A 50-atom thick membrane that separates objects as small as molecules has been developed by researchers at the University of Rochester in New York.

Credit: University of Rochester

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NEW YORK: A newly designed porous membrane, so thin that it's invisible edge-on, might revolutionise the way doctors and scientists manipulate objects as small as molecules.

The 50-atom thick filter can withstand surprisingly high pressures and may be a key to better separation of blood proteins for dialysis patients, speeding ion exchange in fuel cells and purifying air and water at the nanoscopic level.

"It's amazing, we have a material as thin as some of the molecules it's sorting, and riddled with holes - but it can withstand enough pressure to make real-world nano-filtering a practical reality," said Christopher Striemer of the University of Rochester in New York, co-creator of the membrane.

Details of the membrane, which is more than 4,000 times thinner than a human hair - thousands of times thinner than similar filters in use today - are published this week in the British journal Nature. The membrane is a 15-nanometer-thick slice of the same silicon that's used every day in computer-chip manufacturing.

Striemer developed the design while searching for a way to better understand how silicon crystalises when heated. He used such a thin piece of silicon because it would allow him to view the resulting crystal structures with an electron microscope.

As parts of the silicon contracted into crystals, Striemer noticed that holes opened up in their wake. He and colleagues at the University of Rochester realised that - since the membrane's holes were only nanometers in size - it might separate objects as small as proteins much more effectively than existing techniques.

Current molecular-level filters use a polymer-based design that is a jumble of holes and tunnels. The sizes of holes in the polymer vary greatly, and since its 'holes' are really convoluted tunnels through the material, they require much more time for proteins to pass through, and they are prone to clogging.

To test the membrane, the researchers placed a solution of two blood proteins, albumin and IgG, behind the membrane and forced it gently through the nanoscopic holes. In just over six minutes, the albumin had passed through, but the larger IgG protein was stopped. And remarkably, the 50-atom thickness could hold back 10 tonnes per square metre of pressure.

The Rochester team also found a way for the nano-filter to carry a fixed charge, effectively making the hole 'smaller' for molecules of a certain charge than for others. Their membrane can quickly and easily separate molecules by their size and their charge - a serious boon for fuel cell researchers, who wish to move only certain ions from one part of a fuel cell to another.

The Rochester group also foresees many applications for the membrane in medicine, including improved separation of blood proteins for dialysis patients and the nanoscale purification of air and water in hospitals.