Engineers are used to combining two or more materials to make a composite with superior properties, such as the strength and lightness achieved by setting carbon fibres in an epoxy resin.
For those working at the nanoscale, however, the difficulty of combining materials at that level means there have been few composite materials to play with. Now that has changed thanks to chemists in China who have worked out a simple way to add a prime engineering property 鈥� the magnetism of iron particles 鈥� to carbon nanotubes.
Nanotubes have unique electrical, optical and mechanical properties that lend themselves to all sorts of nano-engineering applications, and because they are hollow, they can also be used to transport catalysts or drugs. However, magnetically guiding nanotubes containing drugs, say, to where they are needed has until now been difficult, as the tubes are only very weakly magnetic. Previous attempts to address this have involved filling the tubes with magnetic material, but this has left no space for other cargo.
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Materials scientist Cao Huiqin and colleagues at Donghua University in Shanghai have now worked out a way to give the surface of the nanotubes a magnetic coating. Using a straightforward chemical reaction they peppered the outer wall of the tubes with nanoparticles of magnetic iron oxide 鈥� otherwise known as rust (Journal of Magnetism and Magnetic Materials, vol 305, p 321).
To make their 鈥渕agnetic nanocomposite鈥�, Huiqun鈥檚 team added nanotubes 20 nanometres in diameter to a solution of nitric acid and iron nitrate. They heated the acidic brew to 120 掳C for 4 hours and then stirred in a solution of ammonia until the solution was alkaline, with a pH of 10. This caused the iron to combine with oxygen and precipitate out as iron oxide.
After filtering the nanotubes and washing them with distilled water, the scientists examined them under a transmission electron microscope and found the precipitate had 鈥渄ecorated鈥� the nanotubes along their length with iron oxide particles less than 5 nanometres across, making the nanotubes strongly magnetic.
The team suggest several applications for their idea. Since the tubes remain hollow, they could of course be used to create the sought-after magnetically guided drug-delivery systems, delivering medicines to precise sites in the body.
Also, future generations of super-dense disc drives or memory chips could harness the magnetic particles for data storage. And because iron oxide is itself an important catalyst, the carbon nanotubes could be used to ferry it into reactions where minute amounts of the substance are needed.
