Laser- Written Freestanding Electrodes Based on Laser-Reduced Graphene Oxide/Carboxymethylcellulose for Non-Invasive On-Food Paraquat Detection.

The advancement of sustainable precision agriculture necessitates high-performance, accessible sensing platforms capable of in-field operation. Herein, a scalable, solvent-free Laser Direct Writing (LDW) strategy is reported for fabricating freestanding, seamlessly integrated electrochemical devices based on Laser-reduced Graphene Oxide/Sodium Carboxymethylcellulose (LrGO/CMC) biocomposites. Uniquely, the CMC acts not merely as a matrix but also chemically stabilizes GO sheets via hydrogen bonding, preventing restacking and enabling photothermal processing to selectively convert the film surface into an expanded, highly conductive LrGO architecture. Meanwhile, the unmodified bulk preserves the composite's mechanical integrity. This continuous architecture, in which the active electrode is chemically derived from the substrate itself, effectively eliminates interfacial delamination, thereby enabling flexibility under dynamic stress. Analytically, the device leverages the high electroactive surface area of the rough LrGO network. Using optimized Differential Pulse Voltammetry (DPV) coupled with a rapid electrochemical conditioning step, the sensor demonstrates sensitivity to the Paraquat herbicide (limit of detection of 5.7 ± 1.2 μmol L-1 and a limit of quantification of 18.7 ± 4.1 μmol L-1) and is suitable for field monitoring, while showing selectivity against complex matrix interferents. As a ″lab-on-a-fruit″ proof-of-concept, the flexible device was successfully applied to the rapid (<1 min), noninvasive detection of pesticide residues directly on orange peels using a hydrogel interface. This work establishes LrGO/CMC composites as a versatile and resource-efficient platform for next-generation wearable electronics.