Enveloped ion-selective membranes for robust multi-ion sensing

Abstract

Accurate monitoring of physiological ions in biological fluids is crucial for health assessment; however, traditional devices based on ion-selective membranes (ISMs) face several key limitations, including poor control over ISM thickness and inadequate sealing of the membrane–substrate interface, which results in insufficient long-term stability and selectivity. Herein, we report enveloped ISMs with optimized thickness for robust multi-ion sensing through integration with a vertical organic electrochemical transistor (vOECT) array. By utilizing a detachable polydimethylsiloxane (PDMS) mold, quantitative ISM thickness regulation from 1.5 to 3.9 µm is achieved, where a ∼2.6 µm ISM demonstrates balanced ionic sensitivity and selectivity. Therefore, vOECTs based on such enveloped ISMs exhibit high sensitivities [944.9 µA dec⁻¹ (Na⁺), 763.7 µA dec⁻¹ (K⁺), and 659.4 µA dec⁻¹ (Ca²⁺)], strong selectivity , and good reversibility. Notably, such devices outperform those with drop-casted ISMs in terms of stability, presenting 72.3–98.1% current retention after 18 scanning cycles (vs. only 7.3–28.9% current retention of those with drop-casted ISMs after 6 scanning cycles) and <3% sensitivity loss after 1 month storage. Moreover, a miniaturized multi-ion sensing system is constructed and presents accurate monitoring capability of Na⁺/K⁺/Ca²⁺ concentrations in mixed ionic solutions with coefficients of determination (R²) ≥ 0.98 by further incorporating a least-squares fitting framework. This work provides a scalable strategy for synergistic optimization of ion sensing performance, advancing ISM-enveloped OECTs toward clinical point-of-care testing and implantable multi-ion monitoring.