Graphene oxide-Cu(II) composite electrode for non ...

Journal of Electroanalytical Chemistry 776 (2016) 59–65

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Graphene oxide-Cu(II) composite electrode for non-enzymatic determination of hydrogen peroxide S. Muralikrishna a,b, Sarawut Cheunkar b,c, Benchaporn Lertanantawong b, T. Ramakrishnappa a,d,⁎, D.H. Nagaraju a,⁎, Werasak Surareungchai b,c, R. Geetha Balakrishna a, K. Ramakrishna Reddy e a

Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagaram, Bangalore 562112, India Pilot Plant Development and Training Institute, King Mongkut's University of Technology Thonburi, Bangkhuntien-chaitalay Road, Thakam, Bangkok 10150, Thailand c School of Bioresources and Technology, and Nanoscience & Nanotechnology Graduate Programme, King Mongkut's University of Technology Thonburi, Bangkhuntien-chaitalay Road, Thakam, Bangkok 10150, Thailand d Dayananda Sagar Academy of Technology and Management, Udayapura, Opp Art of Living, Kanakapura Road, Bangalore -560082, India e Department of Chemistry, Government Science College, Bangalore, India b

a r t i c l e

i n f o

Article history: Received 9 June 2016 Received in revised form 23 June 2016 Accepted 26 June 2016 Available online 29 June 2016 Keywords: Graphene oxide-Cu (II) composite Non-enzymatic sensor Amperometric method Hydrogen peroxide

a b s t r a c t We report a novel graphene oxide-Cu(II) (GO-Cu2+) composite electrode for electrochemical determination of hydrogen peroxide (H2O2) in neutral solution (pH = 7.4). Oxygen functional groups such as carboxylic acid, hydroxyl and epoxy present on GO are utilized for the formation of composite with Cu2+ ions. The synthesized GOCu2+ composite was characterized by X-ray diffraction (XRD) studies and cyclic voltammetry (CV). The surface morphology of the GO and GO-Cu2+ composite materials was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Elemental analysis was performed using EDS. The prepared electrode showed a good electrocatalytic performance for reduction of H2O2 and mechanistic pathway has been discussed. Amperometric determination for H2O2 was carried out at a applied potential of −0.25 V vs Ag/AgCl and the result showed a linear response range of 5 μM to 85 μM (correlation coefficient = 0.999) with a detection limit of 0.5 μM (S/N = 3). The sensitivity of the electrode was found to be 0.072 μA μM−1 and the electrode selectively detected H2O2 in the presence of other potential interferences such as oxygen, glucose, ascorbic acid (AA), dopamine (DA) and uric acid (UA). © 2016 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen peroxide (H2O2), a simple molecule acts a strong oxidizing agent and widely used in many industrial applications such as chemical, pharmaceutical, clinical, textile, food and mining industries. It is also generated as a side product of various enzymatic reactions, for example, glucose oxidase, cholesterol oxidase, lactate oxidase, glutamate oxidase, alcohol oxidase, urate oxidase, D-amino acid oxidase, lysine oxidase, oxalate oxidase etc. [1,2]. Although the small amount of H2O2 produced would not toxicological effects due to rapid decomposition of the chemical by the enzyme in the intestinal cells, however, N3% H2O2 solutions generally would result in several diseases such as cancer, Alzheimer's, myocardial infarction, atherosclerosis, Parkinson's,

⁎ Corresponding authors at: Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagaram, Bangalore 562112, India. E-mail addresses: [email protected] (T. Ramakrishnappa), [email protected] (D.H. Nagaraju).

http://dx.doi.org/10.1016/j.jelechem.2016.06.034 1572-6657/© 2016 Elsevier B.V. All rights reserved.

etc. [3,4]. Therefore, the determination of H2O2 concentration is of great importance in both biological and environmental systems. The various analytical methods have been developed for the determination of hydrogen peroxide including titrimetry, spectrophotometry, fluorimetry, fluorescence, chromatography, chemiluminescence and electrochemical methods [5,6]. Among all the above methods, electrochemical methods offer a simple, sensitive, rapid and economically favourable method for the detection of H2O2 [7]. Recently, different types of electrodes have been developed based on enzymes, noble metals, metal alloys, metal nanoparticles and metal-carbon composite [1,8–10]. For example, Jonsson et al. reported horseradish peroxidase adsorbed graphite electrode [11], Xiao et al. reported horseradish peroxidase labeled Au colloids immobilized on gold electrode [12] and Zhang et al. reported poly-L-lysine functionalization single-walled carbon nanotube electrode for H2O2 sensing applications [13]. However, the availability of enzymes are difficult and also highly sensitive to temperature, pH and toxic chemicals [14]. On the other hand, high cost of noble metal electrodes limits its usage in many applications. Hence, the development of a highly sensitive and selective electrode without an enzyme or noble metal is needed.

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S. Muralikrishna et al. / Journal of Electroanalytical Chemistry 776 (2016) 59–65

Fig. 1. Schematic representation for the synthesis GO-Cu2+ composite.

Graphene has attracted the most attention from past few years due to the exceptional electronic, thermal and mechanical properties of its nanosized 2D conducting carbon atoms [15,16]. It has been widely used for various applications including sensors, energy conversion, energy storage, ‘paper-like’ materials, water purification and biological applications [17–19]. Graphene oxide (GO) is a oxidised form of graphene, which contains various oxygen functional groups such as carboxylic acid, hydroxyl and epoxy functional groups at the edges and basal planes [20]. The chemical reduction of graphene oxide to graphene is a economically advantageous method for bulk production [21,22]. However, the chemically reduced GO (rGO) undergoes restacking of some of its layers and the rGO has a low conductivity than prestine graphene due to the impurities of sp3 hybridized functional groups [23]. Literature shows that chemical functionalization of organic molecules or intercalation of metal species prevents the restacking of these layers [18,24–26] and enhances the conductivity of GO/rGO, which has been widely used for the electrochemical applications such as sensors and energy related materials [27–34]. Recently, copper oxide/copper sulphide-graphene based composites are reported for the non-enzymatic detection of H2O2, but these methods require energy and time to synthesize the materials [2,10,14,35]. Herein, we have used for the first time the novel GO-Cu2+ composite material for selective determination of H2O2 concentration in the presence of different interfering analytes at biological pH. The GO-Cu2 + composite was synthesized by adjusting the pH of the GO dispersion followed by mixing of copper sulphate solution. The synthesized material was characterized by XRD and CV. The morphology studies were performed by SEM and AFM. Elemental analysis was performed using EDS. The electrode was utilized for the sensitive determination of H2O2 at a applied potential −0.25 V vs Ag/AgCl by amperometric technique. The electrode showed a linear range between 5 μM to 85 μM (correlation coefficient = 0.999) with a detection limit of 0.5 μM (S/N = 3). The sensitivity of the electrode was found to be 0.072 μA μM− 1. The H2O2 was selectively determined in presence of oxygen as well as other important bioanalytes such as glucose, ascorbic acid (AA), dopamine (DA) and uric acid (UA).

2.2. Synthesis of graphene oxide Graphene oxide was synthesized from oxidation of graphite powder using modified Hummers and Offmann method [36]. Graphite powder (1 g), NaNO3 (0.5 g) and Conc. H2SO4 (23 mL) were mixed by constant stirring and the mixture was cooled to below 5 °C in an ice bath. Then, KMnO4 (3 g) was gradually added under vigorous stirring and the temperature was maintained below 5 °C. The reaction mixture was stirred for 30 min at room temperature followed by addition of 46 mL deionised water under stirring condition. The temperature of the mixture was increased to 98 °C and maintains at this temperature for 15 min. Finally, the reaction was terminated by adding 1 mL H2O2 followed by continuous stirring for another 30 min at room temperature. The resultant precipitate was washed with 5% HCl to remove metal ions followed by copiously washing with distilled water until the supernatant of the solution become neutral. The obtained graphene oxide was sonicated in deionised water (0.1 mg/mL) for 30 min to get GO, then was centrifuged and dried at room temperature.

2.3. Synthesis of GO-Cu2+ composite GO-Cu2+ composite was prepared by simple mixing of pH adjusted GO and copper sulphate solution [29]. In a typical procedure, 40 mg GO was adjusted to pH 7 using 0.01 M KH2PO4 and K2HPO4 followed by sonication for 30 min. The final concentrations of that pH adjusted GO was maintained at 4 mg/mL by centrifugation. After that, once again we tested the pH of the GO suspension then added 30 mM CSPH in 1:1 ratio with stirring and allowed stand for 30 min. to get GO-Cu2+ composite.

2. Experimental 2.1. Materials and reagents All the chemical reagents were used as received without any further purification. Graphite powder (b 20 μm diameter) was purchased from Aldrich. Potassium permanganate (99%) was purchased from SD Fine chemicals. Sodium nitrate (≥98%), hydrogen peroxide (≥30%), hydrochloric acid (≥35%), copper sulphate pentahydrate (CSPH) (≥98%), potassium dihydrogen phosphate (≥ 98%), di-potassium hydrogen phosphate anhydrous (≥98%),and sulphuric acid (95–98%) were analytical grade reagents (AR) purchased from Merck. Deionised water was used in all the experiments.

Fig. 2. XRD pattern of a) GO, b) GO-Cu2+ composite (as prepared) and c) GO-Cu2+ composite (after electrochemical stability test).


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Fig. 3. SEM images of a) GO and b) GO-Cu2+ composite. EDS analysis of c) GO and d) GO-Cu2+ composite.

Fig. 4. a) CVs of GO, CSPE and GO-Cu2+ composite (hybrids) modified GC electrodes in 0.1 M PBS (pH = 7.4) at a scan rate of 50 mV/s. b) CVs of GO-Cu2+ composite modified GC electrode at different scan rates between 10 and 100 mV/s in 0.1 M PBS (pH = 7.4). c) Plot of current vs scan rate obtained from different scan rates of GO-Cu2+ composite modified GC electrode in 0.1 M PBS (pH = 7.4). d) Stability of GO-Cu2+ composite modified GC electrode for 100 cycles in 0.1 M PBS (pH = 7.4) at a scan rate of 100 mV/s.

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2.4. Instrumentation All electrochemical measurements were recorded using a CHI660D potentiostat (CH instruments, Austin, USA) in a standard three electrode system with modified glassy carbon as working electrode, platinum wire as counter electrode and Ag/AgCl (1 M KCl) as reference electrode. Powder X-ray diffraction data was recorded using Philips X'pert PRO PANalytical X-ray diffractometer with graphite monochromatized Cu-Kα (1.5418 Ao) radiation. Scanning electron microscopy was carried out using FEI (Nova nano) for generating images of GO and GO-Cu2+ composite. Atomic force microscopy was carried out using Bruker Dimension Icon AFM equipped with a Nanoscope V SPM controller operated in Tapping Mode. A silicon cantilever with resonance frequency of 300–400 kHz and spring constant 42 N/m was used. AFM height images were presented after simple flattening using V 9.0 Nanoscope software. 2.5. Electrode modification Prior to electrode modification, 3 mm diameter of glassy carbon (GC) electrode was polished with alumina slurry of different particle size (1, 0.3 and 0.05 μm) followed by rinsing with copious amounts of distilled water. Then the electrodes were sonicated in ethanol-water (1:1) mixture for about 5 min. Finally, it was rinsed with distilled water and dried at room temperature. For the electrode modification, 20 μL of 5% Nafion solution was added into 1 mL GO-Cu2 + composite. 10 μL of that suspension was drop cast on the surface of GC electrode and dried normally under room temperature. Similarly, we have been modified with GO and CSPH solution. Prior to electrochemical measurements the electrolyte

was saturated with nitrogen gas and electrode was subjected to potential cycling between the potential window of +0.3 to −0.6 V vs Ag/AgCl for about 50 cycles to achieve stable performance. 3. Results and discussion The schematic representation for the synthesis GO-Cu2+ composite shown in Fig. 1. The GO containing various oxygen functional groups are responsible for the formation of composite with Cu2+ ions. 3.1. XRD studies Fig. 2 depicts the XRD pattern of GO and GO-Cu2+ composite (before and after electrochemical stability test). The reflection peak at 2θ = 11.3° corresponding to the interlayer basal spacing, attributed due to the presence of oxygen functional groups present on the surface of carbon nanosheet (Fig. 2a.). The reflection peak shifted from 11.3° to 10.7° in the case of GO-Cu2+ composite (Fig. 2b). This clearly indicates that basal spacing between the GO layers increases due to the intercalation of Cu2+ ions. The reflection peak of GO-Cu2+ composite retained even after electrochemical stability test without any further additional peaks (Fig. 2c.). This suggests that prepared composite also electrochemically stable. 3.2. Surface morphology and elemental analysis The surface morphology of GO and GO-Cu2 + composite was observed by SEM shown in Fig. 3a and b respectively. The GO shows the presence of sheets and these sheets remains unaltered even after the incorporation of Cu2 + ions. We further analysed surface morphology

Fig. 5. CVs of a) GO-Cu2+ composite (hybrids) modified GC electrode in the presence and absence of 10 mM H2O2 in 0.1 M PBS (pH = 7.4) at a scan rate of 50 mV/s. b) GO modified GC electrode in 0.1 M PBS (pH = 7.4) at a scan rate of 50 mV/s. in the presence and absence of 10 mM H2O2 c) CSPH modified GC electrode in 0.1 M PBS (pH = 7.4) at a scan rate of 50 mV/s. in the presence and absence of 10 mM H2O2 d) CVs of GO-Cu2+ composite modified GC electrode with different concentrations of H2O2 ranging from 2.5 mM to 10 mM.


Fig. 6. The schematic representation of the electrochemical reduction of H2O2 by GO-Cu2+ composite modified GC electrode.

using atomic force microscopy (AFM). The incorporation of Cu2+ ions results in aggregation of the GO sheets as shown in the fig. S1 and S2 in the electronic supplementary information. The root mean square roughness of the GO-Cu2+ is found to be about 106 nm which is about 6.6 times higher than the GO sheets (rms roughness of GO is about 16 nm). This confirms that the incorporation of Cu2+ into GO sheets increases the roughness, indicating that the aggregation of the GO sheets. Elemental analysis of GO and GO-Cu2+ composite was performed using EDS and the results are shown in Fig. 3c and d respectively. The atomic percentage of C, O, S and Cu in GO and GO-Cu2+ composite are shown in the inset of Fig. 3c and d. GO contains about 0.69% Cu, which could be due to impurities. The percentage of Cu increased to 7.78% upon addition of Cu2+ into the GO. The atomic percentage of O remains same in both GO and GO-Cu2+ composite and S peak arises from GO, could be due to the sulphate impurity [37,38] and this impurity decreases in the case of GO-Cu2+ composite due to the additional cleaning of composite using ethanol and water. The atomic percentage of both O and S reveals that GO binds Cu2+ ions through electronegative oxygen groups without exhibiting the CuSO4 complex form. 3.3. Cyclic voltammetry studies CV was used for the characterization of GO-Cu2+ composite, GO and CSPH modified GC electrodes in a potential window of +0.3 to −0.6 V vs Ag/AgCl at a scan rate of 50 mV/s. Fig. 4a. displays the cyclic voltammograms (CVs) of GO-Cu2+ composite GO and CSPH modified GC electrodes recorded in 0.1 M phosphate buffer solution (PBS) pH = 7.4. Among the three electrodes, GO-Cu2+ composite electrode showed excellent electrochemical response compared to GO and CSPH modified GC electrodes with a pair of redox peaks. The cathodic peak at − 0.21 V vs Ag/AgCl due to reduction of Cu(II) to Cu(I) and anodic peak at − 0.03 V vs Ag/AgCl due to oxidation of Cu(I) to Cu(II) [39].

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The similar redox peaks are also observed for the CSPH modified GC electrode at the same peak potentials. However, the GO modified GC electrode not shown redox peaks due to the absence of copper ions. We further measured electrochemical response current of GO-Cu2 + composite modified GC electrode at different scan rates from 10 to 100 mV/s with the increment of 10 mV/s in the same buffer solution as shown in Fig. 4b. The corresponding cathodic and anodic linear plots are shown in Fig. 4c. The current increases linearly with increase in the scan rate. This confirms that the redox peak corresponds to a surface confined species. The ratio of anodic and cathodic peak current is approximately equal to one, which indicates that the electrochemical reaction is highly reversible. The stability of the GO-Cu2+ composite modified GC electrode is observed by continuous running for 100 cycles in the same buffer solution at a scan rate of 100 mV/s as shown in Fig. 4d. There is no decrease in the current response even after 100 cycles; this clearly indicates that the oxygen functional groups present on surface of GO are tightly bonded to Cu2+ ions through dative bonds. Hence, the modified electrode is electrochemically highly stable. We have further investigated the H2O2 sensing using CV with the GO-Cu2+ composite modified GC as working electrode in PBS (pH = 7.4) at a scan rate of 50 mV/s as shown in Fig. 5a. The GO-Cu2+ composite modified GC electrode showed a pair of redox peaks for the reduction and oxidation of copper ions, upon the addition of 10 mM H2O2 the cathodic peak current increases due to the reduction of H2O2 at the electrode surface. The electrochemical reduction mechanism of hydrogen peroxide on the surface of GO-Cu2+ composite modified GC electrode was shown in Fig. 6 [2]. We also performed the control experiments with the GO and CSPH modified GC electrodes for the determination of H2O2 concentration with the same electrolyte, analyte concentration and at the same scan rate. There is no change in electrochemical current response with GO modified GC electrode in the presence and absence of H2O2 shown in Fig. 5b. CSPH modified GC electrode also showed a pair of redox peaks which are similar to GO-Cu2+ composite modified GC electrode in the absence H2O2. After adding 10 mM H2O2, the cathodic peak current slightly increases due to the reduction of H2O2 at the electrode surface as shown in Fig. 5c. However, the increased electrochemical current response is less compared to GO-Cu2+ composite modified GC electrode. This suggests that GO-Cu2+ composite electrode exhibits excellent electrocatalytic response for the reduction of H2O2. Fig. 5d shows the electrochemical response of GO-Cu2+ composite modified GC electrode with different concentrations ranging from 2.5 mM to 10 mM H2O2. The current increases linearly with the increasing H2O2 concentration.

Fig. 7. a) Amperometric response of the GO-Cu2+ composite modified GC electrode upon injection of H2O2 into 0.1 M PBS (pH = 7.4) at applied potential −0.25 V vs Ag/AgCl. b) Amperometric response of the GO-Cu2+ composite modified GC electrode upon injection of H2O2 and other bioanalytes such as glucose, AA, DA and UA in oxygen saturated 0.1 M PBS (pH = 7.4) at applied potential −0.25 V vs Ag/AgCl.

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Table 1 Analytical performance of the GO-Cu2+ composite modified GC electrode compared with the Cu2O, CuO and Cu2O-rGO composites. Electrode materials

Applied potential (V)

Detection limit (μM)

Reference

Cu2O microcubes CuO CuO nanoparticles-ionic liquid CQDs/octahedral Cu2O Cu2O-nitrogen doped graphene Cu2O nanocube-rGO Cu2O micro spheres-rGO Cu2O-rGO Poly-L-lysin-SWCNT-HRP GO-Cu2+ composite

−0.2 −0.2 −0.2

1.5 1.6 0.5

[40] [6] [41]

−0.2 −0.6

2.8 0.8

[42] [35]

−0.4 −0.24 – 0.20 −0.25

20.8 10.8 21.7 0.5 0.5

[10] [14] [2] [13] Present studies

freshly prepared 10 mM H2O2 was used for electrochemical measurements. Amperometric measurements were recorded once in five days and the corresponding results are shown in Fig. 8. The current response of the GO-Cu2+ composite modified GC electrode retained above 95% even after forty days. This clearly demonstrates that the modified electrode is quite stable for longer time for the determination of H2O2. 4. Conclusions In summary, we demonstrate a simple method for synthesis of GOCu2 + composite for the electrochemical detection of H2O2 in 0.1 M PBS (pH = 7.4). The electrode detects selectively hydrogen peroxide in presence of other bioanalytes with a sensitivity of 0.072 μA μM−1. The linear range obtained for this electrode is 5 μM to 85 μM with a detection limit 0.5 μM. The observed detection limit is lower compared to previously reported CuO, Cu2O and Cu2O-rGO composites.

3.4. Amperometric studies

Acknowledgment

Fig. 7a illustrates the amperometric response of GO-Cu2+ composite modified GC electrode with the successive addition of 5 μM H2O2 into the nitrogen saturated 0.1 M PBS (pH = 7.4) solution under stirring condition at a applied potential of −0.25 V vs Ag/AgCl. The current response increases linearly with the increasing concentration of H2O2. The corresponding calibration plot shown in inset of Fig. 7a. The linear current responses can be obtained for the H2O2 concentration ranges between 5 and 85 mM with a correlation co-efficient of 0.999. The limit of detection (LOD) was found to be 0.5 μM based on the signal-to noise ratio of three. The observed detection limit is low as compared to previously reported CuO, Cu2O and Cu2O-rGO composites as listed in Table 1. We further investigated the interference of the other potential analytes such as glucose, AA, DA and UA in oxygen saturated 0.1 M PBS (pH = 7.4) solution under the same electrochemical condition with 100 fold higher concentration of bioanalytes as compared to H2O2 shown in Fig. 7b. There is no change in H2O2 current response after the injection of interference analytes. This suggests that the modified electrode was selectively sensing for H2O2 analyte in the presence and absence of oxygen and nitrogen. Stability is one of the most important factor used to evaluate electrochemical sensor. The GO-Cu2+ composite modified GC electrode was stored in vacuum desiccator and PBS solution (pH 7.4) containing

SM acknowledges Jain University, KMUTT for short term research visit, NRU and Nanomission (SR/NM/NS-20/2014) for financial support. TR acknowledges DST-SERB (YSS/2015/000075), India for financial support. We also wish to extend our gratitude to Dr. Rajalaxmi Dash for critical reading of the manuscript.

Fig. 8. Stability of the GO-Cu2+ composite modified GC electrode stored in vacuum desiccator over 40 days (Amperometric currents obtained by adding 10 mM of H2O2 in 0.1 M PBS (pH 7.4), applied potential −0.25 V vs Ag/AgCl electrode).

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