Radiofrequency sensing systems based on emerging two-dimensional materials and devices

As one of the most promising platforms for wireless communication, radiofrequency (RF) electronics have been widely advocated for the development of sensing systems. In particular, monolayer and few-layer two-dimensional (2D) materials exhibiting extraordinary electrical properties not only can be integrated to improve the performance of RF circuits, but also to display exceptional sensing capabilities. This review provides an in-depth perspective of current trends and challenges in the application of 2D materials for RF biochemical sensing, including: (i) theoretical bases to achieve different sensing schemes; (ii) unique properties of 2D materials for reasoning their applications in RF sensing; (iii) developments in 2D RF sensors to facilitate the practice of biochemical sensors with ever-demanding sensitivities, as well as their potential uses in meeting the requirements and challenges of biochemical sensors in the Internet-of-Things era.


Introduction
Since 1980, radiofrequency (RF) technology has been widely adopted for sensing applications and developed tremendously [1]. By avoiding direct connections between sensing elements and data processing electronics, wireless RF technologies can significantly reduce precision space and enhance practicality * Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. for practical applications. Remarkable efforts have been put forward on the integration of diverse sensors into wireless systems with extensively explored RF sensing schemes composed of either passive or active circuits for transducing biochemical stimuli into readable electronic signals [2,3]. Nowadays, wireless RF detecting systems based on LC resonator, RF identification (RFID) technology and etc., have become an indispensable part in all walks of life, including food safety, environmental monitoring, life science research, and medical diagnosis satisfying the ambitious demands of Internet of Things (IoT) [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].
The concept of RF (ranging from 3 kHz to 300 GHz) sensing encompasses all relevant elements such as materials, devices, circuits and systems that are used in wireless transmission. Realization of such an RF sensing platform should firstly address the issue of optimizing ultrasensitive functional materials compatible with production of the RF sensing systems. With all atoms exposed to the external environment and all charge carriers flowing on the confining surface [21,22], two-dimensional (2D) materials emerge as one of the most ideal building blocks of electronic sensors with ultrahigh sensitivities and free of short channel effect [23][24][25]. Importantly, these atomically thick 2D materials possess excellent electrical properties such as high carrier mobility (e.g. 10 3 -10 6 cm 2 V −1 s −1 for graphene) [26,27]. In addition, 2D materials possess ideal van der Waals (vdW) surfaces, which are chemically inert and can be further regulated through chemical modification to achieve specific detection of foreign stimuli. Furthermore, benefiting from the excellent mechanical properties of 2D materials, ultrathin 2D flexible biochemical sensors are able to integrate intimately on the curvilinear surfaces of biological tissues for wearable devices. Despite the numerous and exciting advantages that 2D materials possess, not every needed sensing device characteristic can be guaranteed separately. Appropriate heterostructures open the door to significantly improving their performance thanks to the layered bulk structure and inherent heterogeneous feature. To this end, since the discovery of graphene by mechanical exfoliation in 2004 [28], 2D materials obtained by diverse synthesis methods have been actively pursued. Recent progresses in high-quality, large-area chemical vapor deposition (CVD) [29], as well as screen-printing technique [7], shed light on the long-term applications by making 2D materials and devices more cost-efficient. To date, advances in 2D materials have made them highly suitable for integrated RF circuits and sensing applications [30][31][32].
This review summarizes the contents to elucidate RF sensor mechanisms, and critically assesses the current trends and challenges in the innovations of RF sensing systems based on 2D materials and devices. We present a comprehensive and balanced analysis first on the theoretical underpinnings of RF sensing to achieve a variety of sensing schemes. Second, we highlight the diverse and unique properties of 2D materials, which hold great promise of acceptance in RF biosensors. Finally, we offer insights into the challenges and prospects for potential commercialization of 2D RF biochemical sensors which are defined as constituent parts of wireless sensing systems for IoT applications.

Principle of RF sensors based on 2D materials
Generally, a sensor device is composed of a sensing part and a transducer [1,33]. The sensing material is the main functional element. It can interact with particular stimuli which can be in a form of a physical irritation such as mechanical, optical, and magnetic signals, a chemical target (e.g. gas, or ion), or a biological perturbation including nucleic acid, protein, etc. and then alter the intrinsic properties of the material (e.g. conductivity σ, permittivity ε, or work function φ). In pursuit of the ever-demanding sensitivity, selectivity and stability, sensing materials take on heavy responsibility. More specifically, 2D materials such as graphene and its derivatives [24,29,[34][35][36], transition metal dichalcogenides (TMDs) [22,37], MXenes and their vdW heterostructures [38][39][40], appear great potential for highly sensitive detection. Interactions between analytes and 2D materials differ from each material due to their different chemical structures, surficial active sites, etc. For example, charge transfer acts between charged targets and conductors/semiconductors. Metal oxides that possess oxygen-rich surfaces can interact with gas molecules via redox reactions. The transducer with arbitrary architectures such as resistors, capacitors, and/or field-effect transistors (FETs) transforms the changes to readable electrical signals, reflecting the variations of capacitance (C), resistance (R), and inductance (L). We note here that despite all the progress, the origin of the sensor response is not always fully clear, particularly due to the difficulty in understanding/distinguishing the adsorption of target analytes at the sensor surface from nonspecific binding, surface chemical instability and drift. In addition, the diversity of 2D material categories, device forms and operating mechanisms also contributes to the complexity. Finally, we accept information wirelessly incorporated with RF communication systems. Figure 1 gives a brief description of RF biochemical sensors based on 2D materials. That is, the introduction of various gases, ions, nucleic acids, proteins, bacteria or pathogens, induces multiparametric changes in the 2D materials, which can be monitored and transduced by RF sensing circuits. Depending on the device configurations (including but not limited to, resistor and varactor), the transducers connected with RF communicators aim to identify electrical signals such as resistance and/or capacitance variations and then transmit them to either the frequency shifts or changes in the full width at half maximum (FWHM).
During the development of RF sensors based on 2D materials, sensing schemes including RLC circuits based on mutual inductance [41,42], RFID tag-antenna coupling [43,44] and near-field communication (NFC) [45,46], were routinely adopted both in the laboratory and in practical applications (figure 2). Attempts were made to develop graphene-like nanosheets gas sensors based on the surface acoustic wave wireless technology in 2008 [47]. Furthermore, in 2010, a combination of graphene/ionic liquid composites with quartz crystal microbalance (QCM) was achieved [48]. The LC resonator was supplemented by graphene resistors in 2012, greatly improving the categories of 2D RF sensors [20]. In 2014, a micro gravimetric wireless ammonia sensor based on reduced graphene oxide (rGO) was fabricated [49]. It is mentioned that RF technologies (for example, QCM and the micro gravimetric system) show mass-related resonant performance, while more are electrical signals-related we mainly discussed in this review. Commercialization of RFID began in the 1980s, and dramatically enriched the categories of 2D RF sensors. For example, ultrasensitive smart graphene RFID hydrogen sensor tags were reported [8]. Moreover, apart from Summary of RF sensors based on 2D materials. The introduction of various gases, ions, nucleic acids, proteins, bacteria or pathogens, induces multiparametric changes in the 2D materials, which can be monitored and transduced by RF sensing circuits. Inset, depending on the device configurations (including but not limited to, resistor and varactor), the transducers connected with RF communicators aim to identify electrical signals such as resistance and/or capacitance variations and then transmit them to either the frequency shifts or changes in the full width at half maximum (FWHM).   unprecedented detection limit down to 1 ppm, this device exhibited a long lifetime on a flexible substrate owing to the passive configuration and strongly immobilized Pt-rGO. Additionally, it was also expanded to wearable CuO-SWCNT (single-walled carbon nanotube) RFID gas sensors with long life-time, advancing the development of next-generation medical and environmental-monitoring electronics [7]. NFC was developed in 2003 upon the evolution success in integration of RFID and interconnection technology. It was applied on graphene NFC sensors soon in 2017 [45]. Another attempt on development of the rGO SRR ammonia detector also showed contactless application with lower costs compared to RFID or NFC tags [46]. Recent advances in smart contact lens sensors [50,51], neural probes in the brain [52] and electronic skin [40,53] have broadened the potential use of 2D RF biochemical sensors. The basic principle of the whole RF sensing platform will be described in detail as follows. The upper panels in figures 3(a) and (b) illustrate the structures of RLC resonant circuits in parallel and series, respectively, where the sensor schemes are simplified as a combination of an inductive and a capacitive element with an added resistor. Generally, either the capacitive element or the resistive element is subject to the influence of the adsorbed analytes. For the RLC series resonant circuit (figure 3(b)), its resonance frequency can be formulated as ω 0 = 1 √ LC with a quality factor Q = 1 Rω0C and an FWHM BW 0.7 = ω0 2π Q . For wireless sensing applications based on RLC resonant circuits, it is clear that their resonant frequency is a function of the capacitance, resistance and/or inductance. Beneficially, 2D materials with confined charge transport could induce profound changes in the quantum capacitance and/or resistance with respect to analytes adsorption, therefore initializing distinct frequency shifts and/or alterations in S parameters with high sensitivity. All these changes in the electrical properties can be transduced through an inductive coil coupling to a wireless readout system (upper panel, figure 3(a)). As placed at a suitable distance, the sensor is coupled to the reader working near the resonant frequency. The coil geometry, distance from the sensor to reader coils, and orientation relationship between the two coils which impact the coupling coefficient should be considered [54,55]. A fair approximation of the coupling coefficient k thus can be expressed by (when r S ≪ d) [56]: , where r 1 is the radius of reader coil, r s is the radius of the sensor coil and d is the distance between them. It is notable that the coupling coefficient plays a dominant role in determining the sensitivity of sensors in accordance with variations in resonant frequency [57]. Additionally, the tensile strain characteristics and light transmittance of the coils can be adjusted according to the needs of the wearable biosensing system.

RFID and NFC tag.
In recent years, RFID technology has developed rapidly based on the principle of LC resonant tanks. An RFID system consists of an interrogation reader (vector network analyzer (VNA)) with an antenna and an active or passive RFID sensor tag, which includes a dipole tag antenna, sensing material, and an integrated circuit (IC) chip (used to provide a unique ID). Compared with active tags, passive sensor tags have a smaller detection range. However, due to the advantages of long service life, portability, and low cost of non-contact tags, passive tags show great application potential. Recently, passive ultra-high frequency (UHF) RFID sensor tags have been applied to wireless sensor systems to sensing distance and signal strength [15,16]. The VNA issues an interrogation signal at a certain power (P 1 ) to activate the sensor tag ( figure 4(a)). Based on the impedance matching between the antenna and the IC chip, the reflected signal morphology (i.e. amplitude and phase) of the sensor tag varies with the target analyte. Finally, the backscattered sequence signal is reflected to the RFID reader antenna at a power level of P 2 . However, RFID usually requires the use of bulky, expensive and complicated VNA or impedance analyzers to receive and process data, which is not conducive to the portability of realtime detection.
NFC is a radio frequency communication technology extended from RFID and has been widely integrated into smartphones and other mobile devices in recent years. Since large-scale detection equipment is not required, wireless passive NFC tag biosensors have broad applications in the detection of gases, ions, proteins and other analytes [45,58,59]. Any NFC-enabled device can power a wireless NFC tag through an alternating magnetic field and receive data from the tag through inductive coupling and signal modulation. NFC tags are based on a resonant circuit, which consists of an IC chip, a chip capacitor (C) and an inductor (L). By employing a varistor in the resonant circuit, accurate semi-quantitative detection of different analytes in the sensor can be achieved. Additionally, NFC tag biosensors communicate with smartphones by transmitting radio waves, thus realizing unsigned identification of the types and concentrations of biochemical molecules. For example, an NFC tag biosensor was designed for the detection of ammonia (NH 3 ) and hydrogen peroxide (H 2 O 2 ) [60]. The commercial NFC tags were modified with a chemiresistive SWCNTs composite thin film. In contrast with other RF technologies, NFC tag biosensors often do not need to rely on expensive, cumbersome and complex impedance and network analyzers for testing. More importantly, this powerful method of converting analog input signals into digital output signals (logical sequences of '0' and '1') and transmitting them wirelessly to smartphones can realize the selective identification of multiple analyte concentrations in more complex situations without data analysis [61]. For example, carbon-interdigitated electrodes covered by a single layer of CVD-grown graphene are used for gas detection [45]. As shown in figure 4(b), if the impedance (Z) of the electrode sensor is less than the threshold resistance (R t ), the smartphone can read the NFC tag sensor, making it 'on' (step 1). When exposed to the target analyte, the R sensor exceeds R t , causing the IC chip to receive insufficient power supply, and the sensor is in the 'off' state (step 2). In summary, the analyte can be detected by changing the state of the tag sensor from 'on' to 'off' (step 3).
In addition to the rapid development of RFID and NFC in recent years, many RF technologies based on mechanical, acoustic and electromagnetic resonance principles have been integrated into sensors using 2D materials. QCM sensing systems typically operate in the low-frequency region to measure changes in mechanical resonance frequencies [48]. They are suitable for determining the affinity of biomolecules to functionalized 2D material surfaces. SRR is a chipless passive device, which is mainly used for high-frequency biosensing by designing the required resonant frequency [46]. However, the SRR sensing device can only measure in the dry state due to the interference of the solution effect. For small biomolecular detection in solution environments, nuclear magnetic resonance-based RF biosensors are of great significance for realizing integrated wireless active sensing systems featuring label-free and high sensitivity.
In addition to the core sensors, the design of the physical connection (i.e. the way of communication) between the sensor and the data processing device is very important for the integration of wearable wireless sensor systems. IoT communication technologies, such as local area network communication, Bluetooth, long range radio, etc., have been fully applied in wireless biosensing systems. Through . Schematic diagrams of two wireless sensor systems based on RFID or NFC tags. (a) Working principle of the RFID sensor system. The RFID-based sensor system consists of RFID sensor tags and RFID antennas connected to a VNA. Reproduced from [15] with permission from the Royal Society of Chemistry. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Scientific Reports [16], Copyright (2018). (b) Construction process and equivalent circuit of the NFC tag sensor. The smartphone powers the NFC tag sensor and receives the signal. There are three main steps in building an NFC tag sensor for different detections. The target analyte is detected by the state change of the label from 'on' to 'off'. Reprinted from [45], Copyright (2017), with permission from Elsevier. the customized writing of the IoT chip and the smartphone application (APP), the wireless connection between the sensor and the smartphone is realized for real-time monitoring on the smartphone. Furthermore, IoT and cloud computing are closely linked. While realizing wireless communication, data can be uploaded to the cloud terminal for big data analysis.

Transmission line theory.
In low-frequency circuits, we generally do not consider the electromagnetic field distribution and loss in the wires. However, in a typical millimeter-size RF circuit, the wavelength of the RF signal is much larger than the length of the wire, leading to significant amplitude changes within the range of the wire. In this case, it is necessary to consider the influence of the electromagnetic field distribution. Therefore, there are distributed resistance, distributed capacitance, distributed conductance, and distributed inductance existing [64].
With the consideration of transmission line theory, we can derive the wave equations from Kirchhoff's laws with d 2 V(z) and I(z) represent the potential and current at z position, respectively. γ is the complex propagation constant given by γ = α + iβ = √ (R + iωL) (G + iωC), where the attenuation constant α is a function of frequency depending on the type of transmission line, and the phase constant β is wavelength related.
In the microwave frequency band, it is difficult to directly and accurately measure current and voltage. Instead, it is much easier to measure the magnitude and phase of electromagnetic waves. In general, an RF circuit based on 2D materials is considered as a two-port network to describe its electrical properties. The most commonly used parameter to describe electrical parameters at the ports is the scattering matrix (or scattering parameter, S) [64]. The S-matrix of a two-port network can be expressed as S = , where S 11 and S 22 refer to the reflection coefficient of the input and output ports of the circuit, respectively. S 21 and S 12 are the transmission coefficient in the two-port network. The measured Sparameter can be transformed into various electrical parameters such as admittance (Y), impedance (Z), hybrid parameter (H) and transfer parameter (T). In an RF biosensor, the concentration of analyte is usually proportional to one of the parameters, so that detection results can be obtained by measuring S-parameters.

Sensing mechanisms of RF sensor systems based on 2D materials
Over the past decade, researches in 2D RF biochemical sensors have been at their heights and the working principle relies on the detection of impedance changes including resistance and capacitance (especially quantum capacitance of 2D materials). Associated with adsorbing targets on the surface, interactions (i.e. charge transfer [52,53,65], capacitive effect [66] and gating effect [67]) happen near targets/2D materials interface to modulate electrical properties. The other new sensing platform is based on homodyne/heterodyne to detect frequency mixing response between molecular dipoles and the biochemical sensor [63]. Current efforts have been widely devoted into 2D RF sensors relying on the impedance monitoring. In a study on wireless in-vivo glucose sensing [50], a compact sensor with a highly transparent antenna (>91%) made of graphene-silver nanowire was integrated into a wearable contact lens. Via detecting the drift of reflection coefficient at a fixed resonant frequency (4.1 GHz) wirelessly, the change of sensor resistance can be identified and related to glucose-graphene interaction. Along with demonstrated biocompatibility with rabbit tests, the combination of 2D sensors and a wireless antenna on the contact lens provided a strategy for non-invasive, highly sensitive glucose detection in body fluids (∼0.4 µM). For another example, an established graphene-based wireless bacteria-detecting biosensor has been demonstrated for bioselective detection of bacteria on teeth via resistance change (figure 3(a)) [20]. Resistive responses to bacteria can lead to frequency variation, based on the theory of LC resonator. Regarded as a resistor, graphene functionalized with stable, selective antimicrobial peptides [68], shows resistance changes when combined with characteristic bacteria [69]. The conductance signal can be further read by a VNA through S parameters transduced by an antenna. Another kind of sensing element utilizes the quantum capacitance effect of graphene for biosensing as shown in figure 3(b) [62]. This work proved that frequency shifts were caused by the quantum capacitance change of graphene in response to glucose. Notably, a multi-finger geometry graphene varactor with thinner highk dielectrics was developed to generate high capacitance, increased tuning range with minimized series resistance, minimized 1/f noise, high-quality factor and fast response [66,70]. The quantum capacitance effect is a manifestation of the Pauli exclusion principle in a system with a finite density of states and varies with the energy-dependent density of states [71]. Quantum capacitance can also be greatly impacted by doping, defects, etc. Besides, graphene quantum capacitance sensing has an important place even in an electrolyte solution because the electrical double layer capacitance of the electrolyte does not shield the quantum capacitance [72]. Compared with alternative passive sensing methods, quantum capacitor sensors with a monotonic dependence between resonance frequency shift and glucose concentration show great noise immunity, fast response and the potential for further integration.
A homodyne or heterodyne detector is an effective sensing scheme to reduce noise interference and increase sensitivity. The heterodyne sensing principle is based on the mixing responses of two signals with different frequencies to gain a new frequency of their sum or difference. With radio communication and amplitude-modulated excitation voltage in the measurement of heterodyne sensors, reduced noise interference is achieved because the emitted monotonic highfrequency modulated signal depresses 1/f noise [73]. A notable work on the graphene nano-electronic heterodyne sensor for polar gas molecule detection has been demonstrated in figure 4(c) [63]. During heterodyne operation, the dipole of the polar molecule is excited by an alternating current (AC)driving voltage (figure 3(c) left). In turn, a conductance modulation on the graphene channel is induced, mixing with the AC excitation and yielding a detectable heterodyne mixing current (figure 3(c) right). This work provided an improved response speed (down to 0.1 s compared to several seconds or less of conventional biosensors) due to the higher AC field switching speed than the dynamics of the interface states and an achieved limit of detection (LoD) down to 1 ppb at higher frequencies. A homodyne detector generally has a similar mechanism, while the distinction is that the mixed signals have the same frequency. With a balanced homodyne detector at the surface plasmon resonance angle, the oxygen gas sensor achieved improved sensitivity (∼10 ppm) and fast response (∼0.42 s) [74].

RF sensing in physiological conditions
Sensing in a physiological environment represents a challenge compared to air conditions. Due to its high dipole moment, water has a high dielectric constant (the ability of the solvent to neutralize attraction between ions of opposite charge) up to 80ε 0 compared to that of 1ε 0 for air. The resulting high capacitive coupling puts an obstacle to precision detection. Apparently, there are also more losses in liquid due to the absorption and scattering of electromagnetic waves [75]. On the other hand, for low-frequency chargedetection-based devices operated in high-salt solutions, the so-called Debye screening effect causes ionic screening of electrostatic field originating from the charged analytes over a short distance [73,76,77]. Therefore, electrostatic interactions and sensing signals decay exponentially with distance away from the sensor surface in electrolyte solutions according to the abovementioned theories. The characteristic distance at which the electric field decreases to 1/e is defined as the Debye screening length L D . At room temperature, the L D can be formulated as [78]: ∈0∈rkBT , ρ i is the ion density and z i is the ion valency. For physiological solutions with ionic strength ( ∑ i ρ i z 2 i ) of 150 mM, L D can be calculated to be approximately 0.7 nm. Therefore, for conventional field-effect biosensors (operated at direct current or relatively low frequencies), it is unlikely that the (charged) target biomolecules binding onto receptors of several nanometers can approach the sensor surface within the Debye screening length to be recorded by the biosensors [79].
Instead, RF sensing at high frequencies (>10 MHz) can penetrate the electrolyte solution to a large distance and probe directly the dielectric properties of targeted biomolecules [80,81]. Consequently, RF sensing eliminates the limit of the Debye screening effect imposed on conventional fieldeffect biosensors based on charge detection, representing a feasible and unique approach to circumvent the interference of Debye screening. A recent study demonstrated this technique by achieving highly sensitive detection under UHF (around 2 GHz) using a custom-designed RF box with an electrolyte-gate graphene field-effect transistor (GFET, lower panel, figure 5(a)). Such an RF-operated GFET (see figure 5(b) for the sensing scheme) enabled real-time monitoring of dielectric-specified biomolecular interactions (figure 5(c)) and cell contractility activities (figure 5(d)), which were fully suppressed under conventional conductance measurements because of Debye screening.

Innovations in RF sensing platforms based on 2D materials and devices
Over the past few years, outstanding progress has been achieved in the field of RF sensing using 2D materials. The atomic thickness of the 2D materials provides unique electrical, mechanical, and chemical properties. This results in a high surface-to-volume ratio with high sensitivity and functionalization capabilities for 2D sensors. Furthermore, a lack of short-channel effect, and low sheet resistance contribute to the restricted carrier transport within planes. Additionally, 2D materials exhibit excellent mechanical strength and flexibility, good optical transparency, and processability of fabrications. Different chemical structures of 2D materials exhibit tunable physical, chemical, and electronic properties.
Graphene, widely investigated for its extremely high conductivity and ideal carrier mobility approximately 10 6 cm 2 V −1 s −1 , exhibits chemical stability, biocompatibility and large-scale CVD synthesis capability [82]. Its defects and sp 2 hybridized surface allow for both covalent and noncovalent bonding, enabling the adsorption of analytes through forces such as vdW forces and π-π interactions [27,83]. In addition, graphene devices have a cut-off frequency in the hundreds of gigahertz, making them suitable for next-generation RF devices [24,84]. Apart from graphene, graphene oxides (GOs) are electrically insulating materials with sp 3 hybridized carbons [85]. They can be reduced to obtain rGOs with more C-O surface sites via thermal, chemical or electrical methods. Solution-phase exfoliation is the most common synthesis method for graphene derivatives [86]. It cannot achieve well-controlled mass production with high quality and uniformity. TMDs can be divided into two categories of metallic and semiconductive TMDs based on their structures [87]. For example, 1T and 1T ′ structures provide metallic properties while 2H phase gives TMDs more semiconductive characteristics. Actually, semiconductive TMDs with 2H phase are more useful and attractive for sensing applications due to their acceptable carrier mobility ranging from dozens to hundreds, high on-off ratio, good thermal, mechanical and chemically stability [88]. Additionally, TMDs RF electronics are essential to be widely studied with an example of high-performing RF MoS 2 (several gigahertz) [22,30]. To date, progress in TMDs sensors requires advances in scalable synthesis of high-quality TMDs, and deeply understanding of their distinct properties which are not yet fully exploited in the applications now. Metal oxides have variable electrical properties from metallic to insulating, and are popular in the sensor field due to their oxygen-rich surfaces, particularly for chemical sensors that interact with analytes through redox reactions [89][90][91]. However, several drawbacks of metal oxides are fatal for the development of RF metal oxides sensors, especially high operating temperatures. MXenes, with their good conductivity and rich functional surface sites, are an emerging opportunity for sensing platforms [31,92]. There should be long-term efforts to controllably synthesize MXenes distinguished from the toxic etching-assisted approach now, and better understand their properties for the new but rapidly expanding RF sensing area. Despite the many impressive benefits of 2D materials, they may not meet all of the requirements for every sensing device individually. However, the use of suitable heterostructures can significantly enhance their performance due to their layered bulk structure and unique heterogeneous characteristics.
As we have already understood the circuit configuration, sensing mechanisms and sensing materials responsible for the sensitivity, selectivity and stability of RF sensors, here we will now proceed to summarize the performance of 2D RF sensors based on different 2D materials in table 1. Furthermore, we will briefly introduce the applications of 2D wireless sensors classified as a group of 2D materials.

RF biosensors based on graphene and its derivatives
Among the diverse 2D materials applied for RF biosensors, graphene and its derivatives have drawn great attention because of their excellent performance [66,72,99,100]. First of all, the extraordinary electrical properties of graphene, which include ultrahigh carrier mobility with a value of 200 000 cm 2 V −1 s −1 and high carrier density with a value as high as 1012 cm −2 , introduce graphene itself to be a favorite candidate for use in sensing electronics [101,102]. Secondly, each carbon atom in graphene is a surface atom and thus a large surface-to-volume ratio is provided, showing a relatively high sensitivity to its facing environment. Graphene is also easily functionalized for the enhancement of surface chemistry and selectivity in addition to its good electrical properties [21,27,29,36,82,83]. It is essential to achieve wearable, flexible electronic sensing devices due to the good mechanical characteristics of graphene [20,50]. On the other hand, graphene is cost-effective and has little impact on the environment [84]. These superior properties make graphene become an ideal material for a wide variety of RF biosensors with different working principles. As summarized in table 1, various sensing mechanisms based on quantum capacitance [62], resistance [93,94], heterodyne sensing [63,73], etc., have been applied to graphene RF sensors, achieving both biological molecules and specific gases detection. GO and rGO have also received considerable interest. Due to their relatively large surface area, delocalized π electrons on the surface which provide connectivity to certain targets through π-π interaction and the presence of various oxygencontaining functional groups, GO and rGO demonstrate excellent performance on biocompatibility and affinity for specific biomolecules [103,104]. Until now, GO or rGO has taken a vital proportion in biochemical sensors.

Graphene wireless biosensors.
A great deal of previous studies has been employed on GFETs. Design ideas for high-frequency electronic devices, such as frequency multipliers, amplitude demodulators and amplifiers, based on graphene have been proposed [35,[105][106][107]. Therefore, the development of graphene RF biosensors has a more mature foundation to expand into more complex areas of the wireless sensors. Wireless graphene sensors have been already designed to detect diverse biological elements, e.g. glucose [50,62,108], bacteria [5,20], lactate [43] and biomarkers [109] both in-vitro or in-vivo. An in-vitro glucose sensing experiment was studied on a metal-oxide-graphene (MOG) varactor connected to an inductor in the form of an LC circuit [62]. Based on the quantum capacitance effect especially for single-layer graphene, the MOG varactor is typically used to explore capacitance modulation. Numerous superiorities, for example, high anti-noise capability, extremely advanced size scalability, rapid response and achievability for large-scale sensing species, acts on the graphene varactor [66]. To improve the performance, a multi-finger graphene varactor is used, which greatly reduces series resistance [99,110]. Figure 6(a) shows a typical structure of a multi-finger graphene varactor. Pyrene-1-boronic acid (PBA) binding on the graphene surface here aims to develop non-covalent surface functionalization of graphene, making it selective for glucose detection while holding quantum effect [108]. An unique insulating layer on contact to vanish following parasitic capacitance in electrolyte and a flow-cell setup are established. A stable and reproducible sensing signal is shown in figure 6(b), proving capacitance at V G = 0 V that alters subsequently as glucose concentration varies. Recently, a work presented a graphene-based portable biosensing scheme for a few disease-related biomarkers detection in human saliva [109]. Here, the nano-sensing system was composed of a buried-gate GFET with functionalization of specific aptamers to detect cytokine specifically (figure 6(c)) and an IC component. Connected to a smartphone through the Wi-Fi module, this portable sensing system was possible to implement real-time intelligent monitoring applications. Results in figure 6(d) showed a detection limit of 12 pM for IL-6 in human saliva, appearing vast prospects for non-invasive early-stage diagnosis of diseases via saliva. Although relatively good performance has been shown in in-vitro sensing work, our concern focuses on the possibility of achieving in-vivo sensing using wireless technology. Further research for continuous, sensitive and large-range dynamic glucose sensing systems with biocompatibility should be developed. In 2013, rabbit experiments with the graphenebased contact lenses were conducted aiming at in-vivo tests [111]. Graphene-silver nanowire hybrid contacts reported here showed superiorities in negligible transconductance, great mechanical properties, high draft ability, high transparency and harmless properties that are appropriate for contact lenses. Further application of graphene-silver nanowire antenna was achieved in 2017. In this configuration, researchers fabricated a wearable sensor-integrated contact lens with a sensor-based silver nanowire antenna fabricated on the contact lens read by a coupling RLC circuit [50]. This work advanced intraocular biocompatibility, stretchability, transparency of the wearable contact lens glucose sensor which was proposed to detect glucose in tears after ∼5 h rabbit wearing. In 2021, a recent research achieved a real-time, remote contact lens sensing system both for monitoring and therapy for chronic ocular surface inflammation (OSI) (figure 6(e)) [112]. A GFET sensor is integrated with an antenna, resistors, capacitors and an NFC chip to form the whole wireless contact lens monitoring system. Heat patch attached acts as a therapeutic device. In-vivo validation experiments were conducted on both rats and human subjects confirming the wearability and reliability. The S 11 values showed differentiation among the OSI-positive group, the natural-healing group and the dexamethasonetreated fast OSI recovery group during the recovery time, as shown in figure 6(f). This work advances potential in the clinical application of OSI diagnosis. In the future, studies in this area can be broadened to investigate various biomarkers for early diagnosis of human diseases in tears, blood, etc.

rGO and GO wireless sensors.
GO and rGO have always been popular biocompatible materials, and they also have good development prospects in RF sensing according to their good electrical properties and high selectivity with surface modification [34,82,85,113,114]. As an example of an rGO-based biosensor, a wrinkled, stretchable, nanohybrid fiber (WSNF) of rGO/polyurethane (PU)-Au can be used for real-time glucose detection [115]. In this biosensing scheme, Au nanowrinkles in WSNF cover the rGO/PU composite fiber, which promotes the dehydrogenation step in glucose oxidation. As shown in figure 7(a), the free-standing WSNF electrodes were aligned in the Teflon mold and then embedded on a polydimethylsiloxane substrate or sewn on the fabric. Finally, the flexible RF biosensor was attached to the forehead of the user and the detection results can be read by a smartphone. In this work, continuous on-body monitoring of glucose levels for a whole day was performed, and the overall trend in biosensor test results throughout the day was similar to the results obtained through glucose assays and blood glucose meters ( figure 7(b)).
Besides, we discuss two configurations of GO-integrated RFID humidity wireless sensors: sensitive films GO coated antenna and GO sensor connected chip. Here, RFID acting as a non-connection communication system has the potential to couple with sensors and is viable to achieve practical portable RF sensing devices in our real life [6,14]. The GO sensor, integrated with a silicon-based substrate and Au electrodes, demonstrates a stable and linear response to relative humidity. Integrating the GO sensor on an RFID tag is reported to be used for breath monitoring [116]. The results indeed illustrate a regular response to humidity. However, the response seems not so sensitive and the application of this sensor structure for other molecules may be difficult. Deposition of GO film on graphene antenna can also be used for humidity sensing [16]. The relative permittivity of GO changes with humidity and then alters the antenna impedance, reflected by the measured phase shift of the backscattering wave. Compared to the first one mentioned, films coated on the antenna seem to be reported more frequently [7,10,15]. Designed sensitive and conductive materials pastes printed on the antenna on a flexible substrate present a way to fabricate wearable flexible sensors which catches our eye nowadays.

Heterostructure wireless sensors.
Although remarkable electrical, optical, and mechanical characteristics in 2D materials have been observed, sometimes, single 2D materials are hard to satisfy omnibearing needs of excellent electricity, sensitivity, selectivity, stability, etc. For example, graphene owns excellent conductivity but zero bandgap semiconducting properties. Therefore, hybrid heterogeneous 2D materials junctions with improved features and interesting tunable band alignments could be a good candidate for biochemical sensors [117,118]. Researchers reported a considerable sensing response which is larger than 1000 times in the resistance of the graphene/MoS 2 junction area upon its exposure to 1 ppm NO 2 gas, as a result of Schottky barriers modulation [119]. In another vdW heterojunction configuration, rGO/WS 2 junction exhibited greater sensitivity and selective performance to NH 3 with improved response/recovery speeds (60 s and 300 s, respectively) [120]. The high performance can be ascribed to the vital role of Lewis extra acid centers in WS 2 and hydroxyls in rGO, facilitating the chemisorption of NH 3 . General metal oxides are modified to achieve room temperature sensing [121]. The modified TMDs, MXenes and metal oxides heterojunctions are discussed in sections 3.2-3.4 respectively. Here we focus on graphene-based hybrid heterojunction wireless sensors.
A tunable graphene-polymer humidity wearable sensor was designed with an ultrafast response for highly sensitive detection of humidity over a wide range [122]. The device confined a humidity-sensitive polymer into graphene nanochannels with tunable thickness. By taking advantages of the tunable nanometer-scale thickness, excellent electrical characteristics of graphene, and an enlarged nanoscale-polymer surface with functional groups, four orders of improved resistance magnitude can be achieved for tracing humidity fluctuations. In addition, fast response/recovery speed of this wireless graphene-polymer sensor, advances opportunities for continuous human health monitoring. Furthermore, rGO/MoS 2 p-n junction shows 200 times higher humidity sensitivity than pristine rGO ascribed to heterojunction-induced electronic sensitization and porous formed structure [123]. Recently, a notable advance fabricated a self-powered 2D wormlike polypyrrole/rGO (w-mPPy@rGO) gas sensor (figure 7(c)) [67]. The w-mPPy@rGO heterojunction provides outstanding anti-humidity properties, selectivity and high sensitivity toward NH 3 sensing and exhibits a 5 mg-40 mg NH 4 NO 3 high response ( figure 7(d)). Novel triboelectric nanogenerators (TENGs) coupled with resonance wireless techniques to achieve self-powered wireless gas sensors are impressive. To date, 2D materials TENGs has attracted considerable interests in the area of biomedical, environmental monitoring and so on [124]. This work demonstrates an innovative strategy for selfpowered wireless biochemical sensors which can be further developed.
Except for typical graphene and graphene derivatives, 2D thin films that are composed of TMDs, metal oxides and hybrid materials are also used frequently. Recent researches also proved metal sulfides such as SnS 2 and MoS 2 , are common sensing materials for biological or chemical sensing, especially gas sensing [37,95]. Alternatively, hybrid materials, i.e. metal oxides/carbon nanotubes (CNTs) [98] and TMDs/MXenes [125] with tunable electrical properties and operating temperatures, are general choices for RF sensors.

TMDs wireless sensors
2D TMDs semiconductors possess a generalized composition of MX 2 (M represents a transition metal typically from IV to VII groups elements such as Mo, W and X represents chalcogen i.e. S, Se, or Te). Monolayer TMDs highlight similarities in the structure of graphene, holding a covalentlybonded X-M-X hexagonal quasi-2D lattice [126]. Distinct from graphene, layered TMDs have sizeable bandgaps introducing a high on-off ratio in FET category sensors. Novel electronic, optical, mechanical, and chemical properties of several TMDs especially those in group IVB such as SnS 2 , MoS 2 , WS 2 , or MoSe 2 , have attracted considerable attention in the sensing field [87]. To date, enormous attempts have been made in fabricating wireless TMDs biochemical sensors. For example, figure 8(a) highlights a MoSe 2 on-skin toxic gas sensor with a fast response [53]. SnS 2 nanosheet is also a kind of perfect material for gas sensing according to its superiorities in enhanced sensing response due to its sulfide compound with an extremely high polarizability [37,127,128]. Harsh environments, such as toxic gas sensing applications, urgently desiderate wireless sensing applications, for example, NH 3 sensing in mine [98]. Additionally, NO 2 is a common toxic gas that needs to be monitored to be lower than a level that can be harmful to humans, 53 ppb according to US EPA [15]. Figure 8(b) demonstrates the physisorption of toxic gas NO 2 molecules on 2D SnS 2 nanoflakes (its transmission electron microscope (TEM) image is shown in figure 8(b) inset) which exhibits high sensitivity to NO 2 presenting as resistance changes of SnS 2 [37]. SnS 2 nanoflakes are placed in the center of a low-temperature co-fired ceramic (LTCC) substrate and then coated by an interdigitated electrode. An integrated square spiral inductor surrounding the center sensor is present on the same substrate and the capacitor and inductor are connected through vias in the LTCC substrate. This kind of device structure enables 0.5 ppm sensitive and fast sensing of NO 2 with stable, wireless and reproducible performance in harsh environments [37].
To boost the sensing response, the application of MoS 2 and SnS 2 /MoS 2 hybrid has also been recently researched [52,127,129], which also has the potential for use in RF applications. However, the disadvantages of poor selectivity, relatively slow recovery, or damaged carrier mobility due to strong phonon scatter, are challenging [88,130]. To solve these problems, a synergistic combination of TMDs with other nanomaterials has been made to improve sensing performance. It was proved that a multi-walled carbon nanotubes/WS 2 LC wireless humidity sensor was more sensitive than every single unit with a high response reflected by 2.562 MHz frequency shift to 15%-95% relative humidity [131]. Moreover, the screen printingfabricated humidity sensor demonstrated fast response in 7.8 s, good stability and repeatability with accurate information transmission within 28 mm (a more desired transmission distance was confirmed to be 6 mm). In hetero-TMDs-Fe nanoparticles (NPs) wireless neurochemical system, figure 8(c) shows the promotion of dopamine charge transfer of both 2Hand 1T-TMDs-Fe hybrid structures compared to pure TMDs  [65]. Copyright (2020) American Chemical Society. [52]. Due to the strong electronegative potential on the hetero-TMDs-Fe surface, promotion of the adsorption of positively charged dopamine can be realized. This work advances a nondissolvable hetero-TMDs-Fe NPs silicon-based neural probe with long-term duration, stability, selectivity and high sensitivity compared to Fe NPs-silicon system, which are potentially applicable in clinical use. MoS 2 -SWCNT cooperation in ethylene wireless monitoring was demonstrated with a low LoD down to 100 ppb and enhanced selectivity (figure 8(d)) [65]. To this end, improvements in the performance of sensitivity, LoD and response time should still be concerned.

2D metal oxides wireless sensors
Emerging layered metal oxides, with a majority of atoms exposed to the external environment, attract extensive attention from sensors [90]. This kind of advanced materials can be synthesized through molecular beam epitaxy, hydrothermal, exfoliation and wet chemical methods (figure 9(a)) [91]. Relying on outstanding physical, chemical and electrical properties along with an enlarged surface ratio, layered metal oxides are widely used in sensing applications, especially gas sensors, with different reaction mechanisms: physisorption, chemisorption, oxidation and reduction [89,[132][133][134], etc. However, using of metal oxides is always accompanied by a high operation temperature which is not favorite for RF sensing [135,136]. Besides, some metal oxides which are selective to specific gas molecules have poor conductivity, limiting their sensing capabilities. As one of the examples, Fe 3 O 4 can react with NO 2 but conducts electricity badly. Therefore, the hybrid of metal oxides with nanomaterials such as CNTs and MXenes to realize selective sensing is generally used to solve the problems [137][138][139][140]. Besides, these cooperated nanomaterials have a large surface-to-volume ratio with more active sites for the adsorption of molecules [7,15] and are promising materials for manufacturing wearable electronic sensors [141,142]. Hence, to solve the same problem in Fe 3 O 4 , the Fe 3 O 4 -CNT hybrid structure is useful and the hybrid has been reported with good sensing response [98]. It is advised that synthesizing nanomaterials in a form of a 2D thin film will also solve the poor repeating issue, measuring the mean performance of numerous nanostructures [143]. The drop-casting CuO-SWCNT hybrid RFID sensor solves the problems of repeatability and duration (30 days) and performs a low LoD of 100 ppb H 2 S (figure 9(b)) [7]. The high sensitivity for metal oxides hybrids is controlled by internal morphologies of nanostructures which are extremely different for preparation via various parameters. It is worth studying CuO-SWCNT hybrid synthesis under different temperatures and with/without a second growing step, as it can result in different morphologies. Obviously, the surface-to-volume ratio alters a lot during the process. This work demonstrated that the flower-like structure obtained at re-150 • C exhibited the most favorable performance with a lower LoD and higher response. Figure 9(c) indicates two types of GO/metal oxidebased urea biosensors with improved stability, linearity and precision sensing performance [144]. TiO 2 or NiO sensing films with the capability of charge transfer and chemical stability were deposited by an RF sputtering system [145][146][147][148]. Here, a 0.3 wt% GO solution and a magnetic beads (MBs)-urease composite solution were sequentially dropped on the sensing films. In this device, the plentiful oxygencontaining functional groups of GO and the high surfacevolume ratio of MBs provide more reaction sites for TiO 2 or NiO, which increase biosensing performance. Heterojunction of SnO 2 with MXene proposed room temperature NH 3 detection in contrast to conventional 200 • C-400 • C metal oxide gas sensors. The fabricated heterojunction sensor achieves more sensitive and stable detection of molecular adsorption than MXene (figure 9(d)) [137,147].

MXenes wireless sensors
Transition metal carbides/nitrides are usually called MXenes. With a 2D layered structure, great conductivity, and rich functional groups, semiconducting MXenes are highly applicable in versatile sensors [92,149]. Recently, inspired advances have been made in the development of MXene-based biochemical sensors. For example, a portable device fabricated with Mo 2 TiC 2 T x /MoS 2 heterojunction monitored NO 2 more selectively and high-sensitively with a 2.5 ppb detection limit, against pure MXene [125]. Nevertheless, the development of MXene wireless sensors is immature with limitations in confined synthesis methods mostly using a top-down etching method. In addition, the common HF etching agent is not only harmful but also causes uncontrollable surface terminal groups. Biocompatibility of MXenes is nearly seldom mentioned. In the future, research on the controlled, large-scale and environmentally friendly synthesis of MXenes, and their biocompatibility should be advised to be intensively strengthened.
In summary, wireless RF sensors of 2D materials are highly desirable for breaking the region limitations of wired layout and achieving improved sensitivity due to 2D materials. As mentioned above, compared to generally wired sensors, RF wireless sensors have an irreplaceable role in various sensing fields, including in-vitro, in-vivo and harsh environments, satisfying the needs of IoT. Coupled with RFID technology or LC circuit designs, commercial applications are advancing now. However, there is still potential space for improvement based on the optimization of all aspects of sensing-related properties of 2D materials and device designs to achieve long-distance transmitted wireless sensors with preferable repeatability, long duration and high sensitivity.

Conclusions and prospective outlook
Over the past decade, the needs of IoT accelerate the development for wireless biochemical 2D sensors in a wide range of applications of wearable electronics, in-vivo clinical diagnosis, remote environmental monitoring and real-time surveys. Tremendous works and great progress have been made on 2D wireless sensors with various advanced 2D materials including graphene and its derivatives, TMDs, 2D metal oxides, MXenes and hybrid heterojunctions. There is great potential for innovation of the RF 2D biochemical sensors in commercialization. However, recognizable challenges still remain in the application of integrated 2D RF sensors from lab to fab. Practical products should be a comprehensive system at least in a form of a 2D sensor, a power supply device, a remote communication system, and user-friendly software to read out. Urgent concerns for commercial promotion are addressed below.
First of all, sensor performance is the primary requirement. However, there are still challenges in achieving stability, reproducibility, uniformity and security for commercial use particularly in in-vivo applications [150]. Especially, research is ongoing in improving the stability of materials such as MXenes, with notable progress being made through the use of functionalization and heterostructures. For example, cetyltrimethylammonium bromide (CTAB) functionalized Nb 2 CT x NO 2 sensor demonstrated a higher sensing response with a threefold increase in the resistance change compared to pristine Nb 2 CT x , and showed reliable performance for 30 days [151]. Improvement of MXenes stability through MoO 3 /TiO 2 /Ti 3 C 2 T x ternary heterostructure was also applicable, resulting in a 20-40 times magnitude higher volatile organic compounds (VOCs) response due to enhanced conductivity from introduced MoO 3 with abundant oxygen vacancies [152]. Nafion-coated Yb 2 O 3 offered long-term detection of urea over a seven-week period [153]. Moreover, restrictions in fabricating large-scale 2D materials with uniform and well-controlled performance are critical to achieving desired device performance. To date, significant efforts have been made to achieve high-quality 2D sensors. High-quality CVD graphene on copper film has already been produced to industrial-scale [154][155][156]. Furthermore, the quality of direct grown graphene on insulators was further improved with better device performance because of little wrinkles and adlayers. In 2022, wafer-scale single-crystal graphene was successfully grown on insulating sapphire substrates with excellent carrier mobility (6.6 × 10 3 cm 2 V −1 s −1 for electrons and 8.0 × 10 3 cm 2 V −1 s −1 for holes) that was as good as graphene on copper [157]. Wafer-scale TMDs were also applicable. For example, 2-in tungsten disulfides (WS 2 ) synthesized on sapphire with a relatively large carrier mobility (50 cm 2 V −1 s −1 ) was recently achieved [158]. Achievement in single-crystal MoS 2 with carrier mobility of 102 cm 2 V −1 s −1 in a wafer was proclaimed [159]. In addition, impressive homogeneity, both 94% device yield and 15% mobility variation were developed for integration. Until now, research in material production is still devoted to develop scalable, consistent, reliable, and commercially available 2D sensing platforms because numerous materials remain inability for wafer-scale growth. Moreover, the lack of RF sensors with minimized device size that are noninvasive, biocompatible, and durable in body fluids deserves attention in the future.
For a commercially successful lab-on-a-chip sensing platform, a stable and minimized power supply is absolutely necessary. The power supplies used in biosensor systems are classified into active and passive types to accommodate the demands of detection procedures, wireless communication and data delivery. Lithium-ion batteries are currently the most common active power source for portable electronic devices due to their advantages of high energy density and low self-discharge [160,161]. However, safety concerns arising from their excessive volume and toxic materials limit their further application in wearable electronic products and implantable integrated devices [162]. Recently, advancements in novel battery technology, such as flexible thin film batteries, offer a promising solution for biosensors. One example is the ultra-thin (0.1 mm), flexible and high-power density (16.3 mW cm −2 ) sweat activated battery (SAB) which provides sufficient power for long-term Bluetooth wireless operation of flexible electronic device [163]. However, on the one hand, the new battery represented by SAB still needs to make greater breakthroughs due to the challenges of large size (not conducive to long-term wear) and complex manufacturing process (not beneficial to commercialization). On the other hand, wearable biosensors, especially the power supply of implantable devices, need more reasonable packaging strategies in the future [164]. For instance, proper packaging can prevent electrolyte leakage or solvent evaporation in batteries or supercapacitors, preserving their energy storage capacity and extending battery life. It is worth mentioning that the passive power supply strategy represented by selfpowering and inductive resonance technology has attracted more attention on the commercialization of biosensor systems. TENGs and piezoelectric nanogenerators (PENGs), as commonly used self-powered technologies, set up new possibilities for on-chip integrated biosensor systems. An example of a PI/PVDF-TrFE composite nanofiber TENG, can collect mechanical energy from different parts of the human body (palm, foot, knee and elbow) and convert it into electrical energy [165]. Similarly, a PENG-based biosensor array integrated in electronic skin was designed and prepared for monitoring the concentration of lactic acid in human joints [166]. It can be used as both energy sources and sensors in self-powered sensing systems. However, their shortcomings of low output current and high output impedance still need to be solved. Currently, the miniaturized wireless power supply system that transmits energy through inductive resonance is one of the most common power supply schemes for implantable equipment. However, the scope of wireless power supply is still limited to the depth of the tissue surface, due to the limitations of radio frequency wireless transmission technologies. Longer transmission distance will lead to low energy transmission efficiency or require excessively large coils (at least 1 cm in diameter), which does not meet the standards for equipment implantation [167]. The future wireless power supply technology needs more advanced technological innovation.
The limited range of communication systems promotes greater technical requirements. Wireless communication systems have ability to transmit and receive information at high data rates and over long distances, leading to high energy consumption. For example, wireless biosensors based on Bluetooth or ZigBee technology that can communicate with reader devices over a relatively long distance (10-100 m) by active transceivers, consume more power and have a larger volume, hindering their use in wearable or implantable biosensors [168,169]. In contrast, passive RF technologies (such as RFID and NFC) do not require an additional power supply, making them attractive for biosensors with compact size and ultra-low energy consumption [170,171]. However, passive sensors cannot automatically record data into memory in real time. More importantly, the data reading range of the passive communication system is very short, usually ranging from only a few centimeters to dozens of centimeters. Therefore, on the premise of ensuring the miniaturization and integration of biosensors, expanding the communication range of wireless communication system is an important technical direction in the future.
A further consideration for commercialization is focused on portable readout systems with user-friendly operation. It should not be confined by expensive and bulky analyzer devices. A notable achievement in intelligent 2D wireless sensors incorporated with a portable readout is exciting, in which an acetylcholinesterase modified graphene chiral pesticide detector was demonstrated [172]. The modified graphene allowed for a 1000 times lower LoD than common circular dichroism. This research established a real-time monitoring platform with the integration of a sensor, lithium battery power supply, and Bluetooth on a print circuit board. Wireless transmission to a smartphone through the Bluetooth module provided easy and user-friendly in situ measurement. Attempts have also been made in 2D material-based smart contact lenses reading by a smartphone, advancing promising clinical applications in the future [12,51,112]. For example, a passive NFC-supported graphene contact lens reading by a smartphone, has been developed and evaluated for cortisol detection in tears with a low enough LoD of 10 pg ml −1 [12]. The battery-free network composed of an NFC chip, a biocompatible graphene contact lens immunosensor, a capacitor and a resistor, was powered wirelessly by mobile devices and transferred data back, eliminating the need for redundant external equipment apart from a smartphone. Besides the efforts in wearable contact lens sensor development, promotion of commercialized neural probes is attracting interest. More recently, industrial-scale graphene neutral probe arrays combined with a quasi-commercial wireless headstage to perform electrophysiological detection have been developed [173]. In this research, wafer-scale GFETs with superior sensitivity, long-term stability, and uniformity, were fabricated. Remarkably, these devices were stable over four weeks without sensitivity loss, and exhibited good biocompatibility during 12-week implantable animal behavior and histological assessment. Additionally, the centimeterscale headstage allowed fora relatively long wireless transmission range (10-15 m depending on environment), minimized power consumption with embedded batteries, and employed a specific signal transmission system with reduced noise which enabled undisturbed GFET sensitivity. This impressive work paves the way for large scale fabrication of 2D RF sensors for practical applications.
Overall, 2D materials promote the integration of usage with RF communication systems due to their unique physical and chemical properties, including a large surface-to-volume ratio, excellent electrical performance and outstanding biocompatibility. Thus, further in-depth exploration of mature 2D RF sensor chips is of great significance.