Microscopic and spectroscopic study of interactions between peptides and single -walled carbon nanotubes

Nano-1, a designed polypeptide, has been demonstrated to efficiently disperse individual single-walled carbon nanotubes (SWNTs). Nano-1 folds into an amphiphilic α-helix wherein the phenylalanine (Phe) residues on the hydrophobic face of the helix interact via π-stacking with the aromatic surface of the SWNT. In this study, the ability of electron donating (hydroxyl) and withdrawing (nitro) groups on the phenyl ring of Phe to affect the interactions between the peptide and SWNTs are examined by substituting the Phe residues in the Nano-1 sequence with tyrosine and p-nitro phenylalanine, respectively. In order to examine the effect of the electron donating/withdrawing character of the peptide in the absence of secondary structure effects, circular dichroism was utilized to determine concentrations at which the peptides displayed an analogous secondary structure, both alone and in the presence of SWNTs. Subsequently, atomic force microscopy measurements and optical absorption spectroscopy revealed that the ability to disperse individual SWNTs increases with increasing electron density of the aromatic residue on the hydrophobic face of the amphiphilic helical peptides.;Scanning tunneling spectroscopy (STS) and Raman analyses were used to examine the effect of noncovalent protein functionalization on the electronic properties of SWNTs. STS dI/dV spectra of the three peptide/SWNT dispersions displayed von Hove singularity (VHS) peaks characteristic of one-dimensional SWNTs, and exhibited both vanishing and non-vanishing density of states around the Fermi energy (Ef) suggesting that the peptide coated SWNTs can exhibit both semiconducting and metallic behavior. STS dI/dV spectra of Nano-1-coated portions of a SWNT closely resembled that of uncoated regions on the same SWNT with some slight modifications. Specifically, weak features that appeared on the valence band side near the Ef of the peptide-coated portion of a SWNT, as well as a shift of the Raman G band peak to higher frequencies for the Nano-1/SWNT composite, are suggestive of a weak charge transfer interaction in which Nano-1 acts as an electron acceptor and the SWNT acts as an electron donor. Likewise, Raman and STS analyses suggest a weak p-doping interaction between Nitro-nano-1 and SWNTs. A small downshift of the Raman G band of the Tyr-nano-1/SWNT composite, compared to the Nano-1/SWNT dispersion, as well as weak features on the conduction band side of Ef in the DOS of Tyr-nano-1 coated SWNTs, suggest a very weak n-doping interaction between Tyr-nano-1 and the SWNT.