Abstract
The presence of various toxic and polluting gases has driven the development of increasingly innovative detection systems. Among these, chemiresistive gas sensors have emerged as the most widely used technology in gas sensing applications. In particular, metal oxide semiconductor based chemoresistive sensors have been extensively studied because of their excellent sensitivity and low production costs. However, despite their advantages, these materials exhibit some limitations, such as poor selectivity and the need for high operating temperatures, necessitating further research and development. To address these challenges, this PhD thesis focused on the explorative use of advanced materials, specifically single-walled carbon nanotubes (SWCNTs) and quantum dots (QDs), aiming to overcome the high-temperature requirements of traditional metal oxide-based chemiresistive sensors. SWCNTs were chosen for their ability to operate at relatively low temperatures, while CuInS2@ZnS (Z-CIS) QDs were selected for their potential to work in light-activation mode. To further enhance selectivity, the project integrated a compact gas separation device designed to separate and concentrate individual components in a mixture before they reached the sensing device. Both the chemoresistive gas sensors and the separation devices were fabricated using micro-electromechanical system (MEMS) technology. Gas sensors based on SWCNTs were fabricated using two distinct suspensions of SWCNTs, stabilized with different surfactants: carboxymethyl cellulose (CMC) and sodium dodecyl sulfate (SDS). These sensors were tested for their electrical responses to NO2 (an oxidizing gas) and NH3 (a reducing gas). The objective was to test the response of the sensors to both oxidizing and reducing gases in order to demonstrate their versatility. Under optimal conditions (150 ◦C operating temperature and 40% relative humidity), the SDS-based sensors achieved a response of 62.5%, while the CMC-based sensors reached 78.6%. These results represent significant performance improvements compared to previously reported SWCNT-based gas sensors. For QD-based sensors, Z-CIS QDs were used to functionalize SnO2, enabling activation through light exposure. The resulting composite material was tested in photoactivation mode using visible-light, achieving a light response of 34.7%. The sensor demonstrated a response of 4% to 300 ppm of H2, showcasing its suitability for hydrogen detection at room temperature under light activation. Finally, the micro separation device, incorporating a micro preconcentrator and a microcolumn, was fabricated and tested. This system included a preconcentrator and a micro column embedded in a 400 µm silicon wafer with dimensions of 1 x 2 cm2 . However, the microcolumn, fetured with a single serpentine channel design caused blockages, preventing the creation of a working device. Consequently, the project shifted its focus to optimizing the micro-preconcentrator. Two different stationary phases were tested, but only the SWCNT-based preconcentrator demonstrated effective adsorption and desorption properties when exposed to xylene. In conclusion, this Ph.D. project highlighted the potential combination of innovative materials with MEMS technology to develop compact and efficient gas detection and separation systems. The insights gained during this PhD project, provide a solid foundation for future advancements, paving the way for the creation of portable, high-performance devices suitable for environmental and industrial applications.