Fabrication of syringes electrodes The step-by-step procedure is schematically presented in Fig. . First, pistons were removed from 2 mL plastic syringes and the syringe bodies were hanged in an upright position. 50 μL of membrane solution was deposited into the tip of the syringe using an automatic pipette. In case of bubble formation the syringe was discarded and another body was filled with the membrane. Electrodes were dried in room temperature for at least 3 hours. Later they were filled with 0.1 M KCl as an internal solution, and the body was closed with a rubber plug equipped with Ag/AgCl wire. Silver wire was coated with silver chloride layer in saturated solution of FeCl3 and later cleaned through sonication in DI water. The electrodes were conditioned till the next day. Potentiometric measurements All measurements were carried out using PalmSens 4 potentiostat in a standard potentiometric system containing the respective ion selective electrode as a working electrode and Ag/AgCl (3 M KCl) as a reference electrode (IJ Cambria Scientific Ltd.). A calibration curve from which we have determined the linear range and sensitivity was prepared for each tested electrode. The selectivity coefficient was determined by Separated Solution Method (SSM) using 0.1 M aqueous solutions of corresponding salts -LiCl, NaCl, KCl, NH4Cl, MgCl2, CaCl2.