Detection of pH and Enzyme-Free H₂O₂ Sensing Mechanism by Using GdO_x Membrane in Electrolyte-Insulator-Semiconductor Structure

A 15-nm-thick GdO_x membrane in an electrolyte-insulator-semiconductor (EIS) structure shows a higher pH sensitivity of 54.2 mV/pH and enzyme-free hydrogen peroxide (H₂O₂) detection than those of the bare SiO₂ and 3-nm-thick GdO_x membranes for the first time. Polycrystalline grain and higher Gd content of the thicker GdO_x films are confirmed by transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS), respectively. In a thicker GdO_x membrane, polycrystalline grain has lower energy gap and Gd²⁺ oxidation states lead to change Gd³⁺ states in the presence of H₂O₂, which are confirmed by electron energy loss spectroscopy (EELS). The oxidation/reduction (redox) properties of thicker GdO_x membrane with higher Gd content are responsible for detecting H₂O₂ whereas both bare SiO₂ and thinner GdO_x membranes do not show sensing. A low detection limit of 1 μM is obtained due to strong catalytic activity of Gd. The reference voltage shift increases with increase of the H₂O₂ concentration from 1 to 200 μM owing to more generation of Gd³⁺ ions, and the H₂O₂ sensing mechanism has been explained as well.