Shaky idea. The movement of a gold nanoparticle (left) in a set of optical tweezers was used to detect sound waves triggered by the expansion of other nanoparticles nearby.

Ohlinger et al., Phys. Rev. Lett. 108, 018101 (2012)

Scientists Create World's Tiniest Ear

Have you ever wondered what a virus sounds like? Or what noise a bacterium makes when it moves between hosts? If the answer is yes, you may soon get your chance to find out, thanks to the development of the world's tiniest ear. The "nano-ear," a microscopic particle of gold trapped by a laser beam, can detect sound a million times fainter than the threshold for human hearing. Researchers suggest the work could open up a whole new field of "acoustic microscopy," in which organisms are studied using the sound they emit.

The concept of the nano-ear began with a 1986 invention known as optical tweezers. The tweezers use a laser beam focused to a point with a lens to grab hold of tiny particles and move them around. They've become a standard tool in molecular biology and nanotechnology, helping researchers inject DNA into cells and even manipulate it once inside. Optical tweezers can also be used to measure minuscule forces acting on microscopic particles; once you've grabbed hold of your particle with the laser beam, instead of moving it yourself, you simply use a microscope or other suitable monitoring apparatus to watch whether it moves of its own accord. That's where the nano-ear comes in.

Sound waves travel as a forward and backward displacement of the particles of the medium they pass through. So to detect sound, you need to measure this back-and-forth motion. Optical physicist Jochen Feldmann and colleagues in the Photonics and Optoelectronics Group at the University of Munich in Germany used a particle of gold 60 nanometers in diameter, immersed in water, and held in optical tweezers.

Feldmann's team recorded and analyzed the movements of this particle in response to acoustic vibrations caused by the laser-induced heating of other gold nanoparticles in the water nearby. As well as having unprecedented sensitivity, their nano-ear could also calculate the direction the sound had come from. They suggest three-dimensional arrays of nano-ears working together could be used to listen in on cells or microorganisms such as bacteria and viruses, all of which emit very faint acoustic vibrations as they move and respire. "There are definitely medical opportunities which we can tackle together with the right people," Feldmann says, "but we just have to see how it works first."

Biophysicist Lene Oddershede of the Optical Tweezer Laboratory at the Niels Bohr Institute in Copenhagen is impressed and suspects the paper will inspire others in the field to look for sound waves when studying microorganisms. "It's a really interesting idea, and we could easily do that, but we have never made any attempt to do so," she says. "I would say this paper's very inspiring in that sense." She cautions, however, that the experimental setup will need to be significantly refined to improve its ability to separate sound waves from random molecular movement before the suggested acoustic microscope can become a reality. But she is optimistic: "I do believe they can relatively quickly improve the equipment."