Sensitivity to pressure and methane of a cryptophane-A doped polymer

Abstract

The principle focus of this thesis is the characterization of an on-chip methane sensor based on a waveguide interferometer. It incorporates cryptophane-A molecules in the waveguide cladding to enhance sensitivity and selectivity towards methane.

First, the sensor was characterized for sensitivities to ambient conditions, in particular its temperature and pressure sensitivity. The measurement results show that a symmetric waveguide interferometer, with the same material on both arms, is almost insensitive to uniform changes in temperature and pressure. On the other hand, an asymmetric waveguide interferometer, with different materials on the arms, is highly temperature and pressure sensitive. However, numerical simulations revealed that a symmetric device can be sensitive to asymmetric heating of the top surface.

Second, the methane sensitivity of the sensor was tested with both pure polymer and polymer doped with cryptophane-A as the sensing medium. Using pure polymer resulted in a moderate sensitivity to methane, which linearly increased with pressure. While polymer doped with cryptophane-A resulted in more than 50-fold enhancement in sensitivity. Furthermore, the sensitivity was shown to be directly proportional to the concentration of cryptophane-A and increasing with pressure. A detection limit of 5 ppm was achieved, which is 1-2 orders of magnitude better than reported for comparable small and low-cost methane sensors.

As a greenhouse gas, methane has a high global warming potential and its atmospheric concentration has increased drastically over the past centuries. Hence, the interest in measuring and mapping the methane sources and atmospheric concentration has increased. The work in this thesis is paving the way for a high sensitive methane sensor, but still low-cost and compact enough to be mounted on drones and employed in poorly accessibly areas.