They might “sound” infected (Image: Eye of Science/Science Photo Library)

MOVE over microphones, nanophones have arrived. A gold sphere just 60 nanometres in diameter is the most sensitive listening device ever created, paving the way for soundtracks to formerly silent movies of bacteria and other single-celled organisms.

Alexander Ohlinger at Ludwig Maximilian University in Munich (LMU), Germany, and colleagues suspended gold nanoparticles in a drop of water. They trapped one sphere in a laser beam and then fired rapid pulses of light from a second laser at others a few micrometres away. The pulses heated the nanoparticles, which disturbed the water around them, generating pressure, or sound, waves.

The single laser-trapped nanoparticle then began to jiggle back and forth, as if it were reacting to the sound waves. To make sure the jiggling was not due simply to the random motion of water molecules, the researchers varied the frequency of the sound waves. The trapped particle matched the frequency every time, and the direction of its movement lined up with the sound waves’ direction, providing further evidence that it was reacting to the waves (Physical Review Letters, DOI: 10.1103/physrevlett.108.018101).


The tiny microphone picked up sounds down to some minus 60 decibels – a level one-millionth of that detectable by the human ear. That makes it more sensitive than any other listening device, says team member Andrey Lutich, also at LMU. “We could not find any other sound detector capable of detecting acoustic waves with such a high sensitivity,” he says.

The technique could one day let us listen in on the tiniest living structures, including cells and viruses, according to the team. Changhuei Yang of the California Institute of Technology in Pasadena, who was not a member of the team, agrees. He says living cells have been seen vibrating under the microscope, but no one has yet set up a microphone to record their sounds. “It would be interesting to try to build upon the technology along this direction,” he says.

Doing that could also tell us more about the mechanical properties of cells and how they change as a result of disease.

In 2008, researchers led by YongKeun Park and Monica Diez-Silva of the Massachusetts Institute of Technology found that red blood cells vibrated less when they were infected with the malaria parasite, apparently because the infection made the cells stiffer than normal (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0806100105).

The gold-nanoparticle technique might eventually allow us to probe such changes, Yang says: “The novelty of the technology holds promise that it can open new ground.”