**3.5 Electrochemical co-polymerization of aniline and aniline derivates**

Owing to its conductivity and redox activity, polyaniline is considered for practical application in various fields. Unfortunately, beside its unique properties, application of polyaniline in biochemical systems is limited as a consequence of the lost of activity at pH above 4 (Karyakin et al., 1994; Malinauskas, 1999; Mu 2011). This problem might be overcome by introduction of so- called pH functional groups into polyaniline chain (Mu, 2011). This could be achieved either by sulfonation (Wei et al., 1996) or by copolymerization, which is more efficient way to alter the properties of parent polymer. Electrochemical polymerization of aniline and aniline derivates with pH functional groups, sulpho, carboxyl or hydroxyl was reported. It was observed that self-doped polyanilines, obtained by electrochemical co-polymerization of aniline with: *o-*aminobenzoic acid, *m*aminobenzoic acid, or *m –* aminobenzensulfonic acid had exhibited redox activity at high pH (Karyakin et al., 1996).

Apart from aniline and aniline derivate, electrolyte solution also contains acid necessary for protonation of nitrogen atom. The obtained co-polymers are often called self-doped polyanilines, since the introduced negatively charged functional groups plays role of an intermolecular dopant which is able to compensate the charge on positively charged nitrogen atoms of the polymer. The presence of intermolecular anion alters properties of "ordinary" polyaniline, and has influence on the polymerization process as well. It was shown that upper switching potential limit had important influence on self-doping, the limit of 0.9 V was proven to be optimal, and while in the case of un- substituted aniline, upper limit was lower.

The problem related to electrochemical activity of self-doped polyaniline is its rapid lost. Recently, it was showed that electrochemical polymerization of aniline and 5-aminosalycylic acid, which nears two acidic functional groups, had lead to co-polymer with satisfactory redox activity (Mu, 2011).
