Nanoparticle Tracking for Fluidic Self-Assembly

A sizable library of nanoscale "building blocks" has been developed over the last couple of decades. These nanostructures — particles, disks, rods - are frequently synthesized in solution. One of the main challenges in nanomanufacturing is to place these solution-based nanoparticles at precise locations on a substrate with nanometer control. The key to achieving that control is understanding the forces and interactions that the particles experience. These can be measured by careful analysis of a nanoparticle’s motion in 3D as it approaches the substrate. Once the interactions are understood, researchers can then design processes to assemble macroscale objects from nanoscale components.

It is straightforward to track a particle in 2D using a digital video camera connected to a microscope. This works well in the case of particles confined in 2D geometries, but does not provide the information needed to understand a real-life 3D process. Getting that 3D track, however, is not so straightforward; up until now it has involved fitting to the ring-shaped diffraction patterns (also called Airy patterns) that result when a particle is out of focus in a microscope.

We have developed a method for measuring a 3D nanoparticle trajectory from a series of 2D digital video images, without fitting to Airy patterns. Our method exploits a microfabricated silicon substrate containing etched wells whose walls are tiny mirrors. The wells are shaped like inverted pyramids. We have studied 190 nm particles using the special substrate with a normal optical microscope. When a nanoparticle is in one of the wells, the microscope sees one or more reflections in addition to the direct image of the particle. If the particle moves in or out of focus, its lateral position doesn’t change, but its reflected image slides up and down the mirror. The mirror provides a side view (or an "orthogonal" view) of the particle. The movie linked below shows a particle with two reflections because it is in the upper left corner of a well. Since the spots are all geometrically related, it is simple to convert the 2D positions of the spots into a fully 3D particle track (see figure 2).

Figure 1. Orthogonal tracking of 190-nm fluorescent polystyrene particle. The nanoparticle is in the top left corner of a well, so it has reflections on the top and left.
Click here or on Figure 1 image above for the video clip (3.60 MB AVI file).
Note: There is no audio in the video. Media player software is needed to open the file. Several media players are available for free download.

Figure 2. 3D track of the particle in the movie above, where position markers are color-coded to the time. The track is constructed by finding the centers of the three spots in the movie and using geometry to correlate them in three dimensions.

It is computationally simpler to calculate the center of the reflected spot than it is to fit the appropriate mathematical function to a diffracted image. This means that the orthogonal tracking technique may be useful as a fast measurement for real-time particle control experiments, in which the motion is controlled by electric fields or fluid flow. If the nanoparticles can be controlled in real-time, then researchers should be able to assemble 3D microstructures step-by-step, in addition to designing massively parallel processes to assemble macroscale structures.

Matthew McMahon - NIST
J. Alexander Liddle - NIST
Andrew Berglund - NIST
Peter Carmichael - NIST
Jabez McClelland - NIST

Online: December 2007
Last Updated: May 2008

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