A low-hysteresis coaxial pH sensor with UV irradiation-pretreated iridium oxide film for continuous soil monitoring

A low-hysteresis coaxial pH sensor was presented for continuous soil monitoring. The electrolyte was irradiated with ultraviolet (UV) light for different durations, and then used for the electrodeposition of the IrO_x film onto the front end of a stainless-steel wire, forming the working electrode. The reference membrane/AgCl/Ag paste (reference electrode) was coaxially integrated onto the insulated middle section of the working electrode, creating the pH sensor. It is found that the UV irradiation treatment generates sufficient hydroxyl radicals in the electrolyte, which increases the proportion of IrO₂ on the surface of the formed IrO_x film and further enhances its reversible surface reactions with H⁺. The reference electrode is reliably integrated with the working electrode, which effectively miniaturizes the pH sensor for soil monitoring. With the UV irradiation duration optimized to be 2 days, the sensitivity and hysteresis voltage of the pH sensor were determined to be -74.8 mV/pH and 3 mV, respectively. The pH sensor maintained a stable potentiometric response even when bent to a 180° angle or subjected to 50 bending cycles. Moreover, the pH values of six soil samples were reliably determined, and the pH variations in a soil sample were monitored over an 11-day testing period. These results benefit accurate and continuous pH monitoring in agricultural and livestock applications.