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Notes Polyanion Protection of Silane Bonds to Silicon Oxide Revealed by Electrokinetic Measurements
Table 1. Structure of the Investigated Silane Monolayers, Maleic Acid Copolymers, and Polypeptides Grafted to Aminosilanized Surfaces
Toshihisa Osaki,† Ralf Zimmermann,† Thomas Kratzmu¨ller,‡ Ru¨diger Schweiss,† and Carsten Werner*,†,§ Department of Biocompatible Materials, Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden, Dresden, Germany, FRIZ Biochem GmbH, Staffelseestr. 6, D-81477 Mu¨ nchen, Germany, and Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada, M5S 3G8 Received August 14, 2003. In Final Form: October 29, 2003
Introduction Alkylsiloxane coupling of organic molecules to silicon dioxide surfaces has been found in widespread applications as adhesion promoters in composite materials,1 chemical functionalization of glass fibers,2 and modification of silica particles3 and gels4 in recent years. ω-Aminosilane coatings have been proven to be very versatile for chemical attachment of single molecules,5 proteins,6 DNA,7 or polymers8-10 to standard silicon dioxide substrates such as glass or silicon wafers and dispersed silica particles. However, the instability of the Si-O bonds against hydroxide ions is often crucial for the use of the surface modifications; whereas in organic media silane monolayers were observed to be stable,11 the layers are prone to hydrolyze upon exposure to aqueous solutions. In this communication, we report on electrokinetic experiments clarifying how far negatively charged polymers attached * To whom correspondence should be addressed. Institut fu¨r Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany. Telephone: +49-351-4658-531. Fax: +49-351-4658-533. E-mail:
[email protected]. † Institute of Polymer Research Dresden and The Max Bergmann Center of Biomaterials Dresden. ‡ FRIZ Biochem GmbH. § University of Toronto. (1) (a) Silane and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, 1992. (b) Plueddeman, E. P. Silane Coupling Agents; Plenum: New York, 1982. (2) Watson, H.; Norstroem, A.; Torrkula, A.; Rosenholm, J. B. J. Colloid Interface Sci. 2001, 238, 136. (3) Wirth, M. J.; Fairbank, R. W. P.; Fatunmbi, H. O. Science 1997, 275, 44. (4) de Campos, E. A.; da Silva Alfaya, A. A.; Ferrari, R. T.; Costa, C. M. M. J. Colloid Interface Sci. 2001, 240, 97. (5) Zhang, Z.; Hu, R.; Liu, Z. F. Langmuir 2000, 16, 1158. (6) Sorribas, H.; Padeste, C.; Tiefenauer, L. Biomaterials 2002, 23, 893. (7) Oh, S. J.; Cho, S. J.; Kim, C. O.; Park, J. W. Langmuir 2002, 18, 1764. (8) Beyer, D.; Bohanon, T. M.; Knoll, W.; Ringsdorf, H.; Elender, G.; Sackmann, E. Langmuir 1996, 12, 2514. (9) (a) Bayer, T.; Eichhorn, K. J.; Grundke, K.; Jacobasch, H. J. Macromol. Chem. Phys. 1999, 200, 852. (b) Bayer, T. Ph.D. Thesis, Dresden University of Technology, Dresden, Germany, 1999. (10) Osaki, T.; Werner, C. Langmuir 2003, 19, 5787. (11) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991.
* Attached to APDMES on glass. **Grafted to APTES on glass.
on top of silane monolayers on glass prevent alkaline hydrolysis of the Si-O bonds and delamination. Experimental Section Polished glass slides (20 × 10 mm, Berliner Glas GmbH, Germany) were used as substrates for electrokinetic measurements. Silicon wafers (SiCo GmbH Jena, Germany) were used as carriers for ellipsometric experiments. 3-Aminopropyltriethoxysilane (APTES) and 3-aminopropyldimethylethoxysilane (APDMES) were obtained from ABCR Gelest (Karlsruhe, Germany). Silanization was performed by immersion of clean glass slides or silicon wafers into 3% (v/v) solutions of the corresponding silane in absolute ethanol at least for 12 h. The slides were rinsed thoroughly with absolute ethanol and annealed at 90 °C for 2 h. Layer thicknesses of 2.1 ( 0.3 nm for APTES and 0.8 ( 0.1 nm for APDMES were determined by ellipsometry in the dry state. Alternating maleic anhydride copolymers with octadecene (POMA, Polysciences Inc., Warrington, PA) or with propylene (PPMA, Leuna-Werke AG, Leuna, Germany) spin-coated from THF and annealed (120 °C for 2 h) on the APDMES layer formed thin films covalently bonded between the maleic anhydride groups and the amino groups, in which the free maleic anhydride groups were hydrolyzed by autoclaving before measurements.10 Grafting of poly-L-glutamic acid (PLGA) and poly-L-lysine (PLL) onto similar aminosilane-modified glass carriers was performed as previously reported.12 The structures of the investigated silane monolayers, maleic acid copolymers, and polypeptides grafted to aminosilanized surfaces are shown in Table 1. (12) Kratzmu¨ller, T. Ph.D. Thesis, Dresden University of Technology, Dresden, Germany, 2001.
10.1021/la0354943 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/03/2003
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Figure 1. Zeta potentials (subsequent cycles) of APDMES and APTES monolayers on glass.
Figure 2. Zeta potentials of maleic anhydride copolymers attached to APDMES monolayers on glass.
Figure 3. Zeta potentials vs pH in 1 mM KCl of PLGA and PLL grafted to APTES monolayers on glass. Electrokinetic measurements were performed using the streaming potential/streaming current technique with an inhouse-built microslit electrokinetic setup (MES).13 Details can be found in the recent literature.13 Zeta potentials were calculated from pressure-dependent measurements of the streaming potential and the streaming current using the Smoluchowski (13) (a) Werner, C.; Ko¨rber, H.; Zimmermann, R.; Dukhin, S. S.; Jacobasch, H. J. J. Colloid Interface Sci. 1998, 208, 329. (b) Zimmermann, R.; Dukhin, S. S.; Werner, C. J. Phys. Chem. B 2001, 105, 8544.
equations. Zeta potential-pH titrations were performed starting from alkaline solutions and rinsing with deionized water between the cycles. Each pH value was equilibrated for about 40 min prior to the electrokinetic measurements.
Results and Discussion Unmodified Silane Monolayers. Figure 1 shows the zeta potential versus pH of APTES and APDMES monolayers on glass. It becomes obvious that the isoelectric
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Figure 4. Survey of stability in alkaline solutions of different layer systems investigated in this work. (A) APTES and APDMES monolayers on glass. (B) PLL grafted to APTES/glass. (C) POMA and PPMA attached to APDMES/glass. (D) PLGA grafted to APTES/glass.
point (IEP) of the amino-functionalized layers shifts toward the acidic range after exposure to alkaline solutions. The shift is most pronounced for the trifunctional silane (APTES). This can be explained by the structural features of this system. As revealed in the higher thickness, the structure of the silane with 3 coupling units resembles a surface-attached network rather than a monolayer. Bayer et al.9 reported on thick APTES layers (>100 nm) by deposition from aqueous solutions at 80 °C. Likewise, the IEP of the layers in this work shifted from 9.5 to about 5 and is consistent with a higher initial density of amino functions at the surface as compared to the APDMS SAMs. Therefore, it can be assumed that hydrolysis of the Si-O bonds preferentially occurs in the network and not near the surface anchoring. After several consecutive cycles, a stable IEP of about 5 is obtained for both types of aminosilanes, which is significantly above the IEP of the pure glass surface14 and which is regardless of the electrolyte concentration. The drift of the isoelectric point of APTES and the differences in the magnitude of the zeta potential between the first and second cycles agree well with results of previous streaming potential studies.15-17 (14) It was observed that the isoelectric point was very sensitive to the surface pretreatment. The isoelectric points of the borofloat glass slides were found in a range from 2.2 (alkaline oxidizer) to 3.5 (acidic oxidizer) and 4.5 (untreated, clean surface). (15) Walker, S. L.; Bhattacharajee, S.; Hoek, E. M. V.; Elimelech, M. Langmuir 2002, 18, 2193.
Aminosilane Layers Converted with Maleic Acid Copolymers. Figure 2 shows the zeta potentials of the APDMES SAM covered by thin maleic acid copolymer layers as a function of pH. Both PPMA- and POMA-covered films were surprisingly stable for the repeated measurements between pH 9 and 3 presenting a great contrast to the bare APDMS monolayer; they indicated a rather lower IEP of 1.9 ( 0.2 for PPMA and 3.0 ( 0.2 for POMA due to the carboxylic acids on the maleic acid groups, while the two titration/plateau features in the middle- and highpH regions confirmed that the dissociation of the acids progresses in two steps. Also, distinctive swelling of the PPMA copolymer layer was suggested from in situ ellipsometric experiments at alkaline pH where the maleic acid group fully dissociates.11 The characteristic of high security from delamination of the SAMs covered by the copolymers is clearly attributed to the electrostatic repulsion between the dissociated acid groups and hydroxide ions since we hardly observed any effects of the hydrophobicity/hydrophilicity of the comonomers10 on the stability. This long-distance interaction effectively prevents hydroxide ions from penetrating into the aminosilane layer and attacking even if the covering PPMA swells and the siloxane anchor groups are revealed in alkaline solution. To verify this hypothesis, we refer to another (16) Giesbers, M.; Mieke-Klejn, J.; Cohen Stuart, M. A. J. Colloid Interface Sci. 2002, 252, 138. (17) Masuda, Y.; Sugiyama, T.; Seo, W. S.; Koumoto, K. Chem. Mater. 2003, 15, 2469.
Notes
study in which we analyzed two oppositely charged grafted polypeptides. Aminosilane Layers Covered with Grafted Polypeptides. The preparation of polypeptide brushes by grafting from aminosilanized glass surfaces was recently reported.12,18 Likewise, it was demonstrated by means of electrokinetic, ellipsometric, and FTIR experiments that PLGA brushes on glass are stable versus hydrolyzation and show ionization-driven helix-coil transitions which are highly reversible.12 Conversely, grafted layers of PLL prepared by a similar method on similar substrates could not be studied thoroughly as these films did not give stable zeta potentials and isoelectric points during the electrokinetic experiments. This clearly confirms the aforementioned stabilization mechanism. Similar to the copolymers, the grafted polypeptides are easily penetrated by water. As the cationic polyelectrolyte (PLL) cannot repel the hydroxide ions from the Si-O anchor groups, these (18) (a) Hartmann, L.; Kratzmu¨ller, T.; Braun, H. G.; Kremer, F. Macromol. Rapid Commun. 2000, 21, 814. (b) Kratzmu¨ller, T.; Appelhans, D.; Braun, H. G. Adv. Mater. 1999, 11, 555.
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grafts hydrolyze much easier in alkaline solutions (see Figure 4). Conclusions It was demonstrated by means of electrokinetic measurements that thin films of anionic copolymers or anionic polypeptide brushes chemically attached to aminosilane layers on silicon are stable against shear forces and hydrolysis in alkaline solutions whereas the unreacted aminosilane is gradually displaced during streaming potential experiments. This manifests itself in a shift of the isoelectric point of the surface toward the acidic range. Likewise, cationic polyelectrolytes such as PLL are delaminated in alkaline solutions. A mechanism of electrostatic repulsion of hydroxide ions by negatively charged polyanions to explain the stability of copolymers tethered to aminosilane layers is proposed. Due to this enhanced stability in aqueous environments, polyanions grafted to aminosilane layers can be concluded to provide a superior basis for surface-confined biomolecular architectures as compared to simple aminosilane coatings. LA0354943
