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
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
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.
1. Waghe, A., Rasaiah, J.C., Hummer, G. J. "Filling and
emptying kinetics of carbon nanotubes in water," J. Chem. Phys. 117, 10789
2. Hummer, J.C., Rasaiah, J.C., Noworyta, J.P. "Water
conduction through the hydrophobic channel of a carbon nanotube," Nature 414,
3. Koga, K., Gao, G.-T., Tanaka, H., Zeng, X.C.
"Formation of ordered ice nanotubes inside carbon nanotubes," Nature 412, 802
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,
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).
Copyright AZoNano.com, Dr. Amanda Ellis (Flinders