A Multimodal Sensing Device for Simultaneous Measurement of Dissolved Oxygen and Hydrogen Ions by Monolithic...

We examined the possibility of measuring dissolved oxygen by using a potentiometric solid-state semiconductor sensor. Thin films of tin (IV) oxide (SnO₂) are widely used in oxygen gas sensors. However, their ability to detect dissolved oxygen (DO) in solutions is still unknown. In this paper, we present a method for investigating the dissolved oxygen-sensing properties of SnO₂ thin films in solutions by fabricating a SnO₂-gate field-effect transistor (FET). A similarly structured hydrogen ion-sensitive silicon nitride (Si₃N₄)-gate FET was fabricated using the same method. The transfer characteristics and sensitivities were experimentally obtained and compared. The transfer characteristics of the FET show a shift in threshold voltage in response to a decrease in DO concentration. The SnO₂-gate FET exhibited a sensitivity of 4 mV/ppm, whereas the Si₃N₄-gate FET showed no response to DO. Although the SnO₂-gate FET responds to pH changes in the solution, this sensitivity issue can be eliminated by using a Si₃N₄-gate FET, which is capable of selectively sensing hydrogen ions without DO sensitivity. The experimental results indicate the promising properties of SnO₂ thin films for multimodal sensing applications.