Detection of pH and Enzyme-Free H ₂O ₂ Sensing Mechanism by Using GdO _x Membrane in Electrolyte-Insulator-Semiconductor Structure

Abstract

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 ₂O 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 ₂O 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 ₂O 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 ₂O ₂ concentration from 1 to 200 μM owing to more generation of Gd ³⁺ ions, and the H ₂O ₂ sensing mechanism has been explained as well.