Optical Signatures of Small Nanoparticles in a Conventional Microscope

Abstract

An elegant method is proposed and demonstrated for tracking the location and movement of nanoparticles in a conventional optical microscope based on exploiting the optical phenomena of caustics and diffraction, generated as a result of light scattering. A simple and reversible adjustment to the microscope generates 3D scatter patterns several orders of magnitude larger than the particles themselves, thus offering a simple and inexpensive method for visualising the behaviour of nanoparticles in situ, without any need for labelling or complex measurement instrumentation. In earlier work also published this year [Patterson & Whelan, Nanotechnology, 19, 105502, March 2008, IF 3.31], the method was successfully applied to track particles with a diameter in the range of 50 to 300nm diameter, which is below the resolution limit of an optical microscope configured in the standard manner. This Small paper however describes the generation of optical signatures from single gold particles as small as 3nm, almost 300 times smaller that the wavelength of the light used to observe them.