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

PhD student Ke Qu and Professor Xiangqun Zeng from the Department of Chemistry at Oakland University (USA) carried out a research to show the feasibility of ionic liquids being the capable dopants for the polyaniline and how they can be effectively incorporated into the polymer backbone to change its microstructure during electrolysis process, thus rendering it to be conductive at the near-neutral pH conditions and broadening its application in the bioanalytical fields.

The lower conductivity of polyaniline under the neutral pH conditions is caused by the missing mobile charge carriers within the polymeric system. Doping with the acids (the proton source) is required to produce the polarons or the bipolarons to facilitate these charge carriers’ mobility. The polyaniline’s sublattice structure and conformation are affected by the monomer structure, solvent and ions used in the film preparation. It is reasonably hypothesized that the ionic liquids could be utilized as the unique dopants to finely tune the properties of the resulting polyaniline. Due to the larger sizes of the organic cations of the ionic liquids, the authors expected that the porosities of the polymer change accordingly after doping, allowing the polymeric chains to be aligned more favorably in terms of the electrical conductivity. Imidazoline molecule forms a large group of imidazolium-ring-based ionic liquids, which are acidic. It was demonstrated that the electroactive range of polyaniline doped by the imidazolium-based ionic liquids moves towards the near-neutral conditions in the biologically important MOPS solution.

Qu and Zeng in their experiment, studied five dopants: [BMIM][BF₄], [BMIM][PF₆],[BMIM][NTF₂], [EMIM][ES] and [HMIM][FAP]. They identified [BMIM][BF₄] as hydrophilic, [BMIM][PF₆], [BMIM][NTF₂] and [HMIM][FAP] as hydrophobic while [EMIM][ES] as intermediary between hydrophilic and hydrophobic. They used cyclic voltammetry technique for electropolymerization of the aniline in ionic liquids. After the electropolymerization of the aniline in the ionic liquids, a film of polymer formed on the electrode surfaces.

As imidazolium-based ionic liquids are acidic in nature, they were selected as the electrolyte for the electropolymerization of aniline, avoiding the usage of extra inorganic or organic acids, which are often needed for the polymerization of aniline in the traditional aqueous solutions. Moreover, the ionic liquids not only serve as the reaction media, exerting the unique favorable p-p interactions between the imidazolium rings and benzene rings of aniline monomer or the growing polymer, but also act as the dopants to give different properties to the resulting polyaniline. Specifically, polyaniline doped by the hydrophobic ionic liquid [BMIM][PF₆] keeps its electroactivity in the MOPS solution with a pH of 5, extending the working pH range of polyaniline, which is typically electroactive only in the solutions with the pH values less than 3.

During the course of the research, the scientists have been able to achieve the direct electropolymerization of aniline in the imidazolium-based ionic liquids and move the working pH ranges of the resulting polyaniline towards the near-neutral conditions. Scientists hope that their findings, described in the journal Electrochimica Acta, will pave the way for developing more versatile polyaniline materials for the bioanalytical and other relevant applications.