Poly(acrylamide) derivatives for QCM-based HCl gas sensor applications

Quartz resonators coated with three kinds of poly(acrylamide) derivatives were studied for simply but accurately detecting HCl gas in air. The exposure of the resonator to HCl gas reversibly decreased the oscillation frequencies. The sensitivity, response time, and reversibility were found to depend on the structure of the amide group. Among the polymers used, poly(N,N-dimethylacrylamide) (PDMAA) showed the most relevant data for the HCl sensor. The HCl sensitivity obtained for PDMAA was ca. 250 ppb/Hz. On the other hand, the irreversible response toward NO₂ gas was considerably high, and great interference was also produced by changes in the test gas humidity.     

A new optical sensing reaction for nitric oxide

Based on the reaction between the Cu(II) complex of Eriochrome cyanine R (ECR) and nitric oxide (NO) in phosphate buffer (pH 7.4), a new colorimetric method for the determination of NO concentration has been developed. The linear calibration range for NO was 0–60 μM with a detection limit of 1.24 μM. This reaction was then used as the basis in the development of a fibre optic chemical sensor for NO gas. The copper complex was incorporated into silicone rubber membranes and exposed to NO gas after incubating the films in phosphate buffer (pH 7.4). A linear calibration for NO gas between 0 and 6 ppm was obtained with a detection limit of 0.227 ppm (1 μM 0.031 ppm NO in solution). The sensor response was shown to be reproducible and reversible (2.77%, R.S.D., n=4) upon exposure to aqueous phosphate buffer (pH 7.4). The sensor response was also found to be pH independent between 7 and 10.     

Novel ammonia-sensing phenomena in sol–gel derived Ba0.5Sr0.5TiO3 thin films

In this paper, ammonia-sensing behavior of barium strontium titanate (BST) thin films have been reported for the first time. Thin films of BST deposited by sol–gel spin coating technique have been found to show an increase in resistance when exposed to ammonia gas. The sensitivity variation was from 20 to 60%, with lowest detection limit of about 160 ppm. The films were prepared with different pre-sintering temperatures and thickness and effect of these parameters on the ammonia-sensing have been studied. The optimum temperature for operation was found to be close to 270 °C. The ammonia-sensing studies were also performed for other gases like ethanol, NO₂ and CO; but the sensitivity in these cases was negligibly smaller than that in case of ammonia.     

EPR and DRS evidence for NO2 sensing in Al-doped ZnO

Zinc oxide (ZnO) is a well-known semiconducting multifunctional material wherein properties right from the morphology to gas sensitivity can be tailor-made by doping or surface modification. Aluminum (Al)-incorporated porous zinc oxide (Al:ZnO) exhibits good response towards NO₂ at low-operating temperature. The NO₂ gas concentration as low as 20 ppm exhibits S = 17% for 5 wt.% Al-incorporated ZnO. The NO₂ response increases with operating temperature and concentration and reaches to its maximum at 300 °C without any interference from other gases such as SO₃, HCl, LPG and alcohol. Physico-chemical characterization likes differential thermogravimetric analysis (TG-DTA) electron paramagnetic resonance (EPR) and diffused reflectance spectroscopy (DRS) have been used to understand the sensing behavior for pure and Al-incorporated ZnO. The TG-DTA depicts formation of ZnO phase at 287 °C. The EPR study reveals distinct variation for O⁻ (g = 2.003) and Zn interstitial (g = 1.98) defect sites in pure and Al:ZnO. The DRS studies elucidate signature of adsorbed NO_x species in aluminium-incorporated zinc oxide indicating its tendency to adsorb these species even at low temperatures. This paper is an attempt to correlate the gas sensing behavior with the physico-chemical studies such as EPR and DRS.     

Highly sensitive detection of HCl gas using a thin film of meso-tetra(4-pyridyl)porphyrin coated glass slide by optochemical method

The meso-tetra(4-pyridyl)porphyrin (MTPyP) deposited on glass slide by dip coating was used as a solid state sensor for HCl gas detection by optochemical method. Exposure of MTPyP coated glass slide to HCl gas results in the formation of protonated meso-tetra(4-pyridyl)porphyrin (PMTPyP). UV-vis and fluorescence spectral methods were used to study the protonation of MTPyP both in solution and on solid state. The absorption spectrum of MTPyP modified glass slide shows an intense Soret band at 427 nm, which is shifted to 470 nm upon exposure to HCl gas. The concentration of HCl gas was monitored from the absorbance changes of Soret band of PMTPyP at 470 nm. The detection limit of the solid state sensor was found to be 0.01 ppm. The recovery rate of the solid state was very fast and it was monitored by UV-vis and fluorescence techniques with successive exposure to HCl gas and ammonia vapor with nitrogen gas. The planarity and energy of the molecule have changed after exposed to HCl gas which was confirmed by ab-intio calculation using Gaussian software. The response of the solid state sensor towards HCl gas was highly stable for several months.     

Sol–gel derived polycrystalline Cr1.8Ti0.2O3 thick films for alcohols sensing application

The powder sample of Cr_1.8Ti_0.2O₃ (CTO) was obtained by a sol–gel method. The thick films were developed on identical ceramic tubes of 4 mm length comprising of two Au-electrodes and printing an eight-layer film prepared by mixing CTO with glass powder and -terpinol as an organic vehicle. X-ray powder diffraction (XRD) patterns showed the formation of a single phase. The scanning electron microscope (SEM) images of the ceramic sensor treated at 850 °C revealed that the grain size was larger than 400 nm for the individual isolated grains on the surface, and the agglomerated dense spheroidal platelets had the size of 1–4 μm in diameter. The AC impedance measurement in ambient air showed that the resistance decreased nearly by two orders of magnitude with an increase in temperature in the range of 400–600 °C for both the powder sample and the thick film, and the activation energy E_a derived from the measurement was found to be 0.35 and 0.36 eV for the powder and the film, respectively. The films were exposed to various concentrations of alcohols (0.4–1.2 ppm of methanol and 1.0–5.0 ppm of ethanol), followed by determination of sensor response, sensitivity and reversibility and reproducibility. The origin of the gas response was attributed to the surface reaction of R-OH (R = methyl and ethyl group) with O⁻_(ads) to form adsorbed R-CHO, which was desorbed as a gas at 400 °C after the sensor departing from the gas.     

Effect of magnesium doping on the structural and piezoelectric properties of sputtered ZnO thin film

Structural and piezoelectric characteristics of magnesium-doped ZnO films were investigated. Magnesium-doped ZnO films with a c-axis preferred orientation were deposited on ST-cut quartz by radio frequency magnetron sputtering. The crystalline structure and surface morphology of films were studied by X-ray diffraction, scanning electron microscopy and atomic force microscopy. The electromechanical coupling coefficient and temperature coefficient of frequency of the filters were then determined using a Love wave filter. A uniform crystalline structure and smooth surface of the ZnO films were obtained when magnesium dopant level was 1.5 mol%. The grain size of the ZnO film increased when magnesium doped. It has been found that the temperature coefficient of frequency declines to +0.44 ppm/°C at 1.5 mol% magnesium-doped ZnO film.     

Bulk Lateral MEM Resonator on Thin SOI With High $Q$ -Factor

The fabrication, design, and characterization of high-quality factor microelectromechanical (MEM) resonators fabricated on thin-film silicon-on-insulators (SOIs) are addressed in this paper. In particular, we investigate laterally vibrating bulk-mode resonators based on connected parallel beams [parallel beam resonators (PBRs)]. The experimental characteristics of PBRs are compared to disk resonators and rectangular plate resonators. All the reported MEM resonators are fabricated on 1.25-mum SOI substrates by a hard mask and deep reactive-ion etching process, resulting in transduction gaps smaller than 200 nm. Additionally, this fabrication process allows the growth of a thermal silicon dioxide layer on the resonators, which is used to compensate the resonance-frequency dependence on temperature. Quality factors Q, ranging from 20 000 at 32 MHz up to 100 000 at 24.6 MHz, are experimentally demonstrated. The motional resistances R _m are compared for different designs, and values as low as 55 kOmega at 18 V of bias voltage are obtained with the thin SOI substrate. The thermal sensitivity of the resonance frequency is investigated from 200 K to 360 K, showing values of -15 ppm/K for the PBRs, with a possible compensation of 2 ppm/K when using 20 nm of SiO₂.     

CdO activated Sn-doped ZnO for highly sensitive, selective and stable formaldehyde sensor

Sn-, Ni-, Fe- and Al-doped ZnO and pure ZnO are prepared by coprecipitation method, and characterized by scanning electron microscope (SEM), energy diffraction spectra (EDS) and X-ray diffraction (XRD). Their formaldehyde gas sensing properties are evaluated and the results show that 2.2 mol% Sn dopant can increase the response of ZnO by more than 2 folds, while other dopants increase little response or even decrease response. Further, CdO is used to activate ZnO based formaldehyde sensing material. It is demonstrated that 10 mol% CdO activated 2.2 mol% Sn-doped ZnO has the highest formaldehyde gas response, with a linear sensitivity of ∼10/ppm at lowered work temperature of 200 °C than 400 °C of pure ZnO, and high selectivity over toluene, CO and NH₃, as well as good stability tested in 1 month.     

Optimization and characterization of a ZnO biosensor array

Practical usage of acoustic biosensors has revealed that high quality factor, Q, is an important attribute of a highly sensitive acoustic sensor. In this research, we present performance optimization of ZnO thin-film bulk acoustic resonators (FBARs) operating in the thickness shear mode through characterization of a variety of electrode geometries. The resulting average Q and K² from each of the electrode geometries were calculated and compared. Based on these results, a preferred electrode configuration was selected, and fabricated into an 8-device array. The arrays were tested in physiologically relevant environments to illustrate the effects of temperature and solution conductivity on the stability of the resonators. The devices demonstrated a temperature coefficient of frequency of 25 ppm/°C. The resonators also exhibited reasonable stability under varying levels of solution conductivity as tested by exposing the devices to solutions containing a varied amount of NaCl in deionized H₂O.     

Development of an optical hydrogen sulphide sensor

A hydrogen sulphide (H₂S)-sensitive optode film has been fabricated by immobilising tetraoctylammonium fluorescein mercury(II) acetate (TOFMA) and tri-n-butyl phosphate in a poly(vinyl chloride) (PVC) matrix. The optode film, coated on an overhead transparency film, was employed as a sensing device for fluorimetric detection of H₂S. The fluorescence intensity monitored at 553 nm (excitation at 503 nm) increased with increasing H₂S concentrations. The optode film showed a good, linear and reversible response to H₂S from 0 to 15 ppm (v/v). It was optically stable and the reproducible response of the film on exposure to 10 ppm (v/v) H₂S was extremely good. There was no sign of degradation after 8 h of continuous use. The response to H₂S levelled off at about 27.5 ppm The response and recovery times of the optical H₂S sensor were fast and less than 2 and 5 s, respectively. An optically-based sensor for H₂S determination was successfully developed. It was anticipated that the system could be used to monitor H₂S with a concentration range of 0–25 ppm (v/v) with satisfactory results. A proposed mechanism for the detection of H₂S by the optode films is described.     

Fabrication of gas sensing devices with ZnO nanostructure by the low-temperature oxidation of zinc particles

A method for low-temperature synthesis of a mixture of high-density ZnO nanoflakes and nanowires was developed to produce low-cost and high-efficiency gas sensors with ZnO nanostructures. ZnO nanoflakes and nanowires were grown on glass substrates by the RF sputter deposition of Zn particles and localized oxidation at a low temperature of 300 °C. The synthesized ZnO nanoflakes and nanowires were polycrystalline and had nanometer dimensions, as revealed by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) measuring. A gas sensor based on the mixture of ZnO nanoflakes/nanowires responded rapidly and sensitively to ethanol. The sensing properties of the ZnO nanostructure sensor were approximately 72% for 50 ppm ethanol gas at an operating temperature of 100 °C. The response to 10 ppm of ethanol gas was 42% at the same temperature.     

A colorimetric response to hydrogen sulfide

Two simple Cu(II) chromophores [Cu(acac)(dmp)](NO₃) (1) and [Cu(acac)(dpph)](NO₃) (2) (acac = acetylacetone, dmp = 2,9-dimethyl-1,10-phenanthroline and dpph = 4,7-diphenyl-1,10-phenanthroline) have been synthesised and immobilized into a highly permeable polymer matrix and subsequently used in the optical detection of low levels of hydrogen sulfide (10 ppm) over a 2 min period. The system is responsive to a concentration one-tenth that quoted as being immediately dangerous to life and health (IDLH) value (100 ppm) by the National Institute for Occupational Safety and Health (NIOSH). The UV–vis spectral change is a consequence of the hydrogen sulfide gas reducing the Cu(II) to Cu(I) in the polymer matrix. A crystal structure of [Cu(acac)(dpph)](NO₃) (2) was obtained and will also be discussed.     

Micro-hotplate based temperature stabilization system for CMOS SAW resonators

Due to the sensitivity of the piezoelectric layer in surface acoustic wave (SAW) resonators to temperature, a method of achieving device stability as a function of temperature is required. This work presents two methods of temperature control for CMOS SAW resonators using embedded polysilicon heaters. The first approach employs the oven control temperature stabilization scheme. Using this approach, the device’s temperature is elevated using on-chip heaters to T_max = 42°C to maintain constant device temperature. Both DC and RF measurements of the heater together with the resonator were conducted. Experimental results have indicated that the TCF of the CMOS SAW resonator of −97.2 ppm/°C has been reduced to −23.19 ppm/°C when heated to 42°C. The second scheme uses a feedback control circuit to switch the on-chip heaters on and off depending on the ambient temperature. This method provided reduction of the TCF from −165.38 ppm/°C, to −93.33 ppm/°C. Comparison of both methods was also provided.     

A compact tri‐band microwave resonator for ethanol gas detection

This article presents the design, simulation, fabrication, and testing of a compact two‐port microwave resonator coated with nanomaterials for ethanol gas sensing applications. The proposed gas sensor consists of a transmission line loaded with three triangular split ring resonators for ethanol detection at three frequency bands viz. 2.2, 4.6, and 6.3 GHz. The transmission line has all‐pass characteristics in which band gaps are introduced using three split ring resonators. The TiO₂ and ZnO nanorods are used as sensitive layers for the proposed sensing application. The nanorods, which are grown on a glass substrate of thickness 1 mm, are loaded on to the two‐port microwave resonator making the device sensitive to ethanol. The microwave behavior of the sensor is analyzed using the scattering parameters. The absorption of the ethanol gas causes frequency detuning which is used to analyze the presence of ethanol and its concentration. From the experiments, it is understood that there is an increase in the frequency shift with an increase in the concentration of ethanol gas. The sensing device with ZnO as a sensitive layer showed a higher average sensitivity of 2.35 compared to TiO₂ whose average sensitivity is 1.29.     

Highly sensitive gas sensors based on hollow SnO2 spheres prepared by carbon sphere template method

Hollow SnO₂ spheres were prepared in dimethylfomamide (DMF) by controlled hydrolysis of SnCl₂ using newly made carbon microspheres as templates. The phase composition and morphology of the material particles were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The gas sensing properties of sensors based on the hollow SnO₂ spheres were investigated. It was found that the sensor exhibited good performances, characterized by high response, good selectivity and very short response time to dilute (C₂H₅)₃N operating at 150 °C, especially, the response to 1 ppb (C₂H₅)₃N attained 7.1 at 150 °C. It was noteworthy that the response to 0.1 ppm C₂H₅OH of the sensor was 2.7 at 250 °C.     

Enhancing the sensitivity of ionic liquid sensors for methane detection with polyaniline template

Flammable gas sensors are essential in occupational health and safety to prevent fire or explosion in gas facilities and underground mining. Our early study demonstrated that ionic liquid (IL)/quartz crystal microbalance (QCM) gas sensors and sensor arrays were excellent for the detection of various organic vapors at both room temperature and elevated temperatures. In this paper, we developed a general method that significantly enhanced the sensitivity of the IL/QCM sensors for flammable gases detection by immobilizing IL on a conductive polymer polyaniline (PAn) template. Studies were performed to optimize the PAn oxidation states, thickness, and IL concentrations. Results showed that the sensitivity increased with increasing the PAn film thickness and the amount of IL immobilized within the PAn film. The sensitivity depended also on the oxidation state and doping state of PAn. With doped and partially oxidized PAn (emeraldine salt) the IL/QCM sensor showed the best performance. The current detection limit for methane was as low as about 115 ppm at room temperature. The sensitivity also depended on the structure of the IL used. Among the four ILs tested, two of them showed excellent sensitivities after being immobilized in the PAn film.     

沸石分子筛修饰的QCM类神经毒气传感器

研究了Cu2 -Beta型纳米分子筛对类神经毒剂DMMP有机气体的敏感特性,并结合高灵敏的石英谐振微天平(QCM)研制了DMMP气体的传感器.研究结果表明,选择Cu2 -Beta纳米分子筛作为敏感膜对DMMP气体的检测灵敏度大大提高,达到14.481 1 Hz/lg(C/ppm).在0.2 ppm DMMP气体浓度下,传感器的响应时间和恢复时间分别为40 s和100 s,响应达到103Hz.同时经过高温和水汽吹扫脱附处理,该传感器表现了较好的重复性.     

Plasma-deposited copper phthalocyanine: A single gas-sensing material with multiple responses

Copper phthalocyanine (CuPc) thin films have been deposited by glow discharge-induced sublimation (GDS). This physical technique allows to produce very high porosity films, whose response to gases is much more intense than evaporated films. It has been found that both electrical and optical properties of these films change upon gas exposure due to the gas/film interaction. Electrical response of the films has been tested by exposing the samples to NO_x-containing atmospheres and by measuring the slope of the electrical surface current versus gas concentration. This way NO₂ and NO concentrations down to 0.1 ppm and 10 ppm have been measured, respectively, with response times shorter than 2 min. Optical responses have been tested by measuring the change of light reflectance at a fixed wavelength upon exposure to ethanol-containing atmospheres down to concentrations of few thousands of ppm. Response times of less than 10 s have been obtained.     

Comprehensive study of hydrogen sensing characteristics of Pd metal–oxide–semiconductor (MOS) transistors with Al0.24Ga0.76As and In0.49Ga0.51P Schottky contact layers

The interesting hydrogen sensing characteristics of two transistors with an Al_0.24Ga_0.76As (device A) and In_0.49Ga_0.51P (device B) Schottky layer are demonstrated and studied. Experimentally, device A shows a lower hydrogen detection limit of 4.3 ppm H₂/air, a higher current variation of 7.79 mA and a shorter adsorption time of 10.95 s in a 9970 ppm H₂/air at room temperature. On the other hand, device B exhibits more stable hydrogen-sensing characteristics at high temperatures. Even at a low concentration of 14 ppm H₂/air the hydrogen sensing properties of device B can be obtained as the temperature increases from 30 to 160 °C. Because the Al_0.24Ga_0.76As and In_0.49Ga_0.51P materials are lattice-matched to the GaAs substrate, the studied devices can be integrated as sensor arrays to obtain superior hydrogen sensing characteristics including higher sensing signals, lower detection limit, shorter response time, and widespread detection and temperature regimes.