Optimization of photocurable polyurethane membrane composition for ammonium ion sensor

The composition of ammonium-ion sensitive membrane based on a photocurable aliphatic urethane diacrylate oligomer has been optimized. Membranes were prepared with exposure to UV and studied in a traditional ion-selective electrode configuration with liquid inner contact. The optimum composition is found to be: urethane diacrylate (45 to 50%), hexandiol diacrylate (10%), photoinitiator (0.8 to 1.2%), plasticizer (bis(2-ethylhexyl) sebacate or di-5-nonyladipate (35 to 40%), nonactin (2%), and KTpC1PB (0.5%). The resulting ion-sensors show a sensitivity of 55.5 mV/decade, a range of linear response from 1 to 4.5 · 10-4 mol/liter of NH4Cl, a limit of detection around 10-5 mol/liter, and good selectivity in the presence of potassium ions. Introduction of 7% PVC into the membrane formulation gives a possibility to enhance the sensitivity up to 58 mV/decade but prevents application of photolithography for membrane patterning in case of ion sensitive field effect transistor-based sensors. The developed polymer composition was used to prepare ISFET-based ammonium sensors with the membrane deposited and structured on a wafer level.