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Hybridization of Co3O4 and #-MnO2 nanostructures for the high performance nonenzymatic glucose sensing Lichchhavi Sinha, Srimanta Pakhira, Prateek Bhojane, Sawanta Mali, Chang Kook Hong, and Parasharam M. Shirage ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02835 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Sustainable Chemistry & Engineering

Hybridization of Co3O4 and α-MnO2 nanostructures for the high performance nonenzymatic glucose sensing Lichchhavi Sinha,a Srimanta Pakhira,a Prateek Bhojane,a Sawanta Mali,b Chang Kook Hongb and Parasharam M. Shiragea,* a

Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology

Indore, Simrol Campus, Khandwa Road, Indore-453552. India. b

Department of Advanced Chemical Engineering, Chonnam National University, Gwangju,

500-757. South Korea. *

Author for correspondence: [email protected], [email protected]

Tel. +91-732-4306739 Abstract This work reports a highly sensitive and selective nonenzymatic detection of glucose that has been achieved by hybridization of 1Dα-MnO2 nanorods modified with surface decoration of Co3O4 nanoparticles. The rational design and controlled synthesis of the hybrid nanostructures are of great importance in enabling the fine tuning of their properties and functions. First principles based periodic hybrid unrestricted HSE06 DFT with Grimme’s long-range dispersioncorrections are employed to compute the equilibrium crystal structures and electronic properties (i.e. band structure, Fermi energy level and density of states) of both the materials. These calculations reveal that both the α-MnO2and Co3O4 materials are indirect band gap semiconductor, and the band gap is about 2.89 eV and 3.18 eV, respectively. The α-MnO2/Co3O4 hybrid nanostructure has been synthesized by simple and economical hydrothermal method. Compared with the performances of pure components MnO2 nanorods and Co3O4 nanoparticles, these hybrid nanostructures demonstrated a maximum electrooxidation toward glucose. The

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glucose sensing performances of fabricated hybrid structures were measured by cyclic voltammetry (CV) and chronoamperometry. The synthesized α-MnO2/Co3O4 electrode exhibited a high sensitivity of 127μAmM-1cm-2 (S/N=3), with a detection limit of 0.03μM, wide linear range of 60μM to 7mM of glucose with a short response time of less than 5 seconds. The favorable properties of the nanostructure fortify its potential utilization in the clinical detection of diabetes. Keywords: Nonenzymatic, amperometric, sensitivity, selectivity, DFT-D, electronic properties Introduction The glucose biosensor has been achieved a considerable breakthrough in the field of research because of its simplicity, excellent selectivity, high sensitivity and potential ability for real time and on-site analysis in various fields.1-4 It is mainly classified in two categories: (1) Glucose oxidase (GOD) based enzymatic sensing (2) non enzymatic glucose sensing. GOD, owing to its high sensitivity and selectivity to glucose has been widely used to construct various amperometric biosensors.5-10 Enzymes based sensors might not be suitable for critical condition, for e.g., ecological, food monitoring and bioprocess controller, etc.11 Therefore, enzyme free glucose sensor has persisted a thrust area of research for the reliable and fast determination in the recent years to overcome the shortcomings of enzyme based glucose sensing. The electrode material is considered as an important and crucial parameter for influencing the analytical skills of enzyme less sensors. Carbon, platinum, gold and palladium have been widely investigated as candidates for improving the sensing performance of enzyme less sensors.12-14 Numerous enzyme free glucose sensors have been established on cobalt oxide nanostructures but those displayed poor limit of detection, low sensitivity, week reproducibility, narrow linear range, and complex electrode fabrication process. However, some problems include poor selectivity, high


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cost and low sensitivity due to the surface poisoning by the adsorbed intermediates or chloride ions still remain challenging.15 Some strategies has been followed for its reasonable cost, facilitating the kinetics of the glucose electro-oxidation like preparing various nanostructures,16 increasing active surfaces,17,72 alloying,14 formation of nanocomposites, etc.18-19 Presently nanostructure metal oxides like Zn based oxide,20-22 graphene oxide,23-25 several composites like Ni-Co based oxides26 and Co-Fe based oxides27 has attracted much interest because of its applications in various fields. Though these materials are sensitive, relatively inexpensive and have the advantage of rapid response associated with specific nanostructures (high surface to volume ratio) such as nanowires, nanorods, nanotubes, nanoparticles, nanofibers, etc. It can also be considered as immobilizing matrices for biosensor.28-29 These materials enhances electron transfer kinetics and strong adsorption capability, providing suitable microenvironments for the immobilizations of biomolecules and resulting in enhanced electron transfer and improved bio-sensing characteristics.30 Previously a lot of literature has been reported on non-enzymatic glucose sensors, based on transition metal oxides such as NiO,31-32 CuO,33-34 Co3O4.35 However very limited literatures are available on MnO2 based glucose sensing. MnO2 is an interesting transition metal oxides, having different structural forms (α, β, , δ) based on its tunnel structure. The high surface area and high redox activity of MnO 2 could deliver a high capacity and good catalytic performance.37 It can be used for biosensors research owing to its superior properties like cheap cost and chemical stability. 36 However MnO2 suffers from poor conductivity (10-5 to 10-6 Scm-1). The major strategy for improving the performance is to integrate other conducting materials to achieve the synergistic effect (carbon based materials, metals and oxides) to MnO237. Cobalt oxides has acquired



substantial research interest due to its diversity of oxidation states for charge transfer and high energy and power density.37 Cobalt oxide is a transition metal oxide with intriguing electronic, optical, electrochemical and electro-catalytic properties.19 Co3O4 based nanocomposites have demonstrated great potentials in several applications up to now. Combined structure of MnO2 with Co3O4 contains high electron conductivity. Co3O4 provides a conducting network for MnO2 such as high surface area (surface to volume ratio), chemical stability and high electrical conductivity and offer high redox features that enhance the direct electron communication through the target analysis. To the best of our knowledge the application of MnO2/Co3O4 has been rarely explored in glucose sensing application. In the present article, we have synthesized α-MnO2/Co3O4 hybrid nanostructures on carbon paper as an electrode material for nonenzymatic amperometric glucose detection by simple hydrothermal process. The Co3O4 nanoparticles were homogenously coated on the α-MnO2 nanorods which were considered as a favourable for the glucose oxidation by providing active sites to the glucose, to get oxidized. The synergistic and interfacial effects between the α-MnO2 nanorods and Co3O4 nanoparticles lead to the enhancement in its physical as well as chemical characteristics. The hetreostructures of α-MnO2 and Co3O4 possesses huge structural importance by demonstrating its tendency to be oxidized and also by offering augmentation or the process of strengthening each other. For comparison we have also synthesized α-MnO2 and Co3O4 separately and have compared the result. Thus, the α-MnO2/Co3O4 heterostructure reveal its potential application for the non-enzymatic biosensor with high sensitivity and low cost, most suitable for practical applications, the detailed results are represented in this manuscript.


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Experimental Section Synthesis of 1D α-MnO2 nanorods α-MnO2 nanorods were synthesized using simple hydrothermal method as reported earlier with some slight modifications.38 Details are provided in the supplementary information(ESI). Synthesis of α-MnO2/Co3O4 hybrid nanostructures For the growth of hybrid nanostructures of α-MnO2/Co3O4, 50 mg of as synthesized αMnO2 nanorods were dispersed in 35 ml of water (deionized water) followed by 30 minutes sonication and 15 minutes stirring to prepare a homogenous suspension. 1mM Co(NO3)2·6H2O was added while stirring to the above suspension and were stirred for additional 30 minutes. Then drop wise ammonia solution was added by maintaining the pH~11 of the suspension. Subsequently, the solution was transferred to a Teflon lined stainless steel autoclave of capacity 100 ml and placed at 150°C temperature for 12 hours in a muffle furnace. The dark brownish product was collected by filtration (using Whatman filter paper) and washed with deionized water (5 times), ethanol (3 times) to remove impurities possibly remaining in the final products. The sample was obtained after drying at 80°C for 12 hours in an aerobic environment. Co 3O4 nanoparticles has also been synthesized by taking 1mM Co(NO3)2·6H2O in 35 ml deionized water followed by the same procedure as in α-MnO2/Co3O4. Electrode fabrication for amperometric glucose sensing:



1 mg each of synthesized α-MnO2 and α-MnO2/Co3O4 and Co3O4 powders were taken in three different tubes. They were sonicated in ethanol for about 20 minutes to make slurry at room temperature. A drop of Teflon suspension (5% Teflon in ammonia) was added as a binding agent. The suspension was drop casted over a 0.8 x 0.8 cm2 carbon paper and left for drying for 2 hours at 80° C in a furnace. The ultimate masses were determined using a balance by the mass difference before and after mass loading. The estimated loaded mass was ~1mg. The schematics for the electrode fabrication are given in the fig.1 (b).

Fig.1 (a)Schematics of α-MnO2/Co3O4 morphology evolution, (b) electrode fabrication for glucose sensing.


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Computational Details The equilibrium crystal structures including optimized unit cell geometries and material/electronic properties (i.e. band structures and density of states calculations) were obtained with the hybrid periodic HSE06 density functional theory (DFT). All the periodic structures as well as properties calculations were performed by ab initio CRYSTAL17 suite code.39-40 This CRYSTAL17 code uses Gaussian type basis sets (Gaussian type orbitals i.e. GTO) rather than plane wave function. 39-40

Here it is

essential to account for the van der Waals (vdW) dispersion effects as there are many weak long-range vdW interactions between the atoms as well as the layers in the crystals.41-44 Thus, Grimme's semi-empirical third order (-D3) dispersion corrections were included in the present DFT computations to account all the long-range vdW interactions. A very accurate geometry and thermochemistry can be obtained by the DFT-D functional when the systems are covalently bonded and weakly bound vdW dominated by longrange dispersion forces.42-44 To specify the spins of the Co and Mn atoms in the crystal structures, unrestricted Scuseria’s DFT, UHSE06-D3 (Grimme’s long-range dispersioncorrected Heyd, Scuseria, and Ernzerhof functional), 45-46 was used in all the calculations of the aforementioned pristine Co3O4 and α-MnO2 materials. For simplicity, HSE06-D3 method was noted as DFT-D in the manuscript. To specify the Gaussian type of atomic orbitals of the atoms, triple-zeta valence with polarization quality (TZVP) basis sets were used for O, Cr, and Mn atoms in the crystal structure calculations for all the systems studied here,47 and the convergence threshold was set at 10 -7 a.u. to evaluate the convergence of the energy, forces, and electron density. The spin contamination effects are reduced in the present computation as the DFT-D method (UHSE06-D3) provides



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quite good quality geometry for the crystal structure and electronic properties also, and the density as well as energy are less affected in the computations. 48-51 After obtaining the optimized crystal structure, the electronic properties (i.e. band structures and density of states (DOSs)) of both the Co3O4and α-MnO2 materials were computed at the same level of theory i.e. using HSE06-D3 method. A set of (8x8x8) Monkhorst-Pack52 k-point grids was carried out for geometric optimization, and similarly a set of (20x20x20) Monkhorst-Pack k-mesh grids was used for the materials/electronic properties calculations i.e. calculations of band structure and DOSs calculations. These kpoints grids are in general used for the integrations inside the first Brillouin zone. The atomic orbitals of the Cobalt (Co) and Oxygen (O), and Manganese (Mn) and Oxygen (O) atoms were used to compute and plot the total DOSs of the pristine Co 3O4 and α-MnO2 materials, respectively. The VESTA, a visualization code, was used to create the graphics and analysis of the crystal structures of all the systems studied here. 53 Result and Discussions Morphology evolution and growth mechanism of the nanostructure Fig. 1(a) shows the growth schematics of hybrid nanostructure of α-MnO2/Co3O4 and its morphological evolution. To grow 1D α-MnO2 nanorods, initially the precursor in double deionized water in the presence of strong hydrochloric acid was kept in an autoclave at a temperature 180°C, oxidized it into manganese oxide with some water molecules [reaction (1)].54 After formation of the MnO2 nuclei at the initial stage, the crystal phase of MnO2 nuclei is mainly determined by the K+ ion concentration. The concentration of K+ ions strongly influences structures/phases of MnO2.

KMnO4 is

enough in solution to stabilize the (2x2) tunnel structure leading to the formation of α-


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MnO2.55 The synthesized MnO2 powder was dispersed into deionized water followed by addition of cobalt nitrate (1mM) as a precursor. The reaction medium was made alkaline by adding ammonium hydroxide (NH4OH) solution, The NH4OH reduces to form NH4+ and OH- [reaction (2)]. We need an alkaline medium for the reaction using ammonium hydroxide. At pH 9~10, we got the precipitation, and the nucleation were initiated at this pH condition. Further addition of ammonia solution reduces the Co2+ ion concentration by producing the complex ion Co(NH3)x2+. So, at pH~11, the bivalent metal ions of Co2+ will be fully coordinated with NH4+ ions that come from NH4OH to form Co(NH3)x2+ ions [reaction (4)].56-57 Therefore, during the precipitation process the numbers of nucleation sites were created, and the number of nucleation center is limited as well as the growth rate of the Co(OH)2 is slow due to the existence of Co(NH3)x2+ ions which leads to the formation of Co3O4 particles [reaction(5)]. The cobalt hydroxide upon reaction with the MnO2 forms MnO2.Co3O4 as a final product. The plausible chemical reactions are shown below: Mn

H2

H l

Mn

2.H2

(a )

l

H2

(a )

1

Structural and Phase confirmation by X-Ray Diffraction (XRD) Fig. 2(a) shows the geometrical structure of α-MnO2 having arrangements of double chains of [MnO6] octahedral forming (2x2) tunnel structure. This structure is anticipated to be the most



favorable structure for electrochemical studies, it provides efficient electrochemical surface by maintaining tunnels size and interlayer separation between the [MnO6] octahedron.58 Crystallinity and crystal phases of the prepared samples were investigated by X-ray diffraction (XRD) as shown in fig. 2 (b-c). The XRD patterns represent the phase pure materials without any impurities. All the peaks in fig. 2(c) can be indexed to a pure tetragonal phase of the of α-MnO2 [space group-I4/m(87)] [JCPDS 44-0141]. The estimated lattice parameters for the α-MnO2 are a=b=9.787(6) Å, c= 2.865(0) Å. For hybrid α-MnO2/Co3O4, the diffraction peaks at 36.85°, 38.54°, 65.2° can be indexed to (311), (222) and (440) planes of Co3O4 [JCPDS 42-1467, space group Fd-3m (227)], respectively. The peaks of Co3O4 showed obvious low intensity, indicating that decorated Co3O4 have a very small crystallite size compare to α-MnO2.59 The XRD pattern of Co3O4 nanoparticles is provided in Supplementary information Fig S1[ESI]. It shows the phase purity of the Co3O4 nanoparticles without any impurity.


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Fig. 2(a) (2x2) tunnel structure of α-MnO2. XRD patterns of the as prepared (b) αMnO2/Co3O4 hybrid and (c) α-MnO2 nanorods.



X-Ray Photoelectron Spectroscopy (XPS) The materials compositional and oxidation state validation of the prepared hybrid structure was done by using X-ray photoelectron spectroscopy (XPS). The full survey spectrum of MnO2 (Fig. 3(a)) shows distinctive XPS peaks of manganese, oxygen and carbon as their auger peaks whereas the survey spectrum of MnO 2/Co3O4(Fig. 3(b)) shows distinctive peaks of cobalt, manganese, oxygen and carbon. Fig.3(c) shows the Mn2p spectra of MnO2/Co3O4. Two distinctive peaks having spin orbital splitting of 11.5 eV at binding energies 642.1 and 653.5 eV in Mn2p spectrum signifies Mn2p3/2 and Mn2p1/2 in α-MnO2/Co3O4, respectively. The deconvolution of complex Co2p spectrum in fig. 3(d) shows the presence of two distinctive peaks i.e., Co2+ and Co3+ respectively. Particularly two peaks at 779.9 eV and 795.1 eV, signified as 2p3/2 and 2p1/2 of Co3+. Peaks at 803.5 eV and 788.9 eV corresponds to 2p3/2and 2p1/2 of Co2+ respectively.37 Two small peaks at 786.3 eV and 804.8 eV are Co2+ shake-up satellite peaks of Co3O4. The energy difference between the peak of Co2p 3/2 and the peak of Co2p1/2 is approximately ~15 eV. This data provides direct evidence for the presence of Co3O4 phase in the as synthesized α-MnO2/Co3O4 hybrid structure. The XPS O1s spectra (Fig. 3(e)) consist of at least three components. The peak at about 529.6 eV is due to oxygen in the Co 3O4 and αMnO2 crystal lattice, a value typical for cobalt oxide networks, while the peaks at about 531.4 eV and 532.6 eV are due to chemisorbed oxygen caused by surface hydroxyl. 60


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Fig. 3 Survey scan spectrum of XPS of (a) α-MnO2 and (b) α-MnO2/Co3O4 hybrid. The deconvoluted peak of (c) Mn2p (d) Co2p and (e) O1s of α-MnO2/Co3O4 hybrid.

To evaluate the electronic structure of both the α-MnO2 and Co3O4 materials, periodic hybrid DFT-D (here HSE06-D3) methods have been employed. The electronic structure calculations with the properties i.e. band structures and density of states (DOS) are shown in Figure 4 and 5, respectively. The lattice constants (a, b, c) and symmetry of both the α-MnO2and Co3O4materials are a=b=9.615 Å, c=2.863 Å and space group symmetry I4/m(87) of the former one and a=b=c=8.053 Å, c=2.863 Å and space group symmetry Fd-3m(227)of the second one, which are well harmonized with our experiment. Our DFT-D calculations found that both the materials have an indirect band gap around 2.89 - 3.18 eV as shown in band structure and DOSs calculations (see Fig 4(c-d) and 5(c-



d)). The computed band gaps (2.89 and 3.18 eV) of both the α-MnO2 and Co3O4 materials are in accord with the previous experimental values. Mulliken charge analysis were performed in the present computation and these calculations indicated that both the ions of the Co atoms (i.e. Co2+ and Co3+) exist in the crystal structure i.e. coexistence of both the ions Co2+ and Co3+are available in the crystal which agrees well with our experiment.


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Fig. E uilibrium crystal structure of α-MnO2 (a) conventional unit cell, (b) primitive unit cell. The band structure and density of states (DOSs) are shown in (c) and (d).

Fig. 5 Equilibrium crystal structure of Co3O4 (a) conventional unit cell, (b) primitive unit cell. The band structure and density of states (DOSs) are shown in (c) and (d).



Transmission electron microscopy (TEM) studies The field emission microscopy was carried out, and the images showed the rod like morphology as shown in supplementary information (ESI) Fig. S2. The decoration of the Co3O4 is clearly displayed in Fig. S2-(d). The more detailed investigation of α-MnO2 and α-MnO2/Co3O4 were carried by the transmission electron microscopy (TEM) and the images are presented in fig. 6(a) confirm the formation of nanorods with diameter ~45nm and length of several micrometers. HRTEM analysis was carried out to confirm the crystallinity and morphology of the nanorods. The selected area electron diffraction pattern (SAED) taken from the α-MnO2 nanorods in fig. 6(b) confirms the single crystalline nature of the prepared sample. Fig 6(c) shows the lattice fringes of α-MnO2 nanorods, from where the inter-planar spacing of the nanorods was found to be 0.50 nm, corresponds to the (200) plane of α-MnO2 which is also confirmed by the XRD data (fig. 2). Similarly, the TEM images shown in fig 6(d), (e) and (f) clearly demonstrate the formation of Co3O4 nanoparticles uniformly distributed over the α-MnO2 nanorods. The lumpy surfaces besides the well-defined 1-D morphology reveal the decoration of isolated Co3O4 nanoparticles upon 1-D α-MnO2 and partly exposure of MnO2 surface. The interplanar spacing of the lumpy spots fringe was 0.25 nm corresponding to the (200) plane of Co3O4 nanoparticles over the 0.50 nm (200) plane of α-MnO2 nanorods. Thus, the SAED pattern having diffraction spots of the plane (200), (400) and (600) of α-MnO2 and (311), (222) of Co3O4 confirms the combination of both the species. For further confirmation regarding the morphology and interaction of both the species α-MnO2 and Co3O4, details materials characterizations are presented in the supplementary information [ES1] (i.e., Fig S2-FESEM and S3-FTIR).


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Fig. 6 (a) TEM images of α-MnO2nanorods (inset) HR-TEM image of α-MnO2nanorods, (b)SAED pattern of α-MnO2nanorods. (c) Lattice fringes of α-MnO2nanorods. (d)TEM images of α-MnO2/Co3O4 hybrid, (inset) HR-TEM image of α-MnO2/Co3O4nanorods. (e) SAED pattern of α-MnO2/Co3O4nanorods. (f) Lattice fringes of α-MnO2/Co3O4nanorods

Electrochemical studies for amperometric glucose sensing The electrochemical properties of the synthesized electrodes, α-MnO2, Co3O4, α-MnO2/Co3O4 on carbon paper and bare carbon paper were studied in 50 mM of NaOH solution in a glass cell. Cyclic voltammetry (CV) experiments were performed in the potential range of -0.1 to 0.6 V at 10 mV/s scan rate to observe the electrocatalytic activity of the nanorods. The electro-oxidation



of glucose on α-MnO2, Co3O4 and α-MnO2/Co3O4 are displayed in fig 7(a). The α-MnO2 reveals the absence of redox peaks, showing typical rectangular shapes, revealing the surface faradaic reaction of the synthesized samples; i.e., the surface electrosorption of Na+ cations and the fastreversible successive surface redox reactions of the electrode by means of interaction/extraction processes of the proton. The quasi-rectangular CV curves indicate the presence of adsorption and desorption processes at the interface of electrode/electrolyte. Moreover, no characteristic redox peaks were observed in the electrodes; indicating that the electrode is displaying a pseudoconstant rate over the complete voltammetric cycle 61. To ascertain the effect of scan rate in CV, measurement was also carried out at different scan rates (10, 20, 30, 40 mV/s) and shown in fig. 7(b), however no redox peaks were appeared. Addition of glucose in the electrolyte, showing negligible peak and can be analyzed as very less response of glucose on α-MnO2. The CV curve of Co3O4 (curve 3), one set of redox peaks can be observed at 0.37 V (oxidation). The CV (curve 4) of α-MnO2/Co3O4 nanomaterial was also investigated in an alkaline solution (50mM NaOH) in the range of -0.1 V to 0.6 V vs. Ag/AgCl, a single well defined and high intensity redox peak (oxidation) at 0.25 V was observed in an alkaline solution Fig.7(a) which can be assigned to the reversible transition between Co3O4 with CoOOH[equation (7) & (8)]. This phenomenon indicates the involvement of OH- in the electrochemical redox reaction of Co3O4. We can see that the oxidation potential of the hybrid αMnO2/Co3O4 electrode has changed with respect to α-MnO2 and Co3O4. So, this shift in negative direction is beneficial for us because it might have arisen from the enhanced contact area and large electron transport channels in the modified α-MnO2/Co3O4 electrode material. This can assist the molecules and ions involved in the glucose oxidation of the system to approach the active sites. As a result, this effect decreases the energy required for the reaction and accelerates


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the reaction dynamics74. The synergistic effects of both the Co3O4 and α-MnO2 owned the higher catalytic activity. MnO2 has unique rod like architecture provides high specific area as shown by BET result. It also improves the conductivity by decreasing the electron transfer resistance by posing a typical rectangular curve behavior. The CV curve behavior of the hybrid nanostructure suggests that Co3O4 supplies a large double layer electron transport mechanism whereas MnO2 acquire large pseudo electron movement mechanism. This effect here causes fast electron transport from the electrolyte to the electrodes. Stability of the well dispersed Co3O4 nanoparticles over MnO2 and high active surface area provides channels to enhance the mass transportation in electrochemical reaction 62,72,73,75. The CV curve of the bare carbon paper substrate was also measured for comparison (to know the effect of carbon paper); a much smaller area under the curve and current value observed in the CV curve (curve-1). The schematics of the glucose sensing mechanism is represented in fig. 7(c) and the plausible reactions involved for the electrode α-MnO2/Co3O4 are as following: The corresponding reactions at each oxidation and reduction point are as following: For the curve 3 and 4 i.e., Co3O4 and α-MnO2/Co3O4, there are two peaks: [6] [7]

The peaks corresponding to each redox couple are not evidently distinguishable, and some are elusive, might be due to surface modification of the Co 3O4 nanostructure63



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[10]

(a) 2

(b)

3

1 0.25 V

Increasing scan rate

(c)

Fig. 7(a) Comparison of cyclic voltammograms of (1) carbon paper(background) (2) αMnO2 (3) Co3O4 and ( ) α-MnO2/Co3O4 in 50 mM NaOH at a scan rate of 10mV/s. (b)


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yclic voltammogram of α-MnO2/Co3O4 hybrid nanorods at different scan rates in 50mM NaOH. (c) Mechanism of electrolytic oxidation on glucose α-MnO2/Co3O4 hybrid nanostructure in the presence of NaOH

The surface area and pore size distribution results from N2-adsorption desorption measurement (BET and BJH analysis) confirmed the mesoporosity, are displayed in the supplementary information Fig. S4. As shown in FESEM images fig. S2 demonstrates the well interconnected nanorods of α-MnO2/Co3O4 hybrid nanostructure which enhances the mobility of electrons in the whole body of nanorods. The augmented amount of electron movement through tunnels of α-MnO2 enhances the delocalization of electrons resulting higher electrochemical performance of prepared α-MnO2/Co3O4 hybrid nanorods.19 The glucose which has been adsorbed on the surface of electrode accelerates further access of analyte on its catalytic active sites for the electrochemical reaction to occur and enhances familiar contact of the electrode and electrolyte with a short diffusion resistance. The high-rate diffusion of analyte (glucose) into the electrode surface oxidized it into gloconolactone because of the role of Mn and Co ions at α-MnO2/Co3O4, leading to an increase in Ipa in an alkaline solution. The increased electrooxidation of glucose which is observed at α-MnO2/Co3O4, is ascribed as the synergistic effect of α-MnO2 and Co3O4 that prompted the electrical conductivity and number of active sites. Fig. 8(a) depicts the V of α-MnO2/Co3O4 as a consequence of various increasing concentration of glucose in 50mM NaOH, measured at fixed scan rate of 20mVs -1. The electrocatalysis of analyte in the favorable (positive) sweep is justified from the noticeable increment in anodic peak current (Ipa) with a very slight shift in the potential by the action of successive addition



of glucose from

0μM to 7mM. The XPS measurement of the modified electrode, before

and after the addition of glucose has been performed given in fig. S7[supplementary information and it depicts no prominent shift in the peak’s binding energy of Mn2p, Co2p and O1s. It clearly demonstrates the diffusion-controlled reaction and the reversible nature of the mechanism. This enhancement in anodic peak current with addition of glucose in glucose sensing cell clearly demonstrates the electrocatalysis of glucose by αMnO2/Co3O4 nanostructure. Additionally, the calibration curve plotted using the data set of Fig. 8(a) is represented in fig 8(b). The curve shows the linear increase in the Ipa with increase in glucose concentration, which reveals the excellent capability of electrocatalytic mechanism of α-MnO2/Co3O4 in glucose oxidation. Fig 8(c) depicts the

V of α-MnO2/Co3O4 at different scan rates

(10mVs-1 – 40mVs-1) measured in 7mM glucose in 50mM NaOH. The Epa and Epc (anodic potential and cathodic potential as shown in (inset) fig. 8(c) were slightly shifted towards negative and positive values as a function of increase in scan rates. The calibration curve shows the linear relation between the Ipa and scan rate in fig. 8(d), implies the typical diffusion controlled electrochemical process. Electrochemical impedance spectroscopy (EIS) was performed to analyze the catalytic action of glucose on the electrode of α-MnO2/Co3O4 and shown in fig. 8(e). The Nyquist plot of the electrode consists of depressed overlapping capacitive semicircle in the high frequency and a linear region in low frequency sides. Nyquist plot for the electrode has been obtained in bare 50mM NaOH solution, 5 mM and 7mM glucose in the parent NaOH solution. The diameter of the semicircle in the high frequency region is related to the charge transfer resistance (Rct) of the redox couple in the existence of glucose, the


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linear portion is attributed to the diffusion limited process. 33 Modification in the electrolyte after the addition of 5 mM and 7mM glucose leads to a smaller semicircle and decreased Rct which is indicative of the efficient mass and charge transfer. This can be described as a uniform and high mass loading distribution of α-MnO2/Co3O4 on carbon paper which is creating a necessary conducting path of electrons.



Figure 8: (a) Cyclic voltammograms of α-MnO2/Co3O4 in 50mM NaOH at 20mVs-1 scan rate (b) Calibration plot of peak current vs. glucose concentration. (c) Cyclic voltammograms of αMnO2/Co3O4 as a function of scan rate for 7mM glucose in 50mM NaOH. (d) Calibration plot of peak current with respect to scan rate.(e) EIS plot of α-MnO2/Co3O4 electrode in NaOH with 5mM and 7mM glucose addition.

Amperometric detection of glucose by α-MnO2/Co3O4

Figure 9: (a) Amperometric current, I(mA) vs time T(s) response of α-MnO2/Co3O4 with increasing glucose concentration in 50mM NaOH at a constant applied potential of 0.55V (vs Ag/AgCl). (b) Current vs. glucose concentration calibration plot of α-MnO2/Co3O4 Amperometric glucose sensing is the simplest and economic technique for estimating the sensitivity. The rapid oxidation of glucose in amperometric makes this technique most suitable for the glucose sensing application with high sensitivity and detection limit due to fast response time. In the current experiment, the glucose addition in 50mM NaOH


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solution is measured at a fixed applied potential of 0.55V vs Ag/AgCl and the obtained curve is represented in fig. 9(a). The amperometric study was carried out using αMnO2/Co3O4 nanostructure with incremental glucose quantity in the cell, resultant current enhanced monotonically, indicating an excellent candidate for glucose sensing. The precise steady state current response towards glucose was found in the ranges of 60μM to 7mM with a fast response time of less than 5 seconds. The advantage of hybrid network of α-MnO2/Co3O4 nanorods causes an enhancement in the number of favourable sites and electron conductivity in the electrode which further ensures an excellent electrocatalytic feature of glucose. This denotes the excellent electrocatalytic behaviour of glucose as a function of number of favourable sites which enhances the electron conductivity of related hybrid network of α-MnO2/Co3O4 nanorods. Fig. 9(b) shows long range linearity from 60 μM to 7mM glucose measured in 50mm Na H. The calibration plot measured in the range of 60 μM to 7 mM fits the e uation of linearity with; I(mA)= 0.13087 + 0.08138 C (mM), Where, 0.130 7 is the intercept, 0.0 13 the slope, ‘ ’ symbolizes concentration of the glucose and ‘I’ the resultant current. Sensitivity is calculated using the e uation: 65 (

)

]

The calculated sensitivity of biosensor is 127 μAmM-1cm-2 (S/N=3) using area of 0.64 cm2, which is 3-4 times higher than MnO2 based bio-sensor reported so far. The lowest limit of detection from the calculated sensitivity value is 0.03μM. For comparing the



performance, the amperometric measurement of the synthesized Co 3O4 electrode has also been measured. The obtained value of sensitivity calculated from the above-mentioned e uation (11) is 0.136 μAmM-1cm-2. The steady state current was found up to 700μM. It is found that the current response reaches saturation gradually in Co 3O4 nanoparticles, indicating that all active sites of the electrode are covered with reaction intermediates at high concentration of glucose in Co3O466. The amperometric current response of the Co3O4 electrode after successive injection of glucose and its calibration curve are given in supplementary information (ES1) Fig.S5 (a & b). These comparatives obtained value clearly showed the synergistic effect of the hybrid structure electrode α-MnO2/Co3O4. It has a major attribute of a glucose sensor, having long range linearity, good response time, high sensitivity and a lower detection limit compared to α-MnO2 and Co3O4. The comparison for the performance of some other materials based on α-MnO2, its composite and Co3O4 are listed in Table 1. This table shows that the current biosensor has 3-4 times higher sensitivity with excellent limit of detection and comparatively longer linearity range order.


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Electrode materials

Linear range (in mM)

Sensitivity (μAmM-1cm2 )

Detection limit

MnO2/MWNT

0.01-28

33.19

0.03μM

References (Publishing Year) [29], 2015

Cu/MnO2

2.5x10-4 -1.02

26.96

0.02μM

[66], 2014

Pt/Au-MnO2

0.1-3.0

58.54

0.02μM

[18], 2013

GOx/MnO2

upto 3.15

31.6

-

[68], 2013

Co3O4 Nanofibers

upto 2.04

36.25

0.97μM

[70], 2010

MnO2 nanowires (enzymes based) MnO2/CNT-air plasma

upto 2

-

1. μM

[71], 2016

0.1-3.2

24.2

3.0mM

[69], 2016

MnO2/Co3O4

upto 7

127

0.03μM

Our Work

Table1: Comparison of other reported MnO2 and Co3O4 based material.



Interference, Long term Stability and reproducibility of the sensing electrode αMnO2/Co3O4

Fig.10: (a) Response of α-MnO2/Co3O4 in NaOH and after the addition of interfering species 0.125mM ascorbic acid, 0.33 mM uric acid and 4mM glucose. (inset) Bar graph for the responses generated by addition of endogenous species (b) Amperometric glucose sensing of two different electrode of α-MnO2/Co3O4 of same batch. (inset) stability test of electrode after 7 days exposure to air.

One of the major challenges in non-enzymatic glucose analysis is to eliminate the interference responses generated by some endogenous species such as Ascorbic acid (AA), Uric acid (UA), etc. In the psychological condition the level of glucose (3-8mM) is much higher than that of these species (

Hybridization of Co3O4 and Î±-MnO2 ... - ACS Publications

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