Selective Diagnosis of Diabetes Using Pt-Functionalized WO3 Hemitube Networks As a Sensing Layer of Acetone in Exhaled Breath

Enhanced Colorimetric Detection of Volatile Organic Compounds Using a Dye-Incorporated Photonic Crystal-Based Sensor Array

Chemoresponsive dyes offer the potential to selectively detect volatile organic compounds (VOCs) unique to certain disease states. Among different VOC sensing techniques, colorimetric sensing offers the advantage of facile recognition. However, it is often challenging to discern the color changes by the naked eye. Here, highly sensitive colorimetric VOC sensor arrays from dye-incorporated colloidal photonic crystals (dye-cPhCs) are reported. cPhCs are scalably fabricated on a 4-inch wafer by spin-coating of silica nanoparticles (NPs) dispersed in a photo-cross-linkable monomer, where the gradient shear flow along the film thickness creates densely-packed square arrays of NPs in the top layers, whereas the bulk is quasi-amorphous with larger periodicities. The broadened reflection peak allows for augmented dye absorption originating from the overlap between the photonic bandgap edge of the cPhC and the dye absorption peak, leading to a more noticeable color change upon exposure to VOCs. The sensor array generates distinct color difference maps for acetaldehyde, acetone, and acetic acid, respectively, without any data amplification. The limit of detection for acetaldehyde, acetone, and acetic acid is 1, 0.1, and 0.02 ppm, respectively. Moreover, VOC can be diagonalized by visually intuitive pattern recognition, and principal component analysis at reduced dimensionality is demonstrated.

Symmetric Catalyst Design Employing Ir Nanoparticles on Black WO3- x Nanofiber Support for Boosting Water Electrolysis

The efficient evolution of gaseous hydrogen and oxygen from water is required to realize sustainable energy conversion systems. To address the sluggish kinetics of the multielectron transfer reaction, bifunctional catalyst materials for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) should be developed. Herein, a tailored combination of atomically minimized iridium catalysts and highly conductive black WO3- x nanofiber supports are developed for the bifunctional electrolyzer system. Atomic Ir catalysts, particularly those that activate the OER, minimize the utilization of precious metals. The oxygen-deficient black WO3- x NF support, which boosts the HER, offers increased electronic conductivity and favorable nucleation sites for Ir loading. The Ir-black WO3- x NFs exhibit increased double-layer capacitance, a significantly reduced onset potential, lower Tafel slope, and stable cyclability for both the OER and HER, compared to large-sized Ir catalysts loaded on white WO3 nanofibers. This study offers a strategy for developing an optimal catalyst material with suitable supports for high-performance and economical water electrolysis systems for achieving carbon-negative targets.

Direct mass spectrometry analysis of exhaled human breath in real-time

The molecular composition of exhaled human breath can reflect various physiological and pathological conditions. Considerable progress has been achieved over the past decade in real-time analysis of exhaled human breath using direct mass spectrometry methods, including selected ion flow tube mass spectrometry, proton transfer reaction mass spectrometry, extractive electrospray ionization mass spectrometry, secondary electrospray ionization mass spectrometry, acetone-assisted negative photoionization mass spectrometry, atmospheric pressure photoionization mass spectrometry, and low-pressure photoionization mass spectrometry. Here, recent developments in direct mass spectrometry analysis of exhaled human breath are reviewed with regard to analytical performance (chemical sensitivity, selectivity, quantitative capabilities) and applications of the developed methods in disease diagnosis, targeted molecular detection, and real-time metabolic monitoring.

Handheld device quantifies breath acetone for real-life metabolic health monitoring

Non-invasive breath analysis with mobile health devices bears tremendous potential to guide therapeutic treatment and personalize lifestyle changes. Of particular interest is the breath volatile acetone, a biomarker for fat burning, that could help in understanding and treating metabolic diseases. Here, we report a hand-held (6 × 10 × 19.5 cm³), light-weight (490 g), and simple device for rapid acetone detection in breath. It comprises a tailor-made end-tidal breath sampling unit, connected to a sensor and a pump for on-demand breath sampling, all operated using a Raspberry Pi microcontroller connected with a HDMI touchscreen. Accurate acetone detection is enabled by introducing a catalytic filter and a separation column, which remove and separate undesired interferents from acetone upstream of the sensor. This way, acetone is detected selectively even in complex gas mixtures containing highly concentrated interferents. This device accurately tracks breath acetone concentrations in the exhaled breath of five volunteers during a ketogenic diet, being as high as 26.3 ppm. Most importantly, it can differentiate small acetone changes during a baseline visit as well as before and after an exercise stimulus, being as low as 0.5 ppm. It is stable for at least four months (122 days), and features excellent bias and precision of 0.03 and 0.6 ppm at concentrations below 5 ppm, as validated by proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). Hence, this detector is highly promising for simple-in-use, non-invasive, and routine monitoring of acetone to guide therapeutic treatment and track lifestyle changes.

The potential of esculin in ameliorating Type-2 diabetes mellitus induced neuropathy in Wistar rats and probing its inhibitory mechanism of insulin aggregation

Diabetic neuropathy encompasses multiple pathological disturbances, many of which coincide with the pathophysiological mechanisms of neurodegenerative disorders. In the present study, various biophysical techniques like Rayleigh light scattering assay, Thioflavin T assay, far-UV Circular Dichroism spectroscopy, Transmission electron microscopy have unveiled the anti-fibrillatory effect of esculin upon human insulin fibrillation. MTT cytotoxicity assay demonstrated the biocompatibility of esculin and in-vivo studies such as behavioral tests like hot plate test, tail immersion test, acetone drop test, plantar test were performed for validating diabetic neuropathy. Assessment of levels of serum biochemical parameters, oxidative stress parameters, pro-inflammatory cytokines as well as neuron specific markers was done in the current study. Rat brains were subjected to histopathology and their sciatic nerves were subjected to transmission electron microscopy to analyze myelin structure alterations. All these results reveal that esculin ameliorates diabetic neuropathy in experimental diabetic rats. Conclusively, our study demonstrates the anti-amyloidogenic potential of esculin in the form of inhibition of human insulin fibrillation, making it a promising candidate in combating neurodegenerative disorders in the near future and the results of various behavioral, biochemical, and molecular studies reveal that esculin possesses anti-lipidemic, anti-inflammatory, anti-oxidative and neuroprotective properties which help in ameliorating diabetic neuropathy in streptozotocin induced diabetic Wistar rats.

Candy-like heterojunction nanocomposite of WO3/Fe2O3-based semiconductor gas sensor for the detection of triethylamine

A highly efficient gas sensor for the detection of triethylamine based on candy-like WO₃/Fe₂O₃ nanocomposite was prepared. The control of morphology and sensing performance of n-n heterojunction WO₃/Fe₂O₃ nanocomposites were successfully achieved by the modulation of Fe element content. When the ratio of Fe to W is 0.4, the candy-like nanocomposite of WO₃/Fe₂O₃ with great performance is obtained. It is interesting that the candy-like nanocomposite of WO₃/Fe₂O₃ with a large specific surface area exhibits better selectivity and sensitivity for sensing TEA gases at a lower operating temperature (260 °C) compared with the gas sensor prepared by using WO₃ alone. To verify the feasibility, the sensing mechanism was investigated and real sample tests were conducted and discussed. Finally, a TEA gas sensor with low limit of detection, short response/recovery time (15/162 s), and high sensitivity was developed. In addition, the prepared gas sensor has satisfactory stability and selectivity and has practical application value.

Temperature-Dependent n-to-p-Type Transition of 2D Mn Oxide Nanosheets toward NO2 for Flexible Gas Sensor Application

Rapid and on-site detection of nitrogen dioxide (NO₂) is important for environmental monitoring as NO₂ is a highly toxic chemical emitted from automobiles and power plants. In this study, we proposed atomically thin two-dimensional (2D) Mn oxide nanosheets (NSs) assembled on a flexible heating substrate for application in flexible and wearable NO₂ sensors. A liquid-phase exfoliation technique was used to obtain individual Mn oxide layers that formed a homogeneous suspension. A flexible heater was fabricated by partially embedding Ag nanotubes (NTs) on the surface of a colorless polyimide (CPI) film for use as a sensor substrate. Temperature-dependent NO₂ sensing properties were investigated via control of the operating temperature using a Ag NT-embedded CPI heating film. As a result, the n-type sensing behavior of the Mn oxide NSs exhibited a response [(R_gas - R_air)/R_air × 100 (%)] of 1.20 ± 0.21% for 20 ppm NO₂ at room temperature (25 °C). Meanwhile, n-p transition occurred with p-type sensing property as the operating temperature increased to 150 °C with an improved response [(R_air - R_gas)/R_air × 100 (%)] of 4.10 ± 0.42% for 20 ppm NO₂. The characteristic n-p transition of Mn oxide NSs at different operating temperatures was evidenced by the surface-adsorbed oxygen ions (i.e., O₂^- and O^-) and nitrate species (NO₃^- and NO₃^2-).

Exploitation of Schottky-Junction-based Sensors for Specifically Detecting ppt-Concentration Gases

Gas species and concentrations of human-exhaled breath correlate with health, wherein disease markers contain volatile organic compounds (VOCs) of concentrations in parts per billion. It is expected that a gas-sensing strategy possesses a gas specificity and detection limit in the parts per trillion (ppt) range; however, it is still a challenge. This investigation has exploited the Schottky junction of gas sensors for detecting the reactance signal of ppt VOC, aiming for a specific and rapid detection toward disease marker acetone. In this new sensing paradigm, formed by the engineered energy band between metal-semiconductor contact, the Schottky junction is accessed to specific modulation of different adsorbate dopings and the corresponding reactance signal is measured. Regarding the detection toward ppt concentration of acetone, this sensing paradigm possesses rapid (∼100 s) and room-temperature response, molecular specificity, and 34 ppt of detection limit. The proposed detection paradigm is demonstrated to show a high feasibility toward detection of disease marker acetone.

Naringin Attenuates the Diabetic Neuropathy in STZ-Induced Type 2 Diabetic Wistar Rats

The application of traditional medicines for the treatment of diseases, including diabetic neuropathy (DN), has received great attention. The aim of this study was to investigate the ameliorative potential of naringin, a flavanone, to treat streptozotocin-induced DN in rat models. After the successful induction of diabetes, DN complications were measured by various behavioral tests after 4 weeks of post-induction of diabetes with or without treatment with naringin. Serum biochemical assays such as fasting blood glucose, HbA1c%, insulin, lipid profile, and oxidative stress parameters were determined. Proinflammatory cytokines such as TNF-α and IL-6, and neuron-specific markers such as BDNF and NGF, were also assessed. In addition, pancreatic and brain tissues were subjected to histopathology to analyze structural alterations. The diabetic rats exhibited increased paw withdrawal frequencies for the acetone drop test and decreased frequencies for the plantar test, hot plate test, and tail flick test. The diabetic rats also showed an altered level of proinflammatory cytokines and oxidative stress parameters, as well as altered levels of proinflammatory cytokines and oxidative stress parameters. Naringin treatment significantly improved these parameters and helped in restoring the normal architecture of the brain and pancreatic tissues. The findings show that naringin's neuroprotective properties may be linked to its ability to suppress the overactivation of inflammatory molecules and mediators of oxidative stress.

Enhanced Response for Foodborne Pathogens Detection by Au Nanoparticles Decorated ZnO Nanosheets Gas Sensor

Listeria monocytogenes is a hazardous foodborne pathogen that is able to cause acute meningitis, encephalitis, and sepsis to humans. The efficient detection of 3-hydroxy-2-butanone, which has been verified as a biomarker for the exhalation of Listeria monocytogenes, can feasibly evaluate whether the bacteria are contained in food. Herein, we developed an outstanding 3-hydroxy-2-butanone gas sensor based on the microelectromechanical systems using Au/ZnO NS as a sensing material. In this work, ZnO nanosheets were synthesized by a hydrothermal reaction, and Au nanoparticles (~5.5 nm) were prepared via an oleylamine reduction method. Then, an ultrasonic treatment was carried out to modified Au nanoparticles onto ZnO nanosheets. The XRD, BET, TEM, and XPS were used to characterize their morphology, microstructure, catalytic structure, specific surface area, and chemical composition. The response of the 1.0% Au/ZnO NS sensors vs. 25 ppm 3-hydroxy-2-butanone was up to 174.04 at 230 °C. Moreover, these sensors presented fast response/recovery time (6 s/7 s), great selectivity, and an outstanding limit of detection (lower than 0.5 ppm). This work is full of promise for developing a nondestructive, rapid and practical sensor, which would improve Listeria monocytogenes evaluation in foods.

Biosensor-Integrated Drug Delivery Systems as New Materials for Biomedical Applications

Biosensor-integrated drug delivery systems are innovative devices in the health area, enabling continuous monitoring and drug administration. The use of smart polymer, bioMEMS, and electrochemical sensors have been extensively studied for these systems, especially for chronic diseases such as diabetes mellitus, cancer and cardiovascular diseases as well as advances in regenerative medicine. Basically, the technology involves sensors designed for the continuous analysis of biological molecules followed by drug release in response to specific signals. The advantages include high sensitivity and fast drug release. In this work, the main advances of biosensor-integrated drug delivery systems as new biomedical materials to improve the patients’ quality of life with chronic diseases are discussed.

Heteronanostructural metal oxide-based gas microsensors

The development of high-performance, portable and miniaturized gas sensors has aroused increasing interest in the fields of environmental monitoring, security, medical diagnosis, and agriculture. Among different detection tools, metal oxide semiconductor (MOS)-based chemiresistive gas sensors are the most popular choice in commercial applications and have the advantages of high stability, low cost, and high sensitivity. One of the most important ways to further enhance the sensor performance is to construct MOS-based nanoscale heterojunctions (heteronanostructural MOSs) from MOS nanomaterials. However, the sensing mechanism of heteronanostructural MOS-based sensors is different from that of single MOS-based gas sensors in that it is fairly complex. The performance of the sensors is influenced by various parameters, including the physical and chemical properties of the sensing materials (e.g., grain size, density of defects, and oxygen vacancies of materials), working temperatures, and device structures. This review introduces several concepts in the design of high-performance gas sensors by analyzing the sensing mechanism of heteronanostructural MOS-based sensors. In addition, the influence of the geometric device structure determined by the interconnection between the sensing materials and the working electrodes is discussed. To systematically investigate the sensing behavior of the sensor, the general sensing mechanism of three typical types of geometric device structures based on different heteronanostructural materials are introduced and discussed in this review. This review will provide guidelines for readers studying the sensing mechanism of gas sensors and designing high-performance gas sensors in the future.

Metal-Oxide Nanomaterials Synthesis and Applications in Flexible and Wearable Sensors

Metal-oxide nanomaterials (MONs) have gained considerable interest in the construction of flexible/wearable sensors due to their tunable band gap, low cost, large specific area, and ease of manufacturing. Furthermore, MONs are in high demand for applications, such as gas leakage alarms, environmental protection, health tracking, and smart devices integrated with another system. In this Review, we introduce a comprehensive investigation of factors to boost the sensitivity of MON-based sensors in environmental indicators and health monitoring. Finally, the challenges and perspectives of MON-based flexible/wearable sensors are considered.

The Combination of Two-Dimensional Nanomaterials with Metal Oxide Nanoparticles for Gas Sensors: A Review

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects.

Breath Analysis: A Promising Tool for Disease Diagnosis-The Role of Sensors

Early-stage disease diagnosis is of particular importance for effective patient identification as well as their treatment. Lack of patient compliance for the existing diagnostic methods, however, limits prompt diagnosis, rendering the development of non-invasive diagnostic tools mandatory. One of the most promising non-invasive diagnostic methods that has also attracted great research interest during the last years is breath analysis; the method detects gas-analytes such as exhaled volatile organic compounds (VOCs) and inorganic gases that are considered to be important biomarkers for various disease-types. The diagnostic ability of gas-pattern detection using analytical techniques and especially sensors has been widely discussed in the literature; however, the incorporation of novel nanomaterials in sensor-development has also proved to enhance sensor performance, for both selective and cross-reactive applications. The aim of the first part of this review is to provide an up-to-date overview of the main categories of sensors studied for disease diagnosis applications via the detection of exhaled gas-analytes and to highlight the role of nanomaterials. The second and most novel part of this review concentrates on the remarkable applicability of breath analysis in differential diagnosis, phenotyping, and the staging of several disease-types, which are currently amongst the most pressing challenges in the field.

Emerging Theranostic Nanomaterials in Diabetes and Its Complications

Diabetes mellitus (DM) refers to a group of metabolic disorders that are characterized by hyperglycemia. Oral subcutaneously administered antidiabetic drugs such as insulin, glipalamide, and metformin can temporarily balance blood sugar levels, however, long-term administration of these therapies is associated with undesirable side effects on the kidney and liver. In addition, due to overproduction of reactive oxygen species and hyperglycemia-induced macrovascular system damage, diabetics have an increased risk of complications. Fortunately, recent advances in nanomaterials have provided new opportunities for diabetes therapy and diagnosis. This review provides a panoramic overview of the current nanomaterials for the detection of diabetic biomarkers and diabetes treatment. Apart from diabetic sensing mechanisms and antidiabetic activities, the applications of these bioengineered nanoparticles for preventing several diabetic complications are elucidated. This review provides an overall perspective in this field, including current challenges and future trends, which may be helpful in informing the development of novel nanomaterials with new functions and properties for diabetes diagnosis and therapy.

Black Tungsten Oxide Nanofiber as a Robust Support for Metal Catalysts: High Catalyst Loading for Electrochemical Oxygen Reduction

Black valve metal oxides with low oxygen vacancies are identified to be promising for various industrial applications, such as in gas sensing, photocatalysis, and rechargeable batteries, owing to their high reducibility and stability, as well as considerable fractions of low-valent metal species and oxygen vacancies in their lattices. Herein, the nanofiber (NF) of black oxygen-deficient tungsten trioxide (WO_{3- x} ) is presented as a versatile and robust support for the direct growth of a platinum catalyst for oxygen reduction reaction (ORR). The nonstoichiometric, poorly crystallized black WO_{3- x} NFs are prepared by electrospinning the W precursor into NFs followed by their low-temperature (650 °C) reductive calcination. The black WO_{3- x} NFs have adequate electrical conductivity owing to their decreased bandgap and amorphous structure. Remarkably, the oxygen-deficient surface (surface O/W = 2.44) facilitates the growth of small Pt nanoparticles, which resist aggregation, as confirmed by structural characterization and computational analysis. The Pt-loaded black WO_{3- x} NFs outperform the Pt-loaded crystalline white WO_{3- x} NFs in both the electrochemical ORR activity and the accelerated durability test. This study can inspire the use of oxygen-deficient metal oxides as supports for other electrocatalysts, and can further increase the versatility of oxygen-deficient metal oxides.

Magnetic detection of albuminuria using hematite nanorods synthesized via chemical hydrothermal method

Objectives

Albuminuria is a biomarker in the diagnosis of kidney disease which is due to the presence of high albumin in the urine and is one of the complications of diabetes. In recent years, the methods used to identify albuminuria have been expensive and time-consuming. Furthermore, another problem is the lack of accurate measurement of albuminuria. This problem leads to kidney isolation as well as a decrease in erythropoietin levels. Therefore, the main aim of our work is to design a magnetic nanobiosensor with better sensitivity to detect minimal levels of albuminuria.

Materials and Methods

In the present work, we synthesized Hematite Nano Rods (HNRs) using FeCl₃, NaOH and Cetyltrimethylammonium bromide (CTAB) precursors via the hydrothermal method. Then, HNRs were characterized using UV-vis spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and Vibrating Sample Magnetometer (VSM) techniques.

Results

The obtained results from clinical performance of the HNR nanobiosensor show that the magnetization changes of HNR in interaction with the albumin biomarker can determine the presence or absence of protein in biological samples. The accuracy and repeatability of the HNR nanobiosensor from the value of the R2 coefficient in the standard equation is 0.9743.

Conclusion

We obtained the standard curve through interaction of the HNRs with albumin protein. The standard equation is obtained by plotting the magnetization curve of a non-interacting sample to interacting samples in terms of protein concentration. The Bland-Altman statistical graph prove that the HNR nanobiosensor is as reliable as experimental methods.

Recent Trends in Exhaled Breath Diagnosis Using an Artificial Olfactory System

Artificial olfactory systems are needed in various fields that require real-time monitoring, such as healthcare. This review introduces cases of detection of specific volatile organic compounds (VOCs) in a patient's exhaled breath and discusses trends in disease diagnosis technology development using artificial olfactory technology that analyzes exhaled human breath. We briefly introduce algorithms that classify patterns of odors (VOC profiles) and describe artificial olfactory systems based on nanosensors. On the basis of recently published research results, we describe the development trend of artificial olfactory systems based on the pattern-recognition gas sensor array technology and the prospects of application of this technology to disease diagnostic devices. Medical technologies that enable early monitoring of health conditions and early diagnosis of diseases are crucial in modern healthcare. By regularly monitoring health status, diseases can be prevented or treated at an early stage, thus increasing the human survival rate and reducing the overall treatment costs. This review introduces several promising technical fields with the aim of developing technologies that can monitor health conditions and diagnose diseases early by analyzing exhaled human breath in real time.

Electrospun Metal Oxide Nanofibers and Their Conductometric Gas Sensor Application. Part 2: Gas Sensors and Their Advantages and Limitations

Electrospun metal oxide nanofibers, due to their unique structural and electrical properties, are now being considered as materials with great potential for gas sensor applications. This critical review attempts to assess the feasibility of these perspectives. This article discusses approaches to the manufacture of nanofiber-based gas sensors, as well as the results of analysis of the performances of these sensors. A detailed analysis of the disadvantages that can limit the use of electrospinning technology in the development of gas sensors is also presented in this article. It also proposes some approaches to solving problems that limit the use of nanofiber-based gas sensors. Finally, the summary provides an insight into the future prospects of electrospinning technology for the development of gas sensors aimed for the gas sensor market.

Chemical Gas Sensors: Recent Developments, Challenges, and the Potential of Machine Learning-A Review

Nowadays, there is increasing interest in fast, accurate, and highly sensitive smart gas sensors with excellent selectivity boosted by the high demand for environmental safety and healthcare applications. Significant research has been conducted to develop sensors based on novel highly sensitive and selective materials. Computational and experimental studies have been explored in order to identify the key factors in providing the maximum active location for gas molecule adsorption including bandgap tuning through nanostructures, metal/metal oxide catalytic reactions, and nano junction formations. However, there are still great challenges, specifically in terms of selectivity, which raises the need for combining interdisciplinary fields to build smarter and high-performance gas/chemical sensing devices. This review discusses current major gas sensing performance-enhancing methods, their advantages, and limitations, especially in terms of selectivity and long-term stability. The discussion then establishes a case for the use of smart machine learning techniques, which offer effective data processing approaches, for the development of highly selective smart gas sensors. We highlight the effectiveness of static, dynamic, and frequency domain feature extraction techniques. Additionally, cross-validation methods are also covered; in particular, the manipulation of the k-fold cross-validation is discussed to accurately train a model according to the available datasets. We summarize different chemresistive and FET gas sensors and highlight their shortcomings, and then propose the potential of machine learning as a possible and feasible option. The review concludes that machine learning can be very promising in terms of building the future generation of smart, sensitive, and selective sensors.

Highly Sensitive and Selective NiO/WO3 Composite Nanoparticles in Detecting H2S Biomarker of Halitosis

Indirectly monitoring halitosis via the detection of hydrogen sulfide (H₂S) biomarkers using gas sensors is a newly emerging technique. However, such H₂S sensors are required with critically high selectivity and sensitivity, as well as a ppb-level detection limit, which remains technologically challenging. To address such issues, here, we have developed highly sensitive and selective H₂S sensors with NiO/WO₃ nanoparticles (NPs), which have been synthesized by firstly hydrolyzing WO₃ NPs and subsequently decorating with NiO NPs in a hydrothermal process. Theoretically, the NiO/WO₃ NPs assist in forming a thicker electron depletion layer, adsorbing more oxygen species O₂^- to oxidize H₂S and finally release more electrons. Beneficially, 2.1 wt % NiO/WO₃ NPs show high sensitivity to H₂S (R_a/R_g = 15031 ± 1370 @ 10 ppm, 100 °C), which is 42.6-fold higher than that of the pristine WO₃ NPs (R_a/R_g = 353 ± 5.6 @ 10 ppm, 100 °C). Further, the H₂S sensor shows ppb-level detection limit (R_a/R_g = 4.95 ± 2.9 @ 0.05 ppm, 100 °C) and high selectivity. Practically, NiO/WO₃ NP sensor prototype has been employed to detect the simulated exhaled halitosis compared with that of gas chromatography, revealing a close concentration of H₂S. Our investigation offers an experimental base in future intelligent medical applications.

Exhaled breath analysis in disease detection

Investigating the use of exhaled breath analysis to diagnose and monitor different diseases has attracted much interest in recent years. This review introduces conventionally used methods and some emerging technologies aimed at breath analysis and their relevance to lung disease, airway inflammation, gastrointestinal disorders, metabolic disorders and kidney diseases. One section correlates breath components and specific diseases, whereas the other discusses some unique ideas, strategies, and devices to analyze exhaled breath for the diagnosis of some common diseases. This review aims to briefly introduce the potential application of exhaled breath analysis for the diagnosis and screening of various diseases, thereby providing a new avenue for the detection of non-invasive diseases.

Ultrafast Growth of h-MoO3 Microrods and Its Acetone Sensing Performance

Hexagonal molybdenum trioxide (h-MoO3) was synthesized by microwave-assisted hydrothermal method, allowing an ultrafast growth of unidimensional microrods with well-faceted morphology. The crystalline structure of this metastable phase was confirmed by X-ray diffraction (XRD) and Raman spectroscopy. Scanning electron microscopy (SEM) showed that hexagonal microrods can be obtained in one minute with well-defined exposed facets and the fine control of morphology. Sensing tests of the acetone biomarker revealed that the h-MoO3 microrods exhibit, at low ppm level, good sensor signal, fast response/recovery times, selectivity to different interferent gases, and a lower detection limit of 400 ppb.

Recent Advances in Nanomaterial-Enabled Wearable Sensors: Material Synthesis, Sensor Design, and Personal Health Monitoring

Wearable sensors have gained much attention due to their potential in personal health monitoring in a timely, cost-effective, easy-operating, and noninvasive way. In recent studies, nanomaterials have been employed in wearable sensors to improve the sensing performance in view of their excellent properties. Here, focus is mainly on the nanomaterial-enabled wearable sensors and their latest advances in personal health monitoring. Different kinds of nanomaterials used in wearable sensors, such as metal nanoparticles, carbon nanomaterials, metallic nanomaterials, hybrid nanocomposites, and bio-nanomaterials, are reviewed. Then, the progress of nanomaterial-based wearable sensors in personal health monitoring, including the detection of ions and molecules in body fluids and exhaled breath, physiological signals, and emotion parameters, is discussed. Furthermore, the future challenges and opportunities of nanomaterial-enabled wearable sensors are discussed.

Au Nanoparticles Decorated Mesoporous SiO2 -WO3 Hybrid Materials with Improved Pore Connectivity for Ultratrace Ethanol Detection at Low Operating Temperature

Semiconducting metal oxides-based gas sensors with the capability to detect trace gases at low operating temperatures are highly desired in applications such as wearable devices, trace pollutant detection, and exhaled breath analysis, but it still remains a great challenge to realize this goal. Herein, a multi-component co-assembly method in combination with pore engineering strategy is proposed. By using bi-functional (3-mercaptopropyl) trimethoxysilane (MPTMS) that can co-hydrolyze with transition metal salt and meanwhile coordinate with gold precursor during their co-assembly with PEO-b-PS copolymers, ordered mesoporous SiO₂ -WO₃ composites with highly dispersed Au nanoparticles of 5 nm (mesoporous SiO₂ -WO₃ /Au) are straightforward synthesized. This multi-component co-assembly process avoids the aggregation of Au nanoparticles and pore blocking in conventional post-loading method. Furthermore, through controlled etching treatment, a small portion of silica can be removed from the pore wall, resulting in mesoporous SiO₂ -WO₃ /Au with increased specific surface area (129 m²  g^-1 ), significantly improved pore connectivity, and enlarged pore window (>4.3 nm). Thanks to the presence of well-confined Au nanoparticles and ε-WO₃ , the mesoporous SiO₂ -WO₃ /Au based gas sensors exhibit excellent sensing performance toward ethanol with high sensitivity (R_a /R_g = 2-14 to 50-250 ppb) at low operating temperature (150 °C).

Enhanced performance of an acetone gas sensor based on Ag-LaFeO3 molecular imprinted polymers and carbon nanotubes composite

High performance acetone gas sensors were fabricated with molecular imprinted polymers of Ag-LaFeO₃ (ALFOMMIPs) and multi walled carbon nanotubes (CNTs) composite using the microwave assisted sol-gel method. The crystalline structure, functional groups, grain size and surface appearance of the synthesized materials were analyzed via different characterization techniques and the gas responses of the samples were examined. The detailed acetone gas sensing tests and analysis revealed that the CNTs and ALFOMIPs nanocomposite (CNT/ALFOMIP) sample possessed a higher response than that of the ALFOMIPs sample. Where 0.75 wt% CNTs were added into the ALFOMIPs (0.75% CNT/ALFOMIP nanocomposite) sensor, an excellent gas sensing performance was exhibited. The response of this sensor was up to 59 for 5 ppm acetone vapors and the response and recovery times were 58 and 33 s at low working temperature of 86 °C, respectively. In addition, it had the best selectivity only to acetone vapors due to the use of the molecular imprinting technique.

Superior Acetone Selectivity in Gas Mixtures by Catalyst-Filtered Chemoresistive Sensors

Acetone is a toxic air pollutant and a key breath marker for non-invasively monitoring fat metabolism. Its routine detection in realistic gas mixtures (i.e., human breath and indoor air), however, is challenging, as low-cost acetone sensors suffer from insufficient selectivity. Here, a compact detector for acetone sensing is introduced, having unprecedented selectivity (>250) over the most challenging interferants (e.g., alcohols, aldehydes, aromatics, isoprene, ammonia, H2, and CO). That way, acetone is quantified with fast response (<1 min) down to, at least, 50 parts per billion (ppb) in gas mixtures with such interferants having up to two orders of magnitude higher concentration than acetone at realistic relative humidities (RH = 30-90%). The detector consists of a catalytic packed bed (30 mg) of flame-made Al2O3 nanoparticles (120 m2 g-1) decorated with Pt nanoclusters (average size 9 nm) and a highly sensitive chemo-resistive sensor made by flame aerosol deposition and in situ annealing of nanostructured Si-doped ε-WO3 (Si/WO3). Most importantly, the catalytic packed bed converts interferants continuously enabling highly selective acetone sensing even in the exhaled breath of a volunteer. The detector exhibits stable performance over, at least, 145 days at 90% RH, as validated by mass spectrometry.

ZnO-CuO Core-Hollow Cube Nanostructures for Highly Sensitive Acetone Gas Sensors at the ppb Level

This paper presents a ZnO-CuO p-n heterojunction chemiresistive sensor that comprises CuO hollow nanocubes attached to ZnO spherical cores as active materials. These ZnO-CuO core-hollow cube nanostructures exhibit a remarkable response of 11.14 at 1 ppm acetone and 200 °C, which is a superior result to those reported by other metal-oxide-based sensors. The response can be measured up to 40 ppb, and the limit of detection is estimated as 9 ppb. ZnO-CuO core-hollow cube nanostructures also present high selectivity toward acetone against other volatile organic compounds and demonstrate excellent stability for up to 40 days. The outstanding gas-sensing performance of the developed nanocubes is attributed to their uniform and unique morphology. Their core-shell-like structures allow the main charge transfer pathways to pass the interparticle p-p junctions, and the p-n junctions in each particle increase the sensitivity of the reactions to gas molecules. The small grain size and high surface area of each domain also enhance the surface gas adsorption.

Cr-Doped Urchin-Like WO3 Hollow Spheres: The Cooperative Modulation of Crystal Growth and Energy-Band Structure for High-Sensitive Acetone Detection

Acetone is a biomarker in the exhaled breath of diabetic patients; sensitive and selective detection of acetone in human exhaled breath plays an important role in noninvasive diagnosis. Tungsten oxide (especially for γ-WO₃) is a promising material for the detection of breath acetone. It is generally believed that the stable metastable phase of WO₃ (ε-WO₃) is the main reason for the improved response to acetone detection. In this work, pure and Cr-doped urchin-like WO₃ hollow spheres were synthesized by a facile hydrothermal approach. Analyses of the resulting materials via X-ray photoelectron spectroscopy (XPS) and Raman confirmed that they are mainly composed by γ-WO₃. The gas sensing performances of pure and Cr-doped WO₃ to acetone were systematically tested. Results show that the sensor based on pure WO₃ annealed at 450 °C has a high response of 20.32 toward 100 ppm acetone at a working temperature of 250 °C. After doped with Cr, the response was increased 3.5 times higher than the pure WO₃ sensor. The pure and Cr-doped WO₃ sensors both exhibit a tiny response to other gases, low detection limits (ppb-level) and an excellent repeatability. The improvement of gas sensing properties could be attributed to an optimized morphology of Cr-doped WO₃ by regulating the crystal growth and reducing the assembled nanowires’ diameter. The increasing number of oxygen vacancy and the introduction of impurity energy level with trap effect after Cr doping would lead to the wider depletion layer as well as a better gas sensing performance. This work will contribute to the development of new WO₃ acetone sensors with a novel morphology and will explain the increased response after Cr doping from a new perspective.

Morphology-activity correlation of electrospun CeO2 for toluene catalytic combustion

Herein, CeO₂ catalysts with nanotube, nanobelt, and wire-in-nanotube morphologies were successfully fabricated by a facile single spinneret electrospinning technique. And catalytic activity of these electrospun CeO₂ nanomaterials were evaluated by toluene catalytic combustion reaction. Among the three morphologies of CeO₂ catalysts, CeO₂ nanobelt (CeO₂-NB) presented the best toluene catalytic combustion performance (T_90% = 230 °C) at WHSV = 60,000 mL g^-1 h^-1, also exhibited the lowest activation energy (E_a = 80.2 kJ/mol). Based on the characterization by TEM, XRD, BET, SEM, XPS, Raman spectroscopy, H₂-TPR, and O₂-TPD results, the high catalytic activity of CeO₂-NB catalyst was attributed to its porous nanobelt morphology with larger specific surface area and the abundance of surface oxygen vacancies. Furthermore, the CeO₂-NB catalysts presented an excellent durability by longtime on-stream test (as well as presence of 5% vol. water vapor), suggesting its great potential for practical air pollution control application.

Bimetallic PtCu Nanocrystal Sensitization WO3 Hollow Spheres for Highly Efficient 3-Hydroxy-2-butanone Biomarker Detection

As a foodborne bacterium, Listeria monocytogenes (LM) can cause serious diseases and even death to weak people. 3-Hydroxy-2-butanone (3H-2B) has been proven to be a biomarker for exhalation of LM. Detection of 3H-2B is a fast and effective method for determining whether the food is infected. Herein, we present an excellent 3H-2B gas sensor based on bimetallic PtCu nanocrystal modified WO₃ hollow spheres. The structure and morphology of the PtCu/WO₃ were characterized, and their gas sensitivities were measured by a static testing method. The results showed that the sensor response of WO₃ hollow spheres was enhanced by about 15 times after modification with bimetallic PtCu nanocrystal. The maximum response value of the PtCu/WO₃ sensor to 10 ppm 3H-2B is as high as 221.2 at 110 °C. In addition, the PtCu/WO₃ sensor also exhibited good selectivity to 3H-2B, fast response/recovery time (9 s/28 s), and low limit of detection (LOD < 0.5 ppm). Furthermore, the sensitivity mechanism was studied by monitoring the reaction products by gas chromatography-mass spectrometry. The excellent gas-sensing performance can be attributed to the synergy between PtCu and WO₃, including the unique spillover effect of O₂ on PtCu nanoparticles, the regulated depletion layer by p-type Cu_xO to n-type WO₃, and their selective catalysis to 3H-2B. Hence, this work offers the rational design and synthesis of highly efficient sensitive materials for the detection of LM for food security.

Fabrication of a Micro-Electromechanical System-Based Acetone Gas Sensor Using CeO2 Nanodot-Decorated WO3 Nanowires

Preparation of reliable, stable, and highly responsive gas-sensing devices for the detection of acetone has been considered to be a key issue for the development of accurate disease diagnosis systems via exhaled breath. In this paper, novel CeO₂ nanodot-decorated WO₃ nanowires are successfully synthesized through a sequential hydrothermal and thermolysis process. Such CeO₂ nanodot-decorated WO₃ nanowires exhibited a remarkable enhancement in acetone-sensing performance based on a miniaturized micro-electromechanical system device, which affords high response (S = 1.30-500 ppb, 1.62-2.5 ppm), low detection limit (500 ppb), and superior selectivity toward acetone. The improved performance of the acetone sensor is likely to be originated from the fast carrier transportation of WO₃ nanowires, the formation of WO₃-CeO₂ heterojunctions, and the existence of large amounts of oxygen vacancies in CeO₂. The improved reaction thermodynamics and sensing mechanisms have also been revealed by the specific band alignment and X-ray photoelectron spectroscopy analysis.

Current Applications of Gas Sensor Based on 2-D Nanomaterial: A Mini Review

Gas sensor, as one of the most important devices to detect noxious gases, provides a vital way to monitor the concentration and environmental information of gas in order to guarantee the safety of production. Therefore, researches on high sensitivity, high selectivity, and high stability have become hot issues. Since the discovery of the nanomaterial, it has been increasingly applied to the gas sensor for its distinguishing surface performances. However, 0-D and 1-D nanomaterials, with limited electronic confinement effect and surface effect, cannot reach the requirement for the production of gas sensors. This paper gives an introduction about the current researching progress and development trend of 2-D nanomaterials, analyzes the common forms of 2-D nanoscale structure, and summarizes the mechanism of gas sensing. Then, widely concerned factors including morphological properties and crystalline structure of 2-D nanomaterial, impact of doped metal on the sensibility of gas sensors, impact of symmetry, and working temperature on the selectivity of gas sensors have been demonstrated in detail. In all, the detailed analysis above has pointed out a way for the development of new 2-D nanomaterial and enhancing the sensibility of gas sensors.

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.

A high-performance trace level acetone sensor using an indispensable V4C3Tx MXene

A high-performance acetone sensor utilizing an emerging indispensable V₄C₃T_x MXene is described via combining experimental results with theoretical study.

Noninvasive and Wearable Respiration Sensor Based on Organic Semiconductor Film with Strong Electron Affinity

Interventional medical detection techniques require expensive devices and cause inconvenience and discomfort to the human body, which restricts their application to the frequency and duration of measurements. A noninvasive respiration test is urgently required for the next-generation medical technologies in early disease warning and postoperative monitoring. This article describes a noninvasive and wearable sensing device that shows high sensitivity toward acetone in respiratory gases with excellent stability, low energy consumption, and reliable flexibility. To obtain such a sensor, the organic semiconductor compound La(TBPP)(TBNc) (TBPP = tetrakis(4-tert-butylphenyl)porphyrin; TBNc = tetrakis(4-tert-butylphenyl)naphthalocyanine) was synthesized and further self-assembled into a highly ordered flexible film via a simple solution-vapor annealing method. The fabricated flexible film was deposited on an interdigitated electrode with poly(ethylene terephthalate) substrate and employed as an electrical identification component for a respiration sensor. Thanks to the attractive electron-transfer properties of highly ordered films and strong electron affinity of La(TBPP)(TBNc) molecules, the as-prepared sensor shows a low detection limit (200 ppb) and acceptable selectivity. The wrinkled/rippled structure of films endows the fabricated sensors with the ability of mechanical flexibility. More importantly, the experimental results suggest the potential application of acetone identification in real respiratory gases and provide a new concept for the development of noninvasive and wearable medical diagnostic devices.

Graphene quantum dot-functionalized three-dimensional ordered mesoporous ZnO for acetone detection toward diagnosis of diabetes

The development of a high-performance semiconductor oxide sensor for the accurate detection of trace disease biomarkers in exhaled breath is still a challenge that urgently needs to be addressed. Here, we proposed a self-assembly strategy and spin-coating process to create a graphene quantum dot (GQD)-functionalized three-dimensional ordered macroporous (3DOM) ZnO structure. The strong synergistic effect and the p-n heterojunction between the p-type GQDs and n-type ZnO effectively enlarged the resistance variation due to the change in oxygen adsorption. The specific 3DOM structure induced a hierarchical pore size (286 nm in macroscale and 26 nm in mesoscale) and 3D interconnection, which guaranteed high gas accessibility and fast carrier transportation. As a result, the GQD-modified 3DOM ZnO sensor exhibited a remarkably high response (Rair/Rgas = 15.2 for 1 ppm acetone), rapid response/recovery time (9/16 s), extremely low theoretical detection limit (8.7 ppb), and good selectivity towards acetone against other interfering gases. In particular, the proposed sensor could accurately distinguish trace acetone in the simulated breath of diabetic patients. These results demonstrate a high potential for the feasibility of the GQD-modified 3DOM SMO structure as a new sensing material for the possibility of noninvasive real-time diagnosis of diabetes.

Microcontroller Implementation of Support Vector Machine for Detecting Blood Glucose Levels Using Breath Volatile Organic Compounds

This paper presents an embedded system-based solution for sensor arrays to estimate blood glucose levels from volatile organic compounds (VOCs) in a patient's breath. Support vector machine (SVM) was trained on a general-purpose computer using an existing SVM library. A training model, optimized to achieve the most accurate results, was implemented in a microcontroller with an ATMega microprocessor. Training and testing was conducted using artificial breath that mimics known VOC footprints of high and low blood glucose levels. The embedded solution was able to correctly categorize the corresponding glucose levels of the artificial breath samples with 97.1% accuracy. The presented results make a significant contribution toward the development of a portable device for detecting blood glucose levels from a patient's breath.

Ultrafast Response/Recovery and High Selectivity of the H2S Gas Sensor Based on α-Fe2O3 Nano-Ellipsoids from One-Step Hydrothermal Synthesis

Ultrafast response/recovery and high selectivity of gas sensors are critical for real-time and online monitoring of hazardous gases. In this work, α-Fe₂O₃ nano-ellipsoids were synthesized using a facile one-step hydrothermal method and investigated as highly sensitive H₂S-sensing materials. The nano-ellipsoids have an average long-axis diameter of 275 nm and an average short-axis diameter of 125 nm. H₂S gas sensors fabricated using the α-Fe₂O₃ nano-ellipsoids showed excellent H₂S-sensing performance at an optimum working temperature of 260 °C. The response and recovery times were 0.8 s/2.2 s for H₂S gas with a concentration of 50 ppm, which are much faster than those of H₂S gas sensors reported in the literature. The α-Fe₂O₃ nano-ellipsoid-based sensors also showed high selectivity to H₂S compared to other commonly investigated gases including NH₃, CO, NO₂, H₂, CH₂Cl₂, and ethanol. In addition, the sensors exhibited high-response values to different concentrations of H₂S with a detection limit as low as 100 ppb, as well as excellent repeatability and long-term stability.

Nanostructured Gas Sensors for Medical and Health Applications: Low to High Dimensional Materials

Human breath has long been known as a system that can be used to diagnose diseases. With advancements in modern nanotechnology, gas sensors can now diagnose, predict, and monitor a wide range of diseases from human breath. From cancer to diabetes, the need to treat at the earliest stages of a disease to both increase patient outcomes and decrease treatment costs is vital. Therefore, it is the promising candidate of rapid and non-invasive human breath gas sensors over traditional methods that will fulfill this need. In this review, we focus on the nano-dimensional design of current state-of-the-art gas sensors, which have achieved records in selectivity, specificity, and sensitivity. We highlight the methods of fabrication for these devices and relate their nano-dimensional materials to their record performance to provide a pathway for the gas sensors that will supersede.

Nanostructured Chemiresistive Gas Sensors for Medical Applications

Treating diseases at their earliest stages significantly increases the chance of survival while decreasing the cost of treatment. Therefore, compared to traditional blood testing methods it is the goal of medical diagnostics to deliver a technique that can rapidly predict and if required non-invasively monitor illnesses such as lung cancer, diabetes, melanoma and breast cancer at their very earliest stages, when the chance of recovery is significantly higher. To date human breath analysis is a promising candidate for fulfilling this need. Here, we highlight the latest key achievements on nanostructured chemiresistive sensors for disease diagnosis by human breath with focus on the multi-scale engineering of both composition and nano-micro scale morphology. We critically assess and compare state-of-the-art devices with the intention to provide direction for the next generation of chemiresistive nanostructured sensors.

Floral and lamellar europium(iii)-based metal–organic frameworks as high sensitivity luminescence sensors for acetone

Floral Eu-BDC and lamellar Eu-BTC as high sensitivity luminescence sensor for acetone.