Scientists create the most sensitive listening device ever.
The nano-ear is a microscopic particle of gold that is trapped by a laser beam. It can detect sound a million times fainter than the threshold of our hearing.
The discovery could open up a whole new field – acoustic microscopy – where organisms can be studied using the sounds they emit, ScienceNOW explains.
Sound waves happen when air is compressed and decompressed by pressure waves. They travel as a forward and backward displacement of the particles they pass through, and to detect sound, you need to measure this back-and-forth motion.
Back in 1986, scientists invented optical tweezers – which use a laser beam to grab hold of tiny particles and move them around. (These are pretty useful if you need to inject DNA into cells and manipulate it once it’s inside.) They can also measure the tiny forces acting on tiny particles: grab hold of a particle, watch it with a microscope, and see how it moves.
And to listen at the microscale, Jochen Feldmann and colleagues at the Ludwig-Maximilian University of Munich use a particle of gold 60 nanometers in diameter, immersed in water, and held in optical tweezers.
- They trapped one gold nanoparticle (the nano-ear) in a laser beam, and then fired rapid pulses of light from a second laser at other nanoparticles a few micrometers away.
- The pulses heated the nanoparticles, which disturbed the water around them, generating pressure, or sound, waves (pictured).
- They recorded the movements of the nano-ear in response to acoustic vibrations caused by the other tiny gold particles heating up in the water nearby.
The gold nano-ear jiggled back and forth, as if it were reacting to the sound waves. When they varied the frequency, the trapped particle matched the frequency every time, and the direction of its movement lined up with the sound waves' direction, New Scientist explains.
“With our nano-ear, we have developed a nanomicrophone that allows us to get closer than ever to microscopic objects,” study author Alexander Ohlinger says.
The nano-ear 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 sound detector capable of detecting acoustic waves, says study coauthor Andrey Lutich.
If these nano-ears were to be arranged into a 3D array, they could be used to listen in on microbes like viruses and bacteria – all of which emit very faint acoustic vibrations as they move.
They could also tell us more about how cells change as a result of disease. (Red blood cells, for example, vibrate less when they’re infected with the malaria parasite.)
The work was published in Physical Review Letters this month.
Image from Ohlinger et al.