Synthesis of magnetic core–shell structure Fe3O4@MCM-41 nanoparticle by vesicles in aqueous solutions☆

Weiming Song*,Xuesong Liu,Ying Yang,Xuejia Han,Qigang Deng

College of Chemistry and Chemical Engineering of Qiqihar University,Qiqihar 161000,China

Keywords:Fe3O4@MCM-41 Core–shell Nanoparticles

ABSTRACT In this study,magnetic core–shell structure Fe3O4@MCM-41 nanoparticles were synthesized with vesicles as soft templates.In the preparation,FeCl2 and tetraethy orthosilicate(TEOS)were selected as Fe processor and Si precursor,respectively.Stable vesicles first formed in 0.03 mol·L?1 1:2 mixture of anionic surfactant sodium dodecyl sulfate and cationic surfactant cetyltrimethyl ammonium bromide.Then,TEOS was added in the vesicle aqueous solution,leading to a highly dispersed solution.After high-temperature calcination,Fe3O4@MCM-41 nanoparticles were obtained.Their structure and morphology were characterized by Saturn Digisizer,transmission electron microscope and vibrating sample magneto-meter.The results indicate that the vesicles are spherical and their size could be tuned between 20 and 50 nm.The average grain diameter of synthesize magnetic core–shell Fe3O4@MCM-41 particles is 100–150 nm and most of them are in elliptical shape.The dispersion of magnetic particles is very good and magnetization values are up to 33.44 emu·g?1,which are superior to that of other Fe3O4 materials reported.

1.Introduction

Recently,core–shell nanostructures consisting of metals and metal oxides have become an attractive research area in material chemistry[1–5].Specially,core/shell structure magnetic nanocomposite materials have been widely used in a variety of biological applications such as drug or gene delivery[6,7],magnetic resonance image[8],biomedical diagnosis[9],catalytic applications[10],and biomolecule detection[11].Fe3O4@SiO2magnetic nanocrystals,consisting of an iron oxide core and silica shell,have attracted considerable attention in view of their vast potential for applications in magnetic fluids,bioseparation,enzyme immobilization,and diagnostic,due to their unique magnetic responsivity,low cytotoxicity,and chemically modifiable surface[12–14].However,with two different materials,the synthesis of ordered SiO2shellremains a challenge,because of incomplete cover of cores,multiple oxide domains in each core,or particle aggregation to large clusters.It is necessary to understand the underlying causes and their effects.

Vesicles of anionic and cationic surfactants in aqueous solution have received a great interest for a wide range of applications including colloids,pharmaceuticals and materials with their use as simple model systems for biological membranes[15–17].The methods using vesicles as template provide a powerful strategy to control the size and morphology of composites in this transfer synthesis with various inorganic or macro-molecular materials in aqueous solutions[18].Hao's group[14,18]had prepared nanometer ZnS particles by using vesicles as soft template in ionic liquid at room temperature.Two main approaches for applying vesicle as template to assemble or incorporate micro-and nanostructures materials have been explored:(1)hydrophilic or hydrophobic bilayer membrane of vesicles(opposite vesicles)as reaction space and(2)a template with the outside of bilayer in the formation of vesicles.Studies have shown that silica about 10 nm can be obtained by depositing on vesicle surface,with surfactant monomer as template in this reaction.

In this study,we intend to combine the advantages of magnetic Fe3O4core with SiO2shell to develop novel magnetic core–shell structure Fe3O4@MCM-41 nanoparticles by using vesicles as soft template.The strategy for the formation of core–shell structure composites is shown in Fig.1.(1)Vesicles are formed by mixing cetyltrimethyl ammonium bromide and sodium dodecyl sulfate in an aqueous solution;(2)Fe2+and Fe3+are added into the solution,adsorbing on the outside and inside surfaces of vesicles or inside of vesicles;(3)Fe3O4nanoparticles are obtained inside vesicles when NaOHis introduced in the solution.TEOS is adsorbed on the outer surface of vesicles;and(4)mesoporous ordered MCM-41 shell is on the outside of Fe3O4nanoparticles.

The synthesized magnetic core–shell structure Fe3O4@MCM-41 nanoparticles have good magnetic properties with the uniform dispersion of magnetite in a template.The magnetite is completely covered with silica so that all magnetite is available for effective magnetic separation and the magnetite on silica surface prevents side reactions.Obviously,to combine magnetic nanoparticles and hexagonal mesoporous silica spheres to form magnetic core/mesoporous shell silica spheres will realize high drug loading and magnetic targeting delivery[19,20].

Fig.1.Schematic illustration of the formation mechanism of the Fe3O4@MCM-41 nanoparticles by using vesicles as soft template in solution.

2.Experimental

2.1.Sample preparation

2.1.1.Materials

All chemicals were used without further purification.Cetyltrimethylammonium bromide(CTAB,99%)was obtained from Jining Chemical Engineering Research Institute of China.Sodium dodecyl benzene sulfonate(SDBS,97%)was obtained from Shantou Guanghua Chemical Co.of China.Other reagents were analytically pure,purchased from Shanghai Chemical Co.Ltd.The water used was deionized.

2.1.2.Synthesis of vesicles

Vesicles were prepared by mixing CTAB and SDBS at the total concentration of 0.03 mol·L?1and mixing mole ratio of 1.0:2.0 for 12–48 h at 30°C[16].The vesicles were dyed by 2%phosphorus tungsten acid solution and observed by transmission electron microscope(TEM)after drying at normal temperature.

2.1.3.Preparation of Fe3O4@MCM-41

A typical synthesis of Fe3O4and Fe3O4@MCM-41 nanoparticles was as follows:1.088 g of FeCl3(0.0067 mol)and 0.419 g(0.0033 mol)of FeCl2were added to 50 mL 0.03 mol·L?1of CTAB/SDBS(mole ratio of 1.0:2.0)solution which had been kept for 12 h.After 30 min of mild shaking,equivalent volumes of two separate solutions containing Fe ions were mixed to form a solution with total Fe concentration of 0.1 mol·L?1.The pH value of the solution was adjusted to 8.0 by adding NaOH(0.01 mol·L?1).The resulting gray mixture of Fe3O4nanoparticles was laid aside at room temperature in the dark for 1 h.Then 20 ml of TEOS(0.1 mol)was quickly added to the mixture with ultraphonic shaking for 120 min.Obtained precipitate of Fe3O4@MCM-41 was separated,washed several times with distilled water and ethanol,dried in a vacuum at 25 °C for 4 h,and calcinated at 550 °C for 10 h.

2.2.Material characterization

The microstructure and morphology of samples were examined by scanning electron microscopy(SEM,XL-30 E with an acceleration voltage of 20 kV).TEM images were obtained with a HORIBA EX-250 transmission electron microscope at an acceleration voltage of 120 kV.Powder X-ray diffraction(XRD)patterns were recorded on a Rigaku D/max-II diffractometer(40 kV,40 mA)with Cu Kαradiation at a scanning speed of 0.4(°)·min?1from 0.5 to 10°.N2adsorption–desorption measurements were conducted by using an AUTOSORB-1 chemisorption-physisorption analyzer.X-ray photoelectron spectroscopy(XPS)analysis was carried out by using a VGESCALAB MK II with achromatic X-ray source.The size distribution of vesicles was determined using a Malvern ZEN3600 Zetasizer Nano.The magnetic properties of samples were conducted using a Nanjing University Instrument Factory HH-15(15,000 Oe)vibrating magneto-meter.The stability of magnetite fluid was examined by measuring the setting height of particles from the upper layer of the magnetic fluid in the capillary viscometer at pH 7.The magnetic fluid was prepared by changing the content of particles from 10 to 80 g in 100 ml of kerosene.

3.Results and Discussion

3.1.Composition and morphology of Fe3O4@MCM-41

Fig.2 shows the representative TEM image of the vesicle synthesized in 12 h.The vesicles are composed of double membrane,and at the synthesized temperature for 24 h,the average diameter of sample is 80–100 nm.In Fig.2(b),the mean diameter is estimated to be 80–120 nm for the Fe3O4@MCM-41 nanoparticles.The nanoparticles are well dispersed,suggesting that the coating ofMCM-41 has a little effect on the dispersibility of nanoparticles,which agrees well with the TEM observation.Fig.2(c)shows the representative TEM image of magnetic Fe3O4@MCM-41 core–shell nanoparticles.The particle size is commonly in 80–120 nm and the shell thickness is 30–50 nm.The shell of Fe3O4@MCM-41 presents mesoporous framework with uniform pore distribution and two dimensional hexagonal arrangement[Fig.2(d)].

Fig.3 shows the XRD patterns of MCM-41 and core–shell Fe3O4@MCM-41 nanoparticles.The samples exhibit three sharp diffraction peaks,indexed to(100),(110),and(200)reflections.This indicates that the shell structure of Fe3O4@MCM-41 has a highly ordered hexagonal array.With N2adsorption–desorption isotherms at?196 °C,the Brunauer–Emmett–Teller gives the surface area and pore diameter of Fe3O4@MCM-41 as 513.9 m2·g?1and 2.28 nm,respectively(Fig.S1).

To investigate the chemical identities of as-prepared Fe3O4spheres and Fe3O4@MCM-41 core/shell particles,Fig.4 shows XPS spectra in the region of0–900 eV.The main content of the surface of Fe3O4spheres is Fe,O,and C elements,and the Fe 2p XPS spectrum of Fe3O4spheres exhibits two peaks at 711.1 and 725.1 eV,corresponding to Fe 2p3/2and Fe 2p1/2spin orbit peaks of Fe3O4(Insets of Fig.4).For Fe3O4@MCM-41 core/shell particles,the binding energy for Fe 2p cannot be detected,Si 2s and 2p peaks appear in these spheres after SiO2coating[21].These support that all Fe3O4cores in the composite are confined within a shell of SiO2,in accordance with TEM analyses.

3.2.Formation of vesicles and Fe3O4 particles

Fig.5 shows the representative negative staining-transmission electron microscopy images of the vesicles forming in different storage time.The vesicles are composed of double membrane in each case,with their diameter increasing with storage time,with 20–30 nm for 12 h,80–100 nm for 24 h,and 120–150 nm for 48 h.Fig.S2 shows the size distribution of vesicles with different storage time and complies with TEM results in Fig.5.

Fig.2.TEM images of vesicle(a),Fe3O4@MCM-41(b,c)with the inset of single Fe3O4@MCM-41 nanoparticle,and HR-TEM image of Fe3O4@MCM-41(d).

Fig.3.XRD spectra of samples MCM-41(a).

In order to reveal the growth mechanism of Fe3O4@MCM-41 nanoparticles,Fe3O4was prepared without TEOS.Fig.6 shows the TEM images of Fe3O4nanoparticles in the vesicles of 20–30 nm with Fe ion concentrations of 0.01,0.05,and 0.1 mol·L?1.In each case,Fe3O4nanoparticles of approximately 20–30 nm precipitate in vesicle phase with good dispersity.The average diameter of Fe3O4nanoparticles is more than 50 nm at Fe ion concentration of 0.15 mol·L?1,so in the next experiments,Fe concentration of 0.1 mol·L?1in vesicles would be used.Fig.S3 shows the XRD spectra of Fe3O4nanoparticles synthesized with different Fe concentrations.Their diffraction peaks coincide with the cubic spinel structure of Fe3O4crystal.

Fig.7 shows the representative SEM images of Fe3O4nanoparticles synthesized with different Fe concentrations.Most of Fe3O4particles have appearance with 20–30 nm in diameter,so that the Fe3O4nanoparticles do not aggregate significantly.Although Fe3O4nanoparticles are in mono-disperse state,they closely accumulate with only specific surface area about 20–30 m2·g?1.

3.3.Application of Fe3O4@MCM-41 in magnetic fluids

The magnetic property of stable magnetic fluids made of Fe3O4and Fe3O4@MCM-41 nanoparticles is determined.The relation of Fe ion concentration,average grain diameter and magnetization saturation(Ms)of Fe3O4and Fe3O4@MCM-41 are shown in Table 1.With the increase of Fe ion concentration,the average grain diameter of Fe3O4and corresponding magnetic value increase.At Fe ion concentration of 0.1 mol·L?1,the magnetic value of Fe3O4@MCM-41 nanoparticles decreases from 54.11 emu·g?1to 33.44 emu·g?1with Fe3O4nanoparticles coated by shell of MCM-41.The Ms value of Fe3O4@MCM-41 is much higher than that in literature(about 1.6 emu·g?1)[22].

Fig.8 shows the magnetized rate curve of the three Fe3O4@MCM-41 core–shell nanoparticles with different Fe ion concentrations.The magnetic fluid has a good stability due to the super-dispersibility of Fe3O4nanoparticles coated with MCM-41.

Fig.4.XPS spectra of Fe3O4(a)and as-prepared Fe3O4@MCM-41 core/shell and Fe3O4@MCM-41(b).(b).Inset:high-resolution spectrum of Fe 2p for Fe3O4.

Fig.5.Negative staining-transmission electron microscopy images of vesicle solution at a total surfactant concentration of 0.03 mol·L?1 with different storage times.(a)12 h;(b)24 h;(c)48 h.

Fig.6.TEM images of Fe3O4 nanoparticles in vesicles with different Fe concentrations.a:0.01 mol·L?1;b:0.05 mol·L?1;c:0.1 mol·L?1.

Fig.7.SEM images of Fe3O4 nanoparticles in vesicles with different Fe concentrations.(a)0.01 mol·L?1;(b)0.05 mol·L?1;(c)0.1 mol·L?1.

Table 1 Relation of Fe ion concentration,average grain diameter,and Ms value of Fe3O4 and Fe3O4@MCM-41

Fig.8.Magnetization curve of Fe3O4@MCM-41 in vesicles.

Fig.9.Viscosity of magnetic fluid with Fe3O4@MCM-41 with different Fe concentrations.nanoparticles in kerosene versus concentration of nanoparticles.(a)0.01 mol·L?1;(b)0.05 mol·L?1;(c)0.1 mol·L?1.

Fig.9 shows the viscosity of magnetic fluid consisting of Fe3O4@MCM-41 nanoparticles in kerosene versus the concentration of Fe3O4@MCM-41 nanoparticles from 10 to 70 g dispersing in 100 ml of kerosene as carrier liquid.The presence of colloidal particles in a fluid increases internal friction as it flows,increasing the viscosity.The viscosity of magnetic fluid increases with the concentration of Fe3O4@MCM-41 nanoparticles in kerosene.The viscosity of magnetic fluid with 0.8 g·ml?1of Fe3O4@MCM-41 nanoparticles in kerosene is 42.3 mPa·s(25 °C)[23].

Overall,the Fe3O4@MCM-41 core–shell nanoparticles have good magnetic properties compared with Fe3O4nanoparticles.Modification of Fe3O4@MCM-41 core–shell nanoparticles with medicine will be discussed in our further work.

4.Conclusions

We have demonstrated a facile soft template method to synthesize Fe3O4@MCM-41 nanoparticles with a core/shell structure in vesicles with diameter of 20–30 nm formed by anionic/cationic surfactant in an aqueous solution.The average particle size of core/shell structure Fe3O4@MCM-41 nanoparticles is in the range 80–120 nm in the same pot by water thermal condensation.The Fe3O4@MCM-41 nanoparticles are extremely good in dispersion and excellent in magnetic property.It is expected that the Fe3O4@MCM-41 nanoparticles can be further modified with other materials by using the functional groups such as Si–OH on the surface of MCM-41 for further medical applications.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2015.04.008.