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Detection and Discrimination of Volatile Organic Compounds Using a Single Multi-resonance Mode Piezotransduced Silicon Bulk Acoustic Wave Resonator (PSBAR) as Virtual Sensor Array
Abstract
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.
... VOCs can have short and/or long-term negative effects on human health and life worldwide. 4 VOCs can be emitted from household products such as paints, paint strippers, wood preservatives, wax, pesticides, aerosol sprays, carpets, and many cleaning, disinfecting, cosmetic and degreasing products. Many pollutants are emitted into the environment as a result of industry, traffic, heating, etc. Aromatic and aliphatic hydrocarbons constitute a significant portion of these pollutants. ...
... However, there are still challenges related to their response, stability and reproducibility despite the efforts in materials design of the molecular structure, or device configuration. Thin film materials such as calix [4]arene, 10,11 pillar [5]arene, 12 porphyrin 13 and perylene diimide 14 can be synthesized at relatively low cost and often possess advantages over many physical and chemical properties. Organic materials may be used to produce thin film sensor chips using several techniques. ...
... The film thickness and refractive index of C1 LB thin films were determined by fitting the experimental SPR curves using Winspall software (by the Wolfgang Knoll group).60 Film thickness and refractive index values(4,8,12,16 and 20 layers) are given inTable 1. The thickness and refractive index of the C1 LB thin film show an increase depending on the number of layers. ...
Langmuir-Blodgett (LB) thin films using a N,N'-(L-alanine tert-butylester)-3,4:9,10-perylene diimide derivative were prepared to determine the optimum thin film forming conditions and their sensing properties towards volatile organic vapors. Our results showed highly uniform LB films could be transferred onto gold substrates and were highly sensitive to dichloromethane and chloroform vapors with a high response and short recovery time. Structure-function relationships suggest hydrogen bonding between perylene diimide and the vapor molecules have a significant role on determining the performance of perylene diimide sensing devices. Diffusion processes were investigated using Fick’s second law. The diffusion of dichloromethane and chloroform vapors into the LB film structure yielded larger responses than other vapors. The interactions were reversible, purging caused the vapor molecules to escape from the LB films.
... On the other hand, in addition to monitoring multiple pollutants at the same time, electronic nose is one of the most efficient way to eliminate the influences of interfering gas and improve data accuracy (Speller et al., 2015). In addition, as the alternative and complementary method to electronic nose, virtual sensor array which can produce multiple analyte-specific signals by a single physical sensor is a very promising method in addressing crosssensitivity issues (Li et al., 2021a;Zhao et al., 2018). ...
People generally spend most of their time indoors, making indoor air quality be of great significance to human health. Large spatiotemporal heterogeneity of indoor air pollution can be hardly captured by conventional filter-based monitoring but real-time monitoring. Real-time monitoring is conducive to change air assessment mode from static and sparse analysis to dynamic and massive analysis, and has made remarkable strides in indoor air evaluation. In this review, the state of art, strengths, challenges, and further development of real-time sensors used in indoor air evaluation are focused on. Researches using real-time sensors for indoor air evaluation have increased rapidly since 2018, and are mainly conducted in China and the USA, with the most frequently investigated air pollutants of PM2.5. In addition to high spatiotemporal resolution, real-time sensors for indoor air evaluation have prominent advantages in 3-dimensional monitoring, pollution peak and source identification, and short-term health effect evaluation. Huge amounts of data from real-time sensors also facilitate the modeling and prediction of indoor air pollution. However, challenges still remain in extensive deployment of real-time sensors indoors, including the selection, performance, stability, as well as calibration of sensors. In future, sensors with high performance, long-term stability, low price, and low energy consumption are welcomed. Furthermore, more target air pollutants are also expected to be detected simultaneously by real-time sensors in indoor air monitoring.
... (a) Photograph of ZnTTP based sensor, (b) Diagram of the test bench used for the detection of VOCs[15]. ...
In this work, an innovative approach based on the combination of hierarchical ascending classification (HAC) and principal component analysis (PCA) is proposed to discriminate individual vapors such as methanol, toluene, acetone, isopropanol, and ethanol using dynamic responses of one single sensor based on 5, 10, 15, 20 tetratoloylphenylporphyrinato Zn (II) (ZnTTP) at room temperature. Experimental data of 20 measurements as individuals (i.e., four different concentrations of each vapor) and extraction of new mathematical features associated with dynamic behavior were the basis of two subsets entered into the gas identification method. With PCA, four distinct groups were observed for each subset. The first three principal components have been added as data features for each previous corresponding subset of the HAC process. The classification becomes clearer and unambiguously separates the different clusters in good compliance with PCA. The obtained results have demonstrated the possibility to recognize the studied vapors by exploiting a single gas sensor with an opportunity to apply the developed PCA/HAC-based platform in cost-effective and highly selective gas monitoring systems.
... The extraction of relevant information from the transient response of resistive type gas sensor to identify and/or quantify a certain gas was reported previously for MOX based sensor in many works especially for electronic nose [13]. In fact, many feature extraction methods have been examined in the literature, one cites for example: transient characteristics (descriptors of Fourier and wavelets, parameters extracted as coefficients from curve fitting, integral and derivatives) steady state characteristics (fractional, relative change, difference and log) [14][15][16][17]. Denise M. Wilson and al [18] describe transient responses with eight points at periodic intervals during the transient responses before steady state is reached at eight different operating temperature in a narrow range for discrimination tasks. ...
Selectivity continues to be one of the major challenges for gas sensors. This work reports on transient variations in the resistance of In2S3 sensors following injection of different concentrations of ethanol, methanol, isopropanol, acetone and toluene at 350 °C operating temperature. Volatile Organic Compounds (VOCs) are recognized using the transient evolution of a single sensing element that offers different characteristics that can be direct or indirect signals detection of the target gas. Novel features for twenty individuals were processed by principal component analysis and hierarchical ascending classification, and four obvious clusters were observed. The output of the VOC detection sensor for the gaseous analyte is represented by the result of analyzes of three data matrices having 9, 7 and 8 dimensional vectors. Result is delivered with a sequence of three digits determined after global execution. Based on this approach, the distinction of one reducing gas from another is demonstrated.
... As many detection techniques have been developed, QCM has the advantage of a low limit of detection, along with others described in various sections below. Statisti- Different sensors arrays for gas sensing for several applications have been reported in the literature, such as metal-oxides [44][45][46][47][48][49], conductive polymers [50][51][52][53][54], electrochemical [55,56], optical [57][58][59][60], and acoustic wave sensors [61,62]. Each one of these chemical sensors have different detection mechanisms and present specific advantages and disadvantages. ...
Volatile organic compounds (VOCs) that evaporate under standard atmospheric conditions are of growing concern. This is because it is well established that VOCs represent major contamination risks since release of these compounds into the atmosphere can contribute to global warming, and thus, can also be detrimental to the overall health of worldwide populations including plants, animals, and humans. Consequently, the detection, discrimination, and quantification of VOCs have become highly relevant areas of research over the past few decades. One method that has been and continues to be creatively developed for analyses of VOCs is the Quartz Crystal Microbalance (QCM). In this review, we summarize and analyze applications of QCM devices for the development of sensor arrays aimed at the detection of environmentally relevant VOCs. Herein, we also summarize applications of a variety of coatings, e.g., polymers, macrocycles, and ionic liquids that have been used and reported in the literature for surface modification in order to enhance sensing and selective detection of VOCs using quartz crystal resonators (QCRs) and thus QCM. In this review, we also summarize novel electronic systems that have been developed for improved QCM measurements.
... To overcome these drawbacks, a recent breakthrough comes from the virtual sensor array (VSA), in which one individual sensor can be used to generate multi-dimensional signals similar to those produced by the electronic nose [29]. These multi-dimensional signals could produce unique responsive patterns for different VOCs [30]. To facilitate accurate identification of each VOC, pattern recognition techniques, such as principal component analysis (PCA) [27], linear discriminant analysis (LDA) [31], and partial least squares (PLS) [29] are often applied. ...
Two-dimensional transition metal carbides/nitrides, known as MXenes, have recently received significant attention for gas sensing applications. However, MXenes have strong adsorption to many types of volatile organic compounds (VOCs), and therefore gas sensors based on MXenes generally have low selectivity and poor performance in mixtures of VOCs due to cross-sensitivity issues. Herein, we developed a Ti3C2Tx-based virtual sensor array (VSA) which allows both highly accurate detection and identification of different VOCs, as well as concentration prediction of the target VOC in variable backgrounds. The VSA’s responses from the broadband impedance spectra create a unique fingerprint of each VOC without a need for changing temperatures. Based on the methodologies of principal component analysis and linear discrimination analysis, we demonstrate highly accurate identifications for different types of VOCs and mixtures using this MXene based VSA. Furthermore, we demonstrate an accuracy of 93.2% for the prediction of ethanol concentrations in the presence of different concentrations of water and methanol. The high level of identification and concentration prediction shows a great potential of MXene based VSA for detection of VOCs of interest in the presence of known and unknown interferences.
... The sensing system successfully detected cadusafos and malathion at concentrations as low as 0.1 mmoLÁL À1 . Zhao et al. [47] reported an e-nose sensing array for the detection of volatile organic compounds (VOCs). Each VOC element was carried by nitrogen (N 2 ) gas and delivered to the sensing setup, with the concentration of the VOCs being controlled by the pressure ratio of N 2 gas and VOC vapor. ...
Ultrashort laser pulses possess many advantages for materials processing. Ultrafast laser has a significantly low thermal effect on the areas surrounding the focal point; therefore, it is a promising tool for micro- and submicro-sized precision processing. In addition, the nonlinear multiphoton absorption phenomenon of focused ultrashort pulses provides a promising method for the fabrication of various structures on transparent material, such as glass and transparent polymers. A laser direct writing process was applied in the fabrication of high-performance three-dimensional (3D) structured multilayer micro-supercapacitors (MSCs) on polymer substrates exhibiting a peak specific capacitance of 42.6 mF·cm−2 at a current density of 0.1 mA·cm−2. Furthermore, a flexible smart sensor array on a polymer substrate was fabricated for multi-flavor detection. Different surface treatments such as gold plating, reduced-graphene oxide (rGO) coating, and polyaniline (PANI) coating were accomplished for different measurement units. By applying principal component analysis (PCA), this sensing system showed a promising result for flavor detection. In addition, two-dimensional (2D) periodic metal nanostructures inside 3D glass microfluidic channels were developed by all-femtosecond-laser processing for real-time surface-enhanced Raman spectroscopy (SERS). The processing mechanisms included laser ablation, laser reduction, and laser-induced surface nano-engineering. These works demonstrate the attractive potential of ultrashort pulsed laser for surface precision manufacturing.
Small-sized, high-sensitivity, and low-cost sensors are required for gas-sensing applications because of their critical role in environmental monitoring, clinic diagnosis, process control, and anti-terrorism. Given the rapid developments in micro-fabrication and microelectromechanical system (MEMS) technologies, film bulk acoustic resonator (FBAR) gas sensors have received increased research attention because of their improved working frequency and reliability. This chapter discusses the state-of-the-art and recent developments in FBAR gas sensors. The sensing mechanism and limitations of these sensors are summarized. Recent progress in the development of four major aspects of FBAR gas sensors, namely, FBAR gas sensors using different sensing materials, FBAR gas sensors used in electronic noses, system integration of FBAR gas sensors, and FBAR gas sensors used as micro-GC detectors, is reviewed. The potential future of FBAR sensors used in flexible electronics is also discussed.
Piezoelectric cantilever resonator is one of the most promising platforms for real-time sensing of volatile organic compounds (VOCs). However, it has been a great challenge to eliminate the cross-sensitivity of various VOCs for these cantilever-based VOC sensors. Herein, a virtual sensor array (VSA) is proposed on the basis of a sensing layer of GO film deposited onto an AlN piezoelectric cantilever with five groups of top electrodes for identification of various VOCs. Different groups of top electrodes are applied to obtain high amplitudes of multiple resonance peaks for the cantilever, thus achieving low limits of detection (LODs) to VOCs. Frequency shifts of multiple resonant modes and changes of impedance values are taken as the responses of the proposed VSA to VOCs, and these multidimensional responses generate a unique fingerprint for each VOC. On the basis of machine learning algorithms, the proposed VSA can accurately identify different types of VOCs and mixtures with accuracies of 95.8 and 87.5%, respectively. Furthermore, the VSA has successfully been applied to identify the emissions from healthy plants and "plants with late blight" with an accuracy of 89%. The high levels of identifications show great potentials of the VSA for diagnosis of infectious plant diseases by detecting VOC biomarkers.
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.
The uniformity and repeatability of sensing film will greatly affect the sensor-to-sensor reproducibility. In this paper, an ethanol sensor based on a surface acoustic wave (SAW) resonator and uniform ZnO nanoparticles-reduced graphene oxide (rGO) composite film as sensing material was proposed. A uniform ZnO-rGO composite film was firstly formed by vacuum filtration and reduction and then transferred to the SAW ethanol sensor. This film formation and transfer process has very good repeatability and stability, as evidenced by the response of the sensors. Benefitting from the uniform composite film, the ethanol sensor shows advantages of wide response range (0~70 vol%), excellent short-term repeatability, good stability, fast response (23 s) and recovery (18 s). Moreover, a dual-mode detection method was proposed to discriminate different gases through simultaneously detecting the mechanical (resonant frequency,
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) and electrical (impedance,
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) responses of the same gas adsorption based proposed sensor. To the best of our knowledge, this is the first report on SAW sensor based on uniform ZnO-rGO composite film formed by vacuum filtration and reduction.
Multisensory tactile systems play an important role in enhancing robot intelligence. A competent robotic tactile system needs to be simple in structure and easily operated, especially with multiple sensations, and have good coordinate ability like human skin. A novel multisensory tactile system for humanoid robotic hands is proposed, allowing the hand to identify objects by grasping and manipulating them. Robotic multifunction sensors based on skin‐inspired thermosensation and structured with micro platinum ribbons partially covered with piezo‐thermic silver‐nanoparticle‐doped porous polydimethylsiloxane membrane simultaneously and independently detect contact pressure, local ambient temperature, and thermal conductivity and temperature of an object. Multiple tactile information which is obtained by the robotic sensors is fused comprehensively based on neural network classification to identify diverse objects in uncertain and dynamic gripping with an object recognition accuracy of 95%. This multisensory system enables robotic hand to better interact with its environment, enhances robotic intelligence, and makes various complex tasks feasible for robots, such as sorting material or rescuing from fire or other disasters. A multisensory tactile system enables a robotic hand to intelligently recognize objects. Robotic multifunction sensors based on thermosensation and structured with platinum ribbons partially covered with piezo‐thermic membrane are used to simultaneously and independently measure contact pressure, temperature, and thermal conductivity of object are simultaneously and independently detected objects. Multi‐sensory information is fused based on neural‐network classification to achieve precise recognition in uncertain and dynamic gripping.
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.
This review provides an overview of the current state‐of‐the‐art of the emerging field of flexible multifunctional sensors for wearable and robotic applications. In these application sectors, there is a demand for high sensitivity, accuracy, reproducibility, mechanical flexibility, and low cost. The ability to empower robots and future electronic skin (e‐skin) with high resolution, high sensitivity, and rapid response sensing capabilities is of interest to a broad range of applications including wearable healthcare devices, biomedical prosthesis, and human–machine interacting robots such as service robots for the elderly and electronic skin to provide a range of diagnostic and monitoring capabilities. A range of sensory mechanisms is examined including piezoelectric, pyroelectric, piezoresistive, and there is particular emphasis on hybrid sensors that provide multifunctional sensing capability. As an alternative to the physical sensors described above, optical sensors have the potential to be used as a robot or e‐skin; this includes sensory color changes using photonic crystals, liquid crystals, and mechanochromic effects. Potential future areas of research are discussed and the challenge for these exciting materials is to enhance their integration into wearables and robotic applications. Flexible multifunctional sensors can provide intelligent sensing for wearable and human–machine interacting robots. This review focuses on recent progress in the development and advancement of flexible and multifunctional sensors based on a range of sensory mechanisms and materials including piezoelectric, conductive, and optical materials. Potential future research directions and challenges are discussed.
The accumulation of non-desorbed species or heel buildup reduces the capacity and lifetime of adsorbents used in cyclic adsorption/desorption processes. This study investigates the changes in the physical, chemical, and dielectric properties of beaded activated carbon (BAC) and their relation to the adsorbent residual capacity (i.e., residual lifetime) during cyclic adsorption/desorption using a non-contact, high-resolution microwave resonator sensor. The resonant frequency of the sensor responds to changes in the dielectric properties of the medium surrounding the sensor. BAC samples with different degrees of capacity reduction as indicated 2 by the apparent density, BET surface area, total pore and micropore volumes were tested. A linear correlation (R 2 =0.93) between the apparent density of the BAC samples and the resonant frequency shift of the sensor was observed, irrespective of heel composition indicating that the effect of adsorbent porosity on dielectric property variation is more important than heel composition. The sensor's ability for fast and continuous measurement makes it an ideal candidate for real-time monitoring of adsorbent capacity status in cyclic adsorption/desorption processes.
We report a flexible sensor array electronic tongue system that fabricated on a polymer substrate by the laser direct writing process for multi-flavors decoction. Electronic tongue is a sensing system that is applied to detect different elements with the same sensor array. By analyzing responses from different measurement units, it enables a cross sensitivity, namely the ability of the system responding to a range of different analytes in solution without specific functionalization of sensors. In this paper, a six-unit sensing array system was fabricated by a laser direct writing process. Sensing units were introduced on a flexible polyamide surface. A high surface-volume ratio porous carbon structure was created by a laser-induced carbonization process, which provides stable conductive carbon electrodes with high sensitivity. Different surface treatments such as gold plating, reduced-graphene oxide (rGO) coating and polyaniline (PANI) coating, were accomplished for different measurement units. By applying principal component analysis (PCA), this sensing system shows a promising result for many flavors detection. The detection limits for each element are about 0.1mM for NaCl and sugar solutions. And it is able to detect 10-4 times diluted commercial table vinegar solution, which originally contains 5% of acetic acid. The detection limit is theoretically lower than the human threshold of 10mM for NaCl and sugar. Besides, the sensing system shows a high sensitivity and selectivity for mixed elements. By mapping the data points, the sensor system could detect flavor combinations and provide a reliable prediction of analyte concentration ratios.
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.
In this paper, we have modeled and analyzed affinities and kinetics of volatile organic compounds (VOCs) adsorption (and desorption) on various surface chemical groups using multiple self-assembled monolayers (SAMs) functionalized film bulk acoustic resonator (FBAR) array. The high-frequency and micro-scale resonator provides improved sensitivity in the detections of VOCs at trace levels. With the study of affinities and kinetics, three concentration-independent intrinsic parameters (monolayer adsorption capacity, adsorption energy constant and desorption rate) of gas-surface interactions are obtained to contribute to a multi-parameter fingerprint library of VOC analytes. Effects of functional group's properties on gas-surface interactions are also discussed. The proposed sensor array with concentration-independent fingerprint library shows potential as a portable electronic nose (e-nose) system for VOCs discrimination and gas-sensitive materials selections.
Molecularly-modified gold nanoparticle (GNP)-based flexible sensors were integrated into a dynamic cross-reactive diagnostic sensing array. Each dynamic flexible sensor-bending state gives it unique nanoparticle spatial organization, altering the interaction between GNP ligands and volatile organic compounds (VOCs), which increases the amount of data obtainable from each sensor. Individual dynamic flexible sensor can selectively detect parts per billion (ppb) levels VOCs that are linked with ovarian cancers in exhaled breath and discriminate them from environmental VOCs that exist in exhaled breath samples, but do not relate to ovarian cancer per se. Strain response-related successfully discriminated between exhaled breath collected from control subjects and those with ovarian cancer, with data from a single sensor being sufficient to obtain 82% accuracy, irrespective of important confounding factors, such as tobacco consumption and comorbidities. The approach raises the hope of achieving an extremely simple, inexpensive, portable and non-invasive diagnostic procedure for cancer and other diseases.
Combining vapour sensors into arrays is an accepted compromise to mitigate poor selectivity of conventional sensors. Here we show individual nanofabricated sensors that not only selectively detect separate vapours in pristine conditions but also quantify these vapours in mixtures, and when blended with a variable moisture background. Our sensor design is inspired by the iridescent nanostructure and gradient surface chemistry of Morpho butterflies and involves physical and chemical design criteria. The physical design involves optical interference and diffraction on the fabricated periodic nanostructures and uses optical loss in the nanostructure to enhance the spectral diversity of reflectance. The chemical design uses spatially controlled nanostructure functionalization. Thus, while quantitation of analytes in the presence of variable backgrounds is challenging for most sensor arrays, we achieve this goal using individual multivariable sensors. These colorimetric sensors can be tuned for numerous vapour sensing scenarios in confined areas or as individual nodes for distributed monitoring.
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.
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.
Significance
Morpho butterflies are a brilliant spectacle of nature’s capability for photonic engineering. Their conspicuous appearance arises from the interference and diffraction of light within tree-like nanostructures on their scales. Scientific lessons learned from these butterflies have already inspired designs of new displays, fabrics, and cosmetics. This study reports a vertical surface polarity gradient in these tree-like structures. This biological pattern design may be applied to numerous technological applications ranging from security tags to self-cleaning surfaces, gas separators, protective clothing, and sensors. Here it has allowed us to unveil a general mechanism of selective vapor response in photonic Morpho nanostructures and to demonstrate attractive opportunities for chemically graded sensing units for high-performance sensing.
Although various gas sensors have been developed for the detection of volatile organic compounds (VOCs), the simultaneous detection and identification of several targets by application of biomaterials together with chemicals are rare in one sensor system. In this study a six chips sensor module coated with synthetic polypeptides together with conducting polymers is used for the simultaneous detection and identification of VOCs such as acetic acid, butyric acid, ammonia, dimethyl amine, benzene, chlorobenzene and their mixtures, on a quartz crystal microbalance. The sensor module demonstrated sensitivity and selectivity for detection, and discrimination via characterized odor profiles of all tested VOCs. The relationship between measured values and vapor concentrations indicated a meritorious linear correlation. Principal component analysis of sensor module responses was used to discriminate between diluted or mixed VOCs. These results may useful for VOCs detection and discrimination in sensing applications.
Conventional diagnostic methods for lung cancer are unsuitable for widespread screening because they are expensive and occasionally miss tumours. Gas chromatography/mass spectrometry studies have shown that several volatile organic compounds, which normally appear at levels of 1-20 ppb in healthy human breath, are elevated to levels between 10 and 100 ppb in lung cancer patients. Here we show that an array of sensors based on gold nanoparticles can rapidly distinguish the breath of lung cancer patients from the breath of healthy individuals in an atmosphere of high humidity. In combination with solid-phase microextraction, gas chromatography/mass spectrometry was used to identify 42 volatile organic compounds that represent lung cancer biomarkers. Four of these were used to train and optimize the sensors, demonstrating good agreement between patient and simulated breath samples. Our results show that sensors based on gold nanoparticles could form the basis of an inexpensive and non-invasive diagnostic tool for lung cancer.
Chronic obstructive pulmonary disease (COPD) and asthma can exhibit overlapping clinical features. Exhaled air contains volatile organic compounds (VOCs) that may qualify as noninvasive biomarkers. VOC profiles can be assessed using integrative analysis by electronic nose, resulting in exhaled molecular fingerprints (breathprints).
We hypothesized that breathprints by electronic nose can discriminate patients with COPD and asthma.
Ninety subjects participated in a cross-sectional study: 30 patients with COPD (age, 61.6 +/- 9.3 years; FEV(1), 1.72 +/- 0.69 L), 20 patients with asthma (age, 35.4 +/- 15.1 years; FEV(1) 3.32 +/- 0.86 L), 20 nonsmoking control subjects (age, 56.7 +/- 9.3 years; FEV(1), 3.44 +/- 0.76 L), and 20 smoking control subjects (age, 56.1 +/- 5.9 years; FEV(1), 3.58 +/- 0.78). After 5 minutes of tidal breathing through an inspiratory VOC filter, an expiratory vital capacity was collected in a Tedlar bag and sampled by electronic nose. Breathprints were analyzed by discriminant analysis on principal component reduction resulting in cross-validated accuracy values (accuracy). Repeatability and reproducibility were assessed by measuring samples in duplicate by two devices.
Breathprints from patients with asthma were separated from patients with COPD (accuracy 96%; P < 0.001), from nonsmoking control subjects (accuracy, 95%; P < 0.001), and from smoking control subjects (accuracy, 92.5%; P < 0.001). Exhaled breath profiles of patients with COPD partially overlapped with those of asymptomatic smokers (accuracy, 66%; P = 0.006). Measurements were repeatable and reproducible.
Molecular profiling of exhaled air can distinguish patients with COPD and asthma and control subjects. Our data demonstrate a potential of electronic noses in the differential diagnosis of obstructive airway diseases and in the risk assessment in asymptomatic smokers. Clinical trial registered with www.trialregister.nl (NTR 1282).
Microarrays have been widely used for the analysis of gene expression, but the issue of reproducibility across platforms has yet to be fully resolved. To address this apparent problem, we compared gene expression between two microarray platforms: the short oligonucleotide Affymetrix Mouse Genome 430 2.0 GeneChip and a spotted cDNA array using a mouse model of angiotensin II-induced hypertension. RNA extracted from treated mice was analyzed using Affymetrix and cDNA platforms and then by quantitative RT-PCR (qRT-PCR) for validation of specific genes. For the 11,710 genes present on both arrays, we assessed the relative impact of experimental treatment and platform on measured expression and found that biological treatment had a far greater impact on measured expression than did platform for more than 90% of genes, a result validated by qRT-PCR. In the small number of cases in which platforms yielded discrepant results, qRT-PCR generally did not confirm either set of data, suggesting that sequence-specific effects may make expression predictions difficult to make using any technique.
Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NO(x) = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NO(x) air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40-80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.
An ambipolar poly(diketopyrrolopyrrole-terthiophene)-based field-effect transistor (FET) sensitively detects xylene isomers at low ppm levels with multiple sensing features. Combined with pattern-recognition algorithms, a sole ambipolar FET sensor, rather than arrays of sensors, can discriminate highly similar xylene structural isomers from one another.
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.
Herein, we demonstrate an alternative strategy for creating QCM-based sensor arrays by use of a single sensor to provide multiple responses per analyte. The sensor, which simulates a virtual sensor array (VSA), was developed by depositing a thin film of ionic liquid, either 1-octyl-3-methylimidazolium bromide ([OMIm][Br]) or 1-octyl-3-methylimidazolium thiocyanate ([OMIm][SCN]), onto the surface of a QCM-D transducer. The sensor was exposed to 18 different organic vapors (alcohols, hydrocarbons, chlorohydrocarbons, nitriles) belonging to the same or different homologous series. The resulting frequency shifts (Δf) were measured at multiple harmonics and evaluated using principal component analysis (PCA) and discriminant analysis (DA) which revealed that analytes can be classified with extremely high accuracy. In almost all cases, the accuracy for identification of a member of the same class, i.e. intraclass discrimination, was 100% as determined by use of quadratic discriminant analysis (QDA). Impressively, some VSAs allowed classification of all 18 analytes tested with nearly 100% accuracy. Such results underscore the importance of utilizing lesser exploited properties which influence signal transduction. Overall, these results demonstrate excellent potential of the virtual sensor array strategy for detection and discrimination of vapor phase analytes utilizing the QCM. To the best of our knowledge, this is the first report on QCM VSAs, as well as an experimental sensor array, that is based primarily on viscoelasticity, film thickness, and harmonics.
The use of molecularly modified Si nanowire field effect transistors (SiNW FETs) for selective detection in liquid phase has been successfully demonstrated. In contrast, selective detection of chemical species in the gas phase has been rather limited. In this paper, we show that the application of artificial intelligence on deliberately controlled SiNW FET device parameters can provide high selectivity towards specific volatile organic compounds (VOCs). The obtained selectivity allows identifying VOCs in both single-component and multi-component environments as well as estimating the constituent VOC concentrations. The effect of the structural properties (functional group and/or chain length) of the molecular modifications on the accuracy of VOC detection is presented and discussed. The reported results have the potential to serve as a launching pad for the use of SiNW FET sensors in real-world counteracting conditions and/or applications.
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.
An array of quartz crystals coated with different room-temperature ionic liquids (RTILs) is proposed for the analysis of flavors by quartz crystal microbalance (QCM) measurements. Seven RTILs were adopted as sensing layers, all containing imidazolium or phosphonium cations, differing from one another in the length and branching of alkyl groups, and neutralized by different anions. The array was at first applied to the analysis of 31 volatile organic compounds (VOCs) such as alcohols, phenols, aldehydes, esters, ketones, acids, amines, hydrocarbons and terpenes, chosen as representative components of a wide variety of food flavors. Multivariate data analysis by the principal component analysis (PCA) approach of the set of the corresponding responses led to separated clusters for these different chemical categories. To prove further the good performance of the RTIL coated quartz crystal array as "electronic nose", it was applied to the analysis of headspaces from cinnamon samples belonging to different botanical varieties (cinnamon zeylanicum and cinnamon cassia). PCA applied to responses recorded on different stocks of samples of both varieties showed that they could be fully discriminated.
A surface acoustic wave (SAW) sensor array has been developed in order to discriminate Spanish wine coming from different grape varieties and elaboration processes. Sensors were coated with diverse thicknesses of polyepichlorohydrin (PECH), polyetherurethane (PEUT) and polydimethylsiloxane (PDMS). Linear techniques as principal component analysis (PCA) and linear discriminant analysis (LDA) and non-linear ones as probabilistic neural networks (PNN) have been used for pattern recognition. A classification success rate of 86% has been achieved.
The gas sensing properties of graphene-like nano-sheets deposited on 36° YX lithium tantalate (LiTaO3) surface acoustic wave (SAW) transducers are reported. The thin graphene-like nano-sheets were produced via the reduction of graphite oxide which was deposited on SAW interdigitated transducers (IDTs). Their sensing performance was assessed towards hydrogen (H2) and carbon monoxide (CO) in a synthetic air carrier gas at room temperature (25°C) and 40°C. Raman and X-ray photoelectron spectroscopy (XPS) revealed that the deposited graphite oxide (GO) was not completely reduced creating small, graphitic nanocrystals ∼2.7nm in size.
1 I. 1 II. 7 III. 8 IV. 10 V. 12 13 References 13 SUMMARY: Plants synthesize an amazing diversity of volatile organic compounds (VOCs) that facilitate interactions with their environment, from attracting pollinators and seed dispersers to protecting themselves from pathogens, parasites and herbivores. Recent progress in -omics technologies resulted in the isolation of genes encoding enzymes responsible for the biosynthesis of many volatiles and contributed to our understanding of regulatory mechanisms involved in VOC formation. In this review, we largely focus on the biosynthesis and regulation of plant volatiles, the involvement of floral volatiles in plant reproduction as well as their contribution to plant biodiversity and applications in agriculture via crop-pollinator interactions. In addition, metabolic engineering approaches for both the improvement of plant defense and pollinator attraction are discussed in light of methodological constraints and ecological complications that limit the transition of crops with modified volatile profiles from research laboratories to real-world implementation.
Tropical Morpho butterflies are famous for their brilliant iridescent colours, which arise from ordered arrays of scales on their wings. Here we show that the iridescent scales of the Morpho sulkowskyi butterfly give a different optical response to different individual vapours, and that this optical response dramatically outperforms that of existing nano-engineered photonic sensors. The reflectance spectra of the scales provide information about the nature and concentration of the vapours, allowing us to identify a range of closely related vapours–water, methanol, ethanol and isomers of dichloroethylene when they are analysed individually. By comparing the reflectance as a function of time for different vapours, we deduce that wing regions with scale structures of differing spatial periodicity give contributions to the overall spectral response at different wavelengths. Our optical model explains the effect of different components of the wing scales on the vapour response, and could steer the design of new man-made optical gas sensors.
The adsorption of volatile organic compounds (VOC) onto soils plays an important role in the mobility of these kinds of contaminant through soils. It is therefore of interest to learn more about the mechanisms of interaction between VOC and soil particles. An experimental study has been carried out in order to determine the adsorption isotherms of volatile organic gases of different properties on soil minerals of different characteristics, working in a wide range of compound concentrations. The adsorption of seven organic compounds (n-hexane, n-heptane, n-octane, toluene, xylene, ethylbenzene, and methyl ethyl ketone) and of water vapor on sand, clay, and limestone has been analyzed. The influence of the presence of water on the adsorption of these compounds has also been analyzed, working at levels below the limit of applicability of Henry's law. The levels of relative air humidity used were 20 and 50%. The results show a big difference between the adsorption levels of the three soil minerals and a higher adsorption for polar compounds than for aliphatic and aromatic compounds. The water affects the VOC adsorption by decreasing the retention of these compounds to a greater extent for aromatic and aliphatic compounds than for the polar compound and by linearizing the isotherms. This reduction has been quantified by a simple exponential equation.
Love-wave devices based on quartz piezoelectric substrate and SiO 2 guiding layer coated with a speci®c polysiloxane polymer are used to detect organophosphorus compounds. This allows to study af®nities of the polysiloxane polymer coating towards organophosphorus compounds, and to demonstrate the high sensitivity of love-wave devices for gas detection. We also present a theoretical model which describe wave propagation in love devices and allow to design optimized structures. Then, we discuss experimental results, in terms of interactions between sensor and vapor and in comparison to SAW results. # 2001 Elsevier Science B.V. All rights reserved.
Carbon nanotube field-effect transistors (NTFETs) coated with poly(ethyleneimine) (PEI) and starch polymers exhibit electrical conductance changes upon exposure to CO2 gas in air at ambient temperature (see Figure). This observation has furnished nanoelectronic CO2 sensors. Their small size and low power consumption has enormous potential in wireless sensing for industrial and medical CO2 sensor units.
Wird eine Fremdschicht auf eine zu Dickenscherungsschwingungen angeregte Schwingquarzplatte aufgebracht, so ndert sich die Eigenfrequenz der Platte infolge Vergrerung der schwingenden Masse. Da die Frequenznderung eines Schwingquarzes sehr genau vermessen werden kann, ergibt sich daraus eine sehr empfindliche Methode zur Wgung dnner Schichten.Massenbelegung der Fremdschicht und Frequenznderung sind einander proportional. Die Proportionalittskonstante lt sich aus der Eigenfrequenz des Schwingquarzes berechnen, so da eine empirische Eichung bei der Schichtwgung mit Schwingquarzen entfllt.Die Genauigkeit des Schichtwgeverfahrens ist in erster Linie durch die Temperaturabhngigkeit der Quarzeigenfrequenz begrenzt und betrgt bei 1 C zugelassener Temperaturschwankung etwa 4 10–9 g cm–2. Das entspricht einer mittleren Dicke von 0,4 bei der Dichte =1 g cm–3.Das Verfahren wurde auch zur direkten Wgung einer Masse ausgenutzt (Mikrowgung). Dabei lie sich eine Genauigkeit von 10–10g erreichen.
Rapid aroma profiling of food products is a first step towards at-line flavor quality control and off-flavor assessment. In this paper, the potential of the zNose™ was tested for the first time to address this application. Honey was chosen as the food product because of its characteristic aroma. Both a chromatogram and a spectral approach to the interpretation of the zNose™ signal were established. In the chromatogram approach, the signal was treated as a traditional chromatogram and relative peak areas were calculated and compared, while the whole aroma spectrum was considered in the spectral approach. Shifts in GC-column retention times initially led to misinterpretation of the results in the spectral approach. A data processing algorithm was, hence, developed to correct for these shifts. Data were analyzed with principal component analysis (PCA), and canonical discriminant analysis (CDA). With both relative peak areas and corrected spectra, the aroma of six different honey varieties and two types of sugar solutions were successfully discriminated. A classification model was built and validated externally, which resulted in a correct classification of 15 out of 16 honey aroma profiles (94%).
Flavour analysis is typically performed by human organoleptic analysis, which is often expensive and subjective. A novel approach using a surface acoustic wave sensing electronic nose (zNoseTM) for flavour analysis was explored to characterise 16 types of vegetable oils. Fatty acid composition, iodine value, peroxide value, p-anisidine value and free fatty acid analyses were conducted to determine the quality and characteristics of vegetable oils. The zNoseTM was employed successfully for qualitative distinction of flavour in different vegetable oils. This is achieved using a visual fragrance pattern, called a VaporPrintTM, derived from the frequency of the SAW detector. VaporPrintTM was shown to be particularly useful for assessing vegetable oil aroma profile in its entirety. This image is created by transforming the time variable to a radial angle with the beginning and end of the analysis occurring at 0°, or vertical. A Chemometric method, particularly principal component analysis (PCA), was conducted for electronic nose data processing and identification. Analysis of the score plot of the PCA for the zNoseTM measurement showed that 97% of the total variance in the data was described by PC 1 and PC 2. The loading plot revealed that five compounds (m,k,n,s, and p) were important for differentiate the vegetable oils.
Surface acoustic wave (SAW) devices have been fabricated and tested as sensors of NH3 in gaseous phase. Polypyrrole films, prepared by Langmuir-Blodgett (LB) technique, have been deposited onto the surface of SAW devices as gas absorbent layers. Simultaneous measurements of SAW phase velocity and attenuation have been carried out in order to investigate the sensing mechanisms. The sensor response shows high sensitivity towards NH3 gas and excellent selectivity with respect to main interfering gases (CO, CH4, H2, O2), at room temperature. The low cross-sensitivity permits the recognition of NH3 gas as one component of a mixture of interfering gases.
The mass sensitivity of an AlN thin film resonators operated in air at 6–8 GHz has been investigated in theory and experiment. The 30 × 30 μm wide resonators included a 180–300 nm thick, highly oriented AlN thin film, bottom and top electrode, and an acoustic reflector made of AlN/SiO2 multilayers. The top electrode was loaded by defined polymer layers to test sensitivity and simulation model. The impact of type and thickness of the top electrode was simulated. Ten to twenty nanometers thick PMMA coatings served to demonstrate sensitivity with a gettering gel in form of spontaneous adsorption of acetone from the vapour phase. Finally, a self-assembled monolayer of 11-mercaptoundecanoic acid was formed on a Pt top electrode for the demonstration of a sub-monolayer sensor in air. In both cases, pico-grams could be detected with a sensitivity up to 1000 m2/kg.
By functionalizing the surfaces of ZnO nanobelts (NBs) with a thin self-assembled molecular layer, the electrical and optoelectronic performances of a single NB-based device are drastically improved. For a single NB-based device, due to energy band tuning and surface modification, the conductance was enhanced by 6 orders of magnitude upon functionalization; a coating molecule layer has changed a Schottky contact into an Ohmic contact without sophisticated deposition of multilayered metals. A functionalized NB showed negative differential resistance and exhibited huge improved photoconductivity and gas sensing response. The functionalized molecular layer also greatly reduced the etching rate of the ZnO NBs by buffer solution, largely extending their life time for biomedical applications. Our study demonstrates a new approach for improving the physical properties of oxide NBs and nanowires for device applications.
This paper studies the application of lateral bulk acoustic thin-film piezoelectric-on-substrate (TPoS) resonators in high-frequency reference oscillators. Low-motional-impedance TPoS resonators are designed and fabricated in 2 classes--high-order and coupled-array. Devices of each class are used to assemble reference oscillators and the performance characteristics of the oscillators are measured and discussed. Since the motional impedance of these devices is small, the transimpedance amplifier (TIA) in the oscillator loop can be reduced to a single transistor and 3 resistors, a format that is very power-efficient. The lowest reported power consumption is approximately 350 microW for an oscillator operating at approximately 106 MHz. A passive temperature compensation method is also utilized by including the buried oxide layer of the silicon-on-insulator (SOI) substrate in the structural resonant body of the device, and a very small (-2.4 ppm/ degrees C) temperature coefficient of frequency is obtained for an 82-MHz oscillator.
Olfaction exhibits both high sensitivity for odours and high discrimination between them. We suggest that to make fine discriminations between complex odorant mixtures containing varying ratios of odorants without the necessity for highly specialized peripheral receptors, the olfactory systems makes use of feature detection using broadly tuned receptor cells organized in a convergent neurone pathway. As a test of this hypothesis we have constructed an electronic nose using semiconductor transducers and incorporating design features suggested by our proposal. We report here that this device can reproducibly discriminate between a wide variety of odours, and its properties show that discrimination in an olfactory system could be achieved without the use of highly specific receptors.
The sensing ability of individual SnO(2) nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface.
We observe second-harmonic generation from metamaterials composed of split-ring resonators excited at 1.5-micrometer wavelength. Much larger signals are detected when magnetic-dipole resonances are excited, as compared with purely electric-dipole resonances. The experiments are consistent with calculations based on the magnetic component of the Lorentz force exerted on metal electrons-an intrinsic second-harmonic generation mechanism that plays no role in natural materials. This unusual mechanism becomes relevant in our work as a result of the enhancement and the orientation of the local magnetic fields associated with the magnetic-dipole resonances of the split-ring resonators.
The development of the electronic nose have paved the way for the classification of bacteria, to monitor air quality on the space shuttle, or to check the spoilage of foodstuff. However, the electronic nose still is unable to discriminated between flavors, perfumes, smells and as a replacement for the human nose. Although it has been used to detect some important nonodorant gases, it is not adapted to substances of daily importance in mammalian life such as the scent of other animals, foodstuff or spoilage. Due to such limitations, the electronic nose was developed to mimic the human nose. It turns out that the human nose's unequaled performance is not due to the high number of different human receptor cells, but their selectivity and their unsurpassed sensitivity for some analyte gases. As such, the success of the electronic nose will not rely on increasing the number of individual sensors and creating redundant information by adding more similar sensors, but rather on DNA, molecular, imprinted molecules or even mobilized natural receptors, which promise to increase the sensitivity and importantly selectivity. An increase in the sensitivity can be achieved by appropriate sample pretreatment and preconcentration techniques, whereas filters and separation units can be used to increase the selectivity and reduce interfering substances.