Measurement of nanomolar dopamine diffusion using low-noise perfluorinated ionomer-coated carbon fiber microelectrodes and high-speed cyclic voltammetry

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Abstract
Several improvements in the fabrication and use of carbon fiber voltammetric microelectrodes (CFVMs) are described. These procedure did not involve oxidative treatment, but resulted in sensitivities and selectivities approaching those of treated CFVMs, without the inherent slow response times associated with the latter electrodes. To accomplish this we reduced CFVM noise by (1) improving the adhesive seal between the 8 .mu.m o.d. carbon fiber and the glass insulation using vaccuum, (2) snapping rather than cutting or beveling the fiber to the flush with the glass, and (3) using a concentrated electrolyte solution to make electrical contact with the fiber. System noise was reduced by digital smoothing and signal averaging. Selectivity of the CFVMs for dopamine over ascorbate was enhanced to better than 2000:1 by coating with Naflon, a perfluorinated cation exchange polymer, usng a low (+0.5 V vs Ag/AgCl) electroplating potential. This low voltage also prevented electrode surface oxidation. To demonstrate the performance of our CFVMs, we used them in conjunction with high-speed cyclic voltammetry to accurately measure the diffusion coefficient of iontophoretically released dopamine at concentrations as low as 35 nM over distances of less than 200 .mu.m in agarose gel.