Electrical modeling of an electrochemical sensor operating at the redox cycling mode

In this work, the direct current vs. voltage (DC) characteristics, and the alternate current (AC) impedance of a 1D four-electrode electrochemical cell, operating under redox-cycling conditions, are presented and discussed. The analysis refers to a four-electrode electrochemical system with one positively biased working electrode, one negatively biased working electrode, one auxiliary electrode, and one reference electrode operating in a redox cycling mode. This configuration is useful, for example, for electrochemical biosensors that can benefit from the inherent redox-cycling amplification of the apparatus. An aqueous buffered electrolyte is assumed, containing electrochemically active species that can be oxidized on the anode, with a positive overpotential. Meanwhile, the product of the oxidation reaction can diffuse to the other working electrode which has a negative overpotential, where the product is reduced. The analysis in this paper assumes that oxidation is the dominant mechanism at the anode, reduction is the dominant mechanism at the cathode, and the transport between them is dominated by diffusion. Since it is a 1D model, an ideal lossless redox cycling, i.e. both working electrodes’ currents (anodic and cathodic) are of the same magnitude with opposite signs, is assumed. Next, the small signal (AC) impedance of the electrochemical cell was calculated assuming that the AC signal modifies the oxidation reaction rate at the anode while the reduction reaction rate at the cathode is fixed. The impedance under redox cycling is calculated vs. frequency for various oxidation and reduction rates and both Bodé and Nyquist plots of the impedance are presented. The last part of the paper presents measured data for both DC and AC (i.e. Bodé and Nyquist plots) experiments with interdigitated array (IDA) electrodes, 5 & 10 μm spacing, operating under redox cycling conditions. The DC measurements were conducted in a 6 mM ferricyanide [K₃Fe(CN)₆] in 0.1 M Phosphate Buffer Saline (PBS) solution, pH 7.4. The AC measurements were conducted in 10 mM & 100 nM ferricyanide (K₃Fe(CN)₆) in 100 mM KNO₃ aqueous electrolyte. A critical discussion follows the results section where the highlights and problems of the models are discussed.