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Operating temperature for detection of a gas/ vapour is an important factor to control the selectivity of a chemi-resistive type sensor. By governing the temperature dependent adsorption/desorption process of a gas/vapor on the surface of a sensing prototype, its selectivity can be switched from one target gas to another, which is useful for field applications. In this vision, this study represents a high performance, low temperature, and versatile volatile organic compound (VOC) sensor fabricated by hydrothermally synthesized ZnO nanorod networks. The selectivity of the prepared sensor prototype has been switched from methanol (at low operating temperature i.e < 50 °C) to formaldehyde (at high operating temperature i.e > 50 °C) by varying the operating temperature of the sensor. The sensor can selectively detect methanol vapor (400 ppm) with a response value of 600 operating at room temperature (RT, 27 °C). On the other hand, the same prototype is selective toward formaldehyde (400 ppm) with an extremely high response of 12000 at an operating temperature of 100 °C. Moreover, high sensitivity, selectivity, repeatability, and low response time (4.2 s) towards methanol at room temperature fulfill the purpose of a reliable VOC sensor for practical applications. The mechanism of selectivity switching associated with the present sensor has been correlated with the activation energy values for methanol and formaldehyde. By virtue of temperature driven selectivity switching phenomena, the ZnO nanorod networks based present sensor prototype can be considered as a smart sensing element for a futuristic, versatile, low temperature, dual mode VOC sensor. [Display omitted] • ZnO nanorod networks were synthesized in low temperature hydrothermal method. • ZnO nanorod network sensor shows a unique selectivity switching in VOC detection. • Selectively detect methanol (R ∼ 600) at RT and formaldehyde (R ∼ 12000) at high temperature (≥ 50 °C). • Activation energy values has been correlated to explain the sensing mechanism behind selectivity switching. [ABSTRACT FROM AUTHOR]

The characteristic and effectiveness of pure and modified tin dioxide catalytic pellets as volatile organic compound sensors are discussed. The pure and La-doped SnO2 thick film sensor pellets exhibited highest sensitivity to ethanol, methanol and acetone vapour at sensing temperature 300°C. The highest sensitivity towards 500 ppm of ethanol, acetone and methanol was achieved by 5 at.% of La-doped SnO2 with values of ~57, ~55 and ~59, respectively. The enhancement of the sensitivity of La-doped SnO2 can be explained by the increase of surface areas; decrease of crystallites sizes and the basicity of the lanthanum dopants. The response times for the developed sensors were about 13-15 s for both pure SnO2 and La-doped SnO2 illustrated a fast response sensor that could be used as VOC sensors. [ABSTRACT FROM AUTHOR]

Abstract: A compact optical fibre sensor coated with a fluorosiloxane polymer has been shown to be suitable for monitoring of some different classes of organic volatile compounds (VOCs), namely some chlorinated hydrocarbons, aromatic hydrocarbons, aliphatic hydrocarbons, acetate and alcohols. The sensing component consists of an optical fibre, with the top end surface coated with poly[methyl(3,3,3-trifluoropropyl)siloxane] by dip-coating technique. Variations of the light power guided through the fibre are detected as the organic vapour is sorbed in the thin polymeric film. The experimental set-up is further constituted by an optical source to generate the interrogating signal and a photodiode to measure the intensity modulated signal. In this work some operational conditions such as, temperature of cure of the polymeric material, injection cell temperature, carrier gas flow rate, laser working wavelengths and frequencies were studied in order to achieve higher sensitivity and accuracy concerning sensor system performance. High detection capability for volatile organic compounds, good sensibility, reversibility, reproducibility and linearity were analytical features checked for this sensor system. However, the main advantage of the developed sensor is its very short analytical time (response time and desorption time), found to be less than 20s. Compounds well known as potentially dangerous for human health and environment, such as toluene and benzene showed changes on the reflected optical power up to 26.0 and 4.0dB, respectively. [Copyright &y& Elsevier]