Electrochemical Synthesis of Polypyrrole in the Presence of Congo Red; Application to Selective Voltammetric Determination of Dopamine in the Presence of Ascorbic Acid

The electrosynthesis of polypyrrole (PPy) film has been achieved on glassy carbon electrode (GCE) in aqueous medium of Congo red (CR) by means of cyclic voltammetry (CV). The modified electrode exhibits high electrocatalytic activity toward dopamine (DA) oxidation, with drastic enhancement of the reversibility and peak currents. This modified electrode, due to electrostatic interactions, is capable to mask the response of ascorbic acid (AA) completely and provide an effective method for the detection of minor amounts of dopamine in the presence of high concentrations of AA. The electrochemical response of the film modified electrode is strongly dependent to the switching potential applied in the CV procedure of the electro‐polymerization. The results show that by increasing the switching potential more than 0.75 V, the peak of AA is gradually disappeared. This peak in potential of 0.85 V is reached to capacitive background current. With respect to the destruction of the conjugated structure of ppy and lowering the conductivity of the film at the surface of modified electrode, higher switching potentials cannot be suitable for electropolymerization procedure. The effects of various experimental parameters such as, number of polymerization cycles, switching potential, pH and potential sweep rate on the voltammetric response of dopamine were also investigated. The modified electrode shows a linear response to DA in the range of 0.5 to 100 μM with a detection limit of 0.1 μM. The prepared modified electrode does not show any considerable response toward sulfhydryl compounds, such as, cysteine, penicillamine and glutathione, revealing a good selectivity for voltammetric response to DA. The effective electrocatalytic property, ability for masking the voltammetric responses of the other biological reducing agents together with high reproducibility and stability make the modified electrode suitable for selective and sensitive voltammetric detection of sub‐micromolar amounts of DA in clinical and pharmaceutical preparations.