Enhancing pH Sensing Capabilities Through Hydroxylated Surface Groups on RuOx Flexible EGFET Sensor

The Nernst equation defines the maximum sensitivity of metal oxide pH sensors, which has a theoretical value of 59.2 mV/pH. However, the phenomenon of super-Nernstian pH response of a metal oxide sensing membrane is related to the hydration of the sensing film. In this context, the purpose of this research is to compare the pH sensing capabilities of distilled water (DI)-pretreated RuOx-based extended gate field-effect transistor (EGFET) pH sensors with those of as-deposited RuOx sensing films. The surface composition of the film was explored by X-ray photoelectron spectroscopy. For the as-deposited film, the RuOx sensor exhibited a sub-Nernstian pH sensitivity of 58.91 mV/pH. The DI-pretreated RuOx sample, however, demonstrated a super-Nernstian pH response of 64.13 mV/pH, with a linearity of 0.997. It is possible to attribute the enhanced sensitivity to the hydration process, which increases the amount of effective surface sites available for pH sensing. The result is more proton (H+) adsorption and desorption sites, exceeding the Nernstian limit of 59.2 mV/pH. Additionally, the DI-pretreated RuOx EGFET sensor demonstrated several advantages, including a low hysteresis voltage (~2 mV), a minimal drift rate (0.25 mV/h), good flexibility, long-term stability (120 days), and excellent repeatability (with 97.95% retention).