Spent battery-derived LiCoO₂-modified 3D-printed electrochemical sensors for cellular hydrogen peroxide monitoring

Electrochemical (bio)sensors fabricated by fused deposition modeling (FDM) represent a versatile and cost-effective platform for customizable analytical devices. However, their performance often requires surface modification strategies that improve electrocatalytic activity while maintaining sustainable fabrication approaches. Here, we introduce a waste-to-sensor strategy based on the upcycling of lithium cobalt oxide (LiCoO₂, LCO) recovered from spent lithium-ion batteries as an electroactive modifier for FDM 3D-printed carbon-based electrodes. Although spent batteries represent an increasing environmental challenge, their functional components can be repurposed as valuable secondary resources for high-value technological applications. The developed LCO-modified electrodes were investigated for the electrochemical detection of hydrogen peroxide (H₂O₂), a key reactive oxygen species (ROS) involved in cellular signaling and oxidative stress-related diseases. LCO modification was optimized and characterized by cyclic voltammetry and chronoamperometry, demonstrating enhanced electrocatalytic performance compared with unmodified electrodes. The optimized sensor exhibited a linear response toward H₂O₂ detection in DMEM over the 0-2.5 mM concentration range, with a limit of detection of 0.02 mM and a limit of quantification of 0.06 mM, demonstrating analytical performance suitable for monitoring biologically relevant concentrations. The applicability of the platform was demonstrated using C2C12 muscle and HeLa cervical cancer cell models subjected to oxidative stress conditions, with or without glutathione antioxidant treatment. Electrochemical measurements showed consistent trends with the conventional DCFH-DA fluorescence assay, confirming the ability of the sensor to monitor ROS variations associated with oxidative stress and antioxidant protection. Overall, this work demonstrates that battery-derived LCO can be effectively repurposed as a sustainable electrocatalytic modifier for 3D-printed electrochemical sensors, bridging circular economy principles with biomedical sensing technologies. The proposed platform provides a low-cost, customizable, and environmentally conscious approach for future ROS monitoring and point-of-care biosensing applications.