Relative abundance of different single-walled carbon nanotubes (SWNTs) (n,m)-species in aqueous suspensions is not known due to the lack of experimental data on such photophysical parameters as absorption cross-sections and fluorescence quantum yields. An emission signal S around the peak wavelength l₁₁ from an individual nanotube excited with external radiation of known intensity I(l_exc) is described as S(l₁₁) = h(l₁₁)·a(l₁₁)·I(l_exc)·{s(l_exc)/(s(l₂₂)}·s(l₂₂)·F_FL , where h(l₁₁) is the detection efficiency of the detector, a(l₁₁) is the collection efficiency of the experimental system, s(l_exc) and (s(l₂₂) are SWNTs absorption cross-sections at the laser excitation wavelength l_exc and the l₂₂ absorption maximum, F_FL is the fluorescence quantum yield. By measuring S(l₁₁), h(l₁₁), a(l₁₁), I(l_exc) and s(l_exc)/(s(l₂₂), the product of SWNTs absorption cross-section s(l₂₂) and its F_FL is directly accessible. Photoluminescence properties of different (n,m)-types can be compared once the total emission signal is normalized per unit of carbon nanotube matter, be it the number of carbons or the unit of length. SWNT suspensions containing individual nanotubes with lengths greater than 3 µm were prepared. Nanotube samples were excited with circularly polarized laser beams at 658, 730, 785 and 850 nm. We imaged these “long” individual nanotubes with well-resolved spatial dimensions in a thin aqueous film using near-IR fluorescence microscopy and measured their emission spectra to find the nanotube's (n,m) identity. Only nanotubes whose E₂₂ transitions were nearly resonant with the excitation lasers were selected, so that s(l_exc)/(s(l₂₂) ratio could be determined with sufficient accuracy from bulk spectrofluorimetric measurements. Within examined (n,m)-species, we find variations of s(l₂₂)·F_FL parameter from 0.5×10⁵ to 2.7×10⁵ cm²/mol C. SWNTs of (10,2) type displayed the highest photoluminescence intensity with estimated F_FL of ca. 8%. Using the measured s(l₂₂)·F_FL values, we normalized emission signals from different (n,m)-species in aqueous suspensions to deduce relative abundance of particular SWNT structures.

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