Limited Diffusion and Cell Dimensions in a Micrometer Layer of Solution: An Electrochemical Impedance Spectroscopy Study

The electrochemical impedance behavior observed for a newly proposed electrochemical system [Botasini et al., Analyst, 2016, 141, 5996-6001], in which the complete electrodic system is screen-printed on the same plane and homogeneously covered by a thin layer of solution, was studied in a wide frequency domain (10 mHz to 100 kHz) involving 127 logarithmically spaced frequencies. Two distinctive frequency domain behaviors were considered, with the addition of a series-connected capacitor, representing the physical limit of the electrochemical cell. Two diffusion regimes for the electroactive species were detected: i) semi-infinite linear diffusion close to the electrode surface and limited by a reflecting boundary, followed by ii) distorted diffusion corresponding to the bulk solution enclosed within the thin-layer cell. From the electric circuit parameters, the diffusion length of the semi-infinite linear diffusion process, the heterogeneous rate constant for the charge transfer, and the width of the thin-layer experimental cell can be calculated. For the first time, electrochemical impedance spectroscopy analysis provides a thorough means to study limited diffusion in micrometer-sized layer that includes the physical limit of the electrochemical cell. All these data are fundamental in the design of novel micro-and nano-fluidic systems.