Ion-Sensitive Stainless Steel Vessel for All-Solid- State pH Sensing System Incorporating pH-Insensitive Diamond Solution Gate Field-Effect Transistors

An all-solid-state pH sensing system utilizing a stainless steel vessel (SUS304) and a pH-insensitive diamond solution-gate field-effect transistor (SGFET) is presented here to explain the interaction between stainless steel vessel and field-effect transistor (FET) pH sensors for the first time. A pH-sensitive ion-sensitive field-effect transistor (ISFET) was first used to show the change of the sensing behavior from 47.78 to −4.73 mV/pH when using an Ag/AgCl electrode and stainless steel vessel as the gate. This intriguing sensing behavior was investigated by developing large- and small-signal equivalent circuit models in a transistor circuit for both the Ag/AgCl and the stainless steel vessel gate. The result shows that the targeted ion change
$\Delta {Q}_{h}$
corresponding to the pH sensitivity has been offset, which explains the phenomenon observed when using the pH-sensitive ISFET with the stainless steel vessel. We then hypothesize that combining a pH-insensitive device with the stainless steel vessel gate should show a pH sensitivity close to the Nernst response. To validate this, a pH-insensitive diamond SGFET was then fabricated and combined with the stainless steel vessel for pH measurements. The system demonstrates a high pH sensitivity at −54.18 mV/pH across a wide range of pH solutions (pH 2–12) and remains stable in elevated temperatures when measured with a potentiostat setup at 80 °C. The results also suggest that this all-solid-state sensing system has great potential to be used in the food and beverage industry where stainless steel is widely employed due to its excellent corrosion resistance, low cost, and high sensing capabilities.