Detection of pH and Enzyme-Free H2O2 Sensing Mechanism by Using GdOx 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 (H 2 O 2 ) detection than those of the bare SiO 2 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 Gd 2+ oxidation states lead to change Gd 3+ states in the presence of H 2 O 2 , 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 H 2 O 2 whereas both bare SiO 2 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 H 2 O 2 concentration from 1 to 200 μM owing to more generation of Gd 3+ ions, and the H 2 O 2 sensing mechanism has been explained as well.