A TiO₂/Er₂O₃ stacked electrolyte/insulator/semiconductor film pH-sensor for the detection of urea

This paper describes the physical properties and sensing characteristics of TiO₂/Er₂O₃ stacked membranes deposited on Si substrates through reactive sputtering. We used atomic force microscopy and scanning electron microscopy to investigate the structural and morphological features of these films after they had been subjected to annealing at various temperatures. The electrolyte/insulator/semiconductor (EIS) device incorporating a TiO₂/Er₂O₃ stacked film that had been annealed at 700 °C exhibited a higher sensitivity (58.89 mV/pH in solutions from pH 2 to 12), a lower hysteresis voltage (1.9 mV in the pH loop 7 → 4 → 7 → 10 → 7), and a smaller drift rate (0.26 mV/h in the buffer solution at pH 7) than did those devices prepared at the other annealing temperatures, presumably because of its thinner interfacial SiO₂ and silicate layer at the oxide-Si interface and its higher surface roughness. Moreover, the enzymatic EIS-based urea biosensor incorporating a TiO₂/Er₂O₃ stacked membrane annealed at 700 °C allowed the potentiometric analysis of urea, at concentrations ranging from 3 to 40 mM, with a sensitivity of 9.25 mV/mM.