MnO2

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B: Biophysics; Physical Chemistry of Biological Systems and Biomolecules

Improved Non-enzymatic Glucose Sensing Properties of Pd:MnO2 Nanosheets: Synthesis by Facile Microwave Assisted Route & Theoretical Insight from Quantum Simulations Rajeswari Ponnusamy, Abhijeet Sadashiv Gangan, Brahmananda Chakraborty, Dattatray J Late, and Chandra Sekhar Rout J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01611 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

Improved

Non-enzymatic

Glucose

Sensing

Properties of Pd:MnO2 Nanosheets: Synthesis by Facile Microwave Assisted Route & Theoretical Insight from Quantum Simulations Rajeswari Ponnusamy,1 Abhijeet Gangan,2 Brahmananda Chakraborty,2,* Dattatray J. Late,3 and Chandra Sekhar Rout 1,* 1

Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagaram, Bangaluru-562112, India 2 High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India 3 Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune - 411008, India Email: [email protected] (BC) Email: [email protected], [email protected] (CSR)

ABSTRACT: The electrocatalytic properties of manganese oxide (MnO2) can be improved significantly by making hybrids/composites with noble metals (Au, Pd). Here, efforts have been made to synthesize the MnO2/Au and MnO2/Pd nanocomposites by a facile, rapid microwave irradiation method. The products characterized by X-ray diffraction and Transmission electron microscopy exhibited its tetragonal phase and the nanosheet morphology. The efficiency of the prepared composite materials as glucose sensor was tested by cyclic voltammetry and chronoamperometry measurements and the results are discussed. The study revealed that successful modification of MnO2 by Pd led to excellent sensing performance by the reduction of size and the synergistic effect between MnO2 and PdO which expedites the electron transfer. Besides, the wide detection range, good selectivity and stability demonstrating its robustness in the design of electrochemical sensor platform. In

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order to get theoretical insight on the excellent sensing performance of MnO2/Pd, we have performed detailed Density Functional Theory (DFT) simulations to explore the charge transfer and bonding mechanism of glucose on MnO2 and Pd/Au doped MnO2 surface. Pd is bonded strongly on MnO2 and makes MnO2/Pd more conducting due to enhancement of density of states near Fermi level. The higher binding energy of glucose and enhanced charge transfer from glucose to Pd-doped MnO2 compared to bare MnO2 infer that Pd-doped MnO2 possess superior charge transfer kinetics resulting higher glucose sensing performance which support our experimental observations.

1. INTRODUCTION Fabrication of composite nanomaterials, integration of nanosized particles with distinctly different physical and chemical behaviour into a standard matrix, has captured considerable attention among the scientific community due to their advanced functionality in widespread applications.1 Nanocomposites composed of metal oxides (ZnO, Fe2O3, TiO2, CeO2 and SnO2) and noble metals (Ag, Au, Pt and Pd) are recognized as one of the attractive hybrids among the all.2-5 These hybrid materials have been effectively utilized in the solar energy conversion, photocatalysis, environmental remediation applications and are synthesized mainly from routine chemical methods with different geometries.6 However, preparation of metal oxidenanocomposites by the conventional methods rarely can ended up in the noble metals of neutral charge state to any of its aliovalent oxidized form. As discussed in the recent review by Liu and his co-workers,7 based on the geometrical configuration, metal oxides with noble metals as supporting materials are classified into five categories: (i) noble metal decorated metal oxide NPs (ii) noble metal-decorated metal oxide nanoarrays (iii) noble metal/metal oxide core/shell nanostructures (iv) noble


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metal/metal oxide yolk/shell nanostructures and (v) Janus noble metal–metal oxide nanostructures. Compared with other composite structures, noble metal decorated on metal oxide surface with large surface area is expected to be a perfect biosensor platform due to its surface synergistic effect. Due to their higher electroactivity and cost effectiveness, transition metal based nanomaterials are advantageous for the effective biosensing applications.8 Better nonenzymatic glucose sensing performance of metal oxides can be achieved by making their hybrids with noble metals like Pt, Pd and Au.2,

7-10

The enhancement in the

nonenzymatic electrocatalytic activity towards the detection of different analyte were realized by several researchers.7-10 For instance, one-dimensional nanocauliflower structured Au/CuO composite showed a high sensitivity for the detection of glucose.9 Au decorated on ZnO nanostructures showed enhanced sensitivity for glucose sensing under the visible light illumination due to the surface plasmon resonance effect.10 PtCuO nanocomposite showed improved sensing activity.11 Nayak et al. reported that the glucose sensing properties of NiCo2O4 nanosheets could be improved significantly by electrodeposition of Pd nanoparticles on it. Atiweena et al. reported the enhanced sensing performance of RGO after Pd decoration.13 Manganese dioxide (MnO2), abundant in nature as pyrolusite, with three different polymorphs (α, β, γ) has been studied enormously in the fields of catalysis, lithium ion batteries and magnetic storage. The important feature of the MnO2 relies on its MnO6 octahedron arrangement where the mesoporous channels form during the stacking of octahedrons by sharing the edge and corner oxygen atoms.14 Among all, with a relatively large size of the tunnels (0.48 nm), α-MnO2 polymorph has been extensively investigated in industrial applications as its intrinsic properties can be easily tuned by doping with suitable elements. As an example, Tseng et al. noted the



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antiferromagnetic to ferromagnetic ground state change in α-MnO2 during alkaline metals doping prepared by hydrothermal synthesis.15 The flaws in energy storage performance like low power density and low cycle life of MnO2 can also be modulated by the combination of graphene/activated carbon for its potential use in flexible solidstate supercapacitor.16 Further, Yang et al.17 revealed the anti-tumor immune response in a biodegradable hollow MnO2 which can also serve in tumor-microenvironmentspecific

imaging

and

on-demand

drug

release.

Three

dimensional

MnO2

nanosheets/carbon foam composite was fabricated by Shuijian He et al.18 which is a capable electrode material for the detection of H2O2. It was also found that the C3N4/MnO2 composite photocatalyst can be used for water splitting applications.19 Hollow bipyramids of MnO2 prepared via a template-free hydrothermal method by Zhan et al.20 was studied for lithium ion battery application. Tsubasa et al. reported that Mo-doped α-MnO2 can act as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation.21 In the biosensing domain, both enzymatic and non-enzymatic electrochemical behaviour of MnO2 for different bio species sensing were explored by several researchers. Predominantly, MnO2 based materials like MnO2–HBCs, MnO2/graphene composite, hierarchical MnO2 spheres, MnO2 nanosheets, MnO2/MWCNTs, Ni/ MnO2, Cu/MnO2 are reported to manifest the glucose sensing behaviour.22-28 But, the synthesis of noble metal/MnO2 nanocomposite by direct microwave irradiation route and its electrocatalytic properties are not completely disclosed yet. Microwave heating is not only a promising technique for the synthesis of nanomaterials but is also adopted in other areas like solid-state chemistry, nanotechnology and organic synthesis. Over conventional heating methods, microwave irradiation offers several advantages like instantaneous and rapid heating during the material synthesis.29


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Henceforth, in the present work, microwave heating technique @ 720 W has been employed for the synthesis of bare and noble metal (Au and Pd) nanocomposites of MnO2. The resulted products were systematically analysed through cyclic voltammetric and chronoamperometric measurements for its effective utilization in the electrochemical glucose sensing applications. The overall results from the structural and electrochemical characterizations depicts that the oxidation of glucose by MnO2 system is boosted up by the noble metal insertion and the results are discussed elaborately in the following sections. Density Functional Theory (DFT) simulations have also been carried out for the interaction of glucose on MnO2 and Pd/Au doped MnO2 surface to qualitatively support our experimental observations.

2. EXPERIMENTAL SECTION 2.1 Preparation of MnO2/noble metal nanocomposites Manganese chloride tetrahydrate (MnCl2.4H2O) and potassium permanganate (KMnO4) were used as the starting materials for the synthesis of MnO2 by the microwave irradiation process. To which, 4.68 mM of MnCl2.4H2O and 3.12 mM of KMnO4 were quickly weighed and dissolved in 40 ml of deionized water followed by sonication for 0.5 h. The obtained homogeneous solution was kept for irradiation in a domestic microwave oven at 720 W for 5 minutes. After cooling naturally, the formed MnO2 nanopowder was collected via vacuum filtration and rinsed with double distilled water and ethanol several times for purification. Air-dried powder at room temperature was further annealed at 500° C for 6 hrs. MnO2-metal nanocomposites were also prepared by the same procedure by adding 5 wt.% of each metal precursors [sodium tetrachloroaurate (III) dihydrate (NaAuCl4.2H2O), palladium (II) chloride (PdCl2), for Au and Pd respectively] before keeping the manganese solution for sonication.



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The products were characterized by X-ray diffraction on a Bruker D8 Advanced diffractometer with Cu Kα X-ray source to confirm the crystal structure and phase. Surface morphology and the elemental compositions were examined using Transmission electron microscopy (TEM, JEOL-2100, acceleration voltage-200 kV) with

Energy

Dispersive

X-ray

spectroscopy

(EDAX).

X-ray

photoelectron

spectroscopy (XPS) measurement was done in VG Microtech, England (MultiLab, ESCA-3000. Sr. no-8546/1) with Al kα X-ray source under the ultra-high vacuum condition.

2.2 Preparation of working electrodes for biosensor Nickel foams of size 1 cm x 0.6 cm (working area: 0.3 cm2) were cleaned in acetone, ethanol and deionized water successively for 10 minutes under sonication and dried. 1.0 mg of the synthesized MnO2 and MnO2/metal nanocomposites were added in 1.0 ml of ethanol and homogeneously dispersed by sonication for 15 minutes. Then the suspension was drop casted on the surface of pre-cleaned Ni foam strips and dried at room temperature. Finally, dried Ni foam strips were crimped under 100 MPa pressure for the better binding of the material and stored in desiccator.

2.3 Electrochemical measurements A PG262A potentiostat/galvanostat (Technoscience Ltd., Bengaluru) with a three-electrode system was employed for the all the electrochemical analyses. Using MnO2/metal nanocomposites modified Ni foams as working electrode, a platinum wire as auxiliary electrode and an Ag/AgCl electrode as reference electrode, all the cyclic voltammetric (CV) and chronoamperometric measurements were carried out at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range 10 mHz to 100 kHz in a 0.1 M NaOH solution at an


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AC amplitude of 5 mV. For this, nanocomposites modified glassy carbon electrodes were used as working electrode.

3. RESULTS AND DISCUSSION 3.1 Structural characterization Phase and crystal structure of MnO2 and its metal nanocomposites were confirmed from the XRD patterns shown in Figure 1. All the diffraction peaks observed for pure MnO2 can be indexed to the α-phase of tetragonal MnO2 (JCPDS card no. 44-0141) and the broader peaks indicate that the particles are in nanometre size regime. Absence of impurity peaks from the other Mn-related phases confirmed the purity of single α- MnO2 phase. Moreover, Pd/MnO2 nanocomposite exhibit the (101), (112) and (103) diffraction planes of tetragonal PdO (JCPDS card no. 06-0515) with α-MnO2. The (111), (200) and (220) peaks found in Au/MnO2 are well consistent with the cubic Au (JCPDS card no. 89-3697) which evidenced the formation of composite material.

Figure 1. XRD patterns of MnO2, Pd/MnO2 and Au/MnO2 nanosheets.



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Figure 2. (A, B) TEM images of MnO2 nanosheets, (C, D) Au/MnO2 nanocomposite (red dotted circles are the Au particles) and (E, F) Pd/MnO2 nanocomposite (yellow dotted circles are the PdO particles). Surface morphological features of the MnO2-metal nanocomposites were investigated through the TEM measurements. Low and high magnification micrographs of pristine MnO2 provided in Figure 2A, B confirms the successful formation of very thin 2D graphene-like nanosheets. The surface of the nanosheets contains plenty of wrinkles and folds. It can be also seen that the thickness of the nanosheets are of the order of few nanometers and its lateral dimension varied from 200-500 nm which can provide a large surface area for the quick electron transfer and the adsorption of analyte molecules. As well, Au/MnO2 nanocomposite also exhibits the nanosheet structure with slightly reduced size (

MnO2

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