Flexible Graphene-Based Wearable Gas and Chemical Sensors

Recent Advances in Graphene-Polymer Nanocomposite-Based Flexible Sensors and Triboelectric Nanogenerators

Flexible sensors are uplifting many application segments with their versatility and ease of fabrication and integration. The amalgamation of functional fillers and polymers advances the field of flexible sensors. Various fillers are currently utilized to develop polymer nanocomposites for sensing applications. However, graphene-polymer nanocomposites find widespread applicability in flexible sensing applications due to the excellent properties of graphene, such as high electrical and thermal conductivity, 2D (2 dimensional) layered structure, and high aspect ratio. This review explores the potential of graphene-polymer nanocomposites as various sensors, including physical, chemical, electrochemical, triboelectric, and moisture-electric generator-based sensors. The technological advancements in developing these sensors are thoroughly discussed, followed by the various underlying sensing mechanisms. Also, the broad application areas where these sensors can be utilized are reviewed and discussed. The review critically assesses the advancements in the established sensing technologies based on graphene-polymer composites. Also, it discusses the challenges and new avenues that are yet to be addressed and explored, paving the way to develop next-generation flexible sensors for advanced applications.

Advancements in flexible porous Nanoarchitectonic materials for biosensing applications

The development of nanoporous materials has gained significant attention due to their unique structural properties and multimodalities, which are highly relevant for advanced sensing technologies. The capability to directly grow nanoporous materials on flexible substrates or indirectly integrate them into soft templates through mixing and dispersion opens exciting opportunities for a new class of flexible and stretchable electronics for personalized healthcare applications. This review paper provides a snapshot of recent advancements in flexible nanoporous materials and their applications, emphasizing biological and biomedical sensors. The review highlights the material of choice for flexible and stretchable substrates and effective approaches to synthesize and integrate nanoporous architectures onto soft polymers. Applications from wearable sweat sensors, mechanical sensors for electronic skins, implantable bioelectronics, and gas sensing are also presented. The paper concludes with current challenges and future perspectives within this highly active research paradigm.

Exploring the Sensing Performance of T-Graphene, T-Boron Nitride, and Their Lateral Heterostructure for Toxic CO, NO, NO2, and SO2 Gas Molecules

In this observation, density functional theory calculations were carried out to examine the adsorption performance of T-graphene (TG), T-boron nitride (TBN), and their heterostructure (TG-TBN) toward CO, SO2, NO, and NO2 gas molecules. To observe the sensing performance of the nanosheets, the adsorption energy with adsorption distance, charge transfer, electronic properties, sensitivity, and recovery time have been investigated. The gas molecules were adsorbed in the tetragonal (T) and octagonal (O) sites of the nanosheets, in which we found that the O site was more favorable. In the case of the interaction between TG and gases, low adsorption behavior has been found, but TBN and TG-TBN exhibit favorable interaction behavior with the gases. Among the four gases, SO2 and NO2 interact with the TBN in chemisorption energy, which are -0.911 and -1.75 eV, at the O site, respectively. During their interaction, the gases gain -0.139e and -0.428e charges from the TBN. TG-TBN shows high interaction properties with the NO and NO2 gases with energies -1.21 and -1.35 eV, respectively. The DOS spectra show that extra electronic states are generated at the Fermi level of NO and NO2 gas adsorption on the nanosheets. Low recovery times have been observed during the desorption; in the case of TG-TBN, the recovery times are 0.19 and 1.56 s at the T and O sites for NO and 28.32 and 41.04 s at the T and O sites for the NO2 gas molecule. Therefore, TBN can be used as a gas sensor for SO2 and NO2 gases and TG-TBN can be used as a gas sensor for NO and NO2 gas molecules.

Next-Generation Chemiresistive Wearable Breath Sensors for Non-Invasive Healthcare Monitoring: Advances in Composite and Hybrid Materials

Recently wearable breath sensors have received significant attention in personalized healthcare systems by offering new methods for remote, non-invasive, and continuous monitoring of various health indicators from breath samples without disrupting daily routines. The rising demand for rapid, personalized diagnostics has sparked concerns over electronic waste from short-lived silicon-based devices. To address this issue, the development of flexible and wearable sensors for breath sensing applications is a promising approach. Research highlights the development of different flexible, wearable sensors operating with different operating principles, such as chemiresistive sensors to detect specific target analytes due to their simple design, high sensitivity, selectivity, and reliability. Further, focusing on the non-invasive detection of biomarkers through exhaled breath, chemiresistive wearable sensors offer a comprehensive and environmentally friendly solution. This article presents a comprehensive discussion of the recent advancement in chemiresistive wearable breath sensors for the non-invasive detection of breath biomarkers. The article further emphasizes the intricate development and functioning of the sensor, including the selection criteria for both the flexible substrate and advanced functional materials, including their sensing mechanisms. The review then explores the potential applications of wearable gas sensing systems with specific disease detection, with modern challenges associated with non-invasive breath sensors.

Three-dimensional graphene on a nano-porous 4H-silicon carbide backbone: a novel material for food sensing applications

BACKGROUND

Sensors that are sensitive to volatile organic compounds, and thus able to monitor the conservation state of food, are precious because they work non-destructively and allow avoiding direct contact with the food, ensuring hygienic conditions. In particular, the monitoring of rancidity would solve a widespread issue in food storage.

RESULTS

The sensor discussed here is produced utilizing a novel three-dimensional arrangement of graphene, which is grown on a crystalline silicon carbide wafer previously porousified by chemical etching. This approach allows a very high surface-to-volume ratio. Furthermore, the structure of the sensor surface features a large number of edges, dangling bounds, and active sites, which make the sensor, on a chemically robust skeleton, chemically active, particularly to hydrogenated molecules. The interaction of the sensor with such compounds is read out by measuring the sensor resistance in a four-wire configuration. The sensor performance has been assessed on three hazelnut samples: sound, spoiled, and stink bug hazelnuts. A resistance variation of about ∆R = 0.13 ± 0.02 Ω between sound and damaged hazelnuts has been detected.

CONCLUSIONS

Our measurements confirm the ability of the sensor to discriminate between sound and damaged hazelnuts. The sensor signal is stable for days, providing the possibility to use this sensor for the monitoring of the storage state of fats and foods in general. © 2023 Society of Chemical Industry.

Wearable bioelectronics based on emerging nanomaterials for telehealth applications

Nanomaterial-driven, soft wearable bioelectronics are transforming telemedicine by offering skin comfort, biocompatibility, and the capability for continuous remote monitoring of physiological signals. The devices, enabled by advanced zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) nanomaterials, have achieved new levels in electrical stability and reliability, allowing them to perform effectively even under dynamic physical conditions. Despite their promise, significant challenges remain in the fabrication, integration, and practical deployment of nanoscale materials and devices. Critical challenges include ensuring the durability and stability of nanomaterial-based bioelectronics for extended wear and developing efficient integration strategies to support multifunctional sensing modalities. Telemedicine has revolutionized healthcare by enabling remote health monitoring. The integration of nanomaterials within wearable devices is a central factor driving this breakthrough, as these materials enhance sensor sensitivity, durability, and multifunctionality. These wearable sensors leverage various operating principles tailored to specific applications, such as intraocular pressure monitoring, electrophysiological signal recording, and biochemical marker tracking.

Electrical Contacts to Graphene by Postgrowth Patterning of Cu Foil for the Low-Cost Scalable Production of Graphene-Based Flexible Electronics

Numerous studies have focused on graphene owing to its potential as a next-generation electronic material, considering its high conductivity, transparency, superior mechanical stiffness, and flexibility. However, cost-effective mass production of graphene-based electronics based on existing fabrication methods, such as graphene transfer and metal formation, remains a challenge. This study proposes a simple and efficient method for creating electrical contacts with graphene. The method involves patterning a Cu foil after graphene growth, enabling the low-cost scalable production of graphene-based flexible electronics. The fabricated graphene devices exhibited linear current-voltage characteristics, indicating good electrical contact between the postgrowth-patterned Cu electrodes and graphene. The proposed postgrowth patterning method allows for the fabrication of Cu-contacted graphene devices on large areas and various flexible substrates, including ultrathin and stretchable films (<10 μm). The feasibility of the proposed method for electronic devices was demonstrated by implementing gas and flexible force sensors. The proposed approach advances the field of graphene-based electronics and holds potential for practical applications in various electronic devices, paving the way for scalable, cost-effective, and flexible technology solutions.

Textronic Sensors of Hazardous Gaseous Substances

Toxic materials are a threat in workplaces and the environment, as well as households. In them, gaseous substances are included, especially ones without any colour or fragrance, due to their non-detectability with the human senses. In this article, an attempt was made to find a solution for its detection in various conditions with the use of intelligent textiles. The approach was to perform modification on fifteen materials by screen printing using carbon nanotubes paste with expanded graphite and embroidery with stainless steel thread and then investigate their reaction with risky gases such as acetone, methanol and toluene. Four combinations of samples were tested: before tests, after the washing test and after the alkaline and acidic sweat contact test. Three materials can be highlighted. Para-aramid knitwear which reacted well to all tested gases. The biggest value of sensory percentage response was 144%. Screen-printed linen knitwear showed properly detecting skills after washing test for toluene. The biggest value of sensory percentage response was noted at 186%. The third most promising material was low surface mass cotton knitwear with embroidery which had a visible response at every stage of testing for acetone. The biggest value of sensory percentage response was 94% and the smallest one was 27%. For these three materials, repeated contact with harmful gases was tested. Simulations showed also repeated responses expressed in changes in surface resistance under changed conditions. After analysis, there is a possibility to create textile sensors for the detection of hazardous substances.

WITHDRAWN: Biomarker Detection and Therapy of Parkinson's and Alzheimer's disease using upconversion based approach: A Comprehensive Review

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Room-Temperature Ammonia Sensing Using Polyaniline-Coated Laser-Induced Graphene

The reliable detection of ammonia at room temperature is crucial for not only maintaining environmental safety but also for reducing the risks of hazardous pollutants. In this study, the electrochemical modification of laser-induced graphene (LIG) with polyaniline (PANI) led to the development of a chemo-resistive nanocomposite (PANI@LIG) for detecting ammonia levels at room temperature. The composite is characterized by field emission scanning electron microscopy, Fourier transforms infrared, and Raman and X-ray photoelectron spectroscopy. This work marks the first utilization of PANI@LIG for gas sensing and introduces a simple but effective approach for fabricating low-cost wearable gas sensors with high sensitivity and flexibility.

Selective Identification of Hazardous Gases Using Flexible, Room-Temperature Operable Sensor Array Based on Reduced Graphene Oxide and Metal Oxide Nanoparticles via Machine Learning

Selective detection and monitoring of hazardous gases with similar properties are highly desirable to ensure human safety. The development of flexible and room-temperature (RT) operable chemiresistive gas sensors provides an excellent opportunity to create wearable devices for detecting hazardous gases surrounding us. However, chemiresistive gas sensors typically suffer from poor selectivity and zero-cross selectivity toward similar types of gases. Herein, a flexible, RT operable chemiresistive gas sensors array is designed, featuring reduced graphene oxide (rGO) and rGO decorated with zinc oxide (ZnO), titanium dioxide (TiO2), and tin dioxide (SnO2) nanoparticles (NPs) on a flexible polyimide (PI) substrate. The sensor array consists of four different sensing layers capable of the selective identification of various hazardous gases such as NO2, NO, and SO2 using machine learning (ML). The gas sensor array exhibits a stable response even when mechanically deformed or exposed to high humidity (up to 60%). Each gas sensor, due to the different metal oxide NPs, shows unique responses in terms of sensitivity, responsiveness, response time, and recovery time to different gases. Consequently, the sensor array generates distinct response patterns that effectively differentiate between the target gases. By leveraging these distinctive recovery patterns and employing a data fusion approach in ML, specific concentrations of target gases can be distinguished. Using ML with fused array sensing data, the training and test accuracies achieved were 98.20 and 97.70%, respectively. This innovative combination of sensor arrays and ML offers significant potential for selective gas detection in environmental monitoring and personal safety applications.

Emerging Horizons in Gas Sensing: Exploring the Potential of MXenes and MBenes

This review article is focused on the development and application of two types of emerging two-dimensional (2D) materials, namely MXenes and MBenes, in the field of gas detection. Owing to its excellent electrical conductivity, specific surface area, and tunable surface functionality, MXenes have demonstrated high sensitivity and rapid response in detecting a variety of gases, such as volatile organic compounds (VOCs), nitrogen dioxide (NO2), and ammonia (NH3). MBenes are relatively newly discovered materials with excellent electronic stability and gas adsorption capabilities. Both of these materials demonstrate significant potential in improving sensor selectivity and stability through surface functionalization and heterostructure designs. However, despite the impressive gas detection performance of these materials, challenges remain in achieving long-term stability, cost-effectiveness, and commercialization. We summarize the prominent research insights on these materials and offer an outlook on future research directions and potential applications. With ongoing research and technological advancements, MXenes and MBenes are anticipated to have a substantial impact in critical areas such as environmental monitoring and industrial safety.

Graphene-supported organoiridium clusters catalyze N-alkylation of amines via hydrogen borrowing reaction

Graphene-supported organic iridium clusters (GSOICs) have been designed, prepared, characterized, and used for N-alkylation of amines via hydrogen borrowing reactions. Structural analysis data (including IR, XPS, TEM and EDS) show that organoiridium clusters are uniformly formed on the surface of graphene, and the grain size of GSOIC is between 1 and 3 nm. After being activated by the auxiliary ligand TMPP (tris(4-methoxyphenyl)phosphine), GSOIC showed excellent catalytic performance for hydrogen borrowing reaction, with its turnover frequency (TOF) reaching 13.67 h-1. Multi-cycle catalysis shows that the GSOIC/TMPP catalytic system exhibits high stability and reliability, with the turnover number (TON) of each catalytic cycle reaching 328, and the cumulative TON of 10 consecutive catalytic cycles reaching 3280. These systems exhibit excellent N-alkylation for a variety of substrates under one-pot conditions without the need for bases, solvents, and other additives, representing a sustainable and environmentally friendly catalytic reaction strategy.

Room-Temperature Flexible CNT/Fe2O3 Film Sensor for ppb-Level H2S Detection

Carbon nanotubes (CNTs) had room temperature response, large surface area, and excellent mechanical properties, making them favorable for the design of flexible, wearable, and portable gas sensors. However, CNTs were lacking in response and selective response to different gases, such as H2S. Here, we demonstrated a flexible H2S ppb-level gas sensor based on a carbon nanotube/amorphous Fe2O3 (CNT/Fe2O3) film at room temperature, which was fabricated via a simple one-step solvent-thermal method. The CNT/Fe2O3 film gas sensor exhibited a high selective response to H2S (with a response of 55.1% to 100 ppb H2S), rapid reversible response at room temperature (with a response time of ∼127 s to 100 ppb H2S), and low limit of detection to about 2 ppb. Additionally, the CNT/Fe2O3 film maintained good sensing performance under various bending conditions and could be further fabricated into the fiber gas sensor device via wet stretching, retaining response at the ppb level (with a response of 18.6% to 100 ppb H2S). This research on a flexible gas sensor device based on the CNT film/fiber opened up new possibilities for wearable portable electronic device applications.

Recent Research Progress of Porous Graphene and Applications in Molecular Sieve, Sensor, and Supercapacitor

Porous graphene, including 2D and 3D porous graphene, is widely researched recently. One of the most attractive features is the proper utilization of graphene defects, which combine the advantages of both graphene and porous materials, greatly enriching the applications of porous graphene in biology, chemistry, electronics, and other fields. In this review, the defects of graphene are first discussed to provide a comprehensive understanding of porous graphene. Then, the latest advancements in the preparation of 2D and 3D porous graphene are presented. The pros and cons of these preparation methods are discussed in detail, providing a direction for the fabrication of porous graphene. Moreover, various superior properties of porous graphene are described, laying the foundation for their promising applications. Owing to its abundant morphology, wide distribution of pore size, and remarkable properties benefited from porous structure, porous graphene can not only promote molecular diffusion and electron transfer but also expose more active sites. Consequently, a serious of applications containing gas sieving, liquid separation, sensors, and supercapacitors, are presented. Finally, the challenges confronted during preparation and characterization of porous graphene are discussed, offering guidance for the future development of porous graphene in fabrication, characterization, properties, and applications.

Gas Sensing Properties of Indium-Oxide-Based Field-Effect Transistor: A Review

Excellent stability, low cost, high response, and sensitivity of indium oxide (In2O3), a metal oxide semiconductor, have been verified in the field of gas sensing. Conventional In2O3 gas sensors employ simple and easy-to-manufacture resistive components as transducers. However, the swift advancement of the Internet of Things has raised higher requirements for gas sensors based on metal oxides, primarily including lowering operating temperatures, improving selectivity, and realizing integrability. In response to these three main concerns, field-effect transistor (FET) gas sensors have garnered growing interest over the past decade. When compared with other metal oxide semiconductors, In2O3 exhibits greater carrier concentration and mobility. The property is advantageous for manufacturing FETs with exceptional electrical performance, provided that the off-state current is controlled at a sufficiently low level. This review presents the significant progress made in In2O3 FET gas sensors during the last ten years, covering typical device designs, gas sensing performance indicators, optimization techniques, and strategies for the future development based on In2O3 FET gas sensors.

Synthesis, Modulation, and Application of Two-Dimensional TMD Heterostructures

Two-dimensional (2D) transition metal dichalcogenide (TMD) heterostructures have attracted a lot of attention due to their rich material diversity and stack geometry, precise controllability of structure and properties, and potential practical applications. These heterostructures not only overcome the inherent limitations of individual materials but also enable the realization of new properties through appropriate combinations, establishing a platform to explore new physical and chemical properties at micro-nano-pico scales. In this review, we systematically summarize the latest research progress in the synthesis, modulation, and application of 2D TMD heterostructures. We first introduce the latest techniques for fabricating 2D TMD heterostructures, examining the rationale, mechanisms, advantages, and disadvantages of each strategy. Furthermore, we emphasize the importance of characteristic modulation in 2D TMD heterostructures and discuss some approaches to achieve novel functionalities. Then, we summarize the representative applications of 2D TMD heterostructures. Finally, we highlight the challenges and future perspectives in the synthesis and device fabrication of 2D TMD heterostructures and provide some feasible solutions.

High-Performance Flexible Sensor with Sensitive Strain/Magnetic Dual-Mode Sensing Characteristics Based on Sodium Alginate and Carboxymethyl Cellulose

Flexible sensors can measure various stimuli owing to their exceptional flexibility, stretchability, and electrical properties. However, the integration of multiple stimuli into a single sensor for measurement is challenging. To address this issue, the sensor developed in this study utilizes the natural biopolymers sodium alginate and carboxymethyl cellulose to construct a dual interpenetrating network, This results in a flexible porous sponge that exhibits a dual-modal response to strain and magnetic stimulation. The dual-mode flexible sensor achieved a maximum tensile strength of 429 kPa and elongation at break of 24.7%. It also exhibited rapid response times and reliable stability under both strain and magnetic stimuli. The porous foam sensor is intended for use as a wearable electronic device for monitoring joint movements of the body. It provides a swift and stable sensing response to mechanical stimuli arising from joint activities, such as stretching, compression, and bending. Furthermore, the sensor generates opposing response signals to strain and magnetic stimulation, enabling real-time decoupling of different stimuli. This study employed a simple and environmentally friendly manufacturing method for the dual-modal flexible sensor. Because of its remarkable performance, it has significant potential for application in smart wearable electronics and artificial electroskins.

Theoretical and Experimental Studies of Molecular Interactions between Engineered Graphene and Phosphate Ions for Graphene-Based Phosphate Sensing

Fundamental understanding of the interactions of nanoscale materials with molecules of interest is essential for the development of electronic devices, such as sensors. In particular, structures and molecular interaction properties of engineered graphenes are still largely unexplored, despite these materials' great potential to be used as molecular sensors. As an example of end user application, the detection of phosphorus in the form of phosphate in a soil environment is important for soil fertility and plant growth. However, due to the lack of an affordable technology, it is currently hard to measure the amount of phosphate directly in the soil; therefore, suitable sensor technologies need to be developed for phosphate sensors. In this work, pristine graphene and several modified graphene materials (oxygenated graphene, graphene with vacancies, and curved graphene) were studied as candidates for phosphate sensor materials using density functional theory (DFT) calculations. Our calculations showed that both pristine graphene and functionalized graphene were able to adsorb phosphate species strongly. In addition, these graphene nanomaterials showed selectivity of adsorption of phosphate with respect to nitrate, with stronger adsorption energies for phosphate. Furthermore, our calculations showed significant changes in electrical conductivities of pristine graphene and functionalized graphenes after phosphate species adsorption, in particular, on graphene with oxygen (hydroxyl and epoxide) functional groups. Experimental measurements of electrical resistivity of graphene before and after adsorption of dihydrogen phosphate showed an increase in resistivity upon adsorption of phosphate, consistent with the theoretical predictions. Our results recommend graphene and functionalized graphene-based nanomaterials as good candidates for the development of phosphate sensors.

Chemiresistive Gas Sensing using Graphene-Metal Oxide Hybrids

Chemiresistive sensing lies in its ability to provide fast, accurate, and reliable detection of various gases in a cost-effective and non-invasive manner. In this context, graphene-functionalized metal oxides play crucial role in hydrogen gas sensing. However, a cost-effective, defect-free, and large production schemes of graphene-based sensors are required for industrial applications. This review focuses on graphene-functionalized metal oxide nanostructures designed for gaseous molecules detection, mainly hydrogen gas sensing applications. For the convenience of the reader and to understand the role of graphene-metal oxide hybrids (GMOH) in gas sensing activities, a brief overview of the properties and synthesis routes of graphene and GMOH have been reported in this paper. Metal oxides play an essential role in the GMOH construct for hydrogen gas sensing. Therefore, various metal oxides-decorated GMOH constructs are detailed in this review as gas sensing platforms, particularly for hydrogen detection. Finally, specific directions for future research works and challenges ahead in designing highly selective and sensitive hydrogen gas sensors have been highlighted. As illustrated in this review, understanding of the metal oxides-decorated GMOH constructs is expected to guide ones in developing emerging hybrid nanomaterials that are suitable for hydrogen gas sensing applications.

Ultrahigh Sensitivity for Thermographic Human-Machine Interface via Precious Metals Atomic Layer Deposition on V-MXene: Computational and Experimental Exploration

Global healthcare based on the Internet of Things system is rapidly transforming to measure precise physiological body parameters without visiting hospitals at remote patients and associated symptoms monitoring. 2D materials and the prevailing mood of current ever-expanding MXene-based sensing devices motivate to introduce first the novel iridium (Ir) precious metal incorporated vanadium (V)-MXene via industrially favored emerging atomic layer deposition (ALD) techniques. The current work contributes a precise control and delicate balance of Ir single atomic forms or clusters on the V-MXene to constitute a unique precious metal-MXene embedded heterostructure (Ir-ALD@V-MXene) in practical real-time sensing healthcare applications to thermography with human-machine interface for the first time. Ir-ALD@V-MXene delivers an ultrahigh durability and sensing performance of 2.4% °C-1 than pristine V-MXene (0.42% °C-1), outperforming several conventionally used MXenes, graphene, underscoring the importance of the Ir-ALD innovative process. Aberration-corrected advanced ultra-high-resolution transmission/scanning transmission electron microscopy confirms the presence of Ir atomic clusters on well-aligned 2D-layered V-MXene structure and their advanced heterostructure formation (Ir-ALD@V-MXene), enhanced sensing mechanism is investigated using density functional theory (DFT) computations. A rational design empowering the Ir-ALD process on least explored V-MXene can potentially unfold further precious metals ALD-process developments for next-generation wearable personal healthcare devices.

Smart Sensing of Nerve Agents

The recent international scenario highlights the importance to protect human health and environmental quality from toxic compounds. In this context, organophosphorous (OP) Nerve Agents (NAs) have received particular attention, due to their use in terrorist attacks. Classical instrumental detection techniques are sensitive and selective, but they cannot be used in real field due to the high cost, specialized personnel requested and huge size. For these reasons, the development of practical, easy and fast detection methods (smart methods) is the future of this field. Indeed, starting from initial sensing research, based on optical and/or electrical sensors, today the development and use of smart strategies to detect NAs is the current state of the art. This review summarizes the smart strategies to detect NAs, highlighting some important parameters, such as linearity, limit of detection and selectivity. Furthermore, some critical comments of the future on this field, and in particular, the problems to be solved before a real application of these methods, are provided.

Carbon Micro/Nano Machining toward Miniaturized Device: Structural Engineering, Large-Scale Fabrication, and Performance Optimization

With the rapid development of micro/nano machining, there is an elevated demand for high-performance microdevices with high reliability and low cost. Due to their outstanding electrochemical, optical, electrical, and mechanical performance, carbon materials are extensively utilized in constructing microdevices for energy storage, sensing, and optoelectronics. Carbon micro/nano machining is fundamental in carbon-based intelligent microelectronics, multifunctional integrated microsystems, high-reliability portable/wearable consumer electronics, and portable medical diagnostic systems. Despite numerous reviews on carbon materials, a comprehensive overview is lacking that systematically encapsulates the development of high-performance microdevices based on carbon micro/nano structures, from structural design to manufacturing strategies and specific applications. This review focuses on the latest progress in carbon micro/nano machining toward miniaturized device, including structural engineering, large-scale fabrication, and performance optimization. Especially, the review targets an in-depth evaluation of carbon-based micro energy storage devices, microsensors, microactuators, miniaturized photoresponsive and electromagnetic interference shielding devices. Moreover, it highlights the challenges and opportunities in the large-scale manufacturing of carbon-based microdevices, aiming to spark further exciting research directions and application prospectives.

Single-Step Functionalization Strategy of Graphene Microtransistor Array with Chemically Modified Aptamers for Biosensing Applications

Graphene solution-gated field-effect transistors (gSGFETs) offer high potential for chemical and biochemical sensing applications. Among the current trends to improve this technology, the functionalization processes are gaining relevance for its crucial impact on biosensing performance. Previous efforts are focused on simplifying the attachment procedure from standard multi-step to single-step strategies, but they still suffer from overreaction, and impurity issues and are limited to a particular ligand. Herein, a novel strategy for single-step immobilization of chemically modified aptamers with fluorenylmethyl and acridine moieties, based on a straightforward synthetic route to overcome the aforementioned limitations is presented. This approach is benchmarked versus a standard multi-step strategy using thrombin as detection model. In order to assess the reliability of the functionalization strategies 48-gSGFETs arrays are employed to acquire large datasets with multiple replicas. Graphene surface characterization demonstrates robust and higher efficiency in the chemical coupling of the aptamers with the single-step strategy, while the electrical response evaluation validates the sensing capability, allowing to implement different alternatives for data analysis and reduce the sensing variability. In this work, a new tool capable of overcome the functionalization challenges of graphene surfaces is provided, paving the way toward the standardization of gSGFETs for biosensing purposes.

Selective Detection of NH3 Gas by Ti3C2Tx Sensors with the PVDF-ZIF-67 Overlayer at Room Temperature

In the realm of NH3 gas-sensing applications, the electrically conductive nature of Ti3C2Tx MXene, adorned with surface terminations such as -O and -OH groups, renders it a compelling material. However, the inherent challenges of atmospheric instability and selectivity in the presence of gas mixtures have prompted the exploration of innovative solutions. This work introduces a strategic solution through the deposition of a mixed-matrix membrane (MMM) composed of poly(vinylidene fluoride) (PVDF) as the matrix and zeolitic imidazolate framework-67 (ZIF-67) as the filler. This composite membrane acts as a selective filter, permitting the passage of a specific gas, namely NH3. Leveraging the hydrophobic and chemically inert nature of PVDF, the MMM enhances the atmospheric stability of Ti3C2Tx by impeding water molecules from interacting with the MXene. Furthermore, ZIF-67 is selective to NH3 gas via acid-base interactions within the zeolite group and selective pore size. The Ti3C2Tx sensor embedded in the MMM filter exhibits a modest 1.3% change in the sensing response to 25 ppm of NH3 gas compared to the response without the filter. This result underscores the filter's effectiveness in conferring selectivity and diffusivity, particularly at 35% relative humidity (RH) and 25 °C. Crucially, the hydrophobic attributes of PVDF impart heightened stability to the Ti3C2Tx sensor even amidst varying RH conditions. These results not only demonstrate effective NH3 detection but also highlight the sensor's adaptability to diverse environmental conditions, offering promising prospects for practical applications.

A visualization method for quickly detecting nitrite ions in breath condensate using a portable closed bipolar electrochemical sensor

A portable and non-invasive sensor presents an innovative way to measure inflammation biomarkers in exhaled breath condensate (EBC). This research is focused on developing a miniaturized bipolar electrochemical sensor that can be connected to a smartphone app. This device will be able to detect adding known amounts of nitrite (spikes) to a salt solution and small amounts of nitrite ions in collected real samples in EBC. The sensor was fabricated and tested for its rapid electron transfer capability and ability to detect nitrite ions even at very low concentrations and low real sample levels. In the proposed setup, when the required potential was applied by using a direct power supply, the nitrite ions were oxidized electrocatalytically at amine-functionalized graphene oxide (AGO) decorated with gold nanoparticles on a carbon paper anodic pole. On the other hand, the reduction reaction of Prussian blue occurred at the cathodic pole of the bipolar electrode simultaneously. This strategy led to a change in color from blue to white as a result of the reduction process and the color change is proportional to the concentration of nitrite ions in the analytical solution. The combination of smartphones with the colorimetric method has resulted in a platform for the detection of test strips that is more visual and convenient. The amperometry and voltammetric methods of nitrite detection showed a linear range of up to 1230 μM. The bipolar electrochemical sensor was able to detect the clinically relevant range of nitrite from 0.5 to 85 μM in a buffer with an ultralow detection limit (LOD) of 250 nM (S/N = 3), fast response and excellent selectivity. It was benchmarked by utilizing pre-characterized real EBC samples to differentiate patients with respiratory diseases from healthy volunteers. By tracking the results of nitrite measurements over time, it has become possible to detect trends and changes in an individual's nitrite ion concentration and to potentially identify lung inflammation earlier.

Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

2D nanomaterials for realization of flexible and wearable gas sensors: A review

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

Single-layer graphene based resistive humidity sensor enhanced by graphene quantum dots

Graphene is broadly applied as sensitive sensing material results from its superb features. Concurrently, as a derivative of graphene with 0D structure, graphene quantum dots (GQDs) offer more possibilities as a supportive sensing material due to its adjustable size and functional group modification. In this work, GQDs are introduced to single-layer graphene (SLG) based humidity sensor to enhance the sensing performance. Specifically, consistent resistance response to relative humidity (RH) is extended from the range of 10%-60% to 10%-90% by contrary to original SLG based sensor. Parallelly, effect of the amount of GQDs is investigated by means of multiple GQDs deposition. As the resultant higher binding efficiency between water molecules and the functional groups of GQDs, improved response rate is observed. For the case of 4-time deposition of GQDs, the response rate (ΔR/R) reaches ∼130% in RH range of 10%-90%. Besides, the response time and recovery time are ∼0.7 s and ∼1.1 s, respectively. The fluctuation of the resistance change of the sensor under constant humidity is less than 5% over a month which demonstrates long-term reliability.

Using Machine Learning to Overcome Interfering Oxygen Effects in a Graphene Volatile Organic Compound Sensor

Discriminating between volatile organic compounds (VOCs) for applications including disease diagnosis and environmental monitoring, is often complicated by the presence of interfering compounds such as oxygen. Graphene sensors are effective at detecting VOCs; however, they are also known to be highly sensitive to oxygen. Therefore, the combined effects of each of these gases on graphene sensors must be understood. In this work, we use graphene variable capacitor (varactor) sensors to examine the cross-selectivity of oxygen at 3 concentrations and 3 VOCs (ethanol, methanol, and methyl ethyl ketone) at 5 concentrations each. The sensor responses exhibit distinct shapes dependent on the relative concentrations in mixtures of oxygen and VOCs. Because the entire response shape is therefore informative for distinguishing between each gas mixture, a classification algorithm that utilizes entire sequences of data is needed. Accordingly, a long short-term memory (LSTM) network is used to classify the mixtures and VOC concentrations. The model achieves 100% accurate classification of the VOC type, even in the presence of varying levels of oxygen. When the VOC type and VOC concentration are classified, we show that the sensors can provide VOC concentration resolution within approximately 200 ppm. Throughout this work, we also demonstrate that an effective gas mixture classification can be achieved, even while the sensors exhibit varied drift patterns typical of graphene sensors. This is made possible due to the data analysis and machine learning methods employed.

High-response room-temperature NO2 gas sensor fabricated with thermally reduced graphene oxide-coated commercial cotton fabric

Electronic textile-based gas sensors with a high response for NO2 gas were fabricated using reduced graphene oxide (rGO)-coated commercial cotton fabric (rGOC). Graphene oxide (GO) was coated on cotton fabric by simply dipping the cotton into a GO solution. To investigate the relationship between the degree of reduction and the sensing response, the GO-coated fabrics were thermally reduced at various temperatures (190, 200, 300, and 400 °C). The change in the amount of oxygen functional groups on the rGOCs was observed by x-ray photoelectron spectroscopy, Raman spectroscopy, and x-ray diffraction patterns. The maximum sensing response of 45.90 % at 10 ppm of NO2 gas at room temperature was exhibited by the rGOC treated at 190 °C, which was the lowest heat-treatment temperature. The high response comes from the greater amount of oxygen functional groups compared to other rGOC samples, and the tubular structure of the cotton.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Recent Advances in Nanostructured Materials for Application as Gas Sensors

Many different industries, including the pharmaceutical, medical engineering, clinical diagnostic, public safety, and food monitoring industries, use gas sensors. The inherent qualities of nanomaterials, such as their capacity to chemically or physically adsorb gas, and their great ratio of surface to volume make them excellent candidates for use in gas sensing technology. Additionally, the nanomaterial-based gas sensors have excellent selectivity, reproducibility, durability, and cost-effectiveness. This Review article offers a summary of the research on gas sensor devices based on nanomaterials of various sizes. The numerous nanomaterial-based gas sensors, their manufacturing procedures and sensing mechanisms, and most recent advancements are all covered in detail. In addition, evaluations and comparisons of the key characteristics of gas sensing systems made from various dimensional nanomaterials were done.

A Short Overview on Graphene and Graphene-Related Materials for Electrochemical Gas Sensing

The development of new and high-performing electrode materials for sensing applications is one of the most intriguing and challenging research fields. There are several ways to approach this matter, but the use of nanostructured surfaces is among the most promising and highest performing. Graphene and graphene-related materials have contributed to spreading nanoscience across several fields in which the combination of morphological and electronic properties exploit their outstanding electrochemical properties. In this review, we discuss the use of graphene and graphene-like materials to produce gas sensors, highlighting the most relevant and new advancements in the field, with a particular focus on the interaction between the gases and the materials.

Compact device prototype for turn-on fluorescence detection of sarin based on reactive 4,4-diaryloxy-BODIPY derivatives

4,4-Diaryloxy-BODIPYs were presented for fluorescence turn-on detection of sarin in solution media. A compact tubular sensor and a sensing platform prototype were fabricated for in situ detection of real agents and simulants at the sub-mM level.

Detection of medically relevant volatile organic compounds with graphene field-effect transistors and separated by low-frequency spectral and time signatures

Exhaled human breath contains a mixture of gases including nitrogen, oxygen, carbon dioxide, water vapour and low molecular weight volatile organic compounds (VOCs). Different VOCs detected in human breath condensate have been recently related to several metabolic processes occurring inside body tissues in the pathological state, as candidate biomarkers for monitoring conditions such as lung injury, airway inflammation, immunity dysfunction, infection, and cancer. Current techniques for detecting these compounds include several types of mass spectroscopy, which are highly costly, time-consuming and dependent on trained personnel for sample analysis. The need for fast and label-free biosensors is paving the way towards the design of novel and portable electronic devices for point-of-care diagnosis with VOCs such as E-noses, and based on the measurement of signal signatures derived from their chemical composition. In this paper, we propose a device for VOC detection that was tested inside a controlled gas flow setup, resorting to graphene field-effect transistors (GFETs). Electrical measurements from graphene directly exposed to nitrogen plus VOC vapours involved cyclic measurements for the variation of graphene's resistance and low-frequency spectral noise in order to obtain distinctive signatures of the tested compounds in the time and frequency domains related, respectively, to Gutmann's theory for donor-acceptor chemical species and spectral sub-band analysis.

Enhancement of Thermal and Gas Barrier Properties of Graphene-Based Nanocomposite Films

Poly(vinyl alcohol) (PVA), a naturally occurring and rapidly decomposing polymer, has gained significant attention in recent studies for its potential use in pollution preventive materials. Its cost-effectiveness and ease of availability as well as simple processing make it a suitable material for various applications. However, the only concern about PVA's applicability to various applications is its hydrophilic nature. To address this limitation, PVA-based nanocomposites can be created by incorporating inorganic fillers such as graphene (G). Graphene is a two-dimensional carbon crystal with a single atom-layer structure and has become a popular choice as a nanomaterial due to its outstanding properties. In this study, we present a simple and environmentally friendly solution processing technique to fabricate PVA and graphene-based nanocomposite films. The resulting composite films showed noticeable improvement in barrier properties against moisture, oxygen, heat, and mechanical failures. The improvement of the characteristic properties is attributed to the uniform dispersion of graphene in the PVA matrix as shown in the SEM image. The addition of graphene leads to a decrease in water vapor transmission rate (WVTR) by 79% and around 90% for the oxygen transmission rate (OTR) as compared to pristine PVA films. Notably, incorporating just 0.5 vol % of graphene results in an OTR value of as low as 0.7 cm m-2 day-1 bar-1, making it highly suitable packaging applications. The films also exhibit remarkable flexibility and retained almost the same WVTR values even after going through tough bending cycles of more than 2000 at a bending radius of 2.5 cm. Overall, PVA/G nanocomposite films offer promising potential for PVA/G composite films for various attractive pollution prevention (such as corrosion resistant coatings) and packaging applications.

Highly Breathable, Stretchable, and Tailorable TPU Foam for Flexible Gas Sensors

Continuous real-time monitoring of air quality is of great significance in the realms of environmental monitoring, personal safety, and healthcare. Recently, flexible gas sensors have gained great popularity for their potential to be integrated into various smart wearable electronics and display devices. However, the development of gas sensors with superior sensitivity, breathability, and stretchability remains a challenge. Here, a new high porosity thermoplastic polyurethane (HP-TPU) foam was reported for gas sensors, which exhibited large three-dimensional network structures and excellent mechanical properties. The HP-TPU foam was achieved by using a simple steam-induced method, which was suitable for mass production. The unique structure endowed this foam with 77.5% porosity, 260% strain ability, and 0.45 MPa Young's modulus, which improved 35, 31, and 80%, respectively, compared to previously reported traditional TPU foam (T-TPU) prepared by the drying method. In addition, the foam presented high gas permeability (312 g/m-2, 24 h) and excellent stability, and it remained undamaged even after 2000 cycles at 70% strain. The sensing material was coated on a PET flexible interdigital electrode and sandwiched between two HP-TPU foam layers for a gas sensitivity test. Due to the easy diffusion of gas between the pores and contact with the sensing materials, the HP-TPU foam exhibited a significant reduction of 85% in average response time and 46% in average recovery time, compared to the T-TPU foam. A wearable sensing device, comprising sensing, data processing, and wireless transmission modules, was successfully developed to enable outdoor testing and achieved a detection range at the ppb level. Finally, the cytotoxicity test results confirmed that this flexible gas sensor did not harm human health. These results proved that this HP-TPU foam was an ideal matrix for the flexible gas sensor, exhibiting great application potential in the fields of seamless human-machine integration.

Enhancement of Adsorption Performance of Gases in Oil on a Cr3-Modified SnS2 Monolayer Based on the First Principles

Dissolved gas analysis (DGA) is the most commonly used transformer fault diagnosis technology at present. In this paper, according to the method of first principles of density function theory (DFT), the gas-sensitive mechanisms of four oil-soluble characteristic gases (H2, CO, C2H2, C2H4) on intrinsic SnS2 and Cr3-SnS2 were studied. The adsorption energy and electron transfer were calculated, and the density of states, energy bands, and recovery times were analyzed. It was concluded that H2 and C2H4 were physisorbed on the Cr3-SnS2 monolayer, while CO and C2H2 were chemisorbed. It is believed that the Cr3-SnS2 material can be used in gas sensing for CO and C2H2. Cr3-SnS2 is expected to serve as a gas detector for the detection of CO with both a good response and reusability. Therefore, Cr3-SnS2 has very promising applications in the evaluation of the operation of oil-immersed transformers. This study will provide some help and inspiration for the development of the Cr3-SnS2 monolayer in gas-sensitive materials.

CO2 on Graphene: Benchmarking Computational Approaches to Noncovalent Interactions

Designing and optimizing graphene-based gas sensors in silico entail constructing appropriate atomistic representations for the physisorption complex of an analyte on an infinite graphene sheet, then selecting accurate yet affordable methods for geometry optimizations and energy computations. In this work, diverse density functionals (DFs), coupled cluster theory, and symmetry-adapted perturbation theory (SAPT) in conjunction with a range of finite and periodic surface models of bare and supported graphene were tested for their ability to reproduce the experimental adsorption energies of CO2 on graphene in a low-coverage regime. Periodic results are accurately reproduced by the interaction energies extrapolated from finite clusters to infinity. This simple yet powerful scheme effectively removes size dependence from the data obtained using finite models, and the latter can be treated at more sophisticated levels of theory relative to periodic systems. While for small models inexpensive DFs such as PBE-D3 afford surprisingly good agreement with the gold standard of quantum chemistry, CCSD(T), interaction energies closest to experiment are obtained by extrapolating the SAPT results and with nonlocal van der Waals functionals in the periodic setting. Finally, none of the methods and models reproduce the experimentally observed CO2 tilted adsorption geometry on the Pt(111) support, calling for either even more elaborate theoretical approaches or a revision of the experiment.

Two-Dimensional Conjugated Metal-Organic Frameworks with Large Pore Apertures and High Surface Areas for NO2 Selective Chemiresistive Sensing

The emergence of two-dimensional conjugated metal-organic frameworks (2D c-MOFs) with pronounced electrical properties (e.g., high conductivity) has provided a novel platform for efficient energy storage, sensing, and electrocatalysis. Nevertheless, the limited availability of suitable ligands restricts the number of available types of 2D c-MOFs, especially those with large pore apertures and high surface areas are rare. Herein, we develop two new 2D c-MOFs (HIOTP-M, M=Ni, Cu) employing a large p-π conjugated ligand of hexaamino-triphenyleno[2,3-b:6,7-b':10,11-b'']tris[1,4]benzodioxin (HAOTP). Among the reported 2D c-MOFs, HIOTP-Ni exhibits the largest pore size of 3.3 nm and one of the highest surface areas (up to 1300 m2  g-1 ). As an exemplary application, HIOTP-Ni has been used as a chemiresistive sensing material and displays high selective response (405 %) and a rapid response (1.69 min) towards 10 ppm NO2 gas. This work demonstrates significant correlation linking the pore aperture of 2D c-MOFs to their sensing performance.

MOF-Based Chemiresistive Gas Sensors: Toward New Functionalities

The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.

Near-Field Photodetection in Direction Tunable Surface Plasmon Polaritons Waveguides Embedded with Graphene

2D materials have manifested themselves as key components toward compact integrated circuits. Because of their capability to circumvent the diffraction limit, light manipulation using surface plasmon polaritons (SPPs) is highly-valued. In this study, plasmonic photodetection using graphene as a 2D material is investigated. Non-scattering near-field detection of SPPs is implemented via monolayer graphene stacked under an SPP waveguide with a symmetric antenna. Energy conversion between radiation power and electrical signals is utilized for the photovoltaic and photoconductive processes of the gold-graphene interface and biased electrodes, measuring a maximum photoresponsivity of 29.2 mA W-1 . The generated photocurrent is altered under the polarization state of the input light, producing a 400% contrast between the maximum and minimum signals. This result is universally applicable to all on-chip optoelectronic circuits.

Flexible tactile sensors with biomimetic microstructures: Mechanisms, fabrication, and applications

In recent years, flexible devices have gained rapid development with great potential in daily life. As the core component of wearable devices, flexible tactile sensors are prized for their excellent properties such as lightweight, stretchable and foldable. Consequently, numerous high-performance sensors have been developed, along with an array of innovative fabrication processes. It has been recognized that the improvement of the single performance index for flexible tactile sensors is not enough for practical sensing applications. Therefore, balancing and optimization of overall performance of the sensor are extensively anticipated. Furthermore, new functional characteristics are required for practical applications, such as freeze resistance, corrosion resistance, self-cleaning, and degradability. From a bionic perspective, the overall performance of a sensor can be optimized by constructing bionic microstructures which can deliver additional functional features. This review briefly summarizes the latest developments in bionic microstructures for different types of tactile sensors and critically analyzes the sensing performance of fabricated flexible tactile sensors. Based on this, the application prospects of bionic microstructure-based tactile sensors in human detection and human-machine interaction devices are introduced.

Direct Induction of Porous Graphene from Mechanically Strong and Waterproof Biopaper for On-Chip Multifunctional Flexible Electronics

Graphene with a 3D porous structure is directly laser-induced on lignocellulosic biopaper under ambient conditions and is further explored for multifunctional biomass-based flexible electronics. The mechanically strong, flexible, and waterproof biopaper is fabricated by surface-functionalizing cellulose with lignin-based epoxy acrylate (LBEA). This composite biopaper shows as high as a threefold increase in tensile strength and excellent waterproofing compared with pure cellulose one. Direct laser writing (DLW) rapidly induces porous graphene from the biopaper in a single step. The porous graphene shows an interconnected carbon network, well-defined graphene domains, and high electrical conductivity (e.g., ≈3 Ω per square), which can be tuned by lignin precursors and loadings as well as lasing conditions. The biopaper in situ embedded with porous graphene is facilely fabricated into flexible electronics for on-chip and paper-based applications. The biopaper-based electronic devices, including the all-solid-state planer supercapacitor, electrochemical and strain biosensors, and Joule heater, show great performances. This study demonstrates the facile, versatile, and low-cost fabrication of multifunctional graphene-based electronics from lignocellulose-based biopaper.

Multifunctional Motion Sensing Enabled by Laser-Induced Graphene

The development of flexible sensors based on laser-induced graphene (LIG) has recently attracted much attention. It was commonly generated by laser-ablating commercial polyimide (PI). However, the weak mechanical extensibility of PI limits the development and diversified applications of LIG-based sensors. In this work, we adopted medical polyurethane (PU) tapes to peel off the LIG generated on PI and developed flexible and wearable sensors based on the proposed LIG/PU composite structure. Compared with other methods for LIG transfer, PU tape has many advantages, including a simplified process and being less time-consuming. We characterized the LIG samples generated under different laser powers and analyzed the property differences introduced by the transfer operation. We then studied the impact of fabrication mode on the strain sensitivity of the LIG/PU and optimized the design of a LIG/PU-based strain sensor, which possessed a gauge factor (GF) of up to 263.6 in the strain range of 75-90%. In addition, we designed a capacitive pressure sensor for tactile sensing, which is composed of two LIG/PU composite structures and a PI space layer. These LIG flexible devices can be used for human motion monitoring and tactile perception in sports events. This work provides a simple, fast, and low-cost way for the preparation of multifunctional sensor systems with good performance, which has a broad application prospect in human motion monitoring.

Graphene-Based Wearable Temperature Sensors: A Review

Flexible sensing electronics have received extensive attention for their potential applications in wearable human health monitoring and care systems. Given that the normal physiological activities of the human body are primarily based on a relatively constant body temperature, real-time monitoring of body surface temperature using temperature sensors is one of the most intuitive and effective methods to understand physical conditions. With its outstanding electrical, mechanical, and thermal properties, graphene emerges as a promising candidate for the development of flexible and wearable temperature sensors. In this review, the recent progress of graphene-based wearable temperature sensors is summarized, including material preparation, working principle, performance index, classification, and related applications. Finally, the challenges and future research emphasis in this field are put forward. This review provides important guidance for designing novel and intelligent wearable temperature-sensing systems.

Electrochemical Fine-Tuning of the Chemoresponsiveness of Langmuir-Blodgett Graphene Oxide Films

Graphene oxide has been widely deployed in electrical sensors for monitoring physical, chemical, and biological processes. The presence of abundant oxygen functional groups makes it an ideal substrate for integrating biological functional units to assemblies. However, the introduction of this type of defects on the surface of graphene has a deleterious effect on its electrical properties. Therefore, adjusting the surface chemistry of graphene oxide is of utmost relevance for addressing the immobilization of biomolecules, while preserving its electrochemical integrity. Herein, we describe the direct immobilization of glucose oxidase onto graphene oxide-based electrodes prepared by Langmuir-Blodgett assembly. Electrochemical reduction of graphene oxide allowed to control its surface chemistry and, by this, regulate the nature and density of binding sites for the enzyme and the overall responsiveness of the Langmuir-Blodgett biofilm. X-ray photoelectron spectroscopy, surface plasmon resonance, and electrochemical measurements were used to characterize the compositional and functional features of these biointerfaces. Covalent binding between amine groups on glucose oxidase and epoxy and carbonyl groups on the surface of graphene oxide was successfully used to build up stable and active enzymatic assemblies. This approach constitutes a simple, quick, and efficient route to locally address functional proteins at interfaces without the need for additives or complex modifiers to direct the adsorption process.

Covalently Functionalized MXenes for Highly Sensitive Humidity Sensors

Transition metal carbides and nitrides (MXenes) are an emerging class of 2D materials, which are attracting ever-growing attention due to their remarkable physicochemical properties. The presence of various surface functional groups on MXenes' surface, e.g., F, O, OH, Cl, opens the possibility to tune their properties through chemical functionalization approaches. However, only a few methods have been explored for the covalent functionalization of MXenes and include diazonium salt grafting and silylation reactions. Here, an unprecedented two-step functionalization of Ti3 C2 Tx MXenes is reported, where (3-aminopropyl)triethoxysilane is covalently tethered to Ti3 C2 Tx and serves as an anchoring unit for subsequent attachment of various organic bromides via the formation of CN bonds. Thin films of Ti3 C2 Tx functionalized with linear chains possessing increased hydrophilicity are employed for the fabrication of chemiresistive humidity sensors. The devices exhibit a broad operation range (0-100% relative humidity), high sensitivity (0.777 or 3.035), a fast response/recovery time (0.24/0.40 s ΔH-1 , respectively), and high selectivity to water in the presence of saturated vapors of organic compounds. Importantly, our Ti3 C2 Tx -based sensors display the largest operating range and a sensitivity beyond the state of the art of MXenes-based humidity sensors. Such outstanding performance makes the sensors suitable for real-time monitoring applications.

Nanostructured wearable electrochemical and biosensor towards healthcare management: a review

In recent years, there has been a rapid increase in demand for wearable sensors, particularly these tracking the surroundings, fitness, and health of people. Thus, selective detection in human body fluid is a demand for a smart lifestyle by quick monitoring of electrolytes, drugs, toxins, metabolites and biomolecules, proteins, and the immune system. In this review, these parameters along with the main features of the latest and mostly cited research work on nanostructured wearable electrochemical and biosensors are surveyed. This study aims to help researchers and engineers choose the most suitable selective and sensitive sensor. Wearable sensors have broad and effective sensing platforms, such as contact lenses, Google Glass, skin-patch, mouth gourds, smartwatches, underwear, wristbands, and others. For increasing sensor reliability, additional advancements in electrochemical and biosensor precision, stability in uncontrolled environments, and reproducible sample conveyance are necessary. In addition, the optimistic future of wearable electrochemical sensors in fields, such as remote and customized healthcare and well-being is discussed. Overall, wearable electrochemical and biosensing technologies hold great promise for improving personal healthcare and monitoring performance with the potential to have a significant impact on daily lives. These technologies enable real-time body sensing and the communication of comprehensive physiological information.