TECHNOLOGY

DNA cages hold the key to well-organised nanoparticles 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

It’s time to measure seconds even more accurately ATOMIC clocks, currently the size of fridges, could shrink to the microscale thanks to a new way of measuring the second. The technique could also see aluminium displace caesium as the standard of time. The world’s most accurate atomic clocks are at the National Institute of Standards and Technology (NIST) at Boulder, Colorado. Known as fountain clocks, they send clouds of caesium atoms through a vacuum chamber in a magnetic field. Large atoms like caesium and aluminium have multiple energy levels that are so close together they appear indistinguishable. The magnetic field separates these levels into two “hyperfine” states. The chamber is also filled with 20 | NewScientist | 14 March 2009

microwaves, which excite the atoms. They then emit light as they drop to the lower hyperfine state. The microwave frequency that maximises this fluorescence is used to define the length of a second, currently the time it takes for 9,192,631,770 cycles of microwave radiation. All this takes place in a large vacuum chamber and so fountain clocks are big devices, about a cubic metre in size. That makes it hard to keep the magnetic field and the device’s temperature uniform over the whole area, which can lead to errors of measurement. That’s why Andrei Derevianko and Kyle Beloy of the University of Nevada in Reno and colleagues have come up with the idea of

How to trap a nanoparticle DNA cages form from double helices tipped at each end with complimentary single DNA strands. These stick together in a way that forces the helices to form a cage. It will only close using DNA attached to the nanoparticle, trapping it inside

SINGLE STRANDS OF DNA

DNA HELICES

GOLD NANOPARTICLE

(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. Jessica Griggs ■

trapping the atoms in place using lasers. This means their energy states could be monitored in an area only a few micrometres across, potentially leading to more accurate measurements. This is difficult to get right, though, because the lasers distort an atom’s energy levels in a complex way, making it impossible to define a jump that equates to a second. Derevianko’s team overcome this

levels is the same as if the atoms are in vacuum,” says Derevianko. Using this method, the team has calculated the second to be 1506 million cycles of microwaves for aluminium-27 and 2678 million cycles for gallium-69. Although the atoms can be trapped in an area only a few micrometres across, the lasers, and cooling and computing equipment will add to the bulk. Nevertheless, the team say the clocks may be portable and could be used in space-based experiments that require extremely accurate timekeeping, such as those for detecting gravitational waves or for testing Einstein’s theories. Tom Heavner, who works on fountain clocks at NIST, describes the proposal as forward-thinking and original. “It is a really clever way to meld together the old-style clocks with new laser technology,” he says. Anil Ananthaswamy ■

“It is a really clever way to meld together the old-style clocks with new laser technology” problem by finding a laser frequency that alters both hyperfine states by exactly the same amount – a trick that works in aluminium and gallium but not as well in caesium (www. arxiv.org/abs/0808.2821). “Then, the energy difference between the