Novel e-nose for the discrimination of volatile organic biomarkers with an array of carbon nanotubes (CNT) conductive polymer nanocomposites (CPC) sensors

We report the original design of a new type of electronic nose (e-nose) consisting of only five sensors made of hierarchically structured conductive polymer nanocomposites (CPC). Each sensor benefits from both the exceptional electrical properties of carbon nanotubes (CNT) used to build the conductive architecture and the spray layer by layer (sLbL) assembly technique, which provides the transducers with a highly specific 3D surface structure. Excellent sensitivity and selectivity were obtained by optimizing the amount of CNT with five different polymer matrices: poly(caprolactone) (PCL), poly(lactic acid) (PLA), poly(carbonate) (PC), poly(methyl methacrylate) (PMMA) and a biobased polyester (BPR). The ability of the resulting e-nose to detect nine organic solvent vapours (isopropanol, tetrahydrofuran, dichloromethane, n-heptane, cyclohexane, methanol, ethanol, water and toluene), as well as biomarkers for lung cancer detection in breath analysis, has been demonstrated. Principal component analysis (PCA) proved to be an excellent pattern recognition tool to separate vapour clusters.     

Optical parameters of calix[4]arene films and their response to volatile organic vapors

The Langmuir-Blodgett (LB) technique was employed to produce thin LB films using an amphiphilic calix-4-resorcinarene onto different substrates such as quartz, gold coated glass and quartz crystals. The characteristics of the calix LB films are assessed by UV-visible, quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) measurements. UV-vis and QCM measurements indicated that this material deposited very well onto the solid substrates with a transfer ratio of >0.95. Using SPR data, the thickness and refractive index of this LB film are determined to be 1.14 nm/deposited layer and 1.6 respectively. The sensing application of calixarene LB films towards volatile organic vapors such as chloroform, benzene, toluene and ethanol vapors is studied by the SPR technique. The response of this LB film to saturated chloroform vapor is much larger than for the other vapors. The response is fast and fully recoverable. It can be proposed that this sensing material deposited onto gold coated glass substrates has a good sensitivity and selectivity for chloroform vapor. This material may also find potential applications in the development of room temperature organic vapor sensing devices.     

Ionic liquids used as QCM coating materials for the detection of alcohols

Six imidazolium-based ionic liquids (ILs) were synthesized and employed as sensing materials coated on quartz crystal microbalance for the detection of organic vapors. Acetone, ethanol, dichloromethane, benzene, toluene and hexane were selected as representatives for common environmental pollutants, and good linear responses from 0 to 100% of concentrations were observed. The halogen-anion-containing imidazolium ILs-coated sensors showed fast response, excellent reversibility, and considerable sensitivity and selectivity towards alcohols, and the selective factors were up to 30 times for ethanol versus other VOCs. The existence of water vapor reduced the frequency response of the sensor, but a good linear relationship remained.     

Vapor sensing properties of thermoplastic polyurethane multifilament covered with carbon nanotube networks

The volatile organic compound (VOC) vapor sensing properties of a novel kind of thermoplastic polyurethane multifilament - carbon nanotubes (TPU-CNTs) composites is studied. And the sensing is based on changes in the electrical resistance of the composites due to vapor contact. The composites were readily obtained by adhering CNTs on the surface layer of TPU by means of simply immersing pure TPU multifilament into CNT dispersion. The uniformly formed nanotube networks on the outer layer of composite multifilament are favorable for providing efficient conductive pathways. The resulting TPU-CNTs composites show good reproducibility and fast response (within seconds) of electrical resistance change in cyclic exposure to diluted VOC and pure dry air. The vapor sensing behaviors of the composites are related to CNT content, vapor concentration, and polar solubility parameters of the target vapors. A relatively low vapor concentration of 0.5% is detectable, and a maximum relative resistance change of 900% is obtained for the composite with 0.8 wt.% CNT loading when sensing 7.0% chloroform. It is proposed that both the disconnection of CNT networks caused by swelling effects of the TPU matrix and the adsorption of VOC molecules on the CNTs are responsible for the vapor sensing behavior of TPU-CNTs composite, while the former effect plays the major role.     

A distributed Bragg reflector porous silicon layer for optical interferometric sensing of organic vapor

In this study, we successfully demonstrated the rapid, sensitive, and reversible sensing of organic vapor using a distributed Bragg reflector (DBR) porous silicon (PS) layer. We fabricated the DBR PS layer on a p⁺-type silicon substrate and investigated its reflectance spectra before, during, and after exposure to the different concentrations of various organic vapors. When the DBR PS layer sample was exposed to methanol, acetone, ethanol, and isopropanol vapors, the maximum reflectance peak promptly shifted toward longer wavelengths by about 4.5, 23.2, 26.0, and 38.2 nm, respectively. We determined that the red-shift in the reflectance spectrum could be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the DBR PS layer. The DBR PS layer showed excellent sensing ability under the different concentrations and types of organic vapors. In addition, a slight hysteresis of the red-shift was observed during repeated exposure to organic vapors at different concentrations. After removing the organic vapors, the reflectance spectrum promptly returned to its original state.     

Vapour sensing with conductive polymer nanocomposites (CPC): Polycarbonate-carbon nanotubes transducers with hierarchical structure processed by spray layer by layer

The development of conductive polymer nanocomposite (CPC) sensors for volatile organic compounds (VOC) detection has been carried out using a spray layer by layer (LbL) process. This technique was successfully used to hierarchically structure polycarbonate-multiwall carbon nanotubes (PC-CNT) solutions into a double percolated architecture as attested by atomic force microscopy (AFM) and optical microscopy (OM). PC-CNT vapour sensing behaviour was investigated as a function of CNT content, films thickness, vapour flow and vapours solubility parameter. The response ranking A_r(toluene) > A_r(methanol) > A_r(water) of PC-CNT was found to be coherent with κ₁₂ Flory–Huggins interaction parameters provided that signals are normalised by analyte molecules number. Signals shape was interpreted to the light of Langmuir–Henry–Clustering (LHC) model and found to be proportional to vapour content.     

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.     

Effect of soft-hard segment structure on vapor responsiveness of polyurethane conducting composite thin films loaded with multi-walled carbon nanotubes

A series of multi-walled carbon nanotubes/polyurethane (MWNTs/PU) composite conducting dispersoids were prepared via an in situ coupling reaction among linear hydroxyl-terminated polymer diols, 1,6-hexamethylene diisocyanate (HDI) and various chain extenders. The composite conducting thin films were formed by spin-coating and depositing the dispersoids onto comb-like electrode substrates. The resulting structure and the dispersion quality of MWNTs in the dispersoids were examined by means of FTIR, XRD, TEM, SEM and UV-vis analyses. The response of the as-prepared films toward some volatile organic solvent vapors such as benzene, anhydrous ether, acetone and chloroform was evaluated. The experimental results indicated that the composite conducting films constructed by hydroxyl-terminated poly(butadiene-co-acrylonitrile), trimethylolpropane, and MWNTs-OH bear better vapor responsiveness. The dispersion behavior of MWNTs in the dispersoids, types of MWNTs and soft-hard segmental compositions are believed to be closely related with the sensing properties of the films. In particular, the chemical linkage of MWNT-OH with HDI in the PU matrix is expected to improve the dispersivity and further to enhance the sensing properties of the composite sensors. The vapor sensing properties well reveal that these materials have a possibility as a candidate of volatile organic solvent vapor sensors.     

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.     

Improved reversibility in aged porous silicon NO2 sensors

Porous silicon (PS) conductometric gas sensors can exhibit large sensitivity to gases, due to the large surface versus volume ratio of porous silicon. A possible application is the detection of traces of nitrogen dioxide (NO₂), an air pollutant. ΔG/G signals in excess of 10 in the presence of concentrations as low as 50 ppb in dry air can be demonstrated. Unfortunately, such high sensitivity to NO₂ is achieved, in fresh samples, with poor reversibility. Another problem is the interference of water vapor, which also affects the porous silicon conductivity. However, we show that reversibility is complete in aged samples, and sensitivity to water vapor is lowered. Although in aged samples large ΔG/G signals are harder to achieve, we show that concentration levels of NO₂ at few tens of ppb are still detectable.     

Effects of morphologies on acetone-sensing properties of tungsten trioxide nanocrystals

In this work, triclinic WO₃ nanoplates and WO₃ nanoparticles were comparatively investigated as sensing materials to detect acetone vapors. Single-crystalline WO₃ nanoplates with large side-to-thickness ratios were synthesized via a topochemical conversion from tungstate-based inorganic-organic hybrid nanobelts, and the WO₃ nanoparticles were obtained by calcining commercial H₂WO₄ powders at 550 °C. The acetone-sensing properties were evaluated by measuring the change in electrical resistance of the WO₃ sensors before and after exposure to acetone vapors with various concentrations. The WO₃ nanoplate sensors showed a high and stable sensitive response to acetone vapors with a concentration range of 2-1000 ppm, and the sensitivity was up to 42 for 1000 ppm of acetone vapor operating at 300 °C. The response and recovery times were as short as 3-10 s and 12-13 s, respectively, for the WO₃ nanoplate sensors when operating at 300 °C. The acetone-sensing performance of the WO₃ nanoplate sensors was more excellent than that of the WO₃ nanoparticle sensors under a similar operating condition. The enhancement of the WO₃ nanoplate sensors in the acetone-sensing property was attributed to the poriferous textures, single-crystalline microstructures and high surface areas of the aggregates consisting of WO₃ nanoplates, which were more favorable in rapid and efficient diffusion of acetone vapors than the WO₃ nanoparticles.     

Improvement of gas sensing performance of carbon black/waterborne polyurethane composites: Effect of crosslinking treatment

When carbon black (CB) filled waterborne polyurethane (WPU) composites are exposed to organic solvent vapors, electrical resistance of the materials increases rapidly. They can thus serve as gas sensors. To improve the composites’ performance for practical applications, crosslinking agent was added to the composite latexes, forming intra-molecular crosslinked networks among the matrix polymer of the composites. The method greatly increased the filler/matrix interfacial interaction and reduced the mobility of CB particles. In the composites that had absorbed solvent vapors, reconstruction of conduction paths through re-aggregation of the disconnected filler particulates became difficult. As a result, the unwanted negative vapor coefficient (NVC) effect was significantly weakened, while the gas sensitivity and the performance reproducibility were enhanced as well.     

炭黑填充聚乙烯导电复合材料气敏性能研究

采用溶液共混法成功制备出炭黑/聚乙烯导电气敏复合材料,研究了在苯、甲苯、二甲苯三种有机溶剂蒸汽中的电阻变化.实验结果表明,该复合材料在二甲苯蒸汽中电阻变化最大,在苯蒸汽中电阻变化最小.进一步研究了其在100 ppm至800 ppm(ppm=10-6)二甲苯蒸汽中的气敏响应性,实验结果表明:复合材料的灵敏度从0.04增大至0.11.     

Microwave remote sensing of plant water stress

The sensitivity of microwave (MW) emission to physical conditions of vegetation has been assessed by means of ground-based microwave and infrared radiometers. Measurements on corn and wheat have shown an inverse correlation between the normalized brightness temperature (T_N) from the Ka band (36 GHz) and the atmospheric water vapor pressure (VP) at the top of vegetation. From this observation, we show that a crop water stress index can be calculated by means of down-looking MW sensors, provided air temperature is known. A polarization index (PI) dependent only on microwave measurements was shown to be related to crop water stress.     

柔性高灵敏单壁碳纳米管气体传感器研究

在柔性聚对二甲苯C基底上制作了基于单壁碳纳米管的小型化、高灵敏、反应快速的气体传感器。使用介质电泳集成碳管束,并利用单链脱氧核糖核酸修饰增强器件灵敏度。当传感器暴露在甲醇蒸气中时,会出现明显可重复的反应,它可以检测含量低至4.3×10-6的甲醇,并且在相当宽的体积分数范围有清晰的分辨能力。对于4.3×10-6的含量,未修饰的传感器对应电阻变化率是4.8%;经过脱氧核糖核酸修饰,电阻变化率增加到了12.3%。此外,该传感器还显示了很快的响应速度和很好的测试复验性。研究表明:这种柔性气体传感器在未来环境监测应用中有很好的前景。     

Vapor recognition with an integrated array of polymer-coated flexural plate wave sensors

Preliminary testing of a prototype instrument employing an integrated array of six polymer-coated flexural plate wave (FPW) sensors and an adsorbent preconcentrator is described. Responses to thermally desorbed samples of individual organic solvent vapors and binary and ternary vapor mixtures are linear with concentration, and mixture responses are equivalent to the sums of the responses of the component vapors, which co-elute from the preconcentrator in most cases. Limits of detection as low as 0.3 ppm are achieved from a 60-s (34 cm³) air sample and peak widths at half-maximum range from 1 to 4 s. Tests at different flow rates suggest that the kinetics of vapor sorption in the sensor coating films may limit responses at higher flow rates, however, low data acquisition rates may also be contributory. Assessments of array performance using independent test data and Monte Carlo simulations with pattern recognition indicate that individual vapors and certain binary and ternary mixtures can be recognized/discriminated with very low error. More complex mixtures, and those containing homologous vapors, are problematic. This is the first report demonstrating multi-vapor analysis with an integrated FPW sensor array.     

Non-solvent induced phase separation as a method for making high-performance chemiresistors based on conductive polymer nanocomposites

The fabrication of novel porous conductive composite vapor sensors characterized by different porosities and specific surface areas is described in this study. These samples were obtained by the dry-cast non-solvent induced phase separation (NIPS) method. Porous composite structures have been studied by the SEM, BET and water evaporation methods. Testing different concentrations of several organic vapors, the porous sensors showed improved sensitivities and response times as compared to their dense counterpart. Improved characteristics of the sensor response were correlated to better sorption and diffusion properties of sensing film due to increased porosity and specific surface area obtained by this method of film fabrication. A competition theory was proposed that describes the optimum porosity and thickness of sensing films in which the highest sensitivities were observed.     

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.     

Gas sensing properties of platinum derivatives of single-walled carbon nanotubes: A DFT analysis

The limitations of intrinsic carbon nanotube (CNT) based devices to examine toxic gases motivate us to investigate novel sensors which can possibly overcome sensitivity problems. Pt–CNT assemblies (with Pt deposited externally as well as internally Pt-doped ones) interacting with NO₂ and NH₃ are studied and compared with unmodified CNTs. DFT calculations show that Pt can enhance adsorption and charge transfer processes to a very large degree. Incoming gas molecules cause changes in the electronic structure and charge distribution of the Pt-substituted CNTs that are both larger and more far-reaching than in their unmodified counterparts. Their relatively high stability is unaffected by the complexation with NO₂ and NH₃. CNTs with defective surface were also investigated. The sensing performance of Pt-doped CNT is found to be superior to defected CNTs.     

Room temperature ammonia sensors based on zinc oxide and functionalized graphite and multi-walled carbon nanotubes

In this work, different techniques are proposed to realize ammonia (NH₃) sensors working at room temperature and a preliminary electrical characterization under water vapor and in NH₃ atmospheres is presented. Three families of ceramic planar sensors based on a zinc oxide (ZnO) layer overlapped by screen-printed Pd-doped carboxyl groups functionalized multi-walled carbon nanotubes (Pd-COOH-MWCNTs) or by blocks of vertically aligned MWCNTs or by graphite as such and functionalized with fluorinated or nitrogenous functional groups were studied.These sensors were almost insensitive to humidity, while all of them gave a good response in NH₃ atmosphere, starting from about 45 ppm in the case of zinc oxide with fluorinated or nitrogenous MWCNTs and graphite or 50 ppm for Pd-COOH-MWCNTs sensors. These results are not actually as good as those reported in the literature, but this preliminary work proposes simpler and cheaper processes to realize NH₃ sensor for room temperature applications.