Sonochemical synthesis, structure and magnetic properties of air-stable Fe3O4/Au

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Nanotechnology 18 (2007) 145609 (8pp) doi:10.1088/0957-4484/18/14/145609

Sonochemical synthesis, structure andmagnetic properties of air-stable Fe3O4/AunanoparticlesWei Wu1, Quanguo He1,3, Hong Chen1,2, Jianxin Tang1 andLibo Nie1

1 Green Packaging and Biological Nanotechnology Laboratory, Hunan University ofTechnology, Zhuzhou 412008, People’s Republic of China2 College of Life Science and Technology, Central South University of Forestry andTechnology, Changsha 41004, People’s Republic of China

E-mail: hequanguo@163.com

Received 29 November 2006, in final form 22 January 2007Published 6 March 2007Online at stacks.iop.org/Nano/18/145609

AbstractAir-stable nanoparticles of Fe3O4/Au were prepared via sonolysis of asolution mixture of hydrogen tetrachloroaureate(III) trihydrate (HAuCl4) and(3-aminopropyl)triethoxysilane (APTES)-coated Fe3O4 nanoparticles withfurther drop-addition of sodium citrate. The Fe3O4/Au nanoparticles werecharacterized by x-ray powder diffraction (XRD), ultraviolet–visiblespectroscopy (UV–vis), scanning electron microscopy (SEM) with energydispersive spectroscopy (EDS), transmission electron microscopy (TEM),x-ray photoelectron spectroscopy (XPS) and superconducting quantuminterference device (SQUID) magnetometry. Nanoparticles of Fe3O4/Auobtained under appropriate conditions possess a very high saturationmagnetization of about 63 emu g−1 and their average diameter is about30 nm.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Magnetic nanoparticles are playing an important role in awide range of sophisticated bio-medical applications, suchas targeted drug delivery [1], magnetic cell sorting andimmunoassays [2], biochemical sensing [3], and ultra-sensitivedisease detection [4]. Recently several research groups haverevealed the possibility of passivating the surface of magneticnanoparticles (γ -Fe2O3, Fe3O4, Co, Fe etc) by another inertshell (SiO2, gold, silver, polymer etc) [5]. A diamagnetic layerformalized the shell that could potentially reduce magneticproperties of the magnetic core of the nanoparticle. At thesame time, it is expected that magnetite nanoparticles can bepassivated to avoid oxidation by diamagnetic layer coatingwithout significant effects on magnetic properties such ascoercivity and blocking temperature.

Meanwhile, gold has become one of the most favouredcoating materials due to its specific surface derivative

3 Author to whom any correspondence should be addressed.

properties for subsequent treatment with chemical or bio-medical agents. For example, it is well established that Aunanoparticle surfaces could be functionalized with thiolatedorganic molecules for further applications. Furthermore,the gold coating could provide a platform for opticalabsorption and emission caused by the collective electronicresponse of the metal to light [7] and the gold surface alsocould make these compatible and adaptable for microchiptechnology. Therefore, Au-coated Fe3O4 nanoparticles havebeen developed with an aim towards sensor and technologicalapplications [6]. The Fe3O4 core provides a particle whichallows for a small size with significant magnetic moment. Inaddition, the Fe3O4 core with gold coating can establish a goodplatform for further conjugation of biomolecules. Reportson Au-coated iron oxide particles [8] and dumbbell-like Au–Fe3O4 nanoparticles [9] have appeared recently, and the reversemicelle method for synthesis of iron oxide@Au nanoparticleshas also been developed [10]. However, these methods are notable to maintain the morphology of the magnetic core particlesand the preparation involved is time-consuming.

0957-4484/07/145609+08$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 145609 W Wu et al

OH

OH

OH

+ H2N Si OEt

OE

t

OE

t

O

O

OSi

NH2

Si NH2

SiNH2

O

O

Fe3O4/AuFe3O4

Au3+

Sonochemical

Scheme 1. Illustration of the synthetic chemistry for Fe3O4/Au nanoparticle preparation.

As a competitive alternative, the sonochemical methodhas been extensively used to generate novel materials withunusual properties. The physiochemical effects of ultrasoundarise from acoustic cavitation, that is, the formation, growthand implosive collapse of bubbles in liquid. The implosivecollapse of the bubble generates a localized hotspot throughadiabatic compression or shock wave formation within thegas phase of the collapsing bubble. The conditions formedin these hotspots have been experimentally determined, withtransient temperatures of 5000 K, pressures of 1800 atm andcooling rates beyond 1010 K s−1. These extreme conditionswere beneficial to forming the new phase and have a sheareffect for agglomeration, which is necessary to prepare thehigh monodispersive nanoparticles [11]. This method has beenapplied for the synthesis of various nanocomposites, and itsversatility has been successfully demonstrated in nanoparticlepreparation. For example, Li et al synthesized air-stable Fe–Coalloy nanoparticles by sonochemistry in 2003 [12], Sivakumaret al synthesized LaFeO3 by sonochemistry in 2004 [13]and Nikitenko et al synthesized Fe–Fe3C nanocrystallineparticles [14]. However, reports on the synthesis of Fe3O4/Aumagnetic nanoparticle via sonochemistry method are veryscarce so far.

In this work, we will present a simple, speedy,sonochemical method for the synthesis of very high saturationmagnetization gold-coated Fe3O4 nanoparticles by threestep reactions in the following order: (a) use the co-preparation method to form the Fe3O4 core nanoparticles;(b) functionalize the Fe3O4 nanoparticles with functionalamine group by APTES [(3-aminopropyl)triethoxysilane];(c) reduce Au3+ ions to form the gold coating by sonolysis(scheme 1). Nanoparticles prepared by sonolysis were of ahigh monodispersity, while the morphology and bulk magneticproperties of the Fe3O4 core nanoparticles were substantiallyretained.

2. Experimental procedures

2.1. Chemicals and reagents

Ferric chloride (FeCl3·6H2O, 99.0%) was purchased fromTianjin Bodi Chemicals Co., Ltd. Ferrous chloride (FeCl2·4H2O, 99.0%) was purchased from Tianjin ShuangchuanChemicals Co., Ltd. (3-aminopropyl)triethoxysilane (APTES)was purchased from Sigma. Hydrogen tetrachloroaureate(III) trihydrate (HAuCl4, Au � 47.8%) was a product ofShanghai Platinum Group Metals Chemicals Co., Ltd. Sodiumhydroxide (NaOH) and sodium citrate were purchased fromHunan Huihong Chemicals Co., Ltd. All chemicals usedwere of analytical grade and used directly. Water (T =25◦, 18.2 M�) was purified by SUPER WATER-II waterpurification systems. The NdFeB magnet, purchased locally,was used to separate magnetic particles at the washing andselecting steps.

2.2. Synthesis of Fe3O4 /Au nanoparticles

The synthesis of Fe3O4 nanoparticles. The Fe3O4 nanopar-ticles as seeds were prepared through the chemical co-precipitation of Fe(II) and Fe(III) chlorides (FeII/FeIII ratio =0.5) with 1.5 M NaOH [15]. The black precipitate was col-lected on a magnet, followed by rinsing with water severaltimes until the pH reached 6–7.

The synthesis of APTES-coated Fe3O4 nanoparticles. For thefunctionalization of Fe3O4 nanocrystals with APTES, 2 mlof ferrofluid (10 mg of Fe3O4/ml of ethanol solution) wasdiluted to 50 ml with absolute ethanol and sonicated for 2–3 min. The resulting colloidal solution was transferred to athree-neck flask equipped with a condenser, a thermometer,and a heating mantle. Then, 180 μl of APTES was injectedinto the flask, and the mixture was vigorously stirred at roomtemperature for about 1 h and then heated to reflux for 2 h underargon. After the mixture was cooled to room temperature,the solid product was magnetically separated, washed withethanol five times and then redispersed in 10 ml of ethanolby sonicating for 10 min. To induce positive charges at thesurface of the APTES-coated Fe3O4 nanoparticles, 10 drops ofHNO3 solution (prepared by mixing 0.05 ml of 6 M HNO3 with20 ml of ethanol) was introduced into the ethanolic dispersionof APTES-coated Fe3O4 and then stirred for 4 h. Thenanoparticles were precipitated and separated by a magneticfield and centrifugation. After that, the product was washedwith ethanol and re-dispersed in ethanol.

The synthesis of Fe3O4/Au nanoparticles. The preparationof gold-coated Fe3O4 nanoparticles, involved the formersynthesis of APTES-coated Fe3O4 nanoparticles as seeds anda subsequent reduction of HAuCl4 in the presence of theseeds. Briefly, at 60 kHz (the ultrasonic frequency), 20 mlof the APTES-coated Fe3O4 nanoparticles and 15 ml HAuCl4

solution (1% mass fraction) were mixed, and 20 mM sodiumcitrate was then dropped into the mixture solution until thecolour changed from yellow to black. The resultant darkmaterial was precipitated and separated by a magnetic fieldand centrifugation. The precipitated product was washed withethanol and re-dispersed in ethanol. The nanoparticle solutionappeared dark purple.

2.3. Characterization

X-ray powder diffraction (XRD). The products wereidentified by powder x-ray diffraction. Powder diffractionpatterns were recorded on a Bruker Advanced-D8 powderdiffractometer equipped with a Si (Li) solid state detector anda source of Cu Kα radiation (λ = 1.5418 A). The data werecollected from 2θ = 5◦ to 90◦ at a scan rate of 0.02◦ per stepand 5 s per point.

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Nanotechnology 18 (2007) 145609 W Wu et al

Figure 1. Representative TEM images of Fe3O4 (A) and Fe3O4/Au (B) in a low concentration of ethanol solution; histograms of the particlediameters for (A) 17.5 nm and (B) 29.5 nm. The selected-area electron diffraction (SAED) pattern of the sample is shown in the histogramson the right, respectively.

Ultraviolet–visible spectroscopy (UV–vis). Ultraviolet–visiblespectra were acquired with a Purkinje General T-1901 spec-trophotometer. The spectra were collected over the range of190–660 nm.

Transmission electron microscopy (TEM). TEM wasperformed on a JEOL H-600 Electron Microscope (20 kV).The nanoparticle samples dispersed in ethanol solution werecast onto a carbon-coated copper grid sample holder, followedby evaporation at room temperature.

X-ray photoelectron spectroscopy (XPS). The XPS measure-ments were made using a Kratos Axis Ultra DLD. This systemuses a focused monochromatic Al x-ray (1486.6 eV) source forexcitation and a spherical section analyser. The percentages ofindividual elements detected were determined from the relativecomposition analysis of the peak areas of the bands. The rela-tive peak areas and their corresponding sensitivity factors wereused to provide relative compositions.

Scanning electron microscopy (SEM) with energy dispersivespectroscopy (EDS). SEM studies were carried out using aHitachi 3000-N microscope operated at 25 kV. Samples forSEM were prepared by deposition of the particle onto an

Al substrate by vacuum drying nanoparticles in air. EDSequipment was installed which was a product of EMAX.

Superconducting quantum interference device (SQUID) mag-netometry. Magnetic measurements were performed using aSQUID magnetometer (Quantum Design MPMS XL-7). Mag-netic susceptibility M–H curves were performed at T = 300and 5 K, where M is the magnetization and H is the appliedmagnetic field which were measured as a function of temper-ature in a magnetic field. The zero-field cooling/field cooling(ZFC/FC) measurements were also performed on the SQUID.

3. Results and discussion

3.1. Characterization of Fe3O4/Au nanoparticles

Figure 1 shows the representative TEM micrograph of Fe3O4

(A) and Fe3O4/Au nanoparticles (B). The particles displayedhigh monodispersity in size. The particles are well isolated,which is characteristic of the presence of an inorganic shell onthe particles’ surface. The average size of the Fe3O4 seeds was17.5 nm. There are two major findings from the morphologicalcomparison of (A) and (B) in both figures. First, the particlesthat were coated with Au appeared much darker than the Fe3O4

nanoparticles. Second, for the particles that were coated withAu, the average size of particles changed from 17.5 nm (A)

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Nanotechnology 18 (2007) 145609 W Wu et al

Figure 2. Representative SEM images of Fe3O4 (A) and Fe3O4/Au nanoparticles (B).

Figure 3. XRD for Fe3O4 nanoparticles at different vacuum drying temperature (A) and the regions of the highest peak (34◦–37◦) (B).

to 29.5 nm (B) which shows a representative set of histogramsby comparing Fe3O4 (A) and Fe3O4/Au nanoparticles (B). Thechange of average size diameter is obviously increased sincethe particles had been modified by APTES before they werecoated with Au. The selected-area electron diffraction (SAED)pattern indicates the crystalline characteristics of magnetiteand Fe3O4/Au nanoparticles (see the histograms on the right,respectively). The SAED pattern of Fe3O4 nanoparticles canbe indexed to (111), (220), (311), (400), (422), (511) and(440), and the SAED pattern of Fe3O4/Au nanoparticles can beindexed to (111), (220), (200), (222) and (311), which agreeswell with other reports [8] illustrating bare Fe3O4 and gold-coated structures, respectively. The above results confirmedthe formation of Fe3O4/Au nanoparticles, evident from theinterparticle spacing and the uniformity.

The SEM images further reveal the surface morphologyof the Fe3O4 (A) and Fe3O4/Au (B) nanoparticles. Particleanalysis indicates that the average particle diameter isincreased, but the surface morphology is not changed, whichis similar to the TEM results. From the SEM imaging,we found the average particle diameter was bigger than theTEM results, maybe due to some particle agglomerationsince the sample for SEM was a solid powder after vacuumdrying. Then the influence of the drying method on theproperties and structure of magnetic Fe3O4 nanoparticles

was further investigated. In figure 3, the XRD spectrafor Fe3O4 nanoparticles at different temperatures werecompared. The data (all curves) show diffraction peaksat 2θ = 18.6◦, 30.4◦, 35.7◦, 37.3◦, 43.2◦, 53.7◦, 57.3◦ and62.8◦, which can be indexed to the (111), (220), (311), (222),(400), (422), (511) and (440) planes of Fe3O4 in a cubic phase,respectively. Obviously, the structure and the crystal form arenot changed since all the sampled nanoparticles’ XRD patternsfollow the standard Fe3O4 crystal pattern and the diffractionlines are widened to some degree, confirming that theXRD diffraction line becomes widened as the nanocrystallinematerial size gets smaller. In figure 3(B), the regions of thehighest peak (34◦–37◦) XRD spectra reveal the changes of theparticle. The particle size was calculated to be 25 ± 3 nmusing the Scherer formula4, which is close to the TEM results.The results suggest that the bigger average particle diameterestimated by SEM imaging was caused by agglomeration ofparticles and further demonstrate the formation of Fe3O4/Aunanoparticles.

To confirm the composition of the nanoparticles, EDSspectra in situ composition analysis were collected during theSEM imaging (figure 2(B)) and the result is shown in figure 4.

4 D = Kλβs

H K L cos θH K L(where D is the crystal diameter, βs

H K L is H K L

diffraction line width, or integral width, λ is the wavelength of x-radial, θH K L

is the angle for diffraction peaks and k is the proportion constant).

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Nanotechnology 18 (2007) 145609 W Wu et al

Figure 4. EDS spectrum of the same sample used to obtain figure 2(B), with identification of the observed peaks. The Al peak appears due toscattering caused by the Al SEM sample substrate.

EDS indicated the presence of gold, iron, silicon, oxygen andnitrogen from the sample. The aluminium is from the SEMsample substrate and the spectrum confirms that both Fe andAu exist in the composite nanoparticles, confirming that theiron oxide nanoparticles are successfully coated and passivatedby the gold shell.

The relative surface composition of Fe3O4/Au was furtheranalysed by XPS, and the representative results are shown infigure 5. A very high concentration of carbon (51.676% atom)is found on the surface of Fe3O4 nanoparticles in figure 5(A),due to the binding energy (Eb) for C1s (284.5 eV) as theinternal references. The XPS of Au from the Fe3O4/Aunanoparticles is exhibited in figure 5(B), in which the peakof gold that appeared at 336.21 eV can be assigned to theAu0 (4d). The XPS of Fe from the Fe3O4/Au nanoparticles asshown in figure 5(C) demonstrates the simultaneous existenceof Fe2p1/2 (725.1 eV) and Fe2p3/2 (710.9 eV), which is close tothe standard data of Fe3O4. The XPS of the regions of O(1s) forcomparing Fe3O4 nanoparticles with Fe3O4/Au nanoparticlesin figure 5(D) indicates the existence of O1s (531.4 eV) fromthe Fe3O4 nanoparticles and O1s (530.78 and 532.29 eV)from the Fe3O4/Au nanoparticles, respectively. The peak ofoxygen appearing at 530.78 eV is close to the oxygen ofFe3O4 nanoparticles and the peak at 532.29 eV is close to thebinding energy (Eb) of an Si–O group (532.64 eV) [16]. TheXPS of the regions of C1s from the Fe3O4/Au nanoparticlesis shown in figure 5(E). The peak of carbon at 284.9 eV isclose to the data of the binding energy (Eb) of a long-chainalkyl and it undoubtedly shows that it comes from the long-chain alkyl of APTES. From all the XPS images of the regions,the corresponding element intensities (united by CPS) of theFe3O4/Au nanoparticles are obviously lower than those of theFe3O4 nanoparticles that accord with the shielding effect (orblocking effect) principle of XPS after gold coating. The XPSresults indicate that the magnetic Fe3O4 core has been fullycoated in a gold shell format, thus greatly reducing the intensitysignals of the element inside.

In summary, the APTES-coated Fe3O4 nanoparticles wereinfluenced by the immobilization of the gold shell on theirsurfaces. However, the formation of such a coating may beadvantageous to serve as a protective layer against oxidation

and some extreme chemical environments and could be appliedin further derivative applications in biosensors.

3.2. Optical properties

Another piece of evidence supporting the Fe3O4/Au morphol-ogy is given by measurements of the surface plasmon (SP) res-onance band of the nanoparticles. Figure 6 shows a typical setof UV–vis spectra comparing the freshly prepared Fe3O4 (dis-persed in ethanol), APTES-coated Fe3O4 (dispersed in ethanol)and the solution before and the product after the reaction(Fe3O4/Au nanoparticles dispersed in ethanol). The collectiveoscillations of free electrons, known as the surface plasmon ofpure Au nanoparticles, cause an absorption peak to appear inthe visible region of the electromagnetic spectrum [17]. In par-ticular, Au particles in water have been shown to exhibit an SPpeak at 520 nm [18]. Factors that affect the position of the SPpeak have been investigated on the basis of Mie theory, for Aunanoparticles, the SP has been shown to shift as a function ofparticle size, stabilizing ligand and solvent dielectric.

Thus, the bimetallic core/shell nanoparticles were furthercharacterized by UV–vis absorption spectra to compare theiroptical properties for each process of preparing the Fe3O4/Aunanoparticles. Spectra of as-prepared core/shell nanoparticlesfor Au deposition are collected and shown in figure 6. Theobservable SP peak of Fe3O4 and APTES-coated Fe3O4 whollydisappeared, but the SP peak of the reaction before and aftergold coating was easily distinguished. The former SP peakwas located at 319 nm, while the SP peak of Fe3O4/Aunanoparticles was at 556 nm, characteristic of the uniqueoptical properties of gold nanostructure. The absorptionintensity of APTES-coated Fe3O4 nanoparticles is lower thanFe3O4 nanoparticles, indicating that the electronic propertiesmay be suppressed by APTES. The electronic properties areinduced by the presence of an Fe oxide core and may affectthe SP position; as Au character increases and the Fe oxideis buried beneath the Au shell, these dielectric effects maybe suppressed [19]. A similar shift has been reported forgold-coated iron oxide nanoparticles prepared by differentmethods [20]. These facts prove that our method is useful andversatile for preparing core–shell nanoparticles, especially forefficient shell gold-coating.

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Nanotechnology 18 (2007) 145609 W Wu et al

Figure 5. XPS spectra for Fe3O4 nanoparticles (1), APTES-coated Fe3O4 nanoparticles (2) and Fe3O4/Au nanoparticles (3) comparison (A),the regions for Au(4d) (B), Fe(2p) (C), O(1s) (D) and C(1s) (E) comparison, respectively.

3.3. Magnetic properties

The ZFC/FC curves of the gold-coating Fe3O4 nanoparticlesand the bare Fe3O4 nanoparticles measured in a field of 100 Oeon a SQUID magnetometer are plotted in figure 7(A). Theabsence of a well-defined maximum in the ZFC curve indicatesthat both the bare Fe3O4 and Fe3O4/Au nanoparticles exhibitblocking temperatures (TB) above 100 K. Furthermore, novisible difference between the two curves was detected for

the bare Fe3O4 and Fe3O4/Au nanoparticles. It is knownthat the maximum of the ZFC curve for a collection ofsuperparamagnetic noninteracting single-domain nanoparticlesis dependent on the size of nanocrystals and their degreeof clustering, as well as on the mutual dipolar interactionsbetween them [21]. However, in the case of Fe3O4/Aunanoparticles, the ZFC and FC curves diverge at a muchlower temperature than the temperature observed in the caseof the bare Fe3O4 nanoparticles. This could be associated

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Nanotechnology 18 (2007) 145609 W Wu et al

Figure 6. UV–vis spectra of Fe3O4 nanoparticles (a), theAPTES-coated Fe3O4 nanoparticles (b), the mixture beforereaction (d) and Fe3O4/Au (c) (λmax = 556 nm).

with a lowering of the anisotropic energy barrier for theFe3O4/Au nanoparticles with respect to that of the bare Fe3O4

nanoparticles.SQUID magnetometry reveals that overlaying Fe3O4

nanoparticles surface with a shell of Au has a negligibledecrease on magnetic behaviour. Figure 7(B) shows thehysteresis loops measured at T = 300 K (close to roomtemperature) and T = 5 K for Fe3O4 nanoparticlesand Fe3O4/Au nanoparticles, respectively. The saturationmagnetization (Ms) of Fe3O4 nanoparticles was found to be65 emu g−1 at 300 K and 78 emu g−1 at 5 K, respectively.Like Fe3O4 nanoparticles, Fe3O4/Au nanocomposites are closeto paramagnetic at 300 K, though the Ms was found to be63 emu g−1 at 300 K and 71 emu g−1 at 5 K, respectively.Obviously, the Ms at 300 K is lower than that at 5 K, whichagrees with the formula that increasing the temperature wouldcause the Ms to decrease. The above interpretation in terms

of a surface anisotropy, as a result of the interaction with theligands or outer layers like the gold coating, indicates that thesurface anisotropy probably also affects the moment of theinner Fe3O4 nanoparticles via the exchange interaction withthose at the surface [21]. Thus, the Ms of Fe3O4 nanoparticleswas decreased by the immobilization of the gold shell on theirsurfaces. Moreover, the decrease in magnetic behaviour wasvery close to other reports [22].

3.4. Proposed mechanism

For the synthesis of the nanocomposite material, at firstwe synthesized colloidal solutions of Fe3O4 nanoparticles inethanol that exhibit long sedimentation times, being stableagainst agglomeration for several days. The second stepof the synthetic processes consists of functionalizing theFe3O4 nanoparticles with APTES. The mechanism is thatthe hydroxyl groups on the magnetite surface reacted withthe ethoxy groups of the APTES molecules leading to theformation of Si–O bonds and leaving the terminal −NH2

groups available for immobilization of gold [23]. The APTES-coated Fe3O4 nanoparticles show strong chelating ability forgold metal (Au3+ ions) by the long pair of terminal −NH2

groups of organic entities, and the interactions can be furtherenhanced by mutually attractive electrostatic interactions whenthe two components are oppositely charged. This mechanismcan be explained by the HSAB (hard and soft acids and bases)formula [24], the Au3+ ion is similar to a soft acid which cancombine with the terminal −NH2 groups easily, and thus Auwill coat the surface of APTES-coated Fe3O4 nanoparticles bya reducing agent and the ultrasonic frequency acts as a catalyst,causing the chemical reaction to be effected fast, acceleratingthe speed of gold depositing onto the APTES-coated Fe3O4

nanoparticle surface.The chemical effects of ultrasound arise from acoustic

cavitation, that is, the formation, growth, and implosivecollapse of bubbles in liquid. Compared with thetraditional stirring technology, the sonochemistry is beneficial

Figure 7. ZFC/FC curves of Fe3O4 and Fe3O4/Au nanoparticles (A), hysteresis loops of Fe3O4 and Fe3O4/Au nanoparticles at T = 300 and5 K (B).

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Nanotechnology 18 (2007) 145609 W Wu et al

to get a uniform medium, eliminate the odds of localconditions, increase the speed of reaction and form thenew phase. Moreover, this method has a shear effectfor agglomeration, which is necessary to prepare highmonodispersive nanoparticles. Therefore, this method hasproven to be simple, inexpensive and versatile. Littleemphasis has so far been placed on the design ofnanocomposite architectures with tailorable properties andcomplex functionalities by the immobilization of metalonto different kinds of metal oxide nanoparticles. OurFe3O4/Au nanocomposites represent a relevant system withsuperior magnetic properties and potential widespread use inbiosensors.

4. Conclusions

In summary, a simple, rapid and feasible route to preparemagnetic Fe3O4/Au nanoparticles by sonolysis and the reactionmechanism has been demonstrated. Our approach differsfrom many nanoparticle-to-nanoparticle based approaches bythe ability to control a combination of chemically tunablechelating layer modifications (such as −NH2 groups) for themagnetic core and further deposition of Au on the amine-functionalized Fe3O4 surface. The Fe3O4/Au nanoparticleshave been characterized by TEM for the detection of changesin particle size and morphology, UV spectrophotometry forobservation of the change in SP optical bands, XRD for thedetection of changes in diffraction peaks, XPS for analysisof the surface compositions and SQUID for measurementof the magnetic properties. The Fe3O4/Au nanoparticlesprovide chemically active sites on the surface of magnetitenanocrystals, enabling their potential derivation with differentmultifunctional organic molecules. This effective approachcan readily extend to the immobilization of other noblemetals onto the chemically modified surface of magnetitenanocrystals, which also opens up new potential avenues forthe functionalization of these nanoensembles and their furthermanipulation in specific biochemical applications.

Acknowledgments

The authors gratefully acknowledge financial support fromthe Natural Science Foundation of China Nos 20505020,60571001, 60571032, Natural Science Foundation of HunanNos 04jj40023, 05jj40053, Scientific Research Fund of HunanProvincial Education Department No 05C508 (2005–2006)and the Skeleton Youth Faculty Program of Hunan HigherEducational School No 2005–2008.

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