Probing Interaction between ssDNA and Carbon Nanotubes by Raman Scattering and Electron Microscopy

We report experimental results of Raman scattering showing that single-stranded DNA (ssDNA) effectively modifies the electronic structure of metallic carbon nanotubes with the diameter of 1.5-1.6 nm but not for the same diameter class of semiconducting tubes. Experimental fact of drastic decrease on the G-band Raman intensity associated with only the metallic tubes, which give asymmetric BWF line shape, can be seen when the nanotubes were soaked in aqueous solution of ssDNA. The G-band signal from semiconducting tubes indicates neither change of intensity nor of peak position. To explain these results, we introduce possible energy scheme for metallic tubes that the doping band of ssDNA locates near below the Fermi level, but it is located just in between the first van Hove energy gap of semiconducting tubes. Transmission electron microscopy shows flappinglike motion of ssDNA in the nanotubes. Such motion of ssDNA is probably triggered by a Coulomb interaction between the electron beam and ssDNA, and this motion can be suppressed when ssDNA@nanotube is wrapped in cationic surfactant.