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Progress in Untangling Nanotubes

03 February 2003

Scientists at the University of Pennsylvania claim they have made progress toward a solution for one of the biggest obstacles against implementing carbon nanotubes in electronics, materials and healthcare applications.

Carbon nanotubes have frustrated researchers in every field with their stubborn and unhelpful tendency to clump together in solution. According to the Penn scientists, a readily available chemical, a surfactant called sodium dodecylbenzene sulfonate (NaDDBS), disperses nanotubes in water with remarkable efficiency. The discovery is described in a paper published this month in the journal Nanoletters .

""Scientists have suggested many possible applications for carbon nanotubes, but tube aggregation in solution has obstructed progress,"" said lead author Mohammad Islam, a postdoctoral researcher in Penn's Department of Physics and Astronomy. ""This new approach improves our ability to manipulate single tubes. Single nanotubes can now participate in controlled self-assembly, form fibers and composites, and serve as microfluidic sensors in water.""

When Islam and collaborator Arjun Yodh added NaDDBS to a cocktail of water and nanotubes, the surfactant adhered weakly to the nanotubes, preventing the tubes from clinging to one another. The researchers determined that NaDDBS increased the concentration of single carbon nanotubes in water up to 100-fold. Even at high concentrations, roughly 63 percent of nanotubes in aqueous solution remained unbound.

""Sodium dodecylbenzene sulfonate is pretty non-invasive, so we expect that the nanotubes' unique electronic, thermal, optical and mechanical properties will be preserved in suspension,"" said Yodh, a professor of physics. ""An added bonus of our complete solubilization approach is that it is gentle. Mixing this particular surfactant with nanotubes and water in a low-power, high-frequency sonicator, as we did, resulted in very little breakage of the nanotubes, which has been a problem with other treatments.""

The researchers also found that NaDDBS-treated nanotubes resisted re-aggregation for as long as three months, a feat other surfactants could not match.

Carbon nanotubes tend to cling together because they are subject to substantial van der Waals attractions. While researchers have explored numerous surfactants to counter this attraction, Islam and Yodh suggest that NaDDBS's benzene ring, together with its long alkane tail and charge group, conspire to produce an unusual molecular arrangement on the nanotube surface that reduces aggregation.





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