A CO2 sensor based upon a continuous-wave thermoelectrically-cooled quantum cascade laser

A CO₂ sensor based upon a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser operating between 2305 and 2310 cm⁻¹ and a 54.2 cm long optical cell has been developed. Two approaches for direct absorption spectroscopy have been evaluated and applied for monitoring of the CO₂ concentration in gas lines and ambient laboratory air. In the first approach optical transmittance was derived from the single channel laser intensity, whilst in the second approach a ratio of signal and reference laser intensities (balanced detection) was used. The optimum residual absorption standard deviation was estimated to be 1.9 × 10⁻⁴ for 100 averages of 1 ms duration and 0.1 cm⁻¹ scans over the P(46) CO₂ absorption line of the ν₃ vibrational band at 2306.926 cm⁻¹. A CO₂ detection limit (1 standard deviation) of 36 ppb was estimated for 0.1 s average and balanced detection.     

Using a TiO2/ZnO double-layer film for improving the sensing performance of ZnO based NO gas sensor

NO gas sensors, based on ZnO thin film (ZnO_film), TiO₂ nanoparticulate film (TiO₂NP), and TiO₂NP/ZnO_film double-layer film, were fabricated, and their sensing characteristics towards NO gas were investigated in this study. The maximal response of a ZnO_film deposited onto a rougher Al₂O₃ substrate, towards NO gas, was higher than that of a ZnO_film deposited on a smoother glass substrate. Although the sensing response of the TiO₂NPs itself towards NO gas was minute, the TiO₂NP/ZnO_film double-layer film showed enhanced response as compared with TiO₂NP or ZnO_film single-layer film. In addition, the sensor response of the TiO₂NP/ZnO_film double-layer film was strongly influenced by the annealing time for the film preparation; the maximum response to NO was enhanced about 6.2 times as the annealing time was increased from 30 min to 2 h. Based on the XPS results, the increase in the transition zone between TiO₂NP and ZnO_film along with the appearance of Ti³⁺ state was noticed when the annealing time was increased. With the high sensitive TiO₂NP/ZnO_film/Al₂O₃ electrode, the limit of detection (S/N = 3) can be achieved at 8.8 ppb. The double-layer TiO₂NP/ZnO_film also showed improved selectivities with respect to NO₂ and CO.     

Impedancemetric gas sensor based on Pt and WO3 co-loaded TiO2 and ZrO2 as total NOx sensing materials

Pt-loaded metal oxides [WO₃/ZrO₂, MO_x/TiO₂ (MO_x = WO₃, MoO₃, V₂O₅), WO₃ and TiO₂] equipped with interdigital Au electrodes have been tested as a NO_x (NO and NO₂) gas sensor at 500 °C. The impedance value at 4 Hz was used as a sensing signal. Among the samples tested, Pt-WO₃/TiO₂ showed the highest sensor response magnitude to NO. The sensor was found to respond consistently and rapidly to change in concentration of NO and NO₂ in the oxygen rich and moist gas mixture at 500 °C. The 90% response and 90% recovery times were as short as less than 5–10 s. The impedance at 4 Hz of the present device was found to vary almost linearly with the logarithm of NO_x (NO or NO₂) concentration from 10 to 570 ppm. Pt-WO₃/TiO₂ showed responses to NO and NO₂ of the same algebraic sign and nearly the same magnitude, while Pt/WO₃ and WO₃/TiO₂ showed higher response to NO than NO₂. The impedance at 4 Hz in the presence of NO for Pt-WO₃/TiO₂ was almost equal at any O₂ concentration examined (1–99%), while in the case of Pt/WO₃ and WO₃/TiO₂ the impedance increased with the oxygen concentration. The features of Pt-WO₃/TiO₂ are favorable as a NO_x sensor that can monitor and control the NO_x concentration in automotive exhaust. The effect of WO₃ loading of Pt-WO₃/ZrO₂-based sensor is studied to discuss the role of surface W-OH sites on the NO_x sensing.     

An ultraviolet selective photodetector based on a nanocrystalline TiO2 photoelectrochemical cell

The UV component of solar light is responsible for skin cancer and a number of other skin disorders. An inexpensive and simple ultraviolet (UV) selective photodetector would be convenient to measure the UV exposure. A sealed two-electrode photoelectrochemical cell (PEC) based on a novel double-layer of nanocrystalline TiO₂ has been designed and constructed as a UV-selective-sensor. The properties of the sensor, including spectral response, current-voltage characteristics, sensitivity, linearity and response time are reported. The results demonstrate the potential for the use of this low cost UV-photodetector - which does not require UV selective filters - to provide a warning of harmful solar UV-radiation levels.     

Large area Ag-TiO2 UV radiation sensor fabricated on a thermally oxidized titanium chip

A Schottky-type ultraviolet (UV) light sensor is fabricated on a thermally oxidized titanium chip. The device is of Ag-TiO₂-Ti structure, and the Schottky junction between silver and rutile is formed subsequent to the vacuum deposition of silver on the thermally grown rutile layer by a controlled thermal annealing in air. The device operates at −300 mV biasing established between silver and titanium electrodes. The dark reverse current of this diode increases four orders of magnitude when illuminated with UV light (355 nm) of 10 μW/mm² intensity. The device is almost insensitive to visible light and requires no filtering when used for ambient UV level detection. The operation mechanism of the device is described by photonic electron-hole pair generation in the carrier depleted titanium dioxide layer adjacent to the silver electrode. The electrode-to-electrode distance is 1 μm only affording much faster performance compared to photoconductive UV detectors fabricated based on titanium dioxide; the response and recovery times of the device are 10 ms and 17 ms, respectively. At its standby mode, a 1 mm² active area device consumes less than 10 pW of electric power. Sensors with sensitive areas as large as ∼10 mm² are easy to fabricate. The fabricated devices are rugged, resistant to UV degradation, and cost effective.     

Flexible H2 sensor fabricated by layer-by-layer self-assembly of thin films of polypyrrole and modified in situ with Pt nanoparticles

A novel flexible H₂ gas sensor was fabricated by the layer-by-layer (LBL) self-assembly of a polypyrrole (PPy) thin film on a polyester (PET) substrate. A Pt-based complex was self-assembled in situ on the as-prepared PPy thin film, which was reduced to form a Pt-PPy thin film. Microstructural observations revealed that Pt nanoparticles formed on the surface of the PPy film. The sensitivity of the PPy thin film was improved by the Pt nanoparticles, providing catalytically active sites for H₂ gas molecules. The interfering gas NH₃ affected the limit of detection (LOD) of a targeted H₂ gas in a real-world binary gas mixture. A plausible H₂ gas sensing mechanism involves catalytic effects of Pt particles and the formation of charge carriers in the PPy thin film. The flexible H₂ gas sensor exhibited a strong sensitivity that was greater than that of sensors that were made of Pd-MWCNTs at room temperature.     

Optical CO2 sensor with the combination of colorimetric change of α-naphtholphthalein and internal reference fluorescent porphyrin dye

A new optical CO₂ sensor based on the overlay of the CO₂ induced absorbance change of pH indicator dye α-naphtholphthalein with the fluorescence of tetraphenylporphyrin (TPP) was developed. The observed luminescence intensity from TPP at 655 nm increased with increasing the CO₂ concentration. The ratio I₁₀₀/I₀ values of the sensing films consisting of α-naphtholphthalein in ethyl cellulose layer and TPP in polystyrene layer, where I₀ and I₁₀₀ represent the detected luminescence intensities from a layer exposed to 100% nitrogen and 100% CO₂, respectively, that the sensitivity of the sensor, are more than 53.9. The response and recovery times of the sensing films consisting of α-naphtholphthalein in ethyl cellulose layer and TPP in polystyrene layer were less than 5 s for switching from nitrogen to CO₂, and for switching from CO₂ to nitrogen. The signal changes were fully reversible and no hysterisis was observed during the measurements. The highly sensitive optical CO₂ sensor based on fluorescence intensity changes of TPP due to the absorption change of α-naphtholphthalein with CO₂ was achieved.     

The effect of La2CuO4 sensing electrode thickness on a potentiometric NOx sensor response

In order to further understand the different contributions to NO_x sensing mechanism as well as the importance of electrode geometry, solid state potentiometric sensors with varying La₂CuO₄ sensing electrode thicknesses were studied. These sensors (with a Pt counter electrode) showed a dependence of NO₂ sensitivity which decreased with increasing thickness in the temperature range of 550-650 °C. They also showed NO sensitivity that was independent of thickness at 400 °C and 600 °C, but varied at temperatures between. This behavior was attributed to multiple mechanistic contributions explained by Differential Electrode Equilibria.     

Effect of thickness of platinum catalyst clusters on response of SnO2 thin film sensor for LPG

This paper reports the sensing response characteristics of rf-sputtered SnO₂ thin films (90 nm thick) loaded with platinum catalyst cluster of varying thickness (2-20 nm) for LPG detection. The enhanced response (5 × 10³) was obtained for 200 ppm LPG with the presence of 10 nm thin and uniformly distributed Pt catalyst clusters on the surface of SnO₂ thin film at a relatively low operating temperature (220 °C). The high response for LPG is shown to be primarily due to the enhanced catalytic activity for adsorbed oxygen on the surface of SnO₂ thin film besides the spill over mechanism at elevated temperature.     

Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in the sputtering gas, and working temperature

We report on electrical responses of tungsten oxide thin film ozone sensors based on a tungsten trioxide (WO₃)/tin oxide (SiO₂)/Si structure with interdigitated Pt electrodes. The influence of O₂ concentration in the sputtering gas and working temperature of the sensor are investigated. Sensitivity to ozone increases with O₂ content in the sputtering gas. It reaches its highest value for sensors fabricated with 50% O₂. For these sensors, the best ozone sensitivity and shortest response and recovery times are obtained at a working temperature of 523 K. Ozone sensitivity is compared to other ozone sensors.     

pH sensor based on the crown heteropolyanion K28Li5H7P8W48O184·92H2O immobilized using a layer by layer assembly process

A new sensitive pH sensor based on immobilization of the crown heteropolyanion K₂₈Li₅H₇P₈W₄₈O₁₈₄·92H₂O (P₈W₄₈) on a electrode surface through a layer by layer assembly process is described. The immobilization is based on the electrostatic adsorption of the complex in layers of charged polyelectrolyte poly(allylamine hydrochloride) (PAH). The deposited P₈W₄₈/LBL film was investigated by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. Compared to the electrochemical behavior of dissolved P₈W₄₈, a slight shift in the redox peak towards negative potentials is observed, which have been attributed to a slight decrease in the acidity of the interior of the P₈W₄₈/LBL film compared to the testing buffer solution. The relationship between the peak currents of the deposited P₈W₄₈/LBL film and the number of layers is shown to be linear, which demonstrates that equal amounts of P₈W₄₈ are adsorbed in each deposition layer. The P₈W₄₈/LBL modified electrode showed high sensitivities toward pH. Therefore, such electrodes were tested as pH sensors using the titration method. The resulting pH sensor has a detection range of pH 1–13, a sensitivity of 69 ± 2 mV/pH, high repeatability (<3 mV), fast response time (<7 s), low sensitivity toward change in ionic strength and nature of the supporting electrolyte, low internal resistance and a working life time of at least 3 months. Moreover, the sensor is easy to manufacture and can be easily miniaturized for measurements in micro- and nano-systems.     

Alpha-1-fetoprotein antibody functionalized Au nanoparticles: Catalytic labels for the electrochemical detection of α-1-fetoprotein based on TiO2 nanoparticles synthesized with ionic liquid

A novel strategy for the preparation of amperometric immunosensor for rapid determination of α-1-fetoprotein (AFP) in human serum has been developed. TiO₂ nanoparticles (NPs) were prepared by solvothermal reaction using TiCl₄ as raw materials and the mixture of ionic liquids and doubly distilled water as solvent. α-1-fetoprotein antibody (AFP Ab) was mixed with TiO₂ NPs/chitsotan (CHIT) solution and immobilized onto the surface of a glassy carbon electrode. AFP (Ab) functionalized Au NPs were used as catalytic labels for the amperometric detection of AFP by means of the electrocatalyzed reduction of Au NPs to H₂O₂. The electrochemical behavior of the immunosensor was studied. Other experimental conditions such as pH, immunoreactions temperature and time were also studied. The prepared immunosensor offers an excellent amperometric response for AFP ranging from 1.0 to 160.0 ng/mL with a detection limit of 0.1 ng/mL. The result shows that the immunosensor displays rapid response, high sensitivity, good reproducibility and favorable stability.