Biologically-driven nanowire assembly

1398_web.jpg“If you want to make something, turn to Mother Nature. From skin to sea shells, remarkable structures are engineered using DNA,” says Adam Lazareck, a graduate student in Jimmy Xu’s lab at Brown University.

Xu is a professor of engineering and physics; his research team is the first to use DNA to direct the growth of nanowires, and the first to create uniform arrays of carbon nanotubes, on which the nanowires were grown.

To grow the nanowires, 15-base pair DNA molecules were introduced into the tips of carbon nanotubes arranged in an array on an aluminium oxide base. Zinc arsenide and complementary DNA sequences coupled with gold nanoparticles were introduced to the arrays, which were then baked in a furnace at a temperature of 600 degrees Celcius. Hybridization of the complementary DNA sequences resulted in the formation of zinc oxide nanowires, with a length of 100-200 nanometres, which can detect and create light. With the application of mechanical pressure they can also generate electricity.

“DNA provides an unparalleled instruction manual because it is so specific,” says Lazareck. This ‘bottom-up’ molecular manufacturing would enable the creation of devices far smaller than those currently made using the conventional methods of moulding or etching designs into circuit components, and may eventually be put to use in the design of computer circuits, fibre optic networks or components for medical diagnostics equipment. 

Related to this is the announcement, made earlier this week, that bacteria grown under certain conditions will sprout nanowires. Yuri Gorby and his colleagues at the Pacific Northwest National Laboratory demonstrated this in Shewanella, a soil bacterium that metabolizes iron.   

The nanowires are actually pili, hollow hair-like structures consisting of proteins which bacterial cells use to swap DNA during the process of conjugation, or to inject toxins into host cells.

In Gorby’s experiments, the Shewenella cells sprouted pili arranged in bundles that grew to hundreds of micrometres in length. The pili were capable of transferring electricity between the cells; they “literally reach out and connect cells…to form an electrically integrated community,” Gorby says.

Whereas electricity in most biological system is carried over short distances by ions, the electricity shuttled between the bacteria in Gorby’s experiments is carried by electrons over distances of hundreds of micrometres.

Gorby repeated the work using mutant bacterial strains lacking the cytochrome oxidase complex, which normally transports electrons during respiration. The pili formed by these cells conducted electricity very poorly, suggesting that cytochromes are responsible for transporting electrons along the pili.