Recent Research Developments

Index of Recent Research News

September 28th, 2005

    Ionophore-Based Chemical Sensors: Testing the Limits of Fluoroelectrochemistry
    Chemical sensors based on receptor-doped polymeric membranes have been developed for over 60 analytes. They are routinely used in clinical chemistry for well over a billion measurements per year. Unfortunately, biofouling can seriously limit the lifetime of these sensors. The Buhlmann group is exploring the use of fluorous sensing membranes with a view to reduce biofouling. Earlier this year, graduate student Paul Boswell and Phil Buhlmann demonstrated the first use of fluorous membranes for potentiometric sensors, exhibiting remarkable potentiometric selectivity. Notably, the fluorous character of these membranes explains the extension of the selectivity range of these sensors by eight orders of magnitude as compared to conventional membranes (J. Am. Chem. Soc. 2005, 127, 8958-8959).

    In an extension of this work, the Buhlmann group and their collaborators Drs. Jozsef Rabai (Eotvos University, Budapest) and Elizabeth Amin (University of Minnesota) have now performed the first quantitative characterization of the coordinative properties of a perfluoroether and a perfluorotrialkylamine. In spite of the widespread use of perfluorinated solvents with amino and ether groups in a variety of application fields, the coordinative properties of these compounds are poorly known. It is generally assumed that the electron withdrawing perfluorinated moieties render these functional groups rather inert, but little is known quantitatively about the extent of their inertness. The results from the Buhlmann group describe the interactions between inorganic monocations and perfluorotripentylamine and 2H-perfluoro-5,8,11-trimethyl-3,6,9,12-tetraoxapentadecane. The amine does not undergo measurable association with any ion tested, and its formal pKa is shown to be smaller than –0.5. This is consistent with the nearly planar structure of the amine at its nitrogen center, as obtained with density functional theory calculations.

    The tetraether interacts very weakly with Na+ and Li+. Assuming 1:1 stoichiometry, formal association constants were determined to be 2.6 and 2.0 M-1, respectively. This disproves an earlier proposition that the Lewis base character in such compounds may be non-existent. Due to the extremely low polarity of fluorous solvents and the resulting high extent of ion pair formation, a fluorophilic electrolyte salt with perfluoroalkyl substituents on both the cation and the anion had to be developed for these experiments. In its pure form, this first fluorophilic electrolyte salt is an ionic liquid with a glass transition temperature, Tg, of -18.5 °C. Interestingly, the molar conductivity of solutions of this salt increases very steeply in the high concentration range, making it a particularly effective electrolyte salt.

    Boswell, P. G.; Lugert, E. C.; Rábai, J.; Amin, E. A., Buhlmann, P., J. Am. Chem. Soc., submitted.
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