Two-cell coupled nonlinear chemical oscillator: asymmetric cells

A two-cell coupled chemical oscillator, based on the cubic autocatalator in each cell, isconsidered in the case of different precursor concentrations in each cell. The coupling between the cells is achieved through diffusive interchange of the autocatalyst. It is shown that there is just one stationary state, the stability of which and the positions of Hopf bifurcations are examined for both small and large coupling. In the latter case it isshown that the system is essentially two-dimensional. Numerical results are obtained for general values of the parameters and compared with the previously treated symmetric case.     

An analysis of a two-cell coupled nonlinear chemical oscillator

A two-cell chemical oscillator, based on cubic autocatalysis, is considered, with the reaction in each identical cell being P → A; A + 2B → 3B; B → C. The coupling between the cells is assumed to take place by a linear exchange of either reactant A or autocatalyst B. In the former case, five possible stationary states are found with there being a range of parameter values over which non-symmetric stable stationary states can exist. In the latter case, the only possible stationary state is the same as for the uncoupled system. In both cases, Hopf bifurcations are found, leading to sequences of complex dynamical behaviour     

Contributions of amplitude measurement in QCM sensors

The characteristics of the amplifier and the feedback loop of a quartz crystal series oscillating circuit are investigated. The fact that the change of the vibration amplitude of the quartz crystal is proportional to the change of its motional resistance is derived. The concept of a characteristic damping coefficient is introduced and the behavior of a quartz crystal vibrating in liquids is analyzed. The experiment shows that the effect of mass loading can be distinguished from that of the liquid damping of a quartz crystal microbalance (QCM) sensor in liquids by simultaneously measuring the amplitude and the frequency change     

Higher order sensing using QCM sensor array and preconcentrator with variable temperature

We proposed the higher order sensing method using a preconcentrator with variable temperature in combination with the quartz-crystal microbalance sensor array to extract many features of the samples. The temperature of the preconcentrator tube packed with hydrophobic adsorbent was gradually raised to 200/spl deg/C. The rough separation among the compounds along the time axis enhanced the discrimination capability of the sensor array system. The results of the second-order sensing, such as images of the sensor responses and the loci on the principal component analysis space, gave us information about the apple flavors with different recipes. Moreover, the low-concentration flavors, such as apple and banana in water, were successfully discriminated using the proposed method. It was found that the system was robust against the sample concentration change under the environment of the large humidity.     

Characterization of infrared chemical sensors modified with ZnO nanowires for the detection of volatile organic compounds

In this paper we describe the application and characterization of zinc oxide (ZnO) nanowires in an infrared (IR) chemical sensing system for the detection of volatile organic compounds (VOCs). Under suitable conditions, we grew ZnO nanowires on the surfaces of IR internal reflection elements (IREs) and obtained successful results for the detection of VOCs. ZnO nanowires offer a large surface area to effectively adsorb the examined species; the sensitivity of these IR sensing systems was increased by 3- to 15-fold after surface treatment with the ZnO nanowires. To explore the performance of this type of sensor, we correlated the morphologies of the ZnO nanowires grown on the surfaces of the IREs with the adsorption behavior observed during the sensing of the VOCs. To characterize the properties of the ZnO nanowires during the detection of VOCs having a range of functionalities, we classified the VOCs and examined their enrichment factors by comparing the IR signals detected in the presence and absence of the ZnO nanowires. Our results indicate that the ZnO nanowires exhibited better performance for the detection of aromatic-type VOCs than they did for non-aromatic compounds. For quantitative analyses, we examined several compounds for their responses toward varying quantities of injected VOCs. Our results indicate that the IREs treated with ZnO nanowires display acceptable linearity in their standard curves; the linear regression coefficients were higher than 0.995 for a range of volatile compounds.     

Design considerations of Miller oscillators for high-sensitivity QCM sensors in damping media

In this paper, a new contribution to the design of quartz crystal oscillators for high-sensitivity microbalance sensors used in liquid media is presented. The oscillation condition for a Miller configuration was studied to work in a wide dynamic range of the resonator losses. The equations relating the values of the active and passive components with the maximum supported damping and mass were obtained. Also, the conditions to obtain a stable frequency according to the resonator damping (R(Q)), the static capacity (Cp) and the filter frequency (f(F)) were found. Under these conditions, the circuit oscillation frequency will be proportional to the resonant series frequency and does not depend on the previous parameters (R(Q), f(F), and Cp). If these conditions cannot be satisfied, the expression of the oscillation frequency is given and the discrimination of these effects is obtained through resonator frequency measurements.     

Position and displacement sensing with shack-hartmann wave-front sensors

The use of a Shack-Hartmann wave-front sensor as a position-sensing device is proposed and demonstrated. The coordinates of a pointlike object are determined from the modal Zernike coefficients of the wave fronts emitted by the object and detected by the sensor. The position of the luminous centroid of a moderately extended incoherent flat object can also be measured with this device. Experimental results with off-the-shelf CCD cameras and conventional relay optics as well as inexpensive diffractive microlens arrays show that axial positioning accuracies of 74 mum rms at 300 mm and angular accuracies of 4.3 murad rms can easily be achieved.     

The enhanced humidity sensing performance of calixarene/PMMA hybrid layers: QCM sensing mechanism

Journal of Materials Science: Materials in Electronics - This paper describes the preparation of the calix-4/PMMA hybrid QCM sensor by electrospinning of calix[4]arene derivative having carboxylic...     

Detecting harmful gases using fluctuation-enhanced sensing with Taguchi sensors

Sensing techniques are often required to not only be versatile and portable, but also to be able to enhance sensor information. This paper describes and demonstrates a new approach to chemical signal analysis that we call fluctuation-enhanced sensing. It utilizes the entire bandwidth of the sensor signal in contrast to more conventional approaches that rely on the dc response. The new principle holds prospects for significantly reducing the necessary number of sensors in artificial noses and tongues, and it can provide improved sensitivity.     

A measurement system for odor classification based on the dynamic response of QCM sensors

In this paper, an innovative measurement system for odor classification, based on quartz crystal microbalances (QCMs), is presented. The application proposed in this paper is the detection of typical wine aroma compounds in mixtures containing ethanol. In QCM sensors, the sensitive layer is, e.g., a polymeric layer deposited on a quartz surface. Chemical mixtures are sorbed in the sensitive layer, inducing a change in the polymer mass and, therefore, in the quartz resonance frequency. In this paper, the frequency shift is measured by a dedicated, fully digital front-end hardware implementing a technique that allows reducing the measurement time while maintaining a high-frequency resolution . The developed system allows, therefore, measuring variations of the QCM resonance frequency shifts during chemical transients obtained with abrupt changes in odor concentration. Hence, the reaction kinetics can be exploited to enhance the sensor selectivity. In this paper, some measurements obtained with an array of four sensors with different polymeric sensitive layers are presented. An exponential fitting of the transient responses is used for feature extraction. Finally, to reduce data dimensionality, principal component analysis is used.     

Fiber-optic chemical sensing with langmuir-blodgett overlay waveguides

Fiber-optic chemical sensing has been demonstrated with a side-polished single-mode optical fiber, evanescently coupled to chemically sensitive Langmuir-Blodgett (LB) overlay waveguides. The sensors exhibit a channel-dropping response centered on a wavelength that is dependent on the thickness and the refractive index of the overlay waveguide. It has been shown that pH-sensitive organic dyes proved to be suitable materials for the formation of an overlay waveguide whereas LB deposition provides the required thickness control. A theoretical model of the sensor response, based on the Kramers-Kronig relations and phase matching of the guided modes within the optical fiber and overlay waveguide, shows good agreement with experimental results.     

Signal amplification with multilayer arrangements on chemical quartz-crystal-resonator sensors

Viscoelastic properties of chemically sensitive coatings can enhance the mass sensitivity of quartz-crystal-microbalance (QCM) sensors. If analyte sorption is accompanied by a change of the viscoelastic properties of the coating material, the accumulated mass cannot be calculated from the frequency shift without further information. We developed a sensor concept, which is based on a double-layer arrangement, permitting acoustic amplification and chemical sensitivity to be separated. With a proper selection of materials, the first layer realizes a constant acoustic amplification of the mass effect; the chemically sensitive layer acts purely gravimetrically. Major sensor design parameters are the shear modulus and the thickness of the first layer. From the acoustic point of view, the thickness of the chemically active layer and its material properties are less critical; a glasslike, rigid coating is preferred for a stable sensor transfer function. Simultaneous measurement of the resonant frequency of the quartz crystal and its motional resistance can be exploited to check the acoustic amplification.     

Sensitivity of Lamb wave sensors in liquid sensing

The differences of the sensitivities of liquid- and solid-sensing of Lamb wave sensors are discussed. It is shown that the sensitivity of S₀ mode in liquid sensing is much smaller than that in solid sensing, and also much smaller than that of the A₀ mode     

Cement-based sensors with carbon fibers and carbon nanotubes for piezoresistive sensing

Electrically conductive cementitious composites carrying carbon fibers and carbon nanotubes were developed and their ability to sense an applied compressive load through a measureable change in resistivity was investigated. Two types of cement-based sensors, one with carbon fibers alone and the other carrying a hybrid of both fibers and nanotubes, were considered. Direct comparisons were also made with traditional strain gauges mounted on the sensor specimens.Sensing experiments indicate that under cyclic loading, the changes in resistivity mimic both the changes in the applied load and the measured material strain with high fidelity for both sensor types. The response, however, is nonlinear and rate dependent. At an arbitrary loading rate, the hybrid sensor, containing a combination carbon fibers and nanotubes, produced the best results with better repeatability.     

Switching the conformation of a DNA molecule with a chemical oscillator

pH oscillations generated by a nonequilibrium chemical reaction are used to switch a pH-sensitive DNA structure between two distinct conformations. The utilization of a chemical oscillator represents a novel method for achieving autonomous motion in molecular devices. The oscillatory reaction is a variant of the Landolt reaction and produces pH variations in the range between pH 5 and 7. In this range, a cytosine-rich DNA strand can be switched between a random coil conformation and the folded i-motif structure. The conformational changes are monitored simultaneously with the pH value in fluorescence-resonance energy-transfer experiments.     

Study of BCN compounds prepared by the chemical vapor deposition with dimethylamineborane

B-C-N films were synthesized by using the chemical vapor deposition in which the starting material dimethylamineborane was transported onto a substrate with nitrogen gas, and analyzed with secondary electron microscopy, Raman scattering, infrared spectroscopy, x-ray photoelectron spectroscopy, transmission electron microscopy, electron diffraction, x-ray diffraction, and ultraviolet-visible-rays absorption spectroscopy. Effects of substrate temperature and deposition pressure on the solid-solutions of B-C-N films were investigated by changing substrate temperature between 700 and 1000 °C under deposition pressure between 100 and 760 Torr. It was found that a mixture of microcrystal glassy carbon and turbostratic-BN both with an apparent crystal size of about 10 Å was deposited under the atmospheric pressure at 1000 °C. In deposition where the pressure was reduced from the atmospheric pressure to 600 Torr, BN gradually lost the stoichiometric compositions to form a mixed structure of BNx (x < 1) and glassy carbon. In deposition where the pressure was lowered than 600 Torr, BNxCy : H (x,y < 1) of which dangling bonds wereterminated with hydrogen was produced as a result of further decrease of the BNx nitriding rate and then substitution of carbon for nitrogen. This BNxCy : H (x,y < 1) showed character close to amorphous material and the composition ratio varied continuously for the deposition pressure change.     

Model-based optimal design of polymer-coated chemical sensors

A model-based methodology for optimal design of polymer-coated chemical sensors is developed and is illustrated for the example of infrared evanescent field chemical sensors. The methodology is based on rigorous and computationally efficient modeling of combined fluid mechanics and mass transfer, including transport of multiple analytes. A simple algebraic equation for the optimal size of the sensor flow cell is developed to guide sensor design and validated by extensive CFD simulations. Based upon these calculations, optimized geometries of the sensor flow cell are proposed to further improve the response time of chemical sensors.     

Gas sensing properties of graphene synthesized by chemical vapor deposition

The gas sensing properties of graphene synthesized by a chemical vapor deposition (CVD) method are investigated. Synthesis of graphene is carried out on a copper substrate using a methane and hydrogen gas mixture by a CVD process at the atmospheric pressure. The graphene films are transferred to different substrates after wet etching of the copper substrates. The Raman spectra reveal that the graphene films made on SiO₂/Si substrates are of high quality. The reflectance spectra of graphene were measured in UV/Visible region of the spectrum. Theoretically calculated reflectance spectra based on Fresnel's approach indicates that the CVD graphene has a single layer. The gas sensing properties of graphene were tested for different reducing gasses as a function of measurement temperature and gas concentration. It is found that the gas sensing characteristics such as response time, recovery time, and sensitivity depend on the target gas, gas concentration, test temperature, and the ambient gas composition. The cross sensitivity of few combinations of reducing gasses such as, NH₃, CH_4, and H₂ was also investigated.     

Quantitative real-time monitoring of chemical reactions by autosampling flow injection analysis coupled with atmospheric pressure chemical ionization mass spectrometry

Although qualitative and/or semiquantitative real-time monitoring of chemical reactions have been reported with a few mass spectrometric approaches, to our knowledge, no quantitative mass spectrometric approach has been reported so far to have a calibration valid up to molar concentrations as required by process control. This is mostly due to the absence of a practical solution that could well address the sample overloading issue. In this study, a novel autosampling flow injection analysis coupled with an atmospheric pressure chemical ionization mass spectrometry (FIA/APCI-MS) system, consisting of a 1 μL automatic internal sample injector, a postinjection splitter with 1:10 splitting ratio, and a detached APCI source connected to the mass spectrometer using a 4.5 in. long, 0.042 in. inner diameter (ID) stainless-steel capillary, was thus introduced. Using this system together with an optional FIA solvent modifier, e.g., 0.05% (v/v) isopropylamine, a linear quantitative calibration up to molar concentration has been achieved with 3.4-7.2% relative standard deviations (RSDs) for 4 replicates. As a result, quantitative real-time monitoring of a model reaction was successfully performed at the 1.63 M level. It is expected that this novel autosampling FIA/APCI-MS system can be used in quantitative real-time monitoring of a wide range of reactions under diverse reaction conditions.