Probing carbon nanotube–amino acid interactions in aqueous solution with molecular dynamics simulations

The adsorption of 20 standard amino acids on (6,6) carbon nanotube (CNT) at a concentration of 0.17 M and neutral pH has been studied by molecular dynamics simulations, to assess the suitability of amino acids for CNT aqueous dispersions. Simulation results show that among the 20 amino acids, phenylalanine, tyrosine, tryptophan and arginine exhibit the strongest affinity for CNT(6,6) in terms of adsorption amount and interaction energy. These amino acids adsorb to CNT(6,6) and form very stable aggregates, covering about half of the tube surface. Phenylalanine, tyrosine and tryptophan interact with the tube via the strong π–π stacking of their aromatic rings. Interestingly, the strong attraction of arginine to CNT(6,6) mainly attributes to its guanidinium group, which strongly interacts with the tube and forms multiple salt bridges. The negatively charged carboxylate and positively charged ammonium groups of these adsorbed amino acids extend away from the tube surface and point towards aqueous solution, which facilitates the solubilization of CNTs in water, and may be able to provide electrostatic repulsion forces to prevent CNT agglomeration. The results of this work provide a theoretical support for using amino acids as novel CNT dispersing agents and help to understand CNT–protein interactions.