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Electrochemical Sensors

F.R. Simões^*

M.G. Xavier^**
^* Institute of Marine Sciences, Federal University of São Paulo, Santos, São Paulo, Brazil
^** Center of Biological and Nature Sciences, Federal University of Acre, Rio Branco, Acre, Brazil

Abstract

Electrochemical sensors are a class of sensors in which the electrode is the transducer element. These devices (which appeared in the second half of the 20th century) are now found in a wide range of commercial applications. These sensors are notable because of several factors: the use of the electron for signal acquisition, which is considered a clean model for analytical applications, with no generation of waste; miniaturization in portable devices (analyses with microvolumes of samples); fast analysis; and low production cost, allowing the popularization of these methods (e.g., as commercial glucose sensors). In addition, the development of electrochemical sensors aids in the improvement of other techniques, such as chromatography detectors. Associated with nanotechnology, electrochemical sensors are becoming increasingly precise, selective, specific, and highly sensitive. This chapter introduces basic concepts of electrochemical sensors and their main analysis methods for a better understanding of nanostructured electrochemical sensors.

Keywords

electrochemical sensors

chemical sensors

nanostructured electrodes

potentiometry

chronoamperometry

voltammetric methods

potential pulse methods

electrochemical impedance spectroscopy

interdigitated sensors

electronic tongue

6.1. Introduction

A sensor is a device that responds to a physical stimulus, such as heat, light, sound, pressure, magnetism, or movement, and transmits a resulting electrical impulse as a means of measuring the change of any intrinsic property of the constituent material [1]. The origin of the word sensor comes from the Latin sentire, to feel. Semantically, sensors have the attribute of feeling into their surrounding environment to define a coupling relationship. Electrochemical sensors, in particular, are a class of chemical sensors in which an electrode is used as a transducer element in the presence of an analyte.

Modern electrochemical sensors use several properties to detect various parameters in our everyday lives, whether physical, chemical, or biological parameters. Some examples are environmental monitoring, health and instrumentation sensors, and sensors related to machines, such as automobiles, airplanes, mobile phones, and technology media. In recent decades, modern sensing systems have benefited from advances in microelectronic and microengineering [2], mainly through the manufacturing of even smaller sensors with more sensitivity and more selectivity, and with lower production and maintenance costs.

In this context, the incorporation of techniques and concepts related to nanoscience and nanotechnology (N&N) has a great impact on the fast development of nanosensors, mainly because of the surface versus volume ratio, which is one of the most significant characteristics of the nanometric order. However, this context is far from meeting the growing consumer expectations of societies with social inequalities, rendering the issue of knowledge democratization, and new technologies, including N&N, a great challenge for science in the 21st century, causing the social and political focus to be increasingly present in the scientific context [3].

Currently, the paradigms that involve research and development of electrochemical sensors allow the study of new materials, applications of samples of different natures, new manufacturing methods, and strategies to enhance selectivity and detection limits. Associated with N&N, these paradigms involve interdisciplinary relationships, highlighting scientists’ roles in the scientific revolution of the last few decades.

Historically, the use of electrochemical sensors had its beginnings in the 1950s, through the monitoring of industrial oxygen [4]. Labor laws related to the health and safety of the workers required the monitoring of toxic gases and fuels in confined spaces, prompting a burst of research into electrochemical sensors that could exhibit good selectivity for the detection of different gases. Leland C. Clark proposed the oxygen sensor concept with the use of two electrodes of a cell with an oxygen permeable membrane separating the electrodes and the electrolyte solution. The oxygen diffused through the membrane and was reduced in the indicator electrode. This resulted in a current proportional to the concentration of oxygen in the sample. The Clark oxygen sensor found wide application in medicine and environmental and industrial monitoring. In 1963, during the Cold War, a different oxygen sensor was developed and used for monitoring water quality in the former Union of Soviet Socialist Republics. Despite its commercial popularity, the electric current signal obtained in the Clark oxygen sensors was unstable, and the oxygen analyzers required frequent precalibration, which hindered their use [5].

The determining factor for the contemporary development of electrochemical sensors was the development of the scanning tunneling microscope (STM) and the atomic force microscope (AFM) in 1981 and 1986, respectively, by the Binnig and Rohrer teams [6,7]. These advances in scientific instrumentation enabled the development of new products and processes based on the ability of these instruments to visualize material structures with dimensions on the order of 1.0 × 10⁻⁹ m (1 nm), resulting in the so-called nanoscience and nanotechnology. Nanoscience shortly became a very broad and interdisciplinary research field, as it is based on various types of materials (polymers, ceramics, metals, semiconductors, composites, and biomaterials) structured on the nanometer scale to form nanoscale structures, such as nanoparticles, nanotubes, nanofibers, and nanospheres. Based on material behavior at the nanometric scale, promising new possibilities arose in the areas of research and applications for electrochemical sensors and electroanalytical methods.

6.2. Electroanalytical Methods

6.2.1. Principles

The signal from an electrochemical sensor is usually derived from an electrical response given the presence of an analyte. Table 6.1 lists some of the main electrochemical methods and their respective monitored electrical signals [8].

Table 6.1

Main Electrochemical Methods, Monitored Electrical Properties, and Respective Units

Electrochemical Methods	Monitored Electrical Properties	Units
Potentiometry	Potential difference (volts)	V
Conductometry	Resistance (ohms)	Ω
Amperometry and voltammetry	Current (amps) as a function of applied potential	I
Coulometry (Q)	Current as a function of time (coulombs)	C = I·s
Capacitance (C)	Potential load (farads)	F = C·V⁻¹

Table 6.1 indicates that in general, electrochemical responses monitored by different methods are based primarily on potential, resistance, and electrical current. Coulometry and capacitance methods, for example, have their responses derived from potential, resistance, and electrical current.

When we treat electrochemical techniques as methods of analysis, we can divide them into two main groups: interfacial methods and noninterfacial methods [8] (measuring the solution as a whole). From the examples mentioned in Table 6.1, conductometry is a noninterfacial method, as it is based essentially on a cell of known size, with two equidistant electrodes that measure the electrical conductance (essentially, the resistance) of a solution as a whole. When applying an alternating current signal, there is no electrode polarization and the response is given in terms of the electric resistance of the solution as a function of the cell constant, which is based on the surface area of the electrodes, their spacing, and the solution volume in the electrochemical cell [8]. In turn, the interfacial methods are methods that respond directly or indirectly to the presence of the analyte on the electrode surface (sensory unit), resulting in a disturbance of an electric signal that can be measured [8]. We can divide the interfacial methods into two main groups: the static methods and the dynamic methods. The static methods are defined as those in which there is no disturbance and the electric current is zero (i = 0), and the dynamic methods are those that exploit a redox reaction; that is, electron transfer occurs between the electrode and the analyte [8]. Importantly, dynamic methods employ current flow (and therefore charge transfer reactions involving electron transport) and can thus be used both for analytical determination (sensors) and in the manufacturing of nanostructured materials (Chapter 3, Volume 1, of this collection). This chapter primarily addresses the use of interfacial methods used in electroanalytical determination (sensors).

6.2.2. Potentiometry

Potentiometry is a static interfacial method and the most widely used electrochemical method in analytical applications [8]. A notable example of potentiometry is the pH meter (Fig. 6.1), which is usually based on glass membrane electrodes as indicators of the concentration (more accurately, the activity) of the H⁺ ions in solution.

Figure 6.1 Bench pH meter and glass electrode (Metrohm). Adapted from http://www.metrohm.com/en-us/products-overview/ph_ion-measurement/912_913_914-ph_cond-meters/%7B100FAF11-60F3-4D90-B244-0E438515D92D%7D

In general, a pH meter compares the activity of the H⁺ ions on both sides of the glass membrane for which the internal activity (concentration) is fixed and known, and the external activity (analysis solution) is measured. The responses are obtained in the potential difference (V) of the solution on both sides of the membrane and are converted to a pH scale based on the Nernst equation, which relates potential and concentration [8]:

$E = E^{0} + \frac{R T}{n F} \ln \frac{[RED]}{OXI}$ $E = E^{0} + \frac{R T}{n F} \ln \frac{[RED]}{OXI}$

(6.1)

where E, cell potential; E⁰, standard potential of a half-reaction; R, universal gas constant; T, temperature; n, number of electrons (eq. mol⁻¹) involved in the half-reaction; F, Faraday constant; [RED] = activity of the reduced species; and [OXI] = activity of the oxidized species.

Dynamic interfacial methods are those in which the surface of the working electrode, indicator electrode, or sensor receives an electric stimulus with a nonzero current (i≠0). Among these, we can highlight amperometry, voltammetry, and coulometry as controlled potential techniques, and the coulometric titrations with controlled current. Fig. 6.2 shows the main electrochemical methods and their subdivisions.

Figure 6.2 Main electrochemical methods and their subdivisions. Adapted from F.J. Holler, D.A. Skoog, S.R. Crouch, Principles of Instrumental Analysis, sixth edition, Thomson Brooks/Cole, 2007 [8].

The fundamental principle of any electrochemical sensor is the recognition of the analyte through the active layer of the material that composes it (Fig. 6.3) and the subsequent signal transduction to the recording equipment. Thus, by observing the diagram of electrochemical methods (Fig. 6.3), we can conclude that an electrochemical sensor is based on interface methods.

Figure 6.3 General diagram of an electrochemical sensor.

The electrochemical sensors of analytes in solution consist, in general, of electrochemical cells specifically set between an active material of the working electrode (sensory unit), the analyte, the reaction medium, and the set of electrodes used. The interface methods are electrolyte processes in which the polarization of the electrodes (cathode and anode) induces an electrochemical reaction. In a conventional electrolyte cell with two electrodes, there is a source, a voltmeter, an anode, and a cathode (Fig. 6.4).

Figure 6.4 Electrochemical cell in the system of two electrodes.

Despite the development of electrochemical analysis methods, there was a need for potential control. As previously mentioned, according to Eq. (6.1) (Nernst equation), the potential of an electrochemical cell is governed by the activity (concentration) of the analyte. As a reaction takes place, with the decrease in the analyte activity, the cell potential changes. The potential control also circumvents the problems related to the application of overpotential (η), which is the additional potential in relation to the thermodynamic potential to overcome the effects of electric and kinetic resistance (activation energy) [8]. The equipment that was derived from this need is called a potentiostat, which maintains the cell potential constant based on the constant monitoring of the reversible reaction potential of a reference electrode. All electrochemical potentiometric, voltammetric, and amperometric sensors use reference electrodes.

A reference electrode is an electrode with a known, constant potential and is insensitive to the composition of the solution under study. The ideal reference electrode must be based on a reversible reaction that obeys the Nernst equation and should preferably have low dependence on temperature [8]. Some reference electrodes meet these requirements well. The most common electrodes are the normal (or standard) hydrogen electrode (NHE or SHE), saturated calomel electrode (SCE), and saturated silver/silver chloride electrode (E_Ag/AgCl). The NHE is the historical and universal reference and can be easily constructed in the laboratory; however, its application is limited to certain applications, as it must be prepared each time it is used. The metal electrodes (SCE and E_Ag/AgCl) play a key role in the performance and applications of the current electrochemical sensors, as they are more robust and can be stored. They consist of a metal and a soluble salt of this metal in the electrode body. Table 6.2 shows the reference electrodes, their compositions, and potential constants at 25°C.

Table 6.2

Types of Reference Electrodes, Their Composition, and Potential at 25°C

Reference Electrode	Reversible Reaction	Standard Potential at 25°C
NHE	$2 H^{+} + 2 e^{-} ⇄ H_{2}$ $2 H^{+} + 2 e^{-} ⇄ H_{2}$	E⁰ = 0.0 V (by definition)
SCE	$H g_{2} C l_{2 (S)} + 2 e^{-} ⇄ H g_{(l)} + 2 C l^{-}$ $H g_{2} C l_{2 (S)} + 2 e^{-} ⇄ H g_{(l)} + 2 C l^{-}$	E⁰ = +0.244 V (vs. NHE)
E_Ag/AgCl	$AGC l_{(S)} + e^{-} ⇄ A g_{(S)} + C l^{-}$ $AGC l_{(S)} + e^{-} ⇄ A g_{(S)} + C l^{-}$	$E_{AgCl}^{0} = + 0.199 V (vs . NHE)$ $E_{AgCl}^{0} = + 0.199 V (vs . NHE)$

NHE, Normal hydrogen electrode; SCE, saturated calomel electrode.

In addition to using the reference electrode in its settings so that there is polarization of the working electrode (indicator electrode or sensory unit), the voltammetric and amperometric techniques (dynamic analysis methods) are based on a system of three electrodes (Fig. 6.5). The electric current of the electrochemical system is monitored at a constant potential (amperometric techniques) or as a function of the variation of the analysis potential (voltammetric techniques).

Figure 6.5 Conventional electrochemical cell in a system of three electrodes.

A conventional electrochemical cell of three electrodes consists of a working electrode, on which there is oxidation and reduction reactions of the species involved, the counter electrode (or auxiliary electrode), which serves to complete the electric circuit of the electrochemical system, and the reference electrode, which provides a reference for the assessment of other measured parameters [8].

6.2.3. Chronoamperometry

The potentiostatic technique of chronoamperometry measures the current flowing through the working electrode as a function of time as a constant potential is applied to the working electrode [9]. The current flow is correlated with the concentration of the oxidized or reduced species on the surface of the working electrode through the Cottrell equation:

$I t = \frac{n F A c_{O}^{0} D_{O}^{1 / 2}}{π^{1 / 2} t^{1 / 2}} = b t^{1 / 2} i_{i}$ $I t = \frac{n F A c_{O}^{0} D_{O}^{1 / 2}}{π^{1 / 2} t^{1 / 2}} = b t^{1 / 2} i_{i}$

(6.2)

where i_t is the current at time t (s), n is the number of electrons (eq. mol⁻¹), F is Faraday’s constant (96,485 C eq.⁻¹), A is the geometric area of the electrode (cm²), c₀ is the concentration of the oxidized species (mol cm⁻³), and D_o is the diffusion coefficient of the oxidized species (cm² s⁻¹).

Chronoamperometry allows processes to be studied in which the current is directly measured as a function of a constant potential applied to the electrochemical system. Given that the current detected in electrochemical experiments is governed by two processes, the mass transfer (corresponding to the transfer of species from the solution to the electrode–solution interface) and the charge transfer (corresponding to the electron transfer in the working electrode surface), the occurrence of chemical reactions that can occur simultaneously with the main processes, such as protonation, polymerization, adsorption, desorption, and crystallization, is studied to gain an understanding of the synthesis processes and characterization of materials through charge and mass transfer mechanisms (oxidation and reduction reactions that occur at the electrode-solution interface) [9]. During the 1960s, the potentiostatic chronoamperometry technique was used in several studies to explain the process of the nucleation and growth of polymers considering them as crystalline materials and therefore suitable for the nucleation of crystals with controlled nucleation [10].

Fig. 6.6 shows the chronoamperometry performed in a simultaneous determination in an aqueous solution of a mixture of nitrite and nitrate using an electrochemical sensor based on a composite electrode of silver (Ag), zeolite (ZE), and graphite-epoxy (GE). The authors observed a linear dependence of the nitrate and nitrite concentrations in the concentration ranges of 1–10 mM for nitrate and 0.1–1 mM for nitrite [11]. Historically, the maximum current peaks followed by a sharp drop in current shown in Fig. 6.6 are associated with the charging process of the electrical double layer, followed by its evolution featuring a maximum peak until its gradual decrease over time [9].

Figure 6.6 Chronoamperograms (CAs) of the AgZEGE electrode at two potential levels, that is, −1.35 V and +0.9 V versus SCE, recorded in 0.1 M Na₂SO₄ supporting electrolyte with the addition of a mixture of 0.1–1 mM nitrite and 1–10 mM nitrate. Insets: Calibration plots corresponding to current readings from CAs at 50 s [11].

Despite the difficulty in explaining such phenomena on the surface of the working electrode, in 1974, the pioneering work of Hills, Schiffrin, and Thompson [12] associated the maximum current value found in the chronoamperogram with equations related to the nucleation and growth model of metals. In later work, Scharifker and Hills demonstrated that the growth of the metal core was not described by linear ion diffusion, as previously thought, but by diffusion of spherical nuclei. This description of the growth of individual nuclei was later confirmed by computer simulation and monitoring of metal core growth in microelectrodes.

6.2.4. Voltammetric Methods

Historically, voltammetric methods were developed from the discovery of polarography in 1922 by the Czech chemist Jaroslav Heyrovsky, who received the chemistry Nobel Prize in 1959 [13]. Polarography studies electrolysis solutions and substances that are reduced or oxidized in a dropping mercury electrode and a reference electrode. The potential between these electrodes is varied, and the resulting changes in the current flow are measured. By plotting the changes in current flow against the potential variation, a current versus potential polarographic graph is obtained (I × E) [8].

During the 1960s and 1970s, theories, methods, and instrumentation were developed for the voltammetry field, thus increasing the sensitivity and repertoire of electroanalytical methods [14]. The common characteristic of all voltammetric techniques is that they involve the application of a potential (E) on an electrode and the monitoring of the resulting current (I) flowing through the electrochemical cell. In many cases, the applied potential is varied or the current is controlled for a period of time (t). Thus, all voltammetric techniques can be described as a function of potential, current, and time (E, I, and t). The analytical advantages of the many voltammetric techniques include the following: excellent sensitivity with the detectable concentration range of organic and inorganic species; a large number of useful solvents and electrolytes; a wide range of temperatures; fast analysis times (seconds); simultaneous determination of several analytes; the ability to determine kinetic parameters and estimate unknown parameters; the ease with which different potential wavelength shapes can be generated; and the measurement of small currents [8].

The potential sweep methods, also known as voltammetric methods, consist of the application of a potential varying continuously with time on a working electrode, which leads to the occurrence of oxidation or reduction reactions of electroactive species in the solution (faradaic reactions), in accordance with the adsorption of species with the potential and a capacitive current due to the electrical double layer. The observed current is therefore different from the current in the steady state. These methods are commonly used for the study of processes occurring in the working electrode and can be used with linear, pulse, and cyclic sweep, in addition to cyclic voltammetry (CV). Their main use has been for the diagnosis of electrochemical reaction mechanisms, for the identification of species present in solution, and for semiquantitative analysis of reaction speeds [15], and in addition to these applications, these methods are also widely used for the measurement of kinetic and constant rates, the determination of adsorption processes in surfaces, the study of electron transfers and reaction mechanisms, the determination of thermodynamic properties of solvated species, essential studies of oxidation and reduction processes in several ways, and the determination of complexing values and coordination.

6.2.4.1. Cyclic Voltammetry

In linear sweep voltammetry, the potential sweep is performed in only one direction, stopping at a chosen value E_f, for example, for t = t₁. The sweep direction can be positive or negative, and the sweep speed, at first, can take any arbitrary value [9]. In CV, when reaching t = t₁, the sweep direction is reversed and changed until E_min is reached, then reversed and changed to E_máx, and so on, generating cycles with several sweeps. The basic diagram involving the application of a potential sweep is shown in Fig. 6.7.

Figure 6.7 Potential applied as a function of time in cyclic voltammetry (CV) (E_inic, initial potential; E_f, final potential; E_máx, maximum potential; E_min, minimum potential).
Sweep speed v = dE/dt [12].

In a cyclic voltammogram, the most analyzed parameters are the following:

• initial potential, E_i;

• initial sweep direction;

• sweeping speed, ν;

• maximum potential, E_máx;

• minimum potential, E_min; and

• final potential, E_f.

An example of a cyclic voltammogram, that is, the measured current response as a function of the potential for a reversible system, is shown in Fig. 6.8. A faradaic current (I_f), resulting from the electrode reaction is recorded in the potential window during the reaction of the analyte on the electrode surface. There is also a capacitive contribution (I_c) because when sweeping the potential, the charge of the electrical double layer (C_d) changes. This contribution increases with increasing sweeping speed. The total current is a sum of the capacitive and faradaic currents [9]. Both currents tend to increase with increasing sweeping speed. This relationship limits the technique sensitivity because the high capacitive current can interfere in the sensitivity of the faradaic current, which, in the linearity region, is proportional to the analyte concentration. The potential obeys the Nernst equation (Eq. 6.1) and is therefore characteristic of a given redox process or an analyte. Thus, CV can be used for quantitative determinations; because of its limitations, however, it is more generally used for exploratory purposes, that is, to determine the redox process of different analytes. To minimize the contribution of the capacitive current and therefore increase the sensitivity of voltammetric methods, potential impulse (or pulse) methods were developed, including pulse voltammetry and square wave voltammetry (SWV) [9].

Figure 6.8 Cyclic voltammogram for a reversible system, where E is the potential and I is current. Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

Fig. 6.9 shows CV results in the determination of nitrite and nitrate using the composite electrode of silver, zeolite, and graphite-epoxy (AgGE). As expected, the AgZEGE electrode enables characterization of the redox peaks of the Ag⁰/Ag⁺ pair. Nitrite exhibits a well-defined oxidation peak at approximately 0.9 V compared with the SCE reference electrode. This peak can be attributed to the oxidation process from nitrite to nitrate. Furthermore, during the cathodic direction sweep, no cathodic peak appears, and an increase of cathodic current is observed starting at −1.0 V (vs. SCE) that corresponds to the reduction of nitrate to nitrite. Finally, successive potential sweeps using the AgZEGE electrode in 0.1 M Na₂SO₄ supporting electrolyte and the mixture of nitrate (0.1 − 1 mM) and nitrate (1 − 10 mM) reveal the linear dependence of the anodic and cathodic currents on the concentrations. These dependences form the basis for the use of the amperometric method (Fig. 6.6) [11].

Figure 6.9 Cyclic voltammograms of the AgZEGE electrode recorded in 0.1 M Na₂SO₄ supporting electrolyte with the addition of a mixture of 0.1–1 mM nitrite and 1–10 mM nitrate.
Scan rate: 50 mVs⁻¹ [11].

6.2.4.2. Differential Pulse Voltammetry

Differential pulse voltammetry (DPV) is a technique that involves applying amplitude potential pulses on a linear ramp potential. In DPV, a base potential value is chosen at which there is no faradaic reaction and is applied to the electrode. The base potential is increased between pulses with equal increments. The current is immediately measured before the pulse application and at the end of the pulse, and the difference between them is recorded. Fig. 6.10 shows the pulse shape in DPV [9].

Figure 6.10 Diagram of the application of pulses in the differential pulse voltammetry (DPV) technique. Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

DPV is a differential technique similar to the first derivative of a linear voltammogram in which the formation of a peak is observed for a given redox process. In the linear sweep technique, the voltammogram has a shape similar to a wave, and the first derivative originates a peak. In linear sweep voltammetry, as in polarography (dropping mercury electrode), the qualitative information of an analyte is given by the half-wave potential (E_1/2), which corresponds to the potential at half the wave height. Similarly, in DPV, the peak potential, E_p, can be approximately identified with E_1/2. Increasing the irreversibility, E_p deviates from E_1/2 as the base of the peak widens and its height decreases. The DPV is therefore a graph of differences between measured currents and applied potentials (Fig. 6.11) [9].

Figure 6.11 Typical response of a differential pulse voltammogram. Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

The best responses obtained with the use of DPV, compared to normal pulse voltammetry, are on solid electrodes [9], especially those involving organic compounds. Since they are usually adsorbed by the electrode, it is possible that a differential technique discriminates the effects that are more or less constant before and after the application of pulses.

In general, pulse techniques, such as DPV, are more sensitive than the linear sweep methods because there is minimization of the capacitive current. In turn, CV is most commonly used for exploratory purposes. Thus, in general, it is not unusual in sensor development to use both techniques because CV provides essential information, such as the process reversibility and types of redox processes present in the analysis (matrix, analyte, and electrode), whereas the pulse techniques are used for quantitative determinations. For instance, DPV was developed for detecting methyl parathion (MP) using a glassy carbon electrode modified with a nanocomposite film based on multiwalled carbon nanotubes (MWCNTs) and polyacrylamide (PAAM) [16]. The experimental results demonstrated that the MWCNT-PAAM/GCE nanocomposite film exhibited strong adsorption and high affinity with MP in environmental samples. Fig. 6.12 shows the results obtained from CV, used for verification of the redox behavior of the pesticide, and DPV, used for the analytical determination. The authors obtained a linear calibration curve in the concentration range of 5.0 × 10⁻⁹ to 1.0 × 10⁻⁵ mol L⁻¹, with a detection limit of 2.0 × 10⁻⁹ mol L⁻¹.

Figure 6.12 CV of MWCNTs–PAAM/GCE in the absence (a) and presence (b and c) of 5.0 × 10⁻⁶ mol L⁻¹ MP; (B) mechanism of the electrochemical reaction of MP with MWCNTs–PAAM/GCE; (C) DPV of MWCNTs–PAAM/GCE (a and d), PAAM/GCE (b and e), and GCE (c and f) in the presence (a, b, and c) and absence (d, e, and f) of 1.0 × 10⁻⁵ mol L⁻¹ MP; supporting electrolyte, 0.2 M PBS (pH 7.0); scan rate, 100 mVs⁻¹; adsorption time, 5 min [17].

6.2.4.3. Square Wave Voltammetry

SWV is one of the fastest and most sensitive pulse voltammetry techniques. The detection limits can be compared with those of chromatographic and spectroscopic techniques. In addition, the analysis of the characteristic parameters of this technique also enables the evaluation of the kinetics and mechanism of the electrode process under study [9,17]. In SWV, the shape of the potential current curve is derived from the application of potentials of height ∆E (pulse amplitude), which vary according to a potential step E_step (in mV) and τ duration (period). On the potential–time curve, the pulse width (τ/2) is denoted by t, and the frequency of pulse application is denoted by f and is given by (1/t). The electric currents are measured at the end of the direct (I₁) and reverse (I₂) pulses, and the signal is obtained as an intensity of the resulting differential current (∆I); this technique offers excellent sensitivity and high rejection to capacitive currents. This measurement precedes an initial time (t_i) at which the working electrode is polarized at a potential for which the redox reaction does not occur [17].

Fig. 6.13 shows details of the potential application of SWV, with the definition of the used parameters, whereas Fig. 6.14 shows the theoretical voltammograms associated with (A) a reversible system and (B) an irreversible system, with observed separation of direct, reverse, and resulting currents, and both voltammetric profiles have similarities to those obtained in square wave polarography.

Figure 6.13 Application of potentials in square wave voltammetry (SWV). Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

Figure 6.14 Schematic square wave voltammogram, where (A) represents a redox process of a reversible system and (B) represents that of an irreversible system. Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

Current–potential curves display well-defined profiles and are usually symmetrical. This is due to all currents being measured only at the end of each semiperiod and the variations in the height and width of the potential pulse being always constant for a determined potential range [17]. Thus, electrochemical techniques can be used in the synthesis and characterization of materials through voltammetric methods that relate the current and the electric potential in the electrochemical cell. In amperometric sensors, for instance, a constant electric potential is applied to the electrochemical cell and a corresponding current appears because of the redox reactions that occur on the surface of the working electrode. This current can be used to quantify the reactions involved. Amperometric sensors can be operated through CV, another powerful technique for the synthesis and characterization of different electroactive species, and the relationship between the current–potential characteristic of each oxidation or reduction reaction involved. In general, voltammetric sensors are used in the detection of species in redox reactions that occur in the electrochemical cell. The SWV technique has also been used in the development of sensors and biosensors because of its high sensitivity and selectivity [17]. It is currently of great interest to the pharmaceutical industry for the use of biomarkers in the detection of disease, environmental pollutants, such as heavy metals, and other chemical contaminants that are part of the environmental liability in contemporary societies. Fig. 6.15 shows the SWV results for the quantification of pheniramine in a pharmaceutical formulation using a glassy carbon electrode modified with MWCNTs in the presence of sodium lauryl sulfate [18]. Experimental results suggest that pheniramine in anionic surfactant solution has an electrocatalytic effect, resulting in a significant increase of the peak current. The authors observed a linear dependence of the peak current with the analyte concentration (pheniramine) in the range of 200–1500 μg mL⁻¹, with a correlation coefficient of 0.9987. The detection limit was 58.31 μg mL⁻¹.

Figure 6.15 (A) Plot of current versus different concentrations of pheniramine in 1% SLS (pH 5.3). (B) Calibration of square-wave voltammetric peak current response of pheniramine at different concentrations in 1% SLS (pH 5.3): (a) blank, (b) 200 μg mL⁻¹, (c) 400 μg mL⁻¹, (d) 750 μg mL⁻¹, (e) 1100 μg mL⁻¹, (f) 1300 μg mL⁻¹, and (g) 1500 μg mL⁻¹.

The efficiency and sensitivity of SWV are used in the detection of food contaminants (bacteria, viruses, and parasites) and to verify the therapeutic ingredients of dietary supplements. Other SWV applications are to research the mechanism of enzymatic kinetics and the development of new methods to improve the surfaces of nanomaterial used in high sensitivity sensors and biosensors. Thus, the SWV technique is a powerful tool in the creation of diagnostic devices and environmental/food monitoring with high sensitivity and selectivity for enzymatic studies [19].

6.2.5. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is a technique used in the analysis of electrochemical processes that occur in the electrode/electrolyte solution interface. This is a method of identification and determination of parameters from a model developed based on the frequency response of the electrochemical system under study. A frequency response analyzer coupled with an electrochemical interface is used in such experiments, which measures the current response of the system as it changes the frequency of an input sinusoidal signal [9,20,21], E = E_o sin (ω = 2πf), that is applied to an unknown sample; the response is analyzed through a correlation with the resulting current I = I_o sin (ωt + ϕ), where ϕ is the phase angle displacement.

The impedance (Z) is the vector sum given by [9]

$Z = R - j X_{c}$ $Z = R - j X_{c}$

(6.3)

where j is equal to

\sqrt{1}

$\sqrt{1}$

, (Fourier) R is resistance, X_c is the capacitive reactance and equal to a1/ωC (measured in ohms) with ω = 2πf, and C is the capacitance of the electric double layer. Therefore, it is noted that R is the real part (Z′) and −jX_c is the imaginary part (Z″) of the impedance.

The complex impedance, represented by the displacement vector Z(ω), is obtained by changing the alternating signal ω; with these parameters, the modulus Z_o = E_o/I_o and the phase angle ϕ can be calculated to determine the real and imaginary impedance values, as shown in Fig. 6.16.

Figure 6.16 Vector representation of the complex impedance Z. Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

An example of a simple equivalent circuit of an electrochemical cell, a resistor in parallel with a capacitor (RC), is depicted in Fig. 6.17. The typical resulting spectrum is shown, consisting of a semicircle in the complex impedance plane, with the corresponding imaginary opposite Z″ = −Im = Z_o sin (ω) conventionally represented on the y axis and the real component Z′ = R and Z = Z_o cos (ω) on the x axis.

Figure 6.17 Schematic representation of the impedance analysis response of a parallel RC circuit. Adapted from A.M.O. Brett, C.M. Brett, A Electrochemistry Principles, Methods, and Applications, Oxford University Press, Coimbra, 1994 [9].

The frequency at the top of the semicircle indicates the resonance condition of the circuit [9,20,21], given by the expression 2πf_oRC = 1. Impedance spectroscopy is often used to study films deposited on electrodes because it can distinguish the different conductivity processes that occur in the material. The results are often associated with an electrical circuit, in which it is possible to distinguish and calculate parameters, such as the electrolyte resistance, ionic conductivity, capacitance of the electrical double layer, and electron transfer resistance [9].

Although minimally used for analytical purposes, capacitance can be used for this purpose, enabling capacitive sensors. When an analyte interacts with the receptor (Fig. 6.2), there may be a change in its specific capacitance, and this response change can be interpreted as an analytical signal. There are currently several types of commercial sensors that use capacitance as a measure. Among them are analyte sensors, such as gas sensors, and especially physical sensors, such as of presence or of touch. Further on, we will discuss the sensory network termed the electronic tongue (ET), which relies on electrochemical impedance spectra, whose signals are results obtained from the capacitance values obtained for each sensor (sensory unit).

6.2.6. Electronic Tongue: Concepts, Principles, and Applications

As previously described, chemical sensors are widely used analytical tools. The first ion selective electrode (ISE) based on potentiometric measures, which appeared in 1909, was the pH meter [22]. This sensor, with minor modifications, is still the most commonly used one for liquids. From the beginning, sensor development has always been based on the selectivity of the analytes. Other ion-selective sensors have been developed, and dozens are commercially available. However, the number of existing analytes that could be sensed, combined with the numerous variables in complex matrices, is incalculable, which makes it necessary to develop less specific sensors. One way to increase the analytical application of sensors is to use a combined matrix of sensors and more sophisticated techniques of data manipulation so that the combination of responses constitutes a “fingerprint,” not only of an analyte but also of the combined form between analytes and their matrices, for instance, a pesticide in a particular soil water matrix [23].

Environmental samples are highly complex matrices because of the many substances that may be present; they are different from one environment to another in addition to having seasonal characteristics. If we analyze samples collected at different locations or days from a given lake or dam, we can easily observe different characteristics among them.

The idea of combining responses from different sensors to obtain this “fingerprint” was initially implemented for gas sensors, which are now known as electronic noses [24]. Subsequently, several researchers developed the principle of the electronic nose method for liquid samples, which became known as the ET.

The electronic nose principle was inspired by sensorial biological systems, for example, the mammalian olfactory power. The olfactory system consists of a large number of nonspecific receptors that react to volatile compounds and transfer nervous stimuli to the brain, where a neural network processes the signal pattern. Any animal with olfactory power is capable of recognizing thousands of odors with very low detection limits, despite the high sensitivity and low selectivity of each individual receiver. However, the high performance of olfactory systems is given by the combined response of several sensor units and subsequent conjoint treatment of the responses in the nervous system. Similarly, taste employs an operational principle and interpretation of responses similar to the olfactory system, except that taste involves, in general, significantly fewer receptors.

As with the biological tongue, the ET operates in a liquid medium, but with a much greater power of sensitivity and selectivity. In terms of performance, these advantages allow the ET to be closer to the olfactory system than the gustatory system. The ET can also be used for the quantitative detection of a variety of dissolved compounds, including volatile substances that are responsible for odors but that originate from both liquid and solid phases. A common feature of all electronic noses and tongues is the use of a matrix of nonspecific sensors combined with data processing by pattern recognition methods.

The most often used data processing system is the artificial neural network (ANN) [25], in which the algorithms are based on the learning and recognition processes of the human brain [26]. The most traditionally used methods are statistical methods, such as the principal component analysis (PCA) method, multivariate regression techniques, and factorial analysis [27,28].

Since the mid-1980s, there have been several studies describing the combination of signals from a sensor matrix in liquid media with complex matrices, using several mathematical models for pattern recognition [29–34]. The first studies were conducted with potentiometric sensors to improve the performance of ion-selective conventional electrodes. The first study that can be considered as a multisensory approach for liquid detection was the one by Otto and Thomas in 1985 [29].

In a simplified manner, the ETs respond to the substances responsible for originating the basic flavors analogously to the biological system and can differentiate beverages with similar flavors. For instance, some of them can distinguish between different brands of bottled water or wine brands and harvests, in addition to detecting metal ions in water. There are several detection methods used in ETs. Potentiometry, CV, and alternating current measurements are examples of applications. However, the method choice can affect the sensor performance. For example, measurements of admittance (Y) or its reciprocal, impedance (Z), using sensors based on polymer films are particularly sensitive [35], with very low detection methods for sucrose and other substances related to flavor. Since then, the ET has been used for measurements of several flavors [36–38]. Other advantages of the use of impedance, admittance or even capacitance (which uses a specific frequency) combined with polymer films, compared with potentiometric sensors, are the experimental simplicity and better performance for nonpolar substances, such as caffeine and sucrose. Unlike the sensors based on voltammetry or potentiometry techniques, the solutions to be analyzed do not require any pretreatment and do not use a reference electrode. In general, the basic requirement to assess the performance of the ET is the ability to differentiate substances that create the basic flavors. A single material likely be sensitive to all basic tastes; therefore, a combination of sensors is usually used. For example, Embrapa Agricultural Instrumentation, in São Carlos, in the state of São Paulo, developed the ET for different applications [39]. The developed ET consists of a network of sensory units based on different conductive polymers deposited on substrates of interdigitated gold electrodes, as shown in Fig. 6.18.

Figure 6.18 (A) Sensor network, (B) diagram of the interdigitated sensor, and (C) sensory unit with a PANI and carbon nanotube film used in the electronic tongue (ET) of Embrapa Agricultural Instrumentation.

The sensory units consist of nanostructured films of conductive polymers, such as polyaniline and their derivatives or polypyrrole, deposited on interdigitated microelectrodes. These polymers have the ability to reversibly change their optical and electrical properties when the characteristics of the surrounding environment are modified [40,41], such as the chemical composition, pH, and ionic strength. Different beverage types have been analyzed with the ET based on conductive polymers, including mineral water, coconut water [42], coffee and tea [43], wine [44,45], and sugarcane juice [46]. In mineral waters, taste characteristics measured with the ET might be associated with their physicochemical composition. With this system, it was possible to detect the presence of heavy metals [47] and pesticides [48] in these beverages. In wines, the ET proved able to identify them according to the harvest, producer, and grape type used for their production. The coffee quality was also systematically investigated with the ET, which led to the technological entrepreneurship project “Development of an electronic tongue prototype for the evaluation of coffee quality” with the support of the Brazilian Association of the Coffee Industry (Associação Brasileira de Indústrias de Café—ABIC). There was a good correlation between the sensor results and scores awarded by tasters.

ETs built with electrochemical, optical, or mass sensors often have the disadvantage of low specificity. Thus, in the search for specificity, the ET has also been developed in the manufacturing of biosensors [49–54]. For example, Hu et al. [53] recently evaluated the applicability of germ cells from male mice testes in the manufacture of a cell-based impedance sensor (CIS) to increase the specificity for the bitter taste, which can have many variations as a result of the wide variety of bitter compounds with diverse chemical structures and functions. The cell impedance response profiles of the germ cells for four bitter compounds were analyzed, assessing the response intensity under different concentrations. Finally, the impedance responses for the five basic tastes were examined to assess the biosensor-based detection performance for bitter compounds. The results revealed that this hybrid bitter biosensor could respond to these bitter compounds in a dose-dependent manner. The quinine detection limit was 0.125 μM. The authors stated that this sensor based on germ cells could specifically detect bitter compounds between the five flavor stimuli, which can provide a promising and valuable approach in the detection of several bitter compounds.

Currently, the ET is also a commercial product. The company Alpha MOS manufactures and sells the “ASTREE electronic tongue” device using chemically modified field effect transistor technology and potentiometric measurements with seven selective liquid sensors for ionic, neutral, and chemical compounds responsible for flavor [55].

List of Symbols

∆E Potential pulse amplitude

A Geometric electrode area

Au Gold

C Capacitance

c Concentration

C Coulomb

C_d Capacitance of electric double layer

cm² Square centimeter

cm³ Cubic centimeter

D₀ Diffusion coefficient

dE Differential compared to potential

dt Differential compared to time

e⁻ Electron

E Potential

E⁰ Standard potential

E_1/2 Half-wave potential

E_Ag/AgCl Standard potential of silver/silver chloride saturated electrode

E_f Final potential

E_i Initial potential

E_inic Initial potential

E_máx Maximum potential

E_mín Minimum potential

E_p Peak potential

E_pa Anodic peak potential

E_pc Cathodic peak potential

eq. Equivalent

f Frequency

F Farad

HCl Hydrochloric acid

I or i Current

I_f Faradaic current

I_c Capacitive current

Im Z Imaginary impedance

I_pa Anodic peak current

I_pc Cathodic peak current

jX_c Imaginary part (Z″) of impedance

m Milli (1 × 10⁻³)

mol Mol

mol L⁻¹ Mol per liter

n Number of electrons (eq. mol⁻¹)

°C Degrees Celsius

PANI Polyaniline

Q Charge

R Universal constant of gases

R Electrical resistance/real part (Z′) of the electrochemical impedance

RC Resistance in parallel with a capacitor R

Z Real impedance

s Seconds

T Temperature

t₁ Initial time

V Volt

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Table of Contents for 6: Electrochemical Sensors

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