“After successfully filling multiwalled carbon nanotubes with a variety of polar and nonpolar liquids, such as water, glycerin, alcohols, benzene, and cyclohexane, it was tempting to try filling large-diameter nanotubes with a particulate fluid,” Yury Gogotsi of Drexel University told nanotechweb.org. “Ferrofluid was a natural choice because it contains small particles (about 10 nm in diameter); numerous ferrofluids are commercially available; and the practical benefits from filling nanotubes with magnetic particles were obvious.”

Gogotsi and colleagues made the carbon nanotubes by chemical vapour deposition into the pores of an alumina membrane. The result was 300 nm-diameter tubes that were open at either one or both ends. Next, the researchers filled the nanotubes with either organic- or water-based ferrofluids containing paramagnetic magnetite (Fe3O4) nanoparticles with an average diameter of 10 nm.

Capillary action enabled the ferrofluid to enter the nanotubes. The carrying fluid then dried to leave magnetic particles. The scientists used sodium hydroxide to remove the alumina template and expose the individual carbon nanotubes. They performed this step either before or after filling the nanotubes with ferrofluid.

“Filling of nanotubes with the ferrofluids appeared to be easier than anticipated,” said Gogotsi. “We initially used strong magnets to guide the fluid into the tubes, but the effect of spontaneous penetration of wetting fluids into capillaries was sufficient. We have also recently demonstrated penetration of the same ferrofluid into 40 nm nanotube channels.”

The team did use a magnetic field to control the magnetic anisotropy of the structures. They estimated that around 70 000 nanoparticles entered each tube.

When suspended in liquid, the resulting paramagnetic nanotubes aligned with an applied magnetic field. The researchers were able to use gold electrodes to orient the nanotubes in the plane of a silicon wafer or to make them stand up perpendicular to the surface.

“The filled nanotubes can be used as nanosubmarines externally driven through blood vessels by a magnetic field and transporting attolitres of drugs to specific locations in the body, as well as for medical diagnostics without surgical interference,” said Gogotsi. “They can also be incorporated into smart textiles or films for magnetic recording.”

According to the researchers, other applications for the magnetic nanotubes include cantilever tips in magnetic force microscopes, magnetic stirrers or magnetic valves in nanofluidic devices. And aligned arrays of magnetic nanotubes could be used instead of nanoposts in fluidic chips for DNA separation. “We should also be able to control the properties of nanotube-covered material surfaces by applying a magnetic field,” said Gogotsi.

Now the team is exploring biomedical applications for the nanotubes and carrying out cytotoxicity studies. “We are also working on modification of tube surfaces to control their hydrophilicity,” said Gogotsi. “We believe that we can create a large number of magnetically-controlled and nanopositioned tools for delivery and diagnostics on cellular and subcellular levels using our magnetic nanotubes.”

The scientists, who reported their research in Nano Letters, have filed for a patent on their work.