Chemical Sensors Based on Graphene and 2D Graphene Analogs

The outbreak of the global pandemic has aroused significant attention from the public for healthy living environments. From this point of view, chemical sensors are crucial since these devices or actuators have diverse applications, such as environmental monitoring, food safety, industry, and healthcare. The development of chemical sensors may substitute human senses and precisely identify unspecified substances or discriminate materials accurately. For the implementation of chemical sensors utilized in daily life, there are requirements such as portability, low cost, low power consumption, high selectivity, and sensitivity. The most adequate materials are 2D materials that exactly agree with the described conditions. 2D materials have been studied for sensor applications owing to their unique material characteristics specialized for detecting particular substances. High surface to volume ratio or numerous reaction sites are representative physical properties of 2D materials. Furthermore, high carrier mobility is a typical feature of these substances appropriate for manufacturing advanced chemical sensors. Herein, the history and recent advances of 2D material‐based chemical sensors along with a description of perspectives and future challenges are introduced. This review provides a guideline for preparing chemical sensors based on 2D materials as next‐generation sensing devices.


Introduction
The development of chemical sensors has provided healthy daily life by precisely detecting airborne or liquid-carried chemicals. [1] The specific detection of these chemical species is crucial, related to the food, medical, and environmental industry having DOI: 10.1002/adsr.202200057 a direct connection to human health. [2,3] Organ-on-chip devices were first introduced in 1990, and have been improved to be more sophisticated and applicable as time passed. [4] These electronically replicated human organs have low limit of detection (LOD), high sensitivity, and discrimination abilities to target substances. Human basic senses comprise auditory, visual, tactile, olfactory, and gustatory, which can be classified into five groups: hearing, sight, touch, smell, and taste. The most conspicuous difference between these sense groups is that the former three groups are triggered by physical stimulation and the latter two groups are induced by chemical stimulation. Thus, the improvement of chemical sensing is closely related to developing the electronic nose (E-nose) or electronic tongue (E-tongue) based on gas or liquid sensors, respectively. The first chemical sensor was introduced in the 1960s, [5] based on metal oxide materials having the advantages of high sensitivity, stability, simple operation principles, and low cost. Thus, the metal oxide-based chemical sensors have been extensively researched due to their unique advantages. [6,7] Various methods were introduced to enhance the sensitivity and selectivity of metal oxide-based chemical sensors by nanostructure construction, [8][9][10] doping, [11][12][13] and heterojunctions. [14,15] However, metal oxide-based chemical sensors such as gas sensors suffer from high electrical power consumption due to the high operating temperature (300-450°C ). [16][17][18] The sensitivity of chemical sensors is highly associated with operating temperature due to the high activation energy for adsorption and desorption of gas molecules. On the other hand, 2D material-based chemical sensors can operate at much lower temperature than metal oxide-based chemical sensors. [19,20] 2D materials have high sensitivity and carrier mobility at low temperature and large specific surface areas, which are advantageous in terms of chemical sensing. [21][22][23] Recently, sensor technologies have been integrated into devices used in daily life, leading to the Internet of Things (IoT) era. [24] However, the demerits of metal oxide materials hinder daily life's sensor usage, such as mobile phones and other portable devices. Following the development of sensor technologies, the platform material must be adjusted for real applications. Nevertheless, there still exist several drawbacks to be resolved, such as high power consumption, limitation of miniaturization, and poor compatibility. The data were collected using the Web of Science-Web of Science Core Collection Search: keywords of (chemical sensor*) or (optical sensor*) or (surface acoustic wave sensor*) or (field effect transistor sensor*) or (electrochemical sensor*) and (2D material*) or (graphene) or (MoS 2 ) or (molybdenum disulfide) or (TMDs) or (MXenes) were used.
2D materials are prospective candidates for chemical sensors which can be applied in the IoT platform due to their unique physical and chemical characteristics, including high surface to volume ratio and abundant edge sites. [25][26][27] Moreover, device fabrication is facilitated due to high flexibility, low cost, excellent mechanical strength, and high optical transparency. [28][29][30] Regardless of operation principles, 2D materials have high potential to be utilized in diverse types of chemical sensors due to their ideal characteristics. Thus, to maximize sensing performance of 2D materials, understanding the sensing mechanism and operation principle are significantly important. The interactions between 2D materials and analytes can be explained by physisorption and chemisorption. [31] As target molecules or ions become adjacent to the surface of 2D materials, they interact with each other without any covalent bonding, called physisorption. The physisorption usually includes bonds between 2D materials and the analyte, which can alter the structure or properties. [32] On the other hand, interactions involving any covalent bonding or chemical reaction is a chemisorption. The chemisorption comprises covalent bonding from the formation of defects on the superficial of 2D materials. [33,34] The physisorption and chemisorption can be triggered by the distinctive natures of 2D materials. For example, graphene and graphene oxide (GO) have abundant conjugated electrons on the aromatic rings and large surface area. Moreover, plenty of active edge sites exist in transition metal dichalcogenides (TMDs) and MXenes, which are highly responsive to chemical substances. Further information on various types of chemical sensors based on these extraordinary features will be discussed.
In Figure 1, the publication and citation number of chemical sensors based on graphene, TMDs, and MXenes are depicted. Both increased substantially in the past decade, showing that 2D material-based chemical sensors has become mainstream of sensor research. The history of 2D materials-based chemical sensors is depicted in Figure 2. Since the exfoliation of graphene was firstly reported in 2004, research on graphene has been actively conducted. [35] In 2007, graphene was initially applied in field-effect transistor (FET) gas sensing, which triggered the graphenebased chemical sensor research. [36] In addition, the exfoliation of MoS 2 was introduced in 1986 and marked the beginning of TMDs-based chemical sensors. [37] MXenes, reported in 2011 as a novel 2D material, has gained numerous interests in sensor applications. [38] Besides the research articles, the review articles covering chemical sensors based on 2D materials have also been reported for decades. The chemical sensors based on 2D materials handled in review articles are classified by operation type, including optical sensors, [39] FET sensors, [40][41][42] and chemoresistive sensors. [43][44][45] Usually, other review articles introduce 2D material-based chemical sensors by comparing only several types of chemical sensors with different principles. [30,[46][47][48] To introduce various chemical sensors based on 2D materials, we organized this article by the following sections. In Section 2, a wide range of operation types based on physical and chemical properties of 2D materials are specifically introduced. In the following section, we cover chemical sensors detecting various phases of analytes including gas and liquid, which is a differentiated aspect from other reviews. The practical applications of 2D materialbased chemical sensors which have outstanding compatibility with human beings will be described in Section 4. The final section states the challenges of conventional chemical sensors and proposes the guideline of next-generation chemical sensors based on 2D materials. This review is anticipated to broaden the field of 2D material-based chemical sensors for future research and the extension to real-life application.

Types of Chemical Sensors
Chemical sensors quantitatively evaluate particular chemical analytes and convert the detected chemical data into electronic data. The chemical sensors are broadly applied in various industries, including medical diagnosis, military purposes, and healthcare purposes. These chemical sensors are different by each type, but they share two aspects: receptors and transducers. The receptor accepts the chemical analyte and physically contacts the detector. According to the type of sensor, the receptor acts differently by interacting with the substance as a whole or thinning out specific ions or molecules. Transducers intake the chemical data from the reaction between the chemical analyte and receptor and convert it into electrical information. The types of chemical sensors are classified by their mechanisms and working principles. In a broad classification, chemical sensors can be divided into adsorption, optical, FET, chemoresistive, electrochemical, and surface acoustic wave (SAW) sensors. According to Figure 3, these chemical sensors are applicable using 2D materials due to their promising characteristics. The properties of 2D materials include large surface area, high electrical conductivity, high sensitivity, and facile formation to colloids, films, and composites. Based on these fascinating features, the chemical sensors can be operated by various mechanisms with remarkable performance.
www.advancedsciencenews.com www.advsensorres.com either a gas or a liquid phase. [49] Unsaturated forces drive adsorption at the solid surface that can create bonds with the adsorbate. Typically, these forces are electrostatic or van der Waals interactions. Direct electron transmission between the sorbate and the sorbent results in a stronger contact. The degree of this contact determines how easy or difficult it is to remove the adsorbate for adsorbent renewal and adsorbate recovery. The adsorbent selective characteristic is determined by the relative access and the intensity of the surface contact for a single component in a feed combination. The solid is the mass-separating agent, and the differentiating mechanism is partitioning the fluid and solid phases. An energy-separating agent, often a pressure or temperature change, is employed to reverse the process and renew the sorbent. As a separating technique, adsorption has various advantages. When distillation is difficult or impossible due to the identical boiling point components, vapor-liquid azeotropes, or species with low relative volatilities, adsorption methods allow for large solute loading and perform well in dilute settings. Adsorption methods have drawbacks due to the usage of an adsorbent mass-separating agent. Each adsorbent bed must be eliminated from the process before regeneration, which normally requires more than one column in series or parallel.

Optical Sensors
An optical sensing system comprises a light source, a sample for analysis, and an optical receiver, all of which determine the scope of the sensing. When a light interacts with materials, it can be ab-sorbed or dispersed; sometimes, both absorption and scattering occur simultaneously. During the contact, the light imparts energy to the matter, causing some of the characteristics of both the matter and the light to alter. The wavelength, velocity, intensity, and deflection angle of light are the most significant components to consider. Optical sensors involve electromagnetic radiation to generate electrical signals from the transducer. The concentration of the analyte is measured by the radiation between the receptor and the target substance. [50] The transmission and reception of light are the fundamental basis of optical sensors. Depending on the material, such as wood, metal, plastic, transparent, or colored product, the light reflected or interrupted by the target item is assessed by various optical equipment. Optical sensors can be divided into direct sensing or indicator-mediated detection. Direct optical sensors detect the analyte directly from intrinsic optical properties such as adsorption or luminescence. [39] On the other hand, indicator-mediated systems, the optical response passes through an analyte-sensitive dye molecule to monitor the concentration of target substances. [51,52]

Colorimetric Sensors
The research on colorimetric sensors has been thrived due to their ease of use, low cost, and sensitive properties to numerous analytes. [53] Colorimetric sensors are optical sensors that change color in response to external inputs. A stimulus can be any change in the physical or chemical environment. As a result, a change in a property of the environment that must be www.advancedsciencenews.com www.advsensorres.com detected dictates the design of the specific colorimetric sensor. Colorimetric chemical sensors detection is noticed by the assembly/disassembly of analytes accompanied by a color change. Colorimetric sensing is a representative indicator-mediated method used when spectroscopic properties are inadequate for direct monitoring. Representative detecting analytes are pH and carbon dioxide. Colorimetric chemical pH sensors are operated in a short pH range, and researchers are struggling to improve the pH range by using various pH indicators with diverse acid-base equilibrium constants (pKa). Using multiple pH indicators, a wide range of pH chemical sensors has been reported. [54] Carbon dioxide concentration is also detected by monitoring pKa in the chemical reactions from carbon dioxide and evaluating pH by the generated carbonic acid.

Electroluminescence Sensors
Electroluminescence (EL) is an optical phenomenon that produces photons by the radiative recombination of holes and electrons as electric current passes through a material. EL is widely categorized into two types: 1) low-field EL, which involves the injection of minority charge carriers, and 2) high-field EL, which involves the acceleration of majority charge carriers to high optical energies. [55] Low-field EL is more prevalent in p-n junc-tion semiconductors such as light-emitting diodes (LEDs), while high-field EL is more common in phosphor or organic luminescent EL displays. Low-field EL requires the recombination of electrons and holes in the semiconductor material, which must be isolated from one other prior to recombination. Doping creates a p-n junction, which separates electrons and holes from one another. On the other hand, high-field EL happens in phosphors when a strong electric field accelerates electrons in the material to very high energies capable of stimulating the luminous centers in the phosphor. The excited states of the luminous core relax, resulting in EL emission. High-field EL devices are further classified as AC-operated and DC-operated. Phosphors for EL displays are utilized in powder or thin films. The electric field in the active area of high-field EL devices is quite high, ranging on the order of 10 6 V cm -1 , whereas the electric field in low-field devices, such as LEDs, is just a few V cm -1 . The former is less sensitive to temperature, whereas the latter is heavily impacted by it. [56] The mechanism of 2D material EL sensors is based on the emission intensity change by charge transfer between chemical analytes and 2D materials. [57,58] The charge transfer occurs between the chemical analytes and 2D materials due to the adsorption of analytes on the surface of 2D materials. The analytes can act as electron acceptors or donors, depending on the adsorption sites and operation temperature. Due to the charge transfer, the electric field on the luminescent 2D materials is modulated, www.advancedsciencenews.com www.advsensorres.com resulting in EL intensity change. Thus, the variation of EL intensity is highly related to the amount of charge transfer between chemical analytes and 2D materials. [59]

Surface Plasmon Resonance (SPR) Sensors
SPR is a charge-density oscillation that can occur at the interface of two substances with opposite-sign dielectric constants, such as a metal and a dielectric. [60] The charge density wave is related to an electromagnetic wave, the field vectors that reach their apex at the interface and fade evanescently into both mediums. This surface plasmon wave (SPW) is a TM-polarized wave, and the magnetic vector is perpendicular to the SPW propagation direction and parallel to the interface plane. Several metals are applicable for SPR, especially, Au and Ag are the most often employed. Because of the substantial loss in the metal, the SPW propagates with considerable attenuation in the visible and nearinfrared spectral ranges. The electromagnetic field of an SPW is very asymmetrically distributed, with the great bulk of the field localized in the dielectric. An SPW propagating along the surface of Ag is less attenuated and displays more electromagnetic field localization in the dielectric than an SPW supported by Au. In general, an SPR optical sensor consists of an optical system, a transducing medium that connects the optical and chemical domains, and an electrical system that supports the optoelectronic sensor components and allows data processing. The transducing medium converts variations in the amount of interest into changes in the refractive index, which may be measured by optically interrogating the SPR. The optical portion of the SPR sensor incorporates an optical radiation source and an optical structure in which SPW is stimulated and probed. The electronic system generates and processes an electronic signal while interrogating the SPR. The features of an SPR sensor subsystem define its major attributes. Sensor sensitivity, stability, and resolution are all affected by the parameters of the optical system and the transducing medium. The parameters of the transducing medium essentially dictate the sensor selectivity and response time. [61]

Surface Enhanced Raman Scattering (SERS) Sensors
The increased intensity of Raman spectroscopy enhances the metal surface electric field due to the SERS. [62] The enhancement of surface effect is the strongest for Au and Ag nanostructures due to the interaction between the localized surface plasmon made on the surface of the metal with molecule vibrational levels. Two main conditions are required for SERS to be measured: SERS activation available nanostructured metal surface is needed, and the sample must be immobilized to the surface. The fields at the surface differ from those detected in the distant field when an electromagnetic wave interacts with a metal surface. If the surface is rough, the wave may excite the localized surface plasmons, causing the electromagnetic fields near the surface to be amplified. If the strength of the incident and scattered fields is increased, then the significant increase in Raman scattering intensity exists. Because SERS is distance dependent, one crucial condition for these sensors is that the analyte of focus must be within a few nanometers of the nanostructured surface. This may be accomplished by either drop casting the analyte directly on bare-metal film over nanospheres or utilizing different surface functionalization procedures to get the analyte closer to the noble-metal structure. [63] SERS operation is highly promising for chemical sensing applications since it is selective, sensitive, and has low interference from water. They are used in various fields such as biomedicine, environmental analysis by the facile materials fabrication and enhancing the understanding of plasmonic interaction. [64,65]

Field-Effect Transistor (FET) Sensors
Due to the ultrahigh sensitivity, label-free operation, cost reduction, and miniaturization, FET sensors have received growing interest for ionic sensing. [66] FET sensors detect changes in surface potential that occur during the binding of an analyte to a recognition element. As the surface potential changes, the current moves through a semiconductor channel. The source electrode is located at one end of the channel. The drain electrode is located at the other end of the channel. The electrical parameter of the channel determines the conductivity of the FET. A minor change in gate voltage (V G ) can result in a fluctuation in current from source to drain due to the amplification of impulses in FET. FET is classified into two types, which are junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). The JFET is constructed of either n-type or p-type semiconductor material, while the gate is built of the opposite semiconductor type. Electric charges are conveyed mostly in electron deficits known as holes in p-type materials, while the charge carriers in n-type materials are mostly electrons. The junction in a JFET is the border between the channel and the gate. The p-n junction is normally reverse-biased, so no current flows between the channel and the gate. However, in certain cases, a little current flow across the junction during a portion of the input signal cycle. The gate electrode consists of metal with an oxidized surface. The oxide layer acts as an electrical insulator between the gate and the channel. As a result, the MOSFET was initially referred to as an insulated-gate FET (IGFET), although this name is now seldom used. Because the oxide layer functions as a dielectric, there is almost no current flowing between the gate and the channel at any time throughout the signal cycle. This results in an unusually high input impedance for the MOSFET. In general, FETs are not employed for high-power amplification, which is necessary for large wireless communications and broadcast transmitters. A single integrated circuit (IC) can include thousands of FETs, resistors, capacitors, and diodes. [47]

Chemoresistive Sensors
Chemoresistors are resistive chemical sensors that rely on chemical interactions between analytes and sensing materials. Covalent bonding, hydrogen bonding, or molecular recognition can all be used to interact between the sensing material and the analyte. Chemoresistive sensors typically use a two-electrode system by measuring the resistance of the sensing layer as a function of time following gas exposure, in which the sensing mechanism is similar to FET sensors. The intrinsic resistance of the sensing www.advancedsciencenews.com www.advsensorres.com material can be changed by the presence or absence of the analyte. Analytes interact with the sensing material during exposure. The resistance reading varies as a result of these interactions. Resistance fluctuations in certain chemoresistors merely indicate the presence of the analyte. In others, the resistance variations are proportionate to the amount of analyte present, allowing the analyte concentration to be detected. These materials are often utilized as partly selective sensors in devices such as E-tongues or E-noses. [67]

Electrochemical Sensors
There are diverse types of electrochemical sensors: amperometric, impedimetric, potentiometric, photoelectrochemical, and electrogenerated chemiluminescence. Due to the specific sensing target molecule interactions, and a Nernstian equilibrium is formed at the sensor surface. [68] Electrochemical oxidation and reduction are involved in amperometric sensors evaluating the current as an indicator of the target molecule concentration by employing a voltage among the reference and working electrodes. Conductometric sensors, also known as impedimetric sensors, assess changes in surface impedance to detect and quantify analyte-specific recognition events on the electrode. Amperometric measurements are widely employed as a high-accuracy and sensitivity analytical approach. The applied voltage acts as a driving force for electrocatalytic redox processes that create electrical currents proportional to the analyte concentration. The fundamental instrumentation requires a controlled-potential system, and the electrochemical cell comprises two electrodes immersed in a suitable electrolyte. A three-electrode cell, with one of the electrodes serving as a reference electrode, is a more advanced and frequently used design. [69] On the other hand, a working electrode is defined as an electrode where the reaction happens. A reference electrode such as Ag/AgCl or Hg/Hg 2 Cl 2 is defined as the electrode that maintains a constant potential while working electrodes are variable. [70] The inert conducting materials are frequently used as an auxiliary electrode. In controlledpotential investigations, a supporting electrolyte is required to reduce electromigration effects, minimize the solution resistance, and keep the ionic strength constant. Both theoretical and practical techniques have been thoroughly discussed. [71] Electrochemical impedance spectroscopy (EIS) is an alternating current (AC) approach by typically using sinusoidal stimulation waveforms. The response of the system to a potential or current sinusoidal disturbance is examined as a function of frequency using AC techniques. The frequency sweep makes it available to investigate the charge transfer and mass transport occurring at the electrode. [72] Using a wide frequency range (1 mHz-10 kHz), the electrical resistance (impedance) of the metal/solution interface is measured. [73] There are various advantages of electrochemical sensors, including low cost, low power consumption, linear output, and superb resolution with excellent accuracy and repeatability. Moreover, the electrochemical sensors have low possibility of the contamination by other chemicals, maintaining the lifespan of the sensor. [74] However, the electrochemical sensors must be replaced periodically for a proper operation. Dry atmosphere and extreme temperatures can dehydrate the electrodes, reducing the lifespan. While the majority of electrochemical sensors func-tion effectively in extreme working environments, they are very sensitive to temperature changes. The reaction speed decreases at low temperature and the evaporation of electrolytes occurs at high temperature. Thus, the atmospheric temperature should be maintained constantly to maximize the sensing performance. [75] 2.6. Surface Acoustic Wave (SAW) Sensors SAW sensors reliably monitor physical and chemical information such as temperature, stress, and gas density. Because of its miniaturized size, the surface wave device is recognized as the next-generation wireless and compact sensors. Simultaneously, they have excellent compatibility with integrated circuits and have been widely employed in the field of simulated digital sensing and communication. [76] The SAW sensors concentrate the signal on the substrate surface. It features a high operating frequency and a high information-sensitive accuracy. The realtime information-detecting is facilitated and can rapidly transform the detected information into electrical signal output. SAW sensors also offer the benefits of downsizing, facile integration, low cost, low power consumption, and direct output of frequency signal. [77] Piezoelectric crystals are commonly used as a medium in SAW sensors. The sound waves are then generated by an extra positive voltage and propagated through a substrate before turning into an electrical signal output. The piezoelectric effect is the primary active effect of the SAW sensor. Several elements must be addressed when designing, such as relative proportions, sensitivity, and efficiency. Generally, the wireless passive surface wave sensor's signal frequency spans from 40 MHz to several GHz. The frequency of SAW is altered by the surface wave of the sensitive layer. Wireless passive SAW system components include a transmitter, a receiver, a sound surface wave device, and a communication channel. The transmitter and receiver are two modules of the transceiver or interpreter respectively. The power is transmitted to the sound surface wave device by the reader, which can be a continuous wave or a pulse from the transceiver input. Generally, the power size acquired by the sound surface wave device has a limit in terms of decreasing the maximum transmit power while maintaining the same average power. The received signal is generally radiated by an efficient radiation power antenna, according to the isotropic radiator. The SAW device wave velocity and frequency drift as the external environment changes. The SAW gas sensor will make use of the performance of a gas-sensitive film that is selectively adsorbed to the target gas on the piezoelectric crystal surface. When the gas-sensitive film interacts with the gas to be tested (chemical effects, physical adsorption), the membrane layer mass and conductivity of the gas-sensitive film change, causing the piezoelectric crystal SAW frequency to drift. The gas concentration varies, as does the thickness of the film layer, resulting in a fluctuation in the wave frequency. The change in reaction gas concentration can be precisely determined by detecting the change in SAW frequency. Reproduced with permission. [78] Copyright 2018, Elsevier.
mechanisms. To improve the sensing properties of chemical sensors, various properties such as surface properties, electrical properties, mechanical properties, and optical properties should be comprehensively modulated. The sensing reactions of chemical sensors mostly take place on the surface of the sensing materials. The extremely high surface area-to-volume ratio and high sensitivity of the surface of 2D materials make them the perfect candidate for efficient chemical sensing. In this section, chemical sensors based on 2D materials will be discussed, explaining the fabrication process, optical images, sensing properties, and sensing mechanism.

Graphene and GO
Graphene, a representative 2D material, is composed of a oneatom-thick single layer of sp 2 hybridized carbon atoms in a honeycomb structure. [39,43] Compared to other 2D materials, graphene has its own unique electronic and optical properties. When applied in optical sensors, the unique properties of graphene, including broadband optical absorption in the visible range, nonlinear optical properties, and ultrafast optical response, become vital components. [39] The optical gas sensors can be operated in diverse principles, including colorimetric type, EL type, and SPR type.
Graphene derivatives have gained tremendous attention in colorimetric gas sensing due to high specific surface area and extremely low light absorption. Moreover, the color shift can be observed intuitively, which is suitable for further application in various chemical sensors, including humidity and harmful gas sensors. Gong et al. utilized GO thin film via dip coating method for visualized colorimetric C 2 H 5 OH gas sensor. [78] Figure 4a shows the schematic illustration of the fabrication of GOreflecting films with various GO concentrations. The Si wafer was immersed in GO solution for 2 min to achieve full wetting and was taken out from the solution with a rate of 1 cm min -1 . The GO film showed different colors on the silicon wafer by controlling the solution concentration. The color difference was derived from the visible light interference at the interfaces, as shown in Figure 4b. When incident light is irradiated on GO film, a portion of the light is reflected, as expressed in the red arrow. The other portion of the light is transmitted through GO film, and the light can be transmitted through the Si layer or reflected at GO/Si interface. The thickness of GO determines the total light waves, which induce either constructive or destructive light. The prepared GO films were characterized by scanning electron microscope (SEM), as shown in Figure 4c,d. The thickness of GO film ranged from 200 to 275 nm, depending on GO concentration. The gas sensing properties of GO film with a thickness of 225 nm were tested, as shown in Figure 4e,f. The wavelength shift of the two main peaks to C 2 H 5 OH was 50 nm and 75 nm, respectively. Meanwhile, the wavelength shift of the two main peaks to CHCl 3 was 10 nm and 15 nm, respectively. The film exhibited significant color change and spectral shift upon exposure to C 2 H 5 OH gas without any electrical devices. Chi et al. fabricated colorimetric NO 2 gas sensor operable at room temperature based www.advancedsciencenews.com www.advsensorres.com on GO/polystyrene sulfonate (GO/PSS) reflecting film. [79] The GO/PSS film was deposited by spin-coating-assisted layer selfassembly (SA-LbL) method with high uniformity. To observe the film's color in the visible light range, GO/PSS film with the optimized thickness of 180 nm was utilized. The blue shift was investigated upon exposure to NO 2, and the wavelength shift showed good linearity to NO 2 concentration, which facilitated the color change observation. Moreover, the spectral shift of the GO/PSS film was reversible during the atmosphere shift between NO 2 and N 2 . The gas molecules were intercalated into the GO layers, which was investigated by X-ray diffraction (XRD) analysis. The (002) diffraction peak of GO at 52% RH appeared at 11.09°, corresponding to 0.797 nm of interlayer distance. The interlayer distance was calculated by applying Bragg's law (n = 2dsin ). The (002) peak was shifted to 9.76°at 75% RH, and 10.43°upon exposure to NO 2 at 75% RH, respectively. GO can behave as a ptype semiconductor in the intercalation process, increasing the interlayer distance. The hydrophilic NO 2 gas can interact with the negatively charged GO/PSS acting as electron acceptor, which reduces the film thickness. The intercalation process can be explained by the change of interlayer distance, inducing the expansion of GO.
The graphene-based EL devices with alternating current (AC) can be suitable for gas sensing thanks to high lighting efficiency, highly distributed light emission, and high flexibility. The AC-EL device consists of four layers: a transparent substrate with transparent conducting material, light emitting phosphor layer, a dielectric layer, and backside electrodes. Seekaew and Wongchoosuk fabricated a graphene-based alternating current electroluminescent (AC-EL) gas sensor for CO 2 detection. [80] For CO 2 detection, the graphene sensing layer was deposited instead of the dielectric layer, and the EL gas sensor structure was fabricated. The fabrication starts with the phosphor layer, where ZnS:Cu, Cl was screen-printed on indium tin oxide (ITO) coated polyethylene terephthalate (PET). Then, the Ag back electrodes were screen-printed on the phosphor layer. Finally, the graphene sensing layer synthesized by chemical vapor deposition (CVD) was drop-coated on Ag electrodes. Before graphene deposition, the EL device exhibited a green light emission when the frequency was higher than 100 Hz due to the radiative recombination of excited electrons. After graphene deposition, the EL graphene sensors exhibited blue light emission at frequency over 1000 Hz due to the transition of Cl from the S site to interstitial Cu site. The transition between Cu or Cl and ZnS host material can induce the blue-green emission. The CO 2 gas sensing properties of the fabricated graphene EL device were investigated at room temperature. Due to the adsorption and desorption of CO 2 molecules on the graphene surface, the EL intensity was changed. When CO 2 was exposed to graphene, the EL intensity decreased and increased to the initial upon exposure to air. The response of the EL gas sensor was defined as ((I air -I gas )/I air ) x 100%, where I air is the EL intensity in the air atmosphere and I gas is the EL intensity in the gas atmosphere, respectively. The resistance of graphene is modulated by the charge transfer between CO 2 molecules and graphene, where electrons transfer from CO 2 to graphene. Due to the p-type semiconducting nature of graphene, the electrons from CO 2 recombine with holes, resulting in reduced hole concentration and increased resistance. Thus, the exciting electric field on EL phosphor is weak-ened, leading to the EL intensity reduction. The amount of charge transfer is related to the resistance variation, which can be detected by EL intensity variation. Yakoh et al. introduced a humidity sensor based on an EL display functionalized by GO/Nafion nanocomposite. [81] The ACEL lamp with top emission structure (TES) was fabricated by depositing several layers. The silver paste ink was coated on the substrate, acting as a rear electrode, and the dielectric layer was coated on the silver paste layer. Then, the phosphor layer was coated, followed by the coating of conductive ink for the top electrode. GO has high permeability to water molecules due to the oxygen functional groups. At low relative humidity (RH), the physisorbed water molecules on GO are difficult to move due to the double hydrogen bonding, resulting in restricted hopping to the adjacent sites. At high RH, the mobility of physisorbed water molecules increases, inducing the proton hopping to adjacent sites. The proton hopping of water molecules is occurred by the conductivity derived from the Grotthuss chain reaction. The GO coating on the phosphor layer resulted in poor humidity response, however, the response was improved when Nafion was the major component in the nanocomposite. The sulfonic acid hydrophilic group attached to the backbone enhanced the proton transport, increasing the proton conductivity at humid condition. The improvement of response and light intensity was derived from the high proton conductivity of Nafion. The TES ACEL sensor with GO/Nafion nanocomposite showed no light emission at low relative humidity (RH). However, the light was emitted at high RH condition, which was induced by the mass loading effect of water adsorption. In the TES configuration, the light generated from the phosphor can be directly emitted through the top conducting material. Compared to bottom emission structure (BES), the humidity sensitivity was higher due to the protective phosphor layer. Due to the intuitive light emission, EL gas sensors are highly appropriate in practical applications such as humidity sensor and breath sensor.
SPR gas sensors are refractometric sensors that measure the refractive index changes upon exposure to target gas in the electromagnetic wave field. Especially, fiber optic SPR gas sensors based on graphene have been widely researched due to their small size, low cost, and remote sensing compared to the conventional prism-based SPR sensors. Mishra et al. applied poly(methyl methacrylate)/reduced GO (PMMA/rGO) nanocomposite film for fiber optic SPR NH 3 gas sensor. [82] The Cu film was deposited on an unclad region of optical fiber acting as sensing probes, and PMMA/rGO nanocomposites were coated on Cu film. The SPR measurements were carried out by wavelength interrogation method at different concentrations of target gases. As NH 3 concentration increased, the SPR spectra were red-shifted in the resonance wavelength. This was due to the change of dielectric constant of PMMA during the interaction with NH 3 gas. By fabricating graphene-based nanocomposite film, the SPR sensing performance was improved compared to pristine GO. The oxygen-containing functional groups of GO are reactive with polar gases, changing the dielectric constant and volume of PMMA. The distance between GO is increased which increases the resistance of PMMA/rGO film. Despite good sensing performance, optical fiber SPR sensors suffer from weak mechanical stability due to the asymmetric destruction. Thus, diverse materials and structures have been proposed to couple the fiber core mode into the SPR mode. Wei et al. proposed fiber grating-assisted SPR www.advancedsciencenews.com www.advsensorres.com sensors for methane gas detection with improved mechanical stability and sensing properties. [83] The structural integrity and circular symmetry of the fiber were maintained by integrating metal layers on fiber gratings. The fiber grating platform used in this work is long-period fiber grating (LPFG), which can enhance the excitation efficiency of SPR compared to fiber Bragg grating (FBG). For LPFG sensor fabrication, Ag film was deposited on the processing region of the fiber. The graphene monolayer was deposited by chemical vapor deposition (CVD) on a Cu foil followed by PMMA film coating. After removing the bottom side graphene and Cu layer completely, the floating graphene film was transferred to the sensor probe. Then, the graphene film covered the Ag film, and the supporting PMMA was removed. The methane sensing properties of LPFG SPR sensor were examined by measuring the resonance spectra. The graphene LPFG SPR sensor was red shifted as the methane concentration increased. The sensor showed the sensitivity of 0.34 nm% −1 and LOD of 0.029%, respectively. The improvement of sensitivity was studied by finite element analysis (FEA) method. From the simulation result, the electric field intensity in Ag/graphene structure was higher than that of structure without graphene, resulting in exponential decay of electric field in the sensing medium. Moreover, the penetration depth of the electric field into the sensing medium was larger in Ag/graphene structure. Thus, the graphene layer in LPFG sensor enhanced the electric field around the sensing layer, which can contribute to enhanced sensitivity to methane gas.
Graphene derivatives are highly prospective in FET gas sensors due to their 2D layered structures. The main components of FET gas sensors are the gate electrode, source electrode, drain electrode, insulating layer, and sensing layer. The sensing mechanism of FET gas sensors is based on the surface charge transfer and the modulation of channel conductivity, which can be obtained by measuring the current, on/off ratio, and threshold voltage (V th ). Graphene derivates can be applied in the sensing layer of the FET gas sensor due to the easy formation of the conductive channel between the source and drain electrodes. [47] Schedin et al. first applied graphene as FET gas sensing layer, introducing the sensing characteristics of mechanically prepared graphene to various gas molecules. [36] They investigated the electrical resistance change when gas molecules were adsorbed on the surface of graphene. The carrier doping induced the conductivity modulation, which enabled the detection of gas molecules. Although the sensitivity to various gases at room temperature was examined, the gas sensing performances needed to be improved. Inaba et al proposed graphene FET gas sensor with enhanced response and selectivity to NH 3 . [84] The ionic liquid (IL) covered the graphene sensing channel, and the V G was applied through the double layer of IL. The conventional SiO 2 layer was replaced by an IL double layer, enabling low-voltage operation. The gas sensing properties were evaluated by measuring the voltage shift and current change, which was -0.057 V and -0.44 A per 10-fold increase of NH 3 concentration. The device showed a LOD of 130 ppb and a response time of 33 s, respectively. The selectivity to NH 3 was excellent, which was investigated by comparing the response to other target gases. In addition, the selectivity of graphene FET gas sensor can be controlled by decorating noble metal on graphene. Sakamoto et al. fabricated Pd modified graphene FET gas sensor for H 2 detection. [85] By decorating Pd on graphene, the sensor exhibited negative voltage shift upon exposure to H 2 due to the work function shift. The electrons were transferred from palladium hydride to graphene resulting in negative voltage shift. The minimum voltage shift was 0.2 V and LOD was 100 ppb, respectively. Although the FET gas sensors based on graphene derivatives reveal good sensing properties, several factors must be improved. The carrier injection efficiency of FET sensors is highly affected by the work function of the source/drain electrodes, which significantly influences the sensing performance. Moreover, the dimensions of channel, including channel length and width should be optimized.
Chemoresistive gas sensors are the most widely studied gas sensors thanks to the easy operation principle, low cost, small size, and low power consumption. The electrical resistance in the air and gas atmosphere are recorded to examine the gas sensing properties. Various semiconducting materials were utilized due to the suitable resistance in chemoresistive gas sensing. As mentioned above, metal oxides were broadly used for chemoresistive gas sensors showing outstanding gas sensing performance. However, a high operating temperature was required due to high electrical resistance and insufficient activation energy at low temperatures. Therefore, the external heating systems such as microelectromechanical systems (MEMS) and back-side heaters were integrated into the sensors, increasing the electrical power consumption. To overcome the high electrical power consumption, room temperature operable gas sensors based on graphene and its derivatives without a heating system was researched. [86][87][88] One of the most promising strategies of room temperature operation is utilizing the Joule heating of the graphene layer. [89][90][91] The heat was generated from the graphene layer when the bias voltage was applied. Kim et al. fabricated an all-graphene gas sensor consisting of graphene for electrodes and sensing area. [19] Three-layer graphene (3LG) was deposited on Cu foil and was transferred to PI substrate with the assistance of poly(methyl methacrylate) (PMMA). The self-activation effect was induced in the patterned graphene channel with a gap width of 5 m. The fabricated graphene sensor showed good gas sensing properties to NO 2 at room temperature. The graphene sensor exhibited a response of 4.47% at 1 V and 12.49% at 60 V, respectively. The patterned graphene gas sensor showed improved sensing properties than the non-patterned graphene gas sensor due to the Joule heating effect in the grain boundaries. The LOD of the sensor was calculated by exposure to NO 2 gas ranging from 1 to 10 ppm. The response was linearly increased with NO 2 gas concentration, and the LOD was estimated to be 6.87 ppb. The self-activation effect by applying bias voltage was examined, and the sensor exhibited outstanding flexibility and transparency due to the all-graphene sensor structure. Kim et al. fabricated a flexible gas sensor platform by tailored graphene micropatterns. [92] Figure 5a shows the fabrication process of the graphene gas sensor by direct polymer curing (DPC) transfer. The 3LG was deposited on Cu foil by CVD for self-heating effect and was patterned by the photolithography process. The uncovered graphene and PR layer were completely removed by oxygen-reactive ion etching (RIE) and acetone treatment. Then, polyimide (PI) was coated and thermally annealed at 300°C. The thermal laminating film (TF) was laminated on the PI layer, and Cu foil was etched by ammonium persulfate (APS), resulting in graphene micropatterns formation. Figure 5b shows the optical image of the graphene microchannel on a PI substrate with excellent transparency. The length of the microchannels was 0.05, 0.1, 0.25, and 0.5 mm, while the width was fixed at 5 m, as shown in Figure 5c. The patterning processes were carried out on 4 in. wafer-scale Cu foil with good transparency. The noble metals were deposited by the e-beam evaporator on a graphene layer with a size ranging from 1 to 20 nm. Further gas sensing measurements were conducted by Pt-decorated graphene, as shown in Figure 5d. The response to various concentrations of H 2 from 20 ppm to 1% was plotted as a function of concentration exhibiting a linear relationship. The sensor was attached to metal pipes in a bent state to investigate the flexibility and durability of Ptdecorated graphene, as shown in Figure 5e,f. The response to 500 ppm of H 2 was slightly decreased when the sensor was bent, as shown in Figure 5g. However, the sensor still exhibited reliable H 2 sensing.
Among various graphene derivatives, GO and rGO are the most frequently used materials for chemoresistive gas sensors. Compared to the preparation methods of graphene, such as CVD and epitaxial growth with high production costs, the preparation of GO is much easier and cheaper. [93] Park et al. proposed a NO 2 gas sensor based on bandgap-engineered GO. [94] The amount of the functional groups was controlled by modifying the acid washing process of Hummers' method. The epoxide groups were mainly reduced, due to the low NO 2 adsorption energy. The concentration of hydrogen chloride (HCl) was controlled and used in the acidic washing process in the Hummers' method. As HCl concentration in the washing process increased, the bandgap decreased, due to the decreased functional groups improving the electrical conductivity. The improved gas sensing performance was derived from the carrier transport of the narrowed bandgap. Thus, the bandgap engineering in GO is considered a critical component in high-performance chemoresistive gas sensing. Park et al. fabricated a chemoresistive humidity sensor based on rGO/MoS 2 composites. [95] Both rGO and MoS 2 have 2D layered structures, and heterojunction can be easily formed by the van der Waals force. The hydrophilic properties of rGO and abundant edge sites of MoS 2 can create a synergistic effect in chemoresistive gas sensing. rGO/MoS 2 hybrid composite (RGMS) was prepared by a facile solution process. rGO and MoS 2 solutions in DI water were ultrasonicated for 24 h. When heterojunction was created between rGO and MoS 2 , the response was improved to 872.7% at 50% RH which is 220 times higher than pristine rGO. The extraordinary humidity sensing performance was derived from various factors. RGMS structure provides abundant active sites for water adsorption which increases the response. Moreover, p-n heterojunction is formed between rGO and MoS 2 which narrows the conduction path. When water molecules are adsorbed on the surface of RGMS, the potential barrier is increased. The degree of potential barrier modulation is higher in RGMS compared to pristine rGO and MoS 2 which enhances the sensing performance.
The electrochemical gas sensors based on graphene and GO were extensively researched for detecting various toxic gases due to the large electrochemically active surface area and high carrier mobility. As explained in the previous section, electrochemical gas sensors can be categorized into diverse types based on measurement methods. Ng et al. applied graphene/IL nanocomposites for an amperometric NO gas sensor. [96] The graphene/IL nanocomposite gel was coated on a glassy carbon electrode, and Nafion membrane was coated to prevent the leaching of IL. The NO sensing properties were examined by conducting amperometric measurements at 0.8 V versus Ag/AgCl in phosphate buffer saline (PBS) solution. The exposure to NO induced the increase of the oxidation current with the response time of 4 s, which was due to the oxidation of NO. Akhter et al. proposed impedimetric CO 2 gas sensor based on GO. [97] The EIS was applied to investigate the CO 2 sensing performance. The resistance of GO film was decreased upon exposure to CO 2 with the response time of 3 s and recovery time of 5 s. Miyamoto et al. reported a proton-conducting GO membrane for solid electrolyte potentiometric H 2 gas sensor. [98] GO can be fabricated into a membrane by vacuum filtration process, which is suitable in potentiometric gas sensor. The potentiometric response of GO to H 2 gas was obtained by measuring the electromotive force (EMF). Pt/C was utilized as a sensing electrode, and Pt-black was utilized as a reference electrode, respectively. The EMF value was increased with H 2 concentration following the Nernst equation. The protonic conduction was occurred in the oxygen-containing surface functional groups of GO membrane. This work suggests that GO can be a promising candidate for potentiometric gas sensor operable at room temperature. Other types of electrochemical gas sensors such as photoelectrochemical methane sensor [99] and chemiluminescence H 2 S sensor [100] based on graphene derivates have been reported.
The acoustic devices for gas sensing can be categorized into two groups: bulk acoustic wave (BAW) and SAW. The difference between BAW and SAW is that acoustic waves are transmitted into the bulk structure for BAW, while acoustic waves are concentrated on the surface of the piezoelectric layer for SAW. [101] The SAW gas sensor consists of a piezoelectric substrate, input interdigitated transducer (IDT), output interdigitated electrodes (IDEs), and a delay line between the IDTs. The sensing layer is the key element determining the sensing properties evaluated by the resonant frequency shift. Graphene derivatives are utilized in SAW gas sensors as the sensing layer due to the ultrathin thickness and large surface area. Especially, GO-based SAW sensors are widely used in humidity detection due to the electrically insulating properties of GO, which enables the direct deposition of IDTs. [102] Xuan et al. fabricated ZnO/glass surface SAW humidity sensor with a GO sensing layer. [103] ZnO/glass substrate was applied for the piezoelectric substrate instead of the conventional LiNbO 3 substrate. The prepared GO solutions were drop-casted on aluminum (Al) IDTs, entirely covering the surface between IDTs. Due to the hydrophilic nature of GO, the mass of GO can be modulated by water molecules, which can induce the resonant frequency shift. The GO film with higher thickness exhibited higher sensitivity to RH due to the increased water-absorbing capacity. The optimized thickness of GO film ranged from 100 to 130 nm, which showed the highest sensitivity and fastest response. The extraordinary humidity sensing characteristics were highly associated with GO film thickness, GO-covered surface area, and resonant frequency. At low RH atmosphere, the water molecules are physisorbed on hydrophilic functional groups and vacancies of GO surface, called as the first physisorbed layer. Thus, the mass change is linearly increased by the surface coverage of water molecules on GO. At high RH atmosphere, water molecules are adsorbed by a single hydrogen bonding of hydroxyl groups, called as multiphysisorbed layer. The mobility of water molecules is substantially increased, causing the exponential increase of water molecules and resonance frequency shift. Meanwhile, the wave types can be classified into four groups by the substrates: Rayleigh wave, Lamb wave, shear horizontal wave, and love wave. The research articles introduced gas sensors based on wave modes such as Lamb wave mode and love wave mode. Xuan et al. fabricated flexible Lamb-wave humidity sensor based on GO sensing layer. [104] ZnO piezoelectric layer was deposited on a flexible PI film, followed by Al IDts deposition, and GO dispersions were drop-casted on ZnO layer with different thicknesses. The Lamb waves can be generated in layered structures at low frequencies. The resonant peaks called zero-order antisymmetric (A0) and symmetric (S0) modes indicate the existence of Lamb waves. When the thickness of the GO film was 400 nm, the sensor showed a sensitivity of 145.83 ppm/%RH for the A0 mode and 89.35 ppm/%RH for the S0 mode, respectively. The hydrophilic GO films absorb water molecules, producing the mass loading effect derived from the resonant frequency shift. Sayago et al. introduced love wave gas sensor based on GO sensing layer for chemical warfare agent (CWA), where the Love wave is generated at the interface of two substrate layers. [105] The thick substrates such as LiNbO 3 and quartz are used, and a thin guiding layer such as ZnO and SiO 2 is deposited on the substrate. The guided waves propagate through the thin guiding layer, and Love wave is generated when high acoustic energy is concentrated on the guiding layer. In case of love wave sensors, the waveguide layer fully covers the IDTs to improve the sensing capabilities without any IDTs damage. In this study, quartz was used as the substrate, SiO 2 as the guiding layer, and GO as the sensing layer. The gas adsorption on GO film changes the mass of the guiding layer, which shifts the frequency. The fabricated sensor showed the best sensing performance to dimethylmethylphosphonate (DMMP) gas. The response to 1 ppm DMMP gas was 3.07 kHz per ppm, response time was 15 min, and the recovery time was 13 min, respectively. The high response to DMMP was achieved by strong hydrogen bonding between phosphoryl groups of DMMP and GO.
In addition to diverse types of graphene-based gas sensors, gravimetric type gas sensors are also highly efficient for detecting low-concentration gas analytes. A cantilever is applied to detect the resonant frequency shift, which can be caused by several factors, including mass change, surface modification, and spring constant. [106] During the sensing measurements, the surface state and spring constant of the cantilever are maintained constantly. Thus, the gravimetric sensing properties are mostly dependent on resonant frequency shift by mass change. In case of graphene-based gravimetric gas sensors, Au/GO is the most widely used material. Xu et al. introduced GO/Au hybrid for gravimetric 2,4,6-trinitrotoluene (TNT) sensing. [107] GO/Au hybrid suspension was coated on a resonant microcantilever forming a porous film. The sensor showed a considerable response of 0.5 Hz to 100 ppt of TNT gas by adsorbing 327 fg of TNT molecules. Due to the porous structure and high surface area of GO/Au hybrid, the resonant frequency shift of the cantilever was easily controlled. Based on the sensing capability of GO/Au-based gravimetric gas sensors to sub-ppt level gases, research works were conducted by functionalizing Au NPs. The carboxyl functionalized GO/Au hybrids were applied in NH 3 sensing [108] and trimethylamine (TMA) sensing. [109]

TMDs and Others
TMDs (MX 2 ) have 2D layered structures where M (Mo, W, V, Nb) indicates the transition metal element, and X (S, Se, Te) is the chalcogen element. [24] The transition metal layer is sandwiched between two chalcogen layers forming hexagonal planes. By varying the stacking orders between layers or coordination models, diverse crystal structures such as 1T, 1T', 2H, and 3R phases are generated. [47] 2H phase TMDs are the most widely applied in chemical sensing due to their semiconducting nature. The edge sites of TMDs act as the gas adsorption sites and promote the reaction between the sensing materials and gas molecules.
Among semiconducting TMDs, MoS 2 is the representative material with 1T, 1H, 2H, and 3R phases for various applications. 1T phase is a metallic phase widely applied in hydrogen evolution catalyst, [110][111][112][113] while the 2H phase is a semiconducting phase broadly utilized in gas sensing. [30,48,114,115] The preparation of MoS 2 can be categorized into top-down and bottomup methods. The top-down methods are based on the exfoliation of thin layers from the bulk crystals, including mechanical exfoliation, liquid exfoliation, and intercalation-assisted exfoliation. The bottom-up methods are based on chemical reactions, including CVD growth and chemical synthesis. Liu et al. first proposed CVD-grown monolayer MoS 2 transistors with Schottky contacts for NO 2 and NH 3 detection. [116] The triangle-shaped MoS 2 monolayer with a lateral size of 5-30 m was deposited on SiO 2 /Si substrate by 3-zone CVD. The MoS 2 transistors exhibited a large modulation of conductance to NO 2 and NH 3 gas. When NO 2 is adsorbed on the surface of MoS 2 , electrons are extracted from MoS 2 to NO 2, inducing a positive V th shift. In the case of NH 3 adsorption, electrons are transferred from NH 3 to MoS 2, inducing a negative V th shift. The sensor exhibited a considerable response to 20 ppb of NO 2 and 1 ppm of NH 3, implying that LOD can reach to sub-ppb level. Zhao et al. proposed a humidity sensor array based on a MoS 2 monolayer for the noncontact moisture mapping application. [117] The schematic illustration of the fabrication process is depicted in Figure 6a. The monolayer MoS 2 film was grown on SiO 2 /Si substrate by a CVD process, and monolayer MoS 2 patterns were obtained by the Auassisted exfoliation method. The patterns were formed by the UVlithography process of photoresist (PR), and Au film was used as the supporting layer on MoS 2 . MoS 2 without PR and PR patterns on MoS 2 were both detached via peeling the tape. Then, Ti/Au electrodes were deposited by the lift-off process. As shown in the optical image of Figure 6b, the channel length was 25 m, and the width was 50 m, respectively. The AFM image shows that the ultraclean surface was obtained with no defect or contamination. The ultraclean MoS 2 monolayer exhibited excellent humidity sensing properties examined by various parameters, as shown in Figure 6c-e. The transfer curves in Figure 6c were obtained at RH ranging from 0 to 35%, where drain voltage (V D ) was 1 V, and V G ranged from -80 to 80 V. The curves showed a positive shift when RH was increased. Figure 6d shows the resistance variation at V G of 30 V. The resistance was increased linearly with RH in logarithmic coordinates due to the charge density shift. The V th modulation is also a phenomenon of the reaction between MoS 2 and water molecules, as shown in Figure 6e. The V th shifts to the positive direction with the increased RH derived from the increased electron transfer from MoS 2 to water molecules. Based on the sensing performance, the 6 × 6 MoS 2 FET array was fabricated to demonstrate the moisture distribution in the external atmosphere. Figure 6f shows the resistance plot as a function of the finger distance, which shows an exponential relationship. The resistance was decreased as the finger approached the FET sensor array. The left image of Figure 6g illustrates the resistance distribution plot of individual pixels in the sensor array at a finger height of 3 mm, and the right image is the calculated RH distribution plot based on the resistance plot of Figure 6f. The noncontact humidity sensing experiment verified the potential of the MoS 2 monolayer in the practical wearable applications. The CVDgrown MoS 2 is also a good candidate for chemoresistive gas sensors. The chemoresistive gas sensor based on CVD-grown MoS 2 on flexible substrates were reported by Zhao et al. [118] The bilayer MoS 2 was grown on PI substrate at 200°C by using Mo(CO) 6 and H 2 S as precursors. The fabricated sensor showed a response of 300% to NO 2 and 5% to NH 3 , respectively. The gas molecules prefer to be adsorbed on the defect or basal plane of MoS 2 . Moreover, the p-type behaviors were observed in MoS 2 , where the current increased after the exposure to NO 2 . Due to the ultrathin thickness and 2D nature, the CVD-grown MoS 2 is beneficial in both FET and chemoresistive gas sensors. SnS 2 is also one of the most prospective materials in TMDbased gas sensing applications, even though tin (Sn) is not categorized as a transition metal. SnS 2 also has a 2D layered structure where Sn atoms are sandwiched between two hexagonally coordinated S atoms. The n-type SnS 2 has unique features for FET gas sensors, including high carrier mobility, high on/off current ratio, and tunable optical bandgap. [119] Hayashi et al. reported CVDgrown SnS 2 flakes for selective detection of HCHO. [120] The fabricated FET sensor based on CVD-grown SnS 2 showed an on/off ratio over 2 × 10 5 and electron mobility of 0.1 cm 2 V −1 s −1 , respectively. The HCHO sensing properties were conducted in an N 2 atmosphere to 1 ppb of HCHO, and a dry air atmosphere to 20 ppb of HCHO, respectively. The sensor exhibited considerable sensing behavior to 20 ppb of HCHO in air atmosphere. The HCHO sensing at room temperature was enabled by the sulfur vacancies terminated by oxygen atoms derived from HCHO molecules. This study suggests that sulfur vacancies are the critical components in gas detection which can act as active sites. The role of sulfur vacancies in gas sensing properties is also extremely critical in chemoresistive gas sensors. Qin et al. prepared 2D SnS 2 NSs with sulfur vacancies by lithium intercalation for room temperature NH 3 detection. [121] The chemically exfoliated SnS 2 consisted of 1-3 layers with lateral sizes ranging from 0.6 to 1.8 nm. The bulk SnS 2 had barely any response to NH 3 , while SnS 2 NSs had a significant response to NH 3 at room temperature with a response time of 16 s. The improved response was derived from the sulfur vacancies of SnS 2, which act as the NH 3 adsorption sites.
Various SnS 2 nanostructures can be prepared by solution processes and applied in room temperature chemoresistive NO 2 sensing. [122] Despite promising sensing properties, diverse strategies have been introduced to improve the performance at room temperature. Especially light activation is one of the most desirable strategies, with extremely low electrical power consumption. Thanks to the appropriate bandgap and high visible light absorbing nature, SnS 2 -based light-activated gas sensors are reported based on SnS 2 NSs, [123,124] rGO/SnS 2 , [125] SnS 2 /SnO 2 . [126] Eom et al. synthesized 2D SnS 2 nanoflowers (NFs) by solvothermal method for high-performance NO 2 detection at room temperature. [127] Figure 7a shows the schematic illustration of the fabrication procedure of the SnS 2 NFs-based NO 2 gas sensor. The solvothermal synthesis was conducted at 160°C for 2 h, and 2D SnS 2 NFs were formed. SnS 2 NFs dispersions were drop-casted on IDEs, and rapid thermal annealing (RTA) was performed. The synthesized SnS 2 NFs were confirmed by FESEM image in Figure 7b and transmission electron microscopy (TEM) image in Figure 7c. SnS 2 NFs were consisted of randomly assembled 2D nanosheets with excellent crystallinity. The gas sensing properties of SnS 2 NFs were tested under visible light illumination, as shown in Figure 7d. The response to 5 ppm of NO 2 under blue light illumination was 14.28 times higher than in dark conditions. Moreover, the base resistance was decreased, and the response was improved as the wavelength of light decreased. The reliability of NO 2 sensing under blue light illumination was excellent, as shown in Figure 7e. The base resistance and response maintained for 8 pulses of 5 ppm of NO 2 . To calculate the LOD, the sensor was exposed to low concentration of NO 2 ranging from 400 to 1000 ppb, as shown in Figure 7f. The response was linearly increased with NO 2 concentration, and LOD was calculated to be 0.32 ppb. The selectivity to NO 2 was also outstanding which is shown in Figure 7g. The response to NO 2 was substantially higher than other interference gases. The synthesized SnS 2 NFs had abundant edge sites serving as the gas adsorption sites and light absorbing sites, which significantly improved the gas sensing properties at room temperature. The electron-hole pairs were generated under light illumination, resulting in electron excitation to the conduction band. Due to the increased electron density, more NO 2 molecules can be adsorbed on SnS 2 surface. Moreover, the photogenerated holes can recombine with NO 2 − , triggering the recovery to the initial state. Similar to MoS 2 , WS 2 is also a core member of the TMDs group, where the tungsten layer is sandwiched between two sulfur layers. The bulk WS 2 has an indirect bandgap of 1.4 eV, and the bandgap shifts to 2.1 eV in monolayer, which is in a suitable range for gas sensing. Afzal et al. fabricated 2D WS 2 /GeSe/WS 2 van der Waals heterojunction bipolar transistor (2D-HBT) for NH 3 and O 2 detection. [128] By utilizing the mechanical exfoliation technique, a 2.6 nm thick GeSe layer was sandwiched between a 10 nm thick top WS 2 layer and a 9 nm thick bottom WS 2 layer. The 2D-HBT sensor exhibited a response time of 6.55 ms and recovery time of16.2 ms, respectively, under a light of 633 nm wavelength. Thanks to the heterostructure, the response to NH 3 was substantially higher than that to other gases. Tang et al introduced WS 2 /IGZO heterojunction for NO 2 detection based on tunable polarity transport. [129] The p-n heterojunction was formed by growing the WS 2 layer on a sputtered IGZO layer. The transistor showed ambipolar characteristics in a dry air atmosphere and p-type characteristics as NO 2 concentration increased. The tunable features were derived from the doping effect of NO 2 on WS 2 /IGZO heterojunction and controllable Schottky barrier value. The WS 2 -based heterojunction can be an effective strategy in improving the sensing performance at room temperature. In case of chemoresistive gas sensors, WS 2 -based nanostructures such as 2D nanoflakes, [130,131] and nanosheets [132][133][134][135][136][137] are widely utilized due to the facile control of nanostructure morphology and numerous edge sites. The exposure of edge sites of 2D nanostructures is proven beneficial in chemoresistive gas sensing, which was reported in several research works. [27,138] Suh et al. introduced the edge-exposed WS 2 on 1D nanostructures for room temperature NO 2 detection. [139] SiO 2 nanorods (NRs) were grown by CVD as a porous template for WS 2 deposition. WCl 6 precursor was spin-coated on SiO 2 NRs followed by sulfurization via CVD resulting in WS 2 formation. Due to the highly exposed edge sites and porous 1D nanostructure, the response to 5 ppm of NO 2 reached 151.2%, and LOD was 13.726 ppb at room temperature. In addition to 2D TMDs mentioned above, a ReS 2 -based FET gas sensor for volatile organic compounds (VOCs) detection was reported. [140] Moreover, chemoresistive NO 2 gas sensors based on NbS 2 [141] and TaS 2 [142] were reported showing promising gas sensing properties at room temperature. These materials also have properties of 2D TMDs such as MoS 2 and SnS 2 ; however, the reported research works are restricted to NO 2 detection. Thus, gas selectivity control is needed to detect various harmful gases.
Similar to 2D TMDs, MXenes have a 2D layered structure and can be exfoliated to ultrathin layers. The abundant surface functional groups, high porosity, and large surface area of MXenes trigger the development of MXene-based gas sensors. MXene-based gas sensors are mostly operable at room temperature due to the rapid charge transfer derived from high conductivity. The MXene-based chemoresistive gas sensor was first reported in 2017 for room temperature acetone detection. Lee et al. synthesized Ti 3 C 2 T x by substituting Al atoms from Ti 3 AlC 2 for a flexible gas sensor. [143] They found that Ti 3 C 2 T x showed p-type semiconducting behavior where the resistance was increased to reducing gases. The sensor showed a LOD of 9.27 ppb to acetone gas at room temperature. Thus, the research on MXene-based gas sensors for both FET type and chemoresistive type gained tremendous attraction. Xu et al. introduced Ag NPs functionalized Ti 3 C 2 T x MXene-based FET gas sensor for H 2 S detection. [144] The gas sensing properties are modulated via diverse strategies, including synthesis, etching, and preservation methods. In addition to Ti 3 C 2 T x , the most commonly used MXenes, V 2 CT x and Nb 2 CT x can be synthesized by varying the synthesis procedure and utilized for gas sensing. However, the gas sensing mechanism of metallic MXenes is highly complicated where the resistance shows same behavior to every gas molecule, regardless of oxidizing and reducing gas. This was due to the decreased conductivity derived from the sluggish charge carrier transport and low carrier density during the gas sensing reaction. Kim et al. introduced metallic Ti 3 C 2 T x MXene chemoresistive gas sensor for VOCs detection. [145] The surface of synthesized Ti 3 C 2 T x MXene was terminated with ─OH, ─O, and ─F functional groups. As a result, the sensor showed high response to hydrogen-bonded gases, including CH 3 COCH 3 , C 2 H 5 OH, NH 3 , and C 3 H 6 O. However, the response to acidic gases, such as NO 2 , SO 2 , and CO 2 was negligible compared to hydrogen-bonded gases. Despite low LOD and signal-to noise, the selectivity must be improved to detect individual target gas in practical applications. As the development of MXene-based gas sensors is still in their infancy, diverse approaches are required along with computational studies.
Phosphorene, a monolayer of black phosphorus (BP), has a honeycomb structure with p-type nature and high carrier mobility. [48] The bandgap is about 0.3 eV in bulk and shifts to 1.9 eV in the monolayer state, which is in the range of semiconductors. The anisotropic conductivity can facilitate the charge transfer to gas molecules, and high molecular adsorption energy can lead to a high response. In 2015, Abbas et al. fabricated multilayer BP-based FET for NO 2 detection. [146] The chemically synthesized BP flakes were exfoliated to multilayers by the scotch tape method. The sensor exhibited good sensitivity to 5 ppb of NO 2 . Furthermore, the conductance was increased with NO 2 concentration, implying that NO 2 extracts electrons, resulting in hole doping in BP flakes. The p-type characteristics of BP reveal the promising potential of BP for effective VOCs detection. Up to now, the reported pristine BP-based gas sensors are mostly applied in NO 2 and RH detection, regardless of FET type and chemoresistive type. Thus, diverse strategies are required to improve the sensing performance and modulate the selectivity. The gas sensing performance of various 2D materials is summarized in Table 1.

Liquid Sensors Based on 2D Materials
With the rapid development of industry and technology, the risks and concerns of water pollution caused by various chemicals are increasing. [147][148][149] The water pollution can have a fatal impact on human life, environment, and safety, even with a very little amount. [150] For example, metal ions and radicals easily enter the human body through drinking water causing severe threats. [151,152] Thus, the demand for in-solution chemical detection technology has increased, and interest in liquid sensors using 2D materials with excellent sensing properties has also increased. [153] In addition, since 2D materials have electrical and optical properties suitable for application to liquid sensors, various types of liquid sensors can be developed, such as adsorption, [154] SPR, [155] SERS, [156] electrochemical, [157] and FET. [158]

Graphene and GO
Among numerous detection methods of chemicals in liquid solutions, the sensors using adsorption and desorption are extremely effective due to the reversible nature of noncovalent interactions, great response, and fast recovery rate. The adsorption sensors based on GO effectively detects metal ions and other chemicals because of their large specific surface area, high hydrophilicity, and numerous active adsorption sites with anionic properties. [153] In addition, GO can be applied in various ways, such as forming composite, hybrid, and aerogels with various materials. Moreover, sensing properties can be substantially improved through nanostructure formation and functionalization. [159] Yu et al. developed GO/chitosan (CS) aerogel microspheres (GCAMs, ≈200 m in diameter) with honeycomb-cobweb structure, as shown in SEM images of GCAMs in Figure 8a,b. [160] The GCAMs were fabricated by electrospraying the synthesized suspension at low temperature, as shown in Figure 8c. The amount of CS greatly affected the stability of the aerogel and sensor capability. The structure of GO was not maintained in low CS concentration, while the millimeter-sized composite spheres and large honeycomb-shaped pores were produced in high CS concentration, extending the diffusion pathway and hindering rapid adsorption. The sensing performance optimization was evaluated according to 5 different CS concentration (0, 5, 10, 20, and 50%), and GCAMS showed the best performance to metal ions when CS concentration was 10%, as shown in Figure 8d. Three metal ions, Pb(II), Cu(II), and Cr(VI), were used to evaluate the sensing performance of GCAMs. The adsorption capacity of metal ions can be calculated by following equation: where q t (mg g -1 ) is the adsorption capacity at t min, C 0 and C t are the concentrations of the adsorbate (mg L -1 ) after 0 and t min, respectively. Moreover, V is the volume of the solutions (mL), and m is the mass of the adsorbent (g). The adsorption kinetics are used to analyze the metal ions adsorption sensing performance. Figure 8e shows the kinetic adsorption curves of three different metal ions in GCAMs. During the initial 5 min, the reaction proceeded rapidly, and Pb(II) and Cu(II) reached the equilibrium state after 40 and 10 min, respectively. Langmuir (solid line) and Freundlich (dashed line) isotherm models were applied to the adsorption data of GCAMs to metal ions, as shown in Figure 8f. All R 2 values of the Freundlich isotherm model were higher than 0.99 and higher than R 2 value of the Langmuir isotherm model. The Freundlich isotherm model application of adsorption measurement data proves that the electrostatic attraction between the metal ions and GCAMs cause the adsorption and desorption. The liquid adsorption sensor based on GO/CS shows excellent sensing properties for various pollutants, including organic dyes and metal ions. The surface of GO can be divided into two regions: unoxidized graphene and oxidized graphene with oxygencontaining functional groups. The unoxidized conjugated portion shows great interaction with aromatic compounds due to the -stacking interaction. The oxygen-containing groups on GO act as adsorption sites through electrostatic attraction with cationic dyes. Furthermore, amino groups on CS act as active sites for electrostatic interaction with anionic dyes. Thus, GO is suitable for use as an adsorption sensing material due to the advantage of detecting pollutants harmful to humankind through integration with materials having various functional groups. The GO-based adsorption sensors are also excellent candidates for detecting metal ions and have significant properties Table 1. Summary of gas sensors based on graphene and 2D graphene analogs. a) SPR: surface plasmon resonance; FET: field-effect transistor; SAW: surface acoustic wave b) GO: graphene oxide; PSS: polystyrene sulfonate; PDA: polydiacetylene; PMMA: poly(methyl methacrylate); rGO: reduced graphene oxide; CNT: carbon nanotube; LPFG: long-period fiber grating; IGZO: indium gallium zinc oxide; BP: black phosphorous; PEI: polyethylenimine; IL: ionic liquid; PVA: polyvinyl alcohol c) RH: relative humidity; THF: tetrahydrofuran; HCHO: formaldehyde; TEA: triethylamine; DMMP: dimethyl methylphosphonate; TNT: 2,4,6-trinitrotoluene; TMA: trimethylamine d) NA: not available. e) The values are either mentioned values in the article or taken as minimum measured value. In case of the articles with both detection range and exact concentration value not mentioned, they were denoted as "NA".  [109] for organic molecules detection through strong -stacking interaction and van der Waals forces. [161][162][163] In addition, many studies have been conducted to improve the sensing properties through nanostructure construction or the composite formation with metal oxides. However, in the case of GO and metal oxide composites, metal oxide on GO caused the deterioration of recovery properties due to the reduced adsorption area on GO. [164] The research on nanostructures using GO have been conducted to overcome the drawbacks. Liu et al. reported a new type of adsorption sensor based on 3D GO sponge. [165] The GO sponge was fabricated by employing a centrifugal vacuum system. [154] Dye adsorption experiments were performed by two different concentrations of MB and MV solution (1.2 × 10 −3 and 2.4 × 10 −3 m) with 10 mg 3D GO sponge. The adsorption mechanism of MB and MV on 3D sponge GO is explained by chemical adsorption to evaluate affect of nanostructure. The MB and MV molecules have rich aromatic rings and cationic atoms in their chemical structures, which enables adsorption through -interaction and anioncation interaction with the aromatic ring present in GO. The researches on GO-based adsorption sensors are widely being researched, which have attractive forces for various chemicals for detecting environmental hazards. The SPR sensors are highly suitable for studying the interactions between two different materials, which detect target molecules in real-time by measuring resonance angle shift or wavelength shift without labeling or complex protocols. [166][167][168] The SPR sensor consists of the glass layer at the bottom acts as a substrate through which light passes, the thin metal film such as Figure 8. a,b) SEM image of graphene oxide/chitosan aerogel Microspheres (GCAM) with honeycomb-cobweb structure. Adsorption kinetics of Pb(II), Cu(II), and Cr(II). c) Schematic images of GCAM fabrication process. d) Adsorption of Pb 2+ , Cu 2+ , and Cr 4+ on GCAMs with different CS contents. e) Adsorption kinetics and f) adsorption isotherms of Pb 2+ , Cu 2+ , and Cr 4+ on GCAM (C 0 = 500 mg L -1 ). Reproduced with permission. [160] Copyright 2017, American Chemical Society.
Au or Ag derives the SPR phenomenon, and the recognition layer on the top serves to detect target materials. Research has been conducted on graphene and GO for the recognition layer, which have excellent optical properties and high reactivity with various materials. [169,170] Zhou et al. reported graphene-enhanced optical fiber SPR sensor for detecting NaCl solutions. [171] The structure of reflection optical fiber SPR sensor with graphene is shown in Figure 9a. The sensor consists of four different parts: the coreless fiber with diameter D, Ag film with thickness d 1 , graphene film as recognition layer above the Ag film, and the sensing medium around the graphene film to be measured. Ag film coated sensing part of plastic clad optical fiber with 600 m diameter is deposited by the Tollens' reaction between silver ammonia solution and glucose solution. After coating the cylindrical surface of the sensing part with Ag film, the graphene layer on the Cu foil was transferred onto the optical fiber by the wet transfer method, as shown in Figure 9b. In order to estimate the sensing properties of graphene to NaCl, the sensing probe with and without graphene were put into different concentrations of NaCl solutions. The sensing part of the optical fiber sensor was immersed into the solutions completely, and then the wavelength shifts were measured when the concentration of NaCl solution changed from 0 to 25%. Due to graphene having a high refractive index, the evanescent field around the interface between the graphene and Ag layer increased, the response and sensitivity of the optical fiber SPR sensor with the graphene layer exhibited better performance than optical fiber SPR sensor without the graphene layer. Figure 9c shows the SPR curves of the optical fiber SPR sensor with a graphene layer while the concentration changes from 0 to 25% NaCl solution, exhibiting good linearity and sensitivity. The graphene layer improves the sensor characteristics by amplifying the signal and prevents the oxidation of the Ag layer. Lokman et al. reported an effective GO/CS recognition layer that exhibited large interaction with Pb(II) ions. Unlike graphene-based SPR sensor, which has improved sensor properties through the amplification of evanescent filed, GO/CS can improve SPR sensing properties through interaction with target materials with various functional groups. Due to the amine (─NH 2 ) and hydroxy (─OH) groups, CS has favorable adsorption properties with heavy metal ions. [172] Moreover, GO has a large surface-to-volume ratio and is suitable for a recognition layer of the SPR sensor structure. [173] Figure 9d shows the schematic image of the SPR measurement device based on the Kretschmann configuration. The light of a fixed wavelength is incident on a GO/CS layer, and its reflectance is detected with a photodetector. The SPR sensor measures the change in reflectivity according to the reflection angle. GO/CS composite was fabricated by mixing GO and CS by sonication and stirring for 1 h, and GO/CS mixture was spin coated on the Au Figure 9. a) Schematic image of the end reflection optical fiber SPR sensor with graphene. b) The process of transferring graphene onto end reflection optical fiber. c) SPR reflection curve of six different concentrations of NaCl solution (0, 5, 10, 15, 20, and 25%). d) Fitting curve of the resonance wavelength and NaCl concentration. Reproduced with permission. [171] Copyright 2021, Elsevier. d) Schematic image of the optical system of the surface plasmon resonance (SPR) sensor setup. e) SEM image of the morphologies of the nanostructured Au/CS/GO thin films. f) SPR curves for the Au/CS/GO thin films exposed to DI water and Pb (II) ions with different concentrations (0.03, 0.1, 0.5, 1, and 5 ppm) Reproduced with permission. [155] Copyright 2014, Elsevier.
film at 6000 rev min -1 for 30 s. Figure 9e shows the SEM image of CS on the Au film. Homogeneously coated CS was transformed into rough sheets by mixing with GO nanosheets. The secondary reaction between the amine group of CS and the carboxyl group of GO resulted in this transformation. [174] The SPR response of the GO/CS recognition layer toward different concentrations of Pb(II) solution (30, 100, 500, 1000, and 5000 ppb) is described in Figure 9f. In a pure deionized water, the resonance occurred at 68.89°, and the resonance occurred at 68.33°after the exposure to 30 ppb of Pb(II) solutions. As the Pb(II) concentration was increased, the resonance angle decreased. The increased surface roughness resulting from the addition of GO to CS leads to the improved sensing performance of GO/CS to Pb(II) detection. Liquid sensing using the SPR phenomenon has been actively researched in the field of biosensors, and research on SPR sensors using graphene or GO has begun in the field of chemical sensing through the advantage of being able to detect substances with very low concentrations in real time.
SERS sensors have been actively researched for detecting various chemicals due to the excellent sensitivity, selectivity, and a wide range of applications. [65,175] The fascinating properties of SERS sensors enable practical applications in various fields such as biomedical, biochemistry, food security, and catalyst technologies. [176][177][178][179] However, conventional SERS chemical Figure 10. a) Schematic image of the synthetic process for CuNPs/rGO composite. b) TEM images of CuNPs/rGO. c) SERS spectra of different concentrations of R6G adsorbed on the CuNPs/rGO. Reproduced with permission. [185] Copyright 2016, Royal Society of Chemistry. d) Schematic image and e) SEM image of the fabrication process of Ag @GO hybrids on Si substrate. SERS signals of 1 × 10 −6 m f) R6G dye, and g) CV. SERS spectra of different concentrations of h) R6G and i) CV. Reproduced with permission. [156] Copyright 2014, Royal Society of Chemistry. sensors using the near-field effect of localized surface plasmon resonance (LSPR) of noble metals (Au, Ag, and Cu) exposed limitations such as high cost and poor biocompatibility. [180,181] GO and rGO are emerging materials that can compensate these shortcomings of noble metal NPs due to their stability, chemical resistance, low cost, and powerful SERS enhancement effect. [182][183][184] He et al. reported SERS Rhodamine 6G (R6G) sensing platform using CuNPs/rGO composite to maximize the advantages of metal NPs and rGO. [185] rGO was prepared by reducing GO synthesized by Hummers' method, and Cu NPs were synthesized by the aqueous reduction of CuCl 2 solution. The CuNPs/rGO composites were fabricated by mixing GO ethanol and CuNP precursor. The mixture was stirred for 30 min at room temperature and put into boiling flasks to react at 85°C in water for 90 min. The collected solid products were washed repeatedly with ethanol and deionized water and dried at 60°C, as depicted in Figure 10a. The morphology of CuNPs/rGO composites was investigated by TEM image in Figure 10b. The average size of Cu NPs was 5-10 nm, which were assembled on rGO sheets. CuNPs/rGO assessed the detection performance by measuring the different concentrations of R6G solution in Figure 10c. The SERS intensity of the CuNPs/rGO sensor increased when the concentration of the R6G solution increased. CuNPs/rGO R6G sensor has a LOD of 10 −8 m and a linear relationship between the response and R6G concentrations in the range from 10 −7 to 10 −4 m. Fan et al. reported Ag NPs/GO composites-based SERS sensor platform for dye sensing. [156] They fabricated two different Ag octa/GO composites, which are Ag octa/GO with GO on top of Ag octahedra NPs (Ag octa@GO) and Ag octa/GO with GO beneath Ag octahedra NPs (GO@Ag octa), as shown in Figure 10d. Both composites enhance the SERS signals, however, the enhancement factor of Ag octa@GO is larger than that of GO@Ag octa. In Figure 10e, the SEM image of Ag octa@GO shows the Ag octa NPs wrapped in thin GO, and the inset shows the corresponding schematic illustration of the composite layouts. SERS spectra of both R6G and CV at 1 × 10 -6 m measured from pure Ag octa NPs, pure GO, and Ag octa@GO are shown in Figure 10f,h. The linearity of Ag octa@GO sensor was confirmed by measuring R6G and CV solutions of various concentrations (0.1× 10 -9 m to 10 × 10 -6 m. Detecting very low concentrations of dyes is due to -interaction between the aromatic ring of dye and GO, as shown in Figure 10g,i. [186] The integration of metal NPs and graphene derivatives can derive synergistic effect in SERS sensing performance, exhibiting high reproducibility and enhancement factor. In addition, SERS sensors showed excellent sensing properties to dye molecules due to the enhancement mechanism of GO and the interaction with dye molecules. FET sensors with high sensitivity are useful for detecting ions by reading source-to-drain currents. Especially, graphene is suitable for FET liquid sensor due to the unique physical, chemical, and electrical properties. [187][188][189] Although pH FET sensors have received massive attention for food safety and environmental analysis, low selectivity drawback has to be solved. Remarkable advances have been made in improving the selectivity of pH sensor by using Nafion layers. Lee et al. reported graphene-based FET pH sensor by utilizing Nafion as a sieving layer, which can induce pH selectivity. [190] The measurements were conducted by controlling V G over the drain-source current (I DS ). the charge transport properties of the graphene-based FET sensor can be obtained by measuring the Dirac point shift. The electrodes were fabricated by photolithography and e-beam evaporation with a distance of 5 m. The graphene monolayer was transferred by wet transfer method on the electrodes, and Nafion layer was spin coated on the graphene layer at the rate of 2000 rpm for 30 s. The microfluidic channel-integrated solution gated FET based on graphene monolayer exhibits sensitivity to H + and glucose. However, the Nafion-coated FET sensor exhibited sensitivity only upon H + . The effect of Nafion coating was verified by measuring the sensitivity to 5 × 10 -3 m of HCl, KCl, and NaCl solutions. The Nafion film on the graphene layer acts as a sieving layer and allows only H + penetration by higher diffusion coefficient than other larger cations. Due to the adsorption of selectively penetrated H + through Nafion layer, graphene becomes n-doped in acidic environments. The n-doping effect causes the shift of the Dirac point in the transfer curve of the graphene FET sensor, and the pH detection was performed through the change in the transfer curve. The sensing properties of Nafion-coated graphene FET sensor were also examined in real beverages such as coke and Sprite for practical applications. A massive amount of research has been conducted on the FET sensor, which is considered the most efficient strategy for application in human life.
Electrochemical detection technology for liquid chemical sensor has several advantages, such as low cost, real-time detection, and simplicity. Graphene and related materials are suitable for electrochemical sensors due to their high electrical conductivity. [191] However, due to the aggregation, graphene derivatives are difficult to be used alone. [192,193] To solve this problem, Rahman et al. reported GO and Ag nanowire (AgNW) composites for electrochemical Hg(II) ion sensor with modified Pt electrode. [194] The composites were fabricated by ultrasonicating GO and AgNW suspension for 1 h, and the composite solutions were drop-casted on Pt electrode. The oxygen-containing functional groups in GO can link strongly with Hg(II) ions due to the strong affinity and the deposition of Hg(II) ions applied negative voltage. The AgNW accelerates electron movement between GO and Pt electrode with high sensitivity and low LOD, which could not be implemented in GO without AgNW. The response was calculated from EIS by the square wave anodic stripping voltammetry measurements, which were performed in the presence of KCl solution.
Graphene quantum dots (GQDs) are one of the most attractive materials for photoluminescence (PL) sensors with low toxicity, great stability, and tunable PL characteristics. [195,196] Du et al. reported N-doped GQDs to enhance the emission properties of GQDs by heteroatom doping. [197] Multi-nanometer-sized Ndoped GQDs were well distributed in the solution and were characterized by Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). A strong N 1s signal of the XPS spectrum and various peaks from the FT-IR spectrum proved the N doping and the presence of oxygen-containing functional groups in the N-doped GQDs. The PL sensors detect chemicals through the change of the PL signal generated from GQDs, in which the PL signal of N-doped GQDs was decreased after the addition of Hg(II) ions. Hg(II) ions formed the complex with functional groups of N-doped graphene, such as hydroxyl, carbonyl, and carboxyl groups. These functional groups formed bondings with Hg(II) ions and induced the aggregation of Ndoped GQDs, decreasing PL intensity. The PL intensity change enabled metal ions detection with extremely low detection limit.
This section provides an overview of the recent advance in various liquid chemical sensors based on graphene and GO. In the field of sensors, Graphene and GO can be applied to various types of liquid chemical sensors based on their excellent physical, chemical, and optical properties. [35] The chemical modification and synergistic effects with metal NPs have also shown to be suited to fabricate more stable and sensitive sensors with diverse functional groups and signal enhancement, making them suitable materials for fabricating SPR, and SERS sensor platforms. Moreover, the research on selectivity improvement, the most important component of chemical sensors, has been conducted using the sieving layer.

TMDs and Others
TMDs have numerous fascinating aspects, such as direct and tunable bandgap, and high on/off ratio at room temperature. These aspects also result in unique electrochemical, optical, and physical properties, thus making TMDs applicable in diverse liquid sensors such as FET, SERS, PL, and electrochemical sensors.
For the detection of metal ions to prevent leakage, studies on various methods, including atomic absorption spectroscopy (AAS), [198] atomic emission spectrometry (AES), [199] inductively coupled plasma mass spectrometry (ICP-MS), [200] colorimetry, [201] and electrochemical methods, [202] have been actively conducted. However, these existing methods have limitations in real-life application due to high cost, and difficulties in real-time detection. Among them, FET ion sensors based on 2D materials attracted increasing attention because of their advantages, such as good electronic properties, easy application, fast response, and label-free detection. FET sensors, based on the principle of effective signal transduction near the surface and amplification effect, are operated by monitoring the I DS changes in 2D materials caused by changes in the external environment, such as charged adsorption, light illumination, and chemical reaction. In this aspect, they are spotlighted for detecting various substances due to the capability of real-time detection and easy fabrication. This section briefly describes FET ion sensor based on TMDs and other 2D materials. − concentration (0, 10, 100, 1000, 10 000, 100 000 ppb). Reproduced with permission. [204] Copyright 2016, AIP. e) Schematic image of Ti 3 C 2 T x MXene FET sensor structure. f) SEM image of Ti 3 C 2 T x MXene nanoflakes bridging between Au electrodes. g) TEM image, h) HRTEM image, and i) SAED patterns of Ti 3 C 2 T x MXene nanoflakes. j) Evolutions in I-V curves of the Ti 3 C 2 T x MXene FET sensor for five different concentrations of Ag + (0 × 10 -6 , 0.5 × 10 -6 , 2 × 10 -6 , 5 × 10 -6 , and 10 × 10 -6 m). k) Normalized current (I/I 0, %) after adding Ag + . l) Linearity of relative response (calculated as (I 0 -I)/I 0 , %) after infecting Ag(I) ions for 10, 20, and 30 s. Reproduced with permission. [210] Copyright 2021, American Chemical Society.

Arsenite (AsO 2
− ) is one of the most common and toxic heavy metal ions that can cause severe health problems even with extremely low concentrations, such as peripheral neuropathy, and cancer. [203] Moreover, due to their easy accumulation through the food chain, many people suffer from related diseases. Li et al. demonstrated the effective FET sensor based on MoS 2 with arsenite ionophore for arsenite detection. [204] The arsenite sensor was fabricated by the transfer of few-layer MoS 2 flakes that were mechanically exfoliated by scotch tape and was transferred between the Ti electrodes. And then, the arsenite ionophore solution was spin-coated on MoS 2 flakes at 4000 rpm for 60 s, as depicted in Figure 11a. The arsenite ionophore film on MoS 2 flakes blocks every metal ion except arsenite, as depicted in Figure 11b. Figure 11c shows the SEM image of the transferred MoS 2 flakes between the electrodes, and the inset shows the optical image of the MoS 2 flake. I ds versus V gs characteristics were measured after solution injection with concentrations ranging from 0 to 100 ppm with 0.03 V of V ds to detect arsenite ions, as shown in Figure 11d.
The absorption of ions on MoS 2 flakes can cause a conductance change derived from the density or mobility change of the charge carriers. Ionophore acts as the filter of the FET sensor structure, resulting in improved selectivity by detecting the desired ions. Studies on pH FET sensors based on TMDs, which are the indicators of H + ions and metal ions, were also conducted. Liao et al. reported a multilayered ReS 2 FET pH sensor on a 20 nm thick HfO 2 /Si substrate exhibiting a small V th and a high on/off current ratio of up to 10 7 . [205] The sensor exhibited a sensitivity of 54.8 mV per pH and an extremely low LOD of 0.0132 pH. In addition, they provided a potential solution to decrease or eliminate the electrical noises by using HfO 2 , a high-k dielectric material.
Ag(I) ions, one of the most hazardous metal ions to the human body, when accumulated in the human body, interfere with the function of the enzymes and have toxic effects on the health of organisms. [206,207] However, Ag(I) ions are widely used throughout the industry, and about 80 tons of Ag(I) ions are leaked into the environment every year. [208,209] Liu et al. reported an effective www.advancedsciencenews.com www.advsensorres.com and rapid Ag(I) ion FET sensor without any label or probe based on Ti 3 C 2 T x MXene using high reducing capacity. [210] Figure 11e shows the schematic diagram of the Ti 3 C 2 T x MXene FET sensor. Ti 3 C 2 T x MXene FET sensor was fabricated by drop-casting 0.5 L of 5 g mL -1 Ti 3 C 2 T x MXene suspension onto the sensing area of Au IDEs. Figure 11f shows the SEM image of the Ti 3 C 2 T x MXene, where Ti 3 C 2 T x MXene nanoflakes were coated between the Au IDEs connecting the electrodes, and the size of the Ti 3 C 2 T x MXene nanoflakes was in the order of several hundred nanometers. Figure 11g-i shows the TEM image, HRTEM image, and selected area electron diffraction (SAED) patterns of the Ti 3 C 2 T x MXene nanoflakes, exhibiting the typical laminar structure. Before the measurements, they stabilized the Ti 3 C 2 T x MXene FET sensor with the water medium, and Ag(I) ion solutions with various concentrations (0.5-10 × 10 -6 m) were pipetted onto the Ti 3 C 2 T x MXene FET sensor. As depicted in Figure 11j, the slope of the I-V curves of Ti 3 C 2 T x MXene FET sensor decreased continuously as the concentration increased, indicating an increase in resistance. When a concentration of 10 × 10 -6 m Ag(I) solution was injected, the slope was almost 0. Figure 11k shows a response curve of sensitivity over time. The normalized current (I/I 0 ) could effectively exhibit the response of Ti 3 C 2 T x MXene FET sensor by eliminating the probable effect from the variations of the original resistance of other FET devices. The normalized current decreases immediately after the Ag(I) ions injection, and the change in normalized current increases with the concentration increase. Figure 11l shows the linearity plot of the changes at three different time points (t = 10, 20, and 30 s) shown in Figure 11k. The sensor responses exhibit linear correlation with various concentrations of Ag(I) ions, and the sensor response obtained at 20 and 30 s showed better linearity than the response obtained at 10 s. The LOD was calculated by sensor response obtained at 20 s which was 0.28 × 10 -6 m, which is lower than the maximum reference value selected by World Health Organization (WHO). The selective detection of hazardous ions in water is an important part of the liquid chemical sensor responsible for human safety. This Ti 3 C 2 T x MXene sensor was exposed to various metal ions (Na + , K + , Mg 2+ , Ca 2+ , Zn 2+ , Fe 3+ , Pb 2+ , Cu 2+ , and Al 3+ ) at 1 × 10 -6 m to examine the selectivity. The selectivity measurements indicate that the response to Ag(I) ions was more than 10 times higher than that of other metal ions, showing excellent selectivity. The high selectivity was due to the reaction in which Ag(I) ions react with MXenes to be reduced to Ag NPs, and the Liu group demonstrated the reduction of Ag(I) ions by TEM and EDS analysis. Research on MXenes has begun to be actively studied in recent years, and the sensing performance of MXene-based FET sensors is significantly expected to be improved through various methods.
BP can also be categorized into graphene analog, which has 2D layered structures. [211] Moreover, BP exhibits higher carrier mobility [212,213] and current on/off ratio [214] than TMDs. Despite their attractive features, the application in real devices without a certain protection strategy is difficult due to the low stability in ambient environments. [215] BP can be easily degraded to oxygenated phosphorus (PO x ) without any protection. Li et al. reported FET ion sensor based on BP encapsulated with ionophore. [216] Encapsulation of ionophores protected BP from the ambient air and provides selectivity to the BP base FET sensor by blocking all the other ions and molecules. The passed metal ions served as a positive gate voltage of BP and the Fermi level of BP changes to move away from the valance band. On the other hand, the anion molecule is adsorbed to BP by using the ionophores, the Fermi level shifts toward the valence band, resulting in higher conductivity.
SERS sensor using graphene and related with metal NPs shows great enhancement factors, however, high cost of metal NPs is an important issue to be resolved. In the last few decades, researches on noble-metal-free SERS platforms have been continuously conducted, such as nanostructured semiconductors and TMDs. [217][218][219] Among them, SERS sensors utilized by TMDs as active platforms are one of the most promising candidates due to their atomic uniformity, chemical stability, and various unique properties. [220,221] Yin et al. reported Raman enhancement on 2H-and 1T-MoX 2 (X = S, Se) monolayer through a chemical mechanism and first-principle density functional theory (DFT) calculations. [222] To compare the Raman enhancement from four different substrates, they measured the SERS spectrum of R6G and CuPc in 10 −5 m and confirmed that Raman enhancement of 1T-MoX 2 (X = S, Se) is greater than that of 2H-MoX 2 (X = S, Se), which is consistent with the first-principle DFT calculations. The results can be explained by the phase difference of 1T-MoX 2 (X = S, Se)and 2H-MoX 2 (X = S, Se). According to the characteristics of the calculated band structures, 1T-MoX 2 (X = S, Se) have metallic nature and 2H-MoX 2 (X = S, Se) have semiconducting nature. The larger Raman enhancement appeared in 1T-MoX 2 (X = S, Se) due to the phase transition, which can be ascribed to the highly efficient charge transfer between the target molecule and 1T-MoX 2 (X = S, Se). Li et al. also reported SERS platforms based on SnSe 2 with low cost, high uniformity, and a low LOD. The noble-metal-free SnSe 2 -based SERS platforms exhibited high performance to R6G. [223] The cavity of SnSe 2 nanoplate arrays fabricated by self-assembled growth acts as a light-trapping site, leading to high enhancement factor and excellent uniformity. The light-trapping ability is calculated by finite-difference time-domain (FDTD) and confirmed that the height and asymmetry of SnSe 2 nanoplate arrays are critical to light-trapping ability. The extremely low LOD and large scale are the merits of this study. In addition to the research on sulfurbased TMDs, Se and Te-based TMDs are gaining tremendous attention with improved sensing abilities.
Multi-nanometer-sized MoS 2 quantum dots (QDs) have PL characteristics at a wavelength of 450 nm due to the quantum confinement effects under the excitation of a UV light. The quantum confinement effects are weakened when MoS 2 is oxidized by the external environment, decreasing the PL intensity. Gan Table 2.
In this section, we introduced chemical sensors based on graphene and 2D graphene analogs due to their unique electrochemical, optical, and physical properties. Various types of chemical sensors detecting a wide range of chemical substances were reported for decades. Recently, due to the increased demand of real-time detecting chemical sensors, the sensors must overcome the strict standards. In the next section, we will discuss the advanced chemical sensors specified in real-time detection for use in daily life.

Practical Applications of Chemical Sensors Based on 2D Materials
The progress of the IoTs has promoted the development of accurate sensors capable of real-time detection. Especially chemical sensors are appealing their potential in diverse applications, including food quality monitoring, environmental monitoring, wearable devices, and human health monitoring. [225,226] Among them, due to the rise of a pandemic, human healthcare monitoring has become more vital than ever. Thus, 2D materials-based gas sensors are broadly applied in human breath monitoring, diabetes monitoring, and wearable gas sensing. For proper applications, several factors of gas sensors must be fulfilled, including low concentration detection, fast response and recovery, high accuracy, and low power consumption. Several research works reported breath monitoring gas sensors based on 2D materials by detecting RH or NH 3 gas in exhaled breath. Xu et al. introduced porphyrin-modified rGO films for multifunctional wearable sensing device. [227] Due to the high flexibility of rGO film, the film was easily attached to the skin. The sensor array was capable of detecting eight different VOCs biomarkers and efficiently discriminated by using a pattern recognition approach. Li et al. reported a wearable NO 2 sensor array based on AgNPs/rGO. [228] The sensor was composed of CNT electrodes and AgNPs/rGO composites sensing layer, resulting in a high flexibility. Figure 12a-c shows the fabricated sensor array attached to a living plant leaf, lab cloth, and portable sticker on the human body. Regardless of the substrate, the sensor arrays were well attached with excellent mechanical robustness. The NO 2 sensing performance was evaluated upon exposure to various NO 2 concentrations ranging from 0.5 to 5 ppm, as shown in Figure 12d-f. The sensor on the plant leaf exhibited the fastest response and recovery, which was derived from the high humidity of the plant leaf. Meanwhile, the sensor on the lab cloth showed the highest response due to the high adsorption area of a fibrous surface. The sensing properties of the sensor array attached to a portable sticker on the human body were also promising in wearable gas sensors. The electronic textile (E-textile)-based gas sensors can be fabricated by coating graphene derivatives directly on flexible fiber substrates. [229,230] Other practical applications of gas sensors include breath monitoring by detecting RH or NH 3 and diabetes monitoring by detecting acetone gas. H 2 gas detection is also extremely significant in various applications, including hydrogen storage, hydrogen fuel cell vehicles, and other energy industries. Kim et al. first applied a self-activated Pt-decorated graphene gas sensor for detecting H 2 gas generated from water splitting. Pt NPs were deposited on 3LG by e-beam evapo-rator with the size of 5 nm Then, to detect the H 2 gas from the water, the electrochemical water splitting system cell was fabricated, as shown in Figure 12g. Pt catalyst was used as a cathode, and Fe 60 (CoNi) 30 Cr 10 alloy plates were used as an anode, respectively. The prepared electrodes were immersed into a 1 m NaOH electrolyte, and two electrochemical cells were separated by Nafion membrane. The sensor was inserted into a quartz tube with electrical contacts, and H 2 gas was flowed from the opposite side to the sensor, as shown in Figure 12h. The bias voltage (1.4, 1.6, 1.8, and 2.0 V) was applied to the electrochemical cell to derive H 2 generation, and the response to H 2 gas was recorded, as shown in Figure 12-i. The response increased linearly with the bias voltage, and the sensor exhibited fast response and recovery. This work implies that H 2 gas sensor has tremendous potential in energy conversion and storage systems related to hydrogen energy. For liquid sensors, the applications in sweat monitoring have been most frequently reported and researched. Human sweat contains diverse ions such as Na + , K + , and Cl − , and these ions can be detected by electrochemical measurements. An et al. reported spray-coated graphene films for a multichannel electrochemical sensor for sweat monitoring. [231] The graphene film was deposited on a superhydrophobic paper electrode substrate by the spray-coating method with excellent flexibility. A recruited volunteer was chosen after 1 h of exercise for sweat ion monitoring. The sensor was connected to the volunteer by using a conductor, and the concentrations of K + , Na + , Cl − ions, and pH were measured. The measured concentration was compared with the data from ion-selective electrode measurement, showing high accuracy of 90% for K + , 88.4% for Na + , 87.6% for Cl − , and 88.7% for pH, respectively. As well as sweat monitoring, water quality monitoring is also a prospective application of liquid sensors. Figure 12j displays the real image of graphenebased ion-sensitive field effect transistors (ISFET) for selective ion sensing in the duckweed aquarium. 160 Duckweed, also called as lemnoideae lemna, absorbs various ions such as K + , NH 4+ , Cl − , HPO 4 2− , and SO 4 2− . As shown in the ion concentration plot in Figure 12k, (NH 4 ) 2 SO 4 , K 2 HPO 4 , NH 4 Cl were added to the aquarium after one week from the start date. It can be seen that the concentration of K + , NH 4+ , Cl − , HPO 4 2− , and SO 4 2decreased with time, while the concentration of Na + and NO 3 − from tap water slightly changed. The results imply that graphene can be utilized to real-time ion concentration monitoring in various water quality monitoring fields. Various practical applications and sensing properties of 2D chemical sensors are summarized in Table 3.

Perspectives and Challenges
To date, chemical sensors have been the core of the attention and have been widely researched since the 1960s from the first proposed chemical sensor using oxide thin films. Metal oxide semiconductors were the mainstream due to their advantages, such as high selectivity, sensitivity to target ions or molecules, and low cost. However, metal oxide semiconductor-based chemical sensors are operated at high temperatures far from room temperature, indicating that metal oxide semiconductor-based chemical sensors are difficult to be applied in our real lives. Therefore, alternative materials must be proposed with higher suitability in practical applications in daily life. Among diverse Table 2. Summary of liquid sensors based on graphene and 2D graphene analogs. a) SPR: surface plasmon resonance; SERS: surface-enhanced Raman scattering; FET: field-effect transistor; b) GO: graphene oxide; rGO: reduced graphene oxide; CNT: carbon nanotube; GQDs: graphene quantum dots; QDs: quantum dots; c) MB: methylene blue; MO: methylene orange; R6G: Rhodamine 6G; CV: crystal violet; MG: malachite green; RhB: rhodamine B; d) The values are either mentioned values in the article or taken as minimum measured value.  Reproduced with permission. [228] Copyright 2018, American Chemical Society. g) Schematic image of the water splitting cell for hydrogen generation. h) Photographs of gas sensing setup for the sensor. i) The response curves of sensor for hydrogen gas generated from water splitting cell. Reproduced with permission. [20] Copyright 2021, Elsevier. j) The optical image of graphene ISFET array in duckweed aquarium. k) The measured concentration plot of various ions. Reproduced with permission. [158] Copyright 2020, Springer Nature. candidates, 2D materials have become the best candidates for room temperature operable chemical sensors due to their fascinating properties. The important parameters of conventional chemical sensors were high sensitivity, selectivity, fast response, and recovery. However, more parameters are considered these days, including low cost, power consumption, and operation temperature, and miniaturization. To fulfill this requirement, chemical sensors must have reliable product design, high scalability, and low-cost process. In this regard, 2D materials are the most emerging candidates for practical sensor applications. Researchers are making efforts to utilize 2D material-based chemical sensors for various purposes and finally replace human senses. However, there are still many obstacles to fulfilling the advent of genuine artificial human sensing because the precise one-to-one correspondence of the device and target substance is yet invalid. To solve this issue, there are two premises that must be solved. In terms of materials, authentic large-area 2D material production is vital. In terms of devices, the highly selective detectors with good reliability are essential prerequisites. The potential of 2D materials by their unique natures and facile device integration is highly anticipated to further develop chemical sensors to be applied in our everyday lives.