Unexpected Electrochemical Transformation of Aminobenzene Sulfonic Acid Isomers to Respective Surface‐Confined‐Redox Active Quinones Bypassing Polyaniline on a MWCNT Surface

Abstract

Electrochemical oxidation of aniline and its derivatives, such as aminobenzene sulfonic acid (SA), on solid electrodes [Pt, glassy carbon (GCE), indium tin oxide (ITO), and Au] leads to the formation of conducting polyaniline (PANI) and its derivatives as the only products in aqueous solution. We report here, for the first time, the unique and unexpected electrochemical transformation of ortho‐, meta‐ and para‐SA into highly redox‐active, adsorption‐controlled quinone isomers [catechol (CA), resorcinol (Re), hydroquinone (HQ)], without any PANI‐like product formation on a multiwalled carbon nanotube (MWCNT)‐modified GCE (GCE/MWCNT@Q, where Q=quinone) within a restricted potential cycling window (−0.4 to 0.4 V versus Ag/AgCl) in a neutral pH solution. It is proposed that upon the potential cycling of a 2.1 wt% iron‐impurity‐containing MWCNT‐modified GCE with a millimolar concentration of SA isomers, benzene (after deamination and desulfonation of SA) and hydrogen peroxide (from the oxygen reduction reaction at −0.4 V vs. Ag/AgCl) are formed as intermediate species, which are then converted to quinones and quinone‐like products through the electro‐Fenton reaction (utilizing the iron impurity and H2O2). One of the reaction intermediates, benzene, was identified (Mw=77.8±0.5 g mol−1) by using an in situ electrochemical quartz crystal microbalance coupled cyclic voltammetric technique. By using GCE/MWCNT@HQ as a model system, efficient electrocatalytic oxidation and sensing of the reduced form of nicotinamide adenine dinucleotide (NADH) was successfully demonstrated.