Gold nanoparticles have a range of biomedical uses, from detecting tumours to delivering drugs. However, their size is important because it affects their optical and mechanical properties, as well as their toxicity. Martin Ploschner and his colleagues at the University of St Andrews, UK, report an efficient way to sort gold nanoparticles by size using laser light.

The team aimed green and red lasers at a thin layer of water containing a mixture of gold nanoparticles 100 and 130 nanometres in diameter. The green light's frequency matched that of the electrons in the smaller nanoparticles. This resonance enhanced forces acting on the particles, pushing them in one direction. The red light interacted with the larger particles, moving them in the opposite direction.

The researchers suggest that the method could sort nanoparticles more finely than current methods, which rely on centrifugation.

Nano Lett. http://dx.doi.org/10.1021/nl204378r (2012)