Printed Potentiometric Sensors for Soil Nitrogen Monitoring

Managing nitrogen is a central concern for precision agriculture and environmental science in order to maximize fertilizer use efficiency and minimize nitrate leaching and greenhouse gas emissions. However, measurement methods for in-soil nitrogen are limited. State-of-the-art soil nitrogen analysis requires taking soil or liquid samples to laboratories for chemical or spectrographic analysis. These methods are accurate, but costly, labor intensive, and cover limited geographic scope. Printed potentiometric nitrate and ammonium sensors are a promising alternative method for nitrogen monitoring because they are small, low power, involve no moving parts, and are mass-producible. Printing techniques are scalable and compatible with unconventional materials used for environmentally-benign sensors.<br/>Potentiometric sensors are composed of two electrodes: an ion-selective electrode (ISE) and a reference electrode (RE). The signal output is the potential difference between the two electrodes at zero-current conditions. Potentiometric ion sensors have been widely studied for health monitoring applications, however, several unique challenges arise when developing potentiometric sensors for environmental applications. Sensors used for soil monitoring must 1) exhibit very low drift to enable functional lifetimes of several months, 2) be insensitive to other chemicals at concentrations commonly found in soils 3) be paired with stable printed references, and 4) be robust to biofouling and damage from microbial activity in soil.<br/>Here we present work towards achieving these aims. Materials for the sensing electrode, ion-to-electron transducer layer, and ion selective membrane are compared and chosen for sensitivity, selectivity, and stability. The reference electrode transducer and membrane layers are likewise optimized for stability minimal drift. Encapsulation techniques are explored for robust production of sensitive electronics, enabling long lifetimes.