FET configuration has been widely applied in many gas sensing devices due to the simple preparation process, high sensitivity, and portability, as well as the easy miniaturization to the nanoscale ^[1][57]. A typical graphene-based FET humidity sensor is comprised of three main parts, graphene as the channel material, two conductive electrodes as the source and drain electrodes, and a gate electrode with a thin dielectric layer (Figure 12). A bias voltage is applied on the gate electrode through the dielectric layer, which can modulate the conductivity of the graphene channel. Humidity sensing is conducted by measuring the current change of the graphene channel before and after exposing it to a humidity environment under a constant applied gate voltage. When adsorbing the water molecules, the electronic structure of the sensing graphene will be changed, leading to a change of the conductivity. In this type of humidity sensor, the sensing material, graphene, can be easily functionalized with different groups or complexed with other components to improve the sensitivity of the humidity sensors. Notably, many gases, including CO, NO, NH₃, NO₂, H₂, SO₂, H₂S, and ethanol, can be detected by such a type of gas sensor due to the different interactions between the gas molecules and sensing materials ^[2][3][48,68].

Resistive humidity sensors are one of the most widely investigated types of humidity sensing devices because of their many intrinsic advantages, such as the facile fabrication, simplicity of operation, cost effectiveness, reusability, low driven power, and easy miniaturization. The sensing mechanism is based on the change of the electrical resistance of graphene materials caused by the water molecules adsorbed on the surfaces, and the humidity can be effectively determined by measuring the resistance change (Figure 23) ^[4][5][46,69]. Equivalently, the humidity sensor can also detect the current change. The configuration of resistive humidity sensors is relatively simple, which consists of sensing materials located between two conductive electrodes on an inert substrate. The electrodes always employ an interdigitated structure to improve the sensing area and thus the sensitivity. The sensing graphene materials can easily be coated onto the substrate with patterned electrodes through spin-coating, drop-casting, spray-coating, dip-coating, printing, or laser transfer deposition. Generally, the sensitivity of such types of humidity sensors can be improved by increasing the materials, surface areas, as well as the functional groups sensitive to water molecules through constructing porous structures, surface modification, or compositing with other sensitive components. Sensors in this type are also widely employed to detect different kinds of gases, such as CO, H₂, NO_x, SO_x, O₂, Cl₂, and organic vapors, by changing the sensing materials ^{[6][4][2][7][8][9][10]}[38,46,48,52,70,71,72].

Impedance humidity sensors are a type of mixed potential sensors, which work based on impedance change. The humidity sensor is applied with a sinusoidal voltage in the frequency domain, and the impedance is calculated by measuring the current. In such a type of humidity sensor, an impedance spectroscopy is utilized to measure the response of the humidity sensor over frequencies ranging from subhertz to megahertz ^[11][73]. There is an advantage in such-type humidity sensors that it has the potential to accurately detect a relatively low humidity, but generally, it requires professional high-performance impedance spectroscopy. The structure of the impedance humidity sensor is similar to the resistive one, in which sensing materials are deposited on two conductive electrodes with gaps on an inert substrate (Figure 23). In impedance humidity sensors, GO and rGO based materials rather than pristine graphene are utilized as sensing materials, because the abundant functional groups on the surfaces as well as porous structures can provide more active sites for water molecules, which induces a simultaneous resistance and capacitance change of the graphene materials, i.e., comprehensive impedance change ^[12][74]. Similarly, GO or rGO can be composited with other active components to further improve the sensitivity. Impedance sensors have also been developed for the efficient detection of various gases, including NO_x, CO, and hydrocarbons, with low concentrations ^[5][69].

Capacitive humidity sensors utilize the special properties of GO materials, which are insulating, but show proton conductivity when the surfaces adsorb water molecules, with the proton conductivity being related to the water concentration in the environment. This induces the change of the capacitance ^[13][75]. The sensor detects the humidity by measuring the capacitance of the device. Therefore, GO can work as a water molecule-sensitive dielectric in a double-layer electrochemical capacitor to detect humidity in the atmosphere. A typical capacitive humidity sensor is generally the in-plane type to expose the maximum active surface area, which consists of two parts, interdigitated conductive electrodes and GO-based sensing materials as the dielectric (Figure 3) ^[14][76]. In capacitive humidity sensors, interdigitated conductive electrodes can be metal materials, as well as rGO, which can form all-graphene based humidity sensors. For the dielectric, GO can be composited with other proton conductive components to improve the sensitivity and response ^[15][77].

Principally, SAW humidity sensors are a type of microelectromechanical system, which detect the humidity change depending on the modulation of surface acoustic waves. An SAW sensor basically comprises a piezoelectric substrate, an input interdigitated transducer (IDT) on one side, a second output IDT on the other side of the substrate, and a delay line in the space between the two IDTs (Figure 34) ^[16][17][78,79]. The input IDT produces a mechanical SAW under the sinusoidal electrical signal using the piezoelectric effect of the piezoelectric substrate, and the SAW propagates across the decay line, where the SAW is influenced by the environment. The SAW is converted into an electric signal by the piezoelectric effect in the output IDT, and any changes made to the mechanical wave are reflected in the output electric signal. The sensing can be accomplished by measuring changes in the amplitude, phase, frequency, or time-delay between the input and output electrical signals. Any changes of the physical or chemical properties of the sensor surface, such as the mass, conductance, or viscoelasticity, affect the acoustic wave velocity or attenuation ^[18][80]. Graphene-based SAW humidity sensors generally employ graphene materials as the acoustic layer and utilize the mass change induced SAW change to detect the humidity because hydrophilic GO or GO-based composites enhance the water adsorption capability through the oxygen-rich groups, strengthening the mass loading effect. Based on the above mechanism, SAW sensors towards other gases or targets can also be designed by modulating the properties of the sensing materials and their interaction with sensing targets.

Balashov et al. investigated the sensing performance of SAW humidity sensors by coating a submicro-thick film of GO or a polyvinyl alcohol (PVA) thin film on the decay line, and the GO-based humidity sensor showed a sensitivity of 1.54 kHz/% RH (relative humidity), much higher than the 0.47 kHz/% RH and 0.13 kHz/% RH for the PVA coated and uncoated one, respectively ^[16][78]. The SAW humidity sensor prepared on the LiNbO₃ surface coated with a CVD-growth graphene layer showed a frequency downshift of 1.38 kHz/% RH in the RH range lower than 50% due to the mass loading and a frequency downshift of 2.6 kHz/% RH in the RH range higher than 50% due to the change in the elastic properties of graphene film ^[19][81]. The SAW humidity sensor based on GO coated LiNbO₃ plate showed a super high sensitivity by employing a high-order lamb wave with a large coupling constant, standard LiNbO₃ plate, and graphene oxide sorbent film, showing a minimal detectable level as low 0.03% RH, and reproducibility of ±2.5% ^[20][82]. Xuan et al. demonstrated SAW humidity sensors based on a ZnO piezoelectric thin film on a glass substrate with GO as the sensing layer, which exhibited a high sensitivity from 0.5% RH to the 85% RH range with very fast responses (rise time of 1 s and fall time of 19 s) ^[21][83]. Then, they realized a high-performance flexible SAW humidity sensor with the same GO sensing layer on a piezoelectric ZnO thin film deposited on a flexible polyimide substrate ^[22][84]. Similarly, Kuznetsova et al. reported an SAW humidity sensor based on a GO film/ZnO film/Si substrate with an enhanced sensitivity of about 91 kHz/% RH and a linear response towards relative humidity in the range of 20–98% ^[23][85]. Recently, Xie’s group proposed an SAW humidity sensor based on an AlN/Si (doped) layered structure with a GO sensing layer for a high sensitivity and low temperature coefficient of frequency ^[17][79]. The humidity sensor coated with the GO layer exhibited an improved sensitivity of up to 42.08 kHz/% RH in the RH higher than 80% and worked well at both low (<10% RH) and high (>90% RH) humidity. Moreover, it showed low hysteresis, outstanding short-term repeatability, long-term stability, and improved thermal stability.

The QCM technique has been widely used to analyze the mass, membrane structure, molecular interaction, and viscoelasticity changes on the surface of electrodes. QCMs coated with water-sensitive materials can act as sensors for humidity detection, which have the advantages of being ultra-sensitive towards water molecules adsorbed on the crystal surface and a digital frequency output. A QCM humidity sensor can be directly connected to a digital control system to simplify the signal processing circuit. A typical QCM humidity sensor consists of a bare quartz crystal resonator deposited with a humidity sensing material on its electrode, forming a composite resonator (Figure 45) ^[24][86]. The composite resonator shows an oscillating frequency shift when the water molecules adsorb or desorb on the sensing material, reflecting the ambient humidity change. There are two types of interaction between water molecules and sensing materials in a QCM humidity sensor, the mass change due to the adsorption and desorption on the surface (mass effect) and the viscoelastic change of the adsorbed film caused by the penetration of water molecules (viscosity effect) ^[25][87].

GO is recognized as a promise humidity sensing material because of its abundant hydrophilic oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups. Yao et al. demonstrated the first GO thin film-coated QCM humidity sensors, which exhibited excellent humidity sensitivity properties with a sensitivity of up to 22.1 Hz/% RH and a linear frequency response in the wide detection range of 6.4–93.5% RH. They found that, at the low RH range (<54.3%), mass changes caused by water molecules’ adsorption/desorption accounted for the frequency response of the sensor, while in the high RH range (>54.3%), both the water adsorption/desorption mass change and expansion stress of the GO thin film induced by the swelling effect caused the frequency change ^[26][88]. They also demonstrated that the GO coated QCM humidity sensor showed better stability than the PEG-coated one ^[27][89]. The incorporation of other components into the GO film has also been widely investigated to improve the humidity sensing performance by addressing the mass effect or viscosity effect. Other carbon materials, such as multi-walled carbon nanotubes (MWCNTs), nanodiamond, or fullerene, were introduced into the GO film to prevent the viscosity effect, because the intercalation may expand the GO film and improve water molecule diffusion ^[25][28][29][87,90,91]. Many hygroscopic or hydrophilic components, such as poly (diallyl dimethyl ammonium chloride) ^[30][18], poly ethyleneimine and protonated poly ethylenimine ^[31][32][92,93], polyethylene oxide ^[33][94], polyaniline ^[24][86], SnO₂ ^[34][95], and ZnO ^[35][96], have been introduced into GO or rGO films by the direct mixing method or layer-by-layer assembly, which can provide more active sites for water adsorption to improve the humidity sensing performance, including the sensitivity, response/recovery time, hysteresis, repeatability, selectivity, and long-term stability. Additionally, diamine- and β-cyclodextrin-functionalized graphene oxide films exhibited a good response to the low humidity region due to the strong sensitivity of the –CONHC₂H₄NH₂ groups to water molecules ^[36][97].

Optical fiber humidity sensors detect changes of the optical properties inside the fiber caused by water molecules, such as changes of the transmitted optical power, dielectric properties, or the refractive index. Compared to electronic humidity sensors, optical fiber humidity sensors show several advantages, such as the possibility of working in harsh conditions, including flammable environments; higher temperature and pressure ranges; and electromagnetic immunity. However, some facts, including the fabrication repeatability and the high cost of the optical equipment, have prevented the commercial applications of optical fiber humidity sensors. According to the working principle, optical fiber humidity sensors can be classified into several groups, such as humidity sensors based on the optical absorption of materials; humidity sensors based on fiber Bragg gratings and long-period fiber gratings; humidity sensors based on interference (Fabry-Pérot, Sagnac, Mach-Zehnder, Michelson, and modal interferometers); humidity sensors based on micro-tapers, micro-ring, micro-knot resonators, and whispering galleries modes; and humidity sensors based on electromagnetic resonances, specifically lossy mode resonances ^[37][98]. The classification is shown in Figure 56. More details can be found in a comprehensive and specialized review on optical fiber humidity sensors summarized by Ascorbe et al. ^[37][98], thus this review will not describe the detailed mechanisms.

Graphene materials have the potential to be applied in optical fiber humidity sensors due to the good adsorption towards water molecules, which will induce changes of the optical properties in the optical fiber. For example, Xiao et al. reported an rGO based optical fiber humidity sensor by coating an rGO film on the polished surface of a side-polished fiber, which achieved a power variation of up to 6.9 dB in the high relative humidity range (70–95%) and displayed a linear response with a correlation coefficient of 98.2%, sensitivity of 0.31 dB/% RH, response speed of faster than 0.13% RH/s, and good repeatability in the 75–95% RH ^[38][99]. Recently, they demonstrated an improved optical fiber humidity sensor based side-polished single-mode fiber coated with GO film, which exhibited a better performance ^[39][100]. The water adsorbed on the GO film caused the dielectric property change, which could influence the TE-mode absorption at 1550 nm of the polymer channel waveguide, constructing the relation between optical absorption and relative humidity ^[40][101]. Wang et al. developed humidity sensors based on tilted fiber Bragg grating coated with GO film, which showed a maximum sensitivity of 0.129 dB/% RH with a linear correlation coefficient of 99% under the RH range of 10–80% due to the dependence of the cladding mode resonances on the water absorption and desorption on the GO film ^[41][102]. They also investigated optical fiber humidity sensors based on an in-fiber Mach-Zehnder interferometer coated with GO or GO/PVA composite, showing high sensitivity, good stability, and linearity ^[42][43][103,104]. An optical fiber humidity sensor based on a Fabry–Perot resonator was also demonstrated by depositing rGO on the other surface of a hollow core fiber, and the light leakage at the resonant wavelength, which depended on the refractive index of rGO, could be measured to reflect the humidity ^[44][105]. Optical fibers coated with graphene materials can be applied not only in humidity sensors, but also in chemical and biological sensors ^[45][106].