**2. Electrochemical impedance spectroscopy**

Electrochemical impedance spectroscopy (EIS) is an analytical tool, which has been used for studying electrochemical systems including corrosion [20–22], battery development [23], electrodeposition [24], fuel cells [25, 26], and charge transport through membranes [27]. For impedance measurements, the alternating current (AC) voltage applied is typically small (up to 10 mV) so that the voltagecurrent response is linear, allowing simple equivalent circuit analysis [28]. Different waveforms of the AC voltage *V*(*t*) varying in time can be used [29]. The simplest but best-known waveform among them is a pure sine wave *V*(*t*) = *V*<sup>0</sup> sin (*t*), which varies periodically (oscillates) with angular frequency ω = 2π *f*, rad/s, where *f*, (1/s ≡ Hz), is the repetition frequency of oscillation periods. The current response *I*(*t*) to the applied voltage *V*(*t*) is also the sine wave at exactly the same frequency ω = 2π *f*. In addition, the current response *I*(*t*) is shifted over the time interval (Δ t) against the applied voltage *V*(*t*) because of containing inert energy saving components (capacitance C and/or inductance L) of impedance Z. In practice, it is reasonable to use the phase shift φ = 2*f*(Δ t), rad, instead of the time interval (Δt). Predominantly, the impedance handling assumes that there are no changes in impedance value during the observation time interval. Therefore, we can exclude time dependence from the mathematical expression of impedance and use the frequency dependent impedance *Z*(ω) instead of *Z*(*t*,ω). Mathematical equation for the impedance *Z*(ω) is the ratio between the voltage-time function *V*(*t*) and the resulting current-time function *I*(*t*) (Eq. (1)): *<sup>Z</sup>*(*t*) = \_

$$\begin{array}{l} \text{FumCronH} \ I(t) \ \text{(Eq. (1))}: \\\\ \text{Z}(t) = \frac{V(t)}{I(t)} = \frac{V\_0 \sin\left(2\pi ft\right)}{I\_0 \sin\left(2\pi ft + \varphi\right)} = \frac{V\_0 \sin\left(\alpha t\right)}{I\_0 \sin\left(\alpha t + \varphi\right)} \end{array} \tag{1}$$

**55**

**Figure 1.**

*Challenges and Applications of Impedance-Based Biosensors in Water Analysis*

are usually analyzed using Bode or Nyquist plots [30, 31].

different frequencies, the data obtained allows characterizing the complex electrode systems on layers, surfaces, or membranes where electrical charge transfer and ion diffusion processes take place [7]. To evaluate and interpret the results, the EIS data

Based on the methodologies of signal collection, impedimetric detection can be categorized in two ways: capacitive faradaic or non-faradaic. It is important to distinguish between those approaches. In electrochemical terminology, a faradaic process is the one where charge is transferred across an interface. In the case of nonfaradaic, the transient currents can flow without charge transfer (e.g., charging a capacitor). In faradaic EIS, a redox probe is alternately oxidized and reduced by the transfer of an electron to and from the metal electrode. Thus, faradaic EIS requires the addition of a redox probe and direct current (DC) bias conditions such that it is not depleted. In contrast, no additional reagent is required for non-faradaic impedance spectroscopy, rendering non-faradaic schemes somewhat more amenable to

In the case of faradaic impedimetry, the electrode surface is partially or fully covered with a non-isolating layer or with an isolating layer able to catalyze a redox probe [34]. Non-faradaic approach is also known as the direct measurement manner (without chemical reactions). In the case the redox probe is missing, the impedance depends on the conductivity of the supporting electrolyte and electrode interfacial properties. Capacitive approach means that the surface of the electrode is completely covered with a dielectric layer. In this type of sensors, no redox probe is present in the system; and the current is measured under a small amplitude sinusoidal voltage signal, at low frequencies [34]. Capacitive biosensors are mainly based on a non-faradaic approach, because the transient current flows without charge

Briefly in faradaic approach, the charge is transferred across the electrified interface as a result of an electrochemical reaction, and in non-faradaic approach, the charge is associated with movement of electrolyte ions, reorientation of solvent dipoles, adsorption/desorption, etc. at the electrode-electrolyte interface. Detailed overviews about faradic and non-faradaic systems are given in Refs. [31, 34, 35]. In order to present information about surfaces, layers, or membranes after the immobilization of biomolecules, EIS experimental data is often analyzed using an equivalent circuit of electrochemical cell [30]. The Randle's circuit (**Figure 1**) is a frequently used equivalent for modeling the impedance [32]. The non-faradaic sensor comprises the uncompensated resistance of the electrolyte (*R*s) and the constant

*Simplified circuit models for (A) non-faradaic and (B) faradaic systems. Abbreviations: Rs, resistance of the electrolyte; CPE, constant phase element; Rct, charge-transfer resistance; W, the Warburg impedance.*

*DOI: http://dx.doi.org/10.5772/intechopen.89334*

point-of-care applications [32, 33].

transfer and no additional reagent is required.

More complicated voltage signal waveforms are required for the fast performance of EIS by generating the signal components at several frequencies simultaneously [29]. As EIS measures the response of an electrochemical cell to a voltage at

#### *Challenges and Applications of Impedance-Based Biosensors in Water Analysis DOI: http://dx.doi.org/10.5772/intechopen.89334*

different frequencies, the data obtained allows characterizing the complex electrode systems on layers, surfaces, or membranes where electrical charge transfer and ion diffusion processes take place [7]. To evaluate and interpret the results, the EIS data are usually analyzed using Bode or Nyquist plots [30, 31].

Based on the methodologies of signal collection, impedimetric detection can be categorized in two ways: capacitive faradaic or non-faradaic. It is important to distinguish between those approaches. In electrochemical terminology, a faradaic process is the one where charge is transferred across an interface. In the case of nonfaradaic, the transient currents can flow without charge transfer (e.g., charging a capacitor). In faradaic EIS, a redox probe is alternately oxidized and reduced by the transfer of an electron to and from the metal electrode. Thus, faradaic EIS requires the addition of a redox probe and direct current (DC) bias conditions such that it is not depleted. In contrast, no additional reagent is required for non-faradaic impedance spectroscopy, rendering non-faradaic schemes somewhat more amenable to point-of-care applications [32, 33].

In the case of faradaic impedimetry, the electrode surface is partially or fully covered with a non-isolating layer or with an isolating layer able to catalyze a redox probe [34]. Non-faradaic approach is also known as the direct measurement manner (without chemical reactions). In the case the redox probe is missing, the impedance depends on the conductivity of the supporting electrolyte and electrode interfacial properties. Capacitive approach means that the surface of the electrode is completely covered with a dielectric layer. In this type of sensors, no redox probe is present in the system; and the current is measured under a small amplitude sinusoidal voltage signal, at low frequencies [34]. Capacitive biosensors are mainly based on a non-faradaic approach, because the transient current flows without charge transfer and no additional reagent is required.

Briefly in faradaic approach, the charge is transferred across the electrified interface as a result of an electrochemical reaction, and in non-faradaic approach, the charge is associated with movement of electrolyte ions, reorientation of solvent dipoles, adsorption/desorption, etc. at the electrode-electrolyte interface. Detailed overviews about faradic and non-faradaic systems are given in Refs. [31, 34, 35].

In order to present information about surfaces, layers, or membranes after the immobilization of biomolecules, EIS experimental data is often analyzed using an equivalent circuit of electrochemical cell [30]. The Randle's circuit (**Figure 1**) is a frequently used equivalent for modeling the impedance [32]. The non-faradaic sensor comprises the uncompensated resistance of the electrolyte (*R*s) and the constant

#### **Figure 1.**

*Simplified circuit models for (A) non-faradaic and (B) faradaic systems. Abbreviations: Rs, resistance of the electrolyte; CPE, constant phase element; Rct, charge-transfer resistance; W, the Warburg impedance.*

*Biosensors for Environmental Monitoring*

high selectivity and sensitivity.

Biosensors can be a useful tool for the detection of pollutants in the water. In comparison with traditional monitoring techniques, biosensors are portable, need minimal sample preparation, and are also rapid and reliable [4]. According to the International Union of Pure and Applied Chemistry (IUPAC) definition, a biosensor is a self-contained, integrated receptor transducer device, which is capable of providing selective quantitative or semiquantitative analytical information and which uses a biological recognition element (bio-receptor) and a transducer in direct special contact [5]. Biosensors can be used for continuous monitoring with

Biosensors are classified according to their biorecognition element or signal transducer into various categories. Electrochemical biosensors based on impedance are among the most promising ones due to their portability, rapidity, and label-free operation. Label-free sensors register changes in the electrical properties due to interactions between biological molecule attached to the sensor and an analyte present in the sample, and as these sensors generate rapid response, they can be used to track molecular events in a real-time manner [6]. The main advantage of labelfree detection is that it is possible to acquire direct information of the interactions between native proteins and ligands [6, 7]. In environmental analysis most of the biosensors used are enzyme-based biosensors [8–12] or antibody-based immunosensors [13–16]. In recent years also the development of aptasensors has increased [17–19]. The present chapter gives a critical overview of the development issues and

applicability of different impedimetric biosensors used for water analysis.

but best-known waveform among them is a pure sine wave *V*(*t*) = *V*<sup>0</sup> sin

varies periodically (oscillates) with angular frequency ω = 2π *f*, rad/s, where *f*, (1/s ≡ Hz), is the repetition frequency of oscillation periods. The current response *I*(*t*) to the applied voltage *V*(*t*) is also the sine wave at exactly the same frequency ω = 2π *f*. In addition, the current response *I*(*t*) is shifted over the time interval (Δ t) against the applied voltage *V*(*t*) because of containing inert energy saving components (capacitance C and/or inductance L) of impedance Z. In practice, it is reasonable to use the phase shift φ = 2*f*(Δ t), rad, instead of the time interval (Δt). Predominantly, the impedance handling assumes that there are no changes in impedance value during the observation time interval. Therefore, we can exclude time dependence from the mathematical expression of impedance and use the frequency dependent impedance *Z*(ω) instead of *Z*(*t*,ω). Mathematical equation for the impedance *Z*(ω) is the ratio between the voltage-time function *V*(*t*) and the

Electrochemical impedance spectroscopy (EIS) is an analytical tool, which has been used for studying electrochemical systems including corrosion [20–22], battery development [23], electrodeposition [24], fuel cells [25, 26], and charge transport through membranes [27]. For impedance measurements, the alternating current (AC) voltage applied is typically small (up to 10 mV) so that the voltagecurrent response is linear, allowing simple equivalent circuit analysis [28]. Different waveforms of the AC voltage *V*(*t*) varying in time can be used [29]. The simplest

(*t*), which

**2. Electrochemical impedance spectroscopy**

resulting current-time function *I*(*t*) (Eq. (1)):

*V*(*t*) *I*(*t*)

<sup>=</sup>*V*<sup>0</sup> sin

(2 *ft*) \_\_\_\_\_\_\_\_\_\_\_\_ *<sup>I</sup>*<sup>0</sup> sin (2 *ft* + φ)

More complicated voltage signal waveforms are required for the fast performance of EIS by generating the signal components at several frequencies simultaneously [29]. As EIS measures the response of an electrochemical cell to a voltage at

= *V*<sup>0</sup> sin (*t*) \_\_\_\_\_\_\_\_\_\_\_ *I*<sup>0</sup> sin

(*t* + φ)

(1)

*<sup>Z</sup>*(*t*) = \_

**54**

phase element (CPE) having capacitive-like properties in parallel with the chargetransfer resistance (*R*ct).

Sometimes simplifications are introduced, and the CPE is replaced by a doublelayer capacitance (*C* dl), which introduces the constant phase shift of -π/2 rad (−90°) at all the frequencies. In reality, the CPE introduces the phase shift φ less than π/2 [29, 36].

The faradaic sensor model includes the Warburg impedance (*W*), which describes diffusion phenomenon taking place due to chemical redox processes. The ideal Warburg impedance introduces the phase shift of π/4. Values of the charge transfer *Rct* and *W* depend on physicochemical parameters of a system. In real systems, impedance spectra are usually more complicated, and, therefore, the Randle's circuit with a corresponding plot may not give proper results [31].
