We measured the UV optical absorption of single-stranded DNA bound to single-walled carbon nanotubes (DNA/SWNT). The nucleotide absorbance from DNA/SWNT provides the first experimental confirmation that DNA binds to nanotubes through π-stacking. Because the hypochromic absorbance typical of π-stacked structures are expected to occur primarily for DNA dipole transitions that lie along the axis of the optically anisotropic SWNTs, the absorbance changes following binding of DNA to nanotubes reveal the preferred orientation assumed by each of the four bound nucleotides with respect to the nanotube’s long axis.
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