Doping Mechanism in Self-Doped Polymers

Recently, Fabre et al. [31] and Freund et al. [7, 8] used electro-chemically deposited, self-doped, boronic-acid-substituted, conducting polymers for saccharide and fluoride detection. Freund et al. prepared a potentiometric sensor for saccharides using self-doped PABA [7, 8]. The transduction mechanism in that system is reportedly the change in pKa of polyaniline that accompanies complexation, and the resulting change in the electrochemical potential. Sensors produced with this approach exhibit reversible responses with selectivity to various saccharides and 1,2-diols (Figure 3.22) that reflect their binding constants with phenylboronic acid observed in bulk solutions. The sensitivity... 

As discussed by Han et al [76], protonic acids are not suitable dopants for PTs or P3ATs, although the optical and mechanical properties of P3ATs, mixed with protonic acids, to some extent resemble those of doped PTs. The reason is the protonation of the thiophene rings, which prevents the conduction of the electric current, in analogy with the case of the self-doped polymers. 

The necessary porosity for thicker layers was introduced by appropriate current densities [321-323], by co-deposition of composites with carbon black [28, 324] (cf. Fig. 27), by electrodeposition into carbon felt [28], and by fabrication of pellets from chemically synthesized PPy powders with added carbon black [325]. Practical capacities of 90-100 Ah/kg could be achieved in this way even for thicker layers. Self-discharge of PPy was low, as mentioned. However, in lithium cells with solid polymer electrolytes (PEO), high values were reported also [326]. This was attributed to reduction products at the negative electrode to yield a shuttle transport to the positive electrode. The kinetics of the doping/undoping process based on Eq. (59) is normally fast, but complications due to the combined insertion/release of both ions [327-330] or the presence of a large and a small anion [331] may arise. Techniques such as QMB/CV(Quartz Micro Balance/Cyclic Voltammetry) [331] or resistometry [332] have been employed to elucidate the various mechanisms. 

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