Molecularly engineered graphene surfaces for sensing applications: A review.

G Model ACA 233386 No. of Pages 19

Analytica Chimica Acta xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Review

Molecularly engineered graphene surfaces for sensing applications: A review Jingquan Liu a, * , Zhen Liu b , Colin J. Barrow b , Wenrong Yang b, * a College of Chemical Science and Engineering, Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao, China b Centre for Chemistry and Biotechnology, Deakin University, Geelong, VIC 3217, Australia

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

The importance of surface chemistry of graphene materials is clearly described. We discuss molecularly engineered graphene surfaces for sensing applications. We describe the latest developments of these materials for sensing technology.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 April 2014 Received in revised form 9 July 2014 Accepted 20 July 2014 Available online xxx

Graphene is scientifically and commercially important because of its unique molecular structure which is monoatomic in thickness, rigorously two-dimensional and highly conjugated. Consequently, graphene exhibits exceptional electrical, optical, thermal and mechanical properties. Herein, we critically discuss the surface modification of graphene, the specific advantages that graphene-based materials can provide over other materials in sensor research and their related chemical and electrochemical properties. Furthermore, we describe the latest developments in the use of these materials for sensing technology, including chemical sensors and biosensors and their applications in security, environmental safety and diseases detection and diagnosis. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Graphene Chemical sensors Biosensors

Contents 1. 2.

Surprising carbon: an introduction to graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Why graphene is important for sensor applications? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Abbreviations: 2D, two-dimensional; CVD, chemical vapor deposition; CNT, carbon nanotube; GO, graphene oxide; RGO, reduced graphene oxide; GOD, glucose oxidase; FRET, fluorescence resonance energy transfer; GSs, graphene sheets; SiC, silicon carbon; GNR, graphene nanoribbon; AChE, acetylcholinesterase; MGF, mesocellular graphene foam; DPV, differential pulse voltammetry; HA, Hypocrellin A; TMB, 3,30 ,5,50 -tetramethylbenzidine; GH, graphene oxide–hemin; TRAP, telomerase repeat amplification protocol; PCR, polymerase chain reaction; IEP, isoelectric point; 1D, one-dimensional; GQD, graphene quantum dot; EDC, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide; DCC, N,N0 -dicyclohexylcarbodiimide; NHS, N-hydroxysuccinimide; ssDNA, single stranded DNA; PVA, polyvinyl alcohol; PDDA, poly(diallyldimethylammonium chloride); PEI, polyetherimide; PVP, polyvinylpyrrolidone; GCE, glassy carbon electrode; AuNps, Au nanoparticles; H2O2, hydrogen peroxide; TBHP, tert-butylhydroperoxide; ILgraphene, liquid-functionalized graphene; SLG, single-layer graphene; SiNW, silicon nanowires; 3D, three-dimensional; SEM, scanning electron microscope; FET, field-effect transistors. * Corresponding authors. E-mail addresses: [email protected] (J. Liu), [email protected] (W. Yang). http://dx.doi.org/10.1016/j.aca.2014.07.031 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: J. Liu, et al., Molecularly engineered graphene surfaces for sensing applications: A review, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.07.031


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J. Liu et al. / Analytica Chimica Acta xxx (2014) xxx–xxx

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Surface chemistry and functionalization . . . . . . . . . . . . . . . . . . . . . . . . . Covalent modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Non-covalent modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Other methods to decorate graphene with inorganic molecules 3.3. Functionalized graphene as a new platform for chemical/biosensors . . Graphene-based electrochemical sensors . . . . . . . . . . . . . . . . . . 4.1. Graphene-based electrical sensors . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Graphene-based optical sensors . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Future challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Prof. Jingquan Liu received his bachelor from Shandong University in 1989. His master and Ph. D. were obtained from the University of New South Wales (UNSW) in 1999 and 2004, respectively, where his Ph.D. was undertaken under the guidance of Prof. Justin Gooding. In 2004, he worked as a CSIRO-UTS post-doctoral fellow prior to returning to UNSW with Prof. Tom Davis as a Vice-Chancellor’s Research Fellow in 2006. In 2010, he took up a professorship at Qingdao University. He has coauthored over 80 peer-reviewed research papers and 2 book chapters. His research interests focus on the synthesis of various bio- and nano-hybrids of versatile polymeric architectures.

Zhen Liu received his master degree of materials processing and engineering under the supervision of Prof. Jingquan Liu from Qingdao University in 2013 and has co-authored 5 papers. He is currently a Ph.D. candidate at Deakin University under the supervision of Dr. Wenrong Yang. His project focuses on the preparation of graphene and furthering the application of graphene-based materials in energy devices.

Prof. Colin Barrow is Chair of Biotechnology at Deakin University. He is also Director of the Centre for Chemistry and Biotechnology (CCB). Professor Barrow’s research is primarily focused on food biotechnology and the application of nanomaterials for industrial purposes. Professor Barrow has a Ph.D. in chemistry from the University of Canterbury in New Zealand and an MBA from Penn State in the USA. Prof. Barrow has approximately 180 peerreviewed publications, several patents, and has presented at numerous conferences and workshops. He has served as a member of the Expert Advisory Committee for Canadian Natural Health Product Directorate (NHPD), is on the Executive and is a founding member of International Society for Nutraceuticals and Functional Foods (ISNFF).

Dr. Wenrong Yang received his Ph.D. degree in Chemistry from the University of New South Wales (UNSW) in Australia in 2002. After several years as a Research Fellow at CSIRO, UNSW, and University of Sydney, he joined Deakin University as a Lecturer in 2010. His current research interests are centered on the synthesis and surface functionalization of Carbon-based soft materials and their applications in biosensor and green energy, bionics, and environmental protection.

1. Surprising carbon: an introduction to graphene Graphene is a relatively new member of the nanocarbon family, composed of well separated two-dimensional (2D) layers composed of aromatic carbon atoms, first reported in 2004 by Novoselov et al. [1]. Graphene’s unique structure and properties has made it an attractive candidate for sensor applications and like other nanomaterials possessing desirable bulk properties, does not have the required surface characteristics necessary for particular applications. Functionalization of the surface is thus essential for sensor applications [2–4] and various covalent and non-covalent chemistries have been reported affording graphene-based materials the surface properties needed for such devices [5–9]. Although relatively new, graphene has already been extensively utilized in various fields because of its distinctive physical and chemical properties, which include superior electrical conductivity, excellent mechanical flexibility, large surface area plus high thermal and chemical stability [10]. For instance, graphene has been exploited for energy applications, due to its high conductivity, transparency and ultra-thin sheets [4,11–13]. Because of graphene’s high surface area (2630 m2 g1) [14], excellent mechanical strength and aromatic-rich structure, it has been employed as a pollutant adsorbent due to the attraction of small molecules to its surface. These properties also contribute to its use as a catalyst or catalytic support for fuels and photo

degradation of organics [15–19]. Moreover, graphene plays a crucial role in sensing applications which utilize its exceptional electrical properties (e.g., extremely high carrier mobility and capacitance), electrochemical properties (e.g., high electron transfer rate), optical properties (e.g., excellent ability to quench fluorescence) and structural characteristics. In this review, we place an emphasis on surface chemistry and functionalization of graphene used as electrochemical, electrical and optical sensors. The methods for the preparation of graphene and their relative advantages and disadvantages are also discussed. Graphene’s properties can be controlled by chemical derivatization, with important parameters being the synthetic conditions, dimensions, number of layers and doping, which provide chemical flexibility for various sensing purposes [20]. Therefore, graphene preparation methods should be carefully selected according to the specific sensing target and mechanism to be utilized, with a balanced consideration on performances (e.g., detection limit and dynamic range), reproducibility, cost and manufacturability [21]. Generally speaking, these methods can be classified as exfoliation, thermal decomposition, chemical vapor deposition (CVD), opening carbon nanotubes (CNTs), thermal reduction and oxidation– reduction, plus others [22,23]. Each of these preparation methods has advantages and drawbacks over the other methods. For example, the oxidation–reduction method for producing graphene from graphite can be used for mass production, whereas the as-




prepared graphene nanosheets usually retain smaller size, high defects and lower electrical conductivity. Graphene prepared by mechanical exfoliation and cleavage exhibits high carrier mobility and low density of defects [1,24], therefore it can be used for fabrication of electronics [25]. However, mechanical exfoliation is a time consuming process that cannot be applied for bulk manufacture of graphene. The mainstream methods of graphene preparation have been summarized in Fig. 1. Redox chemistry is the primary method for the large scale preparation of graphene [31,32]. Using strongly acidic oxidants, graphite is converted to graphene oxide (GO) [33,34] and subsequently into graphene in the presence of various reductants such as hydrazine, sodium borohydride, hydroiodic acid, and L-ascorbic acid [31,35–37]. Some other reductants have also been exploited to efficiently reduce GO, such as polymers, sulfurcontaining compounds and hot strong alkaline solutions (e.g., KOH and NaOH) [38–40]. It has been shown that chemical reduction can restore the electrical conductivity of GO close to the level found for graphite [41,42]. In addition to chemical transformation of GO, other methods including thermal [43], microwave [44],

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electrochemical [45] and UV-induced photocatalytic reductions [46] have been utilized for preparing graphene sheets (GS). In comparison, when using the exfoliation technique, graphene nanosheets obtained from highly-oriented pyrolytic graphite in organic solvents like N-methyl-pyrrolidone [47] were virtually free of crystal defects, resulting in high carrier mobility [48]. A similar approach was adopted to obtain GS by sonication of natural graphite crystals in dimethylformamide (DMF) for large-scale production of high-quality single-layer GS [49,50]. Moreover, much research has focused on matrix-assisted direct exfoliation of graphene from graphite via non-destructive p–p stacking interactions between the aromatic molecules and graphite micro platelets [27,51–53]. Thermal decomposition of SiC is a technique employed for the fabrication and processing of graphene materials [29,54–57]. Sutter et al. [58] found that interaction between the first epitaxial graphene layer and metal substrate of ruthenium (Ru) was fairly strong, while the second layer was almost completely detached, showing weak electronic coupling to the metal. This result showed that GS obtained through this approach retained the inherent

Fig. 1. Summary of mostly utilized methods for graphene fabrications by (A) oxidation–reduction method (reprinted with permission from ref. [26]. Copyright 2010 American Chemical Society); (B) matrix-assisted direct exfoliation (reprinted with permission from ref. [27]. Copyright 2013 Elsevier); (C) chemical vapor deposition (reprinted with permission from ref [28]. Copyright 2011 American Chemical Society); (D) thermal decomposition of silicon carbon (SiC) (reprinted with permission from ref. [29]. Copyright 2009 Elsevier) and (E) unzipping carbon nanotubes (reprinted with permission from ref. [30]. Copyright 2009 Nature Publishing Group).



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electronic structure of graphene. In addition to the thermal decomposition of SiC, graphene with high structural quality and large area can also be obtained through low-pressure CVD, in which carbon is supplied in the gas phase and a metal is used as both catalyst and substrate to grow graphene layer(s) [59,60]. Likewise, versatile single- or multi-layered GS with high transmittance, electrical properties and large size have been successfully prepared by the CVD methods on different substrates [61–64]. Graphene ribbons and sheets can also be prepared by unzipping CNT with intercalation of ammonia and lithium with subsequent exfoliation [65]. Kosynkin et al. [30] first prepared an extremely high yield of graphene nanoribbon (GNR) by longitudinally slicing and unraveling CNT. Their ribbon structures have good water solubility, and the following chemical reduction of the nanoribbons from multiwalled carbon nanotube leads to recovering of electrical conductivity. In addition, Jiao et al. [66] developed a protocol to produce narrow GNR by controlled undoing of CNT using argon plasma etching whilst they were partially fixed to a polymeric substrate. These ribbons synthesized by the plasma approach showed a narrow width distribution and smooth edges (10–20 nm). 2. Why graphene is important for sensor applications? In the past decade or so, various zero-dimensional and onedimensional (1D) nanomaterials have been the main impetus for novel and better sensor developments [67–70]. These include quantum dots [71,72], nanoparticles [73,74], nanowires [75–78] and notably, carbon nanotubes [79–82], the one-dimensional cylinders of carbon sheets. Ever since the first isolation of freestanding graphene sheets [1], this 2D carbon crystal has been anticipated to provide new opportunities for sensing applications [20,83]. In fact, in spite of its short research history, graphene has already demonstrated enormous potential for various novel sensors which utilize its extraordinary physical and chemical properties. These include extraordinary carrier mobility and capacitance, high electron transfer rate, excellent transparency and ability to quench fluorescence, exceptionally large surface-tovolume ratio and single atom thickness, plus its robustness and flexibility. As noted earlier, graphene is a two-dimensional nanosheet comprised of sp2 bonded carbon atoms, which results in the properties summarized above. Interestingly, the large surface area of 2630 m2 g1 is twice than that of single-walled carbon nanotubes, and its mechanical strength is approximately two orders of magnitude greater than steel [84]. Graphene also exhibits a tunable band gap, a quantum Hall effect at roomtemperature and a high thermal conductivity [85]. Additionally, graphene’s properties can be adjusted by regulating the synthetic conditions, dimensions, number of layers and doping constituents [23]. These features provide numerous opportunities for graphene’s surface modification, which would endow additional functions to already versatile GS. Therefore, graphene-based materials are playing and will continually play a significant role in sensing applications. 3. Surface chemistry and functionalization The surface of nanoparticles play a crucial role with respect to interaction with other molecules [86]. Graphene derivatives have many superior surface properties, which make them suitable for the applications of various sensing system [87]. Firstly, graphene, which is a single-atom-thick sheet of sp2 hybridized carbon atoms that are packed in a hexagonal honeycomb crystalline structure, has a conjugated structure with vast delocalized p-electrons, making it highly conductive [88], and accelerate the electron transfer, leading to fast response time and strong response signals

[89,90]. Furthermore, the large surface coverage can be achieved via either direct absorption or binding with other functional groups on graphene derivatives [91,92]. Obviously, higher concentration of probe molecules would make the detection system more sensitive. By smart processing, such as oxidation and heteroatom doping, graphene based materials become rich in functional groups at their surfaces or edges, which helps in molecular level tuning and fabrication of hybrid sensing materials [93]. Therefore, tuning the surface chemistry of graphene materials may be the most primary and direct approach to adjust graphene materials for different purposes. Functionalized graphene materials are apt to conjugate with different recognition molecules as well as incorporate other functional materials (e.g., metal nanoparticles, proteins or conducting polymers) for electrochemical bioanalysis [10]. Graphene is able to be functionalized by covalent, noncovalent and also other methods, which meet the specific requirements of different kinds of sensors. Fig. 2 presents some alternative approaches for surface modifications of GS. 3.1. Covalent modifications Covalent interaction plays an important role in graphene’s functionalization for sensor applications [97]. It primarily takes place with the help of covalent bond formation, which can be formed either on the basal planes or at the edges. Graphene can be covalently functionalized by reaction with unsaturated p-bonds of graphene, the oxygens on GO and organic functional moieties [97], and heteroatom doping. In order to carry out couplings with the C C bonds of GS, several diazonium salts have been used to produce highly reactive free radicals for addition reactions, which vertically immobilize various aryl-addends [98–100] on the surface. These reactions convert sp2 carbon atoms to sp3 hybridization, generating nonconducting and semiconducting regions in the graphene layers [100]. Apart from that, dienophile compounds can also react with the sp2 carbons. Many types of dienophiles, such as aryne [101], nitrene [102,103] and azomethine ylide [104–106] have been used for the addition reaction with graphene, creating versatile terminated groups for further functionalization and modification of graphene materials. Additionally, several groups focused on the covalent linkage between oxygen moieties (from GO) and other functional molecules [107–109]. The carboxyl groups on GS are reacted with the amino groups of molecules or proteins via a wellknown carbodiimide procedure. Specifically, GO first reacts with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) or N,N0 -dicyclohexylcarbodiimide (DCC) under ambient conditions, producing a stable active ester in the presence of N-hydroxysuccinimide (NHS). The ester reacts with the amine group of a target molecule (DNA or enzyme, etc.) to form an amide bond. For instance, graphene derivatives are covalently bonded with betaaminocyclodextrin [94], poly-L-lysine [109], protein [110,111], and DNA [112,113], which can be further employed for the development of electrochemical sensing platforms. The transformation of carboxylates to acyl chlorides is another common method for graphene modification [114]. By this means, graphene (or graphite) oxide is activated with thionyl chloride to obtain an acyl chloride– graphene derivative, which subsequently reacts with hydroxyl or amino group(s), such as neutral red [115], cyclodextrin [116], poly (vinyl alcohol) [117], poly(3-aminobenzene sulfonic acid) [118], amino terminated polypyridyl ruthenium(II) complexes and amino group modified silica spheres [119]. On the other hand, heteroatom doping is a common approach to tailor the electronic properties of graphene materials [120]. The nitrogen and boron atom are mostly used to synthesize heteroatom-doped graphene as they have similar structure with carbon and can act as electron donor or acceptor, respectively [121,122]. When nitrogen atoms are




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incorporated into the basal plane of graphene, they denote electrons into graphene leading to n-type doping of graphene while boron-doped graphene would exhibit p-type behavior. As a result of nitrogen doping, three types of nitrogen bonding configurations are introduced into graphene’s basal plane as ‘pyridinic’, ‘pyrolic’ and ‘graphitic’ nitrogen, which are highly active sites for redox reduction [123]. There are a few reports demonstrated that nitrogen doping can significantly increase the electron conductivity, improve the electron-donor properties and binding ability [124–126], and also enhance the biocompatibility and sensitivity of graphene in biosensing applications [127,128].

molecules (such as sodium dodecyl sulfate and hexadecyltrimethylammonium bromide) are also good dispersing agents for graphene in aqueous solution with the hydrophobic tails attached to the surface of graphene. Additionally, GO (as well as RGO) can be capped by hydrophilic polymers to prevent aggregation in aqueous solution. Several polymers including polyvinyl alcohol [135], poly (diallyldimethylammonium chloride) [136], polyetherimide [137] and polyvinylpyrrolidone [138] have been widely used as dispersants due to their affinities arising from hydrophilic interactions with and without electrostatic attraction.

3.2. Non-covalent modifications

Recently some inorganic materials were used to modify GS and induced an additional electrochemical catalytic ability that may provide further facility for functionalization [139]. The combination of graphene and metal nanoparticles in solution (especially in aqueous solution) has attracted research effort due to their convenient synthesis and considerable potential as enhanced materials for electrochemical and analytical applications. Currently, diverse strategies have been employed to prepare graphene– metal hybrids, particularly for noble metals. Specifically, Hassan et al. [140] reported the microwave-assisted synthesis of metal nanoparticles (Pd, Cu and PdCu) dispersed on the graphene surface in oleylamine and oleic acid. Their method allows the simultaneous reduction of GO and various metal salts, produced new nanocatalysts supported on the large surface area of the thermally stable graphene. Guo et al. [141,142] demonstrated a wet-chemical approach for the synthesis of Pt nanoparticles and Pt-on-Pd nanodendrites attached to graphene nanosheets in an aqueous phase. A “clean” synthesis, involving reduction of Pd and Au, Pt

Non-covalent reactions are frequently utilized to modify graphene materials. Compared with covalent ones, these methods do not destroy the original sp2 conjugated structure of graphene, which furthest retains the high electro conductivity. This is very important for electrical sensor applications with unbroken graphene. Non-covalent linkages between graphene and other functional molecules are mainly achieved through p–p stacking, hydrophilic and hydrophobic interactions. A typical example for p–p stacking interactions is that the GS are held by pyrene derivatives which are readily dissolved or dispersed in solution and this property is subsequently utilized to make graphene-based conducting films or sensors [129–132]. Besides pyrene derivatives, single stranded DNA can also attach to the graphene surface via p–p stacking [132]. Nucleic acid bases and graphene can act as a platform for different categories of DNA related detection techniques [133,134]. Several amphiphilic

3.3. Other methods to decorate graphene with inorganic molecules

Fig. 2. (A) Reduced graphene oxide sheets chemically modified with b-cyclodextrin via an amide bond. From the underside, we can see improvement of water-solubility around pH 2–7 and change of surface charge at different pH value (reprinted with permission from ref. [94]. Copyright 2012 American Chemical Society). (B) Schematic illustrating the modification of GS with diazonium salts (a and b) and via non-covalent p–p stacking (c) (reprinted with permission from ref. [95]. Copyright 2012 American Chemical Society). (C) Protein-based decoration and reduction of GO, resulting in a general nanoplatform for nanoparticle assembly (reprinted with permission from ref. [96]. Copyright 2010 American Chemical Society).



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precursors by GO provides a convenient and direct way to fabricate metal–graphene nanocomposites without capping agents and any additives [143,144]. Additionally, self-assembly approaches provide an alternative strategy for preparing high-quality graphene–metal hybrids with the desired nanoparticles [145,146]. This can be achieved using numerous methods for the synthesis of metal nanoparticles with different sizes, components and shapes. For example, Zhu et al. [147] successfully prepared a hybrid 3D nanocomposite film by alternatively assembling the graphene nanosheets modified by ionic liquid and Pt nanoparticles. In this method, an imidazolium salt-based ionic liquid (IS-IL)-functionalized graphene was synthesized by covalently binding 1-(3-aminopropyl)-3-methylimidazolium bromide onto graphene nanosheets. By introduction of IS-IL on the surface of graphene nanosheets, positive charged graphene nanosheets can be well dispersed in aqueous solution. Self-assembly is a facile and regular approach, making the structure of as-prepared multilayer film highly uniform. Furthermore, the electrocatalytic activity of the films could be further tailored by simply choosing different cycles in self-assembly process, offering a smart route to build electrochemical nanodevices. In another work, Deng and co-workers [96] employed a general approach for GO reduction and decoration using bovine serum albumin (BSA). They prepared graphene–metal (Au, Ag, Pt, Pd) hybrid nanosheets through the affinity between these noble metals and the amine, thiol and imidazole groups on BSA. This is an environmentally friendly, one-step reduction/decoration strategy to assemble nanoparticles with controllable size, shape, composition, and surface property, which are meaningful in graphenebased materials modifications. Also, metal or semimetal oxide nanomaterials have gained considerable attention in the electrochemistry field [13,148]. Oxides generally exhibit low electrical conductivity therefore converting them to conductive graphene materials can decrease the over potential and increase the current density. However, the utilization of oxides may bring additional means for further modifications of graphene materials. Compounds including Fe3O4 [149,150], Co(OH)2/Co3O4 [151,152], and MnO2 [153] can bond with GO via various methods. For instance, Yang et al. [154] fabricated mesoporous SiO2 coated GO using a wet chemical method. In detail, cationic surfactants, such as cetyltrimethyl ammonium bromide, were selected to electrostatically adsorb and selfassemble onto the surface of GO which is highly negatively charged in alkaline solution. This procedure directed the formation of mesoporous silica around the surface of single-layer GO. Utilizing suitable cationic surfactants can not only effectively resolve the incompatibility and aggregation problems between GO and inorganic materials, but also provide the molecular template for controlled nucleation and growth of mesoporous silica on the surface of GO sheets. In addition, Dong et al. [151] used hydrothermal procedure to synthesize Co3O4 nanowires on three-dimensional graphene foam. They indicated that it is capable to deliver high specific capacitance of 1100 F g1 at a current density of 10 A g1 with good cycling stability, moreover, this device can detect glucose with a high sensitivity of 3.39 mA mM1 cm2 and a remarkable lower detection limit of less than 25 nM (S/N = 8.5). Hydrothermal synthesis is a simple and robust method to process functional materials, but the high temperature and high pressure make it dangerous to be performed in the experiment. 4. Functionalized graphene as a new platform for chemical/ biosensors Due to their unique physical and chemical properties, graphene-based materials have become important candidate for sensing application in aqueous environment. Herein, we aim to

provide a critical and comprehensive overview covering the recent developments, more importantly to offer insights on the merits of graphene in comparison with other materials (e.g., CNTs), providing a report of graphene’s electrochemical, optical and electronic properties because these are most relevant to sensor applications. Furthermore, different categories of graphene precursors, such as chemically RGO, large-sized graphene grown by CVD method and graphene nanoribbon, their effects on the device fabrication and sensing performances are also discussed in this section. 4.1. Graphene-based electrochemical sensors As noted above, graphene is an excellent material for electrochemistry [10,155–157] and compared to CNT, it possesses several noteworthy advantages. Firstly, graphene materials do not contain metallic impurities as CNT does [158]. These impurities can interfere with the electrochemistry of CNT, as reported for hydrazine [159,160], amino acids [161], hydrogen peroxide (H2O2) [162–164], halothane [165], glucose [166,167], and short peptides [168] even at levels below 100 ppm [169]. Moreover, the fabrication approaches of graphene employ natural graphite, which is extremely accessible and inexpensive. The applications of graphene for electrochemical sensing including biosensors, have developed very quickly. The first article appeared in 2008 [170], and since then there has been explosive growth in the number of published papers. Since graphene has a large electrochemical potential window (approximately 2.5 V in 0.1 mM phosphate buffer saline solution) [171], therefore the detection of molecules with either high oxidation or reduction potentials becomes feasible. Edges and defects on graphene provide a high electron transfer rate, suggesting that graphene sheets or small flakes of pristine graphene are very suitable for electrochemical detection. As we have known, the electrochemistry of MWCNTs could be controlled by the addition of nanographite or graphite microparticles impurities, which have much higher edge-plane/weight ratio than CNTs [172]. Therefore, compared with Fe3+/2+ at bare glassy carbon electrode (GCE), redox species of Fe3+/2+ at RGO/GCE have 10 times greater electron transfer rate. Fe3+/2+ is an inner-sphere model, and its k app value is very sensitive to the surface carbon– oxygen functionalities, specifically surface carbonyl groups provided by RGO [173,174]. GO is able to support the efficient electrical wiring the redox centers of several heme-containing metalloproteins to the electrode because GO can facilitate electron transfer [175]. As the redox centers of proteins are often concealed in folded polypeptide shells with poor charge transfer, metalloproteins suffered from poor electron transfer rate at various surface. GO is an ideal substrate for accommodating proteins and promoting protein electron transfer due to its high electron mobility and excellent protein adsorption ability because of strong hydrophobic interaction. Therefore, GO can promote electron transfer between redox centers and electrode surface. Furthermore, vast oxygen functional groups on the surface of GO facilitate electrochemical activities [176]. Recently, we developed new electrochemical detection approach toward single protein molecules (microperoxidase-11, MP-11), which are attached on the surface of graphene nanosheets [177] (Fig. 3). The non-covalently functionalized graphene nanosheets exhibited enhanced electroactive surface area, where amplified redox current are produced when graphene nanosheets collide with the electrode. We observed stepwise changes in redox current and the charge transferred in electrochemical processes, which was amplified by repeatedly reducing and oxidizing functionalised graphene nanosheets as they randomly diffuse on the electrode surface. We estimated the number of the MP-11




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Fig. 3. (a) Schematics of the self-assembly of MP-11 with graphene sheets. (b) Schematics of the single graphene sheet becoming a nanoelectrode upon contact with the Au electrode. (Reproduced by permission from ref. [177] of The Royal Society of Chemistry.)

molecules on a single graphene sheet is in the range of 105 18. This facile and highly sensitive detection method may be useful for future biosensing research and investigating single-molecule reactions. Actually, RGO has intrinsic catalytic activity toward some small enzymatic products such as H2O2 and NADH, making it attractive for enzyme-based sensors. Given its extremely high surface area, graphene based electrodes provide a large effective reaction region and high capacity for enzyme loading [178,179]. The high surfaceto-volume ratio also makes it ideal for functional composite (e.g., specific redox mediators) in which a small percentage of graphene is able to provide percolating pathways for charge conduction [180]. Most graphene based electrochemical sensors utilize RGO because: (1) its abundant defects and chemical groups can facilitate charge transfer and thus ensure high electrochemical activity [10,174]; (2) the numerous chemical moieties (hydroxy and carboxyl groups) on the surface afford flexibility and convenience for functionalization to enhance sensor performance [107–109]; (3) the chemical and electrical properties are highly tunable [181] and (4) as compared to insulated GO, RGO can efficiently transfer electrons [182]. Wu et al. [183] utilized a one-pot solvent exfoliation to obtain high quality graphene sheet and further applied it in electrochemical detection of various analytes. Compared with RGO, graphene prepared in this work exhibited stronger signal enhancement, which means better sensing properies. Enzymes have been widely utilized as an important bioelement for electrochemical sensing for. For example, the quantitative determination of glucose levels in blood has been well known and clinically significant for the diagnosis and management of diabetes [83]. Their electrochemical detections have been well demonstrated using glucose oxidase (GOD) as the mediator or recognition element [184,185]. GOD has been immobilized on graphene sheets for fabricating glucose biosensors via various approaches [125,186–188]. Immobilization of GOD on graphene with different types of noble metal nanoparticles for highly sensitive and stable glucose sensors have been developed [90,189– 193]. Wang et al. [194] designed a metal-free fabrication of mesocellular graphene foam (MGF) for direct glucose oxidase electrochemistry and sensitive glucose sensing. Due to its high conductivity, ultra-large surface area and pore volume, MGF can greatly accelerated the immobilization ability of electrocatalytic active GOD, and the direct and fast electron transfer (with rate

constant of 4.8 s1). This work provides a facile and low-cost approach to detect glucoses with a detection limit of 0.25 mM. Further, Liu et al. [195] fabricated a multi-layered GOD enzyme electrode with controlled alternate enzyme and graphene layers, where GOD was first modified with pyrene functionalities in order to be self-assembled onto a graphene basal plane via non-covalent p–p stacking interaction. Using an alternate layer-upon-layer of self-assembled graphene and pyrene functionalized GOD; monoand multi-layered enzyme electrodes with controlled biocatalytic activity were readily fabricated. (Fig. 4A) The biocatalytic activity of these enzyme electrodes increased with the number graphene and GOD layers but reached a maximum at four. (Fig. 4B) Such multi-layered enzyme electrodes with controlled nanostructure exhibited reliable application for the analysis of human serum samples with high sensitivity, good stability and repeatability. Apart from devices based on GOD, some non-enzymatic glucose sensors utilizing metal nanomaterial-modified graphene electrodes have been developed. Copper nanoparticles [167,196–198], platinum [199], Co2Nx [200], nickel oxide [201–203] and alloy material [204,205] have been employed to fabricate non-enzymatic glucose biosensors exhibiting both excellent sensitivity and selectivity. Various graphene-based sensors developed for glucose determination are summarized in Table 1. Mobilization of acetylcholinesterase (AChE) on graphene for pesticides detection have been developed [206–208]. AChE is able to catalyze the acetylcholine (ACh) hydrolysis [209] in which the reaction is: ACh + H2O ! choline + acetate. The inhibition of AChE by organophosphates, a type of pesticides, is recognized as a result of the serine phosphorylation in the active site of the enzyme. The hydrolysis of ACh will be blocked as the phosphorylated enzyme is highly stable and inactive [209]. Graphene-based materials can provide a highly conductive precusor which has large surface area and irreversible absorptive capacity for AChE, therefore, they are ideal substrate for AChE immobilization to monitor harmful organophosphates pesticides, such as monocrotophos [210], paraoxon [211–213], dichlorvos [214] and carbaryl [215]. For example, Zhai et al. [216] developed an AChE biosensor based on graphene–gold nanocomposite and calcined layered double hydroxide. The detection limit for chlorpyrifos was 1.4 1010 M, which is more sensitive than some previous AChE based biosensors [217,218]. In addition, Wang et al. [219] synthesized highly dispersed zinc oxide nanoparticles on carboxylic graphene to provide a hydrophilic



8


Fig. 4. (A) Schematic diagram of the modification of GOD with pyrene and the subsequent fabrication of mono- and multi-layered enzyme electrodes. (B) Cyclic voltammograms of the enzyme electrodes fabricated with mono- and multi-layered graphene and pyrene functionalized GOD: (a) 1 layer, (b) 2 layers, (c) 3 layers, and (d) 4 layers. Data were recorded in 80 mM glucose solution in 0.1 M pH 7.4 phosphate buffer at room temperature and potential scan rate of 10 mV s1. (Reproduced by permission from ref. [195] of The Royal Society of Chemistry.)

surface for AChE adhesion. They used this system to detect chlorpyrifos and carbofuran, and got the detection limits as 5 1014 M and 5.2 1013 M, respectively, which showed even higher sensitivity for pesticides monitoring than Zhai’s work [216]. The excellent conductivity, catalytic activity and biocompatibility were attributed to the synergistic effects of zinc oxide, carboxylic graphene and nafion. Zinc oxide is biocompatible with a high

isoelectric point (IEP), making it suitable for absorbing low IEP proteins such as AChE. The carboxylation of graphene extremely improves the conductivity and dispersive property. Nafion can improve conductivity of zinc oxide–carboxylic graphene and also help to build a protective membrane on electrode surface to prevent the loss of the enzyme molecules. The graphene based enzymatic electrochemical sensors have been widely developed for detection of a variety of biomolecules, such as nitrate [136], trichloroacetic acid [220], cholesterol [221], maltose [222] and so on. There are advantages and also limitations for different systems. For example, Sun et al. [220] used graphene/ TiO2/hemoglobin composite modified carbon ionic liquid electrode to detect trichloroacetic acid. This electrode owns some merits such as easy preparation, high electrochemical stability, good ionic conductivity and wide electrochemical windows. However, the detection limit, which is 0.22 mmol L1, might be promoted via a few modifications, such as increase of electron transfer rate and protein loading. Furthermore, a two-enzyme/GO elelctrode was described by Park et al. [223] to detect cancer antigen 15–3 which is a biomarker of breast cancer. This is measured using the twoenzyme (b-galactosidase and tyrosinase) scheme and GO/ITO electrodes, and the calculated detection limit is 0.1 U mL1. Heavy metal ions in potable water and food are major health concerns and so their detection is critical to maintaining a safe food supply [20,83]. Therefore, the highly sensitive, selective and rapid detection of heavy metal ions has been explored investigated in depth using various analytical methodologies [224–227]. Graphene-based sensors have been employed to detect and monitor the presence of heavy metal ions such as Pb2+, Cd2+, Cu2+ and Hg2+ [226,228–233]. For example, a graphene modified gold electrode was produced via non-covalent interaction for the determination of Pb2+ and Cu2+ [232]. This modified gold electrode was prepared by first functionalizing with benzoic acid moieties followed by coupling of pyrene, and finally immobilization of the graphene nanosheets via non-covalent p–p stacking interactions (Fig. 5). A graphene-based sensor for the detection of Hg2+ ions was reported by Gong et al. [235] based on distributing monodispersed Au nanoparticles regularly across the graphene sheet. Surface modified graphene with 1-octadecanethiol was also described for the detection of Hg2+ ions [236]. Supported by a silicon oxide substrate, the 1-octadecanethiol molecules underwent self-assembly to form extensive, highly ordered monolayers on the 2D the graphene sheet. Alkanethiol modified graphene field effect transistors exhibited a sensitivity for Hg2+ detection at 10 ppm, which provides new opportunities for graphene based electronics as heavy metal sensors. Sensors based on graphene have also been

Table 1 Analytical figures of merit for various electrode materials employed for glucose sensing. Limit of detection

Linear range

Ref.

Electrode materials (based on GOD) N-doped graphene RGO/polypyrrole (GCE) RGO/polyvinylpyrrolidone (GCE) GO/Fe3O4 RGO/PdPt (Au) RGO/Ag (GCE) Graphene/polyaniline/Au (GCE) AuNPs/GO/CNTs (GCE) RGO/multi-layered GOD (GE)

0.01 mM 3 mM NA 0.2 mM 1.0 mM 0.16 mM 0.6 mM 4.8 mM 0.154 mM

0.1–1.1 mM 2–40 mM 2–14 mM 0.5–600 mM 2–12 mM 0.5–12.5 mM 4.0 mM-1.12 mM NA 0.2–40 mM

[125] [186] [187] [189] [190] [192] [90] [193] [195]

Electrode materials (based on metal) RGO/Cu nanowires (GCE) N-doped graphene/Cu (GCE) Pt/poly(glutamic acid) (GCE) Pt/Ni–Co nanowires (GCE) RGO/Ni(OH)2 (Au)

1.6 mM 1.3 mM 11 mM 1 mM 15 mM

0.005–6 mM 0.004–4.5 mM 0.05–5.95 mM 0–0.2 mM and 0.2–8 mM 0.02–30 mM

[196] [167] [199] [201] [203]

Categories of electrodes are given in the bracket while the others are not applicable. Here, GCE stands for glassy carbon electrode and GE stands for graphite electrode.




9

Fig. 5. Schematic of the chemical modification of a gold electrode with a graphene sheet. (Reprinted with permission from ref. [234]. Copyright 2013 Elsevier.)

reported for the detection of Cd2+, Cu2+ and Ag+ [237–240]. A SnO2/ RGO nanocomposite modified glassy carbon electrode was developed for the simultaneous and selective determination of ultra-trace concentrations of Cd2+, Cu2+ and Hg2+ in drinking water [237]. The detections of DNA, proteins, metal ions, cysteine using graphene–DNA based biosensors can be carried out on both fluorescence resonance energy transfer and electrochemical sensors [241]. Graphene, as a perfect two dimensional material with large surface area, can work as a carrier for DNA molecules loading either by directly absorption via p–p interactions or by other binding approaches via functional groups from modified graphene surface. Furthermore, the excellent conductivity of graphene makes it possible for fast electron transfer between DNA and electrodes. Recently the electrochemical detection for target DNA molecules has been reported [242–246]. For example, Huang et al. [246] synthesized a 2D graphene analog tungsten sulfide–graphene (WS2–Gr) composite for DNA biosensor application. Graphene served as a 2D conductive support for a highly electrolytic accessible surface. However, the electronic conductivity of WS2 is much lower compared to graphene materials, which may limit its application for sensitive detection. On the other hand, DNA detections can also be achieved by non-DNA modified electrodes [247–249]. For example, Lu et al. [247] described a sensor system using graphene and Hypocrellin A (HA) which is a natural perylene quinonoid drug and can kill tumor cells efficiently. The HA–graphene based electrode was successfully applied to monitor the telomere DNA in human blood serum and abdominal fluid. In particular, the interaction between HA and telomere DNA was investigated by electrochemical methods for the first time. The fact that no probe DNA sequence was used on the electrode greatly saved the cost, however, the insolubility of HA in water indicated the challenge for combination with graphene sheets, which may limit the quantity of HA fixed on graphene surfaces. The precise determination of H2O2 is of great importance in biology, chemistry, clinical control and environmental protection [250]. Electrochemical methods are useful for H2O2 detection [251,252]. For example, a sensor with excellent response to organic peroxide was created by applying enzyme-free amperometry for the reduction of tert-butylhydroperoxide (TBHP) utilizing nanocobalt phthalocyanine on graphene [253]. The sensor was used for the determination of TBHP in body lotion and had a linear calibration range between 0.026 mM and 4.81 mM with a detection limit of 5 mM, and a sensitivity of 13.64 A M1. In addition, many useful and novel non-enzyme H2O2 biosensors have been reported

[221,254,255]. For instance, Dye et al. [221] developed a highly sensitive amperometric biosensor based on the hybrid material derived from graphene and nanoscale Pt particles for the detection of H2O2 and cholesterol. This sensor shows high sensitivity and linear response toward H2O2 up to 12 mM with a detection limit of 0.5 nM (S/N = 3) in the absence of any redox mediator or enzyme. The connection of super conductive graphene sheets and catalytically active Pt nanoparticles accelerated electron transfer for the oxidation of H2O2. The plasma membrane-bound protein NADH oxidase, is sensitive to activation by epidermal growth factor and insulin and other molecular influences [256]. Shan et al. [257] showed that graphene electrodes functionalised using ionic liquids could be used for both NADH detection and the biosensing for ethanol. The ionic liquid modified-graphene-based sensor exhibited acceptable analytical figures of merit for the determination of NADH together with convenient and low cost preparation. As an important detection method, electrochemiluminescence (ECL) is becoming well recognized in analytical chemistry due to its simplicity, label-free, low background signal, and high sensitivity. Graphene based ECL sensors have been demonstrated as highly sensitive sensors due to the large surface area and excellent electrical conductivity of graphene [258–261]. For example, Qu and co-workers [262] described a label-free ECL system for telomerase activity detection based on porphyrin–graphene nanocomposite modified electrode. The advantages of this work include free PCR, low cost, easy to operate, and ultrasensitivity which is much higher than that of colorimetric sensors. Recently they designed ECL sensors for cancer cell detection by carbon nanodot@Ag/graphene hybrid [263] and RGO nanocomposite-based peptide sensor [264]. In fact, graphene-based materials have been applied for electrochemical sensors to detect a variety of biologically and commercially important molecules, including cancer cells [265,266], dopamine [267], telomerase [268], and in diagnostics and therapeutic applications [269]. 4.2. Graphene-based electrical sensors Since the first single-layer of graphene was obtained by the mechanical cleavage of graphite [1], this pristine material has been an excellent model system for condensed-matter physicists and has enabled them to unravel some of its fundamental properties [85,87]. Single layer graphene is a semi-metal with exceptional properties that are particularly useful for the development of electronic sensors, and these have been summarized in Table 2.



10


Table 2 Summary of reported electronic parameters of graphene. Item Carrier mobility Carrier density Hall effect Intrinsic noises Field effect

Data or status 2

1 1

20,000 cm V s 1013 cm2 Half-integer quantum Dr/r 104 Ambipolar

Ref. [270] [85] [271] [272] [1]

Graphene’s zero energy band gap exhibits remarkably high carrier mobility [270], high carrier density [85], room temperature Hall effect [271], low intrinsic noises as compared with other nanostructured materials [272–274] and ambipolar field-effect characteristics. Electronic sensors based on 1D semiconductor materials such as silicon nanowires and carbon nanotubes represent a paradigm shift in technology [76,275]. This approach offers the potential for high sensitivity, label-free detection, high temporal resolution and compatibility with lab-on-a-chip devices [67,75,81]. Silicon nanowires are perhaps the most studied and successfully applied 1D material for nanoelectronic sensing [276–279]. However, a major limitation of silicon nanowire sensors is that their detection relies essentially on the induced field-effect [75]. Therefore, they are only suitable for the detection of either charged analytes or electrogenic events [76,279]. Graphene has been innovatively utilized for nanoelectronic sensors to achieve a broader range of applications, taking advantage of its exceptional electrical properties [83]. The electrical response of graphene to various chemical species has been reported to be similar to CNT [280]. Given that CNTs have been widely employed in gases detection [281,282], gas sensors based upon graphene materials have drawn considerable attention for the detection of several analytes in the vapor phase [272,283–285]. A graphene-based sensor for NO2 gas detection was demonstrated for the first time by Novoselov and co-workers [272]. It is a sensor based on the micromechanical cleavage of graphite at the surface of oxidized Si wafers detected NO2 gas by measuring the change of source–drain resistance. Several other studies have reported the detection of NO2 using graphene-based devices [285–289]. For instance, reduced graphene oxide conjugated Cu2O nanowire mesocrystals were explored as a NO2 sensor, and were synthesized under hydrothermal conditions using non-classical crystallization in the presence of GO and o-anisidine [287]. This composite sensor

device combined the advantages of the enhanced electronic conductivity of RGO and the interdendritic space within the mesocrystals, which formed a 3D conducting network (Fig. 6). Apart from applications in NO2 sensing, graphene-based materials have been utilized for the detection of NH3 [290]. Lu et al. [291] developed a RGO sensor as a fast, repeatable and low temperature detection tool of NH3. They suggested that the RGO field-effect transistors (FET) operated in n-type mode by applying sufficiently positive gate voltage (Vg) exhibited faster instantaneous response and faster recovery for NH3 sensing in comparison with their performance in p-type mode, which was attributed to the ambipolar transport of RGO and the Vg-induced effects. This Vgdependent NH3 sensing facilitated RGO based sensors application for the room temperature gas detection. In addition to the detection of NO2 and NH3, graphene based gas sensors were also employed for H2 [292–294], CO2 [295], NO [296] and H2O(g) [297]. For example, Johnson et al. [293] used Pdfunctionalized multi-layer GNR networks for the detection of H2 gas. It was demonstrated that high specific surface area and a porous structure for these nanonetworks facilitated effective modification together with good sensitivity toward H2 gas at room temperature (the relative resistance response for H2 ranges from 55% at 40 ppm to 77% at 8000 ppm). Jiang et al. [298] produced Al2O3/graphene nanocomposite from a supercritical CO2 solution of GO using a single step, eco-friendly, and low-cost procedure. We have summarized various selected sensors used the detection of gases in Table 3. As said before, graphene’s exceptional electronic characteristics promise a high signal-to-noise ratio in detection and the conductance of graphene is highly sensitive to the local electrical and chemical perturbations as every atom of a graphene film is exposed to the external environment. Moreover, the Fermi level of zero-bandgap graphene can be modulated by the gate voltage, which in turn determines whether charge carriers will be holes or electrons. When detection is based on the field-effect, a large bandgap is desirable [299]. The bandgap of graphene can be opened by reducing its dimension(s) to the nanoscale [300,301], or by introducing atomic or molecular dopants [302–304]. Moreover, compared to 1D nanostructured sensing elements, 2D graphene can provide a larger detection area, and homogeneous surface for uniform and effective functionalization [14]. Graphene’s 2D structure makes it more suitable for interfacing with flat cell membranes [305,306]. It has been shown that

Fig. 6. Representative SEM images illustrating the time-dependent morphology of the Cu2O nanowire mesocrystal at 200 C, schematic illustration of the Cu2O crystallization process assisted by o-anisidine and GO and the schematic for mechanism of NO2 sensing of RGO–Cu2O. (Reprinted with permission from ref. [287]. Copyright 2012 American Chemical Society.)




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Table 3 Analytical figures of merit for various electrode materials employed gas sensing. Analyte

Electrode materials

Limit of detection

Percentage response

Ref.

NO2 NO2 NO2 NH3 CO2 NO Ethanol

Ozone treated graphene (Au) RGO-conjugated Cu2O S-G or EDA-G (Pt) Ethanol–graphene-based (Ti/Au) GS CVD-grown graphene (Ni) Al2O3/RGO

1.3 ppb 64 ppb 3.6 ppm 160 ppb NA 2 ppb 1.5 mg mL1

17% 67.8% NA 0.71%/ppm 0.17%/ppm 28% NA

[286] [287] [289] [290] [295] [296] [298]

Categories of electrodes are given in the bracket while the others are not applicable. Here, S-G stands for sulfonated reduced graphene oxide while EDA-G represents ethylenediamine-modified reduced graphene oxide.

graphene is able to support cell adhesion and growth, demonstrating its biocompatibility [307,308]. Electronic biosensors based on 1D semiconducting nanostructures have been interfaced to living cells to detect their dynamic activities [309], such as detection of bioelectricity [277,310], adipocytokines [278], triggered secretion of proinflammatory cytokines [279], circulating breast cancer cells [311]. The unique properties of graphene add a new dimension to the nanoelectronic–cell interface. As cell membrane is also a 2D structure (5 nm-thick lipid bilayer), it can intimately interact with flat graphene [312,313]. In contrast, when a cell membrane interfaces with other nanostructures, the interaction may not be tight and homogeneous and the local curvature induced on the thin cell membrane by nanotopographic structures may alter cell functions in intriguing ways [314]. Given the strong interactions between graphene and the cell membrane any cellular activity that induced local electrical and chemical fluctuations in the nanogap between the two surfaces could significantly change its conductance. Lieber and co-workers recently demonstrated that a graphene FET can extracellularly detect action potentials from single electrogenic cardiomyocytes [315]. In this work, mechanically exfoliated graphene was used to fabricate devices by e-beam lithography. They reported that the sensitivity of graphene FET was superior to conventional metallic microelectrodes [316–318] and comparable to a silicon nanowire FET [319]. The device response was triggered by the field-effect due to the short-lived potential

variations across the nanointerface between the membrane and the graphene resulting from the current through the membrane ion channels. Although the field-effect of graphene is less prominent than for silicon nanowires, a comparable signal-tonoise ratio was obtained by a graphene FET [320]. This may be attributed to graphene’s larger interfacing area with the cell [14]. Graphene nanorods possess a large bandgap and can play a key role in the detection of cellular bioelectricity as they offer high sensitivity via of their enhanced field-effect plus excellent spatial resolution due to their nanoscale lateral dimension. 4.3. Graphene-based optical sensors Graphene oxide exhibits interesting optical properties [321]. Unlike zero-gap graphene or other carbonaceous materials, GO can fluoresce at a wide range of wavelength (from near-infrared to ultraviolet) [322]. This is because the disordered oxygenated functional groups on GO confine p-electrons within the sp2carbon nanodomains, thereby giving rise to a local energy gap that inversely scales with the domain size. Therefore, GO has the potential to serve as a universal fluorescence label for optical imaging [323]. Interestingly, just like other graphitic materials, GO is also capable of quenching fluorescence [324]. The quenching efficiency of GO is superior to the conventional organic quenchers. It has been shown that quenching even at a distance of 30 nm is attainable by GO [325]. On the basis of its fluorescence and

Fig. 7. Schematic representation of the sensing mechanism for the detection of Pb2+ ions based on accelerated leaching of gold nanoparticles on the surface of graphene. (Reprinted with permission from ref. [231]. Copyright 2012 American Chemical Society.)



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Table 4 Summary of analytical figures of merit for selected heavy metal ions sensor. Analyte 2+

Pb Pb2+ Pb2+ Pb2+ Pb2+ Hg2+ Hg2+ Hg2+ Cu2+ Cu2+ Ag+ Cd2+

Electrode materials

Limit of detection

Linear range

Ref.

RGO (Au) RGO/polypyrrole (GCE) GQD-DMA RGO/Au SnO2/RGO (GCE) Photoluminescent GO Polyvinylpyrrolidone protected graphene/Au (GCE) SnO2/RGO (GCE) RGO (Au) SnO2/RGO (GCE) Photoluminescent GO SnO2/RGO (GCE)

0.4 nM 4 pM 9 pM 10 nM 0.18 nM 1 nM 6 ppt 0.28 nM 1.5 nM 0.23 nM 10 mM 0.10 nM

0.4–20 nM NA 0.01–1 nM 50–1000 nM NA NA 10–60 ppb 0.1–1.3 nM 1.5–20 nM 0.2–0.6 mM NA NA

[232] [333] [334] [230] [237] [335] [329] [237] [232] [237] [335] [237]

Categories of electrodes are given in the bracket while the others are not applicable. Here, GQD-DMA stands for 3,9-dithia-6-monoazaundecane functionalized graphene quantum dot and GCE stands for glassy carbon electrode.

quenching abilities, GO can serve as either an energy donor or acceptor in fluorescence resonance energy transfer (FRET). The optical characteristics of GO such as fluorescence wavelength and quenching efficiency are tunable by controlling the extent and type of its oxygenation [326,327]. Recently, the selective and highly sensitive fluorescent detection of Pb2+ by graphene-based sensors has gained attention [231,328]. For example, a graphene–DNAzyme sensor for the amplified turn on fluorescence detection of Pb2+ was reported [329]. The sensor operation is based upon the phenomena that ssDNA and the duplex with its target exhibit different affinity to GO. The fluorophore carboxy fluorescein labeled DNAzyme substrate hybrid acted as both a signal reporter and molecular identification element with GO as a super quencher. A novel and effective “turn-on” fluorescence sensor for the detection of Pb2+ in aqueous solution was reported using- graphene functionalized with gold nanoparticles [231]. This device exploited both the optical characteristics of graphene and the ability of AuNPs to quench luminescence. By the addition of Pb2+, graphene fluorescence could reappear and increase, which was attributed to the analyte accelerating the leaching of AuNPs from the graphene surface with the aid of either thiosulfate (S2O32) or 2-mercaptoethanol (2-ME) (Fig. 7). Therefore, this offered an innovative alternate approach to the selective detection of Pb2+. For other metal ions, Zhang et al. developed a highly sensitive and selective graphene based biosensor with DNA duplexes of poly(dT) for rapid fluorescence detection of Hg2+ [330]. A detection limit of 0.5 nM for Hg2+ was obtained under optimal experimental conditions and the methodology showed no interference in the presence of other metal ions. In addition, Liu et al. [239] designed a “turn-on”

fluorescent Cu2+ biosensor employing a DNA cleavage-dependent graphene-quenched DNAzyme. In this sensor, graphene facilitated the configuration of a self-assembled graphene-quenched DNAzyme entity by functioning as both “scaffold” and “quencher” of the Cu2 + -dependent DNAzyme. Qu and co-workers [331,332] described the functionalised graphene quantum dots (GQD) in sensitive optical detection of Ag+ and Cu2+. In comparison with organic dyes and semiconductor quantum dots, GQDs have some advantages, such as high photostability against photobleaching and blinking, good biocompatibility, and low toxicity. Furthermore, due to the quantum confinement and edge effects, GQDs with a graphene structure, exhibit high photoluminescence and slow hot-carrier relaxation, making them distinct from those of conventional graphene sheets. For a comparison we have summarized parameters of selected graphene-based sensors for heavy metal ions detections in Table 4. Apart from the detection of heavy metal ions, graphene-based fluorescence sensors were also used to monitor other important molecules. For example, Lin et al. [336] used combination of GO and thiol-activated DNA metalization for sensitive fluorescence turn-on detection of cysteine. A GO–FRET sensor to detect Cyclin A2 (an early stage cancer indicator) has been reported [337]. However, the achieved LOD of 0.5 nM is approximately 10-fold lower than that achieved by single wall carbon nanotube based sensors. Additionally, fluorescence sensors have been employed to detect some organic molecules [338–340]. Dong et al. [338] prepared surface-passivated GQDs with good fluorescence activity by using incompletely carbonized citric acid. Since free chlorine can destroy this surface passivation resulting in significant quenching of the fluorescence signal a green and facile GQDs sensor for free chlorine in drinking water was developed based on this effect (Fig. 8). Another

Fig. 8. The detection of free chlorine based on the fluorescence quenching of GQDs. (Reprinted with permission from ref. [338]. Copyright 2012 American Chemical Society.)




fast, sensitive, label-free, and general dye-sensor was developed for the detection of synthetic organic dyes by competitive adsorption based on fluorescein/reduced graphene oxide complex [339]. This assay principle relied on the reversible interaction (adsorption and desorption) between RGO and fluorescent dye, and so can be generally utilized for the detection of numerous synthetic organic dyes. Likewise, a fluorescent “turn-on” sensor based on 8-aminoquinoline functionalized GO through a photo-induced electron transfer signaling mechanism was prepared for detection of D-glucosamine with a good sensitivity and selectivity [340]. Interestingly, Xiang et al. [341] demonstrated a fluorescence-based colorimetric droplet sensor that enabled simultaneous DNA determination by using GO nanoprobe labeled with carboxy fluorescein as a quencher for the ssDNA probes labeled with 6carboxy-X-rhodamine. Analysis of the fluorescence intensities of the dyes in droplet provides a quantitative determination of target DNAs. This sensor platform has a low consumption of reagents and provides a promising universal tool for high throughput applications. This technique for fabricating DNA biosensors with high selectivity has the potential to facilitate an innovative approach for the development of novel, robust and highly sensitive electrochemical biosensors. However, most of fluorescence sensors inevitably suffer from requirement of tedious fluorophore labeling processes and timeconsuming purification steps [299]. In order to overcome the limitations of fluorescence detection, some simpler and straightforward optical sensors have been developed [342]. The intrinsic advantages of colorimetry, such as simplicity, low cost, and labelfreeness, make it very attractive for target detection [343]. In recent years, functionalized graphene derivatives have been successfully introduced to fabricate colorimetric biosensors for detection of cancer cell [344,345], glucose [346], intracellular H2O2 [347]. Carboxyl-modified GO has peroxidase-like activity that can catalyze the reaction of peroxidase substrate 3,30 ,5,50 -tetramethylbenzidine (TMB) in the presence of H2O2 to produce a blue color reaction [346]. This offers fresh ideas for the application of GO nanomaterials in medical diagnosis and biosensing. Base on this finding, Qu and coworkers [344] achieved the synergistic GO–Au nanoclusters hybrid which has excellent peroxidase-like activity by simple electrostatic interaction. Compared with natural enzymes, GO-Au nanocluster is a promising enzyme replacer with merits of facile preparation, lowcost, and stability. And GO–Au nanocluster shows fairly high catalytic activity over a broad pH range, even in neutral media. Furthermore, with various functional oxygen groups on GO surface, GO has become more attractive as it is easy to be modified and capable to absorb fluorescence molecules or drugs. Furthermore, GO can be used as a robust sensing platform for label-free colorimetric detection of a broad range of targets, including metal ions, DNA and small molecules, by utilizing the DNA-mediated assembly of GO–hemin (GH) hybird. Hemin has been used as a mimetic enzyme for labeling antigen and antibody reactions [348]. The DNA–GH hybrids was separated at the bottom of the tube after centrifugation, leaving behind a transparent supernatant. After incubation with TMB and H2O2, the colorimetric signal of the centrifugal supernatant will be significantly lower compared to that in the absence of targets. This might provide a universal method for quantitative detection of a variety of analytes.

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for the detection of gas, heavy metal ions, biological molecules and proteins are described and discussed. The unusual and useful properties of graphene can still be applied in many areas of science and technology, and help to provide useful solutions to some of the world’s key problems, such as water and food security and rapid and sensitive medical analyses. However, graphene, as a newly discovered material, faces several challenges including improving synthesis methods, extensive understanding of graphene surface and extending the applications in various practical fields. In the case of synthesis, some fundamental approaches are developing including mechanical and matrix assisted exfoliations, thermal deposition, CVD, unzipping CNT, and reduction of graphite oxide. The output of graphene fabrication can differ significantly, even if there are only minor variations in the preparation method, and so it is necessary to select an appropriate way to fabricate graphene for different purposes. Each of these preparation methods has its own advantages for specific applications, but also some disadvantages, as discussed earlier. The scientific community will continue to improve graphene processes and materials, including more efficient methods for preparing high quality, large-sized and defect-free graphene. There are currently two key challenges for the development of a new graphene technology for large-scale manufacturing of graphene-based products: access to large quantities of high-quality uniform graphene, and on- demand tailoring of graphene properties. It is necessary to better understand the chemistry and physics at the graphene surface, and also the interactions of chemicals or biomolecules at the interface of graphene when applied as nano-scaffold in catalysis and chemical/biosensing (e.g., the mechanisms of absorption molecules on graphene surface, the orientation of biomolecules on the graphene, and how these interactions impact the transport properties of graphene). An improved understanding of graphene and its interaction with molecules, will lead to advances in graphene science and its application in catalysis and sensor development. There is still considerable development remaining in the design and fabrication of graphene-based sensors. Some graphene materials (e.g., GQD or bilayered graphene) have not been fully investigated as sensing materials, despite their potential due to their useful and unusual properties. Novel graphene-related materials and structures (e.g., graphene and graphene foam grown by CVD method) are continually being created by scientists. Hybridizing or compositing graphene with other organic or inorganic materials (such as polymers, CNTs, nanoparticles) also is considerably extending the possibilities for graphene sensor development. Improved modification methods are being investigated and developed, although there is still much work to do to understand and fully utilize graphene in sensor development and application. Acknowledgements We sincerely thank Prof. Neil Barnett for his valuable comments on this manuscript. J. Liu thank the NSF of China (51173087), NSF of Shandong (ZR2011EMM001) and Taishan Scholars Program for financial support. The Australian Research Council (DP130101714) and Deakin Central Grant Scheme are thanked for financial support by W. Yang. References

5. Future challenges Graphene, as a star among nanocarbon materials and widely studied by researchers throughout the world, has drawn considerable interest in many fields. Its unique structure contributes to its exceptional chemical and physical properties, which lead to a broad range of applications in sensing. In this review, recent developments in graphene-based chemical and biological sensors

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