In conclusion, we observed a surprising enhancement of the SWNT QY in aqueous suspensions upon addition of reducing agents, which raises the upper limit of the QY for the brightest individual SWNTs to over 30%. This significant enhancement was attributed to the removal of unintentionally doped holes at defect sites along the sidewall through electron donation from reducing agents. Because most optical studies of SWNTs use samples that have been isolated using a similar procedure to the one employed here, but in absence of the reducing agents, our studies indicate that nearly all linear and nonlinear optical measurements performed on carbon nanotubes have, to this point, been performed largely on defective SWNTs. SWNTs have been commonly referred to as intrinsically poor emitters, especially in water; yet our findings suggest that the brightest SWNTs are indeed highly emissive. Intriguingly, it may be possible to increase the QY of SWNTs even further through use of milder sample preparation processes or application of a magnetic field that would brighten dark excitons.(33)