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Building Assemblies from High Molecular Weight Polyelectrolytes Luis Kolarik,* D. Neil Furlong, Helen Joy,† Corien Struijk,‡ and Rebecca Rowe CSIRO Molecular Science, Bag 10, South Clayton, Victoria 3169, Australia Received April 9, 1999. In Final Form: June 29, 1999 Bulky, open assemblies, prepared from the sequential adsorption of oppositely charged polyelectrolytes (PE), could potentially be used as receptor matrixes in biosensing devices. We have examined the building of such structures onto gold surfaces from water-soluble cationic and anionic polyelectrolytes of different chemical structures, molecular weights (5 × 104 to 1.5 × 107), and charge densities (20-100%) using quartz crystal microgravimetry and atomic force microscopy. The effects of molecular weight were found to be small compared with the effects of charge density. When the charge density on the cationic PE was decreased, the number of PE layers that could be reliably deposited also decreased. At the same time the film became much more open and heterogeneous. The integrity of film fabrication improved with increasing solution ionic strength, confirming the key role of counterions.
Introduction 1
Recently, Decher summarized the preparation, structure, and applications of nanoassemblies formed by the sequential adsorption of oppositely charged polyelectrolytes. Numerous previous studies2-9 had shown that ultrathin films could be prepared by alternately dipping a solid substrate in aqueous solutions of polycations and polyanions. The spontaneous sequential adsorption of these dissolved polyelectrolytes leads to ordered multilayer assemblies. Film buildup is thought to be driven by the electrostatic attractions between opposite charges on the polyelectrolyte (PE) chains, although clearly other enthalpic and entropic drivers will also operate. Complete charge reversal of the surface, denoted as overcompensation6,10,23 at each adsorption stage, is the key to subsequent adsorption of the oppositely charged PE. Typically, a linear increase of the adsorbed amount of polyelectrolyte is observed with an increasing number of layers. In previous studies, while polyelectrolyte structures and functional groups were varied, molecules of high charge density (CD) were usually used and their molecular weights were invariably small in polyelectrolyte terms; usually less than 105. Polyelectrolyte CD was regulated by pH (via dissociation of functional groups) and electrostatic interactions by the addition of salts.4,5,8,9 The cationic poly(allylamine hydrochloride) (PAH) and anionic * To whom correspondence should be addressed at CSIRO Molecular Science. Fax 61 3 9545 2515. † Currently at the School of Physical Science, University of Surrey, Guildford, Surrey, GU2 5XH, United Kingdom. ‡ Currently at the Department of Physical and Colloid Chemistry, Wageningen Agricultural University, The Netherlands. (1) Decher, G. Science 1997, 277, 1232. (2) Decher, G.; Hong, J. D. Macromol. Chem., Macromol. Symp. 1991, 46, 321. (3) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (4) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (5) Decher, G.; Schmitt, J. D. Prog. Colloid Polym. Sci. 1992, 89, 160. (6) Lvov, Y.; Decher, D.; Mo¨hwald, H. Langmuir 1993, 9, 481. (7) Decher, D.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (8) Tronin, A.; Lvov, Y.; Nicolini, C. Colloid Polym. Sci. 1994, 272, 1317. (9) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir, 1997, 13, 3422. (10) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. Langmuir 1996, 12, 3675.
poly(styrene sulfonate) (PSS) polyelectrolytes, which form reproducible, well-defined, regular architectures, were most frequently used as a model system. Their potential for biosensing applications has been signaled via the incorporation of proteins11-13 and extensive characterization using quartz crystal microgravimetry (QCM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) techniques.14 The use of the low molecular weight polyelectrolytes leads to tightly bound layers. These structures, with perhaps limited “porosity” for antibodies and target analytes such as bacteria, are probably not ideal as sensor receptor matrixessopen but robust structures are more likely to allow the necessary rapid and unhindered access. It is the possibility of constructing more open films that led to the current study involving high molecular weight polyelectrolytes of varying charge density. Films were fabricated onto gold electrodes of the QCMs to allow direct monitoring of film buildup and subsequent stability; AFM provided in situ evaluation of film topology, particularly roughness and heterogeneity. Experimental Section Selection of Polyelectrolytes. The PEs are categorized in a manner familiar to all PE practitioners, viz., by molecular weight (low molecular weight (L), 105 Da) and charge density (low charge density (L), 1040% charged; medium charge density (M) 40-60% charged; high charge density (H) 60-100% charged). Charge density (CD) is defined as the number of ionized functional groups per monomer unit. Henceforth we will use a two-letter sequence when categorizing PE used in this study. For example the designation (HL) will denote a PE of high molecular weight and low charge density. Table 1 details the PE structures and the range of molecular weight (MW) and charge density they represent. The PSS and PAH (both LH) were obtained from Aldrich Chemical Co. The Zetag and Magnafloc (11) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (12) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, Langmuir 1994, 10, 4232. (13) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427. (14) Caruso, F.; Furlong, D. N.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 4559.
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Table 1. Description of the Polyelectrolytes Used
polyelectrolytes were used as supplied by Allied Colloids Australia. They are frequently used in water, wastewater, and sludge treatment.15 The preparation of aqueous solutions of low molecular weight PE such as PAH and PSS at 3-4 mg mL-1 is straightforward and quick. However, the dissolution of the high molecular weight PE is slow, and so the solutions of the cationic Zetag 32, 64, and 89 and the anionic Magnafloc LT 30 were only 1.0 mg mL-1. The pH was adjusted with HCl and NaOH and ionic strength with NaCl, MgCl2, MnCl2, and LaCl3 as indicated in the text. Other Materials. All reagents and solvents were AR grade (Sigma Chemical Co.). Water was purified first by activated carbon, reverse osmosis, and ion exchange and then via a threestage Millipore “Milli-Q” system (conductivity < 1 µS cm-1). Substrate Preparation and PE Film Formation. The PE layers were formed on the gold electrodes of a QCM. The gold was first cleaned (2 min) with piranha solution (one part of 30% H2O2 in three parts of concentrated H2SO4), thoroughly washed with Milli-Q water, and blown dry with a stream of nitrogen gas. (Piranha solution should be handled with extreme care, prepared (15) Bolto, B. A. Prog. Polym. Sci. 1995, 20, 987.
in small quantities ( Zetag 32, and the mass of the deposits correspondingly increases. Molar concentrations of salt in the PSS solution are mandatory for the formation of stable, heterogeneous structures. Overall ionic strength is the most important parameter rather than any salt specificity. AFM images show that film roughness increases with salts present. Globular surface subunits are suggestive of PE coiling induced by salt. LaCl3 and combined high molecular weight cationic and anionic polyelectrolytes produced the most irregular, bulky deposits.
Figure 14. AFM image of 11-layer Zetag64 + PSS deposit. PSS was dissolved in solution of 0.25 M LaCl3. Vertical dimensions 1 and 0.05 µm.
can be readily achieved AFM images reveal that these films can be quite rough. High molecular weight cationic polyelectrolytes, such as Zetag 32, 64, and 89, can also be used in conjunction
Acknowledgment. We thank Drs. Brian Bolto, Rob Eldridge (CSIRO), and Frank Caruso (Max Planck Institute for Colloids and Interfaces, Berlin) for useful discussions. We are grateful to Elke Rodda (CSIRO) for her expert advice on the QCM measurements and her help with AFM imaging, Thomas Gengenbach (CSIRO) for the XPS measurements, and Robert Considine (CSIRO) for his help with the AFM image processing. We would also like to acknowledge Dr. Gero Decher (CNRS, Strasbourg) for valuable insights and access to unpublished material. We also readily acknowledge the technical advice and polyelectrolyte samples received from Allied Colloids (Australia) Pty Ltd. LA990413H
