Fullerene Bilayer Membrane Is Watertight

A Japanese study reveals that bilayer membranes formed from a water-soluble fullerene derivative possess properties very different from those of conventional lipid bilayer membranes such as cell membranes. The findings suggest that fullerene membranes could open new possibilities in separations and sensing technologies (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0705010104).

In their study, chemists Hiroyuki Isobe of Tohoku University, in Sendai, and Tatsuya Homma and Eiichi Nakamura of the University of Tokyo, found that the fullerene membrane is several orders of magnitude less permeable to water than a lipid membrane and that the permeability decreases as the temperature is raised.

The researchers believe that this unusual behavior is due to the fact that the water molecules interact with the fullerene molecules as they pass through clefts in the rigid fullerene bilayer and, as a result, have difficulty leaving the bilayer. The water-retarding effect, they believe, is similar to what happens when crystalline proteins or metal salts become hydrated by binding to water molecules.

"Given the uniqueness of the mechanism of water permeation through the fullerene bilayer membrane, we expect that fullerene-based membranes will show molecular recognition properties thus far unavailable in conventional membranes," which are built from hydrocarbon molecules such as those found in lipid membranes and polymer membranes, Nakamura tells C&EN. He predicts that fullerene-based membranes will lead to new technologies for separating water from gases such as methane. Nakamura also envisions that gas-selective fullerene membranes will make new types of sensors possible.

The researchers used spherical bilayer vesicles that assemble spontaneously when the potassium salt of a pentaphenylfullerene (Ph₅C₆₀K) is dissolved in water. They note that the phenyl groups could be chemically modified and metal atoms could be installed on the molecule to produce bilayers tailored for sensing or catalysis.

Nakamura and colleagues first reported the formation of these fullerene membranes more than six years ago (C&EN, March 12, 2001, page 12). They discovered the bilayers??? unusual permeability about five years ago, Nakamura says, but publication was held up for years, because some referees could not accept that the fullerene membrane could be so water-tight. The team has now accumulated a lot more data on the properties of the fullerene membrane and will be publishing them soon, he says.