Other plausible mechanisms for the SWNT fluorescence enhancement were considered but are not consistent with our observations. For example, upon addition of reducing agents, the absorption spectrum did not change significantly in the spectral region corresponding to (6,5) SWNTs (Figure S6, Supporting Information), although the fluorescence intensity increased dramatically (Figure 5a). These observations indicate that simple doping of p-type SWNTs into the valence band, expected to restore the strong excitonic absorption at the E11 energy, is not a likely cause of the fluorescence enhancement.(7) It is also possible that covalent interactions could occur between a defect site and the reducing agent. However, the reversible nature of the fluorescence enhancement (Figure S2, Supporting Information) indicates that the reductants did not permanently bind to the SWNTs.