In impedance spectroscopy, the frequency of an electrical signal is measured. The frequency is then used to determine the impedance of the device. There are two basic types of impedance plots, the Nyquist plot and the Bode plot. The Nyquist plot is used to measure the impedance of a single frequency, while the Bode plot is used to measure the impedance of multiple frequencies.
Electrochemical impedance spectroscopy has many applications. It is particularly important in studying the interface between an electrode and a solution. The results are usually represented in equivalent circuits, and describe the electrical behavior between the electrodes. It is important in electrochemistry, which includes batteries and fuel cells.
The total impedance is the sum of all the real and imaginary components. The real and imaginary components act like parallel combination of resistors and capacitors. The real component of the total impedance has high resistive behavior, while the imaginary component behaves like a capacitor with high capacitance.
In addition to the Bode impedance, Warburg impedance is another example. It is a frequency dependent impedance that is small at high frequencies. In contrast, at low frequencies, the reactants must diffuse more and thus increase their Warburg-impedance. On the Bode plot, a Warburg-impedance can be seen as a 45-degree sloped line.
A Bode Plot of the same data is shown in Figure 24. The lower frequency limit is set to 1 mHz in order to better illustrate the differences in magnitude and phase. The results from EIS are often analyzed using an equivalent circuit model. The Echem Analyst attempts to find a model with an impedance that matches the measured values.
The Nyquist plot is also useful because it can reveal the characteristic shapes of elements and combinations of elements. It also allows rapid data fitting and helps understand how elements work in a system. Moreover, it provides an insight into the electrochemical performance of a device. This makes impedance spectroscopy a valuable tool in electrochemical research.
One recent study developed an impedimetric calcium sensor based on gold nanoparticles. This sensor has high sensitivity and a wide linear range. In addition, it has a lower detection limit than ELISA. Therefore, it should be considered as an alternative to ELISA.
