Noncovalent porphyrin-functionalized single-walled carbon nanotubes: Solubilization and spectral behaviors

We describe the solubilization/dispersion of as-produced and purified single-walled carbon nanotubes (raw-SWNTs and p-SWNTs) with protoporphyrin IX derivatives and tetraphenylporphyrin iron(III) chloride, the spectral behavior of the octaalkylporphyrins-solubilized shortened-SWNTs (s-SWNTs), and the electrochemistry of a p-SWNTs/FePP cast film on a glassy carbon electrode. Transmission electron and atomic force microscopies, as well as UV-visible-near IR spectroscopy, revealed that the protoporphyrin IX derivatives individually dissolved the p-SWNTs in polar solvents under mild conditions of sonication using a bath-type sonicator, followed by centrifugation at 1000 g. The raw-SWNTs were more easily dissolved than the p-SWNTs with protoporphyrin IX zinc(II) (ZnPP), whereas the amount of the solubilized/dispersed p-SWNTs did not depend on the concentration of the solubilizer and sonication time. The absorption and fluorescence spectral measurements of the octaalkylporphyrins in dimethylformamide containing 1 vol% tetrahydrofuran with various amounts of the s-SWNTs showed that the absorption maxima of the octaalkylporphyrins decreased with an increase in the concentration of the s-SWNTs without wavelength shift and the fluorescence of the porphyrins was quenched by the addition of the s-SWNTs. These spectral behaviors are direct evidence for the interaction between the nanotube sidewall and the porphyrins in the solutions. The cyclic voltammograms of the p-SWNTs/hemin (FePP) and free hemin (FePP) suggest that the nanotubes act as a conduction passage for electrons between hemin (FePP) and the glassy carbon electrode.