Two different single-walled carbon nanotubes (SWNTs), the so-called HiPco and CoMoCAT, have been individually dissolved in aqueous solutions of double-stranded DNA (dsDNA). Atomic force microscopy (AFM) revealed the fine structures of the dsDNA-wrapped SWNTs. The near-IR absorption and photoluminescence (PL) spectra of aqueous solutions of dsDNA-wrapped SWNTs were recorded and, in pure water, we observed only a single two-dimensional PL spot from (6,5) SWNTs for both HiPco and CoMoCAT. In sharp contrast, when Tris-EDTA (TE) buffer was used in place of pure water, the PL-mapping images of the solutions showed chirality indices of (6,5), (7,5), (7,6), (8,4), (9,4), and (10,2) for HiPcoSWNTs, and (6,5) and (7,5) for CoMoCAT-SWNTs. The first semiconducting bands in the near-IR absorption spectra of solutions of dsDNA-wrapped SWNTs are different. To explain the observed differences in the near-IR absorption and PL behavior we conducted several experiments and found that the near-IR optical properties of the SWNTs can be modulated by changing the pH of the solutions. The pH breakpoints for near-IR absorption bleaching and PL quenching are different and the phenomena are explained by differences in the numbers of holes generated on the SWNTs. These findings are important from both fundamental and applied viewpoints.
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