Gas sensing properties of a clad modified fiber optic sensor with Ce, Li and Al doped nanocrystalline zinc oxides

Spectral characteristics of a clad modified fiber optic gas sensor are studied for various concentrations (0-500 ppm) of ammonia, methanol and ethanol at room temperature. Cerium, aluminum and lithium doped (6 at.%) nanocrystalline zinc oxides are replaced with a clad and used as gas sensing materials. The study shows that the spectral intensity increases linearly with concentration for ammonia whereas it decreases for methanol and ethanol. The Ce doped ZnO exhibits higher gas sensitivity compared to Al and Li doped zinc oxides. The time response of the sensor is presented for a Ce doped ZnO with ammonia gas. A model is proposed for understanding the spectral intensity variations.     

Detection of ethanol and methanol vapors using polymer-coated piezoresistive Si bridge

A novel alcohol vapor sensor based on the swelling effect of polymer film is proposed for detection of alcohol such as methanol and ethanol vapor, which is composed of a Si bridge embedded with piezoresistive Wheatstone bridge and an epoxy acrylate thin polymer layer-coated Si bridge. When the polymer layer absorbs alcohol vapor molecules from ambient air, the swelling of the thin polymer layer leads to bending of the Si bridge which causes the piezoresistive Wheatstone bridge to produce an output voltage. A theoretical model of the interaction between Si bridge and polymer layer is presented based on elastic mechanics theory. An equivalent transverse gas load is presented. The output expression of sensor is obtained. The model of the sensor is verified by the experimental results. The experimental results show that the alcohol vapor sensor has good linearity, selectivity and reproducibility. The proposed alcohol vapor sensor has simply principle, mature technology, low cost, and can be integrated with signal processing circuit using the MEMS processes.     

Sensitivity to alcohols of a planar waveguide ring resonator fabricated by a sol–gel method

A planar waveguide ring resonator was fabricated by organic–inorganic hybrid sol–gel materials; its sensitivity to ethanol vapor was experimentally investigated. It was found that dips in the transmission spectrum of the device shifted to longer wavelengths with increasing the ethanol concentration, and its sensitivity showed a linear relation with the ethanol concentration, showing a coefficient of 1.13 pm/ppm. In addition, the transmission loss of the ring resonator decreased with increasing the ethanol concentration. The measured characteristics suggest that the device may be considered as one of the candidates of alcohol vapor sensors.     

Analysis of volatile alcohols in apple juices by an electrochemical biosensor measuring in the headspace above the liquid

An electrochemical biosensor was optimised for the analysis of volatile alcohols directly from the gas phase without prior absorption or pre-concentration. The sensor is based on the alcohol oxidase (Pichia pastoris) catalyzed conversion of ethanol and the amperometric detection of the generated hydrogen peroxide. Key part of the three-electrode set-up was a gas-diffusion working electrode (potential: +600 mV vs. Ag/AgCl) that consisted of a porous Teflon membrane coated with a thin platinum layer. Headspace samples were analysed for alcohols and used to derive alcohol concentrations in the liquid phase. The biosensor had a sensitivity of 3.43 μA/mM for ethanol, a response time of 69 s, a linear dynamic range of 0.10-30 mM, a theoretical detection limit (3 < S/N) of 9.9 μM, and a stability of 86% during continuous operation (18 h @ 1 mM ethanol). Using one sensor on three consecutive days, the mean coefficient of variation was 1.3% (three measurements each day @ 10 mM ethanol). Alcohol contents of three apple juices determined with the biosensor were in the range 0.30 g/l-0.67 g/l (equivalent to 6.51 mM-14.5 mM). However, ethanol contents determined by high pressure liquid chromatography coupled to refractive index detection (HPLC-RI) and by a commercial enzyme test kit based on alcohol dehydrogenase ranged from 0.12 g/l to 0.38 g/l (equivalent to 2.60 mM-8.25 mM). Both indicate that the biosensor detected alcohols other than ethanol in the apple juices. HPLC-RI coupled to the biosensor in a flow-through configuration demonstrated that the biosensor detected methanol concomitant to ethanol. Thus, the biosensor could perform a qualitative analysis of the total content of volatile alcohols in apple juices by analysing the gas phase above the sample. This offers the additional advantage that possible, non-volatile interfering substances in the liquid sample cannot impair the measurement.     

Influence of Cs doping in spray deposited SnO2 thin films for LPG sensors

Detection of low concentrations of petroleum gas was achieved using transparent conducting SnO₂ thin films doped with 0–4 wt.% caesium (Cs), deposited by spray pyrolysis technique. The electrical resistance change of the films was evaluated in the presence of LPG upon doping with different concentrations of Cs at different working temperatures in the range 250–400 °C. The investigations showed that the tin oxide thin film doped with 2% Cs with a mean grain size of 18 nm at a deposition temperature of 325 °C showed the maximum sensor response (93.4%). At a deposition temperature of 285 °C, the film doped with 3% Cs with a mean grain size of 20 nm showed a high response of 90.0% consistently. The structural properties of Cs-doped SnO₂ were studied by means of X-ray diffraction (XRD); the preferential orientation of the thin films was found to be along the (3 0 1) directions. The crystallite sizes of the films determined from XRD are found to vary between 15 and 60 nm. The electrical investigations revealed that Cs-doped SnO₂ thin film conductivity in a petroleum gas ambience and subsequently the sensor response depended on the dopant concentration and the deposition temperature of the film. The sensors showed a rapid response at an operating temperature of 345 °C. The long-term stability of the sensors is also reported.     

Thin film characterization and vapor sensing properties of a novel perylenediimide material

A novel N,N′-(glycine tert-butylester)-3,4,9,10-perylenediimide was chosen for the study of Langmuir–Blodgett (LB) thin film characterization and the sensing properties against selected volatile organic vapors. Different number of LB film layer was deposited onto a glass and quartz crystal substrate. The thin film fabrication process was monitored with UV–vis and quartz crystal microbalance (QCM) measurement techniques. The results indicated that absorbance increased linearly with the number of the layers on film. A similar linear relationship between frequency shift and number of the layers was observed by the QCM measurement. It can be concluded that high quality and uniform LB films were produced by using this novel perylenediimide material. Chloroform, toluene, benzene, ethyl alcohol and isopropyl alcohol vapors were selected to test this material's applicability in room temperature as a vapor sensor. This novel material showed a fast, large and reproducible response to chloroform and isopropyl alcohol vapor.     

Flexible sensorial system based on capacitive chemical sensors integrated with readout circuits fully fabricated on ultra thin substrate

In this paper we present the design and fabrication of a fully flexible sensorial system, composed of three different sensor units implemented on an ultrathin polyimide substrate of 8 μm thick. Each unit is composed by a capacitive chemical sensor integrated with readout electronics. The sensors are parallel plate capacitors with the top electrode properly patterned to allow analytes diffusion into the dielectric that acts as chemical interactive material. Three different polymers, poly(tetrafluoroethene) (PTFE), poly(methyl 2-methylpropenoate) (PMMA) and benzocyclobutene (BCB), were used as dielectrics. A ring oscillator circuit, implemented with polysilicon thin film transistors (PS-nTFT), was used to convert the capacitance variations into frequency shifts. The electronic tests show oscillating frequencies of about 211 ± 2 kHz and negligible frequency shifts under different bending radius conditions. Furthermore, system response to some alcohols concentrations (Methanol, ethanol, 1-butanol, and 1-propanol) is reported and data analysis proves that the system is able to discriminate methanol from ethanol.     

Electrocatalytic oxidation of methanol and other short chain aliphatic alcohols on glassy carbon electrodes modified with conductive films derived from Ni-(N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene)

Complexes of nickel(II) with the ligand N,N′-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene (Ni^II-DHS) can be electropolymerized onto glassy carbon surfaces in alkaline solution to give electroactive films strongly adhered on the electrode surface. In alkaline solution, these poly-[Ni^II-DHS]/GC films present the typical voltammetric response of a surface-immobilized redox couple, as can be anticipated for the Ni²⁺/Ni³⁺ transitions into the film. In addition, the films exhibit a potent and persistent electrocatalytic activity towards the oxidation of methanol. The electrocatalytic currents are, at least, 80 times higher than those obtained for the oxidation of methanol at electrodes modified with nickel hydroxide films in alkaline solutions. In addition, the current is proportional to the concentration of methanol from 0.050 to 0.30 μM. The detection limit and the sensitivity were found to be 26 ± 2 nM and 7.4 × 10⁻² ± 6 × 10⁻³ A cm² mol⁻¹ M⁻¹, respectively. Electrodes modified with poly-[Ni^II-DHS]/GC films show a moderate electrocatalytic activity towards the oxidation of other aliphatic short chain alcohols, such as: ethanol, 1-propanol, 2-propanol and n-butanol. In all cases the catalytic currents present linear dependences with the concentration of alcohol in alkaline solution. The analytical properties of these potential alcohol sensors have also been studied.     

掺杂三乙醇胺铜配合物的PVA复合薄膜/K+交换玻璃光波导器件检测SO2气体

三乙醇胺酮配合物气敏性研究发现,该配合物与二氧化硫气体作用时透光率发生变化。以此结果为依据在K 交换玻璃光波导表面固定三乙醇胺铜配合物掺杂的聚乙烯醇复合薄膜,研制出了二氧化硫气体传感元件,并在光波导传感检测系统中测定其传感特性。实验结果表明,本传感元件对低浓度(500×10-9～15×10-6)二氧化硫气体有良好的线性快速可逆响应,低浓度氯化氢、硫化氢、二氧化氮、氨气以及挥发性有机物蒸汽对二氧化硫气体的检测没有干扰,具有灵敏度和选择性高、可逆性好,结构简单和易制作等特点。     

Electrical response of electrospun PEDOT-PSSA nanofibers to organic and inorganic gases

Electrospun isolated nanofibers of poly(3,4-ethylenedioxythiophene) doped with (poly styrene sulfonic acid)-PEDOT-PSSA were used to sense vapors of several aliphatic alcohols. Due to the large surface to volume ratio and small quantity of active material used in their fabrication, these sensors have a similar or faster response time when compared to alcohol sensors based on PEDOT. Increasing the size of the alcohol molecule leads to longer response times, which is attributed to slower diffusion of the larger molecule into the polymer. The sensors were annealed in air at 70 °C and used to sense NH₃, HCl and NO₂ gas. The response time for NH₃ was faster than HCl, and the sensors showed a large initial response to NO₂ at room temperature which is very desirable, as some NO₂ gas sensors only operate at elevated temperatures. Electrospinning is a simple and inexpensive method of preparing PEDOT-PSSA nanofibers making it an attractive technique to fabricate polymer based low cost, rapid response and reusable gas sensors.     

Preparation of chemical vapor sensing materials from composites of esterified poly(vinyl alcohol) and carbon black

Poly(vinyl alcohol) was modified by esterification to prepare poly(vinyl alcohol) copolymers. The degree of esterification on poly(vinyl alcohol) was elucidated by FTIR, ¹H NMR, and elemental analysis. The obtained products were poly(vinyl benzoate)-co-poly(vinyl alcohol) (B-PVA) and poly(vinyl p-toluoate)-co-poly(vinyl alcohol) (P-PVA). The chemical vapor sensors were fabricated by the mixtures of polymer and carbon black in dimethyl sulfoxide and their subsequent preparation as thin films onto the interdigited electrodes by the application of the spin-coating technique. The chemical vapor sensing properties of the sensors were examined with various solvents, such as hexane, toluene, methanol, ethanol, isopropanol, tetrahydrofuran, ethyl acetate, acetonitrile, dimethyl sulfoxide, and water. The experimental results indicated that modifying the chemical structure of PVA results in the decreased polarity of the obtained products. The composites of modified PVA consequently responded well to low polarity solvents, such as THF or ethyl acetate.     

Ceria-doped SnO2 sensor highly selective to ethanol in humid air

In our earlier study, we reported that at 300 °C, a 2.0 wt.% CeO₂-doped SnO₂ sensor is highly selective to ethanol in the presence of CO and CH₄ gases [F. Pourfayaz, A. Khodadadi, Y. Mortazavi, S.S. Mohajerzadeh, CeO₂ doped SnO₂ sensor selective to ethanol in presence of CO, LPG and CH₄, Sens. Actuators B 108 (2005) 172–176]. In the present investigation, we report the influence of ambient air humidity on the ethanol selective SnO₂ sensor doped with 2.0 wt.% CeO₂. Maximum response to ethanol occurs at 300 °C which decreases with the relative humidity. The relative humidity was changed from 0 to 80% for different ambient air temperatures of 30, 40 and 50 °C and the response of the sensor was monitored in a 250–450 °C temperature range. As the relative humidity in 50 °C air increased from 0 to 30%, a 15% reduction in the maximum response to ethanol was observed. A further increase in the relative humidity no longer reduced the response significantly. The presence of humidity improved the sensor response to both CO and CH₄ up to 350 °C after which the extent of improvement became smaller and at 450 °C was almost diminished. The sensor is shown to be quite selective to ethanol in the presence of humid air containing CO and CH₄. The selectivity passes a maximum at 300 °C; however it declines at higher operating temperatures.     

Using a PEDOT:PSS modified electrode for detecting nitric oxide gas

PEDOT:PSS thick films, prepared by the drop-coated method, were used in this study for sensing nitric oxide (NO) gas. The thickness of PEDOT-PSS film was controlled by dropping different volumes of PEDOT-PSS solution to improve the response of PEDOT-PSS film. Due to its porous structure, the thicker the PEDOT-PSS film is, the higher the noticeable surface area. Thus, a larger response is found. However, since the concentration of NO gas used was low (10 ppm), the effect of the surface area was not noticeable when the thickness of the film was greater than 5 μm. In the range of 2.5–10 ppm NO, the relationship between the response of the PEDOT-PSS film and the NO concentration was linear. The limit of detection (S/N = 3), response time (t₉₅), and recovery time (t₉₅) were about 350 ppb, 527 s, and 1780 s, respectively. The response of PEDOT-PSS film to 10 ppm NO gas was dramatically affected by the presence of either O₂ or CO. The standard deviation, with respect to the sensitivity of the NO gas sensor based on PEDOT:PSS film, was 2.2%. The sensitivity of the sensor remained at about 74.5% that of a fresh one.     

硅酸镧镓压电传感器监测离子液体膜的粘度

以硅酸镧镓晶体为压电材料,研制的微天平传感器不仅对质量变化高度敏感,也可用于液体粘度传感,并能在高粘性介质中使用.随传感器所接触的液体粘度增加,它的谐振频率下降,而动态电阻增加.基于粘度响应模式,该传感器应用于实时检测溴代1-甲基-3-正己基咪唑膜吸附乙醇过程中的粘度及乙醇吸附量的动态变化,为研究离子液体粘度及其吸附性能提供了一种新的测量方法.     

Highly responsive fluorescent sensing of explosives taggant with an organic nanofibril film

Efficient sensing of an explosives taggant, 2,3-dimethyl-2,3-dinitrobutane (DMNB), has been performed with an organic nanofibril film through monitoring the fluorescence quenching of the film upon exposure to the DMNB vapor. The nanofibril film was fabricated from an alkoxycarbonyl-substituted carbazole-cornered conjugate tetracycle molecule, namely ACTC, which possesses a planar, rigid molecular geometry that favors cofacial π–π stacking between the molecules, leading to the formation of nanofibril structures with extended one-dimensional molecular stacking. The nanofibril film thus fabricated demonstrated unprecedented efficiency in detecting DMNB vapor, likely due to the extended one-dimensional molecular packing and the highly porous structure thus formed within the film. The former facilitates the exciton migration along the long-axis of nanofiber, while the latter enhances the adsorption of DMNB vapor and the expedient diffusion of the analyte through the film. The enhanced adsorption and diffusion of DMNB molecules within the matrix of the film also accounts for the improved response time of the film in fluorescence quenching upon exposure to DMNB vapor.     

Sol–gel derived polycrystalline Cr1.8Ti0.2O3 thick films for alcohols sensing application

The powder sample of Cr_1.8Ti_0.2O₃ (CTO) was obtained by a sol–gel method. The thick films were developed on identical ceramic tubes of 4 mm length comprising of two Au-electrodes and printing an eight-layer film prepared by mixing CTO with glass powder and -terpinol as an organic vehicle. X-ray powder diffraction (XRD) patterns showed the formation of a single phase. The scanning electron microscope (SEM) images of the ceramic sensor treated at 850 °C revealed that the grain size was larger than 400 nm for the individual isolated grains on the surface, and the agglomerated dense spheroidal platelets had the size of 1–4 μm in diameter. The AC impedance measurement in ambient air showed that the resistance decreased nearly by two orders of magnitude with an increase in temperature in the range of 400–600 °C for both the powder sample and the thick film, and the activation energy E_a derived from the measurement was found to be 0.35 and 0.36 eV for the powder and the film, respectively. The films were exposed to various concentrations of alcohols (0.4–1.2 ppm of methanol and 1.0–5.0 ppm of ethanol), followed by determination of sensor response, sensitivity and reversibility and reproducibility. The origin of the gas response was attributed to the surface reaction of R-OH (R = methyl and ethyl group) with O⁻_(ads) to form adsorbed R-CHO, which was desorbed as a gas at 400 °C after the sensor departing from the gas.     

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.     

Thin film polypyrrole/SWCNTs nanocomposites-based NH3 sensor operated at room temperature

A PPY/SWCNTs nanocomposite-based sensor with relatively high sensitivity and fast response–recovery was developed for detection of NH₃ gas at room temperature. The gas-sensitive composite thin film was prepared using chemical polymerization and spin-coating techniques, and characterized by Fourier transformed infrared spectra and field-emission scanning electron microscopy. The results reveal that the conjugated structure of the PPY layer was formed and the functionalized SWCNTs were well-embedded. The effects of film thickness, annealing temperature, and SWCNTs content on gas-sensing properties of the composite thin film were investigated to optimize the gas-sensing performance. The as-prepared thin film PPY/SWCNTs composite sensor with optimized process parameters had a response of 26–276% upon exposure to NH₃ gas concentration from 10 to 800 ppm, and their response and recovery times were around 22 and 38 s, respectively.     

Improving the ethanol sensing of ZnO nano-particle thin films—The correlation between the grain size and the sensing mechanism

ZnO and Sn doped ZnO (ZnO:Sn) thin films at various doping concentrations from 1 to 10 at.% were prepared by the sol-gel method for an ethanol sensing application. The Sn doping significantly influenced the film growth, grain size and response of the films. The XRD patterns showed that the hexagonal wurtzite structure of the ZnO film was retained even after the Sn doping. The crystallite grain sizes of the ZnO:Sn thin films at 0, 2 and 4 at.% were estimated by using the typical Scherrer's equation. The crystalline quality of the films at 6, 8 and 10 at.% of Sn was degenerated. Typical FESEM images demonstrated the different morphologies for the ZnO:Sn thin films at various Sn concentrations; many pores of various dimensions were observed depending on the doping level. A TEM analysis of the ZnO:Sn thin films at 0, 2 and 4 at.% was performed to verify the grain size. The optimum Sn doping level of ZnO:Sn thin film for ethanol sensing was estimated to be 4 at.%. The 4 at.% sample obtained the highest response to ethanol vapor in the 10-400 ppm level range at a low operating temperature of 250 °C. The sensing mechanism was explained by a variation in the sensitivity model from a neck-grain-boundary controlled sensitivity to a neck-controlled sensitivity. Our work demonstrates the ability to reduce the working temperature as well as to increase the response of ZnO thin film based gas sensors to detect ethanol, which would be of great merit for commercialized applications.