Fe₃O₄ nanoparticles modified with APTES as the carrier for (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid (Naproxen) and (RS) 2-(3-benzoylphenyl)-propionic acid (Ketoprofen) drug

Introduction

In the past decade , targeted drug delivery technology has been enormous attention in  medicine and pharmaceutical industries due to  much advantage compared to conventional  such as low toxicity , biocompatibility [1], improving existing drugs’, therapeutic efficacy, alleviating their side effects, reducing the cost and so on[2].  Nanotechnology has firmly entered the domain of drug delivery. Different Nano carriers including dendrimers [3], micelles [4], emulsions [5] liposomes [6] and magnatic nanoparticles [7-10] and etc, are used to target specific areas in the body. Because of the unique characteristics of magnetic nanoparticles such as superparamagnetism, high coercivity, low Curie temperature, and high magnetic susceptibility have gained much scientific interest [11-12]. Drug-carrying magnetic nanoparticles can be concentrated in cancer tissue by external magnetic fields [13]. Internalization of magnetic nanoparticles strongly depends upon the particle size. These applications require the magnetic nanoparticles in size smaller than 100 nm and a narrow particle size distribution. Larger particles with a diameter higher than 200 nm are easily isolated by the spleen and finally eliminated by the cells of the phagocyte system, thus it leads to a reduction of blood circulation times. Small particles with diameters less than 10 nm are rapidly removed through extravasations and renal clearance. Particles with a diameter ranging from 10 to 100 nm might be considered optimal for intravenous injection and have the most prolonged blood circulation time. These particles are small enough to evade the RES of the body as well as to penetrate small capillaries of the tissues and offer the most effective distribution in targeted tissues. [14]. When the magnetic nanoparticles is used uncoated as drug carriers, they have lower performance  because of some limitations in drug loading, retention time ,and release rates in the blood stream [15,16].  Coated magnetic nanoparticles with silica, gold, or polymers [14] not only overcome these problems but also to avoid the formation of aggregates and provide functional groups (amines or carboxylic acid) for help in binding various biological ligands [17]. Types of polymeric surface coatings(organic and inorganic) have been used such as dextran, carboxymethylated dextran, carboxydextran, starch, arabinogalactan, glycosaminoglycan, sulfonated styrene-divinylbenzene, polyethylene glycol (PEG), polyvinyl alcohol (PVA), poloxamers, polyoxamines [14], Polyvinylpyrrolidone-iodine [18] and chitosan [19]. The natural polymers are more important because these materials are more biocompatibility [14]. Silica shells are appropriate options to be employed as protective coatings on iron oxide nanoparticles thanks to their stability under aqueous conditions and ease of synthesis [20]. Trialkoxysilanes , bifunctional molecules ,entail a trialkoxy group that they are granted to modify the surface of nanoparticles . (3-Aminopropyl) triethoxysilane is intended to be done through the grafting of aminopropylsilane groups (–O)₃ Si–(CH₂)₃–NH₂ via formation of covalent bonds which are bound to the particle surface  and makes basic surface. Following prior step, it would be regarded as nanocarrier attracting acidic drugs resulted in an ionic interaction [21, 22]. Modified magnetic nanoparticles have been synthesized by two methods. In the first method, nanoparticles are coated during the synthesis that is in situ coatings. [23]. The post-synthesis coating method consists of grafting the polymer on the magnetic particles once synthesized [24-26] (polymeric surfactants). This paper provides a detailed study of the preparation iron oxide nanoparticles modified with APTES by post grafting method and the anti-inflammatory drug: (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid –Naproxen and (RS) 2-(3-benzoylphenyl)-propionic acid –Ketoprofen were loaded onto them. The morphology/size and magnetization was determined for these nanoparticles using Field Emission Scanning Microscopy (FE-SEM), X-ray powder diffraction and VSM respectively. Fourier transform infrared spectroscopy was employed in order to identify the presence of APTES, ketoprofen and Naproxen drugs on Fe₃o₄ nanoparticles surface. Hydrodynamic size of ketoprofen-APTES-nanoparticles was designated by Dynamic light scattering (DLS).

Materials and Methods

Reagents and Materials

Ferric chloride hexahydrate (FeCl₃, 6H₂O), (3-aminopropyl) triethoxysilane (APTES), (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid (Naproxen), and (RS) 2-(3-benzoylphenyl)-propionic acid (Ketoprofen) were obtained from Sigma-Aldrich. Iron (III) sulfate heptahydrate (FeSO₄, 7H₂O) and ammonium hydroxide 25 wt% were purchased Fluka (Buchs, Switzerland).

Synthesis of Fe₃O₄ Nanoparticles

Iron oxide magnetic nanoparticles were prepared by a conventional co-precipitation. In summary, Sodium hydroxide solutions (250 mL, 1M) were added to a three-neck round-bottomed flask under protection of argon flow. The solution was heated to 85°C. Then 12 ml of deionized water containing 4.04 g of Iron(III) nitrate and 1.39 g of Iron(III) sulfate (FeSO₄,7H₂O) were added dropwise, while stirring vigorously until a black precipitate was formed. The mixture was kept at this condition for 1 h. To remove the remaining ions, the generated precipitate was centrifuged and washed at least three times until a pH value of 7 was achieved. The powder was dried at 60°C for 24 hours.

Modification of Fe₃O₄ Nanoparticles By (3-Aminopropyl) Triethoxysilane

The obtained magnetite nanoparticles powder (1 g) was dispersed in 150 mL ethanol/water (volume ratio, 1:1) solution by sonication for 30 min. After that, (3-aminopropyl) triethoxysilane (APTES) (99%, 3 mL) were added to the mixture.

Then resulting mixture was stirred under argon atmosphere and 40⁰c for 24 hours. The final product was separated from the solution and washed for 5 times by water, acetone and ethanol. The precipitated product (APTES– Fe₃O₄) was dried at room temperature under vacuum.

Adsorption of Naproxen and Ketoprofen Drugs on APTES- Fe₃O₄ Nano Carrier’s Surface

2.0 g of APTES- Fe₃O₄ introduced to 100 mL of 2-propanol solution containing drug (10 mg/ mL). The adsorption was carried out at room temperature for 24 h. After magnetic separation, drug nanocarriers were perfectly washed by 2-propanol solution and dried at room temperature (24 h). Figure 1 indicates a schematic of these approaches.

 

Fig1: schematic image of syntheses of the Ketoprofen-APTES-Fe3O4 nanoparticles  Click here to View figure

 

Results and Discussion 

Preparing of iron oxide nanoparticles was carried out by the co precipitation method in an aqueous medium, through reaction (1). If the nanoparticles are exposed in the presence of oxygen or air, might undergo oxidation to Fe(OH)₃ or as shown in reaction (2) [27], or  to Fe₂O₃ phase according to reaction (3) [28]. So the reaction was carried out under nitrogen gas continuously.

 

 

Iron oxide nanoparticles surface modified by the process Silanization.  This reaction involves the covering of a surface iron oxide nanoparticles through self-assembly with (3-aminopropyl)-triethoxysilane molecules. During this reaction ,hydroxyl  groups on the surface of iron oxide nanoparticles attack and replace  ethoxy  groups  of APTES ,thus is formed a covalent -Si-O-Si- bond  and  amino propyl-terminated surface((see Figure1). The surface coating of nanoparticles by APTES depends on experimental parameters such as reaction time, temperature and silane concentration. Interaction of ketoprofen and Naproxen drugs (carboxylic acid) with basic amino propyl-terminated surface of iron oxide nanoparticles is an ionic interaction. Rosenholm and Lindén [29] show that in polar solvents like 2-propanol used in this research was possible.

Characterization of the Samples  

X-ray Powder Diffraction

Fig. 2 shows the results of  X-ray diffraction analysis for naked Fe₃o₄ and APTES @Fe₃O₄ nanoparticles.  This figure indicates that the predominant phase of constituted iron oxide is Fe₃o₄ (magnetite). Because the position and relative intensities of all peaks in  XRD obtained patterns are in good agreement with the standard diffraction spectrum (JCPDS Card No. 19-0629) [30] and  Peaks of Fe(OH)₃ (d= 3.376 at 2θ= 26.38⁰  ), goethite (d =4.183 A^o at 2 θ =21.22⁰), hematite (d=2.700 A^o  at 2  =33.15⁰ ) [28] were not observed. A weak broad band (2θ = 17–26◦) can be seen in XRD pattern of APTES- Fe3O4   can be devoted to amorphous silane shell formed Surrounding magnetic core [31].  The average particle size was estimated by Sherrer’s equation: D = Kλ/ (βCosθ). Where D is equivalent of particles average core diameter; K is the grain shape factor (K=0.94); λ is X-ray wavelength (1.54060A⁰); β denotes the full width at half-maximum or FWHM (in radians) of the highest intensity 311 powder diffraction reflection, and θ is the Bragg angle. FWHM and 2θ values ​​for naked Fe₃O₄ and APTES -Fe₃O₄ nanoparticles, are respectively included 1.38, 35.63A^o and 1.59, 35.73 A^o. , Considering these data, both naked Fe3O4 and APTES-Fe3O4 exhibited sizes approximately equal to 6 nm. Although thermal treatment can grow in size and modify nanoparticles physical properties but the same size observed for naked Fe₃O₄ and APTES -Fe₃O₄ nanoparticles, show that thermal treatment in during the silanization reaction was not enough to  cause  growth and  accordingly dramatic  effect  on the physical properties of the iron oxide particles [28].

 

Fig2: X-ray powder diffraction patterns of Naked Fe3O4 nanoparticles and APTES-Fe3O4 composite particle  Click here to View figure

 

Fourier Transforms Infrared Spectra

Fig. 3 indicates the FTIR spectra of the naked Fe₃O₄ and APTES -Fe₃O₄ carrier before and after ketoprofen and naproxen drugs adsorption. The Sharp and revealing peak at around 580-594 cm^-1 can be observed in (a), (b), (e), and (f) spectra is relates to the absorption peak Fe- O -Fe bond of Fe₃O₄ nanoparticles. This peak appears for bulk Fe₃O₄ at 570 and 575 cm⁻¹.This blue shift is a result of decrease in the size of iron oxide [32, 33]. APTES presence on the surface of Fe₃o₄ nanoparticles is proven by the bands at 996 and 1126cm⁻¹   that dedicated to the Si –O stretching vibrations and the broad band at 3401cm⁻¹ that is assigned to the N–H stretching vibration (Fig 3b) [34]. The presence of the propyl group of APTES was confirmed by C–H stretching vibrations that appeared at 2862 cm^-1. Adsorption Of ketoprofen and naproxen drugs on APTES-Fe3O4 nanoparticles resulted in disappearance of the absorption band at 1720 and 1728 cm⁻¹ (Fig. 3c and d) characteristic to carbonyl stretching vibrations in carboxylic groups in adsorbed ketoprofen and naproxen drugs respectively and appearance of them  characteristic bands at 1638  and 1630 cm^-1 (Fig. 3g and h)  related to stretching vibrations of ionized carboxylic groups were seen [35]. This observation confirms the ionic interaction and conjugtion between the drug and the APTES-Fe₃O₄. Moreover, in drug- conjugated Fe₃o₄ nanocomposite presence of many characteristic peaks of ketoprofen and naproxen drugs  such as c=c stretching vibration peak of aromatic group at 1375,1430 and 1605,1462 cm^-1(Fig. 3g and h)   corroborate the conjugtion of drug to the APTES-Fe₃O₄ carrier. To compare the absorption peaks corresponding to Figure 3 are listed in Table 1. The part of FTIR spectrum show exhibiting absorption band of c=c stretching vibration of aromatic group of naproxen (g) and ketoprofen (h) loaded on APTES-Fe3O4 nanoparticles.

 

Fig3: FTIR Spectra naked Fe3O4 (a), APTES- Fe3O4 (b), Naproxen(c), Ketoprofen (d), Naproxen-APTES- Fe3O4 (e) and Ketoprofen-APTES- Fe3O4 (f).The part of FTIR spectrum exhibiting absorption band of c=c stretching vibration of aromatic group of naproxen (g) and ketoprofen (h) on APTES-Fe3O4 nanoparticles   Click here to View figure

 

Table1: Assignment of FTIR spectra of Fe3O4 (a) APTES-Fe3O4 (b), naproxen (e) and ketoprofen (f) – APTES-Fe3O4 nanoparticles, naproxen(c) and ketoprofen (d)  Click here to View table

 

Field Emission Scanning Microscopy

The surface morphology of naked Fe₃O₄, APTES-Fe₃O₄, naproxen and ketoprofen – APTES-Fe₃O₄ nanoparticles, was observed by scanning electron microscopy. Fig. 4(a – d) shows the FE-SEM images of these nanoparticles respectively. As shown in from these images, the formation of nanoparticles is nearly uniform and spherical shape with homogeneously dispersed. In other words, during the silanization reaction and drug loading, morphological properties of nanoparticles do not noticeably change.

 

Fig4: Field emission scanning electron microscopy images of Fe3O4 (a), APTES Fe3O4 (b), naproxen(c) and ketoprofen (d) – APTES-Fe3O4 nanoparticles  Click here to View figure

 

Energy Dispersive X-ray Analysis (EDX)

The surface composition of naked Fe₃O₄, APTES-Fe₃O₄, naproxen and ketoprofen – APTES-Fe₃O₄ nanoparticles was designated by energy-dispersive X-ray spectroscopy as shown in Figure 5 and table 2. The presence of iron and oxygen can be seen in all of the samples, with iron abundance more than oxygen. APTES presence on the surface of Fe₃O₄ nanoparticles was proven by increase of percentage C and Si (b). Also Ketoprofen and naproxen drug adsorption on the surface of APTES-Fe₃O₄ nanoparticles is confirmed by the increase in carbon atomic and weight percent.

 

Fig5: Edx result of naked Fe3O4 (a), APTES-Fe3O4 (b), naproxen(c) and ketoprofen (d) – APTES-Fe3O4 nanoparticles  Click here to View figure

 

Table2: EDAX quantification element normalized  Click here to View table

 

Dynamic Light Scattering (DLS) Diagrams

The size histogram of naproxen (a) and ketoprofen (b) -APTES- Fe₃O₄ is shown in Fig. 6. Particles size was further identified by Zetasizer using DLS. These figures suggest that more than 50% of the atoms have hydrodynamic size below 100 nm.  In drug delivery systems, the entry of nanoparticles to target tissue strongly relies on the size of the particles. Particles with a diameter ranging from 10 to 100 nm might be considered optimal for intravenous injection and have the most prolonged blood circulation time [14].

 

Fig6: particle size distribution of naproxen (a) and ketoprofen (b) -APTES- Fe3O4 nanoparticles  Click here to View figure

 

Vibrating scanning Magnetometry (VSM)

The magnetic properties of naked iron oxide and naproxen-APTES- iron oxide nanoparticles were characterized by vibrating sample magnetometry. VSM graphs of these samples are presented in Fig. 7. As it is obvious from this figure, naked iron oxide nanoparticles and drug-APTES- iron oxide nanoparticles have a hysteresis loop with zero coercivity and remanence values  or super paramagnetic behaviors, super paramagnetism occurs when the particles  sufficiently small so that thermal fluctuations can overcome the magnetic anisotropy. The saturation magnetization value of naked iron oxide and naproxen -APTES- iron oxide nanoparticles were found to be 55.4and 45.5 electromagnetic units per gram (emu/g) respectively. The reduction in saturation magnetization was likely due to the existence of APTES on surface of Fe₃O₄ nanoparticles.

 

Fig7: Magnetic curves of naked-Fe3O4 (a) and naproxen -APTES-Fe3O4(b)- nanoparticles at room temperature  Click here to View figure

 

Conclusions

Iron oxide magnetic nanoparticles were prepared by a conventional co-precipitation and modified by (3-aminopropyl) triethoxysilane (APTES) .The modification of Fe₃O₄ nanoparticles leads to the formation of nanocarriers with surface basic properties. Two anti-inflammatory drug: (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid –Naproxen and (RS) 2-(3-benzoylphenyl)-propionic acid –Ketoprofen were loaded on the surface of nanocarriers The adsorption of drugs is due to ionic interactions between the amine functional group of APTES and the carboxylic group of drugs, that confirmed by Fourier transform infrared spectra. The most part of  nanocarriers loaded with drug has size less than 100 nm and due to inherent magnatic characteristic(45.5 emu/g )  they are able to penetrate the target tissue in attended of external magnetic fields.

References

1. Muhammad Irfan Majeed, Qunwei Lu, Wei Yan, Zhen Li , Irshad Hussain, Muhammad Nawaz Tahir, Wolfgang Tremele and Bien Tan , Highly water-soluble magnetic iron oxide (Fe3O4) nanoparticles for drug delivery: enhanced in vitro therapeutic efficacy of doxorubicin and MION conjugates . J. Mater. Chem. B, 1)2013(2874–2884
2. Li Yan, Xianfeng Chen ,Nanomaterials for Drug Delivery Nanocrystalline Materials (Second Edition) ( 2014) 221-268
3. Ali Pourjavadi, Seyed Hassan Hosseini, Mahshid Alizadeh, Craig Bennett Magnetic pH-responsive nanocarrier with long spacer length and high colloidal stability for controlled delivery of doxorubicin Original Research Article Colloids and Surfaces B: Biointerfaces 116 ( 2014) 49-54
4. Lin Zhu, Federico Perche, Tao Wang, Vladimir P. Torchilin  Matrix metalloproteinase 2-sensitive multifunctional polymeric micelles for tumor-specific co-delivery of siRNA and hydrophobic drugsOriginal Research Article Biomaterials, Volume 35, Issue 13 ( 2014) 4213-4222.
5. Noraini Ahmad, Roland Ramsch, Meritxell Llinàs, Conxita Solans, Rauzah Hashim, Hairul Anuar TajuddinInfluence of nonionic branched-chain alkyl glycosides on a model nano-emulsion for drug delivery systems Original Research Article Colloids and Surfaces B: Biointerfaces, Volume 115, (2014) 267-274.
6. Kazuaki Ninomiya, Takahiro Yamashita, Shinya Kawabata, Nobuaki Shimizu, Targeted and ultrasound-triggered drug delivery using liposomes co-modified with cancer cell-targeting aptamers and a thermosensitive polymer Original Research Article Ultrasonics Sonochemistry, Volume 21, Issue 4, (2014) 1482-1488.
7. Omid Veiseh, Jonathan W. Gunn, Miqin Zhang, Design and fabrication of magnetic nanoparticles for targeted drug delivery and imaging Review Article Advanced Drug Delivery Reviews, Volume 62, Issue 3(2010) 284-304
8. Conroy Sun, Jerry S.H. Lee, Miqin Zhang, Magnetic nanoparticles in MR imaging and drug delivery Review Article Advanced Drug Delivery Reviews, 60, Issue 11, ( 2008) 1252-1265
9. Magdalena Hałupka-Bryl, Kei Asai, Sindhu Thangavel, Magdalena Bednarowicz, Ryszard Krzyminiewski, Yukio Nagasaki, Synthesis and in vitro and in vivo evaluations of poly(ethylene glycol)-block-poly(4-vinylbenzylphosphonate) magnetic nanoparticles containing doxorubicin as a potential targeted drug delivery systemOriginal, Research Article Colloids and Surfaces B: Biointerfaces, In Press, Accepted Manuscript,Available online 8 April 2014
10. Marcu, A.; Pop, S.; Dumitrache, F.; Mocanu, M.; Niculite, C. M.; Gherghiceanu, M.; Lungu, C. P.; Fleaca, C.; Ianchis, R.; Barbut, A.; Grigoriu, C.; Morjan, I. Magnetic iron oxide nanoparticles as drug delivery system in breast cancer Original Research Article Applied Surface Science, Volume 281 ( 2013) 60-65.
11. Y. Tai, L. Wang, G. Yan, J.-m. Gao, H. Yu and L. Zhang, Recent research progress on the preparation and application of magnetic nanospheres Polym. Int., 60 ( 2011) 976–994.
12. A.-H. Lu, E. L. Salabas and F. Sch¨uth, Angew. Chem., Int. Ed. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application, 46(2007) 1222–1244.
13. Rainer Tietze, Stefan Lyer, Stephan Dürr, Tobias Struffert, Tobias Engelhorn, Marc Schwarz, Elisabeth Eckert, Thomas Göen, Serhiy Vasylyev, Wolfgang Peukert, Frank Wiekhorst, Lutz Trahms, Arnd Dörfler, Christoph Alexiou , Efficient drug-delivery using magnetic nanoparticles — biodistribution and therapeutic effects in tumour bearing rabbits Original Research Article Nanomedicine: Nanotechnology, Biology and Medicine, Volume 9, Issue 7,( 2013) 961-971.
14. Sophie Laurent, Delphine Forge, Marc Port, Alain Roch, Caroline Robic, Luce Vander Elst, and Robert N. Muller, Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications Chem. Rev. 108( 2008) 2064–2110
15. J.L. Arias, V. Gallardo, S.A. Gomez-Lppera, R.C. Plaza, A.V. Delgado, Synthesis and characterization of poly (ethyl-2-cyanoacrylate) nanoparticles with a magnetic core, Journal of Controlled Release 77 (3) (2001) 309–321.
16. G. Strom, S.O. Belliot, T. Daemen, D.D. Lasic, Surface modification of nanoparticles to oppose uptake by the molecular phagocyte system, Advanced Drug Delivery Reviews 17 (1) (1995) 31–48.
17. K. Morimoto, S. Chono, T. Kosai, T. Seki, Y. Tabata, Design of cationic microspheres based on aminated gelatin for controlled release of peptide and protein drug, Drug Delivery 15 (2) (2008) 113–117.
18. Guangshuo Wang, Ying Chang, Ling Wang, Zhiyong Wei, Jianyun Kang, Lin Sang, Xufeng Dong, Guangyi Chen, Hong Wang, Min Qi ,Preparation and characterization of PVPI-coated Fe3O4 nanoparticles as an MRI contrast agent Original Research Article Journal of Magnetism and Magnetic Materials, 340 ( 2013) 57-60.
19. Zhongli Lei, Xiaolong Pang, Na Li, Lin Lin, Yanli Li,A novel two-step modifying process for preparation of chitosan-coated Fe3O4/SiO2 microspheres Original Research Article Journal of Materials Processing Technology, Volume 209, Issue 7, 1 ( 2009) 3218-3225
20. Jerry S.H. Lee b, Miqin Zhang a,Magnetic nanoparticles in MR imaging and drug delivery Conroy Sun a, Advanced Drug Delivery Reviews 60 (2008) 1252–1265
21. B. Feng, R.Y. Hong, L.S.Wang, L. Guo, H.Z. Li ,“Synthesis of Fe3O4/APTES/PEG diacid functionalized magnetic nanoparticles for MR imaging”, Colloids and Surfaces A: Physicochem. Eng. Aspects, 328 (2008) 52–59.
22. Michał Moritz, Marek Łaniecki ,SBA-15 mesoporous material modified with APTES as the carrier for2-(3-benzoylphenyl)propionic acid ,Applied Surface Science 258 (2012) 7523– 7529
23. S. Asgari, Z. Fakhari, S. BerijaniSynthesis and Characterization of Fe₃O₄ Magnetic Nanoparticles Coated  with Carboxymethyl Chitosan Grafted Sodium Methacrylate , JNS 4 (2014) 55-63
24. L. Li , K.Y. Mak , C.W. Leung , K.Y. Chan , W.K. Chan , W. Zhong , P.W.T. Pong, effect of synthesis conditions on the properties of citric-acid coated iron oxide nanoparticles, Microelectronic Engineering 110 (2013) 329–334
25. A. Tomitaka, T. Koshi, S. Hatsugai, T. Yamada, Y. Takemura, Journal of Magnetic characterization of surface-coated magnetic nanoparticles for biomedical application , Magnetism and Magnetic Materials 323 (2011) 1398–1403.
26. Okassa, L. N.; Marchais, H.; Douziech-Eyrolles, L.; Cohen-Jonathan, S.; Souce, M.; Dubois, P.; Chourpa I, Development and characterization of sub-micron poly(D,L-lactide-co-glycolide) particles loaded with magnetite/maghemite nanoparticles.. Int. J. Pharm. 2005, 302 (1-2), 187.
27. D.K. Kim, Y. Zhan, W. Voit, K.V. Rao, M. Muhammed, Synthesis and characterization of surfactant-coated super paramagnetic mono dispersed iron oxide nanoparticles, Journal of Magnetism and Magnetic Materials 225 (2001) 30-36
28. M. Yamauraa, R.L. Camiloa, L.C. Sampaiob, M.A. Mac#edoc, M. Nakamurad, H.E. Tomad ,Preparation and characterization of (3-aminopropyl) triethoxysilane-coated magnetite nanoparticles ,Journal of Magnetism and Magnetic Materials 279 (2004) 210–217
29. J.M. Rosenholm, M. Lindén, Towards establishing structure-activity relationships for mesoporous silica in drug delivery applications, Journal of Controlled Release 128 (2008) 157–164.
30. Bragg W.L. Nature. 1915, 95: 561.
31. Y. Jiang, J. Jiang, Q. Gao, M. Ruan, H. Yu, L. Qi, Nanotechnology 19 (2008) 75714.
32. Z.M. Rao, T.H. Wu, S.Y. Peng, Acta Phys. Chim. Sin. 11 (1995) 395–399.
33. R.D. Waldron, Infrared Spectra of Ferrites ,Phys. Rev. 99 (1955) 1727–1735.
34. Z. Xu, Q. Liu, J.A. Finch, Silanation and stability of 3-aminopropyl triethoxy silane on nanosized superparamagnetic particles: I. Direct silanation, Appl. Surf. Sci. 120 (1997) 269–278.
35. E. Pretsch, P. Bühlmann, C. Affolter, Structure Determination of Organic Compounds.Tables of Spectral Data, third ed., Springer, Berlin, 2000.

---

This work is licensed under a Creative Commons Attribution 4.0 International License.