Article

Detection of Volatile Organic Compounds Using Microfabricated Resonator Array Functionalized with Supramolecular Monolayers

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Abstract

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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... 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. ...
... The detailed fabrication process of FBARs was reported previously. 22 Figure 1(a) presents a cross-sectional view of the FBAR structure coated with an HFBI film layer. Numerous researchers have attempted to exploit FBAR devices for sensor applications with remarkable results. ...
... Further details of the detection system were reported previously. 22,24 Figures 2(a) and 2(b) present the real-time responses of the bare FBAR and HFBI-coated FBAR sensors upon exposure to seven different concentrations of ethanol vapor. The sensors were first flushed with pure nitrogen to obtain a stable baseline. ...
Article
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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.
... Acoustic e-noses have been used for the detection of volatile organic compounds (VOCs), chemical warfare agents (CWAs), volatile biomarkers, and odors. Correspondingly, gaseous samples have consisted of indoor, outdoor, or artificial air, breath, and headspace of liquid or solid samples, such as wastewater, food, and plants [25,[37][38][39][40][41][42][43][44]. Further applications include the use as detector for gas chromatography (GC) instead of mass spectrometry (MS) [45,46] and the use as sensor node in sensor networks [47]. ...
... Similarly, an array of four FBARs, each one coated with another type of supramolecular monolayer, was used for the selective detection of six aliphatic compounds from four classes, i.e., two hydrocarbons, one chlorinated hydrocarbon, two alcohols, and one ketone. Furthermore, kinetic and thermodynamic constants were calculated out of the response curves to quantify the interactions between the respective gas molecules and supramolecular monolayers [37]. ...
Article
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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.
... As a result, the properties of sensitive coating determine the molecule recognitions, and directly affect the sensitivity, stability and reversibility of the sensor. Up to now, a variety of sensitive coatings, such as polymers [23,24], proteins [25][26][27], aptamers [28][29][30][31], enzymes [32,33], supramolecular monolayers [34,35], hydrophilic film [23] and CNTs [36] have been employed for FBAR sensors to achieve the selectivity for different analytes. ...
... Each sensor is sensitive to some analytes with different responses; thus, a fingerprint pattern is generated for specific target recognition from the complex environment. For example, Yao el al. [34] demonstrated an e-nose type gas sensor for the selective detection of volatile organic compounds based on the FBAR sensor array functionalized with four supramolecular monolayers (p-tert-butyl calix [8]-arene, porphine, β-cyclodextrin, and cucurbit [8]uril.). ...
Article
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We demonstrate a promising strategy to combine the micro-electromechanical film bulk acoustic resonator and the nanostructured sensitive fibers for the detection of low-concentration formaldehyde vapor. The polyethyleneimine nanofibers were directly deposited on the resonator surface by a simple electrospinning method. The film bulk acoustic resonator working at 4.4 GHz acted as a sensitive mass loading platform and the three-dimensional structure of nanofibers provided a large specific surface area for vapor adsorption and diffusion. The ultra-small mass change induced by the absorption of formaldehyde molecules onto the amine groups in polyethyleneimine was detected by measuring the frequency downshift of the film bulk acoustic resonator. The proposed sensor exhibits a fast, reversible and linear response towards formaldehyde vapor with an excellent selectivity. The gas sensitivity and the detection limit were 1.216 kHz/ppb and 37 ppb, respectively. The study offers a great potential for developing sensitive, fast-response and portable sensors for the detection of indoor air pollutions.
... Based on specific "host-guest" interactions, the specificity and sensitivity of sensors can be highly increased with supramolecular coatings. An e-nose-type inte- grated FBAR array in which each sensor was coated with different supramolecular monolayers has been proposed for VOC detection ( Lu et al. 2015). The supramolecular monolayers used in this work were fabricated using the Langmuir-Blodgett techniques as a specific sensing interface for VOC recognition ( Lu et al. 2015). ...
... An e-nose-type inte- grated FBAR array in which each sensor was coated with different supramolecular monolayers has been proposed for VOC detection ( Lu et al. 2015). The supramolecular monolayers used in this work were fabricated using the Langmuir-Blodgett techniques as a specific sensing interface for VOC recognition ( Lu et al. 2015). The obtained affinity constants for each VOC indicate that vapor molecule adsorption occurs inside and outside of the supramolecular cavities. ...
... 348−351 A recent example was an e-nose made with an integrated microfabricated sensor array, with the resonators being coated with LB films of four supramolecular structures, namely, p-tertbutyl calix [8]arene (calix [8]arene), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (porphyrin), β-cyclodextrin (β-CD), and cucurbit [8]uril (CB [8]). 351 Figure 26 shows a schematic diagram of the coated resonators and the device architecture, including an SEM image. High sensitivity was obtained with film bulk acoustic resonator (FBAR) transducers for various VOCs, which could be discriminated by treating the frequency shift signals of the mass transducers. ...
... High sensitivity was obtained with film bulk acoustic resonator (FBAR) transducers for various VOCs, which could be discriminated by treating the frequency shift signals of the mass transducers. 351 The use of LB films in e-tongues started in the first decade of this century (the 2000s), using mainly impedance spectroscopy and electrochemical methods as detection principles and with semiconducting polymers, phthalocyanines, and ruthenium complexes being the main types of materials used. 352−357 The variety of materials employed increased significantly, including lignins 358,359 and perylenes 360,361 (see the reviews 362−364 ). ...
... The quality factor is as high as 2427 when operating at ultrahigh frequency of 1.44 × 10 9 Hz, indicating high acoustic energy density and well-trapped waves within the piezoelectric layer. In addition, according to Sauerbrey's equation [25], the frequency shift of masssensitive FBAR for a certain mass load increases proportional to the resonance frequency squared; hence, ultrahigh-frequency FBAR shows markedly enhanced sensitivity for humidity sensing. ...
... The quality factor is as high as 2427 when operating at ultrahigh frequency of 1.44 × 10 9 Hz, indicating high acoustic energy density and well-trapped waves within the piezoelectric layer. In addition, according to Sauerbrey's equation [25], the frequency shift of mass-sensitive FBAR for a certain mass load increases proportional to the resonance frequency squared; hence, ultrahigh-frequency FBAR shows markedly enhanced sensitivity for humidity sensing. ...
Article
We developed a highly sensitive humidity sensor based on the combination of ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET) thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity, linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal microbalance (QCM). Additionally, temperature dependence was investigated with the result that PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the selective detection of water vapor from volatile organic compounds (VOCs) with respect to the polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency, PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless sensor networks (WSNs).
... On the other hand, the acoustic wave-based devices are less expensive without compromising their sensitivity. Bulk acoustic wave (BAW) resonators have been widely employed as sensors for detecting variables such as mass García-Gancedo et al., 2013;He et al., 2011;Mai et al., 2004;Nagaraju et al., 2014;Rey-Mermet et al., 2006), ultra-violet light (Bian et al., 2015), infrared light (Wang et al., 2011a), ozone (Wang et al., 2011b), pressure (Giangu et al., 2015), humidity (Zhang et al., 2015a), volatile organic compounds (Chang et al., 2016;Lu et al., 2015), air pollution and biomarkers (Chen et al., 2015b;Dickherber et al., 2008;Guo et al., 2015;Tukkiniemi et al., 2009;Weber et al., 2006;Wingqvist et al., 2008) owing to their high sensitivity, real-time detection, label-free and wireless capabilities (Voiculescu and Nordin, 2012). Table 1 compares the mostly reported technologies used in biosensors. ...
Article
Biosensors play important roles in different applications such as medical diagnostics, environmental monitoring, food safety, and the study of biomolecular interactions. Highly sensitive, label-free and disposable biosensors are particularly desired for many clinical applications. In the past decade, film bulk acoustic resonators (FBARs) have been developed as biosensors because of their high resonant frequency and small base mass (hence greater sensitivity), lower cost, label-free capability and small size. This paper reviews the piezoelectric materials used for FBARs, the optimisation of device structures, and their applications as biosensors in a wide range of biological applications such as the detection of antigens, DNAs and small biomolecules. Their integration with microfluidic devices and high-throughput detection are also discussed.
... FBARs are typical piezoelectrical MEMS devices with a thin piezoelectrical layer sandwiched between top and bottom electrodes (Fig. 5). Similar to SAW gas sensors, coating sorptive materials on FBARs can improve sensitivity and selectivity, and sensor arrays using FBARs can quantitatively and qualitatively identify GC eluates [32][33][34][35]. In contrast to SAW, FBAR is much smaller in size with a high resonating frequency (1-10 GHz). ...
Article
Micro gas chromatography (μGC) has been continuously gaining attention since the last century owing to multiple favorable characteristics, such as its small size, low power consumption and minimal production and maintenance costs. μGC has the potential to provide practical solutions to emerging analytical challenges in security, health, and environment. In this review, we summarize recent advances in micro detectors for μGC, including the study of the miniaturization of conventional detectors and the development of novel detectors for μGC chromatography.
... The basic principle of the technique can be summarized as this: When the surface of the quartz is exposed to VOCs, there is a mass uptake actualized by sensing thin film layer and the frequency change created by this mass uptake generates a signal in terms of wave propagation. [44,45] It means that the QCM systems allow the measurement of very small frequency changes. Acikbas et al. studied the organic vapor sensing properties of a QCM sensor coated with poly[Styrene (ST)-co-Glycidyl Methacrylate (GMA)] copolymer by LB technique. ...
Article
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Determination of organic vapor sensing properties of α-Naphthylmethacrylate (α-NMA) monomer based Langmuir-Blodgett (LB) thin films was aimed in this study. LB thin film fabrication was performed on quartz glass and quartz crystal substrates in order to investigate the characterization and organic vapor properties of α-NMA materials by using UV-Visible, Atomic Force Microscopy (AFM) and Quartz Crystal Microbalance (QCM) techniques. π-A isotherm graph was taken and a suitable surface pressure value were primarily determined as 13 mN m⁻¹ for successful α-NMA LB thin film fabrication. Transfer ratio value was found to be ≥ 0.93 for quartz glass and quartz crystal substrates. The typical frequency shift per layer was obtained as 16.93 Hz/layer and the deposited mass onto a quartz crystal was calculated as 271.30 ng/layer (1.02 ng mm⁻²). The sensing responses of α-NMA LB films against dichloromethane, chloroform, toluene and m-xylene were measured by QCM system. Dichloromethane created the maximum shift in the resonance frequency than other organic vapors used in this study. Results exhibited that α-NMA LB thin films were potential candidates for organic vapor sensing applications, especially high sensitive detection of dichloromethane at room temperature.
... In addition, FBAR sensors are known for providing a simple sensing method based on the measurements of resonator frequency shifts and have been widely used as portable electronic gas sensors. 34,35 Hence, in this work, we are motivated to develop a singlechip VSA system based on the ultrahigh-frequency FBAR. To improve the gas-sensing characteristics of this system, selfassembled poly(sodium 4-styrenesulfonate) (PSS)/poly-(diallyldimethylammonium chloride) (PDDA) thin films are adopted as a gas-sensitive layer. ...
Article
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.
... Owning to the development of microsystem and nanotechnology, acoustic devices based on piezoelectric materials have gained increasing attention in biochemical research field [41][42][43][44] which is due to their low cost, batch manufacturing, small volume and noninvasive to biomolecules [45][46][47]. Here, we demonstrated a novel and versatile controlled release approach using gigahertz ultrasound (hypersound) induced by a nano-electromechanical acoustic resonator composed of ultra-thin material layers (several tens to hundreds of nanometers thick). ...
Article
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Background Controllable and multiple DNA release is critical in modern gene-based therapies. Current approaches require complex assistant molecules for combined release. To overcome the restrictions on the materials and environment, a novel and versatile DNA release method using a nano-electromechanical (NEMS) hypersonic resonator of gigahertz (GHz) frequency is developed. Results The micro-vortexes excited by ultra-high frequency acoustic wave can generate tunable shear stress at solid–liquid interface, thereby disrupting molecular interactions in immobilized multilayered polyelectrolyte thin films and releasing embedded DNA strands in a controlled fashion. Both finite element model analysis and experiment results verify the feasibility of this method. The release rate and released amount are confirmed to be well tuned. Owing to the different forces generated at different depth of the films, release of two types of DNA molecules with different velocities is achieved, which further explores its application in combined gene therapy. Conclusions Our research confirmed that this novel platform based on a nano-electromechanical hypersonic resonator works well for controllable single and multi-DNA release. In addition, the unique features of this resonator such as miniaturization and batch manufacturing open its possibility to be developed into a high-throughput, implantable and site targeting DNA release and delivery system.
... When the sensors come in contact with volatile organic compounds (VOCs), the sensor surface undergoes a physical or chemical change of the sensor [4], and generally its resultant signals are converted into digital values. The commonly used sensors include surface plasmon resonance (SPR) [5], quartz crystal microbalance (QCM) [6], surface acoustic wave (SAW) [7], bulk acoustic wave (BAW) [8], conducting polymers (CP) [9], field-effect transistor (FET)-type transducers [10], etc. Although electronic noses are sensitive to odorants in a specific way, most electronic noses face a significant challenge in terms of specificity of the sensors. ...
Article
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A bioelectronic nose, an intelligent chemical sensor array system coupled with bio-receptors to identify gases and vapours, resembles mammalian olfaction by which many vertebrates can sniff out volatile organic compounds (VOCs) sensitively and specifically even at very low concentrations. Olfaction is undertaken by the olfactory system, which detects odorants that are inhaled through the nose where they come into contact with the olfactory epithelium containing olfactory receptors (ORs). Because of its ability to mimic biological olfaction, a bio-inspired electronic nose has been used to detect a variety of important compounds in complex environments. Recently, biosensor systems have been introduced that combine nanoelectronic technology and olfactory receptors themselves as a source of capturing elements for biosensing. In this article, we will present the latest advances in bioelectronic nose technology mimicking the olfactory system, including biological recognition elements, emerging detection systems, production and immobilization of sensing elements on sensor surface, and applications of bioelectronic noses. Furthermore, current research trends and future challenges in this field will be discussed.
... Many works on resonator gas sensors have focused on their adsorption reaction for sensitivity enhancement. The common method was making an additional highly sensitive layer, which was porous [17], nanostructured [18] or had a good chemical reaction with the adsorbate [19], on the surface of the sensor. In the literature, the response of the resonator gas sensors to different gases was presented [20][21][22][23], but the exact relationship between the frequency shift and the concentration of the gas, which was necessary for the detection application, was seldom defined. ...
Article
Full-text available
This paper presents a sensitivity-enhanced gas sensor based on a film bulk acoustic resonator (FBAR). It was designed and fabricated with micro through-holes in its top electrode for sensitivity enhancement. The sensor was driven by a Colpitts oscillator circuit, and the output signal had characteristics of a power of -2.6 dBm@3 V and a phase noise of -90 dBc/Hz@100 kHz. In order to test the performance of the sensor, it was used for the detection of relative humidity (RH) and ethanol. When the relative humidity ranged from 25% to 88%, the frequency shift of the sensor was 733 kHz, which was 3.2 times higher than that of the existing FBAR sensor with a complete top electrode. Fitting results of the frequency shift and the relative humidity indicated that the measurement error was within ±0.8% RH. When the ethanol concentration ranged from 0 to 0.2355 g/L, the frequency shift of the sensor was 365 kHz. The effect of the oscillator circuit on the adsorption reaction and temperature response of the FBAR sensor device was analyzed to optimize its detection application.
... Microwave gas sensors are emerging as cheap and label-free techniques, and the lack of selectivity can be overcome by combining with highly selective materials [16][17][18][19][20]. Among different types of gas sensors, MEMS piezoelectric gas sensors, such as surface acoustic wave (SAW) resonators [21], Lamb wave resonators (LWR) [22] and film bulk acoustic resonators (FBAR) [23][24][25] have triggered a lot research interest due to their low power consumption, micrometer-scaled sizes, and relatively high sensitivities. Compared with quartz crystal microbalance (QCM), however, they suffer from relatively low Q values, which may result in poor limit of detection (LOD), large phase noise, and instability when integrating with oscillating circuits. ...
Article
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This paper demonstrates a novel micro-size (120 µm × 200 µm) piezoelectric gas sensor based on a piezotransduced single-crystal silicon bulk acoustic resonator (PSBAR). The PSBARs operate at 102 MHz and possess high Q values (about 2000), ensuring the stability of the measurement. A corresponding gas sensor array is fabricated by integrating three different self-assembled monolayers (SAMs) modified PSBARs. The limit of detection (LOD) for ethanol vapor is demonstrated to be as low as 25 ppm with a sensitivity of about 1.5 Hz/ppm. Two sets of identification code bars based on the sensitivities and the adsorption energy constants are utilized to successfully discriminate isopropanol (IPA), ethanol, hexane and heptane vapors at low and high gas partial pressures, respectively. The proposed sensor array shows the potential to form a portable electronic nose system for volatile organic compound (VOC) differentiation.
... Both types of arrays provided consistent results when assisted by PCA and discriminant factor analysis (DFA). Lu et al. reported on microfabricated FBAR sensor arrays coated with cavitands (calix [8]arene, porphyrin, β-cyclodextrin, cucurbit [8]uril) for selective VOC detection [232]. The authors assessed the suitability of their devices for an integrated electronic nose, but they did not fabricate the EN or use MDA in their investigation. ...
Article
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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.
... Mass transducers, such as quartz microbalances and surface acoustic waves, are the simplest choice to prepare sensors with nonconductive molecular materials (595). The mass resolution of these sensors is on the order of nanograms, and assuming that each sensitive molecule captures one volatile compound, the number of sensitive molecules in a single sensor can be very large. ...
Article
The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.
... Dunbar et al. reported on colorimetric gas sensors based on 5,10,15,20-tetrakis (3,4-bis(2-ethylhexyloxy) phenyl)-21H, 23H-porphine with six kinds of metal ligands (Mg, Sn, Zn, Au, Co, and Mn) for the PCA of volatile organic compounds (VOCs) [26]. To further increase selectivity, Chang et al. reported on a Langmuir Blodgett film consisting of a combination of supramolecular monolayers and metalloporphyrin [27]. 5,10,15,20-Tetraphenyl-21H, 23H-porphine cobalt (II) (CoTPP) has high affinity to NO gas adsorption [28,29]. ...
Article
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We report on an optical nitrogen oxide (NO) gas sensor device using cobalt tetraphenylporphyrin (CoTPP) dispersed in three kinds of hydrophobic polymer film matrix (polystyrene (PSt), ethylcellulose (EC), and polycyclohexyl methacrylate (PCHMA)) to improve humidity resistance. Our approach is very effective because it allows us to achieve not only high humidity resistance, but also a more than sixfold increase in sensitivity compared with CoTPP film due to the high dispersion of CoTPP in the polymer film. The limit of detection was calculated as 33 ppb for the CoTPP-dispersed EC film, which is lower than that of CoTPP film (92 ppb).
... An FBAR humidity sensor without any sensing film was reported [7], though this type of sensor showed nonlinearity for the humidity response. Although substantial progress has been made in developing Journal of Micromechanics and Microengineering A film bulk acoustic resonator oscillator based humidity sensor with graphene oxide as the sensitive layer high sensitivity acoustic humidity sensors, so far, most of the research has used discrete acoustic resonators for investigation, and the performances of the humidity sensors were characterized using network analyzers [8,9], far from practical application. For application, chip-type sensors and the corresponding integrated electronics for measuring frequency shift and signal processing are necessary, but limited work has been done in this area, not to mention on chip-type sensors with nanomaterials as the sensitive layers. ...
Article
A film bulk acoustic wave resonator (FBAR) is a type of resonator with high frequency and small dimensions, particularly suitable for use as a sensor for physical and biochemical sensing with high sensitivity. FBAR-based sensors have been extensively studied, however they commonly use discrete devices and network analyzers for evaluation, and therefore are far from being able to be used in practical applications. This paper reports the design and analysis of an FBAR-based Pierce oscillator and a field-programmable gate array (FPGA)-based frequency counter, and the use of the oscillator as a humidity sensor with the frequency counter as the measuring circuit. Graphene oxide (GO) is used as the sensitive film to improve the sensitivity. The resonant frequency of the oscillator with a GO film shows a linear decrease with an increase in relative humidity, with a sensitivity of ca. 5 kHz per %RH (relative humidity) in the range of 3%RH to 70%RH, and a higher frequency shift is induced above 70%RH. The FBAR oscillator sensor shows excellent stability and repeatability, demonstrating the feasibility and potential sensing application using the integrated FBAR chip and simple frequency counter, particularly suitable for portable electronics.
... The gravimetric-based measurement leads to a sensing signal, which carries information about the adsorption of analytes [24]. In recent years, rapid progress has been made in the field of gas sensing using thin film bulk acoustic resonators (FBARs) [25][26][27][28][29]. Due to their GHz-level resonant frequency and high quality-factor, FBARs show higher sensitivity than conventional acoustic wave devices [30,31]. ...
Article
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In this paper, we develop a novel dual-mode gas sensor system which comprises a silicon nanoribbon field effect transistor (Si-NR FET) and a film bulk acoustic resonator (FBAR). We investigate their sensing characteristics using polar and nonpolar organic compounds, and demonstrate that polarity has a significant effect on the response of the Si-NR FET sensor, and only a minor effect on the FBAR sensor. In this dual-mode system, qualitative discrimination can be achieved by analyzing polarity with the Si-NR FET and quantitative concentration information can be obtained using a polymer-coated FBAR with a detection limit at the ppm level. The complementary performance of the sensing elements provides higher analytical efficiency. Additionally, a dual mixture of two types of freons (CFC-113 and HCFC-141b) is further analyzed with the dual-mode gas sensor. Owing to the small size and complementary metal-oxide semiconductor (CMOS)-compatibility of the system, the dual-mode gas sensor shows potential as a portable integrated sensing system for the analysis of gas mixtures in the future.
... When the quartz surface is exposed to a gas, a frequency change occurs as a result of the mass uptake on the surface of the thin film layer. This change generates a signal in terms of wave propagation (Kang et al., 2001;Lu et al., 2015). Even very small frequency changes can be detected by this system (Adhikari & Majumdar, 2004). ...
Article
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In the present work, the characterization and gas sensing properties of newly synthesized N ‐(4‐methylpyrimidine‐2‐yl)methacrylamide (N‐MPMA ) monomer Langmuir–Blodgett (LB) thin films were investigated. The UV–visible spectroscopy, quartz crystal microbalance (QCM), and atomic force microscopy were utilized to characterize N‐MPMA LB thin films. The surface behavior of N‐MPMA monolayer was stable and allowed an effective transfer at a surface pressure of 14 mN/m. The mass change/unit area value of the N‐MPMA LB thin film deposited quartz crystal surfaces was investigated. The amount of N‐MPMA LB thin film deposited on the substrate for bilayer was calculated as 228.72 ng (86.31 ng/mm²) and 12.5 Hz frequency shift was observed for each layer of the N‐MPMA film. The kinetic responses of N‐MPMA LB film against chloroform, dichloromethane, benzene, and toluene were measured via QCM system at room temperature. N‐MPMA QCM sensor results displayed that chloroform has the largest frequency shifts compared with the other vapors used in the present work and these results can be illuminating in terms of physical properties of organic vapors.
Article
This study presents the sensing studies of QCM sensors which coated with calix[4]arene derivatives bearing different functional groups towards some selected Volatile Organic Compounds (VOCs). Initial experiments revealed that QCM sensor coated with calix-3 bearing bromopropyl functionalities was relatively more effective sensor for methylene chloride (MC) emissions than the other calix[4]arene coated QCM sensors, in aqueous media. In further experiments, this effective calix-3 coated QCM sensor were used in detailed sensing studies of selected VOCs. However, the results demonstrated that calix-3 coated QCM sensor was most useful sensor for toluene (TOL) emissions among all. Moreover, the sensing of TOLs with calix-3 coated QCM sensor was also evaluated in terms of sorption phenomena. Consequently, calix-3 coated QCM sensor was good sensor for TOL emissions, and thus it demonstrated that the coating of QCM sensor surface with calixarenes was good approach for sensing of the VOCs.
Article
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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Trimanganese tetraoxide (Mn3O4)−silver (Ag) nanocomposites have been synthesized by seed-mediated growth of manganese oxide by alkaline hydrolysis of manganese precursor on preformed silver nanoparticles at water/n-heptane interface. The morphology of the nanocomposites has been varied by individual addition of five size-selective silver nanoparticles for the evolution of the manganese components. Varieties of spectroscopic and microscopic techniques have been used to characterize these materials. Now, the efficacy of these as-synthesized nanocomposites has been demonstrated towards the sensing of various hazardous volatile organic compounds, exhibiting high sensitivity to ethanol compared to other components. Moreover, the structure-function relationship of the Mn3O4−Ag nanocomposites upon variation of the size of silver particles towards the sensory activity of ethanol has been elucidated.
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In this work, we presented a thin-film piezoelectric acoustic gas sensor with enhanced sensitivity by a surface modification strategy of oxygen plasma treated graphene oxide (GO) functionalization. By exposing to ammonia vapor (NH3) of various concentrations at controlled temperature and humidity, the characteristics of the GO-coated acoustic sensor were investigated, i.e. sensitivity, linearity, response, and recovery time. Oxygen plasma treatment of the GO-coated sensor further enhanced the sensitivity compared with the freshly prepared GO-coated sensor. The mechanism of oxygen plasma treatment effect on the GO-coated sensor was discussed based on characterizations of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscope (SEM), and precise weighing of the acoustic sensor. It was found that the oxygen plasma treatment introduces numerous defects to GO flakes, which are uniformly distributed across the GO surface, providing more gas molecule binding sites.
Chapter
This chapter reports on the state of the art of piezoelectric micro-/nano-mechanical devices in frequency control and sensing applications. Recent studies on bulk acoustic wave (BAW) devices are introduced, including investigation of high-coupling materials and filter and oscillator designs. A novel class of frequency devices based on Lamb waves is also reviewed. Micro- and nano-mechanical sensors for various sensing applications and integrated module are outlined.
Article
Trace formaldehyde vapor was detected by a micron-scale AlN film bulk acoustic resonator based on mass-sensitive mechanism. The layer-by-layer carbon nanotubes/polyethyleneimine multilayers were assembled on the resonator surface as the sensitive coating. An almost linear decrease of the resonant frequency was observed as a function of the number of nanotubes/polyethyleneimine periods. The multilayers showed a random and porous structure and thus provided a large specific surface area for gas adsorption and diffusion. At the same time, the amine groups in polyethyleneimine had an strong affinity to formaldehyde with excellent selectivity. When exposed to gaseous formaldehyde, the attachment of gas molecules induced a small decrease in the resonant frequency, which made the sensor easily detect formaldehyde at ppb levels with 1 min response time. A linear relationship was observed between the formaldehyde concentrations and the frequency downshift of the resonator. The layer number had an obvious influence on the absorption/desorption behavior of formaldehyde. The gas sensitivity of FBAR sensors was 1.29-1.90 kHz ppb⁻¹ with the limit of detection of 24-38 ppb.
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Mellitic triimide derivatives with N -benzyl groups form conformations with concave cavities, that are capable of including aromatic molecules such as benzene, nitrobenzene, etc. In addition, they act as supramolecular gelators,...
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We fabricated a ZnO piezoelectric film resonator modified with multi-walled carbon nanotubes/polyethyleneimine bilayer as the sensitive layer for the detection of trace gaseous formaldehyde. The resonator consists of a ZnO piezoelectric stack and an SiO2/W Bragg reflector. The multi-walled carbon nanotubes were self-assembled on the resonator surface using the n-octadecyl mercaptan monolayer and further modified with polyethyleneimine. The formaldehyde molecules are absorbed on the multi-walled carbon nanotubes/polyethyleneimine bilayer based on the reversible nucleophilic addition reaction between formaldehyde molecules and the amine functional groups on polyethyleneimine. The high working frequency (~ 3.1 GHz) of the resonator provided enough mass sensitivity to probe the ultra-small mass change of the sensitive biolayer. The downshift of resonant frequency was linear with the increase of formaldehyde concentration. The experimental results show that our proposed sensor can yield rapid, sensitive, reversible and repeatable responses to formaldehyde in the concentration range of 50–400 ppb at room temperature. The piezoelectric film resonator is a promising and feasible sensor for the indoor pollution monitoring.
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This paper presented a high-performance pressure sensor based on a film bulk acoustic resonator (FBAR). The support film of the FBAR chip was made of silicon nitride and the part under the resonator area was etched to enhance the sensitivity and improve the linearity of the pressure sensor. A micro resistor temperature sensor and a micro resistor heater were integrated in the chip to monitor and control the operating temperature. The sensor chip was fabricated, and packaged in an oscillator circuit for differential pressure detection. When the detected pressure ranged from -100 hPa to 600 hPa, the sensitivity of the improved FBAR pressure sensor was -0.967 kHz hPa⁻¹, namely -0.69 ppm hPa⁻¹, which was 19% higher than that of existing sensors with a complete support film. The nonlinearity of the improved sensor was less than ±0.35%, while that of the existing sensor was ±5%. To eliminate measurement errors from humidity, the temperature control system integrated in the sensor chip controlled the temperature of the resonator up to 75 °C, with accuracy of ±0.015 °C and power of 20 mW.
Article
Frequent assay of hemostatic status is an essential issue for the millions of patients using anticoagulant drugs. In this paper, we presented a micro-fabricated film bulk acoustic sensor for the real-time monitoring of blood clotting and the measurement of hemostatic parameters. The device was made of an Au/ZnO/Si3N4 film stack and excited by a lateral electric field. It operated under a shear mode resonance with the frequency of 1.42 GHz and had a quality factor of 342 in human blood. During the clotting process of blood, the resonant frequency decreased along with the change of blood viscosity and showed an apparent step-ladder curve, revealing the sequential clotting stages. An important hemostatic parameter, prothrombin time, was quantitatively determined from the frequency response for different dilutions of the blood samples. The effect of a typical anticoagulant drug (heparin) on the prothrombin time was exemplarily shown. The proposed sensor displayed a good consistency and clinical comparability with the standard coagulometric methods. Thanks to the availability of direct digital signals, excellent potentials of miniaturization and integration, the proposed sensor has promising application for point-of-care coagulation technologies.
Article
We presented a pure-shear film bulk acoustic resonator (FBAR) and investigated its sensing characteristics in viscous liquids. In the resonator, the electrodes were located on the surface of c-axis-oriented AlN film to generate the lateral electric field and excite the shear acoustic resonance. Compared with the typical quasi-shear film bulk acoustic resonator based on inclined c-axis-oriented AlN or ZnO piezoelectric film, the proposed device exhibits significantly higher Q-factors and a notably improved detection limit, particularly in water and viscous liquids. The frequency shifts show a linear dependency on the square root of the product of the liquid viscosity and density of the glycerol solution in the viscosity range of 1-5 mPa . s. Furthermore, we measured the mass sensitivity through real-time monitoring of the frequency change during the volatilization process of the loaded saline solutions. The proposed device shows the mass sensitivity of 465 Hz . cm(2)/ng and the mass resolutions of 0.17 ng/cm(2) in air, 0.25 ng/cm(2) in water and 2.08 ng/cm(2) in 50% glycerol solution, respectively. The obtained results clearly indicate that the proposed device is capable of using in liquid phase detection with high sensitivity requirements.
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Aluminum nitride offers unique material advantages for the realization of ultra-high frequency acoustic devices due to its high ratio of stiffness to density, compatibility with harsh environments and superior thermal properties. Although to date aluminum nitride thin film has been widely investigated in electrical and mechanical characteristics under alternating small signal excitation, its ultrathin nature under large bias may also provide novel and useful properties. Here we present a comprehensive investigation of electric field stiffening effect in c-oriented aluminum nitride piezoelectric thin film. By analyzing resonance characteristic in a 2.5GHz aluminum nitride based film bulk acoustic resonator, we demonstrate an up to 10% linear variation in equivalent stiffness of aluminum nitride piezoelectric thin film when applied an electric field from -150MV/m to +150MV/m along c-axis. Moreover, for the first time, an atomic interaction mechanism is proposed to reveal the nature of electric field stiffening effect, suggesting the nonlinear variation of the interatomic force induced by electric field modulation is the intrinsic reason for this phenomenon in aluminum nitride piezoelectric thin film. Our work provides vital experimental data and effective theoretical foundation for electric field stiffening effect in aluminum nitride piezoelectric thin film, indicating huge potential in tunable ultra-high frequency microwave devices.
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Gravimetric resonators based on Micro/Nanoelectromechanical System (M/NEMS) are potential candidates in developing smaller, less expensive, and higher performance gas sensors. Metal organic frameworks (MOFs), with high surface areas, recently come into focus as advanced nano-porous sensitive materials in micro gravimetric gas sensors. The surface of MOFs on those sensors is critical in offering water stability and varying absorption behaviors. However, the influences of surface on sensing performance are less explored and the strategy to tune surface properties of MOFs mounted on gravimetric resonators is still rare. In this paper, a straightforward strategy to engineer surface properties of MOFs, specifically Cu3(benzenetricarboxylate)2 (known as HKUST-1), is reported and the surface hydrophilicity/hydrophobicity of HKUST-1 is tuned by chemical vapor deposition combined with monolayer self-assembly. It is found out that the hybrid inorganic and organic surface engineering strategy not only preserves the absorption capacity of inner MOFs but also significantly enhances sensor stability toward water.
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The presence of chemical warfare agents (CWAs) in the environment is a serious threat to human safety, but there are many problems with the currently available detection methods for CWAs. For example, gas chromatography–mass spectrometry cannot be used for in-field detection owing to the rather large size of the equipment required, while commercial sensors have the disadvantages of low sensitivity and poor selectivity. Here, we develop a portable gas sensing instrument for CWA detection that consists of a MEMS-fabricated micro-preconcentrator (μPC) and a film bulk acoustic resonator (FBAR) gas sensor. The μPC is coated with a nanoporous metal–organic framework material to enrich the target, while the FBAR provides rapid detection without the need for extra carrier gas. Dimethyl methylphosphonate (DMMP), a simulant of the chemical warfare agent sarin, is used to test the performance of the instrument. Experimental results show that the μPC provides effective sample pretreatment, while the FBAR gas sensor has good sensitivity to DMMP vapor. The combination of μPC and FBAR in one instrument gives full play to their respective advantages, reducing the limit of detection of the analyte. Moreover, both the μPC and the FBAR are fabricated using a CMOS-compatible approach, and the prototype instrument is compact in size with high portability and thus has potential for application to in-field detection of CWAs.
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Highly selective, sensitive and fast gas sensing has attracted increasing attention in the fields of environmental protection, industrial production, personal safety as well as medical diagnostics. Field effect transistor (FET) sensors have been extensively investigated in gas sensing fields due to their small size, high sensitivity, high reliability and low energy consumption. This comprehensive review aims to discuss the recent advances in FET gas sensors based on materials such as carbon nanotubes, silicon carbide, silicon, metal oxides-, graphene-, transition metal dichalcogenides- and 2-dimensional black phosphorus. We first introduce different types of sensor structures and elaborate the gas-sensing mechanisms. Then, we describe the optimizing strategies for sensing performances, response parameters, FET based dual-mode sensors and FET based logic circuit sensors. Moreover, we present the key advances of the above materials in gas sensing performances. Meanwhile, shortcomings of such materials are also discussed and the future development of this field is proposed in this review.
Chapter
Electronic noses (e-nose) and tongues (e-tongue) are intelligent devices that mimic the human senses, i.e., the electronic tongue is based on our tongue's ability to differentiate between distinct tastes without having to determine all of the substances. In contrast, the electronic nose can analyze gaseous samples, more specifically, volatile compounds. This chapter presents a different perspective on these devices, focusing on the materials used as sensing units. Seven types of materials are addressed: bare electrodes, carbon-based materials, nanomaterials, polymers, metallic films, metal oxide semiconductors, and cyclic macromolecules films.
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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].
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Volatile organic compounds (VOCs) are pervasive in the environment. Since the early 1980s, substantial work has examined the detection of these materials as they can indicate environmental changes that can affect human health. VOCs and similar compounds present a very specific sensing problem in that they are not reactive and often nonpolar, so it is difficult to find materials that selectively bind or adsorb them. A number of techniques are applied to vapor sensing. High resolution molecular separation approaches such as gas chromatography and mass spectrometry are well-characterized and offer high sensitivity, but are difficult to implement in portable, real-time monitors, whereas approaches such as chemiresistors are promising, but still in development. Gravimetric approaches, in which the mass of an adsorbed vapor is directly measured, have several potential advantages over other techniques but have so far lagged behind other approaches in performance and market penetration. This review aims to offer a comprehensive background on gravimetric sensing including underlying resonators and sensitizers, as well as a picture of applications and commercialization in the field.
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We herein provide quantitative measures of sensors’ reliability and sensitivity as a function of the sensor’s capacity (maximum detection signal or saturation state) in addition to other adsorption–desorption parameters that define the detection signals toward volatile organic compounds (VOCs). The measures we have developed show differentiation between irregular dispersed points of sensors with low and high capacities. We show that the sharpest capacity that separates between the two types of distribution points, viz the reliability limit (RL), is tightly linked with the desorption constant kd. Less sharp RLs give interpretations of other reliability indicators. RL also provides information about the reliability of detecting signals of VOCs for a given sensor and sensors for a particular VOC. We show that sensors with high capacities are more reliable and sensitive to detecting signals of VOCs than sensors with lower capacities.
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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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The application of metal-organic frameworks (MOFs) as a sensing layer has been attracting great interest over the last decade, due to their high porosity and tunability, which provides a large surface area and active sites for trapping or binding target molecules. MIL-101(Cr) is selected as a good candidate from the MOFs family to fabricate a quartz crystal microbalance (QCM) nanosensor for the detection of volatile organic compound (VOC) vapors. The structural and chemical properties of synthesized MIL-101(Cr) are investigated by X-ray diffraction (XRD), Fourier-transfer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) and so on. A stable and uniform layer of MOF is coated onto the surface of a QCM sensor by the drop casting method. The frequency of the QCM crystal is changed during exposure to different concentrations of target gas molecules. Here, the sensor response to some VOCs with different functional groups and polarities, such as methanol, ethanol, isopropanol, n-hexane, acetone, dichloromethane, chloroform, tetrahydrofuran (THF), and pyridine under N2 atmosphere at ambient conditions is studied. Sensing properties such as sensitivity, reversibility, stability, response time, recovery time, and limit of detection (LOD) of the sensor are investigated. The best sensor response is observed for pyridine detection with sensitivity of 2.793 Hz ppm-1. The sensor shows short response/recovery time (less than two minutes), complete reversibility and repeatability which are attributed to the physisorption of the gases into the MOF pores and high stability of the device.
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Blood clotting time monitoring is of great importance in predicting the risk of haemorrhage and cardiovascular diseases. In this study, a new P-Lamb wave sensor was developed to measure activated partial thromboplastic time (aPTT) with enhanced sensitivity. The sensor utilizes a piezoelectric and hydrophobic material, Parylene-C, as a waveguide for the P-Lamb wave sensor in order to monitor the aPTT in whole blood samples. With a smooth surface, the P-Lamb wave sensor reduced energy loss and prevented leaky modes, with a decrease of 33% ± 1.25% in the surface roughness and an increase of 2.75% ± 0.15% in the signal. The experimental results demonstrated that, compared to a quartz crystal microbalance (QCM), a P-Lamb sensor with high sensitivity (the frequency shift of a P-Lamb sensor is approximately 200 times greater than that of a QCM) improved stability and enhanced repeatability. Parylene-C worked as a phonon coupling layer, which improved the stability and sensitivity of the Lamb sensor. The relative standards deviation (RSD) of the aPTT measurement was 2.11%, which was 1.22 times lower than reported values. The P-Lamb wave sensor was also tested for its sensitivity to different concentrations of heparin and was found to exhibit an increase in coagulation time with an increasing heparin concentration. Compared to the SYSMEX CS 5100 haematology analyser, the clinical coefficient index (R²) was 0.99467. Considering its compact size, low cost, and mass production of the chip unit, the developed P-Lamb wave sensor is a promising device for point-of-care diagnosis of haemostasis and personal health monitoring.
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As bendable, conformal electronic components, flexible gigahertz resonators are in demand as important building blocks (e.g., oscillators, filters, and signal processors) in future flexible radio‐frequency electronics for efficient wireless communication. Here, a 2.7 GHz piezoelectric thin‐film resonator (i.e., film bulk acoustic wave resonator) is presented that possesses high levels of both electrical performance and mechanical flexibility. The highly bendable resonator fabricated by FlexMEMS technology is essentially a thin‐film composite (i.e., high‐quality metal and inorganic piezoelectric layer stacks) encapsulated in polymer thin films with a total thickness of only 11.6 µm. The experimental series resonance frequency (fS), parallel resonance frequency (fP), quality factor (Q), and effective coupling coefficient ( kt eff 2) are 2.72 GHz, 2.77 GHz, 1398, and 4.39%, respectively. These parameter values are comparable to those of a conventional silicon‐based resonator. The minimum bending radius of the flexible resonator can be reduced to ≈0.5 mm with very slight electrical performance variation. Furthermore, the flexible resonator retains its mechanical and electrical stability after 2000 bending cycles. The superior mechanical flexibility and stability represent a significant advancement toward bendable, foldable, and conformal electronics working in the RF range. A highly flexible gigahertz piezoelectric resonator shows ultrahigh mechanical bendability when encapsulated in a polymer thin film package with a total thickness of only 11.6 µm. The flexible device possesses comparable performance to conventional resonators on a silicon substrate and remains fully functional even when bent into a radius of 0.5 mm and after 2000 bending cycles.
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Macrocyclic molecules, such as cyclodextrins, possess selective host-guest binding properties and can be incorporated into larger architectures for adsorption or sensing applications. An inverse emulsion approach was developed for the synthesis of crosslinked polymer nanoparticles of cyclodextrins (CD-PNP) without the employment of any additional linker or spacer. The macrocyclic cavity of cyclodextrin endows CD-PNP with the ability to selectively bind benzene, toluene, ethylbenzene, and xylenes (BTEX) molecules according to their size and shape, while the cross-linked porous network enables high adsorption capacities (e.g. 378.1 mg/g for benzene). The solution processability of nanoparticles enables feasible incorporation of CD-PNP, together with carbon nanotubes (CNT), into composite chemiresistive sensor devices. These sensors exhibit high sensitivity and excellent selectivity in response to the exposure of various BTEX vapors.
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A self-referenced resonator consisting of two distinct areas of the top electrode made from Mo and a thin (5-30 nm) functional Au layer is shown. The fundamental frequencies for both the shear (~1 GHz) and longitudinal (~2 GHz) are split in two, such that mass attachment on the functional layer region causes frequency shifts in only one of the resonances, allowing a new approach of using the difference between the two frequencies to be used to measure mass attachment; this reduces the importance of device-to-device variability in absolute resonant frequency as a result of device fabrication.
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Monitoring volatile organic compounds (VOCs) is an important issue, but difficult to achieve on a large scale and on the field using conventional analytical methods. Electronic noses (eNs), as promising alternatives, are still compromised by their per-formances due to the fact that most of them rely on a very limited number of sensors and use databases devoid of kinetic in-formation. To narrow the performance gap between human and electronic noses, we developed a novel optoelectronic nose, which features a large sensor microarray that enables multiplexed monitoring of binding events in real-time with a temporal response. For the first time, surface plasmon resonance imaging is demonstrated as a promising novel analytical tool for VOC detection in the gas phase. By combining it with cross-reactive sensor microarrays, the obtained optoelectronic nose shows a remarkably high selectivity, capable of discriminating between homologous VOCs differing by only a single carbon atom. In addition, the optoelectronic nose has good repeatability and stability. Finally, the preliminary assays using VOC binary and ternary mixtures show that it is also very efficient for the analysis of more complex samples, opening up the exciting perspec-tive of applying it to “real-world” samples in diverse domains.
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We report on ultrasensitive molecularly-modified silicon nanowire field effect transistor that marries between the lock-and-key and cross-reactive sensing worlds for the diagnosis of (gastric) cancer from exhaled volatolome. The sensor is able to selectively detect VOCs that are linked with gastric cancer conditions in exhaled breath and to discriminate them from environmental VOCs that would exist in exhaled breath samples but do not relate to the gastric cancer per se. Using breath samples collected from real patients with gastric cancer and from volunteers that have no cancer, blind analysis validated the ability of the reported sensor to discriminate between gastric cancer and control conditions, irrespective of important confounding factors such as tobacco consumption and gender with >85% accuracy. The reported sensing approach paves the way for utilizing the power of silicon nanowires in simple, inexpensive, portable, and non-invasive diagnosis of both cancer and other diseases conditions.
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Detection of volatile organic compounds (VOCs) using weight-detectable quartz microbalance and silicon-based microcantilever sensors coated with crystalline metal-organic framework (MOF) thin films is described in this paper. The thin films of two MOFs were grown from COOH-terminated self-assembled monolayers onto the gold electrodes of sensor platforms. The MOF layers worked as the effective concentrators of VOC gases, and the adsorption/desorption processes of the VOCs could be monitored by the frequency changes of weight-detectable sensors. Moreover, the MOF layers provided VOC sensing selectivity to the weight-detectable sensors through the size-selective adsorption of the VOCs within the regulated nanospace of the MOFs.
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The Quartz Crystal Microbalance (QCM) system is utilized to investigate the relationship between mass uptake and associated swelling for Langmuir-Blodgett (LB) organic thin films obtained from pyrene end-capped polystyrene (PS). The study was carried out using three different molecular weights of polymeric chains. The changes in resonance frequency associated with mass changes can be attributed to the swelling behavior of polymeric thin films during vapor absorption. This swelling is due to the capturing of organic vapor molecules in the sensor environment. To quantify real-time QCM data for swelling, early-time Fick's law of diffusion was adopted to fit the results, and a good linear relationship was observed between the mass uptake and square root of the swelling time. The diffusion coefficients for swelling were thus obtained from the slopes of the fitting curves and was found to be correlation with the amount of organic vapor content in the cell. It was also observed that diffusion of the organic vapor into higher molecular weight polystyrene thin films are much faster than low molecular weight ones in sensor applications. Diffusion coefficients were found to be 0.2–3.0 × 10−16, 5.0–13 × 10−16, and 1.0–1.6 × 10−15 cm2/s for PS1, PS2, and PS3 LB thin films, respectively. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
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The capability to detect traces of explosives sensitively, selectively and rapidly could be of great benefit for applications relating to civilian national security and military needs. Here, we show that, when chemically modified in a multiplexed mode, nanoelectrical devices arrays enable the supersensitive discriminative detection of explosive species. The fingerprinting of explosives is achieved by pattern recognizing the inherent kinetics, and thermodynamics, of interaction between the chemically modified nanosensors array and the molecular analytes under test. This platform allows for the rapid detection of explosives, from air collected samples, down to the parts-per-quadrillion concentration range, and represents the first nanotechnology-inspired demonstration on the selective supersensitive detection of explosives, including the nitro- and peroxide-derivatives, on a single electronic platform. Furthermore, the ultrahigh sensitivity displayed by our platform may allow the remote detection of various explosives, a task unachieved by existing detection technologies.
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This study reports the characterization and organic vapor sensing properties of Langmuir-Blodgett (LB) thin films of calix[4] arene derivatives that contain different numbers of tert butyl groups on their upper rims. Surface pressure-area isotherms show that very stable monolayers are formed at the air-water interface. The LB films are deposited onto different substrates, which allowed us to characterize the films by contact angle measurements, quartz crystal microbalance (QCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results indicate that good quality, uniform LB films can be prepared with transfer ratios of over 0.95. Meanwhile, our QCM results show that the deposition of LB film layers depends heavily on the number of p-tert-butyl groups and calix[4]arene with four p-tert-butyl groups yields the highest slope with a mass value of 1145 ng per layer. Furthermore, our AFM and SEM studies reveal a dense surface morphology for all prepared LB films. The kinetic response of calix[4]arenes containing p-tert-butyl groups and without p-tert-butyl groups as an LB film to chloroform, benzene, toluene, and ethanol vapors were investigated as a function of time. After attaching tert-butyl groups onto the calix[4]arene structure, the response of LB film to chloroform vapor increased. LB films of compounds 1-4 yield a response to all vapors and more often select chloroform with a larger, faster, and more reproducible response. We thus conclude that these calix[4]arenes could be applied to research concerning vapor sensing devices operating at room temperature.
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The analysis of volatile organic compounds in exhaled breath samples represents a new frontier in medical diagnostics because it is a noninvasive and potentially inexpensive way to detect illnesses. Clinical trials with spectrometry and spectroscopy techniques, the standard volatile-compound detection methods, have shown the potential for diagnosing illnesses including cancer, multiple sclerosis, Parkinson’s disease, tuberculosis, diabetes, and more via breath tests. Unfortunately, this approach requires expensive equipment and high levels of expertise to operate the necessary instruments, and the tests must be done quickly and use preconcentration techniques, all of which impede its adoption.
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This paper reviews the range of sensors used in electronic nose (e-nose) systems to date. It outlines the operating principles and fabrication methods of each sensor type as well as the applications in which the different sensors have been utilised. It also outlines the advantages and disadvantages of each sensor for application in a cost-effective low-power handheld e-nose system.
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Monitoring the binding affinities and kinetics of protein interactions is important in clinical diagnostics and drug development because such information is used to identify new therapeutic candidates. Surface plasmon resonance is at present the standard method used for such analysis, but this is limited by low sensitivity and low-throughput analysis. Here, we show that silicon nanowire field-effect transistors can be used as biosensors to measure protein-ligand binding affinities and kinetics with sensitivities down to femtomolar concentrations. Based on this sensing mechanism, we develop an analytical model to calibrate the sensor response and quantify the molecular binding affinities of two representative protein-ligand binding pairs. The rate constant of the association and dissociation of the protein-ligand pair is determined by monitoring the reaction kinetics, demonstrating that silicon nanowire field-effect transistors can be readily used as high-throughput biosensors to quantify protein interactions.
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The development of carbon nanotube-(CNTs-)based gas sensors and sensor arrays has attracted intensive research interest in the last several years because of their potential for the selective and rapid detection of various gaseous species by novel nanostructures integrated in miniature and low-power consuming electronics. Chemiresistors and chemical field effect transistors are probably the most promising types of gas nanosensors. In these sensors, the electrical properties of nanostructures are dramatically changed when exposed to the target gas analytes. In this review, recent progress on the development of different types of CNT-based nanosensors is summarized. The focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).
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A new p-tert-butyl-calix[8]arene-bonded silica gel stationary phase (CABS) was prepared via 3-glycidoxypropyltrimethoxysilane as a coupling reagent for HPLC. Its structure was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), elemental analysis and thermal analysis. The chromatographic performance of new packing was evaluated by using basic, acidic and neutral aromatic compounds as probes compared with conventional ODS. The results show that the new stationary phase has an excellent reversed-phase property and high selectivities for substituted aromatics compared with ODS, because CABS can provide various sites for the analytes, such as hydrogen-bonding interactions, pi-pi interactions, and inclusion complex, besides hydrophobic interactions.
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A label-free biosensor (for detection of DNA sequences) based on film-bulk-acoustic-resonator (FBAR) is presented in this letter. The FBAR's resonant frequency shifts to a lower value when a complementary single-strand DNA sequence is hybridized with a DNA probe sequence on an Au-coated FBAR surface. The sensor is capable of distinguishing a complementary DNA that is mismatched to a probe DNA by a single nucleotide. The label-free, highly sensitive and selective, and real-time detection of DNA sequence could easily be made into an array for combinatory DNA sequencing, and could possibly help geneticists to detect specific DNA sequences accurately and fast, without any expensive optical scanning or imaging.
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Micro/nano scale biosensors integrated with the local adsorption mask have been demonstrated to have a better limit of detection (LOD) and less sample consumptions. However, the molecular diffusions and binding kinetics in such confined droplet have been less studied which limited further development and application of the local adsorption method and imposed restrictions on discovery of new signal amplification strategies. In this work, we studied the kinetic issues via experimental investigations and theoretical analysis on microfabricated biosensors. Mass sensitive film bulk acoustic resonator (FBAR) sensors with hydrophobic Teflon film covering the non-sensing area as the mask were introduced. The fabricated masking sensors were characterized with physical adsorption of bovine serum albumin (BSA) and specific binding of antibody and antigen. Over an order of magnitude improvement on LOD was experimentally monitored. An analytical model was introduced to discuss the target molecule diffusion and binding kinetics in droplet environment, especially the crucial effects of incubation time, which has been less covered in previous local adsorption related literatures. An incubation time accumulated signal amplification effect was theoretically predicted, experimentally monitored and carefully explained. In addition, device optimization was explored based on the analytical model to fully utilize the merits of local adsorption. The discussions on the kinetic issues are believed to have wide implications for other types of micro/nano fabricated biosensors with potentially improved LOD. Copyright © 2015 Elsevier B.V. All rights reserved.
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In this study, two CO2 solid sorbents based on polyethyleneimine, PEI (423 and 10k) impregnated into mesoporous silica (MPS) foam prepared by in kg quantities via a scale-up process were synthesized and characterized by a range of analytical and surface techniques. The lower molecular weight PEI-423/MPS showed higher capacity towards CO2 sorption than the higher molecular weight PEI-10k/MPS. On the other hand, the higher molecular weight PEI-10k/MPS exhibited higher thermal stability than PEI-423/MPS. The kinetics of CO2 adsorption on both PEI/MPS fitted well with a double exponential model. According to this model CO2 adsorption can be divided into two steps: the first is fast and is attributed to CO2 adsorption on the sorbent surface; the second is slower and assigned to the diffusion of CO2 within and between the mesoporous particles. In contrast, the desorption process obeyed first order kinetics with activation energies of 64.3 and 140.7 kJmol-1 for PEI-423/MPS and PEI-10k/MPS, respectively.
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The field of supramolecular chemistry focuses on the non-covalent interactions between molecules that give rise to molecular recognition and self-assembly processes. Since most non-covalent interactions are relatively weak and form and break without significant activation barriers, many supramolecular systems are under thermodynamic control. Hence, traditionally, supramolecular chemistry has focused predominantly on systems at equilibrium. However, more recently, self-assembly processes that are governed by kinetics, where the outcome of the assembly process is dictated by the assembly pathway rather than the free energy of the final assembled state, are becoming topical. Within the kinetic regime it is possible to distinguish between systems that reside in a kinetic trap and systems that are far from equilibrium and require a continuous supply of energy to maintain a stationary state. In particular, the latter systems have vast functional potential, as they allow, in principle, for more elaborate structural and functional diversity of self-assembled systems - indeed, life is a prime example of a far-from-equilibrium system. In this Review, we compare the different thermodynamic regimes using some selected examples and discuss some of the challenges that need to be addressed when developing new functional supramolecular systems.
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In this work we investigate the adsorption characteristics due to exposure to benzene, toluene and chloroform vapor of 2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine metal free thin films fabricated by using the Langmuir–Blodgett (LB) thin film technique and its derivatives containing iron chloride, cobalt and magnesium. By using the surface pressure–surface area (Π–A) isotherm graphs the optimum conditions for the thin film deposition and mean molecular area values of each porphyrin have been determined. Quartz Crystal Microbalance (QCM) system was employed to investigate the gas sensing performances of thin films during the exposure to Volatile Organic Compounds (VOCs). The surface properties have been investigated by using Atomic Force Microscopy (AFM) and analyzed together with the QCM results to understand the adsorption kinetics of the gas sensing mechanism. The rate constants, ka for each thin film interacting with the saturated concentration of vapors have been calculated. The gas sensing interaction has been considered in terms of rate constants in each case. The highest value for ka has been observed for benzene exposure.
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In this work, a new method to tune the resonant frequency of micro-fabricated resonator using molecular layer-by-layer (LbL) self-assembly approach is demonstrated. By simply controlling the polymer concentration and the number of layers deposited, precisely tuning the frequency of micro-fabricated resonators is realized. Due to its selective deposition through specific molecular recognitions, such technique avoids the high-cost and complex steps of conventional semiconductor fabrications and is able to tune individual diced device. Briefly, Film Bulk Acoustic Resonator (FBAR) is used to demonstrate the tuning process and two types of LbL deposition methods are compared. The film thickness and morphology have been characterized by UV-vis reflection spectra, ellipsometer and AFM. As a result, the maximum resonant frequency shift of FBAR reaches more than 20 MHz, meaning 1.4% tunability at least. The minimum frequency shift is nearly 10 kHZ per bilayer, indicating 7 ppm tuning resolution. Besides, Pressure Cooker Test (PCT) is performed to evaluate the reliability of LbL coated FBAR. Furthermore, applications for wireless broadband communication and chemical sensors of LbL coated FBAR have been demonstrated.
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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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We have innovatively developed an electronic nose consisting of only one type of semiconductor metal oxide (SMO) material. The representative SMO material, porous In2O3 microtubes in this work, offered great surface area and large gas penetration channels. By using a solvent casting process, different amounts of porous In2O3 microtubes were coated on Al2O3 substrate, forming a resistometric SMO sensor array-based electronic nose. Each sensing unit in the electronic nose exhibited independent response toward ethanol. We have successfully applied this electronic nose to distinguish four alcohols at the same concentrations (100ppm), and also utilized the electronic nose for the discrimination of 14 volatile organic compounds (VOCs). Clear differentiation among all the 14 VOCs both at their immediately dangerous to life or health (IDLH) and the permissible exposure limit (PEL) concentrations has been achieved with no errors or misclassifications. We expect that this method will expand the application of SMO sensor array-based electronic nose which has been largely limited by the selection of commercially available SMOs and dopants.
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This paper describes the detection of volatile organic compounds (VOCs) through analyses of two output signals from integrated microcantilever sensor arrays coated with organic-inorganic hybrid sensing layers. The surface of TiO2 porous films was modified by amphiphilic terthiophene monomers and the adsorbed monomers were polymerized at the surface of TiO2 nanoparticles. The TiO2 porous films covered with polythiophene layers worked as highly sensitive sensing interfaces to provide two output signals for weight and resistance changes during exposure to VOC vapor. When the TiO2 porous films onto the sensor arrays were dyed with various kinds of amphiphilic monomers with different substituents, the resulting films provide exact information on VOC concentration from the mass changes as well as VOC classification from the analyses of response patterns.
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The gate adsorption mechanism and kinetics of an elastic layer-structured metal–organic framework (ELM), [Cu(bpy)2(BF4)2]n (ELM-11), that shows typical single-step CO2 gate adsorption/desorption isotherms accompanied with dynamic structural transformation in a wide temperature range were investigated. Adsorption of quite a small amount of CO2 on the external surface of ELM-11 crystals was observed at the pressure just below a gate adsorption pressure and induced a slight structural change in ELM-11. The structural change should start occurring at the outer parts of ELM-11 and transmit to more inner parts with rising pressure. The adsorption provides the stabilization of the framework through the interaction between fluid–solid and fluid–fluid and enables the framework to expand largely along the stacking direction. The CO2 adsorption rate of ELM-11 is almost comparable to that of Zeolite 5A at around ambient temperatures and shows temperature dependence with an anti-Arrhenius trend: higher adsorption rate with lower temperature.
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We present the engineering of cucurbituril/hydroxyapatite based theranostic nanoparticles with a high aspect ratio and a needle shaped morphology. These particles with varying sizes, surface charges and tunable degradation profiles manifested the advantages of the presence of cucurbituril with respect to drug loading, encapsulation efficacy and release kinetics. In vitro release profiles with two model drugs, Doxorubicin hydrochloride (Dox, hydrophilic) and Nile Red dye (NR, hydrophobic), were evaluated. It was ascertained that hydrophilic Dox was released at a faster rate compared to hydrophobic NR over similar time periods. The concomitant presence of samarium (Sm3+) and CB[7] confers theranostic potential to the synthesized nanoparticles. Cellular toxicity effects systematically assessed using MTT and live/dead assay protocols indicate inappreciable toxicity. The nanoparticles further reveal excellent blood compatibility and cellular internalization properties as visualized by fluorescence microscopy. Particles excited at 300 nm revealed Dox emission in the green channel (470 nm) as well as Sm3+ emission in the red channel (590 nm). These studies unravel the potential of these nanoparticles for effective theranostic applications.
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In this study, a label-free graphene-based fluorescence probe used for detection of volatile organic liquids was fabricated by a simple, efficient and low-cost method. To fabricate the probe, a bio-based β-cyclodextrin (β-CD) was firstly grafted on reduced graphene surfaces effectively and uniformly, as evidenced by various characterization techniques such as Ultraviolet/Visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy. The subsequent inclusion of Rhodamine B (RhB) into the inner cavities of the β-CD grafted on the graphene surfaces was achieved easily by a solution mixing method, which yielded the graphene-based fluorescent switch-on probe. In addition, the gradual and controllable quenching of RhB by Fluorescence Resonance Energy Transfer from RhB to graphene during the process of stepwise accommodation of the RhB molecules into the β-CD-functionalized graphene was investigated in depth. A wide range of organic solvents was examined using the as-fabricated fluorescence probe, which revealed the highest sensitivity to tetrahydrofuran with the detection limit of about 1.7 μg/mL. Some insight into the mechanism of the different responsive behaviors of the fluorescence sensor to the examined targets was also described.
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Characterization and organic vapor sensing properties of Langmuir–Blodgett (LB) thin films of calix[n]arene (n = 4, 6, 8) derivatives are reported in this work. Surface pressure–area isotherm graph shows that very stable monolayers are formed at the water surface. The results indicate that good quality, uniform LB films can be prepared with a transfer ratio of over 0.95. Calix[n]arene LB films have been characterized by contact angle measurements, quartz crystal microbalance (QCM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). LB film of calix[8]arene which has the largest cavity yields a gradient with a mass value of 773 ng per layer according to the QCM results. AFM and SEM images showed a dense surface morphology obtained for all samples. QCM system was used for the measurement of sensor response against chloroform, benzene, toluene and ethanol vapors. These LB film samples yield a response to all vapors with a large, fast, and reproducible due to the adsorption of vapors into the LB film structures. Among them, calix[8]arene LB film has higher sensitivity toward the organic vapors because of a large cavity size. This study can be concluded that the cavity size of calix[n]arene molecule could have an important role in the research area of room temperature vapor sensing devices.
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Benzene is an illustrative example of cancerigenic compounds at low ppb level, which can be found as contaminant in petrochemical industry applications. The pre-concentration of this compound is an important step prior to gas detection allowing to reduce significantly the detection limit of current detection systems. Nowadays, huge advancements have been made for developing gas miniaturized pre-concentrators to be used as an injection unit in front of gas microdetection systems for benzene monitoring. Herein, a comparative review about gas microconcentrators for benzene is reported, starting from available microstructures design to the main adsorbents and their application in detection systems.
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A Teflon-like coating is the key for the boost of the sensitivity of quartz microbalances for the tracing of airborne analytes. Since the undesired signals for the interfering compounds are suppressed and the ones for the targeted compounds, e.g. peroxide explosives are enhanced, the PCA out-put is dramatically improved.
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An electrodeless monolithic multichannel quartz crystal microbalance (MQCM) sensor was developed via the direct growth of ZnO nanorod patterns of various sizes onto an electrodeless quartz crystal plate. The patterned ZnO nanorods acted as independent resonators with different frequencies upon exposure to an electric field. The added mass of ZnO nanostructures was found to significantly enhance the quality factor (QF) of the resonator in electrodeless QCM configuration. The QF increased with the length of the ZnO nanorods; ZnO nanorods 5 m in length yielded a 7-fold higher QF compared to the QF of a quartz plate without ZnO nanorods. In addition, the ZnO nanorods offered enhanced sensitivity due to the enlarged sensing area. The developed sensor was used as an electronic nose for detection of vapor mixtures with impurities.
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In the present study, we report about the Langmuir–Blodgett thin film fabrication and gas sensing properties of metal free 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine and its derivatives containing iron chloride, cobalt and magnesium. The isotherm graphs are recorded to investigate the optimum conditions for the thin film deposition. Gas sensing performances of these thin films for volatile organic compounds (VOCs) including chloroform, benzene, toluene and ethyl alcohol are studied using quartz crystal microbalance (QCM) technique. The experimental results showed that these materials are suitable to produce a sensing layer for VOCs compounds. The best response was recorded against toluene vapour for all thin films where the dominating effects for gas sensing mechanism was considered as interaction between the central metal atom and conjugated π electron system.
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A supramolecular cryptophane-A-based quartz crystal microbalance (QCM) gas sensor has been developed. Supramolecular cryptophane-A was synthesized from vanillyl alcohol using a double trimerisation method and deposited on a QCM device via electrospray method. The sensor was exposed to various concentrations of CH4 gas and other gases operated at different temperatures. The influence of humidity was also examined. The sensor's response showed that it is selective to methane gas. A fast response and recovery was observed at room temperature. Detection limit for methane is 0.05% (v/v). The mechanism of the interactions between CH4 gas and cryptophane-A was discussed preliminarily.
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In this work, 2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine (OEP) in its free base form and metalated with iron (III) chloride (FeOEP), magnesium(II) (MgOEP) and cobalt(II) (CoOEP) have been used to fabricate Langmuir–Blodgett (LB) thin films. Using the surface pressure–surface area (Π–A) isotherm graphs optimum conditions for thin film deposition have been determined and by changing the deposition parameters various thin films have been deposited. Quartz Crystal Microbalance (QCM) system was used to investigate their gas sensing performances during exposure to Volatile Organic Compounds (VOCs) including chloroform, benzene and toluene. The surface properties have been investigated using Atomic Force Microscopy (AFM) and analyzed together with the QCM results to understand the effect of the surface properties on gas sensing mechanism. It is observed that larger surface area leads to higher response in gas sensing applications in terms of resonance frequency change.
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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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Absorption spectra of tetraphenylporphine, tetra-(p-methoxyphenyl)-porphine, tetra-p-tolylporphine, tetra-(p-chlorophenyl)-porphine and tetra-(p-nitrophenyl)-porphine in the ultraviolet, visible and infrared regions are reported. The visible and ultraviolet spectra indicate that the para-substituents exert only a small effect on the electronic transitions of the porphine ring system. Assignments of infrared frequencies are made where possible.