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
Graphical abstract
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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