A Tutorial on Electrochemical Impedance Spectroscopy and Nanogap Electrodes for Biosensing Applications

Electrochemical Impedance Spectroscopy (EIS) is a powerful measurement technique used by scientists and engineers to characterize and understand the interfacial behavior of liquid-solid interfaces. Such investigations are important in numerous applications and fields of research, including electrochemical synthesis and deposition of materials, such as metals and conducting polymers, corrosion protection analysis, investigation of molecule-surface interactions, and research on batteries and fuel cells. In recent years, advances in nanotechnology have led to a rise in the use of EIS in chemical and bio-sensing. Areas such as molecular diagnostics and healthcare can greatly benefit from this novel technology, which includes low-cost, rapid response measurements that can be done by non-experts at the point of care. For new entrants, the depth of knowledge and breadth of EIS and its many applications can be rather daunting. This article therefore serves as a tutorial on basic EIS theory, whilst highlighting new applications in advanced bio-sensing. The tutorial begins with a theoretical overview, including electrical AC impedance, formation and structure of electrical double layers at liquid solid-interfaces and equivalent circuit models used to represent interfacial phenomena. This is followed by an introduction to bio-sensing and a review of recent research highlights on resistive and capacitive biosensors based on nanogap electrodes, especially those having novel applications from DNA detection to nanoparticle capturing. Lastly, the article highlights a recent new breakthrough on how the combination of nanogap electrodes with label-free methods, such as aptamer functionalized surfaces, can make capacitive sensors with extremely high sensitivity and specificity.