Environmental Analysis by Electrochemical Sensors and Biosensors

A short introduction to the importance of electroanalytical methods in environmental analysis is given, underlined by some historical aspects and milestones. The main topics of environmental electrochemistry are sketched briefly.

The pedosphere is the total surficial layer of the earth that consists of soil and which has complex and dynamic interactive linkages with the lithosphere, the hydrosphere, the biosphere and the atmosphere. In this chapter, the characteristics of soil and its physical and chemical processes are concisely reviewed, followed by all relevant issues connected to soil analysis with the focus on electrochemical sensors used for this purpose.

Water, as the universal solvent on the surface of our planet, is a key matrix to understand and manage environmental phenomena, e.g., pollutant dynamics, geochemical processes or climate studies. Moreover, an adequate supply of good-quality water is a strategic resource for human development and well-being. In this context, accurate and representative analytical information about chemical composition of water is essential for correct assessment, interpretation and solving of environmental problems. Whereas standardized, lab -based analytical methodologies are still dominant in water analysis, chemical sensors and particularly electrochemical sensors are growing as advantageous alternatives to develop simplified and miniaturized analytical tools applicable for flexible, decentralized measurements capable of providing improved spatial and temporal data resolution that is essential in environmental monitoring. In this chapter, we discuss the fundamental aspects of environmental water chemistry and how this chemistry is linked to relevant chemical substances most often analyzed in the context of environmental studies, with special attention to pollutants. Then, the use of sensors for water analysis, with special focus on electrochemical sensors, is treated. We discuss successful examples of the application of electroanalytical sensing approaches to water component determination and speciation, including relevant inorganic, organometallic and organic substances. Finally, we briefly outline future potentials of electrochemical sensors applied to water analysis.

The atmosphere is the receiver of many by-products of our society, such us products of combustion of fossil fuels and industrial manufacturing. The studies on chemical pathways of trace atmospheric species are often complexes since the life cycles of such species are linked to an elaborate system of chemical and physical processes. The components of air which are essential to analysis, i.e. gaseous constituents and aerosols, are briefly reviewed.

The true biological sensors are all around us. Living world offers a lot of examples of sensors consisting in biological receptors (proteins, nucleic acids, signaling molecules) located everywhere, in the cell (nucleus, mitochondria, cell membrane), in all the tissues, organs or even in circulating bloodstream, the transmission of the nervous stimuli are in fact true electrochemical processes. Electrochemical sensors for vegetal and animal organisms can be considered in different approaches: sensors able to detect the presence, movement and number of organisms in a given environment, or sensors able to detect a large variety of normal or pathologic parameters of living organisms. Some examples of sensors for monitoring agriculture, food and drug quality are presented. Food represents a very important environmental factor with great impact on “life quality” and analytical methods for the assessment of normal constituents, degradation products by alteration or chemical contaminants (pesticides, hormones, antibiotics, etc.), biological contaminants, genetic modification are absolutely necessary. In the same perspective drugs and pharmaceutical formulations constitute a special issue, especially if we define the internal environment, opposite to the external environment.

Electrochemical sensors, especially ion-selective electrodes, are ideally suited for analyses of extraterrestrial environments where comparatively little is known about the chemistry: they have remarkably high sensitivity over a wide dynamic range and are available for a wide range of organic and inorganic cations and anions. In addition, ion-selective electrodes require very little power, have low mass, and can withstand dramatic swings in temperature and pressure without loss of function. Analysis in exosphere environments offers unique challenges caused by the preflight preparations and storage of the sensors, the long cruise to the planetary body, and the harsh environmental conditions in which the analyses must be performed. Currently, only a single set of electrochemical analyses of another planet has been performed, but several new instruments are being developed which will potentially provide insight into the scientific questions surrounding the chemistry and biology of other planetary bodies in our solar system.

This chapter discusses the challenges of performing electrochemical analyses in an extraterrestrial environment such as Mars, with an emphasis on sensor development, characterization, and calibration while addressing lessons learned from the Phoenix mission, and looking to the future of electrochemical analyses of other planetary bodies.

Sensors are reviewed with respect to IUPAC recommendations and compared to biological senses. General strategies, current trends and future prospective for electrochemical sensors and biosensors, particularly focusing on environment analysis, are outlined in this chapter.

Monitoring the environment for contaminants is nowadays closely related with our whole planet and human health. Due to the large variety of pollutants and required environmental analysis, the need for rapid, sensitive, decentralized analysis is continuously increasing. Electrochemical sensors could be well suited for this need but also could complement standard analytical techniques validated for some environmental analysis. Environmental analysis includes atmospheric analysis, groundwater and surface water analysis, ocean monitoring, soil analysis, agriculture, and food and even pharmaceutical monitoring.

In this chapter we present:

This book chapter introduces the principles of membrane electrodes, including potentiometric sensors, in view of their use in environmental analysis. The initial part of the chapter reviews essential response and selectivity theory: phase boundary potential and Nernst equation, selective extraction/permeation, modern selectivity theory and theory of trace-level potentiometry (passive ion fluxes). This is followed by a review of key materials and an overview of ions that can be detected by potentiometry with relevance to the environment: solid-state and polymeric membrane electrodes, corrosion-based potentiometric sensors and chalcogenide sensors. Achievable detection limits are discussed as well. The last part of the chapter covers dynamic electrochemistry approaches with membrane electrodes. Key protocols to be discussed include cyclic voltammetry, stripping voltammetry, exhaustive coulometry and chronopotentiometry, as well as their combination with potentiometry for in situ ion speciation analysis. This last part aims to bring to attention recent developments that will likely have a lasting impact on this class of sensors in the immediate future.

Fundamentals of electrode thermodynamics and kinetics are given, aiming at furnishing a reference to the reader of the following chapters. No previous knowledge of electrochemistry is required; only the basic principles of chemical equilibria are supposed to be known. The suggested readings are given to deepen what is reported here, not constituting a premise in any way.

Effort has been made in order to couple to a rigorous, though simple mathematical treatment, intuitive elements that help the reader pick up the phenomenological aspects of what accounted for by the mathematical expressions. To similar purposes, basic theoretical and experimental aspects of the most frequently used amperometric techniques and of coulometry are also dealt with. Practical considerations are often made throughout the whole chapter.

The first electrochemical biosensor for glucose was developed by Leland C. Clark in 1962. Driven by its huge success, fundamental and applied research has greatly expanded the concept of a biosensor since then. Today biosensors are widely used in biomedical, industrial, and environmental analysis. This chapter is focused on the fundamental concepts of biosensors and the most recent research results from electrochemical biosensors that are used for environmental analysis.

Numerous biosensors based on enzymes have been developed since the first enzymatic biosensor for glucose, which used glucose oxidase attached to the surface of an amperometric oxygen electrode to directly quantify the amount of glucose in a sample. Research of enzyme-based biosensors focuses on choice of proteins, electrochemistry of enzymes, immobilization, and sensor configurations. In addition to purified enzymes, many researchers have also used whole cells and tissue slices as recognition elements with, as well as prokaryotes or slices from plants. There are many environmental applications for these types of sensors ranging from the detection of organic compounds like pesticides to inorganic analytes like heavy metals. This chapter introduces the principles, effectiveness, and limitations of recent biosensors based on enzymes, tissues, and cells for environmental applications.

This chapter reviews the importance and applications of electrochemical DNA biosensors in the field of environmental monitoring and control. The general description of electrochemical DNA biosensors is given herein, and also the applications of these biosensors based on different electrochemical techniques using various electrode materials for environmental analysis are discussed.

Electrochemical DNA sensing protocols based on different modes of nucleic acid interaction possess an enormous potential for environmental analysis. Such devices are extremely attractive for environmental monitoring in a simpler, faster, and cheaper manner compared to traditional methods. In addition, electrochemistry offers innovative routes for monitoring system with the signal-generating element and for amplifying electrical signals. The advances in this technology have led us to face with important developments on DNA sensing based on environmental analysis.

Electrochemical immunosensors combine high sensitivity of electrochemical methods and simple and miniature construction of the required instrumentation, with excellent specificity of antibodies as recognition elements. The current status of this approach applied for environmental analysis is discussed. The various types of biosensors were generally found very suitable for environmental analysis, and the subgroup of immunosensors provided numerous attractive applications in this field, too. This chapter introduces the principles, effectiveness and limitations of immunosensors for environmental applications.

In this chapter, types of electrochemical sensor or biosensor which are based on electrical properties and which cannot be grouped into normal voltammetric or potentiometric sensors are addressed, giving the fundamental principles and selected examples to show how they are implemented for characterisation and for analysis. This will concern sensors based on impedance, solid-state miniaturised sensors and piezoelectric transducer-based sensors.

This chapter covers theoretical and practical aspects of voltammetric microelectrodes, i.e., those that fulfill the following operational definition recommended by IUPAC in Pure and Applied Chemistry: A microelectrode is any electrode whose characteristic dimension is, under the given experimental conditions, comparable to or smaller than the diffusion layer thickness, δ. Under these conditions a steady state or a pseudo steady state is attained.

Various geometry of microelectrodes (disk, spheres, hemispheres, ring, band, cylinder holes, recessed, etc.) and relevant equations that predict steady-state or pseudo-state current are considered. The main properties that make microelectrodes attractive for sensor application, high mass transport, relatively low immunity to ohmic drop, reduced charging current, and enhanced signal-to-noise ration addition, are highlighted. In addition classical and new materials, which are nowadays employed for the fabrication of the microelectrodes, are addressed.

Electrochemical sensors have a long history and they found an important place in analytical chemistry and environmental monitoring thanks to the attractive properties and huge developments in electrode materials. In this chapter the electrode materials used for environmental sensing purposes are presented in the form of two successive parts dealing, respectively, with unmodified and chemically and biologically modified electrodes.

The use of nanosized materials nowadays constitutes one of the most diffused approaches to modify electrode surface when aiming at obtaining efficient amperometric sensors; quite spontaneously, this trend has also involved the field of environmental monitoring. The chapter aims at discussing the properties of nanosized materials, the most widespread strategies for their deposition on the electrode surface as well as the main advantages and limitations of their use in electroanalysis. Metal and carbon nanostructures, and the relevant composite materials, are particularly discussed.

The design and manufacturing of sensors is an important issue for both fields, sensors research and application. For commercialization sensors need to be of constant, reproducible quality and characteristics which are of particular interest for mass-produced one-shot sensors. Apart from these requirements a sufficiently long shelf-lifetime is necessary in order to guarantee the logistic supply with the devices. Sensors research starts usually with laboratory-made or commercially available simple electrodes which are tailored and modified according to the needs and intentions. An important aspect is the miniaturization of sensing devices, which can be achieved by either a diminishing of the dimension of macrosensors or by new concepts of placing micro- and nanosized systems directly on semiconductors and integrating them in the electronic circuits on chips, such as SoC (system on a chip, lab on a chip) and μTAS (micro total analytical system) approaches. In such cases, combination with microsystems and micromachines, also known as MEMS or MOEMS (micro-electro-mechanical systems, micro-optoelectro-mechanical systems), allows the realization of mechanical tasks in more complex analytical approaches, such as pumping, and valve-splitting, in a single microsized chip. Thus, also theoretical considerations concerning micro- and ultramicro-electrodes gain increasing importance. In the chapter here a brief overview is given on the basic transducers and on preparation techniques to create electrochemical sensors. Due to the huge amount of literature in this field, only characteristic examples and review articles are cited.

Gas analysis in the environment is a broad field and includes emission and immission evaluation and control. Furthermore the determination of gases dissolved in water, such as oxygen and carbon dioxide, is a main task for environmental gas analysis. Depending on the matrix to be analysed, the gas concentration and the volume of gases which can be used for sampling and analysis have to be selected. The principles of gas sensors (high and normal temperature) are explained with many illustrative examples, such as lambda probe, Clark cell, fuel cell sensors and Severinghaus electrodes.

This chapter deals with the principles of functioning and electroanalytical usefulness of arrays of micrometer and nanometer-sized electrodes. We discuss arrays of microelectrodes both individually shaped and interdigitated. In the field of nanostructured electrodes, both nanoelectrode ensembles (random arrays) and ordered arrays are presented. A comparison between the fabrication methods, characteristics as well as advantages and limits of each kind of array are critically evaluated.

Fields like miniaturization and nanotechnology have been greatly developed in the last years. Like other fields, analytical chemistry has also been affected by this new technology. In this sense, the possibility to carry out laboratory operations on a small scale using miniaturized devices is very appealing. An overview is given on the micro-processing techniques and on the microfluidic platforms. Principles of sample introduction and preparation, pre-concentration, separation and detection on such platforms are explained. The importance of microfluidic systems for chemical sensing is highlighted.

Artificial olfaction systems stem from the idea that arrays of nonselective gas sensors can mimic the natural olfaction to identify and recognize odors. Electronic noses have been investigated for almost three decades using many different sensor technologies, and the odor identification has been demonstrated in several application fields. In this chapter a review of the main features of electronic noses is given. The discussion is mainly focused on the analogies with natural olfaction and how the current sensor technologies and data processing can be tailored to replicate some of the properties of the natural sense. Finally, examples of applications of electronic noses are given with particular emphasis to food analysis, medical diagnosis, and environmental control.

Remote sensing is the ability to acquire information about an object or phenomenon without physically contacting the object or place from which this information is obtained. Remote sensing is a fast-developing field, which makes it possible to monitor secluded or inaccessible areas. Sensing can be passive, where energy is collected or active whereby energy is emitted by the sensor and perturbs the sensing environment.

Remote electrochemical sensing has many advantages since the electrochemical sensors can be made relatively small and cheap and, nevertheless, are highly sensitive and in many cases possess also high selectivity and robustness. Transduction of the electrochemical response into an electrical signal that can be transmitted over long distances is inherently part of the electrochemical sensor. The major challenges in remote electrochemical systems are sampling and delivery of the sample to the detector. This usually requires introducing a flow system.

Flow systems not only enable the automation of the measurement and ensure relatively easy data collection, but also simplify the entire process in comparison with a static process, which contains various stages of liquid replacements and mixing. Electrochemical methods are known for their high sensitivity, thus enabling the measurement of very low concentration employing small volumes. These make coupling between electrochemical measurements and flow systems ideal for remote sensing.

This chapter describes the concepts of remote sensing in general and remote electrochemical sensing in particular. We review the different approaches and studies dealing with remote electrochemical sensing including voltammetry, potentiometry and other techniques. Conclusions of the advantages and disadvantages of remote electrochemical sensing are discussed and perspectives of this type of sensing are suggested.