The adsorption of the amino acid glycine (NH2CH2COOH) on (3,3) armchair carbon nanotubes has been investigated through calculations based on density functional theory (DFT). A comparison of the adsorption energies for four adsorption models is presented, showing glycine in its zwitterionic form is generally bound stronger to the nanotube sidewall as compared to its nonionic counterparts. Adsorption through the carboxylate group of the glycine zwitterion exhibited the strongest interaction, favourably adsorbing with an adsorption energy of 0.53 eV. Further investigations on the adsorption of glycine on graphite sheets have showed weaker interactions as compared to adsorption onto the nanotube surface, suggesting a decrease in glycine adsorption strength with increasing nanotube diameter. (c) 2005 Elsevier B.V. All rights reserved.