Figure 3. Left: Scanning electron microscopy (SEM) images of E. coli cells exposed to CNTs: (a) cells incubated with MWNTs for 60 min; and (b) cells incubated with SWNTs for 60 min. The bars in both images represent 2 µm. Right: Bacterial cytotoxicity of carbon nanotubes: (a) fluorescence-based toxicity assay of SWNTs and MWNTs for suspended- and deposited-type experiments with E. coli; and (b) metabolic activity test of E. coli cells deposited on a CNT-coated filter and a control PVDF membrane. Obtained from [20], with permission from the American Chemical Society 2008.
With CNTs, it is especially important to define the dispersion state of the fibrous colloids. Unfunctionalized CNTs are amphiphobic, which means that they are nearly insoluble in most solvents. Accordingly, dispersions of CNTs can show a wide range of aggregation states that define the accessible surface area that might interact with bacteria [59]. Some studies differentiate between CNTs that are deposited on a substrate and dispersed CNTs, showing widely diverging bacteria toxicity (Figure 3) [57], while other studies completely fail to address these critical aspects. Furthermore, toxicological assessments of carbon nanotubes exposures should take into consideration significant presence of catalytically active iron embedded within the nanotubes [60], as well as other byproducts of production or processing. Possible interactions with test systems (e.g., MTT assay) have been reported as well [11]. Finally, CNT dispersions seldom contain unfunctionalized nanotubes. As will be discussed later, surface adsorbed molecules or covalent functional groups significantly alter bacterial reactions.
2.3. Fullerenes
Fullerenes are spherical carbon molecules. The most studied fullerene is the Buckminsterfullerene (abbreviated as C60), which consists of exactly 60 carbon atoms that are assembled in a pattern resembling a soccer ball. C60 is strongly hydrophobic and is only marginally soluble in water. However, C60 can be dispersed in water as colloidal aggregates (nano-C60 or nC60) under various conditions. Consequently, nC60 should not be confused with individual C60 molecules with diameters of 1 nm. Instead, the colloidal nC60 suspension consists of crystalline aggregates with sizes between 25 and 500 nm that are expected to have a different set of properties as compared to bulk C60 or individual C60 molecules [61]. Although fullerenes are reported as not being very toxic to eukaryotic cells in comparison to nanotubes and other carbon materials [5], they are found to be potent antibacterial agents. Fullerene water suspensions (nC60) were tested for antibacterial activity using B. subtilis by Lyon et al. [21] This study showed that fractions of nC60 containing smaller fullerene aggregates