SiN Nanopores Enable Electrical Bar-Code Scanning of Tagged DNA
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SiN Nanopores Enable Electrical Bar-Code Scanning of Tagged DNA As the cost of DNA sequencing has decreased dramatically, genome sequences of numerous pathogens are readily available, and their number is growing daily. However, many pathogens are genetically similar, and differentiating between them has proven to be a difficult process with current methods. To facilitate more timely, economical, and accurate investigation of infectious agents, laboratories around the world are investigating various purely electrical schemes of detecting proteins. In the February 10 issue of Nano Letters (DOI: 10.1021/nl00058y; p. 738), A. Meller and M. FrankKamenetskii, professors at Boston University (BU), and their co-workers have introduced just such a technology: By electrostatically pulling a tagged, doublestranded DNA strand through a nanoscale opening on a chip, they detect changes in ion current that discriminate between tagged and untagged sites. The researchers milled single 4–5 nm diameter nanopores in 30-nm thick freestanding SiN membranes using focused electron beam lithography. The 20 μm by 20 μm membrane with the pore is embedded on a 25 mm2 silicon chip that separates two miniature fluid chambers filled with a 1 M solution of KCl. When a positive bias voltage is applied across the membrane, the DNA, which holds a negative charge, threads through the nanopore. This opening is just large enough to allow the molecule to pass through it without bunching (see Figure 1). Senior research associate H. Kuhn prepared DNA molecules, which contain specific sequences bound to peptide nucleic acid (PNA) probes. As the DNA
Study of Mesoscale Epitaxy Using Colloidal Particles Reveals Dynamics Similar to Atomic-Scale Film Growth Epitaxial film growth is a widespread and powerful fabrication method in modern materials research. Research into the physical principles that govern the growth of surface films atop a template crystalline substrate has enabled scientists to more effectively predict and design materials that possess desirable properties. In the January 22 issue of Science (DOI: 10.1126/science.1179947; p. 445), researchers at Cornell University report that many of the same rules that govern atomic-scale epitaxial film growth also apply to surface layers grown epitaxially using colloidal particles, although the difference in size scale introduces some mechanistic dif264
Figure 1. Schematic depicts a DNA molecule with two peptide nucleic acid (PNA) probes threaded through a 4–5 nm nanopore in a 30-nm thick free-standing silicon nitride membrane under a voltage bias. Inset: A transmission electron microscope image of a 4 nm nanopore. Reproduced with permission from Nano Letters 10 (2) (2010) 738; DOI: 10.1021/nl100058y. © 2010 American Chemical Society.
threads through the nanopore, the molecules partially block the opening between the chambers which reduces the ion current between them. Because the PNAtagged regions have a greater crosssectional area than the bare DNA, as these sites pass through the membrane they increase
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