Highly Selective Detection of Silver in the Low ppt Range with Ion-Selective Electrodes Based on Ionophore-Doped Fluorous Membranes

Ionophore-doped sensing membranes exhibit greater selectivities and wider measuring ranges if their membrane matrixes are noncoordinating and solvate interfering ions poorly. This is particularly true for fluorous phases, which are the least polar and polarizable condensed phases known. In this work, fluorous membrane matrixes were used to prepare silver ion-selective electrodes (ISEs). Sensing membranes composed of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and one of four fluorophilic Ag⁺-selective ionophores with one or two thioether groups were investigated. All electrodes exhibited Nernstian responses to Ag⁺ in a wide range of concentrations. Their selectivities for Ag⁺ over interfering ions were found to depend on host preorganization and the length of the −(CH₂)n− spacers separating the coordinating thioether group from the strongly electron withdrawing perfluoroalkyl groups. ISEs based on the most selective of the four ionophores, that is, 1,3-bis(perfluorodecylethylthiomethyl)benzene, provided much higher selectivities for Ag⁺ over many alkaline and heavy metal ions than most Ag⁺ ISEs reported in the literature (e.g., log K_Ag,J^pot for K⁺, −11.6; Pb²⁺, −10.2; Cu²⁺, −13.0; Cd²⁺, −13.2). Moreover, the use of this ionophore with a linear perfluorooligoether as membrane matrix and solid contacts consisting of three-dimensionally ordered macroporous (3DOM) carbon resulted in a detection limit for Ag⁺ of 4.1 ppt (3.8 × 10⁻¹¹ M).