Because of their low polarity and polarizability, fluorous sensing membranes are both hydrophobic and lipophobic and exhibit very high ion selectivities. Here, we report on a new fluorous-membrane ion-selective electrode (ISE) with a wide sensing range centered around physiologically relevant pH values. The fluorophilic tris[perfluoro(octyl)butyl]amine (N[(CH2)4Rf8]3) was synthesized and tested as a new H+ ionophore using a redesigned electrode body that provides excellent mechanical sealing and much improved measurement reliability. In a challenging 1 M KCl background, these fluorous-phase ISEs exhibit a sensing range from pH 2.2 to 11.2, which is one of the widest working ranges reported to date for ionophore-based H+ ISEs. High selectivities against common interfering ions such as K+, Na+, and Ca2+ were determined (selectivity coefficients: logKH, Kpot = - 11.6; logKH, Napot = - 12.4; logKH, Capot < - 10.2). The use of the N[(CH2)4Rf8]3 ionophore with its -(CH2)4- spacers separating the amino group from the strongly electron-withdrawing perfluorooctyl groups improved the potentiometric selectivity as compared to the less basic tris[perfluoro(octyl)propyl]amine ionophore. The use of N[(CH2)4Rf8]3 also made the ISE less prone to counter anion failure (i.e., Donnan failure) at low pH than the use of tris[perfluoro(octyl)pentyl]amine with its longer -(CH2)5- spacers, which more effectively shield the amino center from the perfluorooctyl groups. In addition, we exposed both conventional plasticized PVC-phase pH ISEs and fluorous-phase pH ISEs to 10% serum for 5 days. Results show that the PVC-phase ISEs lost selectivity while their fluorous-phase counterparts did not.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (CHE-1710024). We thank Peter Ness from the College of Science and Engineering Machine Shop, University of Minnesota, for help with the design and fabrication of the electrode bodies and Dr. Aniko Nemes, Department of Chemistry, Eötvös Loránd University, for helpful discussions on the topic of organic synthesis. M.P.S.M. was supported by a University of Minnesota Doctoral Dissertation Fellowship.
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