Graphene Oxide: Preparation, Functionalization, and

Review pubs.acs.org/CR

Graphene Oxide: Preparation, Functionalization, and Electrochemical Applications Da Chen,†,‡ Hongbin Feng,† and Jinghong Li*,† †

Department of Chemistry, Tsinghua University, Beijing 100084, China College of Materials Science & Engineering, China Jiliang University, Hangzhou 310018, China 1. INTRODUCTION

‡

Graphene, which consists of a one-atom-thick planar sheet comprising an sp2-bonded carbon structure with exceptionally high crystal and electronic quality, is a novel material that has emerged as a rapidly rising star in the field of material science.1−4 Ever since its discovery in 2004,5 graphene has been making a profound impact in many areas of science and technology due to its remarkable physicochemical properties. These include a high specific surface area (theoretically 2630 m2/g for single-layer graphene),1,6,7 extraordinary electronic properties and electron transport capabilities,8−10 unprecedented pliability and impermeability,11,12 strong mechanical strength12 and excellent thermal and electrical conductivities.13,14 These unique physicochemical properties suggest it has great potential for providing new approaches and critical improvements in the field of electrochemistry. For example, the high surface area of electrically conductive graphene sheets can give rise to high densities of attached analyte molecules. This in turn can facilitate high sensitivity and device miniaturization. Facile electron transfer between graphene and redox species opens up opportunities for sensing strategies based on direct electron transfer rather than mediation. It is not surprising, therefore, that graphene has recently attracted great attention worldwide from the electrochemical community. Despite its short history, this 2D material has already revealed potential applications in electrochemistry, and remarkably rapid progress in this area has already been made. In recent years, many reviews covering graphene and related materials have been published.1,4,7,15−27 In addition, several reviews with particular emphasis on graphene-based electrochemical applications have also appeared.28−35 One specific branch of graphene research deals with graphene oxide (GO). This can be considered as a precursor for graphene synthesis by either chemical or thermal reduction processes. GO consists of a single-layer of graphite oxide and is usually produced by the chemical treatment of graphite through oxidation, with subsequent dispersion and exfoliation in water or suitable organic solvents.36,37 With respect to its structure, there have been several structural models37−40 proposed over the years. These assume the presence of various oxygencontaining functional groups in the GO. The oxygen functional groups have been identified as mostly in the form of hydroxyl and epoxy groups on the basal plane, with smaller amounts of carboxy, carbonyl, phenol, lactone, and quinone at the sheet edges.41−43 However, currently the precise atomic structure of GO is still uncertain and remains to be fully elucidated. This is

CONTENTS 1. Introduction 2. Structure and Properties of GO 2.1. Structure 2.2. Properties 3. Preparation and Functionalization of GO-based Electrodes 3.1. Preparation of GO-based Electrodes 3.2. Functionalization of GO-based Electrodes 3.2.1. Functionalization with Nanoparticles 3.2.2. Functionalization with Organic Compounds 3.2.3. Functionalization with Polymers 3.2.4. Functionalization with Biomaterials 4. GO-based Electrochemical Applications 4.1. Electrocatalysis 4.2. Electrochemiluminescence 4.3. Electrochemical Gas Sensors 4.4. Electrochemical Biosensors 4.4.1. Enzyme Biosensors 4.4.2. Hemeprotein Biosensors 4.5. Electrochemical Immunosensors 4.6. Electrochemical DNA Sensors 4.7. Other Electrochemical Sensors 5. Conclusions and Perspectives Author Information Corresponding Author Notes Biographies Acknowledgments References

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Received: December 4, 2010 Published: August 14, 2012 © 2012 American Chemical Society

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amount of the GO-based materials is required for solutionprocessed thin film deposition. Moreover, GO-based thin conductive films deposited onto an inexpensive base material can lead to a larger surface area-to-volume ratio, which further lowers the cost of the electrode. Meanwhile, the large effective surface area can also provide a larger number of active sites and often also a higher signal-to-noise ratio. Owing to these advantageous specific properties, GO-based materials have been used to design and prepare GO-based electrodes for a wide range of applications in electrochemical sensors and electroanalysis (Figure 1). To date, although considerable advances in

primarily due to the uncertainty pertaining to both the nature and distribution of the oxygen-containing functional groups,44 its nonstoichiometric atomic composition, and the lack of sufficiently sensitive analytical techniques for characterizing the GO structure. In reality, GO incarnates various nanoscale inhomogeneities in its structure, and the stoichiometry varies depending on the synthesis protocol as well as the extent of the reaction. In fact, the ideal stoichiometry has never been achieved. The oxygenated groups in GO can strongly affect its electronic, mechanical, and electrochemical properties. Hence they account for the differences between GO and pristine graphene.45 Compared with pristine graphene, on the one hand, the covalent oxygenated functional groups in GO can indeed give rise to remarkable structure defects. This is concomitant with some loss in electrical conductivity,46 which possibly limits the direct application of GO in electrically active materials and devices. On the other hand, the presence of these functional groups can also provide potential advantages for using GO in numerous other applications. The reasons are as follows: first the polar oxygen functional groups of GO render it strongly hydrophilic. This gives GO good dispersibility in many solvents, particularly in water.7,47−49 This is important for processing and further derivatization. The resulting GO-stable dispersion can be subsequently deposited on various substrates in order to prepare thin conductive films by means of common methods 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 groups serve as sites for chemical modification or functionalization of GO, which in turn can be employed to immobilize various electroactive species through covalent or noncovalent bonds for the design of sensitive electrochemical systems. Therefore, the chemical composition of GO, which can be chemically, thermally, or electrochemically engineered, allows the tunability of its physicochemical properties.41,50,51 For example, by appropriately fine-tuning the oxidation or reduction parameters with a view to controlling the structural disorder, GO can be made into an insulating, semiconducting, or semimetallic material. Although the unique relativistic nature of charge carriers and other condensed-matter effects that are observed in “nearly ideal” graphene are absent in GO, accessibility, ease of synthesis, solution processability, and its versatile properties make it attractive for fundamental research as well as in applications.41 The study of GO-based materials in recent years has been popular and extensive, particularly with respect to electrochemical applications. Underpinning the significance of GObased materials in electrochemistry are the very specific properties that although relevant to GO are not typical of pristine graphene. These include its facile synthesis, high dispersibility in a range of solvents, capability of coupling electroactive species onto the surface, and unique optical properties (such as fluorescence labels52). In addition, the use of GO-based materials also provides control over the local microenvironment. This is because in most cases GO-based materials can be deposited, with extremely well-defined surfaces, through solution processing. This can be highly advantageous when incorporating sensitive or electroactive species into an electrochemical system. Another important issue is consideration of the costs when manufacturing an electrode for use in any real system. In terms of the manufacture of GObased devices, costs can be reduced compared with the costs for conventional electrodes. This is because only a fraction of the

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

this area have already been made, clearly there is also a great deal of scope for further study in GO-related electrochemical applications. In this review, we hope to summarize and critically discuss recent advances in the use of GO-based materials in the field of electroanalytical chemistry and electrochemical sensors.

2. STRUCTURE AND PROPERTIES OF GO On a simple level, GO can be considered as consisting of individual sheets of graphene decorated with oxygen functional groups on both the basal planes and edges.53 However, as mentioned in the Introduction, the precise atomic and electronic structure of GO remain largely unknown. Greater details about the GO structure, such as possible dominating structural motifs, are still under investigation. In order to provide helpful information in tailoring the fundamental properties of GO and to unleash its potential applications, it is both critical and desirable to explore the atomic details of the GO structure in depth. 2.1. Structure

The study of the GO structure is derived from the structural analysis of graphite oxide itself. Although graphite oxide was first prepared in the mid-1800s,54 its composition and structure is still under debate because of its nonstoichiometric composition and the strong hygroscopicity of dehydrated graphite oxide. Over the years, both theoretically and experimentally, considerable effort has been directed toward understanding the structure of graphite oxide. The result is that several conflicting models have been successively proposed. Originally, in 1939, Hofmann and Holst55 proposed a simple model, in which graphite oxide was thought to consist of epoxy (1,2-ether) group modified planar carbon layers with a molecular formula of C2O. In 1946, Ruess56 suggested that 6028


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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 and carboxylic acid groups at the edges. (B) AFM image of a GO sheet. The apparent thickness of a single sheet is around 1 nm. Reprinted with permission 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 Chemical Society.

above structural models of graphite oxide. It is recommended that interested readers refer to this review for more specific detailed information. While the studies mentioned above outlined many of the fundamental structural features of graphite oxide, it is clear that a more refined picture of the fine GO structure is necessary. One difference from an ideal graphene sheet, which consists of only trigonally bonded sp2 carbon atoms,61 is the fact that the GO sheet consists of a hexagonal ring-based carbon network having 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 covalently bonded with oxygen functional groups (such as hydroxyl, epoxy, and carboxy) are sp3 hybridized. These can be viewed as oxidized regions, and they disrupt the extended sp2 conjugated network of the original honeycomb-lattice structured graphene sheet. The latter can be viewed as the unoxidized regions.63,64 These sp3 hybridized carbon clusters are uniformly but randomly displaced slightly either above or below the graphene plane.65 To date, in order to explore the GO structure in more depth, various microscopic and spectroscopic techniques have been employed for the investigation of its structural features. For example, atomic force microscopy (AFM) directly gives the apparent thickness of the single-layer GO (around 1 nm, Figure 2B) as well as the number of layers.64,66−68 On the other hand, conductive AFM reveals the electrical defects in GO.69 Scanning tunneling microscopy (STM) has been used to examine the structural features of the GO sheets.64,68,70 Results

the carbon layers were not in fact planar but puckered and that the oxygen-containing groups were hydroxyl and ether-like oxygen bridges between carbon atoms 1 and 3, randomly distributed on the carbon skeleton. Later, in order to explain the acidic properties of graphite oxide, Hofmann and coworkers57 further incorporated an enol- and keto-type structure into their model, which also contained hydroxyls and ether bridges at the 1 and 3 positions. In 1969, Scholz and Boehm58 proposed a new structure with corrugated carbon layers. Here the epoxide and ether groups were completely replaced by carbonyl and hydroxyl groups. Meanwhile, Nakajima et al.59,60 proposed a different model for graphite oxide. This model consisted of two carbon layers linked to each other by sp3 carbon−carbon bonds perpendicular to the layers and in which carbonyl and hydroxyl groups were present in relative amounts depending on the level of hydration. Based on expert NMR studies, Leaf and co-workers38 proposed a structural model having a random distribution of flat aromatic regions with unoxidized benzene rings and wrinkled regions of alicyclic sixmembered rings bearing CC, C−OH, and ether groups (reassigned to the 1 and 2 positions). In light of these previous models, Szabó et al.39 recently proposed a new structural model that involves a carbon network consisting of two kinds of regions: (i) trans-linked cyclohexane chairs and (ii) ribbons of flat hexagons with CC double bonds as well as functional groups such as tertiary OH, 1,3-ether, ketone, quinone, and phenol (aromatic diol). Even more recently, Dreyer et al.37 reviewed the structural analogies and differences among the 6029


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Figure 3. (A) Aberration-corrected TEM image of a GO monolayer. On the bottom, holes are indicated in blue, graphitic areas in yellow and high contrast; red indicates disordered regions with oxygen functionalities. (B) Another aberration-corrected TEM image of a GO monolayer for detailed structural 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 proposed possible atomic structure of this region with carbon atoms in gray and oxygen atoms in red, and finally the average of a simulated TEM image of the proposed structure. Expansion b focuses on the white spot seen in the graphitic region. This spot moved along the graphitic region but stayed stationary 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 (right portion of expansion b). The ball-and-stick figures 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 exit plane wave reconstruction of a focal series of GO and the atomic structure of this region. Reprinted with permission from ref 71. Copyright 2010 John Wiley & Sons, Inc.

include the dominant clustered pentagons and heptagons, as well as the in-plane distortions and strain in the surrounding lattice. In addition, scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) has proven to be very effective for measuring the fine structure of the carbon and oxygen K-edges as well as low-loss electronic excitations in GO.46 These results indicate that the oxygen atoms are attached to the graphene sites randomly and convert the sp2 carbon bonds in graphene to sp3 bonds. In GO, the plasma excitations are related to those in graphene but with a substantial blue-shift occurring due to the presence of the oxygen and increased number of sp3 bonds. The chemical composition of GO and the oxygen functional groups in GO have been identified using various spectroscopic techniques, including solid-state nuclear magnetic resonance (SSNMR),40,74−76 X-ray absorption near-edge spectroscopy (XANES),73,77−81 Raman spectroscopy,68,70,77,81 X-ray photoelectron spectroscopy (XPS),40,50,68,79,81,82 and Fourier transform infrared spectroscopy (FT-IR).75,79,83,84 Typical solidstate 13C magic-angle spinning NMR spectra reveal that there are three main peaks in the 13C NMR spectrum of GO. The peak around 60 ppm is assigned to carbon atoms bonding to the epoxy group, the peak around 70 ppm corresponds to the hydroxyl group connected to the carbon atoms, and the peak around 130 ppm is ascribed to the graphitic sp2 carbon (Figure 4A).40,74 It has been demonstrated that these assignments of the three main peaks are most likely correct.40,74 In addition, in

show it to be distinguishable from pristine graphene by the appearance of bright spots and regions lacking ordered lattice features due to the presence of oxygenated functional groups as shown in Figure 2C. According to the ratio of these unordered regions, the degree of functionalization can be estimated.64 Recently, the direct imaging of lattice atoms and topological defects in single-layer GO has been achieved using highresolution transmission electron microscopy (HRTEM).45,71−73 This is a significant breakthrough in exploring the GO structure. By means of HRTEM, Erickson et al.71 identified the specific atomic scale features of the GO monolayers, which comprise three major regions: holes, graphitic regions, and high contrast disordered regions with approximate area percentages of 2%, 16%, and 82%, respectively (Figure 3). The authors proposed that the holes in GO are formed due to the release of CO and CO2 during the aggressive oxidation and sheet exfoliation. They also suggest that the graphitic regions come from the incomplete oxidation of the basal plane with the preserved honeycomb structure of graphene, while the disordered regions of the basal plane consist of a high-density region of oxygen functionalities. These consist of hydroxyls, (1,2) epoxies, and carbonyls, with each carbon most likely being oxidized with no order between the functionalities. In addition, possible atomic structures for the disordered functionalities were also proposed. Reported by GómezNavarro and co-workers,45 aberration-corrected HRTEM was further used to unravel the topological defects in GO. These 6030


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Figure 4. (A) Solid-state 13C magic-angle spinning (MAS) NMR spectra of GO. Reprinted with permission from ref 40. Copyright 2009 Nature Publishing 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 2011 American Chemical Society.

the high-resolution 13C NMR spectrum another three small peaks were also found at about 101, 167, and 191 ppm. These three weak peaks were tentatively assigned to lactol, the ester carbonyl, and the ketone groups, respectively.40 XANES has proven to be another powerful tool for characterizing GO materials. It provides information on the degree of bond hybridization in mixed sp2/sp3 bonded carbon, the specific bonding configurations of functional atoms, and the degree of alignment of the graphitic crystal structures within GO.81 The high-resolution C K-edge XANES spectrum of GO shown in Figure 4B demonstrates a clear presence of both unoccupied π* and σ* states around 285.2 and 293.03 eV, respectively. These can be primarily assigned to the 1s → π* and 1s → σ* transitions in the graphitic carbon atoms in GO.78,81 The broadening of the absorption peak at 289.3 eV can be assigned to the 1s → π* transitions in the carbon involved in bonding with the oxygen atoms. In particular, the ratio of π*/σ* peaks at the C K-edge provides an estimate of the relative concentration of sp2 domain configurations in an sp3 matrix consisting of carbon atoms connected to oxygen groups. Hence this indicates the degree of oxidation in GO.81 On the other hand, the typical O K-edge XANES spectrum of GO shows several distinctive absorption peaks at 531.5, 534.0, 535.5,

540.0, 542.0, and 544.5 eV. These have been assigned to π*(CO), π*(C−O), σ*(O−H), σ*(C−O), σ*(CO), and σ*(CO), respectively.73 This O K-edge spectrum thus clarifies the chemical composition of the oxygenated functional groups in GO. This includes carbonyl groups together with the epoxide and hydroxyl groups attached to aromatic rings, and the carboxyl groups most likely attached to the edges of the GO sheets. In addition, XPS further unambiguously reveals the nature of the carbon and oxygen bonds in their various states: unoxidized carbons (sp2 carbon), C−O, CO, and COOH. In a number of reports on GO,50,81,85,86 the C1s signal of pristine GO, as seen in Figure 4C, consists of five different chemically shifted components, which can be deconvoluted into sp2 carbons in aromatic rings (284.5 eV) and C atoms bonded to hydroxyl (C−OH, 285.86 eV), epoxide (C−O−C, 286.55 eV), carbonyl (>CO, 287.5 eV), and carboxyl groups (COOH, 289.2 eV). Other reports,38,77 on the other hand, consider the deconvolution of the C1s spectra using four components, namely, sp2, C−OH, C−O−C, and COOH, while ignoring the presence of the >CO groups. There is still a considerable degree of vagueness regarding the presence of the carbonyl >CO groups. Information provided by analysis of the O1s spectra can complement the information provided by analysis 6031


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Figure 5. (A) Conductivity of thermally reduced GO as a function of the sp2 carbon fraction. The vertical dashed line indicates the percolation threshold at sp2 fraction of 0.6. The 100% sp2 materials are polycrystalline (PC) graphite and graphene. The two conductivity values are for doped by gating (upper triangle) and intrinsic graphene (lower triangle). Reprinted with permission from ref 50. Copyright 2009 John Wiley & Sons, Inc. (B) (a−c) Structural models of GO during different stages of reduction. The smaller sp2 domains indicated by zigzag lines do not necessarily correspond to any specific structure but to small and localized sp2 configurations 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 of evolution 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 confinement effect. Photogeneration of an electron−hole (e−h) pair on absorption of light (Eexc) followed by nonradiative relaxation and radiative recombination resulting in fluorescence (EPL) is depicted. Black arrows denote the transitions of electrons and holes during this process. DOS, electronic density of states. Reprinted with permission from ref 113. Copyright 2010 John Wiley & Sons, Inc.

indicates the oxidation degree and the size of sp2 ring clusters in a network of sp3 and sp2 bonded carbon.50,88 For example, an average graphitic domain size of ∼2.5 nm in pristine GO was calculated.50 After GO thermal reduction, the ID/IG ratio was found to significantly decrease. This indicates considerable recovery of the conjugated graphitic framework upon defunctionalization of the epoxide and hydroxyl groups.77 FTIR spectroscopy is recognized as an important tool for characterization of functional groups, and in the case of GO has supported the presence of hydroxyl (broad peak at 3050− 3800 cm−1), carbonyl (1750−1850 cm−1), carboxyl (1650− 1750 cm−1), CC (1500−1600 cm−1), and ether or epoxide (1000−1280 cm−1) groups.75,79,83 On the other hand, theoretical studies have also received considerable attention in the exploration of the complex structure of GO. This is because theoretical simulations can provide considerable insight into the possible kinetic and thermodynamic mechanisms needed for a greater understanding of the structural evolution of the functional oxygenated groups found in GO. In general, the current efforts employ simple schemes that permit basic structural derivations of GO using common oxygen groups. For instance, based on first-principle calculations, the energetically favorable atomic configurations (building blocks) in GO have been identified as containing epoxide and hydroxyl groups in close proximity to each other.90−94 Different arrangements of these building block units yield a local-density approximation band gap over a range of a few electronvolts. This suggests the possibility of creating and tuning the band gap in GO by varying the oxidation level

of the C1s spectra. Deconvolution of the O1s spectra produces three main peaks around 531.08, 532.03, and 533.43 eV. These have been assigned to CO (oxygen doubly bonded to aromatic carbon),50,83 C−O (oxygen singly bonded to aliphatic carbon),65,87 and phenolic (oxygen singly bonded to aromatic carbon)65,87 groups, respectively. The pristine GO shows an additional peak at a higher binding energy (534.7 eV).86 This corresponds to the chemisorbed/intercalated adsorbed water molecules. Moreover, another important parameter that can be used to characterize the degree of oxidation in GO is the sp2 carbon fraction. This can be estimated by dividing the area under the sp2 peak by C1s peak area. The sp2 fraction of the pristine GO was found to be only ∼40%, and the amount of carbon sp2 bonding was found to increase with the loss of oxygen during the thermal reduction. It was found to reach a maximum value of ∼80% at an oxygen content of ∼8 atom % (C/O ratio 12.5:1).50,81 This suggests that the remaining oxygen is responsible for ∼20% of the sp 3 bonding. Furthermore, Raman and FTIR spectroscopy data corroborate the identification of the degree of oxidation and the oxygenated species in GO. The Raman spectrum of a GO film displays a Dband at ∼1340 cm−1 and a broad G-band at ∼1580 cm−1.85 The G-band, which is characteristic of all sp2-hybridized carbon networks, originates from the first-order scattering from the doubly degenerate E2g phonon modes of graphite in the Brillouin zone center, while the prominent D peak comes from the structural imperfections created by the attachment of oxygenated groups on the carbon basal plane.85,88,89 Thus, the integrated intensity ratio of the D- and G-bands (ID/IG) 6032


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platelets has been estimated as 32 ± 5 kcal/mol.109 By use of theoretical first-principles calculations, the local-density approximation band gap of GO was found to vary over a range of a few electronvolts depending on the oxidation level.91 This suggests a great potential for tuning the energy gap in GO through the use of controlled reduction processes. In addition to these interesting electronic characteristics, GO is also expected to exhibit unique optical properties as evidenced by the recent demonstration of photoluminesence (PL) from GO.110 This luminescence was found to occur from the near-UV-to-blue visible (vis) to near-infrared (IR) wavelength range. This property should prove useful for biosensing, fluorescence tags and optoelectronics applications.111−114 These PL characterisitcs originate from the recombination of electron−hole (e−h) pairs, localized within small sp2 carbon clusters embedded within an sp3 matrix. The PL intensity varies with the nature of the reduction treatment, and can be correlated to the evolution of very small sp2 clusters (Figure 5B).113 In addition, GO also possesses specific ultrafast optical dynamics and nonlinear optical (NLO) properties, which should prove useful for potential applications in optoelectronic devices. In GO, it was found that there are two NLO regimes (sp2 domains and sp3 domains) with different ultrafast optical dynamics. In the sp2 domains, two-photon absorption dominates the nonlinear absorption for picosecond pulses. On the other hand, for nanosecond pulses excited state absorption also influences the nonlinear response in the sp3 domains.115 On the basis of heterogeneous ultrafast dynamics of GO with saturable absorption in sp2 domains and twophoton absorption in sp3 domains, the NLO response can be tailored by manipulation of the degree and location of oxidation on the GO sheets.116 Increasing the degree of reduction in GO causes excited state absorption to gradually switch to saturable absorption for shorter probe wavelengths. For example, Kürüm and co-workers117 demonstrated that both electrochemically induced reversible reduction and optically induced photoreduction in GO resulted in changes in the NLO properties of GO thin films. Recently, it has become popular to explore the electrochemical properties of GO at electrode surfaces. Due to its favorable electron mobility and unique surface properties, such as one-atom thickness and high specific surface area, GO can accommodate the active species and facilitate their electron transfer (ET) at electrode surfaces.118−120 For example, Zuo et al.120 reported that GO supports the efficient electrical wiring of the redox centers of several heme-containing metalloproteins (cytochrome c, myoglobin, and horseradish peroxidase (HRP)) to the electrode. Second, GO possesses excellent electrocatalytic properties.121−123 For example, our group121 has demonstrated the electrocatalytic activity of GO toward oxygen reduction and certain biomolecules. Wang et al.122 reported the electrocatalytic activity of reduced GO (rGO) toward the oxidation of hydrazine. In addition, it has been shown that GO exhibits high electrochemical capacitance with excellent cycle performance and hence has potential application in ultracapacitors.123,124 Shao et al.123 reported that rGO shows much higher electrochemical capacitance and cycling durability than carbon nanotubes (CNTs). The specific capacitance was found to 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 enhanced chemical activity compared with pristine graphene.37 It appears that one of the most important reactions of GO is its reduction.

and the relative amounts of epoxide and hydroxyl functional groups on the surface.91,92 Using density functional theory (DFT) calculations, Lahaye et al.95 revealed that the oxygen atoms 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 the carbon plane. Using a theoretical approach, Li et al.96 further illustrated the graphene oxidative breakup process, which provides a useful insight into the puzzling GO structure. In addition, theoretical calculations have also been used to study the atomistic structure of progressively reduced GO as well as the chemical changes of functional oxygenated groups during the GO reduction treatments.83,84,97,98 Bagri et al.83 reported a molecular dynamics simulation study of the evolution of the GO structure on thermal annealing. Their simulation results reveal the formation of carbonyl and ether groups through transformation of the initial hydroxyl and epoxy groups during the thermal annealing process. These carbonyl and ether groups are thermodynamically very stable and hence hinder the complete reduction of GO to graphene. Based on ab initio calculations, Larciprete and co-workers98 proposed a dual path mechanism in the thermal reduction of GO driven by the oxygen coverage. At low surface densities, the O atoms adsorbed as epoxy groups evolve as O2 leaving the C network unmodified; whereas with higher coverage of oxygen, the formation of other O-containing species opens up competing reaction channels. These consume the C backbone. In addition, theoretical simulations can provide useful information for obtaining the correct spectroscopic assignments, and these in turn are useful for interpreting experimental spectroscopic data.70,82,99 For example, Kudin et al.70 simulated the Raman spectra of GO and proposed an alternating single−double carbon bond model. 2.2. Properties

Due to its specific 2D structure and the existence of various oxygenated functional groups, GO exhibits various excellent properties. These include electronic, optical, thermal, mechanical, and electrochemical properties, as well as chemical reactivity. We will now briefly introduce electronic, optical, and electrochemical properties, as well as address the chemical reactivity of GO. Electronic properties such as conductivity of GO sheets depend strongly on their chemical and atomic structures. More accurately, they depend upon the degree of structural disorder arising from the presence of a substantial sp3 carbon fraction as seen in Figure 5A. In general, “as-synthesized” GO films are typically insulating with an energy gap in the electron density of states,50,90 and they exhibit sheet resistance (Rs) values of about 1012 Ω sq−1 or higher.100 The intrinsic insulating nature of GO is strongly correlated to the amount of sp3 C−O bonding. This in turn represents transport barriers,50 leading to the absence or disruption of percolating pathways among the sp2 carbon clusters, which allows classical carrier transport to occur.41 However, the reduction of GO (i.e., the incremental removal of oxygen) using a variety of chemical and thermal treatments, which facilitates the transport of carriers,51,101 can result in a decrease in Rs by several orders of magnitude and hence transform the material into a semiconductor and ultimately into a graphene-like semimetal.50,63,64,75,102−108 The conductivity of the reduced GO samples can reach ∼1000 S/m,75,108 and by use of resistivity and temperature-programmed desorption (TPD) measurements, the activation energy of the GO 6033


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routes for the preparation and functionalization of GO-based electrodes. Fine control over assembly conditions is responsible for the range of novel properties exhibited by these GO-based electrodes. To date, several methods for the preparation and functionalization of GO-based electrodes are well established. These techniques are employed to achieve an efficient electrochemical communication between the chemical reaction sites and the GO-based electrode interface with high levels of integration, sensor miniaturization, measurement stability, selectivity, accuracy, and precision. Thus, it must be acknowledged that the preparation and functionalization method is critical for the satisfactory electrochemical performances of GO-based electrodes.

GO can be reduced to graphene by various approaches. In the past few years, there have been reports of reducing GO in the solution phase using various reducing agents, such as hydrazine,66,125 sodium borohydride,126 or hydroquinone,127 and in the vapor phase using hydrazine/hydrogen64,85 or just by thermal annealing85 or by electrochemical techniques.119,128 Another important chemical reactivity of GO is its capacity for chemical functionalization. This involves adding other groups to GO platelets using various chemical reactions. It is known that GO has chemically reactive oxygen functionalities, such as carboxylic acid, epoxy, and hydroxyl groups. Thus, an ideal approach to chemical functionalization would utilize orthogonal reactions of these groups to selectively functionalize one site over another.37 Typically, GO can be covalently functionalized using specially selected small molecules or polymers through activation and amidation/esterification of either the carboxyls or hydroxyls in GO via coupling reactions.129−131 For example, the GO carboxylic acid groups were first activated using thionyl chloride (SOCl2), followed by the coupling of octadecylamine via the formation of amides. The result of this strategic sequence renders GO soluble in common organic solvents.129 The epoxy groups can be also used for the covalent functionalization of GO through ring-opening reactions due to a nucleophilic attack at the α-carbon by the amine.37,101,132 For example, octadecylamine was attached to GO via a ringopening reaction with the epoxy groups. This afforded colloidal suspensions 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 end group to GO platelets also via a ring-opening reaction with epoxy groups. Due to the high polarity of the material, the resulting chemically modified graphenes (CMGs) were welldispersed in solvents such as water, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). In addition to the covalent functionalization, noncovalent functionalization of GO can also be accomplished via π−π stacking, cation−π, van der Waals interactions, or hydrogen bonding.37,133,134 Yang and coworkers133 reported the preparation of the doxorubicin hydrochloride (DXR)/GO hybrid material through noncovalent interactions. It was suggested that π−π stacking and hydrophobic interactions between the quinone functionality of DXR and sp2 networks of GO were the primary noncovalent interactions. In addition, strong hydrogen bonding may be also present between the −OH and −COOH groups of the GO and the −OH and −NH2 groups in DXR. Very recently, GO has been found to act as a convenient carbocatalyst for facilitating oxidation and hydration reactions.135−138 This suggests that GO could be useful beyond the realm of displays and electronics. For example, Bielawski’s135 group demonstrated that GO can catalyze the oxidation of various alcohols and alkenes as well as the hydration of various alkynes into their respective aldehydes and ketones in good to excellent yields. In addition, GO has also shown strong oxidizing properties. Bielawski and co-workers139 reported that GO was an effective oxidant for use in a broad range of reactions. These include the oxidation of olefins to their respective diones, methyl benzenes to aldehydes, diarylmethanes to ketones, and various dehydrogenations.

3.1. Preparation of GO-based Electrodes

Starting from a stable colloidal suspension of GO, several strategies can be adapted to prepare hierarchically organized structures of GO-based materials on various underlying electrode substrates (such as Au, glassy carbon, and quartz glass). The first approach, which is the simplest and most straightforward and pragmatic, involves the simple evaporation of a thin film of a GO suspension on the electrode surface. This has to be carried out under well-controlled conditions of temperature and humidity in order to ensure very slow evaporation of the solvent (in most cases water). This allows the colloidal particles to find their energy minima by forming an almost perfect structure on the electrode surface.140,141 It is fair to say that the GO-based electrodes used in most of the hitherto-reported electrochemical studies were simply prepared by randomly dispersing the GO-based materials onto a substrate electrode114,142−147 or by confining GO on a substrate electrode with other functional materials such as Nafion123 or ionic liquids148 through drop-casting, dip-coating, or spraying procedures. These GO-based electrodes have been demonstrated 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 random confinement and aggregation of the GO. This results in poor control over the film quality and thickness.41 As a result, alternative methods for the preparation of GO-based electrodes from 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 electrodes.100,149−153 It has proven to produce much better control of defect density and also of the overall thickness of the GO film layer. A typical process involves depositing a small amount of a liquid GO suspension onto the center of an electrode substrate and then spinning the substrate at high speed. Final film thickness and homogeneity depend on the GO suspension concentration, the process parameters (such as spinning speed and acceleration), and the number of spin-coating cycles. This technique can result in continuous large-area films with the absence of nanometer scale wrinkling, as was found in other GO deposition procedures.100,149 Another important approach for the preparation of GObased electrodes is to use the self-assembly technique. This has recently received considerable interest because of potential applications in sensor fabrication and for the patterned chemical architecture of solid supports.154 This technique not only can provide a facile protocol for the preparation of GO films with controlled thickness but also can efficiently adjust electrode dimension from conventional to nanoelectrode ensemble. This would be very attractive for electrochemical

3. PREPARATION AND FUNCTIONALIZATION OF GO-BASED ELECTRODES The crucial development of GO-related electrochemical applications lies in the design of simple, reliable assembly 6034


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the fabrication of highly uniform patterned GO films on various substrates including flexible PET. They used a method known as “micromolding in capillary”. In their work, a drop of GO aqueous solution was dropped at one end of the open channels of a hard polydimethylsiloxane (PDMS) stamp, which was then placed in close contact with the 3-aminopropyltriethoxysilane (APTES)-modified flat substrates using an applied force. Afterward the whole system was placed in a vacuum oven and degassed for 30 min to ensure that the solution was sufficiently dried. Subsequently the PDMS stamp was carefully peeled off and the GO patterns were thus formed on the substrate. This was followed by a reduction process in hydrazine vapor at 60 °C for 12 h in order to obtain the rGO patterns. Compared with other methods for the fabrication of such GO patterns, this micromolding in capillary method is fast, facile, and substrate independent.

studies on a nanoscale. Due to the strong electrostatic repulsion between the 2D confined layers,67 stable monolayers of GO sheets can be well formed at the air−water interface. By means of Langmuir−Blodgett (LB) assembly, these GO monolayers can be transferred to an electrode substrate, readily creating highly uniform GO thin film-based electrodes.42,67,155 In addition, GO sheets can be also self-assembled onto an electrode substrate by a layer-by-layer technique using a positively charged self-assembled monolayer (SAM). This is possible since the oxygen-rich functional groups on GO are generally negatively charged. It has been demonstrated that GO sheets can be guided to precise locations on an electrode substrate using the electrostatic attraction between the negatively charged GO sheets and a positively charged template.128,156,157 The distribution of the resulting GO sheets depends on the surface functionalization, background passivation, pH, and deposition time. For example, Wei et al.156 reported the self-assembly of GO sheets onto a pretreated Au electrode with amino-terminated 11-amino-1-undecanethiol (AUT) template patterns. It was found that GO sheets did not adhere to the bare Au surface but could be selectively selfassembled onto the AUT patterns due to the electrostatic interaction between GO and AUT. Aside from the electrostatic attraction, the hydrophobic interaction between the GO sheets and the SAM template has also been considered as another possible driving force to direct the self-assembly of GO sheets onto the SAM template-pretreated substrate surface.158 Alternatively, vacuum filtration has also been used to deposit uniform layers of GO for the preparation of GO-based electrodes. This technique involves filtering the suspension that contains the GO nanosheets through a porous membrane with well distributed pores, such as a cellulose ester membrane,51,159−161 an anodic aluminum oxide (AAO) membrane,162 or an anodisc membrane filter.163,164 To summarize, during the filtration of the GO suspension through the porous membrane, the liquid can pass through the pores whereas the GO sheets become lodged, leading to the deposition of the GO films on the membrane. The permeation process is self-regulating and allows reasonably good nanoscale control over the film thickness by simply varying the concentration of the GO suspension or the filtration volume. The deposited GO films can then be transferred onto various substrates by gently pressing the film against the substrate surface and dissolving the porous membrane,41 leaving behind a well-adhered uniform GO thin film. In addition, the direct patterning of GO thin films onto various substrates, especially flexible substrates, is highly desirable for the preparation of the GO-based flexible electrodes. 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 platelets onto poly(ethylene terephthalate) (PET) could be achieved by using aqueous surfactant-supported dispersions of rGO powder as the printing ink. The resulting inkjet-printed film exhibited good electrical conductivities (σ ≈ 15 S cm−1). In this work, the ink cartridge of a commercial inkjet printer was emptied and refilled with a freshly prepared GO suspension, and then the GO ink was printed directly onto a flexible substrate (such as commercially available PET). The patterns were designed on a computer in advance. Inkjet printing allows control of the film thickness by altering the number of passes and also by making it possible to use the gray scale on the computer. In another example, He et al.166 developed a totally different approach to

3.2. Functionalization of GO-based Electrodes

GO-based electrodes are different from other kinds of carbonbased materials used in electrochemistry, in that they consist of a 2D layered structure with a large surface area and also possess a large number of oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups. Such properties make it possible to functionalize such GO-based electrodes using either covalent or noncovalent chemistry in order to modulate the electrode’s structural architecture and intrinsic properties. To date, a broad range of functional materials (such as nanoparticles, organic compounds, polymers, and biomaterials) have been reported for the functionalization of GO-based electrodes. These generally exhibit novel, interesting properties and hold promise for many applications, especially those involving electrochemistry, due to their high electrical conductivity, chemical stability, tunable modification, and multifunctional structures. 3.2.1. Functionalization with Nanoparticles. GO-based nanocomposites, because of their unique structures and superb properties, have emerged as one category of intriguing materials with promising applications in the field of electrochemistry. For GO-based nanocomposites, the unique properties of the GO nanosheets make them particularly useful as the nanoparticle support. This is because the high surface area is essential for the dispersion of the nanoparticles and in order to maintain their electrochemical activities. The GO-supporting materials not only maximize the availability of the nanosized surface area for electron transfer but also provide better mass transport of the reactants to the electroactive sites on the electrode surface. Moreover, the conductive GO support facilitates the efficient collection and transfer of electrons to the collecting electrode surface. On the other hand, the functionalization of GO with nanoparticles has made the realization of nanoscale composite electrodes possible. GO-based nanocomposite-modified electrodes present unusual advantages over macroelectrodes in electrochemical applications. These include excellent catalytic activity, enhancement of mass transport, a high effective surface area, and control over the electrode microenvironment. In addition, the combination of GO with nanoparticles may prove capable of contributing additional performance in some functional electrochemical applications. Earlier efforts were devoted to the preparation of functional GO/inorganic nanocomposites. These are derived from the decoration of GO sheets with inorganic nanoparticles (NPs) such as metal nanoparticles and metal oxide nanoparticles, with a view to their application in electrochemical sensing, catalysis, 6035


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Figure 6. (A) Schematic illustration and images of aqueous dispersions of reduced GO sheets and composites on the surface: (a) reduced GO aqueous dispersion, black precipitate after reduction; (b) reduced GO−PDI aqueous dispersion, without precipitate after centrifugation; (c) reduced GO−PyS 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 hybrid nanocomposites. Reprinted with permission from ref 198. Copyright 2009 John Wiley & Sons, Inc.

In addition to metal and metal oxide nanoparticles, quantum dots (QDs) have also been used to functionalize GO with a view to creating another platform for electrochemical applications.187−190 For instance, our group188 successfully prepared QD-sensitized rGO nanocomposites by the in situ growth of QDs on noncovalently functionalized rGO. These QD-sensitized rGO photoelectrodes were then used as an efficient platform for photoelectrochemical applications. In addition, other nanoparticles, such as Prussian blue,191−193 metal hydroxides,194,195 and polyoxometalates,196,197 have also been used in the functionalization of GO-based electrodes for electrochemical applications. For example, Chen et al.195 reported a facile soft chemical approach for the fabrication of rGO−Co(OH)2 nanocomposites in a water−isopropanol system. They demonstrated that the electrochemical performance of Co(OH)2 was significantly improved after deposition on rGO sheets. 3.2.2. Functionalization with Organic Compounds. The various hybrid materials created by the organic functionalization of GO have generated intense attention. This is largely driven by the possibility of improving its solubility/processability in both water and organic solvents and combining some of the outstanding properties of the GO nanosheets with those of small organic molecules, such as photoactive or electroactive units. Such hybrid materials can be potentially used for preparing durable and functional chemically modified electrodes that have already greatly facilitated various electrochemical studies and applications. Currently, the noncovalent and covalent functionalization of GO with organic compounds has become the subject of intensive research for the fabrication of novel hybrid nanocomposites with new functions and applications. With respect to the noncovalent approach, as described above, the strong adsorption of organic aromatic compounds onto GO nanosheets is primarily

and fuel cells. These functional metal or metal oxide/GO nanocomposites can be prepared using different physical or chemical approaches: a physical attachment approach,167 an in situ chemical reduction process,168−171 electrochemical synthetic processes,172−174 impregnation processes,175 a selfassembly approach,176 ultrasonic spray pyrolysis,177 and so on. By means of these approaches, GO-based nanocomposites with metal nanoparticles (such as Pt,168−170,172,175,178−180 Au,178 or Ru179,180) and oxide nanoparticles (such as TiO2,167,181,182 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 GObased nanocomposite-modified electrodes. For example, Dong et al.180 reported a simple approach for the deposition of Pt and Pt−Ru nanoparticles onto surfaces of GO nanosheets by ethylene glycol reduction. They systematically investigated the effect of GO as a catalyst support on the electrocatalytic activity of Pt and Pt−Ru nanoparticles for both methanol and ethanol oxidation used in fuel cell applications. In comparison to carbon black as a catalyst support, GO effectively enhanced the electrocatalytic activity of Pt and Pt−Ru nanoparticles for the oxidation of methanol and ethanol into CO2. By using an exfoliation-reassembling method, Paek et al.183 prepared SnO2/ GO nanoporous electrodes with three-dimensionally delaminated flexible structures. They were used for the enhancement of cylic performance and lithium storage capacity. From their experimental results, it appears that the dispersed GO nanosheets in the ethylene glycol solution were reassembled and homogeneously distributed between the loosely packed SnO2 nanoparticles. The resulting SnO2/GO nanoporous electrodes were able to limit the volume expansion upon lithium insertion, and this resulted in the superior cyclic performances. 6036


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

attributed to the π-stacking and hydrophobic interactions between the organic molecules and the GO. Where the covalent approach is concerned, the presence of oxygencontaining groups in GO provides a handle for its chemical modification using known carbon surface chemistry. This requires the chemical reagent to be capable of diffusing into the reactive regions of the GO sheet and reacting with the functional groups on the sheet surface. Until now, a wide range of organic compounds, such as porphyrins,130,198,199 aromatic dyes,118 alkylamines,200 doxorubicin hydrochloride,133 ionic liquids,132 pyrene,201 perylenediimide,201 cyclodextrin,202 7,7,8,8-tetracyanoquinodimethane (TCNQ),203 and aryl diazonium compounds,204 have all been noncovalently or covalently attached onto the GO nanosheets to generate functional organic nanocomposites for electrochemical applications. For example, Liu et al.118 proposed a facile method to increase the dispersity of rGO using noncovalent functionalization of the GO with a water-soluble aromatic electroactive dye. They used methylene green (MG) and investigated the electrochemical properties of the resulting rGO/MG nanocomposites. It was demonstrated that the rGO/ MG nanocomposites confined onto a glassy carbon electrode showed lower charge-transfer resistance and better electrocatalytic activity toward the oxidation of β-nicotinamide adenine dinucleotide (NADH) compared with a pristine rGO-based electrode. In another case, Su et al.201 presented an unprecedented approach to noncovalently functionalized rGO by using a large aromatic donor (pyrene-1-sulfonic acid,

PyS) as well as acceptor molecules (3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid, PDI) via π−π interactions as shown in Figure 6A. Their approach gave rise to a novel class of GO nanocomposites with tunable electronic properties. Xu et al.198 reported the covalent functionalization of GO with a porphyrin, namely, 5,4-(aminophenyl)-10,15,20triphenyl porphyrin (TPP-NH2) via an amide bond (Figure 6B). Attachment of TPP-NH2 significantly improved the solubility and dispersion stability of the GO-based material in organic solvents. The resulting TPP-NH2/GO hybrid nanocomposites exhibited excellent optical-limiting properties. 3.2.3. Functionalization with Polymers. Recently, GObased polymer nanocomposites are emerging as new class of materials that hold promise for electrochemical applications. These nanocomposites show considerable improvement in properties that cannot normally be achieved using conventional composites or pristine polymers. Generally, these improvements can be obtained at very low GO filler loadings in the polymer matrix. The extent of the improvement depends very much on the nature of the polymer, the delicate morphological organization, the fine interface control, and the degree of dispersion of the GO nanofillers within the polymer matrix.131,205,206 In addition, functionalization with polymers is an effective route for ameliorating the GO dispersion. Thus it is possible to incorporate GO into an excellent compatible polymer material in the absence of any aggregation.207−209 Furthermore, such functionalization should also prove helpful for the modification of electrode interfaces with potential 6037


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biofunctional GO-based electrodes for biorelated applications. Recent studies reveal that GO shows excellent biocompatibility,114,220−222 enhances the electrochemical reactivity and the electron transfer (ET) rate of biomolecules,120,223 and is able to accumulate biomolecules.224 To take advantage of these remarkable GO properties in biorelated electrochemical applications, a prerequisite is the development of physical or chemical methods to immobilize biological molecules onto the GO-based electrodes in a reliable manner. Both noncovalent and covalent modification methods have been reported. Alhough the immobilization of biomolecules onto GO has been pursued in the past, to the best of our knowledge, most of these biofunctional GO/biomolecules composites were prepared by noncovalent modification methods, such as physical entrapment120,134,223 or adsorption by incubation of the biomolecules with a GO suspension.220,225−229 The noncovalent interaction between GO and the biomolecules involves electrostatic or hydrogen-bonding interactions, hydrophobic interactions, and π−π stacking interactions.227 For example, Zhang et al.228 fabricated enzyme-modified GO-based electrodes through the incubation of enzymes (such as HRP and lysozyme) with a GO suspension. They demonstrated that the electrostatic interactions and hydrogen bonding between the enzymes and the GO contributed to the enzyme immobilization onto the GO sheets. In addition, the catalytic performance of the immobilized enzymes was also related to the interaction of the enzyme molecules with the surface functional groups of the GO substrate. Apart from the noncovalent attachment, also of importance is the covalent attachment for the immobilization of the biomaterials with the GO sheets. Frequently, the hydrophilic oxygen-containing functional groups of the GO have been used to covalently bind biomaterials such as bovine serum albumin (BSA),230 DNA molecules,52,114 and enzymes119,221 via a carbodiimide coupling of the respective amino-functionalized biomolecules. For example, Liu et al.221 reported the direct, efficient fabrication of glucose biosensors through a covalent attachment between the carboxylic acid groups on the GO sheets and the amine groups of the glucose oxidase. The resulting biosensor showed a broad linearity, good sensitivity, excellent reproducibility, and storage stability suitable for glucose biosensing. In addition, the biotin−streptavidin interaction is considered another important covalent attachment method for the linking of streptavidin/biotin biomolecules to biotin/streptavidin-activated GO sheets. For instance, Liu et al.222 prepared the GO/streptavidin complex and used it to capture biotinylated protein complexes via the streptavidin− biotin interaction for affinity purification.

applications in electrochemistry, since GO-based polymer nanocomposites possess high electrical conductivity, chemical stability, the option for tunable modification, and suitable applicable electroactive sites.210,211 Kuilla et al.212 have thoroughly reviewed the wide range of polymers that have been reported to date that have been used to functionalize GO nanosheets for the fabrication of novel nanocomposites with a view to new functions and applications. Of note is the fact that many research groups have been working on the functionalization of GO with electroactive polymer materials, such as polyaniline (PANI),211,213,214 poly(3,4-ethylenedioxythiophene) (PEDOT), 210 poly(styrenesulfonate) (PSS), 210 Nafion, 215,216 and poly(diallyldimethylammonium chloride) (PDDA),217 with the aim of enhancing their electrochemical properties and overall performance. It was found that GO sensors functionalized with electroactive polymers often offer more stability, higher sensitivity, and better selectivity compared with pristine GO or polymer sensors.213,215 There are two main strategies for the surface functionalization of GO using polymers: in situ intercalative polymerization211,213 and solution intercalation.214−217 With respect to the in situ interactive polymerization method, first the GO or rGO is swollen well within the liquid monomer. Subsequently, the polymerization is initiated using a suitable initiator added under controlled experimental conditions. Zhang et al.211 successfully prepared (reduced) GO/PANI nanocomposites by the in situ polymerization of aniline monomer in the presence of GO under acidic conditions (Figure 7). These nanocomposites exhibited a uniform structure with the PANI fibers absorbed onto the GO surface or filled in the GO sheets. When used as supercapacitor electrodes, such uniform structures afforded high specific capacitance and good cycling stability during the charge− discharge process. Alternatively, solution intercalation is based on a solvent system in which the polymer is solubilized and GO sheets are allowed to swell.212,218 Typically, the GO or rGO sheets are well dispersed in a suitable polymer solution with the assistance of sonication or mechanical agitation. The polymer then adsorbs onto the delaminated GO sheets, and after the evaporation of the solvent, the sheets reassemble, sandwiching the polymer to form the nanocomposites.219 In one example, GO nanosheets were stably dispersed in water by noncovalent functionalization with sulfonated polyaniline (SPANI) through solution intercalation. The resulting composite film of SPANIfunctionalized GO sheets showed enhanced electrochemical stability and electrocatalytic activity.214 Choi et al.215 reported the preparation of free-standing flexible conductive rGO/ Nafion (RGON) hybrid films made by a solution chemistry technique using self-assembly and directional convective assembly. The hydrophobic backbone of Nafion provided well-defined integrated structures for the construction of hybrid materials through self-assembly, while the hydrophilic sulfonate groups enabled highly stable dispersibility and long-term stability for the rGO sheets. It is important to note that the synergistic electrochemical characteristics of the RGON were attributed to the high conductivity, facilitated electron transfer, and low interfacial resistance, all of which lead to their excellent performance as electrochemical biosensing platforms for organophosphate detection. 3.2.4. Functionalization with Biomaterials. Because GO has a large specific surface area and abundant functional groups, it provides an ideal platform for the accommodation of biomolecules, which is a key issue in the fabrication of

4. GO-BASED ELECTROCHEMICAL APPLICATIONS The high surface-to-volume ratio of GO, in conjunction with its high dispersibility in both water and organic solvents as well as its wide range of reactive surface-bound functional groups, makes GO-based materials very attractive for electrochemical studies and applications. As described in Section 2, GO-based materials exhibit a moderate conductivity (depending on the extent of reduction), high chemical stability, and excellent electrochemical properties. More remarkably, GO has been demonstrated to be capable of facilitating the direct electron transfer of enzymes and proteins (such as, cytochrome c and horseradish peroxidase) at a GO-based electrode.120 These properties not only make it possible to understand the intrinsic thermodynamic and kinetic electron transfer properties of 6038


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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 Au−rGO paper electrode and Au foil electrode to the successive addition of 0.1, 0.5, 1.0, 2.0, and 4.0 mM glucose in PBS buffer (pH 7.4) under magnetic stirring; insets show the amperometric response of Au−rGO paper and Au foil electrodes at a lower concentration detected. (C) Typical amperometric response of Au−rGO paper electrode to successive addition of 0.02, 0.1, 0.2, and 1.0 mM H2O2 in a stirring PBS; the inset shows the amperometric response to successive addition of 5 and 10 μM H2O2. Reprinted with permission from ref 241. Copyright 2012 American Chemical Society.

rGO/SAM electrode toward ascorbic acid, dopamine, and uric acid. In addition, the introduction of inorganic nanocatalysts may offer GO-based electrodes novel electrocatalytic properties due to the excellent catalytic activities of such inorganic nanocatalysts. In this case, the GO generally acts as a support for the deposition of the inorganic nanocatalysts. In addition, it facilitates or mediates the charge/electron transfer between the electroactive species and the electrode surface of the GOsupported nanocatalysts and modulates the electrochemical reactions in a controlled fashion. To date, GO has been extensively studied as a support for the dispersion of precious metal nanoparticles to enhance their electrocatalytic activities in fuel cells.168−170,172,179,180,235−237 Seger et al.168 reported the deposition of Pt nanoparticles on rGO sheets by means of the borohydride reduction of H2PtCl6 in a GO suspension. Subsequently they were used as a novel electrocatalyst in a proton exchange membrane (PEM) assembly for PEM fuel cells. The partially reduced GO-Pt based fuel cell delivered a maximum power of 161 mW/cm2 compared with 96 mW/cm2 for an unsupported Pt based fuel cell. This suggests that the role of GO as an effective support material in the development of an advanced electrocatalyst is feasible. Similarly, our group169 recently prepared Pt/rGO nanocomposites in a one-step synthesis and demonstrated that these nanocomposites showed superior electrocatalytic performance toward methanol oxidation. In addition to the electrocatalytic applications in fuel cells,

electroactive species at GO-based electrode interfaces but also pave a new route to electrochemical sensors and electroanalysis. To date, GO-based materials have been used to design and prepare GO-based electrodes for a wide range of electrochemical applications, including electrocatalysis, electrochemiluminescence, electrochemical sensors, immunoassays, DNA sensors, and others. 4.1. Electrocatalysis

Electrocatalysis represents one of the most important areas for the application of GO-based materials in electrochemistry. First of all, GO itself possesses excellent electrocatalytic activities toward some important species.121,122,144,147,158,166,202,231−234 For example, our group121 studied the electrochemical and electrocatalytic properties of rGO films, and demonstrated that the rGO-based electrode exhibited fast electron-transfer kinetics and possessed excellent electrocatalytic activity toward oxygen reduction. When compared with bare basal and edgeplane pyrolytic graphite electrodes, Lin et al.231 reported that the rGO-modified basal and edge-plane pyrolytic graphite electrodes exhibited excellent electrocatalytic activity toward the electrocatalytic oxidation of H2O2 and β-nicotinamide adenine dinucleotide (NADH). Interestingly, Yang et al.158 constructed GO-based electrodes with a tunable electrode dimension using the controllable adsorption of rGO onto the SAM of n-octadecyl mercaptan (C18H37SH) at Au electrodes. They demonstrated the excellent electrocatalytic activity of the 6039


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

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

GO-supported precious metal nanocatalysts, such as Au/ GO,238−241 Pt/GO,242,243 Pd/GO,244,245 and PtNi/GO,246 have been also used to fabricate GO-based electrodes for the sensitive and selective detection of specific species due to their excellent catalytic properties. For example, Xiao et al.241 reported a modular approach to fabricating high-performance flexible electrodes by structurally integrating 2D-assemblies of Au nanoparticles with freestanding GO paper. They demonstrated that these Au/GO flexible electrodes exhibited outstanding electrocatalytic activities for the sensitive and selective detection of glucose and hydrogen peroxide (H2O2) (Figure 8). In addition to precious metal nanoparticles, other nanocatalysts, such as oxide nanoparticles,247 hydroxide nanoparticles,248 Prussian blue nanoparticles,191−193,249−251 methylene green,118 and methylene blue, 252 have also been successfully anchored onto the surface of GO sheets for the design and preparation of novel GO-supported electrocatalysts. For example, Li et al.247 successfully prepared MnO2/GO nanocomposites as a novel electrocatalyst for the nonenzymatic detection of H2O2. Due to the electrocatalytic abilities of MnO2 toward H2O2 and the high surface area of GO, MnO2/GObased electrodes showed very high electrochemical activity for the detection of H 2 O 2 in an alkaline medium. The corresponding H2O2 electrochemical sensors displayed good performance 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 of high-quality Prussian blue nanocubes on the surface of reduced

4.2. Electrochemiluminescence

Recently, GO-based materials have emerged as one of the most fascinating alternative electrode materials for applications in electrogenerated chemiluminescence. This is commonly defined as electrochemiluminescence (ECL) and is a light emission that arises from the high-energy electron-transfer reaction between electrogenerated species.253,254 In fact, pristine GO sheets cannot generate the ECL effect due to the absence of ECL merits. Thus, the GO-related ECL properties generally come from the different luminophore moieties anchored to the GO-based electrode, such as the ́ ruthenium( II) tris(2,2 -bip y rid ine) com plex ( Ru187,190,259−261 2+ 255−258 QDs, upconversion NaYF4/ (bpy)3 ), Yb,Er nanoparticles,262 and luminol.263 Due to the GO’s tunable moderate conductivity and distinctive structural properties, on the one hand, the introduction of GO into an ECL-based sensor platform can be extremely helpful in accelerating electron transfer between the lumophores and the electrode, and on the other hand, they increase the surface area and porosity of the platform to make coreactant diffusion faster for the ECL-based sensors. In one example, Li et al.255 developed an ECL sensor based on a Ru(bpy)32+−rGO− 6040


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Figure 10. (A) Schematic illustration of GO-amplified electrochemiluminescence (ECL) of QDs platform. (B) ECL intensity−potential behaviors of (a) background, (b) CdTe QDs, and (c) CdTe QDs with 1.2 μg mL−1 GO in 0.02 M Na2CO3−NaHCO3 (pH 9.5) buffer solution. (C) ECL intensity−time behaviors of CdTe QDs with 1.2 μg mL−1 GO (a) to the commercial glutathione (GSH) drug with different concentrations, from curve b to curve i: 0.04, 0.1, 0.18, 0.21, 0.25, 0.29, 0.32, and 0.36 μg mL−1. 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.

Nafion composite film. As expected, the introduction of rGO into the Nafion facilitated the electron transfer of Ru(bpy)32+ and improved the long-term stability of the sensor by inhibiting the migration of Ru(bpy)32+ into the electrochemically inactive hydrophobic region of the Nafion. This ECL sensor exhibited a good linear range over 1 × 10−7 to 1 × 10−4 M with a detection limit of 50 nM in the determination of tripropylamine (TPA). Our group187 reported a GO-amplified ECL of QDs platform and its efficient selective sensing for glutathione (Figure 10). It was demonstrated that GO facilitated the generation of QD radicals and formed a high yield of QD*, leading to ∼5 times enhancement of ECL intensity compared with the ECL platform without GO. Based on the proposed GO-amplified ECL platform, we realized the sensitive and selective detection of glutathione from thiol-containing compounds and further used 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 GO has been also demonstrated by Bard’s group.264 In their work, an ECL intensity of >4 × 108 photon counts s−1 cm−2 was found with oxidized highly oriented pyrolytic graphite (HOPG) and of >1.8 × 106 photon counts s−1 cm−2 from a 6 ppm suspension of GO platelets in an aqueous phosphate buffer solution (pH = 7.0) containing 0.1 M NaClO4 and 13 mM trin-propylamine (TPrA). A possible explanation of the broad emission was suggested as being the existence of smaller aromatic hydrocarbon-like domains formed on the “graphitic” layers through interruption of the conjugation by the oxidized centers.

atom-thick planar sheets of sp2-bonded carbon atoms decorated with oxygen functional groups. Every atom in a GO sheet can be considered as a surface atom. Thus, electron transport through these ultrathin materials can be highly sensitive to adsorbed molecules.149 This phenomenon has subsequently enabled the fabrication of sorption-based sensors capable of detecting trace levels of vapor using conventional low-power electronics. The corresponding sensing mechanism can be generally ascribed to the changes in conductance or capacitance when gaseous molecules (which act as electron donors or acceptors) interact with the GO-based materials.265,266 Such conductivity or capacitance changes are caused by the formation of a space charge region induced by either gas adsorption or the formation of oxygen vacancies on the GO surface. Consequently, it is easy for GO-based gas sensors to achieve high sensitivities due to the much higher specific surface area and surface-to-bulk ratio of the GO. However, establishing sensor selectivity for specific gases is difficult and challenging, not least because the sensing selectivity of GObased gas sensors requires a detailed understanding of the surface and interfacial processes at the atomic level as well as their relationship with GO material-properties and device performance. Selectivity is dependent on many parameters, such as gas adsorption and coadsorption mechanisms, surface defect sites, surface reaction kinetics, and electron transfer between the adsorbed gas molecules and the modified electrode of GO-based materials. Currently, sensor selectivity remains, for the most part, empirical. In practice, selectivity can be achieved by enhancing gas adsorption or promoting specific chemical reactions at the GO-based electrodes through controlling the surface defect sites using bulk dopants, surface modification methods, and the addition of metallic clusters or oxide catalysts. The optimal defect density balances the gains in sensitivity against the rapid degradation in conductivity due to the defects. In this respect, GO is an ideal material for balancing these effects because it contains a diverse range of surface sites whose

4.3. Electrochemical Gas Sensors

One of the most promising applications of GO-based materials is for electrochemical sensing, especially for gas sensing and biosensing. In gas sensing devices, two of the most important issues are gas sensitivity (detection of gas concentrations at the ppm level) and gas selectivity (detection of specific gases in a mixed gas environment). As is well-known by now, GO is one6041


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

defects, and oxygen functional groups) through single and double hydrogen bonding. It was also found that the response and recovery characteristics of the conductance response and the sensing selectivity could be tailored by adjusting the reduction process. In another work, Fowler et al.151 reported the development of useful chemical sensors from chemical rGO dispersions for the detection of NO2, NH3, and DNT, as shown in Figure 11. The sensor response was consistent with a charge transfer mechanism between the analyte and rGO with a limited role of the electrical contacts. It was demonstrated that the sensing mechanism of NO2 was attributed to hole-induced conduction. This is because it withdraws an electron from rGO. However, the NH3-sensing mechanism was ascribed to electron-induced conduction, since it donates an electron to rGO. The DNT-sensing mechanism was similar to that for NO2, namely, electron-withdrawing. The DNT detection limit was reported to be 28 ppb. In addition, Dua et al.165 described a rugged and flexible sensor using inkjet-printed films of rGO on PET for the reversible detection of NO2 and Cl2 vapors in an air sample at the parts per billion level. When inkjet-printed rGO/PET films were exposed to successively decreasing concentrations of the electron-withdrawing vapors (i.e., NO2 and Cl2) within a specific range, the conductivity increased in a linear fashion. This was consistent with an increase in the

density is easily controlled. This renders GO a promising candidate for use as the active material in molecular gas sensors requiring high sensitivity and selectivity. Until now, there have been several literature reports regarding GO-based sensors for the detection of gaseous molecules, such as NO2,151,165,267 NO,268 NH3,151,267,269 Cl2,165 chemical warfare agents,149 and explosives.149,151,270 Robinson et al.149 demonstrated that rGO is suitable as the active material for high-performance gas sensors. Sensors were fabricated from an ultrathin network of exfoliated GO platelets, which were then further tunably reduced toward graphene by varying the exposure time to a hydrazine hydrate vapor. Such rGO devices readily offered sensitivities at parts-per-billion levels for both chemical warfare agents (e.g., hydrogen cyanide (HCN), chloroethylethyl sulfide (CEES), and dimethylmethylphosphonate (DMMP)) and explosives (e.g., 2,4-dinitrotoluene (DNT)). The sensing mechanism was attributed to the rapid and slow response of the rGO device upon exposure to the analyte vapor, which leads to the relative change in electrical conductance (ΔG/G0). It was found that the rapid response arised 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 with high-erenergy binding sites (such as vacancies, structural 6042


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Figure 12. (A) Schematic illustration of Au NP/rGO hybrid synthesis and AChE/Au NP/rGO nanoassembly generation by using PDDA. Graphite was used for producing graphene oxide with Hummer’s method, and Au NP/cr-G was obtained by reducing HAuCl4 on GO nanosheets. AChE was stabilized 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 to concentration of organophosphate pesticides (OPs); (b) linear relationship between residual activities of AChE and concentration of OPs. Residual activities of AChE was defined with [(imax − is)/imax%] where imax was the current corresponding to 2 mM ATCh and is was current value after exposure to different concentrations of OPs for 15 min. From ref 283, Reproduced by permission of The Royal Society of Chemistry.

4.4.1. Enzyme Biosensors. In recent years, enzyme electrodes based on GO-based materials have attracted great interest for the detection of a wide range of analytes. An extremely important example is in the determination of glucose, which plays a crucial role in the diagnosis and therapy of diabetes. Recent studies have indicated that GO-based materials are very capable of the sensitive and selective detection of glucose.119,178,221,223,272−278 For example, Liu et al.221 prepared a biocompatible GO-based glucose biosensor using the covalent attachment between the carboxyl acid groups of the GO sheets and the amines of glucose oxidase (GOx). They demonstrated that the resulting biosensors exhibited broad linearity, good sensitivity, excellent reproducibility, and storage stability. Lin and co-workers223 studied the GOx/rGO/chitosan nanocomposite-modified electrode for direct electrochemistry and glucose sensing. It was found that the nanocomposite film can provide a favorable microenvironment for GOx to realize direct electron transfer at the modified electrode. The nanocomposite-based biosensor exhibited a wider linearity range from 0.08 to 12 mM glucose with a detection limit of 0.02 mM and much higher sensitivity (37.93 μA mM−1 cm−2) compared with other nanostructured supports. Apart from GOx-based biosensors, similar sensitivities and stability improvements have been found for electrochemical biosensors based on other enzymes, such as horseradish peroxidase (HRP), 2 7 9 − 2 8 1 alcohol dehydrogenase (ADH),145,148 organophosphorus hydrolase (OPH),215 micro-

number of charge carriers. Interestingly, a signal recovery upon exposure to UV irradiation was observed in the vapor detection, and this suggests that there was no covalent bond formation between the vapor molecules and rGO. 4.4. Electrochemical Biosensors

Another important application of GO-based electrodes is to exploit GO-based materials for electrochemical biosensing, and this is an area that has seen significant growth in the past few years. An electrochemical biosensor is an analytical device that converts a biological response into an electrical signal using electrochemical strategies that determine concentrations of substrates and other parameters of biological interest even where they do not utilize a biological system directly.271 From the perspective of electrochemical reactivity, an important part of the success of GO-based materials for electrochemical biosensing is their ability to provide a suitable microenvironment for biomolecule-immobilization while retaining their biological activities. Also they facilitate electron transfer between the immobilized biomolecules and the electrode substrates.30 Thus, since these types of GO-based electrodes promise lower detection limits, fast response time, high sensitivity, and increased signal-to-noise ratios, they have been used for the fabrication of many novel biosensors with potential applications. In general, most GO-based electrochemical biosensors can be fabricated through the incorporation of special bioactive species (such as enzymes, metalloproteins) onto GO-based electrodes. 6043


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Figure 13. (A) Schematic illustration of the multienzyme labeling amplification strategy using HRP−p53392Ab2−GO conjugate. (B) (a) SWV curves acquired at HRP−p53392Ab2−GO/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 mL−1 phospho-p53392 antigen in pH 7.4 PBS containing 25 μM thionine and 2 mM H2O2; (b) calibration plots of the (i) proposed HRP−p53392Ab2−GO/phospho-p53392/Ab1/NHS/AuNPs-SPCE and (ii) traditional HRP−streptavidin−biotin−p53392Ab2/ phospho-p53392/Ab1/NHS/AuNPs-SPCE for detecting phospho-p53392 antigen. Reprinted with permission from ref 297. Copyright 2011 American Chemical Society.

peroxidase-11,233 tyrosinase,282 acetylcholinesterase,283 catalase,284 and urease.285 For example, Shan et al.148 utilized ionic liquids/rGO/chitosan (ILs/rGO/CS) composites as the platform to construct an electrochemical biosensor for the detection of NADH and ethanol. The ILs/rGO/CS-modified electrode showed an obvious decrease in the overvoltage of NADH oxidation and exhibited good linearity from 0.25 to 2 mM and a high sensitivity of 37.43 μA mM−1 cm−2. With the introduction of ADH, the resulting ADH/ILs/rGO/CS-based biosensor demonstrated rapid and highly sensitive amperometric response to ethanol with a low detection limit (5 μM). Recently, our group283 reported an electrochemical sensor based on acetylcholinesterase (AChE)/Au nanoparticle (NPs)/ rGO nanohybrids for the ultrasensitive detection of organophosphate pesticides (OPs) (Figure 12). One mechanism for detecting OPs is based on inhibiting enzyme (AChE) activity by these toxic chemicals. The AChE activity in this system was monitored by measuring the oxidation or reduction current of the enzymatic products, and the extent of AChE activity was

inversely correlated with the amount of chemical inhibitor present in the system. In this work, the nanohybrid of Au NP/ rGO (cr-Gs) was first synthesized by the in situ growth of Au NPs on the surface of rGO nanosheets in the presence of poly(diallyldimethylammonium chloride) (PDDA). This not only improved the dispersion of the Au NPs but also stabilized cholinesterase with high activity and loading efficiency. Subsequently, the enzyme (AChE) was self-assembled on the Au NP/rGO nanohybrid through the electrostatic interaction between negatively charged AChE and positively charged PDDA. The electrochemical biosensor based on this AChE/Au NP/rGO nanoassembly exhibited an ultrasensitive detection of OPs with a detection limit of 0.1 pM. 4.4.2. Hemeprotein Biosensors. Direct electron transfer from heme proteins other than enzymes to electrode substrates can also be achieved by making use of GO-based electrodes. These composite materials can provide a suitable microenvironment to retain the redox bioactivity and facilitate the electron-transfer rate between the active center of the redox 6044


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amplification strategy for the ultrasensitive detection of human IgG (HIgG). The range was from 0.1 to 200 ng/mL with a detection limit of 0.05 ng/mL at 3σ. Du et al.297 developed a sensitive immunosensor for the detection of phosphorylated p53 at Ser392 (phospho-p53392) based on the p53392 signal antibody (p53392 Ab2)/HRP/GO labeling as seen in Figure 13. In addition, in order to avoid the additional steps normally required for labeling, label-free immunosensors are also attracting much interest. This is largely because it is much simpler and more cost-effective compared with labeled immunosensors because there is no need for a secondary antibody. GO-based electrodes have immense potential for the development of label-free immunosensors.302−309 This is because the oxygenated functional groups of GO can be used as an antibody-immobilizing platform. The GO can also be used in the detection probe of an immunosensor. In fact, the performance of the label-free GO-based electrochemical immunosensor seems promising for further use in clinical applications. Loo et al.309 developed a label-free immunosensor based on rGO modified electrodes for the direct electrochemical detection of antigen−antibody binding reactions. Anti-immunoglobulin G (anti-IgG) was covalently immobilized on the rGO-based electrodes and the binding was characterized using electrochemical impedance spectroscopy (EIS). The linear range of detection of the immunosensor was found to be from 0.3 to 7 μg mL−1. Electrodes modified with rGO/thionine nanocomposites have also been used by Wei et al. to fabricate a label-free electrochemical immunosensor for the detection of αfetoprotein (AFP).302 Kong et al.305 described a label-free electrochemical immunosensor based on a Au nanoparticle/ thionine/rGO nanocomposite film modified glassy carbon electrode (GCE) for the sensitive detection of carcinoembryonic antigen (CEA). The nanocomposites, which exhibited good biocompatibility, excellent redox electrochemical activity, and large surface areas, were coated onto the GCE surface, and then the CEA antibody (anti-CEA) was immobilized onto the electrode in order to construct the label-free immunosensor. The linear range of the CEA detection was found to be from 10 to 500 pg/mL with a detection limit of 4 pg/mL.

proteins and the underlying electrode. The mechanism of the direct electron transfer from heme proteins to these electrodes can serve as models for understanding the electron-transfer mechanism in biological systems. Over the past few years, heme-containing metalloproteins, such as myoglobin,120,286,287 hemoglobin,217,288−291 and cytochrome c,292 have been anchored onto GO-based electrodes for electrochemical biosensing purposes. For instance, Yue et al.287 reported a novel nitrite biosensor involving a sensing platform consisting of a GO/myoglobin composite film. A pair of well-defined and quasi-reversible cyclic voltammetric peaks that reflected the direct electrochemistry for the ferric/ferrous coupling of myoglobin were achieved at the composite film-modified electrode. It was revealed that this electrode displayed excellent electrocatalytic ability for the reduction of nitric oxide and exhibited good electrochemical response to nitrite with a linear range from 0.05 to 2.5 mM and a detection limit of 0.01 mM. Liu et al.217 combined the ILs/PDDA/rGO composite with hemoglobin to construct hemoglobin/ILs/PDDA/rGO-based biosensors. The resulting biosensors showed enhanced stabilities and exhibited excellent electrocatalytic activity for the detection of nitrate with a wide linear range from 0.2 to 32.6 μM and a low detection limit of 0.04 μM at 3σ. 4.5. Electrochemical Immunosensors

Immunosensors are important analytical tools based on detection of the binding events between an antibody and an antigen. The recent development of immunoassay techniques focused, in most cases, on decreasing analysis times, improving assay sensitivity, simplification and automation of the assay procedures, and low-volume analysis.293 So far, electrochemical immunosensors are still one of the mostly widely used protocols for the detection of biomolecules with good inherent selectivity and sensitivity. Recently, several strategies have been proposed to develop electrochemical immunosensors with high sensitivity using GO-based electrodes. In the case of GO-based electrochemical immunosensors, primary antibodies that capture the antigen are attached to the GO-based electrode. Antigen binding, washing, and enzyme label detection are all done on the sensor surface. The increasing demand for detection of ultralow amounts of analytes is pushing the boundaries for the enhancement of detection sensitivity by selecting different signal amplification strategies. One of the most popular strategies uses multifunctional nanoparticles as labels to amplify the electrochemical responses by loading a large amount of the electroactive species (such as ferrocene, enzyme) toward an individual sandwich immunological reaction event. To date, with respect to GObased electrochemical immunosensors, different kinds of multifunctional nanomaterials entrapped with antibodies have been investigated as labels for signal amplification. Some examples are ferrocene-functionalized Fe3O4 nanoparticles,294 Au/Fe3O4 nanocomposites,295 HRP-modified Au/GO nanocomposites,296 HRP-functionalized GO,297,298 HRP-modified Au or Pt@SiO2 nanoparticles,299,300 and HRP-modified ionic liquids (ILs)/SiO2 nanocomposites.301 For example, Yang’s group299,300 has achieved greatly enhanced sensitivity using bioconjugates featuring both HRP and signal antibodies linked to Au/SiO 2 , Pt/SiO 2 , or ILs/SiO 2 as labels for the immunodetection of norethisterone, human serum chorionic gonadotrophin (hCG), and the breast cancer susceptibility gene (BRCAl), respectively. Liu et al.296 designed a sensitive amperometric immunosensor using a graphene-assisted dual

4.6. Electrochemical DNA Sensors

The field of DNA biosensors is currently an area of tremendous interest in genetics, clinical medicine, pathology, criminology, pharmacogenetics, food safety, and many other fields. Recently, GO-based materials have been employed for the development of DNA sensors using various optical (e.g., fluorescence)114,134,189,225,310−319 and electrochemical31,52,145,310,320,321 techniques. Relatively speaking, electrochemical DNA sensors offer fast response speed, low cost, high sensitivity, and excellent selectivity for the detection of specific DNA hybridization and DNA damage,52,322−327 selected DNA sequences145,320,328 and specific analytes.329−331 Overall, the main principle of electrochemical DNA biosensors is based on the hybridization event, that is, the formation of the DNA duplex (from the probe and target DNAs), which involves the immobilization of single-stranded DNA (the probe/target DNA) onto the electrode surface, which is labeled with an electrochemical indicator to recognize its complementary target sequence (target/probe DNA). The result is the conversion of this recognition event to a readable electrochemical signal from the electroactive purine or hybridization indicator or changes in the electrode−solution interfacial properties.332 For example, Mohanty and Berry52 6045


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compared with bare GCE. Under optimal conditions, the peak current linearly increased with dsDNA concentrations in the range 1 to 500 μg mL−1 with a detection limit of 0.35 μg mL−1. In another example, a GO-based DNA biosensor was developed for catechol monitoring.330 The unique properties of GO led to a significantly enhanced electro-oxidative current of the guanine and adenine moieties in the DNA sensor without enhancing the background current. In the presence of catechol, the response of the DNA biosensor linearly decreased with an increasing concentration of catechol from 1.0 × 10−6 to 1.0 × 10−4 mol·L−1. This was due to the interaction between DNA and catechol, indicating that the GO-based DNA biosensor provides an effective assessment approach to control the genotoxic pollutant. Another interesting development in DNA electrochemical biosensors is the use of DNA aptamers. These structures are oligonucleotide sequences that can be generated with an affinity for a variety of specific biomolecular targets such as drugs, proteins, and other relevant molecules.310 For example, Loo et al.331 presented a simple and label-free electrochemical impedimetric aptasensor for thrombin based on GO-modified electrodes. The basis of detection relies on the changes in the impedance spectra of the redox probe after the binding of thrombin to the DNA aptamer; this is specific to thrombin. It was determined that the optimum concentration of aptamer to be immobilized onto the GO-modified electrode surface was 10 μM and the linear detection range of thrombin was 10−50 nM. In another example, based on the first clinical trial using aptamer AS1411 and GO-modified electrode, Feng et al.329 developed a GO-based electrochemical aptasensor for the selective label-free detection of cancer cells.

reported the viability of GO as a sensitive building block for electrochemical DNA sensors with excellent sensitivity. In their work, single-stranded DNAs were chemically grafted onto GO, followed by the hybridization of the fluorescent cDNAs. The electrical measurement data demonstrated that selective tethering of the single-stranded DNA onto GO led to an increase in the conductivity. The subsequent DNA hybridization caused a further increase in conductivity, which was completely reversible during multiple hybridization−dehybridization steps. The increase in the conductivity upon DNA tethering and hybridization was attributed to the attachment of the negatively charged DNA onto the p-type GO, which in turn increased the hole density. Hu et al.325 reported a novel Au− IL/3,4,9,10-perylene tetracarboxylic acid (PTCA)/rGO platform for the label-free impedance sensing of DNA hybridization. The immobilization of probe DNA onto the platform caused an increase in the electrochemical impedance value, due to the blocking of [Fe(CN)6]3−/4− redox indicator by the DNA probes. After the probe DNA was hybridized with a cDNA sequence, a further increase in the electrochemical impedance value was observed. The conductivity and negative-charge changes after DNA immobilization and hybridization on this sensitive Au−IL/PTCA/rGO platform were adopted as the signal for label-free DNA hybridization detection. It was found that the dynamic detection range for the sequence-specific DNA was from 1.0 × 10−13 to 1.0 × 10−6 M with the detection limit of 3.4 × 10−14 M. On the other hand, in order to greatly amplify the hybridization signal with a view to improving the detection limits, recent electrochemical DNA biosensors have involved a significant development using indirect detection methods of DNA hybridization requiring hybridization indicators, including intercalators and labels.322,324 For example, our group322 reported a novel electrochemical DNA biosensor based on a DNA-assembled rGO platform for the sensitive and selective detection of DNA. The DNA sensor was fabricated by directly assembling captured ssDNA onto a rGO-modified electrode through the π−π stacking interaction between rGO and the ssDNA bases. Subsequently, the target DNA sequence and oligonucleotide probe-labeled Au NPs were able to hybridize, in a sandwich assay format, followed by the Au NP-catalyzed silver deposition. Owing to the high DNA loading ability of rGO and the distinct signal amplification by Au NPcatalyzed silver staining, the resulting biosensor exhibited a good analytical performance with a wide detection linear range from 200 pM to 500 nM and a low detection limit of 72 pM. In addition, the intrinsic electrochemical activity of the nucleobases (primarily purine) at the GO-based electrodes provides the potential application for the label-free electrochemical detection of nucleobases,145,320,328 DNA,326 or DNA damage.327 For example, Fan et al.328 reported a TiO2/rGO nanocomposite electrode for the electrochemical sensing of adenine and guanine. It was found that the incorporation of TiO2 nanoparticles with rGO significantly improved the electrocatalytic activity and voltammetric response toward adenine and guanine compared with that of the rGO film itself. The TiO2/rGO-based electrochemical sensor exhibited a wide linear range of 0.5−200 μM with detection limits of 0.10 and 0.15 μM for adenine and guanine, respectively. Wang et al.326 investigated the electrochemical oxidation of native double-standed DNA (dsDNA) on a rGO-modified GCE for the electrochemical detection of DNA. The electro-oxidation peak of the guanine residues in dsDNA was obviously enhanced, and the peak potential was significantly lowered

4.7. Other Electrochemical Sensors

In addition to gas sensing and biosensing, GO-based electrodes have also demonstrated electrochemical sensing capabilities for other species, including hydrazine,333 sulfide,334 small organic molecules (such as aminophenol, 335 diphenolic compounds, 336−339 and 1-hydroxypyrene 340 ), heavy metal ions,216,341 drug molecules (such as paracetamol,146 dopamine,342−344 doxorubicin,345 methotrexate,345 caffeine,346 and rutin347), fungicides (such as carbendazim348), and pesticides (such as methyl parathion349,350). For example, based on the electrocatalytic activity of functionalized GO to paracetamol, Lin and co-workers146 constructed an electrochemical sensor for the sensitive detection of paracetamol. This electrochemical sensor showed excellent performance for detecting paracetamol with a detection limit of 3.2 × 10−8 M, a reproducibility of 5.2% relative standard deviation, and a satisfied recovery from 96.4% to 103.3%. Wang’s group216 demonstrated that the chemical rGO/Nafion nanocomposite film can be used as a sensing platform for ultrasensitive determination of cadmium. It was found that this sensing platform exhibited an enhanced response to the determination of the Cd2+ and has been used in real samples with good recovery. In another case, Wang et al.333 presented an ultrasensitive platform based on a chemicallly reduced poly(sodium styrenesulfonate) (PSS)/ GO nanocomposite film for the determination of hydrazine. In their work, the calibration curve for hydrazine was linear in the range 3.0 to 300 μmol L−1, and the detection limit was as low as 1 μmol L−1. 6046


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Biographies

5. CONCLUSIONS AND PERSPECTIVES The study of GO is a very recent research subject that offers enormous possibilities in electroanalytical chemistry and electrochemical sensors. In this review, we have addressed recent advances in this important field. As a graphene derivative, GO has been shown to be very useful in electrochemical studies and electroanalytical applications. With respect to electrochemical properties and applications, the main advantages of GO over other kinds of carbon-based materials, such as pristine graphene, graphite, carbon nanotubes, fullerene, and diamond, include its facile synthesis, substantial solubility and processability, adjustable moderate

Da Chen is currently an associate professor at China Jiliang University, China. He received his M.Sc. in 2004 from Zhejiang University and his Ph.D in 2008 from University of Science & Technology of China under the supervision of Prof. Jinghong Li. In 2011, he carried out postdoctoral research at the Pennsylvania State University in the group of Professor Donghai Wang, where he worked on the synthesis of graphene-based composite materials for lithium ion batteries. His current research interest is mainly focused on the design and synthesis of functional graphene-based materials and semiconductor nanostructures, as well as their applications for electrochemical sensing and energy conversion and storage.

conductivity, high surface area, excellent biocompatibility, and abundance of inexpensive source material. Owing to these advantageous structural and physicochemical properties, GObased materials have been used to design and prepare GObased electrodes for a wide range of applications in electroanalytical chemistry and electrochemical sensors. To date, considerable advances in this area have already been made. Nonetheless, research toward the wider application of GObased materials in electroanalytical chemistry and electrochemical sensors is still in its infancy. Much work remains to be done in facilitating the practical applications of GO-based materials and broadening the scope of their electrochemical applications in the future. Specific attention should be focused on a range of current challenges, such as more facile synthesis, functionalization and controlled processing of GO-based materials with high quality, the complete determination of the GO structure at the molecular level, further exploration of the electronic properties of GO, a better understanding of the effect of defects on the GO conductivity, the interpretation of

Hongbin Feng received his M.Sc. in 2009 from College of Chemistry and Chemical Engineering of Graduate School, Chinese Academy of Sciences. Currently he is pursuing his Ph.D. degree at University of Science & Technology of China under the supervision of Prof. Jinghong Li. His current research interest is mainly focused on the synthesis and application of graphene-based materials.

the electron transport characteristics at the GO/substrate interface or the GO/solution interface, and the design and construction of GO-based devices. Despite the number of remaining challenges, it is clear that GO-based materials offer many advantages for promising electrochemical applications, and researchers are now making rapid progress in this area. Undoubtedly, the future of GO-based electrochemical sensors and electroanalysis remains very bright and exciting.

AUTHOR INFORMATION Corresponding Author

*Tel/Fax: +86 10 6279 5290. E-mail address: jhli@mail. tsinghua.edu.cn. Notes

The authors declare no competing financial interest. 6047


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Jinghong Li is currently a Cheung Kong Professor in the Department of Chemistry at Tsinghua University, China. He received his B.Sc. in 1991 from University of Science and Technology of China and his Ph.D. in 1996 from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. He then spent several years (1997− 2001) at the University of Illinois at Urbana−Champaign, University of California at Santa Barbara, Clemson University, and Evonyx, Inc., New York, as a postdoctoral fellow and research scientist. He returned to Changchun Institute of Applied Chemistry as a Professor of Chemistry in May 2001 and then moved to the Department of Chemistry at Tsinghua University in July 2004. He has received several awards, including National Science Fund for Distinguished Young Scholars, National Excellent Doctoral Dissertation of China, Distinguished Young Scholars for Chinese Academy of Sciences, the Young Electrochemistry Prize of Chinese Chemical Society, and the Li Foundation Prize, USA. His current research interests include electroanalytical chemistry, bioelectrochemistry and sensors, physical electrochemistry and interfacial electrochemistry, electrochemical materials science and nanoscopic electrochemistry, fundamental aspects of energy conversion and storage, advanced battery materials, and photoelectrochemistry. He has published over 230 papers in international, peer-reviewed journals with >10 invited review articles. http://www.researcherid.com/rid/D-4283-2012.

ACKNOWLEDGMENTS The authors are grateful for financial support from the National Basic Research Program of China (Grant No. 2011CB935704), the National Natural Science Foundation of China (Grant Nos. 20975060 and 21003111), Research Fund of Ministry of Education of China (Grant No. 20110002130007), the European Union Seventh Framework Programme (FP7/ 2007-2013) under Grant Agreement No. 260600 (“GlycoHIT”), and Tsinghua University Initiative Scientific Research Program. The authors thank the anonymous reviewers and the ACS language editor for their useful comments and language editing, which have greatly improved the mansucript. REFERENCES (1) Geim, K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Brumfiel, G. Nature 2009, 458, 390. (3) Sykes, E. C. H. Nat. Chem. 2009, 1, 175. (4) Li, D.; Kaner, R. B. Science 2008, 320, 1170. (5) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (6) Stoller, M. D.; Park, S.; Zhu, Y. W.; An, J.; Ruoff, R. S. Nano Lett. 2008, 8, 3498. (7) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217. (8) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (9) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (10) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2007, 315, 1379. (11) Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; van der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Nano Lett. 2008, 8, 2458. (12) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385. (13) Balandin, A. A.; Ghosh, S.; Bao, W. Z.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Lett. 2008, 8, 902. (14) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Solid State Commun. 2008, 146, 351. (15) Katsnelson, M. I. Mater. Today 2007, 10, 20. 6048


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