Polythiophene, carboxylate derivatives

FIGURE 9.7 Carboxylic acid derivatives of regioregular polythiophene (Table 9.1, entry 17) can act as sensors of alkali metals in water through a self-assembly and disassembly mechanism. 

A number of other conducting polymer derivatives have been used for immobilizing enzymes and redox mediators for biosensor applications. Among these polymers are poly (tyramine) [167] and poly(l-(5-aminonaphthylethanoic acid) [168], which have accessible amine and carboxyl functionalities, respectively, for covalently linking to complementary groups on available amino acid residues of enzymes. Polypyrrole and polythiophene derivatives have also been widely exploited for covalent immobilization of enzymes, which then function as the sensing layers in efficient biosensors [169,170]. 

Many substituted thiophenes are commercially available. Thus, the study of the effects of substituents on polythiophene is relatively straightforward in comparison with pyrrole, whose 3-monosubstituted and 3,4-substituted derivatives are not easily synthesized. Table 3 gives many substituted thiophenes that can be electrochemi-cally oxidized to produce conducting polymers. Some derivatives, such as 3-thiophene acetic acid, 3-thiophene malonic acid, 3-thiophene carboxylic acid, and 3-thiophene carboxaldehyde do not polymerize at all [205). 

As an alternative to pendent sulfonate groups, polythiophene derivatives which possess pendent carboxylic acids have been investigated. The lower acidity of the carboxylic acid functionality (/7ATa 4.8) compared to... 

AKI 12] Akitsu K., Kubo T., Uchida S. et ai, Polymer-sensitized solar cells using polythiophene derivatives with directly attached carboxylic acid groups ,... 

Besides the permselective properties, the electrocatalytic properties of ECP films can be also used for the amperometric detection of some target molecules. Accordingly, electrodes modified with PPy, polythiophene (PTh), PAni, and their derivatives were found to catalyze the electrochemical oxidation of ascorbic acid [127-129], NADH [115, 116,130], dopamine [128], pyrrolo-quinoline quinone [131] as a coenzyme of some oxidoreductases, and quinone and derivatives [132, 133]. Selectivity exhibited by these materials could be enhanced by the introduction of an appropriate substituent onto the polymer backbone. So, a facilitated electron transfer between cytochrome c and carboxylic acid or carboxylate-substituted PPy [134] or polyindole [135] has been observed. As such an effect was not obtained with unsubstituted polymer films, the cytochrome c-polymer interaction was e lained on the basis of binding between the polymer substituents and the lysine residues on the redox protein. 

Irregular polythiophenes carrying carboxylic acids [170] have been prepared elec-trochemically and dramatic ionochromism or ionic self-assembly was not reported. However, other reports have looked at irregular PT carboxylic add derivatives in competitive immunoassays for antigens and haptens [171]. The carboxylate function attached directly to the thiophene ring has been reported to be prepared [172] from the 2,5-dichloro-3-methylthiophene carboxylate by the Yamamoto route [8]. 

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