A high-performance, all-solid-state Na+ selective sensor printed with eco-friendly conductive ink

In recent years, the integration of flexible printed electronics and electrochemical sensors has emerged as a new approach for developing wearable biochemical detecting devices. Among the materials utilized in flexible printed electronics, carbon-based conductive inks are considered to be crucial. In this study, we propose a cost-effective, highly conductive, and environmentally friendly ink formulation utilizing graphite and carbon black (CB) as conductive fillers, resulting in a very low sheet resistance of 15.99 Ω sq−1 (conductivity of 2.5 × 103 S m−1) and a printed film thickness of 25 μm. The unique “sandwich” structure of the working electrode (WE) printed with this ink enhances its electrical conductivity, leading to high sensitivity, selectivity, and stability, with almost no water film generated between the WE and the ion-selective membrane (ISM), strong ion selectivity, long-term stability, and anti-interference. The lower detection limit of the sensor for Na+ is 0.16 mM with a slope of 75.72 mV per decade. To validate the sensor's usability, we analyzed three sweat samples collected during physical activity, with Na+ concentrations within the typical range for human sweat (51 ± 4 mM, 39 ± 5 mM, and 46 ± 2 mM).