Spatiotemporally-distributed electrochemical signals at microelectrode arrays to differentially detect catecholamines.

by Lotfi-Marchoubeh, Mahsa; Hu, Mengjia; Abrego, Miguel; Fritsch, Ingrid

Electrochem. sensing offers rapid quantification of chem. species on a microscopic scale with the capability of probing biol. tissue in vivo. For example, fast scan cyclic voltammetry (FSCV) at single carbon fiber electrodes has analyzed neurotransmitters in brain, like the catecholamine dopamine (DA). Catecholamines serve important roles in neurol. function and related diseases. DA undergoes at two-electron transfer oxidn. (E), forming an orthoquinone (OQ), and producing a current proportional to concn. FSCV can detect the other catecholamines, norepinephrine (NE) and epinephrine (EP), too. However, similarity in their chem. structures makes it difficult to identify them in mixts., most notably DA from NE. By fabricating microstructures of arrays of individually-addressable electrodes of well-defined geometries, dimensions, and spacings, we have enabled development of new electrochem. methods to differentiate between these species. Our arrays consist of coplanar, 4-µm-wide band electrodes, and a potentiostat controls voltages and measures current of multiple electrodes simultaneously. The devices take advantage of homogenous chemistries that follow the heterogeneous oxidn. (an ECC' mechanism). The OQ produced at the "generator" electrodes undergoes a 1,4-addn. cyclization (C) to the electroinactive leucoaminochrome (LAC) with different rate consts., k, specific to the catecholamine (kEP > kNE > kDA). Slower rates allow more OQ mols. to survive the travel time from the generator electrodes over greater distances to neighboring collector electrodes where OQ reduces back to catechol. A bimol. reaction (C') between LAC and OQ also returns OQ back to its catechol form, which can oxidize at the generator electrodes again. Understanding the concn. distribution of each species, and how they are affected by each other and by placement and activation of the electrodes is important in developing this generation-collection (redox-cycling) approach for quant. anal. in mixts. We will report results from expts., modeling, and computer simulations and progress toward fabricating arrays onto microprobes for in vivo studies.