Optimization of a polymer layer highly doped with Cryptophane-A for methane sensing

Abstract

The main objective of this thesis is to optimize the thickness of sensitive polymer layer highly doped with Cryptophane-A for methane pre-concentration and to make the existing sensor more stable and selective towards methane. In recent years, methane detection has become a hot topic due to the strong impact of methane on the global warming and the climate change. There has also been rising interest in the development of new methane sensors that can tackle the task of sensitive atmospheric methane detection, but are smaller, lighter, and cheaper than the state of the art. Our approach to this challenge is the development an on-chip waveguide sensor, which compensates for the rather short path lengths possible on a chip with pre-concentration of methane in a thin, specially designed waveguide cladding layer. Our detection technique is based on evanescent refractive index sensing with a Silicon Nitride shallow rib-waveguide Mach-Zehnder interferometer. The waveguide was fabricated with dimensions supporting single TE and TM modes at the wavelength of 785 nm. The reference arm is cladded with SiO2 that is impermeable to methane, and the sensing arm is cladded with Styrene Acrylonitrile (SAN) polymer doped with Cryptophane-A. Cryptophane-A is a molecular compound, which has a high affinity towards methane. The sensor group have previously reported that the presence of cryptophane increases the methane concentration in the SAN layer. The limit of detection of existing set-up is 6 ppm. However, to bring the methane sensor to the field, not only sensitivity but also specificity to methane and the sensor response time need to be quantified. In this thesis, both parameters were targeted. First, the sensor was characterized for sensitivity and response time of different thicknesses of sensitive layer with Cryptophane-A: SAN concentration of 8.5. The measurement result shows that both sensitivity and response time increase with thickness. The sensitivity gets saturated at 400 nm while response time continues to increase. Second, the measurement was done to reach the limit of sensitivity of developed sensor. The measurement result shows that the sensitive layer of Cryptophane-A: SAN concentration of 1:1 gives the highest sensitivity. Then measurement was done for thin layer of highest sensitive layer and PDMS on top of that. The measurement result shows that sensitivity is increased to twice of previous reported value and response time reduced by almost 3 times. At the end, the drift in the sensor was reduced by deposition of SAN on one arm and SAN doped with cryptophane-A on the other. The measurement data shows that the long-term drift which was evident when SiO2 was cladded in one arm is reduced. The deposition of polymer on both arms also provides specificity towards methane. The specificity, high sensitivity, fast response and stability makes it a robust sensor that can be mounted on drone (UAV) for real-time testing.