In-situ electrochemical immobilization of [Mn(bpy)2(H2O)2]2+ complex on MWCNT modified electrode and its electrocatalytic H2O2 oxidation and reduction reactions: A Mn-Pseudocatalase enzyme bio-mimicking electron-transfer functional model

https://doi.org/10.1016/j.jelechem.2018.01.041Get rights and content

Highlights

  • A high redox active [Mn(bpy)2(H2O)2]2+ complex confined MWCNT modified electrode was developed

  • MWCNT@[Mn(bpy)2(H2O)2]2+ showed mediated H2O2 oxidation and reduction reactions

  • MWCNT@[Mn(bpy)2(H2O)2]2+ is identified as Mn-Pseudocatalase enzyme electron-transfer bio-mimic

  • MWCNT@[Mn(bpy)2(H2O)2]2+ has interesting electro-analytical application towards H2O2

Abstract

Mn-Pseudocatalase is a non-heme catalases family enzyme produced by various bacteria that involves in a two-electron H2O2 catalytic cycle in a manner similar to that of the heme-based Catalase enzymes. Herein, we report a bio-mimicking functional model system prepared by in-situ electrochemical oxidation of Mn(bpy)2Cl2 precursor to a surface-confined [MnII(bpy)2(H2O)2]2+ complex, wherein, bpy = 2,2′-bipyridyl, on a carboxylic acid functionalized multiwalled‑carbon nanotube (f-MWCNT)/Nafion modified glassy carbon electrode for biomimicking H2O2 disproportionation reaction. The modified electrode showed a well-defined redox peaks at an apparent standard electrode potentials (Eo′), 0.65 ± 0.05 V and 0.2 V vs Ag/AgCl with surface-excess (ΓMn) values, 6.24 × 10−9 mol cm−2 and 0.43 × 10−9 mol cm−2 for the MnIV/III and MnIII/II sites of the Mn-complex in neutral pH solution respectively. Physico-chemical characterizations of the system by FTIR, UV–Vis and ESI-MS (ethanolic extract of the electrode) confirming the conversion of [Mn(bpy)2(H2O)2]2+ complex (m/z, 403.09). A strong π-π interaction between the f-MWCNT's graphitic sp2 carbons and the bpy's aromatic electrons, hydrogen bonding between the oxygen and water molecules and ionic interaction between the complex and sulphonic site of nafion favor the stability of the complex. The hybrid system showed selective current signals for mediated oxidation and reduction reactions of H2O2 (disproportional reaction) without any dissolved oxygen interference. As an independent study, selective electro-catalytic oxidation and amperometric detection of H2O2 was demonstrated.

Introduction

Design and development of new bio-inspired catalytic system is a challenging research interest in the interdisciplinary areas of chemistry, biochemistry and biomedical systems [1,2]. Catalase, a porphyrin-metal complex containing protein, made up of 500 amino acids and tetramer of four polypeptide chain is found nearly in all the living organism [3]. It functions as an antioxidant system to protect the cell from the oxidative damage by reactive oxygen species like H2O2. Unlike to the horseradish peroxide (HRP) enzyme, which function as a selective reducing system for the conversion of H2O2 to water, catalase enzyme oxidizes and reduces H2O2 to water and dioxygen respectively (disproportionation reaction, Scheme 1) [[3], [4], [5]]. In the literature, substantial number of articles was reported relating to the development of homo-/heterogeneous metal complexes based biomimetic units for HRP enzyme structure and its functionalities [[6], [7], [8], [9], [10], [11]]. For instance, hemin-based composites, Prussian blue (PB) and iron oxide have been referred as a HRP enzyme functional biomimics [[12], [13], [14], [15]]. On the other hand, limited number of reports was available on the biomimicking model and functionality of the Catalase enzyme (Table 1) [[16], [17], [18], [19], [20], [21], [22], [23]]. Difficulty in preparing the high-valent metal redox species stabilized system is found to be the prime reason for the limitation. Herein, we report an in-situ preparation of Mn bipyridyl-aqua complex, [Mn(bpy)2(H2O)2]2+ on functionalized-multiwalled carbon nanotube modified glassy carbon electrode/Nafion system, designated as GCE/f-MWCNT@Mn(bpy)2(H2O)2/Nf, wherein, f-MWCNT = carboxylic acid functionalized MWCNT, bpy = 2,2′-bipyridine and Nf = nafion (cation exchange polymer), as an elegant Pseudocatalase biomimicking functional system for H2O2 disproportionation reaction in neutral pH solution.

The non-heme Catalases (Mn-CAT) family, so-called Pseudocatalases, in which, manganese is an active center, have been known to produce by various kinds of bacteria such as Lactobacillus plantarum, Thermus thermophilus and Thermoleophilum album [24]. The structure of Mn-Catalase (from Lactobacillus plantarum) exhibits a homo-hexamer, wherein, each subunit contains a dimanganese (di-Mn) active site [25]. The di-Mn active site involves in a two-electron catalytic cycle in a manner that is similar to its heme-counterpart found in other Catalases. Although there is no refined structural parameters and electron-transfer feature have been reported, it has been referred that Mn-Catalase enzyme exists in four different oxidation states like a reduced form, Mn(II)/Mn(II); a mixed valence form, Mn(II)/Mn(III); an oxidized form, Mn(III)/Mn(III) and a super oxidized form, Mn(III)/Mn(IV) as key sites for the H2O2 oxidation and reduction reactions (disproportionation reaction) [26]. In general, the peroxide disproportionation reaction follows two step electron-transfer processes at apparent standard electrode potentials, +0.28 (H2O2 reduction) and +1.35 V (H2O2 oxidation) vs Normal Hydrogen Electrode in a neutral pH solution [27]. Based on the literature on the Catalases enzyme biomimetic studies [[27], [28], [29], [30]], it has been claimed that the presence of high-valent metaloxo, peroxo and hydroperoxo species (intermediates) like MnIV/MnIII-O, MnIV/MnIII–OH, MnIV/MnIII–(OH)2, which has Eo′ ~0.65 ± 0.1 V vs Ag/AgCl in pH 7, are responsible for the H2O2 oxidation (H2O2→O2 + 2H++2e) and low-valent metal oxidation states like MnIII/MnII, MnII/MnII has Eo’ ~0.2 V vs Ag/AgCl in pH 7, are responsible for the H2O2 reduction (H2O2 + 2H++2e → 2H2O) reactions of the overall-disproportional step as in the Scheme 1 [29,30]. With this motive, to study the structure, stability factors and reactivity of the high valent Mn oxo-species of the Pseudocatalases enzymes, several synthetic complexes have been reported in the literature [[31], [32], [33]]. It is noteworthy that macro and bulky ligands like corrole and porphyrin [34], which including [Bn-TPEN = N-benzyl-N,N,N′-tris(2-pyridylmethyl)-1,2-diaminoethane [35], dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-2-methyl-quinolin-8-yl-acetamidate and [H3buea]3− = tris[(N′-tert-butylureaylato)-N′-ethylene]aminato [36] have been used as a protecting systems to stabilize the high-valent MnIV oxidation state of the complex. Herein we report, a multi-redox active and stable [MnIV/III/II(bpy) 2(H2O)2]2+ complex that has been modified on GCE/f-MWCNT/Nafion system for elegant electrocatalytic disproportionation reaction of H2O2 in a physiological pH solution.

In the literature, very few manganese complexes based chemically modified electrodes were reported for Mn pseudocatalase enzyme bio-mimicking activity [[20], [21], [22], [23]]. Similarly, scanty attempts were made available to study the biomimetic electron-transfer reactions of the enzyme. For instance, electropolymerized polypyrrole films of polyfluorinated Zn(II) and Mn(III) porphyrins [37], Manganese oxide nanoparticle-cobalt porphyrin based binary catalysts [38], Mn–phenazine complex modified SWCNT [18], Mn based Prussian blue derivative [39] and polydopamine-coated manganese complex/graphene nanocomposite [40] have been reported for the electrochemical reduction of hydrogen peroxide and/or dissolved oxygen in acid or alkaline pH solutions. Meanwhile, cobalt + manganese tetrakis (o-aminophenyl) porphyrin film modified electrode was also reported for electrocatalytic dismutation (disproportionation) of H2O2 mediated by MnIII/II sites (Eo′~0.2 V vs. Ag/AgCl) in 0.1 M H2SO4 medium [41]. Unfortunately, some of the aforesaid modified electrodes suffering from following drawbacks to study as an electron-transfer biomimicking system: (i) electrode surface-fouling, (ii) lack of biomimetic approaches (involve either H2O2 oxidation or H2O2 reduction reaction) [19], (iii) large magnitude of functionalization [40], (iv) dissolved oxygen interference [41] and (v) poor selectivity and non-physiological pH operability [40]. Thus, a Mn complex-based electron-transfer system which can show effective H2O2 electrocatalytic oxidation/disproportionation reaction in a neutral pH solution is highly desirable for the functional biomimetic application. A new [Mn(bpy)2(H2O)2]2+ complex modified functionalized-MWCNT introduced in this work prepared by electrochemical oxidative immobilization of Mn(bpy)2Cl2 on f-MWCNT modified electrode showed an elegant redox peak and electro-catalytic oxidation and reduction responses for H2O2 without any dissolved oxygen influence in pH 7 PBS. In further, as an application, selective electrochemical sensing of H2O2 was successfully demonstrated.

Section snippets

Reagents and materials

MnCl2.4H2O was purchased from SD fine chemicals (India), 2,2′-bipyridyl (>99% purity), carboxylic acid functionalized multiwalled carbon nanotube (f-MWCNT; ~80% purity on carbon basis, > 8% carboxylic acid functionalized, size 9.5 nm × 1.5 μm), multiwall carbon nanotube (MWCNT ∼90% purity on carbon basis, size 7–15 nm × 0.5–10 m), single walled carbon nanotube (SWCNT ∼70% purity on carbon basis, size 0.7–1.1 nm diameter), graphitized mesoporous carbon (GMC, purity assay ≥ 99.95%, <500 nm pore

Electrochemical behaviour of Mn(bpy)2Cl2 on various electrodes

Initial cyclic voltammetry experiment was carried out by drop-casting of a dilute ethanolic solution of the Mn(bpy)2Cl2 complex on unmodified GC electrode in a limited potential window, −0.1 to 1.0 V vs Ag/AgCl in pH 7 phosphate buffer solution. As seen in Fig. 1A curve a, there is no faradaic response of Mn(bpy)2Cl2 on unmodified GC electrode surface indicating the electro-inactiveness of the complex on the solid electrode. Interestingly, when the same experiment was repeated with f-MWCNT

Conclusions

A new GCE/f-MWCNT@Mn(bpy)2(H2O)2/Nf hybrid modified glassy carbon electrode has synergistic property of both Mn(II) complex and MWCNTs was prepared by potential cycling of Mn(bpy)2Cl2 modified f-MWCNT electrode in pH 7 phosphate buffer solution. The modified electrode showed well-defined surface-confined redox peaks at Eo′ ~0.2 V and ~0.65 V vs Ag/AgCl corresponding the MnIII/II and MnIV/III active sites with pH depended electron-transfer feature (tested with MnIV/III site as a model). From the

Acknowledgements

We thank the Department of Science and Technology-Science and Engineering Research Board (DST-SERB) for financial assistance (EMR/2016/002818). N. Saravanan acknowledges the DST-SERB for national postdoctoral fellowship (PDF/2015/000253). ASK also acknowledges the National Taipei University Technology for the support of his distinguished visiting professorship.

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