Scientists in German investigating ways to control the permeability of graphene say new ways to filter water are on the horizon.

Graphene is an extremely thin, flexible and resistant material made of pure carbon. It forms layers that consist of virtually a single layer of carbon atoms. To make graphene as thick as a human hair, thousands of layers would have to be stacked on top of each other.

Researchers around the world are working intensively on graphene because the special properties of the material promise new applications in areas such as water treatment, electronics and energy technologies.

Graphene permeability

It is particularly interesting for scientists to be able to control the permeability of graphene for different substances, says chemistry professor Frank Würthner from Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany.

"So-called defects can be created in the carbon lattice of graphene. These can be thought of as small holes that make the lattice permeable to gases," he explains.

Permeability to other substances, such as ions like fluoride, chloride or bromide, has not yet been observed, according to the scientists.

"However, this would be of fundamental scientific interest for applications such as the desalination of water, the detection or purification of mixtures of substances," explains the Würzburg professor.

Chloride passes

For the first time, a team led by Würthner has created a model graphene system with a defect that allows the halides fluoride, chloride and bromide to pass through, but not iodide. This was achieved in a stable double layer comprising two nanographenes that enclose a cavity. The penetrated halide ions are bound in this cavity so that the time required for entry could be measured.

Chloride is a component of common salt and is found in seawater. It plays an important role in the life processes of all organisms.

"The proof of a high permeability for chloride by single-layer nanographene, and a selective binding of halides in a double-layer nanographene, brings some applications closer," says Dr Kazutaka Shoyama, who initiated and led the project together with Frank Würthner.

Such applications include water filtration membranes`, artificial receptors and chloride channels. The next step for the Würzburg chemists is to build larger stacks of their nanographenes and use them to investigate the flow of ions.

This research was carried out at the Institute of Organic Chemistry and the Center for Nanosystems Chemistry at JMU. The work was funded by the German Research Foundation (DFG) as part of two grants for the development of nanographenes equipped with imide groups.

The results have been published in the journal Nature.