DNA nanomachines: On the right track

Nature 465, 202–205 (2010);

Nature 465, 206–210 (2010)

Credit: © 2010 NPG/Paul Michelotti

DNA is a natural polymer that is frequently employed as a synthetic nanoscale construction element. The molecule has been used to build intricate structures and primitive machines. In particular, bipedal DNA walkers have been created that can walk along one-dimensional tracks. Now, two independent teams of researchers have extended the capabilities of these nanomachines, allowing them to go on autonomous journeys guided only by their surroundings and to pick up cargoes as they move along nanoscale assembly lines.

Milan Stojanovic and colleagues at Columbia University, Arizona State University, the University of Michigan and the California Institute of Technology developed spider-shaped walkers that can travel across a surface. The spiders are composed of an inert protein 'body' and DNA enzyme 'legs' that can cleave a DNA strand from the surface. The surface itself is made from DNA origami — a technique in which a long single strand of DNA is folded into a predetermined shape — and has a track of cleavable DNA strands with base sequences complementary to those of the spider's legs laid out on top of it. The spider seeks out these strands and can thus move along a path programmed into the origami surface.

Nadrian Seeman and colleagues at New York University and Nanjing University have also developed a DNA walker that operates on an origami track. Their walker, however, has single-stranded DNA 'feet' that move along the origami surface with the help of externally supplied strands. Moreover, the walker has 'arms' that can pick up different types of gold nanoparticles from programmable DNA machines positioned along the track.

Data storage: To the limit and beyond

Nature Photon. doi:10.1038/nphoton.2010.90 (2010)

Conventional approaches to magnetic data storage are expected to offer a maximum storage density of about one terabit per square inch. Any attempts to go above this limit by using smaller bit sizes will be thwarted by the problem of superparamagnetism — random changes in the magnetization direction of small particles. It is possible to avoid the superparamagnetic effect, but only at the expense of increasing the switching field to values above those that can be produced by the magnetic write head.

Researchers have been exploring various ways to overcome these challenges, and now Barry Stipe and co-workers at Hitachi research centres in the US and Japan have shown that two new approaches — thermally assisted magnetic recording and the use of bit-patterned recording media rather than continuous media — can be combined to solve many of the problems associated with superparamagnetism. Thermally assisted magnetic recording involves heating the storage medium to reduce the switching field.

Of course, heating very small regions is a technical challenge, and the Hitachi team include a thin-film waveguide and a plasmonic nanoantenna in their recording head for this purpose. The waveguide directs near-infrared light from a laser diode to the nanoantenna, which focuses the light into a region measuring about 30 nm across. Stipe and co-workers report storage densities of one terabit per square inch — which is the limit of conventional approaches — and claim that much higher densities are possible.

Silicene: Flatter silicon

Appl. Phys. Lett. 96, 183102 (2010)

Graphene's many interesting and attractive properties derive from the hexagonal, two-dimensional arrangement of its carbon atoms. Carbon is not alone, however, in being able to form such structures: boron nitride, for example, can also do it. A silicon analogue to graphene is attractive because it could be synthesized and processed using mature semiconductor techniques, and more easily integrated into existing electronics. Although theorists have speculated about silicene, it has not been observed. Now, Bernard Aufray, Hamid Oughaddou and colleagues at CNRS, the University of Central Florida, the University of Cergy Pontoise, and CEA have observed silicon structures that are suggestive of silicene.

Using a scanning tunnelling microscope, the researchers studied self-aligned silicon nanoribbons deposited onto a silver crystal with near-atomic resolution. The images revealed hexagons in a honeycomb structure similar to that of graphene. Density functional theory calculations showed that silicon atoms tend to form such honeycomb structures on silver, and adopt a slight curvature that makes the graphene-like configuration more likely.

In addition to their potential compatibility with existing semiconductor techniques, silicon nanoribbons have the advantage that their edges do not exhibit oxygen reactivity.

Bioimaging: Doing chemistry in tumours

Angew. Chem. Int. Ed. 49, 3375–3378 (2010)

Pre-targeting tumours with an antibody and subsequently binding a radiolabel probe to the tumour-bound antibody provides superior image contrast, but current pre-targeting systems that use biotin–streptavidin suffer from immune reactions. Now Marc Robillard and co-workers at the Philips Research High Tech Campus in the Netherlands have exploited the fast reaction kinetics and selectivity of a Diels–Alder reaction as a chemical pre-targeting approach to imaging tumour antigens in mice.

Robillard and colleagues attached trans-cyclooctene (an eight-membered-ring molecule) to an antibody that binds to the CC49 antigen found on many solid tumours. A second molecule (an electron-deficient 111In-labelled tetrazine), when administered into the mice, reacts with the cyclooctene through the inverse-electron-demand Diels–Alder reaction to form a radiolabelled conjugate on the tumour-bound antibody. Computed tomography imaging showed pronounced localization of radioactivity in the tumour, with low uptake in blood and non-targeted tissues such as the liver. Besides the tumour, radioactivity was seen in the bladder and kidney, suggesting that the probe was eliminated through the urinary tract. Antibodies that did not have the cyclooctene accumulated in the tumour but showed low 111In radioactivity, indicating that the cyclooctene-modified antigen is specific and tetrazine accumulated only in tissues containing the cycooctene-modified antibody.

This reaction can potentially be effective for imaging low concentrations of tumour antigens and for tracking other antibodies inside the body.