Highly Sensitive Ammonia Sensor with Organic Vertical Nanojunctions for Noninvasive Detection of Hepatic Injury

Design and Fabrication of Ultrathin Nanoporous Donor-Acceptor Copolymer-Based Organic Field-Effect Transistors for Enhanced VOC Sensing Performance

The development of organic field-effect transistor (OFET) chemical sensors with high sensing performance and good air stability has remained a persistent challenge, thereby hindering their practical application. Herein, an OFET sensor based on a donor-acceptor copolymer is shown to provide high responsivity, sensitivity, and selectivity toward polar volatile organic compounds, as well as good air stability. In detail, a polymer blend of N-alkyl-diketopyrrolo-pyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT) and polystyrene is coated onto an FET substrate via shearing-assisted phase separation (SAPS) combined with selective solvent etching to fabricate the DPP-DTT-based OFET device having an ultrathin nanoporous structure suitable for gas sensing applications. This is achieved via optimization of the film morphology by varying the shear rate to adjust the dynamic balance between the shear and capillary forces to obtain an ultrathin thickness (∼8 nm) and nanopore size (80 nm) that are favorable for the efficient diffusion and interaction of analytes with the active layer. In particular, the sensor presents high responsivities toward methanol (∼70%), acetone (∼51.3%), ethanol (∼39%), and isopropyl alcohol (IPA) (∼29.8%), along with fast response and recovery times of ∼80 and 234 s, respectively. Moreover, the average sensitivity was determined to be 5.75%/ppm from the linear plot of the responsivity against the methanol concentration in the range of 1-100 ppm. Importantly, the device also exhibits excellent long-term (30-day) air and thermal storage stability, thereby demonstrating its high potential for practical applications.

Charge Transport Manipulation via Interface Doping: Achieving Ultrasensitive Organic Semiconductor Gas Sensors

Organic semiconductor (OSC) gas sensors are receiving tremendous attention with the rise of wearable devices. Due to the complicated charge transport characteristics of OSCs, it is usually difficult to optimize their gas sensitivity by directly tailoring the original signals, as in many other kinds of sensors. Instead, device engineering strategies are frequently centered on enhancing the gas-film interaction. Herein, by introducing interface doping between self-assembled monolayers and triisopropylsilylethynyl-substituted pentacene films, we report a wide tuning of OSC gas sensitivity via charge transport manipulation and achieve an ultrahigh sensitivity of nearly 2000%/ppm to NO₂, simultaneously resulting in a fast square-wave-like response feature. In addition, this sensor demonstrates good humidity stability and operates well in flexible devices. More importantly, we identify that charge transport manipulation tailors the gas sensibility of OSCs by means of electronic structure instead of original signal values: compared to shallow traps, the presence of proper deep traps is conducive to gaining high sensitivity and ultrafast response/recovery speeds. This approach is also effective for tuning the sensitivity to reductive gases, verifying its generality for promoting the performance of OSC gas sensors, as well as a promising strategy for other types of sensors or detectors.

MXene/SnS2 Heterojunction for Detecting Sub-ppm NH3 at Room Temperature

Detection of ultralow concentrations of ammonia is very important in many applications such as fishing, poultry, agriculture, industry, biomedicine, and clinical diagnosis. However, detecting sub-ppm NH₃ remains a challenge for chemiresistive-type gas sensors. Two-dimensional (2D) materials display tremendous potential for effective gas detectors that can be used in these applications. The as-developed MXene/SnS₂ heterojunction-based chemiresistive-type sensor presents superior gas-sensing performance toward sub-ppm ammonia at room temperature. The sensor can detect NH₃ concentrations down to 10 ppb at room temperature. It also displays excellent long-term stability, with a decline in the response at ∼3.4% for 20 days. The developed sensor also displays good selectivity toward NH₃ relative to some potential interferents, such as HCHO, C₂H₅OH, CH₃OH, C₃H₆O, benzene, and NO₂. The measured in situ diffuse-reflectance infrared Fourier transform (DRIFT) spectra confirm that the products of nitric oxides during the chemical reactions occurred at the surface of MXene/SnS₂. Density functional theory (DFT) based on the first principles was implemented to compute the adsorption ability of NH₃ at the surface of the MXene/SnS₂ heterostructure. This indicates that the enhancement in the sensing properties of the MXene/SnS₂ heterostructure-based chemosensor could be ascribed to the stronger NH₃ adsorption, better catalytical activity, and more effective charge transfer bestowed by the formed heterostructure and the electron-redistribution-assisted stronger extraction of electrons from the sensing material.

Biomass-Derived Carbon Dots and Their Sensing Applications

Carbon dots (CDs) can be widely used in the field of sensing because of its good water solubility, low toxicity, high fluorescence stability and excellent biocompatibility. It has become a popular trend to prepare high-value, inexpensive, renewable and environmentally friendly CDs sensors from biomass resources. This article reviewed the research progress of biomass-derived CDs as chemical, physical and biological sensors in recent years and studied their preparation processes and sensing abilities. Furthermore, the prospects and challenges of biomass-CDs sensors were discussed. This article is expected to provide inspirations for the design, preparation and application of biomass-CDs sensors in the future.

A resorufin-based fluorescence probe for visualizing biogenic amines in cells and zebrafish

Biogenic amines (BAs) are a family of nitrogen-bearing natural organic molecules with at least one primary amine, which play an important role in living organisms. Elevated concentration of BAs may cause neuron disorder, Parkinson's disease and many other diseases. Therefore, it is essential to monitor BAs in living organisms. Herein, we reported a resorufin-based fluorescence probe for sensing of various BAs. Upon nucleophilic substitution reaction with BAs, the probe released resorufin, affording to strong fluorescence emission at 592 nm with rapid response (<8 min), good selectivity and a low detection limit (LOD = 0.47 μM). The probe has low cytotoxicity and good membrane permeability, and has been successfully used to visualize BAs in living cells and zebrafish with good performance.

Utilising problematic waste to detect toxic gas release in the environment: fabricating a NiO doped CuO nanoflake based ammonia sensor from e-waste

Using problematic electronic waste to synthesise high-purity nanomaterials can enable sustainable production and create opportunities to divert waste from landfills. Reported here is a simple strategy for the controllable synthesis of in situ NiO doped CuO nanoflakes from waste flexible printed circuit boards (FPCBs) using a chemothermal microrecycling process, and the nanomaterial is then utilised for an ammonia (NH₃) sensor at room temperature. Characterisation of the nanoflakes confirmed the purity of the CuO phase with a monoclinic structure without the formation of the Cu₂O phase. The NiO doped CuO 2D nanoflakes made of an assembly of 1D nanorods with a high surface area of 115.703 m² g^-1 are selectively synthesised from the waste FPCBs and have outstanding gas sensing characteristics such as a high response, a fast response (11.7 s) and a recovery time of (21.5 s), good stability, and superior selectivity towards 200 ppm of NH₃ gas at room temperature (RT, 20 °C). From a broader perspective, the process opens up exciting new avenues explore the production of toxic gas sensing functional materials from toxic and problematic waste.

MoSe2/multiwalled carbon nanotube composite for ammonia sensing in natural humid environment

Herein, we report ammonia sensing in a natural highly humid environment using MoSe₂/multi-walled carbon nanotube (MWCNT) composite as sensing platform. The composite synthesis involved two steps, in the first step, MWCNTs were treated in an acidic medium to obtain -COOH group functionalized MWCNTs. In the second step, functionalized MWCNTs were probe sonicated with MoSe₂ to obtain MoSe₂/MWCNT composite. Proposed device exhibited superior sensing properties at a temperature down to 16^∘ C and relative humidity of 80%. Under these extreme natural environmental conditions, the device exhibited a relative response of 21% for 0.5 ppm of ammonia and superior noise free signal further suggests their use even below this concentration. Composite based device has also displayed better adsorption selectivity towards NH₃ as compared with other reducing and oxidizing gas molecules. Density functional theory simulations were further employed to understand the underlying adsorption process and selectivity behavior of the composite. Simulations predicted lowest negative adsorption energy for ammonia, implying physisorption (-0.387 eV) type exothermic adsorption process. Present results indicate that a composite with the rightly engineered MoSe₂ and MWCNTs weight ratio may serve as a potential candidate for ammonia sensing in a highly humid environment.

A smartphone-adaptable fluorescent sensing tag for non-contact and visual monitoring of the freshness of fish

Fish-based food products play important roles in our daily diet. The related food safety is vitally essential for human health, thus it is very necessary to screen the freshness of fish-based foods. In this work, we presented a ratiometric fluorescent probe PTCN for the determination of cadaverine, a metabolic biomarker of the spoilage of fish. PTCN displayed a ratiometric fluorescence response towards cadaverine with good specificity, high sensitivity (LOD = 46 nM) and ultra-fast response (<15 s), and thus has been successfully utilized to determine cadaverine from the spoilage of fish. PTCN was fabricated into cheap and portable sensing tags, which can visually detect gaseous cadaverine with obvious fluorescence color transformation from red to green and a low detection limit (8.65 ppm). Moreover, the PTCN tags were used as smart fluorescent tags for non-contact and visual monitoring of cadaverine in fish. Furthermore, the ratiometric fluorescence signals were utilized to create a smartphone-adaptable digital sensing profile for indicating cadaverine in fish products.

Facile Strategy for Modulating the Nanoporous Structure of Ultrathin π-Conjugated Polymer Films for High-Performance Gas Sensors

Conventional conjugated polymer (CP) films based on organic field-effect transistors (OFETs) tend to limit the performance of gas sensors owing to restricted analyte diffusion and limited interactions with the charge carriers that accumulate in the first few monolayers of the CP film in contact with the dielectric layer. Herein, a facile strategy is presented for modulating the morphology and charge-transport properties of nanoporous CP films using shearing-assisted phase separation of polymer blends for fabricating OFET-based chemical sensors. This approach enables the formation of nanoporous films with pore size and thickness in the ranges of 90-550 and 7-27 nm, respectively, which can be controlled simply by varying the shear rate. The resulting OFET sensors exhibit excellent sensing performance when exposed to NH₃ gas, demonstrating a high responsivity (≈70.7%) at 10 ppm and good selectivity toward NH₃ over various organic solvent vapors. After a comprehensive analysis of the morphology and electrical properties of the CP films, it is concluded that morphological features, such as film thickness and surface area, affect the sensing performance of nanoporous-film-based OFET sensors more significantly compared to the charge-transport characteristics of the films.

A smartphone-adaptable fluorescent sensing tag for non-contact and visual monitoring of the freshness of fish

Fish-based food products play important roles in our daily diet. The related food safety is vitally essential for human health, thus it is very necessary to screen the freshness of fish-based foods.

A smartphone-adaptable fluorescent sensing tag for non-contact and visual monitoring of the freshness of fish

Fish-based food products play important roles in our daily diet. The related food safety is vitally essential for human health, thus it is very necessary to screen the freshness of fish-based foods.

MXene Ti3C2Tx-Derived Nitrogen-Functionalized Heterophase TiO2 Homojunctions for Room-Temperature Trace Ammonia Gas Sensing

In this work, MXene Ti₃C₂T_x-derived nitrogen-functionalized heterophase TiO₂ homojunctions (N-MXene) were prepared via the urea-involved solvothermal treatment with varying reaction time as the sensing layer to detect trace NH₃ gas at room temperature (20 °C). Compared with no signal for the pristine MXene counterpart, the 18 h-treated sensors (N-MXene-18) achieved a detection limit of 200 ppb with an inspiring response that was 7.3% better than the existing MXene-involved reports thus far. Also, decent repeatability, stability, and selectivity were demonstrated. It is noteworthy that the N-MXene-18 sensors delivered a stronger response, more sufficient recovery, and quicker response/recovery speeds under a humid environment than those under dry conditions, proving the significance of humidity. Furthermore, to suppress the effect of the fluctuation of humidity on NH₃ sensing during the tests, a commercial waterproof polytetrafluoroethylene (PTFE) membrane was anchored onto the sensing layer, eventually bringing about humidity-independent features. Both nitrogen doping and TiO₂ homojunctions constituted by mixed anatase and rutile phases were primarily responsible for the performance improvement with respect to pristine MXene. This work showcases the enormous potential of N-MXene materials in trace NH₃ detection and offers an alternative strategy to realize both heteroatom doping and partial oxidation of MXene that is applicable in future optoelectronic devices.

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

Recent Advances in Perylene Diimide-Based Active Materials in Electrical Mode Gas Sensing

This review provides an update on advances in the area of electrical mode sensors using organic small molecule n-type semiconductors based on perylene. Among small organic molecules, perylene diimides (PDIs) are an important class of materials due to their outstanding thermal, chemical, electronic, and optical properties, all of which make them promising candidates for a wide range of organic electronic devices including sensors, organic solar cells, organic field-effect transistors, and organic light-emitting diodes. This is mainly due to their electron-withdrawing nature and significant charge transfer properties. Perylene-based sensors of this type show high sensing performance towards various analytes, particularly reducing gases like ammonia and hydrazine, but there are several issues that need to be addressed including the selectivity towards a specific gas, the effect of relative humidity, and operating temperature. In this review, we focus on the strategies and design principles applied to the gas-sensing performance of PDI-based devices, including resistive sensors, amperometric sensors, and operating at room temperature. The device properties and sensing mechanisms for different analytes, focusing on hydrazine and ammonia, are studied in detail, and some future research perspectives are discussed for this promising field. We hope the discussed results and examples inspire new forms of molecular engineering and begin to open opportunities for other rylene diimide classes to be applied as active materials.

A new ESIPT-based fluorescent probe for the highly sensitive detection of amine vapors

A new ESIPT-based fluorescent probe has been developed as a rapid, highly sensitive, and selective sensor for amine vapors.

Ultrafast In Situ Synthesis of Large-Area Conductive Metal-Organic Frameworks on Substrates for Flexible Chemiresistive Sensing

The widespread use of electrically conductive metal-organic frameworks (EC-MOFs) in high-performance devices is limited by the lack of facile methods for synthesizing large-area thin films on the desired substrates. Herein, we propose a spin-coating interfacial self-assembly approach to in situ synthesize high-quality centimeter-sized copper benzenehexathiol (Cu-BHT) MOFs on diverse substrates in only 5 s. The film thickness (ranging from 5 to 35 nm) and surface morphology can be precisely tuned by controlling the reaction time. The gas sensor based on the 10 nm thick Cu-BHT film exhibits a low limit of detection (0.23 ppm) and high selectivity value (>30) in sensing NH₃ at ultralow driving voltages (0.01 V). Moreover, the Cu-BHT films retain their initial sensor performance after 1000 repetitive bending cycles at a bending radius of 3 mm. Density functional theory calculations suggest that Cu_2c sites induced by crystal particles on the film surface can improve the sensing performance. This facile and ultrafast approach for in situ synthesis of large-area EC-MOF films on diverse substrates with tunable thickness on a nanometer scale should facilitate application of EC-MOFs in flexible electronic device arrays.

Ammonia Sensor Based on Vapor Phase Polymerized Polypyrrole

The detection of ammonia is an important issue for a lot of applications: leak detection in industry, agriculture, cooling systems, and medical diagnosis (breath biomarker for non-invasive diagnostic of renal disease). Among the possible sensing technologies, chemosensors based on conducting polymers show interesting characteristics. Polypyrrole (PPy) is well known for its sensitivity to ammonia. In the present work, PPy was synthesized by vapor phase polymerization (VPP) and treated with three different reductants. The ammonia sensing performance was investigated. The response of sodium sulfite Na2SO3 treated PPy was found to be much more pronounced when exposed to ammonia, it was twice as high as the grown PPy. A response of 15% at 500 ppb was obtained with an excellent selectivity towards ammonia compared to ethanol, acetone, and isopropanol. The role of chemical reduction of PPy in ammonia gas sensing was studied using different methods such as Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and ultra-violet, visible, near-infrared (UV–Vis–NIR) spectroscopy.

Rapid and Selective NH3 Sensing by Porous CuBr

Fast and selective detection of NH3 at parts-per-billion (ppb) concentrations with inexpensive and low-power sensors represents a long-standing challenge. Here, a room temperature, solid-state sensor is presented consisting of nanostructured porous (78%) CuBr films. These are prepared by flame-aerosol deposition of CuO onto sensor substrates followed by dry reduction and bromination. Each step is monitored in situ through the film resistance affording excellent process control. Such porous CuBr films feature an order of magnitude higher NH3 sensitivity and five times faster response times than conventional denser CuBr films. That way, rapid (within 2.2 min) sensing of even the lowest (e.g., 5 ppb) NH3 concentrations at 90% relative humidity is attained with outstanding selectivity (30-260) over typical confounders including ethanol, acetone, H2, CH4, isoprene, acetic acid, formaldehyde, methanol, and CO, superior to state-of-the-art sensors. This sensor is ideal for hand-held and battery-driven devices or integration into wearable electronics as it does not require heating. From a broader perspective, the process opens exciting new avenues to also explore other bromides and classes of semiconductors (e.g., sulfides, nitrides, carbides) currently not accessible by flame-aerosol technology.

A Reusable Polymeric Film for the Alternating Colorimetric Detection of a Nerve Agent Mimic and Ammonia Vapor with Sub-Parts-per-Million Sensitivity

Thin polymeric films were developed for the vapor-phase sequential colorimetric detection of a nerve agent mimic and ammonia with high sensitivity. N-(4-Benzoylphenyl)acrylamide (BPAm), N,N-dimethylacrylamide (DMA), and (E)-2-(methyl(4-(pyridine-4yldiazenyl)phenyl)amino)ethyl acrylate (MPDEA, M1) were copolymerized via free radical polymerization (FRP) to yield p(BPAm-co-DMA-co-MPDEA), hereafter referred to as P1. P1 exhibits selective sensing properties toward diethyl chlorophosphate (DCP), a nerve agent mimic, in pure aqueous media. Upon the addition of DCP, the pyridine groups of P1 were quaternized with DCP, accompanied by a color change from yellow to pink due to the enhancement of the intramolecular charge transfer (ICT) effect. In situ generated quaternized P1, hereafter referred to as P2, after DCP sensing was used to selectively detect ammonia via dequaternization in an aqueous medium. Ammonia detection was indicated by a color change in the solution from pink back to yellow. A surface-immobilized P1 film was prepared and employed for the vapor-phase detection of DCP, demonstrating that an amount of as low as 2 ppm was detectable. Ammonia vapor was also successfully detected by the P2 film via the ammonia-triggered removal of the quaternized phosphates. Alternating exposure of the film to DCP and ammonia resulted in the corresponding color changes, thereby demonstrating the reversibility of the system. The reusability of the polymeric film for detecting DCP and ammonia in the vapor phase was confirmed by performing four sequential colorimetric detection cycles.

A smartphone-coalesced nanoprobe for high selective ammonia sensing based on the pH-responsive biomass carbon nanodots and headspace single drop microextraction

Ammonia concentration together with pH values are important and closely linked indexes for aqueous systems. Rapid on-site determination of ammonia or pH is of great significance to environmental monitoring. In this work, a pH-switchable nanoprobe based on biomass carbon dots (CDs) is developed using a smartphone as a simple and handy instrument. The CDs demonstrate sensitive pH response in wide linear ranges of 6.1-13.6, and 2.0-13.6 with colorimetric and fluorescent channels, respectively. It is the pH-induced aggregation that governs the color and fluorescence switch. With the pH evolution caused by the dissolution of ammonia, the smartphone-integrated nanoprobe is applied to ammonia detection with a broad range of 0.5-300 mM. Moreover, the headspace single drop microextraction strategy can concentrate ammonia from matrix, offering a remarkably high selectivity for ammonia determination. Finally, the practical applications of this method for ammonia analysis obtained satisfactory results.

A fluorescent sensor based on aggregation-induced emission: highly sensitive detection of hydrazine and its application in living cell imaging

Aggregation-induced emission (AIE) molecules eliminate the aggregation-caused quenching (ACQ) phenomenon effectively and exhibit excellent properties of a fluorescent sensor in the aggregated state. In this paper, an allochroic fluorescent sensor based on AIE molecules with a diphenylacrylonitrile structure was prepared in high yield by a simple procedure. This molecule possessed good AIE properties and exhibited a sensitive sensor ability for aliphatic amines with an obvious color change from orange to blue-green. The detailed investigation on the detection of hydrazine suggested that the detection limit for hydrazine was 3.67 × 10-6 M, and the highly sensitive sensor for hydrazine was not influenced by other species. The sensor mechanism was confirmed by using 1H NMR and MS spectra. The sensor for hydrazine was successfully applied in a test paper, exhibiting good practical application potential for detecting hydrazine. The experiment of living cell imaging suggested that this sensor showed superior bioimaging performance and presented sensitive detection for hydrazine with an obvious color change from orange to blue-green.

Breath Sensors for Health Monitoring

Breath sensors can revolutionize medical diagnostics by on-demand detection and monitoring of health parameters in a noninvasive and personalized fashion. Despite extensive research for more than two decades, however, only a few breath sensors have been translated into clinical practice. Actually, most never even left the scientific laboratories. Here, we describe key challenges that currently impede realization of breath sensors and highlight strategies to overcome them. Specifically, we start with breath marker selection (with emphasis on metabolic and inflammatory markers) and breath sampling. Next, the sensitivity, stability, and selectivity requirements for breath sensors are described. Concepts are elaborated to systematically address these requirements by material design (focusing on chemoresistive metal oxides), orthogonal arrays, and filters. Finally, aspects of portable device integration, user communication, and clinical applicability are discussed.

Layer-by-Layer Nano-assembly: A Powerful Tool for Optical Fiber Sensing Applications

The ability to tune the composition of nanostructured thin films is a hot topic for the design of functional coatings with advanced properties for sensing applications. The control of the structure at the nanoscale level enables an improvement of intrinsic properties (optical, chemical or physical) in comparison with the traditional bulk materials. In this sense, among all the known nanofabrication techniques, the layer-by-layer (LbL) nano-assembly method is a flexible, easily-scalable and versatile approach which makes possible precise control of the coating thickness, composition and structure. The development of sensitive nanocoatings has shown an exceptional growth in optical fiber sensing applications due to their self-assembling ability with oppositely charged components in order to obtain a multilayer structure. This nanoassembly technique is a powerful tool for the incorporation of a wide variety of species (polyelectrolytes, metal/metal oxide nanoparticles, hybrid particles, luminescent materials, dyes or biomolecules) in the resultant multilayer structure for the design of high-performance optical fiber sensors. In this work we present a review of applications related to optical fiber sensors based on advanced LbL coatings in two related research areas of great interest for the scientific community, namely chemical sensing (pH, gases and volatile organic compounds detection) as well as biological/biochemical sensing (proteins, immunoglobulins, antibodies or DNA detection).

A Highly Proton-Conductive 3D Ionic Cadmium-Organic Framework for Ammonia and Amines Impedance Sensing

Lately, the progressive study of metal-organic frameworks (MOFs) for the detection of ammonia and amines has made infusive achievements. Nevertheless, the investigation of proton-conductive MOFs used to detect the low concentrations of ammonia and amine gases at different relative humidities (RHs) at room temperature is relatively restricted. Herein, by solvothermal reaction of Cd(NO₃)₂ with 2-methyl-1 H-imidazole-4,5-dicarboxylic acid (H₃MIDC), a three-dimensional ionic MOF {Na[Cd(MIDC)]} _n (1) bearing ordered one-dimensional channels was successfully synthesized. Our research indicates that the uncoordination carboxylate sites are beneficial to proton transfer and the recognition of ammonia and amine compounds. The optimized proton conductivity of 1 reaches a high value of 1.04 × 10^-3 S·cm^-1 (100 °C, 98% RH). The room temperature sensing properties of ammonia and amine gases were explored under 68, 85, and 98% RHs, respectively. Satisfactorily, the detection limits of MOF 1 toward ammonia, methylamine, dimethylamine, trimethylamine, and ethylamine are 0.05, 0.1, 0.5, 1, and 4 ppm, respectively, which is one of the best room-temperature sensors for ammonia among previous sensors based on proton-conductive MOFs. The proton conducting and sensing mechanisms were highlighted as well.

Indium Nitrite (InN)-Based Ultrasensitive and Selective Ammonia Sensor Using an External Silicone Oil Filter for Medical Application

Ammonia is an essential biomarker for noninvasive diagnosis of liver malfunction. Therefore, selective detection of ammonia is essential for medical application. Here, we demonstrate a portable device to selectively detect sub-ppm ammonia gas. The presented gas sensor is composed of a Pt coating on top of an ultrathin Indium nitrite (InN) epilayer with a lower detection limit of 0.2 ppm, at operating temperature of 200 °C, and detection time of 1 min. The sensor connected with the external filter of nonpolar 500 CS silicone oil to diagnose liver malfunction. The absorption of 0.7 ppm acetone and 0.4 ppm ammonia gas in 10 cc silicone oil is 80% (0.56 ppm) and 21.11% (0.084 ppm), respectively, with a flow rate of 10 cc/min at 25 °C. The absorption of acetone gas is 6.66-fold higher as compared to ammonia gas. The percentage variation in response for 0.7 ppm ammonia and 0.7 ppm acetone with and without silicone oil on InN sensor is 17.5% and 4%, and 22.5%, and 14% respectively. Furthermore, the percentage variation in response for 0.7 ppm ammonia gas with silicone oil on InN sensor is 4.3-fold higher than that of 0.7 ppm acetone. The results show that the InN sensor is suitable for diagnosis of liver malfunction.

A Reduced GO-Graphene Hybrid Gas Sensor for Ultra-Low Concentration Ammonia Detection

A hybrid structure gas sensor of reduced graphene oxide (RGO) decorated graphene (RGO-Gr) is designed for ultra-low concentration ammonia detection. The resistance value of the RGO-Gr hybrid is the indicator of the ammonia concentration and controlled by effective charge transport from RGO to graphene after ammonia molecule adsorption. In this hybrid material, RGO is the adsorbing layer to catch ammonia molecules and graphene is the conductive layer to effectively enhance charge/electron transport. Compared to a RGO gas sensor, the signal-to-noise ratio (SNR) of the RGO-Gr is increased from 22 to 1008. Meanwhile, the response of the RGO-Gr gas sensor is better than that of either a pristine graphene or RGO gas sensor. It is found that the RGO reduction time is related to the content of functional groups that directly reflect on the gas sensing properties of the sensor. The RGO-Gr gas sensor with 10 min reduction time has the best gas sensing properties in this type of sensor. The highest sensitivity is 2.88% towards 0.5 ppm, and the ammonia gas detection limit is calculated to be 36 ppb.

An ammonia detecting mechanism for organic transistors as revealed by their recovery processes

Organic thin film transistor (OTFT) based gas sensors have demonstrated promising applications, owing to their advantages of high selectivity and room temperature operation, accompanied by their low cost, large scale manufacture, and flexibility. However, the understanding of the sensing mechanism is far from clear. Herein, we reveal the sensing mechanism of an organic transistor sensor for ammonia (NH3) detection through studying the recovery behavior in various atmospheres. Inspired by the significant difference in the recovery of the transistor sensor in N2 and in air, we deduced that the operation mechanism should not only involve the NH3-film interaction. Among a series of recovery processes, only upon exposure to wet air can the sensors completely recover in a certain time. Such a phenomenon, coupled with the transistor's performance evolution under vacuum, directly evidenced the existence of a pre-doping effect in the transistor by water (H2O) in ambient air. As a result, the response to the NH3 analyte is actually a de-doping process via reaction with the H2O. The full recovery in wet air is attributable to re-doping by H2O. Density functional theory (DFT) calculations were employed to assist the understanding of such a sensing mechanism. This study could help in the understanding of the sensing processes in many organic semiconductor based sensors.

What is the real utility of breath ammonia concentration measurements in medicine and physiology?

Much effort continues to be devoted to the development of devices to analyse breath ammonia with the anticipation that breath ammonia analyses will be useful in clinical practice. In this perspective we refer to the analytical techniques that have been used to measure breath ammonia, focusing on selected ion flow tube mass spectrometry, SIFT-MS, of which we have special knowledge and understanding. From the collected data obtained using the different techniques, we exam the origins of mouth- and nose-exhaled ammonia and conclude that mouth-exhaled ammonia is always elevated above a concentration that would be equilibrated with blood ammonia and is largely produced by the action of enzymes on salivary urea. Support to this conclusion is given by the reasonable correlation between blood urea concentration and mouth-exhaled ammonia concentration. Further, it is discussed that nose-exhaled ammonia largely originates at the alveolar interface and so its concentration more closely relates to the expected alveolar blood ammonia concentration. Ingestion of proteins results in increased blood/saliva urea and ultimately mouth-exhaled ammonia as does the generation of urease by H. pylori infection. It is also concluded that when mouth-exhaled ammonia is elevated then it may be due to either abnormally high blood urea, a high pH of the saliva/mouth/airways mucosa, poor oral hygiene or a combinations of these.

Organic Gas Sensor with an Improved Lifetime for Detecting Breath Ammonia in Hemodialysis Patients

In this work, a TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-s-butylphenyl)diphenylamine)]) sensor with a cylindrical nanopore structure exhibits a high sensitivity to ammonia in ppb-regime. The lifetime and sensitivity of the TFB sensor were studied and compared to those of P3HT (poly(3-hexylthiophene)), NPB (N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine), and TAPC (4,4'-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine]) sensors with the same cylindrical nanopore structures. The TFB sensor outstands the others in sensitivity and lifetime and it shows a sensing response (current variation ratio) of 13% to 100 ppb ammonia after 64 days of storage in air. A repeated sensing periods testing and a long-term measurement have also been demonstrated for the test of robustness. The performance of the TFB sensor is stable in both tests, which reveals that the TFB sensor can be utilized in our targeting clinical trials. In the last part of this work, we study the change of ammonia concentration in the breath of hemodialysis (HD) patients before and after dialysis. An obvious drop of breath ammonia concentration can be observed after dialysis. The reduction of breath ammonia is also correlated with the reduction of blood urea nitrogen (BUN). A correlation coefficient of 0.82 is achieved. The result implies that TFB sensor may be used as a real-time and low cost breath ammonia sensor for the daily tracking of hemodialysis patients.

Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly(3-hexylthiophene) (P3HT) Organic Field Effect Transistor

A highly sensitive organic field-effect transistor (OFET)-based sensor for ammonia in the range of 0.01 to 25 ppm was developed. The sensor was fabricated by employing an array of single-crystal poly(3-hexylthiophene) (P3HT) nanowires as the organic semiconductor (OSC) layer of an OFET with a top-contact geometry. The electrical characteristics (field-effect mobility, on/off current ratio) of the single-crystal P3HT nanowire OFET were about 2 orders of magnitude larger than those of the P3HT thin film OFET with the same geometry. The P3HT nanowire OFET showed excellent sensitivity to ammonia, about 3 times higher than that of the P3HT thin film OFET at 25 ppm ammonia. The ammonia response of the OFET was reversible and was not affected by changes in relative humidity from 45 to 100%. The high ammonia sensitivity of the P3HT nanowire OFET is believed to result from the single crystal nature and high surface/volume ratio of the P3HT nanowire used in the OSC layer.

One-Minute Fish Freshness Evaluation by Testing the Volatile Amine Gas with an Ultrasensitive Porous-Electrode-Capped Organic Gas Sensor System

In this work, we successfully demonstrate a fast method to determine the fish freshness by using a sensing system containing an ultrasensitive amine gas sensor to detect the volatile amine gas from the raw fish meat. When traditional titration method takes 4 h and complicated steps to test the total volatile basic nitrogen (TVB-N) as a worldwide standard for fish freshness, our sensor takes 1 min to deliver an electrical sensing response that is highly correlated with the TVB-N value. When detecting a fresh fish with a TVB-N as 18 mg/100 g, the sensor delivers an effective ammonia concentration as 100 ppb. For TVB-N as 28-35 mg/100 g, a well-accepted freshness limit, the effective ammonia concentration is as 200-300 ppb. The ppb-regime sensitivity of the sensor and the humidity control in the sensing system are the keys to realizing fast and accurate detection. It is expected that the results in this report enable the development of on-site freshness detection and real-time monitoring in a fish factory.

Electrical gas sensors based on structured organic ultra-thin films and nanocrystals on solid state substrates

Gas sensors, as useful tools to detect specific gas species such as toxic and explosive gases or volatile organic compounds, are the key components for environmental monitoring, fruit maturity and food safety monitoring, health care, and so on. The present commercial products based on porous metal oxide materials still face problems, such as high temperature operation and low level of selectivity. Thin films or nanostructures of organic materials with thickness or grain size down to nanometer scale represent promising candidates for gas sensing due to their potential for achieving high selectivity, portability and low cost. However, there are still challenges related to their stability, reproducibility and response/recovery speed despite the efforts in materials design, morphology control or device configuration, all of which have been expended during the last few decades. In this review, we summarize the progress of recent research on gas sensors based on organic ultra-thin films and nanostructures. We specifically discuss the effect of microstructure in the active layer on the sensing performance and mechanism.

Functionality-Oriented Derivatization of Naphthalene Diimide: A Molecular Gel Strategy-Based Fluorescent Film for Aniline Vapor Detection

Modification of naphthalene diimide (NDI) resulted in a photochemically stable, fluorescent 3,4,5-tris(dodecyloxy)benzamide derivative of NDI (TDBNDI), and introduction of the long alkyl chains endowed the compound with good compatibility with commonly found organic solvents and in particular superior self-assembly in the solution state. Further studies revealed that TDBNDI forms gels with nine of the 18 solvents tested at a concentration of 2.0% (w/v), and the critical gelation concentrations of five of the eight gels are lower than 1.0% (w/v), indicating the high efficiency of the compound as a low-molecular mass gelator (LMMG). Transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy studies revealed the networked fibrillar structure of the TDBNDI/methylcyclohexane (MCH) gel. On the basis of these findings, a fluorescent film was developed via simple spin-coating of the TDBNDI/MCH gel on a glass substrate surface. Fluorescence behavior and sensing performance studies demonstrated that this film is photochemically stable, and sensitive and selective to the presence of aniline vapor. Notably, the response is instantaneous, and the sensing process is fully and quickly reversible. This case study demonstrates that derivatization of photochemically stable fluorophores into LMMGs is a good strategy for developing high-performance fluorescent sensing films.

Ammonium ion detection in solution using vertically grown ZnO nanorod based field-effect transistor

Vertically aligned ZnO nanorods based fabricated FET providing a well-defined large surface area for ammonium ion detection in solution.

Diazapyrenium-based porous cationic polymers for colorimetric amine sensing and capture from CO2 scrubbing conditions

Supramolecular approach to the highly efficient sensing and capture of aliphatic amines.

Achieving a good life time in a vertical-organic-diode gas sensor

In this study, we investigate the keys to obtain a sensitive ammonia sensor with high air stability by using a low-cost polythiophene diode with a vertical channel and a porous top electrode. Poly(3-hexylthiophene) (P3HT) and air-stable poly(5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene) (PQT-12) are both evaluated as the active sensing layer. Two-dimensional current simulation reveals that the proposed device exhibits numerous connected vertical nanometer junctions (VNJ). Due to the de-doping reaction between ammonia molecules and the bulk current flowing through the vertical channel, both PQT-12 and P3HT VNJ-diodes exhibit detection limits of 50-ppb ammonia. The P3HT VNJ-diode, however, becomes unstable after being stored in air for two days. On the contrary, the PQT-12 VNJ-diode keeps an almost unchanged response to 50-ppb ammonia after being stored in air for 25 days. The improved storage lifetime of an organic-semiconductor-based gas sensor in air is successfully demonstrated.

Point of care monitoring of hemodialysis patients with a breath ammonia measurement device based on printed polyaniline nanoparticle sensors

A device for measuring human breath ammonia was developed based on a single use, disposable, inkjet printed ammonia sensor fabricated using polyaniline nanoparticles. The device was optimized for sampling ammonia in human breath samples by addressing issues such as variations in breath sample volume, flow rate, sources of oral ammonia, temperature and humidity. The resulting system was capable of measuring ammonia in breath from 40 to 2993 ppbv (r(2 )= 0.99, n = 3) as correlated with photoacoustic laser spectroscopy and correlation in normal human breath samples yielded a slope of 0.93 and a Pearson correlation coefficient of 0.9705 (p < 0.05, n = 11). Measurement of ammonia in the breath of patients with end-stage kidney disease demonstrated its significant reduction following dialysis, while also correlating well with blood urea nitrogen (BUN) (r = 0.61, p < 0.01, n = 96). Excellent intraindividual correlations were demonstrated between breath ammonia and BUN (0.86 to 0.96), which demonstrates the possibility of using low cost point of care breath ammonia systems as a noninvasive means of monitoring kidney dysfunction and treatment.