DIRECT MEASUREMENTS OF THE PHYSICAL DIFFUSION OF REDOX ACTIVE SPECIES - MICROELECTROCHEMICAL EXPERIMENTS AND THEIR SIMULATION

We report methodology for direct measurement of the 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 eight parallel microelectrodes ∼50 μm long × ∼2.7 μm wide × ∼0.1 μm thick separated from each other by ∼1.4 μm Experimental studies and simulations involve electrochemical generation of as few as 6 × 10-20 mol of redox active species at one microelectrode (generator) and electrochemical detection of a significant percentage of these species at other microelectrodes (collectors) 0.8-26 μm away. The measurements involve the determination of 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. © 1990 American Chemical Society.