Studies on As-synthesized Graphene Oxide Flakes

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ing this process, we obtained the dark brown suspension. 10 ml of hydrogen .... Brown-Anson model [41] as given by equation (6). (6) where n, v and A are same ...

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RESEARCH ARTICLE

Studies on As-synthesized Graphene Oxide Flakes M. Parvaz1,*, Pramod K. Gupta1,2 Pratima Solanki2 and Zishan H Khan1 1

Department of Applied Science & Humanities, Jamia Millia Islamia, New Delhi, India; 2Special Centre of Nanosciences, Jawaharlal Nehru University, New Delhi, India  

ARTICLE HISTORY Received: September 08, 2016 Revised: October 21, 2016 Accepted: October 31, 2016

 

DOI: 10.2174/246818730766616121312 1721

Abstract: Background: Due to the single atomic layer, flexible material, easily tuned of band gap, easily reduced to reduced graphene oxide, graphene oxide (GO) have a lot of application for Electronics, Biosensors, Biomedical application. Objective: The goal of the present work is to study the electrochemical properties of graphene oxide flakes. Methods: In the present work, we synthesis of graphene oxide using modified hummer techniques. The morphological and structural characterization of as-prepared GO was investigated with the help of XRD, SEM, FTIR, UV, CV. Result: The XRD patterns of As-prepared graphene oxide (GO) flakes shows a peak at 11.20, and the calculated value of lattice parameter comes out to be 15.56 A0.The peaks in the Raman spectrum are observed at 1599 cm-1 and 1349 cm-1, which confirms the lattice distortion in the graphene oxide (GO). The peaks in the FTIR spectrum of graphene oxide (GO) observed at, 3636 cm-1 , 1753 cm-1, 1649 cm-1, 1456 cm-1, and 1177 cm-1 are attributed to O-H, C=O, C=C, C-OH, C-O bonds, respectively. The electrochemical studies of graphene oxide (GO) modified ITO electrode shows an improvement in the electrochemical parameters. Conclusion: We have prepared the graphene oxide by modified Hummers method. According to the electrochemical studies, graphene oxide (GO) modified ITO electrode is an excellent medium for the fast electron transfer between an electrode surface and electrolyte. On the basis of these studies, it is suggested that graphene oxide may be a best material for the fabrication of biosensors.

Keywords: Graphene oxide, raman spectrum, FTIR spectrum, electrochemical studies. 1. INTRODUCTION The idea of graphene and its electronic properties using tight binding model was first introduced by Wallace [1]. Graphite is a semiconductor without activation energy due to the expansion of a small part of the valence band to conduction band. During the last few years, a lot of work has been carried out on the physical properties of graphite [2-5]. Graphene, a two dimensional honeycomb like structure with sp2 hybridization of carbon atom was discovered in 2004 [6]. Graphite, as a raw material, subsists in nature and it was stiff to exfoliate graphene from graphite, few precursors are required to exfoliate graphene from graphite physically and chemically. It has drawn a lot of attention due to its excellent electronic [7-12], thermodynamic [13] and mechanical properties [14-18], which arise from the monolayer structure of sp2 hybridization carbon atom. Recently, it has become an ideal material for many technological applications such as nanoelectronics [19, 20], sensors [21, 22], batteries [23], supercapacitors and hydrogen storage [24]. *Address correspondence to this author at the Department of Applied Science & Humanities, Jamia Millia Islamia, New Delhi, India; Tel/Fax: ???????????????; E-mail: [email protected]

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Graphene oxide is single atomic layer and flexible material and it is prepared by the energetic oxidation of graphite. Nanostructures of graphene oxide (GO) have a lot of potential due to easy synthesis, less expensiveness and scope for mass scale production. It is one of the important materials for the future memory device also. The band gap of graphene oxide can be easily tuned by varying the oxidation level. It is ideal as an electrical insulator as well as a semiconductor, when it is fully and partially oxidized [25]. Brodie et al. [26] were the first group to synthesize the graphene oxide (GO) using potassium chlorate and a paste of graphite in fuming nitric acid. Staudenmaier et al. [27] reported an improvement in this method by using a mixture of concentrated sulphuric acid and fuming nitric acid followed by addition of chlorate to the reaction mixture. Hummer et al. [28] reported a different method for the synthesis of graphene oxide by using KMNO4 and NaNO3 in concentrated H2SO4 and the method was named after his name. In the present work, we report the synthesis of graphene oxide using modified hummer techniques. The morphological and structural characterization of as-prepared GO has also been presented. The work also includes the electrochemical properties of as-prepared GO.

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2. EXPERIMENTAL DETAILS Graphite powder was purchased from Sigma Aldrich (USA), hydrochloric (HCl), hydrogen proxide (30% H2O2), concentrated sulfuric acid (98.09% H2SO4), potassium permanganate (KMnO4) was purchased from Fisher Scientific. Graphene oxide was synthesized using graphite powder by modified Hummers method. According to the modified Hummers method, 2 gm of graphite powder was added in concentrated sulfuric acid (98.09% H2SO4) and was stirred for 2 hour. After this process, 6 gm of potassium permanganate (KMnO4) was added to this solution and then the solution was placed in an ice bath to maintain the temperature of the solution below 200 C and simultaneously stirred for 10 minutes. After removing the ice bath the solution was stirred continuously for 2 hours at room temperature. After completing this process, we obtained the dark brown suspension. 10 ml of hydrogen peroxide (30% H2O2) solution, 150 ml of DI water was added in the above mentioned suspension and was stirred again for 30 minutes. Finally, 50 ml hydrochloric acid (HCl) was added in the as-prepared solution and measured the pH value of this solution, which was found to be around one. To increase its ph value to 6 , we again added some DI

Fig. (1). SEM Images of the as-prepared graphene oxide flakes.

Parvaz et al.

water in this solution. The graphene oxide (GO) solution was washed with DI water and centrifuged at 10000 rpm for 40 minutes. After centrifuging, we again added the DI water in the obtained precipitate and sonicated it for 30 minute followed by drying in the vacuum oven at 500 C for 2 days. We finally obtained the flakes of graphene oxide. The morphology of as-prepared graphene oxide (GO) was studied by SEM, (NOVA NANOSEM 450). The XRD pattern of graphene oxide was recorded by using a Rigaku Ultima IV Diffractometer [copper target was used as the source with λ = 1.54056Ao (Cukα1)] with a scanning range of 5o – 450. UVVis spectroscopy (SHIMADZU, UV-1800 UV-Vis-NIR Spectrophotometer) and Raman spectroscopy,(Renishaw Laser Raman spectrometer, Invia II) were employed to study the optical absorbance, vibrational, rotational, band stretching and other low-frequency modes of the as- prepared graphene oxide (GO). Also, the electrochemical properties of graphene oxide (GO) were studied using the cyclic voltammetry technique (model ZC-2000, Microtec, Japan). 3. RESULT AND DISCUSSION (Fig. 1) shows the SEM images of the as prepared graphene oxide flakes. The XRD patterns of the graphene oxide (GO) are shown in (Fig. 2). As-prepared graphene oxide

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(GO) flakes shows a peak at 11.20, corresponding to the plane (0 0 2), and the calculated value of lattice parameter of as-prepared graphene oxide (GO) comes out to be 15.56 A0 , which agrees with the reported results [29]. Raman spectroscopy is a non-destructive technique which is widely used for the structural information in the case of the carbon based materials [30]. The main indication in the Raman spectrum of graphitic carbon-based materials is the presence of G and D bands. Both G and D band peaks arising from the vibration of sp2 carbon, appear at 1599 cm-1 and 1349 cm-1 respectively. The G band is related to the optical E2g phonon at the brillion zone centre resulting from the stretching of sp2 carbon pairs in ring and chains. The D band characterizes the breathing mode of atomic ring which arises due to the defect in the sample. Raman spectrum of graphene oxide (GO) shown in the (Fig. 3). The peaks are observed at 1599 cm-1 and 1349 cm-1 which are in agreement with the reported results [31]. This confirms the lattice distortion in the graphene oxide (GO). (Fig. 4) shows the UV-Vis spectrum of asprepared graphene oxide (GO). It is observed that graphene oxide (GO) shows the maximum peak at 227 nm, which is attributed to π-π* transition of the aromatic C-C bonds and it is also reported in the literature [32]. FTIR spectrum of graphene oxide (GO) is shown in the (Fig. 5). The peaks observed at, 3636 cm-1, 1753 cm-1, 1649 cm-1, 1456 cm-1, and 1177 cm-1 are attributed to O-H, C=O, C=C, C-OH, C-O bonds, respectively.

function of scan rate varying from 10-100 mV/s (Fig. 7) to evaluate the electrochemical parameters like surface area (Ae), diffusion constant (D), heterogeneous electron transfer rate constant (Ks) and surface concentration of ionic species (I*) of respective electrodes. The top inset of (Fig. 7) shows that the electrodes peak value of anodic (Ipa) and cathodic (Ipc) currents increases linearly (regression constant R2 = 0.99) with the (scan rate = v), which reveals the presence of diffusion-controlled redox process at the interface of electrode and electrolyte [36, 37]. The anodic (Epa) and cathodic (Epc) peak potentials for both electrodes also shifts towards higher potential side with an increase in (lower inset of Fig. 7). This behaviour shows the characteristics of quasiirreversible electron transfer kinetics between electrode surface and electrolyte [38, 39]. The equations (1 to 4) with the values of the slope, intercept and correlation coefficient of ITO and GO/ITO electrodes are given below.

Fig. (2). X-ray diffractogram of the as-prepared graphene oxide.

Fig. (3). Raman spectra of the as-prepared graphene oxide.

The charge-transfer properties of GO/ITO surface studies has been carried out by employing cyclic voltammetry (CV) using sodium phosphate buffer (0.1 M, pH 7.0, 0.9% KCl) as an electrolyte. Cyclic voltammogram of ITO and graphene oxide modified (GO/ITO) electrodes recorded at a scan rate of 50 mV/s with the potential ranges from −0.3 V to + 0.6 V is shown in (Fig. 6). It has been observed that the magnitude of anodic peak current is increased about three times [curve (ii); 1.8 µA] as compared to bare ITO electrode [curve (i); 0.69 µA] [33]. This result reveals that the electrophoretic deposition of graphene oxide (GO) on the ITO surface leads to an increase in electro-catalytic activity and surface areato-volume ratio of electrode [34, 35]. Cyclic voltammograms of both ITO and GO/ITO electrodes have been studied as a

To evaluate the value of diffusion coefficient (D) of the redox species between electrolyte and electrode, we have used Randles–Sevcik equation [40]. This equation shows the dependence of redox event. The peak current (Ip) also depends upon the properties of electroactive species such as concentration, scan rate and diffusion coefficient.

(1) (2) (3)

(4)

A

(5)

where n is the number of electrons involved, D is the diffusion co-efficient, A is the surface area of electrode (0.25 cm2), C is the concentration of redox species and v is the scan rate (50 mV s-1). The estimated value of D for GO/ITO electrode (4.05 х10-19) is greater than for ITO electrode (5.69 х10-20).

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indicates towards the fast electron exchange through the graphene modified electrode surface. Ks= mnFv/RT

(7)

where F, n, v, R, T are same as in equations (6) and m is the peak potential difference. To evaluate the electro-active surface area (Ae) for ITO and GO/ITO electrodes, we use the estimated value of diffusion co-efficient obtained by Randles–Sevcik equation (8) in the equation given below. (8)

where S is the value of the slope of the straight line plotted between Ip versus (v)1/2 n, C, D are same as in mentioned in equations (5-7).

(6)

The calculated values of all the electrochemical parameters such as anodic peak current (Ipa), cathodic peak current (Ipc), diffusion coefficient (D), charge transfer rate constant (Ks), electroactive surface area (Ae) and average surface concentration (I*) of ionic species for respective electrodes are listed in (Table 1). Improvement in electrochemical parameters indicates that GO modified ITO electrode is an excellent promoter for the electron transfer between an electrode surface and electrolyte.

where n, v and A are same as in equation (5), F is the Faraday constant (96485 C mol-1), T is the temperature in K, and R is the gas constant in (VC K−1 mol−1). The value of I* for GO/ITO electrode is greater than that of ITO (Table 1), suggesting that the redox species is adsorbed more on GO/ITO electrode surface as compared to ITO electrode. The value of heterogeneous electron transfer rate constant (Ks) describes the electron exchange process between the electrode surface and redox species in the electrolyte. Electron transfer kinetics depends on the value of scan rate (v) and Ks. We have calculated the value of Ks for GO/ITO and ITO electrodes, using Laviron model, the calculated values are 0.88 s-1 and 0.64 s-1 respectively [42] as given by equation (7), which

Electrochemical impedance spectroscopy is a nondestructive tool, used to study electrochemical changes, mechanisms and reaction kinetics between the electrode surface and electrolyte with the modification of electrode [43]. Electrochemical impedance (Z) is a complex resistance (real part Z’ + imaginary part Z’’) and is equivalent to the impedance of an RLC circuit, which encounters on flowing the current through this circuit. We commonly used an equivalent Randles circuit (as given in the inset of Fig. 8) to measure the electrochemical impedance containing an active electrolyte resistance (RS) in series with charge transfer resistance (Rct) and in parallel combination of the double-layer capacitance (Cdl) or constant phase element (CPE). The

Fig. (4). UV-Vis spectrum of the as-prepared graphene oxide.

The value of surface concentration (I*) of electrodes can be estimated from CV plots (potential Vs current) using the Brown-Anson model [41] as given by equation (6).

Fig. (5). FTIR spectrum of the as-prepared graphene oxide.

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semicircular diameter of EIS spectra gives a value of the charge transfer resistance (RCT) that reveals the electron transfer kinetics of the redox probe at the electrode– electrolyte interface [44]. The linear part of the lowfrequency region corresponds to the diffusion process. (Fig. 8) shows the electrochemical impedance spectra (EIS) of (i) bare ITO (ii) the GO/ITO electrode in phosphate buffer (50mM, pH 7.0, 0.9% KCl) containing 3.3 mM [Fe(CN)M63/4] at zero volt polarization potential in the frequency range of 0.1–105 Hz. The Nyquist plot of the GO/ITO electrode (curve ii) and ITO electrode shows a charge transfer resistance (RCT) value of 3.73kΩ and 5.21 kΩ (Table 2) respectively. The low value of RCT for GO/ITO indicates the good electron transfer between the electrode surface and electrolyte. Fig. (6). CV for (i) ITO and (ii) GO/ITO electrodes at scan rate of 50 mV/s sodium phosphate buffer (0.1 M, pH 7.0, 0.9% KCl).

Fig. (7). Cyclic voltammograms (CV) of (a) ITO and (b) GO/ITO electrode as a function of scan rate of 10-100 mV/s in sodium phosphate buffer (0.1 M, pH 7.0, 0.9% KCl). Upper and lower inset showed the graph of current and potential with respectively.

Table 1.

Estimated values of electrochemical parameters for electrodes.

Electrodes

Ipa (µA)

Ipc (µA)

Ks (s-1)

D (cm2s-1)

Ae (mm2 )

I* (mol cm-2)

ITO

0.69

0.63

0.64

5.69 х10-20

1.18

5.93 х10-11

GO/ITO

1.84

1.69

0.88

4.05 х10-19

11.3

1.58 х10-10

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ACKNOWLEDGEMENTS Declared none. REFERENCES [1] [2] [3] [4] [5]

Fig. (8). EIS spectrum for (i) ITO and (ii) GO/ITO electrodes in frequency range of 0.1 - 105 Hz using PBS containing 3.3 m M[Fe(CN)6]3-/4- as electrolyte. Table 2. Values of parameters associated within equivalent randles circuit.

[6]

[7] [8] [9]

Electrodes

Cdl(µF)

Rct (kΩ)

Rs(Ω)

CPE

ITO substrate

2.45

5.21

74.10

0.999

GO/ITO

4.56

3.73

62.33

0.998

[10]

Ae: surface area, D: diffusion constant, Ks: heterogeneous electron transfer rate con-

[11] [12]

stant, I*: surface concentration of ionic species of respective electrodes, Ipa: electrodes peak value of anodic currents, Ipc: electrodes peak value of cathodic currents, RS: active electrolyte resistance, Rct: charge transfer resistance, Cdl: double-layer capacitance,

[13]

CPE: constant phase element

CONCLUSION We have prepared the graphene oxide flakes by modified Hummers method. Which was confirmed by XRD, Raman spectroscopy, UV-Vis and FTIR spectroscopy. According to the electrochemical studies, it is concluded that the graphene oxide (GO) modified ITO electrode shows an improvement in the electrochemical parameters which suggested that graphene oxide (GO) modified ITO electrode is an excellent medium for the fast electron transfer between an electrode surface and electrolyte. The value of charge transfer resistance of GO/ITO and ITO electrode calculated by electrochemical impedance technique is 3.73 kΩ & 5.21 kΩ respectively. The low value of charge transfer resistance for GO/ITO electrode further confirms the good electron transfer between the electrode surface and electrolytes. On the basis of these studies, it is suggested that graphene oxide may be the best material for the fabrication of biosensors.

[14] [15] [16] [17] [18]

[19] [20] [21] [22]

CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest.

[23]

PR Wallace. The band theory of graphite. Phys Rev 1947; 7: 622– 34. Boehm H P, Clauss A, Fischer G O, Hofmann U. Dunnste kohlenstoff-folien. Verlag der Zeitschrift fur Naturforschung B: J of Chem Sci 1962. Bommel A J Van, Crombeen J E, Tooren A Van et al. LEED and Auger electron observations of the SiC(0001) surface. Surf Sci 1975; 48: 463–472. Forbeaux I, Themlin J M, Debever J M et al. Heteroepitaxial graphite on 6H-SiC(0001): interface formation through conductionband electronic structure. Phys Rev B 1998; 58: 16396–16406. Oshima C, Itoh A, Rokuta E, Tanaka T, Yamashita K, Sakurai T et al. Hetero-epitaxial-double-atomic-layer system of monolayer graphene/monolayer h-BN on Ni(111). Solid State Comm 2000; 116: 37–40. Novoselov K S, Geim A K, Morozov S V, Jiang D, Katsnelson M I, Grigorieva I V, Dubonos S V, Firsov A A et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005; 438: 197–200. Zhang Y, Tan Y W, Stormer H L, Kim P et al. Experimental observation of the quantum Hall effect and Berry’s phase in graphene. Nature 2005; 438: 201–4. Jannik C M, Geim A K, Katsnelson M I, Novoselov M I, Booth T J, Roth S et al. The structure of suspended graphene sheets. Nature 2007; 446: 60–3. Heersche H B, Jarillo-Herrero P, Oostinga J B, Vandersypen L M K, Morpurgo A F et al. Bipolar supercurrent in graphene. Nature 2007; 446: 56–9. Ohta T, Bostwick A, Seyller T, Horn K, Rotenberg E e al., Controlling the electronic structure of bilayer graphene. Science 2006; 313: 951–4. Westervelt RM et al. Graphene nanoelectronics. Science 2008; 320: 324–5. Novoselov K S, Jiang Z, Zhang Y, Morozov S V, Stormer H L, Zeitler U, Maan J C, Boebinger G S, Kim P, Geim A K et al. Room-temperature quantum Hall effect in graphene. Science 2007; 315: 1379. Balandin A A, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, Lau C N et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008; 8: 902–7. Stankovich S, Dikin D A, Dommett G H B, Kohlhaas K M, Zimney E J, Stach E A, Piner R D, Nguyen S T, Ruoff R S et al. Graphenebased composite materials. Nature 2006; 442: 282–6. Dikin D A, Stankovich S, Zimney E J, Piner R D, Dommett G H B, G Evmenenko, Nguyen S T, Ruoff R S et al. Preparation and characterization of graphene oxide paper. Nature 2007; 448: 457–60. D Li, Kaner R B et al. Graphene-based materials. Science 2008; 320: 1170–1. Lee C, Wei X, Kysar J W, Hone J et al. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008; 321: 385–8. Bunch J S, Zande A M Vander, Verbridge S S, Frank I W, Tanenbaum D M, Parpia J M, Craighead H G, McEuen P L et al. Electromechanical resonators from graphene sheets. Science 2007; 315: 490–3. Avouris A, Chen Z, Perebeinos V et al. Carbon-Based Electronics. Nat Nanotechnol 2007; 2: 605–615. Son Y W, Cohen M L, Louie S G et al. Half-Metallic Graphene Nanoribbons. Nature 2006; 444: 347–349. Stankovich S, Dikin D A, Dommett G H B, Kohlhaas K M, Zimney E J, Stach E A, Pinen R D, Nguyen S T, Ruoff R S et al. GrapheneBased Composite Materials. Nature 2006; 442: 282–286. Watcharotone S, Dikin D A, Stankovich S, Piner R, Jung I, Mommett G H B, Evmenenko G, Wu S E, Chen S F, Liu C P et al. Graphene-Silica Composite Thin Films as Transparent Conductors. Nano Letts 2007; 7: 1888–1892. Takamura T, Endo K, Fu L, Wu Y P, Lee K J, Matsumoto T et al. Identification of Nano-Sized Holes by TEM in the Graphene Layer

Studies on As-synthesized Graphene Oxide Flakes

[24] [25] [26] [27] [28] [29]

[30] [31] [32] [33] [34]

of Graphite and the High Rate Discharge Capability of Li-Ion Battery Anodes. Eletrochim Acta 2007; 53: 1055–1061. Novoselov K S, Jiang D, Schedin F, Booth T J, Khotkevich V V, Morozov S V, Geim A K. Two Dimensional Atomic Crystals, Proceeding National Academy of science. USA, July 26, 2005. Loh K P, Bao Q, Eda G, Chhowalla M et al. Graphene oxide as a chemically tunable platform for optical applications. Nat Chem 2010; 2: 1015. Brodie B C et al. On the atomic weight of graphit. Philos. Trans. R. Soc. London 1859; 14: 249–259. Staudenmaier L et al. Verfahren zur Darstellung der Graphitsaure. Ber Dtsch Chem. Ges 1898; 31: 1481-1487. Hummers W S, Offeman R E et al. Preparation of graphitic oxide. J Am Chem Soc 1958; 80: 1339-1339. Khandelwal M, Kumar A et al. One-step chemically controlled wet synthesis of graphene nanoribbons from graphene oxide for high performance supercapacitor applications. J Maters Chem A 2015; 3: 22975-22988. Ferrari A C, Robertson J et al. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2010; 61: 1409514107. Tuinstra F, Koenig J L et al. Raman spectrum of graphite. J Chem Phys 1970; 53: 1126-1130. Krishnamoorthy Karthikeyan, Mohan Rajneesh, Kim S J et al. Graphene oxide as a photocatalytic material. Appl Phys Letts 2011; 98: 244101. Chauhan N, Narang J, Jain U et al. Highly sensitive and rapid detection of acetylcholine using an ITO plate modified with platinum-graphene nanoparticles. Analyst 2015; 140: 1988-1994. Zeng H, Cao Y, Xie S, Yang J, Tang Z, Wang X, Sun L et al. Synthesis, optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures. Nanoscale Res Letts 2013; 8: 1-6.

Current Nanomaterials, 2016, Vol. 1, No. 3 [35] [36]

[37] [38] [39] [40] [41] [42]

[43]

[44]

7

Xu M Q, Wu J F, Zhao G C et al. Direct electrochemistry of hemoglobin at a graphene gold nanoparticle composite film for nitric oxide biosensing. Sensors 2013; 13: 7492-7504. Radhapyari K, Kotoky P, Das M R, Khan R et al. Graphene– polyaniline nanocomposite based biosensor for detection of antimalarial drug artesunate in pharmaceutical formulation and biological fluids. Talanta 2013; 111: 47-53. Sharma A, Baral D, Rawat K, Solanki P R, Bohidar H et al. Biocompatible capped iron oxide nanoparticles for Vibrio cholerae detection. Nanotechnology 2015; 26: 175302. Holze R, Zanello Piero et al. Inorganic electrochemistry: theory, practice and applications. J. Solid State Electrochem 2006; 10: 512513. Wang j. Analytical electrochemistry. John Wiley & Sons 2006. Reichman B, Bard A J, Laser D et al. A digital simulation model for electrochromic processes at WO3 electrodes. J Electrochem Soc 1980; 127: 647-654. Solanki P R, Srivastava S, Ali M A, Srivastava R K, Srivastava A, Malhotra B et al. Reduced graphene oxide–titania based platform for label-free biosensor. RSC Advances 2014; 4: 60386-60396. Kang X, Wang J, Wu H, Aksay I A, Liu J, Lin Y et al. Glucose oxidase–graphene–chitosan modified electrode for direct electrochemistry and glucose sensing. Biosensors and Bioelectronics 2009; 25: 901-905. Puri N, Niazi A, Srivastava A K et al. Biointerfacial impedance characterization of reduced graphene oxide supported carboxyl pendant conducting copolymer based electrode. Electrochim Acta 2014; 123: 211-218. Erdem A, Muti M, Papakonstantinou P, Canavar E, Karadeniz H, Congur G, Sharma S et al. Graphene oxide integrated sensor for electrochemical monitoring of mitomycin C–DNA interaction. Analyst 2012; 137: 2129-2135.