Highly sensitive and selective room temperature NH3 gas microsensor using an ionic conductor (CuBr) film

A new ammonia gas microsensor was developed, based on the large resistance change of an ionic conductor (CuBr) film when exposed to low NH₃ concentrations. The detection is based on specific interactions between ammonia molecules contained in the gas atmosphere and mobile copper ions in the copper(I) bromide layer. The sensor is operating at ambient temperature and allows highly sensitive and specific ammonia detection. The sensor works at ammonia concentrations between 1 and 500 ppm. There are no significant cross-effects to acetylene and carbon monoxide and only a weak cross-sensitivity to hydrogen sulfide gas (200 ppm). The selectivity was experimentally compared with commercial tin dioxide sensors (TGS 826). The sensor fabrication is a simple process, allowing low cost device production.     

Development of a monitoring tape for phosgene in air

A porous cellulose tape impregnated with a processing solution that includes 4-p-nitroben-zylpyridine, N-benzylaniline and methanol is a highly sensitive means of detecting phosgene and maintains stable sensitivity for at least three months in air in a desiccator. When the sample including phosgene was passed through the tape, the color of tape changed to red. The degree of color change was proportional to the concentration of phosgene at a constant sampling time and flow rate. The degree of color change could be recorded by measuring the intensity of reflecting light (555 nm). The detection limit was 6 ppb for phosgene with a sampling time of 60 sec and a flow rate of 400 ml/min. Reproducibility tests showed that the relative standard deviation of response (n = 10) was 2.6% for 0.2 ppm phosgene. No interference was observed from ethanol (1 vol.%), trichloroethylene (1 vol.%), acetone (1 vol.%), carbon dioxide (4.9 vol.%), carbon monoxide (100 ppm), nitrogen dioxide (100 ppm), sulfur dioxide (50 ppm), hydrogen chloride gas (5 ppm), chlorine (3 ppm), acetic acid gas (24 ppm), ammonia (40 ppm), or benzyl chloride (20 ppm).     

Improvement of a monitoring tape for nitrogen dioxide in air

A porous cellulose tape containing a silica gel that was previously impregnated with a processing solution containing p-toluenesulfonic acid, sulfanilic acid, N-1-naphthyl ethylene diamine dihydrochloride, ethylene glycol and methanol has been developed to provide a highly sensitive detection of nitrogen dioxide in air. When the sample including nitrogen dioxide was passed through the tape, the color of tape changed to red, and the degree of color change could be recorded by measuring the intensity of reflecting light (555 nm). The calibration graph was linear up to approximately 0.10 ppm. The detection limit was 0.5 ppb for nitrogen dioxide with a sampling time of 8 min and a flow rate of 60 ml min(-1). No interferences were observed from ammonia (40 ppm), sulfur dioxide (51 ppm), carbon dioxide (21%), ozone (0.75 ppm), hydrogen sulfide (27 ppm) or nitrogen monoxide (99 ppm).     

Miniaturized measurement system for ammonia in air

The development of a miniaturized ammonia sensor made using microsystem technology is described. Gas is sampled in a sampler comprising two opposite channels separated by a gas permeable, water repellent polypropylene membrane. Subsequently, the acid sample solution is pumped into a selector where an alkaline solution is added to ionize all sampled ambient acid gasses, resulting in an enhanced selectivity. In the selector, the ammonia can diffuse through a second membrane into a purified water stream where an electrolyte conductivity sensor quantifies the resulting ammonium concentration. The realized system is shown to be selective enough not to be influenced by normal ambient carbon dioxide concentrations. Experiments with a gas flow of 3 ml/min, containing ammonia concentrations ranging from 9.8 to 0.3 ppm in a nitrogen carrier flow, into a 15 μl/min sample solution flow and finally into a 5 μl/min purified water stream have been carried out and show that the system is sensitive to ammonia concentration below 1 ppm.     

Ammonia gas sensor based on electrosynthesized polypyrrole films

In this work, design and fabrication of micro-gas-sensors, polymerization and deposition of poly(pyrrole) thin films as sensitive layer for the micro-gas-sensors by electrochemical processing, and characterization of the polymer films by FTIR, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), are reported. The change in conductance of thin polymer layers is used as a sensor signal. The behaviours, including sensitivity, reproducibility and reversibility, to various ammonia gas concentrations ranging from 8 ppm to 1000 ppm are investigated. The influence of the temperature on the electrical response of the sensors is also studied. The experimental results show that these ammonia gas sensors are efficient since they are sensitive to ammonia, reversible and reproducible at room temperature.     

A new sensor for ammonia based on cyanidin-sensitized titanium dioxide film operating at room temperature

Design and fabrication of an ammonia sensor operating at room temperature based on pigment-sensitized TiO₂ films was described. TiO₂ was prepared by sol–gel method and deposited on glass slides containing gold electrodes. Then, the film immersed in a 2.5 × 10⁻⁴ M ethanol solution of cyanidin to absorb the pigment. The hybrid organic–inorganic formed film here can detect ammonia reversibly at room temperature. The relative change resistance of the films at a potential difference of 1.5 V is determined when the films are exposed to atmospheres containing ammonia vapors with concentrations over the range 10–50 ppm. The relative change resistance, S, of the films increased almost linearly with increasing concentrations of ammonia (r = 0.92). The response time to increasing concentrations of the ammonia is about 180–220 s, and the corresponding values for decreasing concentrations 240–270 s. At low humidity, ammonia could be ionized by the cyanidin on the TiO₂ film and thereby decrease in the proton concentration at the surface. Consequently, more positively charged holes at the surface of the TiO₂ have to be extracted to neutralize the adsorbed cyanidin and water film. The resistance response to ammonia of the sensors was nearly independent on temperature from 10 to 50 °C. These results are not actually as good as those reported in the literature, but this preliminary work proposes simpler and cheaper processes to realize NH₃ sensor for room temperature applications.     

A Conductimetric System Based on Polyaniline for Determination of Ammonia in Fertilizers

Abstract

A simple conductimetric system to determine ammonia concentration using a sensor based on a conductor polymer was developed. The sensitive element to ammonia is a thin polyaniline film deposited by chemical synthesis in an acrylic substrate prepared before hand with two graphite electrodes. The conductance of the polyaniline film decreases when exposed to the ammonia gas and this variation can be related to the ammonia concentration. To determine ammonia in fertilizer samples a system consisting of a measurement cell, a conductivity meter and a strip chart recorder was used. The results were compared with those obtained by three different laboratories employing a Kjeldahl method and are in good agreement. The detection range of the system was 0.6 to 3.7 μg.mL^?1 with a response time of 4 minutes. The relative standard deviation of the proposed method was about 5%.     

Optical Gas Sensor Fabrication Based on Porphyrin-Anchored Electrostatic Self-Assembly onto Tapered Optical Fibers

Tapered optical fibers with nano-assembled coatings of thicknesses of order tens of nanometres were used for the detection of ammonia gas. The film coating was composed of alternate layers of tetrakis-(4-sulfophenyl) porphine (TSPP) and poly(allylamine hydrochloride) (PAH), which were deposited using the electrostatic self-assembly process (ESA). Exposure of a PAH/TSPP nano-assembled non-adiabatic tapered optical fiber with a waist diameter of 10 µm to ammonia induced significant optical changes in the transmission spectrum of the optical fiber. The fiber optic sensor showed a linear sensitivity to the concentration of ammonia in the range of 10–100 ppm, with response and recovery times less than 100 and 240 sec, respectively. The 3σ limit of detection (LOD) was estimated to be ca. 2 ppm.     

Optical ammonia gas sensor based on a porous silicon rugate filter coated with polymer-supported dye

An ammonia gas sensor chip was prepared by coating an electrochemically-etched porous Si rugate filter with a chitosan film that is crosslinked by glycidoxypropyltrimethoxysilane (GPTMS). The bromothylmol blue (BTB), a pH indicator, was loaded in the film as ammonia-sensing molecules. White light reflected from the porous Si has a narrow bandwidth spectrum with a peak at 610 nm. Monitoring reflective optical intensity at the peak position allows for direct, real-time observation of changes in the concentration of ammonia gas in air samples. The reflective optical intensity decreased linearly with increasing concentrations of ammonia gas over the range of 0–100 ppm. The lowest detection limit was 0.5 ppm for ammonia gas. At optimum conditions, the full response time of the ammonia gas sensor was less than 15 s. The sensor chip also exhibited a good long-term stability over 1 year. Therefore, the simple sensor design has potential application in miniaturized optical measurement for online ammonia gas detection.     

Nanostructured Platinum‐iridium Alloy Microelectrode for Ammonia Determination

Nanostructured platinum‐iridium alloy microelectrode with high surface area was successfully prepared by applying successive potential cycles to a conventional PtIr microdisc in ionic liquid electrolyte containing ZnCl₂ at elevated temperature. Scanning‐electron microscope studies show that a very thin nanostructured film was created on the electrode upon 20 potential cycles between −2.0 and 0.75 V versus a Ag pseudo‐reference electrode. The film nanostructures are characteristic of regular hill‐like nano‐spacings separated by valley‐like nano‐cracks, and a roughness factor of approximately 40. The nanostructured electrode is highly active towards electrochemical oxidation of ammonia, and generates a linear relation between voltammetric peak currents (or chronoamperometric currents), and logarithm of ammonia concentration in a range of approximately 1 ppm to 10000 ppm. It has been proposed that the Temkin adsorption of ammonia from the bulk solution onto the electrode surfaces was involved in its electrochemical oxidation and could be responsible for the linear current‐logarithmic concentration relation.     

Ion chromatographic determination of ammonia in air using a sampling tube of porous carbon.

The ion chromatographic determination of ammonia in air using a sampling tube of porous carbon carbonized at 500 degrees C was examined. When the mean recovery and the reproducibility for a series of five determinations were examined for 1 and 10 ppm ammonia gases, the mean recovery (n = 5) and the relative standard deviation were 97.0% and 3.5% for 1 ppm and 86.9% and 2.8% for 10 ppm, respectively. Furthermore, the recovery from 10 ppm ammonia gas increased with an increase in the extraction time, and a recovery of 99.7% was obtained for 180 min of extraction time. The mean collection efficiency for 1 and 10 ppm ammonia gas was 102.5% and 96.5%, respectively. The relation between the sampling volume and the peak area was linear, and the linearity was 0.999 of the correlation coefficient. The ammonia gas concentration in an actual goat shed could be determined by this sampling device for a sampling volume of 5 L at a flow rate of 1 L/min; 0.79 ppm of the determination value practically agreed as compare with 0.78 ppm from collection by a boric acid solution.     

Determination of sulfur dioxide in solutions by pyridinium bromide perbromide and titrimetric and flow injection procedures

Sulfur dioxide can be determined by its reaction with pyridinium bromide perbromide using photometric titrations and flow injection procedures. Both methods are useful down to 30 ppm, and are unaffected by ammonia or nitrogen dioxide. Both mercury(II) and EDTA interfere under some circumstances.     

Thin films sensitive to pH changes prepared by sol-gel method

The light absorption change of phenolphthalein entrapped in a silica or titania matrix coated as a thin film onto a quartz optical fibre or on planar substrate by the sol-gel technique has been studied. The possibility to use this effect together with evanescent wave principle for the monitoring of pH changes has been tested. The film drying conditions necessary for the film to be sensitive to pH changes were determined experimentally. The stability and reversibility of the colour change was found to be better in the case of titanium matrix.     

The detection of ammonia and nitrogen dioxide at the parts per billion level with coated piezoelectric crystal detectors

Ucon-75-H-90,000 and Ucon-LB-300X are the new substrates used on the piezoelectric crystal detectors. On exposure to nitrogen dioxide these substrates form new compounds on the crystal which are sensitive to nitrogen dioxide and ammonia. With these coatings it is possible to detect nitrogen dioxide and ammonia in the parts per billion range. Some problems are caused by atmospheric moisture and high concentrations of organic pollutants.     

Fabrication of a gas sensor using pentacene thin films as a detector

Highly sensitive gas sensors for both acidic and basic gases were fabricated based on conducting thin films of polyacene compounds. Gas sensors formed with pentacene thin films deposited on various kinds of substrates were found to exhibit high sensitivity in detecting subppm concentrations of NO₂ or Cl₂ by monitoring the conductivity of the thin film. Improvements in the conductance and duration period for detection were achieved by changing the shape of electrodes and substrate. The sensors with PEN thin films initially doped with iodine could detect ppm concentrations of ammonia gas, since iodine molecules were dedoped upon exposure to ammonia, causing the reduction of the conductivity.     

Plastic colorimetric film sensors for gaseous ammonia

The preparation and characterization of three different plastic thin-film colorimetric sensors for gaseous ammonia is described. In the film sensors, the neutral form of a pH-sensitive dye (Bromophenol Blue, Bromocresol Green or Chlorophenol Red) was encapsulated in a plastic medium, either poly(vinyl butyral) or ethylcellulose plasticized with tributyl phosphate. Each of these film optodes gave a reproducible and reversible response towards gaseous ammonia. The sensitivity of the film sensors towards ammonia was found to be strongly dependent upon the pK _a of the encapsulated dye. Thus, the film with Chlorophenol Red (pK _a = 6.25), proved to be very insensitive (operating range: 0.29% < %NH₃ < 100%), whereas the film with Bromophenol Blue (pK _a = 4.1), was much more sensitive (operating range: 0.0003% < %NH₃ < 0.11%). The sensitivity of a plastic film sensor decreased markedly with increasing operating temperature and the 90% response (15–38 s) and recovery (820-127 s) times were slow and activation-controlled.     

Thick silicate glass film by an interfacial polymerization

A silicate glass film of 2–20 µm in thickness has been formed on a fused quartz substrate by a sol-gel process using an interfacial polymerization technique. A partially hydrolyzed silicon alkoxide was dissolved in hexane and brought into contact with ammonia water in a cylindrical Teflon container. A silica gel film formed at the interface between the two immiscible liquids by the polycondensation of the partially hydrolyzed silicon alkoxide was gently placed on a substrate at the bottom of the container, by draining the liquid. The crack free gel film was dried in an ambient atmosphere and dip-coated with boron ethoxide, followed by sintering in an oven at 1250°C for 1 h. The glass film thus obtained was highly transparent and 2–20 µm in thickness depending on the concentration of the precursor solution and the pH of ammonia water.     

溶胶-凝胶薄膜光纤传感器法测定空气中二氧化氮

以四乙氧基硅烷合成溶胶-凝胶薄膜,包埋偶氮试剂制备得到对二氧化氮具有灵敏响应的传感膜;与分支光纤等元件耦合成光纤传感器,通过累积吸收法能够现场测定空气中的低浓度二氧化氮．其检出限为每小时5ng／L;测定相对标准偏差为4．4％（n=6,C（NO2）=200ng／L,1h）。实验表明,CO2、NO、SO2、NO等共存气体在低浓度下对传感器测定NO2无明显干扰。     

A galvanic gas sensor using poly (ethylene oxide) complex of silver trifluoromethane sulphonate electrolyte

Summary A galvanic sensor for monitoring nitrogen dioxide was developed by using a poly(ethylene oxide) complex of silver trifluoromethanesulphonate electrolyte. The sensor, which is expressed as Au/P(EO)_4.5 AgCF₃SO₃/Ag, is a small disk (i.d. 13 mm). The polymeric electrolyte film was made by casting the mixture of acetonitrile solutions of both P(EO) (MW 6×10⁵) and AgCF₃SO₃. The working electrode was made by sputtering of gold in argon. The thicknesses of the desposited gold, polymeric electrolyte film and silver are 25 nm, 30 m and 1 mm, respectively. When the sample gas containing nitrogen dioxide impinges at 20 ml min^–1 on the gold cathode, the current flowing in the external circuit was linearly related to the concentration of nitrogen dioxide from 20 ppb to about 10 ppm. The current efficiency of the cell was 0.051%. The cell's response time was about 2 min for 0.5 ppm of nitrogen dioxide.     

Characteristics of Sol-Gel SnO2 Films Treated by Ammonia

Tin dioxide thin films prepared by sol-gel dip-coating method with ammonia treatment have been studied. By using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy, detailed investigation on the structure and morphology of the films has shown the condensation of Sn-OH and the strengthening of gel network net through ammonia treatment, which leads to the improvement of the adhesion of the films. From the spectral transmission, angular distribution of reflectance and absorption spectrum, the optical properties of the ammonia treated films indicate that the ammonia treated films have a favorable optical performance, and the films are more suitable for acting as antireflective films than the heat-treated films. The ammonia treated films also exhibit higher conductivity compared with the non-treated films.