Experimental and Computational Study of Dopamine As an Electrochemical Probe of the Surface Nanostructure of Graphitized N-Doped Carbon

N-doped carbon nanomaterials have received increased attention from electrochemists because of their applications in the metal-free electrocatalysis of important redox processes. In this work, a series of graphitized undoped and nitrogen-doped carbon electrodes prepared by thermal annealing of sputtered amorphous carbon films were prepared and characterized using a combination of X-ray photoelectron spectroscopy and Raman spectroscopy. Adsorption of the surface-sensitive redox probe dopamine at each electrode surface was then studied using cyclic voltammetry and the results correlated to the physicochemical characterization. Results indicate that dopamine adsorption is influenced by both the nitrogen surface chemistry and the degree of graphitization of the carbon scaffold. N-doping, with predominantly graphitic N-sites, was found to increase adsorption of dopamine more than 6-fold on carbon surfaces when the introduction of N atoms did not result in substantial alterations to the sp(2) network. However, when an identical type and level of N-doping is accompanied by a significant increase in disorder in the carbon scaffold, adsorption is limited to levels comparable to those of nitrogen-free carbon. Density functional theory studies of dopamine adsorption on graphene and N-doped graphene model surfaces showed that dopamine interacts via pi-stacking at the graphene surface. The Gibbs free energy of adsorption on N-doped graphenes was estimated at 12-13 kcal mol(-1) and found to be approximately twice that of undoped graphenes. Results suggest that chemical changes resulting from N-doping enhance adsorption; however, high coverage values depend on the availability of sites for pi-stacking. Therefore, the structurally disruptive effects of N-incorporation can significantly depress the dopamine response by limiting the availability of basal sites, ultimately dominating the overall electrochemical response of the carbon electrode.