Ionic liquid-doped polyaniline and its redox activities in the zwitterionic biological buffer MOPS

The electropolymerization of aniline in several common imidazolium-based ionic liquids has been accomplished successfully with the potentiodynamic method. Considering the fact that imidazolium-based ionic liquids are acidic, they have been selected as the electrolyte for the electropolymerization of aniline, eliminating the usage of extra inorganic or organic acids. The ionic liquids not only serve as the reaction media, exerting the unique favorable π-π interactions between the imidazolium rings and benzene rings of aniline monomer or the growing polymer, but also act as the dopants to render different properties to the resulting polyaniline. Among the tested imidazolium-based ionic liquids, [BMIM][BF4], [BMIM][PF6], [BMIM][NTf2], [EMIM][ES] and [HMIM][FAP], polyaniline doped by the hydrophilic ionic liquid [BMIM][BF4] displays the good electrochemical responses in the biologically important MOPS (3-(N-Morpholino)-propanesulfonic acid) solution with 2.34 × 10−3 M of sulfuric acid additive. NMR, UV–vis and electrochemical impedance experiments were performed to further characterize the polyaniline/[BMIM][BF4] composite. In contrast, polyaniline that is doped by the hydrophobic ionic liquid [BMIM][PF6] is electroactive in the MOPS solution in the absence of the acid additive, with a pH of 5, extending the working pH range of polyaniline, which is typically electroactive in the solutions with the pH values less than 3. It is suggested that the effective hydrogen bonding interactions between BF4 anion and water facilitate its hydrolysis in the microenvironment of the polymer backbone to provide the acidic protons, which are beneficial to the adjustment of the microenvironments of the polyaniline system and thus renders its observed well-resolved reversible pair of redox peaks in the MOPS solution. PF6 anion, on the other hand, with its larger size and less basicity, has the weaker interaction with water, thus releasing the protons in a relatively slow manner, which gives some “buffer” capacities and rendering the [BMIM][PF6]-doped polyaniline electroactive in the pH 5 of MOPS solution without the acid additive.

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