Single walled carbon nanotubes (SWCNT) exhibit unique physical, chemical and electrical properties that have made them an attractive material for use in industry, medical diagnostics and drug delivery 1
; however, enthusiasm for their use has been tempered by relevant concerns regarding their toxicity. The high ratio of the length to the diameter (aspect ratio) of SWCNTs and multi-walled carbon nanotubes (MWCNTs) has led some investigators to compare these particles with asbestos fibers 2-4
, which are also characterized by a large aspect ratio but with a much larger mean diameter (20-250 nm) 5
. Furthering the analogy with asbestos fibers, it has been found that decreasing the length of MWCNTs resulted in reduced toxicity 6-8
. These studies hypothesized that the failure of resident macrophages to clear and eliminate carbon nanotubes results in activation of pro-inflammatory pathways in these cells that induce lung fibrosis and increase the susceptibility to pulmonary malignancies 3, 4, 6-9
While individual carbon nanotubes do possess high aspect ratios, van der Waals interactions between nanotubes in air or in aqueous solutions cause them to form large aggregates, which can be more than one hundred microns in diameter 10
and it is the administration of these aggregates that has been associated with lung toxicity in rodents 11-14
. While accidental industrial exposure to these large aggregates is certainly relevant15
, effective dispersal of SWCNTs at the nanoscale is required for them to exhibit many of their desirable physical properties 12, 14, 16
. Indeed, investigators who have suggested that carbon nanotubes be used in biomedical applications have used several methods to achieve improved dispersion 17
. With this motivation, we sought to test the hypothesis that the observed toxicity associated with exposure to SWCNTs is induced by large nanotube aggregates rather than the large aspect ratio of the individual nanotubes. To do this, we exposed alveolar epithelial cells and mice to aggregated SWCNTs or highly dispersed solutions of SWCNTs having a mean diameter of ~1 nm using a biocompatible block copolymer, Pluronic F 108NF (Pluronic). Despite the substantially increased aspect ratio of the individually dispersed SWCNTs compared with the aggregated SWCNTs, we observed that the individually dispersed SWCNTs exhibited no toxicity in vivo
and appeared to be cleared from the lung over time. These results suggest that comparisons of SWCNTs with asbestos fibers based on the similarities between their aspect ratios may not be justified. We suggest that highly dispersed SWCNTs in a biocompatible block copolymer can be safely handled to take advantage of their unique physiochemical processes for industrial and biomedical applications.
Stable nanoscale dispersions of as-produced HiPco SWCNTs (Unidym, Inc.) were produced by ultrasonication of nanotube powder in 1 wt% aqueous solution of Pluronic F 108NF (BASF Corporation) 18-20
followed by ultracentrifugation, which eliminated large SWCNT bundles and dense impurity species. Optical absorbance, photoluminescence, and microscopy measurements were used to determine the dispersion quality of the SWCNT/Pluronic solutions. The SWCNT optical absorbance prior to ultracentrifugation was marked by a broad background signal that varied inversely with wavelength, characteristic of carbonaceous materials (). The significant degree of bundling in this dispersion, led to weak and broad SWCNT-related absorbance peaks. In contrast, the absorbance of well-dispersed SWCNTs following ultracentrifugation revealed strong absorbance peaks arising from metallic and semiconducting SWCNT species. These SWCNT-related peaks were significantly sharper than those prior to centrifugation as a result of the increased proportion of individual SWCNTs in the solution 21
. Photoluminescence measurements conducted on the well-dispersed SWCNT solutions also showed clear photoluminescence peaks arising from different semiconducting SWCNT chiralities (). These measurements provide further evidence for individual dispersion of SWCNTs in the Pluronic solution as the fluorescence of semiconducting SWCNTs is known to be quenched by metallic SWCNTs when they are in bundled form 21
. Using inductively coupled plasma atomic emission spectroscopy (ICP-AES), we determined that the raw and well-dispersed HiPco SWCNTs had iron impurity levels of 20.6 and 8.5 wt%, respectively. Using different methods, other investigators have achieved lower levels of metal contamination without a change in the observable toxicity 14, 22
, thus the reduction in iron impurity levels between the raw and well-dispersed samples is unlikely to influence their relative toxicity.
Characteristics of the nanoscale dispersed SWCNTs
To assess nanotube dispersion and length, the SWCNTs in Pluronic were deposited onto SiO2
. Atomic force microscopy of such samples revealed high aspect ratio features with heights of ~1-2 nm, which correspond to the expected height of individual and small bundles of SWCNTs (). In addition, the length of the nanotubes was found to vary between ~100 to 2000 nm with a mode length of ~500 nm (). The SWCNTs dispersed in Pluronic formed a homogenous black mixture that was markedly different from SWCNTs dispersed in saline (Supporting Information Figure S1A versus S1B
). Similar dispersion was not observed when particulate matter air pollution less than 2.5 μm in diameter (PM2.5
) or crocidolite asbestos were dispersed in Pluronic (Figure S1C-F).
These results suggest that centrifugal processing might provide a scalable method to generate nanoscale dispersed, purified preparations of SWCNTs. Modification of this procedure using multiple block copolymers can also be used to sort the SWCNTs based on their diameter and electrical properties as we have previously described 24