In Search of a Biosensing Biocide

MATERIALS ENGINEERING

Imagine this: a simple lipid molecule forms a bilayer, the bilayers curl up to form nanotubes, and bunches of nanotubes assemble into a "nanocarpet." Furthermore, the nanotubes respond to different substances by changing color, and they kill bacteria to boot!

No need to imagine all this--such a molecule has been synthesized, and its remarkable capabilities have been explored by a team at the University of Pittsburgh led by Alan J. Russell, a professor of surgery and of chemical and bioengineering [J. Am. Chem. Soc., published online Sept. 24, http://dx.doi.org/10.1021/ja048463i].

"The work is an outgrowth of our interest in developing materials that both sense and decontaminate chemical or biological weapons," says Russell, who directs the university's McGowan Institute for Regenerative Medicine.

Earlier research in his and other labs suggested that the antimicrobial biosensor molecule they were seeking would likely be a cationic lipid with a long-chain diacetylene tail and a quaternary ammonium salt headgroup. Russell's coworker Sang Beom Lee synthesized such a lipid, but it failed to perform as hoped. However, a closely related secondary amine salt surprised the researchers by forming remarkably uniform nanotubes in a single step in 100% yield.

The nanotubes are always 89 nm across, and the walls, which consist of five lipid bilayers, are 27 nm thick. The tube length varies but typically is around 1 µm.

Exposing the nanotubes to ultraviolet light induces them to polymerize through the triple bonds of the diacetylene groups on adjacent molecules. When these polymerized nanotubes are exposed to detergents or strong acids (stand-ins for chemical weapons), they change color from blue to red or yellow.

The researchers also found that solutions of polymerized nanotubes respond to high levels of Escherichia coli by changing from dark blue to reddish hues. The nanotubes also reduce the concentration of the bacteria. "We can likely do better" in terms of improving the biocidal efficiency by modifying the nanotube-forming molecule, Russell tells C&EN.

Much remains to be done if this project, which is funded by the U.S. Army Research Office, is to fulfill its goal of leading to a coating that can simultaneously detect and destroy biological or chemical agents. For example, the nanostructured system would have to be modified to be active against agents such as anthrax, spores, and fungi. Many questions also remain about the mechanism, extent, and specificity of the biocidal activity.