Article

Detection and Discrimination of Volatile Organic Compounds using a Single Film Bulk Acoustic Wave Resonator with Temperature Modulation as a Multiparameter Virtual Sensor Array

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Abstract

This paper describes the detection and discrimination of volatile organic compounds (VOCs) using an e-nose system based on a multiparameter virtual sensor array (VSA), which consists of a single-chip temperature-compensated film bulk acoustic wave resonator (TC-FBAR) coated with 20-bilayer self-assembled PSS / PDDA thin films. The high-frequency and microscale FBAR multiparameter VSA was realized by temperature modulation, which can greatly re-duce the cost and complexity compared to those of a traditional e-nose system and can allow it to operate at different temperatures. The discrimination effect depends on the synergy of temperature modulation and the sensing material. For proof-of-concept validation purposes, the TC-FBAR was exposed to six different VOC vapors at six different gas par-tial pressures by real-time VOC static detection and dynamic detection. The resulting frequency shifts and impedance responses were measured at different temperatures and evaluated using principal component analysis (PCA) and linear discriminant analysis (LDA), which revealed that all analytes can be distinguished and classified with more than 97% accuracy. To the best of our knowledge, this report is the first on an FBAR multiparameter VSA based on temperature modulation, and the proposed novel VSA shows great potential as a compact and promising e-nose system integrated in commercial electronic products.

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... 17 Among these methods, resonator sensors are most competitive in terms of sensitivity and resolution owing to the high quality factor of electromechanical resonators. 16 Film bulk acoustic wave resonators (FBARs), as piezoelectric transducers operating in the gigahertz regime, have been exploited for VOC detection based on the mass loading effect [18][19][20][21][22][23][24][25] and chemical reaction. 26 These sensors feature high sensitivity, miniature size, and low power consumption. ...
... [22][23][24] A single FBAR can also function as a virtual sensor array with temperature modulation to detect and discriminate VOCs. 25 The surfaces of FBARs are typically functionalized with polymers, 18 supramolecular monolayers, 25 or self-assembled monolayers 23,24 to realize selectivity and improved adsorption. Surface functionalization of the sensors plays a key role in recognition, sensitivity, selectivity, stability, and reversibility. ...
... FBAR arrays have also been used as electronic noses to achieve discrimination of VOCs. [22][23][24] A single FBAR can also function as a virtual sensor array with temperature modulation to detect and discriminate VOCs. 25 The surfaces of FBARs are typically functionalized with polymers, 18 supramolecular monolayers, 25 or self-assembled monolayers 23,24 to realize selectivity and improved adsorption. Surface functionalization of the sensors plays a key role in recognition, sensitivity, selectivity, stability, and reversibility. ...
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Film bulk acoustic wave resonators have demonstrated great potential in the detection of volatile organic compounds owing to their high sensitivity, miniature size, low power consumption, capacity for integration, and other beneficial characteristics. However, it is necessary to functionalize the surfaces of these resonators to enhance the adsorption and discrimination of volatile organic compounds. Here, we report a convenient and reliable method for functionalizing the surfaces of film bulk acoustic wave resonators with hydrophobins via self-assembly to enable highly sensitive and polarity sensitive detection of volatile organic compounds. Experiments conducted using various concentrations of five volatile organic compounds possessing different polarities demonstrated that the hydrophobin coating enhanced the responsivity of the proposed sensor. The obtained results were in good agreement with the Brunauer–Emmett–Teller model of multilayer physisorption, which suggests that the hydrophobin coating enhanced the sensitivity by improving the monolayer adsorption capacity. Our work demonstrates that the combination of multifunctional biosurfactants and microelectromechanical devices can permit high-performance gas sensing.
... Previous work using bulk acoustic wave resonators to sense VOCs has taken advantage of many of these sensitizers [30]- [38]. Of these, only Chang et al. used self-assembled monolayers as the functionalizing material [33]. ...
... THESE RESULTS DEMONSTRATE APPROXIMATELY THE SAME RELATIVE RESPONSE AND SELECTIVITY AS WAS MEASURED USING A RESIS-TIVE APPROACH FOR THE SAME RECEPTOR IN PREVIOUS WORK [16] the previous demonstrator of the thiourea monolayer for cyclohexanone detection [9]. Similar functionalized bulk acoustic wave sensors have shown limits of detection between 500 and 4 ppm, so a limit of detection of 1.11 ppm is competitive [33], [35], [37], [38], [40]. ...
... Integration of a sensing and control device to a printed circuit board eliminates some of the frequency drifting that plagues quartz crystal microbalance based sensors [73]. Finally, the limit of detection demonstrated here (1.11 ppm) is the lowest demonstrated for cyclohexanone among comparable chemiresistor and MEMs devices [9], [19], [24], [25], [38]. In particular, Heil et al and Zeng et al used gravimetric approaches resulting in limits of detection for cyclohexanone of 5 and 40 pppm respectively so our demonstration of 1.11 ppm is a significant improvement on this previous work. ...
Article
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Functionalized gravimetric sensors are a promising path to small, versatile, real-time vapor sensors for volatile organic compounds. Many of these compounds can be dangerous to human health, but their nonreactive nature makes them notoriously difficult to sense. Unlike bulk acoustic resonators, chemiresistive devices have been investigated extensively and many researchers have used innovative synthesis strategies to functionalize these devices. In this work, we demonstrate how modifying a particular sensitizer for use with a bulk acoustic resonator significantly improves the sensitivity of the device (5 ppm vs. 1.11 ppm). Additionally, readout circuitry is described to avoid some problems that typically plague gravimetric sensors while simplifying the overall system. These strategies create a playbook for simple, fast, and sensitive systems for sensing volatile organic compounds, while also demonstrating the lowest limit of detection for cyclohexanone outside of gas chromatography/mass spectrometery in the literature. [2020-0053].
... Detection and discrimination of volatile organic compounds (VOCs), which are classified as hazard vapors with adverse short-and long-term consequences on the environment and human health 31 , is important in the continuous monitoring of indoor air quality, which therefore requires reliable sensors. Moreover, exhaled VOCs can serve as biomarkers to assist in the non-invasive identification of various diseases. ...
... Moreover, exhaled VOCs can serve as biomarkers to assist in the non-invasive identification of various diseases. For example, acetone indicates diabetes 14,[32][33][34][35] , whereas isoprene, toluene, and acetic acid are signals of lung cancer, 31,[36][37][38] and breath testing is therefore a highly promising approach for non-invasive cancer screening 39 . VOC analysis in patient breath offers insight into the metabolic processes in the anatomy/physiology that are altered by underlying diseases 40,41 , although a detailed impression of the metabolic route leading to these molecules is still under investigation 39 . ...
... Furthermore, dual or multiple signal transductions have been exploited, e.g., by evaluating both phase and attenuation shifts of a SAW sensor [87], by determining SAW sensor frequency shifts at different temperatures [88], or by measuring the frequency and dissipation shifts of a QCM sensor at multiple harmonics [89]. When conductive films are used as sensor coatings, changes of both the acoustic wave and the electrical properties of the coating can be evaluated [90][91][92]. As both multi-sensor arrays and virtual sensor arrays turned out to be well suited for multi-analyte detection, both were combined to virtual multi-sensor arrays to further enhance the performance [93,94]. ...
... The FBARs were coated with conductive polymer films, and combinations of frequency with resistance or impedance readouts were evaluated, partly with additional modulation of the temperature. These setups allowed the differentiation between five aliphatic compounds or one aromatic and five aliphatic compounds [91,92]. A similar approach was performed when a one-port SAW resonator made of 128 • YX-LiNbO 3 was coated with a conductive material. ...
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Bulk acoustic wave (BAW) and surface acoustic wave (SAW) sensor devices have successfully been used in a wide variety of gas sensing, liquid sensing, and biosensing applications. Devices include BAW sensors using thickness shear modes and SAW sensors using Rayleigh waves or horizontally polarized shear waves (HPSWs). Analyte specificity and selectivity of the sensors are determined by the sensor coatings. If a group of analytes is to be detected or if only selective coatings (i.e., coatings responding to more than one analyte) are available, the use of multi-sensor arrays is advantageous, as the evaluation of the resulting signal patterns allows qualitative and quantitative characterization of the sample. Virtual sensor arrays utilize only one sensor but combine it with enhanced signal evaluation methods or preceding sample separation, which results in similar results as obtained with multi-sensor arrays. Both array types have shown to be promising with regard to system integration and low costs. This review discusses principles and design considerations for acoustic multi-sensor and virtual sensor arrays and outlines the use of these arrays in multi-analyte detection applications, focusing mainly on developments of the past decade.
... Thin film bulk acoustic resonators (FBAR) [1][2][3][4][5] are a newer generation of products in piezoelectric acoustic wave device industry, which is vigorously replacing some applications of surface acoustic wave (SAW) resonators [6,7] and quartz crystal resonators [8] due to its higher resonance frequency and smaller size. FBAR have the typical layered structure, that is a piezoelectric thin film is sandwiched by metal electrodes on the Bragg reflection layers and the substrate [9][10][11]. ...
... where ρ (1) is the density of a piezoelectric thin film. A comma followed by two indexes denotes second-order partial differentiation with respect to the coordinate associated with the indexes. ...
... Complementing somewhat the virtual screening of receptor materials, Speller et al. devised a hardware virtual sensor array whose virtual sensors were the overtone responses of single Q-TSMR GGSs [236,237]. A virtual sensor array for VOCs was obtained by Zeng et al., modulating the temperature of a film bulk acoustic wave (FBAW) transducer covered with self-assembled organic films [238]. The exposure events to the same analyte at different partial pressures appeared as straight lines in the first two principal components (PC). ...
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Within the framework outlined in the first part of the review, the second part addresses attempts to increase receptor material performance through the use of sensor systems and chemometric methods, in conjunction with receptor preparation methods and sensor-specific tasks. Conclusions are then drawn, and development perspectives for gravimetric sensors are discussed.
... Zeng et al. performed PCA on the pattern data of a multiparameter virtual sensor array (VSA) for the discrimination of six VOCs. They found that two principal components could account for more than 97% of the accuracy [36]. PCA reduces redundancy within the sensor sensitivities for each class and projects the sensitivity data orthogonally to several unrelated dimensions to identify the maximum variable component that can be used to classify groups of odorants. ...
Article
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Artificial olfactory sensors that recognize patterns transmitted by olfactory receptors are emerging as a technology for monitoring volatile organic compounds. Advances in statistical processing methods and data processing technology have made it possible to classify patterns in sensor arrays. Moreover, biomimetic olfactory recognition sensors in the form of pattern recognition have been developed. Deep learning and artificial intelligence technologies have enabled the classification of pattern data from more sensor arrays, and improved artificial olfactory sensor technology is being developed with the introduction of artificial neural networks. An example of an artificial olfactory sensor is the electronic nose. It is an array of various types of sensors, such as metal oxides, electrochemical sensors, surface acoustic waves, quartz crystal microbalances, organic dyes, colorimetric sensors, conductive polymers, and mass spectrometers. It can be tailored depending on the operating environment and the performance requirements of the artificial olfactory sensor. This review compiles artificial olfactory sensor technology based on olfactory mechanisms. We introduce the mechanisms of artificial olfactory sensors and examples used in food quality and stability assessment, environmental monitoring, and diagnostics. Although current artificial olfactory sensor technology has several limitations and there is limited commercialization owing to reliability and standardization issues, there is considerable potential for developing this technology. Artificial olfactory sensors are expected to be widely used in advanced pattern recognition and learning technologies, along with advanced sensor technology in the future.
... Guang Zeng et al. fabricated a multiparameter virtual sensor array (VSA) using a temperaturecompensated FBAR with 20-bilayer self-assembled PSS/PDDA thin films to improve Fractal Fract. 2022, 6, 491 2 of 11 molecule absorption efficiency [24]. The authors used a DC heater for temperature control and VOC discrimination. ...
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This paper presents the FEM modeling and simulation of a thin-film bulk acoustic resonator (FBAR) for a tetrachloroethene (PCE) gas-sensing application. A zinc oxide layer is used as a piezoelectric material; an aluminum layer is used as the electrode material in the structure of the FBAR. Polyisobutylene (PIB) is used as the sensitive layer for PCE gas detection. The study was carried out in commercially available FEM-based COMSOL software. The proposed structure was exposed to six different organic gases with concentrations ranging from 0 to 1000 ppm. The structure showed high selectivity for PCE gas. Incorporating the 3rd-order Hilbert fractal geometry in the top electrode of the FBAR increased the sensitivity of the sensor which showed high selectivity for PCE gas detection. A sensitivity enhancement of 66% was obtained using fractal geometry on the top electrode of the FBAR without alteration in size or cost. In addition, a reduction in the cross-sensitivity was achieved. Further, the PIB layer thickness and active area of the FBAR were optimized to obtain high sensitivity. The equivalent circuit was also analyzed to understand the behavior of the sensing effect and mechanism.
... FBAR works at a frequency much higher than any other gravimetric sensor hence its use as an electronic nose gives much higher sensitivity. Chang [40], Lu [41], Sabani [42] and Zeng [43]. designed a FBAR sensor array using different sensitizers for detection of VOCs. ...
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Research in the area of breath biomarkers for non-invasive diagnosis continues to grow with the developments in Electronic Nose (E-Nose) sensing techniques. Sensors are embedded in E-Nose to detect different biomarkers present breath, tissue, urine or blood. These biomarkers are used in preliminary diagnosis for fatal diseases. E-nose mechanism can be related with that of olfactory system of humans. Gravimetric sensors which sense change in mass are predominantly used in E-Nose application. This review paper provides a detailed overview of gravimetric sensors, their principle of operation, fabrication methodology, important technical parameters, and their application in E-Nose for breath bio markers detection. Four types of gravimetric sensors are covered in this review paper, 1) Quartz Crystal Microbalance (QCM) sensor 2) Surface Acoustic Wave (SAW) sensor 3) Thin-Film Bulk Acoustic Resonator (FBAR) 4) Microcantilever. The summary of sensitizers and data processing algorithms which is important for the development of gravimetric E-Nose systems is also presented in this paper.
... With the rapid development of society, deteriorating environmental problems and personal healthcare have been widely concerned. Therefore, effective detection of gases, including volatiles, toxic gases, moisture, and explosives, becomes more and more important in human daily life health and future production [1][2][3][4][5][6][7][8][9][10]. Specifically, in addition to the moisture, there are more than 3000 kinds of volatiles in the samples from human exhaled breath and skin headspace, which provide abundant information about metabolic disorders or dysfunctions of the human body [11][12][13]. ...
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A novel vapor-sensitive composite film comprising cellulose acetate and a representative compound (1-n-butyl-2,3-dimethylimidazolium hexafluorophosphate) from a Group of Uniform Materials Based on Organic Salts (GUMBOS) has been developed and characterized. The vapor sensing characteristics of the film is investigated using a quartz crystal microbalance (QCM) transducer. The material exhibited greatly improved performance characteristics toward a number of organic vapors. It is demonstrated that the ratio of the change in resonance frequency ([capital Delta]f) to the change in motional resistance ([capital Delta]R) is a concentration-independent quantity proportional to the molecular weight of the absorbed chemical species. To the best of our knowledge, this is the first study to show a direct relationship between [capital Delta]f/[capital Delta]R and the molecular weight of analytes. This unique finding should prove extremely useful for easy identification and molecular weight determination of a broad range of chemical vapors.
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Microarrays have been widely used for the analysis of gene expression, but the issue of reproducibility across platforms has yet to be fully resolved. To address this apparent problem, we compared gene expression between two microarray platforms: the short oligonucleotide Affymetrix Mouse Genome 430 2.0 GeneChip and a spotted cDNA array using a mouse model of angiotensin II-induced hypertension. RNA extracted from treated mice was analyzed using Affymetrix and cDNA platforms and then by quantitative RT-PCR (qRT-PCR) for validation of specific genes. For the 11,710 genes present on both arrays, we assessed the relative impact of experimental treatment and platform on measured expression and found that biological treatment had a far greater impact on measured expression than did platform for more than 90% of genes, a result validated by qRT-PCR. In the small number of cases in which platforms yielded discrepant results, qRT-PCR generally did not confirm either set of data, suggesting that sequence-specific effects may make expression predictions difficult to make using any technique.
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Improvements in the responses of semiconductor gas sensors and reductions in their detection limits toward volatile organic compounds (VOCs) are required in order to facilitate the simple detection of diseases, such as cancer, through human-breath analysis. In this study, we introduce a heater-switching, pulse-driven, micro gas sensor composed of a microheater and a sensor electrode fabricated with Pd-SnO2-clustered nanoparticles as the sensing material. The sensor was repeatedly heated and allowed to cool by the application of voltage to the microheater; the VOC gases penetrate into the interior of the sensing layer during its unheated state. Consequently, the utility factor of the pulse-driven sensor was greater than that of a conventional, continuously heated sensor. As a result, the response of the sensor to toluene was enhanced; indeed, the sensor responded to toluene at levels of 1 ppb. In addition, according to the relationship between its response and concentration toluene, the pulse-driven sensor in this report can detect toluene at concentrations of 200 ppt and even lower. Therefore, the combination of a pulse-driven microheater and a suitable material designed to detect toluene resulted in improved sensor response, and facilitated ppt-level toluene detection. This sensor may play a key role in the development of medical diagnoses based on human breath.
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This paper reported a high-performance e-nose type chemiresistive gas sensor composed of graphene oxide (GO) doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) nanowires. Large scale and well-defined sub-100 nm nanowires were prepared using nanoscale soft lithography in a highly efficient and facile way, facilitating subsequent device integration. The responses of the nanowire sensors to volatile organic compounds (VOCs) can be tuned by the different polymer components, which are utilized to constitute unique identification codes for ethanol, n-hexane, acetone and p-xylene and realize the discrimination of different VOCs. Besides, the score plot and classification matrix obtained respectively from the principal component analysis (PCA) and linear discriminant analysis (LDA) provide sufficient information to differentiate different VOCs.
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This paper describes the detection of volatile organic compounds (VOCs) using an e-nose system based on a virtual sensor array (VSA). The VSA was realized by seven different resonant modes of a single piezotransduced silicon bulk acoustic wave resonator (PSBAR) which can greatly reduce the complexity of a conventional e-nose system. The PSBAR was designed and fabricated using standard CMOS compatible process. The resonant modes of the PSBAR and its capability of VOCs discrimination were theoretically explored through finite element analysis. The discrimination effect was enhanced by the non-uniform adsorption of gas molecules on the top, side and bottom regions of the resonator. Score plots and radar fingerprints obtained respectively from principal component analysis (PCA) and fitted coefficient of adsorption isotherms for each VOC compose sufficient information to successfully discriminate different VOCs. The proposed novel VSA shows great potential as a compact and promising e-nose system.
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Herein, a simplistic quartz crystal microbalance (QCM) approach for discrimination of petroleum based fuels is presented. In this regard, a quartz crystal microbalance (QCM) virtual multisensor array (V-MSA) was employed to discriminate between different petroleum based fuels and to detect gasoline adulteration with high accuracy. First, an ionic liquid based V-MSA was used to discriminate between four fuel types (petroleum ether, gasoline, kerosene, and diesel). Subsequently, the system was used to successfully discriminate between three gasoline grades as a precursor for studies of gasoline adulteration. Finally, the system was used to detect and determine the nature of several gasoline adulterants at different v/v ratios (1%, 10%, 20% and 40%). Excellent accuracy (100%) was achieved for each study extolling the potential of this approach. This report represents the first example of a QCM sensor array utilized for detection of gasoline adulteration.
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This paper describes the detection of volatile organic compounds (VOCs) using an e-nose type integrated microfabricated sensor array, in which each resonator is coated with different supramolecular monolayers: p-tert-butyl calix[8]arene (Calix[8]arene), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (Porphyrin), beta-cyclodextrin (β-CD) and cucurbit[8]uril (CB[8]). Supramolecular monolayers fabricated by Langmuir-Blodgett techniques work as specific sensing interface for different VOCs recognition which increase the sensor selectivity. Microfabricated ultra-high working frequency transducers (4.4 GHz) enable their high sensitivity towards monolayer sensing which facilitate the analyses of VOCs adsorption isotherms and kinetics. Two affinity constants (K1, K2) are obtained for each VOC, which indicate the gas molecule adsorption happen inside and outside of the supramolecular cavities. Additional kinetic information (adsorption/desorption rate constants (ka, kd)) are obtained, thus enrich the sensing matrix (△f, K1, K2, ka, kd) which can be used as fingerprint patterns for highly specific detection and discrimination of VOCs.
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Herein, we demonstrate an alternative strategy for creating QCM-based sensor arrays by use of a single sensor to provide multiple responses per analyte. The sensor, which simulates a virtual sensor array (VSA), was developed by depositing a thin film of ionic liquid, either 1-octyl-3-methylimidazolium bromide ([OMIm][Br]) or 1-octyl-3-methylimidazolium thiocyanate ([OMIm][SCN]), onto the surface of a QCM-D transducer. The sensor was exposed to 18 different organic vapors (alcohols, hydrocarbons, chlorohydrocarbons, nitriles) belonging to the same or different homologous series. The resulting frequency shifts (Δf) were measured at multiple harmonics and evaluated using principal component analysis (PCA) and discriminant analysis (DA) which revealed that analytes can be classified with extremely high accuracy. In almost all cases, the accuracy for identification of a member of the same class, i.e. intraclass discrimination, was 100% as determined by use of quadratic discriminant analysis (QDA). Impressively, some VSAs allowed classification of all 18 analytes tested with nearly 100% accuracy. Such results underscore the importance of utilizing lesser exploited properties which influence signal transduction. Overall, these results demonstrate excellent potential of the virtual sensor array strategy for detection and discrimination of vapor phase analytes utilizing the QCM. To the best of our knowledge, this is the first report on QCM VSAs, as well as an experimental sensor array, that is based primarily on viscoelasticity, film thickness, and harmonics.
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The scope of the applications of breath sensors is abundant in disease diagnosis. Lung cancer diagnosis is a well-fitting health-related application of this technology, which is of utmost importance in the health sector, because lung cancer has the highest death rate among all cancer types, and it brings a high yearly global burden. The aim of this review is first to provide a rational basis for the development of breath sensors for lung cancer diagnostics from a historical perspective, which will facilitate the transfer of the idea into the rapidly evolving sensors field. Following examples with diagnostic applications include colorimetric, composite, carbon nanotube, gold nanoparticle-based, and surface acoustic wave sensor arrays. These select sensor applications are widened by the state-of-the-art developments in the sensors field. Coping with sampling sourced artifacts and cancer staging are among the debated topics, along with the other concerns like proteomics approaches and biomimetic media utilization, feature selection for data classification, and commercialization.
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The use of molecularly modified Si nanowire field effect transistors (SiNW FETs) for selective detection in liquid phase has been successfully demonstrated. In contrast, selective detection of chemical species in the gas phase has been rather limited. In this paper, we show that the application of artificial intelligence on deliberately controlled SiNW FET device parameters can provide high selectivity towards specific volatile organic compounds (VOCs). The obtained selectivity allows identifying VOCs in both single-component and multi-component environments as well as estimating the constituent VOC concentrations. The effect of the structural properties (functional group and/or chain length) of the molecular modifications on the accuracy of VOC detection is presented and discussed. The reported results have the potential to serve as a launching pad for the use of SiNW FET sensors in real-world counteracting conditions and/or applications.
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A micromachined metal oxide gas sensor operated with temperature modulation allows discrimination between air, H2 and CO in less than 1s. Furthermore, the actual humidity level and gas concentration can be estimated. We tested gas concentrations of 3–12ppm H2 and 12–100ppm CO at five relative humidity levels between 30 and 70%. While H2 can be identified for all concentrations, certain identification of CO requires concentrations above 20ppm and also determination of the relative humidity level from the sensor data.
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Molecularly modified silicon nanowire field effect transistors (SiNW FETs) are starting to appear as promising devices for sensing various volatile organic compounds (VOCs). Understanding the connection between the molecular layer structure attached to the SiNWs and VOCs is essential for the design of high performance sensors. Here, we explore the chain length influence of molecular layers on the sensing performance to polar and nonpolar VOCs. SiNW FETs were functionalized with molecular layers that have similar end (methyl) group and amide bridge bond, but differ in their alkyl chain lengths. The resulting devices were then exposed to polar and nonpolar VOCs in various conditions. Our results showed that the sensing response to changing the threshold voltage (ΔVth) and changing the relative hole mobility (Δµh/µh-a) have a proportional relationship to the VOC concentration. On exposure to a specific VOC concentration, ΔVth response increased with the chain length of the molecular modification. In contrast, Δµh/µh-a did not exhibit any obvious reliance on the chain length of the molecular layer. Analysis of the responses with an electrostatic-based model suggests that the sensor response in ΔVth is dependent on the VOC concentration, VOC vapor pressure, VOC-molecular layer binding energy, and VOC adsorption induced dipole moment changes of molecular layer. Understanding the relationship between the silicon nanowires' molecular layer structure and the VOCs is essential for the design of high performance sensors.
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Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from solution can be used only with certain classes of molecules. An alternative approach—fabrication of multilayers by consecutive adsorption of polyanions and polycations—is far more general and has been extended to other materials such as proteins or colloids. Because polymers are typically flexible molecules, the resulting superlattice architectures are somewhat fuzzy structures, but the absence of crystallinity in these films is expected to be beneficial for many potential applications.
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Tropical Morpho butterflies are famous for their brilliant iridescent colours, which arise from ordered arrays of scales on their wings. Here we show that the iridescent scales of the Morpho sulkowskyi butterfly give a different optical response to different individual vapours, and that this optical response dramatically outperforms that of existing nano-engineered photonic sensors. The reflectance spectra of the scales provide information about the nature and concentration of the vapours, allowing us to identify a range of closely related vapours–water, methanol, ethanol and isomers of dichloroethylene when they are analysed individually. By comparing the reflectance as a function of time for different vapours, we deduce that wing regions with scale structures of differing spatial periodicity give contributions to the overall spectral response at different wavelengths. Our optical model explains the effect of different components of the wing scales on the vapour response, and could steer the design of new man-made optical gas sensors.
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An emerging approach for diagnosing LC relies on volatile organic compounds (VOC), viz. organic compounds with relatively high vapor pressure or volatility, that can be detected in the headspace of cancer cells or blood samples, and/or in the exhaled breath. Identification, separation, and integration of the peaks in measured chromatograms for each sample. This might involve the use of Gaussian and non-Gaussian peak-fitting software, algorithms for the numerical calculation of the peak area, and algorithms for background compensation. Using internal standards might improve the reliability of the results, allowing a compensation of spectral shifts prior to the peak integration. The available statistical tests differ in the assumptions concerning the tested groups or populations: Gaussian and non-Gaussian populations, paired and unpaired groups, comparison between two or more groups.
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Carbon nanotube field-effect transistors (NTFETs) coated with poly(ethyleneimine) (PEI) and starch polymers exhibit electrical conductance changes upon exposure to CO2 gas in air at ambient temperature (see Figure). This observation has furnished nanoelectronic CO2 sensors. Their small size and low power consumption has enormous potential in wireless sensing for industrial and medical CO2 sensor units.
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In this study, responses of a sensor array were employed to establish a quality index model able to describe the different picking date of peaches. The principal component regression (PCR) and partial least-squares regressions (PLS) model represent very good ability in describing the quality indices of the selected three sets of peaches in calibration and prediction. The results showed that the PLS model represents a good ability in predicting quality index, with high correlation coefficients (R = 0.86 for penetrating force [CF]; R = 0.83 for sugar content [SC]; R = 0.83 for pH) and relatively low standard error of prediction (SEP; 8.77N, 0.299 °Brix, and 0.2 for CF, SC, and pH, respectively). The PCR model had high correlation coefficients (R = 0.84, 0.82, 0.78 for CF, SC, and pH, respectively) between predicted and measured values and a relatively low SEP (7.33N, 0.44 °Brix, 0.21 for CF, SC, and pH, respectively) for prediction. These results prove that the electronic noses have the potential to assess fruit quality indices. KeywordsPeach–Quality–Principal component regression (PCR)–Partial least-squares regressions (PLS)–Electronic nose
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The human nose is still the primary ‘instrument’ used to assess the smell or flavour of various industrial products today, despite considerable and sustained attempts to develop new electronic instrumentation capable of mimicking its remarkable ability. In this paper we review the research effort that has been carried out over the past 25 years or so to create an electronic nose. In doing so, we first provide a definition for the term electronic nose, and then discuss some of the technologies that have been explored in what is essentially an intelligent chemical array sensor system. Finally, we summarize the applications of electronic noses to date and suggest where future applications may lie.
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A novel gas sensor composed of electrospun nanofibrous membranes and quartz crystal microbalance (QCM) was successfully fabricated. The electrospun nanofibers with diameter of 100–400 nm can be deposited on the surface of QCM by electrospinning the homogenous blend solutions of cross-linkable poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). A series of nanofibrous membranes with various weight percentage of PAA to PVA were fabricated and characterized regarding their morphology and sensitivity to NH3. Sensing experiments were examined by measuring the resonance frequency shifts of QCM which due to the additional mass loading. The results showed that the sensing properties were mainly affected by the content of PAA component in nanofibrous membranes, concentration of NH3, and relative humidity. Additionally, the sensitivity of nanofibrous membranes coated (NMC) QCM sensor was much higher than that of continuous films coated QCM sensor.
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Silicon nanowire field-effect transistors (Si NW FETs) have been used as powerful sensors for chemical and biological species. The detection of polar species has been attributed to variations in the electric field at the conduction channel due to molecular gating with polar molecules. However, the detection of nonpolar analytes with Si NW FETs has not been well understood to date. In this paper, we experimentally study the detection of nonpolar species and model the detection process based on changes in the carrier mobility, voltage threshold, off-current, off-voltage, and subthreshold swing of the Si NW FET. We attribute the detection of the nonpolar species to molecular gating, due to two indirect effects: (i) a change in the dielectric medium close to the Si NW surface and (ii) a change in the charged surface states at the functionality of the Si NW surface. The contribution of these two effects to the overall measured sensing signal is determined and discussed. The results provide a launching pad for real-world sensing applications, such as environmental monitoring, homeland security, food quality control, and medicine.
Article
An eight-sensor array coupling a chemoselective room-temperature ionic liquid (RTIL) with quartz crystal microbalance (QCM) transduction is presented in this work in order to demonstrate the power of this approach in differentiating closely related analytes in sensory devices. The underlying mechanism behind the specific sensory response was explored by (i) studying mass loading and viscoelasticity effects of the sensing layers, predominantly through variation in damping impedance, the combination of which determines the sensitivity; (ii) creation of a solvation model based on Abraham's solvation descriptors which reveals the fact that polarizability and lipophilicity are the main factors influencing the dissolution of gas analytes into the RTILs; and (iii) determination of enthalpy and entropy values for the studied interactions and comparison via a simulation model, which is also effective for pattern discrimination, in order to establish a foundation for the analytical scientist as well as inspiration for synthetic pathways and innovative research into next-generation sensory approaches. The reported sensors displayed an excellent sensitivity with detection limit of <0.2%, fast response and recovery, and a workable temperature range of 27-55 °C and even higher. Linear discriminant analysis (LDA) showed a discrimination accuracy of 86-92% for nitromethane and 1-ethyl-2-nitrobenzene, 71% for different mixtures of nitromethane, and 100% for these analytes when thermodynamic parameters were used as input data. We envisage applications to detecting other nitroaromatics and security-related gas targets, and high-temperature or real-time situations where manual access is restricted, opening up new horizons in chemical sensing.
Article
By functionalizing the surfaces of ZnO nanobelts (NBs) with a thin self-assembled molecular layer, the electrical and optoelectronic performances of a single NB-based device are drastically improved. For a single NB-based device, due to energy band tuning and surface modification, the conductance was enhanced by 6 orders of magnitude upon functionalization; a coating molecule layer has changed a Schottky contact into an Ohmic contact without sophisticated deposition of multilayered metals. A functionalized NB showed negative differential resistance and exhibited huge improved photoconductivity and gas sensing response. The functionalized molecular layer also greatly reduced the etching rate of the ZnO NBs by buffer solution, largely extending their life time for biomedical applications. Our study demonstrates a new approach for improving the physical properties of oxide NBs and nanowires for device applications.
Article
Olfaction exhibits both high sensitivity for odours and high discrimination between them. We suggest that to make fine discriminations between complex odorant mixtures containing varying ratios of odorants without the necessity for highly specialized peripheral receptors, the olfactory systems makes use of feature detection using broadly tuned receptor cells organized in a convergent neurone pathway. As a test of this hypothesis we have constructed an electronic nose using semiconductor transducers and incorporating design features suggested by our proposal. We report here that this device can reproducibly discriminate between a wide variety of odours, and its properties show that discrimination in an olfactory system could be achieved without the use of highly specific receptors.
Article
A QCM device employing ionic liquids as the sensing materials for organic vapors has been developed and evaluated. The sensing mechanism is based on the fact that the viscosity of the ionic liquid membrane decreases rapidly due to solubilization of analytes in the ionic liquids. This change in viscosity, which varies with the chemical species of the vapors and the types of ionic liquids, results in a frequency shift of the corresponding quartz crystal. The QCM sensor demonstrated a rapid response (average response time of less than 2 s) to organic vapors with an excellent reversibility because of the fast diffusion of analytes in ionic liquids. Furthermore, the ionic liquids, with zero vapor pressure and stable chemical properties, ensure a long-term shelf life for the sensor.
Article
The sensing ability of individual SnO(2) nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.
Article
The development of the electronic nose have paved the way for the classification of bacteria, to monitor air quality on the space shuttle, or to check the spoilage of foodstuff. However, the electronic nose still is unable to discriminated between flavors, perfumes, smells and as a replacement for the human nose. Although it has been used to detect some important nonodorant gases, it is not adapted to substances of daily importance in mammalian life such as the scent of other animals, foodstuff or spoilage. Due to such limitations, the electronic nose was developed to mimic the human nose. It turns out that the human nose's unequaled performance is not due to the high number of different human receptor cells, but their selectivity and their unsurpassed sensitivity for some analyte gases. As such, the success of the electronic nose will not rely on increasing the number of individual sensors and creating redundant information by adding more similar sensors, but rather on DNA, molecular, imprinted molecules or even mobilized natural receptors, which promise to increase the sensitivity and importantly selectivity. An increase in the sensitivity can be achieved by appropriate sample pretreatment and preconcentration techniques, whereas filters and separation units can be used to increase the selectivity and reduce interfering substances.
Conference Paper
Pellistors are well known for the detection of combustible gases. In this paper we present a simple preparation technique for micro pellistors based on a commercially available Si substrate, which is normally used for semiconducting gas sensitive layers. Catalytically active layers are achieved by drop deposition of very low volumes of noble metal containing solutions. Temperature cycling and intelligent signal processing are used to achieve high selectivity. Due to low thermal time constants discrimination between gasoline and diesel fumes is achieved within two seconds, allowing the development of a fuel sensor for integration in fuel nozzles which can prevent filling the car tank with the wrong fuel.
Article
To improve the selectivity of semiconductor gas sensors, temperature modulation is often used. We present a study showing the potential of this technique with information gained comparable to multisensor systems. A dynamic operating mode coupled with a low-complexity evaluation strategy allows the identification of six organic solvent vapors over a wide concentration range (2-200 ppm) with a single sensor, for example, for leak detection systems. The system features low false alarm rates; in addition, interference by other gases, such as CO or NO<sub>2</sub>, can be suppressed. For even higher identification power, switching on-line between different temperature cycles was studied, which provides better information for critical decisions.
Differential Solute Gas Response in Ionic-Liquid-Based QCM Arrays: Elucidating Design Factors Responsible for Discriminative Explosive Gas Sensing
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Molecular Weight Sensing Properties of Ionic Liquid-Polymer Composite Films: Theory and Experiment
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New Method for Selectivity Enhancement of SiC Field Effect Gas Sensors for Quantification of
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