Conversion of THz refractive index variation to detectable voltage change realized by a graphene-based Brewster angle device.

The Brewster effect has been previously reported as an essential mechanism for terahertz (THz) wave sensing application. However, generally in a sensing application, a complex rotation apparatus is required for detecting the slight change in Brewster angle. Here, we propose a graphene-based Brewster angle device operating at a specific terahertz frequency capable of sensing the refractive index at a fixed incident angle. In other words, our sensing device could avoid the impact of Brewster angle shift and eliminate the need for high-precision rotating equipment, which is usually required in traditional sensing applications. The conversion from the refractive index to a Volt-level detectable voltage roots from the tunability of graphene's Fermi level in the external electrical field. A linear correlation between the output voltage and the background refractive index is observed and theocratically analyzed. Furthermore, we present the improvement of our device in terms of sensing range and sensitivity by adjusting the permittivity of the dielectric substrate. As a demonstration of our proposed device, a detection range of 1.1-2.4 and a sensitivity of 20.06 V/RIU for refractive index is achieved on a high-resistance silicon substrate operating at 0.3 THz.

High sensitivity graphene based health sensor with self-warning function.

In order to reduce the damage to people's health from diseases that attack the respiratory system such as COVID-19, asthma, and pneumonia, it is desired that patients' breathing can be monitored and alerted in real-time. The emergence of wearable health detection sensing devices has provided a relatively good response to this problem. However, there are still problems such as complex structure and poor performance. This paper introduces a laser-induced graphene (LIG) device that is attached to PDMS. The LIG is produced by laser irradiation of Nomex and subsequently transferred and attached to the PDMS. After being tested, it has demonstrated high sensitivity, stable tensile performance, good acoustic performance, excellent thermal stability, and other favorable properties. Notably, its gauge factor (GF) value can reach 721.67, which is quite impressive. Additionally, it is capable of emitting an alarm sound with an SPL close to 60 dB when receiving signals within the range of 5-20 kHz. The device realizes mechanical sensing and acoustic functions in one chip, and has a high application value in applications that need to combine sensing and early warning.

Enhancing the efficiency of graphene-based THz modulator by optimizing the Brewster angle.

The gate-controllable electronical property of graphene provides a possibility of active tuning property for THz modulator. However, the common modulation technology which only depends on voltage cannot solve the problem of power consumption limitation in communication applications. Here, we demonstrated a Brewster angle-controlled graphene-based THz modulator, which could achieve a relatively high modulation depth with low voltage. First, we explored the complex relationships among the Brewster angles, reflection coefficients and the conductivities of graphene. Then, we further investigated the optimal incident angle selection based on the unusual reflection effect which occurs at Brewster angle. Finally, an improved scheme by dynamically adjusting the incident angle was proposed in this paper. It would make it possible that the modulator could achieve a modulation depth of more than 90% with a Fermi level as low as 0.2eV at any specific frequency in the range of 0.4THz-2.2THz. This research will help to realize a THz modulator with high-performance and ultra-low-power in quantities of applications, such as sensing and communication.

Theoretical analysis of the absorption of CO2 and CO on pristine and Al-doped C3B.

CO2 and CO, the by-products of fossil fuels; one of them is a major cause of global warming and the other endangers the nervous and cardiovascular systems of humans. Therefore, real-time monitoring towards those harmful gases is of practical significance. Nano-structured materials have attracted the attention of scholars for their enormous potential for harmful gas detection. In this work, the adsorption and sensing behavior of C3B and Al-doped C3B monolayers for these two typical hazardous gases were investigated theoretically. The most stable doping model was obtained, and the adsorption process for CO and CO2 was simulated based on this model. The adsorption system shows that the gas molecules are all deformed and that the charge transfer and adsorption energy are significantly increased. Moreover, the adsorption mechanism was investigated by analyzing the electronic behavior of the adsorbent, and the physical adsorption between the hazardous gas and the adsorbent was more favorable for desorption. The good adsorption performance and sensing mechanism suggest that the CO/CO2 sensor prepared using Al-C3B has great potential for application. Our work may provide some guidance for the application of toxic gas monitoring and adsorption.

An investigation of the positive effects of doping an Al atom on the adsorption of CO2 on BN nanosheets: a DFT study.

Nowadays, climate problems caused by greenhouse gases are becoming more and more serious. Motivated by reducing carbon dioxide emissions from fossil fuel power generation, scientists are devoting themselves to developing novel materials or technologies for capturing carbon dioxide. Nanostructure materials, which show great potential for this application, have come to the attention of scientists. Herein, the effects of doping an aluminum atom (replacing one boron atom by one aluminum one) on the adsorption of carbon dioxide on boron nitride nanosheets are theoretically investigated through computational analysis based on density functional theory. The results show that the binding between oxygen and aluminum atoms, which comes from classical Lewis base (CO2)-Lewis acid (Al) interactions, can provide a considerable gain to the mutual effect between the carbon dioxide molecule and the doped substrate. Compared with pristine boron nitride nanosheets, the adsorption energy value of the carbon dioxide molecule is markedly increased to 0.4784 eV (about 2.5-fold) after the doping process, which is in the range of the ideal adsorption energy of 0.415-0.829 eV. More importantly, the essence of physisorption signifies that carbon dioxide can be released by means of specific physical desorption, and, sequentially, this is more conducive for achieving reversible adsorption.

A Miniaturized On-Chip Colorimeter for Detecting NPK Elements.

Recently, precision agriculture has become a globally attractive topic. As one of the most important factors, the soil nutrients play an important role in estimating the development of precision agriculture. Detecting the content of nitrogen, phosphorus and potassium (NPK) elements more efficiently is one of the key issues. In this paper, a novel chip-level colorimeter was fabricated to detect the NPK elements for the first time. A light source-microchannel photodetector in a sandwich structure was designed to realize on-chip detection. Compared with a commercial colorimeter, all key parts are based on MEMS (Micro-Electro-Mechanical System) technology so that the volume of this on-chip colorimeter can be minimized. Besides, less error and high precision are achieved. The cost of this colorimeter is two orders of magnitude less than that of a commercial one. All these advantages enable a low-cost and high-precision sensing operation in a monitoring network. The colorimeter developed herein has bright prospects for environmental and biological applications.

Multi-characteristic tannic acid-reinforced polyacrylamide/sodium carboxymethyl cellulose ionic hydrogel strain sensor for human-machine interaction.

Big data and cloud computing are propelling research in human-computer interface within academia. However, the potential of wearable human-machine interaction (HMI) devices utilizing multiperformance ionic hydrogels remains largely unexplored. Here, we present a motion recognition-based HMI system that enhances movement training. We engineered dual-network PAM/CMC/TA (PCT) hydrogels by reinforcing polyacrylamide (PAM) and sodium carboxymethyl cellulose (CMC) polymers with tannic acid (TA). These hydrogels possess exceptional transparency, adhesion, and remodelling features. By combining an elastic PAM backbone with tunable amounts of CMC and TA, the PCT hydrogels achieve optimal electromechanical performance. As strain sensors, they demonstrate higher sensitivity (GF = 4.03), low detection limit (0.5 %), and good linearity (0.997). Furthermore, we developed a highly accurate (97.85 %) motion recognition system using machine learning and hydrogel-based wearable sensors. This system enables contactless real-time training monitoring and wireless control of trolley operations. Our research underscores the effectiveness of PCT hydrogels for real-time HMI, thus advancing next-generation HMI systems.

Laser directed lithography of asymmetric graphene ribbons on a polydimethylsiloxane trench structure.

Recently, manipulating heat transport by asymmetric graphene ribbons has received significant attention, in which phonons in the carbon lattice are used to carry energy. In addition to heat control, asymmetric graphene ribbons might also have broad applications in renewable energy engineering, such as thermoelectric energy harvesting. Here, we transfer a single sheet of graphene over a 5 µm trench of polydimethylsiloxane (PDMS) structure. By using a laser (1.77 mW, 1 µm diameter spot size, 517 nm wavelength) focusing on one side of the suspended graphene, a triangular shaped graphene ribbon is obtained. As the graphene has a negative thermal expansion coefficient, local laser heating could make the affected graphene area shrink and eventually break. Theoretical calculation shows that the 1.77 mW laser could create a local hot spot as high as 1462.5 °C, which could induce an asymmetric shape structure. We also find the temperature coefficient (-13.06 cm(-1) mW) of suspended graphene on PDMS trench substrate is ten times higher than that reported on SiO2/Si trench substrate. Collectively, our results raise the exciting prospect that the realization of graphene with asymmetric shape on thermally insulating substrate is technologically feasible, which may open up important applications in thermal circuits and thermal management.

Gesture Recognition System Using Reduced Graphene Oxide-Enhanced Hydrogel Strain Sensors for Rehabilitation Training.

Gesture recognition systems epitomize a modern and intelligent approach to rehabilitative training, finding utility in assisted driving, sign language comprehension, and machine control. However, wearable devices that can monitor and motivate physically rehabilitated people in real time remain little studied. Here, we present an innovative gesture recognition system that integrates hydrogel strain sensors with machine learning to facilitate finger rehabilitation training. PSTG (PAM/SA/TG) hydrogels are constructed by thermal polymerization of acrylamide (AM), sodium alginate (SA), and tannic acid-reduced graphene oxide (TA-rGO, TG), with AM polymerizing into polyacrylamide (PAM). The surface of TG has abundant functional groups that can establish multiple hydrogen bonds with PAM and SA chains to endow the hydrogel with high stretchability and mechanical stability. Our strain sensor boasts impressive sensitivity (Gauge factor = 6.13), a fast response time (40.5 ms), and high linearity (R2 = 0.999), making it an effective tool for monitoring human joint movements and pronunciation. Leveraging machine learning techniques, our gesture recognition system accurately discerns nine distinct types of gestures with a recognition accuracy of 100%. Our research drives wearable advancements, elevating the landscape of patient rehabilitation and augmenting gesture recognition systems' healthcare applications.

Humidity-Based Human-Machine Interaction System for Healthcare Applications.

Human-machine interaction (HMI) systems are widely used in the healthcare field, and they play an essential role in assisting the rehabilitation of patients. Currently, a large number of HMI-related research studies focus on piezoresistive sensors, self-power sensors, visual and auditory receivers, and so forth. These sensing modalities do not possess high reliability with regard to breathing condition detection. The humidity signal conveyed by breathing provides excellent stability and a fast response; however, humidity-based HMI systems have rarely been studied. Herein, we integrate a humidity sensor and a graphene thermoacoustic device into a humidity-based HMI system (HHMIS), which is capable of monitoring respiratory signals and emitting acoustic signals. HHMIS has a practical value in healthcare to assist patients. For example, it works as a prewarning system for respiratory-related disease patients with abnormal respiratory rates, and as an artificial throat device for aphasia patients. Achieved based on a laser direct writing technology, this wearable device features low cost, high flexibility, and can be prepared on a large scale. This portable non-contact HMMIS has broad application prospects in many fields such as medical health and intelligent control.

Tunable mechanical, self-healing hydrogels driven by sodium alginate and modified carbon nanotubes for health monitoring.

Conductive hydrogels featuring a modulus similar to the skin have flourished in health monitoring and human-machine interface systems. However, developing conductive hydrogels with self-healing and tunable force-electrical performance remains a problem. Herein, a hydrogen bonding cross-linking strategy was utilized by incorporating silk sericin-modified carbon nanotubes (SS@CNTs) into sodium alginate (SA) and polyvinyl alcohol (PVA). Hydrogels synthesized with desirable tensile strength and self-healing ability (67.2 % self-healing efficiency in fracture strength) assembled into strain sensors with a low detection limit of 0.5 % and a gauge factor (GF) of 4.75 (0-17 %). Additionally, as-prepared hydrogels exhibit high sensitivity to tiny pressure changes, allowing recognition of complex handwriting. Notably, resulting hydrogels possess self-powered property, generating up to 215 V and illuminating 100 commercial green LEDs. This work stems from the pressing need for multifunctional hydrogels with prospective applications in human motion sensing and energy harvesting.

Sea urchin-like microstructure pressure sensors with an ultra-broad range and high sensitivity.

Sensitivity and pressure range are two significant parameters of pressure sensors. Existing pressure sensors have difficulty achieving both high sensitivity and a wide pressure range. Therefore, we propose a new pressure sensor with a ternary nanocomposite Fe2O3/C@SnO2. The sea urchin-like Fe2O3 structure promotes signal transduction and protects Fe2O3 needles from mechanical breaking, while the acetylene carbon black improves the conductivity of Fe2O3. Moreover, one part of the SnO2 nanoparticles adheres to the surfaces of Fe2O3 needles and forms Fe2O3/SnO2 heterostructures, while its other part disperses into the carbon layer to form SnO2@C structure. Collectively, the synergistic effects of the three structures (Fe2O3/C, Fe2O3/SnO2 and SnO2@C) improves on the limited pressure response range of a single structure. The experimental results demonstrate that the Fe2O3/C@SnO2 pressure sensor exhibits high sensitivity (680 kPa-1), fast response (10 ms), broad range (up to 150 kPa), and good reproducibility (over 3500 cycles under a pressure of 110 kPa), implying that the new pressure sensor has wide application prospects especially in wearable electronic devices and health monitoring.

Integrated Sensing and Warning Multifunctional Devices Based on the Combined Mechanical and Thermal Effect of Porous Graphene.

Wearable devices with integrated alarm functions play a vital role in daily life and can help people prevent potential hazards. Although many wearable sensors have been extensively studied and proposed to monitor various physiological signals, most of them are needed to integrate with the external alarm elements to realize warning, such as light-emitting diodes and buzzers, resulting in the system complexity and poor flexibility. In this paper, an integrated sensing and warning multifunctional device based on the mechanical and thermal effect of porous graphene is proposed on a bilayer asymmetrical pattern of laser-induced graphene (LIG). Compared with the strain sensor with nonpatterned LIG, the mechanical performance is greatly improved with the highest gauge factor value of up to 950 for the strain sensor with mesh-patterned LIG. On the contrary, the heating performance of the heater with nonpatterned LIG is better than that with mesh-patterned LIG. Combining the performance differences of different LIG patterns, the integrated wearable device with a bilayer asymmetrical LIG pattern is demonstrated. It can generate enough heating energy to warn the person when the detected signal meets the threshold condition measured in real time by the ultrasensitive strain sensor. This work will provide a new way to construct an integrated wearable device for realizing multifunctional applications. This integrated multifunctional device shows great potential toward the applications in healthcare monitoring and timely warning.

Tellurene Nanoflake-Based NO2 Sensors with Superior Sensitivity and a Sub-Parts-per-Billion Detection Limit.

Industrial production, environmental monitoring, and clinical medicine put forward urgent demands for high-performance gas sensors. Two-dimensional (2D) materials are regarded as promising gas-sensing materials owing to their large surface-to-volume ratio, high surface activity, and abundant surface-active sites. However, it is still challenging to achieve facilely prepared materials with high sensitivity, fast response, full recovery, and robustness in harsh environments for gas sensing. Here, a combination of experiments and density functional theory (DFT) calculations is performed to explore the application of tellurene in gas sensors. The prepared tellurene nanoflakes via facile liquid-phase exfoliation show an excellent response to NO2 (25 ppb, 201.8% and 150 ppb, 264.3%) and an ultralow theory detection limit (DL) of 0.214 ppb at room temperature, which is excellent compared to that of most reported 2D materials. Furthermore, tellurene sensors present a fast response (25 ppb, 83 s and 100 ppb, 26 s) and recovery (25 ppb, 458 s and 100 ppb, 290 s). The DFT calculations further clarify the reasons for enhanced electrical conductivity after NO2 adsorption because of the interfacial electron transfer from tellurene to NO2, revealing an underlying explanation for tellurene-based gas sensors. These results indicate that tellurene is eminently promising for detecting NO2 with superior sensitivity, favorable selectivity, an ultralow DL, fast response-recovery, and high stability.

An ultrasensitive strain sensor with a wide strain range based on graphene armour scales.

An ultrasensitive strain sensor with a wide strain range based on graphene armour scales is demonstrated in this paper. The sensor shows an ultra-high gauge factor (GF, up to 1054) and a wide strain range (ε = 26%), both of which present an advantage compared to most other flexible sensors. Moreover, the sensor is developed by a simple fabrication process. Due to the excellent performance, this strain sensor can meet the demands of subtle, large and complex human motion monitoring, which indicates its tremendous application potential in health monitoring, mechanical control, real-time motion monitoring and so on.

Graphene-Paper Pressure Sensor for Detecting Human Motions.

Pressure sensors should have an excellent sensitivity in the range of 0-20 kPa when applied in wearable applications. Traditional pressure sensors cannot achieve both a high sensitivity and a large working range simultaneously, which results in their limited applications in wearable fields. There is an urgent need to develop a pressure sensor to make a breakthrough in both sensitivity and working range. In this paper, a graphene-paper pressure sensor that shows excellent performance in the range of 0-20 kPa is proposed. Compared to most reported graphene pressure sensors, this work realizes the optimization of sensitivity and working range, which is especially suitable for wearable applications. We also demonstrate that the pressure sensor can be applied in pulse detection, respiratory detection, voice recognition, as well as various intense motion detections. This graphene-paper pressure sensor will have great potentials for smart wearable devices to achieve health monitoring and motion detection.

Self-adapted and tunable graphene strain sensors for detecting both subtle and large human motions.

Conventional strain sensors rarely have both a high gauge factor and a large strain range simultaneously, so they can only be used in specific situations where only a high sensitivity or a large strain range is required. However, for detecting human motions that include both subtle and large motions, these strain sensors can't meet the diverse demands simultaneously. Here, we come up with laser patterned graphene strain sensors with self-adapted and tunable performance for the first time. A series of strain sensors with either an ultrahigh gauge factor or a preferable strain range can be fabricated simultaneously via one-step laser patterning, and are suitable for detecting all human motions. The strain sensors have a GF of up to 457 with a strain range of 35%, or have a strain range of up to 100% with a GF of 268. Most importantly, the performance of the strain sensors can be easily tuned by adjusting the patterns of the graphene, so that the sensors can meet diverse demands in both subtle and large motion situations. The graphene strain sensors show significant potential in applications such as wearable electronics, health monitoring and intelligent robots. Furthermore, the facile, fast and low-cost fabrication method will make them possible and practical to be used for commercial applications in the future.

An intelligent artificial throat with sound-sensing ability based on laser induced graphene.

Traditional sound sources and sound detectors are usually independent and discrete in the human hearing range. To minimize the device size and integrate it with wearable electronics, there is an urgent requirement of realizing the functional integration of generating and detecting sound in a single device. Here we show an intelligent laser-induced graphene artificial throat, which can not only generate sound but also detect sound in a single device. More importantly, the intelligent artificial throat will significantly assist for the disabled, because the simple throat vibrations such as hum, cough and scream with different intensity or frequency from a mute person can be detected and converted into controllable sounds. Furthermore, the laser-induced graphene artificial throat has the advantage of one-step fabrication, high efficiency, excellent flexibility and low cost, and it will open practical applications in voice control, wearable electronics and many other areas.

A Flexible 360-Degree Thermal Sound Source Based on Laser Induced Graphene.

A flexible sound source is essential in a whole flexible system. It's hard to integrate a conventional sound source based on a piezoelectric part into a whole flexible system. Moreover, the sound pressure from the back side of a sound source is usually weaker than that from the front side. With the help of direct laser writing (DLW) technology, the fabrication of a flexible 360-degree thermal sound source becomes possible. A 650-nm low-power laser was used to reduce the graphene oxide (GO). The stripped laser induced graphene thermal sound source was then attached to the surface of a cylindrical bottle so that it could emit sound in a 360-degree direction. The sound pressure level and directivity of the sound source were tested, and the results were in good agreement with the theoretical results. Because of its 360-degree sound field, high flexibility, high efficiency, low cost, and good reliability, the 360-degree thermal acoustic sound source will be widely applied in consumer electronics, multi-media systems, and ultrasonic detection and imaging.

High performance flexible strain sensor based on self-locked overlapping graphene sheets.

Strain sensors have been widely used in the fields of wearable devices, robot arms, medical sensing, bio-sensing, artificial skin and so on, but the existing strain sensors have some shortcomings such as a limited gauge factor (GF) or strain range. We fabricate a novel and flexible strain sensor with high performance based on self-locked overlapping graphene sheets (SOGS) which can be used for wearable devices. Polydimethylsiloxane (PDMS) is used to lock the overlapping graphene sheets, and then the graphene can be stretched and achieve an ultrahigh GF. In addition, a new theory is put forward to explain the GF changes with strain range for the SOGS strain sensor. In this work, graphene oxide (GO) film is reduced to reduced GO (rGO) by a laser. Then, the SOGS and electrodes are encapsulated by PDMS. The SOGS strain sensor has a high GF up to 400 and strain range over 7.5%, and this SOGS strain sensor achieves a balance between high sensitivity and large strain range compared with other existing strain sensors. Furthermore the theoretical equation based on the new theory agrees well with the experimental results. And this strain sensor can be used in many applications because of its high sensitivity. Some applications of the SOGS strain sensors are demonstrated for the detection of various human motions and human sounds. The SOGS strain sensor can exhibit great potential in wearable electronics because of its good balance between high sensitivity and large strain.