Chemiresistors

The distinguishing features of these sensors are the simplicity of their preparation and simple instrumentation. Typically, they are implemented as interdigitated electrodes (Fig. 8.4), which are patterned on an insulating substrate, and the signal is at zero frequency, that is, dc. The purpose of interdigitation is to increase the contact area. The selective layer is then deposited by solvent casting, evaporation sublimation, and so on, over the pattern of these electrodes. In most cases, the contact area for the positive and the negative terminal is the same. This fact does not aid the interpretation of the results. 

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A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

Fig.4.16. Heating approaches for monolithicaUy integrated microhotplates (pHP) (a) shows a resistive heater with power transistor and (b) shows a PMOS transistor heater fiheat denotes the heating resistor R is the metal-oxide chemiresistor, and Rj is a resistor used as temperaturesensor (see Fig. 4.4)...

A form of surface effect sensor that exploits altered surface resistance, or chemiresistors, forms the surface from a mixture of tailored polymers and a finely divided conductive material, such as carbon black, as a thin film on a substrate. They use a number of polymers, 32 in one implementation, with different properties to form an array of chemiresistors. When a vapor is passed over the array,... 

Its unit is Siemens-1 cm-1. These terms are used throughout this section interchangeably. These sensors are popularly called chemiresistors, which clearly implies their function. They are simple to fabricate, but the interpretation of their responses and the mechanism of their operation are anything but simple. The fundamental equivalency between current and resistance, which is inherent in the generalized Ohm s law (8.1), sometimes blurs the line between amperometric and conductometric sensors. Nevertheless, it is important to remember that chemiresis-... 

Fig. 8.1 General chemiresistors and their equivalent circuits (a) lateral configuration in which any of the five resistances can be modulated by chemical interaction (b) impedimetric chemiresistor in which capacitance Cg is chemically modulated...

The discussion of the origin of the signal helps in the classification of conduc-tomettic sensors. Chemiresistors (Fig. 8.1a) are normally measured in DC mode and the capacitances in the equivalent circuit can be neglected. Thus, the analytical information is obtained from the modulation of surface (Rs), contact (Rc), or... 

Here V is the applied voltage, 0b is the average barrier height, d is the insulator thickness, h is Planck s constant, m is the mass, e is the charge of electron, and B is a constant dependent on barrier height and thickness. Both barrier height and the thickness of the oxide can be chemically modulated, giving rise to chemiresistive behavior. Such barrier junctions apparently exist in most chemiresistors, which do not use noble metals for the contact. 

One of the simplest, albeit least discussed, chemiresistors is the mercury vapor sensor. The heart of this device is a thin (7-40 nm) film of gold evaporated on a ceramic or glass substrate. At such thicknesses, the films are continuous, with the resistivity between 300 and l,500Qcm. The sensors are usually connected as a pair in a DC bridge configuration with one sensor acting as the reference. 

The emphasis on the thickness of the selective layer is linked to the mode of interaction between the analyte and the selective layer. If this interaction takes place exclusively at the surface of the selective layer, then the bulk conductivity does not contribute and represents only a shunt which decreases the signal-to-noise ratio. This is a typical case of chemiresistors based on inorganic materials (Fig. 8.6). On the other hand in chemiresistors based on organic semiconductors, the signal usually originates in the bulk of the selective layer. In that case, the response time of the sensor is affected by its thickness. 

When a third electrode is added to the structure shown in Fig. 8.12a, below the substrate, it creates an electric field in the insulating substrate, which is perpendicular to the path of the current between the two contacts to the selective layer. Such a structure became known as the Organic Field-Effect Transistor (OFET). It is discussed here and not in Chapter 6 because it belongs to the family of organic chemiresistors. 

When a periodically changing excitation signal is chosen for the operation of chemiresistors, they can be used to detect changes of capacitance (Fig. 8.1b). Therefore, the dielectrometric sensors rely on the chemical modulation of one or more equivalent circuit capacitors, either through the change of the dielectric constant of the chemically sensitive layer or through the chemical modulation of the interfacial charge. 

Chemical sensors can be of the type that sense the species directly, such as ion-selective electrodes, or the type that sense the species indirectly through the change in another physical property produced by the species. An example of this latter type is the so-called "chemiresistor", in which the presence of the species being sensed modifies the electrical resistance of a transducer... 

The results illustrated above show that the CFT method is suitable for making chemical-sensor measurements using both bulk polymers and, in particular, thin film materials that are intrinsically weak conductors. Therefore, the CFT looks premising for such materials as poly(phenylacetylene) derivatives 24., for which carefully shielded electrometer measurements have been required in the past because of current levels at the threshold of detectability. Furthermore, the fact that the CFT always makes AC measurements reduces the problem of DC polarization of electrodes. In addition, the CFT approach should be suitable for other "chemiresistor" applications, such as the metal-substituted phthalocyanines proposed by Jarvis et. al. 2 and for Langmuir—Blodgett films 26. which, because they are so thin, may prove impossible to use in parallel-plate form, but which can be routinely used with the high-sensitivity interdigi-tated-electrode approach provided by the CFT. 

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