Organophilic Colloidal Particles with a Synthetic Polypeptide Coating Brian Fong and Paul S. Russo* Department of Chemistry and Macromolecular Studies Group, Louisiana State University, Baton Rouge, Louisiana 70803-1804 Received November 5, 1998. In Final Form: April 6, 1999 Composite colloidal particles with a silica core and a synthetic, homopolypeptide shell have been produced by initiation of benzyl-L-glutamate N-carboxyanhydride monomer from primary amine functionalized silica particles. The resulting poly(γ-benzyl-R,L-glutamate)-coated spheres were characterized by electron microscopy, dynamic light scattering, infrared spectroscopy, and thermogravimetry. The polypeptide shell accounts for about 20% of the total mass of the particles, which are reasonably uniform in size. Infrared spectra show an R-helical secondary structure, but other conformations are not excluded. A geometrical analysis is applied to calculate the maximum number of amino groups that might realistically participate in initiation. The actual shell thickness is smaller than expected on the basis of these geometrical considerations, which reflects undesired termination steps or the conversion of some monomer to unattached polymer by trace initiator impurities. Introduction Uniform particles have been synthesized for a variety of fundamental studies, including colloid stability, 1,2 colloid-polymer interactions, 3-12 and probe diffusion in polymer solutions and gels. 13-24 Such particles also find many practical applications, ranging from coatings to novel optical devices. Typical random coil polymers, such as polystyrene or poly(dimethylsiloxane), 25,26 are often at- tached to the surface of colloidal particles to provide steric stabilization 27 against the natural tendency toward ag- gregation. Colloidal particles coated with homopolypeptides have not been studied extensively, which is surprising given the special properties of these polymers. In addition to a very versatile chemistry, polypeptides feature well-defined secondary structures. In the R-helical conformation, the polymers form extended, stiff rods. Close approach of particle pairs and the resultant destabilization could be prevented efficiently with a minimal coating of stiff rods. It is possible to produce nearly uniform polypeptides of large size, and the biotechnology to produce perfectly monodisperse peptides of small size continues to develop. 28 Thus, the minimum distance between cores might be set precisely by a polypeptide coating. In addition to being among the most rigid of polymers, homopolypeptides can also be among the most responsive of polymers; their secondary structure can change with temperature, pH, or salt. Such changes might be used to control stability phenomena or to make active colloids or colloid-based materials. Finally, the chiral nature of the chains may be useful for various separation processes. We report the preparation and characterization of nearly uniform, organophilic colloidal silica coated with poly(γ- benzyl-R,L-glutamate). The overall strategy, Scheme 1, is to prepare silica cores by the classical method of Sto ¨ber, 29 followed by surface amino functionalization with 3-ami- nopropyltrimethoxysilane. The free primary amino groups initiate N-carboxyanhydride (NCA) ring-opening polym- * To whom correspondence should be addressed. (1) Monovoukas, Y.; Gast, A. P. J. Colloid Interface Sci. 1989, 128, 533-548. (2) Schatzel, K.; Ackerson, B. J. Phys. Rev. E 1993, 48, 3766-3777. (3) Gast, A. P.; Hall, C. K.; Russel, W. B. J. Colloid Interface Sci. 1983, 96, 251-267. (4) Gast, A. P.; Russel, W. B.; Hall, C. K. J. Colloid Interface Sci. 1986, 109, 161-171. (5) Sperry, P. R.; Hopfenberg, H. B.; Thomas, N. L. J. Colloid Interface Sci. 1981, 82, 62-76. (6) Sperry, P. R. J. Colloid Interface Sci. 1984, 99, 97-108. (7) Pusey, P. N.; Pirie, A. D.; Poon, W. C. K. Physica A 1993, 201, 322-331. (8) Ilett, S. M.; Orrock, A.; Poon, W. C. K.; Pusey, P. N. Phys. Rev. E 1995, 51, 1344-1352. (9) Rohm, E. J.; Horner, K. D.; Ballauf, M. Colloid Polym. Sci. 1996, 274, 732-740. (10) Ye, X.; Tong, P.; Fetters, L. J. Macromolecules 1997, 30, 4103- 4111. (11) Ye, X.; Narayanan, T.; Tong, P.; Huang, J. S.; Lin, M. Y.; Carvalho, B. L.; Fetters, L. J. Phys. Rev. E 1996, 54, 6500-6510. (12) Faers, M. A.; Luckham, P. F. Langmuir 1997, 13, 2922-2931. (13) Allain, C.; Drifford, M.; Gauthier-Manuel, B. Polym. Commun. 1986, 27, 177-180. (14) Turner, D. N.; Hallett, F. R. Biochem. Biophys. Acta 1976, 451, 305-312. (15) Brown, W.; Rymden, R. Macromolecules 1988, 21, 840-846. (16) Onyenemezu, C. N.; Gold, D.; Roman, M.; Miller, W. G. Macromolecules 1993, 26, 3833-3837. (17) Phillies, G. D. J.; Clomenil, D. Macromolecules 1993, 26, 167 (18) Konak, C.; Bansil, R.; Reina, J. C. Polymer 1990, 31, 2333- 2337. (19) Reina, J. C.; Bansil, R.; Konak, C. Polymer 1990, 31, 1038- 1044. (20) Phillies, G. D. J.; Richardson, C.; Quinlan, C. A.; Ren, S. Z. Macromolecules 1993, 26, 6849-6858. (21) Won, J.; Onyenemezu, C.; Miller, W. G.; Lodge, T. P. Macro- molcules 1994, 27, 7389-7396. (22) Gold, D.; Onyenemezu, C.; Miller, W. G. Macromolecules 1996, 29, 5700-5709. (23) Ngai, K. L.; Phillies, G. D. J. J. Chem. Phys. 1996, 105, 8385- 8397. (24) Phillies, G. D. J.; Quinlan, C. A. Macromolecules 1992, 25, 3110- 3116. (25) Cosgrove, T.; Heath, T. G.; Ryan, K. Langmuir 1994, 10, 3500- 3506. (26) Edwards, J.; Lenon, S.; Toussaint, A. F.; Vincent, B. In Polymer Adsorption and Dispersion Stability; Goddard, E. D., Vincent, B., Eds.; American Chemical Society: Washington, DC, 1984; pp 281-296. (27) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983; (28) Zhang, G.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A. Macromolecules 1992, 25, 3601-3603. (29) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69. 4421 Langmuir 1999, 15, 4421-4426 10.1021/la9815648 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/04/1999