Polymeric Sensors and Actuators

By Johannes Karl Fink

This book covers in-depth the various polymers that are used for sensors and actuators from the vantage point of organic chemistry. Since many chemists may not be familiar with the physics and operational specifics of sensors, the book has a general chapter dealing with the overall physics and basic principles of sensors. Also included are methods of fabrication, as well as information on smart textiles, actuators, and the processing of data. The range of sensors covered include humidity, temperature, chemical, mechanical, optical, electrode, electronic nose, switchable devices, biosensors, and others.

• Language: English
• Publisher: Wiley
• Release date: Nov 13, 2012
• ISBN: 9781118547625

Johannes Karl Fink

Dr. Fink is a Professor of Macromolecular Chemistry at Montanuniversit Leoben, Austria.

Book preview

Part I

Basics of Sensors

Chapter 1

Sensor Types and Polymers

In this chapter a survey of the various types of sensors and the basic physical principles accorded the working of these sensors is given. The sensor types are presented in more detail in subsequent chapters.

In addition, the basic polymer types that are used for sensors are presented. For special task of analysis, these polymers are varied in a highly specific way. In the same way, as for sensor types, details of polymer modifications and specific polymeric contrapositions are given in special chapters.

Common commercially available sensors include temperature sensors, pressure sensors, flow sensors, stress/strain sensors, accelerometers, dielectric sensors, conductivity sensors, shock sensors, and vibration sensors (1). Useful characteristics of such sensors include their (2):

Fast response,

High sensitivity with large response signal of the transducer elements,

High selectivity so that the sensor can recognize a specific chemical species,

Capability of detecting and recognizing as many chemicals as possible,

Low temperature operation, and

Small size, lightweight, compact and convenient to use.

1.1 Sensor Types

Basically, sensor types may be subdivided into optical and electrical sensors, depending on whether the signal is eventually monitored: In an electrical or in an optical way. However, there is still an ambiguity, as an optical signal can be transformed into an electrical signal in a modern spectrometer. If we would use this strict classification, then an optical sensor remains only a device that is evaluated by the naked eye, such as a classical test strip.

The forgoing discussion clearly illustrates the problems in attempting a strict classification of sensor types. So, subsequently we will start with an explanation of the basic principles of sensing rather than with a strict classification.

1.1.1 Optical Sensors

Optical sensors rely on a change in their optical properties in the presence of an analyte. These changes may be a change in visible color, a change in the fluorescence spectrum, or changes of the IR or UV spectrum. Optical sensors have a large field of use.

1.1.2 Acoustic Wave Sensors

Surface acoustic waves were discovered in 1885 by Lord Rayleigh (3). Therefore, they are also addressed as Rayleigh waves. They have a longitudinal and a vertical shear component that can couple with some media that are in contact with the surface. This type of coupling affects the amplitude and velocity of the wave. Sensors based on this property directly sense the mass and the mechanical properties.

For a maximum sensitivity, the thickness of the sorption layer should be maximized. The higher the thickness of the layer, the more analyte can be absorbed in the layer, so an increase in the shift of the frequency will be obtained.

On the other hand, the slope of the phase decreases with the increasing thickness of the sensitive layer. Now, for a good signal-to-noise ratio the film thickness should be rather minimal. So, in order to find the optimum conditions, a balance is needed (4).

The aging effect of surface acoustic wave (SAW) sensors that are coated with viscoelastic polymers is often caused by a dewetting of the surface of the substrate (5). This dewetting often leads to an unwanted decrease of the slope of the phase curve.

1.1.2.1 Love Wave Sensors

Love waves are horizontally polarized shear waves, i.e., SH waves, which are guided by an elastic layer. They are named after their discoverer, A. E. H. Love, who was engaged in the theory of surface waves created by earth quakes (6).

The basic Love wave structure consists of a delay line on a piezoelectric substrate, such as quartz, covered by a rigid overlay which acts as the guiding layer. In order to generate Love waves which have a pure SH polarization, interdigital electrodes are placed parallel to the crystallographic axis on the quartz surface. When the cut angle of the quartz is in a specific direction, the SH wave can be coupled into the guiding layer without a change of polarization. A Love wave device for a chemical sensing application is coated with a sensitive layer, as usual.

The good sensitivity of the Love waves compared with other acoustic devices results from the confinement of the wave energy in the guiding layer with a low thickness (7). Love wave sensors have been described for a variety of applications, including and immunosensor for whole E. coli bacteria (8), and also sensors for hydrogen (9).

1.1.2.2 Dynamic Chemical Vapor Sensing

Ultrafine poly(ethylene oxide) fibers with a thickness of 100–300 nm and controlled porosity were applied by an electrospinning process on the surface of a SAW sensor (10). The film from nanofibers provides a high surface area to volume ratio. This effects more adsorption sites for vapor molecules. In addition, it shortens the diffusion length of vapor molecules into the polymeric material.

In comparison to conventional thin films, this sensor type exhibits a higher sensitivity and a faster response. It has been revealed that the nanofiber film holds a great potential in enhancing the sensor performance for trace level detection of chemical analytes.

1.1.2.3 Inductively Coupled Surface Acoustic Wave Sensor

The design of an inductively coupled, polymer-coated SAW sensor has been presented (11). This design differs from conventional devices as the sensor is inductively coupled to the RF circuit. Therefore no bonding wires and sockets are necessary.

Poly(isobutylene) (PIB) has been chosen as coating material. This polymer is used frequently for SAW sensors. It has a low glass transition temperature. Thus, fast vapor diffusion and reversible response is exhibited. PIB is soluble in many volatile solvents which is important for the coating process and it exhibits a high partition coefficient to many vapors (11).

1.1.2.4 Network Analyzer

The performance of SAW gas sensors can be measured with a network analyzer. This is basically an instrument that measures the network parameters of electrical networks (12).

A network analyzer consists of a signal generator, the object to be tested, and receiver units. The use of network analyzers for the characterization of SAW sensors have been described in detail (11).

In the field of radio frequency engineering, network analyzers are used to measure scattering parameters of the means by which components or networks are described. There exist network analyzers having two test ports each which is in most cases are linked to two measuring points so that four measured values are respectively obtained from a test object linked between the two test ports.

There are also so called unidirectional network analyzers, which also comprise two test ports one of which is linked to two measuring points while the other one is linked to only one measuring point. Finally, there are also network analyzers which operate as reflectometers and which include only a single test port linked to two measuring points (13).

In SAW sensors the surface of the substrate is coated with chemically active thin layers, which are capable of sorbing gas specific molecules. This results in a change of mass.

The sensitivity, response time, selectivity and reversibility depend on the thermodynamics and kinetics of the interactions between sensor material and the analyte (14).

1.1.2.5 Acoustic Emission Sensors

In the course of acoustic emission an elastic wave is generated by the rapid release of energy caused by the growth of a crack, a phase transformation, a dislocation in a crystal lattice, or other internal deformations in a material (15, 16). In an acoustic emission stress wave, the released energy escapes from the region of e.g., a crack.

The acoustic emission of an elastic wave technique provides an effective nondestructive technique for investigating failures in of gas pipes, welds and storage tanks. Further, the performance of structures under cyclic loading can be monitored (17, 18).

A conventional acoustic emission of an elastic wave sensor is made from a piezoceramic core. It has high sensitivity when it is operated at resonant frequency. Unfortunately, due to its brittleness, complex geometries cannot be fabricated easily.

Capacitive acoustic emission an elastic wave sensor detects a motion by the change of the electrical capacitance. The capacitance is detected by two electrodes. One electrode is attached to the substrate, another electrode is attached to a plate that is mechanically coupled via a spring to the substrate. In practice, such as sensor is made up of a large number of such elemental structures, where each individual element has its own resonant frequency (19). Therefore, the frequency ranges of all of the elements must be integrated.

An alternative approach is based on poly(vinylidene fluoride) (PVDF) as the piezoelectric material. PVDF is a highly non-reactive semicrystalline fluoropolymer with piezoelectric properties after molecular electrical polarization. This is introduced using a high voltage difference as PVDF is tempered above its Curie temperature of about 150°C.

An advantage of PVDF is its low acoustic impedance, which permits an efficient coupling to other low impedance media such as polymers, biological tissue, or water. In addition, in comparison to a piezoceramic material, PVDF is flexible so that those sensing elements can be attached directly to bent surface.

The basic elements of a PVDF-based acoustic emission elastic wave sensor are shown in Figure 1.1. The steps of fabrication include (19):

Figure 1.1 Basic elements of an acoustic emission elastic wave sensor (19)

Fabrication of the core sensing element,

Deposition of the layers including PVDF, silver electrodes, Parylene, etc.,

Embedding into epoxy resin, and

Final embedding into the metal housing.

Details of the methods of fabrication have been described elsewhere (19). As the housing component, an aluminum tube with an inner diameter of 12 mm, an outside diameter of 16 mm and a height of 12 mm is used. The sensing element is placed into the tube and as the final step, the element is fixed by curing a liquid epoxy resin directly in the tube.

As a potential application for this sensor, a drilling process has been monitored. A hole with a diameter of 5 mm is drilled in a plate consisting of two layers of either plastics and copper. The response of the sensor is as follows:

When the motor of the drilling machine is switched on, the sensor shows stability due to the vibration passing through the drill stand to the plate on the working platform of the drilling machine. When the drill touches the top plastic layer of the composite plate, the signal changes into an increased signal pattern.

When the drill passes from the plastic plate to the copper plate, the signal changes dramatically. Thus, the sensor can distinguish between the two materials in the course of drilling. Finally, when the drill has completely passed through the laminate, the signal reverts to its original level and character (19). The signals are shown in Figure 1.2.

Figure 1.2 Signals during drilling. Reprinted from (19) with permission from Elsevier

1.1.2.6 Ultra-thin Polymer Films

Polymer-coated SAW were exposed to various gases, such as carbon dioxide, methane, and ethane. The polymers used for coating are polycarbonate poly(carbonate) (PC), PIB, and poly(dimethyl siloxane) (PDMS) (20).

The frequency shift of the SAW coated with PC and PIB could be described by the Wohltjen equation for acoustically thin, perfectly elastic films (21). However, the PDMS coated sensors cannot be described in this way.

Obviously, the operation at high frequencies results in a change in the oscillation frequency resulting from a change in the modulus that is in the order of the frequency change resulting from the mass loading due to gas absorption. Plasticization was reported to be responsible for anomalous frequency shifts (22).

The sorption C of a small gas molecule in a glassy polymer such as PC can be described by (23):

(1.1) equation

Here, C is the sorption in the unit volume of gas under standard conditions per volume polymer. CD is the term reflecting Henry’s law and CH is the sorption term according to Langmuir. b reflects the affinity of a certain gas to absorb in a certain polymer. Thus, the first term in Eq. 1.1 reflects Henry’s law and the last term in Eq. 1.1 is a modified Langmuir equation (20).

This type of sorption is also addressed as dual mode sorption (24). The solubility coefficient S(O) is C/p and emerges as

(1.2) equation

In contrast to glassy polymers, the sorption occurs rather as a regular solution mechanism. Therefore, the solubility of a gas is related rather to its ability to condense. In this case, the solubility coefficient reads as (20)

(1.3) equation

Tc is the critical temperature and a and b are coefficients that have to be determined experimentally (20).

1.1.2.7 Multilayered Acoustic Wave Sensors

In a multilayered material it has been found that the velocity of the acoustic wave increases with decreasing surface conductivity. This fact can explain the abnormal response of acoustic wave sensors, in which the central frequencies of acoustic wave sensors increase after they sorb the detected gases.

The conductivity is related to the dielectric constants of a multilayered material as well as to the electromechanical coupling coefficients. The sensitivity of a multilayered acoustic wave sensor can be optimized by taking into account the surface conductivities and thicknesses of the individual layers (25).

1.1.2.8 SAW hydrogen sensor with a bilayer structure

Hydrogen can be detected using a SAW sensor system bilayer structure consisting of a metal free phthalocyanine layer and a palladium layer. The system is suitable for a concentration of 1.5–4% in air (26).

Another bilayer SAW system for the detection of hydrogen is based on ZnO and WO3 films (27).

Usually and mostly, the method of detection is based on changes of the amplitude of the signal with the concentration of the gas to be detected. However, it has been claimed that this method suffers from various drawbacks (26):

Low resolution,

Lengthy response time, and

Disadvantages in neural network systems.

An improved method of detection is based on the great dependence of the interaction speed of the frequency shift on the hydrogen concentration. However, the amplitude of the signal is in the same frequency range. Further, the interaction speed is dependent on the major component of gas. Thus, the interaction speed is different for the detection of hydrogen in pure nitrogen and in synthetic air.

The development of the frequency shift in time with different concentrations of hydrogen is shown in Figure 1.3. Further, the rate of frequency change with the concentration of hydrogen is given in Table 1.1.

Figure 1.3 Frequency shift with time (26)

Table 1.1 Frequency change with hydrogen concentration (26)

The mechanism of interaction of hydrogen with the components in the bilayer structure has been proposed to be as follows (26,28):

1. Molecular hydrogen dissociates on the palladium surface.

2. The hydrogen atoms diffuse into the inner region of the palladium.

3. The absorbed hydrogen atoms act as dipoles at the metal semiconductor surface.

4. This changes the work function of the palladium at the interface.

5. In addition the surface conductivity of the metal free phthalocyanine layer changes.

1.1.3 Electronic Noses

Chemically sensitive sensors that are capable of detecting the presence of a particular chemical in a gas are often addressed as electronic noses. These sensors are often fabricated from a polymeric organic material that is capable of specifically absorbing a certain chemical compound (29).

The absorbance of the compound causes the polymeric material to expand or change, thereby modifying the electrical properties of the sensor. A variability in the ability to absorb a chemical compound results in a variability in the produced signal.

Applications include devices that function as analogs of the mammalian olfactory system (30–32), bulk conjugated polymer (CP) films (33), SAW devices (34), fiber optic micromirrors (35), and dye impregnated polymeric coatings on optical fibers (36).

Previously, many of the sensors employed in those devices have been fabricated from a limited number of polymeric components and are thus limited in the responses they are capable of producing (29).

Arrays of chemically sensitive sensors that are formed from a library of expandable insulating organic polymers containing a conductor such as carbon black are broadly responsive to a variety of analytes (30). However, they allow the classification and identification of organic vapors through application of pattern recognition methods (37). These array elements have been fabricated from a comparatively small number of approximately 10–20 organic polymers, with a single distinct polymer backbone composition in each sensor element (29).

Tunable properties of polymer based sensor can be imparted by the addition of plasticizers (29). In this way, a sensor can be tailored with respect to its response to certain chemical compounds. In general, plasticizers are organic compounds added to polymers to facilitate processing and to increase the flexibility and toughness of the polymeric product. A plasticizer has a low volatility and is compatible with the polymer.

In particular, most important plasticizers are nonvolatile organic liquids or low melting solids such as phthalates, adipate and sebacate esters, and polyols such as ethylene glycol and tricresyl phosphate.

The organic mixture may be composed of a plasticizer and a polymeric blend. Likewise an interpenetrating network of two polymer types may be used. Typical organic polymers are poly(methyl methacrylate) (PMMA), poly(styrene), and poly(vinyl chloride) (PVC). A plasticizer is chosen from dioctylphthalate, diethylene glycol dibenzoate, glycerol triacetate, tributyl phosphate, chloroparaffin, and tricresyl phosphate (29).

An electrically conductive material such as carbon black may be added. The sensors can be arranged in regions of conducting material and insulating material in a matrix. When the electrically conductive material is added to the organic mixture, the resulting sensor has a first electrical response in the absence of the analyte and another electrical response in the presence of the analyte.

A detector, which is an electrical measuring device is electrically coupled to the sensor to measure the electrical response (29). Sensor responses to various solvents are analyzed through the use of a Fisher linear discriminant method (38). This method provides a measure of the resolving power of a given sensor array for the set of solvents. The result is a matrix of resolution factors describing the ability of the array to distinguish between pairs of solvents (29).

The average resolution factor for unplasticized PVC is 2.02. The corresponding numbers for some plasticized PVC formulations are 2.62, 5.19, and 5.78, respectively. Combining all these sensors, to make an array with five sensors provides an average resolution factor of 13.11 (29). In Table 1.2, typical compositions of solutions that are used to either spin or dip coat the sensor substrates are reproduced.

Table 1.2 Compositions for spinning or dip coating sensor substrates (29)

1.1.4 Ion Selective Electrodes

A classical ion selective electrode is based on an external reference electrode and an internal reference electrode. At least, the internal reference electrode is separated to the sample solution by an ion selective membrane.

There are variants of this classical design, e.g., coated wire electrodes or hydrogel separated electrodes. Polymer-based ion selective electrodes contain an conductive polymer as a separate film in between the ion selective membrane, or else, the ion selective membrane may contain the conductive polymer directly. Still another approach is to modify the conductive polymer with an ionophore to impart ion selective properties.

Ion selective electrodes based on conductive polymers have a wide application and selectivity for various inorganic cations and anions and have been reviewed extensively (39).

1.1.5 Tunneling Sensors

The research of building the first scanning tunneling microscope by utilizing the tunneling current by Binnig and Rohrer was awarded with the Nobel Prize in 1986.

In electron tunneling transducers, a 1% change in 1.5 nA current between tunneling electrodes corresponds to displacement fluctuation of less than 0.01 nm. This high sensitivity is independent of the lateral size of the electrodes because the tunneling current occurs between two metal atoms located at opposite electrode surfaces.

Due to its high sensitivity and miniature size, micromachined tunneling transducers make it possible to fabricate a high performance, small size, light mass, inexpensive accelerometer, which is in great demand in applications such as micro-gravity measurements, acoustic measurements, seismology, and navigation (40). Consequently, a promising area of research are polymer-based tunneling sensors.

A process for fabricating a polymer-based circuit has been described. A silicon mold of a comb drive structure is formed by a lithography process. This design is then transferred to a polymer substrate using a hot embossing process.

The hot embossing process can be carried out with any conventional embossing device. A blank of polymer material, such as 0.5 mm sheet of PMMA is positioned in the embossing device and pressed against the lower silicon mold. During embossing, the shrinkage of the polymer must be taken into account. This shrinkage can be compensated by scaling up the silicon molds by the expected shrinkage. Afterwards, a metal layer is deposited over the surface of the design and at least one electrical lead is connected to the deposited metal layer (40).

1.1.6 Potentiostats

Basically, a potentiostat is an electronic device that controls the voltage difference between a working electrode and a reference electrode (41). A potentiostat has two tasks: it measures the potential difference between a working electrode and reference electrode and it injects a current flowing from a counter electrode to a working electrode in order to counter-act the difference between the preset voltage and the actual potential difference.

Many researchers have developed single-chip potentiostats in order to reduce the chip size and the costs (41). Trends in potentiostats are focused in their portability and in situ use. Recent issues are summarized in Table 1.3.

Table 1.3 Recent developments in potentiostats

A portable amperometric potentiostat was designed and implemented. Further, a SOC-based chip (system-on-a-chip) was used for the portable potentiostat to improve the system performance as well as to reduce the costs of fabrication. As a result, the measurement of the proposed potentiostat can be carried out in daily life (48).

1.1.7 Microelectromechanical Systems

Polymers find wide applications in microelectromechanical systems. This area has been extensively developed in the past (49, 50). Initially the technology was based on wet anisotropic chemical etching processes for forming three-dimensional silicon geometries. Subsequently, the metal oxide semiconductor was used for polycrystalline silicon micromachining. In this way, surface micromachined devices can be fabricated, such as drives (51).

Meanwhile, many commercially successful products based on microelectromechanical principles were developed, such as a digital light processor and ink jet printer nozzles (52). Polymers that are used for microelectromechanical devices are explained in detail in a special chapter.

As an example for a mechanical sensor a flow sensor is presented. A scanning electron microscopic image of an artificial haircell flow sensor is shown in Figure 1.4.

Figure 1.4 Scanning electron microscopic image of an artificial haircell flow sensor. Reprinted from (52) with permission from Wiley

1.1.8 Multidimensional Sensing Devices

Multidimensional sensing devices are constructed either by mechanically incorporating several different transducer types on a single-chip microsensor system (53) or by chemically integrating multiple reporter units in a molecule (54).

For example, a sensor for mercury ions has been developed that has a chromogenic, fluorogenic, and a redox response (55). The sensor combines a ferrocene unit and a rhodamine block via the linkage of a carbohydrazone binding unit.

Various metal ions can be discriminated via complexes with aza crown ethers by UV/Vis, photoluminescence, electrogenerated chemiluminescence, and redox properties (56).

1.1.8.1 Sensor Arrays

The state of the art in molecularly imprinted polymer (MIP) sensor arrays has been reviewed (57, 58). A challenge is the adaptation of MIPs for sensing applications is developing an accurate and easily measuring method for monitoring effective binding events. Mostly, such sensors and binding assays have been monitored by UV or by fluorescence. These events can be grouped into three general categories, c.f. Figure 1.5.

Figure 1.5 General methods for monitoring binding in molecular imprinted polymers (57):

1) Direct monitoring of the analyte in solution;

2) Incorporation of spectroscopically responsive units;

3) Competition assays using labeled ligands.

The use of imprinted materials in sensor arrays is comparatively new. However, the reported applications cover a wide range of imprinted polymeric materials, such as sol-gel technology and self-assembled monolayers. Sensor arrays use essentially the full range of established physical mechanisms of detection, such as optical and electrical mechanisms. In particular, sensor arrays based on synthetic receptors may give still more accuracy than sensors that are tailored for specific analytes.

The first sensor array that used MIPs in nowadays common sense was reported in 2004 (59). A MIP sensor array with eight channels was designed to analyze six different aryl amines including diastereomers. The polymer matrix was synthesized from methacrylic acid and ethylene glycol dimethacrylate. The analytes are shown in Figure 1.6.

Figure 1.6 Amine analytes (59)

Remarkably, even aromatic amines that differ only by a single methyl group or the diastereomers can be differentiated with a high accuracy. For protein recognition, antibodies and enzymes are conventionally used in sensors. However, these substances are sometimes difficult to isolate. Therefore for the analysis of proteins, protein imprinted synthetic polymers are a natural choice (60).

For the analysis of proteins, water-soluble monomers have been used, such as acrylic acid, N,N-methylene bis(acrylamide), and glycosyloxyethyl methacrylate. Despite the complexity of the protein templates, MIPs can be prepared by completely analogous procedures as small molecules are imprinted. Since the momomers are soluble in water, the polymerization takes place in aqueous solution together with the respective protein analytes (60,61).

1.2 Basic Polymer Types

Certain aspects and the basic principles of the use of polymers in sensors have been discussed in detail by experienced contributors (62–64). The concept of how to tailor polymers to become intelligent macromolecules for the application in so called smart devices has been described in detail (65,66).

1.2.1 Conjugated Polymers

Conjugated polymers are frequently used in optical and electrical systems and are explained in detail in the chapters dealing with optical sensors or electrical sensors. Conjugated polymers belong to the class of poly(fluorene)s, poly(phenylene vinylene)s, poly(phenylene ethynylene)s, and poly(thiophene)s.

1.2.1.1 Poly(aniline)

Poly(aniline) (PANI) has been studied for electronic and optical applications, such as lightweight battery electrodes, electromagnetic shielding devices, anticorrosion coatings, and sensors (67).

PANI is a unique conjugated polymer since PANI can be tailored for specific applications using non-redox acid and base doping processes. One-dimensional PANI nanostructures, including nanowires, nanorods, and nanotubes exhibit exceptionally small sizes as well as very good electric conductivity.

The research on PANI-based sensors has focused on changing the polymer structure to facilitate the interaction between vapor molecules and the polymer either by modifying the polymer backbone or the interchain connections.

However, poor diffusion can readily outweigh any improvements made to the polymer chains because most of the material other than the limited number of surface sites, is not available for interacting with a chemical vapor, thus degrading sensitivity.

One way to enhance diffusion is to reduce film thickness, such as by producing monolayers of conventional polymer materials, which leads to a trade-off between sensitivity and robustness. Coating PANI on porous substrates can increase the surface area, but the chemistry and physics involving polymer support and polymer electrode interfaces is not very well defined for practical use (67).

Doping. Conductively functionalized, or in other words doped PANI is known as emeraldine. When doped with an acid, the acid protonates the imine nitrogen moieties on the polymer backbone and induces charge carriers (67). The mechanism of doping is shown in Figure 1.7.

Figure 1.7 Mechanism of doping of PANI (68) (top) Emeraldine (Bottom) fully doped form

The conductivity σ of PANI increases with the extent of doping from the undoped insulating emeraldine basic form to the fully doped, conducting emeraldine salt form, in the range of 0.1 n Scm−1 to 1 S cm−1. Dopants can be added in any desired quantity until all imine nitrogen moieties that are half of the total nitrogen atoms, are doped, by controlling the pH of the dopant acid solution. Dopants can be removed by interacting the emeraldine salt form with common bases such as ammonium hydroxide.

The conductivity depends on both the ability to transport charge carriers along the polymer backbone and the ability of the carriers to hop between polymer chains through interpolymer conduction. Any interactions with PANI that will alter either of these conduction processes will affect the overall conductivity.

This property enables PANI to be used as the selective layer in a chemical vapor sensor, such as, a resistance detector generally known as a chemiresistor. Due to room temperature sensitivity, the base of deposition onto a wide variety of sensor substrates and due to the various structures, CPs are potential materials for sensor applications (67).

1.2.2 Conducting Polymers

Most polymers are electrical insulators. However, certain polymers can conduct electric charges. Basically, we may address graphite as a polymer. Its conductivity arises from its structure, which is a perfectly conjugated system. The situation is quite similar in the case of conducting polymers.

Conducting polymers can be used in sensors to detect optical, electrochemical and conducting properties. Conducting polymers are unique to due their changing properties when chemically treated with oxidizing or reducing agents. After chemical treatment with protonating, deprotonating, oxidizing or reducing agents, a CP may reversibly change from an initially electrically insulating state to a conducting state. This transition can be used in such applications as optical sensors, chemical sensors, and biosensors (67).

Conducting polymers include PANI, poly(pyrrole) (PPY), poly-(thiophene), and their derivatives. PANI is a CP that is environmentally stable and can react with chemical species at room temperature. As such, PANI may be suitable for gas sensing applications using processes that create a uniform thin film of the PANI. This thin film may then react with protonating and deprotonating agents to create a conduction pathway that can easily be measured.

1.2.2.1 Doped Conducting Polymers

Conducting polymers including PPY and poly(3-methylthiophene) (P3MT) have been electrochemically synthesized (69). The PPY and P3MT films were deposited directly onto platinum interdigitated electrodes that were formed on an alumina substrate. The interdigitated electrodes were used as working electrodes in an electro-chemical cell containing a pyrrole or a 3-methylthiophene aqueous solution.

The deposited polymers were subsequently doped with copper and palladium. Copper microparticle inclusions could be fabricated with a sequence of cathodic pulses in an aqueous CuCl2 solution, or an aqueous PdCl2 solution, respectively. Gas sensor devices based on these doped organic films can be used to detect reducing gases such as NH3, H2 and CO (69).

The exposure of PPY and Cu doped PPY sensors to H2 and CO showed an increase of the resistance of the film. In contrast, the electrical response of the Pd-PPY sensor to H2 and CO shows a drop in the resistivity, which is a behavior quite opposite to that of other PPY-based sensors. The trend of change in the electrical resistance for different systems is shown in Table 1.4.

Table 1.4 Trend of resistance for reducing gases (69)

The responses of the Pd-PPY sensor to H2 and CO are highly reversible and reproducible (69).

1.2.2.2 Electropolymerization

Electropolymerization is also addressed as electrochemical polymerization. The results of this method are CPs. The principles and fields of applications of CPs have been summarized in reviews and monographs (70–74). The electrodeposition of CP films on electrodes opened new fields of applications in electrocatalysis, energy storage and sensor technology.

In an electrochemical polymerization process, an electrical potential is applied across a solution containing the π-conjugated monomer and an electrolyte. In this way an electrical conductive polymer film at the anode surface is produced. As a representative example, the synthesis of dithienylpyrrole compounds is shown in Figure 1.8.

Figure 1.8 Synthesis of dithienylpyrrole compounds (73)

The side groups can be modified ion a wide number of ways (73). Further, rotaxanes with 2,5-di(2-thienyl)pyrrole moieties have been synthesized. The polymerization of this rotaxane takes place at low potentials (75). An example for a rotaxane is shown in Figure 1.9.

Figure 1.9 Rotaxane with a dithienylpyrrole group (75)

Interfacial Polymerization of Aniline. The synthesis method is based on the interfacial polymerization of aniline. Camphorsulfonic acid and ammonium peroxydisulfate are the initiators. Aniline is dissolved in an organic solvent, such as carbon tetrachloride, benzene, toluene, or carbon disulfide.

In contrast, ammonium peroxydisulfate is dissolved in water together with camphorsulfonic acid. The two solutions are transferred into a reaction polymerization vessel for generating an interface between the two solutions.

After a short period, such as a few minutes, green PANI forms at the interface which then diffuses into the aqueous phase. After several hours, the entire water phase is homogeneously filled with dark green PANI, while the lower organic layer appears red-orange, due to the formation of aniline oligomers.

The by-products in the aqueous phase are removed by dialysis against deionized water. Dedoped PANI can then be obtained by dialysis using 0.1 M ammonium hydroxide and then deionized water. This method yields 6–10% of nanofibers (67).

1.2.3 Electrostrictive Polymers

Electrostrictive