Why does pulsed laser-deposited amorphous NiOx serve as an excellent electrode material for revolutionizing glucose detection?

Abstract

Amorphous materials offer distinct properties, such as specific electronic states, a multitude of surface dangling bonds, unsaturated coordination, and enhanced charge transfer, that may improve their electrochemical performance compared with their nano and bulk counterparts. Given their intriguing characteristics, herein, we report a revolutionary electrochemical response of amorphous NiO_x to glucose, which is confirmed through cyclic voltammetry measurements in an alkaline medium. To investigate this, amorphous NiO_x was grown on a commercially available screen-printed electrode using pulsed laser deposition. The amorphous nature of NiO_x was confirmed using transmission electron microscopy, while nickel with multi valances was confirmed using an X-ray photoelectron microscopy. As the glucose concentration varied in the alkaline medium, a negligible change in current values is observed, whereas a systematic change is observed in oxidation potential E_p in the CV plot. The diffusion coefficient estimated by fitting the experimental data to the Randles–Sevcik expression is observed to be one order higher than that of the reported NiO_x electrodes. The negligible change in current values and the high diffusion coefficient observed herein indicate a charge transfer process that might occur on the electrode surface. However, changes observed in the oxidation potential indicate a surface concentration of active species/phases (viz. α-Ni(OH)₂, β-Ni(OH)₂, β-NiOOH and γ-NiOOH) that drives the electrochemical reactions responsible for glucose oxidation. Various mechanisms proposed herein based on the theoretical models suggest that the surface modifications of the electrode material are a first-order process that gradually evolve due to the formation of multiple phases, i.e., the formation of various nickel hydroxide species.