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The recent development of the Internet of Things (IoT) in healthcare and indoor air quality monitoring expands the market for miniaturized gas sensors. Metal oxide gas sensors based on microhotplates fabricated with micro-electro-mechanical system (MEMS) technology dominate the market due to their balance in performance and cost. Integrating sensors with signal conditioning circuits on a single chip can significantly reduce the noise and package size. However, the fabrication process of MEMS sensors must be compatible with the complementary metal oxide semiconductor (CMOS) circuits, which imposes restrictions on the materials and design. In this paper, the sensing mechanism, design and operation of these sensors are reviewed, with focuses on the approaches towards performance improvement and CMOS compatibility.

Heterogeneous photocatalytic oxidation (PCO) has shown great potential for indoor air purification of gaseous pollutants. Research attention has been drawn to the synthesis of new functional photocatalysts, as well as to kinetic studies of the influences of various reaction parameters, e.g., relative humidity. Nevertheless, when applied for practical use, the coating method and coating stability of are also important factors that will directly affect the removal efficiency. In the present study, a simple and economical spray coating method was developed to effectively immobilize TiO 2 nanoparticles onto a polyester fiber filter at low temperature. Colloidal silica was added as a binder in the coating suspension. PCO efficiency evaluation of the coating was carried out using gaseous formaldehyde. The results indicate that the formaldehyde removal rate is associated with the amount of binder added. The highest removal rate was achieved for a coating with the TiO 2 to binder (SiO 2 equivalent) mass ratio of 1:1, with the results supported by XRF, SEM, FTIR and BET analyses. As compared to the conventional dip coating method, the spray-coated sample showed much higher PCO efficiency and stability, which may be mainly attributed to the more uniform dispersion of the catalyst and the stronger binding formed during high pressure spray coating. This coating method has great potential for large-scale applications of immobilized photocatalyst for indoor air purification.

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Mechanochemical high energy ball milling approach was used to synthesize monoclinic BiVO4 (m-BiVO4) nanoparticles in an attempt to simultaneously reduce the particle size and improve the throughput for practical photocatalytic applications.