Perfluorinated ionomer membranes

Abstract

The effect of casting solvent choice and thermal curing on the permeability and diffusion coefficients of substrates In the perfluorinated sulfonic add Ionomer, Nafion, has been examined for the neutral species glucose [pH 13], hydroqd-none (QH2) [pH 7.4], and ascorbic acid [pH 1.3], as well as the cation [(trlmethylammonlo)methyl]ferrocene (FcTMA+) [pH 7], Glucose and ascorbic acid exhibit effective diffusion coefficients, aDm (a Is the partition coefficient of substrate between solution and Nafion) of 3.4 and 22 X 10"* cm2/s, respectively, In Nafion films cast from alcohol solutions and cured at room temperature. Curing films cast from alcohol at 120 °C reduces aDm for QH2 from 5.4 to 2.5 X 10~* cm2/s, and from 1.8 to 1.1 X 10"* cm2/s for FcTMA+. There Is no significant change In film thickness as a result of the curing procedure. The use of dlmethylformamlde as the coating solvent gives aD" values of 2.7 X 10~* cm2/s for QH2 when cured at room temperature and 0.7 X 10"* cm2/s when cured at 120-130 °C. These results Indicate the role both casting solvent and temperature play In affecting permeation char-acterlstlcs. In addition, elllpsometry was shown to be an effective means of measuring the film thickness, by cross correlation to surface profllometry measurements on the same coatings. The use of semipermeable polymeric overcoatings to protect electrodes in complex sample matrices, or to otherwise tailor their reactivity and selectivity characteristics, is an attractive and successful approach to electrode design.* 1 11The DuPont perfluorinated sulfonate ionomer Nafion has been used as a protective or selective coating material for a number of applications. These include coating of C electrodes used for neurotransmitter determination in cerebral fluids2·3 and for metal ion analysis in whole blood,4 enzyme electrodes used for glucose assays in whole blood,6 Cu2+ ion selective electrodes used in seawater,6 and a number of other applications.7"12 These applications often utilize Nafion recast from its dissolved form due to the ease of film preparation. It is now generally recognized that Nafion prepared in this manner has substantially different characteristics than the solid form supplied by the manufacturer.13-17 Differences in the morphology of the films, such as crystallinity and long-range order, are observed, as well as significant solubility of recast films in aqueous and organic solvents.13·14 It is also believed there are differences in the uptake of solvent and ions between the two forms.18"20 Importantly, the diffusion coefficients in re-cast films appear to be greater than in the solid form received from DuPont.16 Thermal curing of the recast films and judicious choice of casting solvent have been found to increase order in the films and decrease or eliminate their solubil-ity.131417 Yet, many of the applications of Nafion coatings are likely to rely on the less ordered structure of recast Nafion and the larger diffusion coefficients. Consequently, it is of considerable importance to understand how treatments used to affect Nation's stability will affect transport characteristics in the films. Nafion coatings were shown to be very effective as protective coatings for glucose oxidase based glucose sensors.6·21·22 This application involves transport of neutral substrates and supporting electrolyte across the Nafion coating. The study of neutral molecule transport in Nafion has been very limited, and the effect of the film morphology on their permeability is unknown. One report giving the average fluxes of the pro-tonated form of a number of organic acids in the solid form of Nafion has appeared.23 The authors indicated quantitative comparisons could not be made, but concluded that molecular size influenced transport rates in the expected manner. The transport rates of neutral species were lower than those of simple cations such as Na+ and Li+, but much higher than those of anions.23·24 Transport of 02 in the solid form has also been studied and it is quite rapid, exhibiting a diffusion coefficient approaching that of ions in solution.25·26 Transport of 02 in the recast form was found to be about 10-fold faster than in the solid form.16 It has also been shown that fer-(13) Moore, R. B.; Martin, C. R.