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TRAPPING nanoparticles in cages made of DNA could finally allow them to self-assemble into transistors, metamaterials and even tiny robots. The technique should prevent the nanoparticles clumping together at random, one of the biggest problems with nanoscale self-assembly. One idea for making nanoscale building kits is to coat gold nanoparticles with short sequences of single-stranded DNA. The idea is to design the DNA strands in such a way that they will bond with other strands and join the nanoparticles together in a 3D structure. But the technique has never worked well because the random position of the DNA strands on the nanoparticles makes them tend to stick together in clumps. Now, Alexei Tkachenko and Nicolas Licata from the University of Michigan, Ann Arbor, have come up with a solution: trap the nanoparticles in a cage where the bars are made of DNA, and then stack the cages to form nanostructures. To create the cage, they start with DNA helices tipped at each end with a short sequence of single-stranded DNA. Because these strands only stick to their complementary sequences on the other DNA helices, the strands bind together in a way that forms a cage structure. Only when the single strand attached to the nanoparticle binds to its partner will the cage close. The result is a nanoparticle trapped inside a tetrahedral cage which has a single strand of DNA sticking out at each vertex. This symmetrical arrangement of strands is important because it prevents the cages clumping together at random. Instead, the strands bind to strands on other cages, linking the cages together forming a 3D structure (Physical Review E, DOI: 10.1103/PhysRevE.79.011404). Eventually, says Tkachenko, you should be able to feed a design into a computer which will choose the type of cage and the sequences of DNA needed to build your structure. Oleg Gang from the Brookhaven National Laboratory in New York state says this avoids random clumping because the strands on the vertices can only bind when the relative positions and orientations of the cages are correct. Tkachenko is confident that they can now move from the drawing board to the test tube. That will mean tackling various challenges such as ensuring that the mixture produces caged particles rather than other possible structures, perhaps by controlling the temperature of the mixture to favour certain structures over others. It may then be possible to make devices such as nano-circuits and metamaterials. If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
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Wednesday, March 18, 2009
DNA cages guide nanoparticle self-assembly
Hubble snapshot shows the moons transiting in front of planet
MSNBC A new Hubble photograph captured a rare alignment of four of Saturn's moons lining up in front of their planet. The snapshot, taken on Feb. 24 with the Hubble Space Telescope, shows the moons transiting in front of Saturn. The moons, from far left to right, are the white icy moons Enceladus and Dione, the large orange moon Titan, and icy Mimas. Due to the angle of the sun, they are each preceded by their own shadow. These rare moon transits only happen when the tilt of Saturn's ring plane is nearly "edge on" as seen from the Earth. Saturn's rings will be perfectly edge on to our line of sight on Aug. 10 and Sept. 4, 2009. Unfortunately, Saturn will be too close to the sun to be seen by viewers on Earth at that time. This "ring plane crossing" occurs every 14-15 years. In 1995-96 Hubble witnessed the previous ring plane crossing, as well as many moon transits, and helped to discover several new moons of Saturn. Early 2009 was a favorable time for viewers with small telescopes to watch moon and shadow transits crossing the face of Saturn. Titan, Saturn's largest moon, crossed Saturn on four separate occasions: Jan. 24, Feb. 9, Feb. 24 and March 12, although not all events were visible from all locations on Earth. Italian Galileo Galilei — often referred to as the father of astronomy — was the first to observe Saturn through a telescope in 1610. Dutch mathematician and astronomer Christian Huygens discovered Titan in 1655 and, 350 years later, the ESA probe named for him touched down on Titan (on Jan. 14, 2005), giving the world its first views of the surface of the mysterious, icy world. Giovanni Domenico Cassini, a French/Italian astronomer, discovered Dione (in addition to others) and the German-born Englishman, William Herschel, discovered Mimas and Enceladus. These pictures were taken with Hubble's Wide Field Planetary Camera 2 when Saturn was at a distance of roughly 780 million miles (1.25 billion kilometers) from Earth. Hubble can see details as small as 190 miles (300 kilometers) across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet.Hubble snapshot shows the moons transiting in front of planet
These pictures were taken with Hubble's Wide Field Planetary Camera 2 when Saturn was at a distance of roughly 780 million miles from Earth.
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