However, macroscopic samples of SWNTs have extremely low fluorescence efficiencies (<0.1%), initially attributed to the presence of nonemissive metallic nanotubes and residual bundles among the isolated SWNTs.(3) Even with the recent availability of higher quality presorted samples in which these undesired species are significantly removed, the fluoresecence quantum yield (QY) remains poor.(8, 9) Excluding one recent study,(10) the brightest SWNTs(8, 11) and, importantly, individual SWNTs in water(9, 12, 13) display fluorescence QYs of only a few percent, much lower than other emissive nanomaterials.(14) Although SWNT fluorescence is potentially much brighter in certain organic solvents,(10) poor nanotube fluorescence efficiencies in water remain an issue of central importance because aqueous environments are required for any applications in vivo. To address this challenge, it must first be understood whether the QY is limited by the intrinsic photophysical pathways of all SWNTs or an extrinsic quenching mechanism.(15-17)
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