Highly Sensitive and Contactless Ammonia Detection Based on Nanocomposites of Phosphate-Functionalized Reduced Graphene Oxide/Polyaniline Immobilized on Microstrip Resonators

MXene Free Standing Films: Unlocking the Impact of Flake Sizes in Microwave Resonant Structures in Humid Environments

Microwave communication devices necessitate elements with high electrical conductivity, a property which was traditionally found in metals (e.g., copper). However, in applications such as satellite communications, metals prevent the payload from achieving lightweight and flexible characteristics. Here, we demonstrate the development of MXene film microwave resonators, leveraging MXene's high electrical conductivity and unique mechanical properties. To investigate resonant performance in humid conditions and study the effects of MXene's processing and treatment, MXene films with different flake sizes are prepared and exposed to cyclic humidity. For the large- and small-flake Ti3 C2 MXene films in cyclic humidity, the large-flake film demonstrates higher electrical conductivity, higher resonance quality factor (150 and 35 as unloaded, and loaded), and less fluctuation of performance (≈1.7% total shift in resonance frequency). Further, by implementing MXene films of two different diameters, the correlation between film size and resonant frequency is demonstrated. By introducing an active resonant configuration, the effect of MXene degradation and microwave losses can be compensated. This active feedback loop demonstrates a ≈300 times increase in the quality factor of MXene resonators. As a building block for terrestrial and satellite communication modules, MXene resonators potentiate the replacement of metals in achieving unique electrical and mechanical properties.

A Microwave Voyage into Swelling Phenomenon: Investigation of Polydimethylsiloxane and VOCs Interaction

When exposed to specific gases, polymers undergo swelling, leading to physiochemical changes that can significantly affect their performance. Monitoring this swelling phenomenon requires innovative approaches. This study focuses on investigating the real-time resonant microwave behavior of two polydimethylsiloxane (PDMS) structures (solid and porous) in interaction with tetrahydrofuran (THF) and acetone, which are primary swelling agents. A microwave measurement method is proposed using an 8.63 GHz planar split ring resonator (SRR). The device's resonant frequency downshifts to 7.75 and 8.42 GHz when solid and porous PDMS blocks are placed on the split ring gap. Interaction of the solid PDMS and porous PDMS with target gases caused a change in PDMS structure resulting in alterations in the dielectric properties of the PDMS/gas system, as evidenced by the resonator's transmission amplitude and resonant frequency shifts. The magnitude of these shifts depends on the type and concentration of the solvent gas. The PDMS-integrated SRR exhibits a sensitivity of 25.3 MHz/1 ppt THF and 7 MHz/1 ppt acetone. Additionally, the solid block demonstrates response times of 6800 and 4200 s for swelling and deswelling, respectively, when in exposure to 25 ppt concentrations of THF and acetone. Overall, this study underscores the substantial potential of microwave resonators as versatile tools for investigating the physical changes in polymers during their interaction with gases, contributing to the understanding of polymer-gas interactions and opening avenues for further research and diverse applications.

Multiplexed DNA-functionalized graphene sensor with artificial intelligence-based discrimination performance for analyzing chemical vapor compositions

This study presents a new technology that can detect and discriminate individual chemical vapors to determine the chemical vapor composition of mixed chemical composition in situ based on a multiplexed DNA-functionalized graphene (MDFG) nanoelectrode without the need to condense the original vapor or target dilution. To the best of our knowledge, our artificial intelligence (AI)-operated arrayed electrodes were capable of identifying the compositions of mixed chemical gases with a mixed ratio in the early stage. This innovative technology comprised an optimized combination of nanodeposited arrayed electrodes and artificial intelligence techniques with advanced sensing capabilities that could operate within biological limits, resulting in the verification of mixed vapor chemical components. Highly selective sensors that are tolerant to high humidity levels provide a target for "breath chemovapor fingerprinting" for the early diagnosis of diseases. The feature selection analysis achieved recognition rates of 99% and above under low-humidity conditions and 98% and above under humid conditions for mixed chemical compositions. The 1D convolutional neural network analysis performed better, discriminating the compositional state of chemical vapor under low- and high-humidity conditions almost perfectly. This study provides a basis for the use of a multiplexed DNA-functionalized graphene gas sensor array and artificial intelligence-based discrimination of chemical vapor compositions in breath analysis applications.

Advanced polymeric/inorganic nanohybrids: An integrated platform for gas sensing applications

Rapid industrial development, vehicles, domestic activities and mishandling of garbage are the main sources of pollutants, which are destroying the atmosphere. There is a need to continuously monitor these pollutants for the safety of the environment and human beings. Conventional instruments for monitoring of toxic gases are expensive, bigger in size and time-consuming. Hybrid materials containing organic and inorganic components are considered potential candidates for diverse applications, including gas sensing. Gas sensors convert the information regarding the analyte into signals. Various polymeric/inorganic nanohybrids have been used for the sensing of toxic gases. Composites of different polymeric materials like polyaniline (PANI), poly (4-styrene sulfonate) (PSS), poly (3,4-ethylene dioxythiophene) (PEDOT), etc. with various metal/metal oxide nanoparticles have been reported as sensing materials for gas sensors because of their unique redox features, conductivity and facile operation at room temperature. Polymeric nanohybrids showed better performance because of the larger surface area of nanohybrids and the synergistic effect between polymeric and inorganic materials. This review article focuses on the recent developments of emerging polymeric/inorganic nanohybrids for sensing various toxic gases including ammonia, hydrogen, nitrogen dioxide, carbon oxides and liquefied petroleum gas. Advantages, disadvantages, operating conditions and prospects of hybrid composites have also been discussed.

Flexible, robust, and high-performance gas sensors based on lignocellulosic nanofibrils

Gas detection in flexible electronics demands novel materials with superior sensing performance that have high mechanically strength, are flexible, low-cost, and sustainable. We explore a composite sensing nanopaper based on lignocellulosic cellulose nanofibrils (LCNF) as a renewable and mechanically strong substrate that enables the fabrication of flexible, and highly sensitive gas sensors. In the system the hydrophobic lignin covalently bonds to cellulose in the nanofibrils, increasing the nanopaper water-resistance and limiting sensing materials response to humidity. The sensor is composed of polyaniline (PANI) grown on flexible LCNF and reduced graphene oxide (rGO) nanosheets. The proposed structure, at 10 wt% rGO, demonstrated a 10-fold improvement in sensitivity to volatile amines (i.e. ammonia detection down to 1 ppm) while maintaining an acceptable selectivity. Furthermore, we demonstrated the application of the sensing nanopaper in a microwave sensor that paves the path toward flexible, wireless, and high-performance sensing devices.

High-Frequency TiO2 Nanotube-Adapted Microwave Coplanar Waveguide Resonator for High-Sensitivity Ultraviolet Detection

Ultraviolet (UV) sensors are a key component in growing applications such as water quality treatment and environmental monitoring, with considerable interest in their miniaturization and enhanced operation. This work presents a passive gold coplanar waveguide split ring resonator integrated with anodic self-organized TiO₂ nanotube (TNT) membranes with a thickness of 20 μm to provide real-time UV detection. The resonator operated as a one-port device to capture the reflection coefficient (S₁₁) signal, with a center frequency of 16 GHz and a notch amplitude of -88 dB. It was experimentally analyzed for its UV sensing capability in the range of 36.5-463 μW/cm². The high-frequency resonator was improved through design choices including the addition of a tapered input transmission line, wire bonding for practical device design, and an interdigitated capacitive ring gap. The high frequency also helped mitigate noise due to water vapor or environmental contaminants. S₁₁ amplitude variation was found through both experiments and modeling to follow a linear trend with UV illumination intensity. The resonator exhibited over 45 ± 2 dB shift in the resonant amplitude under the highest UV illumination conditions, with a sensitivity of 0.084 dB/μW cm^-2 and the potential to sense UV intensity as low as 2.7 μW/cm². The presented device enabled a repeatable and accurate microwave response under UV illumination with very high sensitivity, entirely through the use of passive circuit elements.

Graphene oxide/polyaniline-based microwave split-ring resonator: A versatile platform towards ammonia sensing

Ammonia gas sensors have always received significant attention as robust platforms for emission control, food safety, and monitoring human exhaled breath for the early diagnosis of diseases such as dysfunction of the kidney and liver. This study explores the development of a microwave-based split-ring resonator (SRR) sensor with enhanced sensitivity to detect ammonia gas at low concentrations. The sensor is based on a nanocomposite fabricated by incorporating 10 wt% of graphene oxide (GO) into polyaniline (PANI) via the in-situ polymerization of aniline monomers over the surface of the GO sheets. The addition of GO to PANI results in a high sensitivity of 0.038 dB ppm^-1 for low concentrations (1-25 ppm) and 0.0045 dB ppm^-1 for high concentrations (> 25 ppm) of ammonia gas, in a 150-400 s time interval at room temperature. The prepared sensor can selectively sense ammonia gas in the presence of other higher concentrations of hazardous gases and a wide range of relative humidity levels (15-90%). The response signal is repeatable after 30 days with less than 0.32% deviation. The developed low-cost and robust sensor has the potential to monitor ammonia gas in various applications, including medical, environmental, food, and agricultural sectors.

Rapid and real-time monitoring of bacterial growth against antibiotics in solid growth medium using a contactless planar microwave resonator sensor

Infection diagnosis and antibiotic susceptibility testing (AST) are pertinent clinical microbiology practices that are in dire need of improvement, due to the inadequacy of current standards in early detection of bacterial response to antibiotics and affordability of contemporarily used methods. This paper presents a novel way to conduct AST which hybridizes disk diffusion AST with microwave resonators for rapid, contactless, and non-invasive sensing and monitoring. In this research, the effect of antibiotic (erythromycin) concentrations on test bacterium, Escherichia coli (E. coli) cultured on solid agar medium (MH agar) are monitored through employing a microwave split-ring resonator. A one-port microwave resonator operating at a 1.76 GHz resonant frequency, featuring a 5 mm² sensitive sensing region, was designed and optimized to perform this. Upon introducing uninhibited growth of the bacteria, the sensor measured 0.005 dB/hr, with a maximum change of 0.07 dB over the course of 15 hours. The amplitude change decreased to negligible values to signify inhibited growth of the bacteria at higher concentrations of antibiotics, such as a change of 0.005 dB in resonant amplitude variation while using 45 µg of antibiotic. Moreover, this sensor demonstrated decisive results of antibiotic susceptibility in under 6 hours and shows great promise to expand automation to the intricate AST workflow in clinical settings, while providing rapid, sensitive, and non-invasive detection capabilities.

Oleophobic textiles with embedded liquid and vapor hazard detection using differential planar microwave resonators

Protective clothing must repel hazardous liquids such as oils, acids, and solvents, which often exhibit low surface tension. The low surface tension liquid repellency of textiles is currently characterized qualitatively, considering only the first thirty seconds of wetting. This study demonstrates that embedded sensors within protective fabrics can more fully characterize liquid repellency while simultaneously detecting the hazardous substance. The liquid repellency of oleophobic textiles was detected in-situ using differential planar microwave resonator structures. A differential split ring resonator was designed with resonant responses at 4.4 and 4.6 GHz with a sensitivity of 50 MHz per unit ε. Fabrics were rendered oleophobic by dip-coating. The liquid repellency was monitored in-situ using droplets of heptane, octane, decane, dodecane, and water. Wetting transitions and droplet evaporation were identified in real time. The 4.4 GHz resonance peak's shift was used to measure the liquid repellency, whereas the 4.6 GHz resonator monitored the liquid's vapor as it absorbed into a gas-sensitive elastomer. The microwave response was tracked over 10 h every 15 s, and this transient data could identify the liquids based on their wetting and evaporation rates. Such sensors could be readily embedded in oleophobic textiles and enhance personal protective equipment.

Microwave Sensors for In Situ Monitoring of Trace Metals in Polluted Water

Thousands of pollutants are threatening our water supply, putting at risk human and environmental health. Between them, trace metals are of significant concern, due to their high toxicity at low concentrations. Abandoned mining areas are globally one of the major sources of toxic metals. Nowadays, no method can guarantee an immediate response for quantifying these pollutants. In this work, a novel technique based on microwave spectroscopy and planar sensors for in situ real-time monitoring of water quality is described. The sensors were developed to directly probe water samples, and in situ trial measurements were performed in freshwater in four polluted mining areas in the UK. Planar microwave sensors were able to detect the water pollution level with an immediate response specifically depicted at three resonant peaks in the GHz range. To the authors’ best knowledge, this is the first time that planar microwave sensors were tested in situ, demonstrating the ability to use this method for classifying more and less polluted water using a multiple-peak approach.

Microwave resonator array with liquid metal selection for narrow band material sensing

A microwave resonator array is integrated with liquid metal to select an individual resonator response within a resonator array, enabling simple and accurate analysis for dielectric sensing. Galinstan, a liquid metal, acts as a multiplexer by inducing a capacitive load to the nearby resonator, lowering its resonant frequency, and thereby isolating its resonant response from other resonators in the array. The liquid metal could be positioned within a fluidic channel to be above any of the resonators, which tuned the resonant frequency from 3.9 to 3.3 GHz where it can be analyzed individually. The resonators showed a consistent response to liquid metal tuning, with tuning error measured below 30 MHz (5%). The sensor also exhibited stable sensitivity to test materials placed on the selected resonator, with a maximum resonant frequency shift of 300 MHz for a dielectric test material (ε = 10.2) and almost no variation in resonant amplitude. The selected resonant response was only sensitive to materials on the selected resonator, and was unaffected by test materials, even when placed on other resonators. The presented design enabled robust and accurate detection of materials using planar microwave resonators that can be controlled at a user’s convenience, specifically for use in systems where multiple parameters or system settings may need to be individually determined.

Lignin Cellulose Nanofibrils as an Electrochemically Functional Component for High-Performance and Flexible Supercapacitor Electrodes

The increasing demand for wearable electronics has driven the development of supercapacitor electrode materials toward enhanced energy density, while being mechanically strong, flexible, as well as environmentally friendly and low-cost. Taking advantage of faradaic reaction of quinone groups in natural lignin that is covalently bound to the high-strength cellulose nanofibrils, the fabrication of a novel class of mechanically strong and flexible thin film electrodes with high energy storage performance is reported. The electrodes were made by growing polyaniline (PANI) on flexible films composed of lignin-containing cellulose nanofibrils (LCNF) and reduced graphene oxide (rGO) nanosheets at various loading levels. The highest specific capacitance was observed for the LCNF/rGO/PANI electrode with 20 wt% rGO nanosheets (475 F g^-1 at 10 mV s^-1 and 733 F g^-1 at 1 mV s^-1 ), which represented a 68 % improvement as compared to a similar electrode made without lignin. In addition, the LCNF/rGO(20)/PANI electrode demonstrated high rate performance and cycle life (87 % after 5000 cycles). These results indicated that LCNF functioned as an electrochemically active multifunctional component to impart the composite electrode with mechanical strength and flexibility and enhanced overall energy storage performance. LCNF/rGO(20)/PANI electrode was further integrated in a flexible supercapacitor device, revealing the excellent promise of LCNF for fabrication of advanced flexible electrodes with reduced cost and environmental footprint and enhanced mechanical and energy storage performances.

Modified Microwave Sensor with a Patterned Ground Heater for Detection and Prevention of Ice Accumulation

Ice accumulation on aircraft is known to negatively impact the aerodynamic and mechanical operation, sometimes resulting in catastrophic failure. Recently, microwave resonators have gained interest as durable and reliable frost and ice detectors. Here, a microwave resonator sensor with built-in heating capability patterned into the ground plane was designed, fabricated, and tested to investigate real-time ice and frost growth. Sensing was performed on surfaces with anti-icing coatings to quantitatively analyze the effectiveness of these materials. The sensor was also tested to determine its ability to evaluate different deicing methods. The sensor itself was a split-ring resonator (SRR) operating at 5.82 GHz, which could effectively distinguish between water and ice by detecting changes in the dielectric properties on or around its surface. This application was particularly suited for an SRR due to the extreme difference between the relative permittivity of water (ε = 90) and ice (ε = 3.2) at 5 GHz and 0 °C. The results from this sensor can be used to determine the holdover time of various coatings to resist ice formation. This study validates the use of SRRs as ice detection sensors for applications where ice and frost are of great interest, such as on aircraft, roads, or walkways.

Detection of food spoilage and adulteration by novel nanomaterial-based sensors

Food industry is always looking for more innovative and accurate ways to monitor the food safety and quality control of final products. Current detection techniques of analytes are costly and time-consuming, and occasionally require professional experts and specialized tools. The usage of nanomaterials in sensory systems has eliminated not only these drawbacks but also has advantages such as higher sensitivity and selectivity. This article first presents a general overview of the current studies conducted on the detection of spoilage and adulteration in foods from 2015 to 2020. Then, the sensory properties of nanomaterials including metal and magnetic nanoparticles, carbon nanostructures (nanotubes, graphene and its derivatives, and nanofibers), nanowires, and electrospun nanofibers are presented. The latest investigations and advancements in the application of nanomaterial-based sensors in detecting spoilage (food spoilage pathogens, toxins, pH changes, and gases) and adulterants (food additives, glucose, melamine, and urea) have also been discussed in the following sections. To conclude, these sensors can be applied in the smart packaging of food products to meet the demand of consumers in the new era.

Kirigami-Enabled Microwave Resonator Arrays for Wireless, Flexible, Passive Strain Sensing

Wireless and highly sensitive flexible strain sensors would have widespread application across a number of different fields. Here, the novel combination of two different metamaterials, one mechanical and one electronic, is demonstrated for its potential as such a sensor. An array of split-ring resonators (SRRs) were mounted on a bespoke kirigami sheet. The hybrid kirigami structure was designed specifically for the resonator array, in terms of both its physical dimensions and elastic response. Mechanical tests in concert with finite element modeling confirmed that the hybrid kirigami structure, containing two disparate kirigami motifs, exhibited a high range of strain and out-of-plane rotation without plastic deformation. The microwave sensing was designed to monitor variations in the S₁₁ response of the resonators as a function of out-of-plane kirigami hinge rotation. The mounted array of SRRs on the hybrid kirigami sheet could wirelessly detect changes in strain with high sensitivity (>30 MHz shift in resonant frequency; >30 dB shift in resonant amplitude) over a large range of strain, from 0.6 to 21.3%.

Green light-driven enhanced ammonia sensing at room temperature based on seed-mediated growth of gold-ferrosoferric oxide dumbbell-like heteronanostructures

Since there is excellent synergy between heterostructures and noble metals due to their unique electro-optical and catalytic properties, the introduction of noble metals into metal oxide semiconductors has substantially improved the performance of gas sensors. However, most of the reported noble metal-metal oxide composites are generally prepared as simple hybrids; hence, there is lack of control over their structure, morphology and dimension. Herein, we report a seed-mediated growth of dumbbell-like Au-Fe₃O₄ heteronanostructured gas sensors for ammonia detection under green light illumination, in which the particle sizes of Au and Fe₃O₄ were readily tuned in a wide range. The ammonia gas-sensing performances of Au-Fe₃O₄ heteronanostructures were greatly improved at room temperature by regulating their dimensions. In particular, the sensitivity improved by 30% while the response and recovery time shortened by 20 s and 50 s for the 7.5 nm Au-loaded Fe₃O₄-based sensor toward 5 ppm ammonia under 520 nm green light illumination as compared to that in the absence of light. This can be ascribed to the localized surface plasmon effect of Au and the Schottky junction formed at the interface between Au and Fe₃O₄. Interestingly, the Au-Fe₃O₄ heteronanostructure exhibits a unique p-type to n-type reversible transition for ammonia detection due to the nature of Fe₃O₄ NPs related to the trade-off between oxygen vacancies and electron transfer caused by ammonia adsorption. In addition, the calculation based on first-principle theory reveals enhanced adsorption capacities of Fe₃O₄ for ammonia after Au-doping.

Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe₂) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO₂ surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO₂-like layer, which is finally transformed into a SnO₂ skin. Remarkably, the self-assembled SnO₂/SnSe_2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe₂ sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO₂/SnSe_2-x at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm]^-1 for NO₂ and H₂, respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO₂ and H₂, respectively. No detectable changes in gas-sensing performances are observed in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.

Recent Trends and Developments in Graphene/Conducting Polymer Nanocomposites Chemiresistive Sensors

The use of graphene and its derivatives with excellent characteristics such as good electrical and mechanical properties and large specific surface area has gained the attention of researchers. Recently, novel nanocomposite materials based on graphene and conducting polymers including polyaniline (PANi), polypyrrole (PPy), poly (3,4 ethyldioxythiophene) (PEDOT), polythiophene (PTh), and their derivatives have been widely used as active materials in gas sensing due to their unique electrical conductivity, redox property, and good operation at room temperature. Mixing these two materials exhibited better sensing performance compared to pure graphene and conductive polymers. This may be attributed to the large specific surface area of the nanocomposites, and also the synergistic effect between graphene and conducting polymers. A variety of graphene and conducting polymer nanocomposite preparation methods such as in situ polymerization, electropolymerization, solution mixing, self-assembly approach, etc. have been reported and utilization of these nanocomposites as sensing materials has been proven effective in improving the performance of gas sensors. Review of the recent research efforts and developments in the fabrication and application of graphene and conducting polymer nanocomposites for gas sensing is the aim of this review paper.