Meanwhile, the extinction ratio of bistable states in the absorber reaches an ultrahigh value of 47.3 dB at 1545.3 nm. Moreover, the influence of changing the structural parameters on the bistable behaviors is discussed in detail, showing that the structure can tolerate structural parametric deviation to some extent. The proposed bistable structure with ultra-compact size, low thresholds, high extinction ratio, and ultrafast response time could be of great applications for fabricating high-performance all-optical-communication devices.A kind of graphene-based perfect absorber which can generate low-threshold and high-extinction-ratio optical bistability in the near-IR band is proposed and simulated with numerical methods. The interaction between input light and monolayer graphene in the absorber can be greatly enhanced due to the perfect absorption. The large nonlinear coefficient of graphene and the strong light-graphene interaction contribute to the nonlinear response of the structure, leading to relatively low switching thresholds of less than 2.5 MW/cm Q factor is required to generate bistable states, which limits the applications of OB. Therefore, materials with large nonlinear coefficients are needed to overcome this drawback.Optical bistability (OB) is a typical phenomenon of nonlinear optics. In some resonant structures with third-order nonlinear optical materials, one given input state has two corresponding output states. Based on this unique property, OB can be applied to the fabrication of all-optical switches, memory cells, and transistors ,3,4,5,6,−12 or even 10−11 m2/W, which is about 107 to 109 times larger than that of bulk dielectrics [Graphene, as a novel 2D material, has been studied intensively over the years due to its outstanding physical properties ,14, suchlectrics ,16,17. Slectrics ,24,25,26lectrics ,32,33,34lectrics ,37,38,39lectrics ,52,53,54lectrics ,56, and 2, and the extinction ratio is over 40 dB. The proposed structure is easy to fabricate and has a toleration to structural parametric deviation, making it practical for the real applications of high-performance graphene-based bistable devices.In this article, we demonstrate graphene-based OB with a low threshold and high extinction ratio by using a graphene-based perfect absorber. The absorber supports guided-mode resonance (GMR) in the near-IR band, which strongly enhances the electric field intensity in the graphene. With the significant light-graphene interaction as well as the large nonlinear coefficient of graphene, the switching threshold of the bistable states is cut down to a relatively low level. Moreover, by optimizing structural parameters to satisfy the critical coupling condition, the extinction ratio of bistable states reaches an ultrahigh level. As is demonstrated by the simulation, the switching threshold can be lower than 2.5 MW/cmp = 1138 nm, the grating thickness h = 67 nm, and the grating width w = p/2. The thickness of the silica layer d = 1200 nm, and the thickness of the gold substrate is 200 nm. The thickness of the graphene 16 s−1 and γ = 8.17 × 1013 s−1 [fE is the Fermi level, τ is the relaxation time and T is the temperature. In the simulation of this work, fE is set to 0.1 eV, considering that graphene might be slightly doped even if gate voltage is absent [τ and T are set to 200 fs and 300 K, respectively, which are typical values for graphene relaxation time and environmental temperature. The total surface conductivity of graphene is 2/W, which is a moderate value for Z-scan measurement in the near-IR band [−14 m2/V2. The nonlinear coefficient of graphene is several orders larger than that of bulk silica, so here silica can be treated as a linear dielectric. Besides the optical Kerr effect, we also consider the saturable absorption effect of graphene, which is necessary for intrinsic or lowly doped graphene [2 and the modulation depth of saturable absorption is k (which can be derived from the conductivity of graphene) with I is treated as time-average power flow and is calculated by I = |E|·|H|/2. Apparently, the saturable absorption effect of graphene leads to a decrease in graphene’s absorption coefficient with increasing incident intensity, which should be carefully considered when optimizing structural parameters.1013 s−1 . The int1013 s−1 :(1)σintrs absent . τ and T-IR band ,16, and graphene . The absurements . The sumxy plane is built since the structure is uniform along the z direction. Periodic boundary condition is added to the x direction of the model so that only one structural period needs to be simulated. A port is added above the grating to give a normal plane wave incidence with a certain intensity and scattering boundary condition is added below the gold layer.The model of the proposed structure is defined and simulated in the FEM software COMSOL Multiphysics. In the simulation, a 2D model of the structural cross-section in the x or y component, so only the z component of normalized electric field , the reflectance spectrum almost coincides with the linear one. When the incident intensity is strong enough (1.5 and 1.75 MW/cm2), we see a red shift of the resonant wavelength which is caused by the shift of graphene’s relative permittivity, and meanwhile, the reflectance suddenly jumps up as the incident wavelength is swept over the resonant wavelength from left to right, indicating the appearance of bistable states.It is known that the rise of incident intensity has a trend to break the spectral symmetry of a resonant structure with Kerr material, with the resonant wavelength shifting away from the initial value and the spectral curve becoming steeper and steeper on one side of the resonant wavelength. 2 (1.68 MW/cm2) for 1545.1 and 1545.6 nm, respectively. These relatively low thresholds are the main contributions of the strong GMR as well as the large nonlinear coefficient of graphene. The simulation results show that when the incident wavelength is between 1545.1 and 1545.6 nm, the extinction ratio of bistable states is greater than 30 dB, and when the wavelength is 1545.3 nm, the extinction ratio reaches the maximum value of 47.3 dB. As mentioned before, the absorption coefficient of graphene has a trend to decrease with increasing light intensity, so the critical coupling condition  within the simulation range. The peak absorption of the structure shows better stability to the deviation of d compared with the resonant wavelength. As is shown in the graph, the structure maintains a considerable peak absorption of over 98% within the simulation range, which means that the extinction ratio of bistable states at 1.75 MW/cm2 is very stable, even if d deviates 30 nm away from the optimized value. d = 1170, 1200, and 1230 nm at their corresponding working wavelengths, which are 1541.7, 1545.6, and 1549.2 nm, respectively. Bistable characteristics are clear enough for all cases. Apparently, the switching-off-to-on thresholds are maintained at nearly 1.7 MW/cm2, and the extinction ratios at 1.75 MW/cm2 are still very high, which are 18.8 and 19.7 dB for d = 1170 and 1230 nm. The switching-on-to-off threshold decreases with d increasing, which has a trend to slightly cut down the structural Q factor.h varying from 52 to 82 nm with a step of 5 nm. Other structural parameters are still the same as those in 2. The change of h has only a slight influence on the resonant wavelength, which shifts no more than 1.5 nm within the simulation range. Its influence on the peak absorption is much stronger compared with that of the silica layer thickness. The peak absorptions drop to 93.4% and 95.7% for h = 52 and 82 nm, respectively. Moreover, the jump of reflectance at the right side of the resonant wavelength is weakened with h increasing, indicating that the structural bistable behaviors will be affected. h = 52 nm, 67 and 82 nm at their corresponding working wavelengths, which are 1544.3, 1545.6, and 1546.5 nm, respectively. The extinction ratios at 1.75 MW/cm2 are 13.5 and 13.7 dB for h = 52 and 82 nm, respectively. Apparently, the hysteresis loop sharply shrinks with h increasing since h directly determines the mode leakage rate of the structure and has a tremendous influence on the structural Q factor. It is predictable that the hysteresis loop is going to vanish if h further increases unless a higher δ varying from 0.3 to 0.7. Likewise, other structural parameters are the same as those in 2. The structure shows significant stability to the change of δ since the spectrum only shifts a little with δ varying from 0.4 to 0.65. δ = 0.3, 0.5, and 0.7 at their corresponding working wavelengths, which are 1544.9, 1545.6, and 1545.9 nm, respectively. Even if δ extremely deviates away from the value of 0.5, the structure still possesses clear bistable characteristics. Similar to that in 2.Q factor of the absorber and can be calculated by Q factor is 530 at a low incident intensity and close to 1000 at extremely high incident intensity when the absorption of graphene is saturated, so Finally, we would like to discuss the response time of the OB in the proposed graphene-based absorber, which is decided by both the graphene’s Kerr effect and the photon lifetime of the absorber. The photon lifetime τR=Q/2πv . For thes 100 fs , as was The proposed perfect absorber can be easily fabricated with standard nanofabrication technology. The gold layer can be deposited onto a chromium-coated silicon substrate by magnetron sputtering. The silica layer can be deposited onto the gold layer by plasma-enhanced chemical vapor deposition (PECVD). The monolayer graphene can be obtained by mechanical exfoliation and can be transferred onto the top of the silica layer. A PMMA film can be spin coated onto the graphene, and the designed grating patterns can be formed by E-beam lithography ,30. We b2 according to the simulation. Meanwhile, the extinction ratio of bistable states exceeds 40 dB at certain incident intensities. The structure is compact and can be easily fabricated with standard nanofabrication technology, which is practical for designing and fabricating graphene-based bistable devices and all-optical switches in the near-IR band.This work demonstrates a graphene-based perfect absorber that possesses bistable characteristics in the near-IR band. By coupling monolayer graphene with dielectric grating patterns, the electric field intensity is highly enhanced within the graphene. The strong light-graphene interaction, together with the large nonlinear coefficient of graphene, highly contributes to low switching thresholds, which can be lower than 2.5 MW/cm


Among the state-of-the-art ammonia detection materials, graphene-based polymeric nanocomposites have gained significant attention. Despite the ever-increasing number of publications on graphene-based polymeric nanocomposites for ammonia detection, various understandings and information regarding the process, mechanisms, and new material components have not been fully explored. Therefore, this review summarises the recent progress of graphene-based polymeric nanocomposites for ammonia detection. A comprehensive discussion is provided on the various gas sensor designs, including chemiresistive, Quartz Crystal Microbalance (QCM), and Field-Effect Transistor (FET), as well as gas sensors utilising the graphene-based polymer nanocomposites, in addition to highlighting the pros and cons of graphene to enhance the performance of gas sensors. Moreover, the various techniques used to fabricate graphene-based nanocomposites and the numerous polymer electrolytes , the ion transport models, and the fabrication and detection mechanisms of ammonia are critically addressed. Finally, a brief outlook on the significant progress, future opportunities, and challenges of graphene-based polymer nanocomposites for the application of ammonia detection are presented. The increasing demand to mitigate the alarming effects of the emission of ammonia (NH Graphene has been considered an excellent nanofillers for constructing high performance composite materials due to its unique 2D structure and the remarkable physicochemical properties. Among its exceptional properties include the high mechanical strength with a high Young’s modulus of 1 TPa and a high tensile stress of 130 GPa [2/Vs, which is higher compared to those of the broadly used semiconductors and silica [2/g [2. In view of this, graphene has shown tremendous potential to improve the properties of sensing and biosensing devices. Thus, graphene-based materials, such as Graphene Oxide (GO) and reduced Graphene Oxide (rGO), have been extensively utilised in ammonia gas sensing due to the fact of their unique structures and functionalities [Graphene originates from the word “graphite”, while the suffix “-ene” reflects the allotrope of carbon that consists of stacked graphene layers. This form of carbon derivative consists of two-dimensional 2D) sheets of spD sheets d silica , a largeica [2/g , and theica [2/g . Graphennalities ,7,8. Granalities ,10,11. Tnalities ,13.3) gas is one of the largest produced toxic Volatile Organic Compounds (VOCs) in the industrial sector, apart from benzene, toluene, and others. It is a ubiquitous gas that is produced naturally in soils from bacterial metabolism and the decaying processes of plants, animals, and animal wastes. It can also be observed in the natural environment, such as in soil, air, and water, due to the involvement of NH3 in the nitrogen cycle, as shown in 3 has been broadly utilised in the production of explosives, fertilisers, plastics, fabrics, pesticides, dyes, and as an industrial coolant. Nevertheless, the widespread use of NH3 on farms, such as in fertilizer to cultivate soil and farmland, in cleaning products for households, and in industrial and commercial applications, such as glass cleaning, cooking grease solution, wine stain beakers, waste and wastewater treatment, cold storage, and stabilisers, signifies a frequent exposure of NH3 through accidental discharge, erosion, mechanical failure, construction defect, nitrification by nitrogen-fixing bacteria, and combustion of fossil fuels in both chemical and transportation industries [Ammonia –(3) [Generally, NHn humans ,17. An a (1)–(3) ,19. Usua (1)–(3) .2NH3 + 3 gas sensors, research on graphene-based sensors is still ongoing and various aspects, including the process, the mechanism of sensors, and new materials, have not been explored extensively. Recently, Tang et al. (2021) reported the development and progression of functionalised graphene sensors for NH3 detection at room temperature (RT) and its sensing mechanism using graphene and other nanoparticles and polymers. Several challenges that hinder the mass production of such sensors were highlighted along with a number of proposals that address these problems as well as the potential opportunities and prospective applications of graphene-based NH3 sensors [3 gas sensors, including GO, graphene, carbon nanofibers and related materials. The paper discussed the sensing characteristics of carbon nanomaterials-based gas sensors, analysed the various techniques of NH3 gas sensors, the problems related to the sensors recovery, and the effect of relative humidity on the sensing behaviour of carbon nanomaterials [Sensors are analytical tools, which consist of an active sensing material with a signal transducer, that detect changes in their environment and send the information to other electronics, usually a computer processor, for data acquisition and interpretation. The first developed sensors for the detection of organic vapours, methanol, and formaldehyde were introduced by Sumner et al. 1923) . To date sensors . In anotaterials . Another . To dat3 gas sensor application, which includes a thorough explanation of the principle and designs of gas sensors, a summary of the role of graphene-based materials for enhancing a gas sensor’s performance for NH3 detection based on recent studies, and the synthesis of graphene-based materials. The diverse range of methods used to fabricate NH3-based graphene–polymer nanocomposites is also critically addressed. In addition, this review provides an extensive analysis of the mechanism of graphene-based polymer nanocomposites and the various polymer electrolytes applied for NH3 detection. Finally, this review concludes by addressing the future perspectives and challenges regarding gas sensors for a safe and sustainable environment.Based on the discussion above, the fabrication of graphene-based polymeric nanocomposite remains a challenge mainly attributed to the agglomeration of graphene 2D materials particularly at higher concentrations. To overcome this, several solution processing techniques can be used to achieve the homogenous distribution of graphene within the polymeric matrices. In addition, the synthesis of high-quality graphene plays an important role in the dispersion and, hence, improves the final sensitivity of the sensor. In an attempt to provide a comprehensive insight of the recent progress of graphene-based polymeric nanocomposites, this review was conducted. It summarises the latest updates on the use of graphene-based polymer nanocomposites in gas sensing applications, specifically its latest application, gas sensor development, synthesis of graphene, and mechanism. While various past reviews have addressed graphene-based polymer nanocomposites, this paper particularly discusses the recent advances in graphene-based polymer nanocomposites for NH2 2s2 2p2 in which the energy gaps between the 2s and 2p orbitals are narrow, promoting one 2s electron to jump to a vacant higher energy 2p orbital. The electron excitation allows carbon to hybridise into sp, sp2, and sp3 configurations, leading to a variety of molecular structures. Every orbital configuration has a specific molecular geometry. For instance, the sp orbital configuration forms circular regions or nodes to represent the s orbital and the dumbbell structure of the px, py, and pz orbital, which forms chain structures. Meanwhile, the sp2 and sp3 orbitals form planar and tetrahedral structures, respectively. All graphene derivatives exhibit the sp2 and sp structure due to the fact of their hexagonal ring structure with layers of the crystalline honeycomb lattice. The 2pz orbital of the carbon atoms can imbricate successfully if they are parallel or out-of-plane π bonds, contributing to the lowest energy of the graphene sheet when it is completely flat. Furthermore, the π orbital in graphene located specifically at the double-bonded C=C region is scattered throughout the graphene sheet, making it highly mobile and thermally and electrically conductive. The distribution of π orbital is known as the delocalisation of electrons. The hexagonal lattice with π electrons tends to undergo electron delocalisation to stabilise the force due to the spreading energy over a larger area rather than confining it to a small area. The graphene layers in graphite are usually separated by a minimal gap of 0.335 nm from each other [Graphene is an allotrope of carbon that consists of unique hybridisation properties ,25. Genech other , as showSince its discovery in 2004, graphene and its derivatives have been constantly studied for their synthesis, functionalities, and applications. Over the years, various methods have been developed to synthesize graphene layers. For example, the mechanical exfoliation, or Scotch tape method, is a very straightforward process to produce graphene layers. Adhesive tape is the most important material needed for this method a. Basica4), hydrogen, and argon. A square inch of metal, such as Ni, Cu, Co, platinum (Pt), or iridium (Ir), is placed in a quartz-tube furnace. The metal is coated with sustainable and waste materials, such as butter, camphor (C10H16O), tea tree extraction , waste plastic in solid form, cookies, and chocolates [Chemical Vapour Deposition (CVD) is another essential method that can produce high-quality graphene layers. ocolates ,31. Withz orbitals and intense interactions of neighboring graphene layers. In addition, GO can be synthesised via the self-assembly process and hydrothermal method. Meanwhile, GO can be reduced to form rGO using several methods including the mechanical exfoliation of a single sheet of graphene film, CVD of graphene monolayers, epitaxial growth of graphene films, chemical or thermal reduction of graphene derivatives, and longitudinal “unzipping” of CNTs [Generally, GO consists of carbon allotropes with hydroxyl and epoxy groups on the interior side and edges of the structure, making it more hydrophilic. This would also provide GO with high solubility and reactive sites for the nucleation and growth of polymer in many organic solvents, especially water, leading to the formation of graphene-based hybrids. Nevertheless, the defection of the graphene is directly proportional to the electrical conductivity, which would support the widening application of GO in electrical materials and devices. The reduction process would increase the amount of unsaturated carbon, resulting in conjugation among benzene structures and a high electrical conductivity ,35. GO c of CNTs ,37.Liquid phase exfoliation is also known as graphite oxide exfoliation which usually originates from graphite oxide before obtaining GO and finally rGO b. The we3, nitrogen dioxide (NO2), carbon monoxide (CO), sulphur dioxide (SO2), and others, but also chemical warfare agents [The remarkable properties of graphene nanocomposites, including the large specific surface area and excellent conductivity, contribute to improving the ions’ mobility and, thus, the sensitivity of the sensors. Graphene nanomaterials provide excellent surface functionalities to various materials including fibres, films, etc. which improve the electroactivity of these composites. According to recent studies, graphene can detect not only well-known toxic gases such as NHe agents ,67,68,69e agents . AdditioGraphene/polymer nanocomposites can be fabricated through several methods, namely, in situ polymerisation, melt intercalation, electropolymerisation, self-assembly, and solution mixing ,72, as pSolution mixing on the other hand is an effective method to form composite graphene-based polymer composites. In this method, re the polymer is well-mixed in a solvent with graphene nanomaterials leading to an increase in the curvature of its curvilinear surface, the dispersity, and the solubility of the graphene . HoweverThe most commonly used graphene-based reinforcement materials include polycarbonate, polyester, epoxy, polyethene, polypropylene, polystyrene, polymethylmethacrylate, nylon, and Teflon ,83. The Polymers’ functionalised graphene nanofillers have received significant traction in the nanofabrication of polymer composites due their unique properties, such as mechanical, optical, magnetic, and electronic properties . The intD/IG = 0.25 or less) and a high carbon-to-oxygen (C/O) ratio (>20) compared to rGO and electrochemical exfoliated graphene [On the contrary, intercalated graphene occurs when fabricated materials/atoms are inserted between the graphene sheet layers. The intercalation structure prevents the agglomeration of the material. The addition of graphene sheets also assists in providing active sites and retaining the size growth of the material. Pure graphene sheets with great π-π interaction tend to collapse with each other. However, the intercalation structure can reduce the π-π stacking interaction between graphene sheets, making it favourable for three-dimensional (3D) mesoporous structures ,94. Meangraphene . Regardlgraphene .The exceptional properties of graphene highly contribute to the development of optical electronics , composi3 gas sensors possess several unique structural functionalities that enables the absorbance of gaseous molecules from the environment with improved sensing capabilities, such as low Limit of Detection (LOD), rapid response time, reproducibility, and stability. Among the proposed methods to effectively fabricate NH3 gas sensors using graphene-based polymer nanocomposites include sol-gel, hydrothermal or solvothermal, layer-by-layer deposition, template-assisted deposition, and physical vapour deposition. The aforementioned methods have been used to fabricate graphene-based compounds that contribute to its wide range of applications. Graphene nanocomposites based NH4-nSiXn compounds . In the organic route, “sol” is obtained by dissolving an alkoxide precursor in a specific solvent, meanwhile “gel” is acquired by the supervised addition of gelatine agents, such as water, that is initially under acidic or basic conditions to initiate the condensation reactions that lead to the formation of a 3D oligomeric network. Apart from the aforementioned addition process, the formation of a gel can be formed by the condensation process, solvent evaporation, and syneresis of a species. The transformation of films on the sensor substrate from the resultant gel is possible via dipping, spin-coating, or spraying techniques. The deposited film is attached to the substrate by treating it with a low-temperature annealing process and t prolonged heating to evaporate all organic residues and water molecules. The reaction starts with solvation, where the metal cation, Mz+, attracts water molecules to form the solvent of M (H2O)nz+ and is highly inclined to release proton (H+) to maintain the equilibrium.The sol-gel method is a wet chemistry-based synthesis route that involves the solidification of a compound containing a highly chemically active component through a solution in the form of sol or gel under mild conditions, followed by heat treatment. The method has recently been applied to fabricate glass, oxide coatings, and functional ceramic powders, especially high critical-temperature oxide superconductors and composite oxide materials that are complicated to prepare through conventional methods. Generally, the technique employs the hydrolysis and polycondensation of alkoxide-based metal precursors, for instance Rn), react with water.The next step in the sol-gel method is the hydrolysis reaction. The nonionising molecular precursors, such as metal alkoxide (M(OR)In the final step, the polycondensation reaction takes place, which depends on the type of removed molecules that gives off two routes of reactions: dehydration polycondensation and dealcoholisation polycondensation.3. The advantages of using the sol-gel method have been thoroughly discussed by Khorramshahi et al. (2018). The sol-gel method is said to be an inexpensive and low-temperature method, which is very suitable for the preparation of zinc oxide (ZnO) nanoparticles. The simplicity, large substrate coating area, and easy control of the film thickness also contribute to the convenience properties of the sol-gel method [3 nanocomposites were developed using a similar method [2) nanoparticles were also fabricated using the sol-gel method to detect NO2 gas at RT with the help of ultraviolet (UV) light [2 was prepared to obtain a colloidal suspension through the hydrolysis and polymerisation of a metal–organic precursor. Tung et al. (2019) also employed the sol-gel method to synthesise rGO in which the poly(ionic liquid) promoted the effective stabilisation and capping agent to control the nucleation growth and prevented the excessive agglomeration of nanoparticles during the synthesis [3O4 nanoparticles formed simultaneously via the sol-gel approach (in situ technique) or blending of the pre-synthesised Fe3O4 nanoparticles with an rGO matrix. In the following year, Wang et al. (2020) synthesised black-TiO2 (B-TiO2) carbon composite powders and fabricated thin film using the sol-gel technique [2 demonstrated an enhanced photocatalytic activity and gas sensing performance. The sol-gel method is practical and commonly used to fabricate chemical sensors for the detection of gas other than NHl method . In the r method . The resV) light . The TiOynthesis . In theiechnique . The preThe hydrothermal or solvothermal method is usually performed in the presence of polar solvents, such as water, ethanol, methanol, and acetic acid. The synthesis reaction can be conducted at a temperature range of 100–200 °C and a pressure range of ~1 atmosphere. The technique can go on below the supercritical temperature of the water, which is 374 °C. This method is suitable for the fabrication of a composites with specific morphologies and controlled hybrid nanostructure size. The synthesis route is also environmentally friendly because the reaction is carried out at low temperatures using a Teflon-lined stainless-steel autoclave to stimulate the reaction process with very minimal energy consumption. The advantages of using this technique include a greatly increased chemical reaction kinetics with minimal temperature change, the formation of new metastable materials, highly pure final products even from impure feedstock, eco-friendly, and able to prepare hybrid hydroxylated clays and zeolite, which cannot be prepared using other synthesis methods.2/prGO), which was also fabricated using the hydrothermal method, showed an incredible methane detection at RT with a response and recovery time of 5 and 7 min, respectively [2/partially rGO and Pd-doped SnO2/rGO thin film sensors.The hydrothermal process is a well-known method, especially in synthesising graphene-based composite materials. The ZnO nanoparticles that loaded onto 3D rGO (ZnO/3D-rGO) for carbon monoxide sensing, which were synthesised using the hydrothermal method, possessed a unique porous structure that exhibited the inherent properties of rGO flakes . The as-ectively . The aut2 nanorods–nanoporous graphene (NPG) synthesised using a similar method to detect CH4 for a detection limit as low as 1000 ppm at relatively low temperature (100–200 °C) [2O3 composite using the hydrothermal method for nitrogen dioxide (NO2) gas detection [2O3 can operate at RT, with a staggering sensor response of up to 1038%. Meanwhile, the use of a 3 wt% rGO-In2O3 composite sensor showed a better NO2 detection at an operating temperature of 74 °C and a sensor response of 1337%. In the same year, Ye et al. (2017) introduced the rGO-TiO2 hybrid material that was synthesised via the same method for NH3 detection at RT [2 hybrid material showed excellent sensing properties due to the porosity surface of the graphene, with a sensor response of 75%.Additionally, the hydrothermal method is considered as the most ordinary and simplest method for the combination of metal oxides on the graphene nanosheets. The one-pot hydrothermal method contributes to the generation of high crystallized nanostructures without post-synthetic annealing or calcination ,119. In –200 °C) . On the etection . The senon at RT . The syn2-rGO hierarchical porous nanosheets via the hydrothermal route for the detection of ethanol. Hydrothermal or solvothermal is considered the most reliable and controllable route to achieve the preconceived nanostructures. This method introduces GO flakes with a large surface area and abundant functional groups that provide a huge number of attachment sites on the GO surfaces and enable the easy nucleation of SnO2 [2-rGO nanosheets have shown a remarkable sensor response, response time, and recovery time of 77.1%, 9 s, and 457 s, respectively, at an operating temperature of 250 °C. Later in the same year, Wang et al. (2018) developed 2D rGO/WS2 heterojunction nanostructures for constructing NH3 gas sensor. Based on their research, a one-step hydrothermal method was used to effectively synthesise the material, which could perform at RT with an excellent selectivity and stability response to 10 ppm NH3 [3 gas detection via the simple surfactant-free method. The developed sensor exhibited a fast response and recovery time of 12 and 90 s [3 nanocomposites via the hydrothermal route for NH3 gas sensing application. The study revealed that the capability of the sensor to detect 100 ppm of NH3 gas within 37 s and recover within 711 s at 300 °C [3 gas at RT. It was stated that the rGO sheets conjoined to another through π-π stacking and hydrogen bonding creating a 3D scaffold during the hydrothermal process. The PANI nanowires interacted with and attached to some of the vicinity of GO sheets via a similar interaction forming a vast interconnected porous network and, thus, were beneficial to the diffusion of the gas molecules into the hybrid material [Furthermore, Zhao et al. (2018) synthesised SnO of SnO2 . The SnO ppm NH3  Meanwhiland 90 s , respectt 300 °C , as listmaterial .The Layer-by-Layer (LbL) deposition method is a highly versatile, simple, cost-effective, and efficient fabrication method of controlled layered structures from numerous component materials. Originally, this method was developed for planar structures and substrates. However, recent studies have used this approach for the development of spherical nanoplates, which produce an LbL assembly freely suspended in water . The synThe LbL deposition method has various merits as a surface modification technique for polymer fabrication such as rapidity, environmentally friendly, and economical process. Furthermore, the surface functionality can be directly tuned by selecting the proper polymer or polyelectrolytes, such as poly- (PET), poly  (PAH), poly(sodium styrenesulfonate) (PSS), polyethene glycol) (PEG), and poly (PDDA), as shown in Template-assisted synthesis method is an advanced technique for the fabrication of highly crystalline mesoporous and hybrid materials. It can also be used for the fabrication of hybrid nanomaterials with different dimensions, such as 2D or 3D structures. Common 3D structures that can be obtained from this method include hexagonal rod-like structures, lamellar structures, or honeycomb interconnected networks. This method can be further classified as hard-template, soft-template, and colloidal templates, which is the recent approach ,138. BasTo date, several templates have been widely used, including colloidal monolayer, Anodic Aluminium Oxide (AAO), Block Copolymer (BCP), and monoprint mould, as shown in 2 hybridised content of carbon containing BCPs in the hydrophobic segments. This newly designed copolymer is relatively stable, and it practically converts in situ into residual carbon to support the mesoporous structure through free radical polymerisation. This novel strategy forms the sp2-hybridised carbon-containing BCPs, such as the ligand-assisted assembly and resolve-assisted assembly strategies, to support the structure of mesoporous metal oxide under extreme calcination temperatures of greater than 400 ℃ and to achieve a controllable and multipurpose mesoporous semiconducting metal oxide synthesis with excellent gas sensing performance [Zou and his co-workers have used soft template synthesis based on the BCP strategy, which is considered the most efficient and flexible method to develop ordered mesoporous materials through the controllable interfacial-induced co-assembly process. The newly designed amphiphilic BCPs consist of high spformance . The plu3@graphene aerogel nanocomposites for the detection of low-temperature acetone. The graphene aerogel was used as the mesoporous substrate with a uniformly coated mesoporous WO3 on both sides of the graphene sheets through the self-assembly solvent evaporation-induced strategy using di-BCP poly(ethylene oxide)-b-polystyrene as the template [3@graphene aerogel nanocomposites possessed a large pore volume that resulted in high sensitivity with a good response (13 s) and recovery (12 s) to acetone at low temperatures of 150 °C. In the same year, Zhao and his co-workers introduced the mesoporous WOtemplate . The fabThe choice of fabrication method for graphene and polymer material is crucial to fabricate efficient gas sensing platforms with a high density, high surface-to-volume ratio, and convex-rich morphology. However, the limitation of the above-mentioned method is that the fabricated materials tend to form internal structural stress defects and deterioration, which could destabilise the sensor. Although this method has a broad-sized distribution, the morphology parameter of the nanostructure-array-based sensing platform is adjustable.The PVD method is an atomistic deposition process in which the material is vapourised from a solid or liquid source in the form of atoms or molecules and transported through a vacuum or low-pressure gases or plasma environment in the form of vapour to the substrate where it is condensed . Commonl3 gas showed an excellent response (~42%) at RT, a fast response and recovery time (115 and 44 s), good selectivity, low cost, flexibility, and wearable characteristics [3 gas sensor was introduced by Hasani and coworkers. The gas sensor was highly environmentally friendly, easy to fabricate, and very cost effective, as it used a low-cost solution processing method. The ohmic back contact to n-GaAs (1 cm2) was made of a Au-Ge alloy through PVD on the reverse side of the GaAs wafers. Furthermore, the GaAs appeared much cleaner prior to the PEDOT: PSS:GO deposition using a piranha solution. As a result, the NH3 gas sensor showed high potential as an active sensing material that fulfilled the industrial need for high speed, high sensitivity, and excellent selectivity. The as-synthesised PEDOT:PSS:GO films demonstrated a high detection sensitivity (194) with a high response and recovery time (95 and 121 s) towards 20 ppm of NH3 gas at RT [3@3D-(N)GFs/PANI) hybrid flexible gas sensor. The sensing material was fabricated using a simple in situ oxidative polymerisation process. Based on the results, the NiNP3@3D-(N)GFs/PANI) hybrid gas sensor demonstrated a high selectivity (750.2) towards 1000 ppm of NH3 with a good response and recovery time (95 and 25 s) at RT [Previously, PVD has been used to deposit interdigitated Au electrodes with 400 µm interdigitated spacing, 100 nm thickness, and 100 µm width on a flexible substrate. The S and N co-doped graphene quantum dots/polyaniline (PANI) hybrids were developed by Gavgani and coworkers by loading flexible polyethene terephthalate thin film through the chemical oxidative polymerisation. The gas sensor’s performance towards 100 ppm of NHeristics . In the as at RT . Tbar ets) at RT . Table 7Various detection principles are commonly used for the determination of as ammonia gas including chemiresistive, Quartz Crystal Microbalance (QCM), and Field-Effect Transistor (FET). Specifically, the detection method can be divided into variations, namely, the solid-state sensing method, optical method, and other methods. The solid-state sensing method involves the use of metal oxide-based and conducting polymer sensors. On the contrary, the sole example of the optical method is tunable diode laser spectroscopy, while other less prevalent methods include the electrochemical, surface acoustic waves, and FET sensors. This review highlights three significant gas sensing designs, which include the chemiresistive, QCM, and FET gas sensors. A summary of the recent works on graphene-based polymer nanocomposite gas sensors is presented in 2, hydrogen sulphide (H2S), and NH3. The gas sensor comprises sensing material, interdigitated electrodes, and electrical resistance. An active layer is placed over an array of electrodes to evaluate the change in electrical resistance in the presence of the target analytes. Subsequently, the chemical information is translated by the change in two-point contact electrical resistance, which is simply an electrical signal that is examined and need minimum supportive electronics to construct a mobile, compact, and free-standing system. The resulting sensors use a full composition and structure based on the charge transport and adsorption to obtain a good sensor performance. The performance of a chemiresistive gas sensor depends on many vital factors, such as response, selectivity, sensitivity, response time, repeatability, operating temperature, and LOD.The chemiresistive gas sensor is one of the prominent types of sensors that can detect harmful gases, such as carbon monoxide (CO), NOChemiresistors are usually fabricated by coating an interdigitated electrode with a thin film or other sensing material that acts as a bridge between the single gap of two electrodes. Common electrodes used in gas sensors include conductive metals, such as gold and chromium. The interdigitated electrodes acts as a greater substrate surface area that directly interact with the electrode, ultimately increasing the electrical connections and enhancing the overall system conductivity. Additionally, interdigitated electrodes have finger spacing and sizes on the order of microns that can only be arranged via photolithography. The interdigitated electrode and single-gap systems can be aligned in parallel to detect several analytes using a single device. Recent upgrading trials of the existing gas sensor technology have taken place due to the high demand for a wireless, portable, low-cost, and low-power consumption gas sensor, such as low-power Light-Emitting Diodes (LEDs), noble-metal functionalisation, and hybrid materials ,156,157,0, ∆f, ∆m, A, ρq, and µq represent the resonant frequency of the fundamental mode (Hz), normalised frequency change, mass change (g), the area between the electrodes (cm2), the density of quartz (ρq = 2.648 g/cm3), and shear modulus of quartz (µq = 2.947 × 1011 g/cm·s), respectively. The equation is derived by assuming the deposited mass as an extension of the thickness of the underlying quartz. Based on Sauerbrey’s equation, the mass to frequency correlation does not depend on the electrode geometry, which allows the mass determination without calibration, thus offering an affordable and time-efficient approach. QCM sensors are the most employed mass-sensitive sensors. Typically, the mass of the sensing coating of the QCM changes after the target gases are absorbed. Then, the computer or the signal receiver transforms the mass change (∆m) into the frequency change (∆f) using the Sauerbrey equation (Equation (9)) to depict the sensing performance, as shown in FET gas sensors have primarily been developed due to their inherent superior features, including effective operation under harsh and corrosive environmental conditions, ultralow power consumption, high-speed operation, and integrated wireless systems. An FET gas sensor displays vivid changes before and after exposure to the gas analyte, demonstrating the suitability of the 2D FET material for sensitive gas sensing applications. Various types of FET gas sensors have been established, such as tunnel, heterostructure electrostrictive, organic, and polymer FET gas sensors. In addition, an organic FET exhibits check-in electric characteristics, such as the threshold voltage, saturation current, and field-effect charge carrier mobility when the sensors are exposed to the targeted gas. Apart from being lightweight and flexible, FET gas sensors possess numerous advantages, such as excellent selectivity, remarkable repeatability and response, and low-cost production ,165,166.Tunnel FETs consist of a conventional Complementary Metal-Oxide Semiconductor (CMOS) that enables supply voltage (V) scaling in ultralow-power and energy-efficient competition. A strong interaction between the device level and circuit level with some modifications of the CMOS circuits is the main requirement of a tunnel FET-based circuit design, to obtain the desired functionalities with optimal energy efficiency. Over the past decades, tunnel FETs have been used to experimentally demonstrate the current and the steep slope in sensor devices. The recent advances in tunnel FETs enable Band-To-Band Tunnelling (BTBT) at the source-channel junction, where the carriers at the high-energy tail of the Fermi–Dirac distribution are filtered by a tunnelling window .on/Ioff) ratio, and reduce the interface traps to suppress the TAT. A low band gap can contribute to both the low effective mass and freedom to obtain hetero-band-alignment, which assists in improving the tunnelling probability at low voltage levels. The TAT reduction and steep are caused by the tunnel junctions with a steep doping profile and low defects. Additionally, the gate electrostatics with steep and high current on (Ion) can be improved by enhancing the gate-control, which converts from the planar device structure into the gate-all-around nanowire structure. The most crucial factor to obtain effective gate control in transistor operation is to determine the quality of the gate dielectrics.Nevertheless, tunnel FET designs face many challenges, specifically the state current, which is restricted by the tunnelling probability and the steep subthreshold slope. Both parameters can be degraded by the thermal energy to determine the Trap-Assist Tunnelling (TAT). The design of a tunnel FET requires the material selection of the system to reduce the tunnelling barrier, achieve good gate electrostatic for the steep and current-on-to-current-off  dry-Solid Polymer Electrolytes (SPEs), (ii) plasticised polymer electrolytes, (iii) Gel Polymer Electrolytes (GPEs), and (iv) Composite Polymer Electrolytes (CPEs). SPEs have drawn wide interest as an alternative to replacing liquid electrolytes due to the fact of their impressive abilities, such as great mechanical strength, ease of fabrication with desirable shapes, and excellent electrode/electrolyte contact formation. They are synthesised through the dissolution of inorganic salt in a polar polymer matrix. Polymer electrolytes consist of both crystalline and amorphous parts in which the ion transport preferably takes place in the amorphous part. The degree of crystallinity and the viscosity of the polymer electrolytes depends mainly on the ionic conductivity of the polymer electrolytes. The low mobility of the ions in the crystalline area would lead to low ionic conductivity. Meanwhile, polymer electrolytes with low viscosity tend to produce more voids and, thus, lead to high conductivity. Several approaches have been proposed to improve the ionic conductivity of polymer electrolytes, such as polymer blending, incorporation of dopants and ionic liquids, utilisation of comb-branched copolymers, and the addition of inorganic filler, plasticisers, and nanomaterials.i)Dry-solid Polymer Electrolytes (SPEs)Research on SPEs was first reported by Wright et al. (1975)  with theDespite its promising properties, the main disadvantages of SPEs include their high crystallinity, which leads to low ionic conductivity, as well as the ambient temperature that contributes to low conductivity and high interfacial resistance ,212,213.ii)Plasticised Polymer ElectrolytesPlasticised polymer electrolytes are developed by combining the polymer host with low molecular weight organic compounds, such as ethylene carbonate , dimethyOne of the main concerns in utilizing these plasticisers is that the resultant electrolyte may demonstrate low mechanical strength . Additioiii)Gel Polymer Electrolytes (GPEs)A GPE is also known as plasticised polymer electrolyte due to its similar structural properties to plastics. A GPE is a plasticised or gelled polymer matrix that results in the expansion of the polymer matrix in a liquid electrolyte. The GPE is easily produced by heating the polymer matrix mixture comprising PEO and an alkali metal salt, for instance lithium salt and zinc salt, and a large amount of organic solvent or plasticiser. The viscous clear solution is then cast in a hot state and left to cool under the electrode’s pressure to generate a thin film. The polymer acts as a host matrix to trap the liquid constituents. Subsequently, the combination of plasticiser molecules generates a interconnected network in which the ion conduction connected to the host polymer and, thus, provides structural support.GPEs display high ambient conductivity, poor volatility, safe operation, low reactivity, and great chemical, mechanical, photochemical, electrochemical, and structural stabilities ,220,221.However, few studies have reported the drawbacks of GPEs, such as the impregnation of liquid electrolytes and volatile solvent residual discharge, which leads to poor mechanical strength and abrupt metal electrode reactivity increment that may hinder the wider application of GPEs ,227,228.iv)Composite Polymer Electrolytes (CPEs)CPEs are developed to overcome the limitations of SPEs mainly the presence of ion pairs in SPEs, called the ion association, and ion triplets, which contribute to the weakening of the dielectric constant of the host polymers . InorganVarious methods have been introduced to fabricate CPEs, for instance, binary salt systems , polymer3 sensing devices is crucial for human and environmental safety. Conventional detection techniques, such as calorimeter, optical spectroscopy, and chromatography, have been frequently used to detect NH3 over the past several years. However, these conventional techniques have failed to overcome certain limitations, such as time-consuming sample pre-treatment and preparation, the requirement of complex instrumentation, and considerably high operating cost. Despite the development of various sensors with excellent sensitivity and selectivity, quick response, and remarkable LOD, they are somehow hindered from applying the conventional detection methods. In that respect, recently developed nanotechnology sensors have enabled the fabrication of novel materials with outstanding sensing performance.The rapid development of effective NH3 gas sensors. PANI, Ppy, and their combination as well as the PEDOT:PSS combinations have received the most attention among researchers given their unique redox properties, low cost, good electrical conductivity, good stability, ease of synthesis, a good response at RT, highly sensitive, and offer a high potential to be applied in many fields, such as corrosion protection [Polymers are promising materials in the development of sensor devices. These large molecules are composed of chains or rings of repeating subunits of monomers and tend to exhibit high melting and boiling points due to the strong bonding of the high molecular-weight molecules. Polymers can be grouped into two types, namely, organic (polymers with carbon as their backbone) and inorganic (polymers with other elements as their backbone). Organic conducting polymers are frequently used in the development of gas sensors due to their ion mobility and conductivity properties. As shown in otection ,248,249,otection ,251,252,otection ,254,255,otection ,257,258,otection ,260,261,otection ,263,264,otection ,266,267.otection ,157,268.Conducting polymers comprises several long chains with extended π-systems permittable to chemical and electrochemical oxidation or reduction. Their electrical and optical properties could be tuned and modified by controlling the oxidation and reduction (redox) process, which are both often reversible. Thus, the electrical and optical properties can be systematically controlled by switching from a highly conductive semiconductor to a less conducting or insulative one with excellent precision. Hence, conducting polymers can form conductive macromolecules with a fully conjugated sequence of bonds and backbones that contain both positive and negative charges via the redox reaction. In recent years, multiple studies have been attempted to incorporate graphene into conducting polymer matrices to develop nanocomposites with novel properties, such as large surface-to-volume ratio, enhanced electrical, mechanical, thermal, and to suit and enable exploration various applications ,261,267.3 gas at RT. The rGO aerogel was synthesised via the hydrothermal method and exhibited high porosity, which is favourable for gas adsorption. The PANI nanofibers interconnect with the fibrillar network, which grows vertically on the surface of the 3D-rGO nanosheets. The high surface area of the synthesised material enhanced the exposure to the NH3 gas which, in turn, contributes to the high sensing performance. The PANI/3D-rGO hybrid sensor has a high response of 111% to 5 ppm of NH3, with a response and recovery time of 85 and 187 s [3 gas. The rGO provides an electron transfer path to PANI and enhances the electron transfer speed using the π-π conjugated bond reaction of rGO and PANI. Thus, the synthesised PANI-rGO nanocomposites demonstrated an incredible sensing response of 0.86% for the detection of 15 ppm of NH3 with 8 and 48 min of response and recovery time [The PANI-based gas sensor has received the highest attention compared to the other conducting polymers used for fabricating gas sensors. Different types of nanoparticles in PANI polymeric matrix could produce different responses to ammonia gas, mainly due to the synergistic effect. For instance, PANI is a p-type polymer with excellent properties as an extrinsic semiconductor with a majority of hole carriers and a minority of electron carriers, a large electron concentration compared to the hole concentration, and acceptor energy levels that are very close to the valence band. Generally, a PANI-based sensor consists of different structures, such as films, thin films, nanofibrous thin films, nanowires, nanofibres, and pellets, which have acquired response values below 100% at a gas concentration of 20–1000 ppm ,156,189.nd 187 s , respect3, the Ppy/rGO nanocomposites recorded a sensing response of 6.1%, with a response and recovery time of 1 and 5 min, respectively, at RT [3, with a considerable response value of 45% at RT [3 molecule on the Ppy surface induced the bonding interactions between NH3 molecules and the Ppy. The electron transfer during the adsorption of NH3 molecules led to the reduction of the hole concentration of the Ppy layer, subsequently increasing the Ppy resistance. On the other hand, the graphene acts as a p-type semimetal. Overall, the functionalisation with the Ppy has made it possible to receive the electron transfer from NH3 molecules, thus increasing the graphene resistance as well.The Ppy/graphene sensor is one of the most widely utilized conducting polymers for ammonia sensing due to its high conductivity, easy synthesis process, and great environmental stability . The oxiy, at RT . The moby, at RT . The in-5% at RT . The syn5% at RT . Since t3 gas sensors due to the fact of its unique features, such as excellent solution-fabrication capability and miscibility, good controllable conductivity, exceptional chemical and electrochemical stability and good biocompatibility and optical transparency [3 gas detection. Previously, the combined graphene quantum dots dopant sensor with PEDOT:PSS showed an excellent response (212.32%) towards 1500 ppm of NH3 molecules at RT [3 gas. There are greater electron hopping which occurr at the shorter PEDOT chains than in between the longer PSS chains. The graphene quantum dots are conductive pathways and act as a charge carrier that causes swelling and increases the PEDOT distance in the film. In one study, the concentration of carbon-based material embedded in PEDOT: PSS highly affected the type of NH3 adsorption that occurred during the adsorption and desorption process [The PEDOT:PSS-based conducting polymer sensor is one of the most extensively researched NHsparency ,271. Thees at RT . The sen process . −17 and ~10−8 S/cm [All conducting polymers can easily endure redox reactions compared to conventional polymers mainly because conducting polymers possess double bonds of a π-bond and σ-bond. The electrons in σ-bonds usually originate from the backbone chain that forms a long-saturated chain, which provides the conducting polymers with considerable mechanical properties. Meanwhile, the electrons in the π-bonds are delocalised along the chain due to the overlapping of the π-orbital with the neighbouring molecules of the conjugated bonds, which contributes to the conducting and semiconducting properties. The presence of π-conjugated structures in the conducting polymers generates charge carriers in the form of free electrons or holes. In turn, the charge carriers tend to delocalise over the conjugated polymer chain and make it easy for the polymer to form a conjugated backbone ,273. In 0−8 S/cm , respect2, H2S, NH3, and NO2 [2 and NO2, can eliminate the electrons from the conjugated benzene rings of the conducting polymers, forming more holes and reducing the electrical resistance. In the case of p-type conducting polymer, the holes are filled up with the electrons from electron-donating gases, such as NH3 molecules, which increases the resistance. Nevertheless, the purging of the conducting polymer sensing layer with dry nitrogen or air would partially or fully recover the original resistance.3, NH4BF4, HClO4, I2, SO3CF3, AsF5, HNO3, HCl, H3PO4, and H2SO4. On the other hand, commonly known n-type dopants or reductants include Na, Li, and K. The general expression for the doping of a conjugated polymer is as follows:The doping and undoping processes play an important role in the gas sensing mechanism fabricated using conducting polymer and graphene fillers . Doping  and NO2 ,277,278.3, water, and hydrazine. The chemical reaction of the process is shown in Equations (14)–(16):On the other hand, the undoping or dedoping process which is also known as the compensation or electrical neutralisation of a doped polymer is the process in which p-type doped polymer reacts with several reducing agents and regains its insulating state. The undoping agents diffuse into the polymer matrix and neutralise the charges of the system through a charge-transfer reaction. The undoping process involves chemical reactions between the undoping agents and dopant or carbonium ion, which leads to neutralisation by charge transfer. The undoping process has a high possibility of being affected by thermal treatments. The common undopant agents used for the undoping process include NH3 gas. The presence of certain functional groups on the surface of the polymers may affect the sensing signal. The intercoiled and compressed polymeric structure would result in a low response due to the lack of unexposed functional sites, while the strong response from porous or mesoporous structures may due to the large amount of functional group exposure. The involvement of graphene nanomaterials and polymeric systems highly affects the surface area of the hybrid nanocomposites. The electrochemical deposition of the conducting polymers promotes a greater porosity and high surface area. In contrast, the solution-cast films of conducting polymers usually introduce low porosity ,280,281.The temperature and humidity are the two main environmental factors that affect the sensing performance of conducting polymer-based sensors ,282. An 3 due to the fact of its large surface area. NH3 can generate the formation of hydrogen bonds between its hydrogen atoms and the oxygen atoms of graphene and its derivatives such as GO, rGO, etc. [3 induces the collapse in metal oxide structures due to the various similarities between NH3 and water in terms of chemistry. Firstly, both inorganic molecules have a strong propensity to form hydrogen bonds with oxygen atoms of any compound in the formation of hybrid nanomaterials. Hydrogen bonds are intermolecular forces that form a unique dipole–dipole interaction when a hydrogen atom is bonded to a strongly electronegative atom that exists in the vicinity of electronegative atoms with a lone pair of electrons. Comparatively, hydrogen bonds appear to be noteworthy among other types of bonding due to the fact of their strong, attractive, and directional characteristics. Hence, the complex hydrogen-bonded networks enhance the adsorption of NH3 on amenable surfaces, which is very intriguing. Finally, both molecules have a high potential for future efforts toward a hydrogen-based economy. While water is essential to living things due to the fact of its ubiquity in the chemistry of living systems, NH3 is widely used in manufacturing fertilisers, making it the most important industrial chemical production. Both of them, NH3 and water molecules, are mainly used for hydrogen production (water–gas shift reaction) or hydrogen storage (NH3 as a hydrogen vector). NH3 is likely adsorbed on the composites via three mechanisms: (1) the intercalation between the graphene layers from the GO; (2) adsorption at the interface between the nanomaterial segments and the graphene layers, where the dispersive forces are enhanced, specifically the synergic effect of the structure; and (3) hydrogen bonding to the metal oxide tetrahedral via the distortion of the graphene materials structure, as shown in Graphene exhibits the highest potential in adsorbing NHGO, etc. ,285,286.3 gas sensing. When Ppy interacts with NH3 molecules, the doublet of nitrogen in the polymer backbone loses its electron, which is transferred between the positive holes of the Ppy and NH3 molecules. This transfer causes the charge-carrier concentration that reduces the conductivity of the sensor. The interaction mechanism between Ppy and NH3 is shown in Equation (17):Numerous gas sensors use conducting polymers to functionalise in the redox reaction for NH3 and increases the conductivity of Ppy. When Ppy reacts with the electron-donating gases, such as NH3, the hole concentration reduces and the resistance increases. New holes are formed within the Ppy structure if and only if the electron-acceptor analytes, such as alcohols, are used to react with Ppy, which leads to decreased electrical resistance [The reverse reaction takes place in the air in which sistance ,290,291.3 environment. During the NH3 gas sensing, a single pair of electrons or an electron pair from a nitrogen atom is donated to the initial oxidised Ppy, reducing the electrical conductivity, which then forms a neutralised Ppy. There is one possibility of a reversible interaction mechanism occurring between the Ppy and the NH3 that may cause a decrement in the doping level of the Ppy through the compensation of the original dopant effect [3 takes place during the desorption of the NH3 molecules, which provides an opportunity for the NH3 molecules to attack the proton of Ppy and forms ammonium ion [3 molecules. As the NH3 molecules are absorbed into the sensor, the resistance is reduced due to the release of electrons into the conduction band, which is transferred through the highly mobile graphene [The mechanism indicates that the p-type Ppy is expected to be undoped under the NHt effect ,293,294.graphene , as show3 molecules, the gas sensing response is similar to the combined adsorption and desorption process and electrical compensation as a side reaction. This process involves chemical bonding and weak forces, such as hydrogen bonding, and van der Waals forces, due to the fact that it is much easier for weak interactions compared to strong interactions [3 gas [3 molecules is similar to that of acid/base reactions in which, at a high concentration of NH3, the neutralisation of the dopant acid occurs quickly, which leads to the formation of a nonconducting emeraldine base. Nonetheless, at low concentrations of NH3, the chemisorption of NH3 molecules takes place on the positively charged nitrogen atoms, as depicted in the schematic mechanism in As for the PANI-based nanocomposite sensing mechanism of NHractions ,297. Thes [3 gas . The intSince its first synthesis over the last two decades, graphene is globally known for its high potential for various applications. Graphene has all the reasons to be considered a promising material with high potential in gas sensor-based applications. The introduction of graphene into polymers has further enhanced the potential of these nanocomposites to improve the performance of gas sensors.3 molecules. As discussed in this paper, NH3 gas can harm the environment and human health, which necessitates the need for the development of a specific gas sensor.The sensing mechanism should be implied and well understood to synthesise exceptional combinations of graphene based polymeric nanocomposites with desired functionalities and morphologies. The sensors can be embedded to form a variety of sensors, for instance, gas sensors, wearable sensors for ubiquitous monitoring of physiological parameters and other chronic diseases, a remote sensor, electrochemical sensor, real-time electronic monitoring sensor, piezoresistive pressure sensor, biosensor, strain sensor, human health medical monitoring sensor, optical sensor, and other published sensors. The development of sensors is highly vital to safeguard human health and environmental sustainability, upgrade recent technology, and in medical diagnosis, industrial manufacture, and national defence. The fabrication of graphene based polymeric nanocomposites enables that design of highly sensitive, reusable, low-cost, selective, and long-lasting gases sensing systems. A diverse range of topics have been discussed throughout this review to improve the quality of the sensor design, functionality and address the drawbacks of the previous sensors. The functionalised graphene–polymer sensors should be extensively studied and utilised to capture specific targeted molecules, especially NH3 gas sensor device has the biggest marketing industry with the prediction of all academician and corporates sectors to benefit the information of graphene–polymer nanocomposites-based ammonia gas sensor. The use of graphene–polymer nanocomposites is highly in demand and increasing with time, and it is expected to greatly impact the quality of life for the future generation. Based on the above discussions, more studies are still required to further study the interfacial interactions and synergetic effects that occur between graphene and polymer matrices which results in improving the sensing performance of gas sensors. In addition, ways achieve homogenous distribution of graphene nanofillers within the polymer matrices and the role of graphene fillers in improving the sensitivity of these sensors require further in-depth experimental studies.There are many ways to improve the quality of sensor devices, such as using less complex and low input power devices, fabricating efficient sensors with consistent performance, and selecting appropriate materials and methods that consume less production cost with multiple distinctive parameters in a single sensing system. The addition of polymer into graphene not only enhances the sensor performance but the properties of the polymer itself, such as being lightweight, durable, flexible, and cheap, contribute to energy conservation and cost reduction. The challenges of graphene–polymer nanocomposites are associated with various parameters, such as the orientation of graphene layers, the type of graphene used, the preparation method, and the ratio of graphene to the polymer matrix. The use of graphene–polymer nanocomposites in improving gas sensors performance as well the methods used to fabricate graphene and graphene polymeric nanocomposites have been clearly explained in this review paper. The promotion of graphene–polymer nanocomposites as a NH3 adsorbing mechanism has been reviewed and illustrated. Although various applications of graphene have been discovered and reported, there are still many aspects that need to be further explored, specifically the application and the potential of both graphene and its hybrid nanomaterials. In conclusion, the suitable selection of the methods and techniques for the synthesis of graphene and its hybrid nanomaterials affects the commercialisation of graphene as next-generation gas sensors.The unique electrical, physical, and chemical properties of graphene and its hybrid nanomaterial-based sensors have driven the interest of researchers from all over the world in various fields. Graphene is easy to synthesise via notable methods, such as mechanical exfoliation, chemical or thermal reduction, CVD, epitaxial growth, and longitudinal “unzipping” of CNTs. GO can be produced from graphite as the precursor following Hummers’, Staudenmaeir’s, and Brodie’s methods. Likewise, polymer electrolyte has been introduced to a variety of applications, and the combination of graphene and polymers can enhance the ability of each material depending on the targeted application. A full explanation of the NH


Sarcoidosis, a multi-systemic granulomatous disease, is a predominantly T-cell disease but evidence for a role for humoral immunity in disease pathogenesis is growing. Utilizing samples from the Genomic Research in Alpha-1 anti-trypsin Deficiency and Sarcoidosis (GRADS) study, we examined the prevalence of autoantibodies in sarcoidosis patients with pulmonary-only and extra-pulmonary organ involvement compared to normal controls.We analyzed serum samples from sarcoidosis patients who participated in the GRADS study utilizing an autoantigen microarray platform for both IgM and IgG antibodies. The cohort included sarcoidosis patients with pulmonary-only disease , sarcoidosis patients with extra-pulmonary disease  and a normal control cohort . Organ involvement was assessed following a standardized format across all GRADS participating centers.Sarcoidosis patients overall had increased levels of IgM and IgG autoantibodies compared to normal controls. In addition, several autoantibodies were elevated in the POS and EPS cohorts compared to the NC cohort. Differences in autoantibody levels were also noted between the POS and the EPS cohorts. When comparing organ involvement with sarcoidosis, bone, spleen and ear, nose and throat involvement had higher IgM expression than other organs.Sarcoidosis patients have elevated IgM and IgG autoantibody levels compared to normal controls. In addition, individuals with pulmonary as well as additional organ involvement had higher IgM expression. Further research is needed focusing on specific organ-autoantibody pairs and role of autoantibodies in disease pathogenesis. Sarcoidosis is a disease characterized by the formation of non-necrotizing granulomas that can involve any organ in the body in an unpredictable manner . The curThere is an increased interest in the role of humoral immunity and autoimmunity in sarcoidosis –7. SarcoMultiplex autoantigen arrays have been used to identify a wide spectrum of autoantibodies in patients with rheumatoid arthritis , systemiWe utilized the GRADS sarcoidosis cohort for this study . GRADS whttps://microarray.swmed.edu/products/product/autoantigen-microarray-super-panel-128-antigen-pan/). The serum samples were pretreated with DNAse-I to remove free DNA and then incubated with the autoantigen array at 1:50 dilution in PBS. Autoantibodies binding to the antigens were detected with cy3 labeled anti-human IgG and cy5 labeled anti-human IgM  and Tiff images were generated using GenePix 4000B scanner with laser wavelengths 532nm for IgG and 635nm for IgM. Genepix Pro 6.0 software was used to analyze the images. The net fluorescence intensity (NFI) and signal-to-noise ratio (SNR) for each antigen were calculated by subtracting the background and negative control (PBS) effects and the averaged NFI and SNR were generated. SNR ≥ 3 were considered true signal from background noise and the antibodies with SNR < 3 in larger than 90% of all samples were filtered out. Antibody score (ABS) value was generated for each antibody using the following formular: ABS = log2[(NFI x SNR) + 1]Autoantibody reactivities against a panel of autoantigens were measured using a pre-designed autoantigen microarray platform developed by University of Texas Southwestern Medical Center. We utilized an autoantigen microarray super panel which contains 128 autoantigens and internal controls. The autoantigens are listed on University of Texas Southwestern Medical Center Microarray and Immune Phenotyping core facility website  was used for principal component analysis (PCA) with the Vegan (2.5) package. Adonis paired tests were performed to test the differences between groups. GraphPad prism (version 8.1) was also used for statistical tests. Leave-one-out cross-validation was performed, which involves using one observation as the validation set and the remaining samples as the training set. This is repeated until all samples are predicted by a stepwise multiple logistic regression model. Prediction performance was evaluated by the area under the curve (AUC) in the receiver operating characteristic (ROC) curve.A total of 327 individuals, including 101 normal controls (NC), 106 pulmonary-only sarcoidosis (POS) and 120 extra-pulmonary sarcoidosis (EPS), were included in this analysis. A summary of the basic demographic characteristics for the cohorts is described in Out of the 120 autoantibodies assayed, 23 IgG autoantibodies and 35 IgM autoantibodies showed a low signal (SNR<3) in over 90% samples and were excluded from the analysis . There wExamining the IgM autoantibody profile revealed more significant differences. The combined sarcoidosis cohort had 58 significantly elevated IgM autoantibodies compared to the control group, while the POS and EPS alone showed 69 and 32 elevated IgM autoantibodies, respectively, comparing to the NC group , and amoPrinciple component analysis (PCA) of IgG and IgM autoantibodies showed clustering of the POS cohort separated from the EPS and NC cohorts . The sepStepwise multiple logistic regression analysis on the differentially expressed autoantibodies identified 11 IgM autoantibodies which can efficiently distinguish sarcoidosis patients from NC, with AUC 0.8984 and 95% CI: 0.8644~0.9325 . Similar−4), spleen (p = 4.5×10−3), bone and ear nose (p = 4.35×10−5) and throat (ENT) (p = 4.9×10−3) involvement had significantly higher IgM autoantibody values whereas those with skin (p = 1.3×10−2) and salivary/lacrimal gland (p = 4.4×10−9) involvement had significantly lower signal intensities . Sarcoidosis subjects with liver  is produced by dendritic cells and monocytes/macrophages and is important for survival of immature B-cell and mature B-cell proliferation . BAFF leIn our cohort, we detected both IgM and IgG autoantibodies against several antigens. Some of those targets overlapped and some were unique for each autoantibody class. IgM autoantibodies have been associated with several autoimmune diseases suggesting a potential role in disease manifestaiton –36. WhatSeveral of the autoantibodies noted in our study are pathogenic and have been associated with other systemic autoimmune diseases. For example, anti-Myelin Basic Protein (MBP) have been reported in multiple sclerosis and systemic lupus erythematosis (SLE) . Anti-chThe differential autoantibodies can be potential biomarkers for prediction of sarcoidosis and clinical subtypes. Using an regression algorithm, we identified a panel of 11 IgM autoantiboides including BPI, Fibrinogen IV, Hemocyanin, Histone H2B, Histone H3, Matrigel, Prothrombin protein, Sm, SmD, TPO and Vimentin, which can efficiently distinguish between sarcoidosis and healthy controls. Interestingly, anti-vimentin antibody has been previously proved to be associated with sarcoidosis . A panelOur data shows ovelap in the antibody profile between healthy controls and the extra-pulmonary sarcoidosis cohort. Extra-pulmonary sarcoidosis is very diverse in its presentation with variable organ manifestations. As the etiology of extra-pulmonary sarcoidosis is yet unknown ), it is plausible that some extra-pulmonary manifestations are autoimmune driven and some are not. As such, future studies focusing on specific-organ manifestations would be better powered to detect associations .Our study has several limitations. First, although our autoantibody panel included over 100 antibodies, it is not an exhaustive panel. Second, the effect of immunosuppressive therapy may impact antibody levels although none of the patients in this cohort were on B-cell depleting therapies. Finally, the organ assessment was systematic and unified across the GRADS cohort but the assessment depended on clinically available tests and not on a thorough systemic organ assessment by advanced imaging or testing which leaves the door open to potential misclassification of organ involvement. All patients were seen and cared for at sarcoidosis specialty centers and underwent standard organ assessment evaluations and performing advanced studies, beyond standard of care, to fully investigate organ involvement is not feasible. Also, none of the sarcoidosis subjects had clinically known autoimmune disease as it was an exclusion criteria for the GRADS study.In summary, humoral immunity is active in sarcoidosis and several autoantibodies are present in sarcoidosis patients and may play a role in the pathogenesis of pulmonary and extra-pulmonary sarcoidosis. Further research is needed to identify specific autoantibody-organ combinations if autoantibodies are to be used as a diagnostic and/or prognostic biomarker in sarcoidosis.S1 Table(XLSX)Click here for additional data file.S2 Table(XLSX)Click here for additional data file.S3 Table(XLSX)Click here for additional data file.S1 FigEach row in the graphics represent an antigen for serum specimens organized into columns classified as normal control (n = 101), extra pulmonary sarcoidosis (n = 120), and pulmonary only sarcoidosis (n = 106). The reactivity intensity ranges from blue (low) to white (moderate) or red (high).(PPTX)Click here for additional data file.S2 FigEach row in the graphics represent an antigen for serum specimens organized into columns classified as normal control (n = 101), extra pulmonary sarcoidosis (n = 120), and pulmonary only sarcoidosis (n = 106). The reactivity intensity ranges from blue (low) to white (moderate) or red (high).(PPTX)Click here for additional data file.S3 Fig(PPTX)Click here for additional data file.


In this study, wereport on the self-assembly of the organic electrondonor 2,3,6,7,10,11-hexamethoxytriphenylene (HAT) on graphene grownepitaxially on Ir(111). Using scanning tunneling microscopy and low-energyelectron diffraction, we find that a monolayer of HAT assembles ina commensurate close-packed hexagonal network on graphene/Ir(111).X-ray and ultraviolet photoelectron spectroscopy measurements indicatethat no charge transfer between the HAT molecules and the graphene/Ir(111)substrate takes place, while the work function decreases slightly.This demonstrates that the HAT/graphene interface is weakly interacting.The fact that the molecules nonetheless form a commensurate networkdeviates from what is established for adsorption of organic moleculeson metallic substrates where commensurate overlayers are mainly observedfor strongly interacting systems. Gating allows rapid control over both charge carrier concentrationand type but suffers from large parasitic capacitance.13 Chemical covalent functionalization, with, forexample, nitrogen or hydrogen, offers a scalable path to controlleddoping, but the chemical bonds formed this way often break the sp2 hybridization of the graphene lattice, which drasticallylowers charge carrier mobility.14 Noncovalentadsorption of small molecules  avoids this problem but is highly sensitiveto changes in the environment.15 Adsorptionof metal atoms16 or nanoparticles17 should be a more robust approach, but the mostpromising results also show signs of chemisorption, which would bringthe same problem as covalent functionalization.18Depending on the substrate it isgrown on, a varying charge carrierconcentration and work function can be obtained.21 since these moleculesnoncovalently interact with graphene through π–πinteractions which do not perturb the graphene sp2 lattice,leaving its high charge carrier mobility intact. It has been shown,both on metal surfaces22 and on graphene,24 that the orientation of the molecules—both the in-plane andthe out-of-plane orientation—in the first layer on the surfacestrongly affects the intermolecular and molecule–surface interactionsand thereby the level of doping and change in work function. The interactionstrength at the graphene/metal interface also plays a role as moleculestend to bind more strongly to graphene if the graphene–substrateinteractions are stronger.26 Of the recent studieson this topic, many have focused on p-type doping of graphene by adsorptionof the organic acceptor tetracyano-p-quinodimethane(TCNQ), whereas a variety of substrates has been used for graphene.29 The degree of doping can be controlled by the coverage,the replacement of TCNQ by its derivatives,25 or a combination with molecular donors to form charge-transfer complexes.30 The so far most commonly employed donor molecules are phthalocyanines31 and tetrathiafulvalene.32 This leaves the class of triphenylene derivatives underexplored.Triphenylene can be easily functionalized and thus can act as bothacceptor or donor molecules.36 To the best of our knowledge, only the acceptor hexacyano-hexaazatriphenylenehas been studied on graphene to date.37Recently, noncovalent functionalization of grapheneby adsorptionof planar pi-conjugated organic molecules has attracted increasingattention40 and charge-transfer complexeson metal surfaces.41 Here, we use scanningtunneling microscopy (STM), low-energy electron diffraction (LEED),and ultraviolet/X-ray photoelectron spectroscopy (UPS/XPS) to studyits structural and electronic properties on slightly p-type dopedgraphene grown on Ir(111).42In thiscontribution, we investigate the behavior of the triphenylene-deriveddonor molecule 2,3,6,7,10,11-hexamethoxytriphenylene HAT, see 1. HAT ha43HATwas synthesized in accordance with literatureprocedures.–10 mbar). Clean Ir(111) crystals were preparedby cycles of argon ion sputtering and annealing at 1300 K. Single-layergraphene was grown by chemical vapor deposition by exposing the Ir(111)substrate to a partial pressure of 4 × 10–7 mbar of ethylene (C2H4) gas for 4 min whilethe substrate was held at 1200 K. The graphene/Ir(111) samples werethen transferred through ambient air to the preparation chamber (basepressure below 2 × 10–9 mbar) of a two-chamberUHV system. After transfer, the graphene/Ir(111) samples were annealedat 750 K for 60 min to ensure clean surfaces. The HAT molecules weresublimed using a commercially available Knudsen cell evaporator (CreaPhys)with the deposition rate monitored by a water-cooled quartz crystalmicrobalance (QMB). The sample was kept at room temperature duringdeposition.The graphene on the Ir(111) substratewas prepared in an ultrahigh vacuum (UHV) system (base pressure below10–9 mbar)of the aforementioned two-chamber UHV system, with the samples heldat room temperature. The analysis chamber houses a variable temperaturescanning tunneling microscope , LEED optics (SPECS), a hemispherical analyzer (PREVACEA15), a twin anode X-ray source, and a He discharge lamp. The STMmeasurements were carried out in the constant current mode with amechanically cut Pt/Ir tip. Bias voltages are reported with respectto a grounded sample. The STM data were processed using WSxM software.44 LEED patterns were simulated using LEEDpat.45 UPS measurements were performed using He I radiation(21.2 eV) at an emission angle of 40° with respect to surfacenormal  and 0° (secondary electron cutoff). XPSmeasurements were performed using Mg Kα radiation (1253.6 eV)at normal emission. A few measurements were performed at the sameUHV system at which the graphene preparation was carried out. Thesemeasurements were performed using a low-temperature STM instrument and a microchannel plateLEED (Scienta Omicron GmbH).Most measurements werecarried out inthe analysis chamber . The tentative structure model isshown in S3–S7 in the Supporting Information). The FFT confirms the size of theunit cell , and both the FFT and LEED data agree exceptionally wellwith the simulated LEED pattern. The network, a commensurate For a molecular coverage up to one monolayer of HAT on graphene/Ir(111),the molecules assembled in a hexagonal close-packed network, as canbe seen in the STM image in 41The molecular arrangement and unit cell are very similar to theones found for HAT on Ag(111), where the molecules were also foundto assemble in a hexagonal packing with a lattice parameter of 1.32nm.Figures S8 and S9).To gain insights into the electronicproperties and molecule–substrateinteractions, we performed photoelectron spectroscopy measurements.The UPS data are displayed in 47 No additional features  are observed,and the relative positions of the HOMO levels agree well with thoseof HAT on Ag(111), a weakly interacting interface41 (for UPS data of HAT on Ag(111), see Figure S10). From this, we can conclude that HAT also interactsweakly with graphene, which can be most likely classified as physisorption.For a coverage of closeto a monolayer  as the substrate41 but 0.7 eV higher than reported for HAT on SiO2.49 This difference might originatefrom the different energy level alignments of HAT on these differentsubstrates. More details on the peak parameters can be found in We also obtained XPS spectra for the C 1s 4 and O 1As the HAT coverage increased fromthe monolayer to multilayer,no significant binding energy shifts were observed for the C 1s orO 1s peak positions within our resolution of ±0.1 eV. Note thata core-hole screening effect similar to the UPS data cannot be observeddue to the higher information depth of XPS, that is, the multilayerspectrum also obtains information from the molecule substrate interfaceand larger line width of the X-ray source compared to the UV source.52 In our data, the peaks originatingfrom the HAT molecules appeared at the same energy for both mono-and multilayer coverage. Moreover, there is also no significant dopingof the graphene present as this would induce a shift in the grapheneC 1s peak.25 In particular, additionalpeaks in the vicinity of the HOMO level would indicate charge transfer;the absence of such features in our data therefore indicates thatno charge transfer takes place.54From our UPS and XPS data, we conclude that no charge transferoccurred between the HAT molecules and the graphene/Ir(111) substrate.A possible charge transfer could be identified by a shift of the peakpositions between mono- and multilayer coverage, and the value ofsuch shifts should be on the order of hundreds of meVs and apply toboth C 1s and O 1s peak positions.push-back or pillow effect) between the electrons of theHAT molecule and the electron cloud of the substrate extending intothe vacuum, well known for molecules adsorbed on metal surfaces.55 This effect has also been predicted for molecule/graphene/metalsystems but tends to be overshadowed by charge-transfer-induced dipoles.25 As no charge transfer occurs in the HAT/graphene/Ir(111)system, the measured work function shift is due to only the push-backeffect. The energy level alignment of HAT on graphene on Ir(111) issummarized in We then finallyturn to the decrease in the work function of graphene/Ir(111)upon adsorption of a monolayer of HAT. We attribute this decreaseto the Pauli repulsion  only formedwhen the molecule substrate interactions exceed the intermolecularinteractions, that is, the molecules want to interact in clearly preferredways with the substrate.66 Possibly, it is a coincidence that the size of the molecular unitcell is very close to a multiple of the graphene unit cell. However,if that was the case, we should observe many rotational domains. Instead,we observe just one domain . Evenif the structure is not exactly commensurate, it still forms a point-on-linecoincidence.68 This speaks for a molecule–substrateinteraction that is optimized by the HAT on graphene on the Ir(111)system, resulting in a single orientation. Given that the molecularlattice appears to be commensurate with the graphene lattice (andnot with the Ir(111) support or the Moiré pattern), it seemslikely that this interaction occurs between the HAT molecules andthe graphene layer. Possibly, π–π interactionsbetween graphene and the aromatic backbone of HAT are responsible.69Our photoelectron spectroscopy data suggest that there is littleinteraction between the HAT molecules and the graphene on the Ir(111)surface. This is not too unusual, as epitaxial graphene has been often—althoughnot alwaysWe studied the structuraland electronic properties of an organicelectron donor on graphene on Ir(111) using STM, LEED, UPS, and XPS.At sub-monolayer coverages, the molecules assembled in an, apparentlycommensurate, close-packed hexagonal structure. The two domains weobserved are mirror images of each other with the [1–10] directionof graphene as the mirror direction. From the photoelectron spectroscopymeasurements, we find neither evidence for charge transfer betweenHAT and graphene nor HAT and Ir(111), all together indicating a weakinteraction (physisorption) between the HAT molecules and the grapheneon the Ir(111) substrate. The fact that we nonetheless have clearindications for a commensurate molecular overlayer is in contrastto the established knowledge for metal–organic interfaces.This shows that even at weakly interacting interfaces, the substratecan play a significant role in guiding self-assembled structures.