Tiny hole guides atoms against tide

By Kimberly Patch, Technology Research News

One of the most elegant and important processes of life is the ion channel, which underlies the nerve signals that carry communications throughout our bodies.

Nerve cells transmit signals to other nerve cells by allowing positively-charged atoms, or ions, of sodium, potassium and calcium on the outside of a neural membrane to switch places with negatively-charged chloride ions on the inside. This depolarizes the membrane, releasing the energy stored in the original arrangement in order to signal an adjoining neuron.

The trick is getting the ions to flow back so the nerve cell can do it again. Nature uses chemistry to coax the ions to make the return trip against their electrochemical potential through special pores, or channels, within the membrane.

Researchers from the Silesian University of Technology and Jagellonian University in Poland have made a synthetic device that uses an electrical field and an extremely small, conical pore in a thin film of material to coax potassium ions through the artificial membrane against their electrochemical potential.

The device can be used to study and better understand the biological ion pump. It could also eventually be used to power microscopic machines.

It was already known that cone-shaped pores that are as small as molecules produce an asymmetric electrical effect similar to a cell membrane's ion pump. The researchers proved, however, that this effect could be used to pump ions as well. "[We thought] that perhaps our conical pore could work according to the same principle," Zuzanna Siwy, an assistant professor at the Silesian University of Technology in Poland and a guest scientist at the Institute for Heavy Ion Research (GSI) in Germany.

The device works by ratcheting the molecules through the widening, and therefore sloping channel, said Andrzej Fulinski, a professor of physics at Jagellonian University in Poland. The oscillating, or periodic electric field drags the ion to and fro, said Fulinski. The net effect is that ions are pushed through the channels and out the wide side of the pore, concentrating the ions on that side of the device.

A key to making the device pump ions against their natural direction is that once they enter the cone-shaped channel, it is easier to go down the widening sloping of the cone than up the walls of the channel. "It is easier to go uphill along a less steep slope," said Fulinski. This, together with friction, leads to the pumping effect. "This is the principal on which both [the] pump and molecular motors, or ratchets work," he said.

The challenge was making a conical pore small enough for ions. The researchers' pore had an opening that widened from two nanometers to 500 nanometers. A nanometer is one millionth of a millimeter, and two nanometers is the width of 20 hydrogen atoms.

To fabricate such a small opening, the researchers bombarded a tiny bit of polymer, or plastic, film with a high-energy ion beam, then chemically etched the remainder of the tapered hole.

When the researchers put the device in a salt solution, they found that more potassium ions flowed from the narrow toward the wide opening of the cone, increasing the concentration of ions on that side of the plastic membrane. The reaction kept going even when there was a 100-fold concentration difference between the two sides of the membrane, according to Siwy.

The researchers found, however, that as concentration changes, the reaction gets less efficient in terms of the energy used for the field per ion pumped. The method is 40 percent efficient when the concentration of ions is the same on both sides of the membrane, and drops to 10 percent when the concentration is 7.5 to 1.

The pumping phenomenon is determined by the size of the narrow side of the pore, the surface change of the cone, and the frequency of the alternating electrical field, according to Siwy. When the narrow side of the cone is 15 nanometers or larger, the reaction does not work.

The researchers originally made the device in order to study synthetic models of biological ion channels, said Fulinski. "These enable measurements which are impossible to perform on living material," he said.

While constructing the synthetic pores, however, [Siwy] realized that the electrical characteristic of the pores would allow them to pump ions using an electric field, said Fulinski. "The measurements... confirmed the suggestion, and we were able to show that indeed such a device works," he said.

The pump can be used as a sort of diode that works in a watery environment, said Fulinski. An electrical diode guides current in only one direction. "The pump can be viewed as a rectifier of ionic currents," he said.

The next step in the research is to make the pump work faster and more efficiently, said Siwy. The researchers are looking to decrease the length of the pore, which is currently 1,000 times longer than biological pores. It will take at least two years before the pump can be used in practical devices, Siwy said.

Siwy and Fulinski published the research in the November 4, 2002 issue of Physical Review Letters. The research was funded by the Alexander Von Humboldt Foundation in Germany, The Institute for Heavy Ion Research (GSI) and the Foundation for Polish Science.

Timeline:   Now
Funding:   Government, Institutional
TRN Categories:   Biotechnology; Chemistry
Story Type:   News
Related Elements:  Technical paper, "Fabrication of a Synthetic Nanoporo Ion Pump," Physical Review Letters, November 4, 2002.


January 29/February 5, 2003

Page One

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Tiny hole guides atoms against tide


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