EGFET-based sensor for pH and nitrate sensing applications

Ensuring soil health is crucial for sustainable agriculture and optimal crop yield. However, conventional laboratory soil tests face substantial limitations, including high costs, complicated sampling procedures, and lengthy processing times. Commercial onsite soil sensors present a practical alternative, yet their adoption is hindered by reliability and selectivity challenges, primarily due to interference from unwanted soil ions. To address these issues, this study aimed to develop highly sensitive, selective, and reliable pH and nitrate sensors using Extended-gate Field Effect Transistor (EGFET) configuration. The EGFET configuration involves an extended sensing film connected to a commercial MOSFET, provides advantages such as improved stability, reduced cost, and simplified fabrication. Titanium dioxide-polyaniline (TiO₂-PANI) composite thin films and titanium dioxide/nitrate ionophore-dibutyl phthalate (TiO₂NI6/DBP) thin films were selected based on their superior performance. The TiO₂PANI composite demonstrated outstanding performance as a pH sensor, achieving a super-Nernstian sensitivity of 66.1 mV/pH with linearity (R² = 0.9931), low hysteresis, excellent reproducibility, repeatability, and stability. Additionally, its low nitrate sensitivity (12.9 mV/dec) confirms its high selectivity toward pH detection. Conversely, the TiO₂-NI6/DBP thin film exhibited exceptional nitrate sensitivity (129.1 mV/dec) and linearity (R² = 0.9994), combined with minimal drift rates at low nitrate concentrations, ensuring sensor stability. Its low pH sensitivity (24.7 mV/pH) indicated high nitrate selectivity. The effects of interference from common fertilizer ions (nitrogen, phosphate, and potassium—NPK) were investigated, revealing minimal interference in pH sensing but notable interference in nitrate sensing. This highlights the need for further optimization of nitrate-selective sensing films. A correlation study comparing MARDI laboratory analytical results and EGFET-based soil measurements validated the practical applicability of the developed sensors. Results showed that the TiO₂-PANI composite film had minimal deviation from MARDI laboratory reference values, confirming its reliability for direct soil pH measurements.