World’s first nanofluidic device with complex 3-D surfaces
Analysts at the Commerce Department’s State Institute of Standards and Technology and Cornell School have capitalized on a method for manufacturing integrated circuits at the nanometer level and used it to develop a strategy for engineering the very first nanoscale fluidic device with complicated three-dimensional surfaces. As explained in a paper published online today in the book Nanotechnology, the Lilliputian chamber is a prototype for future tools with custom-designed surfaces to manipulate and measure various kinds of nanoparticles in solution. Among the potential applications for this technology : the processing of nanomaterials for producing ; the separation and measuring of complicated nanoparticle mixtures for drug delivery, gene treatment and nanoparticle toxicology ; and the isolation and confinement of individual DNA strands for systematic study as they are compelled to unwind and lengthen ( DNA often coils into a ball-like shape in solution ) in the shallowest passages of the device. Nanofluidic devices are customarily built by etching small channels into a glass or silicon wafer with the same lithographic procedures used to make circuit patterns on PC chips.
These flat rectangular channels are then crowned with a glass cover that is bonded in effect.
Due to the limitations inherent to conventional nanofabrication processes, just about all nanofluidic devices so far had straightforward geometries with just a few depths. This limits their ability to split mixtures of nanoparticles with different sizes or observe the nanoscale behavior of biomolecules ( like DNA ) in detail. To unravel the difficulty, NIST’s Samuel Stavis and Michael Gaitan grouped with Cornell’s Elizabeth Strychalski to develop a lithographic process to fabricate nanofluidic devices with complicated 3-D surfaces. As a demonstration of their strategy, the analysts made a nanofluidic chamber with a “staircase” geometry etched into the floor. The “steps” in this staircaseeach level giving the device a gradually increasing depth from ten nanometers ( roughly six thousand times smaller than the width of a human hair ) at the top to 620 nanometers ( marginally smaller than an average micro-organism ) at the bottomare what give the device its capability to manipulate nanoparticles by size in the same way a coin sorter separates nickels, cents and quarters. The NIST-Cornell nanofabrication process implements grayscale photolithography to build 3-D nanofluidic devices.
Photolithography has been used for years by the semiconductor industry to harness the power of light to engrave microcircuit patterns onto a chip. Circuit patterns are outlined by templates, or photomasks, that allow different amounts of light to turn on a photosensitive chemical, or photoresist, sitting on top of the chip material, or substrate.
Traditional photolithography uses photomasks as “black-or-white stencils” to get rid of either all or not one of the photoresist according to a set pattern.
The “white” parts of the patternthose that permit light throughare then etched to a single depth into the substrate. Grayscale photolithography, on the other hand, uses “shades of gray” to turn on and sculpt the photoresist in 3 dimensions. To explain, light is broadcast through the photomask in varying degrees according to the “shades” outlined in the pattern. The quantity of light allowed thru determines the quantity of exposure of the photoresist, and, in turn, the quantity of photosensitive chemical removed after development.
The NIST-Cornell nanofabrication process takes advantage of this characteristic, permitting the analysts to transfer a 3-D pattern for nanochannels of many depths into a glass substrate with nanometer precision employing a single etch. The result’s the “staircase” that gives the 3-D nanofluidic device its flexibility.
Size exclusion of nanoparticles and confinement of individual DNA strands in the 3-D nanofluidic device is accomplished using electrophoresis, the technique of moving charged particles thru a solution by causing them forward with an applied electrical field. In these novel experiments, the NIST-Cornell analysts tested their device with 2 different solutions : one containing 100-nanometer-diameter polystyrene spheres and the other containing 20-micrometer ( millionth of a meter ) -length DNA molecules from a pathogen that infects the common micro-organism 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 less deep levels.
Both the spheres and DNA strands were tagged with fluorescent dye so that their movements might be tracked with a microscope. In the trials using stiff nanoparticles, the area of the 3-D nanofluidic device where the channels were less than a hundred nanometers in depth stayed freed from the particles. In the viral DNA trials, the genetic material appeared as coiled in the deeper channels and lengthened in the more shallow ones. These results show the 3-D nanofluidic device successfully excluded stiff nanoparticles based mostly on size and misshapen ( uncoiled ) the flexible DNA strands into distinct shapes at different steps of the staircase. Now , the analysts are working to split and measure mixtures of different-sized nanoparticles and research the behavior of DNA caught in a 3-D nanofluidic environment.
In a prior project, the NIST-Cornell analysts used heated air to make nanochannels with curving funnel-shaped entrances in a technique they dubbed “nanoglassblowing.” Like its new 3-D cousin, the nanoglassblown nanofluidic device helps the study of individual DNA strands.
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