"Visualization" Gas-Gas Sensors Based on High Performance Novel MXenes Materials.

The detection of toxic, harmful, explosive, and volatile gases cannot be separated from gas sensors, and gas sensors are also used to monitor the greenhouse effect and air pollution. However, existing gas sensors remain with many drawbacks, such as lower sensitivity, lower selectivity, and unstable room temperature detection. Thus, there is an imperative need to find more suitable sensing materials. The emergence of a new 2D layered material MXenes has brought dawn to solve this problem. The multiple advantages of MXenes, namely high specific surface area, enriched terminal functionality groups, hydrophilicity, and good electrical conductivity, make them among the most prolific gas-sensing materials. Therefore, this review paper describes the current main synthesis methods of MXenes materials, and focuses on summarizing and organizing the latest research results of MXenes in gas sensing applications. It also introduces the possible gas sensing mechanisms of MXenes materials on NH3 , NO2 , CH3 , and volatile organic compounds (VOCs). In conclusion, it provides insight into the problems and upcoming challenges of MXenes materials for gas sensing.

Sensing Mechanisms of Rough Plasmonic Surfaces for Protein Binding of Surface Plasmon Resonance Detection.

Surface plasmon resonance (SPR) has been utilized in various optical applications, including biosensors. The SPR-based sensor is a gold standard for protein kinetic measurement due to its ultrasensitivity on the plasmonic metal surface. However, a slight change in the surface morphology, such as roughness or pattern, can significantly impact its performance. This study proposes a theoretical framework to explain sensing mechanisms and quantify sensing performance parameters of angular surface plasmon resonance detection for binding kinetic sensing at different levels of surface roughness. The theoretical investigation utilized two models, a protein layer coating on a rough plasmonic surface with and without sidewall coatings. The two models enable us to separate and quantify the enhancement factors due to the localized surface plasmon polaritons at sharp edges of the rough surfaces and the increased surface area for protein binding due to roughness. The Gaussian random surface technique was employed to create rough metal surfaces. Reflectance spectra and quantitative performance parameters were simulated and quantified using rigorous coupled-wave analysis and Monte Carlo simulation. These parameters include sensitivity, plasmonic dip position, intensity contrast, full width at half maximum, plasmonic angle, and figure of merit. Roughness can significantly impact the intensity measurement of binding kinetics, positively or negatively, depending on the roughness levels. Due to the increased scattering loss, a tradeoff between sensitivity and increased roughness leads to a widened plasmonic reflectance dip. Some roughness profiles can give a negative and enhanced sensitivity without broadening the SPR spectra. We also discuss how the improved sensitivity of rough surfaces is predominantly due to the localized surface wave, not the increased density of the binding domain.

Controllable synthesis of BiPr composite oxide nanowires electrocatalyst for sensitive L-cysteine sensing properties.

BiPr composite oxide nanowires with rhombodedral Bi1.35Pr0.65O3, monoclinic Bi2O3and monoclinic Pr5O9phases were synthesized via a facile sodium dodecyl sulfate (SDS) assisted hydrothermal route. The obtained nanowires were characterized by x-ray diffraction, electron microscopy, x-ray photoelectron spectroscopy and electrochemical measurements. The BiPr composite oxide nanowires possess poly-crystalline structure, semi-circular tips, diameter and length of 20-100 nm and several micrometers, respectively. SDS is essential for the formation of the BiPr composite oxide nanowires which can be explained by a SDS assisted hydrothermal growth process. Electrochemical impedance spectroscopy shows that the electrons are easier to transfer by the surface of the BiPr composite oxide nanowires modified glassy carbon electrode (GCE) than bare GCE. The BiPr composite oxide nanowires modified GCE possesses good electro-catalytic activity for L-cysteine detection with a pair of quasi-reversible cyclic voltammetry peaks at +0.04 V and -0.72 V for the oxidation and reduction of L-cysteine, respectively. The roles of the scan rate, electrolyte species and L-cysteine concentration on the electrochemical responses of L-cysteine at the nanowires modified GCE were systematically analyzed. The BiPr composite oxide nanowires modified GCE presents a linear response range from 0.001 to 2 mM and detection limit of 0.27µM, good reproducibility and stability.

Cocrystallization Enabled Spatial Self-Confinement Approach to Synthesize Crystalline Porous Metal Oxide Nanosheets for Gas Sensing.

Crystalline metal oxide nanosheets show exceptional catalytic performance owing to the large surface-to-volume ratio and quantum confinement effect. However, it is still a challenge to develop a facile and general method to synthesize metal oxide nanosheets. Herein, we report a cocrystallization induced spatial self-confinement approach to synthesize metal oxide nanosheets. Taking the synthesis of SnO2 as an example, the solvent evaporation from KCl and SnCl2 solution induces the cocrystallization of KCl and K2 SnCl6 , and the obtained composite with encapsulated K2 SnCl6 can be in situ converted into SnO2 nanosheets confined in KCl matrix, after water washing to remove KCl, porous SnO2 nanosheets can be obtained. Notably, a series of metal oxide nanosheets can be obtained through this general and efficient green route. In particular, porous CeO2 /SnO2 nanosheets with improved surface O- species and abundant oxygen vacancies exhibit superior gas sensing performance to 3-hydroxy-2-butanone.

Synergistic effects ofα-Fe2O3nanoparticles and Fe-doping on gas-sensing performance of NiO nanowires and interface mechanism.

High surface area nickel oxide nanowires (NiO NWs), Fe-doped NiO NWs andα-Fe2O3/Fe-doped NiO NWs were synthesized with nanocasting pathway, and then the morphology, microstructure and components of all samples were characterized with XRD, TEM, EDS, UV-vis spectra and nitrogen adsorption-desorption isotherms. Owing to the uniform mesoporous template, all samples with the same diameter exhibit the similar mesoporous-structures. The loadedα-Fe2O3nanoparticles should exist in mesoporous channels between Fe-doped NiO NWs to form heterogeneous contact at the interface of n-typeα-Fe2O3nanoparticles and p-type NiO NWs. The gas-sensing results indicate that Fe-dopant andα-Fe2O3-loading both improve the gas-sensing performance of NiO NWs sensors.α-Fe2O3/Fe-doped NiO NWs sensors presented the highest response to 100 ppm ethanol gas (55.264) compared with Fe-doped NiO NWs (24.617) and NiO NWs sensors (3.189). The donor Fe-dopant increases the ground state resistance and the absorbed oxygen content in air.α-Fe2O3nanoparticles in electron depletion region result in the increasing resistance in ethanol gas and decreasing resistance in air. In this way,α-Fe2O3/Fe-doped NiO NWs sensor presents the excellent gas-sensing performance due to the formation of heterogeneous contact at the interface.

Enhanced acetone sensing properties based onin situgrowth SnO2nanotube arrays.

Large-scale and well-alignedin situgrowth SnO2nanotube (NT) arrays have been synthesized directly on the surface of the Al2O3ceramic tube by a cost-effective template self-etching method. The morphology ofin situSnO2NTs can be adjusted by changing the concentration of urea. The structure and morphology characteristics of SnO2NT were examined via x-ray diffraction, BET, and scanning electron microscopy, respectively. A series of detections were carried out to evaluate the gas sensing performances. The results indicated thatin situgrowth SnO2NT arrays sensor exhibited an excellent response (S = 20.3), good linearity under the concentration range of ppm level (5-300 ppm), and outstanding selectivity to 100 ppm of acetone gas. Compared with the sensors fabricated by a slurry-coating method, the controllablein situassembled SnO2NT arrays exhibited a more stable structure and easier fabrication process. The high acetone sensing performance might due to the unique hollow structure and favorable orientation growth. The dominant sensing mechanism about thein situgrowth SnO2NT arrays sensor has been discussed in detail. It is expected thatin situgrowth SnO2NT arrays sensor with the general working principle and controllable growth strategy will become a promising functional material in monitoring and detecting acetone.

Morphology genetic 3D hierarchical SnO2microstructures constructed by Sub 5 nm nanocrystals for highly sensitive ethanol-sensor.

SnO2is widely used for ethanol-sensing applications due to its excellent physicochemical properties, low toxicity and high sensitivity. However it is a challenge to construct 3D-hierarchical structures with sub 5 nm primary grain particle, which is the optimized size for ethanol sensor. Herein, genetic tri-level hierarchical SnO2microstructures are synthesised by the genetic conversion of 3D hierarchical SnS2flowers assembled by ultrathin nanosheets. The SnS2nanosheets are morphology genetic converted to porous nanosheets with sub 5 nm SnO2nanoparticles during the calcination process. When used for the detection of ethanol, the sensor exhibits a high sensitivity of 0.5 ppm (Ra/Rg = 6.8) and excellent gas-sensing response (Ra/Rg= 183 to 100 ppm) with short response/recovery time (12 s/11 s). The excellent gas sensing performance is much better than that of the previous reported SnO2-based sensors. The highly sensitivity is attributed to the large surface area derived from the recrystallization and volume changes, which offers more active sites during the morphology genetic conversion from SnS2to SnO2. Furthermore, the flower-like 3D structure enhances the stability of the materials and is beneficial for the mass diffusion dynamics of ethanol.

Analysis of Effects of Surface Roughness on Sensing Performance of Surface Plasmon Resonance Detection for Refractive Index Sensing Application.

This paper provides a theoretical framework to analyze and quantify roughness effects on sensing performance parameters of surface plasmon resonance measurements. Rigorous coupled-wave analysis and the Monte Carlo method were applied to compute plasmonic reflectance spectra for different surface roughness profiles. The rough surfaces were generated using the low pass frequency filtering method. Different coating and surface treatments and their reported root-mean-square roughness in the literature were extracted and investigated in this study to calculate the refractive index sensing performance parameters, including sensitivity, full width at half maximum, plasmonic dip intensity, plasmonic dip position, and figure of merit. Here, we propose a figure-of-merit equation considering optical intensity contrast and signal-to-noise ratio. The proposed figure-of-merit equation could predict a similar refractive index sensing performance compared to experimental results reported in the literature. The surface roughness height strongly affected all the performance parameters, resulting in a degraded figure of merit for surface plasmon resonance measurement.

Passive Wireless Dual-Tag UHF RFID Sensor System for Surface Crack Monitoring.

The generation and propagation of cracks are critical factors that affect the performance and life of large structures. Therefore, in order to minimize maintenance costs and ensure personal safety, it is necessary to monitor key structures. The sensor based on ultra-high frequency radio frequency identification (UHF RFID) antenna has the advantages of passive wireless, low cost, and great potential in the field of metallic structure health monitoring. In this paper, aimed at the key parts of a metallic structure, a dual-tag system is used for crack monitoring. In conjunction with mode analysis, the principles of the sensing tag and the coupling principles of the dual-tag are analyzed. Considering that the dual-tag is placed in different methods, the effect of mutual coupling on the sensing performance of the tag is studied. The results show that the frequency of the sensing tag can be tuned by adding the interference tag, and the dual-tag sensor system has reasonable sensitivity. The results also provide potential guidance for the optimal placement of multiple tags in the near-field region.

The Field Monitoring Experiment of the Roof Strata Movement in Coal Mining Based on DFOS.

Mining deformation of roof strata is the main cause of methane explosion, water inrush, and roof collapse accidents amid underground coal mining. To ensure the safety of coal mining, the distributed optical fiber sensor (DFOS) technology has been applied in the 150,313 working face by Yinying Coal Mine in Shanxi Province, north China to monitor the roof strata movement, so as to grasp the movement law of roof strata and make it serve for production. The optical fibers are laid out in the holes drilled through the overlying strata on the roadway roof and BOTDR technique is utilized to carry out the on-site monitoring. Prior to the on-site test, the coupling test of the fiber strain in the concrete anchorage, the calibration test of the fiber strain coefficient of the 5-mm steel strand (SS) fiber, and the test of the strain transfer performance of the SS fiber were carried out in the laboratory. The approaches for fiber laying-out in the holes and fiber's spatial positioning underground the coal mine have been optimized in the field. The indoor test results show that the high-strength SS optical fiber has a high strain transfer performance, which can be coupled with the concrete anchor with uniform deformation. This demonstrated the feasibility of SS fiber for monitoring strata movement theoretically and experimentally; and the law of roof strata fracturing and collapse is obtained from the field test results. This paper is a trial to study the whole process of dynamic movement of the deformation of roof strata. Eventually the study results will help Yinying Coal Mine to optimize mining design, prevent coal mine accidents, and provide detailed test basis for DFOS monitoring technique of roof strata movement.

Highly Enhanced Inductance Sensing Performance of Dual-Quartz Crystal Converter.

This paper presents ways of inductance sensitivity improvement in a quartz crystal converter for low inductance measurement. To improve the converter's sensitivity, two quartz crystals that were connected in parallel and additional capacitance connected to the two quartz crystals in the oscillator's circuit are used. The new approach uses a converter with special switchable oscillator and multiplexer switches to compensate for the crystal's natural temperature-frequency characteristics and any other influences, such as parasitic capacitances and parasitic inductances, which reduce them to a minimum. The experimental results demonstrate improved sensitivity and well-compensated dynamic temperature influence on the converter's output frequency. The fundamental quartz crystal frequency-temperature characteristics in the temperature range between 0-40 °C are simultaneously compensated. Furthermore, the converter enables the measurement of the influence of its own hysteresis at different values of inductances at the selected sensitivity by parallel capacitances connected either to the single- or dual-quartz crystal unit. The results show that the converter converting inductances in the range between 85-100 µH to a frequency range between 1-150 kHz only has ±0.05 ppm frequency instability (during the temperature change between 0-40 °C), which gives the converter a resolution of 1 pH. As a result, the converter can be applied where low inductance measurement, nondestructive testing, impedance change measurement, and magnetic material properties measurement are important.

What Exactly Can Bionic Strategies Achieve for Flexible Sensors?

Flexible sensors have attracted great attention in the field of wearable electronic devices due to their deformability, lightness, and versatility. However, property improvement remains a key challenge. Fortunately, natural organisms exhibit many unique response mechanisms to various stimuli, and the corresponding structures and compositions provide advanced design ideas for the development of flexible sensors. Therefore, this Review highlights recent advances in sensing performance and functional characteristics of flexible sensors from the perspective of bionics for the first time. First, the "twins" of bionics and flexible sensors are introduced. Second, the enhancements in electrical and mechanical performance through bionic strategies are summarized according to the prototypes of humans, plants, and animals. Third, the functional characteristics of bionic strategies for flexible sensors are discussed in detail, including self-healing, color-changing, tangential force, strain redistribution, and interfacial resistance. Finally, we summarize the challenges and development trends of bioinspired flexible sensors. This Review aims to deepen the understanding of bionic strategies and provide innovative ideas and references for the design and manufacture of next-generation flexible sensors.

Performance Optimization of Ionic Polymer Sensors through Characteristic Regulation of Chemically Prepared Interfacial Electrodes.

Ionic polymer sensors (IPSs) have broad application prospects in health monitoring, environmental perception, and human-computer interaction. The performance of IPSs with chemically prepared electrodes is generally superior to that with physically prepared electrodes due to the area difference of the electric double layer (EDL), but the effects of the electrode characteristics prepared by chemical methods on the performance of IPSs have not been revealed. Therefore, in this paper, we studied the impact of the characteristics of chemically prepared electrodes on the performance of IPSs and realized the performance optimization of IPSs through electrode characteristic regulation. By controlling the matrix surface roughening, immersion reduction plating (IRP) cycles, and electroplating (EP) time, the sensing performances of IPS samples with different electrode interface roughnesses, electrode penetration depths, and surface resistances were investigated, respectively. The experimental results indicated that the response voltage of the IPS can be improved by increasing the electrode interface roughness and the electrode penetration depth and reducing the surface resistance. In addition, we have proven that the sensing performance of the IPS is determined by its intrinsic capacitance characteristics. Through coupling electrode characteristic regulations such as roughening and increasing IRP cycles and EP time, a high-performance IPS was obtained, and its response amplitude was improved by 237.8%. The obtained high-performance sensor has been applied in human motion detection, which has good potential to develop wearable devices with high stability for physiological activity monitoring.

Lossy Mode Resonance Sensors Based on Anisotropic Few-Layer Black Phosphorus.

Lossy mode resonance (LMR) sensors offer a promising avenue to surpass the constraints of conventional surface plasmon resonance (SPR) sensors by delivering enhanced label-free detection capabilities. A notable edge of LMR over SPR is its excitation potential by both transverse electric (TE) and transverse magnetic (TM) polarized light. Yet this merit remains underexplored due to challenges to achieving high sensing performance under both TM and TE polarization within a singular LMR model. This study introduces a theoretical model for an LMR prism refractive index sensor based on a MgF2-few layer black phosphorus-MgF2 configuration, which can achieve angular sensitivity nearing 90° refractive index unit-1 (RIU-1) for both polarizations. Leveraging the distinct anisotropic nature of black phosphorus, the figure of merit (FOM) values along its two principal crystal axes (zigzag and armchair) show great difference, achieving an impressive FOM of 1.178 × 106 RIU-1 along the zigzag direction under TE polarized light and 1.231 × 104 RIU-1 along the armchair direction under TM polarized light. We also provide an analysis of the electric field distribution for each configuration at its respective resonant conditions. The proposed structure paves the way for innovative applications of anisotropic-material-based LMR sensors in various applications.

Application of MXene composites for target gas detection in food safety.

Food contamination has long plagued agriculture, posing significant health risks to consumers. The use of volatile gases for food safety detection has proven highly effective, with composite gas sensors that leverage the two-dimensional material MXene exhibiting notable advancements in detecting various target gases. This paper reviews the progress of MXene-based composite gas sensors in the detection of food safety-related gases. The review begins by examining MXene material synthesis methods and then presents an overview of techniques aimed at enhancing MXene-based sensor detection capabilities. Recently, advancements in MXene composite gas sensors tailored for food safety gases have been highlighted. Finally, challenges encountered in gas-sensing applications of MXene-based composites are outlined, alongside predictions for their future development, aiming to offer insights for the application and advancement of intelligent gas sensors for target gases in food safety.

Enhancing Stability of n-Type Doped Organic Semiconductors through Low Boiling Point Solvent-Assisted Room Temperature Drying.

The development of n-type organic semiconductors (OSCs) has been lagged behind that of p-type OSCs, mainly due to the limited availability of the electron deficient π-conjugated backbones and facile electron trapping by ambient oxidants. Improving the performance of n-type OSCs through n-doping is essential for realizing p-n junction diodes and complementary circuits. Conventional vacuum deposition doping is costly and time-consuming, while solution doping risks thermal damage through necessary annealing. Therefore, the development of a simpler, more affordable n-doping method is crucial. In this study, we have developed a solution-processed n-doping method using an organic cationic dye in a low boiling point solvent that can be dried at room temperature in 1 h, which eliminates the need for annealing. The effects of different organic cationic dyes and reducing agents on the n-type OSC were evaluated. After n-doping, electron mobility and photoresponsivity in the sample increased by 5.5 and 20 times, respectively, compared to undoped samples. Furthermore, there was no significant degradation in the electron mobility of the n-doped samples under ambient conditions after 15 days. Studying n-doping with various organic cationic dyes in different OSC materials, embracing further research into their applications and mechanisms, would advance the field of organic electronics.

Current On-Skin Flexible Sensors, Materials, Manufacturing Approaches, and Study Trends for Health Monitoring: A Review.

Due to an ever-increasing amount of the population focusing more on their personal health, thanks to rising living standards, there is a pressing need to improve personal healthcare devices. These devices presently require laborious, time-consuming, and convoluted procedures that heavily rely on cumbersome equipment, causing discomfort and pain for the patients during invasive methods such as sample-gathering, blood sampling, and other traditional benchtop techniques. The solution lies in the development of new flexible sensors with temperature, humidity, strain, pressure, and sweat detection and monitoring capabilities, mimicking some of the sensory capabilities of the skin. In this review, a comprehensive presentation of the themes regarding flexible sensors, chosen materials, manufacturing processes, and trends was made. It was concluded that carbon-based composite materials, along with graphene and its derivates, have garnered significant interest due to their electromechanical stability, extraordinary electrical conductivity, high specific surface area, variety, and relatively low cost.

Hard Magnetic Graphene Nanocomposite for Multimodal, Reconfigurable Soft Electronics.

Soft electronics provide effective means for continuous monitoring of a diverse set of biophysical and biochemical signals from the human body. However, the sensitivities, functions, spatial distributions, and many other features of such sensors remain fixed after deployment and cannot be adjusted on demand. Here, laser-induced porous graphene is exploited as the sensing material, and dope it with permanent magnetic particles to create hard magnetic graphene nanocomposite (HMGN) that can self-assemble onto a flexible carrying substrate through magnetic force, in a reversible and reconfigurable manner. A set of soft electronics in HMGN exhibits enhanced performances in the measurements of electrophysiological signals, temperature, and concentrations of metabolites. All these flexible HMGN sensors can adhere to a carrying substrate at any position and in any spatial arrangement, to allow for wearable sensing with customizable sensitivity, modality, and spatial coverage. The HMGN represents a promising material for constructing soft electronics that can be reconfigured for various applications.

Enhanced sensing performance of tapered profile in the apodized fiber Bragg grating for detection of cancerous cells utilizing their refractive index.

Tapered profiles are introduced in Bessel and Blackman apodized fiber Bragg grating, and their sensing performance is theoretically estimated. Reflectivity equation of proposed apodized tapered profiles is obtained using transfer matrix method and coupled mode theory. Since the effective refractive index (RI) of proposed waveguides varies with core radius therefore amplitude distribution and penetration depth of light in surroundings are modified. It is found that the exponential tapered profile with Bessel apodization shows a higher sensitivity of 682.5 nm/RIU, detection accuracy of 3858.45, and quality parameter 1718.02/RIU with minimum full width at half maxima (FWHM) 0.3972 nm of the reflection spectra. The spectral response of the tapered fiber Bragg grating is also analyzed through a group delay study. Again, exponential profile was found to be particularly effective, producing less group delay ripples 2.98 ps and is maximum slope of 42.43°. Hence, exponential tapered profile is demonstrated for detection of cancer cells lies between the refractive index of 1.3333-1.4412.

Enhancement of NO2 Gas Sensing Properties of Polypyrrole by Polarization Doping with DBS: Experimental and DFT Studies.

A new technique of polarization doping was adopted to improve NO2 gas sensing properties of the polypyrrole (PPy) sensor. PPy nanosheets polarization doped with sodium dodecyl benzenesulfonate (SDBS) were synthesized by low-temperature polymerization. The semiagglomerated PPy nanosheets were well-dispersed and a large specific surface areas due to the introduction of dodecyl benzenesulfonate (DBS). The DBS doped PPy sensor shows excellent NO2 sensing performance. Polarization of sulfonate ions to PPy enhanced the adsorption ability of NO2 with the synergistic effect of NO2. The adsorption energy (-0.676 eV) and electron transfer (0.521 |e|) of PPy to NO2 increased greatly after doping. An unoccupied electron state is observed in the valence band electron structure of PPy/DBS after the adsorption of NO2 by calculations of Density Functional Theory (DFT). The intermolecular synergy between NO2 and PPy/DBS causes a strong polarization, resulting in a high polarization potential, which enhances the NO2 sensing performance of PPy sensor. It is of great significance to develop NO2 detection device based on PPy that works at room temperature.