Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow

Flexible Humidity Sensor Based on a Graphene Oxide-Carbon Nanotube-Modified Co3O4 Nanoparticle-Embedded Laser-Induced Graphene Electrode

To meet evolving humidity monitoring needs, the development of flexible, high-performance humidity sensors is crucial. This study introduces an innovative flexible humidity sensor using a single-step laser scribing technique to fabricate a flexible in situ Co3O4 nanoparticle-embedded laser-induced graphene (Co3O4-LIG) composite electrode. Compared to conventional LIG electrodes, the Co3O4-LIG electrode exhibits improved conductivity and hydrophilicity, enhancing charge transfer and water molecule affinity. The unique two-dimensional structure and exceptional water permeability of graphene oxide (GO) combine with the rapid water response and high specific surface area of carboxylated multiwalled carbon nanotubes (MWCNTs), thereby assuming a crucial function in the modification and optimization of the performance of humidity sensors. Through the application of a homogenously blended aqueous solution comprising GO and MWCNTs in precise proportions onto the Co3O4-LIG composite electrode, an excellent humidity-responsive layer is established, culminating in the realization of a cutting-edge GO-MWCNTs@Co3O4-LIG flexible humidity sensor. Noteworthy attributes of this sensor include a heightened sensitivity [959.1% (ΔR/R0)], rapid response and recovery times (within 5 and 26 s, respectively), and a noteworthy linearity (R2 = 0.994) across a relative humidity range of 14 to 95%. The findings presented herein offer valuable insights and a practical blueprint for the design and production of flexible humidity sensors.

Self-powered humidity sensors based on zero-dimensional perovskite-like structures with fast response and high stability

With the rapid development of technology, the development of self-powered sensors has garnered significant attention. The importance of monitoring humidity has grown significantly in various technological contexts, from environmental monitoring to biomedical applications. In this work, we have fabricated a low-cost and self-powered humidity sensor using zero-dimensional perovskite-like structures. Switching tests at different relative humidity levels have shown that the zero-dimensional perovskites have visible coloration at high humidities and discoloration upon reducing the humidity. The humidity sensor was fabricated by spin coating the zero-dimensional perovskites on a patterned fluorine doped tin oxide (FTO) substrate and the sensor not only shows high response values of around 500 mV and few micro amperes of short circuit current densities, but also shows good cycling performance and stability. Also high selectivity to humidity is observed in comparison to different gases and volatile organic compounds. The high selectivity to humidity arises due to the fact that the exclusion of MAI from the MA4PbI6 strucuture does not happen with all the other analytes which has been confirmed from the XRD studies. In addition, due to the low temperature fabrication they can be deposited on flexible substrates and the sensor displayed excellent resistance to bending and durability. Furthermore, the study explored the humidity monitoring capabilities of this sensor, revealing an outstanding response performance to human respiration. This observation suggests that the sensor holds significant potential for practical applications in the monitoring of human health and environmental conditions. This work paves the way for developing organic-inorganic hybrid perovskite materials for self-powered sensing applications.

Humidity Sensing with Supramolecular Nanostructures

Precise monitoring of the humidity level is important for the living comfort and for many applications in various industrial sectors. Humidity sensors have thus become one among the most extensively studied and used chemical sensors by targeting a maximal device performance through the optimization of the components and working mechanism. Among different moisture-sensitive systems, supramolecular nanostructures are ideal active materials for the next generation of highly efficient humidity sensors. Their noncovalent nature guarantees fast response, high reversibility, and fast recovery time in the sensing event. Herein, the most enlightening recent strategies on the use of supramolecular nanostructures for humidity sensing are showcased. The key performance indicators in humidity sensing, including operation range, sensitivity, selectivity, response, and recovery speed are discussed as milestones for true practical applications. Some of the most remarkable examples of supramolecular-based humidity sensors are presented, by describing the finest sensing materials, the operating principles, and sensing mechanisms, the latter being based on the structural or charge-transport changes triggered by the interaction of the supramolecular nanostructures with the ambient humidity. Finally, the future directions, challenges, and opportunities for the development of humidity sensors with performance beyond the state of the art are discussed.

Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Fast Response Facile Fabricated IDE-Based Ultra-sensitive Humidity Sensor for Medical Applications

An eco-friendly, biodegradable, flexible, and facile fabricated interdigital electrode-based capacitive humidity sensor with applications in health and medicine has been reported here. Several sensors use copper tape as electrodes on the polyethylene terephthalate (PET) substrate, with non-woven paper as the sensing layer. Two different configurations of sensors were tested, i.e., with and without pores in the PET substrate. The sensing performance of both sensors has been tested for relative humidity ranging from 35 to 100% at temperatures ranging from 20 to 50 °C. The capacitance of the sensor varies linearly in response to the change in humidity. The sensor with pores shows a response from 28 to 630 pF as the humidity varied from 35 to 100%, whereas the sensor without pores responded from 22 to 430 pF. The response and recovery times of the fabricated sensor are observed as ∼2.4, and ∼1.8 s, respectively, and the sensitivity is 9.67 pF/% RH. The sensors are tested multiple times, and repeatable results are achieved each time with an accuracy of ±0.22%. Further, the sensor's response is also stable for different ranges of temperatures. Finally, to demonstrate an application of the proposed sensor, it has been utilized to monitor respiration through nose and mouth breathing. The low-cost, stable, repeatable, and highly sensitive response makes our fabricated sensor a promising candidate for practical field applications.

Emulating Ebbinghaus forgetting behavior in a neuromorphic device based on 1D supramolecular nanofibres

Mimicking synaptic functions in hardware devices is a crucial step in realizing brain-like computing beyond the von Neumann architecture. 1D nanomaterials with spatial extensions of a few μm, similar to biological neurons, gain significance given the ease of electrical transport as well as directionality. Herein, we report a two-terminal optically active device based on 1D supramolecular nanofibres consisting of CS (coronene tetracarboxylate) and DMV (dimethyl viologen) forming alternating D-A (donor-acceptor) pairs, emulating synaptic functions such as the STP (short-term potentiation), LTP (long-term potentiation), PPF (paired-pulse facilitation), STDP (spike-time dependent plasticity) and learning-relearning behaviors. In addition, an extensive study on the less explored Ebbinghaus forgetting curve has been carried out. The supramolecular nanofibres being light sensitive, the potential of the device as a visual system is demonstrated using a 3 × 3 pixel array.

Fabrication of High-Performance Visible-Blind Ultraviolet Photodetectors Using Electro-ionic Conducting Supramolecular Nanofibers

The detection of ultraviolet (UV) light is vital for various applications, such as chemical-biological analysis, communications, astronomical studies, and also for its adverse effects on human health. Organic UV photodetectors are gaining much attention in this scenario because they possess properties such as high spectral selectivity and mechanical flexibility. However, the achieved performance parameters are much more inferior than the inorganic counterparts because of the lower mobility of charge carriers in organic systems. Here, we report the fabrication of a high-performance visible-blind UV photodetector, using 1D supramolecular nanofibers. The nanofibers are visibly inactive and exhibit highly responsive behavior mainly for UV wavelengths (275-375 nm), the highest response being at ∼275 nm. The fabricated photodetectors demonstrate desired features, such as high responsivity and detectivity, high selectivity, low power consumption, and good mechanical flexibility, because of their unique electro-ionic behavior and 1D structure. The device performance is shown to be improved by several orders through the tweaking of both electronic and ionic conduction pathways while optimizing the electrode material, external humidity, applied voltage bias, and by introducing additional ions. We have achieved optimum responsivity and detectivity values of around 6265 A W^-1 and 1.54 × 10¹⁴ Jones, respectively, which stand out compared with the previous organic UV photodetector reports. The present nanofiber system has great potential for integration in future generations of electronic gadgets.

Aromatic Zipper Topology Dictates Water-Responsive Actuation in Phenylalanine-Based Crystals

Water-responsive (WR) materials that reversibly deform in response to relative humidity (RH) changes are gaining increasing interest for their potential in energy harvesting and soft robotics applications. Despite progress, there are significant gaps in the understanding of how supramolecular structure underpins the reconfiguration and performance of WR materials. Here, three crystals are compared based on the amino acid phenylalanine (F) that contain water channels and F packing domains that are either layered (F), continuously connected (phenylalanyl-phenylalanine, FF), or isolated (histidyl-tyrosyl-phenylalanine, HYF). Hydration-induced reconfiguration is analyzed through changes in hydrogen-bond interactions and aromatic zipper topology. F crystals show the greatest WR deformation (WR energy density of 19.8 MJ m^-3 ) followed by HYF (6.5 MJ m^-3 ), while FF exhibits no observable response. The difference in water-responsiveness strongly correlates to the deformability of aromatic regions, with FF crystals being too stiff to deform, whereas HYF is too soft to efficiently transfer water tension to external loads.  These findings reveal aromatic topology design rules for WR crystals and provide insight into general mechanisms of high-performance WR actuation. Moreover, the best-performing crystal, F emerges as an efficient WR material for applications at scale and low cost.

A self-powered sound-driven humidity sensor for wearable intelligent dehydration monitoring system

Self-powered wearable sensing systems have attracted great attention for their application in continuous health monitoring, which can reveal real-time physiological information on the body. Here, an innovative self-powered sound-driven humidity sensor for wearable intelligent dehydration monitoring system has been proposed. The sensor is primarily comprised of PTFE membrane, ZnO nanoarrays and Ti thin film. The piezoelectric/triboelectric effect of ZnO nanoarrays/PTFE membrane is coupled with the humidity sensing process. Sound wave can drive PTFE membrane to vibrate, and the contact and separation between PTFE and ZnO can generate electrical signals through piezoelectric/triboelectric effect. At the same time, the surface of the nanostructures can absorb the water molecules, which will influence the electrical output of the device. The device can convert sound energy into electrical output without any external electricity power supply, and the outputting voltage decreases with increasing relative humidity, acting as the sensing signal. The sensor has been integrated with data processing unit and wireless transmission module to form a self-powered wearable intelligent dehydration monitoring system, which can actively monitor the humidity of exhaled breath and transmit the information to the mobile phone. The results can open a possible new direction for the development of sound-driven gas sensors and will further expand the scope for self-powered nanosystems.

Evanescent-Field Excited Surface Plasmon-Enhanced U-Bent Fiber Probes Coated with Au and ZnO Nanoparticles for Humidity Detection

We report the design, fabrication, and testing of a humidity sensor based on an optical fiber-based evanescent wave probe. The fiber was bent into a U-shape and de-cladded at the location of the bending. The de-cladded section was coated either with Au or with ZnO nanoparticles. Humidity is detected based on the interaction in the surface plasmon resonance of the Au/ZnO nanoparticles excited by an evanescent wave of light passing through the optical fiber. The response of the U-bent fibers to humidity was investigated using a specifically designed low-voltage portable interrogation box. We found that the fibers coated with ZnO nanoparticles were able to detect a minimum 0.1% change in humidity with an average sensitivity of 143 µV/%RH and 95% linearity over the 10% to 80% humidity range. In comparison, samples coated with Au and Au + ZnO nanoparticles demonstrated a minimum change detection of 0.3% RH and 2% RH respectively. The response and recovery time of the sensor were measured to be 3 s and 4 s, respectively, for a 60% change in humidity from 20% to 80%. The entire measurement system was operated by consuming an electrical power of 1.62 W at an input voltage of 12 Vdc.

KIT-5-Assisted Synthesis of Mesoporous SnO2 for High-Performance Humidity Sensors with a Swift Response/Recovery Speed

Developing highly efficient semiconductor metal oxide (SMOX) sensors capable of accurate and fast responses to environmental humidity is still a challenging task. In addition to a not so pronounced sensitivity to relative humidity change, most of the SMOXs cannot meet the criteria of real-time humidity sensing due to their long response/recovery time. The way to tackle this problem is to control adsorption/desorption processes, i.e., water-vapor molecular dynamics, over the sensor's active layer through the powder and pore morphology design. With this in mind, a KIT-5-mediated synthesis was used to achieve mesoporous tin (IV) oxide replica (SnO₂-R) with controlled pore size and ordering through template inversion and compared with a sol-gel synthesized powder (SnO₂-SG). Unlike SnO₂-SG, SnO₂-R possessed a high specific surface area and quite an open pore structure, similar to the KIT-5, as observed by TEM, BET and SWAXS analyses. According to TEM, SnO₂-R consisted of fine-grained globular particles and some percent of exaggerated, grown twinned crystals. The distinctive morphology of the SnO₂-R-based sensor, with its specific pore structure and an increased number of oxygen-related defects associated with the powder preparation process and detected at the sensor surface by XPS analysis, contributed to excellent humidity sensing performances at room temperature, comprised of a low hysteresis error (3.7%), sensitivity of 406.8 kΩ/RH% and swift response/recovery speed (4 s/6 s).

Deep Learning Image Analysis of Nanoplasmonic Sensors: Toward Medical Breath Monitoring

Sensing biomarkers in exhaled breath offers a potentially portable, cost-effective, and noninvasive strategy for disease diagnosis screening and monitoring, while high sensitivity, wide sensing range, and target specificity are critical challenges. We demonstrate a deep learning-assisted plasmonic sensing platform that can detect and quantify gas-phase biomarkers in breath-related backgrounds of varying complexity. The sensing interface consisted of Au/SiO₂ nanopillars covered with a 15 nm metal-organic framework. A small camera was utilized to capture the plasmonic sensing responses as images, which were subjected to deep learning signal processing. The approach has been demonstrated at a classification accuracy of 95 to 98% for the diabetic ketosis marker acetone within a concentration range of 0.5-80 μmol/mol. The reported work provides a thorough exploration of single-sensor capabilities and sets the basis for more advanced utilization of artificial intelligence in sensing applications.

Toward Continuous Breath Monitoring on a Mobile Phone Using a Frugal Conducting Cloth-Based Smart Mask

A frugal humidity sensor that can detect changes in the humidity of exhaled breath of individuals has been fabricated. The sensor comprises a humidity-sensitive conducting polymer that is in situ formed on a cloth that acts as a substrate. Interdigitated silver electrodes were screen-printed on the modified cloth, and conducting threads connected the electrodes to the measurement circuit. The sensor's response to changing humidity was measured as a voltage drop across the sensor using a microcontroller. The sensor was capable of discerning between fast, normal, and slow breathing based on the response time. A response time of ∼1.3 s was observed for fast breathing. An Android-based mobile application was designed to collect sensor data via Bluetooth for analysis. A time series classification algorithm was implemented to analyze patterns in breathing. The sensor was later stitched onto a face mask, transforming it into a smart mask that can monitor changes in the breathing pattern at work, play, and sleep.

Polyelectrolyte/Graphene Oxide Nano-Film Integrated Fiber-Optic Sensors for High-Sensitive and Rapid-Response Humidity Measurement

Optical fiber humidity sensors have sparked enormous interests in many fields because of their excellent features. However, it remains a great challenge to balance sensitivity, humidity response, temperature crosstalk, and wet hysteresis for real-world application. To overcome this trade-off, an optical fiber humidity sensor is developed here by coating functional graphene oxide (GO)/polyelectrolyte nanocomposite film on the excessively tilted fiber grating (ex-TFG), in which GO/polyelectrolyte nanocomposite film is employed for enhancing the hydrophilicity and accelerating the adsorption/desorption of water molecule, while the ex-TFG is utilized for improving the sensitivity of refractive index and eliminating the crosstalk of temperature. By this design, optical fiber humidity sensors achieve high sensitivity, rapid response and recovery, low hysteresis, and temperature crosstalk as well as excellent repeatability and stability in large relative humidity (RH) range. Our work provides a promising platform for effective RH monitoring systems that can be widely applied in rapid diagnostics, pharmacy, precision medicine, and so forth.

Highly Sensitive and Ultra-Responsive Humidity Sensors Based on Graphene Oxide Active Layers and High Surface Area Laser-Induced Graphene Electrodes

Ultra-sensitive and responsive humidity sensors were fabricated by deposition of graphene oxide (GO) on laser-induced graphene (LIG) electrodes fabricated by a low-cost visible laser scribing tool. The effects of GO layer thickness and electrode geometry were investigated. Sensors comprising 0.33 mg/mL GO drop-deposited on spiral LIG electrodes exhibited high sensitivity up to 1800 pF/% RH at 22 °C, which is higher than previously reported LIG/GO sensors. The high performance was ascribed to the high density of the hydroxyl groups of GO, promoted by post-synthesis sonication treatment, resulting in high water physisorption rates. As a result, the sensors also displayed good stability and short response/recovery times across a wide tested range of 0–97% RH. The fabricated sensors were benchmarked against commercial humidity sensors and displayed comparable performance and stability. Finally, the sensors were integrated with a near-field communication tag to function as a wireless, battery-less humidity sensor platform for easy read-out of environmental humidity values using smartphones.

NaCl Ionization-Based Moisture Sensor Prepared by Aerosol Deposition for Monitoring Respiratory Patterns

A highly polarizable moisture sensor with multimodal sensing capabilities has great advantages for healthcare applications such as human respiration monitoring. We introduce an ionically polarizable moisture sensor based on NaCl/BaTiO₃ composite films fabricated using a facile aerosol deposition (AD) process. The proposed sensing model operates based on an enormous NaCl ionization effect in addition to natural moisture polarization, whereas all previous sensors are based only on the latter. We obtained an optimal sensing performance in a 0.5 µm-thick layer containing NaCl-37.5 wt% by manipulating the sensing layer thickness and weight fraction of NaCl. The NaCl/BaTiO₃ sensing layer exhibits outstanding sensitivity over a wide humidity range and a fast response/recovery time of 2/2 s; these results were obtained by performing the one-step AD process at room temperature without using any auxiliary methods. Further, we present a human respiration monitoring system using a sensing device that provides favorable and stable electrical signals under diverse respiratory scenarios.

Century Impact of Macromolecules for Advances of Sensing Sciences

Impact of macro molecular theory on the progress of sensing sciences and technology has been presented in the light of materials developments, advances in physical and chemical properties. The chronological advances in the properties of macromolecules have significantly improved the sensing performances towards gases, heavy metals, biomolecules, hydrocarbon, and energetic compounds in terms of unexplored sensing parameters, durability, and working lifetime. In this review article, efforts have been made to correlate the advances in structure and interactivity of macro-molecules with their sensing behavior and working performances. The significant findings on the macromolecules towards advancing the sensing sciences are highlighted with the suitable illustration and schemes to establish it as a potential “microanalytical technique” along with existing challenges.

Highly Sensitive and Stable Humidity Sensor Based on the Bi-Layered PVA/Graphene Flower Composite Film

Two-dimensional (2D) materials and their composites have gained significant importance as the functional layer of various environmental sensors and nanoelectronics owing to their unique properties. This work reports for the first time a highly sensitive, fast, and stable humidity sensor based on the bi-layered active sensing area composed of graphene flower (GF) and poly (vinyl alcohol) PVA thin films for multifunctional applications. The GF/PVA humidity sensor exhibited stable impedance response over 15 days, for a relative humidity (RH) range of (40–90% RH) under ambient operating conditions. The proposed bi-layered humidity sensor also exhibited an ultra-high capacitive sensitivity response of the 29 nF/%RH at 10 kHz and fast transient response of 2 s and 3.5 s, respectively. Furthermore, the reported sensor also showed a good response towards multi-functional applications such as non-contact skin humidity and mouth breathing detection.

Electrical Method for In Vivo Testing of Exhalation Sensors Based on Natural Clinoptilolite

Natural substances with a complex chemical structure can be advantageously used for functional applications. Such functional materials can be found both in the mineral and biological worlds. Owing to the presence of ionic charge carriers (i.e., extra-framework cations) in their crystal lattice, whose mobility is strictly depending on parameters of the external environment (e.g., temperature, humidity, presence of small gaseous polar molecules, etc.), zeolites can be industrially exploited as a novel functional material class with great potentialities in sensors and electric/electronic field. For fast-responding chemical-sensing applications, ionic transport at the zeolite surface is much more useful than bulk-transport, since molecular transport in the channel network takes place by a very slow diffusion mechanism. The environmental dependence of electrical conductivity of common natural zeolites characterized by an aluminous nature (e.g., chabasite, clinoptilolite, etc.) can be conveniently exploited to fabricate impedimetric water-vapor sensors for apnea syndrome monitoring. The high mechanical, thermal, and chemical stability of geomorphic clinoptilolite (the most widely spread natural zeolite type) makes this type of zeolite the most adequate mineral substance to fabricate self-supporting impedimetric water-vapor sensors. In the development of devices for medical monitoring (e.g., apnea-syndrome monitors), it is very important to combine these inexpensive nature-made sensors with a low-weight simplified electronic circuitry that can be easily integrated in wearable items (e.g., garments, wristwatch, etc.). Very low power square-wave voltage sources (micro-Watt voltage sources) show significant voltage drops under only a minimal electric load, and this property of the ac generator can be advantageously exploited for detecting the small impedimetric change observed in clinoptilolite sensors during their exposition to water vapor coming from the human respiratory exhalation. Owing to the ionic conduction mechanism (single-charge carrier) characterizing the zeolite slab surface, the sensor biasing by an ac signal is strictly required. Cheap handheld multimeters frequently include a very low power square-wave (or sinusoidal) voltage source of different frequency (typically 50 Hz or 1 kHz) that is used as a signal injector (signal tracer) to test audio amplifiers (low-frequency amplifies), tone control (equalizer), radios, etc. Such multimeter outputs can be connected in parallel with a true-RMS (Root-Mean-Square) ac voltmeter to detect the response of the clinoptilolite-based impedimetric sensors as voltage drop. The frequency of exhalation during breathing can be measured, and the exhalation behavior can be visualized, too, by using the voltmeter readings. Many handheld multimeters also include a data-logging possibility, which is extremely useful to record the voltage reading over time, thus giving a time-resolved voltage measurement that contains all information concerning the breathing test. Based on the same principle (i.e., voltage drop under minimal resistive load) a devoted electronic circuitry can also be made.

Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review

Respiratory monitoring is a fundamental method to understand the physiological and psychological relationships between respiration and the human body. In this review, we overview recent developments on ultrafast humidity sensors with functional nanomaterials for monitoring human respiration. Key advances in design and materials have resulted in humidity sensors with response and recovery times reaching 8 ms. In addition, these sensors are particularly beneficial for respiratory monitoring by being portable and noninvasive. We systematically classify the reported sensors according to four types of output signals: impedance, light, frequency, and voltage. Design strategies for preparing ultrafast humidity sensors using nanomaterials are discussed with regard to physical parameters such as the nanomaterial film thickness, porosity, and hydrophilicity. We also summarize other applications that require ultrafast humidity sensors for physiological studies. This review provides key guidelines and directions for preparing and applying such sensors in practical applications.

Preparation and Research of a High-Performance ZnO/SnO2 Humidity Sensor

A high-performance zinc oxide/tin dioxide (ZnO/SnO₂) humidity sensor was developed using a simple solvothermal method. The sensing mechanism of the ZnO/SnO₂ humidity sensor was evaluated by analyzing its complex impedance spectra. The experimental results prove that the ZnO/SnO₂ composite material has a larger specific surface area than pure SnO₂, which allows the composite material surface to adsorb more water to enhance the response of the ZnO/SnO₂ humidity sensor. ZnO can also contribute to the generation of oxygen-rich vacancies on the ZnO/SnO₂ composite material surface, allowing it to adsorb a large amount of water and rapidly decompose water molecules into conductive ions to increase the response and recovery speed of the ZnO/SnO₂ humidity sensor. These characteristics allowed the Z/S-2 humidity sensor to achieve a higher response (1,225,361%), better linearity, smaller hysteresis (6.6%), faster response and recovery speeds (35 and 8 s, respectively), and long-term stability at 11–95% relative humidity. The successful preparation of the ZnO/SnO₂ composite material also provides a new direction for the design of SnO₂-based resistance sensors with high humidity-sensing performance.

Review of Optical Humidity Sensors

Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.

Emerging MXene-Polymer Hybrid Nanocomposites for High-Performance Ammonia Sensing and Monitoring

Ammonia (NH3) is a vital compound in diversified fields, including agriculture, automotive, chemical, food processing, hydrogen production and storage, and biomedical applications. Its extensive industrial use and emission have emerged hazardous to the ecosystem and have raised global public health concerns for monitoring NH3 emissions and implementing proper safety strategies. These facts created emergent demand for translational and sustainable approaches to design efficient, affordable, and high-performance compact NH3 sensors. Commercially available NH3 sensors possess three major bottlenecks: poor selectivity, low concentration detection, and room-temperature operation. State-of-the-art NH3 sensors are scaling up using advanced nano-systems possessing rapid, selective, efficient, and enhanced detection to overcome these challenges. MXene-polymer nanocomposites (MXP-NCs) are emerging as advanced nanomaterials of choice for NH3 sensing owing to their affordability, excellent conductivity, mechanical flexibility, scalable production, rich surface functionalities, and tunable morphology. The MXP-NCs have demonstrated high performance to develop next-generation intelligent NH3 sensors in agricultural, industrial, and biomedical applications. However, their excellent NH3-sensing features are not articulated in the form of a review. This comprehensive review summarizes state-of-the-art MXP-NCs fabrication techniques, optimization of desired properties, enhanced sensing characteristics, and applications to detect airborne NH3. Furthermore, an overview of challenges, possible solutions, and prospects associated with MXP-NCs is discussed.

Evolution of Supramolecular Systems Towards Next-Generation Biosensors

Supramolecular materials, which rely on dynamic non-covalent interactions, present a promising approach to advance the capabilities of currently available biosensors. The weak interactions between supramolecular monomers allow for adaptivity and responsiveness of supramolecular or self-assembling systems to external stimuli. In many cases, these characteristics improve the performance of recognition units, reporters, or signal transducers of biosensors. The facile methods for preparing supramolecular materials also allow for straightforward ways to combine them with other functional materials and create multicomponent sensors. To date, biosensors with supramolecular components are capable of not only detecting target analytes based on known ligand affinity or specific host-guest interactions, but can also be used for more complex structural detection such as chiral sensing. In this Review, we discuss the advancements in the area of biosensors, with a particular highlight on the designs of supramolecular materials employed in analytical applications over the years. We will first describe how different types of supramolecular components are currently used as recognition or reporter units for biosensors. The working mechanisms of detection and signal transduction by supramolecular systems will be presented, as well as the important hierarchical characteristics from the monomers to assemblies that contribute to selectivity and sensitivity. We will then examine how supramolecular materials are currently integrated in different types of biosensing platforms. Emerging trends and perspectives will be outlined, specifically for exploring new design and platforms that may bring supramolecular sensors a step closer towards practical use for multiplexed or differential sensing, higher throughput operations, real-time monitoring, reporting of biological function, as well as for environmental studies.

Unexpected three-state hydrochromism of a donor-acceptor self-complex with fluctuations in relative humidity

Water in our environment is ever present, particularly in our atmosphere, from which it may be adsorbed by materials hygroscopically. At the molecular level, the binding of water molecules to various materials is driven by weak interactions but can have profound effects on physical properties, including the donor-acceptor interactions in charge transfer (CT) salts. Herein we present the unexpected three-state hydrochromatic switching of a bipyridinium-based donor-acceptor self-complex with changes in relative humidity (RH) and subsequent stable hydrate formation. RH is typically an overlooked variable that can vary greatly. These findings suggest that care should be taken to consider fluctuations in RH when characterizing the solid state optical band gap and CT absorption bands for organic donor-acceptor CT salt complexes.

Flexible, Highly Sensitive Paper-Based Screen Printed MWCNT/PDMS Composite Breath Sensor for Human Respiration Monitoring

Accurate measurement and monitoring of respiration is vital in patients affected by severe acute respiratory syndrome coronavirus - 2 (SARS-CoV-2). Patients with severe chronic diseases and pneumonia need continuous respiration monitoring and oxygenation support. Existing respiratory sensing techniques require direct contact with the human body along with expensive and heavy Holter monitors for continuous real-time monitoring. In this work, we propose a low-cost, non-invasive and reliable paper-based wearable screen printed sensor for human respiration monitoring as an effective alternative of existing sensing systems. The proposed sensor was fabricated using traditional screen printing of multi-walled carbon nanotubes (MWCNTs) and polydimethylsiloxane (PDMS) composite based interdigitated electrodes on paper substrate. The paper substrate was used as humidity sensing material of the sensor. The hygroscopic nature of paper during inhalation and exhalation causes a change in dielectric constant, which in turn changes the capacitance of the sensor. The composite interdigitated electrode configuration exhibited better response times with a rise time of 1.178s being recorded during exhalation and fall time of 0.88s during inhalation periods. The respiration rate of sensor was successfully examined under various breathing conditions such as normal breathing, deep breathing, workout, oral breathing, nasal breathing, fast breathing and slow breathing by employing it in a wearable mask, a mandatory wearable product during the current COVID-19 pandemic situation.Thus, the above proposed sensor may hold tremendous potential in wearable/flexible healthcare technology with good sensitivity, stability, biodegradability and flexibility at this time of need.

Unique Noncontact Monitoring of Human Respiration and Sweat Evaporation Using a CsPb2Br5-Based Sensor

Respiration monitoring and human sweat sensing have promising application prospects in personal healthcare data collection, disease diagnostics, and the effective prevention of human-to-human transmission of fatal viruses. Here, we have introduced a unique respiration monitoring and touchless sensing system based on a CsPb₂Br₅/BaTiO₃ humidity-sensing layer operated by water-induced interfacial polarization and prepared using a facile aerosol deposition process. Based on the relationship between sensing ability and layer thickness, the sensing device with a 1.0 μm thick layer was found to exhibit optimal sensing performance, a result of its ideal microstructure. This sensor also exhibits the highest electrical signal variation at 0.5 kHz due to a substantial polarizability difference between high and low humidity. As a result, the CsPb₂Br₅/BaTiO₃ sensing device shows the best signal variation of all types of breath-monitoring devices reported to date when used to monitor sudden changes in respiratory rates in diverse situations. Furthermore, the sensor can effectively detect sweat evaporation when placed 1 cm from the skin, including subtle changes in capacitance caused by finger area and motion, skin moisture, and contact time. This ultrasensitive sensor, with its fast response, provides a potential new sensing platform for the long-term daily monitoring of respiration and sweat evaporation.

Performance of the highly sensitive humidity sensor constructed with nanofibrillated cellulose/graphene oxide/polydimethylsiloxane aerogel via freeze drying

A kind of capacitive humidity sensor with high sensitivity constructed with nanofibrillated cellulose (NFC), graphene oxide (GO) and polydimethylsiloxane (PDMS) is presented in this work, via a simple ultrasonic dispersion and freeze drying technology. The NFC and GO with a strong adsorption for water molecules were used as a substrate for the promotion of capacitive response of the humidity sensor. Moreover, anhydrous ethanol was added to inhibit the generation of big cracks in the humidity sensor in the freeze drying process, so as to obtain a regular network porous structure, then providing a great deal of conduction channels and active sites for molecular water. Also, the addition of PDMS can effectively enhance the flexibility and stability of its porous structure. The results confirmed that the humidity sensor with 30 wt% GO showed an excellent humidity sensitivity (6576.41 pF/% RH), remarkable reproducibility, low humidity hysteresis characteristic in 11-97% relative humidity (RH) at 25 °C, and short response/recovery times (57 s/2 s). In addition, the presented sensor exhibited small relative deviation of the measured relative humidity value compared with the commercial hygrometer. The realization of the high sensitivity can be attributed to the theories about interaction of the hydrophilic group, proton transfer of water molecules and the three-dimensional network transport structure model. Therefore, the NFC/GO/PDMS humidity sensor finally realizes stable, reproducible and fast humidity sensing via an eco-friendly process, exhibiting promising potential for wide practical application.

High-Performance Colorimetric Humidity Sensors Based on Konjac Glucomannan

High-humidity conditions (85-100% relative humidity (RH)) have very diverse effects on many aspects of people's daily lives. Despite remarkable progress in the development of structural coloration-based humidity sensors, how to significantly improve the sensitivity and visual humidity resolution of these humidity sensors under a high-humidity environment remains a great challenge. In this study, high-performance colorimetric humidity sensors based on environment-friendly konjac glucomannan (KGM) via thin-film interference are developed using a simple, affordable, and scalable preparation method. An effective strategy is demonstrated for substantially improving the sensor sensitivity and visual humidity resolution under a high-humidity environment via synergistic integration of multiorder interference peaks, sensor array technology, and superior water-absorbing polymer. The KGM full-range humidity sensors exhibit fast and dynamic response toward the humidity change without power consumption, and they also show high sensitivity and selectivity, little hysteresis, and excellent stability against high-humidity conditions. The KGM humidity sensors display extraordinary red shift of the reflection peak (e.g., 385 nm) and the visual humidity resolution as high as 1.5% RH in the visible range from 85 to 100% RH, which represent the largest spectra shift and highest visual humidity resolution, respectively, for structural coloration-based humidity sensors in high-humidity conditions.

High-Performance Ultrafast Humidity Sensor Based on Microknot Resonator-Assisted Mach-Zehnder for Monitoring Human Breath

Monitoring the dynamic humidity requires sensors with fast response and anti-electromagnetic interference, especially for human respiration. Here, an ultrafast fiber-optic breath sensor based on the humidity-sensitive characteristics of gelatin film is proposed and experimentally demonstrated. The sensor consists of a microknot resonator superimposed on a Mach-Zehnder (MZ) interferometer produced by a tapered single-mode fiber, which has an ultrafast response (84 ms) and recovery time (29 ms) and a large dynamic transmission range. The humidity in dynamic ambient causes changes in the refractive index of gelatin coating, which could trigger spectral intensity transients that can be explicitly distinguished between the two states. The sensing principle is analyzed using the traditional transfer-matrix analysis method. The influence of coating thickness on the sensor's trigger threshold is further investigated. Experiments on monitoring breath patterns indicate that the proposed breath sensor has high repeatability, reliability, and validity, which enable many other potential applications such as food processing, health monitoring, and other biomedical applications.

TiO2/(K,Na)NbO3 Nanocomposite for Boosting Humidity-Sensing Performances

Nanomaterials of TiO₂, (K_0.5Na_0.5)NbO₃, and the TiO₂/(K_0.5Na_0.5)NbO₃ nanocomposite were successfully synthesized by a hydrothermal method. Impedance-type humidity sensors were fabricated based on these materials. Our results reveal that the impedance of the TiO₂/(K_0.5Na_0.5)NbO₃ sensor changes by 5 orders of magnitude with an ultrahigh sensing response of S_f = 166 470 recorded at 100 Hz in the tested relative humidity (RH) range of 12-94%. This value is almost 2 and 4 orders of magnitude larger than that of the (K_0.5Na_0.5)NbO₃ and TiO₂ sensors, respectively. Interestingly, satisfactory response/recovery time (25/38 s, within 5 min), very small hysteresis (<5%), excellent stability, and good repeatability were also achieved in the TiO₂/(K_0.5Na_0.5)NbO₃ sensor. The improved sensing properties are ascribed to the synergistic effect of TiO₂/(K_0.5Na_0.5)NbO₃ heterojunction, which contributes the impedance that is susceptible to environmental humidity. This work underscores that it is a facile way to boost humidity-sensing performance by constructing proper nanocomposites.

Respiratory rate on exercise measured by nanoparticle-based humidity sensor

In this study, we show the measurement of respiratory rate on exercise using a nanoparticle-based humidity sensor. A portable respiratory rate sensor is comprised of a colloidal silica nanoparticle-based humidity sensor chip. The impedance of the silica nanoparticle film is dependent on humidity and it is used for the detection of humid exhaled air. The respiratory rate sensor can be attached on an oxygen mask and the sensor signal is remotely monitored via Bluetooth. We show that the sensor follows a respiratory rate up to 60 bpm. We compare the sensor signal with that of a conventional respiratory measurement unit, which monitors a respiratory volume. The nanoparticle-based sensor can monitor a respiratory rate of an exercising person on a treadmill. The sensor operates stably for almost one year.

Wearable Devices for Precision Medicine and Health State Monitoring

Wearable technologies will play an important role in advancing precision medicine by enabling measurement of clinically-relevant parameters describing an individual's health state. The lifestyle and fitness markets have provided the driving force for the development of a broad range of wearable technologies that can be adapted for use in healthcare. Here we review existing technologies currently used for measurement of the four primary vital signs: temperature, heart rate, respiration rate, and blood pressure, along with physical activity, sweat, and emotion. We review the relevant physiology that defines the measurement needs and evaluate the different methods of signal transduction and measurement modalities for the use of wearables in healthcare.

Ultrafast freestanding microfiber humidity sensor based on three-dimensional graphene network cladding

An all-fiber humidity sensor is proposed and fabricated by depositing three-dimensional graphene network (3DGN) around the surface of a freestanding microfiber (MF). The high specific surface area and porosity of 3DGN enhances its interaction with water molecules, allowing high performance of the humidity sensor. The sensor can operate in a wide relative humidity (RH) range of 11.6%RH-90.9%RH with a high sensitivity of -2.841 dB/%RH in the RH range (80.3%RH - 90.9%RH). The response and recovery times of this type of microfiber sensor are measured respectively to be 57 ms and 55 ms, which are one order magnitude faster than those of other fiber RH sensors activated by two-dimensional materials coating. Such an all-fiber RH sensor with high sensitivity and fast response property possesses great potential of application in widespread fields, such as biology, chemical processing and food processing.

High-performance humidity sensor using Schottky-contacted SnS nanoflakes for noncontact healthcare monitoring

Humidity sensors based on flexible sensitive nanomaterials are very attractive in noncontact healthcare monitoring. However, the existing humidity sensors have some shortcomings such as limited sensitivity, narrow relative humidity (RH) range, and a complex process. Herein, we show that a tin sulphide (SnS) nanoflakes-based sensor presents high humidity sensing behaviour both in rigid and flexible substrate. The sensing mechanism based on the Schottky nature of a SnS-metal contact endows the as-fabricated sensor with a high response of 2491000% towards a wide RH range from 3% RH to 99% RH. The response and recovery time of the sensor are 6 s and 4 s, respectively. Besides, the flexible SnS nanoflakes-based humidity sensor with a polyimide substrate can be well attached to the skin and exhibits stable humidity sensing performance in the natural flat state and under bending loading. Moreover, the first-principles analysis is performed to prove the high specificity of SnS to the moisture (H₂O) in the air. Benefiting from its promising advantages, we explore some application of the SnS nanoflakes-based sensors in detection of breathing patterns and non-contact finger tips sensing behaviour. The sensor can monitor the respiration pattern of a human being accurately, and recognize the movement of the fingertip speedily. This novel humidity sensor shows great promising application in physiological and physical monitoring, portable diagnosis system, and noncontact interface localization.

Effects of pH on High-Performance ZnO Resistive Humidity Sensors Using One-Step Synthesis

In this paper, we prepared a high-performance zinc oxide (ZnO) humidity sensor in an alkaline environment using one-step hydrothermal method. Experiments showed that the pH value of the precursor solution affects the performance of ZnO humidity sensors. There are abundant hydroxyl group and oxygen vacancies on the surface of ZnO with a precursor pH value of 10. Abundant hydroxyl groups on the surface of ZnO can adsorb a large number of water molecules and rich oxygen vacancies can accelerate the decomposition of water molecules, thus increasing the number of conductive ions (H₃O⁺) and further improving the performance of the sensor. So, such a ZnO humidity sensor exhibited high sensitivity (14,415), good linearity, small hysteresis (0.9%), fast response/recovery time (31/15 s) in the range from 11% to 95% relative humidity (RH). Moreover, the ZnO-2 humidity sensor has good repeatability and can be effectively used for a long time. This study provides a new idea for the development of low-cost, high-performance and reusable ZnO resistive humidity sensors.

A fully inkjet-printed transparent humidity sensor based on a Ti3C2/Ag hybrid for touchless sensing of finger motion

Inkjet-printing was used to prepare a flexible and transparent humidity sensor with a Ti₃C₂/Ag hybrid as the humidity-sensitive film and poly(diallyldimethylammonium chloride) (PDDA) as the adhesive layer. The sensor demonstrates an ultrahigh sensitivity (106 800%), a rapid response (80 ms), and excellent bending resistance. We demonstrate that an array of sensors can track moving fingers in a non-contact way and map the distance of the fingers away from the sensor surface. Therefore, our humidity sensors have great potential for novel human-machine interfacing such as touchless control of electronics and collision control between robots and humans in a cobot setting.

3D hybrid networks of gold nanoparticles: mechanoresponsive electrical humidity sensors with on-demand performances

We have engineered macroscopic 3D porous networks of gold nanoparticles (AuNPs) chemically interconnected by di-thiolated ethylene glycol oligomers. The formation of such superstructures has been followed by means of UV-Vis spectroscopy by monitoring the aggregation-dependent plasmonic band of such nanomaterials. The controlled chemical tethering of the AuNPs with di-thiolated linkers possessing a well-defined contour length rules the interparticle distance. The use of ad-hoc linkers ensures charge transport via direct tunneling and the hygroscopic nature of the ethylene glycol backbone allows interaction with moisture. Upon interaction with water molecules from the atmosphere, our 3D networks undergo swelling reducing the tunnelling current passing through the system. By exploiting such a behavior, we have devised a new approach for the fabrication of electrical resistive humidity sensors. For the first time we have also introduced a new strategy to fabricate stable and robust devices by covalently attaching our 3D networks to gold electrodes. Devices comprising both 4 (TEG) or 6 (HEG) ethylene glycol repetitive units combined with AuNPs exhibited (i) unprecedentedly high response speed (∼26 ms), (ii) short recovery time (∼250 ms) in the absence of any hysteresis effect, and (iii) a linear response to humidity changes characterized by a highest sensitivity of 51 kΩ per RH(%) for HEG- and 500 Ω per RH(%) for TEG-based devices. The employed green solution processing in water and the extreme robustness of our 3D networks make them interesting candidates for the fabrication of sensors which can operate under extreme conditions and for countless cycles.

Skin-Inspired Humidity and Pressure Sensor with a Wrinkle-on-Sponge Structure

Sensors with multifunctions have attracted great attention for their extensive application value, among which humidity sensing and pressure sensing are necessary to electronics undoubtedly because of the complex physical environment we live in. Inspired by the structure of skin, in this article, we design a new method to combine wrinkle structure with porous sponge structure and achieve a novel, flexible, compressible, and bifunctional sensor based on carbon nanotube-polydimethylsiloxane (CNT-PDMS) with functions of humidity sensing and pressure sensing. The performance of the humidity sensing part can be controlled by the ultraviolet and ozone (UVO) treatment time and CNT concentration, while the sensitivity of the pressure sensing part can be controlled by the CNT concentration and grinding time of sugar granules. The bifunctional sensor can easily sense approaching and touching of a hand, which shows great potential of alarming and protecting some electronics. Moreover, the bifunctional sensor can also be used in detecting human joint motions and breath conditions as a wearable and flexible health monitor.

Biofriendly and Regenerable Emotional Monitor from Interfacial Ultrathin 2D PDA/AuNPs Cross-linking Films

Well-designed 2D materials with ultrathin structures show great potential for humidity-sensing performance owing to their high surface-volume ratio and a great number of exposed atoms on the surface. However, some sensing elements employed for healthcare applications may be considered as potentially risky, such as inflammation, granuloma formation, and carcinogenesis. Herein, we explored biofriendly humidity-sensing characteristics inspired by the great biocompatibility and conductivity of hyperbranched polyethyleneimine-capped gold nanoparticles and cross-linked with polydopamine from the adhesive proteins in mussels. It was successfully employed into two kinds of wearable devices, sports watches and breathing masks, for real-time recording humidity's fluctuation in expiration and sweat with changes of individual's crying, laughing, nervous, sleeping, training, and cold states. The wearable devices allow us to monitor individual's physical activities and emotional states well, suggesting a promising prospect in safe, reusable, long term, and noncontact human health monitoring applications.

Formation of Two-Dimensional Network of Organic Charge-Transfer Complexes at the Air-Water Interface

The air-water interface is an ideal platform to produce two-dimensional (2D) structures involving anything from simple organic molecules to supramolecular moieties by exploiting hydrophobic-hydrophilic interactions. Here, we show, using grazing incidence X-ray scattering, the formation of a 2D ordered structure of a charge-transfer (C-T) complex, namely, dodecyl methyl viologen (DMV) as acceptor and coronene tetracarboxylate potassium salt (CS) as donor, at the air-water interface. We have observed a phase transition in the 2D ordered structure as the area per molecule is decreased with increasing surface pressure in a Langmuir trough. The high-pressure ordering of the hydrocarbon chains associated with DMV destroys long-range C-T conjugation of DMV and CS at the air-water interface. Our results also explain the formation of DMV-CS cylindrical reverse micelles and eventually long nanowires that get formed in the self-assembly process in the bulk medium to preserve both the C-T conjugation and the organic tail-tail organization.

Flexible Smart Noncontact Control Systems with Ultrasensitive Humidity Sensors

The development of noncontact humidity sensors with high sensitivity, rapid response, and a facile fabrication process is urgently desired for advanced noncontact human-machine interaction (HMI) applications. Here, a flexible and transparent humidity sensor based on MoO₃ nanosheets is developed with a low-cost and easily manufactured process. The designed humidity sensor exhibits ultrahigh sensitivity, fast response, great stability, and high selectivity, exceeding the state-of-the-art humidity sensors. Furthermore, a wearable moisture analysis system is assembled for real-time monitoring of ambient humidity and human breathing states. Benefiting from the sensitive and rapid response to fingertip humidity, the sensors are successfully applied to both a smart noncontact multistage switch and a novel flexible transparent noncontact screen for smart mobile devices, demonstrating the potential of the MoO₃ nanosheets-based humidity sensors in future HMI systems.

Recent development of respiratory rate measurement technologies

Respiratory rate (RR) is an important physiological parameter whose abnormality has been regarded as an important indicator of serious illness. In order to make RR monitoring simple to perform, reliable and accurate, many different methods have been proposed for such automatic monitoring. According to the theory of respiratory rate extraction, methods are categorized into three modalities: extracting RR from other physiological signals, RR measurement based on respiratory movements, and RR measurement based on airflow. The merits and limitations of each method are highlighted and discussed. In addition, current works are summarized to suggest key directions for the development of future RR monitoring methodologies.

Self-Powered, Rapid-Response, and Highly Flexible Humidity Sensors Based on Moisture-Dependent Voltage Generation

Most advanced humidity sensors are powered by batteries that need regular charging and replacement, causing environmental problems and complicated management issues. This paradigm has been overcome through the development of new technology based on the concept of simple, self-powered, rapid-response, flexible humidity sensors enabled by the properties of densely packed titanium dioxide (TiO₂) nanowire networks. These sensors eliminate the need for an external power source and produce an output voltage that can be readily related to ambient humidity level over a wide range of ambient conditions. They are characterized by rapid response and relaxation times (typically 4.5 and 2.8 s, respectively). These units are mechanically flexible and maintain a constant voltage output after 10 000 bending cycles. This new type of humidity sensor is easily attached to a human finger for use in the monitoring of ambient humidity level in the environment around human skin, near wet objects, or in the presence of moist materials. The unique properties of this new self-powered wearable humidity sensor technology open up a variety of new applications, including the development of electronic skin, personal healthcare products, and smart tracking in the future Internet-of-things.

Bio-compatible organic humidity sensor based on natural inner egg shell membrane with multilayer crosslinked fiber structure

In this paper, we propose a novel bio-compatible organic humidity sensor based on natural inner egg shell membrane (IESM) with multilayer cross linked fiber structure that can be used as a substrate as well as a sensing active layer. To fabricate the proposed sensors, two different size inter digital electrodes (IDEs) with 10 mm × 4 mm for sensor 1 and 12 mm × 6 mm for sensor 2 are printed on the surface of the IESM through Fujifilm Dimatix DMP 3000 inkjet material printing setup, which have finger width of 100 μm and space of 100 μm. The fabricated sensors stably operates in a relative humidity (RH) range between 0% RH to 90% RH, and its output impedance and capacitance response are recorded at 1 kHz and 10 kHz. The response time (T_res) and recovery time (T_rec) of sensor 1 are detected as ~1.99 sec and ~8.76 sec, respectively and the T_res and T_rec of sensor 2 are recorded as ~2.32 sec and ~9.21 sec, respectively. As the IESM for the humidity sensor, the natural materials can be implemented in our daily life as they open a new gate way for bio-compatible devices.

Nanostructured P3HT as a Promising SensingElement for Real-Time, Dynamic Detection ofGaseous Acetone

The dynamic response of gas sensors based on poly(3-hexylthiophene) (P3HT) nanofibers (NFs) to gaseous acetone was assessed using a setup based on flow-injection analysis, aimed at emulating actual breath exhalation. The setup was validated by using a commercially available sensor. The P3HT NFs sensors tested in dynamic flow conditions showed satisfactory reproducibility down to about 3.5 ppm acetone concentration, a linear response over a clinically relevant concentration range (3.5-35 ppm), excellent baseline recovery and reversibility upon repeated exposures to the analyte, short pulse rise and fall times (less than 1 s and about 2 s, respectively) and low power consumption (few nW), with no relevant response to water. Comparable responses’ decay times under either nitrogen or dry air suggest that the mechanisms at work is mainly attributable to specific analyte-semiconducting polymer interactions. These results open the way to the use of P3HT NFs-based sensing elements for the realization of portable, real-time electronic noses for on-the-fly exhaled breath analysis.

Ultrafast Response Polyelectrolyte Humidity Sensor for Respiration Monitoring

Respiration monitoring is important for evaluating human health. Humidity sensing is a promising way to establish a relationship between human respiration and electrical signal. This work describes polymer humidity sensors with ultrafast response for respiration monitoring. The humidity-sensitive polyelectrolyte is in situ cross-linked on the substrate printed with interdigitated electrodes by a thiol-ene click reaction. The polyelectrolyte humidity sensor shows rapid water adsorption/desorption ability, excellent stability, and repeatability. The sensor with ultrafast response and recovery (0.29/0.47 s) when changing humidity between 33 and 95% shows good application prospects in breath monitoring and touchless sensing. Different respiration patterns can be distinguished, and the breath rate/depth of detection subjects can also be determined by the sensor. In addition, the obtained sensor can sense the skin evaporation in a noncontact way.