“The greatest challenge in this field is to learn how the complex structures [that is, the nanowire assemblies] can be structurally coded at the level of building blocks,” Nick Kotov of the University of Michigan told nanotechweb.org. “Biology offers a wonderful solution in the large variety of protein structures self-organizing in cells.”

Kotov and colleagues used two types of biological connectors to assemble cadmium tellurium (CdTe) nanowires - a complimentary antigen-antibody pair (bovine serum albumin (BSA) and the anti-BSA antibody IgG) and biotin-streptavidin. The wires themselves had self-assembled from CdTe nanoparticles in an aqueous environment.

The team conjugated each biological connector to a set of nanowires. Then they mixed sets of nanowires with complementary connectors. The nanowires showed a preference for assembling in a crossbar or end-to-side configuration.

“Our greatest challenge in this work was to prove that the nanowire junctions that one can observe in TEM or AFM were indeed formed due to protein-protein interaction and not by the random (evaporation) process,” said Kotov. “The fact that the CdTe nanowires made by self-organization of nanoparticles have strong luminescence was a great advantage in this case. We were able to observe the formation of nanowire-nanowire assemblies in solutions in real-time by monitoring Förster energy transfer from the green-emitting nanowires to the red-emitting ones.”

The team also measured the electrical characteristics of a triangular nanowire structure containing BSA-IgG connectors. Two of the nanowires had diameters of 10-12 nm and the third was about 5 nm in diameter.

Conductive atomic force microscopy revealed that the junctions between the nanowires did not conduct, presumably because of the presence of the protein molecules. According to the researchers, using DNA oligomers as nanowire connectors could reduce the effective separation between the nanowires and lower the tunnelling barrier. Annealing the structures at high temperature to carbonize or remove the biological coatings could also help.

That said, the scientists believe that the gaps between the wires could also provide a site for introducing nanoparticles to create hybrid nanoparticle-nanowire devices.

“Our opinion about the possibility of making viable computer circuits by bottom-up assembly was and still is sceptical,” said Kotov. “However, the idea itself is appealing and should be verified experimentally.”

Now the scientists say they are working on “the bioassembly of metallic nanowires and design of nanowire-nanowire and nanowire-nanoparticle structures with controlled dimensions of the gap between them”.

The researchers reported their work in Nano Letters.