Journal of Electroanalytical Chemistry 776 (2016) 59–65
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Journal of Electroanalytical Chemistry
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Graphene oxide-Cu(II) composite electrode for non-enzymaticdetermination 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, Indiab Pilot Plant Development and Training Institute, King Mongkut's University of Technology Thonburi, Bangkhuntien-chaitalay Road, Thakam, Bangkok 10150, Thailandc School of Bioresources and Technology, and Nanoscience & Nanotechnology Graduate Programme, King Mongkut's University of Technology Thonburi, Bangkhuntien-chaitalay Road, Thakam,Bangkok 10150, Thailandd Dayananda Sagar Academy of Technology and Management, Udayapura, Opp Art of Living, Kanakapura Road, Bangalore -560082, Indiae Department of Chemistry, Government Science College, Bangalore, India
⁎ Corresponding authors at: Centre for Nano andMaterGlobal Campus, Kanakapura, Ramanagaram, Bangalore 56
Article history:Received 9 June 2016Received in revised form 23 June 2016Accepted 26 June 2016Available online 29 June 2016
We report a novel graphene oxide-Cu(II) (GO-Cu2+) composite electrode for electrochemical determination ofhydrogen peroxide (H2O2) in neutral solution (pH= 7.4). Oxygen functional groups such as carboxylic acid, hy-droxyl and epoxy present on GO are utilized for the formation of composite with Cu2+ ions. The synthesized GO-Cu2+ composite was characterized by X-ray diffraction (XRD) studies and cyclic voltammetry (CV). The surfacemorphology 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 electrodeshowed a good electrocatalytic performance for reduction of H2O2 andmechanistic pathway has been discussed.Amperometric determination for H2O2was carried out at a applied potential of−0.25V vsAg/AgCl and the resultshowed a linear response range of 5 μMto85 μM(correlation coefficient=0.999)with a detection limit of 0.5 μM(S/N= 3). The sensitivity of the electrodewas found to be 0.072 μA μM−1 and the electrode selectively detectedH2O2 in the presence of other potential interferences such as oxygen, glucose, ascorbic acid (AA), dopamine (DA)and uric acid (UA).
Keywords:Graphene oxide-Cu (II) compositeNon-enzymatic sensorAmperometric methodHydrogen peroxide
1. Introduction
Hydrogen peroxide (H2O2), a simple molecule acts a strong oxidiz-ing agent and widely used in many industrial applications such aschemical, 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, gluta-mate oxidase, alcohol oxidase, urate oxidase, D-amino acid oxidase,lysine oxidase, oxalate oxidase etc. [1,2]. Although the small amountof H2O2 producedwould not toxicological effects due to rapid decompo-sition of the chemical by the enzyme in the intestinal cells, however,N3% H2O2 solutions generally would result in several diseases such ascancer, Alzheimer's, myocardial infarction, atherosclerosis, Parkinson's,
etc. [3,4]. Therefore, the determination of H2O2 concentration is ofgreat importance in both biological and environmental systems. Thevarious analytical methods have been developed for the determina-tion of hydrogen peroxide including titrimetry, spectrophotometry,fluorimetry, fluorescence, chromatography, chemiluminescence andelectrochemical methods [5,6]. Among all the above methods, electro-chemical methods offer a simple, sensitive, rapid and economicallyfavourable method for the detection of H2O2 [7]. Recently, differenttypes of electrodes have been developed based on enzymes, noblemetals, metal alloys, metal nanoparticles and metal-carbon composite[1,8–10]. For example, Jonsson et al. reported horseradish peroxidaseadsorbed graphite electrode [11], Xiao et al. reported horseradish per-oxidase labeled Au colloids immobilized on gold electrode [12] andZhang et al. reported poly-L-lysine functionalization single-walled car-bon nanotube electrode for H2O2 sensing applications [13]. However,the availability of enzymes are difficult and also highly sensitive to tem-perature, pH and toxic chemicals [14]. On the other hand, high cost ofnoble metal electrodes limits its usage in many applications. Hence,the development of a highly sensitive and selective electrode withoutan enzyme or noble metal is needed.
Fig. 2. XRD pattern of a) GO, b) GO-Cu2+ composite (as prepared) and c) GO-Cu2+
composite (after electrochemical stability test).
Fig. 1. Schematic representation for the synthesis GO-Cu2+ composite.
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Graphene has attracted the most attention from past few years dueto the exceptional electronic, thermal and mechanical properties of itsnanosized 2D conducting carbon atoms [15,16]. It has been widelyused for various applications including sensors, energy conversion, en-ergy storage, ‘paper-like’materials, water purification and biological ap-plications [17–19]. Graphene oxide (GO) is a oxidised formof graphene,which contains various oxygen functional groups such as carboxylicacid, hydroxyl and epoxy functional groups at the edges and basalplanes [20]. The chemical reduction of graphene oxide to graphene isa economically advantageousmethod for bulk production [21,22]. How-ever, the chemically reduced GO (rGO) undergoes restacking of some ofits layers and the rGO has a low conductivity than prestine graphenedue to the impurities of sp3 hybridized functional groups [23]. Literatureshows that chemical functionalization of organicmolecules or intercala-tion of metal species prevents the restacking of these layers [18,24–26]and enhances the conductivity of GO/rGO, which has been widely usedfor the electrochemical applications such as sensors and energy relatedmaterials [27–34]. Recently, copper oxide/copper sulphide-graphenebased composites are reported for the non-enzymatic detection ofH2O2, but these methods require energy and time to synthesize thema-terials [2,10,14,35].
Herein,we have used for thefirst time thenovel GO-Cu2+ compositematerial for selective determination of H2O2 concentration in the pres-ence of different interfering analytes at biological pH. The GO-Cu2+
composite was synthesized by adjusting the pH of the GO dispersionfollowed by mixing of copper sulphate solution. The synthesized mate-rial was characterized by XRD and CV. The morphology studies wereperformed by SEM and AFM. Elemental analysis was performed usingEDS. The electrode was utilized for the sensitive determination ofH2O2 at a applied potential−0.25 V vs Ag/AgCl by amperometric tech-nique. The electrode showed a linear range between 5 μMto 85 μM(cor-relation 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. TheH2O2 was selectively determined in presence of oxygen as well asother important bioanalytes such as glucose, ascorbic acid (AA), dopa-mine (DA) and uric acid (UA).
2. Experimental
2.1. Materials and reagents
All the chemical reagents were used as received without any furtherpurification. Graphite powder (b20 μm diameter) was purchased fromAldrich. Potassium permanganate (99%) was purchased from SD Finechemicals. Sodium nitrate (≥98%), hydrogen peroxide (≥30%), hydro-chloric acid (≥35%), copper sulphate pentahydrate (CSPH) (≥98%), po-tassium dihydrogen phosphate (≥98%), di-potassium hydrogenphosphate anhydrous (≥98%),and sulphuric acid (95–98%)were analyt-ical grade reagents (AR) purchased from Merck. Deionised water wasused in all the experiments.
2.2. Synthesis of graphene oxide
Graphene oxide was synthesized from oxidation of graphite powderusing modified Hummers and Offmann method [36]. Graphite powder(1 g), NaNO3 (0.5 g) and Conc. H2SO4 (23 mL) were mixed by constantstirring 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 tem-perature was maintained below 5 °C. The reaction mixture was stirredfor 30 min at room temperature followed by addition of 46 mLdeionised water under stirring condition. The temperature of the mix-ture was increased to 98 °C and maintains at this temperature for15 min. Finally, the reaction was terminated by adding 1 mL H2O2
followed by continuous stirring for another 30 min at room tempera-ture. The resultant precipitate was washed with 5% HCl to removemetal ions followed by copiously washing with distilled water untilthe supernatant of the solution become neutral. The obtained grapheneoxide was sonicated in deionised water (0.1 mg/mL) for 30 min to getGO, 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 adjustedGO and copper sulphate solution [29]. In a typical procedure, 40 mg GOwas adjusted to pH7 using0.01MKH2PO4 andK2HPO4 followed by son-ication for 30 min. The final concentrations of that pH adjusted GO wasmaintained at 4mg/mLby centrifugation. After that, once againwe test-ed the pH of the GO suspension then added 30 mM CSPH in 1:1 ratiowith stirring and allowed stand for 30 min. to get GO-Cu2+ composite.
Fig. 4. a) CVs of GO, CSPE and GO-Cu2+ composite (hybrids)modified GC electrodes in 0.1M PBS (pH=7.4) at a scan rate of 50mV/s. b) CVs of GO-Cu2+ compositemodifiedGC electrodeat different scan rates between 10 and 100mV/s in 0.1M PBS (pH=7.4). c) Plot of current vs scan rate obtained from different scan rates of GO-Cu2+ compositemodified GC electrode in0.1 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.
Fig. 3. SEM images of a) GO and b) GO-Cu2+ composite. EDS analysis of c) GO and d) GO-Cu2+ composite.
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2.4. Instrumentation
All electrochemical measurements were recorded using a CHI660Dpotentiostat (CH instruments, Austin, USA) in a standard three elec-trode system with modified glassy carbon as working electrode, plati-num wire as counter electrode and Ag/AgCl (1 M KCl) as referenceelectrode. Powder X-ray diffraction data was recorded using PhilipsX'pert PRO PANalytical X-ray diffractometer with graphitemonochromatized Cu-Kα (1.5418 Ao) radiation. Scanning electron mi-croscopywas carried out using FEI (Nova nano) for generating images ofGO and GO-Cu2+ composite. Atomic force microscopy was carried outusing Bruker Dimension Icon AFM equipped with a Nanoscope V SPMcontroller operated in Tapping Mode. A silicon cantilever with reso-nance frequency of 300–400 kHz and spring constant 42 N/m wasused. AFM height images were presented after simple flattening usingV 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 particlesize (1, 0.3 and 0.05 μm) followed by rinsing with copious amounts ofdistilled water. Then the electrodes were sonicated in ethanol-water(1:1) mixture for about 5 min. Finally, it was rinsed with distilledwater and dried at room temperature.
For the electrode modification, 20 μL of 5% Nafion solution wasadded into 1 mL GO-Cu2+ composite. 10 μL of that suspension wasdrop cast on the surface of GC electrode and dried normally underroom temperature. Similarly, we have been modified with GO andCSPH solution. Prior to electrochemical measurements the electrolyte
Fig. 5. CVs of a) GO-Cu2+ composite (hybrids) modified GC electrode in the presence and abseelectrode in 0.1MPBS (pH=7.4) at a scan rate of 50mV/s. in the presence and absence of 10mthe presence and absence of 10 mM H2O2 d) CVs of GO-Cu2+ composite modified GC electrod
was saturated with nitrogen gas and electrode was subjected to poten-tial cycling between the potentialwindowof+0.3 to−0.6 V vsAg/AgClfor about 50 cycles to achieve stable performance.
3. Results and discussion
The schematic representation for the synthesis GO-Cu2+ compositeshown in Fig. 1. TheGO containing various oxygen functional groups areresponsible for the formation of composite with Cu2+ ions.
3.1. XRD studies
Fig. 2 depicts the XRDpattern of GO andGO-Cu2+ composite (beforeand after electrochemical stability test). The reflection peak at 2θ =11.3° corresponding to the interlayer basal spacing, attributed due tothe presence of oxygen functional groups present on the surface of car-bon 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 thatbasal spacing between the GO layers increases due to the intercalationof Cu2+ ions. The reflection peak of GO-Cu2+ composite retained evenafter electrochemical stability test without any further additionalpeaks (Fig. 2c.). This suggests that prepared composite also electro-chemically stable.
3.2. Surface morphology and elemental analysis
The surface morphology of GO and GO-Cu2+ composite was ob-served by SEM shown in Fig. 3a and b respectively. The GO shows thepresence of sheets and these sheets remains unaltered even after the in-corporation of Cu2+ ions. We further analysed surface morphology
nce of 10 mM H2O2 in 0.1 M PBS (pH= 7.4) at a scan rate of 50 mV/s. b) GO modified GCMH2O2 c) CSPHmodifiedGC electrode in 0.1M PBS (pH=7.4) at a scan rate of 50mV/s. ine 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.
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using atomic force microscopy (AFM). The incorporation of Cu2+ ionsresults in aggregation of the GO sheets as shown in the fig. S1 and S2in the electronic supplementary information. The root mean squareroughness of the GO-Cu2+ is found to be about 106 nm which is about6.6 times higher than the GO sheets (rms roughness of GO is about16 nm). This confirms that the incorporation of Cu2+ into GO sheets in-creases the roughness, indicating that the aggregation of the GO sheets.
Elemental analysis of GO and GO-Cu2+ composite was performedusing EDS and the results are shown in Fig. 3c and d respectively. Theatomic percentage of C, O, S and Cu in GO and GO-Cu2+ composite areshown in the inset of Fig. 3c and d. GO contains about 0.69% Cu, whichcould be due to impurities. The percentage of Cu increased to 7.78%upon addition of Cu2+ into the GO. The atomic percentage of O remainssame 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 de-creases in the case of GO-Cu2+ composite due to the additional cleaningof composite using ethanol and water. The atomic percentage of both Oand S reveals that GO binds Cu2+ ions through electronegative oxygengroups without exhibiting the CuSO4 complex form.
3.3. Cyclic voltammetry studies
CVwas used for the characterization of GO-Cu2+ composite, GO andCSPH modified GC electrodes in a potential window of +0.3 to−0.6 Vvs Ag/AgCl at a scan rate of 50 mV/s. Fig. 4a. displays the cyclic voltam-mograms (CVs) of GO-Cu2+ composite GO and CSPHmodified GC elec-trodes recorded in 0.1 M phosphate buffer solution (PBS) pH = 7.4.Among the three electrodes, GO-Cu2+ composite electrode showed ex-cellent electrochemical response compared to GO and CSPH modifiedGC 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 anodicpeak at −0.03 V vs Ag/AgCl due to oxidation of Cu(I) to Cu(II) [39].
Fig. 7. a) Amperometric response of the GO-Cu2+ composite modified GC electrode upon inAmperometric response of the GO-Cu2+ composite modified GC electrode upon injection of H(pH = 7.4) at applied potential −0.25 V vs Ag/AgCl.
The similar redox peaks are also observed for the CSPH modified GCelectrode at the same peak potentials. However, the GO modified GCelectrode 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 to100 mV/s with the increment of 10 mV/s in the same buffer solutionas shown in Fig. 4b. The corresponding cathodic and anodic linearplots are shown in Fig. 4c. The current increases linearly with increasein the scan rate. This confirms that the redox peak corresponds to a sur-face confined species. The ratio of anodic and cathodic peak current isapproximately equal to one, which indicates that the electrochemicalreaction is highly reversible. The stability of the GO-Cu2+ compositemodified GC electrode is observed by continuous running for 100 cyclesin 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 sur-face 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 theGO-Cu2+ composite modified GC as working electrode in PBS (pH =7.4) at a scan rate of 50mV/s as shown in Fig. 5a. The GO-Cu2+ compos-itemodifiedGC electrode showed a pair of redox peaks for the reductionand oxidation of copper ions, upon the addition of 10 mMH2O2 the ca-thodic peak current increases due to the reduction of H2O2 at the elec-trode surface. The electrochemical reduction mechanism of hydrogenperoxide on the surface of GO-Cu2+ composite modified GC electrodewas shown in Fig. 6 [2]. We also performed the control experimentswith the GO and CSPH modified GC electrodes for the determinationof H2O2 concentration with the same electrolyte, analyte concentrationand at the same scan rate. There is no change in electrochemical currentresponsewith GOmodified GC electrode in the presence and absence ofH2O2 shown in Fig. 5b. CSPH modified GC electrode also showed a pairof redox peaks which are similar to GO-Cu2+ composite modified GCelectrode in the absence H2O2. After adding 10 mM H2O2, the cathodicpeak current slightly increases due to the reduction of H2O2 at the elec-trode surface as shown in Fig. 5c. However, the increased electrochem-ical current response is less compared to GO-Cu2+ composite modifiedGC electrode. This suggests that GO-Cu2+ composite electrode exhibitsexcellent electrocatalytic response for the reduction of H2O2. Fig. 5dshows the electrochemical response of GO-Cu2+ composite modifiedGC electrode with different concentrations ranging from 2.5 mM to10 mM H2O2. The current increases linearly with the increasing H2O2
concentration.
jection of H2O2 into 0.1 M PBS (pH = 7.4) at applied potential −0.25 V vs Ag/AgCl. b)2O2 and other bioanalytes such as glucose, AA, DA and UA in oxygen saturated 0.1 M PBS
Table 1Analytical performance of the GO-Cu2+ composite modified GC electrode compared withthe Cu2O, CuO and Cu2O-rGO composites.
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3.4. Amperometric studies
Fig. 7a illustrates the amperometric response of GO-Cu2+ compositemodified GC electrode with the successive addition of 5 μM H2O2 intothe nitrogen saturated 0.1 M PBS (pH = 7.4) solution under stirringcondition at a applied potential of −0.25 V vs Ag/AgCl. The current re-sponse increases linearly with the increasing concentration of H2O2.The corresponding calibration plot shown in inset of Fig. 7a. The linearcurrent responses can be obtained for the H2O2 concentration rangesbetween 5 and 85 mM with a correlation co-efficient of 0.999. Thelimit of detection (LOD) was found to be 0.5 μM based on the signal-tonoise ratio of three. The observed detection limit is low as comparedto previously reported CuO, Cu2O and Cu2O-rGO composites as listedin Table 1. We further investigated the interference of the other poten-tial analytes such as glucose, AA, DA and UA in oxygen saturated 0.1 MPBS (pH = 7.4) solution under the same electrochemical conditionwith 100 fold higher concentration of bioanalytes as compared toH2O2 shown in Fig. 7b. There is no change in H2O2 current responseafter the injection of interference analytes. This suggests that the mod-ified electrode was selectively sensing for H2O2 analyte in the presenceand absence of oxygen and nitrogen.
Stability is one of themost important factor used to evaluate electro-chemical sensor. The GO-Cu2+ composite modified GC electrode wasstored in vacuum desiccator and PBS solution (pH 7.4) containing
Fig. 8. Stability of the GO-Cu2+ composite modified GC electrode stored in vacuumdesiccator over 40 days (Amperometric currents obtained by adding 10 mM of H2O2 in0.1 M PBS (pH 7.4), applied potential−0.25 V vs Ag/AgCl electrode).
freshly prepared 10 mM H2O2 was used for electrochemical measure-ments. Amperometric measurements were recorded once in five daysand the corresponding results are shown in Fig. 8. The current responseof the GO-Cu2+ composite modified GC electrode retained above 95%even after forty days. This clearly demonstrates that the modified elec-trode is quite stable for longer time for the determination of H2O2.
4. Conclusions
In summary, we demonstrate a simple method for synthesis of GO-Cu2+ composite for the electrochemical detection of H2O2 in 0.1 MPBS (pH = 7.4). The electrode detects selectively hydrogen peroxidein 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 μMwith a de-tection limit 0.5 μM. The observed detection limit is lower compared topreviously reported CuO, Cu2O and Cu2O-rGO composites.
Acknowledgment
SM acknowledges Jain University, KMUTT for short term researchvisit, NRU and Nanomission (SR/NM/NS-20/2014) for financial support.TR acknowledges DST-SERB (YSS/2015/000075), India for financial sup-port.We alsowish to extendour gratitude toDr. RajalaxmiDash for crit-ical reading of the manuscript.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jelechem.2016.06.034.
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