Reductive cleavage of methyl orange under formation of a redox-active hydroquinone/polyaniline nanocomposite on an electrode modified with MWCNTs, and its application to flow injection analysis of ascorbic acid at low potential and neutral pH value

The authors report on the electrochemical degradation of the dye Methyl Orange (MO) on a glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes. Continuous electrochemical cycling of the modified electrode in pH 7 solution leads to reductive cleavage of the azo bond of MO to form intermediate amines such as aniline-4-sulfonic acid and 1,4-diaminobenzene. These are further converted to a highly redox-active composite consisting of quinone and polyaniline derivative respectively on MWCNT. Cyclic voltammetric experiments display two well-defined redox peaks at an equilibrium potential (E1/2) of about 0 V (A1/C1) and 0.2 V (A2/C2) vs Ag/AgCl. Physicochemical characterizations such as FT-IR and in-situ UV-vis spectroelectrochemistry support the mechanism of cleavage of MO. The composite modified electrode is shown to be a viable sensor for use in flow injection amperometric analysis of ascorbic acid (AA; vitamin C) at a potential of −0.15 V (vs Ag/AgCl). No interferences were observed with cysteine, glucose, dopamine, citric acid, nitrite, and uric acid. The measured current is linearly related to the AA concentration in the range from 1 μM to 700 μM, with a 115 nM limit of detection (at an S/N ratio of 3). The method was successfully applied to the selective quantification of AA in two pharmaceutical samples.