Development of reusable screen-printed ion-selective electrodes with calibration-free operation

The reusable solid-state calibration-free screen-printed ion-selective electrodes (SP-ISEs) have been developed with a carbon paste and PEDOT: PEDOT-S (S=sulfonated) back contact. The calibration-free Na⁺ and Ca²⁺ SP-ISEs showed a near Nernstian response of 52.1 ± 2.0 mV/log [a_Na+] and 27.3 ± 0.8 mV/log [a_Ca2+] respectively while holding a stable intercept for multiple calibrations across batches for 12 h and over 7 days. The unique carbon paste and PEDOT: PEDOT-S copolymer combined back contact on carbon substrate allowed us to fix the intercept with a constant low current treatment across different ionophores for both monovalent Na⁺ and divalent Ca²⁺ cations. This treatment imparts exceptional potential stability, leading to highly overlapping calibration curves for individual ISEs across multiple measurement cycles, demonstrating their reusability in a calibration-free mode. These SP-ISEs maintained their respective selectivity against major interfering ions and were able to operate reproducibly despite prolonged dry storage without any curing solution, as tested for a total of 28 days. The practical utility of these sensors was validated by analyzing environmental samples, determining Na⁺ and Ca²⁺ concentrations in tap water (934 ± 37 μM and 194 ± 11 μM respectively) and hydroponics solutions (757 ± 43 μM and 7047 ± 565 μM respectively). This integration of high-performance characteristics on an economical carbon-based substrate creates sensors that are simultaneously affordable enough to be disposable yet stable enough to be reusable, effectively bridging the critical gap between sophisticated laboratory systems and scalable, field-deployable solid ISEs for environmental monitoring applications.