Materials Science and Engineering C 32 (2012) 1043–1049

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Materials Science and Engineering C

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Synthesis and functionalization of SiO2 coated Fe3O4 nanoparticles with amine groupsbased on self-assembly

Mohammad E. Khosroshahi ⁎, Lida GhazanfariLaser and Nanobiophotonics Lab., Biomaterial Group, Faculty of Biomed. Eng., Amirkabir University of Technology, Tehran, Iran

⁎ Corresponding author. Tel.: +98 21 64542398; fax:E-mail address: Khosro@aut.ac.ir (M.E. Khosroshahi)

0928-4931/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.msec.2011.09.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 March 2010Received in revised form 19 July 2010Accepted 6 September 2011Available online 16 September 2011

Keywords:MagnetiteSilicaFunctionalizationAmine groupsCore-shell nanoparticlesBiomedical applications

The purpose of this research was to synthesize amino modified Fe3O4/SiO2 nanoshells for biomedical applica-tions. Magnetic iron-oxide nanoparticles (NPs) were prepared via co-precipitation. The NPs were thenmodifiedwith a thin layer of amorphous silica. The particle surface was then terminated with amine groups. The resultsshowed that smaller particles can be synthesized by decreasing the NaOH concentration, which in our casethis corresponded to 35 nm using 0.9 M of NaOH at 750 rpm with a specific surface area of 41 m2 g−1 foruncoated Fe3O4 NPs and it increased to about 208 m2 g−1 for 3-aminopropyltriethoxysilane (APTS) coatedFe3O4/SiO2 NPs. The total thickness and the structure of core-shell was measured and studied by transmissionelectronmicroscopy (TEM). For uncoated Fe3O4 NPs, the results showed an octahedral geometrywith saturationmagnetization range of (80–100) emu g−1 and coercivity of (80–120) Oe for particles between (35–96) nm,respectively. The Fe3O4/SiO2 NPs with 50 nm as particle size, demonstrated a magnetization value of30 emu g−1. The stable magnetic fluid contained well-dispersed Fe3O4/SiO2/APTS nanoshells which indicatedmonodispersity and fast magnetic response.

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1. Introduction

The increasing availability of nanostructures with highly controlledmagnetic properties in the nanometer size range has created wide-spread interest in the use of NPs in biological systems for diagnosticand therapeutic applications. In recent decades, magnetic NPs, espe-cially Fe3O4 and γ-Fe2O3, have attracted increasing interest becauseof their outstanding properties including superparamagnetism andlow toxicity and, as a result, their potential applications in variousfields, especially in biotechnology and biomedicine, such as cell sort-ing, enzymeimmobilization [1], biosensing and bioelectrocatalysis[2], separation and purification [3,4] and tumor therapy [5]. Variousapproaches have been explored for synthesis and characterization ofhigh-quality magnetic iron oxide NPs [6–14]. The properties of mag-netic particles, including chemical stability, dispersivity in liquidmedia, and uniformity in size, must be considered. In ferrofluid, sta-bility is maintained by electrostatic and repulsive interactions be-tween counterions and amphoteric hydroxyl ions (H3O+ or OH−).In recent years, much effort has been devoted to the synthesis andcharacterization of silica coated iron-oxide NPs [15–21]. The magneticNPs were easily coated with amorphous silica via the sol–gel processbecause the iron-oxide surface has a strong affinity for silica [22]. Ithas been demonstrated that the formation of silica coating on the sur-face of iron-oxide particles can prevent their aggregation and keep

their chemical stability [23]. Another advantage is that this surface of sil-ica coating is often terminated by a silanol (Si\OH) group. These groupscan be easily coupled with organosilanes by formation of Si\O\Si co-valent bonds. The importance of the field is highlighted by the use ofbiomolecules which control the self-assembly of nanodevices [24–26].Molecular self-assembly is a key concept in supramolecular chemistry[27] since assembly of the molecules is directed through noncovalentinteractions (e.g., hydrogen bonding, metal coordination, hydrophobicforces, van derWaals forces, or electrostatic) as well as electromagneticinteractions. Common example includes the formation of Langmuirmonolayers by surfactantmolecules [28]. These technologies commonlyrequire the interaction between biomolecules and materials by meansof a stable intermediate layer. The interlayer should provide adequatefunctional groups that react with biomolecules. The most commonfunctional groups are \NH2 (amines) that interact readily with bio-molecules by covalent bonding [29]. Due to its relevance, the develop-ment of different biomedical applications will rely on advances in thepreparation of functionalized surfaces. The functionalization of nano-shells with self assembled monolayer (SAM) would facilitate nanoshellpurification and offer new strategies for nanoshell manipulation in sub-sequent investigations and applications [30]. SAM-coated nanoparticlesare insensitive to air and moisture and are soluble in a wide range oforganic solvents [31]. Among the most promising substrates, silica hasreceived a great deal of attention in the last years [23,32–36]. In this arti-cle, we report the synthesis and characterization of amino-functionalizedFe3O4/SiO2 NPs using chemical co-precipitation [15,37,38] and Stöber[39] methods for biomedical applications.

Table 1pH measurements.

Materials pH value

Magnetite (at room temperature) 14Magnetite (after watering) 9Magnetite (after adding HCl) 6Magnetite+ethanol+ammonia+water 8Magnetite+TEOS (MS) {after 12 h} 9Magnetite+TEOS+APTS 8

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2. Materials and methods

2.1. Materials

All of analytic reagents were purchased from the indicated suppliersand used without further purification: ferric chloride hexahydrate(FeCl3·6H2O) (99%), ferrous chloride tetrahydrate (FeCl2·4H2O) (99%),sodium hydroxide (NaOH, 99%), hydrochloric acid (HCl, 37%), triso-dium citrate (TSC), absolute ethanol, ammonia aqueous (25 wt.%),Si(OC2H5)4(tetraethyl orthosilicate, TEOS) were purchased fromMerck. 3-aminopropyltriethoxysilane (APTS) was obtained fromSigma-Aldrich (St. Louis, MO). Milli-Q water (specific conductance0.1 μ cm−1) was deoxygenated by bubbling N2 gas for 1 h prior tothe use.

2.2. Synthesis of Fe3O4

Stock solutions of 1.28 M of FeCl3·6H2O, 0.64 M of FeCl2·4H2Oand 0.4 M of HCl were prepared as a source of iron by dissolvingthe respective chemicals in milli-Q water under vigorous stirring.In the same way, stock solutions of (0.9–1.5) M of NaOH wereprepared as the alkali sources and the synthesized Fe3O4 sampleswere classified as S1–S4 where each sample were synthesizedusing different NaOH concentration i.e. 0.9, 1.1, 1.3 and 1.5 M ofNaOH at 750 rpm corresponds to S1, S2, S3 and S4, respectively.Aqueous dispersion of magnetite NPs was prepared by alkalinizingan aqueous mixture of ferric and ferrous salts with NaOH at roomtemperature. 25 ml of iron source was added drop-wise into250 ml of alkali source under vigorous magnetic stirring (450 and750 rpm) for 30 min at ambient temperature. In order to preventFe3O4 NPs from possible oxidation in air as well as from agglomera-tion, they were modified with citrate groups. The precipitated blackpowder was isolated by applying an external magnetic field andthe supernatant was removed from the precipitate by decantation.After the powder being washed, the solution was decanted twice at5000 rpm for 15 min. Then 0.01 M of HCl was added to neutralizethe anionic charge on the particle surface. The cationic colloidalparticles were separated by centrifugation and peptized by watering.The obtained magnetic mud was then redispersed in a 200-mlportion of TSC solution (0.5 M) and heated at 90 °C for 30 minunder magnetic stirring (750 rpm). All the main synthesis stepswere carried out by passing N2 gas through the solution media toavoid possible oxygen contamination during the synthesis. Anappropriate amount of acetone was added to remove the excessivecitrate groups adsorbed on the NPs and collected with a magnet.After coating, the product was washed to remove the physicallyadsorbed surfactant on the particle. The powder was washed toremove the physically adsorbed surfactant on the particle. Aftercentrifugation the solution twice, the resultant mud was freeze-dried at −60 °C. The obtained powder as redispersed suspension inwater was sonicated and homogenized using a probe sonicator(Branson Sonic Power, Danbury, CT equipped with a needle probe).Finally, the resultant dispersion was adjusted to 2.0 wt.%.

2.3. Coating citrate-modified Fe3O4 with silica

Following the Stöber method, with some modifications, coating ofcitrate-modified Fe3O4with silicawas carried out in a basic ethanol/watermixture at room temperature using the obtained magnetite dispersion(only sample S1 at 750 rpm as an optimized value) as seeds. Magnetitedispersion (2 g) was first diluted with water (40ml), ethanol (120 ml),and then concentrated aqueous ammonia (3 ml) was added as suggestedin [22]. Deng et al. [15] studied the volume ratio of ethanol/water (VE/W)on the coating process. They found it should be between 2–4. In our caseVE/W ratio was roughly higher than 3. The resultant dispersion was well-dispersed by ultrasonic vibration for 15 min. Finally, 0.9 g of TEOS diluted

in ethanol (20 ml) was added drop-wisely to this dispersion under con-tinuous mechanical stirring. Finally, under continuous mechanical stir-ring, 0.9 g of TEOS diluted in ethanol (20 ml) was added drop-wisely tothis dispersion. The reaction mechanism is explained as:

SiðOC2H5Þ4 þ 4H2O ¼ SiðOHÞ4 þ 4C2H5OH

SiðOHÞ4 ¼ SiO2 þ 2H2O: ð1ÞAfter stirring for 12 h, the obtained magnetic product was collect-

ed by magnetic separation and washed twice with ethanol. Subse-quently, Fe3O4/SiO2 NPs was obtained through the sol–gel approach.For the TEOS concentration used in our case, the induction periodwas approximately 30 min.

2.4. Functionalization of Silica NP Surfaces with APTS

Silanol groups are the predominant functional groups at the sur-face of unmodified silica nanoshells. The amount of APTS solutionneeded for surface functionalization is estimated according to the ap-proximate concentration and surface area of the silica nanoshells [40].Consequently, we added an excess of APTS (65 μl) to a 200 ml of themagnetic NP solution and the mixture has been vigorously stirredfor 2 h. The feasibility of functionalization reaction could be con-firmed visually by observing the precipitation of APTS functionalizedmagnetic NPs while remaining a clear ethanolic solution at the top.After that, covalent bonding between the APTS groups and the mag-netic NPs was enhanced by gently refluxing the solution [41] for 1 hat 90 °C. The magnetic NPs was centrifuged and redispersed in200 ml of ethanol for future use.

2.5. Instrumentation

The analysis of synthesized material was performed by using thefollowings: pH (EUTECH510pHmeter), XRD (FK60-04), FTIR (EQUINOX55), BET (Quantachrome TPR Win v1.0) to calculate the specific surfaceareas, thermogravimetric analysis-TGA (PL analyzer), TEM (Phillips CM-200-FEG), magnetization measurements (VSM-PAR 155) and particlesize analyzer (Malvern Zetasizer).

3. Results and discussion

A complete precipitation of Fe3O4 should be expected between(7.5–14) pH. It has been proven that silica coated magnetite NPshave an excellent biocompatibility and can homogeneously dispersein aqueous solutions with a wide range of pH values. The NH2 adsorp-tion strongly depends on pH value of the feed solution (Table 1). Inour case (pH=8), relatively large number of NH2 groups exists onthe surface of the silica adsorbents. At pH b10, protonation of theamines may give a positive surface charge which accounts for thehigh stability of these dispersions.

Fig. 1. TEM image of uncoated Fe3O4 NPs (samples S1, S4) at 750 rpm.

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3.1. Morphological properties

As shown in Fig. 1, the mean Fe3O4 particle size examined by TEMimaging exhibited an almost dispersed state. The powder size mea-surement showed a gradual increasing trend with increasing NaOH%which in our case this corresponded to 35 nm at 0.9 M of NaOH(S1) to 100 nm at 1.5 M of NaOH (S4) at 750 rpm, see Fig. 2. The influ-ence of the chemical potential on the shape evolution of crystals hasbeen elucidated by Liu et al. [7]. In the case of crystal growth, itwould be beneficial to have a higher chemical potential, which ismainly determined by the NaOH concentration. Octahedral Fe3O4

with high quality and crystallinity could be obtained in concentratedsolution, because higher OH− ion concentration and higher chemicalpotential in the solution favor the growth of octahedral structuresover other possible iron-oxide crystal forms. The reason may be dueto the reaction mechanism of magnetite:

Fe3þ þ 3OH− ¼ FeðOHÞ3 ð2Þ

FeðOHÞ3 ¼ FeOðOHÞ þ H2O ð3Þ

Fe2þ þ 2OH− ¼ FeðOHÞ2 ð4Þ

2FeOðOHÞ þ FeðOHÞ2 ¼ Fe3O4 þ 2H2O: ð5Þ

Fig. 2. Particle size vs NaOH concentration for uncoated Fe3O4 NPs at different rpmvalues.

A decrease in molarity affects reactions 2 and 4 and may affect thekinetics of the nucleation and growth of Fe3O4 particles. The reduc-tion in particle size creates negative pressure on the lattice whichconsequently leads to a lattice cell volume expansion. Furthermore,the reduction can also decrease the magnetic transition temperature.The increase in unit cell volume with reduction in particle sizeof Fe3O4 particles, perhaps implies an increase in Fe2+ content inthe sample [ionic radius of Fe2+ (0.74 Å) is larger than that of Fe3+

(0.64 Å)]. So the particle size of magnetite colloidal nanocrystal clus-ters (CNCs) is influenced by changes in the base molarity.

To observe the agglomeration state, particle size distribution,polycrystalline electron diffraction pattern (EDP) of Fe3O4/SiO2 nano-particles TEM was used. Fig. 3 shows a typical TEM image (Fig. 3a)and EDP of Fe3O4/SiO2 NPs (Fig. 3b) where the reflection correspondsto diffraction plane (311) characteristic of the magnetite phase. Thetypical size of core-shell structure was measured about 50 nmwhich is comparable with the results obtained by Deng et al. [15].The uniform dispersion would make them as a suitable candidatefor biomedical applications.

Each dried sample (0.01–0.02 g) after being accurately weighed,was placed in a sample tube. The surface area of all samples was de-termined by the BET method. Fig. 4 shows NaOH concentration versusspecific surface area for Fe3O4 NPs. N2 adsorption–desorption isothermsdemonstrated that highest specific surface area of 41 m2 g− 1 wasachieved at 750 rpm using 0.9 M of NaOH. Using the BET values, onecan calculate the amount of particle size, DBET, considering the followingEq:

DBET ¼ 6ρAS

.ð6Þ

Where ρ is the density (5.18 g cm−3) and As is the specific surfacearea of uncoated Fe3O4. According to this formula one can calculatethe particle size. A theoretical value for uncoated magnetite NPs is28 nm which is comparable to the value obtained experimentallyi.e. 35 nm. Table 2 demonstrates that SiO2 coated NPs have numerousnanopores in thewalls,which results in a high BET surface (81 m2 g−1).The BET value increased to about 208 m2 g−1 for Fe3O4/SiO2/APTSnanoshells.

3.2. Magnetic measurements

Saturation magnetization, Ms, residual magnetization, Mr and co-ercivity, Hc, bulk saturation (Mb=ρMs) are among the main magneticparameters that have to be characterized when considering the

Fig. 3. (a) TEM micrograph of Fe3O4/SiO2 NPs, (b) polycrystalline electron diffractionpattern of the magnetite phase corresponding to Fe3O4/SiO2 NPs.

Fig. 4. Specific surface area vs particle size for uncoated Fe3O4 NPs at different rpmvalues.

Table 2BET measurements.

Materials BET value (m2 g−1)

Magnetite 41Magnetite+TSC 112Magnetite+TSC+TEOS 81Magnetite+TEOS+APTS 208

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magnetism of a magnetic material. The value of Ms for these particles,was measured about 82 emu g−1 which steadily increased by in-creasing their size to 97 emu g−1 for S4 (Fig. 5). It can be seen thatthe saturation magnetization of Fe3O4 NPs are close to the bulkvalue of magnetite which is about 85–100 emu g−1 [6]. It approvesthat the synthesized particles are in acceptable crystalline state butin smaller size. In our case, the Ms value for S1 is slightly higherthan that reported by Yang et al. [22]. The formation of magneticbeads results in a decrease in the saturation magnetization, whileCNCs lead to an increase in saturation magnetization. Since theCNCs are composed of small primary nanocrystals, the CNCs can

have bigger saturation magnetization than individual nanoparticlesof the same size. The corresponding values of coercivity with 119 Oefor 35 nm and 85 Oe for 96 nm particles were measured and plottedin Fig. 6.

These results suggest that Hc is strongly size dependent and bulksamples with sizes larger than the domain wall width can cause mag-netization reversal due to domain wall motion. As domain walls movethrough a sample, they can become pinned at grain boundaries andadditional energy is needed for them to continue moving. Pinning isone of the main sources of the Hc. Grain size dependence of Hc andpermeability (GSDCP) theory [42] predicts:

Hc ¼ P1

ffiffiffiffiffiffiffiAK

p

MsDg∝1=Dg ð7Þ

Where A denotes the exchange constant, K is anmagnetocrystallineanisotropy constant, P1 and P2 are dimensionless factors. Therefore,reducing the grain size, Dg, creates more pining sites and increases Hc.For ultrafine particles, the modified form of theory predicts:

Hc ¼ P2K4D6

g

MsA∝D6

g : ð8Þ

The difference between Eqs. (7) and (8) is defined by ferromag-netic exchange length as:

Lex ¼ffiffiffiffiffiffiffiA

K=

qð9Þ

Using the following parameters for magnetite (K=1.35×104 J m−3,A=10−11 J m−1), the exchange length can be estimated as Lex=27 nm. Bulk saturation (Mb) of magnetite NPs is ρMs. Fig. 7 shows thesquareness, SQ=(Mr/Ms) versus particle size. It is worth mentioningthat the amount of SQ has a significant impact on the configuration ofhysteresis loop in magnetic NPs. The non-linear variation of squareness

Fig. 5. Variation of saturation magnetization with particle size for uncoated Fe3O4 NPsat 750 rpm.

Fig. 7. Variation of squareness with particle size for uncoated Fe3O4 NPs at 750 rpm.

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of particles with their size can be attributed first of all to that fact thatthere is a non-uniform distribution of particles sizewithin a given samplegroup (eg. S1,….S4) which implies a random selection of a particle forTEM purpose, and secondly, some magnetite can be changed intomaghemite (γ-Fe2O3) due to oxidization.

The magnetic moment of the unit cell comes only from Fe2+ ionswith a magnetic moment of 4μB. For superparamagnetic particles, thetrue magnetic moment at a particular temperature can be calculatedusing the Langevin function:

M ¼ Ms CothμHKBT

� �−KBT

μH

� �: 10

Where μ ¼ MsπD3

6 is the true magnetic moment of each particle, T isthe absolute temperature. A detailed description of the Langevinfunction can be found in the literature [43]. The SiO2 shell allowseach Fe3O4 particle to behave independently, and interparticleinteractions are therefore not important. Fig. 8 shows the room-temperature magnetization curve of the Fe3O4/SiO2 NPs obtainedusing a VSM. The M (H) hysteresis loop for the Fe3O4/SiO2 NPs wasalmost completely reversible. It means the magnetization curveexhibits zero remanence and coercivity, which proves that Fe3O4/SiO2

NPs has superparamagnetic properties. The Ms value for the Fe3O4/SiO2

NPs was 30 emu g−1 at an applied magnetic field of 6000 Oe, which isabout 3.6% of the Ms for Fe3O4. Taking these results together, we con-clude that the decrease in Ms is most likely the result of a large volumeof SiO2 in the coated NPs. When the external magnetic field was

Fig. 6. Variation of coercivity with particle size for uncoated Fe3O4 NPs at 750 rpm.

removed, the particles could redisperse rapidly, which is an advantageto their biomedical applications.

3.3. Structural analysis by XRD

Fig. 9 shows the XRD patterns for (a) uncoated Fe3O4 NPs, (b)citrate-modified Fe3O4 NPs, (c) SiO2 coated Fe3O4 NPswith each patternnormalized to its maximum intensity. The peaks are indexed with thefcc structure corresponding to magnetite phase. The average size ofthe crystals was estimated using Scherrer's formula [44]:

Dhkl ¼ kλ. ffiffiffiffiffiffiffiffiffiffiffi

B2M�B2

S

pcos θhkl

� � ð11Þ

where k is the shape factor, λ the X-ray wavelength, BM the half maxi-mum line width (FWHM) in radians, BS the half maximum line widthof the instrument, θ the Bragg angle and Dhkl the mean size of the or-dered (crystalline) domains. The dimensionless shape factor has a typ-ical value of about 0.9, but varies with the actual shape of the crystallite.Fig. 9a shows the result for S1 since we have used it as the least valuethroughout the synthesis process. The XRD results showed that thecitrate-modified Fe3O4 NPs are about 8 nm (Fig. 9b). Fig. 9c showsthat the pattern of silica coated magnetite displayed the same lines asuncoated magnetite.

3.4. FTIR study

The main absorption peaks around 591 and 3422 cm−1 in Fig. 10acorresponds to Fe\O and O\H stretching vibrationmodes, respectively.The presence of magnetite is evident at 418 and 462 cm−1. The absorp-tion peaks of citrate-modified samples at 1622 cm−1 and 1397 cm−1

Fig. 8. Variation of of Fe3O4/SiO2 NPs magnetization with applied field.

Fig. 9. X-ray diffraction patterns for: (a) uncoated Fe3O4 NPs, (b) citrate-modifiedFe3O4 NPs, (c) SiO2 coated Fe3O4 NPs. All indicated Miller indices in (a), (b) and (c) cor-responds to magnetite.

Fig. 11. TGA graphs of (a) SiO2 coated Fe3O4 NPs, (b) Fe3O4/SiO2/APTS nanoshells.

1048 M.E. Khosroshahi, L. Ghazanfari / Materials Science and Engineering C 32 (2012) 1043–1049

are due to the COO\Fe carboxylate bond, (Fig. 10b). The band at1092 cm−1 and 802 cm−1 are characteristic peaks of the symmetricaland asymmetrical vibrations of Si\O\Si. The band at 466 cm−1 is an in-dication of the presence of Si\O\Fe (Fig. 10c). Fig. 10d is the IR spectra ofAPTS-coated Fe3O4 NPs. The broad Band near 1092 cm−1 and 798 cm−1

is the contribution of Si\O. The adsorption bands in 2930 and2850 cm−1 are due to stretching vibration of \CH2. Bands near 3430and 1633 cm−1 exhibit the existence of\NH2.

3.5. SiO2 and APTS adsorption on magnetite nanoparticles

The heat endurance of Fe3O4/SiO2/APTS nanoshells was evaluatedby thermo-gravimetric (Fig. 11). Fe3O4/SiO2 NPs indicated three dis-tinct weight loss stages: i — a small weight loss in the range of 40–180 °C mainly due to the evaporation of residual alcohol, physicallyadsorbed water and slight dehydration of silanol groups, ii — a largeweight loss is in the range of 200–450 °C which is attributed to thedecomposition of organic substances, iii — a minor weight loss atthe higher temperatures, 500–550 °C, due to the complete

Fig. 10. FT-IR spectra of: (a) uncoated Fe3O4 NPs, (b) citrate-modified Fe3O4 NPs,(b) SiO2 coated Fe3O4 NPs, (c) Fe3O4/SiO2/APTS nanoshells.

dehydration of silica species which there after remain to be stable.According to TGA analysis, the residual mass percent of silica coatedNPs is 89% at 550 °C, while that of SiO2\NH2 is 87% which indicatesthe grafting percentage of APTS is approximately 6%.

The particle size distribution obtained at room temperature usingdynamic light scattering test is shown in Fig. 12. As it is seen theaverage diameter of the APTS functionalized magnetic NPs is about60 nm with relative maximum intensity of 35% based on opticaldensity of 1.046 and the refraction index of 1.358.

4. Conclusion

In conclusion, this synthesis provides a rapid and effective route tomagnetic core-shell particles that are soluble in aqueous media. Wehave presented a controlled coprecipitation method, which

Fig. 12. Particle size distribution of Fe3O4/SiO2/APTS nanoshells.

1049M.E. Khosroshahi, L. Ghazanfari / Materials Science and Engineering C 32 (2012) 1043–1049

demonstrates the feasibility of synthesizing monocrystalline NPs ofFe3O4. By virtue of the complexion occurring between the iron ionson the magnetic NPs and the citrate groups, highly stable magneticfluid containing well-dispersed magnetite NPs can be obtained. Dur-ing the hydrolysis and condensation of TEOS, the formed primary par-ticles are effective to suppress the dipole–dipole interactions amongthe magnetic particles. Monodisperse Fe3O4/SiO2 core–shell sphereswith a size of about 50 nm can be prepared by first modifying outersilica shells terminated with silanol groups and the final Fe3O4/SiO2/-APTS nanoshells showed a specific surface area of 208 m2 g−1. Thegrafting percentage of NH2 functional groups on surfaces of silicashells is 6%. Fe3O4 is covered by TSC, TEOS and APTS layers that exhib-it their characteristic IR vibration bands. The XRD results confirm thatthe iron-oxide NPs before and after coating have the phase of Fe3O4.These results provide us a better understanding of the structuraland magnetic properties for silica coated magnetite NPs and thusare very valuable for evaluating the future applications of this class ofmagnetic NPs in biomedical applications.

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