Graphene is a sheet of carbon just one atom thick in which the atoms are arranged in a honeycomb lattice. Graphene oxide (GO) is like ordinary graphene but is covered with molecules such as hydroxyl groups, and GO sheets can easily be stacked on top of each other to form extremely thin, but mechanically strong, membranes. These membranes consist of millions of small flakes of graphene oxide with nanosized empty channels (or capillaries) between the flakes.

In 2012, a team of researchers led by Sir Andre Geim of the University of Manchester in the UK – who discovered graphene in 2004 – found that GO membranes were impermeable to all gases and vapours except for water. In fact, Geim’s team found that water passes through a film of GO extremely fast while all other gases and liquids are completely blocked by the film.

HLGO's perfectly laminar structure

Until now, it was thought that graphene oxide was impermeable to organic solvents, for reasons that were little understood. Researchers led by Rahul Nair and Yang Su, also at Manchester, have now found that ultrathin graphene oxide membranes (known as highly laminated GO, or HLGO) are in fact extremely permeable to organic solvents (such as alcohol) too thanks to their perfectly laminar structure. The result is good news since conventional polymer membranes used in organic solvent nanofiltration (OSN), in which solute molecules are separated from organic solvents, are often not very resistant to organic chemicals. And ceramic inorganic membranes used for ONS are, for their part, costly and inefficient.

The HLGO membranes fabricated by the team contain smooth, 2D capillaries comprising large (10–20 micron-thick) flakes. They can be made extremely thin (down to just 10 nm) without losing any of their filtering and sieving characteristics.

HLGO membrane outperforms state-of-the-art membranes for OSN

“Using these membranes, we filtered several organic dye molecules (as small as 1 nm in size) containing benzene rings dissolved in organic solvents,” explains Nair. “We found that they only allow the solvent to permeate through while blocking the dye molecules depending on their molecular (physical) size. We were able to see that no dye molecules had filtered through by looking at the colour of the solution after sieving – it is colourless since it no longer contains any dye molecules but consists only of the solvent.”

For membranes to be efficient filters, they need to allow solutions to pass through at high flow (flux) rates and they must also have a precise and small sieve size, he adds. “Our GO membranes satisfy both of these criteria thanks to the unique structure of the membrane. Indeed, the dye molecule rejection and solvent flux we obtained from our membrane outperforms state-of-the-art membranes for OSN.”

Bridged pinholes allow the membrane to become an atomic-scale sieve

When we fabricate the membranes, we assemble the GO flakes so that they form a layered structure, with each layer containing many pinholes,” he tells nanotechweb.org. “Below a critical thickness, these pinholes pierce through the membrane and it thus cannot sieve anything. However, above a thickness of 8 nm (or eight layers of GO), these pinholes are bridged by 1-nm-wide graphene nanochannels that allow the membrane to become an atomic-scale sieve.”

Given how resistant these membranes are to chemicals, they could be used in a variety of filtration applications, say the researchers. “For example, they could recover smaller molecules of interest from organic solvents or separate solvents from unwanted solute molecules,” explains Nair. “This brings applications in the chemical, pharmaceutical and petrochemical industries. In pharmaceuticals, for example, adequate high-quality separation and concentration processes are critical to obtaining very high purity products from dilute suspensions. These processes add to most of the production costs of therapeutic molecules, so more efficient separation membranes might help significantly bring this cost down.”

The team, reporting its work in Nature Materials DOI: 10.1038/nmat5025, says that it is now looking to scale up its membranes and test them for specific applications.