A new organic redox species-indole tetraone trapped MWCNT modified electrode prepared by in-situ electrochemical oxidation of indole for a bifunctional electrocatalysis and simultaneous flow injection electroanalysis of hydrazine and hydrogen peroxide
Graphical abstract
Introduction
Indole and its derivatives are the core structure in several natural and biological relevant compounds like amino acids-tryptophan, neurotransmitters-serotonin/melatonin and plant auxins-indole-3-acetic acidetc [[1], [2], [3], [4], [5]]. In addition, they are constituents of many therapeutic drugs such as anticancer [6], antioxidant [7], anti-rheumatoidal [8], anti-HIV [9], anti-tuberculosis [10], anti-inflammatory [11], and anti-bacterial [12] in pharmaceutical chemistry (Scheme 1). Investigation on the electron-transfer behaviour of the indole and its derivatives like indole quinones by means of electrochemical techniques is very important for understanding its pharmco-kinetics and biomedical aspects [[1], [2], [3],13,14]. Herein, we report an electrochemical oxidation of electro-inactive indole to a multi-redox active indole-tretraone (1H-Indole-2,3,4,7-Tetraone; Ind-Tetraone) and entrapment as a surface-confined redox active species on multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE/MWCNT@Ind-Tetraone)in physiological pH solution. The Ind-Tetraone trapped chemically modified electrode showed two redox peaks at equilibrium potential, E1/2, −0.270 V (A1/C1) and +0.270 V (A2/C2) suitable for bifunctional electrochemical mediation reactions discreetly in physiological pH.
General questions arise about this work are; what is special with MWCNT? and why do we need MWCNT as a base material? From our recent electrochemical studies with various electro-inactive organic molecules such as anthracene [15], pyrene [16], benzene [17], 1,10-phenonthroline [18], phenothiazine [19] and quinolone [20] on different carbon nanomaterials like MWCNT, graphite nanopowder and graphitized mesoporous carbon and serious of reports on iron impurity containing CNT by other research groups [[21], [22], [23], [24]], it has been found that a trace metal and carbonaceous impurity containing CNT is the efficient material for the electrochemical oxidation reactions [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. The interactions between the iron impurity and N/S atom and the graphitic carbon pi electrons and aromatic units of the test organic compounds allow the electrochemical oxidation as a diffusion restricted reaction (avoids collision) and in turn to unusual redox active organic product formation. Although other forms of the carbon nanomaterials like carbon nano fibre, graphite, graphite oxide and mesoporous materials show qualitatively similar electrochemical features, with respect to quantitative current signal and amount of electro-active species trapped on the surface, MWCNT is found to be the best choice for the surface electrochemical oxidation reaction. In this work, our primary aim is to electro-oxidize the indole in a controlled way and to trap the redox active indole oxidized intermediates on MWCNT surface in a physiological condition.
Regarding to the indole electrochemistry, electrochemical oxidative polymerization of indole to yellow coloured polyindole (PIn) in acid or organic or aqueous medium has been often reported [[25], [26], [27], [28], [29], [30], [31]]. When compared with conventional electro-active polymers like polyaniline (PANI) and polypyrrole, the PIn films have showed better thermal stability, capacitor activity with less-degradation rate. Owing to its unique electrochemical and physical properties [25], PIn has been utilized in batteries [26], super capacitors [27], anti corrosive coatings [28], electro-chromic devices [29] and electro-catalysis applications [30,31]. Meanwhile, Enache and Brett have studied the electrochemical oxidation of indole and its derivatives on an unmodified GCE in neutral pH [32]. It was speculated that two and/or three oxygenated indole derivatives as intermediate species were formed as products upon the electrochemical oxidation reaction [32]. Since, the amount of the product formed is about nanogram and intermediate species involved are short-lived in nature, no attempts were made to characterize the electrochemical reaction product/s [32]. Herein, we report a electrochemical oxidation of indole to surface-confined Indole-Tetraone product (Ind-Tetraone) on GCE/MWCNT surface (GCE/MWCNT@Ind-Tetraone) in pH 7 phosphate buffer solution (PBS). The Ind-Tetraone system showed well-defined redox peaks at equilibrium potential, E1/2 (Epa+Epc/2, Epa and Epc are anodic and cathodic peak potential respectively), −0.27 V (A1/C1) and +0.270 V (A2/C2) vs Ag/AgCl, due to the electron-transfer behaviours of indole 2,3-quinone and 4,7-quinone sites of the Ind-Tetraone respectively. Several electrochemical control experiments and analytical characterization studies were performed to precisely identify the product formed on the electrode surface. In further, selective and simultaneous electrocatalytic oxidation and reduction of hydrazine and hydrogen peroxide at two discreet potentials respectively using flow injection analysis (FIA) coupled with a bipotentiostat was demonstrated as a model system for the application of the indole tetraone-MWCNT modified electrode.
Section snippets
Chemicals and materials
MWCNTs (>90% carbon basis, outer diameter: 10–15 nm; inner diameter: 2–6 nm; length 0.1–10 μm), indole and hydrazine were purchased from Sigma Aldrich. Hydrogen peroxide (30%) was obtained from Rankem and stored in a refrigerator. All of the other chemicals were analytical grade and used without further purification. Aqueous solutions were prepared using deionized and alkaline potassium permanganate distilled water (DD water). The supporting electrolyte pH 7 phosphate buffer solution (PBS) of
Electrochemical oxidation of indole and selective quinone immobilization
Initial electrochemical oxidation experiment was carried out with indole adsorbed on unmodified GCE (GCE@Indads) in a potential window, −0.6 to +0.4 V vs Ag/AgCl at v = 50 mV s−1. As can be seen in Fig. 1A, curve a, featureless voltammetric response was observed indicating electro-inactivity of the unmodified GCE for indole oxidation reaction in pH 7 PBS (i.e., GCE@Ind-Oxid). Interestingly, when the same experiment was repeated with pristine-multiwalled carbon nanotube (MWCNT) modified GCE,
Conclusion
Electrochemical potential cycling treatment of indole adsorbed MWCNT modified electrode in a window, −0.6–0.4 V vs Ag/AgCl resulted in the formation of a new organic redox active species, Indole-Tetraone as an oxidized intermediate and gets immobilized as a surface-confined multi-redox system on MWCNT surface (GCE/WCNT@Ind-Tetraone). The GCE/MWCNT@Ind-Tetraone hybrid electrode showed couple of redox peaks at −0.270 V (A1/C1) and +0.270 V (A2/C2) vs Ag/AgCl in pH 7 PBS due to the
Acknowledgements
We thank the Department of Science and Technology-Science and Engineering Research Board (DST-SERB) for financial assistance (EMR/2016/002818).This work was partially supported by the National Taipei University of Technology (NTUT), Taiwan.
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