NIST Nanofluidic ‘Multi-tool’ Separates and Sizes Nanoparticles

A wrench or a screwdriver of a single size is useful for some
jobs, but for a more complicated project, you need a set of tools of
different sizes. Following this guiding principle, researchers at the
National Institute of Standards and Technology (NIST) have engineered a
nanoscale fluidic device that functions as a miniature “multi-tool” for
working with nanoparticles—objects whose dimensions are measured in
nanometers, or billionths of a meter.

A 3D nanofluidic "staircase" channel with many depths was used to separate and measure a mixture of different-sized fluorescent nanoparticles. Larger (brighter) and smaller (dimmer) particles were forced toward the shallow side of the channel (fluorescence micrograph on left). The particles stopped at the "steps" of the staircase with depths that matched their sizes. View hi-resolution image Credit: S.M. Stavis, NIST

First introduced in March 2009 (see “NIST-Cornell Team Builds World’s First Nanofluidic Device with Complex 3-D Surfaces”,
the device consists of a chamber with a cascading “staircase” of 30
nanofluidic channels ranging in depth from about 80 nanometers at the
top to about 620 nanometers (slightly smaller than an average bacterium)
at the bottom. Each of the many “steps” of the staircase provides
another “tool” of a different size to manipulate nanoparticles in a
method that is similar to how a coin sorter separates nickels, dimes and
quarters.

In a new article in the journal Lab on a Chip*, the NIST
research team demonstrates that the device can successfully perform the
first of a planned suite of nanoscale tasks—separating and measuring a
mixture of spherical nanoparticles of different sizes (ranging from
about 80 to 250 nanometers in diameter) dispersed in a solution. The
researchers used electrophoresis—the method of moving charged particles
through a solution by forcing them forward with an applied electric
field—to drive the nanoparticles from the deep end of the chamber across
the device into the progressively shallower channels. The nanoparticles
were labeled with fluorescent dye so that their movements could be
tracked with a microscope.

As expected, the larger particles stopped when they reached the steps
of the staircase with depths that matched their diameters of around 220
nanometers. The smaller particles moved on until they, too, were
restricted from moving into shallower channels at depths of around 110
nanometers. Because the particles were visible as fluorescent points of
light, the position in the chamber where each individual particle was
stopped could be mapped to the corresponding channel depth. This allowed
the researchers to measure the distribution of nanoparticle sizes and
validate the usefulness of the device as both a separation tool and
reference material. Integrated into a microchip, the device could enable
the sorting of complex nanoparticle mixtures, without observation, for
subsequent application. This approach could prove to be faster and more
economical than conventional methods of nanoparticle sample preparation
and characterization.

The NIST team plans to engineer nanofluidic devices optimized for
different nanoparticle sorting applications. These devices could be
fabricated with tailored resolution (by increasing or decreasing the
step size of the channels), over a particular range of particle sizes
(by increasing or decreasing the maximum and minimum channel depths),
and for select materials (by conforming the surface chemistry of the
channels to optimize interaction with a specific substance). The
researchers are also interested in determining if their technique could
be used to separate mixtures of nanoparticles with similar sizes but
different shapes—for example, mixtures of tubes and spheres.

Media Contact: Michael E. Newman, michael.newman@nist.gov, 301-975-3025