Fluorous Polymeric Membranes for Ionophore-Based Ion-Selective Potentiometry: How Inert Is Teflon AF?

Abstract

Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (87% dioxole monomer content; known as Teflon AF2400) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+-ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability and maintained their selectivity over at least four weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie–Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 103.5, 101.8, 106.8 and 104.4 M−1, respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride (COF) groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(=O)F groups can be eliminated chemically, or they can be used to readily introduce new functionalities.

Electrochemical Mechanism of Ferrocene-Based Redox Molecules in Thin Film Membrane Electrodes

“…However, membranes that are much thinner than the diffusion length of Fc molecules are highly desirable in electrochemical applications due to the rapid surface controlled kinetics/fast response attributes of this thin film electrochemistry. Here, it may be possible to utilize alternative plasticizers in thin membrane systems that do not permit the permeability of aqueous electrolytes embodying the undesirable Cl -species and/or provide a high concentration of innocuous anion to outcompete Cl -in forming an irreversible ion association complex, with hydrophobic ILs appearing to be ideal candidates, although the class of fluorous membranes proposed by Buhlmann and co-workers [37][38][39] that suppress water non-droplet formation may be equally relevant.…”

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].…”

Application of computational fluid dynamics in tissue engineering

“…(13) where D N is the diffusivity of the solute in the solvent. Although MackieeMeares model has shown to agree for a variety of conditions, it underestimates the diffusivities with higher solute concentrations (based on water as the solvent) (71). MackieeMeares model is based on the lattice model for liquids and assumes that the polymer fibers are of the same size as the solutes and the polymer network only blocks the pathway of solutes without affecting their mobility.…”