Applications of Self-Doped Polymers

A distinctive property of self-doped polymers is their water solubility in the neutral (insulating) and doped (conducting) states. This solubility is due to the covalently attached negatively charged groups on the polymer backbone. Solubility allows a deposition of conductive and electroactive layers onto any, even a nonconducting, surface by a simple casting of self-doped polymers. Such layers could find numerous applications... 

Polyaniline is the conducting polymer most commonly used as an electrocatalyst and immobilizer for biomolecules [258-260]. However, for biosensor applications, a nearly neutral pH environment is required, since most biocatalysts (enzymes) operate only in neutral or slightly acidic or alkaline solutions. Therefore, it has been difficult or impossible to couple enzyme catalyzed electron transfer processes involving solution species with electron transport or electrochemical redox reactions of mostly polyaniline and its derivatives. Polyaniline is conducting and electroactive only in its protonated (proton doped) form i.e., at low pH valnes. At pH values above 3 or 4, polyaniline is insulating and electrochemically inactive. Self-doped polyaniline exhibits redox activity and electronic conductivity over an extended pH range, which greatly expands its applicability toward biosensors [209, 210, 261]. Therefore, the use of self-doped polyaniline and its derivatives could, in principle. 

These first examples of self-doped conducting polymers that are stable as aqueous solutions will no doubt lead to new applications as well as to new insights in the behaviour of charge carriers in conducting polymers. 

Self-doped polymers are poly(thiophene)s bearing sufonic acid on the end of an alkyl substituent. These polymers are doped by their own sulfonic acid without any external dopants and are very stable in the doped state. Due to attached sulfonic acid, they are soluble in water. Anion-doping of the polymers is controlled by the motion of cations, thus quick responses are observed in anion-doping and undoping. The polymers are useful for dissipating charge on photoresists and other applications. 

The conducting and physical properties can be modified by the use of 3-/4-substituents, or A-substituents in the case of pyrrole. The counter-ions can be incorporated into a side-chain (self-doping), as in the polymer of 3-(thien-3-yl)propanesulfonic acid. Oligo(thiophenes) are also useful in these applications and have been specifically synthesised up to 27 units long by palladium(0)-catalysed couplings or via the diacetylene synthesis (17.12.1.1). ... 

These novel materials exhibit many properties that are different from those of the parent PANI and show promising applications in various fields. For instance, self-doped PANI is highly soluble in alkaline aqueous solutions and many nonaqueous media unlike PANI. Good solubility is essential for a polymer in order to facilitate postsynthetic processing. The solubility of PANI is greatly improved by the presence of the substituted acidic group. ... 

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