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Luminescent and Mesoporous Europium-Doped Bioactive Glasses (MBG) as a Drug Carrier Yong Fan,†,‡ Piaoping Yang,† Shanshan Huang,† Jinhua Jiang,‡ Hongzhou Lian,† and Jun Lin*,† State Key laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and State Key Laboratory of Inorgnic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130021, Jilin/People’s Republic of China ReceiVed: January 18, 2009; ReVised Manuscript ReceiVed: March 11, 2009
Luminescent and mesoporous europium-doped bioactive glasses (MBG:Eu) were successfully synthesized by a two-step acid-catalyzed self-assembly process combined with hydrothermal treatment in an inorganic-organic system. The obtained MBG was performed as a drug delivery carrier to investigate the drug storage/release properties using ibuprofen (IBU) as a model drug. The structural, morphological, textural and optical properties were well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption/ desorption, and photoluminescence (PL) spectra, respectively. The results reveal that the MBG exhibit the typical ordered characteristics of the hexagonal mesostructure. This composite shows sustained release profile with ibuprofen as the model drug. The IBU-loaded samples still show red luminescence of Eu3+ (5D0-7F1, 2) under UV irradiation, and the emission intensities of Eu3+ in the drug carrier system vary with the released amount of IBU, thus making the drug release be easily tracked and monitored by the change of the luminescence intensity. The system demonstrates a great potential in the drug delivery and disease therapy fields. 1. Introduction Bioactive glasses (BG) and glasses ceramics have been widely studied and used since the pioneering work by Hench et al. in 1971. Because such materials have the ability to chemically bond with living bone tissue, these biomaterials have been used in a variety of medical applications, such as implants in clinical bone repair and regeneration materials, bioactive coating of metallic implants in tissue engineering, tumor treatment, protein and/or cell activation, etc.1-10 The inorganic part of the human bone is hydroxycarbonate apatite (HCA), and when BGs are implanted in the human body, a HCA layer with the ability to bond with living bone is formed on the surface of the bioactive material.5,10 Studies accelerated after the development of new family of biomaterials mesoporous bioactive glasses (MBG) by Zhao et al. in 2004.11 Zhao et al. successfully synthesized highly hexagonally ordered MBGs (SiO2 • CaO • P2O5 system) by templating with a triblock copolymer, EO20PO70EO20 (P123). Compared to conventional BGs, MBGs show different structure and composition and have more specific surface area and pore volume, which may greatly accelerate the kinetic deposition process of hydroxycarbonate apatite and therefore enhance their bone-forming bioactivity.11-13 The high pore volume of the well-ordered mesoporous structure of MBG makes it possible for a direct and highly efficient immobilization of proteins and/or drug molecules within MBGs, which are practically important in medical applications.14 Chang et al. reported a well-ordered mesoporous bioactive glass (MBG) with high specific surface area synthesized in aqueous solution by a two-step acid-catalyzed self-assembly process combined with hydrothermal treatment and studied its drug release property which opens up a new opportunity in drug * To whom correspondence should be addressed. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Jilin University.
delivery.15 So far a large number of drug storage/release systems based on ordered mesoporous materials have been reported such as biodegradable polymers, hydroxyapatite (HA), calcium phosphate cement (CFC), xerogels, hydrogels, and mesoporous silica.16-20 Recently, ordered mesoporous materials have gained enhanced interest with particular attention as drug storage and release hosts due to their unique surface and textural properties, including stable mesostructure, tunable pore size, and easymodified surface features for site-specific delivery.1,2 Several research groups such as Vallet-Regi Maria, Victor S.-Y. Lin et al. have reported the drug delivery systems using the ordered mesoporous materials as carriers.1,21 Current research is focused on the design of ordered mesoporous materials with certain functional groups that respond to environmental changes and thus modify the adsorption and release characteristics.1 Mesoporous materials functionalized with photoluminescence (PL) can be of potential application in the fields of drug delivery and disease diagnose and therapy. Our group’s studies show that these drugs controlled delivery systems not only have high pore volume for the storage and delivery of drugs but also possess photoluminescence properties which can be tracked to evaluate the efficiency of the drug release.18,22-27 Herein, we report the synthesis of a novel luminescence functionalized mesoporous MBG material. This biomaterial may have potential use in a variety of medical applications, such as implants in clinical bone repair and regeneration materials, bioactive coating of metallic implants in tissue engineering and so on. It shows stable mesoporous structure, large pore volume and pore diameter, and large specific surface area with a large amount of Si-OH groups on the surface, which are suitable for loading high amount of drug molecules and possessing high drug sustained release property. This material with rare earth element have the ability of photoluminescence, and its emission intensity will change with the cumulative released amount of the drug (IBU) in the system. Thus the extent of drug release
10.1021/jp900515x CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
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SCHEME 1: Experimental Process for the Loading and Release of IBU on the MBG:Eu Composite
can be easily identified, tracked, and monitored by the change of luminescence. 2. Experimental Section 2.1. Synthesis of Luminescence Functionalized MBG (SiO2 · CaO · P2O5 system). All the reagents including (EO)20(PO)70(EO)20 (P123, Mw ) 5800, Aldrich), Triethyl phosphate (TEP, Sinopharm Chemical Reagent Co., Ltd.), Ca(NO3)2 · 4H2O (Beijing Chemical Regent Co., Ltd.), tetraethyl orthosilicate (TEOS, 99%, Beijing Chemical Regent Company, Beijing), Eu2O3 (99.999%, Science and Technology Parent Company of Changchun Institute of Applied Chemistry), and HNO3 (A. R., Beijing Beihua Chemicals Co., Ltd.) were received without further purification. The doping concentration of Eu3+ was 5 mol % to Ca2+ in MBG: Eu. Eu(NO3)3 was obtained by dissolving stoichiometric Eu2O3 in dilute HNO3 with vigorous stirring. The superfluous HNO3 in the solution was driven off until the Eu(NO3)3 powders were obtained. The luminescence functionalized MBG. was prepared according to the literature with some modifications.15 In a typical process, 3.0 g of P123 was dissolved in 120 mL of 2 M HNO3 and 30 mL of distilled water solution and stirred at 35 °C in water bath until the solution became clear. TEOS (8.50 g), 0.98 g of TEP, and 5.94 g of Ca(NO3)2 · 4H2O and 0.676 g of the as-prepared Eu(NO3)3 were then added into the solution. The mixture was stirred at 35 °C for 12 h, and then was hydrothermalized at 100 °C for 48 h. Without any filtering and washing, the resulting precipitate was directly dried at 100 °C for 20 h in air. The as-synthesized material was calcined from room temperature to 550 °C with a heating rate of 1 °C/min, and kept at 550 °C for 6 h to remove the templates. 2.2. Preparation of Drug Storage/Release System. The drug storage/release system using luminescence functionalized MBG as a carrier was prepared according to the previous report.28 Ibuprofen (IBU, Nanjing Chemical Regent Co., Ltd.) was selected as the model drug. Typically, 0.4 g of MBG:Eu sample was added into 50 mL of hexane solution with a IBU concentration of 60 mg/mL at room temperature, and soaked for 24 h with stirring in a vial which was sealed to prevent the evaporation of hexane. The IBU-loaded MBG:Eu sample, denoted as IBU-MBG:Eu, was separated by centrifugation, and then dried at 60 °C for 12 h. The in Vitro delivery of IBU was performed by immersing 0.2 g of the sample in the release media of simulated body fluid (SBF) with slow stirring under the immersing temperature of 37 °C. The ionic composition of the as-prepared SBF solution was similar to that of human body plasma with a molar composition of 142.0/5.0/2.5/1.5/147.8/4.2/1.0/0.5 for Na+/K+/ Ca2+/Mg2+/Cl-/HCO3-/HPO42-/SO42- (pH ) 7.4).28 The ratio of SBF to adsorbed IBU was kept at 1 mL/mg. The amount of IBU adsorbed onto the MBG was monitored by thermogravimetry (TG) analysis. The solution of IBU in SBF has a strong absorb band at a wavelength of 220 nm by UV-vis spectroscopy, so the amount of IBU released at certain set times can
determined by UV-vis spectroscopy at the wavelength of 220 nm. The experimental process for the loading and subsequent release of the IBU on the luminescent MBG: Eu carrier are shown in Scheme 1. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a Bruker AXS -D8 FOCUS diffractometer using Cu KR radiation (λ ) 0.15405 nm). IR spectra were recorded on a Perkin-Elmer 580B IR spectrophotometer using KBr pellet technique. Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images were recorded on a FEI Tecnai G2 S-Twin with an acceleration voltage of 200 kV. Nitrogen adsorption/ desorption analysis was measured using a Micromeritics ASAP 2010 M apparatus. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method using the data between 0.05 and 0.35. The pore volume was obtained from the t-plot method. Thermogravimetry (TG) measurement (Netzsch Thermoanalyzer STA 409) was used to determine the loading amount of IBU on the materials in an atmospheric environment with a heating rate of 10 °C min-1 from room temperature to 650 °C. The UV-vis excitation and emission spectra were obtained on a Hitachi F-4500 spectrofluorimeter equipped with a 150 W xenon lamp as the excitation source. The UV-vis adsorption spectral values were measured on a TU-1901 spectrophotometer. All the measurements were performed at room temperature. 3. Results and Discussion 3.1. Structure, Formation, Morphology, and Luminescent Properties of Eu:MBG, and IBU-Eu:MBG. Figure 1 show the wide-angle XRD patterns of MBG:Eu. The typical characteristic diffraction peaks of Eu2O3 (JCPDS 34-0392) can be observed at 2θ ) 28.4, 32.9, 47.3, and 56.0° respectively, suggesting the successful crystallization of Eu2O3 on the
Figure 1. Wide-angle XRD patterns and low-angle XRD (inset) patterns of MBG:Eu.
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Figure 2. TEM images of MBG:Eu (a, b, c), and the HRTEM image of MBG:Eu (d).
framework of mesoporous MBG. The low-angle powder X-ray diffraction (SAXRD) patterns (Figure 2 insert) of Eu-MBG powders exhibit three well-resolved peaks that can be indexed as (100), (110), (200) reflections associated with p6mm hexagonal symmetry, which indicates the existence of a high degree of hexagonal mesoscopic organization.15,29 Figure 2 displays the TEM images and the HRTEM image of MBG: Eu, respectively. From the TEM image of MBG: Eu (Figure 2c), the typical characteristics of hexagonally packed mesostructure are present. The observed distance 8.60 nm between the two adjacent fringes agrees well with that 8.65 nm calculated from the low-angle XRD (Figure 1 insert). From the HRTEM image of MBG: Eu (Figure 2d) the crystalline phase of Eu2O3 with well-resolved lattice fringes can be observed. The distances (0.31 nm) between the adjacent lattice fringes agree well with that of d222 spacing values (0.3138 nm) (JCPDS No. 34-0392). The IR spectra for IBU, MBG:Eu and IBU-MBG:Eu are displayed in Figure 3, respectively. As shown in Figure 3a for the MBG:Eu, the obvious broad absorption bands, assigned to OH (3433 cm-1) and H2O (1632 cm-1), indicate that a large number of -OH groups and H2O are present on the surface of MBG, which are important for bonding drug (IBU) molecules. The peaks centered at 806, 461 cm-1 are assigned to the Si-O-Si bands. The peaks centered at 1037 cm-1 are assigned to the P-O bands.30 For the IBU-MBG:Eu (Figure 3b), the sharp band centered at 1713 cm-1 is attributed to the vibration of -COOH which is same as the IBU Figure 3c. The absorption bands assigned to the quaternary carbon atom located at 1460 and 1510 cm-1, tertiary carbon atom at 1330 cm-1, O-H bending vibration at 1423 cm-1, and C-Hx bond at 2929 and 2960 cm-1 can also be observed (Figure 3b),18,31 which confirm the successful adsorption of IBU onto the surface of the mesoporous hydroxyapatite. The nitrogen sorption isotherm of the MBG:Eu is shown in Figure 4, which is type IV with type H1 hysteresis loops typical
Figure 3. IR spectra of MBG:Eu (a), IBU-MBG:Eu (b), and IBU (c).
of mesoporous materials with 1-D cylindrical channels. The results reveal that the doping of Eu3+ has not altered the basic pore structure of mesoporous MBG. The MBG:Eu has a narrow pore distribution and the average pore size is 6.29 nm. This material has BET surface area of 598.49 m2/g, pore volume of 0.8481 cm3/g. After the adsorption of IBU, the BET surface area, pore volume, and pore size drop to 226.93 m2/g, 0.335 cm3/g, and 5.905 nm, respectively (Figure 5). These results further prove that IBU has been successfully incorporated into the channels of mesoporous MBG:Eu, which is well consistent with above IR results. Under UV lamp irradiation (365 nm), the MBG:Eu, IBUMBG:Eu, and IBU released IBU-MBG:Eu samples can show red luminescence. The experimental process for the loading and release of IBU on the MBG:Eu composite is shown in Scheme 1. Figure 6 and 7 show the PL excitation and emission spectra of MBG:Eu and IBU-MBG:Eu, respectively. In the excitation
Bioactive Glasses as a Drug Carrier
Figure 4. N2 adsorption/desorption isotherms for MBG:Eu and pore size distribution (insert) of MBG:Eu.
Figure 5. N2 adsorption/desorption isotherms and corresponding pore size distribution (insert) for IBU-MBG:Eu.
Figure 6. Excitation (a) and emission (b) spectra for MBG:Eu.
spectra monitored by the Eu3+5D0-7F2 transition at 612 nm, the broadband with a maximum near 254 nm may arise from the charge transfer transition between Eu3+ and O2-, and some sharp peaks originating from the f-f transitions of Eu3+ can also be observed in the longer wavelength region (Figure 6a and Figure 7a).31 Upon excitation at 254 nm, the characteristic transition lines from the excited 5D0 level of Eu3+ can be detected in the emission spectra (Figure 6b, Figure 7b), and the locations of the emission lines with their assignments are labeled as well.32 The two main characteristic peaks from 5 D0f7F1 (590 nm) and 5D0f7F2 (612 nm) are dominant. It is worth noting that the characteristic emission lines are still obvious in the emission spectrum for IBU-MBG:Eu (Figure 7b),
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Figure 7. Excitation (a) and emission (b) spectra for IBU-MBG:Eu.
Figure 8. TG-DTA curves for IBU-MBG:Eu.
showing the potential application to be tracked or monitored by the luminescence. A detailed relationship between the emission intensity and extent of IBU drug release in the IBUMBG:Eu system are to be discussed in next section. 3.2. Drug Loading and Release Properties. During the loading and release process, the IBU molecules can be adsorbed onto the surface of mesoporous materials in the impregnation process and liberated by diffusion-controlled mechanism. The OH groups on the surface could form hydrogen bonds with the carboxyl groups of IBU when IBU is adsorbed on the surface. During the release process, the solvent enters the IBU-matrix phase through the pores. The drug is then slowly dissolved into SBF from the surface and channels of the MBG:Eu. The IBU loading for IBU is 35 wt% determined by TG analysis (Figure 8). The cumulative drug release profiles for the IBU-MBG:Eu systems as a function of release time in SBF are shown in Figure 9. This system shows a quick release of about 65% within 6 h followed by the relatively slow release and completely release after 100 h. The initial quick release may be attributed to the IBU weakly adsorbed on the outer surface of mesoporous hydroxyapatite, and the slow release of the rest of IBU can be due to the strong interaction between IBU molecules and the surface. It should be noted that the luminescence functionalized MBG shows much similar release characteristic in comparison with that of pure MBG due to their similar surface and textural properties for the two samples. The PL emission intensity of IBU-MBG:Eu as a function of cumulative released amount of IBU is given in Figure 10. It can be seen that the PL intensity (defined as the integrated area
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Figure 9. Cumulative ibuprofen release from IBU-MBG:Eu systems as a function of release time in the release of media of SBF.
Figure 10. PL emission intensity of Eu3+ in IBU-MBG:Eu as a function of cumulative release amount of ibuprofen.
intensity of 5D0-7F2 and 5D0-7F1 of Eu3+) increases with the cumulative released IBU, and reaches a maximum when IBU is completely released. It is well-known that the emission of Eu3+ will be quenched to some extent in the environments where high phonon frequency is present, such as OH groups with a vibrational frequency near 3450 cm-1.29 The organic groups in IBU with high vibration frequencies from 1000 to 3250 cm-1 will greatly quench the emission of Eu3+ in IBU-MBG:Eu. The quenching effect will be weakened with the release of IBU, resulting in the increase of emission intensity. This relationship between the PL intensity and drug release extent can be potential as a probe for monitoring or tracking the drug release during the drug release process. Acknowledgment. This project is financially supported by National Basic Research Program of China (2007CB935502), and the National Natural Science Foundation of China (NSFC 50702057, 50872131, 00610227, 20871035).
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