Europium-doped amorphous calcium phosphate porous nanospheres: preparation and application as luminescent drug carriers
© Chen et al; licensee Springer. 2011
Received: 11 June 2010
Accepted: 12 January 2011
Published: 12 January 2011
Calcium phosphate is the most important inorganic constituent of biological tissues, and synthetic calcium phosphate has been widely used as biomaterials. In this study, a facile method has been developed for the fabrication of amorphous calcium phosphate (ACP)/polylactide-block-monomethoxy(polyethyleneglycol) hybrid nanoparticles and ACP porous nanospheres. Europium-doping is performed to enable photoluminescence (PL) function of ACP porous nanospheres. A high specific surface area of the europium-doped ACP (Eu3+:ACP) porous nanospheres is achieved (126.7 m2/g). PL properties of Eu3+:ACP porous nanospheres are investigated, and the most intense peak at 612 nm is observed at 5 mol% Eu3+ doping. In vitro cytotoxicity experiments indicate that the as-prepared Eu3+:ACP porous nanospheres are biocompatible. In vitro drug release experiments indicate that the ibuprofen-loaded Eu3+:ACP porous nanospheres show a slow and sustained drug release in simulated body fluid. We have found that the cumulative amount of released drug has a linear relationship with the natural logarithm of release time (ln(t)). The Eu3+:ACP porous nanospheres are bioactive, and can transform to hydroxyapatite during drug release. The PL properties of drug-loaded nanocarriers before and after drug release are also investigated.
The development of multifunctional nanosystems, for maximum therapeutic benefit including early diagnoses of the diseases and delivery of suitable therapeutic drugs, holds a promise for the future of clinical treatment to enhance therapeutic efficacy [1, 2]. Therefore, it is highly desirable to develop novel methods that can achieve simultaneous in vivo imaging and treatment based on nanotechnology. Nanoparticles show unique size-dependant physical and chemical properties and have great potential for clinical use [3, 4]. Dual or multifunctional nanosystems can be constructed based on nanostructures with well-designed structures and constituents, which are simultaneously capable of diagnosis and treatment . Many kinds of nanostructures have been fabricated from a multitude of materials and used in bio-imaging, including quantum dots [6–8], silica particles , gold , carbon nanotubes , dendrimers [12–14], magnetic nanoparticles [15–18], and so on. However, in the search for the nanosystems for bio-imaging and therapy, a couple of the major concerns are their biodegradability and toxicity [19–21]. To date, the development of multifunctional inorganic nanosystems with both biocompatible and biodegradable properties has been little reported, and more research is needed in this regard.
Calcium phosphates including hydroxyapatite (HAp) are the most important inorganic constituents of biological tissues such as bone and tooth [22–25]. Thus, synthetic calcium phosphates are of great significance because of their biodegradability and biocompatibility, and have been investigated for applications in bone repair/tissue engineering [26, 27], drug loading and release [28, 29], gene delivery [30, 31], and other biomedical areas. Owing to its chemical nature, calcium phosphate nanostructures may serve as an ideal candidate for both bio-imaging and drug delivery. The investigation on rare earth-doped calcium phosphates has become a hot research topic [32–37]. However, up to now, little work has been reported on dual or multifunctional calcium phosphate nanostructures for biomedical applications. HAp, a kind of chemically stable calcium phosphate, has been used for drug storage and luminescence . The development of bi-functional nanosystems of other calcium phosphates such as amorphous calcium phosphate (ACP) is still scarce. Compared with HAp, ACP is bioactive with better biodegradability, and can promote osteoblast adhesion and osteoconductivity [39, 40].
In this article, the authors report a facile method for the fabrication of ACP/polylactide-block-monomethoxy(polyethyleneglycol) (PLA-mPEG) hybrid nanoparticles and ACP porous nanospheres. ACP is an important kind of calcium phosphate and is present in natural bone . PEG and PLA are widely used as biocompatible polymers and currently possess the approval of the US Food and Drug Administration for use in a variety of biomaterials applications [42, 43]. Europium doping was performed to enable photoluminescence (PL) function of ACP porous nanospheres. In vitro cytotoxicity experiments showed that the as-prepared porous nanospheres were biocompatible. In vitro drug loading and release experiments indicated that ibuprofen-loaded europium-doped ACP (Eu3+:ACP) porous nanospheres had a slow and sustained drug release profile in simulated body fluid (SBF). The cumulative amount of the released drug had a linear relationship with the natural logarithm of release time (ln(t)). The Eu3+:ACP porous nanospheres were bioactive, and could transform to HAp after drug release.
The block copolymer PLA-mPEG (M w = 8000 Da) was purchased from Jinan Daigang Co., Ltd., Jinan, China, and the molecular weight of the PEG segment was 5000 Da. Other chemicals were purchased from Sinopharm Chemical Reagent Co., Shanghai, China, and used as received without further purification. For the preparation of ACP/PLA-mPEG hybrid nanoparticles, 1.775 g of Na2HPO4 12H2O and 0.025 g of PLA-mPEG was dissolved in 60 mL of distilled water, and the pH value was adjusted to 10 using ammonia, followed by magnetic stirring for 1 h to form a clear solution. Then, 60 mL of calcium chloride aqueous solution containing 0.33 g of CaCl2 and 0.025 g of PLA-mPEG was added dropwise (10 mL/min) to the above solution, and the pH value was maintained at 10 by slow addition of ammonia. The white precipitate was collected and washed by centrifugation-redispersion cycles with distilled water and ethanol. For the preparation of ACP porous nanospheres, 0.1 g of the as-obtained precipitate was transferred into a 70-mL Teflon autoclave with 50 mL N, N-dimethylformamide (DMF) and treated at 200°C for 1 h by a microwave-solvothermal system (MDS-10, Sineo, Shanghai, China). The product was washed with distilled water and ethanol and dried at 60°C. For the preparation of europium-doped Eu3+:ACP porous nanospheres, europium nitrate aqueous solution was added into the calcium source solution before mixing with phosphate source solution. The doping concentration of Eu3+ was 1, 5, and 10 mol% relative to Ca2+.
PL measurements of Eu3+:ACP porous nanospheres were carried out on a spectrofluorometer (Fluorolog-3, Jobin Yvon, France) at room temperature.
Porcine iliac artery endothelial cells (PIECs) were obtained from the Institute of Biochemistry and Cell Biology (Chinese Academy of Sciences). All the culture media and reagents used in cytotoxicity tests were purchased from Gibco Life Technologies (USA). PIECs were seeded in 96-well flat-bottom microassay plates at a concentration of 2 × 104 cells/mL and cultured with 200 μL/well Dulbecco's Modified Eagle's Medium (DMEM) with 10% fetal serum, 100 U/mL penicillin, and 100 U/mL streptomycin for 48 h in a humidified incubator (BB-15, Heraeus, Germany) with 5% CO2 at 37°C. The samples were diluted with DMEM without fetal bovine serum at a concentration of 6 mg/mL. Then, 50, 100, and 150 μL of each sample solution was added into each well, and the cells were cultured for 24 h. To evaluate cytotoxicity, cell viability was quantified using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (Sigma, USA) and Enzyme-labeled Instrument (MK3, Thermo, USA). Data were measured in three independent parallel experiments, and all data points were plotted as means ± standard deviation (SD) (n = 3).
The typical drug loading and in vitro drug release experiments were performed as follows: 0.5 g of dried powder of Eu3+:ACP porous nanospheres was added into 50 mL hexane solution with an ibuprofen concentration of 40 mg/mL. The suspension was then stirred for 24 h in a sealed vessel at 37°C. The Eu3+:ACP porous nanospheres with loaded drug was centrifuged, and 2 mL supernatant was analyzed by UV-Vis absorption spectroscopy at a wavelength of 263 nm to calculate the ibuprofen storage. The drug loaded Eu3+:ACP porous nanospheres were washed with fresh hexane, dried at 60°C in air, and compacted into disks (0.3 g each, diameter 10 mm) at a pressure of 3 MPa. Each disk was immersed into 200 mL of SBF at 37°C under shaking at a constant rate using a desk-type oscillator (THI-92A, China). Two milliliter of solution was removed for UV-Vis analysis at 263 nm at given time intervals to measure the amount of ibuprofen released, and this quantity of solution was replaced with the same volume of fresh SBF.
Transmission electron microscopy (TEM) micrographs were obtained using a JEOL JEM 2100 field emission electron microscope. X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max 2550 V X-ray diffractometer with a graphite monochromator (Cu K α radiation, λ = 1.54178 Å). Thermogravimetry (TG) curves were obtained at a heating rate of 10°C in nitrogen using a STA 409/PC simultaneous thermal analyzer (Netzsch, Germany). The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were measured with an accelerated surface area and porosimetry system (ASAP 2010).
Results and discussion
We investigated the drug loading and release behaviors of Eu3+:ACP porous nanospheres using a typical anti-inflammatory drug, ibuprofen. The drug loading capacity of Eu3+:ACP porous nanospheres was calculated from the absorbance values of ibuprofen hexane solution before and after drug loading. The drug loading capacity of Eu3+:ACP porous nanospheres reached 70 mg/g carrier.
This result is similar to that of our previous report on the drug release system of porous microspheres of calcium silicate hydrate . For the drug delivery system of Eu3+:ACP porous nanospheres, the constant values of A and k are determined to be 15.47 and 19.46, respectively, with a regression factor of 0.999. Figure 8b shows the XRD pattern of the drug delivery system of Eu3+:ACP porous nanospheres after drug release in SBF, from which one can see that the product after drug release was HAp instead of ACP, indicating that the phase transformation from ACP to HAp occurred during the drug release process in SBF.
It is well acknowledged that the Higuchi model (C = K·t 1/2) can well describe the kinetics of drug release from carrier materials [49–51], with a linear relationship between the cumulative amount of the released drug (C) and the square root of time (t 1/2), and that the drug release is governed by a diffusion process. However, for the drug delivery system of Eu3+:ACP porous nanospheres, Eu3+:ACP porous nanospheres are bioactive, and they can transform from ACP to HAp in SBF solution during the drug release process. Therefore, for the drug delivery system of Eu3+:ACP porous nanospheres in SBF, the drug release kinetics is different from that of the model for common carrier materials.
In a previous study , the authors prepared HAp nanostructured porous hollow ellipsoidal capsules which were constructed by nanoplate networks using inorganic CaCO3 template. CaCO3 ellipsoids were synthesized by the reaction between Ca(CH3COO)2 and NaHCO3 in mixed solvents of water and ethylene glycol at room temperature, and they were used as the Ca2+ source and cores. Then, the PO4 3- source was added to react with CaCO3 to form a HAp shell on the surface of CaCO3 ellipsoids. Dilute acetic acid was used to remove the remaining CaCO3 cores. The drug loading and release behaviors of HAp hollow ellipsoidal capsules were also investigated. The ibuprofen-loaded HAp hollow ellipsoidal capsules showed a slow and sustained release of ibuprofen in SBF. A linear relationship between the cumulative amount of the released ibuprofen and the square root of time was found for the first 12 h. Compared with HAp nanostructured hollow ellipsoidal capsules, the Eu3+:ACP porous nanosphere drug delivery system also showed a slow and sustained drug release profile. However, the IBU/Eu3+:ACP porous nanosphere drug delivery system exhibited a faster drug release rate than that of HAp nanostructured hollow ellipsoidal capsules. On the other hand, the drug release kinetics is different for the two drug delivery systems. For the IBU/Eu3+:ACP porous nanosphere drug delivery system, the cumulative amount of the released drug has a linear relationship with the natural logarithm of release time (ln(t)), instead of the square root of time. However, for IBU/HAp nanostructured hollow ellipsoidal capsule drug delivery system, a linear relationship between the cumulative amount of the released ibuprofen and the square root of time was found. The different drug release properties for the two drug delivery systems may be explained by the different chemical compositions, structures, and morphologies of the two samples.
Figure 8c shows PL emission spectra of drug-loaded Eu3+:ACP porous nanospheres before and after drug release. The characteristic emission peaks are still obvious in the emission spectra for both the samples. There were essentially no shifts in the spectral positions for the characteristic emission peaks, but the PL peak relative intensity and the shape of peaks changed after drug loading. The change of PL properties may be explained by the effect of ibuprofen. However, after in vitro drug release, it is worth noting that the intensities of the emission peaks increased. This phenomenon may be explained by the transformation of carrier material from ACP to HAp during the process of drug release in SBF.
The as-prepared Eu3+:ACP porous nanospheres have a porous structure, which is favorable for a sustained drug release. The Eu3+:ACP porous nanospheres empowered by luminescent properties show a great potential for applications in both drug delivery and in vivo bioimaging. Considering the biocompatible and biodegradable nature, Eu3+:ACP porous nanospheres are a new kind of promising biomaterial.
Calcium phosphate is the most important inorganic constituent of biological hard tissues. ACP, as one of the most important calcium phosphates, is bioactive with biocompatibility and biodegradability. In this study, the authors report a facile method for the preparation of ACP/PLA-mPEG hybrid nanoparticles, which were successfully used as the precursor for the preparation of ACP porous nanospheres. PL function of ACP porous nanospheres was achieved by europium doping. The BET-specific surface area, the BJH-desorption cumulative pore volume, and the average pore size were 126.7 m2/g, 0.53 cm3/g, and 16.7 nm, respectively, for the Eu3+:ACP porous nanospheres. The experimental results of PL, cytotoxicity, as well as in vitro drug loading and release showed that the as-prepared Eu3+:ACP porous nanospheres were biocompatible and bioactive with favorable properties of PL, drug loading and drug release, implying that Eu3+:ACP porous nanospheres are a new kind of promising biomaterial with bi-functions of both luminescence and drug delivery.
amorphous calcium phosphate
porcine iliac artery endothelial cells
simulated body fluid.
Financial supports from the Science and Technology Commission of Shanghai (1052nm06200), Shanghai-Unilever Research and Development Fund (09520715200), the National Natural Science Foundation of China (50772124, 50821004), and the State Key Laboratory of High Performance Ceramics and Superfine Microstructure are gratefully acknowledged.
- Shi D: Integrated multifunctional nanosystems for medical diagnosis and treatment. Adv Funct Mater 2009, 19: 3356.View Article
- Park K, Lee S, Kang E, Kim K, Choi K, Kwon IC: New generation of multifunctional nanoparticles for cancer imaging and therapy. Adv Funct Mater 2009, 19: 1553.View Article
- Sanvicens N, Marco MP: Multifunctional nanoparticles--properties and prospects for their use in human medicine. Trends Biotechnol 2008, 26: 425.View Article
- Gunasekera MP, Pankhurst QA, Douek M: Imaging applications of nanotechnology in cancer. Target Oncol 2009, 4: 169.View Article
- Yezhelyev MV, Gao X, Xing Y, Al-Hajj A, Nie S, O'Regan RM: Emerging use of nanoparticles in diagnosis and treatment of breast cancer. Lancet Oncol 2006, 7: 657.View Article
- Bruchez M Jr, Moronne M, Gin P, Weiss S, Alivisatos AP: Semiconductor nanocrystals as fluorescent biological labels. Science 1998, 281: 2013.View Article
- Seydel C: Quantum dots get wet. Science 2003, 300: 80.View Article
- Gao X, Cui Y, Levenson RM, Chung LW, Nie S: In vivo cancer targeting and imaging with semiconductor quantum dots. Nat Biotechnol 2004, 22: 969.View Article
- Choi J, Burns AA, Williams RM, Zhou Z, Flesken-Nikitin A, Zipfel WR, Wiesner U, Nikitin AY: Core-shell silica nanoparticles as fluorescent labels for nanomedicine. J Biomed Opt 2007, 12: 064007.View Article
- Kim D, Park S, Lee JH, Jeong YY, Jon S: Antibiofouling polymer-coated gold nanoparticles as a contrast agent for in vivo X-ray computed tomography imaging. J Am Chem Soc 2007, 129: 7661.View Article
- Liu Z, Cai W, He L, Nakayama N, Chen K, Sun X, Chen X, Dai H: In vivo biodistribution and highly efficient tumour targeting of carbon nanotubes in mice. Nat Nanotechnol 2007, 2: 47.View Article
- Tomalia DA, Reyna LA, Svenson S: Dendrimers as multi-purpose nanodevices for oncology drug delivery and diagnostic imaging. Biochem Soc Trans 2007, 35: 61.View Article
- Kobayashi H, Kawamoto S, Sakai Y, Choyke PL, Star RA, Brechbiel MW, Sato N, Tagaya Y, Morris JC, Waldmann TA: Lymphatic drainage imaging of breast cancer in mice by micro-magnetic resonance lymphangiography using a nano-size paramagnetic contrast agent. J Natl Cancer Inst 2004, 96: 703.View Article
- Shi X, Wang S, Meshinchi S, Van Antwerp ME, Bi X, Lee I, Baker JR Jr: Dendrimer-entrapped gold nanoparticles as a platform for cancer-cell targeting and imaging. Small 2007, 3: 1245.View Article
- Yu MK, Jeong YY, Park J, Park S, Kim JW, Min JJ, Kim K, Jon S: Drug-loaded superparamagnetic iron oxide nanoparticles for combined cancer imaging and therapy in vivo . Angew Chem Int Ed 2008, 47: 5362.View Article
- Landmark KJ, Dimaggio S, Ward J, Kelly C, Vogt S, Hong S, Kotlyar A, Myc A, Thomas TP, Penner-Hahn JE, Baker JR, Holl MM, Orr BG: Synthesis, characterization, and in vitro testing of superparamagnetic iron oxide nanoparticles targeted using folic Acid-conjugated dendrimers. ACS Nano 2008, 2: 773.View Article
- Gupta AK, Naregalkar RR, Vaidya VD, Gupta M: Recent advances on surface engineering of magnetic iron oxide nanoparticles and their biomedical applications. Nanomedicine 2007, 2: 23.View Article
- Lee H, Yu MK, Park S, Moon S, Min JJ, Jeong YY, Kang HW, Jon S: Thermally cross-linked superparamagnetic iron oxide nanoparticles: synthesis and application as a dual imaging probe for cancer in vivo . J Am Chem Soc 2007, 129: 12739.View Article
- Park S, Lee YK, Jung M, Kim KH, Chung N, Ahn EK, Lim Y, Lee KH: Cellular toxicity of various inhalable metal nanoparticles on human alveolar epithelial cells. Inhal Toxicol 2007, 19(Suppl 1):59.View Article
- Tian F, Cui D, Schwarz H, Estrada GG, Kobayashi H: Cytotoxicity of single-wall carbon nanotubes on human fibroblasts. Toxicol In Vitro 2006, 20: 1202.View Article
- Papageorgiou I, Brown C, Schins R, Singh S, Newson R, Davis S, Fisher J, Ingham E, Case CP: The effect of nano- and micron-sized particles of cobalt-chromium alloy on human fibroblasts in vitro . Biomaterials 2007, 28: 2946.View Article
- Driessens FCM, van Dijk JWE, Borggreven JMPM: Biological calcium phosphates and their role in the physiology of bone and dental tissues I. Composition and solubility of calcium phosphates. Calcif Tissue Int 1978, 26: 127.View Article
- Vallet-Regí M, González-Calbet JM: Calcium phosphates as substitution of bone tissues. Prog Solid State Chem 2004, 32: 1.View Article
- Weiner S, Addadi S: Design strategies in mineralized biological materials. J Mater Chem 1997, 7: 689.View Article
- Cai YR, Tang RK: Calcium phosphate nanoparticles in biomineralization and biomaterials. J Mater Chem 2008, 18: 3775.View Article
- Stupp SI, Braun PV: Molecular manipulation of microstructures: biomaterials, ceramics, and semiconductors. Science 1997, 277: 1242.View Article
- Suchanek W, Yoshimura M: Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants. J Mater Res 1998, 13: 94.View Article
- Ma MY, Zhu YJ, Li L, Cao SW: Nanostructured porous hollow ellipsoidal capsules of hydroxyapatite and calcium silicate: preparation and application in drug delivery. J Mater Chem 2008, 18: 2722.View Article
- Tang QL, Zhu YJ, Duan YR, Wang Q, Wang KW, Cao SW, Chen F, Wu J: Porous nanocomposites of PEG-PLA/calcium phosphate: room-temperature synthesis and its application in drug delivery. Dalton Trans 2010, 39: 4435.View Article
- Maitra A: Calcium phosphate nanoparticles: second-generation nonviral vectors in gene therapy. Expert Rev Mol Diagn 2005, 5: 893.View Article
- Wang KW, Zhou LZ, Sun Y, Wu GJ, Gu HC, Duan YR, Chen F, Zhu YJ: Calcium phosphate/PLGA-mPEG hybrid porous nanospheres: A promising vector with ultrahigh gene loading and transfection efficiency. J Mater Chem 2010, 20: 1161.View Article
- Al-Kattan A, Dufour P, Dexpert-Ghys J, Drouet C: Preparation and physicochemical characteristics of luminescent apatite-based colloids. J Phys Chem C 2010, 114: 2918.View Article
- Doat A, Fanjul M, Pelle F, Hollande E, Lebugle A: Europium-doped bioapatite: a new photostable biological probe, internalizable by human cells. Biomaterials 2003, 24: 3365.View Article
- Doat A, Pelle F, Gardant N, Lebugle A: Synthesis of luminescent bioapatite nanoparticles for utilization as a biological probe. J Solid State Chem 2004, 177: 1179.View Article
- Han YC, Wang XY, Li SP: Biocompatible europium doped hydroxyapatite nanoparticles as a biological fluorescent probe. Curr Nanosci 2010, 6: 178.View Article
- Kim EJ, Choi SW, Hong SH: Synthesis and photoluminescence properties of Eu 3+ -doped calcium phosphates. J Am Ceram Soc 2007, 90: 2795.View Article
- Lebugle A, Pelle F, Charvillat C, Rousselot I, Chane-Ching JY: Colloidal and monocrystalline Ln 3+ doped apatite calcium phosphate as biocompatible fluorescent probes. Chem Commun 2006, 606.
- Yang PP, Quan ZW, Li CX, Kang XJ, Lian HZ, Lin J: Bioactive, luminescent and mesoporous europium-doped hydroxyapatite as a drug carrier. Biomaterials 2008, 29: 4341.View Article
- Nagano M, Nakamura T, Kokubo T, Tanahashi M, Ogawa M: Differences of bone bonding ability and degradation behaviour in vivo between amorphous calcium phosphate and highly crystalline hydroxyapatite coating. Biomaterials 1996, 17: 1771.View Article
- Balasundaram G, Sato M, Webster TJ: Using hydroxyapatite nanoparticles and decreased crystallinity to promote osteoblast adhesion similar to functionalizing with RGD. Biomaterials 2006, 27: 2798.View Article
- Dorozhkin SV: Calcium Orthophosphates in Nature, Biology and Medicine. Materials 2009, 2: 399.View Article
- Ramchandani M, Pankaskie M, Robinson D: The influence of manufacturing procedure on the degradation of poly(lactide-co-glycolide) 85:15 and 50:50 implants. J Control Release 1997, 43: 161.View Article
- Jain A: The manufacturing techniques of various drug loaded biodegradable poly(lactide-co-glycolide) (PLGA) devices. Biomaterials 2000, 21: 2475.View Article
- Xu GF, Aksay IA, Groves JT: Continuous crystalline carbonate apatite thin films. A biomimetic approach. J Am Chem Soc 2001, 123: 2196.View Article
- Horikoshi K, Hata K, Kawabata N, Ikawa SI, Konaka S: Vibrational spectra and conformation of polyethylene glycol complexed with calcium and magnesium chlorides. J Mol Struct 1990, 239: 33.View Article
- Uekawa N, Sukegawa T, Kakegawa K, Sasaki Y: Synthesis of Lead Nickel Niobate-Barium Titanate System by Oxidation of Polyethylene Glycol-Cation Complex. J Am Ceram Soc 2002, 85: 329.View Article
- Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, Siemieniewska T: Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). Pure Appl Chem 1985, 57: 603.View Article
- Wu J, Zhu YJ, Cao SW, Chen F: Hierachically nanostructured mesoporous spheres of calcium silicate hydrate: surfactant-free sonochemical synthesis and drug-delivery system with ultrahigh drug-loading capacity. Adv Mater 2010, 22: 749.View Article
- Higuchi T: Mechanism of sustained-action medication: Theoretical analysis of rate of release of solid drugs dispersed in solid matrices. J Pharm Sci 1963, 52: 1145.View Article
- Vallet-Regi M, Balas F, Arcos D: Mesoporous materials for drug delivery. Angew Chem Int Ed 2007, 46: 7548.View Article
- Andersson J, Rosenholm J, Areva S, Lindén M: Influences of material characteristics on ibuprofen drug loading and release profiles from ordered micro- and mesoporous silica matrices. Chem Mater 2004, 16: 4160.View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.