Inverted U-Shaped Counter Electrode Enables Uniform Current Distribution in Planar PCB Electrochemical Sensors

This article presents systematic optimization of electrode geometry for electrochemical sensors fabricated on flame retardant 4 (FR-4) substrate using conventional printed circuit board (PCB) technology. Through finite element modeling and experimental validation, we demonstrate that counter-to-working electrode (CE/WE) surface area ratio critically determines sensor performance in planar geometries. Twelve configurations with CE/WE ratios from 1:1 to 12:1 were systematically evaluated using COMSOL Multiphysics simulations and cyclic voltammetry (CV) characterization. Results reveal that the optimal 12:1 ratio achieves the best overall electrochemical performance across all discriminating metrics: peak current magnitude (Ipa$= 27.70~\pm ~0.57~\mu $A, mean across three independently fabricated devices, relative standard deviation (RSD) = 2.05%), minimum peak separation ($\Delta $Ep = 63 mV approaching the theoretical 59 mV for reversible one-electron transfer), excellent field uniformity, and near-unity peak current ratio (ipa/ipc = 1.02). While ipa/ipc approaches 1.02 at ratios as low as 4:1, the 12:1 configuration uniquely satisfies all performance criteria simultaneously. CV comparison with a commercial DropSens gold screen-printed electrode (SPE) confirms that the optimized PCB geometry produces electrochemically comparable behavior to an established commercial benchmark.