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.Zheng et al. reported that bundles of SWNTs in water can be effectively dispersed by sonication in the presence of single-stranded DNA (ssDNA). 1,2 Molecular modeling by that group suggested that DNA molecules hybridize with individual nanotubes by wrapping around them, with the interaction strength being provided by π-stacking, with the plane of the aromatic nucleotide bases oriented parallel to the surface of the nanotube. 1 The novel properties of this hybrid structure make it possible to separate a motley collection of nanotubes by electronic type, 1 an essential step toward nanotube electronic devices. The electrical properties of the hybrid may also be of special interest in the development of molecular-based electronic devices. Finally, nanotubes offer the potential of providing a scaffold on which DNA molecules can be oriented, manipulated, and studied without the need for chemical functionalization.Although DNA/SWNT structures have been studied using optical absorption spectroscopy in the past, 1-7 little attention has been paid to the range of wavelengths of most interest for examining DNA, namely the UV region. DNA's UV absorbance has been studied extensively and has been central in understanding conformational changes of nucleic acids, such as the transition from double-stranded to single-stranded DNA. 8 Absorbance studies of DNA/SWNT samples have largely ignored the UV because, until not long ago, there was very little known about the absorbance properties of SWNT in this region. Some authors argue that the UV absorbance is entirely due to amorphous carbon material, 7 while others find the two absorbance peaks in the UV to be intrinsic to the nanotubes and, furthermore, to display a strong polarization dependence. 9 By the methods described here, we have found the UV absorbance peak locations and intensities to be repeatable under a variety of SWNT sample preparation conditions, and we demonstrate that the UV region provides important information about DNA/SWNT structure.We used optical absorption spectroscopy from λ) 190900 nm to explore the DNA-SWNT interaction of SWNT dispersed with DNA homopolymers of poly d(A30), poly d(C30), poly