Diffusion-limited chronoamperometry at conical-tip microelectrodes

In this paper, we present simulated diffusion-limited time-variant currents at conical-tip microelectrodes fabricated by depositing a carbon film in and on pulled quartz capillaries. These mechanically strong microelectrodes are suitable probes for detecting neurotransmitters in vivo. The simulated results show that the currents obtained at conical-tip microelectrodes are larger than those at finite conical microelectrodes (e.g. etched carbon fibres protruding from an insulating plane) of comparable dimensions. The currents at conical-tip microelectrodes and finite conical microelectrodes both converge to that of a microdisk electrode at small cone heights and large cone angles, and to that of a cylindrical electrode portion of equal length and half the radius at large cone heights and small cone angles. At short times (scaled by the electrode dimensions), Cottrellian current is achieved at conical-tip microelectrodes and the current densities collapse to the expected chronoamperometric response at a microdisk electrode, subject to some simulation errors. Comparison between a simulated chronoamperogram and an experimental chronoamperogram then allows an estimate of parameters (such as electrode surface area and dimensions) that define the electrode geometry. Steady-state currents based on empirical functions have also been computed for conical-tip microelectrodes and finite conical microelectrodes.