Nanocarpets are formed by stacking a large number of nanotubes together, with their cylindrical axes aligned vertically. Nanocarpets capable of changing color and of killing bacteria have been assembled from specially designed lipids that spontaneously assemble into a variety of nanostructures depending on the conditions. In water, nanotubes are formed. Partial rehydration of dried nanotubes generates a side-by-side array- the nanocarpet.
The lipid consists of a long hydrocarbon chain (25 carbons) with a diacetylenic group in the nanotubes are about 100 nm in diameter by 1000 nm in length. The walls of the nanotubes consist of five bilayers of the lipid. Both the separate lipid molecules and the assembled nanocarpet kill bacteria. Like other long-chain amino compounds, they act as detergent molecules and disrupt the cell membrane. Consequently, the nanocarpet provides a surface lethal to bacteria. This property could be very useful if nanocarpets are used in biomedical applications.
Diacetylenic compounds have the interesting ability to change color. The nanocarpet starts out white, but if exposed to ultraviolet light, it turns deep blue. UV irradiation causes crosslinks to form by reaction between acetylenic groups on neighboring molecules.
This polymerization stabilizes the nanocarpet. Blue nanocarpets change color on exposure to a variety of reagents. Detergents and acids change them from blue to red or yellow, and the presence of bacteria, such as E. coli, gives red and pink shades. Eventually such materials may be used both as biosensors and for protection against bacterial contamination.