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Onion Phases as Biomimetic Confined Media for Silica Nanoparticle Growth Houssam El Rassy,*,†,‡ Emmanuel Belamie,† Jacques Livage,†,‡ and Thibaud Coradin† Laboratoire de Chimie de la Matie` re Condense´ e, Universite´ Pierre et Marie Curie Paris VI, 4 place Jussieu (T54-E5), 75252 Paris Cedex 05, France, Colle` ge de France, 11 place Marcellin-Berthelot, 75005 Paris, France Received June 3, 2005. In Final Form: July 29, 2005 Phospholipid onion phases were investigated as biomimetic media for the synthesis of silica in a confined environment. Stable multilamellar nanovesicles incorporating sodium silicate solutions could be obtained. Upon aging, silica condensation occurs in the onion interlayer space while preserving the initial multilamellar organization. The hybrid structure consists of an array of apparently unconnected silica nanoparticles in the 20-30 Å size range packed in the vesicular 50 Å interlayer space, suggesting that the silica growth was efficiently controlled by its confinement in the onion lamellar organization.
Introduction The control of inorganic phase growth by means of confinement is a key strategy in biomineralization processes.1,2 In the case of silica, the formation of diatoms frustule or sponge spicules occurs in specific silica deposition vesicles (SDV), where nanoparticles grow and assemble in the presence of biomacromolecules.3 In diatoms, the frustule building-up goes with an expansion of the SDV, in close connection with the cells cytoskeleton.4 Until now, biomimetic approaches of silicification have mainly focused on the understanding of the interactions arising between silica precursors and macromolecules, either extracted from living organisms5,6 or selected/ synthesized as suitable models.7,8 The influence of SDV dynamics has also been mimicked by applying external forces, such as shear stress, to the reaction media.9 However, the possibility to simulate the confined space of the SDV has been far less studied. Phospholipid vesicles can be considered as good models of biological compartments. Such layered systems have been used for the biomimetic formation of silica,10,11 but only deposition of silica on the membrane surface was reported. Intralamellar condensation of silicon alkoxides in multilamellar vesicles was successfully achieved by * Corresponding author. Tel: +33-1-44275517. Fax: +33-144274769. E-mail:
[email protected]. † Laboratoire de Chimie de la Matie ` re Condense´e. ‡ Colle ` ge de France. (1) Lowenstam, H. A.; Weiner, S. On Biomineralization; Oxford University Press: New York, 1989. (2) Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials; Oxford University Press: New York, 2001. (3) Simpson, T. L.; Volcani, B. E. Silicon and siliceous structures in biological systems; Springer-Verlag: New York, 1981. (4) Hildebrand, M. Prog. Org. Coat. 2003, 47, 256. (5) Sumper, M.; Kro¨ger, N. J. Mater. Chem. 2004, 14, 2059. (6) Cha, J. N.; Shimizu, K.; Zhou, Y.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D.; Morse, D. E. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 361. (7) Lopez, P. J.; Gautier, C.; Livage, J.; Coradin, T. Curr. Nanosci. 2005, 1, 73. (8) Patwardhan, S. V.; Clarson, S. J.; Perry, C. C. Chem. Commun. 2005, 1113. (9) Naik, R. R.; Whitlock, P. W.; Rodriguez, F.; Brott, L. L.; Glawe, D. D.; Clarson S. J.; Stone, M. O. Chem. Commun. 2003, 238. (10) Mann, S.; Perry, C. C. Adv. Inorg. Chem. 1991, 36, 137. (11) Be´gu, S.; Durand, R.; Lerner, D. A.; Charnay, C.; Tourne´-Pe´teilh, C.; Devoisselle, J. M. Chem. Commun. 2003, 640.
Pinnavaia et al.12 The template can be extracted with solvent or removed by calcination, leading to multilamellar silica particles. However, the reaction conditions are far from those of biomineralization processes and are incompatible with most biological macromolecules. The aim of the present study was to use onion nanovesicles as a suitable template for silica nanoparticles growth, closely mimicking biological conditions. We have explored the possibility to associate the most abundant biological surfactant, L-R-phosphatidylcholine as a template, and sodium silicate solutions as the source of silica precursors. The reaction can thus be performed in purely aqueous media. Additionally, we have selected a specific multilamellar system, called onion phase, resulting from the reorganization of a lyotropic lamellar phase under a controlled shear force.13 The resulting lamellar nanovesicles can be dispersed in an excess of water to give a colloidal dispersion. A molecule dissolved in the lamellar phase before shearing will be inserted either in the lipid membrane or in the aqueous layers, depending on its hydrophilicity/hydrophobicity balance. Owing to these specific and selective properties, onion phases have already been studied as an aqueous phase microreactor to prepare metallic/metal oxide nanoparticles14 and to encapsulate therapeutic and biological molecules.15 The present study demonstrates that stable onion suspensions can be obtained in the presence of sodium silicate solutions. Upon aging, silicate condensation leads to hybrid structures consisting of silica nanoparticles packed in the interlamellar space of the vesicles. The size of the silica particles appears to be controlled by the interlamellar distance in the templating structure, demonstrating the suitability of onion phases as confined media for the biomimetic synthesis of silica nanostructures. (12) (a) Tanev, P. T.; Pinnavaia, T. J. Supramol. Sci. 1998, 5, 399.(b) Tanev, P. T.; Liang, Y.; Pinnavaia, T. J. J. Am. Chem. Soc. 1997, 119, 8616. (13) (a) Diat, O.; Roux, D. J. Phys. II 1993, 3, 9. (b) Gauffre F.; Roux, D. Langmuir 1999, 15, 3070. (14) (a) Gauffre, F.; Roux, D. Langmuir 1999, 15, 3738. (b) Kim, D. W.; Oh, S. G.; Yi, S. C.; Bae, S. Y.; Moon, S. K. Chem. Mater. 2000, 12, 996. (c) Regev, O.; Backov, R.; Faure, C. Chem. Mater. 2004, 16, 5280. (15) (a) Bernheim-Grosswasser, A.; Ugazio, S.; Gauffre, F.; Viratelle, O.; Mahy, P.; Roux, D. J. Chem. Phys. 2000, 112, 3424; (b) Pott, T.; Roux, D. FEBS Lett. 2002, 511, 150.
10.1021/la051462z CCC: $30.25 © 2005 American Chemical Society Published on Web 08/12/2005
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Figure 3. 29Si MAS NMR spectrum of one week aged hybrid silica onions.
Figure 1. TEM micrographs of hybrid silica onions.
Experimental Section Multilamellar hybrid onions have been synthesized by using L-R-phosphatidylcholine (1,2-diacyl-sn-glycero-3-phosphocholine or L-R-lecithin from dried egg yolk type X-E g 60% (TLC)), noted
PC, and poly(ethylene glycol) dodecyl ether, noted Brij 30, as templates for an aqueous solution of sodium silicate. These surfactants were mixed at 70 °C in a hot water bath in order to obtain a yellow mixture containing 80% of PC and 20% of Brij 30. To synthesize the hybrid particles, the templates mixture was manually sheared, using a spatula, for 30 min at room temperature in the presence of an aqueous acidic sodium silicate solution (pH 1, [SiO2] ) 1.25 M)). This solution was obtained by acidification of a sodium silicate solution initially at pH 11 by using a hydrochloric acid solution. The final sodium silicate solution/surfactants ratio is 1 (w/w). The mixture was left at rest for one week at room temperature in order to allow the condensation of silica. The obtained materials resemble to a viscous paste which can be dispersed in water using a high power probe sonicator.
Results and Discussion After dispersing the hybrid mixture in excess water by sonication, a suspension is obtained where onions are
stable for several weeks. Optical microscopy observation of the suspension showed the presence of nonaggregated small mobile spherical vesicles, a characteristic of stable onions in aqueous solutions (not shown). Transmission electron microscopy (TEM; Figure 1) of the suspension deposited and evaporated on a carbon-coated copper grid revealed vesicles between 150 and 500 nm with a concentric multilamellar structure, where the inorganic matter stands between the organic supramolecular template layers to yield rigid hybrid multilamellar onions. The onions hydrodynamic radii were measured by dynamic light scattering (DLS) performed on the colloidal dispersion. DLS measurements indicate that the onions diameter is mainly comprised between 270 and 460 nm, with the presence of fewer small ones between 100 and 180 nm. To determine the percentage of the silicate solution initially confined in the onion phase, the silicic acid concentration was titrated in the dispersion medium by the silicomolybdic acid colorimetric method.16 The results showed an excellent encapsulation ratio since less than 15% of the initial silicate solution was found in the aqueous phase after dispersion. The thermal stability of the synthesized hybrid onions was studied by thermogravimetric analysis (TGA), in the 25-800 °C temperature range at a heating rate of 10 °C/
Figure 2. FTIR spectra of the hybrid onion phase immediately after shearing and after one week aging.
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Figure 5. X-ray diffraction patterns performed on the onion phase at different aging time.
Figure 4. (a) High-magnification TEM micrograph of hybrid onions. The dark dots correspond to silica nanoparticles (b) Intensity profile performed on defined surface area.
min under an air atmosphere (results not shown). The TG curve showed 44% weight loss at 200 °C, which corresponds to the initial amount of water in the silicate solution. 50% were lost between 200 and 550 °C in agreement with the PC and Brij 30 weight introduced in the mixture. The residual mass after heat treatment at 800 °C for hours was approximately 6% of the initial weight. This weight correlates well with the silicon SiO2 amount mixed to the lamellar phase before shear, added to the residual sodium chloride produced during the acidification of the alkaline sodium silicate with HCl. ATR/FTIR spectroscopy was performed on a thin onion phase film immediately after shear and after one week aging. The corresponding normalized spectra (Figure 2) showed an important absorption band at 1082 cm-1 for the aged sample, but not for the initial mixture. This band is due to the asymmetric Si-O-Si stretching vibration and the greater ionic character of the Si-O group.16 The presence of this band is a proof that the silicate monomers are partially or completely condensed. We mention that the absorption band at 1093 cm-1, better seen for the
sample before aging, is due to the symmetric stretching vibration of the PO2- groups of the PC.17 Solid-state 29Si MAS NMR spectroscopy carried out on the aged sample showed the presence of three peaks at -92, -101, and -111 ppm that can be respectively attributed to Q2, Q3, and Q4 silicon groups (Figure 3). By quantification of these peaks, we deduce that the silicon groups in this sample are mainly tri or tetra coordinated to other silicon atoms (50% Q3 and 42% Q4). However, the high Q3/Q4 ratio probably reveals a high proportion of interfacial uncondensed silanols. This indicates that the network is only partially condensed and suggests that the silica network consists of aggregated silica particles rather than a fully 3D cross-linked gel. In agreement with this hypothesis is the fact that all attempts to withdraw the PC/Brij30 template by calcination led to the collapse of the lamellar structure. TEM imaging of the hybrid onions (Figure 4a) revealed a well-ordered lamellar structure with 50 Å wall-to-wall distance, as induced on intensity profiles (Figure 4b). Moreover, a regular array of unconnected silica nanoparticles with diameters in the 20-30 Å range can be visualized in the interlayer space of the vesicles. Confirmation of the preservation of the onion lamellar structure could also be ascertained by X-ray diffraction, performed on the hybrid materials, 24 and 72 h after shearing the lamellar phase, using a diffractometer with a Cu KR target tube (λ ) 1.5418 Å). The X-ray diffraction patterns (Figure 5) showed broad Bragg peaks at 2θ ) 1.64° and 2θ ) 3.26° corresponding respectively to the 001 and 002 peaks of the lamellar phase with a 54 Å d spacing. The spacing was shifted to 50 Å (2θ ) 1.76° and 2θ ∼ 3.3°-3.4°) after 3 days aging, in agreement with the TEM observations. Such a decrease in the interlamellar space suggests a shrinkage of the onion structure upon silica condensation. Additional peaks can be attributed to accessory lamellar phases formed by phospholipids other than PC present in the egg yolk extract. X-ray scattering experiments thus confirm, as inferred from the TEM (16) Coradin, T.; Eglin, D.; Livage, J. Spectroscopy 2004, 18, 567. (17) Socrates, G. Infrared Characteristic Group Frequencies, 2nd ed.; John Wiley and Sons: New York, 1994.
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observations, that the onions multilamellar structure is preserved after thorough condensation of sodium silicates. Conclusion We show here that phospholipid-based multilamellar onion nanovesicles are a suitable template for the biomimetic growth of silica in a confined environment. In contrast to previous works,12 the inorganic phase does not consist of a condensed network but of an assembly of individual nanoparticles stacked between the layers of the preserved multilamellar structure. Investigations are
currently under progress to test the suitability of these hybrid nanoparticles as encapsulation media. Moreover, the stability of the onion phase in the presence of inorganic precursors opens the route to the investigations of other biogenic phases. Acknowledgment. The authors are grateful for financial support from the National Center for Scientific Research (CNRS) and from the College de France. LA051462Z
