Tracking nanoparticles in an optical microscope using caustics

Abstract

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.

Three-dimensional automated nanoparticle tracking using Mie scattering in an optical microscope

“…Polystyrene particles of nominal diameter 50 nm (L0780‐1ML Amine‐modified polystyrene, fluorescent blue; Sigma‐Aldrich Inc., St Louis, MO, USA), were diluted in 1000‐fold in a phosphate buffered saline (PBS) solution and placed between high‐quality glass slide covers (Menzel‐Gläser, Braunschweig, Germany; 21 × 23 mm, #4), which were placed above the objective lens. The use of these glass slide covers is necessary because typical microscope slides contain imperfections that generate diffraction noise under the conditions used in this study (Patterson & Whelan, 2008a). Gold particles of nominal diameter 10 nm were also used and were synthesized in the laboratory in the following way.…”

“…In parallel, Toprak et al (2007) used defocusing implemented in a bifocal microscope to track polystyrene beads of diameter 200 nm. Subsequently, Patterson & Whelan (2008a) demonstrated that silica particles as small as 50 nm diameter and gold particles as small as 3 nm diameter (Patterson & Whelan, 2008b) generated caustics and optical signatures identical in form to those predicted using Mie theory (Ovryn, 2000) when the condenser aperture was closed to a minimum and the objective defocused in an inverted optical microscope.…”

“…In this study, the potential to track automatically in three dimensions particles of the order of 10–50 nm is demonstrated by motorizing the objective of a conventional inverted optical microscope in such a way that the objective could be repeatedly translated over a distance along the light path while an adjustable field aperture was closed to its minimum as described previously (Patterson & Whelan, 2008a). The objective was repeatedly scanned above and below the point of optimum focus so that the defocused images (Guerrero‐Viramontes et al , 2006; Toprak et al , 2007; Patterson & Whelan 2008a, b) were observed. These images were analysed in the time domain to locate the position of the particle in the xy ‐plane (perpendicular to the light path) and subsequently in the spatial domain to identify their position in the z ‐direction (along the light path).…”

Optical Signatures of Small Nanoparticles in a Conventional Microscope

“…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 2. 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 3, 4.…”

Nanoparticle size determination using optical microscopes

“…However, using the smallest condenser aperture (i.e., using low partial coherence), nanoparticles as small as 3 nm in diameter were clearly imaged (including through-focus images) using conventional bright-field optical microscopes. 15,16 On the other hand, increasing condenser aperture diameter to normal operating diameter (i.e., using high partial coherence) nanoparticles could not be imaged. The increased coherence condition enabled clearer imaging of nanoparticles.…”