Gas sensors for detection of volatile organic compounds at room temperature : Air qualty control & medical diagnosis

Abstract

The need of detecting acetone in ambient environment in laboratories and factories to monitor leakage and prevent accidents for human safety and health protection has involved numerous studies. Also, in human breath, acetone at ppm level is present amongst more than two hundred kinds of other volatile organic compounds. The development of reliable gas sensors for breath analysis is aimed to detect acetone at sub-ppm level in human breath for diabetes diagnosis. Field effect transistor based gas sensors have shown great potentials in detecting analytes at molecular level. In a first approach fluorinated-terminated monolayers have been formed on silicon nanowires, that can detect an organic gas (2-octanone) at 0.5 ppm level. Surface formation and stability of these fluorinated monolayers on silicon nanowires have been characterized; Preliminary results towards VOCs have shown high-quality monolayers in various extreme conditions with good surface passivation. As a second approach new nanogap IDEs structures have been developed for acetone detection, and sensing performance could be enhanced by using a nanocomposite (PVPH) sensing layer. Adding functionalized silicon nanoparticles in the PVPH polymer network, a higher sensitivity has been obtained. The IDE-PVPH devices demonstrated good reproducibility after multiple cycles of gas exposure. By reducing the gap-width to the nanoscale, the sensitivity of the device in acetone detection is increased drastically. A detection limit with nanogap IDEs is found to a level of 10 ppm when using PVPH as the sensing layer. Both micro and nano-gap IDEs operate at room temperature and show a good stability even after long-term storage. At room temperature, the detection limit of nanogap capacitive sensors depends heavily on the thermal and mechanical stability of other (parasitic) components that limit the resolution of the measurement. Emphasis is given on the nanostructure of nanogap IDEs; this type of capacitive sensors provide a larger dynamic range of the capacitance as compared to micro IDEs and herewith the signal to noise ratio is considerably improved. The results illustrate an improvement in measurement resolution from pico farads to femto farads. When using nanogap structures, a larger absolute change in capacitance will improve thus the detection limit. Both micro and nano gap sensors showed a relative change of 0.2 % with respect to the bare IDE capacitance and the limit of detection increased by a factor of 12. Further downscaling feature sizes in micro and nano IDE structures are promising for the development of novel portable gas sensors that are applicable in many fields such as industrial and laboratorial security. They promote a new generation of gas sensors for detection of a variety of VOCs at low concentrations in environmental air monitoring as well as disease diagnosis.