Optical signatures of small nanoparticles in a conventional microscope.

Gold nanoparticles ranging in size from nominally 3 to 50 nm have been observed to generate optical signatures detectable in a conventional inverted optical microscope through a simple adjustment to the illumination optics, despite the particle diameter being substantially less than the wavelength of visible light. At their maximum extent the optical signatures are of the order of a thousand times the diameter of the particle, thus offering the possibility for locating and tracking particles in situ without any requirement for labeling. Acquisition of image stacks by scanning the focus of the microscope along the light path shows that the signatures consist of rings that are sections through two sets of cones arranged apex to apex and parallel to the light path, with the particle at the centre. In earlier work caustics were observed from larger (50–300-nm diameter) transparent particles; however, the generation of optical signatures from opaque particles an order of magnitude smaller was unexpected. It is suggested that the three-dimensional (3D) nature of these optical signatures provides the potential for rapid sizing and 3D tracking of particles within a conventional optical microscope; thus, this technique has a variety of potential applications in nanobiotechnology and nanoengineering. Engineered nanoparticles have a number of potential applications in nanobiotechnology; for example, when such particles are attached to molecules then a ‘‘molecular ruler’’ is effectively generated that allows, for instance, the study of the kinetics of single DNA hybridization events. It is not possible to resolve these particles in an optical microscope because their diameters are below the Rayleigh limit. However, when viewed in an appropriately adjusted optical microscope, transparent particles as small as 50 nm generate optical signatures with diameters several hundred times larger than the particle itself. These signatures have been shown to consist of a 3D envelope of bright light and a shadow zone, which is characteristic of caustics formed by the refraction of light by larger particles and spheres. In scanning electron microscopy(SEM) caustics can also appear but are treated as