DNA cages guide nanoparticle self-assembly

• 17 March 2009 by Jessica Griggs
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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.

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