shows the relative ability of TPGS and Triton to disperse multiwall nanotubes at a fixed nanotube concentration of 15 ug/ml. The nanotubes are successfully dispersed at TPGS concentrations of 3.8 ug/ml and above, while lower concentrations leave a film of undispersed solid at the top and bottom liquid/gas interface. At even higher TPGS concentrations, stable gas bubbles collect at the top surface indicating excess TPGS. Though Triton is commonly used for nanotube dispersion [5
], it is less successful here, leaving excess solid at the top liquid/gas interface at concentrations up to 15 or even 60 ug/ml. The polyaromatic content of TPGS may be responsible for its effectiveness in nanotube dispersion based on the known affinity of polyaromatic structures for carbon surfaces [40
] and trends seen in other surfactant systems [5
FIGURE 2 Effectiveness of TPGS and Triton in dispersing MWNTs. Digital photos of 15 ug/mL MWNT suspensions after 2-hour sonication and 15-hour settling. TPGS studied achieves complete dispersion (“CD”) above 3.8 ug-TPGS/mL. Triton leaves some MWNTs (more ...)
indicates dispersion stability after 15 hr of passive settling. We further tested the stability of the suspensions to centrifugation at a fixed surfactant concentration of 25 uM. The MWNTs were removed from the Triton solution after 30 min centrifugation at 1380×G, while an equivalent separation from 25 uM TPGS solution required 4 hr centrifugation at 1380×G. In the case of SWNTs, the two surfactants performed more similarly, dispersing SWNTs well at concentrations above about 6 ug/mL.
is extremely insoluble in water (~1 ng/mL without the use of co-solvents, oxidation, or prolonged stirring [31
]. gives the results for C60
dispersion with TPGS compared with Triton as a reference material using UV/visible spectroscopy as a dispersion metric. Note that in contrast to nanotube dispersion, this is a solubilization process in which molten pure TPGS is saturated with fullerene prior to introduction of water. The UV-vis light spectra in ,C demonstrate the existence of C60
, with fullerene characteristic peaks present at 331, 358, 379, and 469 nm [43
]. give quantitative absorption data for these peaks that can be used to estimate C60
concentration. As an example, we estimate 210 ug-C/mL in 25 wt% TPGS solution using Beer’s Law and ε = 5.19×104
for the absorption peak at 331 nm in water [44
]. TPGS is more effective than Triton at dispersing fullerenes as clearly seen by direct comparison of the spectra (A vs. B) or the quantitative absorption measurements (C vs. D). The solubility of C60
is linearly dependent of the surfactant concentration, which is consistent with previous work using commercial TPGS 1000 as drug solubilizer [19
]. It is also consistent with our preparation method, in which saturated solutions with a fixed TPGS/C60
ratio are first prepared and then hydrated. Dynamic light scattering experiments show monodisperse nanoparticles of hydrodynamic size 11 nm, which are believed to be fullerene-containing TPGS micelles, and are similar to the pure TPGS micelles with a mean size of 12.7 nm measured by the same technique.
FIGURE 3 Solubility of C60 in surfactant solutions probed by UV-Vis light absorption. A,B UV-Vis absorption spectra: (A) in TPGS solutions (note: 25 wt-% solution was diluted by 5x prior to spectral measurement); (B) in Triton solutions. C,D: Quantitative optical (more ...)
shows the morphology of TPGS/nanotube assemblies by HRTEM. An amorphous TPGS coating is seen on the MWNT surfaces dried from TPGS solution, but not the as-received MWNTs. This tube-by-tube TPGS coating is the most likely colloidal stabilization mechanism. There appears to be an amorphous film on the SWNT bundles as well, though it is not as pronounced in TEM. Neither TEM nor UV-Vis spectroscopy gives evidence for individual, fully unbundled SWNTs in the TPGS or Triton solutions under our conditions [45
]. It is possible that these surfactants do not have a favorable geometry for stabilization of the very small 1–2 nm structures as does single-stranded DNA [46
]. shows SWNTs dried from concentrated TPGS solutions (15 ug/ml). Under these conditions the excess TPGS associates with the SWNT bundles as clearly visible, intact micelles.
FIGURE 4 HRTEM images of carbon nanotubes in TPGS solutions after 2-hour sonication. (A–D) 15 ug/mL MWNTs (A,C) in water or (B,D) in 3.8 ug/mL TPGS solution. Scale bar: 5 nm. (E,F) 15 ug/mL SWNTs (E) in water, (F) in 15 ug/mL TPGS solution. Scale bar: (more ...)
gives the morphologies of TPGS/C60
assemblies at various stages of drying. Initially spherical structures are seen (), which in size and morphology are consistent with pure TPGS micelles. Under the action of the electron beam we observed the appearance of high-electron-density regions within the spheres that suggests phase separation (see arrow in B). During longer air drying at room temperature (3 days), the spherical nanostructures develop facets and transform into nanocubes, some of which have asymmetric electron density (). Finally after 1 week of drying, the sample transforms into a uniform collection of highly ordered asymmetric particles that appear to be fullerene nanocrystals (nC60
) adhered to globular or cubic nanostructures of lower electron density. These cubic structures are equiaxed and about 10 nm in dimension, similar to the length of the TPGS molecule. The ED pattern in confirms the formation of nC60
during air drying, showing a face-centred cubic (fcc) structure with a crystal parameter of 1.4 – 1.5 nm [43
FIGURE 5 HRTEM images of unique TPGS/C60 nanostructures following various degrees of drying from 3.8 ug/mL TPGS/C60 solutions. A,B: 20-hour air drying; (arrow shows appearance of fullerene nanocrystals upon e-beam exposure), C,D: faceted cubic particles after (more ...)
The ED pattern of TPGS/C60 nanostructures after 3-day air drying. The rings are assigned to fcc-structure of nC60, but the (111) ring is not observed due to its very large d value, 0.82 nm.
We interpret these results as follows. Fullerene is dissolved at the molecular level in TPGS micelles, likely concentrated in the hydrophobic α-tocopheryl cores. Because the mass ratio of fullerene to TPGS is low (~ 1:1000) these micelles are similar in size and structure to those in pure TPGS solutions. The TPGS micelles remain amorphous unless dried extensively in air or under the electron beam, at which point the C60 phase separates and the dehydrated TPGS crystallizes. When the amphiphile TPGS crystallizes at the nanoscale, the result are amphiliphilic nanoparticles with one hydrophobic end (tocopheryl) and one hydrophilic end (PEG). In the presence of C60 this produces a set of unique asymmetric particles with fullerene nanocrystals attached at one face to TPGS nanobrushes. The uniform and highly ordered nature of these particles suggests a significant driving force for their formation and some significant stabilizing mechanism, which we believe is the attachment of fullerene crystals to the hydrophobic end of the TPGS nanobrushes, thus stabilizing the structure by tocopheryl/C60 hydrophobic interaction.
shows zeta potentials of nanotube/surfactant suspensions measured at room temperature. Increasing surfactant concentration decreases the magnitude of the zeta potentials as expected for these non-ionic surfactants, which preserve the nanotube surface charge but increase the hydrodynamic particle size. The zeta potential of aqueous C60 in surfactant solutions was −5.0±1.2 mV, and the pure TPGS and Triton micelles did not have a measurable zeta potential. Compared to other methods of preparing aqueous C60 dispersions, the present method has the advantage of avoiding cosolvents or covalent fullerene modification. The fullerene/TPGS solutions are stable indefinitely and non-enzymatic hydrolysis at physiological pH is extremely slow. A disadvantage is the large TPGS/C60 mass ratio required (~ 1000:1), which in turn is limited by the inherent solubility of C60 in molten TPGS in our preparation method. This is in contract to TPGS/MWNT suspensions, where the minimum ratio for good dispersion is only 1:4 (3.8:15 as seen in ).
Zeta potentials of nanotube/surfactant suspensions at: 15 ug-CNT/mL; A: MWNT; B: SWNT.