High performance nonenzymatic electrochemical sensors via thermally grown Cu native oxides (CuNOx) towards sweat glucose monitoring

Diabetes, which is the seventh leading cause of death globally, necessitates real-time blood glucose monitoring, a process that is often invasive. A promising alternative is sweat glucose monitoring, which typically uses transition metals and their oxide nanomaterials as sensors. Despite their excellent surface-to-volume ratio, these materials have some drawbacks, including poor conductivity, structural collapse, and aggregation. As a result, selecting highly electroconductive materials and optimizing their nanostructures is critical. In this work, we developed a high-performance, low-cost, nonenzymatic sensor for sweat glucose detection, using the thermally grown native oxide of copper (CuNOx). By heating Cu foil at 160, 250, and 280 °C, we grew a native oxide layer of approximately 140 nm cupric oxide (CuO), which is excellent for glucose electrocatalysis. Using cyclic voltammetry, we found that our CuNOx sensors prepared at 280 °C exhibited a sensitivity of 1795 μA mM^-1 cm^-2, a linear range up to the desired limit of 1.00 mM for sweat glucose with excellent linearity (R² = 0.9844), and a lower limit of detection of 135.39 μM. For glucose sensing, the redox couple Cu(II)/Cu(III) oxidizes glucose to gluconolactone and subsequently to gluconic acid, producing an oxidation current in an alkaline environment. Our sensors showed excellent repeatability and stability (remaining stable for over a year) with a relative standard deviation (RSD) of 2.48% and 4.17%, respectively, for 1 mM glucose. The selectivity, when tested with common interferants found in human sweat and blood, showed an RSD of 4.32%. We hope that the electrocatalytic efficacy of the thermally grown CuNOx sensors for glucose sensing can introduce new avenues in the fabrication of sweat glucose sensors.