Influence of Charge States on the π–π Interactions of Aromatic Side Chains with Surface of Graphene Sheet and Single-Walled Carbon Nanotubes in Bioelectrodes

Density functional calculations were performed to investigate the interaction of the side chains of histidine, phenylalanine, tryptophan, and tyrosine with the outer surface of different charged graphene sheet (GS)/(7,7) single-walled carbon nanotube (CNT) at the M06-2X-6-31+g­(d,p)//M06-2X-6-31G­(d) level of theory, which can get insights into the π–π interactions in enzyme-modified CNT electrodes. The aromatic rings of the amino acids prefer to orient in parallel with the plane of the CNT at the different charge states, which bears the signature of π–π interactions. The π–π interactions mainly include the dispersion forces, the electrostatic forces, and the H−π bonds. The dispersion force nearly keeps constant for the same aromatic ring interaction with the GS/CNT at the different charge states. However, the electrostatic forces and the strength of H···π bonds are significantly affected by the different charge states. These factors cause the change of the binding order for the four aromatic rings with the GS and CNT. More importantly, the highest doubly occupied molecular orbitals (HDMOs), the singly occupied molecular orbitals (SOMOs), and the lowest unoccupied molecular orbital (LUMO) mostly reside on the CNT moieties for all charged systems, indicating that the negative and positive charges are ready to accumulate on the CNT moiety when the CNT interacts with the aromatic amino acids. These results support that the CNTs can be used to assemble the enzyme-modified CNT electrodes.