Equivalent circuit model of a non-faradaic impedimetric ZnO nano-crystal biosensor

Abstract

• Sensing of C-reactive protein using a novel co-planar electrode PET-ZnO biosensor. • Development of an EEC model of the biosensor platform. • EEC modelling demonstrates predicted properties of a non-faradaic biosensor. • Quantified EDL charge modulations arising due to CRP binding at the ZnO surface. • Dominance of each sensor component identified to support control of parasitics. This paper describes the development of an equivalent electrical circuit (EEC) model of a unique co-planar electrode PET-ZnO biosensor platform. This non-faradaic ZnO biosensor had a PET insulating layer between the electrodes and the active ZnO sensor surface and impedance spectroscopy was used to measure varying concentrations of C-reactive protein (CRP). Analysis of the impedance magnitude bode plot reveals a slope indicative of a capacitive behaviour, where R 2 is 0.99. Furthermore, extraction of the complex impedance parameters shows capacitive reactance (Xc) being the dominant parameter as a result of capacitive transduction mechanism. The EEC was based on the generalized Randles model and adapted to represent the non-faradaic ZnO biosensor. Experimental data was fitted to the model at 100 kHz to establish values for the component parts of the biosensor system. With these values the model showed good correlation between experimental and modelled data across the frequency range studied, within the limitations of the measurement instrument. The model quantified charge modulations arising due to bimolecular binding of CRP within the electrical double layer (EDL) formed at the ZnO-PBS interface. The CRP bio-molecular interaction within the EDL can cause high charge carrier density accumulation in the semiconductor and as a result the capacitance of the EDL is increased. The EEC model quantifies this phenomenon, presented as a parallel capacitance connected to the ZnO-Antibody interfacial capacitance. The model was adapted to two variants of the biosensor design, with differing ZnO nanoparticle concentrations forming the sensor surface. The variations on the components modelled concurred with expected changes established from biosensor surface analysis. Using the EEC model, the dominance of each constituent sensor element on measurement stability is quantified identifying the significant contributors to experimental and sensor-to-sensor repeatability to support control of parasitic effects.