Hydrogel Coated Ion Sensor Array for Continuous Nitrogen Monitoring in Variable Soil Conditions

Continuous monitoring of critical soil nutrients, such as nitrate and ammonium, could play an important role in agricultural data-driven fertilizer application improving nitrogen use efficiency and reducing pollution. The ion potentiometric sensor with an ion-selective electrode (ISE) is a widely accepted standard method in water analysis. Therefore, many researchers aim to utilize potentiometric sensors to detect various ion concentrations in soil moisture due to their low cost and high selectivity. However, a potentiometric sensor requires a water environment to function. The varying moisture content in different soil types (sand, clay, silt, peat, chalk, and loam) and the occurrence of freezing conditions in winter make the applicability of existing sensors challenging. To achieve this, we demonstrate an ISE hydrogel sensor with uniform and ultra-thin (around 5 m) hydrogel coatings by in-situ polymerization onto the protection silicon rubber outer layer applied to the potentiometric sensor. The well-designed ISE hydrogel sensor inherits traditional ISE sensors' wide sensing range (0.01mM-100 mM), high sensitivity (~32.5 mV/dec), and stability (drifting < 0.5 mV/h), effectively measuring ion concentration in soil with moisture content as low as 5%, which is inaccessible to traditional ISE sensors even at 20%. Additionally, we design printed heaters to thaw frozen soil during winter, enabling the measurement of ion concentration. And the printed thermometer can effectively monitor soil temperature and control the operation of the heater. Based on that, the viability of continuous ion sensing at a wide range of expected soil temperatures with closed-loop heating is improved. This expands the application space for low-cost electrochemical ion sensors by proving that environmental factors can mitigated with careful materials and system design.