Paper-based laser-written CoO-graphene biosensor for wireless sweat uric acid detection

Uric acid is a critical metabolic biomarker for gout, kidney dysfunction, and cardiovascular disease. Persistent hyperuricemia promotes monosodium urate crystal deposition, triggering recurrent gout flares, chronic joint damage, and systemic inflammation, while early and continuous uric acid monitoring enables timely therapeutic intervention and improved disease outcomes. However, conventional blood tests and enzymatic sensors, although reliable, remain invasive, laboratory-bound, and unsuitable for continuous or point-of-care monitoring. Herein, we report a sustainable one-step strategy to fabricate a non-enzymatic uric acid sensor by direct laser writing on cobalt-treated paper with 455 nm irradiation, producing cobalt oxide-infused graphene. Unlike conventional metal-functionalized laser-induced graphene (LIG), which typically requires multi-step processing and non-biodegradable polymeric substrates, the present approach employs a biomass-derived paper substrate and simultaneously generates conductive graphene and redox-active cobalt oxide nanostructures in a single photothermal process. Furthermore, the incorporated multivalent Co²⁺/Co³⁺ redox couples act as biomimetic active sites for uric acid oxidation, enabling a flexible low-energy electron-hopping mechanism and enhanced interfacial charge transfer. The resulting porous hybrid electrode provides abundant electroactive sites for efficient sensing performance. Integrated into a flexible near-field communication (NFC) tag, the resulting platform enables wireless, battery-free uric acid monitoring in human sweat. The fabricated sensor achieved a sensitivity of 9.96 µA·μM^-1 and a detection limit of 1.08 μM for uric acid sensing. The mechanical robustness is confirmed by minimal resonance frequency variation under bending, shifting only from 13.525 MHz at 0° to 13.575 MHz at 180° (~ 0.37% relative change). This work establishes a low-cost, scalable, and environmentally sustainable route toward metal oxide-carbon hybrid biosensors, offering a promising pathway for wearable uric acid monitoring and next-generation point-of-care diagnostics.