Faradaic impedance sensors

Faradaic and non-Faradaic impedance spectroscopies are effective methods to probe the interfacial electron-transfer resistance at a functionalized electrode. In Faradaic impedance sensors, the interaction between a biological receptor and a target species... 

Non-Faradaic impedance sensors employ no redox probes in the electrolyte solution, and only non-Faradaic impedance is operative. The electron transfer characterizing parameters, / (-t and become infinite, and the equivalent circuit can be simplified ... 

Label-free detection of ligand-aptamer interaction was also demonstrated by means of impedance spectroscopy technique [52,53]. Simultaneously, Radi et al. [52] and Rodriguez et al. [53] reported application of Faradaic impedance spectroscopy (FIS) in detection of interaction of proteins with DNA aptamers. The detection method is based on the measurement of resistance in presence of redox mediator Fe(CN)6-In absence of target protein, the negatively charged aptamer repulse the redox mediator molecules from the sensor surface. In a paper by... 

Alfonta, L., Bardea, A., Khersonsky, O., Katz, E., Willner, I. (2001). Chronopotentiometry and faradaic impedance spectroscopy as signal transduction methods for the biocatalytic precipitation of an insoluble product on electrode supports routes for enzyme sensors, immunosensors and DNA sensors. Biosens Bioelectron 16, 675-687. 

Boronic acids have been used in the development of surface plasmon resonance (SPR) [174,175] quartz crystal microbalance (QCM) sensors [174,176,177], Faradaic impedance spectroscopy [177], ion-sensitive field effect transistors (ISFET) [178], and chemical exchange saturation transfer (CEST) contrast agents in magnetic resonance imaging (MRl) [179]. The swelling of phenylboronic add polymers has also been used to control the release of insulin (180,181]. 

Measurement of the electric conductivity of an electrochemical cell can be the basis for an electrochemical sensor. This differs from an electrical (physical) measurement, for the electrochemical sensor measures the conductivity change of the system in the presence of a given solute concentration. This solute is often the sensing species of interest. Electrochemical sensors may also involve measuring capacita-tive impedance resulting from the polarization of the electrodes and/or the faradaic or charge transfer processes. 

An ideal electrode-electrolyte interface with an electron-transfer process can be described using Randle equivalent circuit shown in Fig. 2.7. The Faradaic electron-transfer reaction is represented by a charge transfer resistance and the mass transfer of the electroactive species is described by Warburg element (W). The electrolyte resistance R is in series with the parallel combination of the double-layer capacitance Cdi and an impedance of a Faradaic reaction. However, in practical application, the impedance results for a solid electrode/electrolyte interface often reveal a frequency dispersion that cannot be described by simple Randle circuit and simple electronic components. The interaction of each component in an electrochemical system contributes to the complexity of final impedance spectroscopy results. The FIS results often consist of resistive, capacitive, and inductive components, and all of them can be influenced by analytes and their local environment, corresponding to solvent, electrolyte, electrode condition, and other possible electrochemically active species. It is important to characterize the electrode/electrolyte interface properties by FIS for their real-world applications in sensors and energy storage applications. 

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