Graphene dialysis membrane filters fast

This graphic shows the process used to create the pores in the graphene filter membrane
Dialysis is the process by which molecules filter out of one solution, by diffusing through a membrane, into a more dilute solution. Outside of haemodialysis, which removes waste from blood, scientists use dialysis to purify drugs, remove residue from chemical solutions, and isolate molecules for medical diagnosis.

Typically, commercial dialysis membranes separate molecules slowly, in part because they are relatively thick, and the pores that tunnel through such dense membranes do so in winding paths, making it difficult for target molecules to pass through quickly.

Engineers from MIT have fabricated a functional dialysis membrane from a 1cm2 sheet of graphene. Their membrane can filter out nanometre-sized molecules from aqueous solutions up to 10 times faster than state-of-the-art membranes, with the graphene itself being up to 100 times faster.

“Because graphene is so thin, diffusion across it will be extremely fast,” said Piran Kidambi, a postdoc in MIT's Department of Mechanical Engineering. “A molecule doesn't have to do this tedious job of going through all these tortuous pores in a thick membrane before exiting the other side. Moving graphene into this regime of biological separation is very exciting.”

To turn the graphene into a molecularly selective sieve, letting through only molecules of a certain size, the engineers created tiny pores in the material by exposing the structure to oxygen plasma, a process by which oxygen, pumped into a plasma chamber, can etch away at materials.

“By tuning the oxygen plasma conditions, we can control the density and size of pores we make, in the areas where the graphene is pristine,” Kidambi explained. “What happens is, an oxygen radical comes to a carbon atom [in graphene] and rapidly reacts, and they both fly out as carbon dioxide.”

What is left is a tiny hole in the graphene, where a carbon atom once sat. Kidambi and his colleagues found that the longer graphene is exposed to oxygen plasma, the larger and denser the pores will be. Relatively short exposure times, of about 45 to 60 seconds, generate very small pores.

The researchers tested multiple graphene membranes with pores of varying sizes and distributions, placing each membrane in the middle of a diffusion chamber. They filled the chamber's feed side with a solution containing various mixtures of molecules of different sizes, ranging from potassium chloride (0.66nm wide) to lysozyme (4nm), a protein found in egg white. The other side of the chamber was filled with a dilute solution. The team then measured the flow of molecules as they diffused through each graphene membrane.

Membranes with very small pores let through potassium chloride but not larger molecules such as L-tryptophan, which is only 0.2nm wider. Membranes with larger pores let through correspondingly larger molecules.

The team carried out similar experiments with commercial dialysis membranes and found that, in comparison, the graphene membranes performed with higher “permeance”, filtering out the desired molecules up to 10 times faster.

To make the graphene membrane even better, the team plans to improve the polycarbonate support by etching more pores into the material to increase the membrane's overall permeance. They are also working to scale up the dimensions of the membrane. Further tuning the oxygen plasma process to create tailored pores will also improve a membrane's performance - something that Kidambi points out would have vastly different consequences for graphene in electronics applications.

“What’s not great for the electronics field is actually perfect in this [membrane dialysis] field,” Kidambi says. “In electronics, you want to minimise defects. Here you want to make defects of the right size. It goes to show the end use of the technology dictates what you want in the technology. That’s the key.”

Author
Tom Austin-Morgan

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