A practical approach to thickness-dependent cell constant correction for coplanar or interdigitated electrodes

Coplanar microelectrodes, particularly interdigitated electrodes (IDEs), are widely employed in electrochemical sensing owing to low cost, scalable fabrication, and high surface sensitivity. However, accurate and quantitative interpretation requires precise determination of the cell constant (k). This becomes nontrivial when the electrode's electric field is only partially immersed in the electrolyte, as is common in thin film or confined-volume systems. This work presents a fast, reproducible, and low-cost experimental platform for measuring film thickness-dependent correction factors (α) of k. Electrochemical impedance spectroscopy (EIS) was applied on printed circuit board (PCB) IDEs comprising five different electrode finger widths (w) and spacings (s) (w = s: 250 µm, 500 µm, 1000 µm, 1500 µm, and 2000 µm). k was determined while fully immersed, and α was subsequently empirically derived as a function of electrolyte film thickness (α(d)). The results were compared to finite element methods (FEM; COMSOL), revealing consistent trends but notable deviations in absolute values of α(d). These findings highlight the limitations of simplified field models and provide a practical approach for a more accurate characterization of thin films on coplanar electrodes.