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Self-assembling structures grow longer in water as the temperature is raised
Rosette nanotubes, a new class of organic nanotubes, are proving to have unusual properties that may make them important players in future nanotechnologies.
Hicham Fenniri, an assistant professor of chemistry at Purdue University, and his colleagues have found that, contrary to the usual expectations, the self-assembly of these nanotubes in water is promoted by heat [Proc. Natl. Acad. Sci. USA, published online March 12, http://www.pnas.org/cgi/doi/10.1073/pnas.032527099].
The Purdue chemists also have found that rosette nanotubes can serve as scaffolds for the spontaneous formation of channellike assemblies on their periphery. Such assemblies could be used to produce new materials, molecular electronic or photonic devices, and drug delivery systems, for instance.
The rosette nanotubes are assembled from a single building block--a bicyclic molecule that, by design, has a hydrogen-bond donor-donor-acceptor array on one side and a complementary acceptor-acceptor-donor array on the other side. These building blocks hydrogen-bond to form a rosette. The rosettes then stack to form a stable nanotube with a hollow core 11 Å across and up to several micrometers long. The tube's structure is maintained by electrostatic, hydrophobic, and stacking interactions.
Fenniri and coworkers can attach to the building block a variety of functional groups, such as crown ethers, which can bind other molecules. When the rosettes stack to form the nanotube, the crown ethers on alternate rosettes line up, forming 12 additional channels on the outside of the nanotube that can serve, say, as conduits for ions.
By modifying the building block and the functional groups attached to it, the team of Purdue chemists can adjust both the dimensions and properties of the nanotube.
The team's most striking observation is that higher temperatures lead to longer tubes, an example of entropy-driven self-assembly. Such processes are well known in aqueous protein systems and even for certain small molecules in organic solvents. But this is the first time that entropically driven self-assembly has been reported for synthetic molecular systems in water, Fenniri says.
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