Investigating the concept of diffusional independence. Potential step transients at nano- and micro-electrode arrays: theory and experiment.

Microelectrode arrays find broad application in electroanalysis offering the enhanced sensitivity associated with microelectrodes, but with a high total current output. Such arrays are often constructed to make the electrodes 'diffusionally independent'. To emphasize that this is a time dependent property, a two-dimensional simulation, in conjunction with the diffusional domain approach, is used to model potential step transient currents at microelectrode arrays. Two types of array, hexagonal and cubic, are considered. In both cases the absolute (not relative) microelectrode separation distance has a significant effect on transient current. Three different regimes of transient current versus time can be observed at microelectrode arrays. At short times the transient response of isolated microelectrodes is seen, then at intermediate times the steady-state response of independent electrodes can be observed. At longer times planar diffusion to the entire array takes over. It follows that only at timescales corresponding to the first two regimes can the electrodes be considered as diffusionally independent. To verify the theory the potential step experiment is performed at a regularly spaced hexagonal iridium microdisk array. Theory is found to be in a good agreement with the experimental results.