Inkjet printed Ta2O5 on a flexible substrate for capacitive pH sensing at high ionic strength

Many pH sensors on the market today have specific limitations, such as the large and fragile construction of glass electrodes, or the complicated manufacturing processes of silicon-based devices including ion-sensitive field-effect transistors (ISFETs). Furthermore, most pH sensors require a stable reference electrode, which is difficult to miniaturize. In applications where the solution properties are largely understood, the use of an impedimetric sensor without a reference electrode may be sufficient, thereby simplifying the manufacturing of such sensors. In this work, inkjet printed and flash lamp annealed Ta2O5 on interdigitated electrodes, with an approximate sensor area of 4 mm × 4 mm, is investigated as a capacitive pH sensing layer in 0.5 M alkali chloride buffer solutions. By using the equivalent circuit of the insulator-electrolyte interface, the double layer capacitance is shown to decrease with an increase in pH within the range of pH 2 to pH 9, and agrees with prior results for anodic Ta2O5. When using the device as a sensor in both 0.5 M NaCl and 0.5 M LiCl aqueous solutions, the change in capacitance at 100 Hz is approximately - 110 nF/pH. Apart from pH sensing, these results may also prove informative in other applications, such as electrolytic capacitors, electrophysiology, and battery anodes in aqueous electrolyte. Moreover, the use of flexible, gold metallized polyethylene terephthalate (PET) foils as the sensor substrate potentially allows for large-scale production via roll-to-roll manufacturing, and further permits for use of the sensor in flexible applications such as goods packaging.