Detecting nanometre-sized entities in solution is complicated by the fact that they must first slowly diffuse to the sensor.
Stephen Arnold of the Polytechnic Institute of New York University in Brooklyn and his co-workers have found a way to speed things up. By shining a laser on silica microspheres, they produce 'whispering gallery modes' (WGMs) — light trapped inside the microsphere by reflection that circles endlessly. This causes a short-range electrical field just outside the sphere, which can attract nanoparticles, bringing them to the sensor about 100 times faster than diffusion.
On top of this, the nanoparticles, once trapped, start to orbit the sphere. This produces a shift in the resonant frequency in the WGM, which in turn permits estimation of the size and, by extension, mass of the nanoparticle. WGMs could have applications in biosensing.