Adsorption of L-phenylalanine on single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) became soluble in water and formed a stable solution when L-phenylalanine (Phe) was adsorbed. The adsorption selectivity of Phe zwitterions for larger diameter SWNTs was confirmed by analysis of Fourier transform infrared spectra and by differential thermogravimetric analysis. Enhanced adsorption of Phe on the oxidized single-walled carbon nanotubes (OSWNT) was observed in comparison with that of the purified single-walled carbon nanotubes (PSWNT). The Phe zwitterions are thought to adsorb on the surface of OSWNT by joint interaction of the pi-pi stacking, hydrogen bond, and part of the covalent bond. The pi-pi stacking is the dominant interaction in the sidewall of OSWNT without defects. The hydrogen bond and covalent bond formed with oxygen-containing groups becomes dominant on the end of OSWNT. For the PSWNT system, pi-pi stacking is an important factor to realize the adsorption of Phe zwitterions on the sidewall of PSWNT. The intermolecular hydrogen bond between Phe zwitterions is also formed when Phe zwitterions are adsorbed on the PSWNT.