Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

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BaSnO 3 has been prepared as porous screen-printed thick films gas sensors. The sensor material was characterized by scanning electron microscopy and X-ray diffraction. The sensors gas response was investigated by exposure to varying concentrations of CO 2 gas in synthetic air and mixtures of CO and CO 2 all at a variety of different operating temperatures. Finally the gas response was examined to potential interferant gases including humidity. It was found that at an operating temperature of 500 • C and to a 2000 ppm pulse of CO 2 the sensors gave a gas response of 3.10, higher than current literature values for metal oxide materials. It was also found that increasing relative humidity enhanced the gas response in contrast to other approaches.There exists a clear demand for CO 2 concentration monitoring for applications such as the assessment of indoor air quality, 1 food storage 2 and early fire detection. 3 Currently, most CO 2 sensing requirements are met using optical and electrochemical devices. Optically based devices are expensive and present integration problems, 1 while electrochemical sensors present complex fabrication processes and humidity interference. 4 Solid-state sensors based on metal-oxide semiconductors potentially provide an attractive alternative, since they offer potential for a lowcost, easily manufactured, small form factor solution.Studies have focused on mixed oxide systems such as BaTiO 3 -CuO and CuO-BaSnO 3 . 5,6 The sensing mechanism appears due to gas interaction on a p-n heterojunction. However, the sensitivity of these devices to concentrations of interest for indoor air quality (300 ppm to 2000 ppm) is too low.Other studies have reported promising metal oxide candidates such as La-doped SnO 2 and BiTiO 3 , which display reliable CO 2 sensing properties. 7-10 However, the CO 2 sensitivity was found to be significantly reduced in the presence of humidity. Nd 2 O 2 CO 3 has also been reported as a strong candidate for a CO 2 sensor. 11 In this case it was noted that the response of the material was enhanced in the presence of humidity.This study presents the case for thick films of BaSnO 3 as a highly promising material for the sensing CO 2 in the 0 to 2500 ppm range. CO 2 sensing characteristics are investigated over a range of operating temperatures and relative humidity's. It is found that the presence of humidity enhances the CO 2 gas response. ExperimentalCeramic grade BaSnO 3 material was sourced from a commercial supplier [Cerac], and sieved through a 325 mesh (∼45 μm aperture). The raw BaSnO 3 powder was then made into a screen-printable ink by mixing with a printing vehicle made from butyl carbitol-ethyl cellulose and in a ratio such as to give a 55% solids loading. Mixing of the powders and vehicle was done in a planetary ball mill (Speedymill) for 3 minutes using 5 mm ceramic mixing media.The sensor inks were screen printed using a DEK printer (Model 1202) onto alumina tiles patterned with a gold inter-digitated electrode pattern (spacing ∼ 65 μm) on one side and a serp...

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The overall performance of an electronic nose system will depend on the individual performance of its constituent elements. Although often overlooked, it is clear that careful design/selection of the front-end signal conditioning circuit is of critical importance if optimal performance of the odour sensing system is to be achieved. In this paper circuits are reviewed, which have been employed as front-end signal conditioners for resistance-based sensors in electronic nose systems, with many of the conclusions drawn being equally applicable to other resistor sensors. The relevant equations governing the behaviour of each circuit methodology are derived and advantages and disadvantages are discussed. The performance of the circuit is then quantitatively assessed in a speci c test case, in which the maximum sensitivity of the circuit is calculated in relation to the task of interfacing to a theoretical thin-lm conducting-polymer sensor.