To make the nanowires, the researchers modified the major coat proteins of an M13 virus by altering the genome of the virus. They added tetra-glutamate groups to each of the 2700 major coat proteins. The glutamate binds to metal ions by a process of ion exchange.

"Once we've altered the genes of the virus to grow the electrode material, we can easily clone millions of identical copies of the virus to use in assembling our batteries," said Angela Belcher of MIT. "We chose cobalt oxide because it has very good specific capacity, which will produce batteries with high energy density."

To coat the virus with cobalt oxide, the team incubated the virus particles in an aqueous solution of cobalt chloride at room temperature. Then they reduced the mixture with a solution of NaBH4. Following spontaneous oxidation in water, the result was nanowires that consisted of 2–3 nm diameter Co3O4 nanocrystals along the length of the virus.

The wires were 6 nm in diameter and 880 nm long, the same dimensions as the virus. "We can make them in larger diameters," said Belcher, "but they are all 880 nm in length."

The nanowires showed similar behaviour when acting as an electrode to Co3O4 nanoparticles produced at temperatures of more than 500 °C.

The team also modified the virus further by introducing a gold-binding peptide to a proportion of the coat proteins. Treating the virus with 5 nm gold particles in suspension before the cobalt oxide treatment produced composite nanowires containing both cobalt oxide and gold nanoparticles.

According to the researchers, the composite nanowires generated higher initial and reversible lithium storage capacity than the pure cobalt oxide nanowires. The team estimates that the hybrid wires have a specific capacity at least 30% greater than the pure nanowires.

What's more, the negatively charged modified virus particles will form ordered 2D liquid crystals on films of linear poly(ethylene imine)/poly(acrylic acid). By carrying out this step before growing cobalt oxide on the viruses, the team made organized assemblies of nanowires over a length scale of around 10 cm.

The researchers reported their work in Sciencexpress.