Artificial aquaporins filter out small ions

Cells rely on proteins known as aquaporins to carry water molecules across their membranes in 1/100 to 1/10 the time they’d take to diffuse through. The specialized biomolecules help cells process water efficiently without altering the transmembrane potential because they also filter out even small dissolved ions (see Physics Today, December 2003, page 27 ). Despite attempts to mimic the specificity of the cellular filter, researchers have been unable to design artificial nanochannels that prevent small ions, such as potassium, sodium, and chloride, from passing through.

A new nanofluidic device designed by Kalon Gopinadhan, Radha Boya, Andre Geim, and coworkers at the University of Manchester, UK, is selective enough to transmit only water molecules. The device can be made from either atomically flat graphite or boron nitride crystals, which are separated by strips of monolayer graphene to create 130-nm-wide channels, as shown in the figure. The structure is supported by a layer of silicon nitride that separates a reservoir of liquid above the device from the channels’ output below.

Because the graphene layer is so thin, the channels are the smallest constructed to date at only 3.5 Å high—large enough to permit water molecules, whose effective size is 2.8 Å, but small enough to exclude the 6.6 Å hydrated Na⁺, K⁺, and Cl^– ions. On each device, the researchers had to include many narrow channels in parallel to generate a measurable flow of water.

To confirm that water was in fact flowing through the channels, Gopinadhan and coworkers installed a container of water above the pores and were able to verify its weight loss over time. They then checked to see if small ions could also pass through with the water. For several reservoirs of chlorine salt solutions, the conductance between the reservoir and the channel output was below the detectable limit of 50 picosiemens, which indicated that ions were not being transported through.

When the researchers used hydrochloric acid, the device showed a large conductance of nearly 1 nanosiemens. They attributed that not to the permeation of H⁺ ions but rather to proton hopping; unlike other ions, bare protons can hop between water molecules without carrying a hydration shell. That makes the channels different from real aquaporins, which block proton transport using either electric charges or configurational barriers. (K. Gopinadhan et al., Science 363, 145, 2019 .)