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Posted: Nov 15, 2006

Lithography-free formation of nanopores in low-cost plastic materials

(Nanowerk Spotlight) Synthetic nanopores are promising biosensors, possibly as a robust and versatile replacement for their biological counterparts in characterizing DNA, RNA, and polypeptides. In the past few years since their first introduction, synthetic nanopores have been found in a wide range of biological and nonbiological applications, including characterization of double-stranded DNA length and folding, detection of immune complexes, profiling of optical traps, and basic studies of nanoscale ion transport mechanisms. Given the broad technological importance of synthetic nanopores, it is highly desirable to develop a reliable technique for fabricating these devices using low-cost materials. Researchers at Brown University now report a systematic study of nanopore formation in a plastics system. They also developed a lithography-free technique for fabricating nanopores with biomolecular sensing capabilities.
Among the promising methods for fabricating synthetic nanopores, it was found by that one can use transmission electron microscopy (TEM) to drill and shrink holes in SiO2 with single nanometer precision. An intriguing finding was that the pore diameter shrinks linearly with the exposure time. This phenomenon was also found in another study using SiO2 and scanning electron microscopy (SEM). It was proposed that a surface-tension-driven viscous flow mechanism can be used to explain the findings, for example, in predicting a critical radius below which a pore shrinks and above which it expands.
Recent studies on the pore shrinking mechanisms went further by studying the change in the atomic compositions of the materials surrounding the pore region and the effects of varying energies of electron beams in materials modification.
"The fact that the surface-tension-driven mass flow can be used to successfully account for the nanopore shrinking in fluidized SiO2 suggested to us that one should be able to apply the same principle to other glassy materials such as low-cost plastics" Dr. Sean Ling explains to Nanowerk.
Ling, who is an Associate Professor of Physics at Brown University, found that one can use lasers to heat shrink a micropore into a nanopore in plastics. They established that the linear time dependence of the pore diameter during shrinkage is indeed generic to viscous materials. "Also we found that Laplace's principle of surface tension can be used to describe the shrinking dynamics" he says.
The researchers found that an initial hole of hundreds of micrometers in diameter can be reduced to a few hundred nanometers in a controlled manner using the laser transmission as a realtime feedback signal. The radius of the shrinking pore scales linearly with time and the shrinking rate is temperature dependent.
"The model we propose here is generic and should apply to pore shrinking processes in any viscous material where the dynamics is overdamped" says Ling. "A key to nanopore fabrication is a feedback mechanism with which one can stop the pore shrinking by turning off the heating power (laser, here) for fluidization. The optical feedback technique we used for our experiments (see figure on the left) is inadequate below the wavelength limit. A more sensitive feedback control mechanism is needed to shrink the pore further into the nanometer regime."
Image: (a) A schematic of the experiment in air. An argon ion laser (blue) is used for fluidizing the plastic material around the pore. A low-power He-Ne laser (red) covers the pore from the opposite direction for imaging purposes. (b) A pore shrinks in radial direction during the process. After shrinking, a ∼200 µm hole (left, prior to laser heating) becomes ∼1000 times smaller (both images taken from the heating - laser - exposure side). The dotted arrows in the left image indicate the radial shrinking dynamics. (c) SEM image looking into the pore from the large opening, facing the opposite side of membrane from (b). A funnel like or conical 3D configuration is observed. The dark spot in the center is the narrowest opening, i.e., the “neck” of the pore, signified with a dark arrow. (Reprinted with permission from American Chemical Society)
A possible solution suggested by Ling is to use the ionic conductance through the pore to monitor the shrinking process in aqueous conditions. Though he points out there is still a need to understanding the 1/f noise in the ionic current through the nanopores in these new devices.
A major push of the nanopore field is to use these devices for rapid DNA sequencing. Ling and his team demonstrated the potential of this simple method for fabricating biomolecule sensors by demonstrating a detection experiment of λ-DNA molecules using these nanopores.
"For applications such as single molecule analysis, DNA sequencing, etc. our technique (laser) and materials (plastics) will lead to dramatic cost reductions in fabricating such nanopore devices" says Ling.
These findings are reported in a recent paper in Nano Letters ("Lithography-Free Formation of Nanopores in Plastic Membranes Using Laser Heating").
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