Ever since the discovery of carbon nanotubes (CNTs), many groups have endeavored to understand the fundamental properties of the CNTs. The redox properties (ie electronic densities, the Fermi levels, redox potentials) of single-walled carbon nanotubes (SWNTs) are related to the structures of SWNTs that have a specified diameter and chirality angle uniquely related to a pair of integers (n, m); the so-called chiral indices.[1, 2] Many attempts have been made to determine the electronic properties of SWNTs using scanning tunneling spectroscopy,[3] redox titrimetry,[4] photoluminescence (PL) measurements,[5–7] and spectroelectrochemistry;[8–13] however, the success in the determination of the redox properties as already reported has been low. Recently, Paolucci et al.[12] employed Vis–near-IR absorption spectroelectrochemistry to estimate the redox potentials of the SWNTs dissolved in an ultradry dimethylsulfoxide (DMSO) solution; however, it is not easy to determine the redox potentials of isolated (n, m) SWNTs using this method because SWNTs with several different chiral indices have band gaps in the near-IR region that overlap one another. We now describe a simple method for the determination of the redox potentials of many (in this study, fifteen) individual (n, m) SWNTs using near-IR PL spectroelectrochemistry in an aqueous medium.

Strategic approaches toward the solubilization of CNTs are essential for many applications of CNTs [14] and numerous dispersants including carboxymethylcellulose sodium salt (CMC, FigureS1a in the Supporting Information)[15] have been used to individually dissolve SWNTs. In this study, we fabricated a non-fluorescent transparent indium tin oxide (ITO) electrode modified with a cast film of CMC/poly-(diallyldimethylammonium chloride)(PDDA; Figure S1b in the Supporting Information) that contained isolated SWNTs (for details, see Experimental Section in the Supporting Information).