Mirrors help microscope track nanoparticles in 3-D

A new microscope design by researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg, US, allows the motion of nanoparticles in solution to be tracked in three dimensions. A better understanding of the dynamics of nanoparticles in fluids could lead to improved process control techniques to optimise the assembly of nanotech devices.

Self-assembly may be a big thing in nanotechnology, but this bottom up approach to nanofabrication is of little use if you cannot accurately monitor and direct the assembly process. Microscopes typically provide a two-dimensional view of structures under observation. Extending this to three dimensions is possible to a limited degree using a combination of fuzzy optics theory and mathematics, but in practice the analysis is slow and imprecise.

By chemically etching angled sides in a microscope’s silicon sample well so they act as mirrors, it is possible to track the motion of nanoparticles in three dimensions

Matthew McMahon as his NIST colleagues have taken a geometrical approach to the problem. They angle the side walls of their microscopic sample well so that they act as mirrors which reflect side views of the sample volume up to the microscope at the same time as the top view. The microscope sees each particle in the sample twice, with one image in the horizontal plane and the other in the vertical. With the two orthogonal planes sharing one common dimension, it is then a simple task to correlate the images and derive the 3-D path taken by the particle during the observation.

“The idea is to use a mirror to get a side view of the moving particle,” says McMahon. “It’s the same concept that your dentist uses when he wants to get a side view of your molars; he places a small tilted mirror in such a way that he can see the side of the tooth. In our experiments we use a substrate that has tilted mirror surfaces to get a side view of the particle.”

“The cool benefit of this technique is that you get to use the exact same techniques to identify the vertical coordinate,” says Emory University microscopist Eric Weeks, who chaired the session of the American Physical Society meeting at which the NIST development was recently presented. “Normally Z is treated a little differently from X and Y, and often the resolution is worse in tracking a particle in Z.”

While the vertical resolution is improved with the NIST mirrors, the technique requires that a particle being tracked is close to one of the walls of the sample well. But some people prefer to look at particle motions well within the sample volume, away from walls which could influence particle motion through hydrodynamic effects. Weeks raises this point, but adds that in some cases this will probably not be a significant restriction.

There is another limitation to McMahon’s mirrors. “Confocal microscopy is often used to track colloidal particles in X, Y, and Z,” says Weeks. ‘Confocal microscopes typically cost $100k – $300k, so Matthew’s technique is a really good cost saving. On the other hand, with a confocal microscope you might be able to track several thousand particles in 3-D simultaneously, whereas Matthew’s technique is limited to only a few particles.”

The NIST technique is still in the early stages of development, says McMahon: “We have proven the concept, and are presently working to demonstrate that it can illuminate the physics and engineering of particle assembly.”

The researchers plan to patent the technique, and once this has been done they will publish further details and invite interested parties to license the technology through NIST’s Office of Technology Partnerships.

Figure: By chemically etching angled sides in a microscope’s silicon sample well so they act as mirrors, it is possible to track the motion of nanoparticles in three dimensions (source: NIST).

Article first published in Nanomaterials News.