AMPEROMETRIC AND VOLTAMMETRIC SENSORS

An amperometric sensor measures a current at a fixed applied potential, that is at one point on the current-voltage curve. A voltammetric sensor records a number of points on, or a chosen region of, the current-voltage profile. Thus an amperometric sensor is a fixed-potential voltammetric sensor. 

The larger part of this book has been devoted to studies related to electrochemical measurements away from equilibrium. As demonstrated, these permit the determination of kinetic and thermodynamic parameters of the electrode processes, whereas measurements at equilibrium furnish only thermodynamic data. So, whilst potentiometric analysis is a powerful tool in the determination of activities or concentrations, the specificity arising from the electrode material, amperometric or voltammetric analysis permits other parameters besides these to be obtined. 

The most important selectivity parameter of electrodes for voltammetric sensors is the applied potential. Ideally, the electrode potentials of the redox couples would be sufficiently far apart for there to be no interference between different species. Unfortunately this is not the case, and it is necessary to look for greater selectivity. We can discriminate better between the different species present in solution through a correct choice of conditions for the study of the electrode reaction electrode material (Chapter 7) in some cases through surface modification use of hydrodynamic electrodes (Chapter 8) application of potential sweep 

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In this chapter, after describing the useful technique of amperometric titrations, recent developments in amperometric and voltammetric sensors are summarized. Their application as biosensors to the study of biological compounds and in vivo, is described in Chapter 17. 

It is commonly assumed that application of these methods in sensors has started from invention of oxygen Clark electrode,2 and in biosensors from first glucose biosensor.3 At present, main sensor application of amperometric and voltammetric detections include, with wide use of oxygen Clark electrode, amperometric sensors based on modification of working electrodes with various materials, and biosensors employing practically all biorecognition species. With the very wide use of the term sensors, applications of voltammetric detections include also miniaturized screen-printed devices for stripping determinations of, e.g., heavy metal ions. 

An electrochemical sensor is generally an electrochemical cell containing two electrodes, an anode and a cathode, and an electrolyte. Electrochemical sensors in general are classified, based on the mode of its operation, and they are conductivity sensors potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered as a special type of voltammetric sensors. The fundamentals of these sensors operational principles are described exceptionally well in several excellent electro-analytical books. In this entry, only the essential features are included. 

Electrochemical sensors can be classified according to their mode of operation, e.g. conductivity/capacitance sensors, potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered a specific type of voltammetric sensor. The general principles of electrochemical sensors have been extensively described in other chapters of this volume or elsewhere. This chapter will focus on the fabrication of electrochemical sensors of micro or miniature size. 

Currently available amperometric and voltammetric porphyrinic sensors for detection of electroactive analytes are based on their electrochemical oxidation or reduction on polymeric conductive films of metalloporphyrins. If the current generated during the process is linearly proportional to the concentration of an andyte, the current can be used as an analytic signal. This current can be measured in either the... 

Amperometric and voltammetric biosensors rely on an electrochemically active analyte that can be oxidized or reduced at a working electrode. Typical electrode materials are platinum (Pt), gold (Au), and carbon. Nowadays some innovative techniques for electrode preparation, characterized by the possibility of mass production and high reproducibility, have been proposed. Among these, the equipment needed for thick-film technology is less complex and costly, and thus, this is one of the most used for sensor production. Thick-film technology consists of depositing inks on a substrate in a film of... 

Amperometric-voltammetric Sensors Thick-film screen-printed carbon containing electrodes modified with formazan were used for determination of Pb(II), Zn(II), and Cd(II) concentration [400]. 

In the third part of the book areas in which there are important applications of electrochemistry are described. Chapters 13 and 14 look at potentiometric and amperometric/voltammetric sensors respectively, focusing particularly on recent developments such as new electrode materials and miniaturization. Electrochemistry in industry, which produces many materials used directly or indirectly in everyday life, as well as batteries, is described in Chapter 15. The electrochemical phenomenon... 

The adsorptive and voltammetric characteristics of Cu(II) complexes with guanine, guanosine and adenosine were exploited [120] in order to detect these bases after separation by capillary zone electrophoresis, and the enzyme-mimic catalytic activity of a DNA-Cu2+ complex [121] was used to develop an amperometric quinacrine sensor using an oxygen electrode covered by the complex entrapped in polyacrylamide gel. 

Whereas SECM usually uses tips that pass a current in amperometric or voltammetric mode, an important related application uses passive ion-selective sensor tips which can be used for mapping two-dimensional and even three-dimensional ion distributions and concentration profiles over the surface, of species such as protons and zinc ions [53]. 

The current-potential relationship of an electrochemical ceU provides the basis for voltammetric sensors. Amperometric sensors, that are also based on the current-potential relationship of the electrochemical cell, can be considered a subclass of voltammetric sensors. In amperometric sensors, a fixed potential is applied to the electrochemical cell, and a corresponding current, due to a reduction or oxidation reaction, is then obtained. This current can be used to quantify the species involved in the reaction. The key consideration of an amperometric sensor is that it operates at a fixed potential. However, a voltammetric sensor can operate in other modes such as linear cyclic voltammetric modes. Consequently, the respective current potential response for each mode will be different. 

Single-fiber sensors and catheter-protected sensors can operate in an amperometric or voltammetric mode. In both methods a current proportional to NO concentration is measured. Of the several voltammetric methods available, differential pulse voltammetry (DPV) is most suitable for the measurement of NO. In DPV, a potential modulated with rectangular pulses is linearly scanned from 0.4 to 0.8 V. The resulting voltammogram (alternating current versus voltage plot) contains a peak due to NO oxidation. The peak current should be observed at a potential of 0.63-0.67 V which depends on the pulse amplitude. This potential is the characteristic potential for NO oxidation on Nafion coated porphyrinic sensor. 

Several decades of industrialization have changed the enviromnent drastically, leading to all sorts of pollution. Water pollution, being one of most important issues related to daily life, has always been addressed and mtmitored by various means of analytical tools. Different electrochemical sensors for the detection of pollutants in water have been well established, which can be categorized into the following (i) poten-tiometric sensors, (ii) amperometric sensors, (iii) voltammetric sensors, and (iv) conductometric sensors. In this chapter, we will introduce the fundamentals, applications, advantages, limitations, and recent trends for the development of each type of sensors. 

Naturally, in other areas with different technical requirements for the measuring system, preference will be given to other gas sensors with other sets of advantages, which depend not only on the sensor type but also on its version. This facet can be well illustrated using the example of electrochemical sensors, which can be realized either as conductometric, potentiometric, or amperometric (voltammetric) sensors (Brett and Brett 1998 Brett 2001). 

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