“We are trying to mimic life itself,” said Chad Mirkin of Northwestern. “Much like proteins, which must fold into complex structures in order to function properly, we have designed new materials that also form complex structures through the process of self-assembly.”

Mirkin and colleagues made the nanorods by electrodepositing gold into porous alumina templates. Then they carried out electrochemical polymerization of pyrrole to produce a segment of oxidized polypyrrole polymer next to the gold. Once the rods were in place, the team dissolved away the alumina template using sodium hydroxide solution. The gold portions of the rod were 400 nm in diameter on average, while the average diameter for the polymer sections was 360 nm.

Suspending the nanorods in water caused them to self-assemble into unusual structures, including bundles, tubes and sheets, depending on their composition. The gold acted as a hydrophilic region, while the polypyrrole segment was hydrophobic. The researchers believe that the driving force for self-assembly is the strong interactions between the polymer ends of the different rods.

Rods with a gold segment 1.8 microns long and a polymer region 8.8 microns long formed bundle structures. In contrast, nanorods with a total length of 4.5 microns and a gold:polymer ratio of 1:4 assembled into tubes with a diameter of 60 microns. The walls of these tubes consisted of a single layer of nanorods, with the polymer ends of the rods pointing towards the inside of the tube.

“We also discovered that the alumina template we used to build the rods initially is essential in guiding the assembly process,” said Mirkin. “Without the orientation the template provides, the rods do not form bundles, sheets or tubes.”

If the scientists broke up the nanorod assemblies by sonication, the rods dispersed irreversibly, and would not repeat the self-assembly process.

“The research clearly shows that some unnatural building blocks, such as the gold-polymer rods, need assistance in order to form higher-ordered structures,” said Mirkin. “This means that when we work with building blocks that are larger than molecules but smaller than macroscopic objects, we should consider building materials in a completely new way - by using templates to help guide the assembly process and reduce the large number of assembly pathways potentially available to the building blocks.”

The researchers reported their work in Science.