A membrane-based ion-selective electrode (ISE) has been developed for the trace-level determination of Cs þ in aqueous samples. A systematic evaluation of several ionophores was performed, examining the effect of membrane plasticizer and ion exchanger on sensor selectivity. Poly(vinyl chloride) (PVC)-based ion-selective membranes of conventional composition containing calix[6]arene hexaacetic acid hexaethylester as ionophore (Cs I, 10 mmol/kg), bis(2-ethylhexyl) sebacate (DOS, 66 wt%) as plasticizer, and undecaiodinated closo-dodecacarborane (UIC, 4.1 mmol/kg) as ion exchanger exhibited superior performance with respect to selectivity over alkali and alkaline earth metal cations. The unbiased selectivity coefficients, log K ) were determined to be À 2.9, À 4.7, À 8.5, and À 8.7, respectively. DOS-plasticized PVC electrodes containing Cs I and UIC were optimized for trace-level measurements, exhibiting a lower detection limit of 7.9 Â 10 À9 M Cs þ (log a Cs,LDL ¼ À 8.1) in the presence of a 1 Â 10 À5 M Na þ background. The utility of the optimized ISEs was demonstrated by determining the concentration of Cs þ in both drinking water and natural water samples.

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Cs + -selective solvent polymeric membrane-based ion-selective electrodes (ISEs) were developed by doping ethylene glycolfunctionalized cross-linked polystyrene microspheres (P-EG) into a plasticized poly(vinyl chloride) (PVC) matrix containing sodium tetrakis-(3,5-bis(trifluoromethyl)phenyl) borate (TFPB) as the ion exchanger. A systematic study examining the effects of the membrane plasticizers bis(2-ethylhexyl) sebacate (DOS), 2-nitrophenyl octyl ether (NPOE), and 2-fluorophenyl nitrophenyl ether (FPNPE) on the potentiometric response and selectivity of the corresponding electrodes was performed. Under certain conditions, P-EG-based ion-selective electrodes (ISEs) containing TFPB and plasticized with NPOE exhibited a super-Nernstian response between 1 × 10 −3 and 1 × 10 −4 M Cs + , a response characteristic not observed in analogous membranes plasticized with either DOS or FPNPE. Additionally, the performance of P-EG-based ISEs was compared to electrodes based on two mobile ionophores, a neutral lipophilic ethylene glycol derivative (ethylene glycol monooctadecyl ether (U-EG)) and a charged metallacarborane ionophore, sodium bis(dicarbollyl)cobaltate(III) (CC). In general, P-EG-based electrodes plasticized with FPNPE yielded the best performance, with a linear range from 10 −1 -10 −5 M Cs + , a conventional lower detection limit of 8.1 × 10 −6 M Cs + , and a response slope of 57.7 mV/decade. The pH response of P-EG ISEs containing TFPB was evaluated for membranes plasticized with either NPOE or FPNPE. In both cases, the electrodes remained stable throughout the pH range 3-12, with only slight proton interference observed below pH 3.