PH-SWITCHABLE, ULTRATHIN PERMSELECTIVE MEMBRANES PREPARED FROM MULTILAYER POLYMER COMPOSITES

In this report, we describe novel polymeric composite films formed from multilayers of amine-terminated dendrimers and poly(maleic anhydride)-c-(methyl vinyl ether) (Gantrez) grafted onto Au-coated Si wafers. These composite membranes exhibit fully reversible, pH-switchable permselectivity for both cationic and anionic redox-active probe molecules. The pH-dependent change in charge of the dendrimer amine groups and the carboxylic groups derived from the random coil polyanhydride chains serve as independent ion gates. These gates pass cations only at high pH, anions only at low pH, and both cations and anions at intermediate pH (Figure 1). Polymeric materials demonstrating this switchable permselective function have not previously been reported. Dispersions of cationic and anionic polyelectrolytes usually result in spontaneous flocculation and precipitation due to strong electrostatic interactions,1 but the layer-by-layer synthetic approach used here avoids this problem. Responsive or “smart” membrane systems have been studied previously. Such materials have applications in controlled release, selective filtration, electronic devices, and chemical sensors and biosensors.2-9 Anionic or cationic polymeric membranes that have permselectivity toward electroactive counterions due to the membrane charge have also been studied.10-12 For example, Anson et al. have studied membrane systems derived from polyelectrolytes such as the polysulfonated fluoropolymer Nafion, poly(acrylic acid), and poly(4-vinylpyridine) that show electrostatic binding of electroactive counterions.10 These materials are related to those described here, but they are not pH switchable between cationic and anionic forms. Moreover, previous work has typically involved much thicker cast films. Recently, Martin and co-workers developed an electronically switchable, ion-exchange membrane system consisting of cylindrical nanoscopic metal tubules whose surface potential can be electronically manipulated to either pass or exclude anions and cations.13 Our new work uses a nanocomposite polymeric ultrathin film that relies on homogeneously dispersed pH-responsive amine and carboxyl groups to induce fully reversible permselectivity into an electrode-supported membrane. The highly functionalized composite thin films were synthesized in a covalent, layer-by-layer fashion by using a general method recently developed in our laboratories.14 First, a selfassembled monolayer (SAM) of dendrimers on a Au-coated Si wafer was prepared by immersion of the substrate in a methanol solution containing a fourth generation, amine-terminated Starburst (PAMAM) dendrimer.15 The resulting dendrimer SAM, denoted as layer D0, was then allowed to react with Gantrez, to afford a first layer of Gantrez (Gz1) covalently linked to D0 Via amic acid bonds. Repetitive sequential deposition leads to additional covalently linked layers of the G4 PAMAM dendrimer and Gantrez: D1, Gz2, D2, and so on. Although the topmost layer is chemically distinct from those underlying it, we believe that the film interior is more or less homogeneous, as shown in Figure 1. These composite films consist of dendritic units having amphoteric internal and peripheral amine groups mixed with the Gantrez network containing amphoteric carboxyls. This structure constitutes the basis for the permselective, pH-responsive supramolecular gate. The extent of ionic permeability of the D2 films was investigated under different pH conditions by using anionic Fe(CN)6 and cationic Ru(NH3)6 as redox probe molecules. Figure 2 shows cyclic voltammograms of electrodes modified with the D2 films in an aqueous solution containing 5 mM of either Fe(CN)6 or Ru(NH3)6 buffered at pH 3.0, 7.0, and 11.0. The shape and magnitude of these voltammograms are significantly affected by pH. For example, the peak current density resulting from Fe(CN)6 reduction is about 750 μA/ cm2 at pH 3.0 while there is essentially no reduction current at pH 11.0. The voltammetric results clearly indicate that the D2 films are open (“On”) to the negatively charged probe, Fe(CN)6, at pH 3.0, but impermeable (“Off”) to the probe at pH 11.0. This responsive behavior is fully reversible to repetitive switching of the pH for up to at least 10 cycles. * Authors to whom correspondence should be addressed. † Phone: (409) 845-3437. Fax: (409) 845-2964. E-mail: Bergbreiter@ chemvx.tamu.edu. ‡ Phone: (409)-845-5629. Fax: (409) 845-1399. E-mail: Crooks@ chemvx.tamu.edu. (1) Kotz, J. Polymeric Materials Encyclopedia; Salamone, J. C., Ed., CRC Press: Boca Raton, FL, 1996; Vol. 8, pp 5762-5771. (2) Odashima, K.; Koga, K. ComprehensiVe Supramolecular Chemistry; Vogtle, F., Ed.; Elsevier Science Inc.: New York, 1996; Vol. 2, pp 143194. (3) Riggs, J. A.; Smith, B. D. J. Am. Chem. Soc. 1997, 119, 27652766. (4) Fyles, T. M.; James, T. D.; Kaye, K. C. J. Am. Chem. Soc. 1993, 115, 12315-12321. (5) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 27392740. (6) Kent, M. S.; Factor, M. S.; Satija, S.; Gallagher, P.; Smith, G. S. Macromolecules 1996, 29, 2843-2849. (7) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Nature 1995, 374, 240-242. (8) Thomas, J. L.; You, H.; Tirrell, D. A. J. Am. Chem. Soc. 1995, 117, 2949-2950. (9) Barbucci, R.; Casolaro, M.; Magnani, A. J. Controlled Release 1991, 17, 79-84. (10) (a) Yoshikawa, M.; Anson, F. C. J. Phys. Chem. 1996, 100, 41994204. (b) Redepenning, J.; Anson, F. C. J. Phys. Chem. 1987, 91, 45494553. (11) (a) Ewing, A. G.; Feldman, B. J.; Murray, R. W. J. Phys. Chem. 1985, 89, 1263-1269. (b) Burgmayer, P.; Murray, R. W. J. Phys. Chem. 1984, 88, 2515-2521. (12) Oh, S.-M.; Faulkner, L. R. J. Am. Chem. Soc. 1989, 111, 56135618. (13) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. (14) Liu, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. In press. (15) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R.; Crooks, R. M. In preparation. Figure 1. Schematic illustration of the pH-switchable “On/Off” function of the composite film. The polyamine dendrimer units are covalently attached to the Gantrez polymer network. At high pH the film has a net negative charge that excludes anions but passes cations; at low pH it is positively charged and excludes cations but passes anions; and at intermediate pH, it passes both cations and anions. 8720 J. Am. Chem. Soc. 1997, 119, 8720-8721