Introduction to Sensors

Chemical sensing is part of an information-acquisition process in which an insight is obtained about the chemical composition of the system in real-time. In this process, an amplified electrical signal results from the interaction between some chemical species and the sensor. Generally, the interaction consists of two steps recognition and amplification. One common example is the measurement of pH with a glass electrode (Fig. 1.1). 

With so many different sensors described in the literature one may ask which one is the best Of course, there is no simple answer, because there are so many 

Ruggedness is usually the most critical criterion in the selection of a sensor for a given application. Rugged sensors are usually the most commercially successful, because they are reliable or because they are the only ones that can provide information under the conditions in which other sensors would not be able to 

Generally speaking, we can distinguish two types of interactions between the chemical species and the sensor a surface interaction in which the species of interest is adsorbed at the surface, and a bulk interaction in which the species of interest partitions between the sample and the sensor and is absorbed. The classification of the interaction as either surface or bulk is relative with respect to the size of the species. It is the case of chicken and chicken wire. Obviously, a chicken wire fence is impervious to chickens, but presents no barrier whatsoever to mosquitoes. Similarly, large molecules, such as proteins, may adsorb at the surface of the sensor layer, whereas smaller ions can penetrate and absorb in the bulk. 

Reversibility in the context of chemical sensing means that the response follows concentration changes, both up and down. It does not have the usual thermodynamic meaning, despite the fact that a decrease of free energy is always the driving force in all interactions. Thus, sensors can be either thermodynamically reversible, as with ion-selective electrodes, or thermodynamically irreversible, as with enzyme electrodes if, however, they respond to a step up or a step down in the concentration 

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For a superior introduction to this difficult topic, try Peter Rock s now classic book, Chemical Thermodynamics, Oxford University Press, Oxford, 1983. The treatment in Temperature Measurement (second edition), by Ludwik Michalski, Joseph McGhee, Krystyna Eckersdorf and Jacek Kucharski, Wiley, New York, 2001, is aimed at engineers manufacturing temperature-measuring machines, such as electrical and optical sensors, but some of its introductory material might help. 

Cunningham A J (1998), Introduction to Bioanalytical Sensors. New York, Wiley Press. 

Janata, J., Principles of Chemical Sensors, Plenum Press, New York, 1989. Some sensor specialists regard this as the definitive work on the subject. While extremely dated, its introductory sections provide a clear, uncluttered introduction to the different modes of sensor operation. 

Hitchman, M. L. and Hill, H. A. O., Electroanalysis and electrochemical sensors . Chemistry in Britain, 22, 1117-1124 (1986), provides a lively, general introduction to this subject, giving details of sensors based on potentiometry, such as ISEs, together with some historical background. 

A. Mulchandani and O. A. Sadik, eds., Chemical and Biological Sensors for Environmental Monitoring Biosensors (Washington. DC American Chemical Society, 2000) D. Diamond, ed., Principles of Chemical and Biological Sensors (New York Wiley, 1998) A. Cunningham, Introduction to Bioanalytical Sensors (New York Wiley, 1998) G. Ramsay, Commercial Biosensors Applications to Clinical, Bioprocess, and Environmental Samples (New York Wiley, 1998) ... 

These chapters divide the discussion of electrochemical sensors by the mode of measurement. This chapter is an introduction to the general parameters and characteristics of electrochemical sensors. Chapter 6 focuses on potentiometric sensors, which measure voltage. Chapter 7 describes amperometric sensors, which measure current. Chapter 8 examines conductometric sensors, which measure conductivity. 

With this group of electrochemical sensors, information is obtained from the current-concentration relationship. The two most important issues to discuss are (1) the origin of the signal for various types of amperometric sensors and (2) the origins of selectivity. To begin our examination of these issues, we briefly reiterate some of the information presented in the Introduction to Electrochemical Sensors (Chapter 5). 

In order to ensure an adequate quality of products and a safe operation, the monitoring of a batch reactor should include, at least, online measurements of temperature, pressure, and of some composition-related variables. In this context, online measurements may be defined as measurements obtained via instruments strictly connected to the reactor and characterize by response times markedly smaller than the characteristic times of the chemical reaction. In general, this is the case of temperature and pressure, which can be easily measured online by means of reliable, relatively cheap, and poorly intrusive sensors. This allows the introduction of sensor redundancy, a common practice to increase reliability. On the other hand, online... 

In this section, only the surface modification techniques will be discussed, and not the impregnation, sol-gel or co-precipitation techniques. Furthermore, it is not our aim to fully cover all chemical modifications on the silica surface. We merely want to present in introduction to and an insight in the fast expanding field of silica modifications, in order to create new catalysts, sensors and immobilizators. 

Amperometric mode An advantage of the lambda sensor as described above is its sensitivity close to the 2=1 point where the oxygen activity in the exhaust changes rapidly with burn conditions and consequently so too does the cell e.m.f. The disadvantage is that under lean-bum (oxygen-rich) conditions the log dependence on oxygen activity renders the cell insufficiently sensitive to provide effective control. This shortcoming has led to the introduction of sensors used in the amperometric mode discussed below and illustrated schematically in Fig. 4.40. 

Preceding chapters have described the detailed operating principles of acoustic wave (AW) devices and how these devices can function as sensors of various physicochemical phenomena in surrounding media. This chapter describes die extension of these capabilities to the detection and quantitation of chemical and biochemical species. An introduction to the fundamental background of various important physical and chemical interactions is presented for those not especially familiar with these topics. 

A. E. G. Cass, Ed., Biosensors A Practical Approach, Oxford University Press, New York, 1989. A. J. Cunningham, Introduction to Bioanalytical Sensors, John Wiley Sons, Inc., New York, 1998. 

Introduction to Combinatorial Methods for Chemical and Biological Sensors... 

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