Ppb In Air Research Articles

Gas detection in a graphene based dual-mode fiber laser microcavity

Detection with both high sensitivity and component distinguishability is the major goal of any chemical sensor. In the case of gas detection, such capability could enable wide applications ranging from environmental monitoring, medical diagnosis to explosion analysis. But it is still difficult to achieve in label-free photonic devices, which usually lacks versatile freedoms of measurement. Here, we report a scheme to generate both ≈ 100 kHz Q switching and ≈ 2.5 GHz cavity-mode laser oscillations in a graphene based D-shaped fiber microcavity. This allows us to simultaneously measure the two beat notes in the radio frequency. In this condition, we exploit our unique dual-mode gas sensing scheme, and we achieve real-time detection of two gas samples (CO2 and NH3) in mixture, with detect limit down to single nM/L in vacuum, and tens of ppb in air. Such combination of graphene optoelectronics, integrated fiber laser cavity and dual-mode heterodyne technique paves a way toward high performance chemical sensing, and provides inspirations for design of diverse 2D material optical devices.

An Activity-Based Sensing Fluorogenic Probe for Monitoring Ethylene in Living Cells and Plants.

Ethylene (ET) is an important gaseous plant hormone. It is highly desirable to develop fluorescent probes for monitoring ethylene in living cells. We report an efficient RhIII -catalysed coupling of N-phenoxyacetamides to ethylene in the presence of an alcohol. The newly discovered coupling reaction exhibited a wide scope of N-phenoxyacetamides and excellent regioselectivity. We successfully developed three fluorophore-tagged RhIII -based fluorogenic coumarin-ethylene probes (CEPs) using this strategy for the selective and quantitative detection of ethylene. CEP-1 exhibited the highest sensitivity with a limit of detection of ethylene at 52 ppb in air. Furthermore, CEP-1 was successfully applied for imaging in living CHO-K1 cells and for monitoring endogenous-induced changes in ethylene biosynthesis in tobacco and Arabidopsis thaliana plants. These results indicate that CEP-1 has great potential to illuminate the spatiotemporal regulation of ethylene biosynthesis and ethylene signal transduction in living biological systems.

An Activity‐Based Sensing Fluorogenic Probe for Monitoring Ethylene in Living Cells and Plants

AbstractEthylene (ET) is an important gaseous plant hormone. It is highly desirable to develop fluorescent probes for monitoring ethylene in living cells. We report an efficient RhIII‐catalysed coupling of N‐phenoxyacetamides to ethylene in the presence of an alcohol. The newly discovered coupling reaction exhibited a wide scope of N‐phenoxyacetamides and excellent regioselectivity. We successfully developed three fluorophore‐tagged RhIII‐based fluorogenic coumarin‐ethylene probes (CEPs) using this strategy for the selective and quantitative detection of ethylene. CEP‐1 exhibited the highest sensitivity with a limit of detection of ethylene at 52 ppb in air. Furthermore, CEP‐1 was successfully applied for imaging in living CHO‐K1 cells and for monitoring endogenous‐induced changes in ethylene biosynthesis in tobacco and Arabidopsis thaliana plants. These results indicate that CEP‐1 has great potential to illuminate the spatiotemporal regulation of ethylene biosynthesis and ethylene signal transduction in living biological systems.

A compact optical sensor for explosive detection based on NIR luminescent quantum dots

Detection of explosive traces in the vapor phase is of primary importance for safety and security in several environments. Different detection methods with high sensitivity are available in the market, but they are typically expensive and require specialized personnel to be operated. Here, we propose a compact, low-cost sensor for explosive detection based on the photoluminescence (PL) quenching of solid-state PbS quantum dot solids cast from the solution phase on a silicon substrate. We demonstrate the sensor capability to detect nitrobenzene vapor at a concentration as low as 445 ppb in air at room temperature, overcoming the performance of other state-of-the-art quantum dot-based PL sensors for nitroaromatic compounds. Moreover, the proposed system can be realized with off-the-shelf electronics and does not need any additional laboratory equipment to be operated, thus paving the way for its deployment in distributed sensor networks.

Reliable anatase-titania nanoclusters functionalized GaN sensor devices for UV assisted NO2 gas-sensing in ppb level

Internet of Things applications require ultra-low power, integrable into electronic circuits and mini-sized chemical sensors for automated remote air quality monitoring system. In this work, a highly sensitive and selective detection of nitrogen dioxide (NO2) has been demonstrated by functionalizing gallium nitride (GaN) submicron wire with titania (TiO2) nanoclusters. The two-terminal GaN/TiO2 sensor device was fabricated by top-down approach. The photo-enabled sensing makes it possible to operate this sensor at room-temperature, resulting in a significant reduction in operating power. The GaN/TiO2 sensor was able to detect NO2 concentrations as low as 10 ppb in air at room temperature (20 °C) with a quick response-recovery process. The sensor was found highly selective toward NO2 against other interfering gases, such as ethanol (C2H5OH), ammonia (NH3), sulfur dioxide (SO2), methane (CH4) and carbon dioxide (CO2). Furthermore, principal component analysis has been performed to address the cross-sensitive nature of TiO2. The sensor device exhibited excellent long-term stability at room temperature and humidity and was quite stable and reliable at various environmental conditions. Continuous exposure of the device to siloxane for a one-month period has shown a very small degradation in sensor response to NO2. Finally, interaction of NO2 gas molecules with the GaN/TiO2 sensor has been modeled and explained under the light of energy band diagram. The photoinduced oxygen desorption and subsequent charge transfer between TiO2 nanoclusters and NO2 molecules modulate the depletion region width within the GaN, thus contributing to a high performance NO2 gas sensing.

Nanostructured tin oxide materials for the sub-ppm detection of indoor formaldehyde pollution

Interesting sensing performances of indoor formaldehyde pollution were obtained when small amounts of zinc were introduced in tin oxides. Nanostructured Sn oxide-based porous materials doped with Zn or not, were synthesized using hydrothermal routes. The physicochemical properties of the as-prepared metal-oxide materials were characterized using nitrogen adsorption, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Gas sensors were prepared using the aforementioned tin oxide materials and they exhibited a high sensitivity to formaldehyde at 230 °C, as well as a good repeatability over the time. Their limit of formaldehyde detection was as low as 8 ppb in dry air and 50 ppb in air with 60% RH at 25 °C. These results were much better that those reported in the open literature and they were attributed to both higher area BET, around 180 m2/g, and smaller crystallite size, 3.1 nm.

Impact of Temperature and UV Irradiation on Dynamics of NO₂ Sensors Based on ZnO Nanostructures.

The main object of this study is the improvement of the dynamics of NO2 sensors based on ZnO nanostructures. Investigations presented in this paper showed that the combination of temperature and ultraviolet (UV) activation of the sensors can significantly decrease the sensor response and regeneration times. In comparison with the single activation method (elevated temperature or UV), these times for 1 ppm of NO2 decreased from about 10 min (or more) to less than 40 s. In addition, at the optimal conditions (200 °C and UV), sensors were very stable, were fully scalable (in the range on NO2 concentration of 1–20 ppm) and baseline drift was significantly reduced. Furthermore, in this paper, extensive studies of the influence of temperature and carrier gas (nitrogen and air) on NO2 sensing properties of the ZnO nanostructures were conducted. The NO2 sensing mechanisms of the sensors operating at elevated temperatures and under UV irradiation were also discussed. Our study showed that sensor responses to NO2 and response/regeneration times are comparable from sensor to sensor in air and nitrogen conditions, which suggests that the proposed simple technology connected with well-chosen operation conditions is repeatable. The estimated limit of detection of the sensors is within the level of ≈800 ppb in nitrogen and ≈700 ppb in air.

Ruthenium(II) and osmium(II) vinyl complexes as highly sensitive and selective chromogenic and fluorogenic probes for the sensing of carbon monoxide in air.

The detection of carbon monoxide in solution and air has been achieved using simple, inexpensive systems based on the vinyl complexes [M(CHCHR)Cl(CO)(BTD)(PPh3 )2 ] (R=aryl, BTD=2,1,3-benzothiadiazole). Depending on the nature of the vinyl group, chromogenic and fluorogenic responses signalled the presence of this odourless, tasteless, invisible, and toxic gas. Solutions of the complexes in CHCl3 underwent rapid change between easily differentiated colours when exposed to air samples containing CO. More significantly, the adsorption of the complexes on silica produced colorimetric probes for the naked-eye detection of CO in the gas phase. Structural data for key species before and after the addition of CO were obtained by means of single X-ray diffraction studies. In all cases, the ruthenium and osmium vinyl complexes studied showed a highly selective response to CO with exceptionally low detection limits. Naked-eye detection of CO at concentrations as low as 5 ppb in air was achieved with the onset of toxic levels (i.e., 100 ppm), thus resulting in a remarkably clear colour change. Moreover, complexes bearing pyrenyl, naphthyl, and phenanthrenyl moieties were fluorescent, and greater sensitivities were achieved (through turn-on emission fluorescence) in the presence of CO both in solution and air. This behaviour was explored computationally using time-dependent density functional theory (TDDFT) experiments. In addition, the systems were shown to be selective for CO over all other gases tested, including water vapour and common organic solvents. Supporting the metal complexes on cellulose strips for use in an existing optoelectronic device allows numerical readings for the CO concentration to be obtained and provision of an alarm system.

Characterization of changes in gene expression and biochemical pathways at low levels of benzene exposure.

Benzene, a ubiquitous environmental pollutant, causes acute myeloid leukemia (AML). Recently, through transcriptome profiling of peripheral blood mononuclear cells (PBMC), we reported dose-dependent effects of benzene exposure on gene expression and biochemical pathways in 83 workers exposed across four airborne concentration ranges (from <1 ppm to >10 ppm) compared with 42 subjects with non-workplace ambient exposure levels. Here, we further characterize these dose-dependent effects with continuous benzene exposure in all 125 study subjects. We estimated air benzene exposure levels in the 42 environmentally-exposed subjects from their unmetabolized urinary benzene levels. We used a novel non-parametric, data-adaptive model selection method to estimate the change with dose in the expression of each gene. We describe non-parametric approaches to model pathway responses and used these to estimate the dose responses of the AML pathway and 4 other pathways of interest. The response patterns of majority of genes as captured by mean estimates of the first and second principal components of the dose-response for the five pathways and the profiles of 6 AML pathway response-representative genes (identified by clustering) exhibited similar apparent supra-linear responses. Responses at or below 0.1 ppm benzene were observed for altered expression of AML pathway genes and CYP2E1. Together, these data show that benzene alters disease-relevant pathways and genes in a dose-dependent manner, with effects apparent at doses as low as 100 ppb in air. Studies with extensive exposure assessment of subjects exposed in the low-dose range between 10 ppb and 1 ppm are needed to confirm these findings.

WO3 Nanolamella Gas Sensor: Porosity Control Using SnO2 Nanoparticles for Enhanced NO2 Sensing

Tungsten trioxide (WO3) is one of the important multifunctional materials used for photocatalytic, photoelectrochemical, battery, and gas sensor applications. Nanostructured WO3 holds great potential for enhancing the performance of these applications. Here, we report highly sensitive NO2 sensors using WO3 nanolamellae and their sensitivity improvement by morphology control using SnO2 nanoparticles. WO3 nanolamellae were synthesized by an acidification method starting from Na2WO4 and H2SO4 and subsequent calcination at 300 °C. The lamellae were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which clearly showed the formation of single-crystalline nanolamellae with a c-axis orientation. The stacking of each nanolamella to form larger lamellae that were 50-250 nm in lateral size and 15-25 nm in thickness was also revealed. From pore size distribution measurements, we found that introducing monodisperse SnO2 nanoparticles (ca. 4 nm) into WO3 lamella-based films improved their porosity, most likely because of effective insertion of nanoparticles into lamella stacks or in between assemblies of lamella stacks. In contrast, the crystallite size was not significantly changed, even by introducing SnO2. Because of the improvement in porosity, the composites of WO3 nanolamellae and SnO2 nanoparticles displayed enhanced sensitivity (sensor response) to NO2 at dilute concentrations of 20-1000 ppb in air, demonstrating the effectiveness of microstructure control of WO3 lamella-based films for highly sensitive NO2 detection. Electrical sensitization by SnO2 nanoparticles was also considered.

Nitro-Aromatic Explosive Sensing Using GaN Nanowire-Titania Nanocluster Hybrids

Globalization of terrorist attacks in recent years calls for advanced technologies to detect the concealed explosives effectively. We demonstrate a highly sensitive and selective detection of traces of nitro-aromatic explosive compounds by functionalizing gallium nitride (GaN) nanowires with titania (TiO2) nanoclusters to address this issue. The ultraviolet light photo-modulated GaN/TiO2 nanowire-nanocluster (NWNC) hybrids show remarkable selectivity to benzene and related aromatic compounds, with no measureable response for other analytes at room temperature. This paper presents the sensing characteristics of GaN/TiO2 NWNC hybrids towards the different aromatic and nitroaromatic compounds at room temperature. The GaN/TiO2 hybrids are able to detect trinitrotoluene (TNT) concentrations as low as 500 ppt in air and dinitrobenzene concentrations as low as 10 ppb in air in approximately 30 s. The noted sensitivity range of the devices for TNT is from 8 ppm down to as low as 500 ppt. The detection limit of dinitrotoluene, nitrobenzene, nitrotoluene, toluene and benzene in air is 100 ppb with a response time of ~ 75 s. The devices showed very sensitive and selective response to TNT when compared to interfering compounds like toluene with good reproducibility. Integration of different NWNC hybrids on the same substrate could lead to tiny, highly sensitive, selective, low-power and smart explosive detectors that could be manufactured cheaply in large numbers.

A carbon-nanotube-based sensor array for formaldehyde detection

We have fabricated a sensor array consisting of 32 sensor elements with pristine, dopedand metal-loaded single-wall carbon nanotubes as sensing materials. The sensorelements consist of interdigitated electrodes with varying finger widths and gaps.The chemiresistor-type sensors provide a significant response to formaldehyde atconcentrations down to 10 ppb in air with rapid response and recovery times.

Human chemosensory perception of methyl isothiocyanate: Chemesthesis and odor

An unpublished laboratory study by Russell and Rush (1996) showed that human subjects sense the presence of methyl isothiocyanate (MITC) via the eyes at concentrations as low as hundreds of ppb in air, with dependence upon duration of exposure. The longer the stimulation, the lower the concentrations sensed. Application of benchmark concentration (BMC10) modeling indicated a best estimate of 330 ppb by the end of 4 h. With a confidence limit (BMCL) applied, the level dropped to 220 ppb, when employing a probit model. Receptors known as TRPA1 ion channels present in trigeminal and associated peripheral afferent nerves have shown particular sensitivity to isothiocyanates. Sensitivity to these electrophiles, which occur naturally in plants (e.g., capers and mustard greens), most likely derives from a mechanism of reversible covalent bonding. Such sensing can provide warning of potential damage rather than actual damage itself. Based upon its reputation as a lachrymator, Russell and Rush assumed that the eyes would sense MITC, before the upper airways, so gathered no data from the airways, except for odor. Field results from spills and results of acute exposures to animals covered in Dourson et al. (2010) add pertinent information on the matter.

Computational screening of metal–organic frameworks for large-molecule chemical sensing

Grand canonical Monte Carlo simulations were performed to identify trends in low-pressure adsorption of a broad range of organic molecules by a set of metal-organic frameworks (MOFs). While previous simulation studies focused on the adsorption of small molecules such as carbon dioxide and methane, we consider more complicated organic molecules relevant to chemical sensing and detection: small aromatics (o-, m-, and p-xylene), polycyclic aromatic hydrocarbons (naphthalene, anthracene, phenanthrene), explosives (TNT and RDX), and chemical warfare agents (GA and VM). The framework materials include several Zn-IRMOFs (IRMOFs 1-3, 7, 8), a Cr-MOF (CrMIL-53lp), and a Cu-MOF (HKUST-1). A wide range of loading pressures is examined, extending from 100 ppm to 10 ppb in air, thus spanning the entire range of conditions relevant to chemical sensing for security, environmental, and industrial process monitoring. Our results are validated by comparing calculated adsorption energies with experimental values, where available. Many of the larger organics are significantly adsorbed by the target MOFs at low pressure, which is consistent with the high isosteric heats of adsorption (12 kcal mol(-1)-49 kcal mol(-1)) computed for these analytes. These adsorption energies are significantly large that interference from atmospheric components should not interfere with chemical detection at low pressures. We show that pi-pi stacking interactions are an important contributor to these high heats of adsorption. CrMIL-53lp shows the highest adsorption energy for all analytes, suggesting that this material may be suitable for detection of low-level organics. At higher loading pressures, the Zn-MOFs show a much higher volumetric uptake than either CrMIL-53lp or HKUST-1 for all types of analyte considered here. Within the Zn-IRMOF series, analyte loading is proportional to accessible free volume, and loading decreases with increasing analyte size due to molecular packing effects. Overall, the results demonstrate that atomistic simulation can be used as an efficient first step in the screening of MOFs for detection of large molecules. For example, at the 10 ppb level, all of the Zn-IRMOFs are able to distinguish between TNT and the structurally similar xylenes.

Quantitative determination of hexamethylene diisocyanate (HDI), 2,4-toluene diisocyanate (2,4-TDI) and 2,6-toluene diisocyanate (2,6-TDI) monomers at ppt levels in air by alkaline adduct coordination ionspray tandem mass spectrometry

Occupational exposures to isocyanates can lead to occupational asthma. Once sensitized, some workers could react to isocyanate monomers at concentrations below 1% of the Permissible Exposure Limit of 5 ppb in air. Currently available methods are not sufficiently sensitive to adequately evaluate isocyanates present at these levels in workplace air. This article describes a novel method for isocyanate determination allowing the ultratrace quantification in workplace air of hexamethylene diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate monomers. Sampling is performed during a complete workshift at a flow rate of 1 L min(-1) with a cassette containing a 1-(2-methoxyphenyl)piperazine-impregnated 25 mm filter. Analysis is performed using liquid chromatography hyphenated with coordination ionspray tandem mass spectrometry. The analytical method's linearity was measured for a concentration range varying from the limit of detection of 0.04-0.13 ng mL(-1), depending on the monomer, up to approximately 32 ng mL(-1) for every isocyanate monomer, all with correlation coefficients (R(2)) greater than 0.999. The analytical method's lower limit of quantification combined with an adapted sampling strategy allow the quantification of isocyanate monomers down to 0.04 ppt for an 8 h work shift when a lithium adduct is used, which is more than 300 times lower than the most sensitive method currently available. This novel method can be used to confirm the very low level of isocyanate monomers for the safe reassignment of sensitized workers and it is also useful for charting the isocyanate dispersion tail in workplace environments.

Effects of oxygen annealing on gas sensing properties of carbon nanotube thin films

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition have been investigated as resistive gas sensors towards NO 2 oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes’ electrical resistance as determined by volt-amperometric measurements. In particular the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. The electrical response was then measured exposing the films to sub-ppm NO 2 concentrations (100 ppb in air) at 165 °C. Upon exposure to NO 2, the electrical resistance of CNTs was found to decrease. The obtained results demonstrate that nanotubes could find use as a sensitive chemical gas sensor for (a) the fast response accompanied by a high sensitivity to sub-ppm NO 2 exposure, and (b) the precise recover of the base resistance value in absence of NO 2 at a fixed operating temperature, likewise indicating that intrinsic properties measured on as prepared nanotubes may be severely changed by extrinsic oxidative treatment effects.

Reversible oxidation effects on carbon nanotubes thin films for gas sensing applications

Carbon nanotubes (CNTs) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) have been investigated as resistive gas sensors towards NO 2 oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes' electrical resistance as determined by volt-amperometric measurements. In particular, the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. After oxygen annealing, X-ray photoelectron spectroscopy (XPS) proves the increase of oxygen linked to nickel located at the nanotube's cap and a no appreciable variation of oxygen physisorbed on the carbon nanotubes. Tangential mode Raman lines from metallic oxidized CNTs was found to depend sensitively on adsorbed oxidizing molecules exhibiting different line shapes than the as-deposited nanotubes. However, the line shapes became identical after thermal annealing in vacuum at 250 °C, which is attributed to degassing of doping adsorbates. The electrical resistance measured by exposing the films to sub-ppm NO 2 concentrations (100 ppb in air) at 165 °C was found to decrease. The obtained results demonstrate that nanotubes could find use as sensitive chemical gas sensor for the fast response accompanied by a high sensitivity to sub-ppm NO 2 exposure; the precise recover of the base resistance value in absence of NO 2 at a fixed operating temperature likewise indicate that intrinsic properties measured on as-prepared nanotubes may be severely changed by extrinsic oxidative treatment effects.

Chemiluminescence detection of hydrazine vapor

An efficient, real-time chemiluminescence detector for hydrazine vapor, N 2H 4(g), is described, capable of monitoring sub part-per-billion levels of hydrazine in air. The catalytic oxidation of hydrazine by colloidal platinum forms an intermediate, oxidizing agent ( e.g. OH or OOH) which subsequently oxidizes luminol, generating a chemiluminescence signal that is proportional to the hydrazine concentration. Major components of the instrument include a photomultiplier tube (PMT), a short length of glass tubing coiled directly in front of the PMT cathode surface, a vacuum pump for sampling the air, and a peristaltic pump for circulating the liquid reagent. The liquid reagent, a basic solution (pH 13) of luminol and colloidal platinum, is continuously recycled. The detection sequence is initiated by pumping the hydrazine vapor through a short length of teflon tubing that is concurrently transporting the liquid reagent. The liquid is separated from the gas stream in an impinger and quickly pumped to the PMT. We have evaluated the effect of solution pH, luminol and platinum concentrations, and air and liquid flow rates on the analytical characteristics of this system. A linear, dynamic detection range for hydrazine has been obtained from 1 to 2000 ppb in air, with an instrument response that is fully reversible and achieves plateau response in less than 2 min.

An SnO 2 based switching tunnel device for the detection of NO 2 in air at the sub ppm level

A new device has been developed for the detection of low NO 2 concentrations in air. The structure consists of a p +-Si substrate, an n-Si epitaxial layer, a thin tunnel SiO 2 layer and a thin SnO 2 sputtered film. Some of the sensors have been catalyzed with a thin Al sputtered layer to improve the sensitivity and response rate to NO 2. The electrical I— V characteristics are those of a tunnel diode, which shows a bi-stable (low impedance, high impedance) behaviour. The switch may be accounted for by punchthrough and avalanche multiplication at the n-Si layer. It was found that the switching voltage is an increasing function of the NO 2 concentration, as the work function of the SnO 2 increases with the ionosorption of the electronegative NO 2 molecules. The possibility of detecting NO 2 concentrations as low as 50 ppb in air has been demonstrated.

Comparison of H2O2 and O3 content in atmospheric samples in the San Bernardino mountains, Southern California

Concentrations of atmospheric H 2 O 2 were measured in air, rain, cloud and dew samples in forested areas of the San Bernardino Mountains, southern California, from spring through fall of 1987–1990 O 3 measurements in air were also conducted for comparison. Typical ranges of H 2 O 2 concentrations measured were 1–3 ppb in air, 10–90 μM in rain and cloud water, and < μM in dew. The results show that gas-phase H 2 O 2 concentrations were slightly higher at nighttime than at daytime or nearly constant throughout a 24-hr period, whereas O 3 concentrations were highest during the afternoon, when polluted air masses from Los Angeles carried by daily sea breezes reached the mountain region. Afternoon concentrations of gaseous H 2 O 2 and O 3 in the mountain region were compared with those measured in Los Angeles urban sites to elucidate the regional variation of these oxidants. The results show that ambient concentrations of H 2 O 2 and O 3 were about 50–100% higher at the mountains sites than at the Los Angeles sites.