Optical detection of very small nonfluorescent nanoparticles

We discuss an interferometric method for the optical detection of very small nonfluorescent nanoparticles. In particular, we show that single gold nanoparticles with a diameter as small as 5 nm can be detected. We discuss the potential of such tiny particles as optical labels for biological studies. Furthermore, we show that our interferometric method can be also used for the detection and tracking of unlabelled biological nano-entities such as viruses or microtubuli.