Proton-Conductive Gas Sensor: a New Way to Realize Highly Selective Ammonia Detection for Analysis of Exhaled Human Breath

SnTe/SnSe Heterojunction Based Ammonia Sensors with Excellent Withstand to Ambient Humidities

Non-invasive breath testing has gained increasing importance for early disease screening, spurring research into cheap sensors for detecting trace biomarkers such as ammonia. However, real-life deployment of ammonia sensors remains hindered by susceptibility to humidity-induced interference. The SnTe/SnSe heterojunction-based chemiresistive-type sensor demonstrates an excellent response/recovery to different concentrations of ammonia from 0.1 to 100 ppm at room temperature. The improved sensing properties of the heterojunctions-based sensors compared to single-phased SnTe or SnSe can be attributed to the stronger NH3 adsorptions, more Te vacancies, and hydrophobic surface induced by the formed SnTe/SnSe heterojunctions. The sensing mechanisms are investigated in detail by using in situ techniques such as diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), Kelvin probe, and a.c. impedance spectroscopy together with the Density-Function-Theory calculations. The formed heterojunctions boost the overall charge transfer efficiency between the ammonia and the sensing materials, thus leading to the desirable sensing features as well, with excellent resistance to ambient humidities.

Hydrophobic functionalization of a metal-organic framework as an ammonia visual sensing material under high humidity conditions

Since exhaled ammonia (NH3) is one of the metabolic markers of liver and kidney diseases, ammonia visual sensing materials in humid environments have received extensive attention and investigation. Herein, through a tailor-made pore environment provided by metal-organic framework (MOF) materials (CH3-Cu(BDC)), we achieved NH3 anti-interference sensing with apparent color changing under humid conditions. With methyl (CH3-) functionalization, CH3-Cu(BDC) demonstrated a strong response for trace ammonia and showed high selectivity under a humid environment. Grand canonical Monte Carlo (GCMC) simulations indicated that CH3-Cu(BDC) showed stronger attraction towards NH3 molecules than H2O. Benefiting from the target changing coordination environment, CH3-Cu(BDC) showed a rapid response and simple analysis properties for patients' exhaled air. The strategy used in this study not only provides a demonstration case for NH3 colorimetric sensing with high humidity and anti-interference but also introduces a new method for painless and quick exhaled breath analysis for diagnosis of patients with kidney and liver diseases.

Mixed Potential Type Isoprene Sensor for the Application in Real-Time Monitoring of Biomarker Gases

Monitoring of isoprene in exhaled breath is expected to provide a noninvasive and painless method for dynamic monitoring of physiological and metabolic states during exercise. However, for real-time and portable detection of isoprene, gas sensors have become the best choice for gas detection technology, which are crucial to achieving the goal of anytime, anywhere, human-centered healthcare in the future. Here, we first report a mixed potential type isoprene sensor based on a Gd2Zr2O7 solid electrolyte and a CdSb2O6 sensing electrode, which enables sensitive detection for isoprene with sensitivities of -21.2 mV/ppm and -65.8 mV/decade in the range of 0.05-1 and 1-100 ppm. The sensing behavior of the sensor follows the mixed potential sensing mechanism and was further verified by the electrochemical polarization curves. The significant differentiation between the sensor response to exhaled breath of healthy individuals and simulated breath containing different concentrations of isoprene demonstrates the potential of the sensor for the detection of isoprene in exhaled breath. Simultaneously, monitoring of isoprene during exercise signifies the feasibility of the sensor in dynamic monitoring of physiological indicators, which is not only of great significance for optimizing training and guiding therapeutic exercise intervention in sporting scenarios but also expected to help further reveal the interaction between exercise, muscle, and organ metabolism in medicine.

Self-Healing, Laminated, and Low Resistance NH3 Sensor Based on 6,6',6″-(Nitrilotris(benzene-4,1-diyl))tris(5-phenylpyrazine-2,3-dicarbonitrile) Sensing Material Operating at Room Temperature

With the rapid development of the concept of the Internet of Things (IoT), gas sensors with the function of simulating the human sense of smell became irreplaceable as a key element. Among them, ammonia (NH3) sensors played an important role in respiration tests, environmental monitoring, safety, and other fields. However, the fabrication of the high-performance device with high stability and resistance to mechanical damages was still a challenge. In this work, polyurethane (PU) with excellent self-healing ability was applied as the substrate, and the sensor was designed from new sensitive material design and device structure optimization, through applying the organic molecule with groups which could absorb NH3 and the laminated structure to shorten the electronic transmission path to achieve a low resistance state and favorable sensing properties. Accordingly, a room temperature flexible NH3 sensor based on 6,6',6″-(nitrilotris(benzene-4,1-diyl))tris(5-phenylpyrazine-2,3-dicarbonitrile) (TPA-3DCNPZ) was successfully developed. The device could self-heal by means of a thermal evaporation assisted method. It exhibited a detection limit of 1 ppm at 98% relative humidity (RH), as well as great stability, selectivity, bending flexibility, and self-healing properties. The improved NH3 sensing performance under high RH was further investigated by complex impedance plots (CIPs) and density functional theory (DFT), attributing to the enhanced adsorption of NH3. The TPA-3DCNPZ based NH3 sensors proved to have great potential for application on simulated exhaled breath to determine the severity of kidney diseases and the progress of treatment. This work also provided new ideas for the construction of high-performance room temperature NH3 sensors.

Sodium titanate: A proton conduction material for ppb-level NO2 detection with near-zero power consumption

Constrained by the traditional charge transfer sensing mechanism, it is quite challenging to fabricate NO2 sensors that simultaneously exhibit high sensitivity, rapid response/recovery, and low power consumption. Herein, sodium titanate (NTO), a layered material with abundant surface-rooted OH groups (OHR), is demonstrated to be a promising NO2 sensing material. To understand the sensing behavior of NTO, the influences of operating temperature, applied voltage, and relative humidity are investigated, and a novel OHR-enabled proton conduction sensing mechanism is proposed. The sensing process mainly involves selective NO2 adsorption on OHR, thereby lowering the activation energy for proton transportation along the NTO surface. Meanwhile, the moderate intermolecular interaction makes NO2 both easily adsorbed and desorbed at room temperature. Hence, NTO exhibits a highly sensitive, rapid, and fully recoverable response (∼5.7-1 ppm NO2 within 3 s), wide detection range (1 ppb-20 ppm), good stability (>2 months), and near-zero power consumption (0.5 nW). Finally, we demonstrate that NTO has an excellent practical indoor/outdoor NO2 sensing ability. This work offers a new pathway to resolve the inherent conflicts in available NO2 sensors by using NTO via the OHR-enabled proton conduction sensing mechanism, which may also provide insight into designing high-performance sensors for other gases.

Electronic sensitization enhanced p-type ammonia gas sensing of zinc doped MoS2/RGO composites

Zinc (Zn) doping induced synergetic effects of defects engineering and heterojunction in Molybdenum disulphide/Reduced graphene oxide (MoS₂/RGO) effectively enhances the p-type Volatile organic compounds (VOC) gas sensing traits and helps in tailoring the over dependence on noble metals for surface sensitization. Through this work, we have successfully prepared Zn doped MoS₂ grafted on RGO employing an in-situ hydrothermal method. Optimal doping concentration of Zn dopants in the MoS₂ lattice triggered more active sites on the basal plane of MoS₂ with the aid of defects promoted by the zinc dopants. Effective intercalation of RGO further boost up the exposed surface area of Zn doped MoS₂ for further interaction of ammonia gas molecules. Besides, smaller crystallite size brought out by 5% Zn dopants aids in efficient charge transfer across the heterojunctions that further amplifies the ammonia sensing traits with a peak response of 32.40% along with a response time of 21.3 s and recovery time of 44.90 s. The as prepared ammonia gas sensor exhibited excellent selectivity and repeatability. The obtained results reveal that transition metal doping into the host lattice proves to be a promising approach for VOC sensing characteristics of p-type gas sensors and gives insight about the importance of dopants and defects for the development of highly efficient gas sensors in the future.

Wearable Dual-Signal NH3 Sensor with High Sensitivity for Non-invasive Diagnosis of Chronic Kidney Disease

NH₃ gas in human exhaled breath contains abundant physiological information related to human health, especially chronic kidney disease (CKD). Unfortunately, up to now, most wearable NH₃ sensors show inevitable defects (low sensitivity, easy to be interfered by the environment, etc.), which may lead to misdiagnosis of CKD. To solve the above dilemma, a nanoporous, heterogeneous, and dual-signal (optical and electrical) wearable NH₃ sensor mask is developed successfully. More specifically, a polyacrylonitrile/bromocresol green (PAN/BCG) nanofiber film as a visual NH₃ sensor and a polyacrylonitrile/polyaniline/reduced graphene oxide (PAN/PANI/rGO) nanofiber film as a resistive NH₃ sensor are constructed. Due to the high specific surface area and abundant NH₃ binding sites of these two nanofiber films, they exhibit good NH₃ sensing performance. However, although the visual NH₃ sensor (PAN/BCG nanofiber film) is simple without the need of any detecting facilities and quite stable when temperature and humidity change, it shows poor sensitivity and resolution. In comparison, the resistive NH₃ sensor (PAN/PANI/rGO nanofiber film) is of high sensitivity, fast response, and good resolution, but its electrical signal is easily interfered by the external environment (such as humidity, temperature, etc.). Considering that the sensing principles between a visual NH₃ sensor and resistive NH₃ sensor are significantly different, a wearable dual-signal NH₃ sensor containing both a visual NH₃ sensor and resistive NH₃ sensor is further explored. Our data prove that the two sensing signals in this dual-signal NH₃ sensor mask can not only work well without interference with each other but also complement each other to improve the sensing accuracy, indicating its potential application in non-invasive diagnosis of CKD.

Engineering of ZnO/rGO towards NO2 Gas Detection: Ratio Modulated Sensing Type and Heterojunction Determined Response

Nanoscale heterostructured zinc oxide/reduced graphene oxide (ZnO/rGO) materials with p-n heterojunctions exhibit excellent low temperature NO₂ gas sensing performance, but their doping ratio modulated sensing properties remain poorly understood. Herein, ZnO nanoparticles were loaded with 0.1~4% rGO by a facile hydrothermal method and evaluated as NO₂ gas chemiresistor. We have the following key findings. First, ZnO/rGO manifests doping ratio-dependent sensing type switching. Increasing the rGO concentration changes the type of ZnO/rGO conductivity from n-type (<0.6% rGO) to mixed n/p -type (0.6~1.4% rGO) and finally to p-type (>1.4% rGO). Second, interestingly, different sensing regions exhibit different sensing characteristics. In the n-type NO₂ gas sensing region, all the sensors exhibit the maximum gas response at the optimum working temperature. Among them, the sensor that shows the maximum gas response exhibits a minimum optimum working temperature. In the mixed n/p-type region, the material displays abnormal reversal from n- to p-type sensing transitions as a function of the doping ratio, NO₂ concentration and working temperature. In the p-type gas sensing region, the response decreases with increasing rGO ratio and working temperature. Third, we derive a conduction path model that shows how the sensing type switches in ZnO/rGO. We also find that p-n heterojunction ratio (n_p-n/n_rGO) plays a key role in the optimal response condition. The model is supported by UV-vis experimental data. The approach presented in this work can be extended to other p-n heterostructures and the insights will benefit the design of more efficient chemiresistive gas sensors.

Response of Quartz Crystal Microbalance to Liquid Electrical Properties

Quartz crystal microbalance (QCM) operating in liquid can detect not only liquid mechanical properties (liquid density and viscosity) but also liquid electrical properties (liquid dielectric constant and conductivity). However, the relevant research so far has mainly focused on the liquid conductive property and mostly used relative values, which cannot fully reflect the response of QCM to liquid electrical properties. To study the effect of the electrical field scattering on the QCM response to liquid electrical properties and whether there is a coupling between the mechanical, dielectric, and conductive properties, reference groups for excluding the liquid mechanical effect were set up; solutions (isopropanol/water) with different dielectric constants and conductivities were adopted; static capacitance of the QCM covered with the isopropanol/water solutions was measured. The results indicate that the electrical field scattering plays an important role in the sensitivity of QCM to the dielectric property of liquid. There may be no, or very little, coupling between mechanical and electrical properties, but there is a coupling between dielectric and conductive properties at low conductivity, while at high conductivity, the conductive property is the dominant factor. The results are meaningful for understanding the multi-property sensing of QCM in the liquid phase.

Porous Oxide-Functionalized Seaweed Fabric as a Flexible Breath Sensor for Noninvasive Nephropathy Diagnosis

Ever-increasing quality of life demands low-power and reliable gas-sensing technology for point-of-care monitoring of human health by relevant breath biomarkers. However, precise identification is rather challenging due to the relatively small concentration and an abundance of interferents. Herein, a breath sensor that can detect ppb-level ammonia is constructed based on a soft-hard interface design of biocompatible seaweed fabric and nanosheet-assembled bismuth oxide architectures after undergoing heat treatment. Benefiting from abundant defective sites and surface chemical state changes, the flexible sensor can work at room temperature and exhibits superior characteristics for ammonia detection, including ultrahigh response (1296), short response/recovery time (12/6 s), small detection limit (117 ppb), and remarkable anti-interference, even after repetitive mechanical bending and long-term fatigue. Furthermore, the flexible sensor demonstrates a noticeable response to the exhaled breath of a patient with Helicobacter pylori infection. After connecting the sensor with a green-light-emitting diode (LED) in the circuit, an alarm system successfully warns about ammonia levels based on the brightness of the LED. This work provides a potential strategy for wide-range ammonia detection and opens new applications in predictive and personalized healthcare platforms for noninvasive medical diagnosis.

Trends in the Development of Electronic Noses Based on Carbon Nanotubes Chemiresistors for Breathomics

The remarkable potential of breath analysis in medical care and diagnosis, and the consequent development of electronic noses, is currently attracting the interest of the research community. This is mainly due to the possibility of applying the technique for early diagnosis, screening campaigns, or tracking the effectiveness of treatment. Carbon nanotubes (CNTs) are known to be good candidates for gas sensing, and they have been recently considered for the development of electronic noses. The present work has the aim of reviewing the available literature on the development of CNTs-based electronic noses for breath analysis applications, detailing the functionalization procedure used to prepare the sensors, the breath sampling techniques, the statistical analysis methods, the diseases under investigation, and the population studied. The review is divided in two main sections: one focusing on the e-noses completely based on CNTs and one reporting on the e-noses that feature sensors based on CNTs, along with sensors based on other materials. Finally, a classification is presented among studies that report on the e-nose capability to discriminate biomarkers, simulated breath, and animal or human breath.

Emerging Development of Auto-Charging Sensors for Respiration Monitoring

In recent years, the development of biomedical monitoring systems, including respiration monitoring systems, has been accelerated. Wearable and implantable medical devices are becoming increasingly important in the diagnosis and management of disease and illness. Respiration can be monitored using a variety of biosensors and systems. Auto-charged sensors have a number of advantages, including low cost, ease of preparation, design flexibility, and a wide range of applications. It is possible to use the auto-charged sensors to directly convert mechanical energy from the airflow into electricity. The ability to monitor and diagnose one's own health is a major goal of auto-charged sensors and systems. Respiratory disease model output signals have not been thoroughly investigated and clearly understood. As a result, figuring out their exact interrelationship is a difficult and important research question. This review summarized recent developments in auto-charged respiratory sensors and systems in terms of their device principle, output property, detecting index, and so on. Researchers with an interest in auto-charged sensors can use the information presented here to better understand the difficulties and opportunities that lie ahead.

Optical Microfibers for Sensing Proximity and Contact in Human-Machine Interfaces

The monitoring of proximity-contact events is essential for human-machine interactions, intelligent robots, and healthcare monitoring. We report a dual-modal sensor made with two functionalized optical microfibers (MFs), which is inspired by the somatosensory system of human skin. The integrated sensor with a hierarchical structure gradationally detects finger approaching and touching by measuring the relative humidity (RH) and force-triggered light intensity variations. Specifically, the RH sensory part shows enhanced evanescent absorption, achieving a sensitive RH measurement with a fast response (110 ms), a high resolution (0.11%RH), and a wide working range (10-100%RH). Enabled by the transition from guided modes into radiation modes of the waveguiding MF, the force sensory part exhibits a high sensitivity (6.2%/kPa) and a fast response (up to 1.5 kHz). By using a real-time data processing unit, the proximity-contact sensor (PCS) achieves continuous detection of the full-contact events, including finger approaching, contacting, pressing, releasing, and leaving. As a proof of concept, the electromagnetic-interference-free PCS enables a smart switch system to recognize the proximity and contact of bare/gloved fingers. Moreover, skin humidity detection and respiration monitoring are realized. These initial results pave the way toward a category of optical collaborative devices ranging from human-machine interfaces to multifunctional on-skin healthcare sensors.

Controlled preparation and gas sensitive properties of two-dimensional and cubic structure ZnSnO3

Two-dimensional (2D) ZnSnO₃ is a promising candidate for future gas sensors due to its high chemical response and excellent electronic properties. However, the preparation of 2D ZnSnO₃ nanosheets by utilizing soluble inorganic salts and nonorganic solvents remains a challenge. In this work, 2D ZnSnO₃ was synthesized via a facile graphene oxide (GO)-assisted co-precipitation method, in which inorganic salts in the aqueous phase replaced metal organic salts in a non-aqueous system. Meanwhile, a "dissolution and recrystallization" mechanism was proposed to explain the transformation from 3D nanocubes to 2D nanosheets. In comparison, the 2D ZnSnO₃ nanosheets showed a higher response to formaldehyde (HCHO) at low operating temperature (100 °C). The response (R_a/R_g) of the 2D ZnSnO₃ sensor to 10 ppm HCHO was as high as 57, which was approximately 5 times the response of the ZnSnO₃ nanocubes sensor. However, the ZnSnO₃ nanocubes sensor showed better gas sensing performance to ethanol at high temperature (200 °C). Different gas-sensitive properties were attributed to the different gas diffusion and adsorption processes caused by the morphology and nanostructure. Moreover, both sensors could detect either 0.1 ppm HCHO or ethanol at their optimum operating temperature. This work presents a relatively economical method to prepare 2D compound metal oxides, provides a novel "dissolution and recrystallization" mechanism for 2D multi-metal oxide preparation, and sheds light on the great potential of high-efficiency HCHO and/or ethanol gas sensors.

Recent progress of self-powered respiration monitoring systems

Wearable and implantable medical devices are playing more and more key roles in disease diagnosis and health management. Various biosensors and systems have been used for respiration monitoring. Among them, self-powered sensors have some special characteristics such as low-cost, easy preparation, highly designable, and diversified. The respiratory airflow can drive the self-powered sensors directly to convert mechanical energy of the airflow into electricity. One of the major goals of the self-powered sensors and systems is realizing health monitoring and diagnosis. The relationship between the output signals and the models of respiratory diseases has not been studied deeply and clearly. Therefore, how to find an accurate relationship between them is a challenging and significant research topic. This review summarized the recent progress of the self-powered respiratory sensors and systems from aspects of device principle, output property, detecting index and so on. The challenges and perspectives have also been discussed for reference to the researchers who are interested in the field of self-powered sensors.

Flexible, non-contact and multifunctional humidity sensors based on two-dimensional phytic acid doped co-metal organic frameworks nanosheets

The development of high-performance humidity sensors is of great significance to explore their practical applications in the fields of environment, energy saving and safety monitoring. Herein, a flexible, non-contact and multifunctional humidity sensor based on two-dimensional Co-metal organic frameworks (Co-MOF) nanosheets is proposed, which is fabricated by simple bottom-up synthesis method. Furthermore, environmentally friendly, renewable and abundant biomass phytic acid (PA) is modified on the surface of Co-MOF nanosheets, which releases free protons being capable of etching the framework of MOF to improve the hydrophilicity and conductivity of MOF. Compared with Co-MOF-based sensor, the Co-MOF@PA-based sensor exhibits significantly enhanced sensitivity and broadened response range within 23-95% relative humidity (RH). The humidity sensor has an excellent humidity sensing response over 2 × 10³. The Co-MOF@PA-based sensor shows good flexibility and humidity sensing properties, endowing it with multifunctional applications in real-time facial respiration monitoring, skin humidity perception, cosmetic moisturizing evaluation and fruit freshness testing. Four respiration patterns, including slow breath, deep breath, normal breath and fast breath are wirelessly monitored in real time by Co-MOF@PA-based sensor and recorded by mobile phone software. The research work presents potential applications in human-machine interactions (HMI) devices in future.

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.

Theoretical and Experimental Research on Ammonia Sensing Properties of Sulfur-Doped Graphene Oxide

In this paper, gas sensing characteristics of sulfur-doped graphene oxide (S-GO) are firstly presented. The results of the sensing test revealed that, at room temperature (20 °C), S-GO has the optimal sensitivity to NH3. The S-GO gas sensor has a relatively short response and recovery time for the NH3 detection. Further, the sensing limit of ammonia at room temperature is 0.5 ppm. Theoretical models of graphene and S-doped graphene are established, and electrical properties of the graphene and S-doped graphene are calculated. The enhanced sensing performance was ascribed to the electrical properties’ improvement after the graphene was S-doped.

A Microdroplet-Based Colorimetric Sensing Platform on a CMOS Imager Chip

Interest in mobile chemical sensors is on the rise, but significant challenges have restricted widespread adoption into commercial devices. To be useful these sensors need to have a predictable response, easy calibration, and be integrable with existing technology, preferably fitting on a single chip. With respect to integration, the CMOS imager makes an attractive template for an optoelectronic sensing platform. Demand for smartphones with cameras has driven down the price and size of CMOS imagers over the past decade. The low cost and accessibility of these powerful tools motivated us to print chemical sensing elements directly on the surface of the photodiode array. These printed colorimetric microdroplets are composed of a nonvolatile solvent so they remain in a uniform and homogeneous solution phase, an ideal medium for chemical interactions and optical measurements. By imaging microdroplets on the CMOS imager surface we eliminated the need for lenses, dramatically scaling down the size of the sensing platform to a single chip. We believe the technique is generalizable to many colorimetric formulations, and as an example we detected gaseous ammonia with Cu(II). Limits of detection as low as 27 ppb and sensor-to-sensor variation of less than 10% across multiple printed arrays demonstrated the high sensitivity and repeatability of this approach. Sensors generated this way could share a single calibration, greatly reducing the complexity of incorporating chemical sensors into mobile devices. Additional testing showed the sensor can be reused and has good selectivity; sensitivity and dynamic range can be tuned by controlling droplet size.

Porous materials applied to biomarker sensing in exhaled breath for monitoring and detecting non-invasive pathologies

Overview of the use of porous materials for gas sensing to analyze the exhaled breath of patients for disease identification.