Vertically aligned conductive metal-organic framework nanowires array composite fiber as efficient solid-contact for wearable potentiometric sweat sensing

Fiber-type solid-contact ion-selective electrodes (SC-ISE) show unique superiorities for flexible and wearable sweat sensors with excellent wearing experiences. However, it is still challenging to construct high-quality fiber-type SC-ISE with robust sensing properties and facile fabrication methods. Here, we propose a new fiber-type flexible potentiometric sensor platform based on the conductive metal-organic framework (MOF) composite carbon nanotubes fiber (CNTF). The vertically aligned Nickel triphenylene-fused metal catecholate (NiCAT) nanowires arrays are in situ grown on CNTF through a bottom-up one-step hydrothermal synthetic approach. Nafion coating is designed to improve the surface hydrophobicity for avoiding the aqueous layer formation at the solid-contact=ion-selective-membrane interface. The well-defined solid-contact transducer of Nafion/NiCAT@CNTF exhibits the greatly enlarged double-layer capacitance (34.39 mF/cm²) attributed to its ordered porous architecture and low contact impedance. Consequently, the as-prepared Na⁺-ISE displays a sensitive near-Nernstian response of 58.69 ± 0.77 mV/dec, an excellent long-term potential stability with an ultralow drift of 8.1 ± 4.1 μV/h, and a good electrode-to-electrode reproducibility E⁰ of 308.0 ± 3.1 mV. An integrated wearable band-based sweat sensor device was developed for on-body sweat sodium monitoring during exercise. This platform provides an efficient and facile approach to construct reliable fiber-type potentiometric sensors for wearable applications.