“MS-1, rod-shaped bacteria [found] in freshwater, swim and orient themselves along the Earth’s magnetic field due to the iron oxide nanoparticles inside the bacteria,” researcher Jong-in Hahm told nanotechweb.org. “Linearly arranged nanoparticles inside MS-1 are single-domain, single-crystalline and highly monodisperse when compared to synthetically grown nanoparticles of similar sizes.”
Hahm and colleagues coated a silicon substrate with a solution of the bacteria before leaving the culture to grow overnight in a magnetic field. Then the scientists applied distilled water to break open the bacterial cell walls and reveal the iron oxide nanoparticles inside. They removed the cellular membranes by a heat treatment in air and then deposited carbon nanotubes by chemical vapour deposition using the magnetite nanoparticles as catalysts.
The resulting nanotubes typically contained 12-14 concentric walls of carbon tubes and had an average diameter of 13 ± 3.6 nm. They grew preferentially along two directions, one at roughly 60° to a reference axis and one at about 130° to the axis.
“We reasoned that the homogeneous size of the nanoparticles can contribute to carbon nanotube growth with better control over diameter, and the crystallinity and magnetic properties of these nanoparticles can be exploited to control the orientation of carbon nanotube growth,” said Hahm.
According to the scientists, the cuboctahedral iron oxide nanoparticles orient themselves so that their [111] direction is roughly parallel to the magnetic field. This results in four possible carbon nanotube growth directions. The nanotubes appeared to grow along the <100> directions of the nanocrystals.
The carbon nanotubes did not align when grown on nanoparticles whose parent bacteria were not subjected to a magnetic field overnight.
“Our approach may facilitate easy integration of carbon nanotubes for use as nanoelectronic devices and chemical/biological sensors,” said Hahm. “We are interested in identifying new bacteria and viruses, and engineering them in order to assemble easily and inexpensively various one-dimensional nanomaterials with better structural and orientational control.”
The researchers reported their work in Applied Physics Letters.