Are Carbon Nanotubes the Ultimate Water Transporters?

Water flow through a garden hose or a nanochannel (10^-9 m) has incredibly different fluid flow transport mechanisms. In one case the theory of fluid dynamics holds and in the other nanoscale phenomena dominant. In a hose single water molecule transport is not a dominate feature however the unique properties of, in particular, carbon nanotubes make single water molecule transport a reality. Interestingly, it is the entry and exit of water molecules that is believed to be the factor limiting transport . So how does this come about?

For this we need molecular dynamic (MD) simulations which show that hydrogen atoms are attracted to the nanotube while oxygen is repelled such that water is orientated with the hydrogen atom entering the tube first.¹ If the nanotube diameter is small enough (approximately 0.8 nm) water forms a 1-dimensional chain travelling at an average of 17 molecules per nanosecond (or 99 cm s^-1).² However, other MD simulations report dramatically decreased mobility at this nanoscale due to the change in the mechanical properties of water.³ So there are discrepancies in our theoretical understanding of this phenomena.

Experimentally water transport through carbon nanotubes is far from trivial. However, ever since Holt et al.⁴ measured the flow of water through a silicon nitride/double-walled carbon nanotube nanofiltration membrane and discovered flow rates 4-5 orders of magnitude higher than estimated by classic models for Poiseuille permeation⁴ the search has been on to create extended membrane films for use in emerging novel water treatment technologies. The promise of higher transport fluxes, specificity and selectivity through a water treatment membrane is always in our minds, particularly in arid environments such as Australia.

Dr Ellis and her colleagues at Flinders University believe they can make a valuable contribution to the problems facing water challenged environments. Their work is currently funded by an Australian Research Council Discovery grant. Their recent review on "Carbon nanotubes anchored to silicon for device fabrication"⁵ emphasizes the need for control of nanotube orientation in order to reap the diverse range of potential applications they may afford. In particular, their uses in water transport systems.

The efficiency of a membrane may be dictated by structure, composition, and design. Many synthetic chemistry techniques exist which enable the design of a nanohybrid filtration membrane to vastly improve its function. To this end Dr Ellis and her colleagues are using a variety of synthetic chemistries to construct nanohybrid water filtration membranes based on wet chemical (self-assembled) vertically aligned carbon nanotubes (VA-CNTs) and ultrathin nanocrystalline porous silicon.

A variety of alignment chemistries have been achieved using DCC or EDC coupling, and thioester coupling.^{5, 6} Figure 1(a) shows an atomic force micrograph (AFM) image of bundles of single-walled carbon nanotubes (SWCNTs) on a silicon surface.⁶ An increasingly popular method to improve device properties is the employment of "Click" chemistry.

Figure 1. (a) AFM image of bundles of VA-SWCNTs on silicon attached via a dithioester linkage and (b) schematic of SWCNT/polymer nanohybrid membrane synthesis showing click chemistry with subsequent derivatization of SWCNT surface moieties with a RAFT chain transfer agent for controlled 'living' free radical polymerization.

Click chemistry became a popular term by Sharpless et al.⁷ in 2001 due to the resurrection of a small group or class of organic reactions. There are definitive requirements to classify a reaction as a click process, although the synthetic importance is that click reactions proceed in an orthogonal and quantitative fashion.

To advance the applications of VA-SWCNTs, click chemistry has been used to successfully attach SWCNTs vertically to a silicon surface. By utilizing various click reactions in one process, the VA-SWCNTs may be embedded within a polymeric matrix by the most diverse controlled radical polymerisation process, reversible addition-fragmentation chain transfer (RAFT) (Figure 1B). These membranes will not only provide low-energy and high-flux water purification capabilities but will also enable the effective removal of toxins.

Due to the various synthetic combinations possible when utilizing both click and RAFT, many improved properties and applications will continue to arise in water treatment membrane technologies.

References

1. Waghe, A., Rasaiah, J.C., Hummer, G. J. "Filling and emptying kinetics of carbon nanotubes in water," J. Chem. Phys. 117, 10789 (2002).
2. Hummer, J.C., Rasaiah, J.C., Noworyta, J.P. "Water conduction through the hydrophobic channel of a carbon nanotube," Nature 414, 188 (2001).
3. Koga, K., Gao, G.-T., Tanaka, H., Zeng, X.C. "Formation of ordered ice nanotubes inside carbon nanotubes," Nature 412, 802 (2001).
4. Holt, J. K., Noy, A., Huser, T., Eaglesham, D., Bakajin, O. "Fabrication of a carbon nanotube-embedded silicon nitride membrane for studies of nanometer-scale mass transport," Nano Lett. 2004, 4, 2245.
5. Constantopoulos, K.T., Shearer, C. J., Ellis, A. V., Voelcker, N. H., Shapter, J. G. "Carbon nanotubes anchored to silicon for device fabrication," Advanced Materials 21, 1-15 (2009).
6. Poh, Z., Flavel, B.S., Shearer, C. J., Shapter, J.G., Ellis, A.V. "Fabrication and electrochemical behaviour of vertically-aligned carbon nanotube electrodes covalently attached to p-type silicon via a thioester linkage," Mater. Lett. 63, 757-760 (2009).
7. Kolb, H.C., Finn, M.G., Sharpless, K.B. "Click chemistry: Diverse chemical function from a few good reactions," Angew. Chem. Int. Edit. 40, 2004-2021 (2001).

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