ANTHRAQUNIONEDISULFONATE ELECTROCHEMISTRY - A COMPARISON OF GLASSY-CARBON, HYDROGENATED GLASSY-CARBON, HIGHLY ORIENTED PYROLYTIC-GRAPHITE, AND DIAMOND ELECTRODES

The electrochemistry of anthraquinone-2,6-disulfonate (2,6-AQDS) at gl assy carbon (GC), hydrogenated glassy carbon (HGC), the basal plane of highly oriented pyrolytic graphite (HOPG), and boron-doped diamond wa s investigated by cyclic voltammetry and chronocoulometry. Quantitativ e determination of the surface coverage and qualitative assessment of the physisorption strength of 2,6-AQDS adsorption on each of these ele ctrodes were done. The diamond and HGC surfaces are nonpolar and relat ively oxygen-free, with the surface carbon atoms terminated by hydroge n, The polar 2,6-AQDS does not adsorb on these surfaces, and the elect rolysis proceeds by a diffusion-controlled reaction. Conversely, the G C and HOPG surfaces are polar, with the exposed defect sites terminate d by carbon-oxygen functionalities. 2,6-AQDS strongly physisorbs on bo th of these surfaces at near monolayer or greater coverages, such that the electrolysis proceeds through a surface-confined state. Less than 40% of the initial surface coverage can be removed by rinsing and sol ution replacement, reflective of strong physisorption. The results sho w the important role of the surface carbon-oxygen functionalities in p romoting strong dipole-dipole and ion-dipole interactions with polar a nd ionic molecules such as 2,6-AQDS. The results also support the theo ry that diamond electrodes may be less subject to fouling by polar ads orbates, as compared to GC, leading to improved response stability in electroanalytical measurements. The relationship between the 2,6-AQDS surface coverage, the double-layer capacitance, and the heterogeneous electron-transfer rate constant for Fe(CN)(6)(3-/4-) for these four ca rbon electrodes is presented.