Noncovalent functionalization of single-walled carbon nanotubes with water-soluble porphyrins.

We have employed water-soluble porphyrin molecules [meso-(tetrakis-4-sulfonatophenyl) porphine dihydrochloride] to solubilize single-walled carbon nanotubes (SWNTs), resulting in aqueous solutions that are stable for several weeks. The porphyrin-nanotube complexes have been characterized with absorption and fluorescence spectroscopy and with AFM. We find that the porphyrin/SWNT interaction is selective for the free base form, and that this interaction stabilizes the free base against protonation to the diacid. Under mildly acidic conditions nanotube-mediated J-aggregates form, which are unstable in solution and result in precipitation of the nanotubes over the course of a few days. Porphyrin-coated SWNTs can be precisely aligned on hydrophilic poly(dimethylsiloxane) (PDMS) surfaces by combing SWNT solution along a desired direction and then transferred to silicon substrates by stamping. Parallel SWNT patterns have been fabricated in this manner.