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Novel synthesis of Fe-containing mesoporous carbons and their release of ibuprofen
Microporous and Mesoporous Materials 145 (2011) 98–103
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Novel synthesis of Fe-containing mesoporous carbons and their release of ibuprofen Xiufang Wang a, Ping Liu b, Yong Tian a,⇑, Linquan Zang a a b
College of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
a r t i c l e
i n f o
Article history: Received 23 February 2011 Received in revised form 21 April 2011 Accepted 25 April 2011 Available online 3 May 2011 Keywords: Mesoporous carbon Magnetic Synthesis Drug release
a b s t r a c t A simple route has been developed to synthesize magnetic Fe-containing mesoporous carbon by an incipient-wetness impregnation technique without using a solvent. The materials were characterized by using X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, transmission electron microscopy, and physical property measurements. The species of iron in the resulting carbons were metallic a-Fe and magnetite Fe3O4/c-Fe2O3. The ordering of the mesostructure, the specific surface area and the total pore volumes decreased with the increasing amount of FeCl3 used. The saturation magnetization strength could be easily adjusted by changing the amount of FeCl3 used in the synthesis. Ibuprofen (IBU) release behavior from the materials showed that the release rate of IBU increased with the increase of the order and loading degree. Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved.
1. Introduction Ordered mesoporous carbons (OMCs), have attracted great technological interest for the development of adsorption and purification of water, electronic, catalytic and energy storage systems, due to their remarkable properties such as high specific surface area, narrow pore size distribution, tunable pore structure, large pore volume and high thermal and mechanical stability. Moreover, the excellent biocompatibility and controllable pore size distribution of OMCs make them highly prospective and valuable in drug delivery system [1–8]. However, carbon powders are notoriously difficult to be separated from solutions. The conventional approach normally involves a filtration or centrifugation procedure, which is rather complex and expensive [9]. Magnetic mesoporous materials containing Fe, Co, Ni, or alloys magnetic nanoparticles provided an alternative opportunity for the separation. Among them, iron nanoparticles were more preferred candidates for many advanced nanotechnological applications [10], such as magnetic storage media, directed drug delivery, and groundwater remediation or other environmental applications. So far, there are generally two routes widely used to synthesize Fecontaining mesoporous carbons. One is soft-templating synthesis [9–11]. Although the route is simple, the mesoporous materials usually have low pore volumes and surface areas, which limit their applications in many fields [12]. The other is hard-templating synthesis, after preparation of silica template, filling the silica mesopore with appropriate carbon precursors (furfuryl alcohol/FA or pyrrole) and iron sources (FeCl3 or Fe(NO)3), which involve two steps. Some ⇑ Corresponding author. Tel./fax: +86 2039352129. E-mail address: [email protected] (Y. Tian).
researches reported that iron sources (FeCl3 or Fe(NO)3) were firstly introduced into the silica template and then carbon precursors (FA or pyrrole) [2,13–15], while other studies investigated that carbon precursors were firstly introduced into the silica template and then iron sources [16,17]. However, the synthesis involving two steps was fussy and difficult to be controlled. Besides, solvents such as ethanol or water needed for dissolving carbon precursor or iron source were the additional drawbacks for the synthesis. Consequently, an easy and simple synthesis procedure for magnetic mesoporous carbon was in urgent demand for practical applications. Herein, we successfully present a simple and novel route to synthesize magnetic Fe-containing mesoporous carbons by an incipient-wetness impregnation technique without using a solvent. Carbon precursor (FA) and iron source (FeCl3) were introduced simultaneously into the silica template in one step. FeCl3 was directly dissolved in FA which acted dual functions as solvent and carbon sources, and then the above solution containing FA and FeCl3 were impregnated simultaneously into the silica template in one step. IBU, as an anti-inflammatory drug, has been commonly used as the release drug model due to its good pharmacological activity and the small molecule size (1 nm), which could fit easily into the pores of OMCs. The drug loading and release behavior from Fe-containing carbons and the species of iron in the resulting carbons were also reported.
2. Materials and methods 2.1. Synthesis of SBA-15 templates SBA-15 was synthesised according to Zhao and coworkers [18]. In a typical synthesis, 4.00 g of P123 (PEO20-PPO70-PEO20;
1387-1811/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.04.033
X. Wang et al. / Microporous and Mesoporous Materials 145 (2011) 98–103
99
Fig. 1. Wide-angle XRD patterns of the materials.
Aldrich) was added into 144 ml of aqueous HCl solution (1.7 M) and the mixture was stirred for 4 h at 40 °C. Next, 8.00 g of tetraethyl orthosilicate (Fuchen Chemical Regent of Tianjin) was added dropwise. The resulting gels were transferred to the Teflon-lined sealed containers and were kept at 70 °C for 48 h under static hydrothermal conditions. The as-synthesized samples were extracted with a mixture of 200 ml of ethanol 95% (v/v) and 4 ml of concentrated HCl for 24 h twice. All chemicals were used as received without further purification. Ultrapure water was used in all experiments. 2.2. Synthesis of magnetic carbon materials Incipient-wetness impregnation was employed to introduce simultaneously carbon precursor (FA) and iron source (FeCl3) into the SBA-15 template with oxalic acid as a homogeneous catalyst. Typically, 0.5 mmol of FeCl3 and 0.05 g of oxalic acid were dissolved in 2 ml of FA solution. SBA-15 was impregnated with the above solution at room temperature. The mixture thus prepared was heated at 80 °C for 12 h under vacuum for the polymerization of FA and then at 150 °C for 6 h. After cooling to room temperature, the sample was heated to 300 °C (1 °C/min), then to 800 °C (2 °C/ min) for 3 h under flowing argon atmosphere (200 ml/min). The silica template was removed by boiling the materials in 1 M NaOH solution dissolved in 1:1 (v:v) mixtures of water and ethanol for more than 1 h twice. The obtained black powders were filtered, washed with water and dried under vacuum at 60 °C for 24 h. The obtained sample was denoted as M-C-0.5. The same procedures were carried out for the preparation of sample M-C-1.0 and M-C-1.5, except that the amounts of FeCl3 were 1.0 and 1.5 mmol for M-C-1.0 and M-C-1.5.
Fig. 2. XPS spectra of the materials for (a) Fe2p3/2 and (b) O1s.
2.3. Loading and in vitro IBU release 0.25 g of magnetic carbon materials was added into 50 ml of IBU solution in hexane (35 mg/ml), followed by stirring at room temperature for 72 h in a closed batch to prevent the evaporation of hexane. The loaded materials were then filtered and dried under vacuum at 40 °C for 24 h. Twenty milligrams of the resulting materials were immersed into 50 ml of simulated body fluid (SBF, pH 7.4) [19,20] at 37 °C under stirring at 100 rpm. Three milliliters of samples were removed at given time intervals, diluted to 10 ml with SBF, and analyzed by UV–visible spectroscopy at a wavelength of 272 nm. The volume removed was replaced with
Fig. 3. Small-angle XRD patterns of the materials.
the same amount of preheated SBF. The experiments were carried out in triplicate. The loading degrees of IBU were determined by UV–visible spectroscopy analysis.
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2.4. Characterization The X-ray diffraction (XRD) patterns were recorded on a Multi Purpose Diffractometer (PANanalytical. Inc. X’Pert Pro., MPD) with Cu KR radiation (0.1540 nm), using an operating voltage of 40 kV and 40 mA, 0.017° step size, and 4.96 s step time. Nitrogen adsorption isotherms were measured with Micromeritics Tristar 3020 volumetric adsorption analyzer at 196 °C. The samples were degassed at 150 °C for 5 h prior to the measurements. The specific surface area of the samples was calculated by using the Brunauer–Emmett–Teller (BET) method. The pore size distributions were derived from the adsorption branches of isotherms by using the Barett–Joyner–Halenda (BJH) model, and the total pore volumes (Vt) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.99. Transmission Electron Microscopy (TEM) with energy dispersive X-ray analysis (EDX) was performed on a JEOL 2011 microscope operated at 200 kV. The UV–visible absorption spectra values were measured on a U-3010 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra. Calibration of spectra was performed by taking the C 1s electron peak (BE = 284.6 eV) as internal reference. The field-dependent magnetization curve of the materials was measured in a magnetic field of ±5 kOe at 300 K using Physical Property Measurement System (PPMS-9).
3. Results and discussion The wide-angle XRD patterns (Fig. 1) exhibited a well resolved diffraction peak at 2h = 44.6° and four weak diffraction peaks at 35.8°, 57.0°, 62.9° and 64.7° for all samples. 44.6° and 64.7° could
be indexed to the (1 1 0) and (2 0 0) diffraction of body-centered cubic (bcc) a-Fe (JCPDS card No. 06-0696). 35.8°, 57.0° and 62.9° could be assigned to the (3 1 1), (5 1 1) and (4 4 0) characteristic reflections from face centered cubic Fe3O4 (JCPDS card No. 481487) or cubic c-Fe2O3 (JCPDS card No. 39-1346), respectively. No nonmagnetic a-Fe2O3 could be detected in Fig. 1. The main peak of Fe3O4 and c-Fe2O3 for Fe2p3/2 was similar at about 710 eV (Fig. 2a). The O1s spectrum was shown in Fig. 2b. The reported values of O1s binding energy for Fe3O4 were 530.0 eV, slightly lower than that for c-Fe2O3 (530.6 eV) [21]. But the resolution of the spectra (sharpness) is too low to distinguish between Fe3O4 and c-Fe2O3. The O1s spectrum showed also that the iron oxide was Fe3O4/c-Fe2O3. XRD and XPS analysis of the materials revealed that metallic a-Fe co-existed with magnetite Fe3O4/c-Fe2O3. The carbon samples reported by Lee et al. [16] contained a-Fe and Fe3O4. The samples obtained by this one-pot loading method were also a-Fe and Fe3O4/c-Fe2O3. The advantages for the one-pot method were as follows. First, it took two steps in Ref. [16] by introducing carbon precursor and then iron sources. In comparison, we presented a one-pot method, which was simpler than previous reports. Second, solvents such as water were used to dissolve iron source or carbon precursor and a suction filtration process was needed before carbonization, which was the additional drawbacks for the synthesis. Iron nanoparticles (20–100 nm in diameter) could be synthesized by carbothermal reduction of iron compound and carbon under inert atmosphere above 600 °C [13,22]. So, Fe nanoparticles could be produced through carbonization process at a temperature as high as 800 °C under argon atmosphere in this study. While the existence of Fe3O4/c-Fe2O3 might be due to the fact that metallic Fe possessed extremely high reactivity and thus could be easily
Fig. 4. The TEM images of the materials for M-C-0.5 (a), M-C-1.0 (b) and M-C-1.5 (c) and the energy-dispersive X-ray spectroscopy (d).
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oxidized to iron oxides during the NaOH etching process [10,16,23]. The magnetic nanoparticle size was estimated by using the Scherrer equation of diffraction peak widths for the (3 1 1) peak of Fe3O4/c-Fe2O3 and (1 1 0) peak of a-Fe. The average crystal sizes of Fe3O4/c-Fe2O3 were 13.3, 14.0 and 15.2 nm and the sizes of Fe were 26.1, 30.3 and 35.4 nm for M-C-0.5, M-C-1.0 and M-C-1.5, increasing with the increase of FeCl3 loading. According to TG analysis curves recorded in air, the total amount of Fe loading for M-C0.5, M-C-1.0 and M-C-1.5 was calculated to be 2.8%, 5.3% and 8.7%, respectively (see Supporting information, Fig. S1). For the small angle XRD analysis, M-C-0.5 and M-C-1.0 exhibited well-resolved diffraction peaks (Fig. 3), confirming a wellordered mesoporous structure in the materials. M-C-1.5 exhibited poorly-resolved scattering peaks, indicating that the ordering of mesostructure had been partially deteriorated with the increase in amounts of FeCl3. The larger iron oxide particles might penetrate the pore walls through the mesochannels with the increase in amounts of FeCl3 [9], leading to the part loss of the ordering. The ordered arrangement of mesopores was clearly observed in M-C-0.5 and M-C-1.0 (Fig. 4a and b), giving evidence for the presence of ordered hexagonal mesostructure. While M-C-1.5 showed some less ordered arrays, (Fig. 4c), which was consistent with the results of XRD analysis. The dark spots were observed for Fe3O4/c-Fe2O3 or Fe nanoparticles, which were well dispersed into the carbon matrix and no bulk aggregates could be found on the outside surface of the mesoporous carbon. This result indicated that the materials were stable for application in separation. Fig. 4d was the energy-dispersive X-ray spectroscopy (EDS) taken on the round area in Fig. 4a provided the proof for the existence of Fe, O, and C. The element Cu could be ascribed to the Cu grid supporting the sample. N2 sorption isotherms of magnetic mesoporous carbons exhibited type IV curves with an obvious capillary condensation at a relative pressure P/P0 0.2–0.4 (Fig. 5a), indicating a uniform mesoporosity with a narrow pore size distribution (Fig. 5b). The parameters were listed in Table 1. The mean pore diameters for M-C-0.5, M-C-1.0 and M-C-1.5 exhibited 3.3, 3.4 and 3.4 nm, basically showing no variations. The specific surface area and the total pore volumes decreased with the increasing usage of FeCl3, which might arise from the higher density of magnetic iron oxide and Fe. The magnetization curves for the M-C-0.5, M-C-1.0 and M-C-1.5 (Fig. 6a) presented no hysteresis loop, indicating that the samples exhibited superparamagnetic characteristics desirable for their application in separation and targeted drug delivery under an external magnetic field. The corresponding saturation magnetization strengths were 0.8, 7.2 and 15.2 emu/g, increasing with the increase of FeCl3 loadings. This could be ascribed to the growth in size of magnetic nanoparticles with increased FeCl3 loadings. The values of the saturation magnetization strengths were much lower than that of the bulk Fe3O4 (80.7 emu/g) or c-Fe2O3 (83 emu/g) [9,24], mainly due to the much smaller particle size and the nonmagnetic or weak magnetic interfaces with carbon matrix. The magnetic separability of the magnetic Fe-containing carbon materials was tested in a water solution (Fig. 6b). Upon placement of a magnet near the glass bottle, the powder Fe-containing carbon samples were quickly attracted to the side of the bottle within a few seconds. Therefore, this provided an easy and effective way to separate the mesoporous carbons from the solution, which facilitated the materials to be used as magnetic targeting and separation. In addition, the materials could be well redispersed by shaking or ultrasonic vibration. As seen in Table 1, the loading degrees increased with the increase of the specific surface area and pore volume of the mesoporous carbons. For M-C-0.5, a drug loading degree of up to 30% could be reached, whereas 23% and 18% could be reached for MC-1.0 and M-C-1.5, respectively. It was well known that the bigger
the specific surface area and pore volume, the more chance and space for IBU adsorption on the carbons were, thus leading to the higher loading degree. The loading degree had an effect on release rate and the release rate increased with the increase of the loading degree [25,26]. Fig. 7 showed the drug release behavior from M-C-0.5, M-C-1.0 and M-C-1.5. In order to investigate the diffusion mechanism of the drug from the materials, the drug release data was fitted with the Korsmeyer–Peppas equation:
f ¼ kt
n
ð1Þ
where f was the fractional release of drug, k was the rate constant, t was the elapsed time and n was the release exponent describing the drug release mechanism [27–29]. The release exponent n = 0.5 corresponded to a fully Fickian diffusion based transport of drug to the dissolution medium. The diffusion parameters (n and k) could be obtained through a log–log analysis of the above equation under
Fig. 5. N2 sorption isotherms (a) and pore size distributions (b) of the materials.
Table 1 The structural parameters of magnetic mesoporous carbons. Sample
SBET (m2/g)
Vt (cm3/g)
Dpore (nm)
Loading degree
M-C-0.5 M-C-1.0 M-C-1.5
1471 1068 667
0.98 0.80 0.75
3.3 3.4 3.4
30 23 18
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The results indicated that the release rate became slow and the release mechanism deviated still further from Fickian diffusionbased behavior with the increase of FeCl3 loading. This could be ascribed to the deformed channel (the poor order) and big magnetic nanoparticles which sterically hindered the free diffusion of ibuprofen. In a word, the poor order might contribute to the relative slow release rate of IBU. Meanwhile, the low loading degree also slowed down the release rate. 4. Conclusions A simple and novel route has been developed to synthesize magnetic Fe-containing mesoporous carbons by an incipient-wetness impregnation technique without using a solvent. The species of iron in the resulting carbons were metallic a-Fe and magnetite Fe3O4/c-Fe2O3. The materials exhibited superparamagnetic characteristics desirable for their application to separation and targeted drug delivery. IBU release behavior from the materials showed that the release rate of IBU increased with the increase of the order and loading degree. It was believed that the simple and novel route could provide a common path to the synthesis of other metals in porous materials. Acknowledgments The authors acknowledge the financial support from the National Science Foundation of China (No. 50802017), the Medical Science Research Fund of Guangdong Province (No. B2009118) and the Teaching Staff Construction Fund of Guangdong Pharmaceutical University. Appendix A. Supplementary data Fig. 6. The magnetization curves (a) of the materials and images of the magnetic separability (b) for M-C-1.0.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2011.04.033. References
Fig. 7. The drug release behavior from the materials.
the condition of f < 0.6. The model was proved to describe the initial 60% ibuprofen release very well with high correlation coefficients (R2 > 0.99) in all cases. The values of rate constant k for M-C-0.5, M-C-1.0 and M-C-1.5 was 8.1, 5.9 and 2.3, decreasing with the increase of FeCl3 loading. While the release exponent n increased in this order, which was 0.54, 0.62 and 0.67.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Novel synthesis of Fe-containing mesoporous carbons and their release of ibuprofen Xiufang Wang a, Ping Liu b, Yong Tian a,⇑, Linquan Zang a a b
College of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
a r t i c l e
i n f o
Article history: Received 23 February 2011 Received in revised form 21 April 2011 Accepted 25 April 2011 Available online 3 May 2011 Keywords: Mesoporous carbon Magnetic Synthesis Drug release
a b s t r a c t A simple route has been developed to synthesize magnetic Fe-containing mesoporous carbon by an incipient-wetness impregnation technique without using a solvent. The materials were characterized by using X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, transmission electron microscopy, and physical property measurements. The species of iron in the resulting carbons were metallic a-Fe and magnetite Fe3O4/c-Fe2O3. The ordering of the mesostructure, the specific surface area and the total pore volumes decreased with the increasing amount of FeCl3 used. The saturation magnetization strength could be easily adjusted by changing the amount of FeCl3 used in the synthesis. Ibuprofen (IBU) release behavior from the materials showed that the release rate of IBU increased with the increase of the order and loading degree. Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved.
1. Introduction Ordered mesoporous carbons (OMCs), have attracted great technological interest for the development of adsorption and purification of water, electronic, catalytic and energy storage systems, due to their remarkable properties such as high specific surface area, narrow pore size distribution, tunable pore structure, large pore volume and high thermal and mechanical stability. Moreover, the excellent biocompatibility and controllable pore size distribution of OMCs make them highly prospective and valuable in drug delivery system [1–8]. However, carbon powders are notoriously difficult to be separated from solutions. The conventional approach normally involves a filtration or centrifugation procedure, which is rather complex and expensive [9]. Magnetic mesoporous materials containing Fe, Co, Ni, or alloys magnetic nanoparticles provided an alternative opportunity for the separation. Among them, iron nanoparticles were more preferred candidates for many advanced nanotechnological applications [10], such as magnetic storage media, directed drug delivery, and groundwater remediation or other environmental applications. So far, there are generally two routes widely used to synthesize Fecontaining mesoporous carbons. One is soft-templating synthesis [9–11]. Although the route is simple, the mesoporous materials usually have low pore volumes and surface areas, which limit their applications in many fields [12]. The other is hard-templating synthesis, after preparation of silica template, filling the silica mesopore with appropriate carbon precursors (furfuryl alcohol/FA or pyrrole) and iron sources (FeCl3 or Fe(NO)3), which involve two steps. Some ⇑ Corresponding author. Tel./fax: +86 2039352129. E-mail address: [email protected] (Y. Tian).
researches reported that iron sources (FeCl3 or Fe(NO)3) were firstly introduced into the silica template and then carbon precursors (FA or pyrrole) [2,13–15], while other studies investigated that carbon precursors were firstly introduced into the silica template and then iron sources [16,17]. However, the synthesis involving two steps was fussy and difficult to be controlled. Besides, solvents such as ethanol or water needed for dissolving carbon precursor or iron source were the additional drawbacks for the synthesis. Consequently, an easy and simple synthesis procedure for magnetic mesoporous carbon was in urgent demand for practical applications. Herein, we successfully present a simple and novel route to synthesize magnetic Fe-containing mesoporous carbons by an incipient-wetness impregnation technique without using a solvent. Carbon precursor (FA) and iron source (FeCl3) were introduced simultaneously into the silica template in one step. FeCl3 was directly dissolved in FA which acted dual functions as solvent and carbon sources, and then the above solution containing FA and FeCl3 were impregnated simultaneously into the silica template in one step. IBU, as an anti-inflammatory drug, has been commonly used as the release drug model due to its good pharmacological activity and the small molecule size (1 nm), which could fit easily into the pores of OMCs. The drug loading and release behavior from Fe-containing carbons and the species of iron in the resulting carbons were also reported.
2. Materials and methods 2.1. Synthesis of SBA-15 templates SBA-15 was synthesised according to Zhao and coworkers [18]. In a typical synthesis, 4.00 g of P123 (PEO20-PPO70-PEO20;
1387-1811/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.04.033
X. Wang et al. / Microporous and Mesoporous Materials 145 (2011) 98–103
99
Fig. 1. Wide-angle XRD patterns of the materials.
Aldrich) was added into 144 ml of aqueous HCl solution (1.7 M) and the mixture was stirred for 4 h at 40 °C. Next, 8.00 g of tetraethyl orthosilicate (Fuchen Chemical Regent of Tianjin) was added dropwise. The resulting gels were transferred to the Teflon-lined sealed containers and were kept at 70 °C for 48 h under static hydrothermal conditions. The as-synthesized samples were extracted with a mixture of 200 ml of ethanol 95% (v/v) and 4 ml of concentrated HCl for 24 h twice. All chemicals were used as received without further purification. Ultrapure water was used in all experiments. 2.2. Synthesis of magnetic carbon materials Incipient-wetness impregnation was employed to introduce simultaneously carbon precursor (FA) and iron source (FeCl3) into the SBA-15 template with oxalic acid as a homogeneous catalyst. Typically, 0.5 mmol of FeCl3 and 0.05 g of oxalic acid were dissolved in 2 ml of FA solution. SBA-15 was impregnated with the above solution at room temperature. The mixture thus prepared was heated at 80 °C for 12 h under vacuum for the polymerization of FA and then at 150 °C for 6 h. After cooling to room temperature, the sample was heated to 300 °C (1 °C/min), then to 800 °C (2 °C/ min) for 3 h under flowing argon atmosphere (200 ml/min). The silica template was removed by boiling the materials in 1 M NaOH solution dissolved in 1:1 (v:v) mixtures of water and ethanol for more than 1 h twice. The obtained black powders were filtered, washed with water and dried under vacuum at 60 °C for 24 h. The obtained sample was denoted as M-C-0.5. The same procedures were carried out for the preparation of sample M-C-1.0 and M-C-1.5, except that the amounts of FeCl3 were 1.0 and 1.5 mmol for M-C-1.0 and M-C-1.5.
Fig. 2. XPS spectra of the materials for (a) Fe2p3/2 and (b) O1s.
2.3. Loading and in vitro IBU release 0.25 g of magnetic carbon materials was added into 50 ml of IBU solution in hexane (35 mg/ml), followed by stirring at room temperature for 72 h in a closed batch to prevent the evaporation of hexane. The loaded materials were then filtered and dried under vacuum at 40 °C for 24 h. Twenty milligrams of the resulting materials were immersed into 50 ml of simulated body fluid (SBF, pH 7.4) [19,20] at 37 °C under stirring at 100 rpm. Three milliliters of samples were removed at given time intervals, diluted to 10 ml with SBF, and analyzed by UV–visible spectroscopy at a wavelength of 272 nm. The volume removed was replaced with
Fig. 3. Small-angle XRD patterns of the materials.
the same amount of preheated SBF. The experiments were carried out in triplicate. The loading degrees of IBU were determined by UV–visible spectroscopy analysis.
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2.4. Characterization The X-ray diffraction (XRD) patterns were recorded on a Multi Purpose Diffractometer (PANanalytical. Inc. X’Pert Pro., MPD) with Cu KR radiation (0.1540 nm), using an operating voltage of 40 kV and 40 mA, 0.017° step size, and 4.96 s step time. Nitrogen adsorption isotherms were measured with Micromeritics Tristar 3020 volumetric adsorption analyzer at 196 °C. The samples were degassed at 150 °C for 5 h prior to the measurements. The specific surface area of the samples was calculated by using the Brunauer–Emmett–Teller (BET) method. The pore size distributions were derived from the adsorption branches of isotherms by using the Barett–Joyner–Halenda (BJH) model, and the total pore volumes (Vt) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.99. Transmission Electron Microscopy (TEM) with energy dispersive X-ray analysis (EDX) was performed on a JEOL 2011 microscope operated at 200 kV. The UV–visible absorption spectra values were measured on a U-3010 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra. Calibration of spectra was performed by taking the C 1s electron peak (BE = 284.6 eV) as internal reference. The field-dependent magnetization curve of the materials was measured in a magnetic field of ±5 kOe at 300 K using Physical Property Measurement System (PPMS-9).
3. Results and discussion The wide-angle XRD patterns (Fig. 1) exhibited a well resolved diffraction peak at 2h = 44.6° and four weak diffraction peaks at 35.8°, 57.0°, 62.9° and 64.7° for all samples. 44.6° and 64.7° could
be indexed to the (1 1 0) and (2 0 0) diffraction of body-centered cubic (bcc) a-Fe (JCPDS card No. 06-0696). 35.8°, 57.0° and 62.9° could be assigned to the (3 1 1), (5 1 1) and (4 4 0) characteristic reflections from face centered cubic Fe3O4 (JCPDS card No. 481487) or cubic c-Fe2O3 (JCPDS card No. 39-1346), respectively. No nonmagnetic a-Fe2O3 could be detected in Fig. 1. The main peak of Fe3O4 and c-Fe2O3 for Fe2p3/2 was similar at about 710 eV (Fig. 2a). The O1s spectrum was shown in Fig. 2b. The reported values of O1s binding energy for Fe3O4 were 530.0 eV, slightly lower than that for c-Fe2O3 (530.6 eV) [21]. But the resolution of the spectra (sharpness) is too low to distinguish between Fe3O4 and c-Fe2O3. The O1s spectrum showed also that the iron oxide was Fe3O4/c-Fe2O3. XRD and XPS analysis of the materials revealed that metallic a-Fe co-existed with magnetite Fe3O4/c-Fe2O3. The carbon samples reported by Lee et al. [16] contained a-Fe and Fe3O4. The samples obtained by this one-pot loading method were also a-Fe and Fe3O4/c-Fe2O3. The advantages for the one-pot method were as follows. First, it took two steps in Ref. [16] by introducing carbon precursor and then iron sources. In comparison, we presented a one-pot method, which was simpler than previous reports. Second, solvents such as water were used to dissolve iron source or carbon precursor and a suction filtration process was needed before carbonization, which was the additional drawbacks for the synthesis. Iron nanoparticles (20–100 nm in diameter) could be synthesized by carbothermal reduction of iron compound and carbon under inert atmosphere above 600 °C [13,22]. So, Fe nanoparticles could be produced through carbonization process at a temperature as high as 800 °C under argon atmosphere in this study. While the existence of Fe3O4/c-Fe2O3 might be due to the fact that metallic Fe possessed extremely high reactivity and thus could be easily
Fig. 4. The TEM images of the materials for M-C-0.5 (a), M-C-1.0 (b) and M-C-1.5 (c) and the energy-dispersive X-ray spectroscopy (d).
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oxidized to iron oxides during the NaOH etching process [10,16,23]. The magnetic nanoparticle size was estimated by using the Scherrer equation of diffraction peak widths for the (3 1 1) peak of Fe3O4/c-Fe2O3 and (1 1 0) peak of a-Fe. The average crystal sizes of Fe3O4/c-Fe2O3 were 13.3, 14.0 and 15.2 nm and the sizes of Fe were 26.1, 30.3 and 35.4 nm for M-C-0.5, M-C-1.0 and M-C-1.5, increasing with the increase of FeCl3 loading. According to TG analysis curves recorded in air, the total amount of Fe loading for M-C0.5, M-C-1.0 and M-C-1.5 was calculated to be 2.8%, 5.3% and 8.7%, respectively (see Supporting information, Fig. S1). For the small angle XRD analysis, M-C-0.5 and M-C-1.0 exhibited well-resolved diffraction peaks (Fig. 3), confirming a wellordered mesoporous structure in the materials. M-C-1.5 exhibited poorly-resolved scattering peaks, indicating that the ordering of mesostructure had been partially deteriorated with the increase in amounts of FeCl3. The larger iron oxide particles might penetrate the pore walls through the mesochannels with the increase in amounts of FeCl3 [9], leading to the part loss of the ordering. The ordered arrangement of mesopores was clearly observed in M-C-0.5 and M-C-1.0 (Fig. 4a and b), giving evidence for the presence of ordered hexagonal mesostructure. While M-C-1.5 showed some less ordered arrays, (Fig. 4c), which was consistent with the results of XRD analysis. The dark spots were observed for Fe3O4/c-Fe2O3 or Fe nanoparticles, which were well dispersed into the carbon matrix and no bulk aggregates could be found on the outside surface of the mesoporous carbon. This result indicated that the materials were stable for application in separation. Fig. 4d was the energy-dispersive X-ray spectroscopy (EDS) taken on the round area in Fig. 4a provided the proof for the existence of Fe, O, and C. The element Cu could be ascribed to the Cu grid supporting the sample. N2 sorption isotherms of magnetic mesoporous carbons exhibited type IV curves with an obvious capillary condensation at a relative pressure P/P0 0.2–0.4 (Fig. 5a), indicating a uniform mesoporosity with a narrow pore size distribution (Fig. 5b). The parameters were listed in Table 1. The mean pore diameters for M-C-0.5, M-C-1.0 and M-C-1.5 exhibited 3.3, 3.4 and 3.4 nm, basically showing no variations. The specific surface area and the total pore volumes decreased with the increasing usage of FeCl3, which might arise from the higher density of magnetic iron oxide and Fe. The magnetization curves for the M-C-0.5, M-C-1.0 and M-C-1.5 (Fig. 6a) presented no hysteresis loop, indicating that the samples exhibited superparamagnetic characteristics desirable for their application in separation and targeted drug delivery under an external magnetic field. The corresponding saturation magnetization strengths were 0.8, 7.2 and 15.2 emu/g, increasing with the increase of FeCl3 loadings. This could be ascribed to the growth in size of magnetic nanoparticles with increased FeCl3 loadings. The values of the saturation magnetization strengths were much lower than that of the bulk Fe3O4 (80.7 emu/g) or c-Fe2O3 (83 emu/g) [9,24], mainly due to the much smaller particle size and the nonmagnetic or weak magnetic interfaces with carbon matrix. The magnetic separability of the magnetic Fe-containing carbon materials was tested in a water solution (Fig. 6b). Upon placement of a magnet near the glass bottle, the powder Fe-containing carbon samples were quickly attracted to the side of the bottle within a few seconds. Therefore, this provided an easy and effective way to separate the mesoporous carbons from the solution, which facilitated the materials to be used as magnetic targeting and separation. In addition, the materials could be well redispersed by shaking or ultrasonic vibration. As seen in Table 1, the loading degrees increased with the increase of the specific surface area and pore volume of the mesoporous carbons. For M-C-0.5, a drug loading degree of up to 30% could be reached, whereas 23% and 18% could be reached for MC-1.0 and M-C-1.5, respectively. It was well known that the bigger
the specific surface area and pore volume, the more chance and space for IBU adsorption on the carbons were, thus leading to the higher loading degree. The loading degree had an effect on release rate and the release rate increased with the increase of the loading degree [25,26]. Fig. 7 showed the drug release behavior from M-C-0.5, M-C-1.0 and M-C-1.5. In order to investigate the diffusion mechanism of the drug from the materials, the drug release data was fitted with the Korsmeyer–Peppas equation:
f ¼ kt
n
ð1Þ
where f was the fractional release of drug, k was the rate constant, t was the elapsed time and n was the release exponent describing the drug release mechanism [27–29]. The release exponent n = 0.5 corresponded to a fully Fickian diffusion based transport of drug to the dissolution medium. The diffusion parameters (n and k) could be obtained through a log–log analysis of the above equation under
Fig. 5. N2 sorption isotherms (a) and pore size distributions (b) of the materials.
Table 1 The structural parameters of magnetic mesoporous carbons. Sample
SBET (m2/g)
Vt (cm3/g)
Dpore (nm)
Loading degree
M-C-0.5 M-C-1.0 M-C-1.5
1471 1068 667
0.98 0.80 0.75
3.3 3.4 3.4
30 23 18
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The results indicated that the release rate became slow and the release mechanism deviated still further from Fickian diffusionbased behavior with the increase of FeCl3 loading. This could be ascribed to the deformed channel (the poor order) and big magnetic nanoparticles which sterically hindered the free diffusion of ibuprofen. In a word, the poor order might contribute to the relative slow release rate of IBU. Meanwhile, the low loading degree also slowed down the release rate. 4. Conclusions A simple and novel route has been developed to synthesize magnetic Fe-containing mesoporous carbons by an incipient-wetness impregnation technique without using a solvent. The species of iron in the resulting carbons were metallic a-Fe and magnetite Fe3O4/c-Fe2O3. The materials exhibited superparamagnetic characteristics desirable for their application to separation and targeted drug delivery. IBU release behavior from the materials showed that the release rate of IBU increased with the increase of the order and loading degree. It was believed that the simple and novel route could provide a common path to the synthesis of other metals in porous materials. Acknowledgments The authors acknowledge the financial support from the National Science Foundation of China (No. 50802017), the Medical Science Research Fund of Guangdong Province (No. B2009118) and the Teaching Staff Construction Fund of Guangdong Pharmaceutical University. Appendix A. Supplementary data Fig. 6. The magnetization curves (a) of the materials and images of the magnetic separability (b) for M-C-1.0.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2011.04.033. References
Fig. 7. The drug release behavior from the materials.
the condition of f < 0.6. The model was proved to describe the initial 60% ibuprofen release very well with high correlation coefficients (R2 > 0.99) in all cases. The values of rate constant k for M-C-0.5, M-C-1.0 and M-C-1.5 was 8.1, 5.9 and 2.3, decreasing with the increase of FeCl3 loading. While the release exponent n increased in this order, which was 0.54, 0.62 and 0.67.
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