Fluorescence has been observed directly across the band gap of semiconducting carbon nanotubes. We obtained individual nanotubes, each encased in a cylindrical micelle, by ultrasonically agitating an aqueous dispersion of raw single-walled carbon nanotubes in sodium dodecyl sulfate and then centrifuging to remove tube bundles, ropes, and residual catalyst. Aggregation of nanotubes into bundles otherwise quenches the fluorescence through interactions with metallic tubes and substantially broadens the absorption spectra. At pH less than 5, the absorption and emission spectra of individual nanotubes show evidence of band gap-selective protonation of the side walls of the tube. This protonation is readily reversed by treatment with base or ultraviolet light.

Individual single-walled carbon nanotubes (SWNTs) have been suspended in aqueous media using various anionic, cationic, nonionic surfactants and polymers. The surfactants are compared with respect to their ability to suspend individual SWNTs and the quality of the absorption and fluorescence spectra. For the ionic surfactants, sodium dodecylbenzene sulfonate (SDBS) gives the most well resolved spectral features. For the nonionic systems, surfactants with higher molecular weight suspend more SWNT material and have more pronounced spectral features.

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In acidic solution between pH 6 and 2.5, protons react reversibly and selectively in the presence of preadsorbed oxygen at the sidewall of aqueous dispersed single-walled carbon nanotubes suspended in sodium dodecyl sulfate. This reactive complex, which protonates the nanotube sidewall, reversibly diminishes absorption intensity, fluorescent emission, and resonant Raman scattering intensity. The results document the first evidence of electronic selectivity with metallic nanotubes reacting initially near neutral pH, followed by successive protonation of nanotubes with increasing band gap as the solution is increasingly acidified. Preadsorption of molecular oxygen is shown to play a critical role in the interaction, and its desorption kinetics is followed using UV irradiation. The role of the charged electric double layer of the surfactant is discussed. This chemistry, which proceeds under relatively mild conditions, holds promise for separating nanotubes by metal and semiconducting types.