Abstract
This work presents the fabrication and performance evaluation of an extended-gate field-effect transistor (EGFET) nitrate sensor based on zinc oxide/polyaniline (ZnO/PANI) bilayer thin films. ZnO layers were deposited on indium tin oxide (ITO) substrates using spin coating, followed by polyaniline (PANI) deposition via dip coating at varying dipping times. Surface morphology and elemental analysis confirmed that a 15-second dipping time yielded a compact and uniform bilayer structure with enhanced film integrity. The optimized sensor exhibited a sensitivity of 58.13 mV/decade (R² = 0.99558) over the 10–50 ppm nitrate range and a super-Nernstian response of 62.13 mV/decade (R² = 0.99637) from 100 to 250 ppm. The detection limit was estimated at 1.77 × 10⁻⁶ M (0.11 ppm), and the repeatability showed a standard deviation of 0.12 %. Selectivity coefficients against chloride ranged from 0.00149 to 0.13495. In mixed-ion conditions, values of 0.3307, 0.0151, 0.0980, and 0.3495 were obtained for nitrate with Cl⁻, SO₄²⁻, HCO₃⁻, and all combined, respectively. The sensor also showed pH and temperature sensitivities of 3.20 mV/pH and 4.81 mV/°C. Long-term evaluation revealed drift rates of 1.63 mV/day during continuous immersion and 0.18 mV/day after dry storage, indicating stable and consistent performance.