm across, presents a larger target than the 12.5-nm-radius spheres. The irregular shape of the virus particles leads to far more complex interactions between the influenza A and the toroidal surface. Still, team members raised the signal-to-noise ratio for detecting the flu virus to 38:1, versus the 3:1 ratio of previous attempts eight years ago.
The researchers did not set up their experiment to resemble a biological system, Vahala said. However, plenty of research groups are thinking up future ways to use these small resonators for single-cell studies and biochemical assays.
Patricia Daukantas is a freelance science writers who specialize in optics and photonics.