Colloids twist like DNA

French scientists have used magnetic colloids to make self-assembling, helical structures reminiscent of DNA. Similar methods could be used to make tiny, self-propelling objects and colloidal models that mimic the assembly of complex, naturally occurring molecules.

The helices are formed from chains of dumbbell-shaped silica spheres, which are larger at one end than at the other and encircled by a magnetic ’waist’ of iron oxide. In a magnetic field, the waists are attracted to each other, but as a new dumbbell approaches the growing chain it rotates to align its waist with the field. Because the dumbbells are asymmetric, the chain begins to twist into a helix.

Jerome Bibette, who led the team at the Industrial Physics and Chemistry Higher Educational Institution in Paris, says the effect resembles the way that subunits of a polymer approach each other in order to avoid steric hindrance. ’You cannot escape from the direction of polymerisation. It’s imposed by nature. That’s the game of chemistry and we have recreated this game with just silica spheres and magnetic waists.’

The work also shows that nanotechnology can imitate nature by creating structures with left- and right-handedness - chirality. ’This is really opening a gate for a large amount of very smart material,’ says Bibette.

A molecule is chiral if its mirror images can’t be superimposed onto each other. Chirality is important in nature because often one version, or enantiomer, of a molecule - the left- or right-handed version - prevails and receptor sites for the molecule are primed to receive this version. DNA, for instance, is predominantly right-handed.

The team’s chiral colloidal structures coil like DNA, but are larger - so their assembly can be filmed in real time under a microscope. This raises the possibility of generating colloidal replicas of chiral molecules, which could, for instance, be used by scientists to model entantiomeric separation. And because the structures are also light activated, Bibette says his work also paves a route to self-propelling objects.

The researchers are now trying to find a way to make their helices stick together when the magnetic field is turned off. ’We have found that polyacrylic acid is very well absorbed on magnetic oxide and when you put these things together, it creates a sticking mechanism which is irreversible. It’s a sort of glue that you can activate with a field,’ says Bibette.

Tim Deming, who studies biomimetic materials at the University of California, Los Angeles, says, ’The formation of chiral structures from such simple components has great potential for development of sophisticated materials.’ But the researchers are still a long way from being able to fully replicate the way molecules self-assemble in nature, he adds.

Hayley Birch