Researchers at the Commerce Department's National
Institute of Standards and Technology (NIST) and Cornell University have
capitalized on a process for manufacturing integrated circuits at the nanometer
(billionth of a meter) level and used it to develop a method for engineering
the first-ever nanoscale fluidic (nanofluidic) device with complex three-dimensional
surfaces.
(A) Schematic of the NIST-Cornell nanofluidic device with complex 3-D surfaces. Each "step" of the "staircase" seen on the side marks a different depth within the chamber. The letter "E" shows the direction of the electric field used to move the nanoparticles through the device. The green balls are spheres with diameters of 100 nanometers whose size restricts them from moving into the shallower regions of the chamber. The coil in the deep end of the chamber (upper right corner) is a single DNA strand that elongates (upper left corner) in the shallow end. (B) Photomicrograph showing fluorescently tagged spherical nanoparticles stopped at the 100-nanometer level of the chamber, the depth that corresponds to their diameter. (C) Photomicrograph of a single DNA strand that is coiled in the deep end of chamber (box at far right) and elongated in the shallow end (box at far left). Larger boxes are closeups showing the fluorescently tagged strands. Credit: NIST
As described in a paper published online today in the journal Nanotechnology,*
the Lilliputian chamber is a prototype for future tools with custom-designed
surfaces to manipulate and measure different types of nanoparticles in solution.
Among the potential applications for this technology: the processing of nanomaterials
for manufacturing; the separation and measuring of complex nanoparticle mixtures
for drug delivery, gene therapy and nanoparticle toxicology; and the isolation
and confinement of individual DNA strands for scientific study as they are forced
to unwind and elongate (DNA typically coils into a ball-like shape in solution)
within the shallowest passages of the device.
Nanofluidic devices are usually fabricated by etching tiny channels into a
glass or silicon wafer with the same lithographic procedures used to manufacture
circuit patterns on computer chips. These flat rectangular channels are then
topped with a glass cover that is bonded in place. Because of the limitations
inherent to conventional nanofabrication processes, almost all nanofluidic devices
to date have had simple geometries with only a few depths. This limits their
ability to separate mixtures of nanoparticles with different sizes or study
the nanoscale behavior of biomolecules (such as DNA) in detail.
To solve the problem, NIST's Samuel Stavis and Michael Gaitan teamed with
Cornell's Elizabeth Strychalski to develop a lithographic process to fabricate
nanofluidic devices with complex 3-D surfaces. As a demonstration of their method,
the researchers constructed a nanofluidic chamber with a "staircase"
geometry etched into the floor. The "steps" in this staircase—each
level giving the device a progressively increasing depth from 10 nanometers
(approximately 6,000 times smaller than the width of a human hair) at the top
to 620 nanometers (slightly smaller than an average bacterium) at the bottom—are
what give the device its ability to manipulate nanoparticles by size in the
same way a coin sorter separates nickels, dimes and quarters.
The NIST-Cornell nanofabrication process utilizes grayscale photolithography
to build 3-D nanofluidic devices. Photolithography has been used for decades
by the semiconductor industry to harness the power of light to engrave microcircuit
patterns onto a chip. Circuit patterns are defined by templates, or photomasks,
that permit different amounts of light to activate a photosensitive chemical,
or photoresist, sitting atop the chip material, or substrate.
Conventional photolithography uses photomasks as "black-or-white stencils"
to remove either all or none of the photoresist according to a set pattern.
The "white" parts of the pattern—those that let light through—are
then etched to a single depth into the substrate. Grayscale photolithography,
on the other hand, uses "shades of gray" to activate and sculpt the
photoresist in three dimensions. In other words, light is transmitted through
the photomask in varying degrees according to the "shades" defined
in the pattern. The amount of light permitted through determines the amount
of exposure of the photoresist, and, in turn, the amount of photosensitive chemical
removed after development.
The NIST-Cornell nanofabrication process takes advantage of this characteristic,
allowing the researchers to transfer a 3-D pattern for nanochannels of numerous
depths into a glass substrate with nanometer precision using a single etch.
The result is the "staircase" that gives the 3-D nanofluidic device
its versatility.
Size exclusion of nanoparticles and confinement of individual DNA strands in
the 3-D nanofluidic device is accomplished using electrophoresis, the method
of moving charged particles through a solution by forcing them forward with
an applied electric field. In these novel experiments, the NIST-Cornell researchers
tested their device with two different solutions: one containing 100-nanometer-diameter
polystyrene spheres and the other containing 20-micrometer (millionth of a meter)-length
DNA molecules from a virus that infects the common bacterium Escherichia coli.
In each experiment, the solution was injected into the deep end of the chamber
and then electrophoretically driven across the device from deeper to shallower
levels. Both the spheres and DNA strands were tagged with fluorescent dye so
that their movements could be tracked with a microscope.
In the trials using rigid nanoparticles, the region of the 3-D nanofluidic
device where the channels were less than 100 nanometers in depth stayed free
of the particles. In the viral DNA trials, the genetic material appeared as
coiled in the deeper channels and elongated in the shallower ones. These results
show that the 3-D nanofluidic device successfully excluded rigid nanoparticles
based on size and deformed (uncoiled) the flexible DNA strands into distinct
shapes at different steps of the staircase.
Currently, the researchers are working to separate and measure mixtures of
different-sized nanoparticles and investigate the behavior of DNA captured in
a 3-D nanofluidic environment.
In a previous project, the NIST-Cornell researchers used heated air to create
nanochannels with curving funnel-shaped entrances in a process they dubbed "nanoglassblowing."
Like its new 3-D cousin, the nanoglassblown nanofluidic device facilitates the
study of individual DNA strands. More information on nanoglassblowing may be
found in the June 10, 2008, issue of NIST Tech Beat at http://www.nist.gov/public_affairs/techbeat/tb2008_0610.htm#glass.
The work described in the Nanotechnology paper was supported in part by the
National Research Council Research Associateship Program and Cornell's Nanobiotechnology
Center, part of the National Science Foundation's Science and Technology Center
Program. The 3-D nanofluidic devices were fabricated at the Cornell Nanoscale
Science and Technology Facility and the Cornell Center for Materials Research,
and characterized at the NIST Center for Nanoscale Science and Technology. All
experiments were performed at the NIST laboratories in Maryland.