Science 329, 197–200 (2010)

New functional materials can be created by organizing nanoscale building blocks, such as nanoparticles, into precise structures. Assemblies of inorganic nanoparticles can, for example, exhibit collective properties that rely on interactions between the individual species. An appealing strategy for preparing these ensembles is self-assembly, but such organization of nanoparticles in a controllable and predictable manner is difficult to achieve. Eugenia Kumacheva, Michael Rubinstein and colleagues at the University of Toronto and the University of North Carolina at Chapel Hill have now shown that the self-assembly of metal nanoparticles can behave in a similar way to well-known polymerization processes; this allows the shape of the resulting nanostructures to be predicted.

The researchers synthesized gold nanorods with arrowhead-shaped ends that were coated with polystyrene molecules. Dispersed in solution, the rods linked up end-to-end to form chains connected by reversible, non-covalent bonds. This self-assembly could be controlled by reducing the solubility of the polystyrene through changes to the composition of the solvent, allowing linear, branched and cyclic chains to be created.

By examining the growth of the chains, Kumacheva and colleagues found that the self-assembly could be described using the kinetics and statistics of step-growth polymerization. This allowed the architecture and aggregation number of the nanorod structures to be predicted.