A Low-Cost, Reusable All-Solid-State TiN-Based EGFET Soil pH Sensor

The latest farming technology utilizes specialized electrochemical soil sensors for precision agriculture. Soil pH is the primary soil attribute which influences the crop yield by affecting the availability of nutrients to plants. Hence, monitoring and maintaining optimal soil pH is of utmost importance for enhancing the farm output. In this study, a low-cost, all-solid-state electrochemical extended gate field effect transistor (EGFET) based soil pH sensor is fabricated by exploring the pH sensing and specific conductive properties of titanium nitride (TiN) in conjunction with a low-drift planar Ag/AgCl reference electrode (PARE). The thickness dependence and hydrophilicity of sputtered TiN pH sensing films on pH response is investigated. The TiN pH electrode with a thickness of 50 nm demonstrated a higher sensitivity of 57.25 mV/pH and is relatively more hydrophilic with a water contact angle (WCA) of 46.425°. The fabricated TiN-PARE soil pH sensor measured the soil pH and pHCa with a maximum relative error of 2.98% and 4.31%, when compared with a laboratory pH meter, and demonstrated a linear pH response with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}^{{2}} \gt 0.99$ </tex-math></inline-formula>. The reusability and longevity are ensured by postconditioning of the TiN pH electrode with oxalic acid. Hence, the developed TiN-PARE soil pH sensor is promising for reliable pH measurements.