Abstract
Indole and its derivatives are important core constituents of several natural, biological and pharmaceutical relevant compounds. In general, electrochemical oxidation of indole on solid electrodes in acid and non-aqueous conditions results in the formation of polyindole like compounds as an end product. Selective and controlled electrochemical oxidation of indole and its derivatives to redox active intermediate compound/s without over-oxidation to the polymeric product is a challenging research task. Herein, we report an electrochemical oxidation of electro-inactive indole to a multi-redox active Indole Tetraone (1H-Indole-2,3,4,7-Tetraone)-a new organic redox species (Ind-Tetraone) and entrapment as a surface-confined redox active species on multiwalled carbon nanotube modified glassy carbon electrode (GCE/MWCNT@Ind-Tetraone) in physiological pH solution. GCE/MWCNT@Ind-Tetraone showed a well-defined surface-confined redox peaks at E-1/2, -0.270 V (A(1)/C-1) and +0.270 V (A(2)/C-2) vs Ag/AgCl. From the physicochemical characterizations by Raman and IR spectroscopy, XPS, LC-MS (an ethanolic extract) and control electrochemical experiments with various substituted indole derivatives, it is confirmed the formation of Ind-Tetraone species without any polyindole formation upon the electrochemical oxidation of indole on MWCNT surface. Electrochemical oxidation of nitrogen atom as a radical species and subsequent electron-transfer/water addition reaction is proposed as a possible mechanism for the Ind-Tetraone product formation. A simultaneous electrocatalytic oxidation of hydrazine and reduction reaction of hydrogen peroxide at two discreet potentials has been demonstrated as a bifunctional application of the GCE/MWCNT@Ind-Tetraone system. In further, the GCE/MWCNT@Ind-Tetraone as a electrochemical detector, simultaneous flow injection analysis of hydrazine and hydrogen peroxide was also demonstrated as a proof of concept for the bifunctional application. (c) 2018 Elsevier Ltd. All rights reserved.