pubs.acs.org/Langmuir © 2009 American Chemical Society
Carboxyl Enriched Monodisperse Porous Fe3O4 Nanoparticles with Extraordinary Sustained-Release Property Xiaowang Liu,*,† Qiyan Hu,‡ Zhen Fang,† Qiong Wu,† and Qiubo Xie† †
College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, China, and ‡Department of Pharmacy, Wannan Medical College, Wuhu 241002, China Received April 21, 2009. Revised Manuscript Received May 30, 2009 Carboxyl-enriched monodisperse porous Fe3O4 nanoparticles with diameters of about 85-nm have been synthesized via a simple hydrothermal method. The porous structure of the product is confirmed further by transmission electron microscopy (TEM) observation and nitrogen sorption measurement with a Brunauer-Emmett-Teller (BET) surface area about 36.61 m2/g. An IR spectrum of the sample indentifies that abundant caboxylate groups are formed on the surface of the nanoparticles as well as the pore surface. Because of the confined effect of the nanochannels in the nanoparticles and carboxyl-functionalized Fe3O4 nanoparticles, and the strong interaction between ibuprofen and COO-, as-prepared porous nanoparticles show a more extraordinary sustained-release property than that of hollow silica nanoparticles in vitro. This result suggests that as-prepared porous nanoparticles can also be used for the targeted delivery of other aromatic acid drugs.
Introduction Recently, there has been an increasing amount of activity to fabricate nano/micrometer-scale carriers and a growing demand for their use in sophisticated applications in the life and materials science.1 Generally, nano/microcarriers would be inexpensive materials with simple methods to fabricate, and have high medicine loading. A series of nano/microstructures, such as CaCO3 microparticles, porous hollow nanostructured hydroxyapatite and calcium silicatehave, amphiphilic TiO2 nanotube arrays, calcium phosphate nanoparticles, and mesoporous silica have been applied as drug carriers with sustained-release property.2 Among these materials, amorphous mesoporous silica is one of the most promising candidates for the usage of drug carriers because of its nontoxic nature, tunable diameter, and very high specific surface.3 However, it is difficult to guide pure silica materials to target organs or locations in the body as drug carriers. Nanocomposites with the advantages of mesoporous silica and magnetic nanoparticles are feasible to be applied in *To whom correspondence should be addressed. E-mail: xwliu601@yahoo. com.cn. (1) Lvov, Y. M.; Shchukin, D. G.; M€ohwald, H.; Price, R. R. ACS Nano 2008, 2, 814. (2) Volodkin, D. V.; Larionova, N. I.; Sukhorukov, G. B. Biomacromolecules 2004, 5, 1962. Ma, M.-Y.; Zhu, Y.-J.; Li, L.; Cao, S.-W. J. Mater. Chem. 2008, 18, 2722. Song, Y.-Y.; Schmidt-Stein, F.; Bauer, S.; Schmuki, P. J. Am. Chem. Soc. 2009, 131, 4230. Morgan, T. T.; Muddana, H. S.; Altinolu, E.; Rouse, S. M.; Tabakovi, A.; Tabouillot, T.; Russin, T. J.; Shanmugavelandy, S. S.; Butler, P. J.; Eklund, P. C.; Yun, J. K.; Kester, M.; Adair, J. H. Nano Lett. 2008, 8, 4108. Yang, J.; Lee, J.; Kang, J.; Lee, K.; Suh, J.-S.; Yoon, H.-G.; Huh, Y.-M.; Haam, S. Langmuir 2008, 24, 3417. (3) Lu, J.; Liong, M.; Zink, J. I.; Tamanoi, F. Small 2007, 3, 1341. Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, 5083. Mal, N. K.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350. Munoz, B.; Ramila, A.; Pariente, J. P.; Diaz, I.; Vallet-Regi, M. Chem. Mater. 2003, 15, 500. Giri, S.; Trewyn, B. G.; Stellmaker, M. P.; Lin, V. S.-Y. Angew. Chem., Int. Ed. 2005, 44, 5038. Trewyn, B. G.; Slowing, I. I.; Giri, S.; Chen, H.-T.; Lin, V. S.-Y. Acc. Chem. Res. 2007, 40, 846. (4) Zhao, W.; Chen, H.; Li, Y.; Li, L.; Lang, M.; Shi, J. Adv. Funct. Mater. 2008, 18, 2780. Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916. Hu, S.-H.; Liu, T.-Y.; Huang, H.-Y.; Liu, D.-M.; Chen, S.-Y. Langmuir 2008, 24, 239. Huang, S.; Fan, Y.; Cheng, Z.; Kong, D.; Yang, P.; Quan, Z.; Zhang, C.; Lin, J. J. Phys. Chem. C 2009, 113, 1775.
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targeted delivery. Several strategies have been successfully used to synthesize such nanocomposites, and the results demonstrate that as-prepared nanocomposites have a good sustained-release property.4 However, it is worth noting that as-prepared nanocomposites usually have a diameter higher than 200 nm, which means that such nanocomposites are impossible to inject into the body intravenously as drug carriers because particles with such size are easily sequestered by the spleen and eventually removed by the cells of the phagocyte system, resulting in decreased blood circulation time.5 Magnetic nanoparticles, especially iron oxide nanocrystals, have been widely studied for various biomedical applications such as targeted drug delivery, cell sorting, contrast agents for magnetic resonance imaging, and hyperthermia, because of their fascinating properties,6 including biocompatibility and stability in physiological conditions and size-dependent magnetic properties.7 In this letter, carboxyl-enriched monodisperse porous Fe3O4 nanoparticles with diameters less than 100 nm were synthesized using a facile hydrothermal method. Because of abundant carboxyl on the pore of the nanochannels and particle surface of the products, they were dispersed well in aqueous solution, and the obtained colloid solution could exist at least for several weeks. Ibuprofen, a kind of aromatic acid drug with good pharmacological activity, was chosen as a model drug to investigate the in vitro release property of the prepared carboxylenriched monodisperse porous Fe3O4 nanoparticles, which have (5) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Elst, L. V.; Muller, R. N. Chem. Rev. 2008, 108, 2064. Berry, C. C.; Curtis, A. S. G. J. Phys. D: Appl. Phys. 2003, 36, R198. (6) Alexiou, C.; Arnold, W.; Klein, R. J.; Parak, F. G.; Hulin, P.; Bergemann, C.; Erhardt, W.; Wagenpfeil, S.; Lubbe, A. S. Cancer Res. 2000, 60, 6641. Artemov, D. J. Cell. Biochem. 2003, 90, 518. Ito, A.; Tanaka, K.; Kondo, K.; Shinkai, M.; Honda, H.; Matsumoto, K.; Saida, T.; Kobayashi, T. Cancer Sci. 2003, 94, 308. Alexiou, C.; Jurgons, R.; Schmid, R. J.; Bergemann, C.; Henke, J.; Erhardt, W.; Huenges, E.; Parak, F. J. Drug Target. 2003, 11, 139. (7) Cengelli, F.; Maysinger, D.; Tschudi-Monnet, F.; Montet, X.; Corot, C.; Petri-Fink, A.; Hofmann, H.; Juillerat-Jeanneret, L. J. Pharmacol. Exp. Ther. 2006, 318, 108. Petri-Fink, A.; Chastellain, M.; Juillerat-Jeanneret, L.; Ferrari, A.; Hofmann, H. Biomaterials 2005, 26, 2685. Brigger, I.; Dubernet, C.; Couvreur, P. Adv. Drug Delivery Rev. 2002, 54, 631. Wang, L.; Bao, J.; Wang, L.; Zhang, F.; Li, Y. Chem.;Eur. J. 2006, 12, 6341.
Published on Web 06/09/2009
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shown a capacity of ibuprofen loading of about 134.2 mg/g. For comparison, hollow silica nanoparticles (HSNPs) were synthesized according to the published procedure.2e The results show that the as-prepared carboxyl-enriched monodisperse porous Fe3O4 nanoparticles showed more extraordinary sustained-release properties than that of HSNPs under both acid and basic conditions. It is worth mentioning that as-prepared porous Fe3O4 nanoparticles may be optimal for intravenous injection after drug loading because particles with a diameter ranging from 10 to 100 nm have the most prolonged blood circulation time.5a Also, these particles are small enough to evade the reticuloendothelial system of the body as well as penetrate small capillaries of the tissues, and offer the most effective distribution in targeted tissues.
Experimental Methods Materials. All reagents were purchased from Beijing Chemical Reagent Ltd. and used as-received without further purification. Synthesis of Carboxyl-Enriched Monodisperse Porous Fe3O4 Nanoparticles and HSNPs. Carboxyl-enriched monodisperse porous Fe3O4 nanoparticles with mass production were synthesized by a simple solvothernal route. Typically, 0.5 g of ferric ammonium citrate was dissolved in 30 mL water to form a clear solution, followed by adding 10 mL of poly(acrylic acid) (PAA) and 10 mL of hydrated hydrazine (80%). The final mixture was stirred vigorously until it became homogeneous and it was subsequently transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was maintained at 180 o C for 10 h. The black product was collected and washed with ethanol for several times, and then dried in a vacuum oven at 40 o C overnight. HSNPs were synthesized by a reported method.2e Drug Storage and Release. The drug storage and in vitro release experiments were performed using methods reported elsewhere.2b,3a,10 Typically, as-prepared porous Fe3O4 nanoparticles (1.0 g) were added into 50 mL of the ibuprofen/hexane solution (70 mg mL-1) at room temperature. The mixture was irradiated by ultrasonic waves in the air for 10 min, and then was stirred for 24 h after being sealed tightly. The resulting nanoparticles were obtained with the assistance of an external magnet. These products were dried under vacuum at 30 o C, after being washed with dilute HCl solution (pH=1) several times. The final solution was analyzed by ultraviolet-visible (UV-vis) spectroscopy at a wavelength of 263 nm after being diluted. Before the UV-vis spectroscopy measurement, calibration curves of ibuprofen were constructed. We found that the ibuprofen absorbance versus its concentration between 0 and 2.5 mg mL-1 fits the Lambert and Beer’s law well (R = 0.999): A ¼ C 8:7630 þ 0:2173 where A is the absorbance and C is the concentration of ibuprofen (mg mL-1). The dried products were compacted into 0.1 g disks under a pressure of 3 MPa. Three disks were added into 50 mL of release medium at room temperature at different pH values under stirring at a rate of about 100 rpm. Five milliliters of the release medium was extracted, and its absorbance was measured at given intervals after the nanoparticles were removed. It is noteworthy that another 5 mL of fresh release medium was added to the drug release systems. The amount of ibuprofen released from the nanoparticle at different stages could be calculated from the corresponding UV-vis absorbance. The drug storage and in vitro realease property of the HSNPs were investigated similarly. Characterization Methods. The products were characterized by X-ray diffraction (XRD; X-6000), scanning electron microscopy (SEM; S-4800), and high-resolution transmission electron microscopy (HRTEM; JEOL-2010). XPS data were obtained with an ESCALab220-XL electron spectrometer from VG Scientific using 300 W Al KR radiation. Nitrogen adsorption-desorption meaLangmuir 2009, 25(13), 7244–7248
Figure 1. XRD pattern of the as-obtained sample. surements were performed on a Micromeritics ASAP 2020 accelerated surface area analyzer at -196 o C, using the volumetric method. Specific surface areas and pore volumes were calculated by the Brunauer-Emmett-Teller (BET) method. Pore size distributions were estimated from adsorption branches of isotherms by the Barrett-Joyner-Halenda (BJH) method. The magnetic properties of the sample were measured on a BHV-55 vibrating sample magnetometer at room temperature. Infrared (IR) spectra of porous Fe3O4 nanoparticles before and after ibuprofen loading and the sample after the release of ibuprofen were obtained on a Vectortm 22 Fourier Transform spectrometer (FTIR) (Bruke, Germany). The UV-vis absorbance measurements of all samples were performed on a U-4100 spectrophotometer.
Results and Discussion The crystal phase of the product was determined by XRD, as shown in Figure 1. The XRD pattern of the material reveals a typical diffraction pattern of pure Fe3O4 (JCPDS card no.: 86-1354). No evidence of impurities can be found in the XRD pattern. However, it is well-known that XRD measurement is not an essential method to characterize the phase structure of certain iron-containing nanostructures.8 For example, γ-Fe2O3 and Fe3O4 show similar XRD patterns. The sample was further characterized by X-ray photoelectron spectroscopy (XPS) (Figure 2a,b). The binding energies near 710, 724, and 531 eV are the characteristic peaks of Fe 2p3/2, Fe 2p1/2, and O 1s. This result confirms that the product obtained is pure Fe3O4. It is worth noting that the peak of C 1s is also observed, which suggests that a layer of PAA may be on the surface of the product. An SEM image of the product is shown in Figure 3a, from which a large quantity of spherical nanoparticles with uniform size and shape is observed. The average size of these nanoparticles is about 85 nm estimated from the SEM image. Figure 3b shows a low-magnification TEM image of the sample. This image shows that these nanoparticles are dispersed well and have rough surface. A High-magnification TEM image (Figure 3 c) shows the porous nature of the nanoparticle, and indicates that the nanoparticle is composed of a large scale of smaller nanoparticles with diameters ranging from 3 to 7 nm. More detailed information on the porous nanoparticle was obtained by HRTEM. Figure 3d shows the clear lattice of the product, indicating the single-crystalline nature of the particles, which is consistent with the result obtained from selected area electron diffraction (SAED) (8) Zhu, L.-P.; Xiao, H.-M.; Zhang, W.-D.; Yang, G.; Fu, S.-Y. Cryst. Growth Des. 2008, 8, 957.
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Figure 2. (a) XPS spectrum of the as-prepared product. (b) The expanded spectrum of Fe 2p.
Figure 5. Room-temperature magnetization curve of the pore Fe3O4 nanoparticles.
Figure 3. (a) Typical SEM image, (b,c) TEM images with different magnifications, and (d) the HRTEM image of the product. The inset in panel d is the electron diffraction pattern.
Figure 6. (a,b) FTIR spectra of porous Fe3O4 nanoparticles before and after ibuprofen loading. (c,d) The FTIR spectra of Fe3O4 nanoparticles after the release of ibuprofen at pH 2 and pH 8, respectively. Inset is the section of the high-resolution spectra of lines “c” and “d” marked in the figure.
Figure 4. Nitrogen adsorption/desorption isotherm and BJH pore plot (inset) of the porous Fe3O4 nanoparticles.
patterns (inset of Figure 3d). The distance between two adjacent planes is measured to be about 0.254 nm, which corresponds to the separation of the (311) planes of magnetite. Nitrogen sorption measurement was conducted to further investigate the porous structure of the as-prepared Fe3O4 nanoparticles. Figure 4 presents the nitrogen adsorption-desorption isotherms and BJH pore size distribution curves (inset in Figure 4) 7246 DOI: 10.1021/la901407d
of the sample. The isotherms are identified as type IV, which is characteristic of mesopores. The pore size distribution of the sample demonstrates that a few pores are about several nanometers, which arise from the spaces among the nanocrystallites within a porous Fe3O4 nanoparticle. The pores with diameter about 33 nm are attributed to the interparticle spaces. The sharp distribution of the mesopores around 33 nm suggests that the product has high monodispersity.9 The BET surface area of the (9) Hu, J.-S.; Ren, L.-L.; Guo, Y.-G.; Liang, H.-P.; Cao, A.-M.; Wan, L.-J.; Bai, C.-L. Angew. Chem., Int. Ed. 2005, 44, 1269.
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Figure 7. Schematic illustration for ibuprofen loading of as-prepared porous Fe3O4 nanoparticles.
sample was calculated to be as much as about 36.61 m2/g, which is much higher than that of hollow Fe3O4 particles.7 All data strongly support the fact that the product has a nanoporous structure. The magnetic property of the porous Fe3O4 nanoparticles was studied using a vibrating sample magnetometer. The plots of magnetization versus magnetic field (Figure 5) indicate that there is no remanence or coercivity at room temperature. Thus, the products have superparamagnetic behavior, which may attribute to the smaller components as we observed from the TEM images. This phenomenon is consistent with the results reported by other groups.10 As-prepared porous Fe3O4 nanoparticles were dispersed in water well, and the obtained colloid solution could exist at least for several weeks. The main reason is that the surface of the nanoparticles is coated with a layer of carboxylate group, which can be confirmed by the typical IR spectrum of original porous Fe3O4 nanoparticles (curve “a” of Figure 6). Three peaks located at 1635, 1455, and 1404 can be attributed to the C-O stretching mode of carboxylate groups, CH2 bending mode, and symmetric C-O stretching mode of the COO- group.11 This result demonstrates that the particle surface as well as the pore surface contain a large number of COO- groups, which is consistent with the XPS measurement. Drug loading and release experiments were performed by the method reported elsewhere.2b,3b,4a When the porous Fe3O4 nanoparticles were added into the ibuprofen/ hexane solution (70 mg mL-1), the black participates were on the surface of the solution first as a result of the hydrophilic nature of the nanoparticles. However, with the assistance of ultrasonic waves, the black participates were dispersed into the mixture quickly, presumably because ibuprofen molecules would interact with COO- groups on both the surface of the nanoparticles and pore via hydrogen bonds, leading to the formation of a hydrophobic surface, which is illustrated in Figure 7 schematically. The above conclusion can be confirmed further by the IR spectra of porous Fe3O4 nanoparticles after ibuprofen loading (curve “b” of Figure 6); the peaks around 2852-2952 cm-1 are ascribed to the C-H stretching model of the alky chain of ibuprofen. It is worth noting that the curve shows a band at 1710 cm-1, which corresponds to the carboxylic group of ibuprofen and indicates (10) Deng, Y.; Qi, D.; Deng, C.; Zhang, X.; Zhao, D. J. Am. Chem. Soc. 2008, 130, 28. Ge, J.; Hu, Y.; Biasini, M.; Beyermann, W. P.; Yin, Y. Angew. Chem., Int. Ed. 2007, 46, 4342. (11) Lee, D. H.; Condrate, R. A.; Reed, J. S. J. Mater. Sci. 1996, 31, 471. Li, H.; Tripp, C. P. Langmuir 2005, 21, 2585.
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Figure 8. UV-vis absorbance spectra of original (a) and final (b) ibuprofen solutions after being diluted with hexane 40 times.
that a large quantity of ibuprofen molecules have been stored in the pores without any interaction with COO- groups on the pore surface.4a,12 The capacity of ibuprofen loading of carboxyl enriched porous Fe3O4 nanoparticles is about 134.2 mg/g, which can be obtained from the UV absorbance difference before nanoparticle addition and after their removal, as shown in Figure 8. However, the capacity of ibuprofen loading of the HSNPs is about 278.3 mg/g, which is higher than that of porous Fe3O4 nanoparticles, as the density of Fe3O4 is relatively high.12 Figure 9 shows the cumulative drug release behaviors of the ibuprofen-loaded porous Fe3O4 nanoparticles and HSNPs over an 80 h period in simulated body fluid at different pH. It can be seen that porous Fe3O4 nanoparticles have shown a more extraordinary sustained-release property than that of HSNPs under both conditions. For HSNPs, there is a burst release at an early period under both conditions. Specifically, the mean release rates of this period reach 5.85 mg/g 3 h (pH=2) and 7.87 mg/g 3 h (pH=8), which are about 4 times larger than that of porous Fe3O4 nanoparticles under the same conditions. The burst release of drug is not observed in porous Fe3O4 nanoparticles. The ibuprofen release rates in basic solution are much higher than that in acidic condition for both porous Fe3O4 nanoparticles and HSNPs, as ibuprofen has a pH-dependent solubility, that is, its solubility gradually grows as the pH of the solution enhances.3b,4a The difference of the sustained-release property between as-prepared porous Fe3O4 nanoparticles and HSNPs may mainly (12) Zhu, Y.-F.; Shi, J.-L.; Li, Y.-S.; Chen, H.-R.; Shen, W.-H.; Dong, X.-P. Microporous Mesoporous Mater. 2005, 85, 75.
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Figure 9. Ibuprofen release behaviors from porous Fe3O4 nanoparticles and HSNPs in release media of different pH values. A basic and an acidic solution. (a) pH = 2.0 and (b) pH = 8.0 from porous Fe3O4 nanoparticles; (c) pH = 2 and (d) pH = 8 from HSNPs.
attribute the pH-dependent solubility of carboxyl-enriched porous Fe3O4 nanoparticles. It is well-known that carboxylfunctionalized nanoparticles have a pH-dependent solubility as the carboxylate groups can be ionized or deionized under different pH.13 In our drug release systems, drug disks dissolved gradually under mechanical stirring, and ibuprofen diffused from both the pore and the surface of the nanoparticles to the solution as the concentration gradient. In basic solution, ibuprofen molecules release from the surface of nanoparticles easily with the assistance of OH-, and the ionization of carboxylate groups occurs spontaneously, resulting in the quick dispersement of the drug disks. However, under acidic conditions, the ibuprofen molecules slowly release from the surface of the pores of nanochannels and nanoparticles to the solution because of deionization of carboxylate groups caused by H+. Once ibuprofen molecules release from the nanoparticles, carboxylate groups on the surface of them will obtain H+ and form -COOH, which leads to a lower solubility of the as-prepared porous Fe3O4 nanoparticles. Thus, dispersement of the drug disks is rather slow under acidic conditions. The difference of the dispersement rates of drug (13) Ge, J.; Hu, Y.; Biasini, M.; Dong, C.; Guo, J.; Beyermann, W. P.; Yin, Y. Chem.;Eur. J. 2007, 13, 7153.
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disks under different conditions affect the drug release rates seriously. The IR spectra of porous Fe3O4 nanoparticles after the release of ibuprofen at pH=2 and pH=8 are shown as curves “c” and “d” in Figure 6, respectively. From the inset, we can see that a certain amount of ibuprofen molecules still exist on the surface of both nanopartilces, indicating the strong interaction between ibuprofen and -COO-. The density of ibuprofen molecules on the nanoparticles obtained from pH 2.0 solution is higher than that obtained for pH 8.0 solution, as the stretching mode of C-H of ibuprofen in the former is stronger than that of the latter. It is noteworthy that the peak located at 1710 cm-1 disappeared in both curves “c” and “d”, which demonstrate that ibuprofen molecules stored in the pore without any interaction with COOhave diffused into solution mostly. Because of the lack of caboxylate groups bonding on their surface, the release rates of drug of HSNPs are too fast to control under both conditions.
Conclusion Monodisperse porous Fe3O4 nanoparticles with diameters of about 85 nm have been synthesized successfully via a simple hydrothermal method. Nitrogen sorption measurement further indicated that as-prepared nanoparticles have porous structure with a BET surface area of about 36.61 m2/g. The IR spectrum of the sample indentified that an abundance of caboxylate groups are formed on the surface of the nanoparticles as well as the pore surface. As a result of the confined effect of the nanochannels in the Fe3O4 nanoparticles, the pH-dependent solubility of carboxyl-functionalized Fe3O4 nanoparticles, and the strong interaction between COO- and ibuprofen, as-prepared porous Fe3O4 nanoparticles have shown a more extraordinary sustained-release property than that of HSNPs. As-prepared porous nanoparticles may be applied for loading and sustained-release of other aromatic acid drugs. Acknowledgment. Financial support of this work from the College Natural Science Foundation of Anhui Province (KJ2008B168) and the Young Teachers’ Foundation of Wannan Medical College and NSFC (20671003 and 20701002) is gratefully acknowledged.
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