Chemical sensors types

All chemical sensors presently in use or in development are based on a two-step detection mechanism [1]. In the first step, chemical selectivity is achieved by a chemical reaction or chemi- or physisorption on a chemically selective surface as listed in table 1. In the second step, a physical change, which is the result of the first selective chemical step, is transformed by a suitable transducer into an electrical signal. This physical change can be a variation of the chemical potential, caused by the reaction or sorption process, a change in optical properties, a change in mass, conductivity, surface resistance or conductance. The physical effects listed in table 2 in column 1 can now be combined with the various transducers listed in column 2 in a suitable way. They will yield the total spectrum of chemical sensor types presently under investigation. 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

Nowadays all over the world considerable attention is focused on development of chemical sensors for the detection of various organic compounds in solutions and gas phase. One of the possible sensor types for organic compounds in solutions detection is optochemotronic sensor - device of liquid-phase optoelectronics that utilize effect of electrogenerated chemiluminescence. In order to enhance selectivity and broaden the range of detected substances the modification of working electrode of optochemotronic cell with organic films is used. Composition and deposition technique of modifying films considerably influence on electrochemical and physical processes in the sensor. 

In recent years further concepts have been developed for the construction of polymer-based diodes, requiring either two conjugated polymers (PA and poly(A-methyl-pyrrole) 2 > or poly(A-methylpyrrole in a p-type silicon wafer solid-state field-effect transistor By modifying the transistor switching, these electronic devices can also be employed as pH-sensitive chemical sensors or as hydrogen or oxygen sensors 221) in aqueous solutions. Recently a PPy alcohol sensor has also been reported 222). 

We scrutinize issues dealing with requirements of high sensitivity and response selectivity of electrophysical parameters in reference to the gas monitored or the type of active particles under study as well as other requirements put forward to adsorbents of chemical sensors. We discuss principles underlying the basis of solving these problems. We dwell on the issue of the type of crystal of adsorbents examined, which is directly linked to the character of intracrystallite contacts. 

Sensors that operate on the basis of interactive chemical surfaces are chosen for discussion here because they are likely candidates for the detection of chemical agents. However, many other sensor types are possible. 

In practice, surface modifications are restricted to sensors of the ATR- or FEWS-type. For other transducer layouts, the sample - radiation interaction is less localised, making a modification difficult to impossible. Depending on the analytes and the environment of the sensor, two basic surface modification strategies can be used to enhance the function of vibrational spectroscopic optical chemical sensors. The functional layers can either be... 

Lambeck P.V., Hoekstra H.J.W.M., VanLith J., Van Elzakker G., Two novel integrated optical sensor types for measuring chemical concentrations, based on chemically induced changes of modal field profiles, J. Nonlinear Opt Phys. Mat 2004 13 (2) 209-217. 

Figure 5. Schematic depiction of a self-encoded bead array. A mixture of three sensor types fills the fiber tip wells randomly. The sensors are identified by their characteristic responses to a test vapor pulse. Reprinted with permission from ref. 9b. Copyright 1999 American Chemical Society.

Figure 6. Response profiles from three different sensor types, (a) Concatenated Responses of each of the individual sensor types (decoded array), (b) The collective response (undecoded array). Reprinted with permission from ref. 11. Copyright 2003 American Chemical Society.

The goal of this book is to cover the full scope of electrochemical sensors and biosensors. It offers a survey of the principles, design and biomedical applications of the most popular types of electrochemical devices in use today. The book is aimed at all scientists and engineers who are interested in developing and using chemical sensors and biosensors. By discussing recent advances, it is hoped to bridge the common gap between research literature and standard textbooks. 

The history of ion-selective electrodes (ISEs) [1] starts from the discovery of the pH response of thin film glass membranes by Cremer in 1906, thus making ISEs the oldest class of chemical sensors. They still are superior over other sensor types in a variety... 

Osterloh, F. Hiramatsu, H. Porter, R. Guo, T., Alkanethiol induced structural rearrange ments in silica gold core shell type nanoparticle clusters An opportunity for chemical sensor engineering, Langmuir. 2004, 20, 5553 5558... 

Chemical. Exquisitely sensitive chemical agent sensors are available, but work best under laboratory conditions. Environmental chemical sensors suffer many of the same issues as biological detectors. They lack sensitivity, are not sufficiently mobile or flexible, and require trained users. Several types of chemical detectors are in use and are mentioned above. 

Surface acoustic wave (SAW)-type chemical sensors exploit the propagation loss of the acoustic waves along layered structures consisting of at least a substrate covered by the CIM. 

Bulk sensors certainly have a role in chemical sensing of explosives, but the subject of this book is the other basic type sensor, one that seeks molecules released from the bulk of the explosive material in an object. We will refer to these as trace chemical sensors. They are sometimes called vapor sensors, but that seems a less accurate description when they are applied to explosive molecules, which may not always be found in a vapor state. As we shall see in Chapter 5, that requires us to understand where and how to look for these molecules. It will become apparent upon a little reflection that the two types of sensors are complementary and are best used in different situations. Furthermore, even when trace sensors are used, in some situations sampling of particles of soil or vegetation or sampling from surfaces may prove to be more productive that vapor sampling. For underwater sources the term vapor sensing is also inappropriate. 

The ideal (bio)chemical sensor should operate reversibly and respond like a physical sensor (e.g. a thermometer), i.e. it should be responsive to both high and low analyte concentrations and provide a nil response in its absence. One typical example is the pH electrode. In short, a reversible (bio)chemical sensor provides a response consistent with the actual variation in the analyte concentration in the sample and is not limited by any change or disruption in practical terms, responsiveness is inherent in reversibility. An irreversible-non-regenerable (bio)chemical sensor only responds to increases in the analyte concentration and can readily become saturated only those (bio)chemical sensors of this type intended for a single service (disposable or single-use sensors) are of practical interest. On the other hand, an irreversible-reusable sensor produces a response similar to that from an irreversible sensor but does not work in a continuous fashion as it requires two steps (measurement and renewal) to be rendered reusable. Figures 1.12 and 1.13 show the typical responses provided by this type of sensor. Note... 

Bio)chemical sensors can be used in both the batch and the continuous mode. While this is also true of probe-type sensors, flow-through sensors can only be used in a continuous regime coupled on-line to a continuous-flow configuration. 

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