(Nanowerk Spotlight) Nanofluidic channels, confining and transporting tiny amounts of fluid, are the pipelines that make the cellular activities of organisms possible. Nanoscale channels carry nutrients into cells and waste from cells and they also transport water into and out of the cell. Body temperature, digestion, reproduction, fluid pressure in the eye, and water conservation in the kidney are only a few of the processes in humans that depend on the proper functioning of cellular water channels. Special proteins called aquaporins can transport water through the cell membrane at a high rate while effectively blocking everything else - even individual protons, the nuclei of hydrogen atoms. The aquaporin channels are so narrow that no molecule larger than water can pass through, effectively forcing them through like beads on a chain. A unique distribution of amino acid residues along the pore wall also accounts for the channel's ability to move water quickly. To keep out molecules smaller than water there is also a chemical filter, formed by the specific orientation and distribution of the amino acid residues lining the pore. Thus water, and only water, flows freely through the aquaporin nanochannels, the direction of flow depending only on changing relative pressure inside and outside the cell. This intriguing mechanism has attracted the attention of nanotechnology researchers who see it as a blueprint for the construction of nanoscale water pumps. A molecular dynamics simulation conducted by Chinese researchers proposes a design for such a molecular pump constructed with a carbon nanotube.
"Much progress has been made in moving water unidirectionally by designing systems with an imbalance of surface tension or a chemical or thermal gradient, but it is still difficult to make a controllable continuous unidirectional water flow" Dr. Haiping Fang explains to Nanowerk. "Our findings focus on single-walled carbon nanotubes (SWCNT) with appropriate radii in which water molecules exhibit single-file structure. By arranging discrete charges on the exterior of a SWCNT we can rotate the dipoles of water molecules which then leads to a smooth concerted water motion through the center of the nanotube."
Since water is charge-neutral, it is not easy to be driven by an electric field in an environment full of thermal fluctuations. Building a water nanopump that actively transports water through nanochannels is technically difficult or maybe even impossible using mechanisms comparable to those used in the macro world.
Fang, a Professor of physics at the Shanghai Institute of Applied Physics, Chinese Academy of Sciences (CAS), in Shanghai, and his collaborators propose a new strategy for a water nanopump: this pump can push neutral water molecules to move in one direction by precisely putting extra charges on the surface of a nanotube. This strategy is inspired by the structure of aquaporins.
Such a water nanopump would have important applications including the desalination of seawater, chemical separation, water purification, in vivo sensing and drug delivery.
Schematic of a charge-driven molecular water pump. Water molecules move in single file through a single-walled carbon nanotube, driven by three electric charges located on the outside of the nanotube. (Image: Dr. Fang, CAS)
The method proposed by Fang and his colleagues places two positive 0.5e and one 1e charges (e is the charge of an electron) asymmetrically about the vertical center of the SWCNT.
"To understand how the particular charge configuration affects the water dynamics in the channel, we compared the behavior of the main system (i.e. the three charge configuration - 2x 0.5e plus 1e) with systems having only some of the charges" Fang explains. "We found that the method of using two +0.5e charges positioned in the middle of the channel is very important. If we merged them into on +1.0e charge the permeation behavior would be qualitatively different."
Fang cautions that one may argue that the pump they designed looks like a perpetual mobile. "We should point out that extra energy is required to constrain the charges at their original positions, which is the key to the pumping ability" he says. "We found that the electrostatic forces exerted by water molecules on the three charges are 533 pN, 313 pN and 300 pN. Without the constraint, the charges would be forced away and the net flux would vanish."
The researchers note that the pumping ability described here can be directly applied to insulator nanochannels and could also be extended to semiconductor nanopores, such as armchair SWCNTs with finite length or zigzag SWCNTs.
"In these systems, it will be necessary to account for the effects of screening, and the charges we used here should be regarded as effective charges – the realistic charges should be made larger by multiplication of a factor related to the screening effect" says Fang. "The positions of the charges may be left unchanged."
While this work provides a blueprint for the design of a nanoscale (water) pump, there might be technical problems to be overcome in order to move this research from the simulation stage to experimental applications. Nevertheless, this is a first design for a nanoscale waterpump, inspired by the structure of aquaporins, that could potentially be engineered into future nanoscale systems.