Patching sulfur vacancies: A versatile approach for achieving ultrasensitive gas sensors based on transition metal dichalcogenides.

Transition metal dichalcogenides (TMDCs) garner significant attention for their potential to create high-performance gas sensors. Despite their favorable properties such as tunable bandgap, high carrier mobility, and large surface-to-volume ratio, the performance of TMDCs devices is compromised by sulfur vacancies, which reduce carrier mobility. To mitigate this issue, we propose a simple and universal approach for patching sulfur vacancies, wherein thiol groups are inserted to repair sulfur vacancies. The sulfur vacancy patching (SVP) approach is applied to fabricate a MoS2-based gas sensor using mechanical exfoliation and all-dry transfer methods, and the resulting 4-nitrothiophenol (4NTP) repaired molybdenum disulfide (4NTP-MoS2) is prepared via a sample solution process. Our results show that 4NTP-MoS2 exhibits higher response (increased by 200 %) to ppb-level NO2 with shorter response/recovery times (61/82 s) and better selectivity at 25 °C compared to pristine MoS2. Notably, the limit of detection (LOD) toward NO2 of 4NTP-MoS2 is 10 ppb. Kelvin probe force microscopy (KPFM) and density functional theory (DFT) reveal that the improved gas sensing performance is mainly attributed to the 4NTP-induced n-doping effect on MoS2 and the corresponding increment of surface absorption energy to NO2. Additionally, our 4NTP-induced SVP approach is universal for enhancing gas sensing properties of other TMDCs, such as MoSe2, WS2, and WSe2.

Large-Area and Visible-Light-Driven Heterojunctions of In2O3/Graphene Built for ppb-Level Formaldehyde Detection at Room Temperature.

Achieving convenient and accurate detection of indoor ppb-level formaldehyde is an urgent requirement to ensure a healthy working and living environment for people. Herein, ultrasmall In2O3 nanorods and supramolecularly functionalized reduced graphene oxide are selected as hybrid components of visible-light-driven (VLD) heterojunctions to fabricate ppb-level formaldehyde (HCHO) gas sensors (named InAG sensors). Under 405 nm visible light illumination, the sensor exhibits an outstanding response toward ppb-level HCHO at room temperature, including the ultralow practical limit of detection (pLOD) of 5 ppb, high response (Ra/Rg = 2.4, 500 ppb), relatively short response/recovery time (119 s/179 s, 500 ppb), high selectivity, and long-term stability. The ultrasensitive room temperature HCHO-sensing property is derived from visible-light-driven and large-area heterojunctions between ultrasmall In2O3 nanorods and supramolecularly functionalized graphene nanosheets. The performance of the actual detection toward HCHO is evaluated in a 3 m3 test chamber, confirming the practicability and reliability of the InAG sensor. This work provides an effective strategy for the development of low-power-consumption ppb-level gas sensors.

Assembly of Core/Shell Nanospheres of Amorphous Hemin/Acetone-Derived Carbonized Polymer with Graphene Nanosheets for Room-Temperature NO Sensing.

Implementing parts per billion-level nitric oxide (NO) sensing at room temperature (RT) is still in extreme demand for monitoring inflammatory respiratory diseases. Herein, we have prepared a kind of core-shell structural Hemin-based nanospheres (Abbr.: Hemin-nanospheres, defined as HNSs) with the core of amorphous Hemin and the shell of acetone-derived carbonized polymer, whose core-shell structure was verified by XPS with argon-ion etching. Then, the HNS-assembled reduced graphene oxide composite (defined as HNS-rGO) was prepared for RT NO sensing. The acetone-derived carbonized polymer shell not only assists the formation of amorphous Hemin core by disrupting their crystallization to release more Fe-N4 active sites, but provides protection to the core. Owing to the unique core-shell structure, the obtained HNS-rGO based sensor exhibited superior RT gas sensing properties toward NO, including a relatively higher response (Ra/Rg = 5.8, 20 ppm), a lower practical limit of detection (100 ppb), relatively reliable repeatability (over 6 cycles), excellent selectivity, and much higher long-term stability (less than a 5% decrease over 120 days). The sensing mechanism has also been proposed based on charge transfer theory. The superior gas sensing properties of HNS-rGO are ascribed to the more Fe-N4 active sites available under the amorphous state of the Hemin core and to the physical protection by the shell of acetone-derived carbonized polymer. This work presents a facile strategy of constructing a high-performance carbon-based core-shell nanostructure for gas sensing.

Low-Cost Wearable Fluidic Sweat Collection Patch for Continuous Analyte Monitoring and Offline Analysis.

Sweat sensors allow for new unobtrusive ways to continuously monitor an athlete's performance and health status. Significant advances have been made in the optimization of sensitivity, selectivity, and durability of electrochemical sweat sensors. However, comparing the in situ performance of these sensors in detail remains challenging because standardized sweat measurement methods to validate sweat sensors in a physiological setting do not yet exist. Current collection methods, such as the absorbent patch technique, are prone to contamination and are labor-intensive, which limits the number of samples that can be collected over time for offline reference measurements. We present an easy-to-fabricate sweat collection system that allows for continuous electrochemical monitoring, as well as chronological sampling of sweat for offline analysis. The patch consists of an analysis chamber hosting a conductivity sensor and a sequence of 5 to 10 reservoirs that contain level indicators that monitor the filling speed. After testing the performance of the patch in the laboratory, elaborate physiological validation experiments (3 patch locations, 6 participants) were executed. The continuous sweat conductivity measurements were compared with laboratory [Na+] and [Cl-] measurements of the samples, and a strong linear relationship (R2 = 0.97) was found. Furthermore, sweat rate derived from ventilated capsule measurement at the three locations was compared with patch filling speed and continuous conductivity readings. As expected from the literature, sweat conductivity was linearly related to sweat rate as well. In short, a successfully validated sweat collection patch is presented that enables sensor developers to systematically validate novel sweat sensors in a physiological setting.

CarbonIron Electron Transport Channels in Porphyrin-Graphene Complex for ppb-Level Room Temperature NO Gas Sensing.

It is a great challenge to develop efficient room-temperature sensing materials and sensors for nitric oxide (NO) gas, which is a biomarker molecule used in the monitoring of inflammatory respiratory diseases. Herein, Hemin (Fe (III)-protoporphyrin IX) is introduced into the nitrogen-doped reduced graphene oxide (N-rGO) to obtain a novel sensing material HNG-ethanol. Detailed XPS spectra and DFT calculations confirm the formation of carbon-iron bonds in HNG-ethanol during synthesis process, which act as electron transport channels from graphene to Hemin. Owing to this unique chemical structure, HNG-ethanol exhibits superior gas sensing properties toward NO gas (Ra /Rg  = 3.05, 20 ppm) with a practical limit of detection (LOD) of 500 ppb and reliable repeatability (over 5 cycles). The HNG-ethanol sensor also possesses high selectivity against other exhaled gases, high humidity resistance, and stability (less than 3% decrease over 30 days). In addition, a deep understanding of the gas sensing mechanisms is proposed for the first time in this work, which is instructive to the community for fabricating sensing materials based on graphene-iron derivatives in the future.

Carbon Dots Embedded in Cellulose Film: Programmable, Performance-Tunable, and Large-Scale Subtle Fluorescent Patterning by in Situ Laser Writing.

Fluorescent patterns with multiple functions enable high-security anti-counterfeiting labels. Complex material synthesis and patterning processes limit the application of multifunctional fluorescent patterns, so the technology of in situ fluorescent patterning with tunable multimodal capabilities is becoming more necessary. In this work, an in situ fluorescent patterning technology was developed using laser direct writing on solid cellulose film at ambient conditions without masks. The fluorescent intensity and surface microstructure of the patterns could be adjusted by programmable varying of the laser parameters simultaneously. During laser direct writing, carbon dots are generated in situ in a cellulose ester polymer matrix, which significantly simplifies the fluorescent patterning process and reduces the manufacturing cost. Interestingly, the tunable fluorescent intensity empowers the fabrication of visual stereoscopic fluorescent patterns with excitation dependence, further improving its anti-counterfeiting performance. The obtained fluorescent patterns still show ultrahigh optical properties after being immersed in an acid/base solution (pH 5-12) over one month. In addition, the anti-UV performance of the obtained laser-patterned film with transmittance around 90% is comparable to that of commercial UV-resistant films. This work provided an advanced and feasible approach to fabricating programmable, performance-tunable, subtle fluorescent patterns in large-scale for industrial application.

Heart Rate Extraction in a Headphone Using Infrared Thermometry.

The heart rate is a vital indicator of the health state of an individual. By continuously monitoring it, the fitness and health of the cardiovascular system of a user can be analyzed and impending problematic health episodes could be addressed better. Existing techniques to measure heart rate, such as electrocardiogram or photoplethysmography, are either uncomfortable for the user, or are not low-power or sensitive to motion artifacts. Infrared thermography is a non-contact technique with improved user comfort and low power consumption. In this paper, we have analyzed, built, and tested a novel system that uses infrared differential thermometry to detect the heart rate in the auricle. The sensor system was fitted into a commercial headphone since this paper is a first step into integration of the system in a Bluetooth headset. To the best of our knowledge, there has been no previous work on the detection of the heart rate signal in the ear using infrared thermometry. Positive results have been obtained after extraction of the frequency features of the bioheat transfer signal on test persons in rest.