Pores in graphene could lead to better membranes

In their experiments, the researchers found that small molecules like salts passed easily through a graphene membrane’s tiny pores, while larger molecules were unable to penetrate.

The results, the researchers claim, point 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.

The pores ranged in size from about 1 to 12 nanometers

“No one has looked for holes in graphene before,” says Rohit Karnik, associate professor of mechanical engineering at MIT. “There’s a lot of chemical methods that can be used to modify these pores, so it’s a platform technology for a new class of membranes.”

Karnik worked with MIT graduate student Sean O’Hern to look for materials “that could lead to not just incremental changes, but substantial leaps in terms of the way membranes perform.”

In particular, the team cast around for materials with two key attributes, high flux and tunability: that is, membranes that quickly filter fluids, but are also easily tailored to let certain molecules through while trapping others.

The group settled on graphene, in part because of its extremely thin structure and its strength: A sheet of graphene is as thin as a single atom, but strong enough to let high volumes of fluids through without shredding apart.

They used graphene synthesised by chemical vapor deposition. The team then developed techniques to transfer the graphene sheet to a polycarbonate substrate dotted with holes.

They theorised 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.

It appeared that there was a limit to the size of the pores, as larger molecules were unable to pass through the membrane.

We’re in the process of transferring graphene to different substrates and making holes of our own, making a viable membrane for water filtration

They found that pores ranged in size from about 1 to 12 nanometers – just wide enough to selectively let some small molecules through.

“Right now we know from this characterisation how the graphene behaves, and what kind of intrinsic pores it has,” Karnik says. “In some sense it’s the first step to practically realising graphene-based membranes.”

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.

“We’re right now in the process of transferring more graphene to different substrates and making holes of our own, making a viable membrane for water filtration,” O’Hern said.

Scott Bunch, an assistant professor of mechanical engineering at the University of Colorado, said the group’s results are the first demonstration that graphene bears defects.

He added that the material developed by the group “has the potential to be a revolutionary membrane” that separates particles at the molecular scale.