In nature, complexity and order can often result from self-assembly — the spontaneous and reversible aggregation of the components of a disordered system into an ordered one. But so far, researchers have mainly studied systems of one or two components, whereas nature can mix many more — as is the case, for example, with the assembly of ribosomes, the protein-synthesizing organelles in cells. On page 999 of this issue, Erb et al. show that man-made multi-component systems can assemble under the action of a magnetic field (R. M. Erb et al. Nature 457, 999–1002; 2009).
Self-assembly processes are usually driven by intermolecular forces, including hydrophobicity and the formation of hydrogen bonds, as opposed to the more-stable covalent forces. But external effects such as electrical and magnetic forces can also lend a hand. The components of such a system can range from metallic nanoparticles to biopolymers and macroscopic objects.
The system studied by Erb and colleagues is a ferrofluid — a mixture of water and magnetic nanoparticles — to which larger, micrometre-sized colloidal particles of different kinds are added. When the fluid is subjected to a magnetic field, the particles can align their magnetic dipoles either with or opposite to the field depending on their magnetic properties with respect to the ferrofluid. When further magnetic nanoparticles are added, what might previously have behaved as a paramagnetic colloidal particle (one that aligns with the field) can become a diamagnetic one (aligning opposite to the field), and vice versa. Such changes cause the particles to assemble into diverse structures.
The type of structure that forms depends on the size and magnetic properties of the components. Thus, the geometries of the assemblies can feature clusters of smaller particles at the poles of a larger central particle or at its equator (pictured), or even more complex, flower-shaped arrangements.
Erb et al. show that these structures have a remarkably uniform shape, and that they do not aggregate among themselves because their large, central paramagnetic particles repel each other. And they can be made to persist even after the field has been turned off. To do this, their components are made to stick to each other by attaching two kinds of molecule to them that act like Velcro once they are in close proximity. Thus, when the magnetic field is turned off and the structures are dried, they don't fall apart and can be studied individually.
In the future, the use of a larger number of smaller and differently shaped components may help our understanding of complex assemblies to blossom, together with our ability to make them. A possible outcome could be the development of materials with tailored optical properties that would be useful for making biosensors.
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Tonzani, S. Let assemblies bloom. Nature 457, 974 (2009). https://doi.org/10.1038/457974a