Direct Measurements of the Physical Diffusion of Redox Active Species: Microelectrochemical Experiments and Their Simulation.

摘要

This report provides a method for direct measurement of physical diffusion of molecular species in solution. Movement of a small number of species in solution can be tracked as a function of space and time by using an array of individually addressable microelectrodes. The arrays typically consist of 8 parallel microelectrodes approx. 50 microns x 2.7 microns x approx. 0.1 microns separated from each other by approx. 1.4 microns. Experimental studies involve electrochemical generation of as few as 6 x 10 to -20th power moles of redox active species at one microelectrode (generator) and electrochemical detection of a significant percentage of these species at other microelectrodes (collection) 0.8 -26 microns away. Measurements involve determining the time dependence of the collector current associated with detection of the diffusing species produced in short and long pulses at the generator. Microelectrochemical experiments and numerical simulations are presented to show that the time dependence of the collector current allows determination of the diffusion coefficient of the diffusing species. For the geometries of generator and collector used, an important relationship for short generator pulses is t(mt) = 0.22 (d sq)/D where t(mt) is the time associated with the peak in the collector current, d the distance from the center of the generator to the nearest edge of the collector, and D the diffusion coefficient. Thus, by knowing d and measuring t(mt), D can be determined for a species generated at one microelectrode and detected at another. Microelectrochemical arrays; Electrolyte aqueous media. (edc)