Determination of the Concentration of Single-Walled Carbon Nanotubes in Aqueous Dispersions Using UV-Visible Absorption Spectroscopy S. Attal, † R. Thiruvengadathan, † and O. Regev* ,†,‡ Department of Chemical Engineering and The Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel Stable, homogeneous, aqueous dispersions of single- walled carbon nanotubes (SWNTs) are prepared by non- specific physical adsorption of surfactants enhanced by sonication. Upon centrifugation, supernatant and precipi- tate phases are obtained. The initial weights of the SWNTs and the surfactant are divided between these two phases, and the respective SWNT concentration in each phase is unknown. The focus of this work is on the determination of the true concentration of raw, exfoliated HiPCO SWNTs in the supernatant phase. A UV-visible absorption-based approach is suggested for a direct measurement of the SWNT and the surfactant concentration in the superna- tant. UV-visible absorbance spectra of SWNTs-surfactant dispersions and surfactants alone reveal that the intensity of a certain peak, attributed to the π-plasmon resonance absorption, is unaffected by the presence of most surfac- tants. A calibration plot is then made by monitoring the intensity of the peak as a function of the true concentration of the exfoliated SWNTs. Thus, we are able to determine the unknown concentration of surfactant-dispersed HiP- CO SWNTs in the supernatant solution, simply by mea- suring its optical absorbance. Moreover, we can now calculate the surfactant efficiency in dispersing SWNTs. Cryogenic-transmission electron microscopy and thermo- gravimetric analysis techniques are used for the charac- terization of these dispersions and to complement the UV-visible measurements. The discovery of carbon nanotubes 1 (CNTs) in the past decade has immensely contributed to the advancement of nanoscience and technology. The potential of CNTs as transistors, chemical and biological sensors, field emission sources, and filler in polymer matrixes is attributed to their exotic physical properties, such as high aspect ratio, low density, high tensile strength, and aniso- tropic electrical conductivity. 1-5 However, the as-synthesized single-walled carbon nanotubes (SWNTs) are bundled, 6 preventing their efficient use. 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