1
Fundamental Concepts

1.1 Why Electroanalysis?

Electroanalytical techniques are concerned with the interplay between electricity and chemistry, namely the measurements of electrical quantities, such as current, potential, or charge, and their relationship to chemical parameters. Such use of electrical measurements for analytical purposes has found a vast range of applications, including environmental monitoring, biomedical analysis, or industrial quality control. Over the past three decades, we have witnessed major advances in the field of electroanalysis, including the development of ultramicroelectrodes, the design of tailored interfaces and molecular monolayers, the coupling of new biological components and electrochemical transducers, the synthesis of ionophores and biomimetic receptors containing cavities of molecular size, design of new interfaces and tags based on diverse nanomaterials, the development of high-resolution scanning probe microscopies or of solid-contact potentiometric sensors, the introduction of green electrode materials and of paper-based electrochemical sensors, the development of disposable sensors strips coupled to hand-held mobile electrochemical analyzers and smartphone devices, the introduction of flexible skin-worn or wrist-band wearable electrochemical devices or the development of multiplexed bioelectronic assays. Such dramatic changes reflect major technological advances in materials, mobile and wearable devices, microfabrication, additive manufacturing and miniaturization, nanotechnology and biotechnology, along with societal changes and new trends (toward the internet of things, digital health, personalized nutrition, or wellness). These advances have led to a substantial increase in the power and popularity of electroanalysis, and to its expansion into new phases and environments. The recent COV19 pandemic has emphasized why miniaturized electrochemical sensors are critically needed to contain the spread of infectious diseases. The pandemic has also accelerated the role of mobile and wearable sensors for implementing telehealth (remote) systems. Wearable electrochemical sensors, integrated on the human body, have been shown to be extremely useful for monitoring continuously and noninvasively the wearer's health, nutrition, wellness, and performance. Indeed, electrochemical probes are receiving a major share of attention in the development of chemical sensors. Beyond disease diagnosis or environmental monitoring, electrochemical devices, such as batteries, fuel cells, or electrochromic displays, play key roles in diverse parts of our daily life, ranging from wearable electronics to energy management.

In contrast to many chemical measurements, which involve homogeneous bulk solutions, electrochemical processes take place at the electrode-solution interface. The distinction between various electroanalytical techniques reflects the type of electrical signal used for the quantitation. The two principal types of electroanalytical measurements are potentiometric and potentiostatic. Both types require at least two electrodes (conductors) and a contacting sample (electrolyte) solution, which constitute the electrochemical cell. The electrode surface is thus a junction between an ionic conductor and an electronic conductor. One of the two electrodes responds to the target analyte(s) and is thus termed the indicator (or working) electrode. The second one, termed the reference electrode, is of constant potential (that is independent of the properties of the solution). Electrochemical cells can be classified as electrolytic (when they consume electricity from an external source) or galvanic (if they are used to produce electrical energy).

Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples.

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to drive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such controllable parameter can be viewed as "electron pressure," which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). Accordingly, the resulting current reflects the rate at which electrons move across the electrode-solution interface. Potentiostatic techniques can thus measure any chemical species that is electroactive, i.e. that can be reduced or oxidized. Knowledge of the reactivity of functional group in a given compound can be used to predict its electroactivity. Nonelectroactive compounds may also be detected in connection with indirect or derivatization procedures.

The advantages of controlled-potential techniques include high sensitivity, selectivity toward electroactive species, a wide linear range, portable and low-cost instrumentation, speciation capability, and a wide range of electrode materials that allow assays of unusual environments. Several properties of these techniques are summarized in Table 1.1. Extremely low (nanomolar) detection limits can be achieved with very small (5-20?µl) sample volumes, thus allowing the determination of analyte amounts of 10-13 to 10-15 mol on a routine basis. Improved selectivity may be achieved via the coupling of controlled-potential schemes with chromatographic or optical procedures.

This chapter attempts to give an overview of electrode processes, together with discussion of electron transfer kinetics, mass transport, and the electrode-solution interface.

1.2 Faradaic Processes

The objective of controlled-potential electroanalytical experiments is to obtain a current response which is related to the concentration of the target analyte. Such objective is accomplished by monitoring the transfer of electron(s) during the redox process of the analyte:

where O and R are the oxidized and reduced forms, respectively, of the redox couple. Such reaction will occur in a potential region that makes the electron transfer thermodynamically or kinetically favorable. For systems controlled by the laws of thermodynamics, the potential of the electrode can be used to establish the concentration of the electroactive species at the surface [Co(0,t) and CR(0,t)] according to the Nernst equation:

Table 1.1 Properties of controlled-potential techniques.

a DC = direct current; NNP = normal pulse; DP = differential pulse; SW = square wave; AC = alternating current.

b HMDE = hanging mercury drop electrode; MFE = mercury film electrode.

where E° is the standard potential for the redox reaction, R is the universal gas constant (8.314?J/K/mol), T the Kelvin temperature, n is the number of electrons transferred in the reaction, and F is the Faraday constant (96,487 coulombs). On the negative side of E°, the oxidized form thus tends to be reduced, and the forward reaction (i.e. reduction) is more favorable. The current resulting from a change in oxidation state of the electroactive species is termed the faradaic current because it obeys Faraday's law (i.e. the...