This article appeared in a journal published by Elsevier.The attached copy is furnished to the author for internal non-commercial research and education use,including for instruction at the authors institutionand sharing with colleagues.Other uses,including reproduction and distribution,or selling or licensing copies,or posting to personal,institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle(e.g.in Word or Tex form)to their personal website orinstitutional repository.Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:/copyrightElectrochimica Acta 56 (2011) 2712–2716Contents lists available at ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taElectrochemical detection of hydroquinone by graphene and Pt-graphene hybrid material synthesized through a microwave-assisted chemical reduction processJing Li a ,b ,Chun-yan Liu a ,∗,Chao Cheng a ,baKey Laboratory of Photochemical Conversion and Optoelectronic Materials of Technical Institute of Physics and Chemistry,Chinese Academy of Sciences,Zhongguancun,Beijing 100190,PR China bGraduate School of the Chinese Academy of Sciences,Beijing 100806,PR Chinaa r t i c l e i n f o Article history:Received 17June 2010Received in revised form 17September 2010Accepted 14December 2010Available online 21 December 2010Keywords:Hydroquinone GraphenePt-graphene hybrid material Electrochemical detectionDifferential pulse voltammetrya b s t r a c tWe have synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application as electrode materials towards the electrochemical detection of hydroquinone.Graphene modified glass carbon electrode (GCE)showed a good performance for detecting hydroquinone due to the unique properties of graphene which increased the active surface area of the electrode and accelerated the electron transfer.The linear detection range of hydroquinone concentration was 20–115␮M with a sensitivity of 1.38␮A ␮M −1cm −2;the detection limit was esti-mated to be 12␮M (S/N =3).The electrocatalytic activity of the Pt-graphene modified GCE was further improved due to the enhanced electron transfer and the linear detection range was 20–145␮M with the sensitivity of 3.56␮A ␮M −1cm −2,detection limit 6␮M (S/N =3).© 2010 Elsevier Ltd. All rights reserved.1.IntroductionHydroquinone is a phenolic compound which is important in a wide number of biological and industrial processes such as coal–tar production,paper manufacturing and photographic developers,and is considered as an important xenobiotic micropollutant [1].Several analytical methods have been used to detect hydroquinone,including high performance liquid chromatography [2,3],flow injection analysis [4,5],spectrophotometry [6,7],and electrochemi-cal methods [8,9],among which the electrochemical methods have attracted great attentions owing to the advantages such as effi-ciency,simplicity and quick response.In previous work,several carbon-based materials such as mesoporous carbon [10],boron-doped diamond [11],conductive carbon cement [12],carbon fibre [13],and carbon nanotubes (CNTs)[14]have been explored for the electrochemical detection of hydroquinone.Graphene (G),a monolayer of carbon atoms packed into a dense,honeycomb crystal structure as well as being a fundamental build-ing block for fullerenes,carbon nanotubes,and graphite [15],has shown fascinating properties and holds the promise for future carbon-based device architectures [16–19].Recently,graphene-based materials have been explored for electrochemical sensor due to its large surface area,extraordinary electronic transport prop-∗Corresponding author.Tel.:+8601082543573;fax:+8601062554670.E-mail address:cyliu@ (C.-y.Liu).erties,strong mechanical strength,and its lower cost and easier preparation in mass quantities compared with carbon nanotubes,typical examples including detection of glucose by metal deco-rated graphene [20]and nitrogen-doped graphene [21]modified glass carbon electrode,electrochemical detection of paracetamol by graphene modified glass carbon electrode [22],and selective detection of dopamine by graphene modified glass carbon elec-trode [23].However,to the best of our knowledgement,there is no report about the eletrochemical detection of hydroquinone by graphene-based materials.Up to now,numerous methods have been developed to synthe-size graphene such as micromechanical cleavage of graphite [24],chemical vapor deposition technique [25],epitaxial growth on a single-crystal silicon carbide by vacuum graphitization [26],and chemical reduction of graphene oxide [27–31].Among them,the chemical reduction of graphene oxide,involving graphite oxida-tion,exfoliation and reduction,is the most efficient approach to bulk production of graphene-based sheets at low cost.In this con-text,we synthesized graphene and Pt-graphene hybrid material by a microwave-assisted chemical reduction process and evaluated their application to electrochemical detection of hydroquinone.2.Experimental2.1.Preparation of grapheneGraphene was prepared through a microwave-assisted chemi-cal reduction of graphene oxide.In a typical procedure,graphite0013-4686/$–see front matter © 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2010.12.046J.Li et al./Electrochimica Acta56 (2011) 2712–27162713oxide(25mg)prepared from natural graphite powders(Beijing Chemical Factory of China)by Hummer’s method[32]was son-icated in water(50mL)for2h using a JK-300ultrasonic cleaner (300W,40kHz)to achieve a clear,brown dispersion of graphene oxide(0.5mg/mL).5mL of hydrazine hydrate(80%)was added into the graphene oxide and kept stirring for3h at100◦C in a microwave oven with program-control(Sineo MAS-II).The heat-ing power was set to300W at the beginning of the reaction and automatically reduced to as low as about10W when a given tem-perature reached.The stirring rate was set to700rps.The black graphene was obtained after centrifugation,washed with water and ethanol,and naturally dried in air.2.2.Preparation of Pt-graphene hybrid materialThe route to prepare Pt-graphene hybrid material was similar to graphene,except that5mL of hexachloroplatinic acid solution (0.0193M)was added to the graphene oxide solution and kept stirring for15min at room temperature before the addition of hydrazine hydrate.2.3.Preparation of graphene and Pt-graphene modified glass carbon electrodePrior to modification,the glassy carbon electrodes(GCE)of3mm diameter were polished with0.5␮m alumina powder,washed with deionized water and then dried in air.Graphene-based materials were dispersed in N,N-dimethylformamide(DMF)by sonication for30min to achieve a2mg/mL graphene-DMF suspension.Then, 5␮L of the suspension was coated onto GCE and allowed to dry in air.2.4.CharacterizationThe AFM images were recorded using a Multimode Nanoscope III a AFM(Veeco Metrology LLC,Santa Barbara,CA).Tapping-mode imaging was applied to provide the largest amount of structural detail of the graphene oxide sheets as well as to prevent transloca-tion of the sheets on the surface by the tip(Veeco MP-11100silicon cantilevers,force constant k=60N/m,radius of curvature r=10nm, and resonance frequency f=300kHz).The sample was prepared by depositing graphene oxide dispersion in water(0.05mg/mL) onto a new cleaved mica surface and dried under vacuum at room temperature.The TEM images were taken with a JEM2100F trans-mission electron microscope,by using an accelerating voltage of 200kV.The samples were dispersed in deionized water by sonica-tion and dropped onto a conventional carbon-coated copper grid. The XRD pattern was obtained with a Bruker D8Focus under Cu K␣radiation at1.54056˚A with a scanning speed of4◦min−1.The XPS measurements were performed on a MICROLAB MK II spec-trophotometer with Mg K␣radiation.UV–vis absorption spectra were recorded with a Shimadzu UV-1601PC spectrophotome-ter.The samples were dispersed in deionized water for optical measurements.Differential pulse voltammetry(DPV)was car-ried out at room temperature with a CHI660C workstation(CH Instruments,Chenhua,Shang-hai,China)connected to a personal computer.A three-electrode configuration was employed,con-sisting of a modified glassy carbon electrode(3mm in diameter) serving as the working electrode,saturated calomel electrode and platinum wire serving as the reference and counter electrodes, respectively.3.Results and discussionAFM images of graphene oxide revealed the presence of sheets with thickness of1.199nm(Fig.1),which wascharacter-Fig.1.AFM images and height profile of graphene oxide.istic of a fully exfoliated graphene oxide sheet[27,33,34].Such thickness was larger than that of single-layer pristine graphene (0.34nm),due to the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the dis-placement of the sp3-hybridized carbon atoms slightly above and below the original graphene plane[27].The TEM images of graphene sheets and Pt-graphene hybrid material were shown in Fig.2,where the transparent graphene sheets with crumpled silk veil waves on the top of the carbonfilm were observed(Fig.2a),and the rumples was intrinsic to graphene nanosheets[35].A large number of Pt nanoparticles were sup-ported on graphene sheets and few particles resided outside of the support(Fig.2b),which indicated the strong interaction between the Pt nanoparticles and the graphene sheets.The size distribution of Pt nanoparticles was8–45nm(Fig.2c).In Fig.3,graphite showed a sharp diffraction peak at26.2◦cor-responding to(002)plane with d-spacing of0.34nm(Fig.3a). Compared with the graphite,the feature diffraction peak of graphite oxide appeared at10.4◦corresponding to d-spacing of 0.85nm(Fig.3b),which was larger than that of graphite due to the intercalated water molecules between layers[36].After the exfo-liation of graphite oxide by ultrasonic vibration and subsequent chemical reduction,the obtained graphene showed a broadened diffraction peak at24.5◦(Fig.3c),meaning the layers of graphite along c-axis were exfoliated and the carbon sp2bond were restored. The XRD pattern of Pt-graphene hybrid material is shown in Fig.3d. Apart from the peak at24.5◦assigned to graphene,all other peaks can be indexed to Pt nanoparticles(face-centered cubic,JCPDS04-0802).2714J.Li et al./Electrochimica Acta56 (2011) 2712–2716Fig.2.TEM images of (a)graphene sheets and (b)Pt-graphene hybrid material,(c)the size distribution of supported Pt nanoparticles.Compared with graphite oxide,the C 1s XPS spectrum of graphene at 286–289eV corresponding to oxygenated carbon showed a significant decrease (Fig.4a),confirming that most of the epoxide,hydroxyl,and carboxyl functional groups were success-fully removed through the reduction process.This observation was in agreement with that found in previous studies [27,31].Fig.4b represented the XPS signature of the Pt 4f doublet (4f 7/2and 4f 5/2)for the Pt nanoparticles supported on graphene sheets.The Pt 4f 7/2and Pt 4f 5/2peaks appeared at 70.1eV and 73.35eV,respectively,which shifted remarkably to the lower binding energy compared with the standard binding energy of Pt 4f 7/2and Pt 4f 5/2for Pt 0state (70.83eV and 74.23eV)[37]due to the electron transfer from the graphene sheet to Pt nanoparticles.Because the work func-tion of graphene (4.48eV)[38]is smaller than that of Pt (5.65eV)[39],electron transfer from the graphene sheets to Pt nanoparticles would occurred during the formation of the Pt-graphene hybridstructures.Fig.3.XRD patterns of (a)graphite,(b)graphite oxide,(c)graphene,and (d)Pt-graphene hybrid material.The UV–vis spectrum of GO exhibited two characteristic peaks,one was at 230nm corresponding to ␲→␲*transitions of aromatic C–C bonds,and a shoulder at 303nm was attributed to n →␲*transitions of C O bonds (Fig.5a)[40].The peak at 230nm was red shift to 268nm after the chemical reduction treatment (Fig.5b and c),which was an indication of the restoration of the electronic conjugation within the G sheets [41].The activity of the graphene modified GCE and Pt-graphene modified GCE towards electrochemical detection of hydroquinone were investigated by differential pulse voltammetry (DPV).The GCE exhibited a weak and broad peak at 0.09V correspondingtoFig.4.(a)C 1s XPS spectra of graphite oxide and graphene,and (b)Pt 4f spectra of Pt-graphene.J.Li et al./Electrochimica Acta 56 (2011) 2712–27162715Fig.5.UV–vis absorption spectra of (a)graphene oxide,(b)graphene,and (c)Pt-graphene hybrid material.the oxidation of hydroquinone (curve a in Fig.6A).The graphene-modified GCE displayed a well-defined peak at 0.002V with a much higher current intensity as compared with the GCE (the curve b in Fig.6A),indicating the enhanced electrocatalytic activ-ity towards hydroquinone,which could be due to the unique properties of graphene that increased the active surface area of the electrode and accelerated the electron transfer via improved conductivity and the good affinity of graphene to hydroquinone.When Pt nanoparticles were combined with graphene,the cur-rent density further increased,maybe due to the enhanced electron transfer in Pt-graphene hybrid system,which was attributed to the charge hopping through the metallic Pt nanoparticles andthe effective charge migration through the graphene.The effec-tive transport of the electrons to the electrode in the Pt-graphene matrix led to the efficient electrocatalytic oxidation of hydro-quinone.We can observe in Fig.6B and C that the peak current increased with the increase of the hydroquinone concentration (from the curve a to f).It could be seen from the curve a in Fig.6D that the current at graphene modified GCE linearly increased with the increase of the concentration of hydroquinone over the 20–115␮M range with sensitivity of 1.38␮A ␮M −1cm −2;and the detection limit was estimated to be 12␮M (S/N =3).For Pt-graphene modified GCE (the curve b in Fig.6D),a lin-ear detection range was from 20␮M to 145␮M with sensitivity of 3.56␮A ␮M −1cm −2,the detection limit 6␮M (S/N =3).Addi-tionally,the current intensity at the Pt-graphene modified GCE was higher than that at the graphene modified GCE in the whole concentration range.These results indicated that the Pt-graphene modified GCE showed higher current intensity,lower detection limit,and higher sensitivity towards electrochemical detection of hydroquinone compared with the pure graphene modified GCE,possibly due to the enhanced electron transfer in the Pt-graphene hybrid system.The applicability of the graphene modified GCE to the selective detection of hydroquinone in the presence of phenol was stud-ied using DPV.Phenol and hydroquinone showed respectively an oxidation peak at 0.218eV (Fig.7a)and 0.002eV (Fig.7b).For a mixed solution of 0.15mM hydroquinone and 10mM phenol,there were two well-distinguished peaks at the potential of 0.002eV and 0.218eV,corresponding to the oxidation of hydroquinone and phenol,respectively (Fig.7c),indicating that hydroquinone can be selectively detected in the presence of large concentration of phe-nol.Fig.6.DPV obtained at GCE (a),graphene modified GCE (b),and Pt-graphene modified GCE (c),using 115␮M hydroquinone in 0.05M (pH 7.4)phosphate buffer solution (A).DPV obtained at graphene modified GCE (B)and Pt-graphene modified GCE (C)with various concentration of hydroquinone:(a)20␮M,(b)29␮M,(c)65␮M,(d)83␮M,(e)115␮M and (f)145␮M in 0.05M (pH 7.4)phosphate buffer.DPV conditions:pulse amplitude =0.05V,sample width =0.0167s,pulse width =0.15s,pulse period =0.4s,and quiet time =2s.(D)Calibration plots of background subtracted peak current at graphene modified GCE (a)and Pt-graphene modified GCE (b)versus the concentration of hydroquinone.2716J.Li et al./Electrochimica Acta56 (2011) 2712–2716Fig.7.DPV obtained at graphene modified GCE:(a)10mM phenol,(b)0.15mM hydroquinone,and(c)10mM phenol and0.15mM hydroquinone in0.05M(pH7.4) phosphate buffer.DPV condition:pulse amplitude=0.05V,sample width=0.0167s, pulse width=0.15s,pulse period=0.4s,and quiet time=2s.4.ConclusionsIn conclusion,we have synthesized graphene and Pt-graphene hybrid material through a microwave-assisted chemical reduction process.The resulting graphene and Pt-graphene hybrid materials have been used for the electrochemical detection of hydroquinone. Graphene modified GCE showed a good performance for detect-ing hydroquinone due to the unique properties of graphene,which increased the active surface area of electrode and accelerated the electron pared with the pure graphene modified GCE, the electrocatalytic activity of the Pt-graphene hybrid material was further improved possibly due to the enhanced electron transfer in the Pt-graphene hybrid system.AcknowledgementsThe author thanks the support of Chinese Academy of Sciences and973Program.References[1]H.Cui,C.He,G.J.Zhao,J.Chromatogr.A855(1999)171.[2]C.H.Lin,J.Y.Sheu,H.L.Wu,Y.L.Huang,J.Pharm.Biomed.Anal.38(2005)414.[3]N.A.Penner,P.N.Nesterenko,Analyst125(2000)1249.[4]M.E.Rueda,L.A.Sarabia,A.Herrero,M.C.Ortiz,Anal.Chim.Acta479(2003)173.[5]G.N.Chen,J.S.Liu,J.P.Duan,H.Q.Chen,Talanta53(2000)651.[6]L.P.Jia,A.M.Zhang,Chin.J.Anal.Chem.31(2003)1508.[7]T.Gajewska,A.Kaszuba,Polimery44(1999)690.[8]I.C.Vieira,O.Fatibello-Filho,Talanta52(2000)681.[9]L.H.Wang,Analyst120(1995)2241.[10]Y.Hou,L.P.Guo,G.Wang,J.Electroanal.Chem.617(2008)211.[11]D.Sopchak,ler,Y.Avyigal,R.Kalish,J.Electroanal.Chem.538(2002)39.[12]X.J.Huang,J.J.Pot,W.Th.Kok,Anal.Chim.Acta300(1995)5.[13]R.M.Carvalho,C.Mello,L.T.Kubota,Anal.Chim.Acta420(2000)109.[14]Y.P.Ding,W.L.Liu,Q.S.Wu,X.G.Wang,J.Electroanal.Chem.575(2005)275.[15]A.K.Geim,K.S.Novoselov,Nat.Mater.6(2007)183.[16]T.Takamura,K.Endo,L.Fu,Y.P.Wu,K.J.Lee,T.Matsumoto,Eletrochim.Acta53(2007)1055.[17]S.Gilje,S.Han,M.Wang,K.L.Wang,R.B.Kaner,Nano Lett.7(2007)3394.[18]J.S.Bunch,A.M.van der Zande,S.S.Verbridge,I.W.Frank,D.M.Tanenbaum,J.M.Parpia,H.G.Craighead,P.L.McEuen,Science315(2007)490.[19]A.Sakhaee-Pour,M.T.Ahmadian,A.Vafai,Solid State Commun.147(2008)336.[20]T.T.Baby,S.S.J.Aravind,T.Arockiadoss,R.B.Rakhi,S.Ramaprabhu,Sens.Actu-ators B:Chem.145(2010)71.[21]Y.Wang,Y.Y.Shao,D.W.Matson,J.H.Li,Y.H.Lin,ACS Nano4(2010)1790.[22]X.H.Kang,J.Wang,H.Wu,J.Liu,I.A.Aksay,Y.H.Lin,Talanta81(2010)754.[23]Y.Wang,Y.M.Li,L.H.Tang,J.Lu,J.H.Li,mun.11(2009)889.[24]K.S.Novoselov,A.K.Geim,S.V.Morozov,D.Jiang,Y.Zhang,S.V.Dubonos,I.V.Grigorieva,A.A.Firsov,Science306(2004)666.[25]N.G.Shang,P.Papakonstantinou,M.McMullan,M.Chu,A.Stamboulis,A.Potenza,S.S.Dhesi,H.Machetto,Adv.Funct.Mater.18(2008)3506.[26]A.Charrier,A.Coati,T.Argunova,Y.Garreau,R.Pinchaux,I.Forbeaux,J.M.Debever,M.Sauvage-Simkin,J.M.Themlin,J.Appl.Phys.92(2002)2479. [27]S.Stankovich,D.A.Dikin,R.D.Piner,K.A.Kohlhaas,A.Kleinhammes,Y.Y.Jia,Y.Wu,S.T.Nguyen,R.S.Ruoff,Carbon45(2007)1558.[28]H.L.Guo,X.F.Wang,Q.Y.Qian,F.B.Wang,X.H.Xia,ACS Nano3(2009)2653.[29]G.X.Wang,J.Yang,J.Park,X.L.Gou,B.Wang,H.Liu,J.Yao,J.Phys.Chem.C112(2008)8192.[30]H.L.Wang,J.T.Robinson,X.L.Li,H.J.Dai,J.Am.Chem.Soc.131(2009)9910.[31]Y.Zhou,Q.L.Bao,L.A.L.Tang,Y.L.Zhong,K.P.Loh,Chem.Mater.21(2009)2950.[32]W.Hummers,R.Offeman,J.Am.Chem.Soc.80(1958)1339.[33]S.Stankovich,R.D.Piner,X.Chen,N.Wu,S.T.Nguyen,R.S.Ruoff,J.Mater.Chem.16(2006)155.[34]Y.Xu,H.Bai,G.Lu,C.Li,G.Shi,J.Am.Chem.Soc.130(2008)856.[35]J.C.Meyer,A.K.Geim,M.I.Katsnelson,K.S.Novoselov,T.J.Booth,S.Roth,Nature446(2007)60.[36]A.Buchsteiner,A.Lerf,J.Pieper,J.Phys.Chem.B110(2006)22328.[37]J.F.Moudler,W.F.Stickle,P.E.Sobol,K.D.Bomben,Handbook of X-ray Photo-electron Spectroscopy,PerkinElmer,Eden Prairie,MN,1992.[38]X.J.Wu,X.C.Zeng,Nano Lett.9(2009)250.[39]H.B.Michaelson,J.Appl.Phys.48(1977)4729.[40]J.I.Paredes,S.Villar-Rodil,A.Martınez-Alonso,J.M.D.Tascón,Langmuir24(2008)10560.[41]D.Li,M.B.Muller,S.Gilje,R.B.Kaner,G.G.Wallace,Nat.Nanotechnol.3(2008)101.。