Sensor impedimetric

Fig. 8.2 Nyquist plot of impedimetric sensor (Section 8.4.3.3). The redox couple ferro/ ferricyanide recorded at (a) bare Au electrode (b) Au electrode modified with thrombin-binding aptamer (c) Au/aptamer electrode with added 2-mercapto ethanol and (d) electrode with bound complementary thrombin/aptamer complex. Each plot was obtained separately (adapted from Radi etal.,2005)...

Frequency as an experimental variable offers additional design flexibility. This approach has several advantages. The most important one is the lack of polarization of the contacts. The second one is the fact that equivalent electrical circuit analysis can be used that aids in elucidation of the transduction mechanisms. Perhaps the most important distinguishing feature of this class of conductometric sensors is the fact that their impedance is measured in the direction normal to their surface. In fact, there may be no requirement on their DC conductivity and their response can be obtained from their capacitive behavior. In the following section, we examine so-called impedance sensors (or impedimetric sensors see Fig. 8.1b). 

Some examples of thick-film impedimetric sensors can also be found in the bibliography. In Haeusler and Meyer (1996) interdigitated gold electrodes were used and BaTiOj-CuO was obtained from pulverized powders of both materials. The sensing device was printed onto an alumina substrate provided with a heater on its reverse. They found the response strongly dependent on the firing parameters and on the metal oxide additives added to the base material, such as CaCOj and La203. 

Sensitivity of a carbon dioxide BaTiOs-CuO thin film impedimetric sensor at 300°C and frequency of 100 Hz as the resistance change (Sr) and as the capacitance change (Sc) (Mandayo et a ., 2011). 

Mandayo, G.G., Herran, J., Castro-Hurtado, I. and Castano, E. (2011), Performance of a CO2 Impedimetric Sensor Prototype for Air Quality Monitoring , Sensors, 11,5047-57. 

The function of impedimetric sensors consists of the electrical properties of an electrode change, with its svurface properties. The electric properties of surface modified electrodes can be probed by means of impedance spectroscopy. ... 

Impedance spectroscopy measures the frequency response of the electric impedance. In an impedimetric sensor, the molecules to be characterized are adsorbed on the electrodes, or on a surface between a pair of electrodes. This process of absorption changes the electrical performance of the device. By measuring the impedance between the electrodes, some conclusions can be established [166]. 

Figure 14.16 Scheme of the immunosensor reaction for the development of an impedimetric sensor for atrazine detection. An amount of the specific antibody is bounded on the coated antigen layer. The rest is evacuated of the IDyEs which is related to the atrazine concentration. Erom Ref [41 ] with permission from Elsevier... 

As has been shown above, a number of samples can be analyzed at a single time using impedance microbial technology but the sensitivity of electrical impedimetric sensors is less as compared to other sensors. Some studies have demonstrated the promise of interdig-itated microelectrodes in impedance measurements for monitoring the growth of bacteria to improve analytical performance [1]. 

Chemoresistors for hquid phase (impedimetric sensors) have a design similar to that of gas sensors (Fig. 5.7). In contact with electrolytic solution, a specific electrochemical cell is estabhshed. With this cell, the measming set-up cannot be arranged to respond to effects of a single electrode alone, as was possible with classical electrochemical impedance experiments (Sect. 2.2.6). Hence, with chemoresistors the equivalent circuit must consider both electrodes. For a sensitive layer with some intrinsic conductivity, for the low frequency range the conditions can be symbolized approximately by Fig. 5.9. Cf and R symbolize the film s capacity and resistance, respectively. Q and R are the corresponding quantities of the sensor interface. 

Among the class of impedimetric sensors are real electrochemical sensors where a chemical reaction is the source of information. Of special importance are biosensors with SAMs, which are considered in more detail in the following chapter (Knichel et al. 1995, Rickert et al. 1996). 

Layers of synthetic peptides (molar mass ca. 3000 Da) which represent a specific site of an antigen have been bound via Tinkers or spacers at a gold surface. As a result, a SAM-based impedimetric biosensor has been reahzed (Rickert et al. 1996). The signal was obtained by evaluation of the impedance spectra recorded in the presence of the reversible redox system fer-rocyanide/ferricyanide. The redox reaction of this indicator is blocked when the antigen binds with the antibody. For this sensor, the term impedimetric sensor is appropriate. It has been apphed to detect the foot-and-mouth disease virus. Such application justifies the considerable efforts expended on the preparation of synthetic peptides. 

Examples of successful impedimetric sensors (rather than conductivity or capacitance) are few. One example involved the fabrication of lignin-modified glassy carbon electrodes which are sensitive to ozone. Exposing the sensor electrodes to various ozone concentrations resulted in proportional changes in the charge-transfer resistance in the impedance spectmm. More recently, microelectrochemical sensors were prepared which are suitable for use in marine environments and were tailored to voltammetric and impedimetric or conductivity measurements. ... 

Labib M, Zamay AS, Kolovskaya OS, Reshetneva IT, Zamay GS, Kibbee RJ, Sattar SA, Zamay TN, Berezovski MV (2012) Aptamer-based viability impedimetric sensor for bacteria. Anal Chem 84 8966-8969... 

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