A highly stable and flexible ion-selective patch sensor for real-time sweat Na+ and K+ monitoring

Abstract

Wearable electrochemical biosensors based on solid-contact ion-selective electrodes (SC-ISEs) have emerged as a promising platform for non-invasive, real-time monitoring of sweat electrolytes. However, conventional ion-selective biosensors often suffer from potential drift and long-term instability due to the formation of undesired aqueous layers and interference from other ions. To overcome these challenges, we present a flexible and highly stable SC-ISE patch sensor for simultaneous detection of Na⁺ and K⁺ ions in sweat. The sensor employs a laser-induced graphene (LIG) electrode patterned directly onto a Ti₃C₂T_x -MXene/PVDF nanofiber mat, which was fabricated using electrospinning followed by CO₂ laser carbonization. The MPNFs/LIG@TiO₂ hybrid structure exhibits excellent electrical conductivity, high electrochemical surface area, and enhanced hydrophobicity, all contributing to reduced potential drift and improved signal stability.

Ion-selective membranes (ISMs) based on a PVC-SEBS blend were drop-cast onto the LIG electrode to achieve selective ion recognition, while a double-sided PET tape substrate ensured mechanical flexibility and skin conformity. The addition of TiO₂ nanoparticles during the thermal laser oxidation process induced π-π interactions within the composite, resulting in a robust 3D porous electrode architecture with enhanced ion transport and interfacial contact. The fabricated Na⁺ and K⁺ sensors demonstrated near-Nernstian sensitivities of 48.8 mV/decade and 50.5 mV/decade, respectively, within physiologically relevant sweat concentration ranges. Additionally, the sensors showed excellent long-term stability with minimal potential drift (0.04 mV/h for Na⁺ and 0.08 mV/h for K⁺), along with rapid response and high accuracy. The use of scalable, low-cost laser engraving and solution casting techniques enables reliable batch fabrication, making the proposed sensor patch a strong candidate for integration into wearable platforms aimed at continuous electrolyte monitoring during physical activity.