Gaseous Volatile Organic Compounds Research Articles

A new measurement protocol to differentiate sources of halitosis

Objective: Three sources of halitosis exist, potentially in any combination: mouth, nasal cavity or alveolar breath. There has been no universally accepted protocol which differentiates and quantifies each odour source separately. In this study a new gas measurement protocol is described and tested to determine whether each odour source can be separately detected without contamination.Materials and methods: Ninety healthy volunteers were divided into three groups. Hydrogen sulphide (H2S), volatile organic compounds (VOCs) and hydrogen (H2) were artificially generated in the mouth, nose and pulmonary alveoli, respectively. VOC, ammonia (NH3), sulphur dioxide (SO2), H2S and H2 gas readings from mouth, nose and alveolar air were measured and compared. Measurements were taken before and during gas generation.Results: Contamination of nasal air (2.8%) and alveolar air (5.0%) by oral H2S; alveolar air (2.06%) and oral air (4%) by nasal organic gas; nasal air (18.43%) and oral air (9.42%) by alveolar H2 was calculated.Conclusion: The results demonstrated that artificially generated oral H2S nasal VOC and alveolar H2 can be individually quantified. This gas measurement protocol can be used diagnostically or to gauge response to therapy in any medical or dental setting.

A highly sensitive VOC gas sensor using p-type mesoporous Co3O4 nanosheets prepared by a facile chemical coprecipitation method

Mesoporous Co3O4 nanosheets were synthesized via a facile chemical coprecipitation method without using of surfactant or template. Mesoporous Co3O4 material was characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) to examine the morphology and microstructure to find out the cause for the highly sensitive sensing behavior. SEM and TEM analyses reveal that the Co3O4 nanosheets consisting of nanoparticles prepared here are porous in nature, with average pore sizes of approximately 50nm or smaller. The sensor based on porous Co3O4 nanaosheets was used for detecting the volatile organic compounds (VOCs) including ethanol, methanol, acetone, isopropanol, formaldehyde and n-butanol. The results demonstrate that the sensor shows potential for detecting VOCs. The significant improvement of sensitivity is attributed to the porous structure, good contact and relatively small crystal size. The gas sensor shows high response values, fast response and recovery times towards VOCs. So, porous Co3O4 nanosheets are considered as promising sensor material for detecting VOCs.

Pulsed laser deposition of metal oxide nanostructures for highly sensitive gas sensor applications

Nanosecond and picosecond pulsed laser deposition (PLD) was used to prepare metal oxide nanostructures with different morphologies as gas sensing materials on top of oxidized silicon substrates and commercial SGX Sensortech SA MEMS microheater platforms. The layers were formed of different types of nanostructures including nanoparticles, agglomerates, and nanotrees with fractal-like growth. Clear dependencies between the deposition parameters, structural morphology, and gas sensing performance were found. Also, some differences in the morphologies of the layers were seen when picosecond PLD was used instead of nanosecond PLD. Many of the sensing materials were found to be highly sensitive to different types of gaseous species. We investigated inorganic gases in the ppm range (2–400ppm) including NO, CO, and NH3, and the selectivity and sensitivity were shown to be dependent, not only on layer morphology, but also on the measurement temperature. Moreover, an investigation with volatile organic compound gases in the ppb range demonstrated that WO3 layers are highly sensitive and selective towards naphthalene at least down to 2.5ppb.

Carbon quantum dots decorated Bi2WO6 nanocomposite with enhanced photocatalytic oxidation activity for VOCs

We incorporated highly stable carbon quantum dots (CQDs) with Bi2WO6 to sufficiently photocatalytic removal of gaseous volatile organic compounds (VOCs) utilize solar energy. With the facile decoration of CQDs, the composite photocatalysts of CQDs/Bi2WO6 extend the absorption into visible light region and improve the photoexcited charge separation in comparison with pristine Bi2WO6. The CQDs/Bi2WO6 exhibited higher photocatalytic oxidation activities towards acetone and toluene under both UV–vis and visible light irradiation. In all, CQDs could be a promising candidate for visible light photocatalysts due to their superior ability to extend the visible absorption and suppress the photo-excited charge recombination.

Investigation of Silver Catalytic Properties on Reduced Graphene Oxide to Enhance Gas Sensitivity and Selectivity from Mixed VOC Gases

Volatile organic compounds (VOCs) that easily evaporate at room temperature have caused detrimental to respiratory, allergic, and/or immune effects in humans. Increased awareness of toxicity on these VOC gases has led many studies to focus on direct measurement of these harmful VOCs with high sensitivity at lower operating temperature. Gas sensors based on surface chemical reaction of semiconducting metal oxides are readily available commercially. To improve gas sensing performance with lowered operating temperature and high sensitivity, adequate catalysts are commonly used. However, there is still limit to lower sensing temperature of metal oxides to room temperature. It has been reported that graphene oxide (GO), a graphene sheet linked to oxygen functional groups, has potential to realize room temperature operation. Additional advantage of the GO includes large scale and low cost production for commercial application. In this study, GO thin films were deposited by drop coating and thermally reduced under Ar to eliminate the functional groups from GO films. The electrical properties of GO transformed from an insulator to semiconductor support GO to easily be activated for sensing gases. To enhance selective gas sensing at room temperature, many types of noble metals such as Pt and Au can be considered as an activators or sensitizers. We investigated GO with silver catalyst with relatively low cost. The formation of Ag island clusters driven by deposition conditions and their VOC gas sensing was characterized to find optimal condition of catalysts. The morphologies of silver catalyst decoated GO film were analyzed with SEM, TEM, and XRD, and the quantity of the silver catalyst was measured by EDS. Gas sensing response to VOCs was measured at room temperature to extract characteristic feature. In order to determine selective detection of specific VOC gas, 2 types and 5 types of mixed gases such as formaldehyde, benzene, styrene, toluene, and xylene were used. Detailed discussion on and the mechanism of silver catalytic effect on GO will be given. AcknowledgementThis research was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (20158520000210) grant funded by the Korea Government Ministry of Trade, Industry and Energy, a grant from a Strategic Research Project (2013-0132) funded by the Korea Institute of Construction Technology, and Auburn University IGP.References Vlachos, D. S., C. A. Papadopoulos, and J. N. Avaritsiotis. "On the electronic interaction between additives and semiconducting oxide gas sensors." Applied physics letters 69.5 (1996): 650-652.Tyagi, Punit, et al. "Metal Oxide Catalyst assisted SnO2 thin film based SO2 gas sensor." Sensors and Actuators B: Chemical (2015).Ahn, Hosang, et al. "Volatile gas sensing properties of phase and composition gradient SnOx thin films by combinatorial sputter deposition." ECS Solid State Letters 2.1 (2013): P11-P13.Li, Yongjie, et al. "Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation." Carbon 48.4 (2010): 1124-1130.Wang, Jianwei, et al. "Dielectrophoretic assembly of Pt nanoparticle-reduced graphene oxide nanohybrid for highly-sensitive multiple gas sensor." Sensors and Actuators B: Chemical 220 (2015): 755-761.Park, Hyejin, et al. "Transition of gas sensing behavior in non-reduced graphene oxides with thermal annealing." Materials Letters 136 (2014): 164-167.

Fabrication of 3D Graphene and 3D Graphene Oxide Devices for Sensing VOCs

ABSTRACTDetection of volatile organic compounds (VOCs) emitted from cancerous tumor cells in exhaled human breath allows for early diagnosis of various types of cancers. 3D graphene with a large surface area is considered a suitable material for creating novel sensitive VOCs sensors. In this study, 3D graphene and 3D graphene oxide were synthesized from graphene oxide suspension, hydroquinone and formaldehyde by employing polymerization and reduction. The capability of VOC gas sensing was evaluated by measuring the electrical current response in flowing N2 gas over a range of concentrations of acetone or 1-butanol at room temperature. It was observed that the device current correlated well with the VOC concentration. The adsorption of acetone decreased the current, but the adsorption of 1-butanol increased the current during sensing. 3D graphene oxide device was more sensitive than 3D graphene device because of the high concentration of oxygen-containing functional groups on the surface. These results indicated that 3D graphene and 3D graphene oxide may be the suitable materials for VOCs sensing devices.

The effect of Pd on the H2 and VOC sensing properties of TiO2 nanorods

This work presents highly sensitive TiO2 nanorods modified with palladium to investigate the sensing properties of H2 and volatile organic compound (VOC) gases. TiO2 nanorods were synthesized in a mixed solution containing 1ml of titanium n-butoxide (TnBT) by the hydrothermal method. Pd on the surface of the TiO2 nanorods was decorated by heat treatment. The samples were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The electrical properties of the TiO2 and Pd loaded TiO2 nanorods were investigated under a dry air flow at a temperature range from 30°C to 200°C. The H2 sensing properties of the TiO2 nanorods decorated with Pd were investigated in the concentration range of 500–2000ppm at 200°C. Moreover, the effect of Pd on the VOC gas sensing properties of the TiO2 nanorods was examined at 200°C. The results revealed that the nanorods modified with palladium exhibited excellent sensing performance to H2 at 30°C, and also had the appropriate sensor behavior at 200°C with clear response–recovery, good stability, and reproducibility. The maximum VOC response was obtained for ethanol.

Micro structural characterization and gas sensing behaviour of In2O3:Ag nano composite thin films deposited by electron beam evaporation technique

The main objective in this discussion is to investigate the role of metal additives in In2O3 to detect the toxic gases. Ag-In2O3 nano composite thin films were grown by electron beam evaporation. In2O3:Ag nano composite thin films were deposited on quartz substrate by electron beam evaporation technique at a substrate temperature of 350°C, with deposition time 10mts and electron gun power of 75 W. The vacuum chamber was evacuated to a base vacuum of 6x10−6 Torr and oxygen partial pressure was maintained at 0.1 mTorr. GIXRD patterns confirmed the formation of polycrystalline films with cubic structure of (222) plane direction for In2O3 and (111) orientation for Ag2O. Thickness of the films was estimated by stylus profilometer. SEM micrographs of pure In2O3 films shows spherical shaped grain morphology and after adding Ag to In2O3 grain morphology changes to helical structure with an increase in grain size. All the above synthesized thin films have been tested for sensing of volatile organic compound (VOC) gases. The In2O3:Ag nano composite thin films deposited on quartz substrate have shown better response to volatile organic compound gases.

Ag-Functionalized macro-/mesoporous AZO synthesized by solution combustion for VOCs gas sensing application

The aim of this paper is to develop easily manufactured and highly sensitive gas sensors for VOCs (volatile organic compounds) detection.

Self‐Powered, Room‐Temperature Electronic Nose Based on Triboelectrification and Heterogeneous Catalytic Reaction

Herein, a self‐powered electronic nose strategy with highly selective gas detection is described. The electronic nose is a two‐dimensional microarray based on the triboelectrification between ZnO nanowires and the dielectric layers, and the heterogeneous catalytic reaction occurring on the nanowires and on the NiO nanoparticles. These electronic noses show the ability to distinguish between four volatile organic compound (VOC) gases (methanol, ethanol, acetone, and toluene) with a detection limit of 0.1% at room temperature using no external power source.

Au-Polypyrrole Framework Nanostructures for Improved Localized Surface Plasmon Resonance Volatile Organic Compounds Gas Sensing.

In this paper, we propose an Au-polypyrrole (Ppy) nanorod gas sensor for the detection of volatile organic compound (VOC) gases. This gas sensor operates on the principle of localized surface plasmon resonance (LSPR). The Au-Ppy nanorods used in this experiment were synthesized using an anodic aluminum oxide template by the electrochemical deposition method. Using field emission scanning electron microscopy, we confirmed that the Au-Ppy nanorod arrays were successfully fabricated with a uniform size. By depositing gold, the Au-Ppy nanorods exhibited both optical and LSPR interference. The gas sensing properties of the fabricated nanorods were tested for VOCs such as acetic acid, benzene, and toluene with a short response time (~1 min). Moreover, the proposed VOC gas sensing system was tested with three types of VOC gases over a wide concentration range from 10 to 100 ppm. Highest sensitivity was observed for acetic acid gas, which had a linear relation with the gas concentration, indicating that the system can be used as a gas sensor.

VOC 분해 플라즈마-덤프 연소기 특성

VOCs (Volatile Organic Compounds) are generally generated in the painting process, or at the company and laundry where use organic solvents. The VOCs consist of various hydrocarbons and has low calorific value due to its dilution with atmospheric air. Therefore, the VOCs are difficult to burn by a conventional fuel combustor. In this study, a novel plasma dump combustor was proposed for the treatment of low calorific VOC gases. This combustor was designed a combination of the characteristics in a plasma burner, a dump combustor and a 3D matrix burner. The combustor has good structure for maintaining enough residence time and reaction temperature for stable flame formation and VOC destruction. For investigating the performance characteristics of the plasma dump combustor, an experiment was achieved for VOC feed rate, VOC injector position, etc. Toluene was used as a surrogate of VOC. The novel combustor gave better performance than a conventional combustor, showing that VOC destruction rate and energy efficiency were 89.64% and 12.27 kg/kWh respectively, at feeding rate of 450 L/min of VOC of 3,000 ppm of toluene concentration.

VOC Gas Sensors Fabricated with Graphene Oxide Composites for Food Safety and Quality

For food market, off-odors or flavors have become more than a quality issue with the increased awareness of food safety problems. This awareness to solve the safety problems, off-odor detection in foods, packaging materials, and their interactions involving scalping or other flavor system modification have been attracted. One of suggested methods is to use an electronic nose consisting of a gas sensor array to detect the volatile organic compounds (VOCs), released from foods as a byproduct of everyday physiological processes. Monitoring of the specific VOCs by a gas sensor can provide the information on both crop and field conditions. The array structure of gas sensors can enhance selectivity for in-situ detection [1, 2]. In this study, an array of sensors is investigated by employing graphene oxide and nanostructured metal oxide based on SnO2. Their composite films on a series of electrodes are prepared to create a high density of sensing array in a small size chip. A composition gradient sensing layers can produce unique gas response patterns. Figure 1 shows surface morphology of compositionally gradient tin oxides (1:6 ratio of GO to SnO2) and structure property confirmed by XRD result. The results on the growth of nanostructured films using solution process and vacuum process are presented. Figure 2 also shows sensing properties of the composites to formaldehyde gas at room temperature. The combinatorial structure enhances the sensing properties of devices in terms of sensitivity, selectivity and response rate. Additionally, such a sensor can meet the proposed idea, in-situ detection, and also the requirements for low-cost and high-volume production as well as high gas analytical performance. This research was partially supported by a grant from a Strategic Research Project (2013-0132) funded by the Korea Institute of Construction Technology, and Auburn University IGP. [1] A. D. Wilson and M. Baietto. Sensors. 2009, 9:5099-5148. [2] K. Arshak, E. Moore, G. M. Lyons, J. Harris, and S. Clifford. Sensor Review. 2004, 24(2):181-198. Figure 1

Electronic Identification System of Hazardous Volatile Organic Compound Gases using Roselle-Chitosan Blend

This paper presents a study on design and fabrication of an electronic identification system (EIS) for some volatile organic compounds i.e. toluene, ethanol and methanol through vaporization technology. A new sensing material of roselle-chitosan blending thin film based on electrochemical deposition method was used to guarantee the differentiation and the accuracy of the identification system. Chitosan smart gel with 2% acetic acid was prepared and then roselle extract was blended to enhance the sensing properties of the chitosan film. Differentiation ability was studied by exposing 50ppm of toluene, ethanol and methanol vapor to the sensing material. The accuracy of the identification system was tested by IV electrical testing. The analyzed data demonstrated that the electronic identification system is capable of identifying different volatile organic compound gases at room temperature successfully. The roselle-chitosan film sensors showed the characteristic of a reliable sensor i.e. good sensitivity, selectivity, repeatability, recovery and stable.

Meso- and macroporous coral-like Co3O4 for VOCs gas sensor

Meso- and macroporous coral-like Co3O4 was synthesized via a facile self-sustained decomposition using the complexes of metallic salts and organics (metal-organic complexes). Porous Co3O4 material was characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectrum (XPS) to examine the morphology and microstructure to find out the cause. Co3O4 with the meso- and macroporous structure consisting of nanoparticles has a coral-like shape with a size of tens of micrometers and exhibits the hierarchically porous morphology, in which the walls of the macropores contain smaller mesopores. The gas-sensing characteristics of the porous Co3O4 for volatile organic compounds (VOCs) were investigated. The gas response to 1000ppm n-butanol reaches a maximum of 27.7 at an operating temperature of 120°C, and about five times of the Co3O4 particles. The porous Co3O4 also exhibits certain selectivity and response/recovery time.

Test on the Reliability of Gastight Syringes as Transfer/Storage Media for Gaseous VOC Analysis: The Extent of VOC Sorption between the Inner Needle and a Glass Wall Surface

A gastight syringe (GTS) is commonly used as a medium for transfer or storage of gaseous standards (or samples) in the analysis of volatile organic compounds (VOCs). In this study, the sorptive loss patterns of 21 VOCs were examined, using GTS as the transfer medium. The results of the test were evaluated with respect to a number of key variables including concentration, sampling volume, and physicochemical properties (molecular weight and boiling point). The VOCs with relatively high volatility (Group 1: aldehyde, ketone, ester, alcohol, and aromatic hydrocarbons (n = 12)) showed low sorptive losses with a mean (±SD) of 2.56 ± 2.87%, regardless of differences in the aforementioned key variables (p-value by t-test before and after using GTS = mean 0.15 ± 0.13). Conversely, the sorptive losses of seven semi-VOCs (Group 2: carboxyl and cresol (n = 9)) were significantly high, ranging from 18.0 ± 4.10% (propionic acid) to 65.4 ± 10.9% (n-heptanonic acid). In addition, we also measured the sorptive losses on the syringe needle (mean sorptive loss of Group 2 = 5.94 ± 5.63%). A linear regression analysis showed that the sorptive losses for Group 2 increased as molecular weight (or boiling point) increased, exhibiting a highly significant correlation (R(2) value (0.804 ± 0.084) and mean p-value (0.002 ± 0.003).

Identification of Formaldehyde under Different Interfering Gas Conditions with Nanostructured Semiconductor Gas Sensors

Sensor array with pattern recognition method is often used for gas detection and classification. Processing time and accuracy have become matters of widespread concern in using data analysis with semiconductor gas sensor array for volatile organic compound gas mixture classification. In this paper, a sensor array consisting of four nanostructured semiconductor gas sensors was used to generate the response signal. Three main categories of gas mixtures, including single-component gas, binary-component gas mixtures, and four-component gas mixtures, are tested. To shorten the training time, extreme learning machine (ELM) is introduced to classify the category of gas mixtures and the concentration level (low, middle, and high) of formaldehyde in the gas mixtures. Our results demonstrate that, compared to traditional neural networks and support vector machines (SVM), ELM networks can achieve 204 and 817 times faster training speed. As for classification accuracy, ELM networks can achieve comparable results with SVM.

Trace gas VOC Detection Using Metal-organic Frameworks as Pre-concentrators and Semiconductor Gas Sensors

A method for the measurement of low gas concentrations is presented. The method was developed for detection of Volatile Organic Compounds (VOCs) in indoor air quality applications and tested with benzene, a carcinogenic VOC with no safe exposure limit value. Using metal organic framework materials, gas pre-concentration devices for benzene were prepared on ceramic micro hotplates and characterized. Two MOF materials (HKUST-1 and MIL-53) were tested. Both showed good pre-concentrator performance concerning adsorption and desorption of the target gas. A significant temporary decrease of the benzene concentration in the sampling phase and a corresponding increase in the desorption phase were detected using a mass spectrometer. The MOF pre-concentrators were also tested in combination with a semiconductor gas sensor resulting in a significant increase in the sensor response in the desorption phase.

MEMS gas preconcentrator filled with CNT foam for exhaled VOC gas detection

For disease monitoring and diagnostics using breath analysis, the gas preconcentrator is crucial for low-concentration exhaled volatile organic compound (VOC) gas analyses, and overcomes existing detection limits associated with the commercialized gas sensors. In this work, the microelectromechanical system (MEMS) gas preconcentrator chip was designed and fabricated for conducting low-power-operated breath analyses. It consists of a microheater and a gas chamber filled with a carbon nanotube (CNT) foam. The CNT foam, used as a gas adsorbing material, has several advantages including its large gas adsorption capacity due to its large surface-to-volume ratio, a low pressure drop due to its high porosity, and a rapid thermal desorption due to its high thermal conductivity. Using the developed MEMS gas preconcentrator chip, several basic performances were tested for clinically important VOC gases using a commercial gas chromatography-flame ionization detector (GC-FID). For gas preconcentrations over five minute interval, the preconcentration factors for methane and ethane gases were 8.05 and 7.72, respectively. These results suggest that the developed MEMS gas preconcentrator can be potentially utilized to analyze the low-concentration exhaled VOC gases for the purpose of noninvasive medical diagnoses.

Effect of continuous ozone injection on performance and biomass accumulation of biofilters treating gaseous toluene

Biofilters treating high-concentration gaseous volatile organic compounds (VOC) can be subject to bed clogging induced by excess biomass accumulation. In this study, O3 was continuously injected into biofilters to control biomass. Its effects on the performance of the biofilters and on biomass accumulation were investigated. Four identical biofilters designed to treat gaseous toluene were operated for 70days, and three of them were continuously injected with O3 at different concentrations (from 80 to 320mg/m(3)). The results showed that continuous O3 injection could effectively keep the bed pressure drop stable and had no adverse effect on toluene removal when O3 concentrations were 180-220mg/m(3). The maximum toluene elimination capacity of the four biofilters was 140g-toluene/m(3)/h, and the bed pressure drop of the biofilter fed with 180-220mg/m(3) O3 remained below 3 mmH2O/m throughout the operation period. The biomass accumulation rates of the three biofilters with O3 at 80-320mg/m(3) were lowered by 0.15-0.25g/L/day compared with the biofilter without O3. The decreases in biomass accumulation resulted in higher void fractions of the filter beds with O3 injection. Carbon balance analysis indicated that CO2 production had increased while biomass accumulation and leachate waste production decreased in response to O3 injection. Based on the experimental results, it was concluded here that continuous O3 injection can reduce increases in bed pressure effectively, preserve VOC removal capacity, and prevent production of extra leachate waste.