Vapochromic coordination polymer immobilization techniques for ammonia sensors with applications to power transformers

Ammonia detection is important for many applications in the biomedicine, agriculture, and automotive industries. Sensing of ammonia is also crucial in determining the health of power transformers as the presence of ammonia indicates a breakdown in a transformer’s insulating materials. Current methods of gas analysis for detecting ammonia in such applications are costly, complicated, and time consuming. This thesis is concerned with the use of vapochromic coordination polymers (VCPs), which are in this case fluorescence-based gas sensitive polymers, whose emission spectrum changes upon the binding of target gases, e.g., ammonia. VCP materials have shown great promise in ammonia detection due to their superior fluorescence response and selectivity to ammonia but require immobilization to enable their use as a sensor surface. The work presented in this thesis examines several different immobilization techniques for VCPs to create a new class of ammonia sensors. The first immobilization technique explored involves creating a sheet of post arrays in polydimethylsiloxane (PDMS) to trap and adhere the VCPs to the sensing surface. We show that as the shape of the top of the post arrays is changed (e.g. from simple post to mushroom-shaped caps), the sensitivity of the sensing surface changes. Ammonia detection in the amount of 5 ppm is possible with the most pronounced mushroom shaped posts. The second immobilization method involves dissolved polylactic acid (PLA) mixed with VCPs that are deposited on a PLA substrate, resulting in nanoporous membranes (NPMs) that immobilize the VCP. This technique results in ammonia detection of 5 ppm based on available gas concentrations and reveals that a mix ratio of PLA to VCP of 12% wt. to 88% wt. results in a sensor surface with the highest degree of reversibility. This second immobilization technique also makes a sensor surface that is able to directly detect ammonia dissolved in fluids. Because of the ability of multi-phase gas detection with this immobilization technique we determine that it is the more promising of the two immobilization methods. We explore the application of both immobilization methods in the creation of a sealed micro-fluidic ammonia sensor. Our prototypes use a 3D printed cyclic olefin copolymer (COC) micro-fluidic cell, where COC is employed due to its exceptional optical properties and chemical inertness. These sensor cells detect ammonia both in gas and dissolved in fluids (transformer oil) but are limited in a detection of 1,000 ppm using available gas concentrations for testing.