Atomistic simulations of dopamine diffusion dynamics on a pristine graphene surface

Carbon microelectrodes enable in vivo detection of neurotransmitters, and new electrodes are being developed to optimize the carbon surface. However, the work is mainly empirical and there have not been corresponding theoretical studies using molecular-level simulations of the diffusion and orientation of neurotransmitters near these surfaces. Here, we employ molecular dynamics simulations to investigate in atomistic detail the surface diffusion of dopamine (DA), its oxidation product dopamine-o-quinone (DOQ), and their protonated forms on the pristine basal plane of flat graphene. All DA species rapidly adsorb to the surface and remain adsorbed for the full length of the equilibrium simulations, even without a holding potential or graphene surface defects. The diffusivities of the adsorbed and the fully solvated DA are similar, and all molecular diffusion on the surface is slower than that of an adatom of comparable molecular weight. The protonated species diffuse more slowly than their corresponding neutral forms, and the oxidized species diffuse more rapidly. The underlying hexagonal graphene structure has little influence over the molecular adsorbate's lateral position. The vertical placement of the amine group on dopamine is highly dependent upon its charge, and the protonated amine prefers to be above the surface near the solvating waters. Solvation has a large effect on surface diffusivities when diffusion is compared to that in a vacuum. These are the first results of molecular dynamics simulations of dopamine diffusion at the aqueous-graphene interface, and they show that dopamine diffuses quickly on graphene surfaces, even without an applied potential. These calculations provide a basis for future simulations to predict the behavior of neurotransmitter diffusion on advanced carbon materials electrodes.