Fabrication of Ion-Selective Electrodes

The sensing principle is demonstrated in Fig. S19 (Supplemental Material). First, a Na⁺-selective membrane mixture consisting of Na⁺ ionophore X (1 wt.%), Na–TFPB (0.55 wt.%), PVC (33 wt.%), and DOS (65.45 wt.%) was prepared. The mixture (200 mg) was dissolved in 1320 μL tetrahydrofuran and shaken for 30 min on a shaker. The mixture for the K⁺-selective membranes consisted of valinomycin (2 wt.%), NaTPB (0.5 wt.%), PVC (32.7 wt.%), and DOS (64.7 wt.%). The membrane-modification solution was obtained by dissolving the cocktail (200 mg) in 700 μL cyclohexanone. Similarly, the Ca²⁺-selective membrane mixture consisted of ETH 129 (0.46 wt.%), NaTPB (0.48 wt.%), PVC (33.02 wt.%), and NPOE (66.04 wt.%). Membrane-modification solutions were prepared by dissolving 200 mg of the mixture in 1 mL of tetrahydrofuran. All ion-selective solutions were stored in a sealed container at 4 °C until use.
To minimize the potential drift at the ion-selective electrode (ISE), we used poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the ion-to-electron sensor and deposited it on the working electrode. Specifically, electrodes were prepared via constant-current electrochemical polymerization using a solution of 0.01 M PEDOT and 0.1 M NaPSS. A constant current of 14 μA was applied to each electrode for 720 s. Subsequently, ion-selective membranes were prepared by dripping 10 μL Na⁺-selective membrane mixture, 4 μL K⁺-selective membrane mixture, and 5 μL Ca²⁺-selective membrane mixture onto the corresponding electrodes. The electrodes were allowed to dry overnight in an ambient environment and were then ready for testing.