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Nanotubes form an antimicrobial magic carpet
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cell membrane likely destabilizes the E. coli cell membrane. Using transmission electron microscopy, the investigators saw that the majority of the nanotubes were fused to the outer surface of the bacterium. In other cases, nanotubes completely enveloped the E. coli cells. Many nanotechnologists are excited about this work. Kenneth Klabunde of Kansas State University comments, “It is a very nice contribution that shows an unexpected nanotube morphology. They have taken advantage of this and attached quaternary ammonium groups, known biocidal agents, to impart biocidal properties. . . . Russell’s work is promising for making biocidal coatings.” Experts in the field of microbiology remain cautiously optimistic, however. They point out that the nanotubes appear to be nonspecifically bacteriocidal and may be equally dangerous to human cells. “The antimicrobial action of the nanotubes is interesting, but we are not told whether [they] are also toxic to human or animal cells. This is clearly important [to establish] before their use with human or animal systems can even be considered. . . . Many antimicrobial compounds are quite toxic,” explains Tomas Ganz of the University of California, Los Angeles. This novel discovery has opened up a plethora of questions that Russell and his colleagues must tackle. “We are focused on the understanding of the chemistry at this stage. The hard part will be polymerizing the tubes and decorating them with further chemistry,” says Russell. The team also seeks to determine the exact mechanism behind the interaction of the nanocarpet with the bacterial membrane. a —Eurona Earl Tilley ALAN RUSSELL
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same diameter. Russell and colleagues anotechnology may prove to be hypothesized that the addition of small the key to the next generation amounts of chloroform to a disordered of antimicrobial weaponry. Alan surface of nanotubes caused the upper Russell and colleagues at the University surface of the tubes to melt, and thus a of Pittsburgh and the McGowan Instilamellar structure was created from which tute for Regenerative Medicine have developed a novel way to synthesize a nano- disoriented pillars could emerge. The tubes became aligned during the gradual carpet that displays biocidal properties removal of chloroform. (J. Am. Chem. Soc. 2004, 126, 13,400– 13,405). The U.S. Army Research Office 1 m “wants a material that kills biological weapons, such as anthrax,” says Russell. The researchers thought that if they could incorporate their nanostructures into the right kind of material, they could make a material that is biocidal. And that is just what they did. “We took a diacetylene, which is a molecule that has the ability to self-assemble in two dimensions, and attached it chemically to a molecule that has the ability to kill A scanning electron micrograph of an antimicrobial cells,” explains Russell. nanocarpet made out of nanotubes. Russell and his colleagues disNext, the investigators evaluated the solved these assembled molecules in a antimicrobial ability of the nanocarpet. solvent and observed the formation of Could these nanocarpets destroy unwantnanotubes. Then, they discovered that ed bacterial cells? To answer this quesUV light cross-links the assembled dition, they incubated E. coli cells with soacetylenes; these links allow the tubes lutions of nanotubes. They found that to become hardened. The investigators found that they could higher concentrations of nanotube material killed 100% of the bacterial cells withmanipulate the nanotubes into a uniform in 1 h. In addition, a color change from nanostructured material, called a nanodark blue to reddish pink was observed. carpet. After they prepared the aqueous nanotubes and dried them on a glass slide, Russell and colleagues speculated that something that causes the color change they applied chloroform and allowed it is released from the cells after they come to evaporate. Scanning electron microsinto contact with the nanotubes. Thus, copy was used to analyze the material the nanostructure can not only kill bactesurrounding the chloroform droplets. The researchers saw a uniformly aligned rial cells but also act as a biosensor. Although the actual mechanism of display of nanotube pillars, which appeared biocidal activity is unknown, Russell adds as a carpet. The pillars were all ~100 nm in diameter and 1 µm in length, and each that the co-location of charge groups held within the nanocarpet next to the pillar contained 3– 4 nanotubes of the
© 2005 AMERICAN CHEMICAL SOCIETY
