INVESTIGATION OF THE MASS-TRANSPORT PROCESS IN THE VOLTAMMETRY OF CYTOCHROME-C AT 4,4'-BIPYRIDYL DISULFIDE MODIFIED STATIONARY AND ROTATED MACRODISK AND MICRODISK GOLD ELECTRODES

The mass transport mechanism associated with the reduction of cytochrome c at gold electrodes modified by adsorption of 4,4'-bipyridyl disulfide (SS-bpy) has been investigated at a range of electrode configurations. Radial diffusion to the small electroactive sites on the electrode surface leads to the observation of sigmoidal shaped curves when the surface density of the modifier is low at a stationary conventionally sized gold electrode. High modifier coverage causes overlap of the diffusion layers which leaves linear diffusion as the dominant mode of mass transport resulting in peak-shaped cyclic voltammograms. In contrast, at a 5-mum radius gold disk microelectrode, radial diffusion is always the dominant mode of mass transport. However, the current is small or nonmeasurable when a microelectrode is modified ex situ at open circuit, whereas, if a suitable potential is applied to ensure a uniform distribution of SS-bpy over the entire surface of the microelectrode, then an easily measured reversible sigmoidal shaped voltammogram is observed by both in situ and ex situ methods of modification. In agreement with available spectroscopic evidence, data are consistent with electron transfer occurring at electroactive sites which are smaller than the 5-mum microelectrode radius and confirm that SS-bpy and cytochrome c interactions on the electrode surface are dynamic rather than static. Studies on the time dependence of the voltammetry at a rotated gold disk electrode modified by adsorbed SS-bpy under conditions of low surface coverage give calculated values of the relative surface-interaction rate constants, k*f = k(f)GAMMA(SS-bpy). For a 10(-5) M SS-bpy solution k*f is 2.2 X 10(-3) CM S-1 for an accumulation time of 15 s and 3.9 X 10(-3) cm s-1 for an accumulation time of 60 s. As predicted theoretically with this model, a linear relationship between k*f and the square root of the accumulation time is demonstrated for very short times.