NIST researchers carried out simulations of a graphene membrane featuring oxygen-lined pores and immersed in a liquid solution of potassium ions (charged atoms), which under certain conditions can be trapped in the pores. Slight stretching of the graphene greatly increases the flow of ions through the pores. Image: NIST.Researchers at the US National Institute of Standards and Technology (NIST) have conducted simulations suggesting that graphene, in addition to its many other useful features, can be modified with special pores to act as a tunable filter or strainer for ions (charged atoms) in a liquid.
The concept, which may also work with other membrane materials, could have applications in nanoscale mechanical sensors, drug delivery and water purification. It could also lead to sieves or pumps for ion mixtures that are similar to biological ion channels, which are critical to the function of living cells. The research is reported in a paper in Nature Materials.
"Imagine something like a fine-mesh kitchen strainer with sugar flowing through it," said project leader Alex Smolyanitsky. "You stretch that strainer in such a way that every hole in the mesh becomes 1–2% larger. You'd expect that the flow through that mesh will be increased by roughly the same amount. Well, here it actually increases 1000%. I think that's pretty cool, with tons of applications."
If it can be achieved experimentally, this graphene sieve would be the first artificial ion channel to produce an exponential increase in ion flow when stretched, offering possibilities for fast ion separations or pumps and precise salinity control. Collaborators plan laboratory studies of these systems, Smolyanitsky said.
Graphene is a layer of carbon atoms arranged in hexagons, similar in shape to chicken wire, that can conduct electricity. The NIST molecular dynamics simulations focused on a graphene sheet 5.5nm by 6.4nm in size featuring small holes lined with oxygen atoms. These pores are crown ethers – electrically neutral circular molecules known to trap metal ions. A previous NIST simulation study showed that this type of graphene membrane might be used for nanofluidic computing.
In the simulations, the graphene was suspended in water containing potassium chloride, a salt that splits into potassium and chlorine ions in solution. The crown ether pores can trap potassium ions, which have a positive charge, and the trapping and release rates can be controlled electrically. An electric field of various strengths was applied to drive the ions flowing through the graphene membrane.
The researchers then simulated tugging on the membrane with various degrees of force to stretch and dilate the pores, and found that this greatly increased the flow of potassium ions through the membrane. Stretching in all directions had the biggest effect, but even tugging in just one direction had a partial effect.
The researchers found that the unexpectedly large increase in ion flow was due to a subtle interplay of several factors. These include: the thinness of graphene; interactions between the ions and the surrounding liquid; and the ion-pore interactions, which weaken when pores are slightly stretched. There is a very sensitive balance between ions and their surroundings, Smolyanitsky said.
This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.