Journal of Membrane Science 205 (2002) 3–10 Effects of mixture casting Nafion ® with quaternary ammonium bromide salts on the ion-exchange capacity and mass transport in the membranes Matthew J. Schrenk, Robert E. Villigram, Nicholas J. Torrence, Sam J. Brancato, Shelley D. Minteer ∗ Department of Chemistry, Saint Louis University, St. Louis, MO 63103, USA Received 4 September 2001; received in revised form 28 November 2001; accepted 15 January 2002 Abstract Nafion ® has been used to modify electrodes for a wide variety of electrochemical applications. This research stud- ies how the physical properties of Nafion ® are affected by mixture casting Nafion ® with quaternary ammonium bro- mide salts of differing sizes and hydrophobicities. The quaternary ammonium bromides incorporated into Nafion ® have shown up to a (6.73 ± 1.49)-fold increase in the flux of tris(2,2 ′ -bipyridyl) ruthenium(II) (Ru(bpy) 3 2+ ) through a mix- ture cast (C 2 H 5 ) 4 NBr/Nafion ® membrane. The quaternary ammonium bromide incorporation can increase the pore size of Nafion ® to allow for the electrochemical sensing of large redox moieties, such as erythromycin, which cannot be sensed at Nafion ® -modified electrodes. One of the most startling effects of incorporating the quaternary ammonium bromide salts is the loss of selectivity upon incorporation of all tetraalkylammonium bromides. Along with this loss of selectivity, the number of available exchange sites to proton of the Nafion ® membranes decreases with the increase in size and hydrophobicity of the quaternary ammonium bromides. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nafion ® ; Quaternary ammonium salts; Electrochemistry; Ion-exchange membranes; Membrane electrodes 1. Introduction In the last two decades, researchers have made ex- tensive use of Nafion ® -modified electrodes for a wide variety of electrochemical applications from sensors [1–11] to fuel cells [12]. Nafion ® is a perfluorinated cation-exchange polymer that is well known for its se- lectivity. Due to the high ionic charge density at the orifices of the small channels [13], Nafion ® will ex- clude anions from the polymer membrane. Nafion ® ’s ∗ Corresponding author. Tel.: +1-314-977-3624; fax: 1-314-977-2521. E-mail address: minteers@slu.edu (S.D. Minteer). selectivity properties have been widely exploited in sensor development. In recent years, scientists have begun to use Nafion ® to immobilize redox mediators [1–8], an- tibodies [9] and enzymes [10,11] in membranes for sensor development. This immobilization can occur in two ways: the mediator can be ion-exchanged into a pre-cast Nafion ® membrane (adsorbed) [5–8] or co-cast with suspended Nafion ® onto the electrode surface and dried (mixture cast) [1–4,9–11]. It has previously been assumed that mixture casting species with Nafion ® has no effect on the bulk structure of the polymer, the diffusional properties of the polymer or the selectivity of the polymer. 0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0376-7388(02)00031-5