Label-free detection of uric acid using a disposable poly-N- isopropylacrylamide as an encapsulating enzyme material based on high-κ Eu₂Ti₂O₇ electrolyte-insulator-semiconductor devices

In this paper, we proposed a disposable poly-N-isopropylacrylamide (PNIPAAm) as an encapsulating enzyme material based an electrolyte-insulator- semiconductor (EIS) device for uric acid biosensing, prepared from high-κ Eu₂Ti₂O₇ sensing membranes. We used X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy to examine the structural and morphological features of these membranes annealed at different annealing temperatures. The Eu₂Ti₂O₇ EIS device annealed at 900 °C exhibited a higher sensitivity (59.03 mV/pH), a lower drift rate (0.586 mV/h), and a smaller hysteresis voltage (2.8 mV) compared to those of the other annealing temperatures, presumably because of its lower number of crystal defects and its higher surface roughness. Furthermore, for a high-κ Eu₂Ti₂O₇ EIS biosensor, a disposable PNIPAAm was used as an encapsulating enzyme material, making the loading and removal of a biological recognition element on a biosensor easily. PNIPAAm film has reversible solubility to become a solution-gelatin state change in response to thermal condition (∼32 °C). Below the phase transition temperature, such a PNIPAAm film exhibits a higher solubility in water, whereas this film becomes a gel state if the temperature increases. The sensitivity of an uricase-immobilized EIS biosensor was 17.6 mV/mM for uric acid concentrations ranging from 2 to 8 mM.