Impedimetric aptasensing using a symmetric Randles circuit model

Abstract

Electrochemical impedance spectroscopy (EIS) applied for aptasensing is being extensively studied for a variety of targets. Substitution of a symmetric electrode configuration for a conventional three-electrode setup on EIS aptasensing is becoming a trend due to its simple and favorable characteristics for biochip fabrication. However, a single Randles circuit is often used for impedance element parameter fitting in symmetric electrode aptasensing without rationalized basis of validity to date. In this work, an AC impedance model is derived for proving the validity of simplifying a symmetric Randles circuits in series into a single one and is examined with aptasensing applications. With the model, circuit parameter relationships can be quantified and correlated between symmetric and three-electrode configurations. The relationships are verified by comparing the fitted parameters obtained from standard gold electrodes (SGEs), microfabricated symmetric Au electrodes, and interdigitated array (IDA) Au electrode chips with different surface and detection conditions. The model is then found valid for different redox species composition (either Fe(CN)63− or Fe(CN)63−/4- is used) and for both blank and aptamer modified surfaces. From aptasensing of thrombin and tumor marker mucin 1 (MUC1), it is proved that the measured dissociation constants (KD: 129.4 nM for thrombin and 16.3 nM for MUC1) remain the same between a symmetric and three-electrode configuration, but the maximum change of charge transfer resistance (Bmax) is doubled in the symmetric one. This means that the symmetric configuration not only yields the same KD, but also results in a doubled binding signal for EIS aptasensing. In addition to the model derivation and experimental validation, specific EIS aptasensing of MUC1 using a symmetric electrode configuration is successfully demonstrated in this work.