Ecological Uptake and Depuration of Carbon Nanotubes by Lumbriculus Variegatus

Article excerpt

Carbon nanotubes have been the subject of extensive research over the past decade because of potential breakthroughs in a broad range of applications. Discovered by Sumio Iijima in 1991 (Iijima 1991), nanotubes are essentially seamless cylinders composed of sp2-hybridized carbon atoms arranged in a regular hexagonal pattern. Single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) make up the two principal classes of carbon nanorubes. SWNTs are one-layer graphitic cylinders having diameters on the order of a few nanometers, whereas MWNTs are composed of numerous concentric cylinders having much larger diameters.

Although carbon nanotubes have drawn widespread research attention in recent years, their potential environmental and human health impacts have not been well characterized, and the risks they may pose to the welfare of humankind and the environment are largely unknown (Colvin 2003). A number of studies have indicated that carbon nanotubes can, in fact, enter cells (Cherukuri et al. 2004; Heller et al. 2005; Kam et al. 2004, 2006; Kostarelos et al. 2007; Monteiro-Riviere et al. 2005) and cause toxic damage to cells (Pulskamp et al. 2007; Sayes et al. 2006; Shvedova et al. 2003) and to aquatic organisms (Cheng et al. 2007; Roberts et al. 2007; Smith et al. 2007; Templeton et al. 2006). SWNTs have been detected qualitatively in Daphnia magna (Roberts et al. 2007), the estuarine copepod Amphiascus tenuiremis (Templeton et al. 2006), and the fish Oncorhynchus mykiss (Smith et al. 2007), although the extent to which nanotubes accumulate in these organisms was not quantitatively ascertained. One potential approach for predicting such behaviors is via comparison with hydrophobic organic chemicals that share some chemical similarities, such as polycyclic aromatic hydrocarbons (PAHs), counterparts of smaller sizes having between two to seven aromatic rings. PAHs accumulate readily in the fatty tissue of organisms, in large part as a result of the combined hydrophobicities and resistances of these chemicals to microbial degradation (Di Toro et al. 1991; Jager et al. 2003). Taken in combination with the observed facile cellular uptake of nanotubes and their detection in aquatic organisms, this leads to the conjecture that carbon nanotubes may also be bioaccumulable entities. This would, of course, have profound implications for ecological and human health. If organisms do uptake carbon nanoparticles, these materials, like PAHs, might also be transferred through food chains and enter organisms, such as humans, at higher trophic levels in significant amounts. In addition to the risks posed by the carbon nanotubes themselves, these materials strongly adsorb organic and inorganic chemicals and may exacerbate the biological uptake of such pollutants in environmental systems (Li et al. 2003; Yang et al. 2006).

A substantial challenge that has limited investigation of nanotube behaviors in environmental settings is the lack of a method by which to quantify them in biological or environmental media. The polydispersivity of nanotube mixtures with a broad range of diameters and lengths hinders the use of chromatographic techniques. Methods based on elemental analyses and spectroscopic techniques are generally not feasible because of the presence of organic matter. Spectrofluorimetry is one approach that has been used successfully to quantify nanotubes in cells and rabbits (Cherukuri et al. 2004, 2006). Given the unknown typical aggregation state of nanotubes in environmental systems, this technique has limited potential because of its inability to detect metallic SWNTs or nanotube bundles with metallic SWNTs (O'Connell et al. 2002). Raman spectroscopy was used to detect SWNTs qualitatively in the aquatic organism D. magna (Roberts et al. 2007). This approach, however, cannot provide quantitative results and is best suited for only SWNTs. A method used recently to detect nanotubes in biological systems is tagging them with molecules that are either bonded to radioactive isotopes or are themselves fluorescent (Kam et al. …