Since Clark and Lyons published their work in 1962, a number of analytical chemistry researchers have been fascinated by the development of biosensors, which can be a fast and cheap way to conduct clinical, chemical, environmental or pharmaceutical research [1]. Electrochemical biosensors are devices that detect biological reactions happening at the electrode – solution interface and utilise transducers to transform chemical energy into electric signals. These devices could be used to detect various biological molecules, such as: proteins, antibodies, aptamers, or even entire cells [1]. One type of an electrochemical biosensor is an impedimetric sensor. These devices track biological reactions by measuring impedance changes at certain frequencies as biological reactions progress. Electrochemical impedance spectroscopy (EIS) is a widely used method for characterising and quantifying these types of interface reactions [1].
EIS as a method for tracking biological reactions has a few advantages. Firstly, it utilises currents of small amplitudes to disrupt a system from equilibrium, only causing minimal damage to the system in the process, It can be used to measure impedance with or without the presence of a redox pair, measuring faradaic processes in the former variant and non-faradaic processes in the latter. Biosensors that use both types of measurement have been developed in the past [2]. Faradaic sensors typically track changes in charge transfer resistance that stems from electrostatic interactions between charge carriers and biological molecules, while non – faradaic sensors measure changes in the electric double layer capacity that occur during biological reactions at the interface.
EIS is limited by a few requirements that are needed to obtain a valid impedance spectrum. These are causality-meaning the response of our system must be caused by the applied alternating potential. Linearity - the voltammetric curve must be linear in the chosen potential range. Stability- the system must return to its original state after any measurement. And finally, finity- meaning out impedance spectrum has to have finite values as frequency approaches 0 and infinity, as well as all frequencies in between. The biggest issue with biosensors has been their stability, since biological systems in electrochemical cells rarely retain stability for extended periods of time [2].
In this work, we utilise local EIS with scanning electrochemical microscopy (SECM) to analyse anti-human growth hormone (anti - HGH) antibodies modified with gold nanoparticles as a potential tool for developing a biosensor for the HGH protein. A functional HGH biosensor could provide a non – invasive method for testing HGH blood levels in humans.