Sir Andre Geim, co-discoverer of graphene and Nobel laureate, led the team at the University of Manchester’s National Graphene Institute which developed the artificial channels, capable of blocking hydrated ions larger than around 7A (0.7nm) in diameter. “Obviously, it is impossible to make capillaries smaller than one atom in size,” Geim explained. “Our feat seemed nigh on impossible, even in hindsight, and it was difficult to imagine such tiny capillaries just a couple of years ago.”
The channels, made using a technique known as van der Waals assembly or “atomic-scale Lego”, are 3.4A in height, allowing water molecules (2.8A in diameter) to pass through but blocking even the smallest hydrated ions, such as K+ and Cl-, which are 6.6A in diameter because the ions themselves are surrounded by a shell of water. The technique was developed as a result of research on graphene, and the team explains its technique in the journal Science.
“We cleave atomically flat nano crystals just 50 and 200nm in thickness from bulk graphite and then place strips of monolayer graphene onto the surface of these nano crystals,” said Radha Boya, a physicist specialising in condensed matter and atomic physics and a co-author of the paper. “These strips server as spacers between the two crystals when a similar atomically-flat crystal is subsequently placed on top.” The resulting three-layer structure contains a flat void that can accommodate only one atomic layer of water, she said.
Previous studies had suggested that such structures would collapse because of attractive forces between the opposite walls, but the team’s calculations suggested that water molecules inside the void could act as a support and prevent the “ceiling” of the void from falling onto the floor. The team assembled the capillaries on top of a silicon nitride membrane that separated two isolated containers, so the channels were the only way that any material could flow from one container to the other. They built over a hundred channels in parallel, and used plasma etching to remove any potential blockages from the openings of the capillaries.
The researchers then filled one container with water, applied an electric field to the assembly to force charged ions to move through the channels, and monitored the weight of the full container with microgram precision to determine how much water pass through the channels.
“If our capillaries were two atoms high, we found that small ions can move freely though them, just like what happens in bulk water,” said Radha. “In contrast, no ions could pass through our ultimately-small one-atom-high channels. The exception was protons, which are known to move through water as true subatomic particles, rather than ions dressed up in relatively large hydration shells several angstroms in diameter. Our channels thus block all hydrated ions but allow protons to pass.”
The capillaries act like natural structures called aquaporins, proteins found in membranes of biological cells which have been implicated in diseases such as diabetes and multiple sclerosis. Because they allow water to flow through membranes but block ions, copying them has been a target for research for some time.
The research also expands our understanding of how water behaves on the atomic scale, added Geim. "Our work...shows that atomically-confined water has very different properties from those of bulk water,” he said. “For example, it becomes strongly layered, has a different structure, and exhibits radically dissimilar dielectric properties.”
These studies might lead to the development of high-flux membranes that could be used for water desalination, allowing drinking water to be produced faster and using lower energy than existing membrane technologies.
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