Structural properties and sensing performance of high-k Nd₂TiO₅ thin layer-based electrolyte-insulator-semiconductor for pH detection and urea biosensing

For high sensitive pH sensing, an electrolyte-insulator-semiconductor (EIS) device with Nd₂TiO₅ thin layers fabricated on Si substrates by means of reactive sputtering and the subsequent post-deposition annealing (PDA) treatment was proposed. In this work, the effect of thermal annealing (600, 700, 800, and 900 °C) on the structural characteristics of Nd₂TiO₅ thin layer was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy. The observed structural properties were then correlated with the resulting pH sensing performances. For enzymatic field-effect-transistors-based urea biosensing, a hybrid configuration of the proposed Nd₂TiO₅ thin layer with urease-immobilized alginate film attached was established. Within the experimental conditions investigated, the EIS device with the Nd₂TiO₅ thin layer annealed at 800 °C exhibited a higher pH detection sensitivity of 57.2 mV/pH, a lower hysteresis voltage of 2.33 mV, and a lower drift rate of 1.80 mV/h compared to those at other annealing temperatures. These results are attributed to the formation of a thinner low-k interfacial layer at the oxide/Si interface and the higher surface roughness occurred at this annealing temperature. Furthermore, the presented urea biosensor was also proved to be able to detect urea with good linearity (R² = 0.99) and reasonable sensitivity of 9.52 mV/mM in the urea concentration range of 3-40 mM. As a whole, the present work has provided some fundamental data for the use of Nd₂TiO₅ thin layer for EIS-based pH detection and the extended application for biosensing.