(Keynote) Electrochemical Impedance Spectroscopy for Sensor and Biosensor Characterization and for Impedimetric Sensing

Abstract

Electrochemical impedance spectroscopy (EIS) is now becoming extensively used for the characterization of electrochemical sensors and biosensors, particularly with modifier layers applied on electrode substrates. EIS enables the effect of the addition of each layer to be probed, including of nanomaterials, such as carbon nanotubes or graphene or metal nanoparticles, conducting or redox polymers, enzymes or antibodies. The spectra that are obtained can often be complex because of the sensitive nature of EIS, dependent on the detailed structure of the sensor or biosensor architecture. However, the stepwise addition of porous or compact modifier layers can aid in seeing the transformations to the spectra that occur and help in understanding the physical and chemical phenomena and proposing a physical model that represents them. Variations of the assembly composition with time, such as diffusion of species from solution or dissolution of the sensor surface can be probed, and thence its stability, which are pre-requisites for use of the sensor in a reproducible way. EIS measurements should be complemented by voltammetric measurements and surface analysis; further important information can sometimes be obtained by using the electrochemical quartz crystal microbalance. Characterization of sensor and biosensor assemblies and their modelling will be illustrated [1-4], particularly regarding the influence of the build-up of the sensor platform following successive addition of modifier components on the impedance spectra. The choice of the applied potential at which spectra are recorded will come from voltammetric experiments, in the capacitive region or where redox processes occur. Spectra at open circuit potential in the presence of a redox probe with both oxidized and reduced forms present may be used to measure the amount of active surface area available in sensors relying on adsorption phenomena. The impedance characterization of electrodes modified with carbon nanotubes/graphene and/or phenazine redox polymers, modified electrodes reviewed in [1], will be shown, such modified electrodes being used for the measurement of species such as ascorbate, for neurotransmitters or after enzyme immobilization, as enzyme biosensors. EIS can also throw light on the properties of modified electrodes modified with conducting polymers, such as in [2] where poly(3,4-ethylenedioxythoiphene) films were grown in different deep eutectic solvents. EIS is particularly useful for monitoring the process of self-assembly in layer-by-layer (LbL) structures, e.g. in [3], where fitting of the spectra at different stages of sensor fabrication to an equivalent circuit corresponding to the physical model allowed important conclusions to be reached regarding the internal reorganization of the self-assembled structure. In [4], the effect of different graphene functionalisation methods on the properties of graphene incorporated in LbL structures for use in glucose enzyme biosensors was clearly demonstrated. Examples of impedimetric sensors, without the necessity of the commonly-used redox probes, will be illustrated by choline and lactate biosensors. Future perspectives for the use of EIS for the characterization of sensor and biosensor platforms as well as in impedance sensors will be discussed.