Mass transfer in amperometric gas sensors

The mass transfer in amperometric gas sensors intended for atmosphere monitoring is studied theoretically and experimentally. External and internal constituents of the diffusion resistance (DR) of the sensors are determined. The external constituent is defined by conditions of convective diffusion of air under analysis relative to the sensor. The internal constituents are defined by parameters of construction of sensor elements, and electrolyte film, as well as the structure of the indicator electrode, and solubility of the gas under analysis in the electrolyte. Testing a chlorine sensor shows that the DR of the internal constituents is independent of the conditions of convective diffusion of the environment under analysis near the sensor. The similarity criterion of sensors of different types is shown to be the relative share of DRs of individual constituents in the overall DR of the sensor. The results obtained can be used for the development and design of sensors with required range and resolution.     

Organic semiconductors in potentiometric gas sensors

Solid-state potentiometric sensors based on the chemical modulation of the work function of organic semiconductors are discussed. The theory of the chemical work function modulation is briefly reviewed. There are several sensor configurations, in which this transduction principle can be employed. First is the Kelvin probe, second is the chemically sensitive field-effect transistor in which the conventional metal gate of the silicon-based transistor has been replaced by an organic semiconductor. Chemical modulation of work function enters also into the operation of the third type of sensor discussed in this review, on “organic field-effect transistor”. It is shown that in reality such sensors are “field-modulated chemiresistors”, rather than potentiometric sensors.     

Porous metal oxides as gas sensors

Semiconducting metal oxides are frequently used as gas-sensing materials. Apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly desired for improved sensing performance. This article addresses the role of some key structural aspects in porous gas sensors, such as grain size and agglomeration, pore size or crack-free film morphology. New synthesis concepts, for example, the utilisation of rigid matrices for structure replication, allow to control these parameters independently, providing the opportunity to create self-diagnostic sensors with enhanced sensitivity and reproducible selectivity.     

Carbon nanotube gas and vapor sensors

Carbon nanotubes have aroused great interest since their discovery in 1991. Because of the vast potential of these materials, researchers from diverse disciplines have come together to further develop our understanding of the fundamental properties governing their electronic structure and susceptibility towards chemical reaction. Carbon nanotubes show extreme sensitivity towards changes in their local chemical environment that stems from the susceptibility of their electronic structure to interacting molecules. This chemical sensitivity has made them ideal candidates for incorporation into the design of chemical sensors. Towards this end, carbon nanotubes have made impressive strides in sensitivity and chemical selectivity to a diverse array of chemical species. Despite the lengthy list of accomplishments, several key challenges must be addressed before carbon nanotubes are capable of competing with state-of-the-art solid-state sensor materials. The development of carbon nanotube based sensors is still in its infancy, but continued progress may lead to their integration into commercially viable sensors of unrivalled sensitivity and vanishingly small dimensions.     

Electrochemical sensors for environmental gas analysis

The application of electrochemical sensors for measurement of concentration of pollutant gases in air in the part-per-billion (10⁹) range is reviewed. Performance-limiting factors, particularly the effects of extremes and of relatively rapid changes in ambient temperature and humidity, are noted. Variations in composition of the electrolyte in the meniscus at the electrode–gas interface and instability of the solid–liquid–gas contact line, causing important variations in current due to background electrode reactions, are deduced and suggested as the reason for the performance limitations. Suggestions are made for mitigation through instrument design.     

Ionic liquid high temperature gas sensors

An ionic liquid piezoelectric gas sensor was demonstrated for detection of polar and nonpolar organic vapors at high temperature with fast linear and reversible response.     

Phthalocyanine-based field-effect transistors as gas sensors

In this review molecular field-effect transistors are described and compared with their gate-modified inorganic counterparts. The different processes involved in gas sensing are summarized. The advantages of transistors on resistors are demonstrated. The sensitivity of molecular field-effect transistors to strong oxidizing species, for example ozone, is detailed and compared with their sensitivity to humidity and volatile organic compounds. Application to ozone monitoring in urban atmospheres is also described.     

Design of platinum hot wire gas sensors

The design characteristics of lower-power platinum gas sensors are studied. These sensors work by the detection of the positive ions produced during the catalytic oxidation of organic vapours on hot platinum wires and ribbons. These sensors are selective to long-chain hydrocarbons. A prototype sensor design is presented, which uses a small piece of platinum ribbon welded to a ceramic header with a wire mesh cathode to collect the positive ions. The ionic current is measured with a battery-run picoammeter circuit biased to float at −120 V. The ribbon is heated resistively with a standad power supply. At the operating temperature of 800 °C, the power consumption is about 2 W. The prototype is capable of detecting iso-octane vapour down to a concentration of 2 ppm. Platinum ribbon was used rather than wire as the wire had a greater tendency to melt due to thermal runaway in the resistive heating process. It was observed that the number of positive ions produced by the catalytic process decreased during long-term measurements. Scanning electron micrographs showed this to be due to facetting of the platinum surface. A few seconds' exposure at 1300 °C restored the surface and the ionic response. Thus periodic thermal cycling is necessary for the prototype sensor.     

Highly sensitive WO3 hollow-sphere gas sensors

In this paper, we describe how WO(3) hollow spheres have been synthesized in solution phase by the controlled hydrolysis of WCl(6) using novel carbon microspheres as the templates. All of the products were characterized by X-ray powder diffraction (XRD), scanning electronic microscopy (SEM), and transmission electron microscopy (TEM). The as-synthesized spheres had large diameters of about 400 nm and thin shells of about 30 nm composed of numerous small nanocrystals. Prompted by the porous structure and small crystal size of the shell wall, we constructed WO(3) hollow-sphere gas sensors and found that these sensors had good sensitivity to alcohol, acetone, CS(2), and other organic gases.     

Chlorine gas sensors using one-dimensional tellurium nanostructures

Tellurium nanotubes have been grown by physical vapor deposition under inert environment at atmospheric pressure as well as under vacuum conditions. Different techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical absorption have been utilized for characterization of grown structures. Films prepared using both types of tellurium nanotubes were characterized for sensitivity to oxidizing and reducing gases and it was found that the relative response to gases depends on the microstructure. Nanotubes prepared at atmospheric pressure (of argon) showed high sensitivity and better selectivity to chlorine gas. Impedance spectroscopy studies showed that the response to chlorine is mainly contributed by grain boundaries and is therefore enhanced for nanotubes prepared under argon atmosphere.     

Configuration and optimization of semiconductor gas sensors as gas chromatographic detectors

A tin oxide, gas-sensitive semiconductor sensor was configured as a gas chromatographic detector and its performance was optimized. Two sensor housings were compared but little difference was found in performance. The flow rate and temperature of the column and the internal heater voltage of the sensor affected both the sensitivity and peak shape. The temperature of the sensor surface was the most critical parameter. Optimal conditions for the gas chromatographic detection of a mixture of alkanes (C₁–C₅) and hydrogen were identified by using the simplex technique. The detection limit for hydrogen was improved by a factor of five to 20 ppb (v/v), illustrating the value of optimization and the excellent sensitivity of the detector. It is concluded that semiconductor gas sensors offer significant advantages as gas chromatographic detectors for the determination of reducing gases.     

Performance of fibrous bed bioreactor for treating odorous gas

A fibrous bed bioreactor was used for treatment of odorous volatile fatty acid (VFA). The effect of gaseous VFA (acetic, propionic, and butyric acids) mass loading on the bioreactor performance was investigated. The VFA degrading microbial culture was selected from activated sludge by the three VFAs using a shake-flask culture. The selected microorganisms were then immobilized in a biofilter using cotton fabric as packing material. In the biofiltration experiment, the inlet gas flow rates ranged from 1 to 4 L/min, the total VFA concentrations ranged from 0.10 to 0.43 g/m³, and the resulting total mass loadings of VFA studied ranged from 9.7 to 104.3 g/m³/h. At total mass loading of 104.3 g/m³/h, the VFA removal efficiency was 87.7%. Higher removal efficiencies (>90%) were achieved at mass loadings below 50.3 g/m³/h.     

Neutral cuprous complexes as ratiometric oxygen gas sensors

Four neutral mononuclear Cu(I) complexes, [Cu(pyin)(PPh(3))(2)] (1a), [Cu(pyin)(DPEphos)] (1b), [Cu(quin)(PPh(3))(2)] (2a) and [Cu(quin)(DPEphos)] (2b) (Hpyin = 2-(2-pyridyl)indole, Hquin = 2-(2-quinolyl)indole and DPEphos = bis(2-(diphenylphosphino)phenyl)ether) have been synthesized. X-Ray crystal structure analysis revealed that the central Cu(I) ion in all complexes is in a distorted tetrahedral coordination environment. All four complexes display the typical metal-to-ligand charge transfer (MLCT) absorption band at 371, 363, 413 and 402 nm, respectively. No emission was observed from any complexes in the solid state due to triplet-triplet annihilation. However, the complexes show unusual dual-emission originating from intraligand charge-transfer (ILCT) and MLCT transitions, when dispersed in a rigid matrix (e.g. PMMA) or in frozen CH(2)Cl(2). The oxidation potential of Cu(I)/Cu(II) in these neutral complexes, ～0.5 V (vs. Ag/AgCl), is lower than those of cationic Cu(I) complexes. Films containing 10 wt% of these complexes in PMMA shows ratiometric fluorescent oxygen gas sensing property with a response ratio of 0.3-3.2 and response time of 3-4 s. Complex 2b acts as a ratiometric oxygen gas sensor with good reversibility through energy and electron transfer mechanisms under the loss of a counteranion.     

A microfabricated microconcentrator for sensors and gas chromatography

The key component in trace analysis is the concentration step where the analytes are accumulated before the analysis. This paper presents the development of a micromachined microconcentrator that can be used to enhance the sensitivity of microsensors. Another application demonstrated here is a concentrator-injector for a gas chromatograph. The microconcentrators were fabricated on a 6-in. silicon substrate using standard photolithographic techniques (1 in.= 2.54 cm). The channels were lined with a resistive layer, through which an electric current could be passed to cause ohmic heating. The preconcentration was done on a thin-film polymeric layer deposited above the heater in the channel. Rapid heating of the resistive layer caused the "desorption pulse" to be injected into the sensor, or onto a GC column. Due to their small size, the microconcentrators could be fabricated 20 to 50 (depending upon the size) at a time on a 6-in. silicon wafer. This paper presents the development and characterization of the microconcentrator. It was found that the microconcentrator performed well as a concentrator, and as an injector for GC. A 14-fold enrichment factor was achieved. The microconcentrator exhibited long-term stability in response, with typical relative standard deviation of between 3 and 5%.     

Metal-oxide based nanocomposites as materials for gas sensors

Correlations between the composition, structure, and sensor properties of SnO₂-M^IIO (M^IIO = Fe₂O₃, MoO₃, V₂O₅) nanocomposites prepared by wet chemistry synthesis were elucidated. The elemental and phase compositions of the materials, distribution of components between the bulk and surface, particle size, and specific surface area were examined. Surface modification of semiconductor oxides allows controlling the type and density of surface acid centers and redox properties of materials. The result is an increase in the sensor selectivity.     

Thin films of palladium oxide for gas sensors

Thin films of PdO obtained by thermal oxidation of Pd films in air in the temperature range of 240–800°C were characterized using fast electron diffraction, transmission electron microscopy, and optical spectroscopy. The PdO films were found to be non-stoichiometric. With increasing oxidation temperature, the deviation of the PdO film composition from the stoichiometric component ratio becomes less pronounced. The resistivity response of PdO films to the presence of ozone in air was studied for the first time and good prospects for using this material for gas sensors are demonstrated.     

The detection of hydrogen using catalytic flammable gas sensors

The response to hydrogen (0.1 – 2.0% in air) of flammable gas sensors consisting of pellistors coated with supported palladium has been investigated. An optimum diffusion-limited response of 15 – 30 mV/(% H₂), depending on reactor geometry, was obtained with pellistors operated at ≈200 °C under atmospheric pressure. Both ambient and generated water reduced catalytic activity and response, particularly at low temperature. Activity was also greatly reduced by overheating of the pellistor. The influence of these factors on recommended operating procedures is considered.     

Detection of chlorinated methanes by tin oxide gas sensors

Tin oxide thin films prepared by thermal oxidation of Sn films were used for the detection of chlorinated methanes (CH2Cl2, CHCl3 and CCl4). This resulted in better chemical selectivity, sensitivity, response speed and detection limit than seen with previous detectors. The temperature dependence of the sensing of 1% CCl4 gas was studied and the best sensing behavior was observed at 300 degrees C. The films showed different chemical selectivity in both speed and direction of sensing response to each gas and were stable for more than 3 weeks under operating conditions. The films showed rapid gas sensing (<40 s to reach 90% of full response) and low detection limits (< 4 ppm CCl4). The role of oxygen in the detection of chlorinated methanes and in resistance changes without chlorinated methanes was also studied. The changes at the surface of the film after gas sensing were examined using scanning electron microscopy with energy-dispersive X-ray spectrometry.