Oxygen-vacancy-rich NiCo₂O_x nanoflowers for ascorbic acid detection in tear fluid

Ascorbic acid (AA) is a key antioxidant in tear fluid and is closely related to ocular redox homeostasis. Sensitive detection of tear AA is therefore important for evaluating the ocular redox state. Here, we developed oxygen-vacancy-rich NiCo₂O_x nanoflowers featuring a three-dimensional porous architecture assembled from interconnected nanowires for efficient electrocatalytic oxidation of AA. This hierarchical NiCo₂O_x nanostructure provide abundant accessible active sites for AA diffusion, while oxygen vacancies induced local electronic redistribution. Thus, the NiCo₂O_x nanoflowers exhibit excellent electrocatalytic activity toward AA oxidation, achieving a sensitivity of 2.75 μA mM^-1 mm^-2 over a linear range of 0-2 mM, together with good selectivity and electrochemical stability. Density functional theory calculations further revealed that oxygen vacancies enhance AA adsorption and reduce the energetic barrier for AA activation, thereby promoting the oxidation of AA to dehydroascorbic acid. Furthermore, the NiCo₂O_x -based sensing interface enabled AA detection in simulated tear fluid and could be integrated with K⁺ and Na⁺ sensing units for simultaneous tear biomarker analysis. This hierarchical defect-engineered NiCo₂O_x nanostructure provides an effective strategy for enhancing the sensitivity of AA sensing.