Chemiresistive Graphene Sensors for Ammonia Detection

Gas Sensors Based on Semiconductor Metal Oxides Fabricated by Electrospinning: A Review

Electrospinning has revolutionized the field of semiconductor metal oxide (SMO) gas sensors, which are pivotal for gas detection. SMOs are known for their high sensitivity, rapid responsiveness, and exceptional selectivity towards various types of gases. When synthesized via electrospinning, they gain unmatched advantages. These include high porosity, large specific surface areas, adjustable morphologies and compositions, and diverse structural designs, improving gas-sensing performance. This review explores the application of variously structured and composed SMOs prepared by electrospinning in gas sensors. It highlights strategies to augment gas-sensing performance, such as noble metal modification and doping with transition metals, rare earth elements, and metal cations, all contributing to heightened sensitivity and selectivity. We also look at the fabrication of composite SMOs with polymers or carbon nanofibers, which addresses the challenge of high operating temperatures. Furthermore, this review discusses the advantages of hierarchical and core-shell structures. The use of spinel and perovskite structures is also explored for their unique chemical compositions and crystal structure. These structures are useful for high sensitivity and selectivity towards specific gases. These methodologies emphasize the critical role of innovative material integration and structural design in achieving high-performance gas sensors, pointing toward future research directions in this rapidly evolving field.

Building a cm2scale CVD graphene-based gas sensor: modelling the kinetic with a three-site adsorption/desorption Langmuir model

In this article, we aim to develop and study a highly sensitive and selective cm2scale graphene-based gas sensor. We present the technology used to fabricate sensors which integrate monolayer chemical vapour deposition graphene: photolithography and transfer of layers. Characterization techniques (optical microscopy, AFM, micro-Raman spectroscopy, transport electrical measurements) ensure a diagnosis of graphene ribbons and allow good reproducibility of technological processes. We present the results of gas characterizations after a 200 ppm NO2exposure. We propose a novel approach for the modelling of the sensor response with a three-site adsorption/desorption Langmuir model. This innovative way of modelling the sensor response should provide a better understanding of the sensor's kinetic and help to overcome the long response time observed with graphene gas sensors.

Designing State-of-the-Art Gas Sensors: From Fundamentals to Applications

Gas sensors are crucial in environmental monitoring, industrial safety, and medical diagnostics. Due to the rising demand for precise and reliable gas detection, there is a rising demand for cutting-edge gas sensors that possess exceptional sensitivity, selectivity, and stability. Due to their tunable electrical properties, high-density surface-active sites, and significant surface-to-volume ratio, nanomaterials have been extensively investigated in this regard. The traditional gas sensors utilize homogeneous material for sensing where the adsorbed surface oxygen species play a vital role in their sensing activity. However, their performance for selective gas sensing is still unsatisfactory because the employed high temperature leads to the poor stability. The heterostructures nanomaterials can easily tune sensing performance and their different energy band structures, work functions, charge carrier concentration and polarity, and interfacial band alignments can be precisely designed for high-performance selective gas sensing at low temperature. In this review article, we discuss in detail the fundamentals of semiconductor gas sensing along with their mechanisms. Further, we highlight the existed challenges in semiconductor gas sensing. In addition, we review the recent advancements in semiconductor gas sensor design for applications from different perspective. Finally, the conclusion and future perspectives for improvement of the gas sensing performance are discussed.

Detection of PPB-Level H2S Concentrations in Exhaled Breath Using Au Nanosheet Sensors with Small Variability, High Selectivity, and Long-Term Stability

The continuous monitoring of hydrogen sulfide (H2S) in exhaled breath enables the detection of health issues such as halitosis and gastrointestinal problems. However, H2S sensors with high selectivity and parts per billion-level detection capability, which are essential for breath analysis, and facile fabrication processes for their integration with other devices are lacking. In this study, we demonstrated Au nanosheet H2S sensors with high selectivity, ppb-level detection capability, and high uniformity by optimizing their fabrication processes: (1) insertion of titanium nitride (TiN) as an adhesion layer to prevent Au agglomeration on the oxide substrate and (2) N2 annealing to improve nanosheet crystallinity. The fabricated Au nanosheets successfully detected H2S at concentrations as low as 5.6 ppb, and the estimated limit of detection was 0.5 ppb, which is superior to that of the human nose (8-13 ppb). In addition, the sensors detected H2S in the exhaled breath of simulated patients at concentrations as low as 175 ppb while showing high selectivity against interfering molecules, such as H2, alcohols, and humidity. Since Au nanosheets with uniform sensor characteristics enable easy device integration, the proposed sensor will be useful for facile health checkups based on breath analysis upon its integration into mobile devices.

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.

A Highly Sensitive Surface-Modified Porous Carbon Nanotube-Based Sensor for Ammonia Gas Detection

In this work, we compared the gas sensing behaviors of pristine and decorated multi-walled carbon nanotubes (MWCNTs) and examined the response behavior of bare and adorned MWCNTs in gas sensing. According to the data, the decorated response was 144%, which is higher than the bare CNT response of 117% in terms of the sensing response. The RF-sputtering method is used to decorate the carbon nanotubes by pure Indium (In) metal nanoparticles. Every measurement was performed in a temperature-controlled environment. Tests of the entire procedure were conducted at a 10 ppm concentration of ammonia gas. We have observed the quick reaction time (1-10 s) in pristine and (1-7 s) in decorated MWCNTs. The response was obtained 117% for the pristine and 144, 115, and 73% for the second (3 min decoration), third (6 min decoration), and fourth (9 min decoration) MWCNTs, respectively. The as-prepared pristine samples and all the decorated sensors had sensitivity values of 0.45, 0.50, 0.51, and 0.57 for time intervals of 0, 3, 6, and 9 min, respectively. It amounted to 45% for the pure and 50, 51, and 57% for the remaining as-prepared decorated sensors, respectively. Based on the measured sensor response graph, a recovery of between 80 and 85% was achieved. For a period of 10 days at a constant concentration, the stability was also assessed and we have analyzed the structural, electrical, and elemental composition of the prepared CNTs by FESEM, EDX, Raman spectroscopy, FTIR, and XRD.

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.

Machine-Learning-Aided NO2 Discrimination with an Array of Graphene Chemiresistors Covalently Functionalized by Diazonium Chemistry

Boosted by the emerging need for highly integrated gas sensors in the internet of things (IoT) ecosystems, electronic noses (e-noses) are gaining interest for the detection of specific molecules over a background of interfering gases. The sensing of nitrogen dioxide is particularly relevant for applications in environmental monitoring and precision medicine. Here we present an easy and efficient functionalization procedure to covalently modify graphene layers, taking advantage of diazonium chemistry. Separate graphene layers were functionalized with one of three different aryl rings: 4-nitrophenyl, 4-carboxyphenyl and 4-bromophenyl. The distinct modified graphene layers were assembled with a pristine layer into an e-nose for NO2 discrimination. A remarkable sensitivity to NO2 was demonstrated through exposure to gaseous solutions with NO2 concentrations in the 1-10 ppm range at room temperature. Then, the discrimination capability of the sensor array was tested by carrying out exposure to several interfering gases and analyzing the data through multivariate statistical analysis. This analysis showed that the e-nose can discriminate NO2 among all the interfering gases in a two-dimensional principal component analysis space. Finally, the e-nose was trained to accurately recognize NO2 contributions with a linear discriminant analysis approach, thus providing a metric for discrimination assessment with a prediction accuracy above 95 %.

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.

Scalable biomimetic sensing system with membrane receptor dual-monolayer probe and graphene transistor arrays

Affinity-based biosensing can enable point-of-care diagnostics and continuous health monitoring, which commonly follows bottom-up approaches and is inherently constrained by bioprobes' intrinsic properties, batch-to-batch consistency, and stability in biofluids. We present a biomimetic top-down platform to circumvent such difficulties by combining a "dual-monolayer" biorecognition construct with graphene-based field-effect-transistor arrays. The construct adopts redesigned water-soluble membrane receptors as specific sensing units, positioned by two-dimensional crystalline S-layer proteins as dense antifouling linkers guiding their orientations. Hundreds of transistors provide statistical significance from transduced signals. System feasibility was demonstrated with rSbpA-ZZ/CXCR4QTY-Fc combination. Nature-like specific interactions were achieved toward CXCL12 ligand and HIV coat glycoprotein in physiologically relevant concentrations, without notable sensitivity loss in 100% human serum. The construct is regeneratable by acidic buffer, allowing device reuse and functional tuning. The modular and generalizable architecture behaves similarly to natural systems but gives electrical outputs, which enables fabrication of multiplex sensors with tailored receptor panels for designated diagnostic purposes.

An ultra-sensitive NH3 gas sensor enabled by an ion-in-conjugated polycroconaine/Ti3C2Tx core-shell composite

MXenes are emerging sensing materials due to their metallic conductivity and rich surface chemistry for analytes; they, however, suffer from poor stability. Incorporation with functional polymers can largely prevent the performance decay and enhance the sensing performance. Herein, we demonstrate a core-shell composite, Ti₃C₂T_x@croconaine (poly(1,5-diaminonaphthalene-croconaine), PDAC) prepared by a facile in situ polymerization reaction, suitable for NH₃ detection. Compared to pristine Ti₃C₂T_x, the sensor made of a Ti₃C₂T_x-polycroconaine composite exhibits a significantly enhanced sensitivity of 2.8% ppm^-1 and an estimated achievable limit of detection of 50 ppb. The improved sensing performance could be attributed to the presence of PDAC facilitating the adsorption of NH₃ and changing the tunneling conductivity between Ti₃C₂T_x domains. Density functional theory (DFT) calculations reveal that the adsorption energy of NH₃ on PDAC is the highest among the tested gases, which supports the selectivity of the sensor to this analyte. Benefiting from the protection conferred by the PDAC shell, the composite has a reliable operation period of at least 40 days. In addition, we demonstrated a flexible paper-based sensor of the Ti₃C₂T_x@PDAC composite, without attenuated performance upon mechanical deformation. This work proposed a novel mechanism and a feasible methodology to synthesize MXene-polymer composites with improved sensitivity and stability for chemical sensing.

Solution-Processed Functionalized Graphene Film Prepared by Vacuum Filtration for Flexible NO2 Sensors

Large-scale production of graphene nanosheets (GNSs) has led to the availability of solution-processable GNSs on the commercial scale. The controlled vacuum filtration method is a scalable process for the preparation of wafer-scale films of GNSs, which can be used for gas sensing applications. Here, we demonstrate the use of this deposition method to produce functional gas sensors, using a chemiresistor structure from GNS solution-based techniques. The GNS suspension was prepared by liquid-phase exfoliation (LPE) and transferred to a polyvinylidene fluoride (PVDF) membrane. The effect of non-covalent functionalization with Co-porphyrin and Fe-phthalocyanines on the sensor properties was studied. The pristine and functionalized GNS films were characterized using different techniques such as Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and electrical characterizations. The morphological and spectroscopic analyses both confirm that the molecules (Co-porphyrin and Fe-phthalocyanine) were successfully adsorbed onto the GNSs surface through π-π interactions. The chemiresistive sensor response of functionalized GNSs toward the low concentrations of nitrogen dioxide (NO₂) (0.5-2 ppm) was studied and compared with those of the film of pristine GNSs. The tests on the sensing performance clearly showed sensitivity to a low concentration of NO₂ (5 ppm). Furthermore, the chemical modification of GNSs significantly improves NO₂ sensing performance compared to the pristine GNSs. The sensor response can be modulated by the type of adsorbed molecules. Indeed, Co-Por exhibited negative responsiveness (the response of Co-Por-GNS sensors and pristine GNS devices was 13.1% and 15.6%, respectively, after exposure to 0.5 ppm of NO₂). Meanwhile, Fe-Phc-GNSs induced the opposite behavior resulting in an increase in the sensor response (the sensitivity was 8.3% and 7.8% of Fe-Phc-GNSs and pristine GNSs, respectively, at 0.5 ppm NO₂ gas).

Gel Electrolytes for Electrochemical Actuators and Sensors Applications

Advanced functional materials, especially gel electrolytes, play a very important role in the preparation of electrochemical actuators and sensors, and have received extensive attention. In this review, a general classification of gel electrolytes is firstly introduced according to the type of medium. Then, the research progress of gel electrolytes with different types used to fabricate electrochemical actuators is summarized. Next, the current research progress of gel electrolytes used in different types of electrochemical sensors, including strain sensors, stress sensors, and gas sensors is introduced. Finally, the future challenges and development prospects of electrochemical actuators and sensors based on gel electrolytes are discussed. The huge application prospects of gel electrolyte are worthy of further focusing by researchers, which will have an indispensable impact on human life and development.

A Chemiresistor Sensor Array Based on Graphene Nanostructures: From the Detection of Ammonia and Possible Interfering VOCs to Chemometric Analysis

Sensor arrays are currently attracting the interest of researchers due to their potential of overcoming the limitations of single sensors regarding selectivity, required by specific applications. Among the materials used to develop sensor arrays, graphene has not been so far extensively exploited, despite its remarkable sensing capability. Here we present the development of a graphene-based sensor array prepared by dropcasting nanostructure and nanocomposite graphene solution on interdigitated substrates, with the aim to investigate the capability of the array to discriminate several gases related to specific applications, including environmental monitoring, food quality tracking, and breathomics. This goal is achieved in two steps: at first the sensing properties of the array have been assessed through ammonia exposures, drawing the calibration curves, estimating the limit of detection, which has been found in the ppb range for all sensors, and investigating stability and sensitivity; then, after performing exposures to acetone, ethanol, 2-propanol, sodium hypochlorite, and water vapour, chemometric tools have been exploited to investigate the discrimination capability of the array, including principal component analysis (PCA), linear discriminant analysis (LDA), and Mahalanobis distance. PCA shows that the array was able to discriminate all the tested gases with an explained variance around 95%, while with an LDA approach the array can be trained to accurately recognize unknown gas contribution, with an accuracy higher than 94%.

Targeting biomarkers in the gas phase through a chemoresistive electronic nose based on graphene functionalized with metal phthalocyanines

Electronic noses (e-noses) have received considerable interest in the past decade as they can match the emerging needs of modern society such as environmental monitoring, health screening, and food quality tracking. For practical applications of e-noses, it is necessary to collect large amounts of data from an array of sensing devices that can detect interactions with molecules reliably and analyze them via pattern recognition. The use of graphene (Gr)-based arrays of chemiresistors in e-noses is still virtually missing, though recent reports on Gr-based chemiresistors have disclosed high sensing performances upon functionalization of the pristine layer, opening up the possibility of being implemented into e-noses. In this work, with the aim of creating a robust and chemically stable interface that combines the chemical properties of metal phthalocyanines (M-Pc, M = Fe, Co, Ni, Zn) with the superior transport properties of Gr, an array of Gr-based chemiresistor sensors functionalized with drop-cast M-Pc thin layers has been developed. The sensing capability of the array was tested towards biomarkers for breathomics application, with a focus on ammonia (NH₃). Exposure to NH₃ has been carried out drawing the calibration curve and estimating the detection limit for all the sensors. The discrimination capability of the array has then been tested, carrying out exposure to several gases (hydrogen sulfide, acetone, ethanol, 2-propanol, water vapour and benzene) and analysing the data through principal component analysis (PCA). The PCA pattern recognition results show that the developed e-nose is able to discriminate all the tested gases through the synergic contribution of all sensors.

Recent Advances in Graphene-Based Nanocomposites for Ammonia Detection

The increasing demand to mitigate the alarming effects of the emission of ammonia (NH3) on human health and the environment has highlighted the growing attention to the design of reliable and effective sensing technologies using novel materials and unique nanocomposites with tunable functionalities. Among the state-of-the-art ammonia detection materials, graphene-based polymeric nanocomposites have gained significant attention. Despite the ever-increasing number of publications on graphene-based polymeric nanocomposites for ammonia detection, various understandings and information regarding the process, mechanisms, and new material components have not been fully explored. Therefore, this review summarises the recent progress of graphene-based polymeric nanocomposites for ammonia detection. A comprehensive discussion is provided on the various gas sensor designs, including chemiresistive, Quartz Crystal Microbalance (QCM), and Field-Effect Transistor (FET), as well as gas sensors utilising the graphene-based polymer nanocomposites, in addition to highlighting the pros and cons of graphene to enhance the performance of gas sensors. Moreover, the various techniques used to fabricate graphene-based nanocomposites and the numerous polymer electrolytes (e.g., conductive polymeric electrolytes), the ion transport models, and the fabrication and detection mechanisms of ammonia are critically addressed. Finally, a brief outlook on the significant progress, future opportunities, and challenges of graphene-based polymer nanocomposites for the application of ammonia detection are presented.

Machine Learning-Based Rapid Detection of Volatile Organic Compounds in a Graphene Electronic Nose

Rapid detection of volatile organic compounds (VOCs) is growing in importance in many sectors. Noninvasive medical diagnoses may be based upon particular combinations of VOCs in human breath; detecting VOCs emitted from environmental hazards such as fungal growth could prevent illness; and waste could be reduced through monitoring of gases produced during food storage. Electronic noses have been applied to such problems, however, a common limitation is in improving selectivity. Graphene is an adaptable material that can be functionalized with many chemical receptors. Here, we use this versatility to demonstrate selective and rapid detection of multiple VOCs at varying concentrations with graphene-based variable capacitor (varactor) arrays. Each array contains 108 sensors functionalized with 36 chemical receptors for cross-selectivity. Multiplexer data acquisition from 108 sensors is accomplished in tens of seconds. While this rapid measurement reduces the signal magnitude, classification using supervised machine learning (Bootstrap Aggregated Random Forest) shows excellent results of 98% accuracy between 5 analytes (ethanol, hexanal, methyl ethyl ketone, toluene, and octane) at 4 concentrations each. With the addition of 1-octene, an analyte highly similar in structure to octane, an accuracy of 89% is achieved. These results demonstrate the important role of the choice of analysis method, particularly in the presence of noisy data. This is an important step toward fully utilizing graphene-based sensor arrays for rapid gas sensing applications from environmental monitoring to disease detection in human breath.

A Flexible and Attachable Colorimetric Film Sensor for the Detection of Gaseous Ammonia

A cost-effective, simple, flexible, and disposable colorimetric film sensor was constructed for the rapid detection of gaseous ammonia. The sensor was designed to consist of three layers, namely top, middle, and bottom layers of a polymeric elastomer. The bromocresol (BCG) indicator embedded in the middle layer of the film facilitated a change in color of the sensor from yellow-orange to blue upon exposure to gaseous ammonia. The color change was visually observed by the naked eye. The sensitivity of the sensor was verified by a successful detection of gaseous ammonia at concentrations from 4 to 235 ppm within 3 min, and the corresponding visual detection of ammonia gas was at a concentration as low as 11 ppm. The sensor also achieved a selective detection of gaseous ammonia over a variety of alkaline chemicals. The color of the sensor exposed to ammonia reverted from blue to the original yellow-orange upon subsequent exposure to the fume of acetic acid or aeration for 48 h, and it showed reliable performance for the detection of gaseous ammonia even after five repeated uses. The applicability of the sensor was validated by attaching it onto a safety helmet for a simulation of an industrial ammonia gas leak. The results indicated that our colorimetric film sensor is affordable, disposable, and reproducible, and can serve as an effective alternative for simple and rapid recognition of gaseous ammonia in environmental and air quality monitoring as well as in industrial applications.

Research Progress on the Preparation and Applications of Laser-Induced Graphene Technology

Graphene has been regarded as a potential application material in the field of new energy conversion and storage because of its unique two-dimensional structure and excellent physical and chemical properties. However, traditional graphene preparation methods are complicated in-process and difficult to form patterned structures. In recent years, laser-induced graphene (LIG) technology has received a large amount of attention from scholars and has a wide range of applications in supercapacitors, batteries, sensors, air filters, water treatment, etc. In this paper, we summarized a variety of preparation methods for graphene. The effects of laser processing parameters, laser type, precursor materials, and process atmosphere on the properties of the prepared LIG were reviewed. Then, two strategies for large-scale production of LIG were briefly described. We also discussed the wide applications of LIG in the fields of signal sensing, environmental protection, and energy storage. Finally, we briefly outlined the future trends of this research direction.

Critical review and recent advances of 2D materials-Based gas sensors for food spoilage detection

Many people around the world are concerned about meat safety and quality, which has resulted in the ongoing advancement of packaged food technology. Since the emergence of graphene in 2004, the number of studies on layered two-dimensional materials (2DMs) for applications ranging from food packaging to meat quality monitoring has been expanding quickly. Recently, scientists have been working hard to develop a novel class of 2DMs that keep the good things about graphene but don't have zero bandgaps at room temperature. Much work has been done on layered transition metal dichalcogenides (TMDCs) like different metal sulfides and selenides for meat spoilage gas sensors. This review looks at (i) the main indicators of meat spoilage and (ii) the detection methods that can be used to find out if meat has been spoiled, such as chemiresistive, electrochemical, and optical methods. (iii) the role of 2DMs in meat spoilage detection and (iv) the emergence of advanced methods for selective classification of target analytes in meat/food spoilage detection in recent years. Thus, this review demonstrates the potential scope of 2DMs for developing intelligent sensor systems for food and meat spoilage detection with high viability, simplicity, cost-effectiveness, and other multipurpose tools.

Reduction of the Measurement Time of a Chemiresistive Gas Sensor Using Transient Analysis and the Cantor Pairing Function

It is vital to measure the concentration of gas quickly in many gas sensing applications. Predicting the steady-state response from the earlier transient response is the economical and viable solution in this regard. However, existing transient analysis approaches either need huge data and computationally intensive algorithms or are inefficient. Here, we described a method to reduce the measurement time of the concentration of CH₄ with a chemiresistive gas sensor at room temperature (27 °C). The presented method considers the sensor's response at two fixed time intervals after gas exposure and maps their pairing number to the gas concentration. The proposed method measures the gas concentration in just 30 s from the gas exposure time. As the proposed method can quickly measure gas concentrations, it can be employed in widespread applications where quick quantification of gas is necessary.

Theoretical Approach to Evaluate the Gas-Sensing Performance of Graphene Nanoribbon/Oligothiophene Composites

Composite formation with graphene is an effective approach to increase the sensitivity of polythiophene (nPT) gas sensors. The interaction mechanism between gaseous analytes and graphene/nPT composite systems is still not clear, and density functional theory calculations are used to explore the interaction mechanism between graphene/nPT nanoribbon composites (with n = 3-9 thiophene units) and gaseous analytes CO, NH₃, SO₂, and NO₂. For the studied analytes, the interaction energy ranges from -44.28 kcal/mol for (C₅₄H₃₀-3PT)-NO₂ to -2.37 kcal/mol for (C₅₄H₃₀-3PT)-CO at the counterpoise-corrected ωB97M-V/def2-TZVPD level of theory. The sensing mechanism is further evaluated by geometric analysis, ultraviolet-visible spectroscopy, density of-states analysis, calculation of global reactivity indices, and both frontier and natural bond orbital analyses. The variation in the highest occupied molecular orbital/lowest unoccupied molecular orbital gap of the composite indicates the change in conductivity upon complexation with the analyte. Energy decomposition analysis reveals that dispersion and charge transfer make the largest contributions to the interaction energy. The graphene/oligothiophene composite is more sensitive toward these analytes than either component taken alone due to larger changes in the orbital gap. The computational framework established in the present work can be used to evaluate and design graphene/nPT nanoribbon composite materials for gas sensors.

Physical and Chemical Sensors on the Basis of Laser-Induced Graphene: Mechanisms, Applications, and Perspectives

Laser-induced graphene (LIG) is produced rapidly by directly irradiating carbonaceous precursors, and it naturally exhibits as a three-dimensional porous structure. Due to advantages such as simple preparation, time-saving, environmental friendliness, low cost, and expanding categories of raw materials, LIG and its derivatives have achieved broad applications in sensors. This has been witnessed in various fields such as wearable devices, disease diagnosis, intelligent robots, and pollution detection. However, despite LIG sensors having demonstrated an excellent capability to monitor physical and chemical parameters, the systematic review of synthesis, sensing mechanisms, and applications of them combined with comparison against other preparation approaches of graphene is still lacking. Here, graphene-based sensors for physical, biological, and chemical detection are reviewed first, followed by the introduction of general preparation methods for the laser-induced method to yield graphene. The preparation and advantages of LIG, sensing mechanisms, and the properties of different types of emerging LIG-based sensors are comprehensively reviewed. Finally, possible solutions to the problems and challenges of preparing LIG and LIG-based sensors are proposed. This review may serve as a detailed reference to guide the development of LIG-based sensors that possess properties for future smart sensors in health care, environmental protection, and industrial production.

Graphene Biodevices for Early Disease Diagnosis Based on Biomarker Detection

The early diagnosis of diseases plays a vital role in healthcare and the extension of human life. Graphene-based biosensors have boosted the early diagnosis of diseases by detecting and monitoring related biomarkers, providing a better understanding of various physiological and pathological processes. They have generated tremendous interest, made significant advances, and offered promising application prospects. In this paper, we discuss the background of graphene and biosensors, including the properties and functionalization of graphene and biosensors. Second, the significant technologies adopted by biosensors are discussed, such as field-effect transistors and electrochemical and optical methods. Subsequently, we highlight biosensors for detecting various biomarkers, including ions, small molecules, macromolecules, viruses, bacteria, and living human cells. Finally, the opportunities and challenges of graphene-based biosensors and related broad research interests are discussed.

Computational modelling of ammonia addition on partially reduced graphene oxide flakes

Density functional theory is employed to model the chemisorption of ammonia on epoxy-containing polycyclic aromatic hydrocarbons (PAHs) and understand the reaction mechanism of ammonia addition on partially reduced graphene oxide flakes. Coronene (C₂₄H₁₂) and ovalene (C₃₂H₁₄) based four-epoxy group containing molecules are used to mimic the RGO surface properties. The reaction mechanism changing effect of a second ammonia molecule as well as explicit water molecules is considered. The proposed reaction mechanism consists of two steps: the migration of one epoxy group out of the modelled four-epoxy group formation to a thermodynamically less stable one and the nucleophilic addition of the ammonia molecule. The second step involves forming an amine group and reducing an epoxy group to a hydroxyl one. Interestingly, the forming amine group bonds to the carbon atom with the smallest bond order among the available ones and not necessarily to the carbon atom of the opening epoxy ring. Incorporating a second ammonia molecule has a negligible effect on the overall reaction mechanism, while in the presence of one water molecule, the reaction goes through a different pathway involving a trimolecular state during the nucleophilic addition. Including more than one water molecule or applying an implicit solvent model does not cause further changes in the reaction.

Wireless Self-Powered High-Performance Integrated Nanostructured-Gas-Sensor Network for Future Smart Homes

The accelerated evolution of communication platforms including Internet of Things (IoT) and the fifth generation (5G) wireless communication network makes it possible to build intelligent gas sensor networks for real-time monitoring chemical safety and personal health. However, this application scenario requires a challenging combination of characteristics of gas sensors including small formfactor, low cost, ultralow power consumption, superior sensitivity, and high intelligence. Herein, self-powered integrated nanostructured-gas-sensor (SINGOR) systems and a wirelessly connected SINGOR network are demonstrated here. The room-temperature operated SINGOR system can be self-driven by indoor light with a Si solar cell, and it features ultrahigh sensitivity to H₂, formaldehyde, toluene, and acetone with the record low limits of detection (LOD) of 10, 2, 1, and 1 ppb, respectively. Each SINGOR consisting of an array of nanostructured sensors has the capability of gas pattern recognition and classification. Furthermore, multiple SINGOR systems are wirelessly connected as a sensor network, which has successfully demonstrated flammable gas leakage detection and alarm function. They can also achieve gas leakage localization with satisfactory precision when deployed in one single room. These successes promote the development of using nanostructured-gas-sensor network for wide range applications including smart home/building and future smart city.

Metal-Oxide Based Nanomaterials: Synthesis, Characterization and Their Applications in Electrical and Electrochemical Sensors

Pure, mixed and doped metal oxides (MOX) have attracted great interest for the development of electrical and electrochemical sensors since they are cheaper, faster, easier to operate and capable of online analysis and real-time identification. This review focuses on highly sensitive chemoresistive type sensors based on doped-SnO₂, RhO, ZnO-Ca, Sm_x-CoFe_2−xO₄ semiconductors used to detect toxic gases (H₂, CO, NO₂) and volatile organic compounds (VOCs) (e.g., acetone, ethanol) in monitoring of gaseous markers in the breath of patients with specific pathologies and for environmental pollution control. Interesting results about the monitoring of biochemical substances as dopamine, epinephrine, serotonin and glucose have been also reported using electrochemical sensors based on hybrid MOX nanocomposite modified glassy carbon and screen-printed carbon electrodes. The fundamental sensing mechanisms and commercial limitations of the MOX-based electrical and electrochemical sensors are discussed providing research directions to bridge the existing gap between new sensing concepts and real-world analytical applications.

Design and fabrication of green and sustainable vapochromic cellulose fibers embedded with natural anthocyanin for detection of toxic ammonia

Exposure to colorless ammonia gas may lead to damage in human organs or even death. Herein, we describe facile fabrication of an environmentally-friendly, portable, reversible, and sensitive solid-state colorimetric cellulose (Cell)/anthocyanin (Anth) vapochromic sensor that exhibits instant visual color change to both gaseous and aqueous phases of ammonia. The naturally occurring anthocyanin can be easily extracted from the red-cabbage plant and applied as a direct dyestuff onto viscose fibers in the presence of potassium aluminum sulfate as mordant to generate nanoparticles of mordant/anthocyanin coordinated complex. Thus, upon exposure to aqueous ammonia, an instant color change of the smart (Cell-Anth) diagnostic assays, from purple to colorless, was noted and quantitatively probed via both CIE Lab coordinates and UV-Vis spectral measurements. Importantly, the fabricated (Cell-Anth) viscose fabric showed rapid responses, fraction of second, with a good limit of detection (LOD) in the range of 200-1200 mg L^-1. This receptor also demonstrated ratiometric changes in the UV-Vis absorbance spectra, giving a hypsochromic shift from 611 to 375 nm upon increasing the total content of ammonia in an aqueous media. The morphologies of Cell-Anth fabrics as well as particle size of the generated mordant/dye complex on the fabric surface have been characterized by transmission electron microscopic (TEM), scan electron microscopy (SEM), energy-dispersive X-ray patterns (EDX) and Fourier-transform infrared spectroscopic (FT-IR). The comfortability of the dyed cellulose fibers was also investigated in terms of their bend length, air-permeability and colorfastness properties. Significantly, the present study offers a promising onsite vapochromic device that enables detection of ammonia in either aqueous or gas phase in various environments and products.

A Review on Functionalized Graphene Sensors for Detection of Ammonia

Since the first graphene gas sensor has been reported, functionalized graphene gas sensors have already attracted a lot of research interest due to their potential for high sensitivity, great selectivity, and fast detection of various gases. In this paper, we summarize the recent development and progression of functionalized graphene sensors for ammonia (NH3) detection at room temperature. We review graphene gas sensors functionalized by different materials, including metallic nanoparticles, metal oxides, organic molecules, and conducting polymers. The various sensing mechanism of functionalized graphene gas sensors are explained and compared. Meanwhile, some existing challenges that may hinder the sensor mass production are discussed and several related solutions are proposed. Possible opportunities and perspective applications of the graphene NH3 sensors are also presented.

Recent Advances in Ammonia Gas Sensors Based on Carbon Nanomaterials

This review paper is devoted to an extended analysis of ammonia gas sensors based on carbon nanomaterials. It provides a detailed comparison of various types of active materials used for the detection of ammonia, e.g., carbon nanotubes, carbon nanofibers, graphene, graphene oxide, and related materials. Different parameters that can affect the performance of chemiresistive gas sensors are discussed. The paper also gives a comparison of the sensing characteristics (response, response time, recovery time, operating temperature) of gas sensors based on carbon nanomaterials. The results of our tests on ammonia gas sensors using various techniques are analyzed. The problems related to the recovery of sensors using various approaches are also considered. Finally, the impact of relative humidity on the sensing behavior of carbon nanomaterials of various different natures was estimated.

Graphene for Biosensing Applications in Point-of-Care Testing

Graphene and graphene-related materials (GRMs) exhibit a unique combination of electronic, optical, and electrochemical properties, which make them ideally suitable for ultrasensitive and selective point-of-care testing (POCT) devices. POCT device-based applications in diagnostics require test results to be readily accessible anywhere to produce results within a short analysis timeframe. This review article provides a summary of methods and latest developments in the field of graphene and GRM-based biosensing in POCT and an overview of the main applications of the latter in nucleic acids and enzymatic biosensing, cell detection, and immunosensing. For each application, we discuss scientific and technological advances along with the remaining challenges, outlining future directions for widespread use of this technology in biomedical applications.

Chemoresistive Room-Temperature Sensing of Ammonia Using Zeolite Imidazole Framework and Reduced Graphene Oxide (ZIF-67/rGO) Composite

The present work demonstrates the application of a composite of the zeolite imidazole framework (ZIF-67) and reduced graphene oxide (rGO), synthesized via a simple hydrothermal route for the sensitive sensing of ammonia. The successful synthesis of ZIF-67 and rGO composite was confirmed with structural and spectroscopic characterizations. A porous structure and a high surface area (1080 m2 g-1) of the composite indicate its suitability as a gas sensing material. The composite material was coated as a thin film onto interdigitated gold electrodes. The sensor displays a change in its chemoresistive property (i.e., resistance) in the presence of ammonia (NH3) gas. A sensor response of 1.22 ± 0.02 [standard deviation (sd)] is measured for 20 ppm of NH3, while it shows a value of 4.77 ± 0.15 (sd) for 50 ppm of NH3. The fabricated sensor is reproducible and offers a stable response, while also providing tolerance against humidity and some other volatile compounds. The average response and recovery times of the sensor, for 50 ppm NH3 concentration, are found to be 46.5 ± 2.12 (sd) and 66.5 ± 2.12 (sd) s, respectively. The limit of detection of the sensor was found to be 74 ppb.

Mechanistic Insight into the Limiting Factors of Graphene-Based Environmental Sensors

Graphene has demonstrated great promise for technological use, yet control over material growth and understanding of how material imperfections affect the performance of devices are challenges that hamper the development of applications. In this work, we reveal new insight into the connections between the performance of the graphene devices as environmental sensors and the microscopic details of the interactions at the sensing surface. We monitor changes in the resistance of the chemical-vapor deposition grown graphene devices as exposed to different concentrations of ethanol. We perform thermal surface treatments after the devices are fabricated, use scanning probe microscopy to visualize their effects down to nanometer scale and correlate them with the measured performance of the device as an ethanol sensor. Our observations are compared to theoretical calculations of charge transfers between molecules and the graphene surface. We find that, although often overlooked, the surface cleanliness after device fabrication is responsible for the device performance and reliability. These results further our understanding of the mechanisms of sensing in graphene-based environmental sensors and pave the way to optimizing such devices, especially for their miniaturization, as with decreasing size of the active zone the potential role of contaminants will rise.

Functionalized Graphene Surfaces for Selective Gas Sensing

Environmental monitoring through gas sensors is paramount for the safety and security of industrial workers and for ecological protection. Graphene is among the most promising materials considered for next-generation gas sensing due to its properties such as mechanical strength and flexibility, high surface-to-volume ratio, large conductivity, and low electrical noise. While gas sensors based on graphene devices have already demonstrated high sensitivity, one of the most important figures of merit, selectivity, remains a challenge. In the past few years, however, surface functionalization emerged as a potential route to achieve selectivity. This review surveys the recent advances in the fabrication and characterization of graphene and reduced graphene oxide gas sensors chemically functionalized with aromatic molecules and polymers with the goal of improving selectivity toward specific gases as well as overall sensor performance.

Recent developments in carbon-based two-dimensional materials: synthesis and modification aspects for electrochemical sensors

This review (162 references) focuses on two-dimensional carbon materials, which include graphene as well as its allotropes varying in size, number of layers, and defects, for their application in electrochemical sensors. Many preparation methods are known to yield two-dimensional carbon materials which are often simply addressed as graphene, but which show huge variations in their physical and chemical properties and therefore on their sensing performance. The first section briefly reviews the most promising as well as the latest achievements in graphene synthesis based on growth and delamination techniques, such as chemical vapor deposition, liquid phase exfoliation via sonication or mechanical forces, as well as oxidative procedures ranging from chemical to electrochemical exfoliation. Two-dimensional carbon materials are highly attractive to be integrated in a wide field of sensing applications. Here, graphene is examined as recognition layer in electrochemical sensors like field-effect transistors, chemiresistors, impedance-based devices as well as voltammetric and amperometric sensors. The sensor performance is evaluated from the material's perspective of view and revealed the impact of structure and defects of the 2D carbon materials in different transducing technologies. It is concluded that the performance of 2D carbon-based sensors is strongly related to the preparation method in combination with the electrical transduction technique. Future perspectives address challenges to transfer 2D carbon-based sensors from the lab to the market. Graphical abstract Schematic overview from synthesis and modification of two-dimensional carbon materials to sensor application.

Hydrogel-Based Gas Sensors for NO2 and NH3

In this study, an innovative gas sensing mechanism, self-responsive sensing mechanism, has been detected in the supramolecular hydrogel-based sensors. The self-responsive ability of as-fabricated hydrogel-based sensors to the target gas (e.g., NO₂, NH₃, etc.) is determined by three synergetic supramolecular interactions, namely, hydrogen bonding, molecule crystallization, and electrostatic interactions existing in hydroxyls, poly(vinyl alcohol) (PVA) crystallization, and poly(ionic liquids) of the intrinsic hydrogel networks, respectively. On account of unique synergetic supramolecular interactions, the sensors not only exhibit a rapid, reversible, and reproducible response but also show good tensile and compressive properties and excellent recovery property. The results demonstrate the potential of the self-responsive sensing mechanism as a pathway to realize a new generation of highly responsive hydrogel-based gas sensors.

The Role of Functionalization in the Applications of Carbon Materials: An Overview

The carbon-based materials (CbMs) refer to a class of substances in which the carbon atoms can assume different hybridization states (sp1, sp2, sp3) leading to different allotropic structures -. In these substances, the carbon atoms can form robust covalent bonds with other carbon atoms or with a vast class of metallic and non-metallic elements, giving rise to an enormous number of compounds from small molecules to long chains to solids. This is one of the reasons why the carbon chemistry is at the basis of the organic chemistry and the biochemistry from which life on earth was born. In this context, the surface chemistry assumes a substantial role dictating the physical and chemical properties of the carbon-based materials. Different functionalities are obtained by bonding carbon atoms with heteroatoms (mainly oxygen, nitrogen, sulfur) determining a certain reactivity of the compound which otherwise is rather weak. This holds for classic materials such as the diamond, the graphite, the carbon black and the porous carbon but functionalization is widely applied also to the carbon nanostructures which came at play mainly in the last two decades. As a matter of fact, nowadays, in addition to fabrication of nano and porous structures, the functionalization of CbMs is at the basis of a number of applications as catalysis, energy conversion, sensing, biomedicine, adsorption etc. This work is dedicated to the modification of the surface chemistry reviewing the different approaches also considering the different macro and nano allotropic forms of carbon.

Nature of the Synergistic Effect of N and S Co-Doped Graphene for the Enhanced Simultaneous Determination of Toxic Pollutants

N-doped graphene (NG), S-doped graphene (SG), and N and S co-doped graphene nanocatalysts with different doping sequences (N-SG and S-NG) are successfully synthesized by a facile low-temperature hydrothermal method. By changing the synthetic sequence, S-NG significantly increases the electron transport rate of the sensor and the electrocatalytic ability compared to NG, SG, and N-SG due to the optimal proportion of doping element content and suitable N- and S-bonding configurations. The origin of the synergistic effect of N and S co-doped graphene is confirmed. Traces of S doping greatly enhance the electrochemical performance. The large volume of S-O_x groups may prevent the analytes from approaching the catalytic sites of the sensing materials due to a steric hindrance effect. S-NG, which possesses less S-O_x groups, exhibits better performance than N-SG. Pyridinic N plays an important role in enhancing the electrochemical activity and conductivity. The simultaneous determination of aniline (AN), p-phenylenediamine (PPD), and nitrobenzene (NB) as typical toxic pollutants is performed by employing the S-NG nanoarchitecture. The detection limits (S/N = 3) for AN, PPD, and NB are 0.023, 0.051, and 0.216 μM, respectively. In addition, the S-NG sensors also have excellent anti-interference, stability, and reproducibility. The precise control and synthesis of multiheteroatoms into graphene represent a promising strategy to enhance the electrocatalytic performance in energy and environmental fields.

Waterproof molecular monolayers stabilize 2D materials

Two-dimensional van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines n-C _m H_2m+1NH₂, with m = 4 through 11, are highly effective in protecting the optoelectronic properties of these materials, such as black phosphorus (BP) and transition-metal dichalcogenides (TMDs: WS₂, 1T'-MoTe₂, WTe₂, WSe₂, TaS₂, and NbSe₂). As a representative example, n-hexylamine (m = 6) can be applied in the form of thin molecular monolayers on BP flakes with less than 2-nm thickness and can prolong BP's lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, H₂ annealing, and organic solvents, but can be removed by certain organic acids.

Naphthyl End-Capped Terthiophene-Based Chemiresistive Sensors for Biogenic Amine Detection and Meat Spoilage Monitoring

A reliable and sensitive detection of biogenic amines (BAs) is essential to ensure food safety and maintain public health. In this study, two naphthyl end-capped terthiophene derivatives, namely, 5-(naphthalen-1-yl)-2,2':5',2''-terthiophene (NA-3T) and 5,5''-di(naphthalen-1-yl)-2,2':5',2''-terthiophene (NA-3T-NA), were employed to develop chemiresistive sensors for detecting gaseous BAs. In contrast to NA-3T, the NA-3T-NA-based sensor showed a higher sensitivity for trimethylamine (TMA) with an experimental detection limit lower than 22 ppm, and for aromatic BAs, including dopamine, histamine, tryptamine, and tyramine. Additionally, the recovery time for TMA was found to be shorter than 23 s. In addition, both sensors were successfully used for an in situ evaluation of meat freshness by monitoring the concentration of relevant volatile BAs. The difference in the sensing performances of the two chemiresistive sensors was tentatively ascribed to different packing structures of the derivatives and the adlayer structures of the films developed with the compounds.

Laser-Induced Graphene for Flexible and Embeddable Gas Sensors

Laser-induced graphene (LIG) has received much attention since it enables simple and rapid synthesis of porous graphene. This work presents a robust direct-write LIG-based gas sensor, which senses gases based on thermal conductivity, similar to a katharometer sensor. The gas sensors are fabricated by lasing polyimide substrates with a 10.6 μm CO₂ laser to synthesize LIG. This enables the formation of flexible gas sensors which could be incorporated on a variety of surfaces. High surface area and thermal conductivity of the LIG results in rapid response times for all studied gases. The gas sensors are also embedded in cement to form a refractory composite material. These sensors are used to determine composition of various gas mixtures, such as N₂ and CO₂, which are the most abundant gaseous species in flue gas. Thus, LIG based embeddable sensors could be incorporated in composites to enable electronically functional construction materials.

Mixed-Valence Cobalt(II/III) Metal-Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH₃) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt(II/III) metal-organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)amine (H₃L) ligand was synthesized for colorimetric sensing toward ammonia. The activated FJU-56 demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH₃ or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the FJU-56 framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing.