Oct 24, 2012

Pores in Graphene Could Give Rise to Membranes

Tiny pores in graphene offer possibility of membranes that filter microscopic contaminants from water

Graphene Water Filtration

Much has been made of graphene’s exceptional qualities, from its ability to conduct heat and electricity better than any other material to its unparalleled strength. Previous research has also shown that pristine graphene — a microscopic sheet of carbon atoms arranged in a honeycomb pattern — is among the most impermeable materials ever discovered.

But the material may not be as impenetrable as scientists have thought. By engineering relatively large membranes from single sheets of graphene grown by chemical vapor deposition, researchers from the Massachusetts Institute of Technology (MIT), Oak Ridge National Laboratory (ORNL) and elsewhere have found that the material bears intrinsic defects, or holes in its atom-sized armor.

The results, the researchers say, point not to a flaw in graphene, but to the possibility of promising applications, such as membranes that filter microscopic contaminants from water, or that separate specific types of molecules from biological samples.

Rohit Karnik, associate professor of mechanical engineering for MIT, and his colleagues have published their results in the journal ACS Nano.

The team set out to engineer a membrane spanning 25 sq mm — a surface area that is large by graphene standards, holding about a quadrillion carbon atoms. The team then developed techniques to transfer the graphene sheet to a polycarbonate substrate dotted with holes.

Once the researchers successfully transferred the graphene, they began to experiment with the resulting membrane, exposing it to flowing water containing molecules of varying sizes. They theorized that if graphene were indeed impermeable, the molecules would be blocked from flowing across. However, experiments showed otherwise, as researchers observed salts flowing through the membrane.

Through a series of tests, researchers found that pores in graphene ranged in size from about 1 to 12 nm — just wide enough to selectively let some small molecules through.

Karnik added that a near-term application for such membranes may include a portable sensor in which a layer of graphene “could shield the sensor from the environment,” letting through only a molecule or contaminant of interest. Another use may be in drug delivery, with graphene, dotted with pores of a determined size, delivering therapies in a controlled release.

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