Fabrication of stable sweat ion sensor by coupling high-capacitance membrane with hydrophobic layer

Real-time monitoring of ion content variations in humans offers a noninvasive approach to ensuring health, made possible through wearable sensors. However, challenges such as the low capacitance of the solid-contact (SC) layer and the formation of a water layer hinder the low-concentration ion detection and stable performance of these sensors. To address these limitations, a novel design is proposed using graphene quantum dots (GQD)-doped polypyrrole (PPy) nano-dendritic membranes, combined with a hydrophobic Nafion membrane, for sweat ion sensors. The GQD-doped PPy membrane serves as the SC layer, where GQD enhance capacitance and the nano-dendritic structure improves ion-to-electron transduction efficiency. Meanwhile, the hydrophobic Nafion membrane prevents water layer formation, stabilizing the sensor’s performance. This design significantly improves both the sensitivity (from 49 mV/decade to 70 mV/decade) and stability (potential drift decreasing from 1.3 mV/h to 18.7 μV/h) of the sensor. Leveraging this configuration, Na + and K + sensors are integrated onto a flexible electrode using a dispense-coupled inkjet printing technique. This enables real-time, continuous monitoring of Na + and K + concentrations in human sweat simultaneously. The proposed sensing device demonstrates strong potential for noninvasive sweat monitoring, contributing to enhanced health management in daily life.