Ultra-sensitive detection of individual gold nanoparticles: spectroscopy and applications to biology

Gold nanoparticles are now widely used in different fields of chemistry, physics and biology. For many applications, it became crucial to use the smallest ones and at the same time to be able to detect them at the single particle level with versatile optical methods. Such methods should also allow the study of their intimate optical properties. Here we review the ultra-sensitive optical methods based on the photothermal effect which permit the detection of individual gold nanoparticles down to 1.4 nm through their absorption. The absorption spectroscopy of the Surface Plasmon Resonance of individual gold nanoparticles down to 5nm is further presented and revealed intrinsic size effects in the optical response of the smallest nanoparticles. Different applications to biology are also presented such as for new types of gold nanoparticles based DNA microarrays and for live cell molecular imaging owing to the insensitivity of the photothermal methods to scattering environments.