Highlights
The organohydrogel-based O
2
sensor features full concentration detection range (0-100%), ultralow limit of detection (5.7 ppm), high sensitivity (0.2%/ppm), excellent selectivity, tunable ...response/recovery speeds, good linearity, and room-temperature operation.
The oxygen sensor can work normally under various extreme environmental conditions, such as low (below −18 °C) and high (above 40 °C) temperatures, dry (11.3% RH), and humid (90.5% RH) environments.
An electrochemical reaction-based mechanism is proposed to elucidate the oxygen sensing behavior of ion-conducting organohydrogel.
With the advent of the 5G era and the rise of the Internet of Things, various sensors have received unprecedented attention, especially wearable and stretchable sensors in the healthcare field. Here, a stretchable, self-healable, self-adhesive, and room-temperature oxygen sensor with excellent repeatability, a full concentration detection range (0-100%), low theoretical limit of detection (5.7 ppm), high sensitivity (0.2%/ppm), good linearity, excellent temperature, and humidity tolerances is fabricated by using polyacrylamide-chitosan (PAM-CS) double network (DN) organohydrogel as a novel transducing material. The PAM-CS DN organohydrogel is transformed from the PAM-CS composite hydrogel using a facile soaking and solvent replacement strategy. Compared with the pristine hydrogel, the DN organohydrogel displays greatly enhanced mechanical strength, moisture retention, freezing resistance, and sensitivity to oxygen. Notably, applying the tensile strain improves both the sensitivity and response speed of the organohydrogel-based oxygen sensor. Furthermore, the response to the same concentration of oxygen before and after self-healing is basically the same. Importantly, we propose an electrochemical reaction mechanism to explain the positive current shift of the oxygen sensor and corroborate this sensing mechanism through rationally designed experiments. The organohydrogel oxygen sensor is used to monitor human respiration in real-time, verifying the feasibility of its practical application. This work provides ideas for fabricating more stretchable, self-healable, self-adhesive, and high-performance gas sensors using ion-conducting organohydrogels.
A multifunctional sensor comprising humidity, temperature, and flow detection capabilities is fabricated with a facile, single-layered device structure. A microheater based on serpentine Pt ...microlines plays key roles in both humidity and flow sensing at the hot state by introducing an efficient Joule heating effect, and meanwhile functions as a reliable thermistor at the cold state for accurate temperature measurement. For the first time, the strong temperature-dependent humidity-sensing properties of graphene oxide (GO) are revealed using the microheater platform. The GO-based humidity sensor displays ultrahigh sensitivity 124/% relative humidity (RH), fast response time (3 s), wide detection range (8–95% RH) at room temperature, while the sensitivity drops at elevated temperatures, indicating the non-negligible temperature effect. Interestingly, a linear relationship between sensitivity and voltage is observed for the flow sensor, indicating the capability to manipulate sensitivity by conveniently modifying the voltage applied on the microheater. Because the three sensors work independently with distinguishable output signals, multiparametric sensing is enabled to monitor various human activities, such as respiration, noncontact sensation, and so forth. This work develops a simple, cost-effective, and useful multiparametric-sensing platform using a microheater for potential applications in the growing fields of internet of things, healthcare monitoring, and human–machine interfaces.
Conductive hydrogels can be used in wearable electronics integrated with skin, but the bulk structure of existing hydrogel-based temperature sensors limits the wearing comfort, response/recovery ...speeds, and sensitivity. Here, stretchable and transparent temperature sensors based on a novel thin-film sandwich structure (TFSS) are designed, which display unprecedented thermal sensitivity (24.54%/°C), fast response time (0.19 s) and recovery time (0.08 s), a broad detection range (from −28 to 95.3 °C), high resolution (0.8 °C), and high stability. The thin hydrogel layer (12.15 μm) is encapsulated by two thin elastomer layers, which prevent the water evaporation and enhance the heat transfer, leading to the boosted stability and accelerated response/recovery speeds. The nondrying and antifreezing capabilities are further promoted by the hydratable lithium bromide (LiBr) incorporated in the hydrogel, enabling it to avoid dehydration in an extremely arid environment and freeze below subzero temperatures (freezing point below −120 °C). A comparative study reveals that the thermal sensitivity displayed by the TFSS sensor in capacitance mode is several times higher than that in conventional conductance/resistance mode above room temperature. Importantly, a new mechanism based on a horizontal plate capacitance model is proposed to understand the high sensitivity by considering the permittivity and geometry variations of TFSS. The thin TFSS, stretchability and transparency enable the sensor to be conformally and comfortably attached to human skin for real-time and reliable monitoring of various human motions, physical states, skin temperature, etc., without affecting the appearance.
It is essential to impart the thermal stability, high sensitivity, self-healing, and transparent attributes to the emerging wearable and stretchable electronics. Here, a facile solvent replacement ...strategy is exploited to introduce ethylene glycol/glycerol (Gly) in hydrogels for enhancing their thermal sensitivity and stability synchronously. For the first time, we find that the solvent plays a key role in the thermal sensitivity of hydrogels. By adjusting the water content in hydrogels using a simple dehydration treatment, the thermal sensitivity is raised to 13.1%/°C. Thanks to the ionic transport property and water–Gly binary solvent, the organohydrogel achieves an unprecedented thermal sensitivity of 19.6%/°C, which is much higher than those of previously reported stretchable thermistors. The mechanism for the thermal response is revealed by considering the thermally activated ion mobility and dissociation. The stretchable thermistors are conformally attached on curved surfaces for the practical monitoring of minute temperature change. Notably, the uncovered Gly-organohydrogel avoids drying and freezing at 70 and −18 °C, respectively, reflecting the excellent antidrying and antifreezing attributes. In addition, the organohydrogel displays ultrahigh stretchability (1103% strain), self-healing ability, and high transparency. This work sheds light on fabricating ultrasensitive and stretchable temperature sensors with excellent thermal stability by modulating the solvent of hydrogels.
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•3D structured SnS2/Reduced graphene oxide (RGO) heterojunction is synthesized via a facile hydrothermal route.•The 3D SnS2/RGO heterojunction exhibits an order of magnitude higher ...response to NO2 gas than the pristine RGO.•A liquid crystal polymer substrate was used to fabricate flexible NO2 sensor with endurance to mechanical deformation and humidity variation.•A charge transfer mechanism mediated by a depletion layer is proposed for the NO2 sensing mechanism.•The elevated temperature improves the response and recovery kinetics.
Graphene (Gr) has been recognized as a promising candidate for room temperature (RT), high-performance gas sensing due to its unique structure and electronic properties. However, the poor selectivity of pristine Gr hinders its practical application in NO2 detection. SnS2 displays a strong physical affinity to NO2, while the ultrahigh resistance and low sensitivity at RT limit its utility. Here, 3D structured SnS2/reduced graphene oxide (RGO) heterojunction, a novel gas responsive material merging the merits of RGO and SnS2, is synthesized through a facile hydrothermal route. The 3D SnS2/RGO displays impressive NO2 sensing performance, including high sensitivity (6.1 ppm−1), excellent selectivity, low theoretical limit of detection (8.7 ppb), linearity and reversibility. The SnS2/RGO heterojunction exhibits 22.6 times higher response to NO2 than the pristine RGO, indicating the remarkable role of surface decoration in enhancing the gas sensing performance of RGO. The high sensitivity originates from the formation of heterojunction and 3D porous structures, which promote NO2 adsorption, diffusion and charge transfer by providing alternative adsorption sites and charge transport paths. Notably, a liquid crystal polymer substrate is employed to fabricate a flexible NO2 sensor with endurance to mechanical deformation. The elevated temperature improves the response and recovery kinetics.
The traditional human-machine interaction mode of communicating solely with pressure sensors needs modification, especially at a time when COVID-19 is circulating globally. Here, a transparent, ...stretchable, resilient, and high-performance hydrogel fiber-based bimodal sensor is fabricated by using a polyacrylamide-alginate double network hydrogel, which features high sensitivity (3.17% cm
−1
), wide working range (18 cm), fast response/recovery speeds (90/90 ms) and good stability in proximity sensing, and impressive pressure sensing performance, including high sensitivity (0.91 kPa
−1
), short response/recovery time (40/40 ms), low detection limit (63 Pa) and good linearity. Moreover, the response switch between proximity/pressure modes is measured and non-interfering dual-mode detection is achieved. Notably, the stretchable bimodal sensor is capable of working under 100% tensile strain without degrading the sensing performance. Specifically, the proximity sensor shows good immunity to the strain, while the pressure sensitivity is even promoted. Furthermore, the sensor is tough enough to work normally after punctures from a knife and strikes from a wrench. Notably, the sensor can be used for gesture recognition and subtle pressure detection, such as small water droplets (10 mg), wrist pulse,
etc.
A 3 × 3 array is further shown for accurate spatial sensing and location identification, verifying the feasibility of its practical application.
An ultrastretchable hydrogel fiber-based proximity/pressure bimodal sensor with high sensitivity, resilience and low detection limit, and capability for real-time monitoring of physiological signals and human-machine interfaces is fabricated.
A facile, one-step hydrothermal route was exploited to prepare SnO2-decorated reduced graphene oxide hydrogel (SnO2/RGOH) with three-dimensional (3D) porous structures for NO2 gas detection. Various ...material characterizations demonstrate the effective deoxygenation of graphene oxide and in situ growth of rutile SnO2 nanoparticles (NPs) on 3D RGOH. Compared with the pristine RGOH, the SnO2/RGOH displayed much lower limit of detection (LOD) and an order of magnitude higher sensitivity, revealing the distinct impact of SnO2 NPs in improving the NO2-sensing properties. An exceptional low theoretical LOD of 2.8 ppb was obtained at room temperature. The p–n heterojunction formed at the interface between RGOH and SnO2 facilitates the charge transfer, improving both the sensitivity in NO2 detection and the conductivity of hybrid material. Considering that existing SnO2/RGO-based NO2 sensors suffer from great vulnerability to humidity, here we employed integrated microheaters to effectively suppress the response to humidity, with nearly unimpaired response to NO2, which boosted the selectivity. Notably, a flexible NO2 sensor was constructed on a liquid crystal polymer substrate with endurance to mechanical deformation. This work indicates the feasibility of optimizing the gas-sensing performance of sensors by combining rational material hybridization, 3D structural engineering with temperature modulation.
To address the low gas sensitivity of pristine graphene (Gr), chemical modification of Gr has been proved as a promising route. However, the existing chemical functionalization method imposes the ...utilization of toxic chemicals, increasing the safety risk. Herein, vitamin C (VC)-modified reduced graphene hydrogel (V-RGOH) is synthesized via a green and facile self-assembly process with the assistance of biocompatible VC molecules for high-performance NH3 and NO2 detection. The three-dimensional (3D) structured V-RGOH is highly sensitive to low-concentration NH3 and NO2 at room temperature. In comparison with those of the unmodified RGOH, the V-RGOH gas sensors display an order of magnitude higher sensitivity and much lower limit of detection, resulting from the enhanced interaction between VC and analytes. NH3 and NO2 with extremely low concentrations of 500 and 100 ppb are detected experimentally. Notably, imbedded microheaters are exploited to explore the temperature-dependent gas sensing properties, revealing the negative and positive impacts of temperature on the sensitivity and recovery speed, respectively. Notably, the V-RGOH sensor exhibits remarkable selectivity and linearity and a wide detection range. This work reveals the remarkable effects of chemical modification with biodegradable molecules and 3D structure design on improving the gas sensing performance of the Gr material.
Here stretchable, self‐healable, and transparent gas sensors based on salt‐infiltrated hydrogels for high‐performance NO2 sensing in both anaerobic environment and air at room temperature, are ...reported. The salt‐infiltrated hydrogel displays high sensitivity to NO2 (119.9%/ppm), short response and recovery time (29.8 and 41.0 s, respectively), good linearity, low theoretical limit of detection (LOD) of 86 ppt, high selectivity, stability, and conductivity. A new gas sensing mechanism based on redox reactions occurring at the electrode–hydrogel interface is proposed to understand the sensing behaviors. The gas sensing performance of hydrogel is greatly improved by incorporating calcium chloride (CaCl2) in the hydrogel via a facile salt‐infiltration strategy, leading to a higher sensitivity (2.32 times) and much lower LOD (0.06 times). Notably, both the gas sensing ability, conductivity, and mechanical deformability of hydrogels are readily self‐healable after cutting off and reconnection. Such large deformations as 100% strain do not deprive the gas sensing capability, but rather shorten the response and recovery time significantly. The CaCl2‐infiltrated hydrogel shows excellent selectivity of NO2, with good immunity to the interference gases. These results indicate that the salt‐infiltrated hydrogel has great potential for wearable electronics equipped with gas sensing capability in both anaerobic and aerobic environments.
An intrinsically stretchable, self‐healing, transparent, and ion‐conductive hydrogel is utilized to fabricate NO2 gas sensors. The sensing mechanism reveals that the redox reaction of NO2 at the electrode–hydrogel interface induces current variation. The sensor exhibits high sensitivity (119.9%/ppm), ultralow limit of detection (86 ppt), good selectivity. Meanwhile, it can operate in both anaerobic and aerobic environments at room temperature.
Highly stretchable, sensitive and room-temperature nitrogen dioxide (NO2) sensors are fabricated by exploiting intrinsically stretchable, transparent and ion-conducting hydrogels and active metals as ...the novel transducing materials and electrodes, respectively. The NO2 sensor exhibits high sensitivity (60.02% ppm−1), ultralow theoretical limit of detection (6.8 ppb), excellent selectivity, linearity and reversibility at room temperature. Notably, the sensitivity can be maintained even under 50% tensile strain. For the first time, it's found that the metal electrodes significantly impact the sensing performance. Specifically, the sensitivity is boosted from 31.18 to 60.02% ppm−1 by replacing the anodic silver with copper–tin alloy. Importantly, by applying specially designed sensing tests, and microscopic and composition analyses, we have obtained the inherent NO2 sensing mechanism: the anodic metal tends to be oxidized and the NO2 molecules tend to react in the cathode–gel interface. The introduction of glycerol converts the hydrogel into the organohydrogel with remarkably enhanced anti-drying and anti-freezing capacities and toughness, which effectively improved the long-time stability of the sensors. Importantly, we execute sound/light alarms and a wireless smartphone alarm by utilizing a designed circuit board and applet. This work gives an incisive investigation for the preparation, performance improvement, mechanism and application of hydrogel-based NO2 sensors, promoting the evolution of hydrogel ionotronics.
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