3D Hollow Quasi-Graphite Capsules/Polyaniline Hybrid with a High Performance for Room-Temperature Ammonia Gas Sensors

A Flexible Ammonia Gas Sensor Based on a Grafted Polyaniline Grown on a Polyethylene Terephthalate Film

A novel NH3 gas sensor is introduced, employing polyaniline (PANI) with a unique structure called a graft film. The preparation method was simple: polydopamine (PD) was coated on a flexible polyethylene terephthalate (PET) film and PANI graft chains were grown on its surface. This distinctive three-layer sensor showed a response value of 12 for 50 ppm NH3 in a dry atmosphere at 50 °C. This value surpasses those of previously reported sensors using structurally controlled PANI films. Additionally, it is on par with sensors that combine PANI with metal oxide semiconductors or carbon materials, the high sensitivity of which have been reported. To confirm our film's potential as a flexible sensor, the effect of bending on the its characteristics was investigated. This revealed that although bending decreased the response value, it had no effect on the response time or recovery. This indicated that the sensor film itself was not broken by bending and had sufficient mechanical strength.

Wireless Flexible System for Highly Sensitive Ammonia Detection Based on Polyaniline/Carbon Nanotubes

Ammonia (NH3) is a harmful atmospheric pollutant and an important indicator of environment, health, and food safety conditions. Wearable devices with flexible gas sensors offer convenient real-time NH3 monitoring capabilities. A flexible ammonia gas sensing system to support the internet of things (IoT) is proposed. The flexible gas sensor in this system utilizes polyaniline (PANI) with multiwall carbon nanotubes (MWCNTs) decoration as a sensitive material, coated on a silver interdigital electrode on a polyethylene terephthalate (PET) substrate. Gas sensors are combined with other electronic components to form a flexible electronic system. The IoT functionality of the system comes from a microcontroller with Wi-Fi capability. The flexible gas sensor demonstrates commendable sensitivity, selectivity, humidity resistance, and long lifespan. The experimental data procured from the sensor reveal a remarkably low detection threshold of 0.3 ppm, aligning well with the required specifications for monitoring ammonia concentrations in exhaled breath gas, which typically range from 0.425 to 1.8 ppm. Furthermore, the sensor demonstrates a negligible reaction to the presence of interfering gases, such as ethanol, acetone, and methanol, thereby ensuring high selectivity for ammonia detection. In addition to these attributes, the sensor maintains consistent stability across a range of environmental conditions, including varying humidity levels, repeated bending cycles, and diverse angles of orientation. A portable, stable, and effective flexible IoT system solution for real-time ammonia sensing is demonstrated by collecting data at the edge end, processing the data in the cloud, and displaying the data at the user end.

The influence of advanced materials on the analytical performance of semiconductor-based gas sensors

Chemiresistive gas sensors are metal oxide-based sensors that have received significant attention in different fields. Ambient gas sensors are especially important in the fabrication of wearable probes for the real-time detection of biomarkers in human body samples. Usually, room temperature sensors are affordable due to their low power consumption, resulting in simple instrumentation and maintenance. To fabricate versatile gas sensors, i.e. sensitive, selective, ambient temperature operating gas sensors, and improve the sensing performance of the traditionally used sensor, new materials play an important role. In other words, new advanced materials are essential for designing and fabricating new gas sensors. Hence, in this review, the application and impact of new advanced materials in the fabrication of reliable gas sensors are discussed in detail. Special emphasis is given to the effect of new materials in the fabrication of room-temperature operating systems. Finally, future research outlook and possible challenges that may be encountered by reliable gas sensors are also explained.

Highly sensitive Cu-ethylenediamine/PANI composite sensor for NH3 detection at room temperature

Ammonia detection is needed in several sectors including environmental monitoring, automobile industry, and in medical diagnosis. Conducting polymers, such as polyaniline (PANI), have been utilized to develop NH₃ sensors operating at room temperature. However, the performance of these sensors in terms of sensitivity and selectivity need improvement. Functionalization of conducting PANI with metal nanocomposites have shown improved sensor performance. In this work, we report a highly sensitive copper-based nanocomposite for NH₃ detection. The novelty lies in utilization of copper-ethylenediamine (Cu-en) nanocomposite functionalized over PANI for gas sensing. Resistance of the 20 wt% Cu-en with PANI increased 3.8 times upon exposure to 100 ppm of NH₃. The nanocomposite sensor detected NH₃ concentrations as low as 2 ppm. Further, the sensing mechanism was studied by in-situ IV characteristics and impedance spectroscopy during NH₃ exposure. NH₃ showed ionic interaction with PANI, and Cu²⁺. The strong affinity of Cu²⁺ for the lone pair of NH₃ enhanced the sensor response from 0.78 to 3.8 for 100 ppm of NH₃ at 20 °C. The sensor response was completely recovered after heating at 75 °C, which indicates reusability of the sensor. The sensor showed selectivity for NH₃ over ethanol and H₂S. The response was reasonably stable after bending the flexible sensor for 1000 times at a radius of 5 mm.

Gel Electrolytes for Electrochemical Actuators and Sensors Applications

Advanced functional materials, especially gel electrolytes, play a very important role in the preparation of electrochemical actuators and sensors, and have received extensive attention. In this review, a general classification of gel electrolytes is firstly introduced according to the type of medium. Then, the research progress of gel electrolytes with different types used to fabricate electrochemical actuators is summarized. Next, the current research progress of gel electrolytes used in different types of electrochemical sensors, including strain sensors, stress sensors, and gas sensors is introduced. Finally, the future challenges and development prospects of electrochemical actuators and sensors based on gel electrolytes are discussed. The huge application prospects of gel electrolyte are worthy of further focusing by researchers, which will have an indispensable impact on human life and development.

Real-Time Monitoring of Breath Biomarkers with A Magnetoelastic Contactless Gas Sensor: A Proof of Concept

In the quest for effective gas sensors for breath analysis, magnetoelastic resonance-based gas sensors (MEGSs) are remarkable candidates. Thanks to their intrinsic contactless operation, they can be used as non-invasive and portable devices. However, traditional monitoring techniques are bound to slow detection, which hinders their application to fast bio-related reactions. Here we present a method for real-time monitoring of the resonance frequency, with a proof of concept for real-time monitoring of gaseous biomarkers based on resonance frequency. This method was validated with a MEGS based on a Metglass 2826 MB microribbon with a polyvinylpyrrolidone (PVP) nanofiber electrospun functionalization. The device provided a low-noise (RMS = 1.7 Hz), fast (<2 min), and highly reproducible response to humidity (Δf = 46−182 Hz for 17−95% RH), ammonia (Δf = 112 Hz for 40 ppm), and acetone (Δf = 44 Hz for 40 ppm). These analytes are highly important in biomedical applications, particularly ammonia and acetone, which are biomarkers related to diseases such as diabetes. Furthermore, the capability of distinguishing between breath and regular air was demonstrated with real breath measurements. The sensor also exhibited strong resistance to benzene, a common gaseous interferent in breath analysis.

A Highly Sensitive Ammonia Gas Sensor Using Micrometer-Sized Core-Shell-Type Spherical Polyaniline Particles

A highly sensitive NH₃ gas sensor based on micrometer-sized polyaniline (PANI) spheres was successfully fabricated. The PANI microspheres were prepared via a facile in situ chemical oxidation polymerization in a polystyrene microsphere dispersion solution, resulting in a core–shell structure. The sensor response increased as the diameter of the microspheres increased. The PSt@PANI(4.5) sensor, which had microspheres with a 4.5 μm average diameter, showed the largest response value of 77 for 100 ppm dry NH₃ gas at 30 °C, which was 20 times that of the PANI-deposited film-based sensor. Even considering measurement error, the calculated detection limit was 46 ppb. A possible reason for why high sensitivity was achieved is simply the use of micrometer-sized PANI spherical particles. This research succeeded in providing a new and simple technology for developing a high-sensitivity NH₃ gas sensor that operates at room temperature.

Chromatographic Approach to Isolate Exfoliated Graphene

A chromatographic approach for separating exfoliated graphene from natural flake graphite is presented. Graphene is an extremely strong, electrically and thermally conductive two-dimensional hexagonal array of carbon atoms with the potential to transform applications such as supercapacitors, composites, biosensors, ultra-thin touchscreens, and solar cells. However, many of these applications require the use of exfoliated graphene, and the current cost of this material can be prohibitive. The most cost-effective source of graphene is exfoliated graphite, and numerous approaches have been proposed for exfoliating graphite to graphene. Solution approaches are the most common, with graphite often exfoliated by extended sonication treatment followed by separation of graphene from graphite using centrifugation. This time-consuming approach results in low concentrations of small lateral dimension graphene, often in high-boiling-point organic solvents or containing stabilizers. In this study, a chromatographic approach is used in combination with a solvent interface trapping method of graphite exfoliation to isolate graphene. The interface trapping exfoliation approach uses a hydrophobic/hydrophilic solvent interface to spontaneously exfoliate graphite and form a graphene-stabilized water-in-oil emulsion. This emulsion contains both graphene and graphite, and when added to water-wet glass beads, graphene adsorbs onto the glass surface, leaving graphite in the hydrophobic mobile phase, where it is removed by washing with an additional oil phase. The efficiency of this scalable approach to separation is demonstrated by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Tyndall effect scattering.

Superior Room-Temperature Ammonia Sensing Using a Hydrothermally Synthesized MoS2/SnO2 Composite

Layered two-dimensional transition metal dichalcogenides, due to their semiconducting nature and large surface-to-volume ratio, have created their own niche in the field of gas sensing. Their large recovery time and accompanied incomplete recovery result in inferior sensing properties. Here, we report a composite-based strategy to overcome these issues. In this study, we report a facile double-step synthesis of a MoS₂/SnO₂ composite and its successful use as a superior room-temperature ammonia sensor. Contrary to the pristine nanosheet-based sensors, the devices made using the composite display superior gas sensing characteristics with faster response. Specifically, at room temperature (30° C), the composite-based sensor exhibited excellent sensitivity (10%) at an ammonia concentration down to 0.4 ppm along with the response and recovery times of 2 and 10 s, respectively. Moreover, the device also exhibited long-term durability, reproducibility, and selectivity toward ammonia against hydrogen sulfide, methanol, ethanol, benzene, acetone, and formaldehyde. Sensor devices made on quartz and alumina substrates with different roughnesses have yielded almost an identical response, except for slight variations in response and recovery transients. Further, to shed light on the underlying adsorption energetics and selectivity, density functional theory simulations were employed. The improved response and enhanced selectivity of the composite were explicitly discussed in terms of adsorption energy. Lowdin charge analysis was performed to understand the charge transfer mechanism between NH₃, H₂S, CH₃OH, HCHO, and the underlying MoS₂/SnO₂ composite surface. The long-term durability of the sensor was evident from the stable response curves even after 2 months. These results indicate that hydrothermally synthesized MoS₂/SnO₂ composite-based gas sensors can be used as a promising sensing material for monitoring ammonia gas in real fields.

Graphene, an Interesting Nanocarbon Allotrope for Biosensing Applications: Advances, Insights, and Prospects

Graphene, a relatively new two-dimensional (2D) nanomaterial, possesses unique structure (e.g. lighter, harder, and more flexible than steel) and tunable physicochemical (e.g. electronical, optical) properties with potentially wide eco-friendly and cost-effective usage in biosensing. Furthermore, graphene-related nanomaterials (e.g. graphene oxide, doped graphene, carbon nanotubes) have inculcated tremendous interest among scientists and industrials for the development of innovative biosensing platforms, such as arrays, sequencers and other nanooptical/biophotonic sensing systems (e.g. FET, FRET, CRET, GERS). Indeed, combinatorial functionalization approaches are constantly improving the overall properties of graphene, such as its sensitivity, stability, specificity, selectivity, and response for potential bioanalytical applications. These include real-time multiplex detection, tracking, qualitative, and quantitative characterization of molecules (i.e. analytes [H₂O₂, urea, nitrite, ATP or NADH]; ions [Hg²⁺, Pb²⁺, or Cu²⁺]; biomolecules (DNA, iRNA, peptides, proteins, vitamins or glucose; disease biomarkers such as genetic alterations in BRCA1, p53) and cells (cancer cells, stem cells, bacteria, or viruses). However, there is still a paucity of comparative reports that critically evaluate the relative toxicity of carbon nanoallotropes in humans. This manuscript comprehensively reviews the biosensing applications of graphene and its derivatives (i.e. GO and rGO). Prospects and challenges are also introduced.

Hollow Au/Polypyrrole Capsules to Form Porous and Neural Network-Like Nanofibrous Film for Wearable, Super-Rapid, and Ultrasensitive NH3 Sensor at Room Temperature

Wearable conducting polymer-based NH₃ sensors are highly desirable in real-time environmental monitoring and human health protection but still a challenge for their relatively long response/recovery time and moderate sensitivity at room temperature. Herein, we present an effective route to fulfill this challenge by constructing porous and neural network-like Au/polypyrrole (Au/PPy) electrospun nanofibrous film with hollow capsular units for NH₃ sensor. Taking the unique architecture and synergistic effect between Au and PPy, our sensor exhibits not only super-rapid response/recovery time (both ∼7 s), faster than all reported sensors, but also stable and ultrahigh sensitivity (response reaches ∼2.3 for 1 ppm NH₃) at room temperature even during repeated deformation. Furthermore, good selectivity has been also achieved. These outstanding properties make our sensor hold great potential in real-time NH₃-related disease diagnosis and environmental monitoring at room temperature.

Graphene and Perovskite-Based Nanocomposite for Both Electrochemical and Gas Sensor Applications: An Overview

Perovskite and graphene-based nanocomposites have attracted much attention and been proven as promising candidates for both gas (H₂S and NH₃) and electrochemical (H₂O₂, CH₃OH and glucose) sensor applications. In this review, the development of portable sensor devices on the sensitivity, selectivity, cost effectiveness, and electrode stability of chemical and electrochemical applications is summarized. The authors are mainly focused on the common analytes in gas sensors such as hydrogen sulfide, ammonia, and electrochemical sensors including non-enzymatic glucose, hydrazine, dopamine, and hydrogen peroxide. Finally, the article also addressed the stability of composite performance and outlined recent strategies for future sensor perspectives.

Hydrogel-Based Gas Sensors for NO2 and NH3

In this study, an innovative gas sensing mechanism, self-responsive sensing mechanism, has been detected in the supramolecular hydrogel-based sensors. The self-responsive ability of as-fabricated hydrogel-based sensors to the target gas (e.g., NO₂, NH₃, etc.) is determined by three synergetic supramolecular interactions, namely, hydrogen bonding, molecule crystallization, and electrostatic interactions existing in hydroxyls, poly(vinyl alcohol) (PVA) crystallization, and poly(ionic liquids) of the intrinsic hydrogel networks, respectively. On account of unique synergetic supramolecular interactions, the sensors not only exhibit a rapid, reversible, and reproducible response but also show good tensile and compressive properties and excellent recovery property. The results demonstrate the potential of the self-responsive sensing mechanism as a pathway to realize a new generation of highly responsive hydrogel-based gas sensors.

Gas Sensors Based on Chemi-Resistive Hybrid Functional Nanomaterials

Chemi-resistive sensors based on hybrid functional materials are promising candidates for gas sensing with high responsivity, good selectivity, fast response/recovery, great stability/repeatability, room-working temperature, low cost, and easy-to-fabricate, for versatile applications. This progress report reviews the advantages and advances of these sensing structures compared with the single constituent, according to five main sensing forms: manipulating/constructing heterojunctions, catalytic reaction, charge transfer, charge carrier transport, molecular binding/sieving, and their combinations. Promises and challenges of the advances of each form are presented and discussed. Critical thinking and ideas regarding the orientation of the development of hybrid material-based gas sensor in the future are discussed.