Chem. Mater. 2009, 21, 781–783
Direct Assembly of Anisotropic Layered Double Hydroxide (LDH) Nanocrystals on Spherical Template for Fabrication of Drug-LDH Hollow Nanospheres Poernomo Gunawan and Rong Xu* School of Chemical & Biomedical Engineering, Nanyang Technological UniVersity, N1.2, 62 Nanyang DriVe, Singapore 637459 ReceiVed NoVember 26, 2008 ReVised Manuscript ReceiVed January 26, 2009
Layered double hydroxides (LDHs) represent an important class of host-guest materials, and their composition can be expressed by a general formula, [MII1-xMIIIx(OH)2]x+ · An-x/n · mH2O.1 They have been extensively studied owing to their wide applications in catalysis, separation, electrochemistry, and biotechnology.2In particular, remarkable interest has been generated recently on using this type of material in storage and controlled release of biomolecules such as DNA, drugs, or vitamins.2f,g,3 However, the as-prepared LDH materials by conventional methods are usually loose powders of irregular aggregates which render most of these applications restricted.4 Therefore, fabrication of LDHs with wellcontrolled 2D or 3D nanostructures would certainly enhance their possibilities for practical applications. Up to date, the syntheses of LDHs with ordered structures have been reported by some groups. For example, Geraud et al. applied an “inverse opal” method to fabricate 3D macroporous LDHs, using polystyrene bead arrays as the sacrificial template.5 Nanosized LDHs were also formed inside the mesoporous carbon with large pores.6 In addition, growth or assembly of LDH nanocrystals on flat solid surfaces for generation of continuous, noncontinuous, or patterned films
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has been achieved by several groups.7 However, direct assembly of LDH nanocrystals on surfaces of nano- or microsized spherical templates to fabricate core-shell type composites and LDH hollow spheres has not been reported, although Li et al. constructed LDH nanoshells using the exfoliated LDH nanosheets by a layer-by-layer (LBL) process.8 In fact, the LBL process via electrostatic interaction of oppositely charged layers has been frequently used for the assembly of preformed nanoparticles9 or exfoliated nanosheets10 of various materials, such as TiO2, SiO2, SnO2, Au, clay, R-ZrP, niobate, and so forth, on spherical templates. In such a multistep process, polyelectrolytes are often required as binders in each step. In addition, to preserve the spherical shape with a structural integrity during the subsequent thermal treatment, multilayer coating and postsealing are necessary.9,10 Nanomaterials with hollow structures have attracted much attention in recent years, owing to their unique properties such as high surface area, good permeability, light weight, and peculiar optical/electrical/magnetic properties.11 Herein, we demonstrate a simple method for generating LDH hollow nanospheres via direct assembly of preformed anisotropic LDH nanocrystals (30 × 10 nm) on the surface of carbon nanospheres (CNS, ∼800 nm). Closely packed Mg2Al-LDH films were first formed on the CNS template in one step. After removing the core, hollow nanospheres of MgAl oxides with robust shell walls were formed. A key requirement identified to achieve the above results is the use a stable colloidal suspension of finely dispersed LDH nanocrystals. Further, we have shown that the oxide shell can be readily converted to LDHs intercalated with functional anions (e.g., drug anions) based on the well-known memory effect.1 Scheme 1 summarizes the major steps involved in this work. The precursor Mg2Al-LDH nanocrystals (LDH-NCs) were first synthesized using a similar method as reported by Gursky et al. using a coprecipitation method from the
* Corresponding author. E-mail:
[email protected]. Fax: (65) 6794 7553.
(1) Cavani, F.; Trifiro, F.; Vaccari, A. Cat. Today 1991, 11, 173. (2) (a) Braterman, P. S.; Xu, Z. P.; Yarberry, F. Layered doublbe hydroxides (LDHs). In Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2004; p 373. (b) Structure and bonding: Layered Double Hydroxides; Duan, X., Evans, D. G., Eds.; Springer: Berlin/Heidelberg, 2006; Vol. 119. (c) Xiong, Z.; Xu, Y. Chem. Mater. 2007, 19, 1452. (d) Liang, L.; He, J.; Wei, M.; Evans, D. G.; Duan, X. Water Res. 2006, 40, 735. (e) Xu, Z. P.; Xu, R.; Zeng, H. C. Nano Lett. 2001, 1, 703. (f) Choy, J. H.; Kwak, S. Y.; Jeong, Y. J.; Park, J. S. Angew. Chem., Int. Ed. 2000, 39, 4041. (g) Khan, A. I.; Lei, L.; Norquist, A. J.; O’Hare, D. Chem. Commun. 2001, 2342. (h) Sokolova, V.; Epple, M. Angew. Chem., Int. Ed. 2008, 47, 1382. (i) Roto, R.; Villemure, G. J. Electroanal. Chem. 2007, 601, 112. (3) (a) Gunawan, P.; Xu, R. J. Pharm. Sci. 2008, 97, 4367. (b) Xu, Z. P.; Niebert, M.; Porazik, K.; Walker, T. L.; Cooper, H. M.; Middelberg, A. P. J.; Gray, P. P.; Bartlett, P. F.; Lu, G. Q. J. Controlled Release 2008, 130, 86. (c) Gunawan, P.; Xu, R. J. Mater. Chem. 2008, 18, 2112. (d) Aisawa, S.; Higashiyama, N.; Takahashi, S.; Hirahara, H.; Ikematsu, D.; Kondo, I.; Nakayama, H.; Narita, E. Appl. Clay Sci. 2007, 35, 146. (4) Gursky, J. A.; Blough, S. D.; Luna, C.; Gomez, C.; Luevano, A. N.; Gardner, E. A. J. Am. Chem. Soc. 2006, 128, 8376. (5) (a) Geraud, E.; Prevot, V.; Ghanbaja, J.; Leroux, F. Chem. Mater. 2006, 18, 238. (b) Geraud, E.; Rafqah, S.; Sarakha, M.; Forano, C.; Prevot, V.; Leroux, F. Chem. Mater. 2008, 20, 1116. (6) Dubey, A. Green Chem. 2007, 9, 424.
(7) (a) Wang, L. Y.; Li, C.; Liu, M.; Evans, D. G.; Duan, X. Chem. Commun. 2007, 123. (b) Lee, J. H.; Rhee, S. W.; Jung, D. Y. J. Am. Chem. Soc. 2007, 129, 3522. (c) Okamoto, K.; Sasaki, T.; Fujita, T.; Iyi, N. J. Mater. Chem. 2006, 16, 1608. (d) Liu, Z. P.; Ma, R. Z.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872. (e) Zhang, F. Z.; Zhao, L. L.; Chen, H. Y.; Xu, S. L.; Evans, D. G.; Duan, X. Angew. Chem., Int. Ed. 2008, 47, 2466. (8) Li, L.; Ma, R. Z.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Commun. 2006, 3125. (9) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Caruso, F.; Caruso, R. A.; Mohwald, H. Chem. Mater. 1999, 11, 3309. (c) Caruso, F.; Shi, X. Y.; Caruso, R. A.; Susha, A. AdV. Mater. 2001, 13, 740. (d) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (e) Liang, Z. J.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (f) Martinez, C. J.; Hockey, B.; Montgomery, C. B.; Semancik, S. Langmuir 2005, 21, 7937. (10) (a) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (b) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (c) Putlitz, B. Z.; Landfester, K.; Fischer, H.; Antonietti, M. AdV. Mater. 2001, 13, 500. (d) Wang, L. Z.; Ebina, Y.; Takada, K.; Sasaki, T. J. Phys. Chem. B 2004, 108, 4283. (e) Miyamoto, N.; Kuroda, K. J. Colloid Interface Sci. 2007, 313, 369. (11) (a) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (b) Lou, X. W.; Wang, Y.; Yuan, C. L.; Lee, J. Y.; Archer, L. A. AdV. Mater. 2006, 18, 2325. (c) Lin, G. F.; Zheng, J. W.; Xu, R. J. Phys. Chem. C 2008, 112, 7363.
10.1021/cm803203x CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
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Scheme 1. One-Step Direct Assembly Process of Nanocrystals Toward the Formation of LDH Film on the Surface of a Spherical Template and Subsequent Generation of Hollow Nanospheres of LDH Intercalated with Drug Anions
Figure 2. (A) TEM image of LDH-NCs and (B and C) SEM images of LDH-NCs/CNS composites obtained by direct assembly of LDH-NCs on the surface of CNS in a single step without exfoliation.
solution of Mg(NO3)2 and Al(NO3)3 in the presence of NaOH in pure methanol.4 Different from their ambient conditions during the aging, solvothermal treatment at 150 °C for 18 h was applied in our study (see Supporting Information) which resulted in smaller particle sizes. As shown in Figure 1A, a narrow distribution of the particle size around 30 nm was estimated based on dynamic light scattering technique. Our particle sizes are also smaller than those (50-300 nm) of stable LDH nanoparticles prepared in aqueous media by Xu et al. under hydrothermal conditions.12 This should be due to different solvation effects of methanol and water as the synthesis medium.13 The XRD pattern of LDH-NCs shown in Figure S1A (Supporting Information) indicates the formation of LDH structure with a rhombohedral symmetry. The estimated thickness of LDH-NCs based on the fwhm of the (003) basal plane is 10 nm. Besides a strong absorption peak at 1384 cm-1 due to the interlayer NO3- anions, the IR result (Figure S2A, Supporting Information) also shows the presence of methoxide in LDH-NCs by peaks at 1054 and 2950 cm-1 corresponding to C-O and C-H, respectively.4 Since LDH-NCs were synthesized in methanol, methoxide is expected to be intercalated in the interlayer space and possibly adsorbed on the surface of LDH-NCs. On the basis of the elemental and thermal analysis results, the chemical formula determined for LDH-NCs is Mg1.98Al(OH)5.96(NO3)0.71 (CH3O)0.56 · 1.52H2O. The calculated elemental compositions based on this formula agree well with the experimental values (Mg, 18.0%; Al, 10.1%; C, 2.5%; N, 3.7%; and H2O, 10.2%). However, it is noted that there are extra negative charges in this formula, which could be due to the presence of some surface adsorbed methoxide. For comparison, a control-LDH sample was synthesized by a coprecipitation method using
Figure 1. (A) Particle size distribution of LDH-NCs and (B) a translucent and stable suspension of LDH-NCs in methanol/water solvent.
the same amounts and concentrations of the reagents but in an aqueous solution, and the as-formed precipitate was aged at 60 °C for 24 h. The suspension of LDH-NCs in a methanol/water solvent after thorough washing was translucent (Figure 1B) and remarkably stable for more than 2 months without undergoing aggregation. The TEM image in Figure 2A shows that the sample consists of finely dispersed platelet-like nanocrystals. This stable dispersion was then used to directly coat the curved surface of carbon nanospheres as template without going through the exfoliation step which was adopted by Li et al. earlier.8 The measured zeta potential of LDH-NCs suspension was +31.6 mV. Carbon nanospheres (CNS) prepared in this work have a negative zeta potential of -13.4 mV due to its abundant surface hydroxyl groups (Figure S3, Supporting Information).14 Therefore, the coating of CNS with LDHNCs was performed without any additives or binding reagents. The assembly of LDH-NCs from the methanol/ water suspension (∼2 wt %) on the CNS surface was performed simply under the assistance of ultrasonication at room temperature for a short duration of 0.5 h. SEM images of the dried composite sample (Figure 2B,C) indicate the successful deposition of LDH-NCs on CNS by this simple procedure, as the surface of the nanospheres became rough after coating compared to the pristine smooth nanospheres. In addition, the measured zeta potential of the resultant composite was +42.2 mV, indicating a good coverage of the nanocrystals on CNS via electrostatic attraction. The obtained composite nanospheres were further calcined at 500 °C to remove the CNS core. The reconstruction of the resultant oxide product to LDHs intercalated with drug anions was performed in ethylene glycol with the controlled amount of water. Figure 3 shows SEM and TEM images of the MgAl-oxide (Figure 3A,C) and ibuprofen (a model drug)intercalated LDH (Figure 3B,D) products. Both samples exhibit the morphology of hollow nanospheres with their diameters reduced to around 400 nm. The shell thickness of the oxide hollow nanospheres is approximately 40 nm. The oxide shell has a large surface area of 151.7 m2/g and an average pore size of 9.5 nm. After reconstruction, the thickness was increased to about 80 nm because of the formation of the LDH structure with expanded interlayer spacing by the drug anions. The XRD and FTIR results (12) Xu, Z. P.; Stevenson, G. S.; Lu, C. Q.; Lu, G. Q.; Bartlett, P. F.; Gray, P. P. J. Am. Chem. Soc. 2006, 128, 36. (13) Dedonder-Lardeux, C.; Gregoire, G.; Jouvet, C.; Martrenchard, S.; Solgadi, D. Chem. ReV. 2000, 100, 4023. (14) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 597.
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Figure 4. In vitro release profile of ibuprofen from ibp-LDH hollow nanospheres in a buffer solution of pH 7.0. The inset shows the release profiles from ibp-LDH nanoplates (9) which were prepared by a coprecipitation method as described in our earlier work3a and ibuprofen sodium salt (2) under the same conditions. Figure 3. SEM (A) and TEM (C) images of metal oxide hollow nanospheres obtained after calcination of LDH-NCs/CNS composites and SEM (B) and TEM (D) images of ibuprofen-intercalated LDH hollow nanospheres after reconstruction.
(Figzures S1D and S2D, Supporting Information) confirm the formation of ibuprofen-intercalated LDH (ibp-LDH). The d-spacing of ibp-LDH hollow nanospheres calculated from the (003) peak is 22.0 Å which is consistent with that previously reported.3a The intercalated ibuprofen content in the hollow nanospheres was found at 36.8% by UV-vis measurement. On the basis of the ICP/CHN/thermal analysis results, the estimated formula of ibp-LDH hollow nanospheres is Mg1.99Al(OH)5.98(Ibp)0.65(CO3)0.17 · 2.40H2O. The presence of carbonate could be due to contamination from atmospheric CO2. The surface area and average pore size of ibp-LDH hollow nanospheres are 53.9 m2/g and 4.2 nm, respectively. The other general physicochemical properties of the samples prepared at different stages can be found in Supporting Information (Figures S1, S2, and S4 and Table S1). It is very interesting that the spherical shape with good structural integrity is well preserved even with a large volume contraction (50% of shrinkage in diameter) from the composite precursor to the oxide. This should be attributed to the densely packed pristine LDH-NCs on CNS from the stable colloidal suspension. During the heat treatment, the nanocrystals were transformed to oxides which cross-linked to form the continuous and porous shell of the hollow nanospheres. In contrast, the metal oxide prepared under the same conditions but using the control-LDH suspension which is nonstable gave broken hollow nanospheres with fragmented shell walls (Figure S5, Supporting Information). Further, it has been shown that these oxide hollow nanospheres (Figure 3C) are sufficiently robust to withstand ultrasonication, gas bubbling, and magnetic stirring during the reconstruction process in the solution phase. Such characteristics provide good opportunities for forming LDH hollow nanospheres intercalated with functional anions and further entraping other molecules into the hollow interior to
make multifunctional nanospheres. Besides ibuprofen, other drugs such as 4-biphenylacetic acid, 5-fluorouracil, valproic acid, folic acid, and so forth in their anionic forms have been intercalated to form the corresponding LDH hollow nanospheres. Figure 4 shows the in vitro release profile of ibuprofen from ibp-LDH hollow nanospheres, in comparison with those from ibp-LDH nanoplates and the simple ibuprofen salt. It is found that there is no substantial difference in the release profiles for the two LDH samples. At 5 min, a release percentage of 50% was detected and 90% was detected after around 40 min for both samples. However, there still exist several advantages for the hollow nanosphere LDH sample, such as (i) a lower density and less aggregation compared to the nanoplates, which leads to a better dispersion in the liquid phase (see Figure S6, Supporting Information) for potential applications in controlled drug release with an intravenous injection mode; (ii) a higher surface area (53.9 m2/g compared to 14.7 m2/g for ibp-LDH nanoplates) allowing more effective surface modification by functional species (e.g., polymers, silica, etc.); and (iii) the interior space of the hollow nanospheres being used for encapsulation of other molecules or nanoparticles (e.g., dyes, magnetic nanoparticles, etc.) to make multifunctional nanocomposites. In summary, this is the first demonstration of the direct assembly of preformed anisotropic nanocrystals from a stable colloidal suspension onto a spherical template in a single step for the generation of hollow nanospheres. It is anticipated that this approach will open a new avenue to fabricate core/shell composites and hollow structures by avoiding the tedious procedures involving the conventional exfoliation/ LBL-stacking process. Acknowledgment. This work was supported by AcRF Grant RG30/07 from Ministry of Education, Singapore. Supporting Information Available: Detailed experimental procedures for materials synthesis and characterization and SEM/ TEM/XRD/FTIR/TGA results for various samples (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM803203X
