Water-Soluble Carbon Nanotubes
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P.E. Pehrsson,1 and L.J. Buckley1
2University of Connecticut (Postdoc)
Introduction: Single-wall carbon nanotubes (SWNTs, abbreviated as NTs) have been intensively investigated over the last several years because growing interests in their unique chemical and physical properties for molecular electronics1 and chemical/biological sensors.2 However, the widespread use of NTs is severely limited by the difficult nature of processing and handling them in a facile manner because of its insolubility in a process-friendly solvent. One solution for this is the use of polymers, which not only solubilize NTs by encapsulation but also keep the intrinsic property of NTs intact. However, typical polymers tend to randomly wrap around NT bundles3 in solution unless they have a specific binding interaction that drives individual NTs to be encapsulated.
Approach: Our approach is focused on developing a molecularly controlled process of encapsulation of NTs with water-soluble amylose (a glucose-based natural polymer), whose biocompatibility and functionality are particularly attractive for biological applications. Previously, we have developed expertise4 in the supramolecular inclusion of organic molecules with water-soluble amylose. This commonly requires the use of a solvent to permit a loose, interrupted, helical conformation of amylose to allow the molecular inclusion. The solubilization of NTs by amylose requires two important conditions for efficient encapsulation and colloidal stability: a debundled fine dispersion of the NTs, and a strong binding interaction with the amylose. The former can be made by sonication, but the latter requires an optimal solvent condition for the amylose complexation. The following is a unique process that is simple, fast, and efficient to produce a stable colloidal solution of amylose-NTs complexes, with a helically twisted surface morphology.
For solubilization, a small piece of HiPco NT mat is sonicated in water for 15 min; the resulting fine suspension of NTs is treated with amylose in DMSO with additional sonication of a few minutes, during which the suspended NTs are completely dissolved. The colloidal solution is stable over several weeks without separation. This is extremely important for processing the materials into devices in which a colloidal solution enables various solution processing techniques.
The influence of solvent composition on the NT solubilization by amylose was assessed by ultraviolet/visible light absorption at 500 nm (Fig. 3). The inset shows the trend of the solubilization as a function of DMSO volume fraction, which is represented by the absorption. A strong dependence on the solvent composition is indicative of a special binding interaction4 between NTs and amylose. The slightly lowered solubility in 100% water is due to the relatively poor solubility of amylose in water, in contrast to its homologs5 such as CMA, whose solubilization of NTs is somewhat higher in 100% water.
As shown in Fig. 4, the SEM image (A) shows loosely twisted NTs with a diameter of approximately 30 nm. The atomic force microscope (AFM) gives an enhanced image of the helical twist (C), which reveals a locally intertwined multiple twist (B), which is probably caused by the sonication processing step. This twisted structure discloses a new feature where amylose can encapsulate a large diameter (≥1 nm) guest molecule. This is driven by hydrophobic interactions due to the chirality and the linear chain structure.5 Raman spectra (Fig. 5) of amylose-encapsulated NTs indicate a small shift (3-5 cm-1) relative to the pristine NTs in the radial breathing mode frequency. This structure is similar to that described6 previously for polymer-wrapped NTs and is further evidence of the encapsulated structure.
Summary: In conclusion, a simple, efficient process for solubilization of NTs with amylose in aqueous DMSO has been described. This process requires sonication of the NTs in water and subsequent treatment of the fine NT dispersion with amylose with an additional brief sonication. The best solvent condition is 10-20% DMSO, in which amylose assumes an interrupted loose helix, suggesting that the helical state of amylose is not a prerequisite for amylose encapsulation of NTs. The scanning electron microscope and AFM display images of twisted ribbons showing the encapsulation of NTs by amylose.
[Sponsored by ONR]References
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