"These structures for carbon represent a new morphology that, to the best of our knowledge, has not been reported before," Mahendra Sunkara of the University of Louisville told nanotechweb.org. "The nanostructures are externally conical, with a constant-diameter hollow cavity in the centre: the shell is made up of helical graphite sheets."

Sunkara and colleagues discovered the structures while investigating the nucleation of carbon phases. The team immersed platinum wire substrates into the dense part of methane-hydrogen plasmas in a chemical vapour deposition reactor, and found that nanopipettes formed alongside diamond crystals. The structures grew in a methane-hydrogen gas mixture containing 1-2% methane, a composition which Sunkara says is atypical of carbon nanotube growth. The researchers believe that the pipettes grew because of the presence of simultaneous growth and etching modes in the plasma.

Longer growth times resulted in longer nanopipettes, but the structures still maintained a nanoscale tip and a constant-diameter hollow core. The team grew pipettes with lengths of a few hundred nanometres to a few micrometres, external diameters of roughly 200-700 nm and internal diameters of 1-20 nm. Both ends of the pipettes were open.

"[The nanopipettes] should find a unique set of applications in field emission, as atomic force microscope/scanning tunnelling microscope tips, in nanofluidics, transdermal/trans-ocular drug-delivery patches, neurological solute detection, fluid delivery and detection into single cells," added Sunkara. "The most dramatic application might be the drug-delivery one, due to their internal hollow core, rigidity and high aspect ratio."

Now the researchers plan to come up with a technique for growing long, dense arrays of nanopipettes on flat substrates and transferring the array onto desirable substrates such as plastics for applications in transdermal/trans-ocular drug delivery. "We will also be doing detailed structural investigation at both the base and tip of the nanopipette," added Sunkara. "We believe that this information will be of fundamental interest in understanding the nucleation and growth mechanisms involved."

The scientists reported their work in Nano Letters.