graphene oxide: preparation, functionalization, and electrochemical applications

Graphene Oxide: Preparation, Functionalization, and ElectrochemicalApplicationsDa Chen,, Hongbin Feng, and Jinghong Li*,

Department of Chemistry, Tsinghua University, Beijing 100084, ChinaCollege of Materials Science & Engineering, China Jiliang University, Hangzhou 310018, China

CONTENTS

1. Introduction 60272. Structure and Properties of GO 6028

2.1. Structure 60282.2. Properties 6033

3. Preparation and Functionalization of GO-basedElectrodes 60343.1. Preparation of GO-based Electrodes 60343.2. Functionalization of GO-based Electrodes 6035

3.2.1. Functionalization with Nanoparticles 60353.2.2. Functionalization with Organic Com-

pounds 60363.2.3. Functionalization with Polymers 60373.2.4. Functionalization with Biomaterials 6038

4. GO-based Electrochemical Applications 60384.1. Electrocatalysis 60394.2. Electrochemiluminescence 60404.3. Electrochemical Gas Sensors 60414.4. Electrochemical Biosensors 6043

4.4.1. Enzyme Biosensors 60434.4.2. Hemeprotein Biosensors 6044

4.5. Electrochemical Immunosensors 60454.6. Electrochemical DNA Sensors 60454.7. Other Electrochemical Sensors 6046

5. Conclusions and Perspectives 6047Author Information 6047

Corresponding Author 6047Notes 6047Biographies 6047

Acknowledgments 6048References 6048

1. INTRODUCTION

Graphene, which consists of a one-atom-thick planar sheetcomprising an sp2-bonded carbon structure with exceptionallyhigh crystal and electronic quality, is a novel material that hasemerged as a rapidly rising star in the eld of materialscience.14 Ever since its discovery in 2004,5 graphene has beenmaking a profound impact in many areas of science andtechnology due to its remarkable physicochemical properties.These include a high specic surface area (theoretically 2630m2/g for single-layer graphene),1,6,7 extraordinary electronicproperties and electron transport capabilities,810 unprece-dented pliability and impermeability,11,12 strong mechanicalstrength12 and excellent thermal and electrical conductiv-ities.13,14 These unique physicochemical properties suggest ithas great potential for providing new approaches and criticalimprovements in the eld of electrochemistry. For example, thehigh surface area of electrically conductive graphene sheets cangive rise to high densities of attached analyte molecules. This inturn can facilitate high sensitivity and device miniaturization.Facile electron transfer between graphene and redox speciesopens up opportunities for sensing strategies based on directelectron transfer rather than mediation. It is not surprising,therefore, that graphene has recently attracted great attentionworldwide from the electrochemical community. Despite itsshort history, this 2D material has already revealed potentialapplications in electrochemistry, and remarkably rapid progressin this area has already been made. In recent years, manyreviews covering graphene and related materials have beenpublished.1,4,7,1527 In addition, several reviews with particularemphasis on graphene-based electrochemical applications havealso appeared.2835

One specic branch of graphene research deals withgraphene oxide (GO). This can be considered as a precursorfor graphene synthesis by either chemical or thermal reductionprocesses. GO consists of a single-layer of graphite oxide and isusually produced by the chemical treatment of graphite throughoxidation, with subsequent dispersion and exfoliation in wateror suitable organic solvents.36,37 With respect to its structure,there have been several structural models3740 proposed overthe years. These assume the presence of various oxygen-containing functional groups in the GO. The oxygen functionalgroups have been identied as mostly in the form of hydroxyland epoxy groups on the basal plane, with smaller amounts ofcarboxy, carbonyl, phenol, lactone, and quinone at the sheetedges.4143 However, currently the precise atomic structure ofGO is still uncertain and remains to be fully elucidated. This is

Received: December 4, 2010Published: August 14, 2012

Review

pubs.acs.org/CR

2012 American Chemical Society 6027 dx.doi.org/10.1021/cr300115g | Chem. Rev. 2012, 112, 60276053

primarily due to the uncertainty pertaining to both the natureand distribution of the oxygen-containing functional groups,44

its nonstoichiometric atomic composition, and the lack ofsuciently sensitive analytical techniques for characterizing theGO structure. In reality, GO incarnates various nanoscaleinhomogeneities in its structure, and the stoichiometry variesdepending on the synthesis protocol as well as the extent of thereaction. In fact, the ideal stoichiometry has never beenachieved. The oxygenated groups in GO can strongly aect itselectronic, mechanical, and electrochemical properties. Hencethey account for the dierences between GO and pristinegraphene.45 Compared with pristine graphene, on the onehand, the covalent oxygenated functional groups in GO canindeed give rise to remarkable structure defects. This isconcomitant with some loss in electrical conductivity,46 whichpossibly limits the direct application of GO in electrically activematerials and devices. On the other hand, the presence of thesefunctional groups can also provide potential advantages forusing GO in numerous other applications. The reasons are asfollows: rst the polar oxygen functional groups of GO renderit strongly hydrophilic. This gives GO good dispersibility inmany solvents, particularly in water.7,4749 This is important forprocessing and further derivatization. The resulting GO-stabledispersion can be subsequently deposited on various substratesin order to prepare thin conductive lms by means of commonmethods such as drop-casting, spraying, or spin-coating.43

These can be used as excellent electrode materials. In addition,using well-known chemistry strategies, these functional groupsserve as sites for chemical modication or functionalization ofGO, which in turn can be employed to immobilize variouselectroactive species through covalent or noncovalent bonds forthe design of sensitive electrochemical systems. Therefore, thechemical composition of GO, which can be chemically,thermally, or electrochemically engineered, allows the tunabilityof its physicochemical properties.41,50,51 For example, byappropriately ne-tuning the oxidation or reduction parameterswith a view to controlling the structural disorder, GO can bemade into an insulating, semiconducting, or semimetallicmaterial. Although the unique relativistic nature of chargecarriers and other condensed-matter eects that are observed innearly ideal graphene are absent in GO, accessibility, ease ofsynthesis, solution processability, and its versatile propertiesmake it attractive for fundamental research as well as inapplications.41

The study of GO-based materials in recent years has beenpopular and extensive, particularly with respect to electro-chemical applications. Underpinning the signicance of GO-based materials in electrochemistry are the very specicproperties that although relevant to GO are not typical ofpristine graphene. These include its facile synthesis, highdispersibility in a range of solvents, capability of couplingelectroactive species onto the surface, and unique opticalproperties (such as uorescence labels52). In addition, the useof GO-based materials also provides control over the localmicroenvironment. This is because in most cases GO-basedmaterials can be deposited, with extremely well-denedsurfaces, through solution processing. This can be highlyadvantageous when incorporating sensitive or electroactivespecies into an electrochemical system. Another important issueis consideration of the costs when manufacturing an electrodefor use in any real system. In terms of the manufacture of GO-based devices, costs can be reduced compared with the costs forconventional electrodes. This is because only a fraction of the

amount of the GO-based materials is required for solution-processed thin lm deposition. Moreover, GO-based thinconductive lms deposited onto an inexpensive base materialcan lead to a larger surface area-to-volume ratio, which furtherlowers the cost of the electrode. Meanwhile, the large eectivesurface area can also provide a larger number of active sites andoften also a higher signal-to-noise ratio. Owing to theseadvantageous specic properties, GO-based materials have beenused to design and prepare GO-based electrodes for a widerange of applications in electrochemical sensors and electro-analysis (Figure 1). To date, although considerable advances in

this area have already been made, clearly there is also a greatdeal of scope for further study in GO-related electrochemicalapplications.In this review, we hope to summarize and critically discuss

recent advances in the use of GO-based materials in the eld ofelectroanalytical chemistry and electrochemical sensors.

2. STRUCTURE AND PROPERTIES OF GOOn a simple level, GO can be considered as consisting ofindividual sheets of graphene decorated with oxygen functionalgroups on both the basal planes and edges.53 However, asmentioned in the Introduction, the precise atomic andelectronic structure of GO remain largely unknown. Greaterdetails about the GO structure, such as possible dominatingstructural motifs, are still under investigation. In order toprovide helpful information in tailoring the fundamentalproperties of GO and to unleash its potential applications, itis both critical and desirable to explore the atomic details of theGO structure in depth.2.1. Structure

The study of the GO structure is derived from the structuralanalysis of graphite oxide itself. Although graphite oxide wasrst prepared in the mid-1800s,54 its composition and structureis still under debate because of its nonstoichiometriccomposition and the strong hygroscopicity of dehydratedgraphite oxide. Over the years, both theoretically andexperimentally, considerable eort has been directed towardunderstanding the structure of graphite oxide. The result is thatseveral conicting models have been successively proposed.Originally, in 1939, Hofmann and Holst55 proposed a simplemodel, in which graphite oxide was thought to consist of epoxy(1,2-ether) group modied planar carbon layers with amolecular formula of C2O. In 1946, Ruess

56 suggested that

Figure 1. Schematic illustration of GO-based electrodes for electro-chemical applications.

Chemical Reviews Review

dx.doi.org/10.1021/cr300115g | Chem. Rev. 2012, 112, 602760536028

the carbon layers were not in fact planar but puckered and thatthe oxygen-containing groups were hydroxyl and ether-likeoxygen bridges between carbon atoms 1 and 3, randomlydistributed on the carbon skeleton. Later, in order to explainthe acidic properties of graphite oxide, Hofmann and co-workers57 further incorporated an enol- and keto-type structureinto their model, which also contained hydroxyls and etherbridges at the 1 and 3 positions. In 1969, Scholz and Boehm58

proposed a new structure with corrugated carbon layers. Herethe epoxide and ether groups were completely replaced bycarbonyl and hydroxyl groups. Meanwhile, Nakajima et al.59,60

proposed a dierent model for graphite oxide. This modelconsisted of two carbon layers linked to each other by sp3

carboncarbon bonds perpendicular to the layers and in whichcarbonyl and hydroxyl groups were present in relative amountsdepending on the level of hydration. Based on expert NMRstudies, Leaf and co-workers38 proposed a structural modelhaving a random distribution of at aromatic regions withunoxidized benzene rings and wrinkled regions of alicyclic six-membered rings bearing CC, COH, and ether groups(reassigned to the 1 and 2 positions). In light of these previousmodels, Szabo et al.39 recently proposed a new structural modelthat involves a carbon network consisting of two kinds ofregions: (i) trans-linked cyclohexane chairs and (ii) ribbons ofat hexagons with CC double bonds as well as functionalgroups such as tertiary OH, 1,3-ether, ketone, quinone, andphenol (aromatic diol). Even more recently, Dreyer et al.37

reviewed the structural analogies and dierences among the

above structural models of graphite oxide. It is recommendedthat interested readers refer to this review for more specicdetailed information.While the studies mentioned above outlined many of the

fundamental structural features of graphite oxide, it is clear thata more rened picture of the ne GO structure is necessary.One dierence from an ideal graphene sheet, which consists ofonly trigonally bonded sp2 carbon atoms,61 is the fact that theGO sheet consists of a hexagonal ring-based carbon networkhaving both (largely) sp2-hybridized carbon atoms and (partly)sp3-hybridized carbons bearing oxygen functional groups(Figure 2A).39,62 In GO, the carbon atoms that are covalentlybonded with oxygen functional groups (such as hydroxyl,epoxy, and carboxy) are sp3 hybridized. These can be viewed asoxidized regions, and they disrupt the extended sp2 conjugatednetwork of the original honeycomb-lattice structured graphenesheet. The latter can be viewed as the unoxidized regions.63,64

These sp3 hybridized carbon clusters are uniformly butrandomly displaced slightly either above or below the grapheneplane.65 To date, in order to explore the GO structure in moredepth, various microscopic and spectroscopic techniques havebeen employed for the investigation of its structural features.For example, atomic force microscopy (AFM) directly gives theapparent thickness of the single-layer GO (around 1 nm, Figure2B) as well as the number of layers.64,6668 On the other hand,conductive AFM reveals the electrical defects in GO.69

Scanning tunneling microscopy (STM) has been used toexamine the structural features of the GO sheets.64,68,70 Results

Figure 2. (A) Scheme of structural model of graphene and graphene oxide (GO), showing that graphene consists of only trigonally bonded sp2

carbon atoms while GO consists of a partially broken sp2-carbon network with phenol, hydroxyl, and epoxide groups on the basal plane andcarboxylic acid groups at the edges. (B) AFM image of a GO sheet. The apparent thickness of a single sheet is around 1 nm. Reprinted withpermission from ref 67. Copyright 2008 American Chemical Society. (C) STM image of a GO monolayer on a highly oriented pyrolytic graphite(HOPG) substrate. Oxidized regions are marked by green contours. Reprinted with permission from ref 64. Copyright 2007 American ChemicalSociety.



show it to be distinguishable from pristine graphene by theappearance of bright spots and regions lacking ordered latticefeatures due to the presence of oxygenated functional groups asshown in Figure 2C. According to the ratio of these unorderedregions, the degree of functionalization can be estimated.64

Recently, the direct imaging of lattice atoms and topologicaldefects in single-layer GO has been achieved using high-resolution transmission electron microscopy (HRTEM).45,7173

This is a signicant breakthrough in exploring the GOstructure. By means of HRTEM, Erickson et al.71 identiedthe specic atomic scale features of the GO monolayers, whichcomprise three major regions: holes, graphitic regions, and highcontrast disordered regions with approximate area percentagesof 2%, 16%, and 82%, respectively (Figure 3). The authorsproposed that the holes in GO are formed due to the release ofCO and CO2 during the aggressive oxidation and sheetexfoliation. They also suggest that the graphitic regions comefrom the incomplete oxidation of the basal plane with thepreserved honeycomb structure of graphene, while thedisordered regions of the basal plane consist of a high-densityregion of oxygen functionalities. These consist of hydroxyls,(1,2) epoxies, and carbonyls, with each carbon most likelybeing oxidized with no order between the functionalities. Inaddition, possible atomic structures for the disorderedfunctionalities were also proposed. Reported by Gomez-Navarro and co-workers,45 aberration-corrected HRTEM wasfurther used to unravel the topological defects in GO. These

include the dominant clustered pentagons and heptagons, aswell as the in-plane distortions and strain in the surroundinglattice. In addition, scanning transmission electron microscopy(STEM) combined with electron energy loss spectroscopy(EELS) has proven to be very eective for measuring the nestructure of the carbon and oxygen K-edges as well as low-losselectronic excitations in GO.46 These results indicate that theoxygen atoms are attached to the graphene sites randomly andconvert the sp2 carbon bonds in graphene to sp3 bonds. In GO,the plasma excitations are related to those in graphene but witha substantial blue-shift occurring due to the presence of theoxygen and increased number of sp3 bonds.The chemical composition of GO and the oxygen functional

groups in GO have been identied using various spectroscopictechniques, including solid-state nuclear magnetic resonance(SSNMR),40,7476 X-ray absorption near-edge spectroscopy(XANES),73,7781 Raman spectroscopy,68,70,77,81 X-ray photo-electron spectroscopy (XPS),40,50,68,79,81,82 and Fourier trans-form infrared spectroscopy (FT-IR).75,79,83,84 Typical solid-state 13C magic-angle spinning NMR spectra reveal that thereare three main peaks in the 13C NMR spectrum of GO. Thepeak around 60 ppm is assigned to carbon atoms bonding tothe epoxy group, the peak around 70 ppm corresponds to thehydroxyl group connected to the carbon atoms, and the peakaround 130 ppm is ascribed to the graphitic sp2 carbon (Figure4A).40,74 It has been demonstrated that these assignments ofthe three main peaks are most likely correct.40,74 In addition, in

Figure 3. (A) Aberration-corrected TEM image of a GO monolayer. On the bottom, holes are indicated in blue, graphitic areas in yellow and highcontrast; red indicates disordered regions with oxygen functionalities. (B) Another aberration-corrected TEM image of a GO monolayer for detailedstructural examination. The scale bar is 2 nm. Expansion a shows, from left to right, a 1 nm2 enlarged oxidized region of the material, then a proposedpossible atomic structure of this region with carbon atoms in gray and oxygen atoms in red, and nally the average of a simulated TEM image of theproposed structure. Expansion b focuses on the white spot seen in the graphitic region. This spot moved along the graphitic region but stayedstationary for three frames (6 s) at a hydroxyl position (left portion of expansion b) and for seven frames (14 s) at a (1,2)-epoxy position (rightportion of expansion b). The ball-and-stick gures below the microscopy images represent the proposed atomic structure for such functionalities.The simulated TEM image for the suggested structure agrees well with the TEM data. Expansion c shows a 1 nm2 graphitic portion from the exitplane wave reconstruction of a focal series of GO and the atomic structure of this region. Reprinted with permission from ref 71. Copyright 2010John Wiley & Sons, Inc.



the high-resolution 13C NMR spectrum another three smallpeaks were also found at about 101, 167, and 191 ppm. Thesethree weak peaks were tentatively assigned to lactol, the estercarbonyl, and the ketone groups, respectively.40 XANES hasproven to be another powerful tool for characterizing GOmaterials. It provides information on the degree of bondhybridization in mixed sp2/sp3 bonded carbon, the specicbonding congurations of functional atoms, and the degree ofalignment of the graphitic crystal structures within GO.81 Thehigh-resolution C K-edge XANES spectrum of GO shown inFigure 4B demonstrates a clear presence of both unoccupied *and * states around 285.2 and 293.03 eV, respectively. Thesecan be primarily assigned to the 1s * and 1s *transitions in the graphitic carbon atoms in GO.78,81 Thebroadening of the absorption peak at 289.3 eV can be assignedto the 1s * transitions in the carbon involved in bondingwith the oxygen atoms. In particular, the ratio of */* peaksat the C K-edge provides an estimate of the relativeconcentration of sp2 domain congurations in an sp3 matrixconsisting of carbon atoms connected to oxygen groups. Hencethis indicates the degree of oxidation in GO.81 On the otherhand, the typical O K-edge XANES spectrum of GO showsseveral distinctive absorption peaks at 531.5, 534.0, 535.5,

540.0, 542.0, and 544.5 eV. These have been assigned to*(CO), *(CO), *(OH), *(CO), *(CO), and*(CO), respectively.73 This O K-edge spectrum thusclaries the chemical composition of the oxygenated functionalgroups in GO. This includes carbonyl groups together with theepoxide and hydroxyl groups attached to aromatic rings, andthe carboxyl groups most likely attached to the edges of the GOsheets. In addition, XPS further unambiguously reveals thenature of the carbon and oxygen bonds in their various states:unoxidized carbons (sp2 carbon), CO, CO, and COOH. Ina number of reports on GO,50,81,85,86 the C1s signal of pristineGO, as seen in Figure 4C, consists of ve dierent chemicallyshifted components, which can be deconvoluted into sp2

carbons in aromatic rings (284.5 eV) and C atoms bonded tohydroxyl (COH, 285.86 eV), epoxide (COC, 286.55 eV),carbonyl (>CO, 287.5 eV), and carboxyl groups (COOH,289.2 eV). Other reports,38,77 on the other hand, consider thedeconvolution of the C1s spectra using four components,namely, sp2, COH, COC, and COOH, while ignoring thepresence of the >CO groups. There is still a considerabledegree of vagueness regarding the presence of the carbonyl>CO groups. Information provided by analysis of the O1sspectra can complement the information provided by analysis

Figure 4. (A) Solid-state 13C magic-angle spinning (MAS) NMR spectra of GO. Reprinted with permission from ref 40. Copyright 2009 NaturePublishing Group. (B) High-resolution (a) C K-edge and (b) O K-edge synchrotron XANES spectra of GO. Reprinted with permission from ref 81.Copyright 2011 American Chemical Society. (C) High-resolution C1s XPS spectra of GO. Reprinted with permission from ref 81. Copyright 2011American Chemical Society.



of the C1s spectra. Deconvolution of the O1s spectra producesthree main peaks around 531.08, 532.03, and 533.43 eV. Thesehave been assigned to CO (oxygen doubly bonded toaromatic carbon),50,83 CO (oxygen singly bonded to aliphaticcarbon),65,87 and phenolic (oxygen singly bonded to aromaticcarbon)65,87 groups, respectively. The pristine GO shows anadditional peak at a higher binding energy (534.7 eV).86 Thiscorresponds to the chemisorbed/intercalated adsorbed watermolecules. Moreover, another important parameter that can beused to characterize the degree of oxidation in GO is the sp2

carbon fraction. This can be estimated by dividing the areaunder the sp2 peak by C1s peak area. The sp2 fraction of thepristine GO was found to be only 40%, and the amount ofcarbon sp2 bonding was found to increase with the loss ofoxygen during the thermal reduction. It was found to reach amaximum value of 80% at an oxygen content of 8 atom %(C/O ratio 12.5:1).50,81 This suggests that the remainingoxygen is responsible for 20% of the sp3 bonding.Furthermore, Raman and FTIR spectroscopy data corroboratethe identication of the degree of oxidation and the oxygenatedspecies in GO. The Raman spectrum of a GO lm displays a D-band at 1340 cm1 and a broad G-band at 1580 cm1.85 TheG-band, which is characteristic of all sp2-hybridized carbonnetworks, originates from the rst-order scattering from thedoubly degenerate E2g phonon modes of graphite in theBrillouin zone center, while the prominent D peak comes fromthe structural imperfections created by the attachment ofoxygenated groups on the carbon basal plane.85,88,89 Thus, theintegrated intensity ratio of the D- and G-bands (ID/IG)

indicates the oxidation degree and the size of sp2 ring clusters ina network of sp3 and sp2 bonded carbon.50,88 For example, anaverage graphitic domain size of 2.5 nm in pristine GO wascalculated.50 After GO thermal reduction, the ID/IG ratio wasfound to signicantly decrease. This indicates considerablerecovery of the conjugated graphitic framework upondefunctionalization of the epoxide and hydroxyl groups.77 FT-IR spectroscopy is recognized as an important tool forcharacterization of functional groups, and in the case of GOhas supported the presence of hydroxyl (broad peak at 30503800 cm1), carbonyl (17501850 cm1), carboxyl (16501750 cm1), CC (15001600 cm1), and ether or epoxide(10001280 cm1) groups.75,79,83On the other hand, theoretical studies have also received

considerable attention in the exploration of the complexstructure of GO. This is because theoretical simulations canprovide considerable insight into the possible kinetic andthermodynamic mechanisms needed for a greater under-standing of the structural evolution of the functional oxy-genated groups found in GO. In general, the current eortsemploy simple schemes that permit basic structural derivationsof GO using common oxygen groups. For instance, based onrst-principle calculations, the energetically favorable atomiccongurations (building blocks) in GO have been identied ascontaining epoxide and hydroxyl groups in close proximity toeach other.9094 Dierent arrangements of these building blockunits yield a local-density approximation band gap over a rangeof a few electronvolts. This suggests the possibility of creatingand tuning the band gap in GO by varying the oxidation level

Figure 5. (A) Conductivity of thermally reduced GO as a function of the sp2 carbon fraction. The vertical dashed line indicates the percolationthreshold at sp2 fraction of 0.6. The 100% sp2 materials are polycrystalline (PC) graphite and graphene. The two conductivity values are for doped bygating (upper triangle) and intrinsic graphene (lower triangle). Reprinted with permission from ref 50. Copyright 2009 John Wiley & Sons, Inc. (B)(ac) Structural models of GO during dierent stages of reduction. The smaller sp2 domains indicated by zigzag lines do not necessarily correspondto any specic structure but to small and localized sp2 congurations that act as the luminescence centers. The PL intensity is relatively weak for (a)as-synthesized GO but increases with reduction due to (b) formation of additional small sp2 domains between the larger clusters because ofevolution of oxygen with reduction. After extensive reduction, the smaller sp2 domains create (c) percolating pathways among the larger clusters. (d)Schematic band structure of GO. Smaller sp2 domains have a larger energy gap due to a stronger connement eect. Photogeneration of anelectronhole (eh) pair on absorption of light (Eexc) followed by nonradiative relaxation and radiative recombination resulting in uorescence(EPL) is depicted. Black arrows denote the transitions of electrons and holes during this process. DOS, electronic density of states. Reprinted withpermission from ref 113. Copyright 2010 John Wiley & Sons, Inc.



and the relative amounts of epoxide and hydroxyl functionalgroups on the surface.91,92 Using density functional theory(DFT) calculations, Lahaye et al.95 revealed that the oxygenatoms and the adjacent carbon atoms form 1,2-ether groups(epoxides) on the carbon grid. During the oxidation process,the hydroxyl groups,are formed on the opposite side of thecarbon plane. Using a theoretical approach, Li et al.96 furtherillustrated the graphene oxidative breakup process, whichprovides a useful insight into the puzzling GO structure. Inaddition, theoretical calculations have also been used to studythe atomistic structure of progressively reduced GO as well asthe chemical changes of functional oxygenated groups duringthe GO reduction treatments.83,84,97,98 Bagri et al.83 reported amolecular dynamics simulation study of the evolution of theGO structure on thermal annealing. Their simulation resultsreveal the formation of carbonyl and ether groups throughtransformation of the initial hydroxyl and epoxy groups duringthe thermal annealing process. These carbonyl and ether groupsare thermodynamically very stable and hence hinder thecomplete reduction of GO to graphene. Based on ab initiocalculations, Larciprete and co-workers98 proposed a dual pathmechanism in the thermal reduction of GO driven by theoxygen coverage. At low surface densities, the O atomsadsorbed as epoxy groups evolve as O2 leaving the C networkunmodied; whereas with higher coverage of oxygen, theformation of other O-containing species opens up competingreaction channels. These consume the C backbone. In addition,theoretical simulations can provide useful information forobtaining the correct spectroscopic assignments, and these inturn are useful for interpreting experimental spectroscopicdata.70,82,99 For example, Kudin et al.70 simulated the Ramanspectra of GO and proposed an alternating singledoublecarbon bond model.

2.2. Properties

Due to its specic 2D structure and the existence of variousoxygenated functional groups, GO exhibits various excellentproperties. These include electronic, optical, thermal, mechan-ical, and electrochemical properties, as well as chemicalreactivity. We will now briey introduce electronic, optical,and electrochemical properties, as well as address the chemicalreactivity of GO.Electronic properties such as conductivity of GO sheets

depend strongly on their chemical and atomic structures. Moreaccurately, they depend upon the degree of structural disorderarising from the presence of a substantial sp3 carbon fraction asseen in Figure 5A. In general, as-synthesized GO lms aretypically insulating with an energy gap in the electron density ofstates,50,90 and they exhibit sheet resistance (Rs) values of about1012 sq1 or higher.100 The intrinsic insulating nature of GOis strongly correlated to the amount of sp3 CO bonding. Thisin turn represents transport barriers,50 leading to the absence ordisruption of percolating pathways among the sp2 carbonclusters, which allows classical carrier transport to occur.41

However, the reduction of GO (i.e., the incremental removal ofoxygen) using a variety of chemical and thermal treatments,which facilitates the transport of carriers,51,101 can result in adecrease in Rs by several orders of magnitude and hencetransform the material into a semiconductor and ultimately intoa graphene-like semimetal.50,63,64,75,102108 The conductivity ofthe reduced GO samples can reach 1000 S/m,75,108 and byuse of resistivity and temperature-programmed desorption(TPD) measurements, the activation energy of the GO

platelets has been estimated as 32 5 kcal/mol.109 By use oftheoretical rst-principles calculations, the local-density approx-imation band gap of GO was found to vary over a range of afew electronvolts depending on the oxidation level.91 Thissuggests a great potential for tuning the energy gap in GOthrough the use of controlled reduction processes.In addition to these interesting electronic characteristics, GO

is also expected to exhibit unique optical properties asevidenced by the recent demonstration of photoluminesence(PL) from GO.110 This luminescence was found to occur fromthe near-UV-to-blue visible (vis) to near-infrared (IR) wave-length range. This property should prove useful for biosensing,uorescence tags and optoelectronics applications.111114

These PL characterisitcs originate from the recombination ofelectronhole (eh) pairs, localized within small sp2 carbonclusters embedded within an sp3 matrix. The PL intensity varieswith the nature of the reduction treatment, and can becorrelated to the evolution of very small sp2 clusters (Figure5B).113 In addition, GO also possesses specic ultrafast opticaldynamics and nonlinear optical (NLO) properties, whichshould prove useful for potential applications in optoelectronicdevices. In GO, it was found that there are two NLO regimes(sp2 domains and sp3 domains) with dierent ultrafast opticaldynamics. In the sp2 domains, two-photon absorptiondominates the nonlinear absorption for picosecond pulses.On the other hand, for nanosecond pulses excited stateabsorption also inuences the nonlinear response in the sp3

domains.115 On the basis of heterogeneous ultrafast dynamicsof GO with saturable absorption in sp2 domains and two-photon absorption in sp3 domains, the NLO response can betailored by manipulation of the degree and location of oxidationon the GO sheets.116 Increasing the degree of reduction in GOcauses excited state absorption to gradually switch to saturableabsorption for shorter probe wavelengths. For example, Kurumand co-workers117 demonstrated that both electrochemicallyinduced reversible reduction and optically induced photo-reduction in GO resulted in changes in the NLO properties ofGO thin lms.Recently, it has become popular to explore the electro-

chemical properties of GO at electrode surfaces. Due to itsfavorable electron mobility and unique surface properties, suchas one-atom thickness and high specic surface area, GO canaccommodate the active species and facilitate their electrontransfer (ET) at electrode surfaces.118120 For example, Zuo etal.120 reported that GO supports the ecient electrical wiring ofthe redox centers of several heme-containing metalloproteins(cytochrome c, myoglobin, and horseradish peroxidase (HRP))to the electrode. Second, GO possesses excellent electro-catalytic properties.121123 For example, our group121 hasdemonstrated the electrocatalytic activity of GO toward oxygenreduction and certain biomolecules. Wang et al.122 reported theelectrocatalytic activity of reduced GO (rGO) toward theoxidation of hydrazine. In addition, it has been shown that GOexhibits high electrochemical capacitance with excellent cycleperformance and hence has potential application in ultra-capacitors.123,124 Shao et al.123 reported that rGO shows muchhigher electrochemical capacitance and cycling durability thancarbon nanotubes (CNTs). The specic capacitance was foundto be 165 and 86 F/g for rGO and CNTs, respectively.Due to the presence of a large number of oxygen-containing

functional groups and structural defects, GO exhibits enhancedchemical activity compared with pristine graphene.37 It appearsthat one of the most important reactions of GO is its reduction.



GO can be reduced to graphene by various approaches. In thepast few years, there have been reports of reducing GO in thesolution phase using various reducing agents, such ashydrazine,66,125 sodium borohydride,126 or hydroquinone,127

and in the vapor phase using hydrazine/hydrogen64,85 or just bythermal annealing85 or by electrochemical techniques.119,128

Another important chemical reactivity of GO is its capacity forchemical functionalization. This involves adding other groupsto GO platelets using various chemical reactions. It is knownthat GO has chemically reactive oxygen functionalities, such ascarboxylic acid, epoxy, and hydroxyl groups. Thus, an idealapproach to chemical functionalization would utilize orthogonalreactions of these groups to selectively functionalize one siteover another.37 Typically, GO can be covalently functionalizedusing specially selected small molecules or polymers throughactivation and amidation/esterication of either the carboxylsor hydroxyls in GO via coupling reactions.129131 For example,the GO carboxylic acid groups were rst activated using thionylchloride (SOCl2), followed by the coupling of octadecylaminevia the formation of amides. The result of this strategicsequence renders GO soluble in common organic solvents.129

The epoxy groups can be also used for the covalentfunctionalization of GO through ring-opening reactions dueto a nucleophilic attack at the -carbon by the amine.37,101,132

For example, octadecylamine was attached to GO via a ring-opening reaction with the epoxy groups. This aorded colloidalsuspensions of GO in organic solvents.101 Yang et al.132

reported the attachment of an ionic liquid (1-(3-aminopropyl)-3-methylimidazolium bromide; R-NH2) with an amine endgroup to GO platelets also via a ring-opening reaction withepoxy groups. Due to the high polarity of the material, theresulting chemically modied graphenes (CMGs) were well-dispersed in solvents such as water, N,N-dimethylformamide(DMF), and dimethyl sulfoxide (DMSO). In addition to thecovalent functionalization, noncovalent functionalization of GOcan also be accomplished via stacking, cation, van derWaals interactions, or hydrogen bonding.37,133,134 Yang and co-workers133 reported the preparation of the doxorubicinhydrochloride (DXR)/GO hybrid material through non-covalent interactions. It was suggested that stacking andhydrophobic interactions between the quinone functionality ofDXR and sp2 networks of GO were the primary noncovalentinteractions. In addition, strong hydrogen bonding may be alsopresent between the OH and COOH groups of the GO andthe OH and NH2 groups in DXR. Very recently, GO hasbeen found to act as a convenient carbocatalyst for facilitatingoxidation and hydration reactions.135138 This suggests thatGO could be useful beyond the realm of displays andelectronics. For example, Bielawskis135 group demonstratedthat GO can catalyze the oxidation of various alcohols andalkenes as well as the hydration of various alkynes into theirrespective aldehydes and ketones in good to excellent yields. Inaddition, GO has also shown strong oxidizing properties.Bielawski and co-workers139 reported that GO was an eectiveoxidant for use in a broad range of reactions. These include theoxidation of olens to their respective diones, methyl benzenesto aldehydes, diarylmethanes to ketones, and variousdehydrogenations.

3. PREPARATION AND FUNCTIONALIZATION OFGO-BASED ELECTRODES

The crucial development of GO-related electrochemicalapplications lies in the design of simple, reliable assembly

routes for the preparation and functionalization of GO-basedelectrodes. Fine control over assembly conditions is responsiblefor the range of novel properties exhibited by these GO-basedelectrodes. To date, several methods for the preparation andfunctionalization of GO-based electrodes are well established.These techniques are employed to achieve an ecientelectrochemical communication between the chemical reactionsites and the GO-based electrode interface with high levels ofintegration, sensor miniaturization, measurement stability,selectivity, accuracy, and precision. Thus, it must be acknowl-edged that the preparation and functionalization method iscritical for the satisfactory electrochemical performances ofGO-based electrodes.

3.1. Preparation of GO-based Electrodes

Starting from a stable colloidal suspension of GO, severalstrategies can be adapted to prepare hierarchically organizedstructures of GO-based materials on various underlyingelectrode substrates (such as Au, glassy carbon, and quartzglass). The rst approach, which is the simplest and moststraightforward and pragmatic, involves the simple evaporationof a thin lm of a GO suspension on the electrode surface. Thishas to be carried out under well-controlled conditions oftemperature and humidity in order to ensure very slowevaporation of the solvent (in most cases water). This allowsthe colloidal particles to nd their energy minima by forming analmost perfect structure on the electrode surface.140,141 It is fairto say that the GO-based electrodes used in most of thehitherto-reported electrochemical studies were simply preparedby randomly dispersing the GO-based materials onto asubstrate electrode114,142147 or by conning GO on a substrateelectrode with other functional materials such as Naon123 orionic liquids148 through drop-casting, dip-coating, or sprayingprocedures. These GO-based electrodes have been demon-strated to be useful for practical electrochemical applications.However, such a procedure often results in intrinsic defects(such as cracks) and nonuniform deposition due to the randomconnement and aggregation of the GO. This results in poorcontrol over the lm quality and thickness.41 As a result,alternative methods for the preparation of GO-based electrodesfrom an electrochemical point of view are still needed.Spin-coating is one of alternative approaches, which is now

frequently used for the preparation of GO-based electro-des.100,149153 It has proven to produce much better control ofdefect density and also of the overall thickness of the GO lmlayer. A typical process involves depositing a small amount of aliquid GO suspension onto the center of an electrode substrateand then spinning the substrate at high speed. Final lmthickness and homogeneity depend on the GO suspensionconcentration, the process parameters (such as spinning speedand acceleration), and the number of spin-coating cycles. Thistechnique can result in continuous large-area lms with theabsence of nanometer scale wrinkling, as was found in otherGO deposition procedures.100,149

Another important approach for the preparation of GO-based electrodes is to use the self-assembly technique. This hasrecently received considerable interest because of potentialapplications in sensor fabrication and for the patternedchemical architecture of solid supports.154 This technique notonly can provide a facile protocol for the preparation of GOlms with controlled thickness but also can eciently adjustelectrode dimension from conventional to nanoelectrodeensemble. This would be very attractive for electrochemical



studies on a nanoscale. Due to the strong electrostatic repulsionbetween the 2D conned layers,67 stable monolayers of GOsheets can be well formed at the airwater interface. By meansof LangmuirBlodgett (LB) assembly, these GO monolayerscan be transferred to an electrode substrate, readily creatinghighly uniform GO thin lm-based electrodes.42,67,155 Inaddition, GO sheets can be also self-assembled onto anelectrode substrate by a layer-by-layer technique using apositively charged self-assembled monolayer (SAM). This ispossible since the oxygen-rich functional groups on GO aregenerally negatively charged. It has been demonstrated that GOsheets can be guided to precise locations on an electrodesubstrate using the electrostatic attraction between thenegatively charged GO sheets and a positively chargedtemplate.128,156,157 The distribution of the resulting GO sheetsdepends on the surface functionalization, background passiva-tion, pH, and deposition time. For example, Wei et al.156

reported the self-assembly of GO sheets onto a pretreated Auelectrode with amino-terminated 11-amino-1-undecanethiol(AUT) template patterns. It was found that GO sheets didnot adhere to the bare Au surface but could be selectively self-assembled onto the AUT patterns due to the electrostaticinteraction between GO and AUT. Aside from the electrostaticattraction, the hydrophobic interaction between the GO sheetsand the SAM template has also been considered as anotherpossible driving force to direct the self-assembly of GO sheetsonto the SAM template-pretreated substrate surface.158

Alternatively, vacuum ltration has also been used to deposituniform layers of GO for the preparation of GO-basedelectrodes. This technique involves ltering the suspensionthat contains the GO nanosheets through a porous membranewith well distributed pores, such as a cellulose estermembrane,51,159161 an anodic aluminum oxide (AAO)membrane,162 or an anodisc membrane lter.163,164 Tosummarize, during the ltration of the GO suspension throughthe porous membrane, the liquid can pass through the poreswhereas the GO sheets become lodged, leading to thedeposition of the GO lms on the membrane. The permeationprocess is self-regulating and allows reasonably good nanoscalecontrol over the lm thickness by simply varying theconcentration of the GO suspension or the ltration volume.The deposited GO lms can then be transferred onto varioussubstrates by gently pressing the lm against the substratesurface and dissolving the porous membrane,41 leaving behind awell-adhered uniform GO thin lm.In addition, the direct patterning of GO thin lms onto

various substrates, especially exible substrates, is highlydesirable for the preparation of the GO-based exibleelectrodes. To realize the direct patterning process, however,novel methods are still needed. Very recently, Dua et al.165

reported that the inkjet printing of chemically rGO plateletsonto poly(ethylene terephthalate) (PET) could be achieved byusing aqueous surfactant-supported dispersions of rGO powderas the printing ink. The resulting inkjet-printed lm exhibitedgood electrical conductivities ( 15 S cm1). In this work, theink cartridge of a commercial inkjet printer was emptied andrelled with a freshly prepared GO suspension, and then theGO ink was printed directly onto a exible substrate (such ascommercially available PET). The patterns were designed on acomputer in advance. Inkjet printing allows control of the lmthickness by altering the number of passes and also by makingit possible to use the gray scale on the computer. In anotherexample, He et al.166 developed a totally dierent approach to

the fabrication of highly uniform patterned GO lms on varioussubstrates including exible PET. They used a method knownas micromolding in capillary. In their work, a drop of GOaqueous solution was dropped at one end of the open channelsof a hard polydimethylsiloxane (PDMS) stamp, which was thenplaced in close contact with the 3-aminopropyltriethoxysilane(APTES)-modied at substrates using an applied force.Afterward the whole system was placed in a vacuum ovenand degassed for 30 min to ensure that the solution wassuciently dried. Subsequently the PDMS stamp was carefullypeeled o and the GO patterns were thus formed on thesubstrate. This was followed by a reduction process inhydrazine vapor at 60 C for 12 h in order to obtain therGO patterns. Compared with other methods for thefabrication of such GO patterns, this micromolding in capillarymethod is fast, facile, and substrate independent.

3.2. Functionalization of GO-based Electrodes

GO-based electrodes are dierent from other kinds of carbon-based materials used in electrochemistry, in that they consist ofa 2D layered structure with a large surface area and also possessa large number of oxygen-containing functional groups, such ashydroxyl, carboxyl, and epoxy groups. Such properties make itpossible to functionalize such GO-based electrodes using eithercovalent or noncovalent chemistry in order to modulate theelectrodes structural architecture and intrinsic properties. Todate, a broad range of functional materials (such as nano-particles, organic compounds, polymers, and biomaterials) havebeen reported for the functionalization of GO-based electrodes.These generally exhibit novel, interesting properties and holdpromise for many applications, especially those involvingelectrochemistry, due to their high electrical conductivity,chemical stability, tunable modication, and multifunctionalstructures.3.2.1. Functionalization with Nanoparticles. GO-based

nanocomposites, because of their unique structures and superbproperties, have emerged as one category of intriguing materialswith promising applications in the eld of electrochemistry. ForGO-based nanocomposites, the unique properties of the GOnanosheets make them particularly useful as the nanoparticlesupport. This is because the high surface area is essential for thedispersion of the nanoparticles and in order to maintain theirelectrochemical activities. The GO-supporting materials notonly maximize the availability of the nanosized surface area forelectron transfer but also provide better mass transport of thereactants to the electroactive sites on the electrode surface.Moreover, the conductive GO support facilitates the ecientcollection and transfer of electrons to the collecting electrodesurface. On the other hand, the functionalization of GO withnanoparticles has made the realization of nanoscale compositeelectrodes possible. GO-based nanocomposite-modied elec-trodes present unusual advantages over macroelectrodes inelectrochemical applications. These include excellent catalyticactivity, enhancement of mass transport, a high eective surfacearea, and control over the electrode microenvironment. Inaddition, the combination of GO with nanoparticles may provecapable of contributing additional performance in somefunctional electrochemical applications.Earlier eorts were devoted to the preparation of functional

GO/inorganic nanocomposites. These are derived from thedecoration of GO sheets with inorganic nanoparticles (NPs)such as metal nanoparticles and metal oxide nanoparticles, witha view to their application in electrochemical sensing, catalysis,



and fuel cells. These functional metal or metal oxide/GOnanocomposites can be prepared using dierent physical orchemical approaches: a physical attachment approach,167 an insitu chemical reduction process,168171 electrochemical syn-thetic processes,172174 impregnation processes,175 a self-assembly approach,176 ultrasonic spray pyrolysis,177 and soon. By means of these approaches, GO-based nanocompositeswith metal nanoparticles (such as Pt,168170,172,175,178180

Au,178 or Ru179,180) and oxide nanoparticles (such asTiO2,

167,181,182 ZnO,173,174,177 SnO2,171,176,177,183 Cu2O,

174

MnO2,176,184 Mn3O4,

185 NiO,176 and SiO2176,186) have all

been reported recently and used for the preparation of GO-based nanocomposite-modied electrodes. For example, Donget al.180 reported a simple approach for the deposition of Pt andPtRu nanoparticles onto surfaces of GO nanosheets byethylene glycol reduction. They systematically investigated theeect of GO as a catalyst support on the electrocatalytic activityof Pt and PtRu nanoparticles for both methanol and ethanoloxidation used in fuel cell applications. In comparison to carbonblack as a catalyst support, GO eectively enhanced theelectrocatalytic activity of Pt and PtRu nanoparticles for theoxidation of methanol and ethanol into CO2. By using anexfoliation-reassembling method, Paek et al.183 prepared SnO2/GO nanoporous electrodes with three-dimensionally delami-nated exible structures. They were used for the enhancementof cylic performance and lithium storage capacity. From theirexperimental results, it appears that the dispersed GOnanosheets in the ethylene glycol solution were reassembledand homogeneously distributed between the loosely packedSnO2 nanoparticles. The resulting SnO2/GO nanoporouselectrodes were able to limit the volume expansion uponlithium insertion, and this resulted in the superior cyclicperformances.

In addition to metal and metal oxide nanoparticles, quantumdots (QDs) have also been used to functionalize GO with aview to creating another platform for electrochemicalapplications.187190 For instance, our group188 successfullyprepared QD-sensitized rGO nanocomposites by the in situgrowth of QDs on noncovalently functionalized rGO. TheseQD-sensitized rGO photoelectrodes were then used as anecient platform for photoelectrochemical applications. Inaddition, other nanoparticles, such as Prussian blue,191193

metal hydroxides,194,195 and polyoxometalates,196,197 have alsobeen used in the functionalization of GO-based electrodes forelectrochemical applications. For example, Chen et al.195

reported a facile soft chemical approach for the fabrication ofrGOCo(OH)2 nanocomposites in a waterisopropanolsystem. They demonstrated that the electrochemical perform-ance of Co(OH)2 was signicantly improved after depositionon rGO sheets.3.2.2. Functionalization with Organic Compounds.

The various hybrid materials created by the organicfunctionalization of GO have generated intense attention.This is largely driven by the possibility of improving itssolubility/processability in both water and organic solvents andcombining some of the outstanding properties of the GOnanosheets with those of small organic molecules, such asphotoactive or electroactive units. Such hybrid materials can bepotentially used for preparing durable and functional chemicallymodied electrodes that have already greatly facilitated variouselectrochemical studies and applications. Currently, the non-covalent and covalent functionalization of GO with organiccompounds has become the subject of intensive research forthe fabrication of novel hybrid nanocomposites with newfunctions and applications. With respect to the noncovalentapproach, as described above, the strong adsorption of organicaromatic compounds onto GO nanosheets is primarily

Figure 6. (A) Schematic illustration and images of aqueous dispersions of reduced GO sheets and composites on the surface: (a) reduced GOaqueous dispersion, black precipitate after reduction; (b) reduced GOPDI aqueous dispersion, without precipitate after centrifugation; (c) reducedGOPyS aqueous dispersion, without precipitate after centrifugation. Reprinted with permission from ref 201. Copyright 2009 John Wiley & Sons,Inc. (B) Schematic representation of part of the structure of the covalent 5,4-(aminophenyl)-10,15,20-triphenyl porphyrin (TPP-NH2)/GO hybridnanocomposites. Reprinted with permission from ref 198. Copyright 2009 John Wiley & Sons, Inc.



attributed to the -stacking and hydrophobic interactionsbetween the organic molecules and the GO. Where thecovalent approach is concerned, the presence of oxygen-containing groups in GO provides a handle for its chemicalmodication using known carbon surface chemistry. Thisrequires the chemical reagent to be capable of diusing into thereactive regions of the GO sheet and reacting with thefunctional groups on the sheet surface.Until now, a wide range of organic compounds, such as

porphyrins,130,198,199 aromatic dyes,118 alkylamines,200 doxor-ubicin hydrochloride,133 ionic liquids,132 pyrene,201 perylene-diimide,201 cyclodextrin,202 7,7,8,8-tetracyanoquinodimethane(TCNQ),203 and aryl diazonium compounds,204 have all beennoncovalently or covalently attached onto the GO nanosheetsto generate functional organic nanocomposites for electro-chemical applications. For example, Liu et al.118 proposed afacile method to increase the dispersity of rGO usingnoncovalent functionalization of the GO with a water-solublearomatic electroactive dye. They used methylene green (MG)and investigated the electrochemical properties of the resultingrGO/MG nanocomposites. It was demonstrated that the rGO/MG nanocomposites conned onto a glassy carbon electrodeshowed lower charge-transfer resistance and better electro-catalytic activity toward the oxidation of -nicotinamideadenine dinucleotide (NADH) compared with a pristinerGO-based electrode. In another case, Su et al.201 presentedan unprecedented approach to noncovalently functionalizedrGO by using a large aromatic donor (pyrene-1-sulfonic acid,

PyS) as well as acceptor molecules (3,4,9,10-perylenetetracar-boxylic diimide bisbenzenesulfonic acid, PDI) via interactions as shown in Figure 6A. Their approach gave riseto a novel class of GO nanocomposites with tunable electronicproperties. Xu et al.198 reported the covalent functionalizationof GO with a porphyrin, namely, 5,4-(aminophenyl)-10,15,20-triphenyl porphyrin (TPP-NH2) via an amide bond (Figure6B). Attachment of TPP-NH2 signicantly improved thesolubility and dispersion stability of the GO-based material inorganic solvents. The resulting TPP-NH2/GO hybrid nano-composites exhibited excellent optical-limiting properties.3.2.3. Functionalization with Polymers. Recently, GO-

based polymer nanocomposites are emerging as new class ofmaterials that hold promise for electrochemical applications.These nanocomposites show considerable improvement inproperties that cannot normally be achieved using conventionalcomposites or pristine polymers. Generally, these improve-ments can be obtained at very low GO ller loadings in thepolymer matrix. The extent of the improvement depends verymuch on the nature of the polymer, the delicate morphologicalorganization, the ne interface control, and the degree ofdispersion of the GO nanollers within the polymermatrix.131,205,206 In addition, functionalization with polymersis an eective route for ameliorating the GO dispersion. Thus itis possible to incorporate GO into an excellent compatiblepolymer material in the absence of any aggregation.207209

Furthermore, such functionalization should also prove helpfulfor the modication of electrode interfaces with potential

Figure 7. Schematic illustration of the process for preparation of PANI/GO or PANI/rGO nanocomposites by in situ polymerization of anilinemonomer in the presence of GO under acidic conditions. Reprinted with permission from ref 211. Copyright 2010 American Chemical Society.



applications in electrochemistry, since GO-based polymernanocomposites possess high electrical conductivity, chemicalstability, the option for tunable modication, and suitableapplicable electroactive sites.210,211

Kuilla et al.212 have thoroughly reviewed the wide range ofpolymers that have been reported to date that have been usedto functionalize GO nanosheets for the fabrication of novelnanocomposites with a view to new functions and applications.Of note is the fact that many research groups have beenworking on the functionalization of GO with electroactivepolymer materials, such as polyaniline (PANI),211,213,214

poly(3,4-ethylenedioxythiophene) (PEDOT),210 poly-(styrenesulfonate) (PSS),210 Naon,215,216 and poly-(diallyldimethylammonium chloride) (PDDA),217 with theaim of enhancing their electrochemical properties and overallperformance. It was found that GO sensors functionalized withelectroactive polymers often oer more stability, highersensitivity, and better selectivity compared with pristine GOor polymer sensors.213,215 There are two main strategies for thesurface functionalization of GO using polymers: in situintercalative polymerization211,213 and solution intercala-tion.214217 With respect to the in situ interactive polymer-ization method, rst the GO or rGO is swollen well within theliquid monomer. Subsequently, the polymerization is initiatedusing a suitable initiator added under controlled experimentalconditions. Zhang et al.211 successfully prepared (reduced)GO/PANI nanocomposites by the in situ polymerization ofaniline monomer in the presence of GO under acidicconditions (Figure 7). These nanocomposites exhibited auniform structure with the PANI bers absorbed onto the GOsurface or lled in the GO sheets. When used as supercapacitorelectrodes, such uniform structures aorded high speciccapacitance and good cycling stability during the chargedischarge process. Alternatively, solution intercalation is basedon a solvent system in which the polymer is solubilized and GOsheets are allowed to swell.212,218 Typically, the GO or rGOsheets are well dispersed in a suitable polymer solution with theassistance of sonication or mechanical agitation. The polymerthen adsorbs onto the delaminated GO sheets, and after theevaporation of the solvent, the sheets reassemble, sandwichingthe polymer to form the nanocomposites.219 In one example,GO nanosheets were stably dispersed in water by noncovalentfunctionalization with sulfonated polyaniline (SPANI) throughsolution intercalation. The resulting composite lm of SPANI-functionalized GO sheets showed enhanced electrochemicalstability and electrocatalytic activity.214 Choi et al.215 reportedthe preparation of free-standing exible conductive rGO/Naon (RGON) hybrid lms made by a solution chemistrytechnique using self-assembly and directional convectiveassembly. The hydrophobic backbone of Naon providedwell-dened integrated structures for the construction of hybridmaterials through self-assembly, while the hydrophilic sulfonategroups enabled highly stable dispersibility and long-termstability for the rGO sheets. It is important to note that thesynergistic electrochemical characteristics of the RGON wereattributed to the high conductivity, facilitated electron transfer,and low interfacial resistance, all of which lead to their excellentperformance as electrochemical biosensing platforms fororganophosphate detection.3.2.4. Functionalization with Biomaterials. Because GO

has a large specic surface area and abundant functional groups,it provides an ideal platform for the accommodation ofbiomolecules, which is a key issue in the fabrication of

biofunctional GO-based electrodes for biorelated applications.Recent studies reveal that GO shows excellent biocompati-bility,114,220222 enhances the electrochemical reactivity and theelectron transfer (ET) rate of biomolecules,120,223 and is able toaccumulate biomolecules.224 To take advantage of theseremarkable GO properties in biorelated electrochemicalapplications, a prerequisite is the development of physical orchemical methods to immobilize biological molecules onto theGO-based electrodes in a reliable manner.Both noncovalent and covalent modication methods have

been reported. Alhough the immobilization of biomoleculesonto GO has been pursued in the past, to the best of ourknowledge, most of these biofunctional GO/biomoleculescomposites were prepared by noncovalent modicationmethods, such as physical entrapment120,134,223 or adsorptionby incubation of the biomolecules with a GO suspen-sion.220,225229 The noncovalent interaction between GO andthe biomolecules involves electrostatic or hydrogen-bondinginteractions, hydrophobic interactions, and stackinginteractions.227 For example, Zhang et al.228 fabricatedenzyme-modied GO-based electrodes through the incubationof enzymes (such as HRP and lysozyme) with a GOsuspension. They demonstrated that the electrostatic inter-actions and hydrogen bonding between the enzymes and theGO contributed to the enzyme immobilization onto the GOsheets. In addition, the catalytic performance of theimmobilized enzymes was also related to the interaction ofthe enzyme molecules with the surface functional groups of theGO substrate. Apart from the noncovalent attachment, also ofimportance is the covalent attachment for the immobilization ofthe biomaterials with the GO sheets. Frequently, the hydro-philic oxygen-containing functional groups of the GO havebeen used to covalently bind biomaterials such as bovine serumalbumin (BSA),230 DNA molecules,52,114 and enzymes119,221 viaa carbodiimide coupling of the respective amino-functionalizedbiomolecules. For example, Liu et al.221 reported the direct,ecient fabrication of glucose biosensors through a covalentattachment between the carboxylic acid groups on the GOsheets and the amine groups of the glucose oxidase. Theresulting biosensor showed a broad linearity, good sensitivity,excellent reproducibility, and storage stability suitable forglucose biosensing. In addition, the biotinstreptavidininteraction is considered another important covalent attach-ment method for the linking of streptavidin/biotin biomole-cules to biotin/streptavidin-activated GO sheets. For instance,Liu et al.222 prepared the GO/streptavidin complex and used itto capture biotinylated protein complexes via the streptavidinbiotin interaction for anity purication.

4. GO-BASED ELECTROCHEMICAL APPLICATIONSThe high surface-to-volume ratio of GO, in conjunction with itshigh dispersibility in both water and organic solvents as well asits wide range of reactive surface-bound functional groups,makes GO-based materials very attractive for electrochemicalstudies and applications. As described in Section 2, GO-basedmaterials exhibit a moderate conductivity (depending on theextent of reduction), high chemical stability, and excellentelectrochemical properties. More remarkably, GO has beendemonstrated to be capable of facilitating the direct electrontransfer of enzymes and proteins (such as, cytochrome c andhorseradish peroxidase) at a GO-based electrode.120 Theseproperties not only make it possible to understand the intrinsicthermodynamic and kinetic electron transfer properties of



electroactive species at GO-based electrode interfaces but alsopave a new route to electrochemical sensors and electroanalysis.To date, GO-based materials have been used to design andprepare GO-based electrodes for a wide range of electro-chemical applications, including electrocatalysis, electrochemi-luminescence, electrochemical sensors, immunoassays, DNAsensors, and others.

4.1. Electrocatalysis

Electrocatalysis represents one of the most important areas forthe application of GO-based materials in electrochemistry. Firstof all, GO itself possesses excellent electrocatalytic activitiestoward some important species.121,122,144,147,158,166,202,231234

For example, our group121 studied the electrochemical andelectrocatalytic properties of rGO lms, and demonstrated thatthe rGO-based electrode exhibited fast electron-transferkinetics and possessed excellent electrocatalytic activity towardoxygen reduction. When compared with bare basal and edge-plane pyrolytic graphite electrodes, Lin et al.231 reported thatthe rGO-modied basal and edge-plane pyrolytic graphiteelectrodes exhibited excellent electrocatalytic activity towardthe electrocatalytic oxidation of H2O2 and -nicotinamideadenine dinucleotide (NADH). Interestingly, Yang et al.158

constructed GO-based electrodes with a tunable electrodedimension using the controllable adsorption of rGO onto theSAM of n-octadecyl mercaptan (C18H37SH) at Au electrodes.They demonstrated the excellent electrocatalytic activity of the

rGO/SAM electrode toward ascorbic acid, dopamine, and uricacid.In addition, the introduction of inorganic nanocatalysts may

oer GO-based electrodes novel electrocatalytic properties dueto the excellent catalytic activities of such inorganic nano-catalysts. In this case, the GO generally acts as a support for thedeposition of the inorganic nanocatalysts. In addition, itfacilitates or mediates the charge/electron transfer betweenthe electroactive species and the electrode surface of the GO-supported nanocatalysts and modulates the electrochemicalreactions in a controlled fashion. To date, GO has beenextensively studied as a support for the dispersion of preciousmetal nanoparticles to enhance their electrocatalytic activities infuel cells.168170,172,179,180,235237 Seger et al.168 reported thedeposition of Pt nanoparticles on rGO sheets by means of theborohydride reduction of H2PtCl6 in a GO suspension.Subsequently they were used as a novel electrocatalyst in aproton exchange membrane (PEM) assembly for PEM fuelcells. The partially reduced GO-Pt based fuel cell delivered amaximum power of 161 mW/cm2 compared with 96 mW/cm2

for an unsupported Pt based fuel cell. This suggests that therole of GO as an eective support material in the developmentof an advanced electrocatalyst is feasible. Similarly, our group169

recently prepared Pt/rGO nanocomposites in a one-stepsynthesis and demonstrated that these nanocomposites showedsuperior electrocatalytic performance toward methanol oxida-tion. In addition to the electrocatalytic applications in fuel cells,

Figure 8. (A) Schematic illustration of the fabrication of free-standing hybrid electrodes from 2D-assembly of gold nanoparticles and GO paper. (B)Typical amperometric response of AurGO paper electrode and Au foil electrode to the successive addition of 0.1, 0.5, 1.0, 2.0, and 4.0 mM glucosein PBS buer (pH 7.4) under magnetic stirring; insets show the amperometric response of AurGO paper and Au foil electrodes at a lowerconcentration detected. (C) Typical amperometric response of AurGO paper electrode to successive addition of 0.02, 0.1, 0.2, and 1.0 mM H2O2in a stirring PBS; the inset shows the amperometric response to successive addition of 5 and 10 M H2O2. Reprinted with permission from ref 241.Copyright 2012 American Chemical Society.



GO-supported precious metal nanocatalysts, such as Au/GO,238241 Pt/GO,242,243 Pd/GO,244,245 and PtNi/GO,246

have been also used to fabricate GO-based electrodes for thesensitive and selective detection of specic species due to theirexcellent catalytic properties. For example, Xiao et al.241

reported a modular approach to fabricating high-performanceexible electrodes by structurally integrating 2D-assemblies ofAu nanoparticles with freestanding GO paper. They demon-strated that these Au/GO exible electrodes exhibited out-standing electrocatalytic activities for the sensitive and selectivedetection of glucose and hydrogen peroxide (H2O2) (Figure 8).In addition to precious metal nanoparticles, other nano-

catalysts, such as oxide nanoparticles,247 hydroxide nano-particles,248 Prussian blue nanoparticles,191193,249251 methyl-ene green,118 and methylene blue,252 have also beensuccessfully anchored onto the surface of GO sheets for thedesign and preparation of novel GO-supported electrocatalysts.For example, Li et al.247 successfully prepared MnO2/GOnanocomposites as a novel electrocatalyst for the nonenzymaticdetection of H2O2. Due to the electrocatalytic abilities of MnO2toward H2O2 and the high surface area of GO, MnO2/GO-based electrodes showed very high electrochemical activity forthe detection of H2O2 in an alkaline medium. Thecorresponding H2O2 electrochemical sensors displayed goodperformance along with low working potential, high sensitivity,low detection limits, and long-term stability. Cao et al.193

demonstrated a facile strategy for the controlled growth ofhigh-quality Prussian blue nanocubes on the surface of reduced

GO (PBNCs/rGO). They also investigated the electrocatalyticperformance of PBNCs/rGO nanocomposites as amperometricsensors toward the reduction of H2O2 as shown in Figure 9.The sensor showed a rapid and highly sensitive response toH2O2 with a low detection limit (45 nM).

4.2. Electrochemiluminescence

Recently, GO-based materials have emerged as one of the mostfascinating alternative electrode materials for applications inelectrogenerated chemiluminescence. This is commonlydened as electrochemiluminescence (ECL) and is a lightemission that arises from the high-energy electron-transferreaction between electrogenerated species.253,254 In fact,pristine GO sheets cannot generate the ECL eect due tothe absence of ECL merits. Thus, the GO-related ECLproperties generally come from the dierent luminophoremoieties anchored to the GO-based electrode, such as theruthenium(II) t r i s(2 ,2 -b ipyr id ine) complex (Ru-(bpy)3

2+),255258 QDs,187,190,259261 upconversion NaYF4/Yb,Er nanoparticles,262 and luminol.263 Due to the GOstunable moderate conductivity and distinctive structuralproperties, on the one hand, the introduction of GO into anECL-based sensor platform can be extremely helpful inaccelerating electron transfer between the lumophores andthe electrode, and on the other hand, they increase the surfacearea and porosity of the platform to make coreactant diusionfaster for the ECL-based sensors. In one example, Li et al.255

developed an ECL sensor based on a Ru(bpy)32+rGO

Figure 9. (A) Scheme of procedure for the fabrication of Prussian blue nanocube/rGO (PBNCs/rGO) nanocomposite electrocatalyst. (B) TEMimage of PBCNs/rGO nanocomposites (inset, photograph of GO dispersion (left) and PBCNs/rGO nanocomposites dispersion (right)). (C)Cyclic voltammograms of a PBNCs/rGO modied glassy carbon electrode in PBS buer solution in the absence (dotted line) and presence of 0.4mM H2O2 (solid line). (D) Amperometric response to H2O2 concentration. Reprinted with permission from ref 193. Copyright 2010 AmericanChemical Society.



Naon composite lm. As expected, the introduction of rGOinto the Naon facilitated the electron transfer of Ru(bpy)3

2+

and improved the long-term stability of the sensor by inhibitingthe migration of Ru(bpy)3

2+ into the electrochemically inactivehydrophobic region of the Naon. This ECL sensor exhibited agood linear range over 1 107 to 1 104 M with a detectionlimit of 50 nM in the determination of tripropylamine (TPA).Our group187 reported a GO-amplied ECL of QDs platformand its ecient selective sensing for glutathione (Figure 10). Itwas demonstrated that GO facilitated the generation of QDradicals and formed a high yield of QD*, leading to 5 timesenhancement of ECL intensity compared with the ECLplatform without GO. Based on the proposed GO-ampliedECL platform, we realized the sensitive and selective detectionof glutathione from thiol-containing compounds and furtherused it for glutathione drug detection.In addition, it is fascinating to note that even in the absence

of a luminescent species, a fairly intense ECL emission of GOhas been also demonstrated by Bards group.264 In their work,an ECL intensity of >4 108 photon counts s1 cm2 wasfound with oxidized highly oriented pyrolytic graphite (HOPG)and of >1.8 106 photon counts s1 cm2 from a 6 ppmsuspension of GO platelets in an aqueous phosphate buersolution (pH = 7.0) containing 0.1 M NaClO4 and 13 mM tri-n-propylamine (TPrA). A possible explanation of the broademission was suggested as being the existence of smalleraromatic hydrocarbon-like domains formed on the graphiticlayers through interruption of the conjugation by the oxidizedcenters.

4.3. Electrochemical Gas Sensors

One of the most promising applications of GO-based materialsis for electrochemical sensing, especially for gas sensing andbiosensing. In gas sensing devices, two of the most importantissues are gas sensitivity (detection of gas concentrations at theppm level) and gas selectivity (detection of specic gases in amixed gas environment). As is well-known by now, GO is one-

atom-thick planar sheets of sp2-bonded carbon atoms decoratedwith oxygen functional groups. Every atom in a GO sheet canbe considered as a surface atom. Thus, electron transportthrough these ultrathin materials can be highly sensitive toadsorbed molecules.149 This phenomenon has subsequentlyenabled the fabrication of sorption-based sensors capable ofdetecting trace levels of vapor using conventional low-powerelectronics. The corresponding sensing mechanism can begenerally ascribed to the changes in conductance or capacitancewhen gaseous molecules (which act as electron donors oracceptors) interact with the GO-based materials.265,266 Suchconductivity or capacitance changes are caused by theformation of a space charge region induced by either gasadsorption or the formation of oxygen vacancies on the GOsurface. Consequently, it is easy for GO-based gas sensors toachieve high sensitivities due to the much higher specicsurface area and surface-to-bulk ratio of the GO. However,establishing sensor selectivity for specic gases is dicult andchallenging, not least because the sensing selectivity of GO-based gas sensors requires a detailed understanding of thesurface and interfacial processes at the atomic level as well astheir relationship with GO material-properties and deviceperformance. Selectivity is dependent on many parameters,such as gas adsorption and coadsorption mechanisms, surfacedefect sites, surface reaction kinetics, and electron transferbetween the adsorbed gas molecules and the modied electrodeof GO-based materials. Currently, sensor selectivity remains, forthe most part, empirical. In practice, selectivity can be achievedby enhancing gas adsorption or promoting specic chemicalreactions at the GO-based electrodes through controlling thesurface defect sites using bulk dopants, surface modicationmethods, and the addition of metallic clusters or oxide catalysts.The optimal defect density balances the gains in sensitivityagainst the rapid degradation in conductivity due to the defects.In this respect, GO is an ideal material for balancing theseeects because it contains a diverse range of surface sites whose

Figure 10. (A) Schematic illustration of GO-amplied electrochemiluminescence (ECL) of QDs platform. (B) ECL intensitypotential behaviors of(a) background, (b) CdTe QDs, and (c) CdTe QDs with 1.2 g mL1 GO in 0.02 M Na2CO3NaHCO3 (pH 9.5) buer solution. (C) ECLintensitytime behaviors of CdTe QDs with 1.2 g mL1 GO (a) to the commercial glutathione (GSH) drug with dierent concentrations, fromcurve b to curve i: 0.04, 0.1, 0.18, 0.21, 0.25, 0.29, 0.32, and 0.36 g mL1. The inset is the linear relationship between the relative ECL intensity (I0/I) and the concentration of GSH drug. Reprinted with permission from ref 187. Copyright 2009 American Chemical Society.



density is easily controlled. This renders GO a promisingcandidate for use as the active material in molecular gas sensorsrequiring high sensitivity and selectivity.Until now, there have been several literature reports

regarding GO-based sensors for the detection of gaseousmolecules, such as NO2,

151,165,267 NO,268 NH3,151,267,269 Cl2,

165

chemical warfare agents,149 and explosives.149,151,270 Robinsonet al.149 demonstrated that rGO is suitable as the active materialfor high-performance gas sensors. Sensors were fabricated froman ultrathin network of exfoliated GO platelets, which werethen further tunably reduced toward graphene by varying theexposure time to a hydrazine hydrate vapor. Such rGO devicesreadily oered sensitivities at parts-per-billion levels for bothchemical warfare agents (e.g., hydrogen cyanide (HCN),chloroethylethyl sulde (CEES), and dimethylmethylphospho-nate (DMMP)) and explosives (e.g., 2,4-dinitrotoluene(DNT)). The sensing mechanism was attributed to the rapidand slow response of the rGO device upon exposure to theanalyte vapor, which leads to the relative change in electricalconductance (G/G0). It was found that the rapid responsearised from molecular adsorption onto low-energy binding sites(such as sp2-bonded carbon) through weak dispersive forces,and the slow response arised from molecular interactions withhigh-erenergy binding sites (such as vacancies, structural

defects, and oxygen functional groups) through single anddouble hydrogen bonding. It was also found that the responseand recovery characteristics of the conductance response andthe sensing selectivity could be tailored by adjusting thereduction process. In another work, Fowler et al.151 reportedthe development of useful chemical sensors from chemical rGOdispersions for the detection of NO2, NH3, and DNT, as shownin Figure 11. The sensor response was consistent with a chargetransfer mechanism between the analyte and rGO with alimited role of the electrical contacts. It was demonstrated thatthe sensing mechanism of NO2 was attributed to hole-inducedconduction. This is because it withdraws an electron from rGO.However, the NH3-sensing mechanism was ascribed toelectron-induced conduction, since it donates an electron torGO. The DNT-sensing mechanism was similar to that forNO2, namely, electron-withdrawing. The DNT detection limitwas reported to be 28 ppb. In addition, Dua et al.165 described arugged and exible sensor using inkjet-printed lms of rGO onPET for the reversible detection of NO2 and Cl2 vapors in anair sample at the parts per billion level. When inkjet-printedrGO/PET lms were exposed to successively decreasingconcentrations of the electron-withdrawing vapors (i.e., NO2and Cl2) within a specic range, the conductivity increased in alinear fashion. This was consistent with an increase in the

Figure 11. (A) Photograph of the four-point interdigitated electrode sensor. Two serpentine electrodes between the interdigitated electrodes areused for four-point resistance measurements. The electrode array is 2.4 mm 1.9 mm with 20 m serpentine electrode widths, 40 m interdigitatedelectrode widths, and 20 m electrode gaps. (B) A SEM image of spin-coated rGO lm as deposited on the chemical sensor. (C) NO2 and (D) NH3detection using a rGO lm-based sensor, respectively. The NO2 and NH3 concentrations are 5 ppm in dry nitrogen. Reprinted with permission fromref 151. Copyright 2009 American Chemical Society.



number of charge carriers. Interestingly, a signal recovery uponexposure to UV irradiation was observed in the vapor detection,and this suggests that there was no covalent bond formationbetween the vapor molecules and rGO.

4.4. Electrochemical Biosensors

Another important application of GO-based electrodes is toexploit GO-based materials for electrochemical biosensing, andthis is an area that has seen signicant growth in the past fewyears. An electrochemical biosensor is an analytical device thatconverts a biological response into an electrical signal usingelectrochemical strategies that determine concentrations ofsubstrates and other parameters of biological interest evenwhere they do not utilize a biological system directly.271 Fromthe perspective of electrochemical reactivity, an important partof the success of GO-based materials for electrochemicalbiosensing is their ability to provide a suitable microenviron-ment for biomolecule-immobilization while retaining theirbiological activities. Also they facilitate electron transferbetween the immobilized biomolecules and the electrodesubstrates.30 Thus, since these types of GO-based electrodespromise lower detection limits, fast response time, highsensitivity, and increased signal-to-noise ratios, they havebeen used for the fabrication of many novel biosensors withpotential applications. In general, most GO-based electro-chemical biosensors can be fabricated through the incorpo-ration of special bioactive species (such as enzymes, metal-loproteins) onto GO-based electrodes.

4.4.1. Enzyme Biosensors. In recent years, enzymeelectrodes based on GO-based materials have attracted greatinterest for the detection of a wide range of analytes. Anextremely important example is in the determination of glucose,which plays a crucial role in the diagnosis and therapy ofdiabetes. Recent studies have indicated that GO-based materialsare very capable of the sensitive and selective detection ofglucose.119,178,221,223,272278 For example, Liu et al.221 prepareda biocompatible GO-based glucose biosensor using the covalentattachment between the carboxyl acid groups of the GO sheetsand the amines of glucose oxidase (GOx). They demonstratedthat the resulting biosensors exhibited broad linearity, goodsensitivity, excellent reproducibility, and storage stability. Linand co-workers223 studied the GOx/rGO/chitosan nano-composite-modied electrode for direct electrochemistry andglucose sensing. It was found that the nanocomposite lm canprovide a favorable microenvironment for GOx to realize directelectron transfer at the modied electrode. The nano-composite-based biosensor exhibited a wider linearity rangefrom 0.08 to 12 mM glucose with a detection limit of 0.02 mMand much higher sensitivity (37.93 A mM1 cm2) comparedwith other nanostructured supports.Apart from GOx-based biosensors, similar sensitivities and

stability improvements have been found for electrochemicalbiosensors based on other enzymes, such as horseradishperoxidase (HRP),279281 a lcohol dehydrogenase(ADH),145,148 organophosphorus hydrolase (OPH),215 micro-

Figure 12. (A) Schematic illustration of Au NP/rGO hybrid synthesis and AChE/Au NP/rGO nanoassembly generation by using PDDA. Graphitewas used for producing graphene oxide with Hummers method, and Au NP/cr-G was obtained by reducing HAuCl4 on GO nanosheets. AChE wasstabilized on the surface of Au NP/cr-G hybrid by self-assembling. (B) TEM image of Au NP/rGO. (C) (a) Plots of current intensities toconcentration of organophosphate pesticides (OPs); (b) linear relationship between residual activities of AChE and concentration of OPs. Residualactivities of AChE was dened with [(imax is)/imax%] where imax was the current corresponding to 2 mM ATCh and is was current value afterexposure to dierent concentrations of OPs for 15 min. From ref 283, Reproduced by permission of The Royal Society of Chemistry.



peroxidase-11,233 tyrosinase,282 acetylcholinesterase,283 cata-lase,284 and urease.285 For example, Shan et al.148 utilizedionic liquids/rGO/chitosan (ILs/rGO/CS) composites as theplatform to construct an electrochemical biosensor for thedetection of NADH and ethanol. The ILs/rGO/CS-modiedelectrode showed an obvious decrease in the overvoltage ofNADH oxidation and exhibited good linearity from 0.25 to 2mM and a high sensitivity of 37.43 A mM1 cm2. With theintroduction of ADH, the resulting ADH/ILs/rGO/CS-basedbiosensor demonstrated rapid and highly sensitive ampero-metric response to ethanol with a low detection limit (5 M).Recently, our group283 reported an electrochemical sensorbased on acetylcholinesterase (AChE)/Au nanoparticle (NPs)/rGO nanohybrids for the ultrasensitive detection of organo-phosphate pesticides (OPs) (Figure 12). One mechanism fordetecting OPs is based on inhibiting enzyme (AChE) activityby these toxic chemicals. The AChE activity in this system wasmonitored by measuring the oxidation or reduction current ofthe enzymatic products, and the extent of AChE activity was

inversely correlated with the amount of chemical inhibitorpresent in the system. In this work, the nanohybrid of Au NP/rGO (cr-Gs) was rst synthesized by the in situ growth of AuNPs on the surface of rGO nanosheets in the presence ofpoly(diallyldimethylammonium chloride) (PDDA). This notonly improved the dispersion of the Au NPs but also stabilizedcholinesterase with high activity and loading eciency.Subsequently, the enzyme (AChE) was self-assembled on theAu NP/rGO nanohybrid through the electrostatic interactionbetween negatively charged AChE and positively chargedPDDA. The electrochemical biosensor based on this AChE/AuNP/rGO nanoassembly exhibited an ultrasensitive detection ofOPs with a detection limit of 0.1 pM.4.4.2. Hemeprotein Biosensors. Direct electron transfer

from heme proteins other than enzymes to electrode substratescan also be achieved by making use of GO-based electrodes.These composite materials can provide a suitable micro-environment to retain the redox bioactivity and facilitate theelectron-transfer rate between the active center of the redox

Figure 13. (A) Schematic illustration of the multienzyme labeling amplication strategy using HRPp53392Ab2GO conjugate. (B) (a) SWV curvesacquired at HRPp53392Ab2GO/phospho-p53392/Ab1/NHS/AuNPs-SPCE after incubation with (a) 0, (b) 0.01, (c) 0.02, (d) 0.05, (e) 0.1, (f) 0.2,(g) 0.5, (h) 1, (i) 2, and (j) 5 ng mL1 phospho-p53392 antigen in pH 7.4 PBS containing 25 M thionine and 2 mM H2O2; (b) calibration plots ofthe (i) proposed HRPp53392Ab2GO/phospho-p53392/Ab1/NHS/AuNPs-SPCE and (ii) traditional HRPstreptavidinbiotinp53392Ab2/phospho-p53392/Ab1/NHS/AuNPs-SPCE for detecting phospho-p53

392 antigen. Reprinted with permission from ref 297. Copyright 2011 AmericanChemical Society.



proteins and the underlying electrode. The mechanism of thedirect electron transfer from heme proteins to these electrodescan serve as models for understanding the electron-transfermechanism in biological systems. Over the past few years,heme-containing metalloproteins, such as myoglobin,120,286,287

hemoglobin,217,288291 and cytochrome c,292 have beenanchored onto GO-based electrodes for electrochemicalbiosensing purposes. For instance, Yue et al.287 reported anovel nitrite biosensor involving a sensing platform consistingof a GO/myoglobin composite lm. A pair of well-dened andquasi-reversible cyclic voltammetric peaks that reected thedirect electrochemistry for the ferric/ferrous coupling ofmyoglobin were achieved at the composite lm-modiedelectrode. It was revealed that this electrode displayed excellentelectrocatalytic ability for the reduction of nitric oxide andexhibited good electrochemical response to nitrite with a linearrange from 0.0

graphene oxide: preparation, functionalization, and electrochemical applications

Documents

electrochemical sensors

electrochemical biosensors

electrochemical community

electrochemical immunosensors

singlelayer graphene

electrochemical gas

electrochemical dna

based electrodes