Conducting electroactive polymers

As with the majority of ISEs, all of the aforementioned receptors are immobilised within close proximity to the transducer element. However, conducting polymers (electroactive conjugated polymers) are now emerging rapidly as one of the most promising classes of transducer for use within chemical sensors. Here, the receptor can be doped within the polymer matrix, i.e. within the transducer element itself. This will facilitate the production of reliable, cost-effective, miniaturised anion-selective sensors, as it will be possible to move away from plasticiser-based membranes, but allow for ion recognition sites in conjunction with all-solid-state ion-to-electron transducers. 

Deshpande SD, Kim J, Yun SR (2005b) Studies on conducting polymer electroactive paper actuators effect of humidity and electrode thickness. Smart Mater Struct 14 876-880. doi 10.1088/0964-1726/14/4/048... 

Conducting Polymer Blends, Composites, and Colloids. Incorporation of conducting polymers into multicomponent systems allows the preparation of materials that are electroactive and also possess specific properties contributed by the other components. Dispersion of a conducting polymer into an insulating matrix can be accompHshed as either a miscible or phase-separated blend, a heterogeneous composite, or a coUoidaHy dispersed latex. When the conductor is present in sufftcientiy high composition, electron transport is possible. 

The final conclusion from the different kinetic studies that simultaneously followed productivity, consumed current, storage capacity of the obtained films, and the current efficiency in generating electroactive polymer in the final film is that any electropolymerization of conducting polymers occurs together a partial degradation of the electroactive polymer. The final film is a mixed material. From the kinetic studies we know the variables that increase or deplete the degradation reaction in relation to the polymerization reaction. 

The term overoxidation refers to degradation of the conductivity and electroactivity of an oxidized conducting polymer by reaction with a nucleophile. This topic has recently been thoroughly reviewed,33 and so the treatment here will be brief. 

The electrochemistry of a polymer-modified electrode is determined by a combination of thermodynamics and the kinetics of charge-transfer and transport processes. Thermodynamic aspects are highlighted by cyclic voltammetry, while kinetic aspects are best studied by other methods. These methods will be introduced here, with the emphasis on how they are used to measure the rates of electron and ion transport in conducting polymer films. Charge transport in electroactive films in general has recently been reviewed elsewhere.9,11... 

A variety of other techniques have been used to investigate ion transport in conducting polymers. The concentrations of ions in the polymer or the solution phase have been monitored by a variety of in situ and ex situ techniques,8 such as radiotracer studies,188 X-ray photoelectron spectroscopy (XPS),189 potentiometry,154 and Rutherford backscatter-ing.190 The probe-beam deflection method, in which changes in the density of the solution close to the polymer surface are monitored, provides valuable data on transient ion transport.191 Rotating-disk voltammetry, using an electroactive probe ion, provides very direct and reliable data, but its utility is very limited.156,19 193 Scanning electrochemical microscopy has also been used.194... 

Like other ion-exchange polymers, conducting polymers have been used to immobilize electroactive ions at electrode surfaces. Often the goal is electrocatalysis, and conducting polymers have the potential advantage of providing a fast mechanism for electron transport to and from the electrocatalytic ions. 

The electrochemical characteristics of an electroactive ion immobilized in a conducting polymer film depend on whether the film is conductive at the formal potential of the ion.243... 

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

The electrochemistry of polyaniline is more complex than that of other conducting polymers and given the large number of possible structures for the material, it is not surprising that many possible reaction schemes have been suggested [181,182, 195-197,205], Many of the properties of the material are pH-dependent [173,174, 206], including the open circuit potential [207] which is most positive at pH 0, and this is further complicated by the fact that not all the polymer chains are necessarily in exactly the same state at any given time [197]. Above pH 3 polyaniline films do not show any electroactivity, but are not electroactive even at low pH with... 

Another problem that is common for all membrane-based solid-state sensors is the ill-defined membrane-metal interface. A large exchange current density is required to produce a reversible interface for a stable potentiometric sensor response. One approach to improving this interface is to use conducting polymers. Conducting polymers are electroactive n-conjugated polymers with mixed ionic and electronic conductivity. They... 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

A basic property of all conducting polymers is the conjugation of the chain-linked electroactive monomeric units, that is, the monomers interact via a jt-electron system. In this respect, they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated and the interaction between the isolated redox counters is weak. Consequently, redox polymers are nonconductors [17]. They will not be discussed in this context. 

A very important electrochemical phenomenon, which is not well understood, is the so-called memory effect. This means that the charging/discharging response of a conducting polymer film depends on the history of previous electrochemical events. Thus, the first voltammetric cycle obtained after the electroactive film has been held in its neutral state differs markedly in shape and peak position from subsequent ones [126]. Obviously, the waiting time in the neutral state of the system is the main factor determining the extent of a relaxation process. During this waiting time, which extends over several decades of time (1-10 s), the polymer slowly relaxes into an equilibrium state. (Fig. 13) After relaxation, the first oxidation wave of the polymer appears at more... 

The preparation of bis(thienyl)furans of type 33 and 34 has been descibed. These compounds have been proposed as suitable precursors for electroactive molecular species and conducting polymers <99JOC6418>. 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

The third class of polymers used to prepare chemically modified electrodes is the electronically conductive polymers [25]. The polymer chains in this family of materials are themselves electroactive. For example, the polymer redox reaction for polypyrrole (Table 13.2) can be written as follows ... 

Polymer films can also be electropolymerized directly onto the electrode surface. For example, Abruna et al. have shown that vinylpyridine and vinyl-bipyridine complexes of various metal ions can be electropolymerized to yield polymer films on the electrode surface that contain the electroactive metal complex (see Table 13.2) [27]. The electronically conductive polymers (Table 13.2) can also be electrosynthesized from the corresponding monomer. Again, a polymer film that coats the electrode surface is obtained [25]. Electropolymerized films have also been obtained from styrenic, phenolic, and vinyl monomers. 

Conducting polymers based on polymer chains with conjugated double bonds are electroactive materials that have found widespread use also in the field of chemical sensors [11-41], Oxidation of the conjugated polymer backbone is accompanied by anion insertion or cation expulsion, as follows ... 

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