Ionic liquid high-temperature gas sensor array

A novel sensor array using seven room-temperature ionic liquids (ILs) as sensing materials and a quartz crystal microbalance (QCM) as a transducer was developed for the detection of organic vapors at ambient and elevated temperatures. Ethanol, dichloromethane, benzene, and heptane were selected as representative gas analytes for various kinds of environmental pollutants and common industrial solvents. The QCM/IL sensors responded proportionately and reversibly to the organic vapor concentrations (i.e., ethanol, heptane, and benzene) in the gas phase from 0 to 100% saturation at room and elevated temperatures (e.g., 120 degrees C) but deviated from this linear relationship at high concentrations for dichloromethane, a highly volatile compound. Linear discriminant analysis was used to analyze the sensing patterns. Excellent classifications were obtained for both known and unknown concentrations of vapor samples. The correct classifications were 100% for known concentration samples and 96% for samples with unknown concentrations. Thermodynamics and ATR-FT-IR studies were conducted to understand specific molecular interactions, the strength of the interaction between ILs and organic vapors, and the degree of ordering that takes place upon dissolution of the vapors in ILs. The different response intensity of the QCM/IL sensors to the organic vapors depends on the different solubilities of organic vapors in ILs and varying molecular/ion interactions between each organic vapor and IL. The diverse set of IL studied showed selective responses due to structural differences. Therefore, a sensor array of ILs would be able to effectively differentiate different vapors in pattern recognitions, facilitating discrimination by their distinctive patterns in response to organic vapors in both room and high temperatures.     

Volatile organic compound detection using nanostructured copolymers

Regioregular polythiophene-based conductive copolymers with highly crystalline nanostructures are shown to hold considerable promise as the active layer in volatile organic compound (VOC) chemresistor sensors. While the regioregular polythiophene polymer chain provides a charge conduction path, its chemical sensing selectivity and sensitivity can be altered either by incorporating a second polymer to form a block copolymer or by making a random copolymer of polythiophene with different alkyl side chains. The copolymers were exposed to a variety of VOC vapors, and the electrical conductivity of these copolymers increased or decreased depending upon the polymer composition and the specific analytes. Measurements were made at room temperature, and the responses were found to be fast and appeared to be completely reversible. Using various copolymers of polythiophene in a sensor array can provide much better discrimination to various analytes than existing solid state sensors. Our data strongly indicate that several sensing mechanisms are at play simultaneously, and we briefly discuss some of them.     

Chemiresistive Sensing with Chemically Modified Metal and Alloy Nanoparticles

This review describes the use of chemically modified pure and alloyed metal nanoparticles for chemiresistive sensing applications. Chemically modified metal nanoparticles consist of a pure or alloyed metallic core with some type of chemical coating. Researchers have studied the electronic properties of 1D, 2D, and 3D assemblies of chemically modified metal nanoparticles, and even single individual nanoparticles. The interaction with the analyte alters the conductivity of the sensitive material, providing a signal to measure the analyte concentration. This review focuses on chemiresistive sensing of a wide variety of gas‐ and liquid‐phase analytes with metal nanoparticles coated with organothiols, ions, polymers, surfactants, and biomolecules. Different strategies used to incorporate chemically modified nanoparticles into chemiresistive sensing devices are reviewed, focusing on the different types of metal and alloy compositions, coatings, methods of assembly, and analytes (vapors, gases, liquids, biological materials), along with other important factors.     

Multianalyte chemical identification and quantitation using a single radio frequency identification sensor

We demonstrate an approach for multianalyte chemical identification and quantitation using a single conventional radio frequency identification (RFID) tag that has been adapted for chemical sensing. Unlike other approaches of using RFID sensors, where a special tag should be designed at a much higher cost, we utilize a conventional RFID tag and coat it with a chemically sensitive film. As an example, we demonstrate detection of several vapors of industrial, health, law enforcement, and security interest (ethanol, methanol, acetonitrile, water vapors) with a single 13.56-MHz RFID tag coated with a solid polymer electrolyte sensing film. By measuring simultaneously several parameters of the complex impedance from such an RFID sensor and applying multivariate statistical analysis methods, we were able to identify and quantify several vapors of interest. With a careful selection of the sensing film and measurement conditions, we achieved parts-per-billion vapor detection limits in air. These RFID sensors are very attractive as ubiquitous multianalyte distributed sensor networks.     

Ordered‐Assembly Conductive Nanowires Array with Tunable Polymeric Structure for Specific Organic Vapor Detection

An artificial organic vapor sensor based on a finite number of 1D nanowires arrays can provide a strategy to allow classification and identification of different analytes with high efficiency, but fabricating a 1D nanowires array is challenging. Here, a coaxial Ag/polymer nanowires array is prepared as an organic vapor sensor with specific recognition, using a strategy combining superwettability‐based nanofabrication and polymeric swelling‐induced resistance change. Such organic vapor sensor containing commercial polymers can successfully classify and identify various organic vapors with good separation efficiency. An Ag/polymer nanowires array with synthetic polyethersulfone polymers is also fabricated, through molecular structure modification of the polymers, to distinguish the similar organic vapors of methanol and ethanol. Theoretical simulation results demonstrate introduction of specific molecular interaction between the designed polymers and organic vapors can improve the specific recognition performance of the sensors.     

The fabrication of a MWNTs–polymer composite chemoresistive sensor array to discriminate between chemical toxic agents

In this study, a chemoresistive sensor was fabricated by the chemical polymerization and coating of either polyaniline (PANI), poly[2-methoxy-5-(2-ethyloxy)-p-phenylenevinylene], or commercial poly(methyl methacrylate) on MWNTs. We investigated the resistance responsiveness of the multilayer samples to simulated chemical warfare agents, including dimethyl methyl phosphonate (DMMP) and dichloromethane (DCM), as well as to organic agents, such as chloroform, tetrahydrofuran, methyl-ethyl ketone, and xylene. The MWNTs–PANI film was characterized by SEM and FT-IR, and the resistivity values for the six solvents were measured at different temperatures. We observed that the MWNTs-PANI sensing film exhibited a high sensitivity, excellent selectivity, and good reproducibility to the detection of all of the aforementioned agent vapors. In addition, we used atomic force microscopy to demonstrate the MWNTs–PANI absorption of DMMP vapor, wherein the sensing film exhibited a swelling phenomenon, such that the film thickness increased from 0.8 to 1.3 μm. In addition, we used principal component analysis to evaluate the performance of the sensor in detecting DMMP, DCM, and the aforementioned organic agent vapors.     

A study of Langmuir-Blodgett thin film for organic vapor detection

In this work, arachidic acid was deposited onto a quartz crystal using a standard Langmuir-Blodgett (LB) thin film deposition procedure. Quartz Crystal Microbalance (QCM) technique was used to monitor the reproducibility of the LB film monolayer and was employed to study the organic vapor sensing properties of chloroform, toluene, benzene, ethyl alcohol and isopropyl alcohol. QCM results show that arachidic acid monolayer was successfully organised and deposited from the water surface onto a quartz crystal substrate. This LB film is found to be highly sensitive and selective to chloroform vapor than other vapors. The response of the sample against chloroform is fast, large and reversible.     

酸处理对MWCNTs石英晶振微天平VOCs传感器灵敏度的影响(英文)

为分析酸处理过程对石英晶振型碳纳米管(CNTs)有机(VOCs)气体传感器灵敏度的影响,制作了一种多壁碳纳米管(MWCNTs)石英晶振式VOCs传感器.发现酸处理可以明显提高传感器灵敏度,利用毛细凝聚原理与开尔文公式对上述实验现象进行了分析.另外,比较了传感器对不同有机气体(甲苯、乙醇和丙酮)的灵敏度,研究结果表明传感器灵敏度与气体饱和蒸汽压有关,同样,该实验现象可利用毛细凝聚原理与开尔文公式来解释.     

离子液体在有机光电转换器件中的应用研究进展

近年来,离子液体因具有不易挥发、性质稳定、透光性好、导电率高、可设计性,以及易于在界面处形成双电层等物理化学性质,而展现出广阔的应用潜力和前景,逐渐成为国际科学研究的前沿和热点之一。其中,将离子液体应用于染料敏化太阳能电池(Dye-sensitized solar cells,DSSCs)、钙钛矿太阳能电池和有机光电探测器等有机光电转换器件的研究备受关注。 在有机光电转换器件中,离子液体在染料敏化太阳能电池方面的应用最为广泛且完善。高效DSSCs主要是基于有机溶剂的液态电解质结,但有机溶剂在带来较高光电转换效率的同时,其本身存在的易挥发汽化、光热稳定性差等缺点,导致DSSCs的器件寿命与长期稳定性受到影响,离子液体的引入能有效解决以上问题。此外,离子液体还以电子传输层以及界面修饰层的形式引入,具有高电荷迁移率、低功函数以及高稳定性等优点,能在一定程度上改善器件的短路电流、填充因子和光电转换效率等。因此,离子液体成为在DSSCs的实际应用中兼具性价比高、封装难度低、性能好、稳定性高四大优点的辅助材料。在钙钛矿太阳能电池方面,离子液体的低功函数和高电子迁移率以及一些特殊性质如钝化反应、黏度效应等,都能够实现对电子萃取率、电荷转移电阻、钙钛矿结晶情况等方面的控制以满足实际设计要求,进而有助于钙钛矿太阳能电池的光电转换效率、填充因子等性能指标不同程度的提升。在有机光电探测器方面,引入的离子液体能促使在与之接触的界面处形成双电层,双电层的形成及离子液体的高导电率使得入射光不必照射有机光电探测器上下电极的重叠区域仍旧可以产生较大的光电流输出,从而可以有效摆脱有机光电探测器对电极材料透光性要求的局限性。同时双电层的形成还将促进有机光电探测器工作层中的电荷分离,进一步提高有机光电探测器的响应率。 本文主要从染料敏化太阳能电池、钙钛矿太阳能电池、有机光电探测器三个方面,综述了离子液体在有机光电转换器件中的国内外应用研究进展,就离子液体对提升有机光电转换效率及其实现器件新功能的工作机理进行了详细分析,并对其未来的应用研究方向进行了展望,为今后进一步设计出更适合有机光电转换领域应用的离子液体提供参考。     

Quartz crystal microbalances for microscale thermogravimetric analysis

A new method for analyzing the chemical purity and consistency of microscale samples with a quartz crystal microbalance (QCM) sensor platform is described. The QCM is used to monitor submicrogram changes in the mass of a deposited thin film as a function of temperature, in a manner similar to that of a conventional thermogravimetric analyzer (TGA). Results correlated well with TGA measurements for a wide range of representative materials, including organic compounds, ionic detergents, oxidizing and inert powders, carbon nanotubes, and various mixtures of these samples. In each case, the sample mass was on the order of a few micrograms, compared to the need for several milligrams for conventional TGA analysis. This work illustrates the effectiveness of this approach for analysis of nanoparticles, thin films, and highly purified specimens on the microgram scale.     

Quartz crystal microbalance sensor for organic vapor detection based on molecularly imprinted polymers

Molecularly imprinted polymers on quartz crystal microbalances (QCM) are examined for their ability to detect vapors of small organic molecules with greater sensitivity and selectivity than the traditional amorphous polymer coatings. Hydroquinone and phenol serve as noncovalently bound templates that generate shape-selective cavities in a poly(acrylic) or poly(methacrylic) polymer matrix. The imprinted polymers are immobilized on the piezoelectric crystal surface via a precoated poly(isobutylene) layer. The behavior of the imprinted polymer films is characterized by the dynamic and steady-state response of the QCM frequency to pulses of organic vapors in dry air. The apparent partition coefficients are determined for imprinted and nonimprinted polymers prepared by two synthetic methods and for varying mole ratios of template to monomer. The hydroquinone-imprinted polymers and, to a lesser extent, the phenol-imprinted polymers exhibit greater sensitivity and higher selectivity than the nonimprinted polymers toward organic vapors that are structurally related to the templates. These results indicate that molecularly imprinted polymers are promising for the development of selective piezoelectric sensors for organic vapor detection.     

Hazardous industrial gases identified using a novel polymer/MWNT composite resistance sensor array

Hazardous industrial chemical gases pose a significant threat to the environment and human life. Therefore, there is an urgent need to develop a reliable sensor for identifying these hazardous gases. In this work, a silicon wafer microelectrode substrate for a resistance sensor was fabricated using the semiconductor manufacturing process. Conductive carbon nanotubes were then mixed with six different polymers with different chemical adsorption properties to produce a composite thin film for the fabrication of a chemical sensor array. This array was then utilized to identify three hazardous gases at different temperatures. Experimental results for six polymers for chemical gases, such as tetrahydrofuran (THF), chloroform (CHCl₃) and methanol (MeOH) at different temperatures, indicate that the variation in sensitivity resistance increased when the sensing temperature increased. The poly(ethylene adipate)/MWNT sensing film had high sensitivity, excellent selectivity, and good reproducibility in detecting all chemical agent vapors. Additionally, this study utilized a bar chart and statistical methods in principal component analysis to identify gases with the polymer/MWNT sensor.     

Nano Chemical Sensors With Polymer-Coated Carbon Nanotubes

A simple sensor platform consisting of an interdigitated electrode (IDE) pattern has been fabricated for sensing gas and organic vapors. Purified single-walled carbon nanotubes (SWNTs) in the form of a network laid on the IDE by solution casting serve as the sensor material. The electrical conductivity of the SWNT network changes reproducibly upon exposure to various gases and vapors. Selectivity to specific gases, for example, chlorine and hydrochloric acid vapor, is demonstrated by coating the SWNTs with polymers such as chlorosulfonated polyethylene and hydroxypropyl cellulose.     

3,3',5,5'-tetramethyl-N-(9-anthrylmethyl)benzidine: a dual-signaling fluorescent reagent for optical sensing of aliphatic aldehydes

A new optical chemical sensor for continuous monitoring of aliphatic aldehydes has been proposed based on the reversible chemical reaction between a new sensing reagent, 3,3',5,5'-tetramethyl-N-(9-anthrylmethyl)benzidine (TMAB), and the analytes. TMAB, containing two receptors and two fluorescent reporters, can perform dual fluorescence responses corresponding to the reactions of hydrogen ion and carbonyl compound. When immobilized in a plasticized poly(vinyl chloride) membrane, TMAB extracts aliphatic aldehydes from aqueous solution into the bulk membrane phase and reacts with the analyte by forming a Schiff base. Since the extraction equilibrium and chemical reaction are accompanied by fluorescence increase of the sensing membrane, the chemical recognition process could be directly translated into an optical signal. At pH 3.20, the sensor exhibits a dynamic detection range from 0.017 to 4.2 mM n-butyraldehyde with a limit of detection of 0.003 mM. The forward response time (t95) of the sensor is 3-5 min, and the reverse response time is 5-7 min. The responses of the sensor toward different kinds of aldehydes and ketones depend on the lipophilicity and the reactivity of the analytes. Since the fluorescence enhancement of the sensing membrane at 296 nm/410 nm is only related to the formation of Schiff base, the measurement of aldehydes is independent of pH.     

A lateral-field-excited LiTaO3 high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO₃. Prototype LFE LiTaO₃ sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.     

Improved vapor selectivity and stability of localized surface plasmon resonance with a surfactant-coated Au nanoparticles film

Here, we report the use of tetraoctylammonium bromide (TOABr)-coated Au nanoparticles (NPs) for the optical sensing of volatile organic compounds (VOCs). We find that the film responded selectively to the presence of polar and nonpolar vapors by changes in the maximum wavelength (λ(max)) toward higher and lower wavelengths, respectively, as determined by UV-visible spectroscopy. We also observed that the organic coating reorganizes when vapors partition into the film indicated by FT-IR and the film contracts in the presence of water indicated by scanning electron microscopy (SEM). In the present sensor, the metallic Au core serves as the plasmonic signal while the organic coating acts as the receptor material providing vapor selectivity and sensor stability. Correlating changes in (λ(max)) with changes in the refractive index (RI) and nanoparticle-to-nanoparticle separation in the film is important both fundamentally and for improving selectivity in localized surface plasmon resonance (LSPR) sensors.     

A micro-system based on glass-nanoporous silicon for optical sensing of organic solvent vapor

We present a recent experimental study on the application of nanoporous silicon (np-Si) to an optical vapor sensor. We fabricated the micro-system based on a glass-nanoporous silicon layer on a p(+)-type silicon wafer. To check the selectivity and sensitivity of the np-Si layer to organic vapors, we prepared three types of np-Si layer samples--a single layer, distributed Bragg reflector (DBR) layer, and microcavity layer--and investigated its reflectance spectra upon exposure to different concentrations of various organic vapors. When the np-Si layer samples were exposed to the organic vapors, a red-shift occurred in the reflectance spectrum, and we determined that this red-shift can be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the np-Si layer. The np-Si layer samples showed excellent sensing ability to different types and concentrations of organic vapors. After removing the organic vapors, the reflectance spectrum immediately returned to its original state.     

Discrimination of methanol and ethanol vapors by the use of a single optical sensor with a microporous sensitive layer

The sorption of methanol and ethanol vapors by a microporous glassy polycarbonate is studied. The increase of the refractive index of the polymer during analyte sorption is measured by surface plasmon resonance. Both analytes are sorbed into the micropores of the polymer showing different diffusion kinetics. The sensor response during analyte exposure is subdivided into different time channels. By evaluating this additional data dimension by neural networks, a simultaneous multicomponent analysis of binary mixtures of ethanol and methanol vapors is possible using the sensor response of only one single sensor. A feature extraction results in an interpretable model and an improved prediction with errors of 2.0% for methanol and 2.4% for ethanol.     

Dynamics of redox processes in ionic liquids and their interplay for discriminative electrochemical sensing

Motivated by the use of ionic liquids (ILs) as green replacers of traditional electrolytes, a mechanistic study has been systematically conducted to comprehend various design principles responsible for electrochemical profiling of redox-active species in ILs. The full spectrum of properties associated with ILs is exploited to assess the viability of this platform, thus revealing the correlation between the redox properties and the physiochemical parameters of the species involved. This includes the evaluation of (1) the variation of redox responses toward analytes with similar molecular structures or functionalities of ILs, (2) the influence in terms of physical criteria of the system such as viscosity and conductivity as well as chemical structure of ILs, and (3) the sustainability in harsh conditions (high temperature or humidity) and interferences. The principle is exemplified via trinitrotoluene (TNT) and dinitrotoluene (DNT) with inherent redox activity as analytes and IL membranes as solvents and electrolytes using glassy carbon (GC) electrodes. A discrete response pattern is generated that is analyzed through linear discriminant analysis (LDA) leading to 100% classification accuracy even for the mixture of analytes. Quantitative analysis through square wave voltammetry (SWV) gave rise to the detection limits in liquid phase of 190 and 230 nM for TNT and DNT, respectively, with a linear range up to 100 μM. Gas-phase analysis shows strong redox signals for the estimated concentrations of 0.27 and 2.05 ppm in the gas phase for TNT and DNT, respectively, highlighting that ILs adopt a role as a preconcentrator to add on sensitivity with enhanced selectivity coming from their physiochemical diversity, thus addressing the major concerns usually referred to most sensor systems.     

A quartz crystal microbalance-based sensor system coated with functional polymers for SO2 and NO2 detection

In this work, a quartz crystal microbalance (QCM)-based adsorption sensor system with high sensitivity, selectivity, and reproducibility is designed and fabricated. The functional polymers such as polypyrrole, poly(3,4-ethylenedioxythiophene) (PEDOT), and polystyrene are coated on 8 MHz AT-cut quartz crystal surfaces as sensing materials for SO2 and NO2. All sensing materials on the QCM surface are characterized experimentally by SEM and AFM. The frequency shifts of the QCM by adsorption and desorption of gases are measured and analyzed to assess the practical applicability of the sensor system. The overall results show that the QCM coated with polypyrrole is highly selective for SO2 gas and that coated with PEDOT is for NO2. It is proven that the QCM-based adsorption sensor system is possible for monitoring SO2 and NO2 gases in the mixture of ppm level.