Application of Electrochemical Redox Cycling: Toward Differentiation of Dopamine and Norepinephrine.

The electrochemical redox cycling behavior of dopamine (DA), norepinephrine (NE), and their mixture was investigated using coplanar gold microband electrode arrays at four generator-collector gap conditions (4, 12, 20, and 28 μm). This method provides opportunity for differentiating the catecholamines in mixtures by monitoring the current at collector electrodes activated at different distances from generator electrodes. It takes advantage of the ECC' mechanism associated with the electrochemical oxidation of catecholamines, in which DA and NE have rate constants that differ by a factor of 7.5 for the first order intramolecular cyclization (C) following electron transfer (E). Collector electrodes activated at different distances from the generators were used to examine the process of the following chemistry at different time points, because spatial relationships are related to temporal ones through diffusion. Solutions of artificial cerebral spinal fluid containing 50 μM DA, 50 μM NE, and a DA-NE mixture of 50 μM of each were examined. The collection efficiency during redox cycling for NE had a greater dependence on gap width than DA, and the collector current of NE became silent at ∼20 μm. The collector current of the mixture approaches that of DA alone with increasing gap, suggesting that differentiation of DA and NE may be possible. The collector current of the mixture is further affected by the homogeneous reaction (C') between oxidized and cyclized products of DA and NE and drops below that of DA alone. This may be used for differentiation in more complicated chemical systems.