Study On The Electrochemical Properties Of Polyaniline And Fabrication Of Electrochemical Sensor For Tetracycline Detection

At first, this paper investigated the electrochemical properties of polyaniline (PANI). The electrochemically synthesized PANI demonstrated three pairs of redox waves in the cyclic voltammograms, including two pairs of PANI characteristic peaks and one for the transitions of quinine-like molecules (e.g. hydroquinone and benzoquinone). PANI suffered severe overoxidation at high anodic potentials in acidic electrolytes, which led to the degradation of this polymer. The resulting quinine-like species may affect the electrochemical overoxidation of PANI. When hydroquinone (HQ) was intentially added into the electrolyte solutions to magnify the potential influence, the overoxidation reaction was weakened with the increasing HQ. Kinetic study showed the decomposition process was a first-order reaction, and the rate constant k decreased from0.1399s-1to0.1027s-1as the concentration of HQ was improved from0to0.05M. It was found from the FT-IR speactra that the relative content of the reduced part of polyaniline increased in the course of degradation. Among the cyclic voltammograms, the peak potentials of leucoemeraldine/emeraldine (LEM/EM) shifted to more negative values, and the peaks for emeraldine/pernigraniline (EM/PEB) transitions showed a reverse shift to more positive values, while the middle peaks ascribed to the reversible redox convertion of hydroquinone/benzoquinone (HQ/BQ)couple nearly stayed the same. It revealed that the existence of HQ/BQ species made it harder for the intermediate oxidation state EM to convert into the two extreme states, LEM and PEB. On the contrary, both the reduced and the oxidized forms tended to transform into the half-oxidized structure easier. The study on the decomposition mechanism of PANI showed the resistance to overoxidation for polyaniline initiated by the resulting quinone-like molecules. These newly-produced molecules can adsorb on the polymeric chains, have electronic exchanges with the nitrogen atoms, and thus influence the transitions of the three states of PANI. As a result, the amount of PEB, which can be susceptible to nucleophile attacks to cause degradation, decreased and the stability of PANI structure was enhanced.Moreover, the fabrication of electrochemical sensor for tetracycline (TTC) detection with multi-walled carbon nanotubes modified glassy carbon electrode (MWNTs/GCE) was also studied. Compared with the bare electrode, there were two well-defined anodic peaks during the electrochemical oxidation of TTC on the MWNTs/GCE, with improved peak currents and decreased peak potentials. It demonstrated the catalytic effect of MWNTs on the oxidation. When the electrochemical sensor constructed with the modified electrode probed the solutions containing0-80mgL-1TTC, the peak currents of the two waves both had good linear responses with TTC concentration, which displayed the reliability of this kind of sensor. The effects of potential scan rate and pH values on the voltammetric response were investigated. It was concluded that the electrochemical oxidation of TTC was a reaction involved in four electrons and four protons from further data analyses. But it was divided into two steps, a2electrons-lproton reaction and the subsequent2electrons-3protons process with potential increasing and therefore two oxidation waves were formed in the cyclic voltammograms. Both of the two peaks were as a result of the interactions between the phenolic substituent in position ten and the surroundings.