How to Spot Nanoparticles

Spotting nanoparticles is an increasingly popular pursuit. Everybody from the makers of solar cells to beauty products are producing products based on nanoparticles, while the atmosphere is filling with aerosols from factories and car exhaust, many of which pose a threat to human health.

So it’s no surprise then that nanotechnologists are racing to build devices that can detect and characterise these particles.

One way to do it is to label the particles with a fluorescent tag that makes them easy to see. But of course, that’s only possible with prior access to the particles.

Then there are various resonating devices in which a nanoparticle landing on a spring board changes the way it vibrates in a way that can be measured by a laser bouncing off the surface.

The trouble with these is that the position of the particle on the spring board influences the change in vibration. Also, the shift in vibration frequency is comparable in size to that caused by noise such as fluctuations in laser intensity and frequency and thermal noise. That makes unambiguous nanoparticle detection a tricky business.

Today, Jiangang Zhu and buddies from Washington University in St. Louis demonstrate another way of doing it that promises to reveal with much greater reliability, not only a particle’s mass but also its radius.

Their new device is a tiny resonating toroid on a stalk, a kind of vibrating micro-mushroom. What’s cool about a toroid is that it can support clockwise and anticlockwise waves at the same time. That’s important because the two waves effectively calibrate each other so that the change in resonance caused by a nanoparticle can be easily distinguished from background noise. With a bit of signal processing, the team say it is also possible to count the number of particles and determine their size.

The new device has the potential to be not only more accurate and reliable than earlier attempts but smaller too: the entire thing could be built into a single chip.

That should have an important impact for a variety of researchers, such as environmental scientists measuring pollution in real time, biotech companies developing nanoparticle-based drugs and just about anybody else fiddling with things on this scale. Handy!

Ref: arxiv.org/abs/0912.0078: On-chip Single Nanoparticle Detection and Sizing by Modesplitting in an Ultra-high-Q Microresonator