pubs.acs.org/Langmuir © 2010 American Chemical Society
Silica-Polypeptide Composite Particles: Controlling Shell Growth Erick Soto-Cantu,† Sibel Turksen-Selcuk,† Jianhong Qiu,† Zhe Zhou, and Paul S. Russo* Department of Chemistry and Macromolecular Studies Group, Louisiana State University, Baton Rouge, Louisiana 70803
Margaret C. Henk Socolofsky Microscopy Facility, College of Basic Sciences, Louisiana State University, Baton Rouge, Louisiana 70803. †These authors contributed equally to this work. Received June 11, 2010. Revised Manuscript Received August 17, 2010 A method is presented for preparing core-shell silica-polypeptide composite particles with variable and controllable shell growth. The procedure is demonstrated using poly(carbobenzoxy-L-lysine) and poly(benzyl-L-glutamate); after deprotection, these can lead to the most common basic and acidic homopolypeptides, poly(L-lysine) and poly(L-glutamic acid). Control over shell thickness is made possible by sequential addition of N-carboxyanhydride peptide monomer to surfaces that have been functionalized with an amino initiator combined with a surface passivation agent. This results in a series of particles having different shell thicknesses. Variation of shell thickness was evident both in light scattering and in thermogravimetric assays. The shells were visible by transmission electron microscopy; these images along with light scattering measurements suggest the polymers in the shells are highly solvated.
Introduction Of all the composite core-shell colloidal particles that have been made,1-9 few feature a homopolypeptide shell. That is *To whom correspondence should be addressed: E-mail:
[email protected]. (1) Bartholome, C.; Beyou, E.; Bourgeat-Lami, E.; Chaumont, P.; Zydowicz, N. Nitroxide-mediated polymerizations from silica nanoparticle surfaces: “Graft from” polymerization of styrene using a triethoxysilyl-terminated alkoxyamine initiator. Macromolecules 2003, 36(21), 7946–7952. (2) Jordi, M. A.; Seery, T. A. P. Quantitative determination of the chemical composition of silica-poly(norbornene) nanocomposites. J. Am. Chem. Soc. 2005, 127(12), 4416–4422. (3) Joubert, M.; Delaite, C.; Bourgeat-Lami, E.; Dumas, P. Hairy PEO-silica nanoparticles through surface-initiated polymerization of ethylene oxide. Macromol. Rapid Commun. 2005, 26(8), 602–607. (4) Liu, J.; Pelton, R.; Hrymak, A. N. Properties of poly(N-isopropylacrylamide)grafted colloidal silica. J. Colloid Interface Sci. 2000, 227(2), 408–411. (5) Liu, T. Q.; Jia, S.; Kowalewski, T.; Matyjaszewski, K.; Casado-Portilla, R.; Belmont, J. Grafting poly(n-butyl acrylate) from a functionalized carbon black surface by atom transfer radical polymerization. Langmuir 2003, 19(16), 6342– 6345. (6) Maitra, P.; Ding, J.; Huang, H.; Wunder, S. L. Poly(ethylene oxide) silananted nanosize fumed silica: DSC and TGA characterization of the surface. Langmuir 2003, 19(21), 8994–9004. (7) Ogden, A. L.; Lewis, J. A. Effect of nonadsorbed polymer on the stability of weakly flocculated suspensions. Langmuir 1996, 12(14), 3413–3424. (8) Seebergh, J. E.; Berg, J. C. Depletion flocculation of aqueous, electrosterically-stabilized latex dispersions. Langmuir 1994, 10(2), 454–463. (9) Shirai, Y.; Shirai, K.; Tsubokawa, N. Effective grafting of polymers onto ultrafine silica surface: Photopolymerization of vinyl monomers initiated by the system consisting of trichloroacetyl groups on the surface and Mn-2(CO)(10). J. Polym. Sci., Part A: Polym. Chem. 2001, 39(13), 2157–2163. (10) Tipton, D. L.; Russo, P. S. Thermoreversible gelation of a rodlike polymer. Macromolecules 1996, 29(23), 7402–7411. (11) Delong, L. M.; Russo, P. S. Thermodynamic and dynamic behavior of semiflexible polymers in the isotropic-phase. Macromolecules 1991, 24(23), 6139– 6155. (12) Bellomo, E. G.; Deming, T. J. Monoliths of aligned silica-polypeptide hexagonal platelets. J. Am. Chem. Soc. 2006, 128(7), 2276–2279. (13) Breedveld, V.; Nowak, A. P.; Sato, J.; Deming, T. J.; Pine, D. J. Rheology of block copolypeptide solutions: Hydrogels with tunable properties. Macromolecules 2004, 37(10), 3943–3953. (14) Chang, Y.; Frank, C. W. Grafting of poly(γ-benzyl-L-glutamate) on chemically modified silicon oxide surfaces. Langmuir 1996, 12, 5824–5829. (15) Enriquez, E. P.; Gray, K. H.; Guarisco, V. F.; Linton, R. W.; Mar, K. D.; Samulski, E. T. Behavior of Rigid Macromolecules in Self-Assembly at An Interface. J. Vac. Sci. Technol., A 1992, 10(4), 2775–2782.
15604 DOI: 10.1021/la1023955
surprising, because polypeptides combine enormous chemical versatility with useful physical and optical properties.10-20 Combining these properties with the features of colloidal particles (easy manipulation chief among them) would be beneficial. A few reports of polypeptide shells on cores of carbon black21 and fumed silica22 appeared many years ago, but the particles were not highly uniform. About a decade ago, this group improved on the uniformity and characterization of silica-homopolypeptide composite particles, some of which form colloidal crystals.23,24 It was demonstrated that the polypeptide shells were covalently attached to the silica particles; for example, the same rinsing/purification procedures used to isolate the particles removed all detectable (by FTIR and thermogravimetric analysis) polypeptide from preparations made by mixing silica and premade polypeptide. Studies elsewhere confirm the polymerization of polypeptide from functionalized silica spheres, with attachment.25 More recently, it was (16) Machida, S.; Sano, K.; Sasaki, H.; Yoshiki, M.; Mori, Y. Preparation of monolayer of poly(γ-benzyl-L-glutamate) by chemical reaction on a silicon crystal surface. J. Chem. Soc., Chem. Commun. 1992, 1626–1628. (17) Wang, Y. L.; Chang, Y. C. Synthesis and conformational transition of surface-tethered polypeptide: Poly(L-lysine). Macromolecules 2003, 36(17), 6511– 6518. (18) Wieringa, R. H.; Siesling, E. A.; Werkman, P. J.; Angerman, H. J.; Vorenkamp, E. J.; Schouten, A. J. Surface grafting of poly(L-glutamates).2. Helix orientation. Langmuir 2001, 17, 6485–6490. (19) Worley, C. G.; Linton, R. W.; Samulski, E. T. Electric-field-enhanced selfassembly of R-helical polypeptides. Langmuir 1995, 11(10), 3805–3810. (20) Yu, M. E.; Deming, T. J. Synthetic polypeptide mimics of marine adhesives. Macromolecules 1998, 31(15), 4739–4745. (21) Tsubokawa, N.; Kobayashi, K.; Sone, Y. Grafting of polypeptide from carbon-black by the ring-opening polymerization of γ-methyl L-glutamate Ncarboxyanhydride initiated by amino-groups on carbon-black surface. Polym. J. 1987, 19(10), 1147–1155. (22) Dietz, V. E.; Fery, V. N.; Hamann, K. Poyreaktionen an Pigmentoberfl€achen IV. Mitteilung: Polymerisation von N-Carboxy-a-aminos€aureanhydriden auf der Oberfl€ache von Siliciumdioxid. Angew. Makromol. Chem. 1974, 35, 115–129. (23) Fong, B.; Turksen, S.; Russo, P. S.; Stryjewski, W. Colloidal crystals of silica-homopolypeptide composite particles. Langmuir 2004, 20(1), 266–269. (24) Fong, B.; Russo, P. S. Organophilic Colloidal Particles with a Synthetic Polypeptide Coating. Langmuir 1999, 15, 4421–4426. (25) Abelow, A. E.; Zharov, I. Poly(L-alanine)-modified nanoporous colloidal films. Soft Matter 2009, 5(2), 457–462.
Published on Web 09/13/2010
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Article
Scheme 1. Production of Si-PCBL Core-Shell Particles with Sequential Addition of Monomer (Not Drawn to Scale)
demonstrated26 that premade rodlike polypeptides, at least relatively short ones, can be attached to the cores using Huisgen coupling chemistry. Polypeptide-functionalized particles are beginning to find applications; for example, Abelow and Zharov produced silicapolypeptide particles and studied molecular transport through their colloidal films.25 Kar et al. prepared poly-L-lysine grafted silica particles for use as antimicrobials.27 Silica-polypeptide particles also hold much promise for fundamental studies, such as the transport of particles in complex fluids,28-30 stability of mixed particle suspensions,31 and phase relations in rod-sphere mixtures.32 These applications often require particles of controllable and uniform size. In this report, the facile synthesis of silica-polypeptide composite particles with these desirable properties is demonstrated. The general plan is outlined in Scheme 1 using as an example poly(Nε-carbobenzyloxy-L-lysine), PCBL. A similar scheme is used to make particles coated with poly(γ-benzyl-R,L-glutamate), PBLG. Particle production begins with the St€ober method to create a silica core, which is functionalized with reactive amino groups that initiate ring-opening polymerization (26) Balamurugan, S. S.; Soto-Cantu, E.; Cueto, R.; Russo, P. S. Preparation of organosoluble silica-polypeptide particles by “click” chemistry. Macromolecules 2010, 43(1), 62–70. (27) Kar, M.; Vijayakumar, P. S.; Prasad, B. L. V.; Sen Gupta, S. Synthesis and characterization of poly-L-lysine-grafted silica nanoparticles synthesized via NCA polymerization and click chemistry. Langmuir 2010, 26(8), 5772–5781. (28) Brown, W.; Rymden, R. Comparison of the translational diffusion of large spheres and high molecular-weight coils in polymer-solutions. Macromolecules 1988, 21(3), 840–846. (29) Onyenemezu, C. N.; Gold, D.; Roman, M.; Miller, W. G. Diffusion of polystyrene latex spheres in linear polystyrene nonaqueous solutions. Macromolecules 1993, 26(15), 3833–3837. (30) Reina, J. C.; Bansil, R.; Konak, C. Dynamics of probe particles in polymersolutions and gels. Polymer 1990, 31(6), 1038–1044. (31) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (32) Vliegenthart, G. A.; Lekkerkerker, H. N. W. Phase behavior of colloidal rod-sphere mixtures. J. Chem. Phys. 1999, 111(9), 4153–4157.
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of N-carboxyanhydride (NCA) monomers. It was long ago established33,34 and recently confirmed35,36 that initiation of NCAs by small primary amines leads to the polymer molecular weight being controlled by monomer/initiator ratio, with reasonably good uniformity at high degrees of polymerization (sometimes, a low-M fraction must be removed, and the absence of this fraction is one benefit of more sophisticated controlled polymerization schemes37-39). To the extent that the tethered chains remain reactive, and given the accessibility of the extended helical, rodlike polypeptide shell in appropriate solvents,40-42 it seemed reasonable that particle size could be varied just by sequential monomer addition. Strict adherence to linearity (proportional increases in particle size with each generation of added monomer) is hardly guaranteed for growth of polymers on particle surfaces, due to crowding or partial coverage of the core surface by “fallen” (33) Kricheldorf, H. R.; M€ulhaupt, R. Mechanism of the NCA polymerization 7. The primary and secondary amine-initiated polymerization of β-amino acid NCAs. Makromol. Chem. 1979, 180, 1419–1433. (34) Kricheldorf, H. R. R-Amino acid-N-carboxy-anhydrides and related heterocycles; Springer: New York, 1987. (35) Daly, W. H.; Poche, D.; Negulescu, I. I. Poly(γ-alkyl-R, L-glutamate)s derived from long-chain paraffinic alcohols. Prog. Polym. Sci. 1994, 19(1), 79–135. (36) Poche, D. S.; Daly, W. H.; Russo, P. S. Synthesis and some solution properties of poly(γ-stearyl R,L-glutamate). Macromolecules 1995, 28(20), 6745– 6753. (37) Deming, T. J. Transition metal-amine initiator for preparation of welldefined poly(γ-benzyl-L-glutamate). J. Am. Chem. Soc. 1997, 119, 2759–2760. (38) Deming, T. J. Cobalt and iron initiators for the controlled polymerization of R-amino acid N-carboxyanhydrides. Macromolecules 1999, 32, 4500–4502. (39) Deming, T. J. Living polymerization of R-amino acid-N-carboxyanhydrides. J. Polym. Sci., Part A: Polym. Chem. 2000, 38(17), 3011–3018. (40) Block, H. Poly(γ-benzyl-L-glutamate) and other glutamic acid containing polymers; Gordon and Breach Science Publishers: New York, 1983. (41) Masuda, Y.; Miyazawa, T.; Advincula, R. C. Infrared spectra and molecular conformations of poly-γ-benzyl-L-glutamate. Macromol. Chem. 1967, 103, 261–267. (42) Pauling, L.; Corey, R. B. Atomic coordinates and structure factors for two helical configurations of polypeptide chains. Proc. Natl. Acad. Sci. U.S.A. 1951, 37 (5), 235–271.
DOI: 10.1021/la1023955
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polypeptide chains. Yet it may be possible to vary particle size and characteristics over a useful range. Compared to earlier work,23,24 which resulted in very densely packed polymer shells, an important improvement described here is the partial passivation of the initiating core surface with a certain percentage of nonreactive groups. This is intended to prevent crowding of the polypeptides that grow from the aminofunctionalized groups. Also, greater emphasis is placed on surface characterization after functionalizing the particles and on visualizing the polymer shell. Either 3-aminopropyltrimethoxysilane (APTMS) or N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS) can be used to supply the amine functionality. The latter has the longest carbon chain among the commercially available aminosilanes. Methyltrimethoxysilane (MTMS) is a shorter silane lacking an amino functional group, making it useful as a surface passivation agent in this work. Mixing these reactive and passivating silanes is not the only route to amino-functionalized surfaces with controlled functionality; for example, Heise et al.43 mixed 1-(trichlorosilyl)undecane and 1-bromo-11-(trichlorosilyl)undecane to vary the ratios of Br groups on the surface. Later the Br groups were converted to -N3 by NaN3 and then to -NH2 by LiAlH4. The next section is devoted to materials and methods used to make and characterize particles coated with PCBL or PBLG, generally following Scheme 1. In the Results and Discussion section, either type of particle may be used to demonstrate the main features. Not all details for one particle type will be reproduced for the other, and it was not our intention to make a “matched set” of PBLG- and PCBL-coated particles. However, previous literature suggests the main characteristics, such as size uniformity and thermal stability, are the same.23,24,26
Experimental Section Materials. Absolute ethanol 200 proof was purchased from Pharmco AAPER, concentrated ammonia (29%) was purchased from EMD Chemicals, and water was drawn from a Barnstead Nanopure purification system. Tetrahydrofuran (THF), N,Ndimethylformamide (DMF), and hexanes were obtained from a Pure-Solv system and had water contents
