Showing papers in "Sensors and Actuators B-chemical in 2010"

Selective hydrogen gas nanosensor using individual ZnO nanowire with fast response at room temperature

Abstract: In this work, we report on a single ZnO nanowire-based nanoscale sensor fabricated using focused ion beam (FIB/SEM) instrument. We studied the diameter dependence of the gas response and selectivity of ZnO nanowires (NWs) synthesized by chemical vapor phase growth method. The photoluminescence (PL) measurements were used to determine the deep levels related to defects which are presented in the ZnO nanomaterial as well as to evaluate the effect of thermal treatment in H2 atmosphere on the emission from ZnO nanowires. We show that sample annealed in hydrogen leads to passivation of recombination centers thus modifying the NWs properties. We studied the gas response and selectivity of these ZnO nanowires to H2 ,N H 3, i-Butane, CH4 gases at room temperature. Our results indicated that zinc oxide NWs hold a high promise for nanoscale sensor applications due to its capability to operate at room-temperature and its ability to tune the gas response and selectivity by the defect concentration and the diameter of ZnO nanowire. A method is proposed to reduce the nanosensor’s recovery time through the irradiation with an ultraviolet radiation pulse. The sensing mechanisms of ZnO nanowires will be discussed.

Hydrogel-based devices for biomedical applications

Abstract: This review paper presents hydrogel-based devices for biomedical applications. The first part of the paper gives a comprehensive, qualitative, theoretical overview of hydrogels' synthesis and operation. Crosslinking methods, operation principles and transduction mechanisms are discussed in this part. The second part includes applications of hydrogel devices in specific fields of interest. Sensing, fluid control, drug delivery, nerve regeneration and other biomedical applications constitute the main focus of this part. The aim of this paper is to briefly present recent advances of the field, without neglecting older ones, and to discuss important or novel concepts of each. (C) 2010 Elsevier B.V. All rights reserved.

Voltammetric biosensors for the determination of paracetamol at carbon nanotube modified pyrolytic graphite electrode

Abstract: The voltammetric oxidation of paracetamol on single-walled carbon nanotubes (SWNT) modified edge plane pyrolytic graphite electrode (EPPGE) was explored in phosphate buffer solution by using square wave voltammetry. Cyclic and square wave voltammetry studies indicated the oxidation of paracetamol at the electrode surface through a two-electron reversible step and fundamentally controlled by adsorption. Besides semi-infinite planar diffusion, the role of thin layer diffusion at nanotube modified electrodes is also suggested. The sensitivity at SWNT modified EPPGE is ∼2 times more than that at MWNT modified EPPGE. Paracetamol gave a sensitive oxidation peak at ∼187 mV at pH 7.2 (μ = 0.5 M) which was used to quantitate the drug in the range of 5–1000 nM with a detection limit of 2.9 × 10−9 M at SWNT modified EPPGE. The interfering effect of physiologically common interferents on the current response of paracetamol has been reported. The procedure was successfully applied for the assay of paracetamol in pharmaceutical formulations. The applicability of the developed method to determine the drug in human urine samples obtained after 4 h of administration of paracetamol is illustrated.

Synthesis of CuO nanostructures and their application for nonenzymatic glucose sensing

Abstract: CuO flowers and nanorods have been synthesized for the first time by the composite-hydroxide-mediated and the composite-molten-salt method, respectively, with advantages of one-step, ambient pressure, low temperature, template-free and low cost. Both nanostructures have been applied to modify the graphite substrates for nonenzymatic glucose detection. Compared with bare graphite electrode, the new electrodes exhibit excellent catalysis to direct glucose oxidation. Though the electrode based on the CuO flowers has higher sensitivity than that of the CuO nanorods modified electrode, the latter presents a much better linear range of glucose concentration and a shorter response time. Both electrodes exhibit the same detection limit of glucose as low as 4 μM. In addition, the detection of dopamine and ascorbic acid has also been carried out on these CuO nanostructure modified electrodes, indicating good selectivity for glucose detection.

Recent developments on ZnO films for acoustic wave based bio-sensing and microfluidic applications: a review

Abstract: Recent developments on the preparation and application of ZnO films for acoustic wave-based microfluidics and biosensors are reviewed in this paper. High quality and strongly textured ZnO thin films can be prepared using many technologies, among which RF magnetron sputtering is most commonly used. This paper reviews the deposition of ZnO film and summarizes the factors influencing the microstructure, texture and piezoelectric properties of deposited ZnO films. ZnO acoustic wave devices can be successfully used as biosensors, based on the biomolecule recognition using highly sensitive shear horizontal and Love-wave surface acoustic waves, as well as film bulk acoustic resonator devices. The acoustic wave generated on the ZnO acoustic devices can induce significant acoustic streaming, small scale fluid mixing, pumping, ejection and atomization, depending on the wave mode, amplitude and surface condition. The potential to fabricate an integrated lab-on-a-chip diagnostic system based on these ZnO acoustic wave technologies is also discussed.

Metal decorated graphene nanosheets as immobilization matrix for amperometric glucose biosensor

Abstract: Amperometric glucose biosensors have been fabricated by using platinum–gold (Pt–Au) and gold (Au) nanoparticle spacers decorated graphene nanosheets. Functionalized graphene (f-G) sheets have been prepared by exfoliation of graphitic oxide and it has been decorated with crystalline (Pt–Au)/Au metal nanoparticles using a simple chemical reduction method. The immobilization of glucose oxidase (GOD) over Nafion-solubilized metal nanoparticles dispersed graphene f-G-(Pt–Au) and f-G-(Au) electrode has been achieved by physical adsorption. The resultant bioelectrode retains its biocatalytic activity and offers fast and sensitive glucose quantification. The performances of the biosensor have been investigated by electrochemical method at an optimum potential of +0.8 V in pH 7.0 phosphate buffer. The fabricated f-G-(Au) based glucose biosensor exhibits best sensing performance with a linear response up to 30 mM with an excellent detection limit of 1 μM. The elimination of restacking of f-G by using (Pt–Au) and (Au) nanoparticle spacers resulted in the increase in the surface area and glucose sensing performance.

Gas sensing properties of thermally evaporated lamellar MoO3

Abstract: In this work, MoO 3 was thermally evaporated onto gold interdigital fingers on quartz substrates and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques The deposited MoO 3 consist of stratified long rectangles (average length of 50 μm width of 5 μm and thickness of 500 nm) which are predominantly orthorhombic (α-MoO 3 ) Each of these plates was composed of many nano-thick layers (average ∼30 nm) placed by Van der Waals forces on top of each other forming lamellar patterns The devices were used as sensors and exhibited considerable change in surface conductivity when exposed to NO 2 and H 2 gases at elevated temperature of 225 °C The structural and gas sensing properties of thermally evaporated MoO 3 thin films were investigated

Simultaneous electrochemical determination of dopamine and acetaminophen using multiwall carbon nanotubes modified glassy carbon electrode

Abstract: A highly sensitive method was investigated for the simultaneous determination of dopamine (DA) and acetaminophen (AP) using acid functionalized multi-wall carbon nanotubes (f-MWCNTs) modified glassy carbon electrodes (GCEs). Both DA and AP were accumulated at the surface of f - MWCNTs modified GCE (under open circuit condition for 30 s). In differential pulse voltammetry (DPV) technique both DA and AP give sensitive oxidation peaks at 125 mV and 307 mV, respectively. Under the optimized experimental conditions (such as supporting electrolyte pH, accumulation time and scanning rate, etc.) DA and AP give linear response over the range of 3–200 μmol L −1 ( r = 0.992) and 3–300 μmol L −1 ( r = 0.989), respectively. The lower detection limits were found to be 0.8 for DA and 0.6 μmol L −1 for AP. The interfering species such as ascorbic acid (AA), uric acid (UA) and reduced form of Nicotinamide adenine dinucleotide (NADH) showed no interference with the selective determination of DA and AP. The investigated method showed good stability, reproducibility (1.3% (DA) and 2.3% (AP)), repeatability (1.9%) and high recovery in pharmaceutical preparation (1.7% (DA) and 2.7% (AP)), and human serum (1.7% (DA) and 1.9% (AP)).

Long term stability of metal oxide-based gas sensors for e-nose environmental applications: An overview

Abstract: The e-nose technology has enormous potentialities for in site monitoring of off-odours. However a number of limitations are associated with the properties of chemical sensors, the signal processing performances and the real operating conditions of the environmental field. The field experience of the research group included testing of a large amount of sensors in different sensor technologies and among those the metal oxide-based gas sensors (Figaro type) are the best gas sensors for long term application, as stated during more than 1 year of field testing. To be usable for the off-odours field measurement, the e-nose has to deal with the lack of long term stability of these sensors. The drift and the sensors replacement have to be considered. In order to appraise the time evolution of the sensors and the effect on the results of an electronic nose, experimentation has been performed during more than 3 years on two identical sensor arrays. The two arrays contain the same six Figaro sensors and are in the same sensor chamber of the e-nose system. Both arrays have worked continuously, without break. This paper presents the drift of some TGS sensors for 7 years as well as the difference in the temporal behaviour of identical sensors and the consequence on the e-nose results after the sensor replacement in the sensors array. A correction of the drift and of the replacement effect is applied and the classification results are exposed, with and without correction.

Wafer-scale synthesis of graphene by chemical vapor deposition and its application in hydrogen sensing

Abstract: Graphene with a large area was synthesized on Cu foils by chemical vapor deposition under ambient pressure. A 4 �� × 4 �� graphene film was transferred onto a 6 �� Si wafer with a thermally grown oxide film. Raman mapping indicates monolayer graphene dominates the transferred graphene film. Gas sensors were fabricated on a 4 mm × 3 mm size graphene film with a 1 nm palladium film deposited for hydrogen detection. Hydrogen in air with concentrations in 0.0025-1% (25-10,000 ppm) was used to test graphene- based gas sensors. The gas sensors based on palladium-decorated graphene films show high sensitivity, fast response and recovery, and can be used with multiple cycles. The mechanism of hydrogen detection is also discussed.

Ag nanoparticle embedded-ZnO nanorods synthesized via a photochemical method and its gas-sensing properties

Abstract: A facile photochemical method was used to synthesize Ag nanoparticle (Ag NP) embedded-ZnO nanorods in this article. The as-synthesized Ag NP embedded-ZnO nanorod samples were characterized systematically by TEM, XPS, DSC, XRD and SEM. The characterization results confirmed that Ag NPs had been embedded in ZnO nanorods. The gas-sensing properties of Ag NP embedded-ZnO nanorods were also investigated. While the performances of the sensors can be enhanced by embedding Ag NPs onto the surfaces of ZnO nanorods, the response of Ag NP embedded-ZnO nanorod sensors to 50 ppm ethanol is almost three times as high as that of those made from pure-ZnO nanorods. The responses of the sensors have no apparent degradation after being exposed to ethanol of 30 ppm for 100 days. Our Ag NP embedded-ZnO nanorod sensors have long-term stability and exhibit highly enhanced gas-sensing performances in their response and selectivity for detecting ethanol vapor.

Hg(II) sensing based on functionalized carbon dots obtained by direct laser ablation

Abstract: The synthesis of carbon nanoparticles obtained by direct laser ablation [UV pulsed laser irradiation (248 nm, KrF)] of carbon targets immersed in water is described. Laser ablation features were optimized to produce carbon nanoparticles with dimensions up to about 100 nm. After functionalization with NH 2 –polyethylene-glycol (PEG 200 ) and N-acetyl- l -cysteine (NAC) the carbon nanoparticles become fluorescent with excitation and emission wavelengths at 340 and 450 nm, respectively. The fluorescence decay time was complex and a three-component decay time model originated a good fit ( χ = 1.09) with the following lifetimes: τ 1 = 0.35 ns; τ 2 = 1.8 ns; and τ 3 = 4.39 ns. The fluorescence of the carbon dots is sensitive to pH with an apparent p K a = 4.2. The carbon dots were characterized by 1 H NMR and HSQC and the results show an interaction between PEG 200 and the carbon surface as well as a dependence of the chemical shift with the reaction time. The fluorescence intensity of the nanoparticles is quenched by the presence of Hg(II) and Cu(II) ions with a Stern–Volmer constant (pH = 6.8) of 1.3 × 10 5 and 5.6 × 10 4 M −1 , respectively. As such the synthesis and application of a novel biocompatible nanosensor for measuring Hg(II) is presented.

Silicon nanowire biosensor for highly sensitive and rapid detection of Dengue virus

Abstract: The paper presents an innovative silicon nanowire (SiNW)-based sensor for highly sensitive and rapid detection of reverse-transcription-polymerase chain reaction (RT-PCR) product of Dengue serotype 2 (DEN-2). A specific peptide nucleic acid (PNA) was covalently attached onto the SiNW surface. A complementary fragment of DEN-2 (69 bp) was obtained through one-step RT-PCR amplification, and applied to the PNA-functionalized SiNW. The hybridization event was verified by measuring the resistance change of the SiNW before and after binding of the RT-PCR product of DEN-2 to the PNA sequence. It is found that the SiNW sensor can detect below 10 fM concentration of the amplicons within 30 min. The approach shows potential of eliminating the demand for laborious methods by integrating the SiNW sensor with RT-PCR based on silicon technology platform. Consequently, this system-scale integration for a point-of-care medical device will facilitate the diagnostic applications.

Design of selective gas sensors using electrospun Pd-doped SnO2 hollow nanofibers

Abstract: Undoped and Pd-doped SnO2 hollow nanofibers were prepared by single capillary electrospinning and their gas sensing characteristics for 100 ppm H2, 100 ppm CO, 500 ppm CH4, and 100 ppm C2H5OH were investigated. The gas responses and responding kinetics were closely dependent upon the sensor temperature and Pd doping concentration. The undoped and 0.08 wt% Pd-doped SnO2 nanofibers showed selective detection to C2H5OH at 385 and 440 °C. In the 0.4 wt% Pd-doped SnO2 nanofiber sensor, the selective detection to CH4 and H2 was optimized at 440 °C with the minimum cross-sensitivity to C2H5OH. The tuning of gas selectivity via the combinatorial control of Pd doping concentration and sensor temperature in the electrospun hollow nanofibers was discussed in relation to the gas sensing mechanism.

Monodispersed Au nanoparticles decorated graphene as an enhanced sensing platform for ultrasensitive stripping voltammetric detection of mercury(II)

Abstract: We demonstrate a new highly sensitive and selective Hg(II) sensor with a graphene-based nanocomposite film as the enhanced sensing platform. The platform was constructed by homogenously distributing monodispersed Au nanoparticles (AuNPs) onto the two-dimensional (2D) graphene nanosheet matrix. Its surface structure and electrochemical performance were systematically investigated. Such a nanostructured composite film platform could combine with the advantages of AuNPs and graphene nanosheets, greatly facilitate electron-transfer processes and the sensing behavior for Hg(II) detection, leading to a remarkably improved sensitivity and selectivity. The detection limit was found to be as low as 6 ppt ( S / N = 3), much below the guideline value from the World Health Organization (WHO). The interference from other heavy metal ions such as Cu 2+ , Cr 3+ , Co 2+ , Fe 3+ , Zn 2+ and I − ions associated with mercury analysis could be effectively inhibited. The performance of new sensor was also evaluated by the direct detection of Hg(II) in river water specimens, suggesting it is very promising for practical environmental monitoring applications.

Sensor response formula for sensor based on ZnO nanostructures

Abstract: In this paper, we propose a new and general formula to describe ethanol adsorption mechanism underlying the response enhancement of ZnO nanostructure sensors. The derivation of sensor response formula based on basic chemical reaction at the sensor surface is presented. The formula can be used to explain response enhancement due to effect of metal doping, surface-to-volume ratio, and surface depletion layer. Thus, it can be regarded as a general formula to describe the sensor response characteristics of ZnO sensors. This general formula is a powerful tool for designing ZnO sensor at any desired sensor response. Furthermore, it is reasonable to expand this formula to explain other sensing materials and also to explain for different active gases.

Voltammetric selectivity conferred by the modification of electrodes using conductive porous layers or films: The oxidation of dopamine on glassy carbon electrodes modified with multiwalled carbon nanotubes

Abstract: Amperometric detection provides a highly sensitive approach to the electroanalytical determination of many target molecules and is widely used in the laboratory and field as well as in the form of disposable sensors. However, the approach can occasionally be restricted by limitations of selectivity; various species present in the target medium may oxidise or reduce at similar potentials. We show that the use of conducting porous layers on the surface of electrodes can be used to modify the mass transport regime from linear (planar) diffusion to one of approximately ‘thin layer’ character and that this alteration can in favourable circumstances facilitate the amperometric discrimination between species which oxidise or reduce at similar potentials under planar diffusion conditions. The method is illustrated with respect to the detection of dopamine at naked glassy carbon electrodes and at such electrodes modified with a layer of multiwalled carbon nanotubes, and experiments are reported which are consistent with the proposed strategy. The literature for the electroanalytical amperometric detection of dopamine in the presence of interfering molecules such as uric acid, serotonin and ascorbic acid, which often are found to oxidise at potentials close to dopamine, is reviewed and the modus operandi for many chemically modified electrodes apparently designed for the sought resolution of dopamine from these species are found to possibly rely on the physical mechanism proposed.

Large-scale synthesis of flowerlike ZnO nanostructure by a simple chemical solution route and its gas-sensing property

Abstract: A large-scale flowerlike ZnO nanostructure is prepared using a very simple solution method at near room temperature. The flowerlike ZnO nanostructure is self-assembled by thin and uniform nanosheets with a thickness of approximately 18 nm. X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the structure and morphology. The possible growth mechanism is carefully discussed based on the reaction process. The as-prepared ZnO nanoflowers exhibit a good response and reversibility to some organic gases, such as ethanol and n-butanol. The responses to 100 ppm ethanol and n-butanol are 25.4 and 24.1, respectively, at a working temperature of 320 °C. In addition, the sensors exhibit a good response to acetone, 2-propanol, and methanol. The relationship between the gas-sensing properties and the microstructure of the as-prepared ZnO nanoflowers is also investigated.

Novel resistive-type humidity sensor based on multiwall carbon nanotube/polyimide composite films

Abstract: A novel resistive-type relative humidity (RH) sensor based on plasma-treated multiwall carbon nanotube/polyimide (p-MWCNT/PI) composite films is reported. Details of the fabrication process and sensing characteristics such as linearity and sensitivity are described. The electrical resistance of the p-MWCNT/PI composite film shows typical concentration percolation behavior with increasing p-MWCNT loading. At loadings beyond the percolation threshold (0.05 wt%), this sensor exhibits very good linearity with a positive slope over the entire operational RH range. The sensor has a humidity sensitivity of about 0.0047/% RH and a linearity correlation (R2) of 0.9999. A humidity sensing mechanism for the p-MWCNT/PI composite film is proposed on the basis of charge transfer between adsorbed water molecules and the p-MWCNTs.

Pd-doped TiO2 nanofiber networks for gas sensor applications

Abstract: This work presents a new route to enhance the sensitivity of nanocrystalline TiO 2 fibers based gas sensors. Pure and Pd-doped TiO 2 nanofiber mats were synthesized by electrospinning and subsequent calcination. Highly porous fibrillar morphology was observed in the resultant nanocrystalline TiO 2 fibers. In addition to anatase TiO 2 crystallites, tiny (∼2 nm) PdO crystallites were observed in the Pd-doped TiO 2 fibers. The sensors using Pd-doped TiO 2 fibers showed promising gas sensing characteristics, such as low operation temperature (180 °C) and sufficient gas response ( R / R o = 38 to 2.1 ppm NO 2 ). The techniques outlined here offer new means for preparing novel metal-oxide nanofibers with catalytic dopants.

Chemical signal amplification in two-dimensional paper networks.

Abstract: Two-dimensional paper networks (2DPNs) hold great potential for extending the utility of paper-based chemical and biochemical diagnostics at a cost and ease-of-use that are comparable to conventional lateral flow strips. 2DPNs enable the automated sequential delivery of multiple reagents to a detection region with a single user activation step, and therefore have the potential to extend the processing capabilities of inexpensive paper-based assays with comparable ease of use to conventional lateral flow tests. In this communication, we used a simple 3 inlet 2DPN to perform signal amplification of a colloidal gold label using a gold enhancement solution, thus demonstrating the capability of 2DPNs to perform processes that can improve limits of detection.

Synthesis and gas sensing characteristics of highly crystalline ZnO–SnO2 core–shell nanowires

Abstract: The ZnO–SnO2 core–shell nanowires (NWs) were synthesized by a continuous two-step vapor growth method at different synthesis temperatures. A crystalline 15–20 nm-thick, SnO2 shell layer was pseudo-epitaxially coated on ZnO NWs with a diameter of 50–80 nm. The gas response of the ZnO–SnO2 core–shell NW sensor to 10 ppm NO2 reached ∼33 times enhancement compared to that of the ZnO NWs at 200 °C. In addition, the ZnO–SnO2 core–shell NW sensors showed selective detection to NO2 at 200–300 °C and to C2H5OH at 400 °C. The enhanced gas responses to NO2 and C2H5OH are discussed in relation to the thin SnO2 shell layer and core–shell configuration of the NWs.

New strategy for simultaneous and selective voltammetric determination of norepinephrine, acetaminophen and folic acid using ZrO2 nanoparticles-modified carbon paste electrode

Abstract: A novel ZrO2 nanoparticles-modified carbon paste electrode (ZONMCPE) was fabricated and used to study the electrooxidation of norepinephrine (NE), acetaminophen (AC), folic acid (FA) and their mixtures by electrochemical methods. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of NE, AC and FA has been explored at the modified electrode. Differential pulse voltammetry peak currents of NE, AC and FA increased linearly with their concentrations at the ranges of 1.0 × 10−7–2.0 × 10−3 M, 1.0 × 10−6–2.5 × 10−3 M and 2.0 × 10−5–2.5 × 10−3 M, respectively and the detection limits for NE, AC and FA were 8.95 × 10−8, 9.12 × 10−7and 9.86 × 10−6 M, respectively. The modified electrode displayed strong function for resolving the overlapping voltammetric responses of NE, AC and FA into three well-defined voltammetric peaks. In the mixture containing NE, AC and FA, the three compounds can well separate from each other with potential differences of 220, 290 and 510 mV between NE–AC, AC–FA and NE–FA, respectively, which was large enough to determine NE, AC and FA individually and simultaneously.

Sub-ppm H2S sensing at room temperature using CuO thin films

Abstract: We have investigated room temperature sensing characteristics of CuO films to various gases i.e. Cl 2 , H 2 S, NH 3 , CH 4 , CO and NO. CuO films were prepared by oxidation of Cu films, which were deposited on polycrystalline alumina substrates by thermal evaporation technique. CuO films have been found to be highly selective towards H 2 S. We demonstrate that the H 2 S response of CuO films can be divided in to three regions: (a) low concentrations (100–400 ppb), (b) intermediate concentrations (500 ppb to 50 ppm), and (c) high concentrations (>50 ppm). For low concentrations (100–400 ppb), the response curves have been found to be highly reversible with very small response (∼60 s) and recovery (∼90 s) times, indicating suitability of CuO films for sub-ppm sensing of H 2 S. Oxidation of H 2 S by adsorbed oxygen is found to be the responsible sensing mechanism. However, at very high H 2 S concentrations (>50 ppm), surface of CuO grains is found to convert into CuS, resulting in an irreversible response curve. For intermediate concentrations (500 ppb to 50 ppm), the response curve is governed by both H 2 S oxidation and CuS formation mechanisms.

H2 gas sensor performance of NiO at high temperatures in gas mixtures

Abstract: Gas sensors operating at elevated temperatures (T > 500 °C) near combustion engines and power plants are needed to improve combustion control and to monitor and control emission. We investigated the effects of film thickness and operating temperature on the gas sensitivity of NiO films. 50 nm films had the best sensor characteristics and were further characterized for repeatability and selectivity. We present data on the structure, surface morphology, and chemical composition of 30–130 nm thick NiO films using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. This report includes gas sensor responses (GSRs) at operating temperatures from 300 to 650 °C for H2 test gas concentrations ranging from 500 to 10,000 ppm in a synthetic air carrier gas. After annealing, the 50 nm thick films showed the highest surface roughness of 14.6 nm and the highest GSR at all operating temperatures tested. Surface roughness and area appear to be correlated to the gas sensor response of NiO thin films. An average GSR of 55 for 5000 ppm H2 was found at 600 °C with a standard deviation of ±6.23. Repeated measurements for gas sensitivity were collected at 600 °C for 8 h where NiO was exposed to H2 11 times at concentrations varying from 500 to 10,000 ppm and each exposure lasted for 20 min, yielding GSRs that follow a power law behavior. Cross sensitivity of NiO for 1100 ppm CO2, 150 ppm NH3 and 50 ppm NOx was investigated for individual gases and mixtures, all using synthetic air carrier gas. Selectivity was observed to decrease as the operating temperature increased.

Fast response photonic crystal pH sensor based on templated photo-polymerized hydrogel inverse opal

Abstract: Polymer hydrogels can exhibit large reversible volume changes in response to external stimuli, and thus are regarded as excellent materials for chemical sensors. In this report, we demonstrate a mechanically robust and fast response photonic crystal pH sensor fabricated by templated photo-polymerization of hydrogel monomers within the interstitial space of a self-assembled colloidal photonic crystal. Throughout a rigorous optimization of the photo-polymerization, pH sensors showing a response time of less than 10 s upon a pH change were fabricated. Repeated pH changes revealed that the sensor has a long lifetime (>6 months) without degradation of the response time or reproducibility in pH-driven color change.

Preparation of mesoporous In2O3 nanofibers by electrospinning and their application as a CO gas sensor

Abstract: Mesoporous In 2 O 3 nanofibers with a high surface area were synthesized by calcining electrospun polyvinyl alcohol (PVA)/indium acetate composite fibers. A PVA solution and indium acetate were mixed and electrospun. After calcining the PVA/indium acetate composite nanofiber precursor, mesoporous In 2 O 3 nanofibers were successfully formed. These nanofibers had diameters in the range of 150–200 nm and consisted of cubic indium oxide nanocrystals with a primary particle size of 10–20 nm. The Brunauer–Emmett–Teller (BET) surface area of the In 2 O 3 nanofibers was strongly affected by the calcining temperature. The BET surface area of the fibers calcined at 400 °C was significantly higher than the surface area of the nanofibers calcined at 500 °C or 600 °C and of the commercial In 2 O 3 powder. The response of mesoporous In 2 O 3 nanofibers to CO in air is strongly affected by the surface area. The highly elevated response of In 2 O 3 nanofibers calcined at 400 °C could be attributed to the high surface area, which provides a large amount of surface sites for adsorption and reaction of CO. The results demonstrate that the electrospinning approach is an easy and useful method to synthesize metal oxides with mesopores and high surface area, which may enhance their gas sensing properties.

Electrochemical determination of nitrite at a bare glassy carbon electrode; why chemically modify electrodes?

Abstract: The oxidation of nitrite was studied at a bare glassy carbon (GC) electrode in aqueous solution using cyclic voltammetry, square wave voltammetry and chronoamperometry. A mechanism for the electrode reaction is proposed. A limit of detection (LOD) of 4 × 1 0 − 7 M was obtained for amperometry and this is evaluated with reference to literature reports for NO 2 − detection; in particular, the possible merits of using chemically modified electrodes as compared to ‘bare’ unmodified electrodes are critically assessed.

Growth of ZnO tetrapods for nanostructure-based gas sensors

Abstract: Standard vapour phase growth process for ZnO tetrapods has been optimized in order to reach a very large yield, a good reproducibility and a single morphology (tetrapods are separated from the other possible ZnO nanostructures). The large yield of the growth and the simple deposition of these nanostructures on an alumina substrate with contacts and heater, allowed us to realize gas sensor prototypes with a relatively low-cost procedure. The obtained ZnO tetrapods-based gas sensors have been tested with different gases (CH 3 CH 2 OH, NO 2 , CO and H 2 S) and, especially, response values S = 25 and S = 100 have been measured towards 1 ppm and 5 ppm of hydrogen sulphide, respectively.

Humidity sensors based on polyaniline nanofibres

Abstract: Humidity sensor based on polyaniline nanofibres was fabricated and its response to humidity was investigated. It was found that the sensor behaved differently compared to commonly known conducting polymer based sensors. The sensor responded to low relative humidity (