Optimized detection of calcium ion in serum using constant potential coulometry with metastable liquid–liquid contact doping enhanced PEDOT: PSS ink

Highly stable calcium ion selective electrodes (Ca²⁺-ISEs) were developed by drop-casting a layer of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) as an ion-to-electron transfer layer onto Au electrode. The conductive PEDOT: PSS ink was prepared using a metastable liquid–liquid contact (MLLC) doping method, which induced phase separation, removed excess PSS, and significantly enhanced charge transfer kinetics and conductivity. The resulting Ca²⁺-ISEs exhibited excellent electrochemical performance. Potentiometric studies revealed a significant sensitivity of 33.1 ± 0.98 mV/decade (N = 3) with a high potential stability of 3.16 ± 2.53 μV/h. Importantly, Ca²⁺-ISEs combined with the constant potential coulometry method, the lower detection limit was optimized to 8.527 × 10⁻⁸ M (LOD = 3σ/s, N = 3). The performance of the Ca²⁺-ISE system was evaluated in inactivated fetal bovine serum using constant potential coulometry, demonstrating the highest measurement accuracy compared to potentiometric and chronoamperometric. The enhanced PEDOT: PSS-MLLC based Ca²⁺-ISEs combined with the constant potential coulometry method developed in this research demonstrate considerable potential for clinical applications in blood ion analysis.