An elegant method is proposed and demonstrated for tracking the location and movement of nanoparticles in an optical microscope using the optical phenomenon of caustics. A simple and reversible adjustment to the microscope generates caustics several orders of magnitude larger than the particles. The method offers a simple and relatively inexpensive method for visualizing such phenomena as the formation of self-assembled monolayers and the interaction of nanoparticles with chemically functionalized surfaces.

Maurice Whelan, JRC, Italy, and Eann Patterson, Department of Mechanical Engineering, Michigan State University, MI, USA, are the authors of a study that characterizes nanoparticles via a brand-new approach, a light-microscopic method implementing the optical phenomenon called caustics. Whelan is the SRP leader of the "In vitro toxicology" group whose aim is to facilitate the exploitation of emerging nano-biotechnologies by the in vitro testing community. He also takes part in the intersection project "In vitro cytotoxicity testing" that addresses the challenges associated with the conception and design of an integrated device (loosely termed "chip") to carry out cytotoxicity assays based on mammalian cells.

Introduction

Caustics phenomena in everyday occurrences can be found in the bottom of your cup or glass. The first description from 1828 explained the formation of caustic curves and surfaces when light rays are reflected from curved mirrors. The effect of the curvature of a mirror is to cause the light rays to concentrate in some regions in space and to be absent from others so that, when a flat screen is placed in their path, bright curves and corresponding dark areas or shadows are observed. The same phenomenon is seen in transmission of light rays through objects that have curved surfaces.
The approach taken here is derived from the caustic method for the characterization of stress singularities in which a large caustic is used to identify a very small feature, i.e. the formation of a large caustic by a transparent nanoparticle is used to identify and track the location of the particle.

Conclusions

It has been shown that by closing down the in-built field aperture fitted to the halogen light source of an optical microscope, caustics can be generated from spherical particles in the size range from 30 000 nm (30 μm) to 50 nm (0.05 μm). At the nano end of this range the caustics are of the order of hundreds of times larger than the particle thus allowing the location of a particle to located and tracked. The technique has potential in the tracking of nanoparticles during a wide variety of applications in nano-engineering and biology.

Source: Tracking nanoparticles in an optical microscope using caustics, Eann A Patterson1 and Maurice PWhelan2, 1 Department of Mechanical Engineering, Michigan State University, East Lansing, MI 48824, USA, 2 Nanotechnology and Molecular Imaging Unit, Institute for Health and Consumer Protection, European Commission DG Joint Research Center, 21021 Ispra (VA), Italy, IOP PUBLISHING NANOTECHNOLOGY, Nanotechnology 19 (2008) 105502 (7pp), doi:10.1088/0957-4484/19/10/105502