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We report an investigation of the nature and chemical functionalization of nitric acid treated single-walled carbon nanotubes (SWNTs). SWNTs washed with diluted sodium hydroxide solutions were characterized by near-IR, mid-IR, and Raman spectroscopy as well as TEM, and the remaining carboxylic acid content was determined to assess the effect of base washing on the removal of carboxylated carbon fractions, which are generated by the nitric acid treatment. It was found that even after exhaustive washing with aqueous base the purified SWNTs contain carboxylic acid groups in sufficient quantity to prepare high quality soluble SWNT materials by covalent functionalization with octadecylamine.

Using the Bingel reaction as a model for side-wall functionalization of single-walled carbon nanotubes, we report the discovery of highly regular, long-distance (several nanometer) patterns and examine the conditions for the occurrence of such patterns, possibly due to longrange induced reactivity. Varying periodicities of the patterns have been observed via scanning tunneling microscopy and are attributed to nanotube geometry. Patterns are most prominent on medium heavy functionalized nanotubes and likely tied to a nucleophilic reaction mechanism.

The carbon nanotube−polyethylene glycol (PEG) graft copolymer was synthesized by covalent functionalization of electric arc single-walled carbon nanotubes (SWNTs) with the monofunctional, tetrahydrofurfuryl-terminated polyethylene glycol PEG-THFF (MW∼200), to give a material composed of 80 wt % SWNTs. We show that the sequential processing of the resulting material by ultrasonication and high-shear mixing provides a means to disperse the SWNT-PEG-THFF macromolecules on two different length scales and leads to highly viscous solutions; at a concentration of 10 mg/mL the kinematic viscosity (ν) of an aqueous SWNT-PEG-THFF dispersion reaches a value of ν > 1000 cSt (for water ν ∼ 1 cSt). Analysis of this procedure by means of viscosity measurements and atomic force microscopy (AFM), shows that ultrasonication is effective in disrupting the SWNT bundles, while the high shear mixing disperses the individual SWNTs. The kinematic viscosity of aqueous dispersions of SWNT-PEG-THFF was measured as a function of nanotube concentration and compared to that of SWNT-PEG dispersions. The viscosity and AFM measurements show that the SWNT-PEG-THFF and SWNT-PEG graft copolymers form aqueous dispersions with distinct viscous characteristics; the use of monofunctional PEG-THFF for covalent functionalization of the SWNTs prevents cross-linking of the SWNTs in the final product, and this allows the production of more completely dispersed SWNTs than in the case of the SWNT-PEG graft copolymer, which is synthesized from a bifunctional glycol.

The detection of gas analytes relies on a variety of operating mechanisms which include ionization of gases, modulation of optical properties, gas chromatography, mass spectrometry, electrochemistry, conductance modulation in field effect transistors, and chemical resistors. The modulation of the electronic characteristics of a sensor device based on a chemical resistor is a very attractive approach because of its simplicity. In the search for novel sensor materials, which can broaden the range of detected molecules and the limits of detection, single-walled carbon nanotubes (SWNTs) have emerged as a promising candidate. Most of the research in the field has been focused on understanding and exploring the sensitivity of the electronic structure of SWNTs to charge transfer from adsorbed molecules. [1][2][3][4][5][6][7][8] It has been shown that defects, [9,10] non-covalently attached polymers, [11][12][13][14] and metal [15,16] or metal oxide nanoparticles [17] can affect the sensitivity of SWNTs to gas molecules. The research on the ability of SWNTs to act as a sensor material has primarily used gas molecules with electron donating or withdrawing properties such as ammonia and nitrogen dioxide. [1,8,18,19] The mechanism of modulation of the transport properties of SWNTs is usually attributed to charge transfer between the dopant and carbon nanotubes, which is assumed to lead to refilling or depletion of the valence band of the semiconducting SWNTs.Previously we reported that covalent functionalization can dramatically increase the resistance change of SWNT films during exposure to gas molecules. [5,7] The advantages of covalently functionalized SWNT materials for sensor applications are obvious: i) facile dispersibility in solvents and enhanced processability, ii) reproducible and well-defined chemical composition, and iii) possibility of attaching organic moieties specifically engineered to interact with analyte molecules. In our previous work on the sensing mechanism of poly-(m-aminobenzene sulfonic acid)-functionalized SWNTs (SWNT-PABS) toward ammonia, [5,7] we argued that the electron affinity of the attached PABS functionality was modulated by an acid-base equilibrium and that this directly influenced the conductivity by controlling the concentration of holes in the valence band of the semiconducting SWNTs.In the present study we explore the generality of this approach by employing chemically functionalized SWNT materials with a range of basicities and electronic structures and studying the change of their electronic properties on exposure to hydrogen chloride.For this study we synthesized three types of functionalized SWNT materials with covalently attached functional groups of varying basicity (Table 1) and compared the change of their electronic properties to that of purified non-functionalized SWNTs on exposure to HCl. Thin films of the SWNT materials were deposited on a glass substrate with pre-patterned interdigitated gold contacts (Fig. 1a). The SWNT materials were dispersed in a solvent (Table 1) by ...

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