J. Phys. Chem. B 2007, 111, 13909-13913
13909
A Novel Composite: Layered Double Hydroxides Encapsulated in Vesicles Na Du,† Wan-Guo Hou,*,†,‡ and Shu-E Song† Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong UniVersity, Jinan 250100, People’s Republic of China, and College of Chemistry and Molecular Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China ReceiVed: August 3, 2007; In Final Form: October 2, 2007
We report a novel composite: layered double hydroxides (LDHs) encapsulated in vesicles. It was found that positively charged Mg3Al-LDH nanoparticles can induce the spontaneous formation of vesicles in a mixture of a zwitterionic surfactant, dodecyl betaine (C12BE), and an anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and importantly, we obtain simultaneously a novel composite of Mg3Al-LDH encapsulated in vesicles. The obtained composite is very stable and expected to be potentially used in drug delivery and gene therapy.
1. Introduction Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTlc), are a class of synthetic twodimensional nanostructured inorganic materials possessing structural positive charges. They consist of layers, containing the hydroxides of two (sometimes more) different kinds of metal cations and possessing an overall positive charge, which is balanced by intercalation of anions in the hydrated interlayer regions.1 These materials can be represented by the general formula1-3 [MII1-xMIIIx(OH)2]x+(An-)x/n‚mH2O, where MII and MIII are divalent and trivalent metal ions, respectively, An- is the charge compensating anion or gallery anion, m is the number of moles of cointercalated water per formula weight of the compound, and x is the number of moles of MIII per formula weight of the compound. A large number of studies on the preparation and the properties of LDHs have been reported.1-3 Recently, some researchers4-6 focused on the preparation of LDH films from colloidal suspensions deposited on glass or Langmuir-Blodgett films deposited on mica4 and the morphologic controlled synthesis of LDHs using reverse micelles or droplets in emulsions or microemulsions as microreactors.5,6 The lamellar structure and the anion-exchange properties of LDHs make them attractive for both fundamental investigations and technological applications such as adsorbents,7 ion-exchangers,8 pharmaceutical stabilizers,9 precursors of new catalytic materials,10 host structures for nanocomposite materials,10,11 and more recently, drug delivery and gene therapy because of their biocompatibility and nontoxicity,12 etc. A vesicle is one of the amphiphilic molecules’ organized assemblies, consisting of unilamellar or multilamelar closed bilayers. Since it may represent simple model systems for biological membranes and has practical applications such as controlled drug or DNA release,13 investigations of vesicle phases are of considerable interest in different areas, including surfactants, materials, and life sciences. In the past few years, it has been reported that the spontaneous formation of vesicles may be induced by the variation of some * To whom correspondence should be addressed. E-mail: wghou@ sdu.edu.cn. Tel.: +86-531-88564750. Fax: +86-531-88564750. † Shandong University. ‡ Qingdao University of Science and Technology.
environmental factors, such as pH,14 temperature,15 and salinity,16 or addition of organic additives17 or heavy metal ion.18 Furthermore, Hanczyc et al.19 reported that clay minerals can also catalyze the formation of vesicles from fatty acids; in turn, the clay minerals and any other molecules on their surface often become trapped in these vesicles. Herein we report, for the first time to our knowledge, the spontaneous formation of vesicles induced by LDHs nanoparticles in a mixture of a zwitterionic surfactant and an anionic surfactant, and more importantly, a novel composite, LDHs encapsulated in vesicles, was obtained. It was found that the obtained composite of LDHs encapsulated in vesicles is a very stable system, up to now for about 8 months at room temperature. We expect the composite of LDHs encapsulated in vesicles may be potentially used in drug delivery and gene therapy. 2. Experimental Section Materials. Dodecyl carboxyl betaine (C12BE) was doubly purified by recrystallization and extraction. The structural formula is displayed as follows:
Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (>98% purity) was purchased from Fluka Co. and used as received without any further purification. Other regents were A. R. grade purchased from Tianjin Sitong Chemical Co. All experiments were conducted with pure water that was passed through a Milli-Q plus purification system (Millipore), with a resistivity of 18.2 MΩ·cm. Mg3Al-LDH sol of 5 g/L was synthesized by the coprecipitation method.20 Preparation of Samples. Mg-Al-LDH (Mg/Al ) 3:1(mol/ mol)) was synthesized by the coprecipitation method.20 The stock mixed solution of zwitterionic surfactant, C12BE, and anionic surfactant, AOT, was prepared with the total surfactant concentration of 20 mmol/L and the molar ratio of C12BE to AOT of 3:2. In a series of glass tubes, a given volume of C12BE/AOT solution was added followed by adding a given volume of the Mg3Al-LDH sol, respectively, after shaking to obtain
10.1021/jp076230i CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
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Figure 1. Visible change of appearance of mixed systems of Mg3Al-LDH sol and C12BE/AOT/H2O solution with various Q values. The Q values are 0:10 (a), 1:9 (b), 2:8 (c), 3:7 (d), 4:6 (e), 5:5 (f), 6:4 (g), 7:3 (h), 8:2 (i), 9:1 (j), and 10:0 (k), respectively.
Figure 2. Negative-staining TEM photos of Mg3Al-LDH sol (a) and the composites of Mg3Al-LDH encapsulated in vesicles of C12BE/AOT/H2O with Q ) 1:9 (b), 4:6 (c), and 6:4 (d), respectively. The white arrows direct the Mg3Al-LDH particles encapsulated within the water core of the vesicles. The black arrows direct the nonencapsulated Mg3Al-LDHs sheets.
mixed systems. And after brief vortex mixing, the solutions were not subject to any type of mechanical agitation. All samples were equilibrated at room temperature for at least 3 h prior to any experimental treatment. Transmission Electron Microscope. The morphology of organized assemblies was studied by transmission electron microscopy (TEM, JEM-100CXII). The freeze-fracture and negative-staining (with uranyl acetate ethanol solution) techniques were used for TEM sample preparation. Fracturing and replication were carried out in a high-vacuum freeze-etching system (Balzers BAF-400D). High-Resolution Transmission Electron Microscopy. The morphologies and format of vesicles and Mg3Al-LDHs were characterized by high-resolution transmission electron microscopy (HR-TEM) on a JEOL JEM-2100 at 200 kV. A drop of the suspension was spread on a carbon-coated copper grid for the measurements, and the negative-staining method was performed as mentioned above. Dynamic Light Scattering. The diameter of vesicles was determined by the dynamic light scattering (DLS) method which was done with a spectrometer of standard design (Brookhaven model BI-200SM goniometer and model BI-90000AT correlator) and a 300 mW Ar laser (488 nm wavelength). All measurement was made at the scattering angle of 90° at 25 °C, and the intensity of the function was analyzed by the method
of CONTIN. The particle size distribution of aggregation in the solution was obtained through scattering light intensity Γ·G(Γ) as a function of hydrodynamic radius Rh. 3. Results and Discussion The appearances of the mixed systems are shown in Figure 1. It was found that the mixed systems in the range of volume ratio (Q) of Mg3Al-LDH sol to the C12BE/AOT solution of 1:97:3 appeared bluish and turbid to the naked eye, indicating the formation of vesicles, whereas a white cloudy precipitation was observed in the mixed systems with Q g 8:2. After depositing for 5 months at room temperature, the mixed systems with Q of 5:5-7:3 also appeared as a white cloudy precipitation (see the Supporting Information). TEM observation (Figure 2a) shows that the Mg3Al-LDH particles are of hexagonal platelike shape with the lateral size of 60-100 nm. The X-ray diffraction (XRD) pattern of the Mg3Al-LDH sample is not different from that of hydrotalcite (see the Supporting Information), indicating that the Mg3Al-LDH sample synthesized in the work has the same structure as hydrotalcite. The XRD pattern shows that the interlayer distance of the Mg3Al-LDH is 0.79 nm, and the gallery height of the Mg3Al-LDH may be calculated to be about 0.31 nm according to the thickness of the brucite-like layer of LDHs of about 0.48 nm.
Composite of LDHs Encapsulated in Vesicles
Figure 3. FF-TEM photo of the composites of Mg3Al-LDH encapsulated in vesicles of C12BE/AOT/H2O with Q ) 1:9.
To verify the formation of vesicles, negative-staining TEM was performed, and the images of the mixed systems at various Q values are shown in Figure 2b-d; the existence of vesicles with a vivid bilayer can be obviously observed. Most vesicle diameters were about 80 to ∼150 nm except a small quantity of vesicles with diameters of 200 to ∼400 nm fused by two or three small ones. Moreover, an interesting phenomenon was found that Mg3Al-LDH particles were encapsulated within the water core of vesicles, as directed by the white arrows in Figure 2b-d. The size of encapsulated objects in the vesicles is 60 to ∼100 nm, consistent with the scale of Mg3Al-LDHs sheets. In addition, vesicle aggregates and nonencapsulated Mg3Al-LDHs sheets (as directed by black arrows) can be observed in the mixed system with the higher Q value of 6:4 (Figure 2d). Maybe, it is the existence of nonencapsulated Mg3Al-LDH
J. Phys. Chem. B, Vol. 111, No. 50, 2007 13911 sheets that results in the vesicles aggregating in turn to form the white cloudy precipitation for the mixed systems with higher Q values, which may be attributed to the electrostatic interaction between positively charged nonencapsulated Mg3Al-LDH sheets and negatively charged vesicles. In order to corroborate the structures of the obtained composite, freeze-fracture TEM (FF-TEM) was introduced to our experiments (see Figure 3, some FF-TEM photos of the composites are shown in the Supporting Information). It was found that some LDH sheets were encapsulated in vesicles and the surfaces of vesicles were deformed because of the LDH sheets encapsulated within them. Moreover, the sizes of the LDH sheets and composite are in agreement with the above conclusion from the negative-staining TEM results. HR-TEM was employed to provide further insight into the subtle structures of the composite (Figure 4); it can be seen that there were some sheets encapsulated in a large vesicle (Figure 4a). Besides showing the low-magnification images, the crystal lattice fringes can be clearly seen in the composite of Q ) 1:9 (Figure 4b) and pure Mg3Al-LDH (Figure 4d), respectively, and the length of the lattice fringes is measured to be 4 to ∼8 nm. The differences in the orientations of the lattice fringes suggest that this is not a single crystallite but an agglomeration of several small crystallites. Importantly, the distance between two lattice fringes of encapsulated substance is 0.296 nm, which is close to the crystal lattice space of Mg3Al-LDH sheets of 0.297 nm (Figure 4, parts b and d). As far as we know, it is impossible to observe fringes in the surfactant system in lattice fringe (LF) mode. Therefore, we can conclude that Mg3Al-LDH induced the formation of vesicles; Mg3AlLDH sheets were encapsulated within the water core of vesicles to form a novel composite. Dynamic light scattering measurements show that average apparent hydrodynamic radius (Rh) of the composites increases
Figure 4. HR-TEM images of the composites of Mg3Al-LDH encapsulated in vesicles of C12BE/AOT/H2O with Q ) 1:9 (a and b) and Mg3AlLDH sol (c and d).
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Figure 5. Hydrodynamic radius (Rh) distributions of the composites of Mg3Al-LDH encapsulated in vesicles of C12BE/AOT/H2O with various Q values (a-c) and Mg3Al-LDH sol (d).
with the increase of Q values. As can be seen from Figure 5, the Rh values of the composites with Q values of 1:9, 4:6, and 6:4 are 45, 65, and 74 nm, respectively, and the Rh value of the Mg3Al-LDH sample is about 54 nm; those DLS results are consistent with the observation from TEM (Figure 2). The increase of the Rh value with the Q value maybe arises from the increase of the number of Mg3Al-LDH sheets encapsulated in one vesicle and the incorporation of the amphiphilic molecules into the bilayer membrane of the vesicles The mechanism of the spontaneous formation of vesicles induced by LDHs to form the composite of LDHs encapsulated in vesicles is not well-known. According to the suggestion of Hanczyc et al.,19 we hypothesize that the positively charged LDHs surface attracts negatively charged micelles or free amphiphilic molecules, facilitating their aggregation into a bilayer membrane. This process may be similar to the adsorption and aggregation of charged colloidal particles on an oppositely charged surface.21,22 Membranes formed in this way could in turn expand by absorbing additional amphiphilic molecules, coming to encapsulate the LDHs sheets along with any additional vesicles subsequently formed at the LDHs surface. 4. Conclusions In conclusion, we found, for the first time to our knowledge, that positively charged LDHs nanoparticles can induce the spontaneous formation of vesicles in a mixture of a zwitterionic surfactant and an anionic surfactant to obtain simultaneously a novel composite: LDHs encapsulated in vesicles. The obtained composite is very stable and expected to be potentially used in drug delivery and gene therapy. Ongoing work at understanding
the mechanism of the spontaneous formation of vesicles induced by positively charged solid particles is being pursued in our laboratory. Acknowledgment. This work was supported by the National Key Basic Research Program of China (No. 2004CB418504), the National Natural Science Foundation of China (No. 20573065), and the Natural Science Foundation of Shandong Province of China (Nos. Y2004B03, Z2005B02, and Z2006B06). Supporting Information Available: Synthesis and XRD patterns of the Mg3Al-LDH sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Rives, V., Ed. Layered Double Hydroxides: Present and Future; Nova Science Publishers: New York, 2001. (b) Evans, D. G.; Duan, X. Chem. Commun. 2006, 5, 485. (c) Liu, Z. P.; Ma, R. Z.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872. (2) Clearfield, A. Chem. ReV. 1988, 88, 125. (3) Zhao, Y.; Li, F.; Zhang, R.; Evans, D. G.; Duan, X. Chem. Mater. 2002, 14, 4286. (4) (a) Gardner, E.; Huntoon, K. M.; Pinnavaia, T. J. AdV. Mater. 2001, 13, 1263. (b) He, J. X.; Takahashi, K. M.; Villemure, G.; Yamagishi, A. Thin Solid Films 2001, 397, 255. (c) He, J. X.; Yamashita, S.; Jones, W.; Yamagishi, A. Langmuir 2002, 18, 1580. (5) Hu, G.; O’Hare, D. J. Am. Chem. Soc. 2005, 127, 17808. (6) He, J.; Li, B.; Evans, D. G.; Duan, X. Colloids Surf., A 2004, 251, 191. (7) Pavan, P. C.; Gomes, G. D.; Valim, J. B. Microporous Mesoporous Mater. 1998, 21, 659. (8) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (9) Vaccari, A. Catal. Today 1998, 41, 53. (10) Sels, B.; Vos, D. D.; Buntinx, M.; Pierard, F.; Mesmaeker, K. D.; Jacobs, P. Nature 1999, 400, 855.
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