Japanese and US researchers tackle the problems of nanotube hydrophobicity

Kenji Hata and Sumio Iijima at Tsukuba, Japan, have changed the recipe for making carbon nanotubes using chemical vapour deposition (CVD) on iron catalysts by just adding water. While the original method developed by Iijima produces multi-walled nanotubes, the catalytic method results in the single-walled tubes more desirable for applications in electronics. However, impurities and catalyst lifetimes have remained problematic.

A weak oxidating agent could remove the amorphous carbon deposits that tend to clog up the catalyst, so the Japanese researchers introduced a controlled amount of water vapour into the arrangement which also contained ethylene, argon, and hydrogen, and a catalyst such as iron chloride. The researchers found that the correct water dose greatly improved the performance of the catalyst. Vertical nanotube ’forests’ grew to heights of several millimetres within minutes.(1)

The resulting single wall nanotubes are highly pure and can be used directly without the need for difficult purification procedures. Spectroscopic analysis suggests that a significant fraction of the tubes remains unbundled. Moreover, the lithographic patterns in the catalyst are reproduced in the three-dimensional nanotube architecture, such that a pattern of round catalyst dots would produce cylindrical pillars. In a separate approach, researchers in Israel succeeded in producing nanotubes that are lying down instead of standing tall. (2)

But what if the tubes are already bundled up and stuck together, can they still be dispersed and made to function, eg in a biological context? Carolyn Bertozzi and Alex Zettl at the University of California Berkeley, US, have developed a biomimetic approach to making nanotubes water-soluble and biocompatible. Bertozzi’s group recently developed a new kind of highly glycosylated polymer that mimics cell surface mucins (3).

The researchers combined these ’artificial mucins’ with a C18 lipid expected to stick to the surface of the nanotubes. When they sonicated bundles of nanotubes in an aqueous suspension, the nanotubes were fully solubilised and remained in solution for months, indicating that they had achieved a permanent hydrophilic coating. Further, the coated tubes interacted specifically with a lectin protein (as mucin-covered cells would), while rejecting non-specific protein interactions. Thus, solubilised nanotubes could be used for biotechnological applications.

Sticky tubes aren’t always bad news. Ray Baughman’s group at the University of Texas at Dallas, US, has shown that the bundles can be spun into strong threads of any length (4). In this case, hydrophobicity and stickiness are an advantage.

Michael Gross