Researchers are looking to human cells to create new nano-filters that could be used for everything from water purification to drug delivery.
Human cell membranes can regulate what chemicals cross into the cell; that's one of the things that makes many biological processes work. Bing Gong and his colleagues at the State University of New York at Buffalo have constructed a membrane that mimics the ion channels that exist in cells and that has pores less than a nanometer wide.
The artificial membrane is made with molecules called macrocycles. A macrocycle is basically a set of carbon-based molecules strung together, to form a larger, hexagonal ring. The edges of the ring have another set of atoms that can bind to other macrocycles in a uniform way, using hydrogen bonds.
Gong noted that stacking lots of these rings on top of each other resulted in nanometer-size tubes, all of a uniform size. That by itself is a big step, he said, because normally the pores are not uniform. In this case the openings were all about 8.8 angstroms, or 0.88 nanometers.
The team experimented with different ions — lithium, potassium and sodium, and found that potassium ions passed through the openings but lithium and sodium did not. In addition, water passed through the pores easily.
"That last one actually puzzled us," Gong said. It isn't clear why water passes through — it might have something to do with the chemistry of the inner surface of the tube, he speculated.
One reason the pores are so selective is that different ions will stick to water molecules in different ways; some will do so more strongly. So when potassium is passing into the tube, the water molecules get bumped off and the ion goes on its way. Lithium and sodium are harder to separate from the water, so they get stuck.
The ability to make the pores of uniform size, though, also means the size and shape can be adjusted, as well as the surface chemistry. And that means they have a membrane that can allow only the desired ions to pass through.
"The holy grail of this technology is just to allow water without anything else," Gong said. That would mean that separating two mixed chemicals would be a lot easier to do. The next step, he added, is to make the necessary macrocycles in larger quantities.
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