Nanotubes That Form 'Nanocarpets' and Kill Bacteria

By Roland Piquepaille

By mixing a salt compound with an hydrocarbon, researchers at the University of Pittsburgh have created molecules able to sense their environment. Then they used these molecules to develop self-assembling nanotubes which look like that 'nanocarpets'. These nanostructures can change colors when their environment is modified and can be trained to kill bacteria, such as E. coli. Now, they plan to develop products that would both detect and destroy biological weapons. Read more...

Here is how they created these nanocarpets.

The researchers thought that by combining a chemical structure called a quarternary ammonium salt group, known for its ability to disrupt cell membranes and cause cell death, with a hydrocarbon diacetylene, which can change colors when appropriately formulated, the resulting molecule would have the desired properties of both biosensor and biocide. Remarkably, in addition to being able to kill cells, the resulting reaction mixture had the ability to self assemble into beautiful nanotubes of uniform structure.

After searching for what was forming the tubes, the researchers discovered that synthesis of a secondary salt and diacetylene, thereby creating a lipid molecule, also resulted in production of absolutely pure self-assembling nanotubes, all having the same diameter (89 nanometers) and wall thickness (27 nanometers).

When dried from water and other solvents, and under magnification, these nanostructures look much like a heaping serving of Kraft macaroni or ziti pasta. Incredibly, when coaxed with simple processing, the tubes align into the more formal pattern of a nanocarpet. Just like any rug, a backing, also self-assembled from the same material, holds it all together. The nanocarpet measures about one micrometer in height, approximately the same height as the free-form nanotubes.

Here is a photo of the self-assembling nanocarpet (Credit: University of Pittsburgh). You can find other pictures on this page.

Then, the researchers describe how they check the properties of these nanostructures.

To test the nanostructure's potential as a biosensor and antimicrobial, the authors conducted studies using the water-based nanotubes. Normally a neutral color, when exposed to ultraviolet light the nanotubes changed to a permanent deep blue. The process also chemically altered the nanotubes so that they became polymerized, giving them a more firm structure. Polymerized, these nanotubes could change from blue to other colors, depending on its exposure to different materials. For instance, in tests with acids and detergents, they turned red or yellow.

The most critical tests, say the researchers, were those involving E. coli, which were conducted to assess the material's interactions with living cells. In the presence of E. coli, some strains of which are food-borne pathogens, the nanotubes turned shades of red and pink. Moreover, with the aid of an electron microscope, the researchers observed the tubes piercing the membranes of the bacteria like a needle being inserted into the cell. Both the polymerized (those that can change color) and the unpolymerized nanotube structures were effective antimicrobials, completely killing all the E. coli within an hour's time.

The research work has been published by the Journal of the American Chemical Society under the name "Self-Assembly of Biocidal Nanotubes from a Single-Chain Diacetylene Amine Salt." Here is a link to the article if you're a subscriber -- or if you want to buy it.

Source: University of Pittsburgh news release, September 24, 2004