26 February 2010

Nanotubes mimic biological ion channels

Cosmos Online
Tiny tubes of boron nitride – similar to biological ion channels – could provide an alternative to desalination plants and may even lead to future treatments for cancer and cystic fibrosis, scientists said.

Ion channels in cells regulate the flow of ions and water into and out of the cell. Now scientists have found that artificially created nanotubes could mimic this function. Credit: wikimedia

SYDNEY: Tiny tubes of boron nitride – similar to biological ion channels – could provide an alternative to desalination plants and may even lead to future treatments for cancer and cystic fibrosis, scientists said.

Tamsyn Hilder, a computational biophysicist at Australian National University in Canberra, ran simulations on boron nitride ‘nanotubes’, just 1.4 nanometres (nm) long, embedded in a thin membrane of silicon nitride.

She found that the nanotubes let through water but selectively block other particles, as she announced at the International Conference on Nanoscience and Nanotechnology this week in Sydney.

Making fresh water

Hilder said boron nitride nanotube membranes could filter the salt from seawater up to five times faster than membranes currently used in desalination plants.

“The advantage is they get a lot more flow for the same amount of energy, so in effect they are more efficient,” she said.

Work is still required to create membranes strong enough to withstand the pressures used in desalination plants. However, adding a film of porous silicon could strengthen the membranes without affecting water flow and it shouldn’t be long before the technology catches up, according to Hilder.

Mimicking biological processes

The nanotubes used in the simulations are constructed of alternating boron and nitrogen atoms in a hexagonal lattice.

Their diameter affects the type of ions that can pass through, so scientists can mimic the biological ion channels in a cell membrane that control the flow of water and ions in an out of living cells.

Nanotubes that are 0.35 nm in diameter let water flow through them but block the sodium and chloride ions that make up salt. This is similar to the channel, called aquaporin, in human cells, only the flow in the nanotubes is three times faster, at about 10 billion water molecules per second.

The rapid transport of water molecules and rejection of salt is what makes boron nitride nanotubes ideal candidates for filtration devices.

But when the diameter of the tubes is increased slightly to 0.41 nm, the distribution of charge through the tubes changes and they also allow through positively charged ions such as sodium.

This mimics the antibiotic Gramicidin, which kills bacteria by increasing the flow of positive ions through their cell walls and disrupting the cell function.

This shows the potential for nanotubes to be used as antibiotics and as a way to kill cancer cells one day, Hilder speculates.

Ideas for treating cystic fibrosis

If the diameter of the tubes is increased even further to 0.55 nm, they stop sodium ions but allow chloride ions to pass through it.

There is a chloride channel that is disrupted in people suffering from cystic fibrosis, so the nanotubes may also motivate research into treating this disease.

Hilder’s findings are also described in two papers submitted to the journal Small in October and December last year.

Varying diameter

Materials engineer Mikel Duke, who works with membranes and nanotubes at Victoria University in Melbourne, said it was intriguing that varying the diameter of the nanotubes gave different results.

“As an experimentalist myself it’s hard to say what’s really going on if you make these sorts of observations,” he said.

“To actually be able to define a structure and change the parameters to … find new ways of looking at things [means] the results have a lot of significance.”

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