Growth and characterisation of diffusion junctions between paired gold electrodes: diffusion effects in generator-collector mode

A diffusion junction between two paired gold electrodes is created in a bipotentiostatic electro-deposition process. Gold metal is deposited simultaneously on two adjacent disc electrodes (100 mu m diameter, approximately 125 mu m separation) until short-circuit conditions trigger the end point of the electro-deposition. Symmetric gold junctions with typically 5 mu m average inter-electrode gap size, 140 mu m gap length, and approximately 18 mu m junction depth are obtained. These paired gold electrodes are employed in generator-collector mode to give well-defined steady-state feedback currents even for extremely low concentrations of analyte (sub-mu M) and without any contributions from capacitive charging. Four redox systems are investigated spanning a wide range of diffusion coefficients: (1) the one-electron oxidation of iodide to iodine, (2) the two-electron oxidation of hydroquinone to benzoquinone, (3) the two-electron reduction of alizarin red S, and (4) the one-electron oxidation of the redox protein cytochrome c. Consistent results for these redox systems suggest that (1) the junction zone between the two electrodes is dominating the behaviour of the electrode in particular for the slower diffusing systems and (2) the paired gold electrode junction can be calibrated and employed for electroanalysis at very low concentrations and for a wider range of analytically relevant redox systems.