Formation of Uniform Water Microdroplets on Wrinkled Graphene for Ultrafast Humidity Sensing

Portable humidity sensors with ultrafast responses fabricated in wearable devices have promising application prospects in disease diagnostics, health status monitoring, and personal healthcare data collecting. However, prolonged exposures to high‐humidity environments usually cause device degradation or failure due to excessive water adsorbed on the sensor surface. In the present work, a graphene film based humidity sensor with a hydrophobic surface and uniformly distributed ring‐like wrinkles is designed and fabricated that exhibits excellent performance in breath sensing. The wrinkled morphology of the graphene sensor is able to effectively prevent the aggregation of water microdroplets and thus maximize the evaporation rate. The as‐fabricated sensor responds to and recovers from humidity in 12.5 ms, the fastest response of humidity sensors reported so far, yet in a very stable manner. The sensor is fabricated into a mask and successfully applied to monitoring sudden changes in respiratory rate and depth, such as breathing disorder or arrest, as well as subtle changes in humidity level caused by talking, cough and skin evaporation. The sensor can potentially enable long‐term daily monitoring of breath and skin evaporation with its ultrafast response and high sensitivity, as well as excellent stability in high‐humidity environments.     

Transparent and Flexible Cellulose Nanocrystal/Reduced Graphene Oxide Film for Proximity Sensing

The rapid development of touch screens as well as photoelectric sensors has stimulated the fabrication of reliable, convenient, and human‐friendly devices. Other than sensors that detect physical touch or are based on pressure sensing, proximity sensors offer controlled sensibility without physical contact. In this work we present a transparent and eco‐friendly sensor made through layer‐by‐layer spraying of modified graphene oxide filled cellulose nanocrystals on lithographic patterns of interdigitated electrodes on polymer substrates, which help to realize the precise location of approaching objects. Stable and reproducible signals generated by keeping the finger in close proximity to the sensor can be controlled by humidity, temperature, and the distance and number of sprayed layers. The chemical modification and reduction of the graphene oxide/cellulose crystal composite and its excellent nanostructure enable the development of proximity sensors with faster response and higher sensitivity, the integration of which resolves nearly all of the technological issues imposed on optoelectronic sensing devices.     

Hybrid Films of Graphene and Carbon Nanotubes for High Performance Chemical and Temperature Sensing Applications

A hybrid composite material of graphene and carbon nanotube (CNT) for high performance chemical and temperature sensors is reported. Integration of 1D and 2D carbon materials into hybrid carbon composites is achieved by coupling graphene and CNT through poly(ionic liquid) (PIL) mediated‐hybridization. The resulting CNT/PIL/graphene hybrid materials are explored as active materials in chemical and temperature sensors. For chemical sensing application, the hybrid composite is integrated into a chemo‐resistive sensor to detect a general class of volatile organic compounds. Compared with the graphene‐only devices, the hybrid film device showed an improved performance with high sensitivity at ppm level, low detection limit, and fast signal response/recovery. To further demonstrate the potential of the hybrid films, a temperature sensor is fabricated. The CNT/PIL/graphene hybrid materials are highly responsive to small temperature gradient with fast response, high sensitivity, and stability, which may offer a new platform for the thermoelectric temperature sensors.     

Flexible and Transparent Gas Molecule Sensor Integrated with Sensing and Heating Graphene Layers

Graphene leading to high surface‐to‐volume ratio and outstanding conductivity is applied for gas molecule sensing with fully utilizing its unique transparent and flexible functionalities which cannot be expected from solid‐state gas sensors. In order to attain a fast response and rapid recovering time, the flexible sensors also require integrated flexible and transparent heaters. Here, large‐scale flexible and transparent gas molecule sensor devices, integrated with a graphene sensing channel and a graphene transparent heater for fast recovering operation, are demonstrated. This combined all‐graphene device structure enables an overall device optical transmittance that exceeds 90% and reliable sensing performance with a bending strain of less than 1.4%. In particular, it is possible to classify the fast (≈14 s) and slow (≈95 s) response due to sp²‐carbon bonding and disorders on graphene and the self‐integrated graphene heater leads to the rapid recovery (≈11 s) of a 2 cm × 2 cm sized sensor with reproducible sensing cycles, including full recovery steps without significant signal degradation under exposure to NO₂ gas.     

Laser‐Induced Direct Graphene Patterning and Simultaneous Transferring Method for Graphene Sensor Platform

General methods utilized in the fabrication of graphene devices involve graphene transferring and subsequent patterning of graphene via multiple wet‐chemical processes. In the present study, a laser‐induced pattern transfer (LIPT) method is proposed for the transferring and patterning of graphene in a single processing step. Via the direct graphene patterning and simultaneous transferring, the LIPT method greatly reduces the complexity of graphene fabrication while augmenting flexibility in graphene device design. Femtosecond laser ablation under ambient conditions is employed to transfer graphene/PMMA microscale patterns to arbitrary substrates, including a flexible film. Suspended cantilever structures are also demonstrated over a prefabricated trench structure via the single‐step method. The feasibility of this method for the fabrication of functional graphene devices is confirmed by measuring the electrical response of a graphene/PMMA device under laser illumination.     

All‐Carbon Pressure Sensors with High Performance and Excellent Chemical Resistance

An all‐carbon pressure sensor is designed and fabricated based on reduced graphene oxide (rGO) nanomaterials. By sandwiching one layer of superelastic rGO aerogel between two freestanding high‐conductive rGO thin papers, the sensor works based on the contact resistance at the aerogel–paper interfaces, getting rid of the alien materials such as polymers and metals adopted in traditional sensors. Without the limitation of alien materials, the all‐carbon sensors demonstrate an ultrawide detecting range (0.72 Pa–130 kPa), low energy consumption (≈0.58 µW), ultrahigh sensitivity (349–253 kPa^?1) at low‐pressure regime (<1.4 Pa), fast response time (8 ms at 1 kPa), high stability (10 000 unloading–loading cycles between 0 and 1 kPa), light weight (<10 mg), easily scalable fabrication process, and excellent chemical stability. These merits enable them to detect real‐time human physiological signals and monitor the weights of various droplets of not only water but also hazardous chemical reagents including strong acid, strong alkali, and organic solvents. This shows their great potential applications in real‐time health monitoring, sport performance detecting, harsh environment‐related robotics and industry, and so forth.     

Quantitative Principles for Precise Engineering of Sensitivity in Graphene Electrochemical Sensors

A major difficulty in implementing carbon‐based electrode arrays with high device‐packing density is to ensure homogeneous and high sensitivities across the array. Overcoming this obstacle requires quantitative microscopic models that can accurately predict electrode sensitivity from its material structure. Such models are currently lacking. Here, it is shown that the sensitivity of graphene electrodes to dopamine and serotonin neurochemicals in fast‐scan cyclic voltammetry measurements is strongly linked to point defects, whereas it is unaffected by line defects. Using the physics of point defects in graphene, a microscopic model is introduced that explains how point defects determine sensitivity. The predictions of this model match the empirical observation that sensitivity linearly increases with the density of point defects. This model is used to guide the nanoengineering of graphene structures for optimum sensitivity. This approach achieves reproducible fabrication of miniaturized sensors with extraordinarily higher sensitivity than conventional materials. These results lay the foundation for new integrated electrochemical sensor arrays based on nanoengineered graphene.     

Tunable Reverse‐Biased Graphene/Silicon Heterojunction Schottky Diode Sensor

A new chemical sensor based on reverse‐biased graphene/Si heterojunction diode has been developed that exhibits extremely high bias‐dependent molecular detection sensitivity and low operating power. The device takes advantage of graphene's atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si interface barrier height, thus affecting the junction current exponentially when operated in reverse bias and resulting in ultrahigh sensitivity. By operating the device in reverse bias, the work function of graphene, and hence the barrier height at the graphene/Si heterointerface, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity. Such sensitivity control is also possible by carefully selecting the graphene/Si heterojunction Schottky barrier height. Compared to a conventional graphene amperometric sensor fabricated on the same chip, the proposed sensor demonstrated 13 times higher sensitivity for NO₂ and 3 times higher for NH₃ in ambient conditions, while consuming ～500 times less power for same magnitude of applied voltage bias. The sensing mechanism based on heterojunction Schottky barrier height change has been confirmed using capacitance‐voltage measurements.     

Chemical Preparation of Graphene‐Based Nanomaterials and Their Applications in Chemical and Biological Sensors

Graphene is a flat monolayer of carbon atoms packed tightly into a 2D honeycomb lattice that shows many intriguing properties meeting the key requirements for the implementation of highly excellent sensors, and all kinds of proof‐of‐concept sensors have been devised. To realize the potential sensor applications, the key is to synthesize graphene in a controlled way to achieve enhanced solution‐processing capabilities, and at the same time to maintain or even improve the intrinsic properties of graphene. Several production techniques for graphene‐based nanomaterials have been developed, ranging from the mechanical cleavage and chemical exfoliation of high‐quality graphene to direct growth onto different substrates and the chemical routes using graphite oxide as a precusor to the newly developed bottom‐up approach at the molecular level. The current review critically explores the recent progress on the chemical preparation of graphene‐based nanomaterials and their applications in sensors.     

Sensing at the Surface of Graphene Field‐Effect Transistors

Recent research trends now offer new opportunities for developing the next generations of label‐free biochemical sensors using graphene and other two‐dimensional materials. While the physics of graphene transistors operated in electrolyte is well grounded, important chemical challenges still remain to be addressed, namely the impact of the chemical functionalizations of graphene on the key electrical parameters and the sensing performances. In fact, graphene – at least ideal graphene – is highly chemically inert. The functionalizations and chemical alterations of the graphene surface – both covalently and non‐covalently – are crucial steps that define the sensitivity of graphene. The presence, reactivity, adsorption of gas and ions, proteins, DNA, cells and tissues on graphene have been successfully monitored with graphene. This review aims to unify most of the work done so far on biochemical sensing at the surface of a (chemically functionalized) graphene field‐effect transistor and the challenges that lie ahead. The authors are convinced that graphene biochemical sensors hold great promise to meet the ever‐increasing demand for sensitivity, especially looking at the recent progresses suggesting that the obstacle of Debye screening can be overcome.     

Nitrogen‐Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen‐doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen‐doping is carried out under thermal annealing in H₂(4%)/N₂(96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1–66.4% relative humidity (RH). Furthermore, a 1.36‐fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt‐nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real‐time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt‐nRGO fiber integrated on a portable sensing module. The Pt‐nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.     

Multiscale Hierarchical Design of a Flexible Piezoresistive Pressure Sensor with High Sensitivity and Wide Linearity Range

Flexible piezoresistive pressure sensors have been attracting wide attention for applications in health monitoring and human‐machine interfaces because of their simple device structure and easy‐readout signals. For practical applications, flexible pressure sensors with both high sensitivity and wide linearity range are highly desirable. Herein, a simple and low‐cost method for the fabrication of a flexible piezoresistive pressure sensor with a hierarchical structure over large areas is presented. The piezoresistive pressure sensor consists of arrays of microscale papillae with nanoscale roughness produced by replicating the lotus leaf's surface and spray‐coating of graphene ink. Finite element analysis (FEA) shows that the hierarchical structure governs the deformation behavior and pressure distribution at the contact interface, leading to a quick and steady increase in contact area with loads. As a result, the piezoresistive pressure sensor demonstrates a high sensitivity of 1.2 kPa⁻¹ and a wide linearity range from 0 to 25 kPa. The flexible pressure sensor is applied for sensitive monitoring of small vibrations, including wrist pulse and acoustic waves. Moreover, a piezoresistive pressure sensor array is fabricated for mapping the spatial distribution of pressure. These results highlight the potential applications of the flexible piezoresistive pressure sensor for health monitoring and electronic skin.     

Ultrahigh Conductive Copper/Large Flake Size Graphene Heterostructure Thin‐Film with Remarkable Electromagnetic Interference Shielding Effectiveness

To guarantee the normal operation of next generation portable electronics and wearable devices, together with avoiding electromagnetic wave pollution, it is urgent to find a material possessing flexibility, ultrahigh conductive, and superb electromagnetic interference shielding effectiveness (EMI SE) simultaneously. In this work, inspired by a building bricks toy with the interlock system, we design and fabricate a copper/large flake size graphene (Cu/LG) composite thin film (≈8.8 μm) in the light of high temperature annealing of a large flake size graphene oxide film followed by magnetron sputtering of copper. The obtained Cu/LG thin‐film shows ultrahigh thermal conductivity of over 1932.73 (±63.07) W m^?1 K^?1 and excellent electrical conductivity of 5.88 (±0.29) × 10⁶ S m^?1. Significantly, it also exhibits a remarkably high EMI SE of over 52 dB at the frequency of 1–18 GHz. The largest EMI SE value of 63.29 dB, accorded at 1 GHz, is enough to obstruct and absorb 99.99995% of incident radiation. To the best of knowledge, this is the highest EMI SE performance reported so far in such thin thickness of graphene‐based materials. These outstanding properties make Cu/LG film a promising alternative building block for power electronics, microprocessors, and flexible electronics.     

Harnessing the Influence of Reactive Edges and Defects of Graphene Substrates for Achieving Complete Cycle of Room‐Temperature Molecular Sensing

Molecular doping and detection are at the forefront of graphene research, a topic of great interest in physical and materials science. Molecules adsorb strongly on graphene, leading to a change in electrical conductivity at room temperature. However, a common impediment for practical applications reported by all studies to date is the excessively slow rate of desorption of important reactive gases such as ammonia and nitrogen dioxide. Annealing at high temperatures, or exposure to strong ultraviolet light under vacuum, is employed to facilitate desorption of these gases. In this article, the molecules adsorbed on graphene nanoflakes and on chemically derived graphene‐nanomesh flakes are displaced rapidly at room temperature in air by the use of gaseous polar molecules such as water and ethanol. The mechanism for desorption is proposed to arise from the electrostatic forces exerted by the polar molecules, which decouples the overlap between substrate defect states, molecule states, and graphene states near the Fermi level. Using chemiresistors prepared from water‐based dispersions of single‐layer graphene on mesoporous alumina membranes, the study further shows that the edges of the graphene flakes (showing p‐type responses to NO₂ and NH₃) and the edges of graphene nanomesh structures (showing n‐type responses to NO₂ and NH₃) have enhanced sensitivity. The measured responses towards gases are comparable to or better than those which have been obtained using devices that are more sophisticated. The higher sensitivity and rapid regeneration of the sensor at room temperature provides a clear advancement towards practical molecule detection using graphene‐based materials.     

温度对石墨烯膜光纤F-P声压传感器灵敏度影响研究

石墨烯具有优异的机械、电学与光学等传感特性,有希望成为下一代可穿戴电子设备的功能敏感材料。石墨烯膜Fabry-Perot(F-P)声压传感器具有高灵敏度、小型化和抗电磁干扰等优点,但会受到温度漂移的影响。温度对传感器的影响主要体现在F-P腔长变化,引起工作点漂移,导致传感器光学灵敏度发生变化,以及改变石墨烯膜预应力。本文制备了石墨烯膜光纤F-P声压传感器探头,通过声压测试表明,温度改变了悬浮石墨烯膜的机械力学特性,在1 kHz处使其机械灵敏度由1.80 nm/Pa提高至2.44 nm/Pa。     

Cross‐Talk Between Ionic and Nanoribbon Current Signals in Graphene Nanoribbon‐Nanopore Sensors for Single‐Molecule Detection

Nanopores are now being used not only as an ionic current sensor but also as a means to localize molecules near alternative sensors with higher sensitivity and/or selectivity. One example is a solid‐state nanopore embedded in a graphene nanoribbon (GNR) transistor. Such a device possesses the high conductivity needed for higher bandwidth measurements and, because of its single‐atomic‐layer thickness, can improve the spatial resolution of the measurement. Here measurements of ionic current through the nanopore are shown during double‐stranded DNA (dsDNA) translocation, along with the simultaneous response of the neighboring GNR due to changes in the surrounding electric potential. Cross‐talk originating from capacitive coupling between the two measurement channels is observed, resulting in a transient response in the GNR during DNA translocation; however, a modulation in device conductivity is not observed via an electric‐field‐effect response during DNA translocation. A field‐effect response would scale with GNR source–drain voltage (V_ds), whereas the capacitive coupling does not scale with V_ds. In order to take advantage of the high bandwidth potential of such sensors, the field‐effect response must be enhanced. Potential field calculations are presented to outline a phase diagram for detection within the device parameter space, charting a roadmap for future optimization of such devices.     

Wearable gas/strain sensors based on reduced graphene oxide/linen fabrics

Multifunctional wearable e-textiles have been a focus of much attention due to their great potential for healthcare, sportswear, fitness, space, and military applications. Among them, electroconductive textile yarn shows great promise for use as the next-generation flexible sensors without compromising properties and comfort of usual textiles. Recently, a myriad of efforts have been devoted to improving performance and functionality of wearable sensors. However, the current manufacturing process of metal-based electroconductive textile yarn is expensive, unscalable, and environmentally unfriendly. In this work, we report the preparation of multifunctional reduced graphene oxide/linen (RGO/LN) fabrics through the reduction and the followed suction filtration. As-prepared RGO/LN fabric could serve as the methane gas sensor, which exhibited high sensitivity, remarkable reliability and feasibility. Furthermore, the RGO/LN fabric sensor exhibited good moisture permeability and air permeability. The present work reveals that RGO/LN fabric has great potential as wearable smart devices in personal healthcare applications.     

Laminated Object Manufacturing of 3D‐Printed Laser‐Induced Graphene Foams

Laser‐induced graphene (LIG), a graphene structure synthesized by a one‐step process through laser treatment of commercial polyimide (PI) film in an ambient atmosphere, has been shown to be a versatile material in applications ranging from energy storage to water treatment. However, the process as developed produces only a 2D product on the PI substrate. Here, a 3D LIG foam printing process is developed on the basis of laminated object manufacturing, a widely used additive‐manufacturing technique. A subtractive laser‐milling process to yield further refinements to the 3D structures is also developed and shown here. By combining both techniques, various 3D graphene objects are printed. The LIG foams show good electrical conductivity and mechanical strength, as well as viability in various energy storage and flexible electronic sensor applications.     

Molecule–Graphene Hybrid Materials with Tunable Mechanoresponse: Highly Sensitive Pressure Sensors for Health Monitoring

The development of pressure sensors is crucial for the implementation of electronic skins and for health monitoring integrated into novel wearable devices. Tremendous effort is devoted toward improving their sensitivity, e.g., by employing microstructured electrodes or active materials through cumbersome processes. Here, a radically new type of piezoresistive pressure sensor based on a millefeuille‐like architecture of reduced graphene oxide (rGO) intercalated by covalently tethered molecular pillars holding on‐demand mechanical properties are fabricated. By applying a tiny pressure to the multilayer structure, the electron tunnelling ruling the charge transport between successive rGO sheets yields a colossal decrease in the material's electrical resistance. Significantly, the intrinsic rigidity of the molecular pillars employed enables the fine‐tuning of the sensor's sensitivity, reaching sensitivities as high as 0.82 kPa^?1 in the low pressure region (0–0.6 kPa), with short response times (≈24 ms) and detection limit (7 Pa). The pressure sensors enable efficient heartbeat monitoring and can be easily transformed into a matrix capable of providing a 3D map of the pressure exerted by different objects.     

Selective Detection of Target Proteins by Peptide‐Enabled Graphene Biosensor

Direct molecular detection of biomarkers is a promising approach for diagnosis and monitoring of numerous diseases, as well as a cornerstone of modern molecular medicine and drug discovery. Currently, clinical applications of biomarkers are limited by the sensitivity, complexity and low selectivity of available indirect detection methods. Electronic 1D and 2D nano‐materials such as carbon nanotubes and graphene, respectively, offer unique advantages as sensing substrates for simple, fast and ultrasensitive detection of biomolecular binding. Versatile methods, however, have yet to be developed for simultaneous functionalization and passivation of the sensor surface to allow for enhanced detection and selectivity of the device. Herein, we demonstrate selective detection of a model protein against a background of serum protein using a graphene sensor functionalized via self‐assembling multifunctional short peptides. The two peptides are engineered to bind to graphene and undergo co‐assembly in the form of an ordered monomolecular film on the substrate. While the probe peptide displays the bioactive molecule, the passivating peptide prevents non‐specific protein adsorption onto the device surface, ensuring target selectivity. In particular, we demonstrate a graphene field effect transistor (gFET) biosensor which can detect streptavidin against a background of serum bovine albumin at less than 50 ng/ml. Our nano‐sensor design, allows us to restore the graphene surface and utilize each sensor in multiple experiments. The peptide‐enabled gFET device has great potential to address a variety of bio‐sensing problems, such as studying ligand‐receptor interactions, or detection of biomarkers in a clinical setting.