Aquaporins inspire graphene-oxide membrane

Nanoscale water channels found in nature have provided the inspiration at KAUST for a graphene-oxide-based membrane that rapidly separates chemical mixtures.

Solutions of organic dye molecules could be easily separated by the dual-spaced membrane (© 2019 KAUST; Anastasia Khrenova)

In the natural world, aquaporins rapidly shuttle water molecules into and out of cells and mimicking this function could help improve the production of chemicals and pharmaceuticals.

KAUST researchers in Saudi Arabia have tailored the structure of graphene-oxide layers to mimic the hourglass shape of these biological channels, creating ultrathin membranes to rapidly separate chemical mixtures. Their research is published in Journal of Materials Chemistry A.

“In making pharmaceuticals and other chemicals, separating mixtures of organic molecules is an essential and tedious task,” said Shaofei Wang, postdoctoral researcher in Suzana Nuñes lab at KAUST.

One way to make these chemical separations faster and more efficient is through selectively permeable membranes, but these are faced with a compromise called the permeance-rejection trade-off. This means narrow channels may effectively separate the different-sized molecules, but they also have a low flow of solvent through the membrane, and vice versa – they flow fast enough, but perform poorly at separation.

Nuñes, Wang and the team looked at aquaporins to overcome this limitation as they have an hourglass-shaped channel that is wide at each end and narrow at the hydrophobic middle section, a structure that combines high solvent permeance with high selectivity.

Claiming to improve on nature, the team has created channels that widen and narrow in a synthetic membrane.

The membrane is made from flakes of graphene oxide that are combined into sheets several layers thick. Organic solvent molecules are small enough to pass through the narrow channels between the flakes to cross the membrane, but organic molecules dissolved in the solvent are too large to take the same path.

To boost solvent flow without compromising selectivity, the team is said to have introduced spacers between the graphene-oxide layers to widen sections of the channel, mimicking the aquaporin structure. The spacers were formed by adding a silicon-based molecule into the channels that, when treated with sodium hydroxide, reacted in situ to form silicon-dioxide nanoparticles.

“The hydrophilic nanoparticles locally widen the interlayer channels to enhance the solvent permeance,” Wang said in a statement.

When the team tested the membrane’s performance with solutions of organic dyes, they found that it rejected at least 90 per cent of dye molecules above a threshold size of 1.5nm.

Incorporating the nanoparticles is said to have enhanced solvent permeance 10-fold, without impairing selectivity. The team also found there was enhanced membrane strength and longevity when chemical cross-links formed between the graphene-oxide sheets and the nanoparticles.

“The next step will be to formulate the nanoparticle graphene-oxide material into hollow-fibre membranes suitable for industrial applications,” Nuñes said.