Nanoparticles trap themselves

A new type of optical trap that works with a low-intensity laser could safely confine nanosize objects such as viruses.

Credit: © 2009 NPG

Optical trapping — the use of lasers to confine particles — has been very successful for controlling and measuring objects on the micrometre scale, such as cells and bacteria. Now, Romain Quidant at Institut de Ciences Fotoniques in Barcelona and co-workers have achieved optical trapping of nanometre-size particles, through a clever experimental set-up in which the particle itself enhances the trapping forces¹.

Generally, optical trapping forces are much weaker for smaller objects, and thermal motion allows the particles to escape the trap. These problems can be solved by using a stronger laser, but this runs the risk of damaging the particles.

Quidant and co-workers instead directed their laser onto a 310-nm aperture in a gold film, in a chamber with polystyrene beads suspended in water. The researchers were able to detect polystyrene beads just 50 nm in diameter by observing changes in the laser intensity transmitted through the aperture.

What's more, if the particles move away from the aperture, they change the rate of photons travelling through, thus inducing a force back towards the trap. Because of this effect, which the researchers call 'self-induced back-action', nanoparticles can be trapped using a very low-intensity laser. The work therefore paves the way to safely isolating nanoscale objects such as viruses.