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Langmuir 2004, 20, 5418-5424
Pore Assembled Multilayers of Charged Polypeptides in Microporous Membranes for Ion Separation Aaron M. Hollman and D. Bhattacharyya* Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046 Received February 4, 2004. In Final Form: March 15, 2004 In this study, highly permeable ion-selective membranes are prepared via immobilization of polyelectrolyte multilayer networks within the inner pore structure of a microporous (pore size ) 0.2 µm) support. Electrostatic layer-by-layer assembly is achieved through alternate adsorption of cationic and anionic polyelectrolytes under convective flow conditions. To initiate pore assembly, the first layer consists of covalently bound charged polypeptides (poly(L-glutamic acid) (PLGA) or poly(L-lysine) (PLL)) establishing a charged support for subsequent adsorption. Nonstoichiometric immobilization of charged multilayers within a confined pore geometry leads to an enhanced volume density of ionizable groups in the membrane phase. This overall increase in the effective charge density allows for Donnan exclusion of ionic species (especially divalent co-ions) using microporous materials characterized by permeability values that exceed conventional membrane processes. Multilayer assemblies are fabricated using both PLGA/PLL and synthetic polyelectrolytes (poly(styrenesulfonate)/poly(allylamine)) in an attempt to compare the level of adsorption and separation properties of the resulting materials. The role of salt concentration in the carrier solvent on overall polyelectrolyte adsorption was examined to determine its effect on both solute (Cl-, SO42-, As(V)) and water transport. Constriction of the pore size induced by multilayer propagation was monitored through permeability measurements and dextran rejection studies at each stage of the deposition process.
1. Introduction Electrostatic assembly based on the layer-by-layer (LBL) deposition of polyelectrolytes, as first proposed by Iler1 and extended by Decher,2 provides a simple, versatile, environmentally efficient method of preparing thin films on the nanometer scale.3 The LBL approach is based upon the nonstoichiometric adsorption of cationic and anionic polyelectrolytes in alternating sequence. Multiple electrostatic interactions between the adsorbing polymer and the preceding oppositely charged layer result in normally excellent cohesion, as well as adhesion to the substrate. Due to an excess of charge at each adsorption step, enzymes,4-7 nanoparticles,8,9 dyes,10,11 or proteins12 are easily incorporated within assemblies covering a wide range of length scales.13 Most studies up to this point involve LBL film growth on the surface of a charged solid substrate, glass beads,4 or onto a porous support.14,15 In this paper, polyelectrolyte multilayer assemblies are * To whom correspondence may be addressed. E-mail: DB@ ENGR.UKY.EDU. (1) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (2) Decher, G. Science 1997, 277, 5330, 768. (3) Decher, G.; Schmitt, J. Prog. Colloids Polym. Sci. 1992, 89, 160. (4) Santos, J. P.; Welsh, E. R.; Gaber, B. P.; Singh, A. Langmuir 2001, 17, 5361. (5) Caruso, F.; Trau, D.; Mohwald, H.; Renneberg, R. Langmuir 2000, 16, 1485. (6) Li, W.; Wang, Z.; Sun, C.; Xian, M.; Zhao, M. Anal. Chim. Acta 2000, 418, 225. (7) Trau, D.; Renneberg, R. Biosens. Bioelectron. 2003, 18, 1491. (8) Danta, S.; Hou, Z.; Risbud, S.; Stroeve, P. Langmuir 1999, 15, 2176. (9) Sennerfors, T.; Bogdanovic, G.; Tiberg, F. Langmuir 2002, 18, 6410. (10) Das, S.; Pal, A. J. Langmuir 2002, 18, 458. (11) Guan, Y.; Antonietti, M.; Faul, C. Langmuir 2002, 18, 5939. (12) Ladam, G.; Schaaf, P.; Decher, G.; Voegel, J.; Cuisinier, F. Biomol. Eng. 2002, 19, 273. (13) Hammond, P. T. Curr. Opin. Interface Sci. 2000, 4, 430. (14) Stair, J. L.; Harris, J. J.; Bruening, M. L. Chem. Mater. 2001, 13, 2641. (15) Dai, J.; Balachandra, A. M.; Lee, J. I.; Bruening, M. L. Macromolecules 2002, 35, 3164.
integrated within the pore structure of microporous membranes (pore size ) 0.2 µm) based on convective transport of the adsorbing species. Nonstoichiometric immobilization of charged polyelectrolyte assemblies within confined pore geometries leads to an enhanced volume density of ionizable groups in the membrane phase. This increase in the effective charge density allows for Donnan or charge-based exclusion of ionic species using porous materials characterized by hydraulic permeability (A) values well beyond conventional membrane processes. Recent work examining the feasibility of multilayered polyelectrolyte films for nanofiltration (NF) and reverse osmosis (RO) applications have shown a great deal of promise.14-17 Jin et al. investigated the pressure-driven transport of ions across a PAN/PET support membrane surface coated with a 60-bilayer poly(vinylamine)/poly(vinyl sulfate) film.16 While this composite membrane displayed excellent ion separation properties proving proof-of-concept, the permeability (A ) 0.3 × 10-6 cm3/(cm2 s bar)) was insufficient for commercial application. Bruening and co-workers achieved efficient ion separation at much higher permeability (A ) 2.4-4.8 × 10-4 cm3/(cm2 s bar)) by limiting deposition to 4.5-5 bilayers (poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) system) onto the surface of anodized alumina (pore size ) 20 nm, porosity ∼50%, thickness ) 60 µm).17 Fundamental research regarding the consecutive adsorption of strong polyelectrolytes (PSS, PAH, etc.) on charged substrates has revealed the enhancement of film thickness with increased ionic strength.3 It was also determined that electrostatic shielding of the adsorbing polyion and the preceding charged layer at high salt concentration dramatically altered both the degree of polymer interpenetration and the overall surface charge. (16) Jin, W.; Toutianoush, A.; Tieke, B. Langmuir 2003, 19, 2550. (17) Stanton, B. W.; Harris, J. J.; Miller, M. D.; Bruening, M. L. Langmuir 2003, 19, 7038.
10.1021/la049688+ CCC: $27.50 © 2004 American Chemical Society Published on Web 05/15/2004
Charged Polypeptides in Microporous Membranes
Rubner and colleagues observed similar trends by modification of the electrostatic properties of weak polyelectrolytes via pH adjustments of the adsorbing solution.18,19 This work will focus on the pore assembly of polyelectrolyte multilayers within microporous materials using charged polypeptides (poly(L-lysine) (PLL) and poly(Lglutamic acid) (PLGA)). Voegel et al. studied the structural properties and growth mechanisms of PLGA/PLL multilayers formed on the surface of a poly(ethylenimine)coated solid substrate.20,21 In the work presented here, PLGA/PLL multilayer growth occurs within the restricted inner pore domain of a support material. On the basis of the dimensions of the support (pore diameter ) 200 nm), pore assembly requires the pregrafting of an initial covalently bound polymer layer. As shown previously, permeation of poly(amino acids) through activated (i.e., aldehyde, epoxide) supports under appropriate solution conditions yields terminally anchored polymer chains.22,23 Extension of these charged functional groups into the pore cross section allows for subsequent polyelectrolyte adsorption and thus assembly can proceed in a layered fashion until proper pore coverage is achieved. In contrast to stratified surface deposition, film growth using this technique proceeds perpendicular to the direction of solvent flux. Multilayer assemblies were fabricated using both PLGA/PLL and synthetic polyelectrolytes (PSS/PAH) in an attempt to compare the level of adsorption and separation properties of the resulting materials. The role of salt concentration in the carrier solvent on overall polyelectrolyte adsorption was examined to determine its effect on both solute exclusion and water transport. Constriction of the pore size was monitored through permeability measurements and neutral solute rejection studies using dextran (482 kDa) at each stage of the deposition process. 2. Experimental Methods 2.1. Materials. All chemicals were used as received unless otherwise specified. Hydrated Na2SO4‚10H2O and anhydrous NaHCO3, NaCl, NaC2H3O2, and Na2SO3 were supplied by Fisher Scientific. Electroless gold plating was accomplished using anhydrous SnCl2 (Aldrich), AgNO3 (J. T. Baker Corp.), formaldehyde (Mallinckrodt), trifluoroacetic acid (Acros), and a commercially available gold-plating (Na3Au(SO3)2) solution (Oromerase Part B, Technic Inc.). NaIO4, Na2HAsO4‚7H2O, NaBH4, ninhydrin, PAH (70 kDa), PSS (70 kDa), and dextran (482 kDa, lot 111K1647) standard were purchased from Aldrich. Amine-terminated PLGA (Na form, degree of polymerization (DP) ) 356) and PLL (hydrochloride, DP ) 461) were supplied through Sigma. All aqueous feed solutions used in this study were prepared with deionized ultrafiltered water from Fisher Scientific. 2.2. Multilayer Membrane Preparation. Polycarbonate track-etched membranes (PCTE, thickness ) 10 µm, pore diameter ) 200 nm, poly(vinylpyrrolidone) (PVP) coated) (Whatman Inc.) were used as the support material for multilayer assembly. PVP coating within the pore structure allows for electroless Au deposition as described previously (with slight modification) by Martin and co-workers.24,25 In this work, Au deposition was conducted under convective flow (∆P < 0.4 bar) (18) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (19) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (20) Boulmedais, F.; Schwinte, P.; Gergely, C.; Voegel, J. C.; Schaaf, P. Langmuir 2002, 18, 4523. (21) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458. (22) Hollman, A. M.; Bhattacharyya, D. Langmuir 2002, 18, 5946. (23) Hollman, A. M.; Scherrer, N. T., Cammers-Goodwin, A.; Bhattacharyya, D. J. Membr. Sci., in press. (24) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920. (25) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M. J. Phys. Chem. B 2001, 105, 1925.
Langmuir, Vol. 20, No. 13, 2004 5419 conditions. Proper control of contact time allows for more uniform Au coating within the membrane pores. The cross sections of the Au-coated membranes were examined through transmission electron microscopy (TEM). Following Au deposition, the chemisorption of 3-mercapto-1,2-propanediol (Sigma) was accomplished via permeation of a 5 mM solution. The incorporation of these glycol moieties was followed by NaIO4 oxidation (0.02 M solution in 0.05 M Na2HPO4 buffer) yielding surface reactive aldehyde groups. Amine-terminated poly(amino acids) were then permeated through the functionalized supports under suitable reaction conditions (pH ) 9.2) yielding terminally anchored polymer chains. Multilayer formation always began with the single-point covalent attachment of either positively charged PLL (∼2 × 10-3 mM solutions) or negatively charged PLGA (∼3 × 10-3 mM solution). It should be noted that the pKa values associated with the terminal amine and the corresponding amine side chains of PLL are 9.1 and 10.5, respectively. Thus, the amine terminus is more nucleophilic at pH 9.2 and will react more readily with the surface aldehyde moieties. The extent of poly(amino acid) (PA) attachment was measured through total organic carbon (TOC) measurements (TOC 5000A total organic carbon analyzer) of the feed and permeate solutions. Experimental error for TOC analysis in the range of 10-120 mg of C/L was
