Ethanol gas sensor based on Al-doped ZnO nanomaterial with many gas diffusing channels

ZnO nanomaterial with multi-microstructures is synthesized by using normal pressure thermal evaporation and then doped with different Al₂O₃ contents by grinding in an agate mortar. The as-prepared Al-doped ZnO nanomaterials are characterized by X-ray diffraction and scanning electron microscopy. The characterization results show that all the compounds are wurtzite with hexagonal structure and are well crystallized. Channels/connecting holes arising from many kinds of ZnO microstructures are abundant. Both annealing and Al₂O₃-doping contributes to an increase in the quasi-one-dimensional and tri-dimensional microstructures. The as-prepared Al-doped ZnO nanomaterials show excellent gas responses to ethanol. The sensing mechanism of the ZnO-based nanomaterials with multi-microstructures is further analyzed by using the Effective Specific Surface Model. Excellent sensitivity (200) companied with short response time (8 s) and recovery time (10 s) to 3000 ppm ethanol is obtained with a ZnO-based sensor with 2 at.% Al₂O₃ at the operating temperature of 290 °C after the sensor is annealed at 500 °C.     

Hydrogen sensors with double dipole layers using a Pd-mixture-Pd triple-layer sensing structure

This paper reports on new GaN sensors using a Pd-mixture-Pd triple-layer sensing structure to enhance their sensitivity to hydrogen at the tens of ppm level. The proposed hydrogen sensor biased with a constant voltage produced relatively high sensing responses of 4.84 × 10⁵% at 10,100 ppm and 8.7 × 10⁴% at 49.1 ppm H₂ in N₂. The corresponding barrier height variations are calculated to be 220 and 168 mV. When the sensor is biased by a constant current with maximum power consumption of 0.4 mW, a sensing voltage as an output signal showed a voltage shift of more than 17 V (the highest value ever reported) at 49.1 ppm H₂ in N₂. By comparison to Pd-deposited GaN sensors, the improvement in static-state performance is likely attributed to double dipole layers formed individually at the Pd–GaN interface and inside the mixture. Moreover, voltage transient response and current transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the former response time is shorter than the latter one.     

Response time of nanostructured relative humidity sensors

Capacitive relative humidity (RH) sensors were fabricated by coating countersunk interdigitated electrode substrates with nanostructured TiO₂ films produced using glancing angle deposition. Areal capacitance increased from 1 nF cm⁻² to 800 nF cm⁻² as relative humidity was increased from 2% RH and 95% RH. For films deposited at 81° and with a thickness below 4 m, response time was (162±4) ms m⁻¹. Response times increased from 64 ms to 1440 ms as film thickness increased from 280 nm to 8.5 m. The linear dependence of response time with film thickness indicates that device response time is dominated by surface adsorption. Response time decreased with increasing deposition angle, with a slope of (−15.2±1.6) ms degree⁻¹ for the adsorption data, and (−17.3±2.5) ms degree⁻¹ for the desorption data. The optimum operating range of the sensors depends on deposition angle, and can be tuned to different ranges to match application needs.     

Wireless sensor network based wearable smart shirt for ubiquitous health and activity monitoring

Nanofibers of a composite of a silicon-containing polyelectrolyte, polyethylene oxide and polyaniline (PANI) were obtained by electrospinning and heat treatment. The morphologies of the composite nanofibers were characterized by scanning electron microscopy, which showed that the nanofibers with a diameter of 250–500 nm formed a non-woven mat with highly porous structure. It was found that the polymer composite nanofibers showed impedance change from 6.3 × 10⁶ to 2.5 × 10⁴ Ω with the increment of relative humidity (RH) from 22 to 97% at room temperature, exhibiting high sensitivity and good linearity on a semi-logarithmic scale. In addition, they exhibited fast and highly reversible response characterized by very small hysteresis of 2% RH and short response time (t_90%: 7 s and 19 s for adsorption and desorption between 33 and 97%RH, respectively). The modification of the electrode with poly(diallyldimethylammonium chloride) prior to the deposition of nanofibers improved their humidity response, which may be related to the enhanced contact between the nanofibers and underlying substrate. The effect of PANI on the humidity response of the composite nanofibers was also investigated, and it was found that PANI effectively decreased the impedance of the nanofibers. The electrospun polymer composite nanofibers may find applications in the preparation of high performance humidity sensors.     

A technique for monitoring SO2 in combustion exhausts: Use of a non-Nernstian sensing element in combination with an upstream catalytic filter

Detection of sulfur dioxide (SO₂) at high temperature (600–750 °C) in the presence of some interferents found in combustion exhausts (NO₂, NO, CO₂, CO, and hydrocarbon (C₃H₆)) is described. The detection scheme involves use of a catalytic filter in front of a non-Nernstian (mixed-potential) sensing element. The catalytic filter was a Ni:Cr powder bed operating at 850 °C, and the sensing elements were pairs of platinum (Pt) and oxide (Ba-promoted copper chromite ((Ba,Cu)_xCr_yO_z) or Sr-modified lanthanum ferrite (LSF)) electrodes on yttria-stabilized zirconia. The Ni:Cr powder bed was capable of reducing the sensing element response to NO₂, NO, CO, and C₃H₆, but the presence of NO₂ or NO (“NO_x”, at 100 ppm by volume) still interfered with the SO₂ response of the Pt–(Ba,Cu)_xCr_yO_z sensing element at 600 °C, causing approximately a 7 mV (20%) reduction in the response to 120 ppm SO₂ and a response equivalent to about 20 ppm SO₂ in the absence of SO₂. The Pt–LSF sensing element, operated at 750 °C, did not suffer from this NO_x interference but at the cost of a reduced SO₂ response magnitude (120 ppm SO₂ yielded 10 mV, in contrast to 30 mV for the Pt-(Ba,Cu)_xCr_yO_z sensing element). The powder bed and Pt–LSF sensing element were operated continuously over approximately 350 h, and the response to SO₂ drifted downward by about 7%, with most of this change occurring during the initial 100 h of operation.     

H2 sensing characteristics of highly textured Pd-doped SnO2 thin films

The effects of the crystallographic orientation on the H₂ gas sensing properties were investigated in highly oriented polycrystalline Pd-doped SnO₂ films, which were obtained using rf magnetron sputtering of a Pd (0.5 wt%)-SnO₂ target on various substrates (a-, m-, r-, and c-cut sapphire and quartz). All the films had a similar thickness (110 nm), root-mean-square (rms) roughness (1.3 nm), surface area, and chemical status (O, Sn, and Pd). However, the orientation of the films was strongly affected by the orientation of the substrates. The (1 0 1), (0 0 2), and (1 0 1) oriented films were grown on (a-cut), (m-cut), and (r-cut) Al₂O₃ substrates, respectively, and rather randomly oriented films were deposited on (0 0 0 1) (c-cut) Al₂O₃ and quartz substrates. In addition, the oriented Pd-doped SnO₂ films were highly textured and had in-plane orientation relationships with the substrates similar to the epitaxial films. The (1 0 1) Pd-doped SnO₂ films on and Al₂O₃ showed a considerably higher H₂ sensitivity, and their gas response decreased with increasing sensing temperature (400–550 °C). The films deposited on and (0 0 0 1) Al₂O₃ showed the maximum sensitivity at 500 °C. The comparison of the H₂ gas response between undoped and Pd-doped SnO₂ films revealed that the Pd-doping shifted the optimum sensing temperature to a lower value instead of improving the gas sensitivity.     

SnO2 thin films modified by the SnO2–Au nanocomposites: Response to reducing gases

The influence of the SnO₂ surface modification by the SnO₂–Au nanocomposites on conductivity response to such reducing gases as CO and H₂ has been analyzed in the present paper. Both initial SnO₂ films, subjected for surface modification, and SnO₂–Au nanocomposites were deposited by Successive Ionic Layer Deposition (SILD) method. The SnO₂–Au nanocomposites with Au/Sn ratio 1 were synthesized using HAuCl₄ and SnCl₂ precursors. The thickness of the Au-SnO₂ nanolayers varied from 0.7–1.0 nm to 10–15 nm. It was established that the increase in the thickness of the SnO₂–Au nanocomposite layer formed on the surface of the SnO₂ films was accompanied by both the improvement of sensor response and the decrease in response and recovery times. An explanation of the observed effects has been proposed.     

Enhanced H2S sensing characteristics of SnO2 nanowires functionalized with CuO

The CuO-functionalized SnO₂ nanowire (NW) sensors were fabricated by depositing a slurry containing SnO₂ NWs on a polydimethylsiloxane (PDMS)-guided substrate and subsequently dropping Cu nitrate aqueous solution. The CuO coating increased the gas responses to 20 ppm H₂S up to 74-fold. The R_a/R_g value of the CuO-doped SnO₂ NWs to 20 ppm H₂S was as high as 809 at 300 °C, while the cross-gas responses to 5 ppm NO₂, 100 ppm CO, 200 ppm C₂H₅OH, and 100 ppm C₃H₈ were negligibly low (1.5–4.0). Moreover, the 90% response times to H₂S were as short as 1–2 s at 300–400 °C. The selective detection of H₂S and enhancement of the gas response were attributed to the uniform distribution of the sensitizer (CuO) on the surface of the less agglomerated network of the SnO₂ NWs.     

Crossed zinc oxide nanorods for ultraviolet radiation detection

An ultraviolet photosensor has been successfully constructed by the in situ lift-out method in a focused ion beam system. The prototype consists of individual naturally self-assembled crossed ZnO nanorods grown by an aqueous solution process. The current–voltage (I–V) characteristics show linear behavior. The photosensor exhibits a response of 15 mA/W for UV light (361 nm) under 1 V bias. Response measurements showed that such a photosensor is suitable for low levels of ultraviolet detection. The method is simple, rapid and applicable to research prototypes for further studies of crossed ZnO nanorods for nano-device applications.     

Potentiometric sensor based on molecularly imprinted polymer for determination of melamine in milk

A polymeric membrane ion-selective electrode for determination of melamine is described in this paper. It is based on a molecularly imprinted polymer (MIP) for selective recognition, which can be synthesized by using melamine as a template molecule, methacrylic acid as a functional monomer and ethylene glycol dimethacrylate as a cross-linking agent. The membrane electrode shows near-Nernstian response (54 mV/decade) to the protonated melamine over the concentration range of 5.0 × 10⁻⁶ to 1.0 × 10⁻² mol L⁻¹. The electrode exhibits a short response time of 16 s and can be stable for more than 2 months. Combined with flow analysis system, the potentiometric sensor has been successfully applied to the determination of melamine in milk samples. Interference from high concentrations of ions co-existing in milk samples such as K⁺ and Na⁺ can be effectively eliminated by on-line introduction of anion- and cation-exchanger tandem columns placed upstream, while melamine existing as neutral molecules in milk of pH 6.7 can flow through the ion-exchanger columns and be measured downstream by the proposed electrode in an acetate buffer solution of pH 3.7.     

Dynamic evolvement and formation of refractive microlenses self-assembled from evaporative polyurethane droplets

This paper proposed a method of microfabrication for the formation of hemispherical refractive microlenses by depositing a colloid evaporative droplet onto hydrophobic surfaces. The microdroplets made of polyurethane (PU) were self-driven by surface tension to evolve their three-dimensional (3D) shapes on the surface-treated substrate. The substrates were coated with low surface energy material (Teflon) to de-pin the fluids obeying classic Young–Laplace equation until drying. Array and size-variation experiments, corresponding to different placement and droplet volume, were performed for the shaping process in which the polymers of the drops were self-assembled to be hemispherical utilizing general principle of minimal surface energy. Using the hydrophobic surfaces, plano-convex shapes with spherical curvature were fabricated with micrometer dimensions (base radius between 70 and 1016 μm). The formed structures were observed to form themselves hemispherically by the de-wetting (de-pinning) process during most of evaporation. Moreover, the gravity flatting effect was further found for the larger drop (radius = 1016 μm) when compared to that of smaller one (radius = 118 μm). In the cases, both the modeling calculations and experimental results were performed and compared to illustrate the similar geometries with the contact angle (70°) using dimensionless analysis. In addition, one interesting and significant finding, based on close morphological inspection of the SEM picture, showed that the resulting elongational polymer chains (width 200 nm) stretched (extension 5 μm) on the surface nearby the corner of the contact area, indicating a shear stress occurrence. Compared to those previous methods operated on (soft-) photolithographic techniques, this present one could rapidly predict and microfabricate the hemispherical formation in terms of the radius, height, and contact angle. It is also potentially appropriate for smaller and complex placement by using drop-on-demand (DOD) nozzle arrays for mass-production process.     

A SOI-MEMS-based 3-DOF planar parallel-kinematics nanopositioning stage

This paper presents the design, kinematic and dynamic analysis, fabrication and characterization of a monolithic micro/nanopositioning three degrees-of-freedom (DOF) (XYθ) stage. The design of the proposed MEMS (micro-electro-mechanical system) stage is based on a parallel-kinematic mechanism (PKM) scheme that allows for translation in the XY plane and rotation about the Z axis, an increased motion range, and linear kinematics in the operating region (or work area) of the stage. The truss-like structure of the PKM results in higher modal frequencies by increasing the structural stiffness and reducing the moving mass of the stage. The stage is fabricated on a silicon-on-insulator (SOI) wafer using surface micromachining and deep reactive ion etching (DRIE) processes. Three sets of electrostatic linear comb drives jointly actuate the mechanism to produce motion in the X, Y and θ (rotation) directions. The fabricated stage provides a motion range of 18 μm and 1.72° at a driving voltage of 85 V. The resonant frequency of the stage under ambient conditions is 465 Hz. Additionally a high Q factor (66) is achieved from this parallel-kinematics mechanism design.     

Interactions Between PVS and Maple in Symbolic Analysis of Control Systems

This paper presents a decision procedure for problems relating polynomial and transcendental functions. The procedure applies to functions that are continuously differentiable with a finite number of points of inflection in a closed convex set. It decides questions of the form ‘is f0?’, where {=,>,<}. An implementation of the procedure in Maple and PVS exploits the existing Maple, PVS and QEPCAD connections. It is at present limited to those twice differentiable functions whose derivatives are rational functions (rationally differentiable). This procedure is particularly applicable to the analysis of control systems in determining important properties such as stability.     

A localized surface plasmon resonance nanosensor based on rhombic Ag nanoparticle array

A localized surface plasmon resonance (LSPR) nanosensor by using a rhombic Ag nanoparticle array is proposed and developed. The LSPR extinction spectra of the antigen or antibody with lower concentration are detected. The obtained results detected from 100 nM target molecule indicate a larger wavelength shift for the rhombic based nanosensor than the triangular based nanosensor. By experimental detection, we demonstrated that the refractive index sensitivity is 266.2 nm per refractive index unit for Ag rhombic nanoparticle with in-plane width of 140 nm and out-of-plane height of 47 nm.     

Rapid sensing of molybdenum by combined colorimetric solid-phase extraction—Reflectance spectroscopy

A new and selective colorimetric solid-phase extraction (C-SPE) procedure for the determination of Mo(VI) in aqueous samples is proposed. Phenylhydrazine (PHZ) was used as a coloring reagent for molybdenum and its immobilisation onto a commercially available Amberlite XAD-16 resin was achieved. In this analytical technique analogous to a reflectometric sensor, the analytes in samples were extracted onto a solid sorbent matrix loaded with a colorimetric reagent and then quantified directly on the adsorbent surface by using a miniature reflectance spectrometer. The measurements were carried out at a wavelength of 554.05 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with Mo(VI). The solution pH was controlled at pH 2.2 using phosphoric acid-primary phosphate buffer solution. A large number of metal ions known as common matrix elements for Mo(VI) did not interfere with the determination. The amount of resin-concentrated Mo(VI) was then determined in a few seconds by using a miniature reflectance spectrometer, with a total sample workup and readout time of 5 min using only 1-mL sample volumes. The linear dynamic range of Mo(VI) was found within the concentration range of 0.5–12 ppm with its LOD of 0.2 ppm. The C-SPE from different extractions (n = 5) gave a RSD of 3.3% at 9.6 ppm Mo.     

Nanostructured cerium oxide film for triglyceride sensor

Lipase has been immobilized onto sol–gel derived nanostuctured cerium oxide (Nano-CeO₂, 35 nm) film deposited onto indium–tin–oxide (ITO) coated glass plate for tributyrin detection. The Nano-CeO₂/ITO electrode and lipase/nano-CeO₂/ITO bioelectrode have been characterized using scanning electron microscopy (SEM) and cyclic voltammetry (CV). The electrochemical response of lipase/nano-CeO₂/ITO bioelectrode towards tributyrin, investigated using CV studies, exhibits linearity, detection limit and shelf life as 50–500 mg/dL, 32.8 mg/dL and 12 weeks, respectively. The value of the apparent Michaelis–Menten constant () obtained as 22.27 mg/dL (0.736 mM) for lipase/nano-CeO₂/ITO bioelectrode indicates high affinity of lipase with tributyrin. Attempts have been made to detect triglyceride concentration in serum samples.     

Application of m-CNTs/NaClO4/Ppy to a fast response, room working temperature ethanol sensor

A new blended material, m-CNTs/NaClO₄/Ppy, was developed as a gas sensor to detect ethanol concentrations at room temperature. The sensing material polypyrrole (Ppy) was synthesized in situ by UV-photo-polymerization. The multiwall carbon nanotubes (m-CNTs) added enhanced the short-term repeatability of the Ppy-sensing material. The relative resistance variation (R_ethanol/R_air) of m-CNTs/NaClO₄/Ppy was 1.193 when exposed to ethanol of 30,000 ppm. The sensor response and recovery times (both 20 s) were very short to this concentration. An unstable baseline of the sensor was explained by theoretical calculation of molecular dynamics made for ethanol adsorption on polypyrrole, which revealed formation of a new bond, N–HO. Adsorption energy decreased with increasing the number of ethanol molecules adsorbed and was 0.8 kcal/mol at eight adsorbed molecules. Sensor responses of three different Ppy samples were measured to ethanol concentrations of 18,000–40,000 ppm.     

Using electric actuation and detection of oscillations in microcantilevers for pressure measurements

Response characteristics of a microcantilever, such as resonant frequency, amplitude, phase and quality factor, can be used for absolute pressure measurements in the range of 10⁻⁴ to 10³ Torr. To this end, it would be very convenient to have the resonance of the microcantilever actuated and detected electrostatically. Herein, we report the nonlinear dynamics of microcantilevers under varying pressure and different gases using the harmonic detection of resonance (HDR) technique [J. Gaillard, M.J. Skove, R. Ciocan, A.M. Rao, Electrical detection of oscillations in 340 microcantilevers and nanocantilevers, Rev. Sci. Instrum. 77 (2006) 073907]. The HDR technique exploits nonlinearities in the cantilever-counter electrode system to allow electrostatic actuation and detection of the responses of the microcantilever to the pressure and gas composition. In particular, the 2nd and 3rd harmonics of the measured charge on the cantilever are investigated. The microcantilever demonstrates a quality factor of 10,000 at 10⁻³ Torr, and a usable response in the range from 10⁻³ to 10³ Torr. The use of different harmonics can enable us to adjust the range of pressures over which the sensor has an efficacious response, enhancing its sensitivity to a particular environment. The experimental results are in reasonable agreement with theoretical calculations, despite the nonlinearities involved.     

Zeolite-coated interdigital capacitors for humidity sensing

Inter-digital capacitors (IDCs) with electrode gaps of 10 or 50 microns have been coated with zeolite films consisting of different zeolites with Si/Al ratios ranging from 1.5 (zeolite A) to infinite (silicalite). The performance of the sensor in the measurement of humidity has been related to the electrical properties of the zeolites (relative permittivity, ?_r), which in turn is a function of their Si/Al ratio. With zeolites of a high Al content the limit of detection was under 0.5 ppmV.     

Fully Scalable Fault-Tolerant Simulations for BSP and CGM

In this paper we consider general simulations of algorithms designed for fully operational BSP and CGM machines on machines with faulty processors. The BSP (or CGM) machine is a parallel multicomputer consisting of p processors for which a memory of n words is evenly distributed and each processor can send and receive at most h messages in a superstep. The faults are deterministic (i.e., worst-case distributions of faults are considered) and static (i.e., they do not change in the course of computation). We assume that a constant fraction of processors are faulty.  We present two fault-tolerant simulation techniques for BSP and CGM:  1. A deterministic simulation that achieves O(1) slowdown for local computations and O((log_h p)²) slowdown for communications per superstep, provided that a preprocessing is done that requires O((log_h p)²) supersteps and linear (in h) computation per processor in each superstep.  2. A randomized simulation that achieves O(1) slowdown for local computations and O(log_h p) slowdown for communications per superstep with high probability, after the same (deterministic) preprocessing as above.  Our results are fully scalable over all values of p from Θ(1) to Θ(n). Furthermore, our results imply that if pn for 0<<1 and h=Θ((n/p)^δ) for 0<δ1 (which hold in almost all practical BSP and CGM computations), algorithms can be made resilient to a constant fraction of processor faults without any asymptotic slowdown.