High-performance gas sensors based on single-wall carbon nanotube random networks for the detection of nitric oxide down to the ppb-level

Low Part-Per-Trillion, Humidity Resistant Detection of Nitric Oxide Using Microtoroid Optical Resonators

The nitric oxide radical plays pivotal roles in physiological as well as atmospheric contexts. Although the detection of dissolved nitric oxide in vivo has been widely explored, highly sensitive (i.e., low part-per-trillion level), selective, and humidity-resistant detection of gaseous nitric oxide in air remains challenging. In the field, humidity can have dramatic effects on the accuracy and selectivity of gas sensors, confounding data, and leading to overestimation of gas concentration. Highly selective and humidity-resistant gaseous NO sensors based on laser-induced graphene were recently reported, displaying a limit of detection (LOD) of 8.3 ppb. Although highly sensitive (LOD = 590 ppq) single-wall carbon nanotube NO sensors have been reported, these sensors lack selectivity and humidity resistance. In this report, we disclose a highly sensitive (LOD = 2.34 ppt), selective, and humidity-resistant nitric oxide sensor based on a whispering-gallery mode microtoroid optical resonator. Excellent analyte selectivity was enabled via novel ferrocene-containing polymeric coatings synthesized via reversible addition-fragmentation chain-transfer polymerization. Utilizing a frequency locked optical whispering evanescent resonator system, the microtoroid's real-time resonance frequency shift response to nitric oxide was tracked with subfemtometer resolution. The lowest concentration experimentally detected was 6.4 ppt, which is the lowest reported to date. Additionally, the performance of the sensor remained consistent across different humidity environments. Lastly, the impact of the chemical composition and molecular weight of the novel ferrocene-containing polymeric coatings on sensing performance was evaluated. We anticipate that our results will have impact on a wide variety of fields where NO sensing is important such as medical diagnostics through exhaled breath, determination of planetary habitability, climate change, air quality monitoring, and treating cardiovascular and neurological disorders.

Room-Temperature Nitric Oxide Gas Sensors Based on NiO/SnO2 Heterostructures

Nitric oxide (NO) is a very well-known indoor pollutant, and high concentrations of it in the atmosphere lead to acid rain. Thus, there is great demand for NO sensors that have the ability to work at room temperature. In this work, NiO/SnO2 heterostructures have been prepared via the polyol process and were tested against different concentrations of NO gas at room temperature. The structural and morphological characteristics of the heterostructures were examined using X-ray diffraction and scanning electron microscopy, respectively, while the ratio of NiO to SnO2 was determined through the use of energy-dispersive spectrometry. The effects of both pH and thermal annealing on the morphological, structural and gas-sensing properties of the heterostructure were investigated. It was found that the morphology of the heterostructures consisted of rod-like particles with different sizes, depending on the temperature of thermal annealing. Moreover, NiO/SnO2 heterostructures synthesized with pH = 8 and annealed at 900 °C showed a response of 1.8% towards 2.5 ppm NO at room temperature. The effects of humidity as well as of stability on the gas sensing performance were also investigated.

Polyethyleneimine-Starch Functionalization of Single-Walled Carbon Nanotubes for Carbon Dioxide Sensing at Room Temperature

There is an ever-growing interest in the detection of carbon dioxide (CO₂) due to health risks associated with CO₂ emissions. Hence, there is a need for low-power and low-cost CO₂ sensors for efficient monitoring and sensing of CO₂ analyte molecules in the environment. This study reports on the synthesis of single-walled carbon nanotubes (SWCNTs) that are functionalized using polyethyleneimine and starch (PEI-starch) in order to fabricate a PEI-starch functionalized SWCNT sensor for reversible CO₂ detection under ambient room conditions (T = 25 °C; RH = 53%). Field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy are used to analyze the physiochemical properties of the as-synthesized gas sensor. Due to the large specific surface area of SWCNTs and the efficient CO₂ capturing capabilities of the amine-rich PEI layer, the sensor possesses a high CO₂ adsorption capacity. When exposed to varying CO₂ concentrations between 50 and 500 ppm, the sensor response exhibits a linear relationship with an increase in analyte concentration, allowing it to operate reliably throughout a broad range of CO₂ concentrations. The sensing mechanism of the PEI-starch-functionalized SWCNT sensor is based on the reversible acid-base equilibrium chemical reactions between amino groups of PEI and adsorbed CO₂ molecules, which produce carbamates and bicarbonates. Due to the presence of hygroscopic starch that attracts more water molecules to the surface of SWCNTs, the adsorption capacity of CO₂ gas molecules is enhanced. After multiple cycles of analyte exposure, the sensor recovers to its initial resistance level via a UV-assisted recovery approach. In addition, the sensor exhibits great stability and reliability in multiple analyte gas exposures as well as excellent selectivity to carbon dioxide over other interfering gases such as carbon monoxide, oxygen, and ammonia, thereby showing the potential to monitor CO₂ levels in various infrastructure.

Enhanced Nitric Oxide Sensing Performance of Conjugated Polymer Films through Incorporation of Graphitic Carbon Nitride

Organic field-effect transistor (OFET) gas sensors based on conjugated polymer films have recently attracted considerable attention for use in environmental monitoring applications. However, the existing devices are limited by their poor sensing performance for gas analytes. This drawback is attributed to the low charge transport in and the limited charge-analyte interaction of the conjugated polymers. Herein, we demonstrate that the incorporation of graphitic carbon nitride (g-C₃N₄) into the conjugated polymer matrix can improve the sensing performance of OFET gas sensors. Moreover, the effect of graphitic carbon nitride (g-C₃N₄) on the gas sensing properties of OFET sensors based on poly(3-hexylthiophene) (P3HT), a conjugated polymer, was systematically investigated by changing the concentration of the g-C₃N₄ in the P3HT/g-C₃N₄ composite films. The obtained films were applied in OFET to detect NO gas at room temperature. In terms of the results, first, the P3HT/g-C₃N₄ composite films containing 10 wt.% g-C₃N₄ exhibited a maximum charge carrier mobility of ~1.1 × 10^-1 cm² V^-1 S^-1, which was approximately five times higher than that of pristine P3HT films. The fabricated P3HT/g-C₃N₄ composite film based OFET sensors presented significantly enhanced NO gas sensing characteristics compared to those of the bare P3HT sensor. In particular, the sensors based on the P3HT/g-C₃N₄ (90/10) composite films exhibited the best sensing performance relative to that of the bare P3HT sensor when exposed to 10 ppm NO gas: responsivity = 40.6 vs. 18.1%, response time = 129 vs. 142 s, and recovery time = 148 vs. 162 s. These results demonstrate the enormous promise of g-C₃N₄ as a gas sensing material that can be hybridized with conjugated polymers to efficiently detect gas analytes.

Measurement of nitric oxide spectral parameters: Considering the effects of CO2 and H2O

A continuous-wave quantum cascade laser (CW-QCL) based spectrometer is developed to investigate the high-resolution spectral parameters of nitric oxide (NO). Line strengths and nitrogen (N₂)-, carbon dioxide (CO₂)-, water vapor (H₂O)-, broadening coefficients for NO R(6.5) are measured. The spectral region ranging from 1989 cm^-1 to 1901 cm^-1, which is suitable for the in situ laser sensing of trace NO, is investigated. Spectral parameters are determined by fitting absorption spectra with multi-peak Voigt profiles. The measured intensities are compared to the HITRAN2020 database to assess the performance of the system and a good agreement within ±5% is obtained for the established lines. The measurement results are useful for the design of a spectroscopic sensor for monitoring exhaled breath NO, vehicle exhaust, and industrial emissions.

Assembly of Core/Shell Nanospheres of Amorphous Hemin/Acetone-Derived Carbonized Polymer with Graphene Nanosheets for Room-Temperature NO Sensing

Implementing parts per billion-level nitric oxide (NO) sensing at room temperature (RT) is still in extreme demand for monitoring inflammatory respiratory diseases. Herein, we have prepared a kind of core-shell structural Hemin-based nanospheres (Abbr.: Hemin-nanospheres, defined as HNSs) with the core of amorphous Hemin and the shell of acetone-derived carbonized polymer, whose core-shell structure was verified by XPS with argon-ion etching. Then, the HNS-assembled reduced graphene oxide composite (defined as HNS-rGO) was prepared for RT NO sensing. The acetone-derived carbonized polymer shell not only assists the formation of amorphous Hemin core by disrupting their crystallization to release more Fe-N₄ active sites, but provides protection to the core. Owing to the unique core-shell structure, the obtained HNS-rGO based sensor exhibited superior RT gas sensing properties toward NO, including a relatively higher response (R_a/R_g = 5.8, 20 ppm), a lower practical limit of detection (100 ppb), relatively reliable repeatability (over 6 cycles), excellent selectivity, and much higher long-term stability (less than a 5% decrease over 120 days). The sensing mechanism has also been proposed based on charge transfer theory. The superior gas sensing properties of HNS-rGO are ascribed to the more Fe-N₄ active sites available under the amorphous state of the Hemin core and to the physical protection by the shell of acetone-derived carbonized polymer. This work presents a facile strategy of constructing a high-performance carbon-based core-shell nanostructure for gas sensing.

Ferromagnetic Ni1-xVxO1-y Nano-Clusters for NO Detection at Room Temperature: A Case of Magnetic Field-Induced Chemiresistive Sensing

Surface modulation of functional nanostructures is an efficient way of improving gas sensing properties in chemiresistive materials. However, synthesis methods employed so far in achieving desired performances are cumbersome and energy intensive. Moreover, nano-engineering-induced magnetic properties of these materials which are expected to enhance sensing responses have not been utilized until now in improving their interaction with target gases. In particular for gasses with paramagnetic nature such as NO or NO₂, the inherent magnetic property of the chemiresistor might assist in enabling superior sensing performance. In this work, vanadium-doped NiO nano-clusters with ferromagnetic behavior at room temperature have been synthesized by a simple and effective combination of soft chemical routes and employed in efficient and selective detection of paramagnetic NO gas. While NiO is typically anti-ferromagnetic, the nanoscale engineering of NiO- and V-doped NiO samples have been found to tune the inherent anti-ferromagnetic behavior into room-temperature ferromagnetism. Surface modification in terms of formation of nano-clusters led to an increased Brunauer-Emmett-Teller surface area of ∼120 m²/g. The sample Ni_0.636V_0.364O has been observed to exhibit a selective and high response of ∼98% to 1 ppm NO at room temperature with fast response (14 s) and recovery (95 s). The improved sensing response of this sample compared to other doped NiO variants could be explained in terms of lower remnant magnetic moment of the sample accompanied with higher excess negative charge at the surface. The sensing response of this sample was increased by 30% in the presence of an external magnetic field of 280 gauss, highlighting the importance of magnetic ordering in chemiresistive gas sensing between the magnetic sensor material and target analyte. This material stands as a potential gas sensor with excellent NO detection properties.

Hybrid Carbon Nanotubes/Gold Nanoparticles Composites for Trace Nitric Oxide Detection over a Wide Range of Humidity

Composites of functionalized single walled carbon nanotubes (SWCNTs) and gold nanoparticles (Au NPs) of ≈15 nm diameter were drop-cast on a printed circuit board (PCB) substrate equipped with interdigitated electrodes to make a hybrid thin film. Addition of Au NPs decorated the surface of SWCNTs networked films and acted as catalysts which resulted into an enhanced sensitivity and low ppb concentration detection limit. The compositions of the film were characterized by scanning electron microscope (SEM). SWCNTs clusters were loaded with various amount of Au NPs ranging from 1-10% (by weight) and their effect on Nitric oxide (NO) sensitivity was studied and optimized. Further, the optimized composite films were tested in both air and nitrogen environments and as well as over a wide relative humidity range (0-97%). Sensors were also tested for the selectivity by exposing to various gases such as nitrous oxide, ammonia, carbon monoxide, sulfur dioxide and acetone. Sensitivity to NO was found much higher than the other tested gases. The advantage of this sensor is that it is sensitive to NO at low ppb level (10 ppb) with estimated response time within 10 s and recovery time around 1 min, and has excellent reproducibility from sensor to sensor and works within the wide range of relative humidity (0-97%).

Cerebrospinal Fluid Leak Detection with a Carbon Nanotube-Based Field-Effect Transistor Biosensing Platform

Cerebrospinal fluid (CSF) leakage may lead to life-threatening complications if not detected promptly. However, gel electrophoresis, the gold-standard test for confirming CSF leakage by detecting beta2-transferrin (β2-Tf), requires 3-6 h and is labor-intensive. We developed a new β2-Tf detection platform for rapid identification of CSF leakage. The three-step design, which includes two steps of affinity chromatography and a rapid sensing step using a semiconductor-enriched single-walled carbon nanotube field-effect transistor (FET) sensor, circumvented the lack of selectivity that antitransferrin antibody exhibits for transferrin isoforms and markedly shortened the detection time. Furthermore, three different sensing configurations for the FET sensor were investigated for obtaining the optimal β2-Tf sensing results. Finally, body fluid (CSF and serum) tests employing our three-step strategy demonstrated high sensitivity, suggesting its potential to be used as a rapid diagnostic tool for CSF leakage.

Role of Defect Engineering and Surface Functionalization in the Design of Carbon Nanotube-Based Nitrogen Oxide Sensors

Nitrogen oxides (NOx) are among the main atmospheric pollutants; therefore, it is important to monitor and detect their presence in the atmosphere. To this end, low-dimensional carbon structures have been widely used as NOx sensors for their outstanding properties. In particular, carbon nanotubes (CNTs) have been widely used as toxic-gas sensors owing to their high specific surface area and excellent mechanical properties. Although pristine CNTs have shown promising performance for NOx detection, several strategies have been developed such as surface functionalization and defect engineering to improve the NOx sensing of pristine CNT-based sensors. Through these strategies, the sensing properties of modified CNTs toward NOx gases have been substantially improved. Therefore, in this review, we have analyzed the defect engineering and surface functionalization strategies used in the last decade to modify the sensitivity and the selectivity of CNTs to NOx. First, the different types of surface functionalization and defect engineering were reviewed. Thereafter, we analyzed experimental, theoretical, and coupled experimental-theoretical studies on CNTs modified through surface functionalization and defect engineering to improve the sensitivity and selectivity to NOx. Finally, we presented the conclusions and the future directions of modified CNTs as NOx sensors.

Lead-Free Halide Cs2PtI6 Perovskite Favoring Pt-N Bonding for Trace NO Detection

In recent years, the performance research of perovskite materials is not only concentrated in the field of solar cells or optics, but the field of gas sensing has gradually entered the public view. However, the detection of nitric oxide (NO) by lead-free halide perovskites has not yet been reported. Herein, we use Cs₂PtI₆ to realize the first example of a halide perovskite applied to NO sensing. Due to favoring Pt-N binding, the material has some excellent properties such as a NO detection limit as low as 100 parts-per-billion (ppb), ultrahigh selectivity to NO, and can work at room temperature for more than 2 months. In situ sum frequency generation (SFG) spectra and crystal orbital Hamilton population (COHP) analysis reveal that the strong bonding interaction between Pt 5s and N 2s ensure the high adsorption energy, and Pt 5d electron back donation to N 2p_x, N 2p_z antibonding causes the conductive change of the sensors. In addition, its flexible wearable technology shows the application potential of the device and promotes the further development of perovskite materials.

Single-Walled Carbon Nanotube-Based Biosensors for Detection of Bronchial Inflammation

Inflammation is a protective mechanism against invading pathogens and tissue damage. However, the inflammatory process is implicated in a wide range of diseases affecting all organs and body systems. Nitric oxide — a multifunctional signaling molecule that plays a critical role in systemic blood pressure homeostasis, prevention of platelet aggregation, antimicrobial resistance, immunoregulation, tumor suppression and as a neurotransmitter — is used as a surrogate marker for inflammation. However, the most commonly used Griess assay is an indirect and expensive method for the determination of nitric oxide concentration. Hence, single-walled carbon nanotube-based biosensors have been explored as real-time, sensitive, selective and safe methods to determine nitric oxide released during the inflammatory process. In this review, we explore current developments in single-walled carbon nanotube-based biosensors for the detection of nitric oxide in exhaled breath as a direct and noninvasive test for detection of bronchial inflammation.

Selective and self-validating breath-level detection of hydrogen sulfide in humid air by gold nanoparticle-functionalized nanotube arrays

We demonstrate the selective detection of hydrogen sulfide at breath concentration levels under humid airflow, using a self-validating 64-channel sensor array based on semiconducting single-walled carbon nanotubes (sc-SWCNTs). The reproducible sensor fabrication process is based on a multiplexed and controlled dielectrophoretic deposition of sc-SWCNTs. The sensing area is functionalized with gold nanoparticles to address the detection at room temperature by exploiting the affinity between gold and sulfur atoms of the gas. Sensing devices functionalized with an optimized distribution of nanoparticles show a sensitivity of 0.122%/part per billion (ppb) and a calculated limit of detection (LOD) of 3 ppb. Beyond the self-validation, our sensors show increased stability and higher response levels compared to some commercially available electrochemical sensors. The cross-sensitivity to breath gases NH₃ and NO is addressed demonstrating the high selectivity to H₂S. Finally, mathematical models of sensors' electrical characteristics and sensing responses are developed to enhance the differentiation capabilities of the platform to be used in breath analysis applications.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (details on the dielectrophoretic deposition, AuNP functionalization optimization, full range of experimental and model H₂S sensing response up to 820 ppb, and sensing response to NO gas) is available in the online version of this article at 10.1007/s12274-021-3771-7.

Strengthening Mechanism of Electrothermal Actuation in the Epoxy Composite with an Embedded Carbon Nanotube Nanopaper

We assessed an effect of an embedded electro-conductive multiwalled carbon nanotube nanopaper in an epoxy matrix on the release of the frozen actuation force and the actuation torque in the carbon nanotube nanopaper/epoxy composite after heating above its glass transition temperature. The presence of the nanopaper augmented the recovery of the actuation stress by the factor of two in comparison with the pure epoxy strips. We proposed a procedure that allowed us to assess this composite strengthening mechanism. The strengthening of the composite was attributed to the interlocking of the carbon nanotubes with the epoxy. When reheated, the composite samples, which contained stretched mutually intertwined nanotubes and epoxy segments, released a greater actuation stress then the epoxy samples, which comprised of less elastic networks of crosslinked segments of pure epoxy.

Functionalization of Zinc Oxide Nanoflowers with Palladium Nanoparticles via Microwave Absorption for Room Temperature-Operating Hydrogen Gas Sensors in the ppb Level

Microwave-assisted functionalization of zinc oxide nanoflowers (ZnO NFs) with palladium nanoparticles (Pd NPs) is demonstrated to realize high-performance chemiresistive-type hydrogen (H₂) gas sensors operating at room temperature (RT). The developed gas sensors exhibit a high response of up to 70% at 50 ppm and a theoretical detection limit of 10 ppb. The formation of ZnO NFs with an enhanced specific surface area and their functionalization with Pd NPs are investigated through various characterizations. Furthermore, the optimization of microwave absorption upon the structural incorporations between nanostructures (NF-NPs) is investigated for solution-based functionalization at low temperatures (below 120 °C) for short process times (within 1 min), compared to the conventional thermal annealing at 250 °C for 1 h. Highly sensitive and selective ZnO-based gas sensors enabling the detection of H₂ gas molecules at 300 ppb concentration at RT exhibit a short response/recovery time of below 3 min and a good selectivity toward different gases including nitric oxide, carbon monoxide, and oxygen. The successful functionalization of nanostructured metal oxide semiconductors (MOSs) with metal NPs via effective and practical microwave absorption enhances the potential on highly sensitive and selective chemiresistive-type MOS-based gas sensors operating at RT without additional heaters or photogenerators.

Preparation of defected SWCNTs decorated with en-APTAS for application in high-performance nitric oxide gas detection

We demonstrate highly sensitive and selective chemiresistive-type NO gas detection using defected single-walled carbon nanotubes (SWCNTs) decorated with N-[3-(trimethoxysilyl)propyl]ethylene diamine (en-APTAS) molecules. The defected SWCNTs were prepared via furnace annealing at 700 °C and confirmed by transmission electron microscopy. A single en-APTAS molecule has two amine groups acting as adsorption sites for NO gas, which can improve the NO response. The NO response was further enhanced when the defected SWCNTs were utilized because NO sensing reactions could occur on both the inner and outer walls of the defected SWCNTs. The en-APTAS decoration improved the NO response of the SWCNT-based gas sensing devices by 2.5 times; when the defected SWCNTs were used, the NO response was further improved by 3 times. Meanwhile, the recovery performance in a time-resolved response curve was significantly improved (45 times) via a simple rinsing process with ethanol. Specifically, the fabricated device did not respond to carbon monoxide (CO) or BTEX gas (i.e., a mixture of benzene, toluene, ethyl benzene, and xylene), indicating its high selectivity to NO gas. The results show the possibility of a high-performance SWCNT-based NO gas sensor applicable to healthcare fields requiring ppb-level detection, such as in vitro diagnostics (IVDs) of respiratory diseases.

Negatively-Doped Single-Walled Carbon Nanotubes Decorated with Carbon Dots for Highly Selective NO2 Detection

In this study, we demonstrated a highly selective chemiresistive-type NO₂ gas sensor using facilely prepared carbon dot (CD)-decorated single-walled carbon nanotubes (SWCNTs). The CD-decorated SWCNT suspension was characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-visible spectroscopy, and then spread onto an SiO₂/Si substrate by a simple and cost-effective spray-printing method. Interestingly, the resistance of our sensor increased upon exposure to NO₂ gas, which was contrary to findings previously reported for SWCNT-based NO₂ gas sensors. This is because SWCNTs are strongly doped by the electron-rich CDs to change the polarity from p-type to n-type. In addition, the CDs to SWCNTs ratio in the active suspension was critical in determining the response values of gas sensors; here, the 2:1 device showed the highest value of 42.0% in a sensing test using 4.5 ppm NO₂ gas. Furthermore, the sensor selectively responded to NO₂ gas (response ~15%), and to other gases very faintly (NO, response ~1%) or not at all (CO, C₆H₆, and C₇H₈). We propose a reasonable mechanism of the CD-decorated SWCNT-based sensor for NO₂ sensing, and expect that our results can be combined with those of other researches to improve various device performances, as well as for NO₂ sensor applications.

A resonant single frequency molecular detector with high sensitivity and selectivity for gas mixtures

Air quality control is an important task in prevention of human exposure to toxic and harmful gases and requires reliable gas sensors. During last decades many gas sensing mechanisms, based on different physical or chemical interactions with sensitive materials, have been developed, but the problem of precise analysis of gas mixtures still remains. The problem can be solved by introducing new sensing mechanism based on an adiabatically changing electric field interacting with the rotational structure of the molecules with dipole moments. We have theoretically demonstrated a single low frequency gas detector that can be used for sensing of gas mixtures with high selectivity, accuracy, and sensitivity. The enhancement of the population difference between corresponding molecular levels and reached the theoretical maximum of absorption have been shown.

An Analytical Conductance Model for Gas Detection Based on a Zigzag Carbon Nanotube Sensor

Recent advances in nanotechnology have revealed the superiority of nanocarbon species such as carbon nanotubes over other conventional materials for gas sensing applications. In this work, analytical modeling of the semiconducting zigzag carbon nanotube field-effect transistor (ZCNT-FET) based sensor for the detection of gas molecules is demonstrated. We propose new analytical models to strongly simulate and investigate the physical and electrical behavior of the ZCNT sensor in the presence of various gas molecules (CO₂, H₂O, and CH₄). Therefore, we start with the modeling of the energy band structure by acquiring the new energy dispersion relation for the ZCNT and introducing the gas adsorption effects to the band structure model. Then, the electrical conductance of the ZCNT is modeled and formulated while the gas adsorption effect is considered in the conductance model. The band structure analysis indicates that, the semiconducting ZCNT experiences band gap variation after the adsorption of the gases. Furthermore, the bandgap variation influences the conductance of the ZCNT and the results exhibit increments of the ZCNT conductance in the presence of target gases while the minimum conductance shifted upward around the neutrality point. Besides, the I-V characteristics of the sensor are extracted from the conductance model and its variations after adsorption of different gas molecules are monitored and investigated. To verify the accuracy of the proposed models, the conductance model is compared with previous experimental and modeling data and a good consensus is observed. It can be concluded that the proposed analytical models can successfully be applied to predict sensor behavior against different gas molecules.

High-Performance UV-Assisted NO2 Sensor Based on Chemical Vapor Deposition Graphene at Room Temperature

Nitrogen dioxide (NO₂) is one of the most dangerous air pollutants that can affect human health even at the ppb (part per billion) level. Thus, the superior sensing performance of nitrogen dioxide gas sensors is an imperative for real-time environmental monitoring. Traditional solid-state sensors based on metal-oxide transistors have the drawbacks of high power consumption, high operating temperature, poor selectivity, and difficult integration with other electronics. In that respect, graphene-based gas sensors have been extensively studied as potential replacements. However, their advantages of high sensing efficiency, low power consumption, and simple electronic integration have been countered by their slow response and poor repeatability. Here, we report the fabrication of high-performance ultraviolet (UV)-assisted room temperature NO₂ sensors based on chemical vapor deposition-grown graphene. UV irradiation improves the response of the sensor sevenfold with respect to the dark condition attaining 26% change in resistance at 100 ppm NO₂ concentration with a practical detection limit below 1 ppm (42.18 ppb). In addition, the recovery time was shortened fivefold to a few minutes and the excellent repeatability. This work may provide a promising and practical method to mass produce room-temperature NO₂ gas sensors for real-time environment monitoring due to its simple fabrication process, low cost, and practicality.

Probing Ca2+-induced conformational change of calmodulin with gold nanoparticle-decorated single-walled carbon nanotube field-effect transistors

Nanomaterials are ideal for electrochemical biosensors, with their nanoscale dimensions enabling the sensitive probing of biomolecular interactions. In this study, we compare field-effect transistors (FET) comprised of unsorted (un-) and semiconducting-enriched (sc-) single-walled carbon nanotubes (SWCNTs). un-SWCNTs have both metallic and semiconducting SWCNTs in the ensemble, while sc-SWCNTs have a >99.9% purity of semiconducting nanotubes. Both SWCNT FET devices were decorated with gold nanoparticles (AuNPs) and were then employed in investigating the Ca²⁺-induced conformational change of calmodulin (CaM) - a vital process in calcium signal transduction in the human body. Different biosensing behavior was observed from FET characteristics of the two types of SWCNTs, with sc-SWCNT FET devices displaying better sensing performance with a dynamic range from 10^-15 M to 10^-13 M Ca²⁺, and a lower limit of detection at 10^-15 M Ca²⁺.