Filters that select molecules

MIT researchers have a new technique that could produce filters that select molecules according to their chemical properties and dimensions. The team’s ability to produce tiny, uniform pores smaller than 10 nanometers (billionths of a meter) across is itself a significant accomplishment that solves a major problem in existing nanoseparation technology.

This is “a fundamentally different way” of separating molecules, Gleason says. “People usually think of size as being the defining factor,” but by making the pores in the filter small enough so that there is a significant chemical interaction between the pore walls and the molecules passing through them, it becomes possible to discriminate according to other characteristics, she explains. In this case, the selection was based on the molecules’ affinity for water. Because the walls of the pores were hydrophobic (water repelling), other hydrophobic molecules were more easily drawn to the pores and propelled through them than were other, less hydrophobic molecules.

In living organisms, cell walls routinely perform this kind of chemical separation, letting certain specific kinds of molecules — for example, nutrients, enzymes or signaling molecules — pass freely through pores in a cell membrane, while blocking all others. But this is the first time, Asatekin says, that such chemical separation has been demonstrated in a synthetic membrane.

Using a polycarbonate membrane (a type of plastic) treated with a vapor-deposited layer of another polymer, the researchers were able to separate the two dyes very effectively, with more than 200 times more of one type passing through than the other. The coating process they used not only adds the capability for discriminating between molecules based on their differing affinities for water, but by coating the insides of tube-like pores in the material it also provides a way of creating extremely small pores of uniform size — much smaller than can be produced by conventional methods.

In pharmaceutical manufacturing, many processes involve chemical reactions in which both the reactants and the chemical being produced are very similar in molecular size, so being able to separate the two efficiently could be a significant advance in allowing large-throughput processing instead of small-batch production as is done currently, Asatekin says.

In addition to possible applications in drug manufacturing, such membranes could be important for the detection of biologically significant molecules. For example, the U.S. military, which funded this research through the Institute for Soldier Nanotechnology, is interested in their possible use in detectors that could identify a chemical marker the body produces when an inflammatory response is triggered, which could be a way of quickly revealing that the body had been exposed to a toxin even without knowing what the toxin was.

As a next step, Asatekin and Gleason plan to try the technique to separate biomolecules that are of real relevance to biological processes, to demonstrate that it works for materials that would be of interest for actual applications.

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