A Handwriting Method for Low-Cost Gas Sensors

In vitro gastrointestinal gas monitoring with carbon nanotube sensors

In vitro simulators of the human gastrointestinal (GI) tract are remarkable technological platforms for studying the impact of food on the gut microbiota, enabling continuous and real-time monitoring of key biomarkers. However, comprehensive real-time monitoring of gaseous biomarkers in these systems is required with a cost-effective approach, which has been challenging to perform experimentally to date. In this work, we demonstrate the integration and in-line use of carbon nanotube (CNT)-based chemiresitive gas sensors coated with a thin polydimethylsiloxane (PDMS) membrane for the continuous monitoring of gases within the Simulator of the Human Microbial Ecosystem (SHIME). The findings demonstrate the ability of the gas sensor to continuously monitor the different phases of gas production in this harsh, anaerobic, highly humid, and acidic environment for a long exposure time (16 h) without saturation. This establishes our sensor platform as an effective tool for real-time monitoring of gaseous biomarkers in in vitro systems like SHIME.

Superhydrophobic, stretchable kirigami pencil-on-paper multifunctional device platform

Wearable electronics with applications in healthcare, human-machine interfaces, and robotics often explore complex manufacturing procedures and are not disposable. Although the use of conductive pencil patterns on cellulose paper provides inexpensive, disposable sensors, they have limited stretchability and are easily affected by variations in the ambient environment. This work presents the combination of pencil-on-paper with the hydrophobic fumed SiO2 (Hf-SiO2) coating and stretchable kirigami structures from laser cutting to prepare a superhydrophobic, stretchable pencil-on-paper multifunctional sensing platform. The resulting sensor exhibits a large response to NO2 gas at elevated temperature from self-heating, which is minimally affected by the variations in the ambient temperature and relative humidity, as well as mechanical deformations such as bending and stretching states. The integrated temperature sensor and electrodes with the sensing platform can accurately detect temperature and electrophysiological signals to alert for adverse thermal effects and cardiopulmonary diseases. The thermal therapy and electrical stimulation provided by the platform can also deliver effective means to battle against inflammation/infection and treat chronic wounds. The superhydrophobic pencil-onpaper multifunctional device platform provides a low-cost, disposable solution to disease diagnostic confirmation and early treatment for personal and population health.

Identification of Chemical Vapor Mixture Assisted by Artificially Extended Database for Environmental Monitoring

A fully integrated sensor array assisted by pattern recognition algorithm has been a primary candidate for the assessment of complex vapor mixtures based on their chemical fingerprints. Diverse prototypes of electronic nose systems consisting of a multisensory device and a post processing engine have been developed. However, their precision and validity in recognizing chemical vapors are often limited by the collected database and applied classifiers. Here, we present a novel way of preparing the database and distinguishing chemical vapor mixtures with small data acquisition for chemical vapors and their mixtures of interest. The database for individual vapor analytes is expanded and the one for their mixtures is prepared in the first-order approximation. Recognition of individual target vapors of NO₂, HCHO, and NH₃ and their mixtures was evaluated by applying the support vector machine (SVM) classifier in different conditions of temperature and humidity. The suggested method demonstrated the recognition accuracy of 95.24%. The suggested method can pave a way to analyze gas mixtures in a variety of industrial and safety applications.

Role of Defect Engineering and Surface Functionalization in the Design of Carbon Nanotube-Based Nitrogen Oxide Sensors

Nitrogen oxides (NOx) are among the main atmospheric pollutants; therefore, it is important to monitor and detect their presence in the atmosphere. To this end, low-dimensional carbon structures have been widely used as NOx sensors for their outstanding properties. In particular, carbon nanotubes (CNTs) have been widely used as toxic-gas sensors owing to their high specific surface area and excellent mechanical properties. Although pristine CNTs have shown promising performance for NOx detection, several strategies have been developed such as surface functionalization and defect engineering to improve the NOx sensing of pristine CNT-based sensors. Through these strategies, the sensing properties of modified CNTs toward NOx gases have been substantially improved. Therefore, in this review, we have analyzed the defect engineering and surface functionalization strategies used in the last decade to modify the sensitivity and the selectivity of CNTs to NOx. First, the different types of surface functionalization and defect engineering were reviewed. Thereafter, we analyzed experimental, theoretical, and coupled experimental-theoretical studies on CNTs modified through surface functionalization and defect engineering to improve the sensitivity and selectivity to NOx. Finally, we presented the conclusions and the future directions of modified CNTs as NOx sensors.

Convenient CNT-Paper Gas Sensors Prepared by a Household Inkjet Printer

A hydrosoluble light-sensitive polymer named PSAG (poly-styrenesulfonate acrylic acid glycidyl methacrylate) was synthesized by acrylic acid (AA), sodium 4-styrenesulfonate (SS), and glycidyl methacrylate (GMA). PSAG is used to modify multiwall carbon nanotubes (MWCNTs) with a length diameter between 0.004 and 0.016. An inkjet conductive ink was formed by well-dispersed MWCNTs in aqueous and organic solvents, which could adjust the surface tension and viscosity of the ink. Gas sensors were then fabricated using this conductive ink on a household inkjet printer. The sensors demonstrated good reproducibility and acceptable recovery time (<200 s) to ammonia, methanol, and acetone. The resistance of the inkjet-printed sensor electrodes remained stable in the process of bending the sensors to different angles because of ultraviolet curing.

Paper-Based Sensors for Gas, Humidity, and Strain Detections: A Review

Paper, as a flexible, low-cost, lightweight, tailorable, environmental-friendly, degradable, and renewable material, is emerging in electronic devices. Especially, many kinds of paper-based (PB) sensors have been reported for wearable applications in recent years. Among them, the PB gas, humidity, and strain sensors are widely studied for monitoring gas, humidity, and strain from the human body and the environment. However, gas, humidity, and strain often coexist and interact, and the paper itself is hydrophilic and flexible, resulting in that it is still challenging to develop high-performance PB sensors specialized for gas, humidity, and strain detections. Therefore, it is necessary to summarize and discuss them systematically. In this review, we focus on summarizing the state-of-art studies of the PB gas, humidity, and strain sensors. Specifically, the fabrications (electrodes and sensing materials) and applications of PB gas, humidity, and strain sensors are summarized and discussed. The current challenges and the potential trends of PB sensors for gas, humidity, and strain detections are also outlined. This review not only can help readers to understand the development status of the PB gas, humidity, and strain sensors but also is helpful for readers to find out and solve the problems in this field through comparative reading.

Fully Integrated Indium Gallium Zinc Oxide NO2 Gas Detector

We report an amorphous indium gallium zinc oxide (IGZO)-based toxic gas detection system. The microsystem contains an IGZO thin-film transistor (TFT) as a sensing element and exhibits remarkable selectivity and sensitivity to low concentrations of nitrogen dioxide (NO₂). In contrast to existing metal oxide-based gas sensors, which are active either at high temperature or with light activation, the developed IGZO TFT sensor is operable at room temperature and requires only visible light activation to revive the sensor after exposure to NO₂. Furthermore, we demonstrate air-stable sensors with an experimental limit of detection of 100 ppb. This is the first report on metal oxide TFT gas sensors without heating or continuous light activation. Unlike most existing gas sensing systems that take care of identifying the analytes alone, the developed IGZO microsystem not only quantifies NO₂ gas concentration but also yields a 5-bit digital output. The compact microsystem, incorporating readout and analog-to-digital conversion modules developed using only two TFTs, paves the way for inexpensive toxic gas monitoring systems.