Reversible water-solubilization of single-walled carbon nanotubes by polymer wrapping

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Abstract

Single-walled carbon nanotubes (SWNTs) have been solubilized in water by non-covalently associating them with linear polymers, most successfully with polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS). This association is characterized by tight, uniform association of the polymers with the sides of the nanotubes. A general thermodynamic drive for this wrapping is discussed, wherein the polymer disrupts both the hydrophobic interface with water and the smooth tube–tube interactions in aggregates. The nanotubes can be unwrapped by changing the solvent system. This solubilization process opens the door to solution chemistry on pristine nanotubes, as well as their introduction into biologically relevant systems.

Introduction

For all the promise of single-walled carbon nanotubes (SWNTs) implied by their unique architecture and remarkable mechanical [1] and electrical [2] properties, their poor solubility characteristics [3] have hindered their chemical manipulation, and thus their use in applications. The most successful approaches to SWNT solubilization to date have involved chemical modification of the tubes, significantly altering their desirable properties [4], [5]. And the attempts at dissolving the SWNTs in water, the medium required for any eventual interface to biochemistry, have relied on surfactants, an approach severely restricting the concentrations accessible [6] and with the potential to denature biological molecules.

In this study we describe a technique for rendering underivatized SWNTs water soluble in the g/l concentration range. This approach, wrapping water-soluble linear polymers around the tubes, is robust and general, allowing pristine nanotubes for the first time to be manipulated reliably by solution-phase techniques such as chromatography and electrophoresis, and simplifying their use as chemical reagents.

Section snippets

Experimental

SWNTs were produced by both the laser-oven [7] (Tubes@Rice, Houston, TX) and HiPco [8] methods. As received, laser-oven SWNT material (dispersed in toluene) was filtered (Whatman #41) and washed with methanol and then with water. The SWNTs were then homogenized with a high-shear mixer (Polyscience X-520) and refiltered repeatedly until the filtrate was clear and colorless. HiPco SWNT material was purified by gas-phase oxidation, hydrochloric acid extraction, and high-temperature annealing [9].

Solubilization by polymer wrapping

AFM images of PVP–SWNT supramolecular aggregates adsorbed onto amine-functionalized substrates show SWNT height and length distributions consistent with the notion that most of the complexes consist of a single SWNT associated with at most a monolayer of polymer, and a smaller percentage of ropes consisting of more than one SWNT (Fig. 1). The solutions formed are stable for months, and easily pass through a 1μm track-etched polycarbonate filter membrane. When the material is dried, it is easily

Conclusions

SWNTs have been reversibly solubilized in water by wrapping them with a variety of linear polymers. We have demonstrated that the association between the polymer and the SWNT is robust, not dependent upon the presence of excess polymer in solution, and is uniform along the sides of the tubes. A general thermodynamic driving force for such wrapping in an aqueous environment has been identified. This solubilization provides a route to more precise manipulation, purification, fractionation, and

Acknowledgments

We would like to acknowledge Michael Bronikowski and Peter Willis for providing HiPco SWNT material, and Wan-Ting Chiang and Robert Hauge for purifying the material. We thank Larry Alemany for exceptional assistance with the NMR measurements, Robert Hauge, Ken Smith, and Dan Colbert for helpful discussions. We would also like to thank Milton Pierson for elemental analysis, and Deron Walters for aid with the nematic solid work. Funding for this work was provided by NASA, the Robert A. Welch

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