Sensitivity Study of Spin-Coated Metal Oxides Thin Films for Extended Gate Field-Effect Transistor (EGFET) pH Sensor

Increasing demand for accurate, reliable and highly sensitive pH sensors has led researchers to explore various materials for this reason. Metal oxide (MOx) pH sensors have received considerable attention due to their high degree of accuracy and great sensitivity to hydrogen ions. Additionally, this MOx pH sensor overcomes the shortcomings of the glass electrode. Thus, a comparative experimental study on various metal oxides (MOx) of TiO2, ZnO, CuO, and NiO thin films as sensing electrodes for extended-gate field effect transistor (EGFET)-pH sensor was carried out via a facile sol-gel spin-coating method. Here, the thin films were tested as pH sensors in pH 2, 4, 7, 10 and 12 and hysteresis stability for 25 min in pH 7 -> 4 -> 7 -> 10 -> 7. The pH measurements were repeated several times to confirm the sensitivity behaviour. The surface morphology and surface roughness of the films were characterized using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM), respectively. The TiO2 thin films showed the highest sensitivity (53.4 mV/pH, R2 = 0.992) and lowest hysteresis value (1 mV) compared to the other sensing electrodes. Moreover, the thin film showed drift rates of 6.74, 3.52 and 41.18 mV/h for pH 10, 7 and 4. The experimental findings suggested that both surface morphology and surface roughness affect the sensitivity performance of these devices since a smooth surface morphology and low roughness value were observed for TiO2 thin films. Besides, the basic mechanism of MOx pH sensor was presented in this study.