Selected examples of these species are characterized in regard of their properties and possible applications.In the review hybrids of cationic porphyrins ( Porphyrins are planar tetrapyrrolic macrocycles that have interesting properties and value for applications in various fields. They are promising in the design of sensors [e.g. design of solar cells [ sensors  and they sensors ; some me sensors . The rea sensors –10, e.g.ar cells , 12 and ar cells .i.e. porphyrins meso-functionalized by quaternary azaaromatic units, due to the presence of four positive charges are of interest in the construction of next generation photosensitizers [Cationic porphyrins, sitizers . CationiN-methyl-4-pyridinium porphyrin (TMPyP), in this review referred to as A. Among hybrids of cationic porphyrins, those with nanocarbons are a topic of present review.The representative compound of cationic porphyrins is meso-tetrakis, generating singlet oxygen for selective destruction of localized tumors [A growing attention is paid today to interaction of cationic porphyrins with nucleic acids . Cationid tumors .It is noteworthy that antimicrobial photodynamic therapy (aPDT) is now emerging as an alternative to antibiotics; in this procedure the cationic photosensitizers such as cationic porphyrins interact in the excited state with molecular oxygen to produce singlet oxygen that kills the microbial cells .Among a variety of other applications of porphyrins in medicinal field  one may Nanocarbons, belonging to carbon allotropes include zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes and two-dimensional (2D) graphene. Nanocarbons have attracted recently a growing attention due to their unique electronic, optical, thermal and chemical properties, promising for their applications in a variety of areas [of areas .i.e. fullerenes consist of bent sp2 carbon atoms. From fullerenes to graphene the strain at sp2 carbon atoms decreases; the surfaces of fullerenes and graphene have different reactivities. In fullerenes attempts to release strain by formation of sp3 carbon centers exist, therefore fullerenes have anomalously large reactivity, while in graphene no strain at carbon atoms is observed, i.e. no tendency for sp3 hybridization occurs, and consequently, graphene is less reactive than fullerenes [The smallest species, llerenes .Nanocarbons are today a topic of an intense research, and a considerable progress in this area is observed. Due to valuable properties they may find applications in various fields; one should point out the importance of these materials so from theoretical and practical viewpoints.Fullerenes, carbon nanotubes and graphene are of interest in the study of artificial photosynthesis and solar energy conversion, in the construction of electronic, optoelectronic, photovoltaic and sensing devices  and in t2 (produced as an emission from use of hydrocarbon fuels) to hydrocarbon fuels [Carbon nanotubes and graphene may be applied in photocatalytic reduction of COon fuels . Carbon on fuels .Carbon nanotubes and graphene are investigated in order to facilitate charge transport across abiotic-biotic interfaces , they ari.e. fullerenes, single wall carbon nanotubes SWCNTs and graphene), acting as electron acceptors, with photosensitizers, acting as electron donors, are promising for this aim. In the experiments concerning the above theme, the light-induced electron transfer processes of nanocarbon hybridized with photosensitizers, have been shown to be very useful for this purpose.Today the use of solar energy is crucial to solve the energy problems for a sustainable society; molecular devices employing hybrids of nanocarbons,  assemblies by electrostatic and charge-transfer interactions; due to these interactions the obtained assemblies are very stable [It was established that octacationic zinc porphyrin ZnPy stable , 60. One60 dendrofullerenes the compounds 1–3 have been used. The formation of the assemblies was investigated by gel electrophoresis. For example, the aggregation of ZnP8+ with 2 yields the assembly porph·2, and with 3 the assemblies porph·3 and porph2·3 are obtained.As C·+/C60·− are formed. All radical ion-pair states decay to their singlet ground states without passing through the intermediate triplet excited states of ZnP8+ or C60 [The results of experiments have shown that radical ion pairs ZnP+ or C60 , 60.The layer-by-layer (LbL) deposition of positively and negatively charged oligoelectrolyte multilayer (OEM) films was investigated in view of their assembly and disassembly behavior. The LbL technique is a simple and very efficient wet-coating method.e.g. on metal or glass. Formation of iterative dipping circles makes it possible to obtain the deposited thin films composed of alternating monolayers of polycations and polyanions. This easy and cheap method affords advanced materials based on polyelectrolyte multilayer films.The LbL process involves the alternate deposition of cationic and anionic species from a solution –65. Due 8+ was used as a positively charged oligoelectrolyte, and compounds 4–6 served as negatively charged oligoelectrolytes. By use of electrolytes 5 and 6 containing the porphyrin chromophore, the deposition could be easily followed by UV–Vis spectroscopy [The octacationic zinc porphyrin ZnPtroscopy . HoweverBesides the assembly, also the first study of disassembly was made; this investigation is promising in incorporation of bioactive molecules at a given inter-layer position, since after removal of the outer layers the bioactive molecules can be released. Such drug delivery is interesting for biomedical use.8+ and 4, 5 or 6 were deposited on planar silica substrates using the alternate dipping method. For investigation of possible application of OEMs as drug delivery systems, the biologically active pamidronate 7 used in the bone metastasis was employed as an anionic building block. The porphyrin ZnP8+ served as a cationic component and at the same time as a reporter electrolyte, therefore the detection of the anionic component occurred indirectly. It was found that the absorbance of ZnP8+ is enhanced when the number of ZnP8+ layers increases; the correlation is linear. This behavior enables the regulation of the amount of deposited 7 by the number of layers.For LbL assembly of molecular oligoelectrolytes, the layers of porphyrin ZnPIt was established that the use of chromophore containing electrolytes, in particular porphyrins allows the observation of both assembly and disassembly of OEMs by optical spectroscopy; the compounds containing chromophore serve here as reporter electrolytes. One should point out that oligoelectrolytes used are monodisperse, and various synthetic procedures enable modification of properties of OEMs. It is of interest that the investigated OEMs may show the time dependent release of active components, this behavior being promising for drug delivery .Carbon nanotubes have interesting mechanical, electronic and optical properties, therefore they are promising for design of composite materials, drug carriers as well as sensing and energy conversion devices. They are investigated with the aim to obtain photo- and redox-responsive materials. Carbon nanotubes, due to their unique structural and electronic properties are of interest for design of hybrids useful in various areas. The applications of nanotubes are today a topic of an enormous amount of reports –72.SWCNTs may be chemically modified for use as nanosized building blocks. This process may be performed by covalent or noncovalent functionalization. The covalent functionalization is rather disadvantageous because of structural changes of nanotubes and loss of some of their unique properties. To overcome this difficulty the noncovalent functionalization of nanotubes is used, this process leads to hybrids of nanotubes with functionalizing agents.The noncovalent functionalization of nanotubes usually proceeds by π-stacking interactions of an organic molecule or polymer with nanotube walls. It is preferable when the attached molecule has an extended π-electron system of bonds, in this respect the use of porphyrins is convenient. It is noteworthy that in porphyrin/nanotube hybrids, the existence of a light-induced charge transfer between porphyrins and SWCNTs is promising for their application in solar energy conversion.In the study of mechanism of the hybrid formation it was found that the interaction of nanotubes and porphyrins with charged side residues is much stronger than in the case of neutral porphyrins. Quantum-chemical calculations have shown that the hybrids formed by nanotubes with charged molecules are stabilized due to strong cation-π interactions. It is noteworthy that such complexes are highly stable in water . HybridsHybrids of cationic porphyrins with carbon nanotubes are a topic of an intense research, below some examples are presented.8 and anionic sodium copper chlorophyllin 9 were used.It is known that assembling composites of chromophores with carbon nanotubes is a crucial process for construction of photoelectronic devices, biosensors and electron storage devices ; for ass8-SWNT/9-SWNT was built from noncovalently adsorbed composites 8-SWNT and 9-SWNT. For this purpose SWNTs were dissolved in water soluble cationic 8 and anionic 9, and the obtained solution served for electrostatic LbL multilayer preparation. The composites were highly dispersed due to existence of π-π interactions [The LbL film ractions .8 and 9 fluorescence resulting from their interaction with SWNT was observed. Due to the efficient charge separation and electron transfer in the above LbL film electrodes, the photocurrent generation is enhanced as compared with 8/9 film electrodes without SWNT.A strong quenching of meso-5,10,15,20-tetraphenylporphyrin 10 with zigzag single-walled carbon nanotube (SWNT) has been investigated by resonance Raman spectroscopy and by ab initio and molecular dynamic calculations [In the study of hybrids of porphyrins with carbon nanotubes the interaction of porphyrins A and ulations .10 with SWNT, since between A and SWNT the cation-π attraction exists. In the molecule A the strong cation-π attraction leads to the saddling; the saddled structure of A provides a closer contact between the charged groups of A with the SWNT surface. The formation of A/SWNT complex in the aqueous solution has been modeled by the molecular dynamics method, showing its stability in the water environment.The results have shown that the interaction of A with SWNT is stronger than the interaction of 4+ or MS4−, respectively, serving as electron donors, and from noncovalently functionalized SWNTs serving as electron acceptors. In a first step, SWNTs were solubilized by π-π stacking of pyrene functionalized by anionic (COO−) or cationic (NH3+) groups. Cationic or anionic porphyrins MA4+ or MS4− were ion-paired with functionalized SWNT/pyr− and SWNT/pyr+ systems to give porphyrin-SWNT donor–acceptor hybrids MA8+/SWNT/pyr− or MS8+/SWNT/pyr+ [Photoinduced electron transfer in ion-paired porphyrin/SWNT hybrids has been studied. The donor–acceptor hybrids have been built from water-soluble cationic or anionic porphyrins MAWNT/pyr+ . The ste2+ or HV2+, respectively) and an electron–hole shifter 1-benzyl-1,4-dihydronicotinamide (BNAH) in water or DMF, used as solvents. The photoinduced processes resulted in the accumulations of radical cations MV·+ and HV·+, due to the electron-pooling in the presence of a sacrificial electron donor. This behavior shows the photoinduced electron transfer and enables photocatalytic applications of the above systems.The charge separation process was further confirmed by an electron mediator (methyl or hexylviologen dication MVThe immobilization of porphyrin A on the carbon nanohorn (CNH) was investigated in order to design donor–acceptor CNH-based hybrids for managing electronic interactions in environmentally friendly aqueous media. In the experiments the water soluble porphyrin A has been immobilized by π-π stacking interactions on carbon nanohorns (CNHs); this process does not disrupt their π-electronic network .i.e. oxidation of the porphyrin and reduction of the nanohorns were studied by transient absorption spectroscopy. In the presence of MV2+ and a hole trap, the accumulation of the reduced form of methyl viologen was observed by the illumination of A-CNH; this behavior shows the electron migration from the initially formed charge-separated state [The efficient fluorescence quenching of the A unit in the A/CNH assembly suggests charge separations from the photoexcited A to CNH. The photoinduced charge-separation processes within the illuminated A/CNH hybrids, ed state .2 laser ablation of graphite in the absence of metal catalyst under the inert argon atmosphere. The mild sonication of CNH in aqueous solution of A afforded the water soluble A/CNH hybrid. It was found that the morphology of CNH was retained upon immobilization of A, this suggests that the stacking of A onto CNH does not alter the aggregation of CNHs. The high solubilization of CNHs in aqueous solution shows that the surface of CNHs is covered by A.The CNHs were produced by COIt is noteworthy that CNHs are more convenient for design of nanomaterials than carbon nanotubes, since they are of a high purity due to the absence of any metal nanoparticles during the laser ablation production, and they have heterogenous surface structure resulting from highly strained conical ends. It is important that the rough surface structure of CNH aggregates enables the better dispersion of CNHs in liquid media than in the case of tightly bundled carbon nanotubes.In investigation of systems able to mimic natural photosynthesis, the hybrids built from porphyrins and CNHs are of interest. They contain CNHs behaving as electron acceptors and porphyrins as electron donors which act as light-harvesting antennas capturing visible light and transducing the excitation energy. It should be pointed out that the above A/CNH assemblies are promising for use in solar energy conversion.8+, namely FeP8+ and CoP8+ with SWNTs was investigated. For this purpose SWNTs were functionalized with anionic pyrenes from among 11a–d and treated with metalloporphyrins FeP8+ or CoP8+. It was observed that the resulting MP8+/SWNT/pyr− assemblies form stable hybrid structures in aqueous media (pyr− = pyrene). An important feature of this procedure is that an efficient exfoliation of the initial bundles affords isolated nanohybrid structures [Formation of hybrids of cationic, water soluble metalloporphyrins MPructures .8+/SWNT/pyr− hybrid with visible light, the rapid intrahybrid electron transfer from the photoexcited MP8+ to SWNT occurs. This fact causes the reduction of the electron-accepting SWNT and, at the same time, the oxidation of the electron donating MP8+.Upon excitation of the above MPOne should note the importance of SWNT templates bearing carboxylate or sulfonate groups. These negative groups act as promoters for suspending SWNT samples in aqueous media and as anchors interacting with pyridinium headgroups present in metalloporphyrins. Pyrene compounds are strong fluorophores therefore they are useful sensitive markers in the performed experiments, the fluorescence spectroscopy being a very convenient tool for investigation of the above electron donor–acceptor interactions.e.g. high charge carrier mobility, graphene is today a topic of an intense research [Graphene is a single layer of graphite, where carbon atoms are arranged in a honeycomb lattice; owing to its specific electronic and mechanical properties, research –92. One research . In contresearch .The selected techniques for preparation of graphene and related species are presented below . The comGraphene oxide (GO) is the single-layered graphite oxide which can be prepared by the exfoliation of graphite oxide. GO having many functional groups such as hydroxyl, aldehyde, carboxyl, epoxy is obtained by exposing graphite powder to strong oxidant solutions, e.g. KMnO4/H2SO4 followed by H2O2 (30 %). Exfoliation is performed by sonication of GO dispersion.Chemically converted graphene (CCG) is prepared by reduction of GO suspensions e.g. with hydrazine; in this way a majority of the oxygen functional groups undergoes elimination and the conjugated structure of GO is restored.Carboxylic acid modified GO (CGO) is obtained by sonication of GO under basic conditions, followed by neutralization with HCl; it selectively reserves the carboxylic acid functional groups.The unique electronic system of graphene results from its high quality 2D crystal lattice in which electrons can move without being scattered off, therefore high electrical conductivity exists. As drawbacks of graphene may be considered its severe stacking as well as its complete insolubility in all solvents including water. Therefore in the investigation of graphene the main topics are its exfoliation from graphite and its solubilization. Chemical methods of exfoliation involve the use of strong acids , ionic lIn order to improve the chemical compatibility of graphene with diverse media, it is necessary to develop its well-dispersed form. For this purpose covalent and noncovalent functionalization of graphene with various molecules and nanomaterials was investigated in water and in organic solvents –103.covalent functionalization of graphene introduces sp3 defects in its lattice; this behavior decreases its high conductivity [noncovalent functionalization may be used, in this way the electronic network of graphene is not disrupted [Functionalization of graphene offers its new application possibilities. However, the extensive uctivity . To overisrupted . NoncovaIn the noncovalent functionalization the planar aromatic organic molecules are employed; they interact through numerous π–π van des Waals forces with the graphitic framework and promote spontaneous exfoliation . For a ngraphene-coated metal NPs and graphenes modified by metal NPs; four examples concerning this theme will be given (NP = nanoparticle).Describing graphene it seems noteworthy to mention graphene-coated metal NPs two examples will be shown. In the first one the graphene-coated CoNPs 12 which can be functionalized are presented; e.g. they can react with 4-nitrobenzene diazonium tetrafluoroborate to give p-nitrophenyl derivative 13, which upon reduction with elemental sulfur affords aminoderivative 14 useful for peptide coupling [In the study of coupling , 110–11215 undergo “click” reaction with stable radical propargyl ether TEMPO 16 yielding 17 which is an efficient catalyst for oxidation of primary and secondary alcohols [In the next example the graphene-coated CoNPs functionalized by (azidomethyl)benzene alcohols .graphenes modified by NPs, two examples, i.e. graphenes modified by AuNPs and by AgNPs will be shown. Graphene sheets composed from 2 to 10 layers may serve for preparation of graphenes modified by NPs. Chemical exfoliation of graphite in aqueous medium  afforded GO; the subsequent thermal treatment of GO  promoted further exfoliation.In the study of 4 solution, and the subsequent reaction with sodium citrate afforded graphene modified by AuNPs. This material may serve as a substrate for surface-enhanced Raman scattering (SERS) [The oxygen functionalities (hydroxyl and epoxy groups) situated on the graphene surface are reactive sites for chemical modification and for deposition of NPs . The preg (SERS) .2 films. It was observed that upon UV illumination of TiO2/GO films, the photogenerated electrons from TiO2 are captured by GO. [2 network. The stored electrons can serve for reduction of metal ions to metal NPs.Another method was used for preparation of graphene modified by AgNPs. GO sheets have been anchored onto TiOd by GO. , 116. Th2, since GO is able to transport electrons through its plane. The above procedure is possible due to the unique property of GO/RGO to shuttle electrons in a direction orthogonal to its plane, and transfer electrons to Ag+ ions. The formed semiconductor-graphene-metal (SGM) film is valuable for catalysis and sensor use. These SGM films, tested as SERS sensors produced considerable target molecule signal enhancement, in this way enabling detection of their nanomolar concentrations.In the presence of silver ions, the AgNPs begin deposit onto GO surface opposite TiOIt seems that the above examples concerning graphene-coated NPs and graphenes modified by NPs may be considered as a supplement to the wide field of graphene applications –122.i.e. hybrids of cationic porphyrins with graphene [Hybrids of cationic porphyrins with graphene are today intensively studied. Among many works concerning graphenes functionalized by porphyrins –126, a sgraphene –131.i.e. GO, CCG and CGO with porphyrin A was studied. It was observed that the intermolecular interaction occurs immediately upon mixing graphene with A; in this process A is immobilized on the graphene surface via electrostatic and π-π stacking interactions.The noncovalent functionalization of graphene with porphyrins was widely investigated , 131. InIt is worth noting that functional groups on the surface and edge of GO, CCG and CGO play an important role in functionalization of graphenes with A. The strong fluorescence quenching of A upon its interaction with graphene is due to the efficient electron or energy transfer from excited state A to graphene. The red shift of Soret band of A occurring at its interaction with graphene depends on the kind of functional groups of graphene and decreases in the order CCG > CGO > GO .4+. As a result, the aqueous dispersion of graphene/pol/A4+ assembly was formed. It was established by photoluminescence experiments that in this assembly the graphene layers act as electron acceptor, and A is the photoexcited electron donor.In further experiments porphyrin A was integrated via Coulombic interactions to exfoliated graphene, and the possible photoinduced electron/energy transfer phenomena have been investigated . It is kThe absorption and fluorescence spectroscopic measurements have shown the presence of electronic communication between A and graphene in the ground and excited states. Basing on experimental results and taking into account the reported redox potential of graphene , one mayIt is known that the selectivity and improved sensitivity of SERS measurements are desirable for analytical and environmental applications. The RGO-AgNP composite has been studied for improvement of surface-enhanced resonance Raman signal (SERRS) sensitivity of porphyrin A . The SERS-active materials include noble metal NPs, such as silver or gold NPs. These NPs provide a localized surface plasmon resonance leading to a large local electromagnetic field which can enhance Raman scattering signals, enabling single molecule SERS . Metal NThe RGO-AgNP composite is promising for use as a SERS substrate. Graphene has high surface area, therefore is suitable for dispersion of metal NPs. It was observed that metal NPs can be grown directly on graphene by a simple solution-based approach. The intrinsic ability of RGO to adsorb and complex with molecular species facilitates the enhanced SERRS sensing.i.e. porphyrin A interaction; in experiments RGO was shown to be an effective substrate for dispersing AgNPs. The red shift of the Soret band of A in UV–Vis absorption spectrophotometry, observed upon complexation of A with RGO-AgNP confirms this behavior [The increased sensitivity for SERS detection using RGO-AgNP composite was investigated in terms of RGO-AgNP-target molecule, behavior . It was i.e. fullerenes, carbon nanotubes and graphene showed to be promising for various applications, therefore recently they are a topic of a wide investigation. In view of the intense progress in the field of porphyrins [e.g. in the area of electronics or photovoltaics.Hybrids of cationic porphyrins with nanocarbons, rphyrins –138, fulrphyrins –142, carrphyrins –146 and rphyrins –150 one In a summary it should be pointed out that the theme concerning hybrids of cationic porphyrins with nanocarbons is very large, therefore from among a great number of reports only selected works have been shown in the review. However, one may hope that, although not exhaustive, it would to some extent help to gain better insight into this area, promising for novel, valuable applications.


Graphene, a two dimensional structure of carbon atoms, has been widely used as a material for gas sensing applications because of its large surface area, excellent conductivity, and ease of functionalization. This article reviews the most recent advances in graphene hybrid materials developed for gas sensing applications. In this review, synthetic approaches to fabricate graphene sensors, the nano structures of hybrid materials, and their sensing mechanism are presented. Future perspectives of this rapidly growing field are also discussed. Graphene is conductive, transparent and bendable, and is one of the strongest known materials [2/Vs, it has a resistivity of 10−6 Ω·cm, and its carrier density is 1012/cm2 [Graphene is a monolayer of sp in 2004 ,2. Graph in 2004 . The per1012/cm2 .Graphene is an attractive sensing material for fabricating a gas sensing device due to its intrinsic properties such as large surface area, higher interaction with gas molecules, and zero rest mass of its charged carriers . The atoi.e., hybrids of graphene with polymers and hybrids of graphene with metal/metal oxides.This review article is intended to focus on the potential applications of graphene hybrids. It will bring forward most recent advancements in the field of graphene hybrid based gas sensors. Initially, we have included some papers in which graphene was utilized as sensing material to highlight its behavior towards different analytes, its high and low binding energy centers and some methods of its fabrication. Our main focus is on graphene hybrids which were divided into two types, 2) gas sensor [2 gas is a strong oxidizer which extracts electrons after its adsorption on a graphene surface. This effect reduces electron density in n-type material and leads to an increase in resistivity. Resistivity of the NO2 sensor device drastically changed after annealing at 120 °C which removes contaminants that remained on the chip after the fabrication process. There are two different types of adsorption centers on a graphene surface: one with high binding energy and the other is low binding energy centers. The annealing process increases sensor sensitivity towards NO2 down to sub-ppt level by activating high binding energy adsorption centers. Lower concentration of NO2 will occupy high binding energy adsorption centers which results in zero desorption due to strong binding of NO2 molecules. At higher concentration, desorption becomes significant which indicates adsorption centers with low binding energies are also occupied. Ozone treatment of graphene enhances its sensing behavior towards NO2 by introducing a number of functional oxygen groups on the surface [et al. developed a graphene sensor device by mechanical exfoliation of highly ordered pyrolytic graphite with subsequent deposition on silica/silicon (SiO2/Si) substrate [2 gas molecules adsorb/desorb on the graphene surface and there are three possible adsorption configurations: the nitro, nitrite and cyclo-addition configurations. The change or increase in graphene conductivity is attributed to the charge transfer between NO2 molecules and graphene where NO2 molecules act as an acceptor. The p-type semiconducting graphene was also grown by the ethanol-chemical vapor deposition (CVD) technique [2 and ammonia (NH3) toxic gases at room temperature. The electron withdrawing effect of NO2 gas molecules will increase hole-density of p-type semiconductor graphene upon adsorption which leads to a decrease in resistance of sensor device. The responses of GNM were greater than their counterpart films which was attributed to the formation of a large number of edges in the structure that leads to hole doping. The resistance of GNM sensor increases when exposed to NH3 molecules. NH3 is an electron donor which results in depletion of holes in p-type GNM. Ethanol-based CVD grown graphene (Gethanol) expressed a p-type semiconducting behavior due to polycrystallinity in comparison to ambipolar electron transfer behavior of methanol-based CVD grown graphene (Gmethanol). The Gethanol nanomesh contains highly disordered sp3 hybridization, a higher number of edge defects and functionalized oxygen groups affecting its electronic and transport characteristics in comparison to Gmethanol nanomesh. The defects in Gmethanol nanomesh were formed mainly due to edge formation on nanoholes whereas Gethanol nanomesh contains additional unsaturated grain boundary of intrinsic topological defects due to its polycrystalline nature. These higher number of defects in Gethanol nanomesh increase device sensitivity. The response times of both Gethanol and Gmethanol nanomesh devices are almost similar but response intensities are different. The defect sites in Gmethanol nanomesh are less than Gethanol nanomesh which leads to adsorption of a lower number of NO2 molecules.An epitaxial graphene film was grown on semi-insulating 4H silicon carbide (4H-SiC) wafer to fabricate nitrogen dioxide (NOs sensor . Epitaxi surface . Ko et aubstrate . The thiechnique . Nanomes3 gas was fabricated on the basis of graphene/microfiber hybrid waveguide (GMHW) [3 adsorption on graphene. The graphene film was grown on copper (Cu) foil by CVD method with subsequent coating of polymethylmethacrylate (PMMA) film on graphene/Cu foil. The Cu foil was later on etched by ferric chloride (FeCl3) solution and PMMA/G was transferred onto magnesium fluoride (MgF2) substrate. Finally, the PMMA was removed by acetone. Then, a microfiber is attached onto graphene/MgF2 substrate by van der Waals and electrostatic forces. The nitrogen atom of NH3 becomes pentavalent after adsorption on graphene and captures two free electrons of two carbon atoms which leads to an increase in resistance of graphene. A hydrogen plasma was used to reduce graphene oxide (GO) at room temperature for fabricating the carbon dioxide (CO2) gas sensor [2 crystalline interaction sites which in turn increases sensor sensitivity. The CO2 gas molecule acts as a donor and its adsorption on graphene sheet induces an increase in graphene conductance. Therefore, the physical adsorption of CO2 gas on graphene sheet is the dominant sensing mechanism which leads to a change in conductance of graphene sheet. The graphene samples were prepared by stamping method to fabricate CO2 gas sensor [2) gas sensor [2 gas.A soft lithographical method was used to fabricate ammonia gas sensor by using highly oriented reduced graphene oxide (rGO) microbelts . These ue (GMHW) . Mach-Zes sensor . The rads sensor . In thiss sensor . The GO et al. synthesized graphene-poly :poly (styrenesulfonate) (G-PEDOT:PSS) composite by gradually mixing graphene solution in PEDOT:PSS solution [3 gas sensor. The sensitivity of the sensor device was assessed while being exposed to different concentrations of NH3 ranging from 5–1000 ppm. Whereas, selectivity of the sensor was investigated by using ethanol, methanol, toluene, acetone and diethylamine gases. The response of G-PEDOT:PSS sensor towards 500 ppm of NH3 was 9.6% which is higher than the responses of PEDOT:PSS (4.4%) and pristine graphene (2.4%) gas sensors. Similarly, the G-PEDOT:PSS hybrid sensor exhibited a response time of 3 min, which is fast in comparison to the PEDOT:PSS sensor.Seekaew solution . Screen (i)Redox reactions(ii)Charge transfer(iii)Polymer swellingSeveral sensing mechanisms have been proposed for sensors based on conducting polymers:2−) interact with NH3 molecules and release their electrons back to graphene/polymer composite. The holes of the valence band combine with electrons of the conduction band which results in lowering carrier concentrations and, finally, an increase in the resistance of the p-type G-PEDOT:PSS sensing film.(i) According to the first mechanism, the surface of G-PEDOT:PSS trap oxygen species  In the charge transfer process, the holes of conductive G-PEDOT:PSS surface interact with electron donating NH3 molecules’ diffusion. Graphene in G-PEDOT:PSS acted as a conductive pathway that favors the electron hopping process. The swelling process disrupts these conductive pathways as well as increases the PEDOT distance leading to a significant increase in resistance of the G-PEDOT:PSS sensor upon NH3 exposure. The resistance of graphene/polymer composite returns to its original value after NH3 desorption by dry air purging.(iii) A single PSS chain in PEDOT:PSS polymer film interacts with many PEDOT chains over its length. There is a very short interchain distance which favors the electron hopping process. However, this hopping process becomes difficult because of an increase in interchain distance of PEDOT due to swelling of the polymer with NHA surface plasmon resonance (SPR) based fiber optic gas probe was fabricated by utilizing a nanocomposite film based on PMMA and rGO . The perThe response mechanisms of fiber optic sensor based on PMMA-rGO nanocomposite for different gases can be explained as:(i) The rGO surface contains polar oxygen functionalities which adsorb polar gas molecules and change its dielectric constant due to charge transfer; (ii) The incorporation of rGO into the polymer matrix creates conducting pathways, and adsorption of gas molecules may increase contact resistance between rGO sheets by increasing distance between rGO sheets. The change in volume of polymer matrix due to gas adsorption also increases the distance between sheets.3 molecule is a reducing agent and has a lone pair of electrons which can be easily donated to p-type rGO that leads to an increase in sensor resistance. The response behavior of rGO sheets can be divided into two types: a fast response and a slow response. The fast response of rGO sheets is due to the gas adsorption onto binding sites with low energy (sp2-bonded carbon) and a slow response is due to high energy binding sites . The sensing performance of this device also depends on the number of PPy molecules attached on the rGO surface. A lower number of PPy molecules on rGO surface allows complete interaction between NH3 molecules and sp2-bonded carbon atoms of rGO which leads to fast response of devices. A higher number of PPy molecules on rGO surface hinders the interaction between NH3 gas and sp2-bonded carbon, which severely affects the sensing performance.A self-assembly technique was used to create a conductive network of chemically reduced graphene oxide (CrGO) between parallel Au electrodes . Au elec3 gas sensor [i.e., acid-doped PANI attached rGO, dedoped PANI attached rGO and free rGO. The sensing properties of free rGO were much better than acid doped or dedoped rGO. Excellent sensing properties of completely free rGO based sensor is due to intrinsic properties of free rGO. The response of the sensor can be explained because of charge transfer from p-type rGO to adsorb ammonia molecules. In another study, a drop drying method has been utilized to produce ammonia gas sensor based on aniline reduced GO [3 gas interacts with the p-type graphene and causes charge transfer. PPy-rGO composite was prepared by a drop casting method to fabricate an ammonia gas sensor [Aniline was used to reduce GO in order to fabricate NHs sensor . rGO obtduced GO . A condus sensor . The com2 gas molecules was fabricated by a porous conducting polymer PEDOT nanocomposite prepared on rGO film [2 gas molecules increases the number of charge carriers and tremendously decreases resistance of rGO-PEDOT composite.A chemiresistor sensor for NOrGO film . The inc2 gas. A procedure to fabricate rGO/nylon-6 nanofiber mats (FRGO) based gas sensor has been illustrated in 2/Si substrate having platinum (Pt) interdigitated electrode (IDE) arrays [2 gas sensor exhibited a sensor response of 7% to 0.25 ppm of NO2 as shown in A combination of an electrospinning method and GO wrapping on nylon-6 fibers through an electrostatic self-assembly, followed by a low-temperature chemical reduction, was utilized to produce a sensor for detecting NO) arrays . These n2 gas. The pyrrole monomer was absorbed by the GO surface by π-π stacking interactions. Polymerization of pyrrole to PPy and reduction of GO to rGO occur simultaneously under UV illumination. Graphene oxide (GO) was synthesized by acid dissolution method in which graphite powder is heated with sulfuric acid and phosphoric acid in the presence of potassium permanganate. Then, prepared GO was chemically reduced with ascorbic acid to synthesize rGO.The GO was reduced to rGO during the formation of PPy nanofibers by utilizing a one-step redox reaction under UV illumination at room temperature . The rGOIn another study, a paper-like nanocomposite of G/nylon-6 was fabricated by incorporating rGO onto a nylon-6 (N6) membrane via vacuum assisted self-assembly (VASA) method . The diset al. deposited palladium nanoparticles (Pd NPs) on single-layer graphene (SLG) via galvanic displacement between graphene-buffered Cu and Pd ion followed by spin-coating with a PMMA membrane layer to fabricate a hydrogen gas sensor [2 detection can be explained by dissolution and dissociation of H2 molecules into atomic hydrogen in the presence of Pd NPs deposited on graphene. Pd-hydrides (PdHx) were formed (as a function of Pd is diminished) when H2 molecules dissolved into atomic hydrogen after being adsorbed on Pd NPs surfaces. The resistance of the sensor increases as the diminished function of PdHx results in increased electron transfer to graphene from PdHx. Increase in the density of Pd NPs on SLG increases PdHx formation due to the high surface area of Pd. This will change the conduction pathways from graphene to Pd film like structures, which decreases gas response sensitivity. Pak et al. synthesized palladium-decorated graphene nanoribbons (Pd-GNRs) for detecting hydrogen gas sensor [2/silicon wafer by inductively coupled plasma chemical vapor deposition (ICP-CVD) technique. The CVD-grown graphene was transferred to desired substrate by following polymer PMMA transferring technique. A periodically aligned GNR array was synthesized by placing a chromium interlayer under photoresist via laser interference lithography and then, incorporating it into the Pd-decorated hydrogen sensors. The developed sensor exhibited excellent response towards H2 gas with response/recovery time of 60 s/90 s.Hong s sensor . This prs sensor . Laser i3  nanostructures in GO and partially reduced graphene oxide (PrGO) by utilizing a controlled hydrothermal process [3 sol was synthesized from sodium tungstate powder Na2WO4 and its ribbon like nanostructure was formed by adding K2SO4. The GO was synthesized by following Hummer’s method, whereas PrGO suspension was obtained through the hydrothermal method. Pd-WO3/GO and Pd-WO3/PrGO hybrid was produced by adding Pd-WO3 drop by drop to GO and PrGO suspensions under vigorous stirring. In this hybrid structure, Pd NPs and adsorbed oxygen species  play a dominant role in sensing target gas molecules. These oxygen species capture electrons from conduction band of WO3 (as an n-type semiconductor) which generate a depletion layer with low electrical conductivity. Hydrogen as a reducing gas remove oxygen charge after interacting with these oxygen species and the generation of electrons in this reaction results in conductivity increase. The rate of this reaction increases in Pd NPs’ presence due to molecular hydrogen dissociation into active atoms. PdHx hybrids are also formed due to hydrogen dissolution in Pd, which has diminished function. This effect also leads to donation of electrons to the conduction band of WO3 and increases the electrical conductivity of Pd-WO3/graphene hybrid.Another hydrogen gas sensor has been fabricated by incorporating Pd-WO process . The homet al. synthesized graphene-supported platinum/palladium (Pt/Pd) core-shells hybrid to fabricate a hydrogen sensor device [2) is 36% at room temperature with an excellent response/recovery time of 3/1.2 min whereas the detectable range of Pt/Pd core-shell hybrid is 1–40,000 ppm. Another hydrogen sensor was fabricated by using Pt and platinum-iron alloy (Pt3Fe) nanoparticles which were decorated on nitrogen doped graphene [3Fe alloy nanoparticles. Then, ethylene glycol reduction technique was followed to decorate graphene with Pt and Pt3Fe alloy nanoparticles. Chloroplatinic acid (H2PtCl6) solution was used for decorating graphene with Pt nanoparticles whereas a mixture of H2PtCl6 and FeCl3 solutions was used for Pt3Fe alloy decoration. The sensor sensitivities were measured through exposure to 1%–4% of H2 which indicates that sensitivity decreases when alloying Pd with 3D transition metals.Phan r device . At firsgraphene . GO was et al. synthesized a nanocomposite of graphene and zinc oxide (ZnO) by in situ reduction of zinc acetate and GO during refluxing whereas GO was prepared from graphite powder by following Hummer’s method [2 sensor. A number of G-ZnO thick film sensors were fabricated by varying the graphene content  in G-ZnO composite. The G-ZnO composite having 1.2 wt% of graphene content is very sensitive in comparison to ZnO (optimum operating temperature 400 °C) and other composites, and gives a sensor response of 3.5 at an optimum operating temperature of 150 °C. The composites exhibited improved performance even at lower temperatures in comparison to bare ZnO. Mishra et al. synthesized rGO decorated with tin oxide (SnO2) quantum dots (QDs) by a surfactant assisted hydrothermal method [2 QDs were prepared from SnCl4·5H2O by a hydrothermal method. The rGO-SnO2 QDs based sensor was used to detect H2 and liquefied petroleum gas (LPG) in a concentration range of 100–500 ppm and it gives a response of 89.3% to H2 (500 ppm) and 92.4% to LPG (500 ppm) at an operating temperature of 200 °C and 250 °C, respectively.Anand s method . A paste2, NH3, and NO gases at room temperature. The Pt nanoparticles decorated rGO sensor device exhibited an improvement of 100%, 60% and 25% to H2, NH3, and NO gases in comparison to the sensor having an rGO structure without Pt.Multiple gas sensors based on Pt decorated rGO nanostructures were fabricated via DEP. Different parameters of the DEP technique such as applied voltage, frequency and processing time were optimized to assemble Pt-rGO nanocomposite into the electrodes . For thi2, NO2, and CO. The rGO coupled Au NPs’ thin film sensor was exposed to 1 ppm of NO2, 10,000 ppm and 100 ppm of H2 and 10,000 ppm of CO. A change in optical properties was observed while exposing the sensitive surface (graphene flakes-Au NPs) to different gases such as reducing and oxidizing gases. This shift in surface plasmon resonance is attributed to an electron transfer to Au NPs and sp2-hybridized carbons atoms of graphene oxide.A surface plasmon resonance based gas sensor was synthesized depositing graphene flakes over gold (Au) nanoparticles . These n2−, O−, O2− that operate below 100 °C, between 100 and 300 °C and above 300 °C, respectively. The oxygen adsorption reactions can be represented as follows:A chemiresistive type of acetylene gas sensor was synthesized based on silver (Ag) loaded ZnO-rGO hybrid via a chemical route . A chemiNon-ZnO modified graphene exhibited p-type behavior, whereas, in case of ZnO modified graphene, an opposite sensitivity response was observed due to n-type sensitivity nature of ZnO. Ag has better oxygen dissociation ability than ZnO and activates molecular oxygen dissociation on hybrid surface and creates more acetylene sensing active sites on the surface. The oxygen species on sensing material becomes more active with acetylene molecules, which donate electrons to the sensing surface.2-rGO hybrid composite film was fabricated on a printed circuit board (PCB) substrate (having interdigitated electrodes) by utilizing a hydrothermal method [4 with rGO and, later on, autoclave at high temperature. Then, the sensor was fabricated on PCB by microfabrication and sensing film was casted by drop casting. The higher sensing performance of SnO2-rGO hybrid composite can be attributed to three factors:A room temperature acetone gas sensor based on SnOl method . It was 2 nanoparticles in rGO creates a porous nanostructure by reducing aggregation of rGO sheets. The porous structure has greater surface area and more active sites such as defects, vacancies, oxygen functional groups and sp2-bonded carbons which adsorb and diffuse acetone gas molecules.(i) Incorporation of SnO2 nanoparticles and p-type rGO nanosheets. The electron donor ability of acetone gas increases negative charge carriers in n-type thin film and reduces an overall sensor resistance.(ii) The gas sensing performance is due to heterojunction formation at the interface between n-type SnO2− is also responsible for gas sensing. These chemisorbed oxygen molecules are ionized to oxygen species by capturing free electrons from SnO2 nanoparticles. The rGO exhibits p-type semiconducting behavior and contains dominantly positive charge carriers (holes). The adsorbed acetone gas is an electron donor which changes the concentration of holes in rGO, resulting in a change in rGO resistance. The SnO2-rGO hybrid composite shows a higher response to acetone gas, which is attributed to high surface area and porous nanostructure. (iii) The surface oxygen species such as O−, O2−, O2− which leads to a formation of depletion layer and, finally, increases ZnO resistance. The adsorbed oxygen species interacts with formaldehyde and releases trapped electrons back to ZnO conduction band resulting in a decrease in sensor resistance. Thus, ZnO-G hybrid exhibits enhanced sensor response in comparison to ZnO or graphene alone as a sensing material.A thin film of zinc oxide was deposited on graphene film by Atomic Layer Deposition (ALD), whereas graphene film was grown by CVD . A ZnO/G2O4) composite was prepared by solvothermal method and utilized to fabricate acetone gas sensor [2O4 was produced by using Zn(NO3)2·6H2O and Fe(NO3)3·9H2O reagents as precursors. Acetone molecules interact with oxygen species on sensor surface and oxidize them to carbon dioxide and water along with release of free electrons, which results in a decrease in sensor resistance. A gas sensor was fabricated composed of ZnO conductive bottom layer on a metal foil  and a graphene based top conductive electrode [− or O2− depending on operating temperature. The interaction of reducing gas ethanol with ZnO NR substitutes surface-bound oxygen which releases electrons back to ZnO crystals and leads to an increase in electrical conductivity. The higher sensitivity of this sensor is attributed to the existence of pores between monolithic nanorods which means more surface area and greater sensor response.A graphene-zinc ferrite (FOH)2 and possessing high dielectric strength and chemical and thermal stability. Monolayer graphene (MLG) was prepared by CVD on Cu foil and, subsequently, transferred to the desired substrate by a PMMA assisted transfer technique. NH3 molecules have strong electron donating capability and donate electrons to p-type graphene when adsorbed. The effect will shift Fermi level to Dirac point and leads to an increase in resistance upon exposure to NH3 gas. The higher sensitivity on mica substrate can be explained in terms of substrate dependant doping and hydrophilicity. The mica substrate shift graphene Fermi level to downward which leads to form additional states around Fermi level. Thus, graphene orbitals significantly overlap with highest occupied molecular orbitals (HOMO) of NH3 and cause large charge transfer. A gas sensor has been fabricated by using SnO2-G composite which was synthesized by one-pot method with GO and SnCl2 as precursors [2 was n-type semiconductor and its interaction with reducing/oxidizing gas molecules leads to an increase/decrease in conductivity, respectively. The SnO2-G composite also exhibited increased conductivity while in contact with reducing gases such as hydrogen sulfide (H2S) and NH3.In a comparison of G-mica and G-SiOsitivity . Muscoviecursors . The SnOet al. synthesized a composite of PPy and graphene nanoplatelets (GNs) by chemical polymerization and decorated it with titanium dioxide nanoparticles (TiO2 NPs) by sol-gel technique (TiO2@PPy-GN) [2 NPs were in the range of 10–30 nm which were well dispersed on the PPy-GN composite. NH3 sensor was fabricated by drop-coating a suspension of TiO2@PPy-GN in DMF on integrated indium-tin oxide (ITO) electrodes. The sensors of GN, PPy-GN thin films were also fabricated for comparison. TiO2@PPy-GN sensor content showed higher sensitivity to NH3 and faster response and recovery times in comparison to GN and PPy-GN thin films. The response of TiO2@PPy-GN sensor is about 12 and 4.5 times higher in comparison to GN and PPy-GN composite sensors. In another study, a single step synthetic procedure was applied to fabricate a room temperature gas sensor for CO, NH3 and NO gases based on photoluminescence (PL) properties of zinc oxide (ZnO) decorated graphene oxide sheets (GO) as sensitive coatings [Xiang @PPy-GN) . The TiOcoatings . GO was 2) nanoflakes were prepared by one-pot reflux method to fabricate NOx gas sensors [2+ in an aqueous solution through electrostatic interaction to form Co(OH)2 nanoparticles on graphene sheets. The small nanocrystals with high energy recrystallize to form nanosheets and nanopetals, in a manner of energy minimization. The nanopetals arrange themselves to form nanoflowers. Some of the nano-nuclei form the shape of nanowires because of time and space constraints. Gas-sensing performance of this hybrid can be explained as: the chemisorbed oxygen present on Co(OH)2-rGO hybrid interact with NOx molecules which extract electrons from hybrid, leading to an increase of hole conductance, which finally decreases the resistance of p-type hybrid. The oxygen adsorption plays an important role in the gas sensing mechanism of the Co(OH)2-rGO composite sensor which follows the surface charge model. The adsorption of NOx and high electron affinity of NOx molecules lead to electron transfer from the Co(OH)2-rGO layer to NOx. Thus, adsorption of NO2 on graphene hybrid converts NO2 to NO2− and NO to NO−. The target gas molecules NO or NO2 also adsorbed onto graphene hybrid and reacted with O- to generate NO2− or NO3−.A hybrid of rGO and cobalt hydroxide (Co(OH) sensors . The negxO/multilayer graphene composite was synthesized to measure NOx gas molecules at room temperature [xO/multilayer graphene composites (CuMGCs) were synthesized by vacuum assisted reflux method. Copper acetate and cetyltrimethylammonium bromide (CTAB) are incorporated into the layers of aEG by vacuum-assisted technique and, then, react with functional groups by exfoliation of aEG via reflux, which results in formation of 3D nanoflowers of CuxO homogeneously grown on aEG.In another study, a 3D nanoflower-like Cuperature . The thex ranging from 97 ppm to 97 ppb at room temperature as shown in x), the sensor response was 27.1% with a response time of 59 s. The gas molecules extract electrons from CuMGCs when exposed to oxidizing gas NOx, which leads to a decrease in electron density and increases hole carriers on p-type semiconductor surface. This effect results in decreasing resistance of sensor material when NOx gas molecules directly adsorb onto CuMGCs. The adsorption of NO2 on sensor material leads to NO2− and adsorption of NO to NO−.The schematic illustration of the synthesis of CuMGCs is shown in 2O NW-rGO composite mesocrystals were also synthesized by crystallizing copper (II) acetate in the presence of GO and o-anisidine under hydrothermal conditions [2O. GO interacts with Cu2+ ions during the in situ crystal growth and was subsequently reduced to rGO  to form Cu2O mesocrystal interconnected via rGO network as shown in 2O NW and Cu2O-rGO composite were fabricated. These sensors were exposed to different concentrations of NO2 ranging from 0.4–2 ppm as shown in 2O-rGO composite based sensor is greater in comparison to rGO and Cu2O NW based sensors. The calculated LOD of Cu2O-rGO based sensor is 64 ppb which is the best LOD in NO2 sensing at room temperature. The sensing mechanism for rGO-Cu2O composite can be explained as charge doping by adsorbing gas at sensor material which alters conductivity. The rGO prepared by chemical modification contains electron-withdrawing oxygen functionalities.Cunditions . These n2O composite is a p-type semiconductor and releases its electrons to oxidizing gas NO2, which results in an increase in hole conductance. This composite is highly oriented and contains porous structure which gives a much larger surface accessibility in a 3D network. This effect results in increasing the sensor sensitivity towards adsorbed NO2 gas molecules.The rGO-Cu2O3) was homogenously dispersed on rGO nanosheets by a hydrothermal method to synthesize a nanocomposite of In2O3-rGO [3)3 in the presence of hydrazine hydrate to obtain In2O3-rGO. The NO2 sensor was fabricated by coating a suspension of nanocomposite onto the IDEs. Different nanocomposite samples were prepared by varying rGO concentrations . The maximum sensor response of In2O3-rGO composite having 0.72 wt% of rGO was observed towards NO2 at room temperature. Co3O4 intercalated rGO based sensor showed a much higher response to NO2 at room temperature compared to rGO alone [2 at defective sites of rGO. The enhanced sensor response can be explained by two possible mechanisms: increased surface area of rGO thick film by intercalation of Co3O4 and the Co3+-carbon coupling effect for rapid response. The in-between cobalt oxide (Co3O4) nanocrystals behave like nanopillars which leads to an extra macroporous structure between rGO layers. This results in better diffusion of gas molecules driven by capillary force which leads to enhanced response. The other possible mechanism is related to the coupling effect between Co and graphene, leading to enhanced oxygen reduction ability. The coupling between oxygen ions on graphene and Co makes Co-O a more ionic bond and results in Co3+ because extra interaction sites for NO2 molecules and electrons would be extracted indirectly from the p-type graphene through bridging oxygen. However, this composite based sensor showed a fast response and full recovery when exposed to methanol gas due to the interaction of gas molecules with sp2 carbon bonding sites. This interaction with gas molecules results in the release of electrons to graphene leading to a rise in resistance. The methanol gas molecules do not interact with oxygen containing defect sites because these species are negatively charged and reject electron donation from reducing species such as methanol.Indium oxide  and tin chloride (SnCl4) by the hydrothermal method [2 hybrid was dip coated on ceramic substrate  to fabricate NO2 sensor device. The incorporation of CNTs in rGO-SnO2 hybrid not only improve surface area of rGO by preventing its restacking but also improving electron transfer rate during gas sensing. The presence of SnO2 enhanced the recovery rate of hybrid sensor which was actually slow due to defects in the rGO structure. Thus, response and recovery time was greatly improved after incorporation of CNT and SnO2 in the rGO structure. The sensor based on rGO-CNT-SnO2 hybrid was used over a detection range of 1–100 ppm of NO2 gas at room temperature. The sensor exhibited a sensitivity of 2.53 at 5 ppm of NO2 gas with fast response/ recovery time of 8 s/77 s. The response and recovery time of rGO-CNT-SnO2 hybrid based sensor is better than previously proposed NO2 gas sensors [et al. synthesized a nanocomposite based on SnO2-rGO by reacting aqueous dispersion of GO with Sn salt hydrothermally [2 gas at low operating temperatures. The nanocomposite exhibited higher sensor response of 3.31 at 5 ppm of NO2 gas in comparison to response of 1.13 of rGO based sensor. A NO2 gas sensor has been developed on interdigitated Pt electrodes by synthesizing caesium doped graphene oxide (GO-Cs) [2 gas in comparison to pristine GO based sensors with the detection limit of approximately 90 ppb. Li et al. prepared three-dimensional (3D) SnO2/rGO composite by hydrothermal and lyophilization method to measure NO2 gas [2+ and Sn4+ and it was found that NO2 detection behavior of these composites was dependent on precursors. The Brunauer-Emmett-Teller method was used to analyze the surface area and porosity of 3D porous composite. It was found that surface area of porous composite prepared from Sn4+ precursor is large which is beneficial for gas sensing applications. The extremely large surface area of 3D porous composite made it possible to detect as low as 2 ppm of NO2 gas at low temperature.Liu l method . The aqu sensors ,61,62,63hermally . This na (GO-Cs) . The gra NO2 gas . The 3D 2-assisted thermal method was followed to synthesize iron oxide-graphene (Fe2O3-G) hybrid. This hybrid was further converted into paper-like vertically arranged Fe2O3-G nanosheets by a controlled magnetic field [2S gas sensor. Both vertically and horizontally aligned nanosheets were synthesized and used as sensing material for H2S, which showed that nanosheets with vertical arrangements are more beneficial than with horizontal arrangements in terms of sensitivity. In Fe2O3-G nanocomposite, the contents of both carbon and oxygen are decreased in comparison to that of GO. The decarboxylation reaction decreases carbon content, and dehydration is the cause for decreasing oxygen, which are the main reactions that occurred during formation of nanocomposite. The better sensitivity of vertically aligned Fe2O3/graphene is not only due to a large amount of uniformly distributed Fe2O3 on graphene sheets but also the special structure of vertically aligned nanosheets. This special structure provides a large contact area and less resistance which makes it very sensitive to H2S gas. In another study, a 3D array structure was synthesized from 1D SnO2 nanorods and 2D graphene sheets (SnO2-G 3D array structure) via a straightforward nanocrystal-seeds-directing hydrothermal method [2 nanorods on graphene sheets can be controlled by changing seeding concentration and temperature. Graphene sheets were synthesized by CVD method. The 3D array structures were synthesized in two steps: at first nanocrystal seeds were arranged hydrolytically on graphene sheets and then grown into a 3D structure hydrothermally. The gas sensing behavior of SnO2 is an adsorption-oxidation-desorption process which changes the electrical conductance of the sensing material. The electron depletion layer is generated on n-type SnO2 semiconductor due to the presence of surface oxygen species  which increases surface potential barrier. These ions were consumed after exposure to reductive gas such as H2S which generates additional electrons resulting in increased material conductance. The sensing performances of SnO2-G 3D array structures are significantly better than SnO2 flowers due to the following reasons: (1) The uniform distribution and highly aligned nanorods of moderate diameter offer a large surface area which facilitate molecular adsorption, gas diffusion and mass transport; (2) The outstanding electrical conductivity and chemical sensitivity of graphene as a sensing platform lead to better gas sensing behavior. MalekAlaie et al. prepared molybdenum trioxide (MoO3) nanoparticle decorated GO for the detection of H2S [4)6Mo7O24) was used to decorate rGO by calcination at 550 °C in N2 environment to convert Mo to MoO3. MoO3-rGO composite was spin coated between Pt electrodes on alumina substrate to fabricate a sensor device. Sensor response was investigated by adding different wt% of metal precursor in different concentrations of graphene oxide suspensions. The composite of 3 wt% of MoO3 and 5 mg/mL of GO suspension exhibits higher sensitivity at different concentrations (50–500 ppm) of H2S at 160 °C. The operating temperature of MoO3-rGO is lower in comparison to pristine rGO.A COic field . These pl method . The gron of H2S . At firs2 gas sensor based on epitaxial graphene exhibited the lowest LOD of 0.6 ppt (detection range of 0.2 ppb–1 ppm) in comparison to all other graphene based sensors fabricated for NO2 gas detection (LOD in ppb range). The sensitivity of the sensor device towards NO2 down to the sub-ppt level is due to the activation of high binding energy adsorption centers by an annealing process. This annealing process removed contaminants on the sensor surface which remained on the chip after fabrication. The introduction of functional oxygen groups to graphene by ozone treatment increases the sensor response to 17%. The fabrication of highly oriented rGO microbelts for ammonia gas showed the highest detection range of 10–1000 ppm at operating temperatures of 40–100 °C. Ethanol based CVD grown graphene nanomesh (Gethanol) expressed the lowest LOD of 160 ppb which is attributed to highly disordered sp3 hybridized carbon atoms, a higher number of edge defects, and functionalized oxygen groups. The reduction of graphene oxide by hydrogen plasma exhibited a detection range of 0–1500 ppm with a sensor response of 71% to CO2 gas, which is due to the presence of more sp2 crystalline interaction sites.A comparison of graphene based gas sensors is described in 3 gas. However, rGO-PPy is capable of detecting the lowest concentration of NH3 gas (1 ppb) with a response/recovery time of 1.4 s/76 s. The higher sensing performance of rGO device was due to the intrinsic properties of graphene sheets as well as adsorbed PPy molecules. The aniline reduced GO showed higher sensor response of 37% towards NH3 gas but its response time is very high (18 min). PPy-rGO nanofibers exhibited a detection range of 1–50 ppm to NO2. The rGO based NO2 gas sensor which was fabricated on electrospun Nylon-6 nanofiber scaffold is able to detect the lowest concentration down to 0.25 ppm in comparison to all other NO2 gas sensors based on graphene polymer hybrids.A comparison of graphene polymer hybrid based gas sensors has been presented in 3 nanostructures in GO and partially reduced graphene oxide (PRGO) expressed a detection range of 20–10,000 ppm with a response/recovery time of <1 min. Whereas, a higher sensor response of 89.3% towards hydrogen gas was observed by a sensor device based on rGO-SnO2 QDs at an operating temperature of 200 °C. An optical type H2 sensor exhibited excellent response/recovery time of 1–2 s/2–4 s at an operating temperature of 150 °C. TiO2-PPy-G based NH3 gas sensor showed a higher sensor response of 102.2% with an excellent response/recovery time of 36 s/16 s at room temperature in comparison to all other NH3 gas sensors. A chemiresistor type acetylene gas sensor based on Ag-ZnO-rGO exhibited higher sensor response (21.2%) and excellent response/recovery time (25 s/80 s) but it requires an operating temperature of 150 °C. 3D nanoflower of CuMGC exhibited excellent sensor response (95%) but its response time (9.6 s) is a little bit slower than the rGO-CNT-SnO2 based sensor having a response time of 8 s. whereas, Cu2O-rGO nanowires mesocrystals expressed the lowest LOD of 64 ppb to NO2. A chemiresistor based on MoO3-rGO composite exhibited a higher sensor response of 4120 to 50 ppm of H2S with a response/recovery time of 60 s/120 s at a low operating temperature of 160 °C. A 3D array structure (1D SnO2 and 2D graphene sheets) based H2S sensor exhibited better response/recovery time but the operating temperature of this sensor device was 260 °C.Graphene materials and their composites offer an attractive way to fabricate sensor devices for different gases and vapors. These graphene based sensors exhibited excellent performance compared to all other sensors in terms of sensitivity, limit of detection and reversibility. These sensors have the advantage over other sensors due to their low energy consumption and ability to operate at room temperature. The unique properties of graphene such as high surface area, mechanical strength and better electrical/temperature tolerance make this material a promising candidate for developing gas sensors which can operate in extreme conditions.Apart from a large number of advantages, there are still some issues which need to be addressed for fabricating graphene sensors on a commercial scale. Highly sophisticated techniques are required for the fabrication of ultrathin graphene layers. The simple and widely used fabricating techniques such as spin coating, drop casting or inkjet printing cannot be applied for synthesizing ultrathin graphene layers. Moreover, the number of graphene sheets is also very difficult to control.


As is well established, mastery to precise control of the layer number, stacking order of graphene, and the size of single-crystal monolayer graphene is very important for both fundamental interest and practical applications. In this report, millimeter-sized single-crystal monolayer graphene has been synthesized to multilayer graphene on Cu by chemical vapor deposition. The relationship of the growth process between monolayer graphene and multilayer graphene is investigated carefully. Besides the general multilayer graphene with Bernal stacking order, parts of multilayer graphene with non-Bernal stacking order were modulated under optimized growth conditions. The oxide nanoparticle on the Cu surface derived from annealing has been found to play the key role in nucleation. In addition, the hydrogen concentration impacts significantly on the layer number and shape of the graphene. Moreover, a possible mechanism was proposed to understand the growth process discussed above, which may provide an instruction to graphene growth on Cu by chemical vapor deposition.The online version of this article (doi:10.1186/s11671-015-1164-0) contains supplementary material, which is available to authorized users. Graphene, a monolayer of carbon atom arranged in a two-dimensional hexagonal lattice, has attracted increasing attention due to its novel properties , 2 and tHowever, in the CVD growth process, single-crystal monolayer graphene always accompanies multilayer graphene . What isIn this report, we report an approach to synthesize large-sized single-crystal monolayer graphene and multilayer graphene with different stacking orders on Cu by CVD. By controlling the growth parameters, millimeter-sized single-crystal monolayer graphene grew on Cu by CVD. Furthermore, multilayer graphene with Bernal stacking order and non-Bernal stacking order was also synthesized under optimized growth conditions. In addition, the relationship of the growth process between monolayer graphene and multilayer graphene is investigated carefully. The oxide nanoparticle on the Cu surface derived from annealing was found to play an important role in nucleation, while the hydrogen concentration impacted greatly on the layer number and shape of the graphene. Finally, a possible mechanism was proposed to reveal the growth process, which may advance our understanding on the growth of the large-sized single-crystal monolayer graphene and multilayer graphene with different stacking order.2 gas. Next, the Cu foil was placed in a quartz tube in the furnace and heated to 1080 °C with flowing 300 sccm Ar, and then the temperature was held for 4 h with flowing 300 sccm Ar and 50 sccm H2. Graphene growth was carried out by starting the CH4 flow at 0.5 and 50 sccm H2 with 3 h in the same tube furnace following completion of the annealing/reduction step of the Cu foil. The sample was cooled down quickly to room temperature by opening the furnace under 300 sccm Ar and 4 sccm H2 after growth. The process to grow multilayer graphene is similar to the method shown above. The main difference are shown as follows: the annealing time is 3 h in the annealing/reduction step, the CH4 and H2 flow is 0.5 and 25 sccm, respectively, with 0.5 h in the growth process. The method to transfer graphene grown on copper foils is similar to the previous report [The synthesis of graphene was carried out in a split tube furnace using CVD. The typical process to synthesize large-sized single-crystal monolayer graphene is shown as follows. The Cu foils  used as substrate were cut into a small rectangle shape with a size of 2.5 cm. The Cu foil was then etched in dilute hydrochloric acid and cleaned by acetone under ultrasonic and then dried by flowing Ns report .The Raman spectra were recorded at room temperature using HORIBA Jobin Yvon Evolution with laser excitation at 532 nm with power less than 5 mW. The optical microscopy was characterized with the Olympus BX51M in reflection mode at room temperature. The scanning electron microscopy (SEM) was characterized by Hitachi SU-8010. The element analysis was recorded by energy-dispersive X-ray spectroscopy  attached on SEM. Transmission electron microscopy (TEM) attached with selected area electron diffraction (SAED) was characterized by JEOL JEM-2010 TEM.4 and 50 sccm H2 were introduced to synthesize a single-crystal monolayer graphene. A series of experiments were carried out, and the typical results are shown in Fig. In general, the treatment of the Cu foil is found to be a critical step to grow high-quality graphene. At present, various methods have been developed to grow large-sized single-crystal monolayer graphene, for instance, suppressing evaporative loss of Cu , preanne2/Si substrate  [2/Si substrate; it can be seen that the graphene domains are uniform with the size of 1.2 mm except some impurities produced in the transferred process. As is well acknowledged, the Raman spectroscopy is an excellent approach to evaluate the quality, thickness, and uniformity of the graphene [2/Si substrate was performed and the typical result is shown in Fig. −1, respectively. Meanwhile, the I2D/IG is about 1.80 and the 2D band exhibits a single Lorentzian curve with a full width at half maximum (FWHM) of ~32.3 cm−1, confirming that it is a single-layer graphene. On the other hand, the intensity of the disorder-induced D-peak (~1345 cm−1) is very weak, indicating the high quality of the graphene. To further confirm the crystallinity of the graphene domains, selected area electron diffraction (SAED) patterns were performed and the results are shown in Additional file SEM was employed to further reveal the structure of the samples, before which the graphene domains on Cu foil were transferred to the SiO 300 nm) . Figure graphene , 32. TheBesides the large-sized single-crystal monolayer graphene, another interesting phenomenon is found when the hydrogen concentration decreases in the growth process. A series of experiments with hydrogen concentration from high to low in the growth process are performed, and the typical results are shown in Additional file To further characterize the sample, optical microscopy has been conducted and the results are shown in Fig. 2/Si substrate was characterized by Raman spectroscopy, and the typical results are displayed in Fig. −1, respectively, with a weak disorder-induced D-peak located at ~1345 cm−1, indicating the high quality of the multilayer graphene. To identify more details, the 2D band has been deconvoluted by Lorentzian function. Additional file 2D/IG value of ~1.73, and the 2D band exhibits a single Lorentzian curve with the full width at half maximum (FWHM) of ~28.66 cm−1, corroborating its single-layer structure. The red circle area shown in Fig. 2D/IG value of ~0.73, and the 2D band exhibits four fitted Lorentzian curves with the FWHM of ~24.57 cm−1, indicating it is a bilayer graphene. The blue circle area shown in Fig. 2D/IG value of ~0.50, and the 2D band exhibits six fitted Lorentzian curves with the FWHM of ~22.56 cm−1, indicating it is a trilayer graphene. And the pink circle area is a tetralayer structure. From single-layer graphene to tetralayer graphene, the positions of the G band are 1589.3, 1582.8, 1582.3, and 1581.8 cm−1, respectively, decreasing as the layer number increases as shown in Additional file −1, respectively, increasing as the layer number increases as shown in Additional file Then, the multilayer graphene transferred on the SiO2/Si substrate are shown in Additional file −1) is weak, indicating that the quality of the multilayer graphene is very high. From the outer to inner position, the Raman shift of G band are 1589.4, 1585.6, 1583.2, and 1583.2 cm−1  to inner regions were collected and are shown in Additional file 2D/IG value increases from 1.79, 2.97, 3.23, to 3.79. And the intensity of 2D band increases linearly with a slope of 3229.3 as shown in Fig. Surprisingly, besides the Bernal stacking order of the multilayer graphene shown above, some multilayer graphene with another stacking order are observed. The optical microscopy images of the typical sample transferred to the SiOTherefore, as the hydrogen concentration decreases in the growth process, the multilayer graphene domains could be obtained and the size of the multilayer graphene in the center region increases. Most of the multilayer graphene have a Bernal stacking order; however, parts of multilayer graphene have non-Bernal stacking order. This phenomenon is interesting because precise control of the layer number and stacking order of graphene are very important to both fundamental interest and practical applications.From the experiments shown above, large single-crystal monolayer graphene and multilayer graphene with different stacking order could be synthesized on Cu by CVD under optimized growth conditions. However, how can we understand the phenomenon shown above? What is the relationship of the growth mechanism between monolayer graphene and multilayer graphene? Given the findings in our experiments and the facts in previous reports , 27, 35,Large-sized single-crystal monolayer graphene has been synthesized to multilayer graphene with Bernal stacking order and non-Bernal stacking order on Cu by CVD under optimized growth conditions. The oxide nanoparticle derived from the mild oxidation residual on Cu surface plays an important role in nucleation and controls the density of the graphene domains. While the hydrogen concentration impacts greatly on the shape and layer number of the graphene. The relationship of the growth process between monolayer graphene and multilayer graphene is investigated carefully. Furthermore, a possible mechanism based on on-top growth mechanism was proposed to understand the growth process, which may have a great significance on the growth of graphene domains with a different size, layer number, and stacking order.


Social anhedonia (SA), a trait-like disinterest in social contact and diminished capacity to experience pleasure from social interactions, is consistently associated with social impairments in both healthy and clinical populations. However, the mechanisms underlying the relationship between SA and social impairment are poorly understood. Attentional control, selecting and focusing on relevant information and inhibiting irrelevant, may be one such mechanism. We examined individual differences in SA, attentional control, and social impairment in 108 healthy adults. High SA related to low attentional control and high social impairment. Moreover, attentional control mediated the relationship between SA and social impairment, establishing attentional control as one mechanism underlying aberrations in the fundamental human need for social contact. Although both attentional deficits and social impairment have been separately noted in SA, the relationship between SA, attentional control and social impairment in this non-clinical sample reflects a novel contribution. The desire for frequent and meaningful social interactions is a fundamental human motivation . Social why high SA individuals have poor social functioning is not entirely known. One proposal is that the consequences of SA may be related to deficits in attentional control, also termed effortful control, executive control, or cognitive control, and operationalized as the capacity to engage the inhibitory functions necessary to maintain task-relevant processing and goal oriented behavior , the underlying reason for behavior . A core behavior . Social behavior , responsbehavior , and resbehavior , indicatbehavior , and othbehavior , in whicSocially anhedonic but otherwise healthy individuals are impaired on tasks requiring attentional control, such as the Stroop paradigm  and the To test whether attentional control underlies the relationship between SA and social impairment, the present study investigates the relationship between individual differences in SA, attentional control, and social impairment in a large, representative, community sample. Specifically, we investigated whether attentional control mediates the relationship between SA and social impairment. Mediation analysis provides a meaningful statistical method for describing the mechanisms through which one variable exerts an effect on another . We asseOne hundred and eight individuals participated in the study. Participants were recruited from the Greater Boston area via flyers posted in local cafes, community centers and on college campuses, as well as advertisements posted to online billboards . Respondents to these flyers/advertisements contacted the lab and were scheduled for a screening appointment. Two hundred and twenty six individuals were screened for participation using the Structured Clinical Interview for DSM-IV Axis I Disorders , the StrTable 1.Of the 226 individuals screened for participation, 106 were excluded due to current/past Axis I disorders, three were excluded due to current/past personality disorders, and nine did not complete the study (missed appointments/withdrew following consent). Thus, a total of 108 participants met inclusion criteria for the study and completed the three study measures of SA, attentional control, and social impairment (detailed below). Demographics and sample characteristics of these 108 participants are presented in Harvard University Institutional Review Board approved the study. Participants gave written informed consent and were paid for their participation.Participants completed the RSAS  – a 40 iTable 2. We used total ACS score in our primary analyses and conducted follow-up analyses using the three subscale scores.Participants completed the ACS , a 20-itTable 2). The ACS has also been shown to have good convergent and divergent validity with attentional and personality tests respectively , specifically attentional focusing . This provides additional data that the ACS is a valid measure of attentional control mechanisms.The ACS has high internal consistency; coefficient alpha for the ACS total score is high in this sample (α = 0.90), and comparable to prior literature , as are ectively . Additioectively . We condParticipants completed the SAS-SR , a selfrFull scale IQ scores were estimated using the matrix reasoning and vocabulary subtests of the WASI .t-tests were used to assess gender differences and bivariate Pearson correlations were calculated to assess relationships between all variables. Due to three subjects missing data for one or more variables, the sample size used for the mediation analysis was 105.Data analysis was conducted with IBM SPSS 19.0. Chi square analysis and independent sample z-test), bootstrapping does not assume a normal distribution of the indirect effect .Here we assessed two mediation models: a single mediator model testing the effect of SA on social impairment through overall self-reported attentional control, followed by a multiple mediator model to determine the specific indirect effects of the three aspects of attentional control  on the relationship between SA and social impairment. We conducted bootstrap analysis with the SPSS macro INDIRECT from Table 1 presents demographics and sample characteristics. There were no gender differences on any demographic or self-report measures. SA spanned the full range of possible scores (0–40), and had acceptable spread (interquartile range = 19). 35 participants (32%) – 20 female  and 15 male  – were high deviant scorers per the cut offs reported by Figure 1.Table 3 presents bivariate Pearson correlation coefficients between IQ and all self-report variables. SA, attentional control, and social impairment were all significantly intercorrelated in the predicted directions; higher SA related to lower attentional control and greater social impairments, and lower attentional control related to higher social impairment. IQ did not significantly relate to any variables, indicating that these relationships are not due to IQ.Figure 2A): SA had a total positive effect on social impairment , and a total negative effect on attentional control ; attentional control had a direct negative effect on social impairment . Bootstrap analysis of the indirect effect (Table 4A) revealed a bias corrected 95% confidence interval excluding zero , demonstrating that attentional control mediates the relationship between SA and social impairment. The direct effect of SA on social impairment, controlling for attentional control, remained significant , indicating that attentional control only partially mediates the relationship between SA and social impairment. The mediated effect of SA on social impairment through attentional control accounts for 19% of the variance in social impairment .We assessed the single mediator model in which attentional control is hypothesized to mediate the relationship between SA and social impairment. All four paths were significant in the predicted directions . SA had negative effects on all three components of attentional control: focusing , divided , and shifting attention . However, only focusing attention had a significant indirect effect on social impairment ; all other paths between components of attentional control and social impairment were non-significant . Bootstrap analysis (Table 4B) of the specific indirect effect of focusing attention on the relationship between SA and social impairment revealed a bias corrected 95% confidence interval excluding zero . Confidence intervals for the specific indirect effects of divided attention and shifting attention both included zero, indicating that the relationship between SA and social impairment is partially mediated by one specific aspect of attentional control – focusing attention. Pairwise contrasts revealed the specific indirect effect through focusing attention is larger in magnitude than the specific indirect effect through divided attention. All other pairwise contrasts were non-significant (Table 4B).We examined the specific indirect effects of the three components of attentional control in a multiple mediator model (p < 0.001) and a direct negative effect on attentional control . However, attentional control did not have a direct effect on SA in this reverse model  and the confidence intervals for the indirect effect of attentional control on SA through social impairment included zero , indicating that the model in which social impairment leads to SA via attentional control is not supported by the data.Our theoretical model, based on prior literature, proposes that SA leads to social impairment via attentional control deficits. However, it is possible that the opposite is true – that social impairment predicts SA via attentional control deficits. Although we cannot speak to causal directions in the absence of longitudinal data, we tested this reverse mediation model in which social impairment is hypothesized to lead to SA via attentional control. Social impairment had a direct positive effect on SA  and others’ mental states ToM; , processClearly, social functioning is complex and multiply determined; a comprehensive model of how SA impacts social functioning would need to incorporate multiple neurocognitive and social cognitive factors . Here weLimitations must be acknowledged. The sole use of self-report assessments in the current study design could have resulted in inflated relationships due to common measurement bias, thus replication samples and follow-up studies using alternative measures are necessary. In particular, the assessment of attentional control using a self-report questionnaire, although partially validated here in a sub-sample of participants and in prior literature, is less than ideal. Future studies employing behavioral assessments of attentional control are necessary. Similarly, the ability of high SA individuals to self-report their level of SA could be confounded by accompanying deficits in self-reflection/metacognition. As discussed above, future research should include measures of metacognitive processes in order to delineate the unique contribution of SA and metacognition to social impairment. Finally, our sample – although representative of the Greater Boston Community – was comprised solely of healthy individuals, warranting caution when generalizing the results to patient populations. Parallel investigations of the relationship between SA and social impairment in patient samples are needed.physical anhedonia and attention in a longitudinal follow-up of individuals at genetic risk for schizophrenia (It is important to note that the current study is unable to establish causal directions between variables because of its cross-sectional design; a statistically significant mediated effect does not determine the causal direction of a relationship . We condophrenia . ResultsThe current study demonstrates that the relationship between SA and social impairment is partially mediated by attentional control. This has implications for our understanding of a fundamental human desire, the need to belong, and suggests attentional control is one of the mechanisms necessary for successful social interactions. This finding also illuminates one of the mechanisms underlying the relationship between a well-established negative symptom of schizophrenia and social impairment.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Raman spectroscopy is among the primary techniques for the characterisation of graphene materials, as it provides insights into the quality of measured graphenes including their structure and conductivity as well as the presence of dopants. However, our ability to draw conclusions based on such spectra is limited by a lack of understanding regarding the origins of the peaks. Consequently, traditional characterisation techniques, which estimate the quality of the graphene material using the intensity ratio between the D and the G peaks, are unreliable for both GO and rGO. Herein we reanalyse the Raman spectra of graphenes and show that traditional methods rely upon an apparent G peak which is in fact a superposition of the G and D’ peaks. We use this understanding to develop a new Raman characterisation method for graphenes that considers the D’ peak by using its overtone the 2D’. We demonstrate the superiority and consistency of this method for calculating the oxygen content of graphenes, and use the relationship between the D’ peak and graphene quality to define three regimes. This has important implications for purification techniques because, once GO is reduced beyond a critical threshold, further reduction offers limited gain in conductivity. D/IG ratio, which has been validated as a measure of inter-defect distance in graphene, is unreliable when applied to GO and rGO131415D/IG ratio of amorphous carbon (that was still sp2 bonded) would increase with the removal of defects, proportional to the square of the crystallite size2 even in raw GOGraphene has become a primary research focus in many current fields of science as evidenced by the recent announcement from the European Commission of €1 billion funding for the commercialisation of graphene research over the next ten years2357891011app), it is impractical to measure the position or intensity of the D’ mode. The second order transition  does not coincide with other modes and is an allowed mode even without defects, in the same way that the 2D is, and so can be observed even in pristine graphene. After GO is reduced the 2D’ peak occurs at higher energy, shifting by as much as 40 cm−1 from GO to grapheneapp) positions (2D’ − Gapp) increases. Also, by simply halving the energy of the 2D’ mode we can get the energy of the inferred D’ mode (D’inf) as it is not expected to vary as greatly as the 2D and D modes caninf  − Gapp) is more reliable than the absolute 2D’ or D’inf position, which is subject to natural variation within samples, laser spot locations and measurement conditions. D/IGapp ratio is not clearly correlated to the C/O atom ratio for the same samples  for the as-produced GO sample , allowing for some minor dispersion in the actual G peak. Thus, the net dispersion of Gapp is most likely due to the dispersion of the D’ peak.Excitation-energy-dependent position shifts (dispersion) of the Gk of rGO . The disapp peak). This superposition renders the ID/IGapp ratio an unreliable measure for the reduction of GO, and has prevented it from accurately describing some of the recently reported GO and rGO samples. We propose that the best measure of GO reduction is the difference in the positions of the D’inf and Gapp peaks, i.e. D’inf  − Gapp or the equivalent 2D’ − Gapp. The strong correlation that exists between the C/O atom ratio and D’inf − Gapp, illustrated for a range of GO and rGO samples from different laboratories and using different reduction techniques  or 2 accumulations (633 nm and 785 nm). For spectral analysis each peak was fitted to a single Lorentzian  for 2 or 6 s. For the chemical reduction ascorbic acid was mixed with GO in a ratio of 10:1 by mass, and stirred at 85 °C for ~24 h until the reaction reached completion, then heated in the microwave at 150 °C for 30 min (ramp rate 1 °C min−1). The same process was used for reductions with ethylene glycol. Samples of 1 mgml−1 GO in ethylene glycol were prepared and diluted with twice as much deionised water by volume prior to reduction as above. Pristine single layer graphene was purchased and used as is from Graphenea.GO samples were synthesised using the modified Hummers method as described in9How to cite this article: King, A. A. K. et al. A New Raman Metric for the Characterisation of Graphene oxide and its Derivatives. Sci. Rep. 6, 19491; doi: 10.1038/srep19491 (2016).