Transport Effects in Electrochemical Biochip Sensors with Redox Cycling Amplification: Computational and Experimental Analysis
Authors: Dotan Tali , Shacham-Diamand Yosi 
Published: August 2024

In the last few decades, and especially after the COVID-19 pandemic, point-of-care diagnostics and field-deployable biosensors have attracted significant attention. Key components of these instruments are microchips that integrate microfluidics with a sensor (e.g., optical or electrochemical) to provide rapid, low-cost, real-time detection of analyte molecules in very small volumes. Next-generation electrochemical sensors have been miniaturized on solid-state platforms (e.g., arrays and vectors) where the area is limited. However, as the active surface area of the sensors decreases from a few square microns to sub-micron and nanometric scales, the measured currents drop, hence affecting the limit of detection. As the concentration of many analytes in the physiological environment is low, low signal-to-noise ratios have limited the miniaturization of standard electrochemical sensors. One way to increase signal at the sensor output is by redox amplification. To achieve amplification, a second working electrode must be incorporated close to the first working electrode and proper bias must be applied to both. Micrometer-sized interdigitated electrode arrays have been fabricated using robust and scalable processes based on optical lithography and reactive ion-etching techniques. In some cases, IDAs have been integrated into microfluidics devices. The physical and electrochemical phenomena that occur in microfluidic channels under flow have been previously evaluated and showed an increase of current under flow. The effect of redox cycling in microfluidic and nanofluidic systems has also been qualitatively demonstrated. Some studies measured the amplification in the cases with and without flow and showed that the flow itself amplifies the measured electrochemical current, though in most cases, it reduces the amplification factor.
Towards a more quantitative investigation, analytical and computational modeling were used, providing meaningful insights into the physics of IDAs without requiring numerous device fabrications. In this work, we used COMSOL Multiphysics to create a full three-dimensional simulation-based model of a real device in order to study the effect of laminar flow on redox cycling, as well as a simplified one-dimensional model for the analysis of transition velocity between different amplification regimes. IDAs with 5-10 mm for both electrodes width and gap were studied numerically and experimentally. The results of the 3D and 1D simulations were consistent between themselves, and also with those of the physical experiments. The relationship between the two mass transfer methods — diffusion and convection — were analyzed at different flow velocities and for different electrode geometries.
Here we report a study of transport effects on a microfluidic sensor-chip in which electrochemical sensing is amplified by redox cycling using an interdigitated electrode array (IDA). Our models indicate that there exist two dominant regimes: one which is limited by redox cycling diffusion—a unique feature of IDAs— which is dominant at relatively low flow rates, and another which is limited by convection and has dominant flow effects which are effective at high flow rates (see Figure). The transition velocity between the two sensing regimes depends on the width of the electrode and spacing. It was found to be Vflow [mm/sec]=0.335/L[μm] + 0.04 which is similar to the prediction of Vflow∝1/L obtained from the one-dimensional model.
An understanding of the combined effects of redox cycling and convection flow is critical as microfluidic chips with electrochemical sensing amplified by redox cycling using IDAs are becoming widely used in diagnostic assays. The method for prediction of current under any flow rate described here will support technological development of more efficient and accurate devices.

Figure 1