**4. Conclusions**

The electrochemical oxidation of piperazine on platinum electrodes at moderate potentials (roughly below 1.0 V/RHE) preserves the ring structures and produces ketopiperazines as the main reaction product. In situ FTIR spectroscopy strongly suggested that ring opening and overoxidation occur at higher potentials to form both amides and isocyanates. As a result, it was observed that the homopolymerization of piperazine cannot be achieved in perchloric acid aqueous solution under electrochemical conditions.

On the contrary, piperazine can be successfully copolymerized with aniline in acidic medium. The deposited copolymer shows some electrochemical features similar to those of pristine polyaniline, particularly those related with leucoemeraldine-to-emeraldine and emeraldine-to-pernigraniline transitions. However, a key difference arises in the intermediate potential region between both transitions. As shown by in situ FTIR and XPS spectroscopies, the intermediate redox peak is a consequence of the incorporation of piperazine units to the copolymer structure. Most of these piperazine centers undergo electrochemical oxidation during the copolymerization potential scans and, as a result, a new reversible hydroxy - ketopiperazine redox transformation seems to occur as the intermediate voltammetric feature centered at 0.68 V. It should be noted that, owing to the conservative potential program applied during the deposition process, any significant amount of overoxidation products was not incorporated to the copolymer structure. As a result, the deposited material is chemically stable, and presents a well-defined electrochemical behavior.

It was observed that the aniline–piperazine copolymer shows a linear response when applied to the electrochemical determination of dopamine or ascorbic acid in synthetic samples. The sensitivity of the measurement is in both cases high enough to assure the correct quantification of analytes. This electrode material has to be tested in real samples, but according to the results presented in this contribution, it shows potential application in DA and AA sensors, owing to its facile synthesis, high chemical stability, and reproducible linear response.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/11/6/1012/ s1, Figure S1: Linear Sweep Voltammogram showing the oxidation of 3 mM DA on a Pt electrode covered with the aniline-piperazine copolymer. DA oxidation peak is centered at 0.85 V, Figure S2: Linear Sweep Voltammogram showing the oxidation of 30 mM AA on a Pt electrode covered with the aniline-piperazine copolymer. AA oxidation peak is centered at 0.89 V, Table S1: Observed frequencies and proposed assignments for the vibrational bands derived from Figures 2 and 5.

**Author Contributions:** F.H., F.M. and E.M. conceived and designed the experiments; S.D., S.L.-B. and C.N.K. performed the experiments; S.D., C.N.K., F.H., F.M. and E.M. analyzed the data; S.D., F.H. and E.M. wrote the paper. All authors participated in the Investigation and in manuscript preparation. All authors contributed in Writing-Review & Editing of the manuscript and approved the final version.

**Funding:** This research was funded by the Spanish Ministerio de Economía y Competitividad and FEDER funds, gran<sup>t</sup> MAT2016-76595-R. The stay of S. Dkhili at the University of Alicante was funded by the Ministry of Higher Education and Scientific Research of Tunisia.

**Conflicts of Interest:** The authors declare no conflict of interest.
