Fluorophilic Ionophores for Potentiometric pH Determinations with Fluorous Membranes of Exceptional Selectivity

Abstract

Ionophore-doped sensor membranes exhibit greater selectivities and wider measuring ranges when they are prepared with noncoordinating matrixes. Since fluorous phases are the least polar and least polarizable liquid phases known, a fluorous phase was used for this work as the membrane matrix for a series of ionophore-based sensors to explore the ultimate limit of selectivity. Fluorous pH electrode membranes, each comprised of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and one of four fluorophilic H(+)-selective ionophores were prepared. All the ionophores are highly fluorinated trialkylamines containing three electron withdrawing perfluoroalkyl groups shielded from the central nitrogen by alkyl spacers of varying lengths: [CF(3)(CF(2))(7)(CH(2))(3)](2)[CF(3)(CF(2))(6)CH(2)]N, [CF(3)(CF(2))(7)(CH(2))(3)](2)(CF(3)CH(2))N, [CF(3)(CF(2))(7)(CH(2))(3)](3)N, and [CF(3)(CF(2))(7)(CH(2))(5)](3)N. Their pKa values in the fluorous matrix are as high as 15.4 +/- 0.3, and the corresponding electrodes exhibit logarithmic selectivity coefficients for H(+) over K(+) as low as <-12.8. The pKa and selectivity follow the trends expected from the degree of shielding and the length of the perfluoroalkyl chains of the ionophores. These electrodes are the first fluorous ionophore-based sensors described in the literature. The selectivities of the sensor containing [CF(3)(CF(2))(7)(CH(2))(5)](3)N are not only greater than those of analogous sensors with nonfluorous membranes but were of the same magnitude as the best ionophore-based pH sensors ever reported.

Electroanalytical Sensor Technology

“…In each instance improved selectivity is reported coupled with favourable LODs. Buhlmann's group having reported the success of highly fluorinated liquid phases as sensing materials, detailing the design of numerous ion-selective sensors which incorporate fluorinated membranes [129][130][131].…”

Micro- and Nanosensors for Medical and Biological Measurement

“…There are many ionophores now available and others being developed to achieve greater selectivity and stability through the use of fluorous phases. (30) Among the various types of invasive sensor, those designed for intravascular implantation are of major importance. Figure 7 shows an amperometric microsensor fabricated within a flexible tube that is suitable for the intravascular monitoring of PO 2 .…”

Noncovalent associations in fluorous fluids

“…Remarkably, the selectivity was comparable to that of membranes doped with selective ionophores for Cs + based on crown ethers. Very recently, the same group reported the first examples of ionophore-doped ISEs based on fluorous matrices [11]. The fluorophilic amines 15-18 were employed as ionophores for H + (Scheme 6).…”