Mimicking the sense of olfaction: A conducting-polymer-based electronic nose

Abstract— We describe herein the construction of a simple, low-power, broadly responsive vapor sensor. Carbon-black-organic-polymer composites have been shown to swell reversibly upon exposure to vapors. Thin films of carbon-black-organic-polymer composites have been deposited across two metallic leads, with swelling-induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements have been constructed, with each element containing a different organic polymer as the insulating phase. The differing gas-solid partition coefficients for the various polymers of the sensor array produce a pattern of resistance changes that can be used to classify vapors and vapor mixtures. This type of sensor array has been shown to resolve all organic vapors that have been analyzed, and can even resolve H²O from D²O.     

可视嗅觉传感器阵列研究及图像分析

针对目前电子鼻系统普遍存在的检测范围窄、受环境湿度影响较大等缺点,本文依据金属卟啉配合物与有机气体反应会产生颜色变化的原理,研制了一种新型的气体敏感膜,设计了可视嗅觉传感器阵列的实验系统.系统通过分析敏感膜与不同的气体反应所呈现的颜色变化识别气体,具有完全不受外界水蒸气影响和检测精度高等特点.用该传感器敏感膜分别对环己胺、乙腈和四氢呋喃进行了试验,试验结果表明该传感器阵列能够正确地识别这三种有机物.     

Modeling Carbon-Black/Polymer Composite Sensors

Conductive polymer composite sensors have shown great potential in identifying gaseous analytes. To more thoroughly understand the physical and chemical mechanisms of this type of sensor, a mathematical model was developed by combining two sub-models: a conductivity model and a thermodynamic model, which gives a relationship between the vapor concentration of analyte(s) and the change of the sensor signals. In this work, 64 chemiresistors representing eight different carbon concentrations (8-60 vol% carbon) were constructed by depositing thin films of a carbon-black/polyisobutylene composite onto concentric spiral platinum electrodes on a silicon chip. The responses of the sensors were measured in dry air and at various vapor pressures of toluene and trichloroethylene. Three parameters in the conductivity model were determined by fitting the experimental data. It was shown that by applying this model, the sensor responses can be adequately predicted for given vapor pressures; furthermore the analyte vapor concentrations can be estimated based on the sensor responses. This model will guide the improvement of the design and fabrication of conductive polymer composite sensors for detecting and identifying mixtures of organic vapors.     

Orthogonal gas sensor arrays with intelligent algorithms for early warning of electrical fires

The feasibility of utilizing chemical sensor arrays and multivariable analyses as the basis for an early-warning combustion alarm for electrical fires was evaluated. During the pre-combustion phase of electrical fires, electronic components will heat up, resulting in an out-gassing of chemical vapors, which generally will precede the formation of smoke, scorching and fire. A variety of materials (PVC, Teflon^®, Kapton^®, and silicone rubber) that are frequently used as wire insulation were subjected to electrically induced thermal excursions, thereby simulating an electrical failure and possible pre-combustion condition. The off-gassing vapors from the various coatings can serve as chemical signatures for a pending fire and were detected by an array of chemical sensors (e.g., an electronic nose). Principal component analyses and KNN identification algorithms applied to the sensor response patterns successfully identified the various vapor sources. A 20-sensor array including electrochemical sensors, quartz microbalance (QMB) sensors with different polymer coatings, and heated metal oxide sensors (MOXs) was evaluated and the optimal performance was obtained using the electrochemical and MOXs. The use of heterogeneous orthogonal sensors increased the information content of sensor array signals and a diminutive array can still identify fire materials and extent of damage. The small, lightweight, inexpensive and low power sensors used to detect vapors during pre-fire conditions were ideal for space or commercial aircraft applications.     

Estimating gas concentration using a microcantilever-based electronic nose

This paper investigates the determination of the concentration of a chemical vapor as a function of several nonspecific microcantilever array sensors. The nerve agent dimethyl methyl phosphonate (DMMP) in parts-per-billion concentrations in binary and ternary mixtures is able to be resolved when present in a mixture containing parts-per-million concentrations of water and ethanol. The goal is to not only detect the presence of DMMP, but additionally to map the nonspecific output of the sensor array onto a concentration scale. We investigate both linear and nonlinear approaches — the linear approach uses a separate least-squares model for each component, and a nonlinear approach which estimates the component concentrations in parallel. Application of both models to experimental data indicate that both models are able to produce bounded estimates of concentration, but that the outlier performance favors the linear model. The linear model is better suited to portable handheld analyzer, where processing and memory resources are constrained.     

Dew point measurement for organic vapor mixture using a quartz crystal sensor

A dew point measurement device for organic vapor mixtures using a quartz crystal sensor was proposed, and its performance was examined with acetone–methanol mixtures. The device has a sensor module to accommodate the quartz crystal sensor and a cooling system. The variation in resonant frequency of the sensor indicates the condensation of sample vapor, which tells the beginning moment of condensation, i.e., dew point. The measurement results with the proposed device were compared with the predicted dew points from the UNIQUAC equation and other experimental outcome to examine the performance of the device. Though some measurement error was yielded, it is proved that the proposed device is useful for measuring the dew points of organic vapors at different pressures.     

Microfabricated preconcentrator-focuser for a microscale gas chromatograph

The design, fabrication, and testing of a preconcentrator-focuser (PCF), consisting of a thick micromachined Si heater packed with a small quantity of a granular adsorbent material are described. The PCF is developed to capture and concentrate vapors for subsequent focused thermal desorption and analysis in a micro gas chromatograph. The microheater contains an array of high-aspect-ratio, etched-Si heating elements, 520 /spl mu/m (h)/spl times/50 /spl mu/m (w)/spl times/3000 /spl mu/m (l), bounded by an annulus of Si and thermally isolated from the remaining substrate by an air gap. This structure is sandwiched between Pyrex glass plates with inlet/outlet ports that accept capillary tubes for sample flow and is sealed by anodic bonding (bottom) and rapidly annealed glass/metal/Si solder bonding (top). The large microheater surface area allows for high adsorption capacity and efficient, uniform thermal desorption of vapors captured on the adsorbent within the structure. The adsorbent consists of roughly spherical granules, /spl sim/200 /spl mu/m in diameter, of a high-surface-area, graphitized carbon. Key design considerations, fabrication technologies, and results of performance tests are presented with an emphasis on the thermal desorption characteristics of several representative volatile organic compounds as a function of volumetric flow rates and heating rates. Preconcentration factors as high as 5600 and desorbed peak widths as narrow as 0.8 s are achieved from 0.25-L samples of benzene at modest heating rates. The effects of operating variables on sensitivity, chromatographic resolution, and detection limits are assessed. Testing of this PCF with a micromachined separation column and integrated sensor array is discussed briefly.     

Composites of carboxylate-capped TiO2 nanoparticles and carbon black as chemiresistive vapor sensors

Titanium (IV) dioxide (TiO₂) nanoparticles (NPs) with a 1-5 nm diameter were synthesized by a sol-gel method, functionalized with carboxylate ligands, and combined with carbon black (CB) to produce chemiresistive chemical vapor sensor films. The TiO₂ acted as an inorganic support phase for the swellable, organic capping groups of the NPs, and the CB imparted electrical conductivity to the film. Such sensor composite films exhibited a reproducible, reversible change in relative differential resistance upon exposure to a series of organic test vapors. The response of such chemiresistive composites was comparable to, but generally somewhat smaller than, that of thiol-capped Au NPs. For a given analyte, the resistance response and signal-to-noise ratio of the capped TiO₂-NP/CB composites varied with the identity of the capping ligand. Hence, an array of TiO₂-NP/CB composites, with each film having a compositionally different carboxylate capping ligand, provided good vapor discrimination and quantification when exposed to a series of organic vapors. Principal components analysis of the relative differential resistance response of the sensor array revealed a clear clustering of the response for each analyte tested. This approach expands the options for composite-based chemiresistive vapor sensing, from use of organic monomeric or polymeric sorbent phases, to use of electrically insulating capped inorganic NPs as the nonconductive phase of chemiresistive composite vapor sensors.     

Active gas/odor sensing system using automatically controlled gas blender and numerical optimization technique

As measurement of a vapor mixture composition is a difficult technique, no method using a sensing system has yet been established in spite of great effort by many researchers. In this paper, the authors propose a new gas/odor sensing system using a gas blender and a nonlinear numerical optimization algorithm by which the concentration of each component in an unknown vapor can be quantified. The component vapors are internally blended and the mixture ratio is modified by the system so that the sensor array output pattern of the blended vapor can be made equal to that of the unknown one. After several iterations, convergence is obtained and the vapor concentration of each component is determined from the mixture composition of the blended vapor. Although the conventional system is passive, this system is considered as an active one as it performs exploratory behavior prior to recognition. Here, gasoline vapor concentration is measured under the condition that one or two interference vapors exist together. Gasoline vapor has been adopted as an example of odors in the passenger compartment of a car, since it sometimes smells unpleasant. The measurement is essential for designing a car in order to keep it comfortable for passengers. The sensors used here are three semiconductor gas sensors and two electrochemical sensors, which are chosen in order to obtain high sensitivity to gasoline. The nonlinear numerical optimization techniques used are the simplex method and the gradient descent method and these two methods are compared here. It is found that the quantification error is within ten ppm for two- or three-component vapors.     

Effect of trace residual ionic impurities on the response of chemiresistor sensors with dithiol-linked monolayer-protected gold (nano)clusters as sensing interfaces

Column liquid–solid chromatography was used to remove residual impurities of isolated n-octanethiol (C₈H₁₇SH) monolayer-protected gold nano-clusters (MPCs) which were synthesized by a Brust two-phase method. Three-dimensional (3D) cross-linked MPC films were prepared directly on interdigitated electrodes to form chemiresistor sensors through the exchange reactions of the chromatographically purified MPCs with 1,6-hexanedithiol (HDT) or 1,4-benzenedimethanethiol (BDT). Ionic current induced by trace residual ionic impurities in MPCs was qualitatively detected by comparing the resistance responses of the sensors interfaced with the chromatographically purified and unpurified MPC films by employing volatile organic compounds (VOCs) and water vapor as probes, respectively. The existence of the ionic current significantly decreases the sensor sensitivities to VOCs. As for water vapor with high permittivity, the ionic current totally distorted the resistance responses from positive to negative with increasing humidity. Capacitance was also measured to characterize the permittivity change. The effect of ionic current on capacitance was not obvious. The humidity effects on the sensor responses to VOCs were also investigated. Fewer effects were observed on the higher hydrophobic compounds. A ternary sensor array was constructed with C₈Au MPCs, HDT and BDT cross-linked MPC films as sensing interfaces. The response pattern showed that the sensor array could discriminate VOCs with different functional groups. The as-prepared sensor showed the same sensitivities as the acoustic wave sensors.     

Gas sensing properties of a composite composed of electrospun poly(methyl methacrylate) nanofibers and in situ polymerized polyaniline

Poly(methyl methacrylate) (PMMA) nanofibers with different diameters were fabricated by electrospinning and their composites with polyaniline (PANI) were formed by virtue of in situ solution polymerization. The coaxial composite nanofibers so prepared were then transferred to the surface of a gold interdigitated electrode to construct a gas sensor. The structure and morphology of the PANI/PMMA composite fibers were characterized by UV–vis spectroscopy and scanning electron microscopy, which indicated that the coaxial nanofibres of PANI emeraldine salt and PMMA were successfully prepared. The electrical responses of the gas sensor based on the composite nanofibres towards triethylamine (TEA) vapors were investigated at room temperature. It was revealed that the sensor showed a sensing magnitude as high as 77 towards TEA vapor of 500 ppm. In addition, the responses were linear, reversible and reproducible towards TEA vapors ranging from 20 to 500 ppm. The diameters of the electrospun PMMA fibers had an effect on the sensing magnitude of the gas sensor, which is proposed to relate to the difference in the surface-to-volume ratio of the fibers. Furthermore, it was found that the concentration of doping acids only led to changes in resistance of the sensor, but could not affect its sensing characteristics. In contrast, the nature of the doping acids was determinative for the sensing magnitude of the sensor. The gas sensor with toluene sulfonic acid as the doping acid exhibited the highest sensing magnitude, which is explained by taking into account of the sensing mechanism and the interactions of doping acids with TEA vapor.     

准最佳三进阵列偶

最佳二进阵列在通信领域中有着广泛的应用,但由于其存在体积的限制,制约了它的应用范围,针对这种现象,提出了一种新的最佳离散信号,即准最佳三进阵列偶,给出了准最佳三阵列偶的定义和变换性质,研究了它的组合允许条件,分析了它与准最佳二进阵列偶、准最佳屏蔽二进阵列偶以及最佳三进阵列偶之间的关系,并用计算机搜索出部分小体积的准最佳三进阵列偶。实验结果显示准最佳三进阵列偶的存在范围非常广泛,为通信工程的应用提供了更多的选择。     

基于微气体传感器阵列和神经网络的VOCs的辨别

六个由贵金属Au,Cu,Pt做添加剂的SnO2气体传感器构成了微气体传感器阵列.首先研究了这六只传感器对挥发性有机化合物(VOCs)敏感特性,本文中的VOCs 指VOCsmixture和甲醛(HCHO)气体,其中VOCsmixture是10 ppm甲苯、1 ppm丙酮、5 ppm α-派烯和10 ppm乙醇的混合气.然后采用BP神经网络对所获得的传感器信号进行了分析、识别.结果显示微气体传感器阵列与BP神经网络相结合不仅能有效地识别低浓度的单成分VOCsmixture和甲醛气体,而且也能有效地识别两元气体中的VOCsmixture和甲醛气体.     

A high-shear, low Reynolds number microfluidic rheometer

We present a microfluidic rheometer that uses in situ pressure sensors to measure the viscosity of liquids at low Reynolds number. Viscosity is measured in a long, straight channel using a PDMS-based microfluidic device that consists of a channel layer and a sensing membrane integrated with an array of piezoresistive pressure sensors via plasma surface treatment. The micro-pressure sensor is fabricated using conductive particles/PDMS composites. The sensing membrane maps pressure differences at various locations within the channel in order to measure the fluid shear stress in situ at a prescribed shear rate to estimate the fluid viscosity. We find that the device is capable to measure the viscosity of both Newtonian and non-Newtonian fluids for shear rates up to 10⁴ s^?1 while keeping the Reynolds number well below 1.     

Thermal measurement and analysis of micro hotplate array using thermography

A micro hotplate (MHP) array was made by incorporating several MHPs into a single chip as an integrated sensor array. The sensing film must operate at a specific high temperature condition (300–400 °C) with a low power consumption. In addition, each hotplate should be independent and all the hotplates should be uniform thermally. Therefore, the thermal characteristics of the individual hotplate must be measured. In this work, the temperature distribution in a 2×2 MHP array was measured using infrared thermography. The temperature distribution of the small area and other thermal characteristics were obtained, providing reliable experimental information for the design of the MHP array.     

Integrated micro-heat-pipe fabrication technology

This paper presents the design and fabrication of an integrated micro-heat-pipe system consisting of a heater, an array of heat pipes, temperature and capacitive sensors. Taking advantage of the large difference between the dielectric constants of liquid and vapor, the integrated capacitor can be used for void-fraction measurements in two-phase flows. Both CMOS-compatible and glass-based fabrication technologies are reported. In the CMOS-compatible technology, the heat pipes are capped by a thin nitride layer utilizing wafer bonding and etch back technique. In the glass-based technology, the heat pipes are covered by a glass substrate using die-by-die anodic bonding to allow visualization of the two-phase flow patterns. This approach also results in a significant reduction of the parasitic capacitance, thus enhancing the sensitivity of the capacitance sensor. A few particular problems related to this technology are discussed and proper solutions are proposed.     

用于室内有毒气体快速检测的便携式CC/SAW电子鼻

开发了一种用于室内空气质量快速检测的便携式电子鼻气体分析仪.该仪器利用毛细管分离柱(CC)将混合气体选择性分离,实现对气体的分类识别,然后借助声表面波(SAW)传感器频率响应测量每种组分对应的浓度,实现对气体的量化.实验针对17种有毒有害气体进行测试,结果表明该系统能对低浓度复杂混合气体进行快速检测,且具有较高的灵敏度,良好的选择性和重复性.这为不同应用场合下的痕量气体检测提供了一种可行的技术.     

On-chip Fabry-Pérot interferometric sensors for micro-gas chromatography detection

We fabricated and characterized on-chip Fabry-Pérot (FP) vapor sensors for the development of on-column micro-gas chromatography (μGC) detectors. The FP sensors were made by coating a thin layer of polymer on a silicon wafer. The air-polymer and polymer-silicon interfaces form an FP cavity, whose resonance wavelengths change in response to the vapor absorption/desorption, thus allowing for rapid detection and quantification of vapors. For proof-of-concept, two polymers (PDMS and SU-8) were used independently and placed in an array in a microfluidic channel, and showed different sensitivities for different vapors. A sub-nano-gram detection limit and sub-second response time were achieved, representing orders of magnitude improvement over those previously reported. This on-chip design will enable the unprecedented integration of optical vapor sensors with μGC systems.     

Multiple conducting polymer microwire sensors

In this work, conducting polymer microwires of three commonly used conducting polymers were fabricated simultaneously on a common substrate using an intermediate-layer lithography (ILL) method. The three conducting polymers under consideration were polypyrrole (PPy), sulphonated polyaniline (SPANI) and poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulphonate) (PEDOT-PSS). The fabricated microwires were implemented as sensing elements in detecting humidity and two organic vapors (i.e., methanol and acetone). The sensitivity of a single PPy microwire was compared with a rectangular PPy film after both were exposed to 45–85% humidity. The microwire sensor, due to its higher surface-to-volume ratio, was found to be more sensitive than the film sensor at low levels of humidity (between 45 and 58%). Beyond 58% humidity, the responses of the film and microwire sensors were similar. Three different sets of conducting polymer microwires (of PPy, SPANI and PEDOT) were then fabricated and employed as sensors to detect methanol, acetone and their mixtures. These microwires exhibited wave-like responses when they were exposed to these targets. The PPy and PEDOT microwires showed higher sensitivities in detecting methanol and acetone, respectively. The SPANI microwires exhibited similar responses in detecting methanol and acetone. The results demonstrate that microwire sensors were more effective than film sensors in detecting little quantities of target molecules. A sensor platform which integrates multiple microwire detectors is promising to detect multiple targets, and it also provides more information in detecting and distinguishing targets.     

Mass-Sensitive Microfabricated Chemical Preconcentrator

This paper describes a mass-sensitive microfabricated preconcentrator for use in chemical detection microsystems. The device combines mass sensing and preconcentration to create a smart preconcentrator (SPC) that determines when it has collected sufficient analyte for analysis by a downstream chemical microsystem. The SPC is constructed from a Lorentz-force-actuated pivot-plate resonator with an integrated heater. Subsequent to microfabrication, the SPC is coated with an adsorbent for collection of chemical analytes. The frequency of operation varies inversely with the mass of collected analyte. Such shifts can be measured by a back-EMF in the SPC's drive/transducer line. By using a calibrated vapor system, the limit of detection of the SPC was determined to be less than 50 ppb for dimethyl-methyl-phosphonate (DMMP) (actual limits of detection are omitted due to export control limitations). At 1 ppm of DMMP, 1-s collection was sufficient to trigger analysis in a downstream microsystem; other micropreconcentrators would require an arbitrary collection time, normally set at 1 min or longer. This paper describes the theory of operation, design, fabrication, coating, vapor system testing, and integration of the SPC into microanalytical systems. The theory of operation, which is applicable to other torsional oscillators, is used to predict a shear modulus of silicon (100) of $G = 57.0 hbox{GPa} pm 2.2 hbox{GPa}$. $hfill$ [2007-0305]