"We focused on cubic magnetite nanoparticles because we thought the shape may be special - the cube is the simplest shape that can be magnetised along an axis that is unfavourable for maximising van der Waals forces," says Rafal Klajn of the Weizmann Institute of Science in Israel. "We thought this might lead to some interesting assemblies but we did not suspect it would lead to a helical structure."

According to Klajn, magnetite is the most abundant magnetic material on Earth and magnetite nanocrystals have been studied for decades. They're even found in organisms such as birds, helping them to orient themselves with respect to the Earth’s magnetic field. "A recent paper describes iron oxide nanoparticles created nearly 5 billion years ago that have been found in a meteorite – that would make them the oldest known nanoparticles, and now we find they have these interesting properties for self-assembly."

Helical advantage

Although their potential applications have not yet been investigated, Klajn suggests that the optical and magnetic properties of these helical magnetite nanostructures may be interesting. The structures may also prove useful for templating and catalysis.

Helices are chiral and can twist to the right or left; the two mirror-image versions cannot be superposed on each other. The chirality of a molecule can affect how it interacts with other chiral molecules. This is particularly common in living organisms as many of the molecules in the body are also chiral.

"A few colleagues have suggested that since these structures are chiral they may have applications in asymmetric catalysis [where one chiral product is produced in preference to the other]," Klajn tells nanotechweb.org.

The right concentration

Klajn and his colleagues at the Weizmann Institute of Science and the University of Illinois in the US formed the helical structures by dropping an organic solution of relatively monodisperse magnetite nanocubes onto the surface of diethylene glycol in the presence of a magnetic field. When the concentration of nanocubes at the surface was low, the nanocubes assembled into parallel arrays of one-dimensional belts. However when the concentration was increased, helical structures formed. Klajn suggests that the transition is the result of increasing effective magnetic field as the nanocube concentration increased.

Generally the van der Waals interactions between faces of the nanocubes dominate over corner-to-corner attraction. But magnetite nanocubes can be magnetised so that the magnetic forces preferentially align the cubes corner to corner so that the competition with the van der Waals forces, which preferably align the cube faces, is more evenly matched. "We tested lots of different magnetite shapes but only the cubic shapes have this property," said Klajn.

Next steps

The study gives guidelines for the design of this type of material; Klajn suggests that nanoparticles of other magnetic materials, such as octahedral particles of iron, may also have this property. The researchers plan to investigate the self-assembly of different nanoparticle shapes of iron, nickel and cobalt, as well as the effect of functionalising the surfaces of magnetite nanocubes.

Full details of the research are reported in Science Express.