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.