New NIST Microscope Tracks Nanoparticles in 3D

Researchers at the National Institute of Standards and Technology (NIST) have come up a new microscope design--which the agency plans to patent--that will allow users to track the motions of nanoparticles in solution as they dart around in three dimensions.

NIST's hope is that this technology will lead to a better understanding of the dynamics of nanoparticles in fluids and, ultimately, process control techniques to optimize the assembly of nanotech devices.

While some nanoscale fabrication techniques borrow from the lithography and solid state methods of the microelectronics industry, NIST says, an equally promising approach relies on "directed self-assembly"--which capitalizes on physical properties and chemical affinities of nanoparticles in solutions to induce them to gather and arrange themselves in desired structures at desired locations. Potential products include extraordinarily sensitive chemical and biological sensor arrays and new medical and diagnostic materials based on "quantum dots" and other nanoscale materials.

Because a microscope sees a three-dimensional fluid volume as a 2-D plane, NIST says, there's no real sense of the "up and down" movement of particles in its field of view except that they get more or less fuzzy as they move across the plane where the instrument is in focus. To date, attempts to provide a 3-D view of the movements of nanoparticles in solution largely have relied on that fuzziness. Optical theory and mathematics can estimate how far a particle is above or below the focal plane based on diffraction patterns in the fuzziness. The math, however, is extremely difficult and time consuming and the algorithms are imprecise in practice.

One alternative, NIST researchers reported at the annual meeting of the American Physical Society, is to use geometry instead of algebra; specifically, angled side walls of the microscopic sample well act as mirrors to reflect side views of the volume up to the microscope at the same time as the top view. (The typical sample well is 20 microns square and 15 microns deep.) The microscope sees each particle twice, one image in the horizontal plane and one in the vertical. Because the two planes have one dimension in common, it's a simple calculation to correlate the two and figure out each particle's 3-D path.

"Basically, we reduce the problem of tracking in 3-D to the problem of tracking in 2-D twice," explains lead author Matthew McMahon.

For more information, go to www.nist.gov.

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