Tunnel-structured Na0.44MnO2 battery cathode as a label-free amperometric platform for non-invasive sodium ion detection in saliva

Accurate, non-invasive monitoring of sodium ions (Na + ) is essential for assessing electrolyte balance and hydration, yet current methods rely largely on invasive blood tests or potentiometric ion-selective sensors. Here, we report the first label free amperometric salivary sensor for Na + detection. The platform employs tunnel-structured Na 0.44 MnO 2 as the active cathode material, leveraging its inherent Na + intercalation capability, structural stability, and scalable synthesis. The active material Na 0.44 MnO 2 was embedded in a β-cyclodextrin/polyvinylidene fluoride (PVDF) composite and deposited onto printed silver electrodes on flexible substrates via direct ink writing. The sensor achieved low limits of detection of 1.2 mM in synthetic solutions and 1.27 mM in sodium-spiked saliva, within physiologically relevant salivary ranges. High sensitivity and selectivity toward Na + were observed, with negligible interference from K + , Ca 2+ , Mg 2+ , glucose, lactate, and uric acid. Real-time monitoring of salivary Na⁺ following salty food intake demonstrates the platform’s practical applicability. This work introduces amperometric salivary sodium sensing as a novel approach and offers a robust, flexible strategy for non-invasive electrolyte and hydration monitoring.