Journal of Magnetism and Magnetic Materials 322 (2010) 3519–3526

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

Mainz,

+49 613

E-m

(M. Ma

journal homepage: www.elsevier.com/locate/jmmm

Magnetic polyorganosiloxane core–shell nanoparticles: Synthesis,characterization and magnetic fractionation

Stefanie Utech a,b, Christian Scherer a,c, Korinna Krohne a, Luca Carrella d, Eva Rentschler d, Teuta Gasi d,Vadim Ksenofontov d, Claudia Felser d, Michael Maskos a,b,c,n

a University of Mainz, Jakob-Welder-Weg 11, 55128 Mainz, Germanyb Institut fur Mikrotechnik Mainz GmbH (IMM), Carl-Zeiss-Str. 18-20, 55129 Mainz, Germanyc BAM—Bundesanstalt fur Materialforschung und -prufung, Unter den Eichen 87, 12205 Berlin, Germanyd University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany

a r t i c l e i n f o

Article history:

Received 8 April 2010

Received in revised form

18 June 2010Available online 14 July 2010

Keywords:

Nanoparticle

Magnetic

Encapsulation

Polyorganosiloxane

Magnetic separation

53/$ - see front matter & 2010 Elsevier B.V. A

016/j.jmmm.2010.06.056

esponding author at: University of Mainz, Ja

Germany. Tel.: +49 30 8104 1610, +49 6131 2

1 22970.

ail addresses: Michael.Maskos@bam.de, mask

skos).

a b s t a c t

Here, we present the synthesis, characterization and magnetic separation of magnetic polyorganosi-

loxane nanoparticles. Magnetic iron oxide nanoparticles with average particle radii of 3.2 nm had been

synthesized by a simple coprecipitation process of iron(II) and iron(III) salt in basic solution.

Afterwards, the particles were successfully incorporated into a polyorganosiloxane network via a

polycondensation reaction of trimethoxymethylsilane (T), diethoxydimethylsilane (D) and the

functional monomer (chloromethylphenyl)trimethoxysilane (ClBz-T) in aqueous dispersion. A core–

shell system was chosen to increase the flexibility of the system concerning size, composition and

functionalization possibilities. The magnetic nanocapsules with particle radii below 60 nm were

separated from non-magnetic material with a high effectiveness by the use of commercially available

separation columns which are commonly used for isolation of microbeads and subsequently

characterized via transmission electron microscopy (TEM), asymmetrical flow field-flow fractionation

(AF-FFF), superconducting quantum interference device (SQUID) and Mossbauer spectroscopy.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

For the last decades, the interest in iron oxide based magneticnanoparticles is steadily increasing due to the broad range ofpossible applications in biomedicine and in technical areas [1,2].To prevent aggregation and undesired biodegradation the parti-cles have to be stabilized by a coating material. Multipleapproaches can be found in literature using several coatingmaterials e.g. inorganic oxides like TiO2 or SiO2 as well as organicpolymers like polystyrene, polyethylene glycol or dextran [3].Independent from the materials used intensive care has to betaken concerning the following points: (i) the particles have to bestable in solution in order to prevent aggregation, (ii) theencapsulation must be complete and permanent so that themagnetic particles are harboured from environmental influencesthat could cause oxidation or degradation, (iii) the magneticcontent must be high enough for a magnetic response atroom temperature and (iv) in case of biomedical applications

ll rights reserved.

kob-Welder-Weg 11, 55128

4190; fax: +49 30 8112029,

os@uni-mainz.de

biocompatibility has to be warranted [4]. Regarding the size of theparticles, the magnetism of the inorganic material is a crucialfactor. Magnetic iron oxide nanoparticles show superparamag-netic behaviour, which means that the particles show highsaturation magnetizations (MS) in the presence of a magneticfield that disappears as soon as the magnetic field is switched off.This phenomenon is highly preferred for biomedical applicationsbecause an easy distribution and transport of the particles in thebiological system is guaranteed since the particles show nospecific interaction to each other during the absence of themagnetic field [5–7]. Another size-determining parameter is theenvisioned application. A compromise between the requiredstrength of the magnetic properties and the interaction with thetargeted biological system has to be found [8]. By this means,micron-sized particles can be used for magnetic separationwhereas imaging applications typically require materials below100 nm. The kind of material used for encapsulation depends onthe envisaged application; however, iron oxides are preferredfor the use in biomedicine due to their low cytotoxicity [9]. Inthis paper, we report about a novel magnetically functionalizedpolymeric material consisting of magnetic iron oxidenanoparticles incorporated into a polyorganosiloxane core–shellsystem. For the synthesis of the magnetic iron oxide nanoparticles(FexOy-I), we performed a simple coprecipitation process [13].

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–35263520

The polyorganosiloxane system (POS) we used as a protectivecoating of the iron oxide nanoparticles had been reported severaltimes by our group [10]. In comparison to other materialspresented in literature the polyorganosiloxane system has theadvantage of a highly flexible composition and architecture due tothe use of various monomers and the core–shell structure thatoffers the possibility of several independent functionalizations atthe core as well as on the particle surface. The polyorganosiloxanenanoparticles form network structures that can easily be accessedand, therefore, filled with appropriate molecules, for example bycovalent linking. In earlier publications, we also investigatedthe loading with different dye molecules via electrostaticinteractions as well as various inorganic materials like gold, silverand palladium [11]. We were also successful in biologicalmodification of the system and cell culture experiments showedpromising results concerning cell uptake, biocompatibility andcytotoxicity [12].

2. Experimental section

2.1. Materials

Water was purified with a milliQ deionizing system (Waters,Germany). All chemicals were used as received: diethoxydimethylsi-lane (D), trimethoxymethylsilane (T), ethoxytrimethylsilane (M),dodecyl benzenesulfonic acid (DBS) (Wacker Chemie), toluene,methanol (Acros), iron(II)-chloride tetrahydrate, iron(III)-chloridehexahydrate, sodium hydroxide, TWEEN 20 (Fluka) and (p-chloro-methylphenyl)trimethoxysilane (ClBz-T) (ABCR, Germany). Magneticseparation columns were received from Miltenyi Biotec.

2.2. Instrumentation

Transmission electron microscopy (TEM) was performed witha Philips CM-12 microscope at 120 kV. Samples were prepared byair-drying drops of aqueous solutions on carbon films supportedby copper grids.

For zeta potential measurements of the aqueous iron oxidedispersions a Malvern Zetasizer Nano ZS was used.

The asymmetrical flow field-flow fractionation (AF-FFF) fromConsenxus consisted of an AF-FFF channel system 2.0. Poly(ethersulfone) membranes were utilized as semipermeable walls(MWCO: 4 kDa). As eluent degassed milliQ water with NaCl(5 mmol/L) and TWEEN 20 (100 mg/L) was used. A Waters 486-UV detector operating at 254 nm monitored the eluting particles.Commercially available poly(styrene) nanoparticles were used forcalibration.

Dynamic light scattering (DLS) was performed at 20 1C using amulti-angle light scattering setup with an ALV/CGS-8F DLS/SLS5022F goniometer, an ALV-7004 Correlator, a 35 mW HeNe-laser(l0¼633 nm) and an APD avalanche photodiode optic detectionsystem. Measurements were performed between 301 and 1501 at8.51-intervals. To each sample a 5 millimolar solution of NaBr wasadded and was filtered with Millex-LG filters (pore size 0.2 mm,Millipore).

The superconducting quantum inference device (SQUID)measurements of the dried samples were performed in a gelatincapsule using a quantum design squid-magnetometer MPMS XLat 5 and at 300 K with magnetic field strengths between �80 000and 80 000 Oe.

A constant acceleration Mossbauer spectrometer with a 57Co-source in Rh was used for the measurements of the dried samples.The isomeric shift was measured relative to a-Fe at room

temperature and at 80 K. For data analysis the software ‘‘Recoil’’was used.

2.3. Synthesis

For all samples, the sample code FexOy-Iw%@POSz is usedwherein x and y referring to a mixture of different magnetic ironoxides, namely maghemite (g-Fe2O3) and magnetite (Fe3O4), w

corresponds to the amount of iron oxide added during synthesisand the z to the surface properties of the particles and the type ofsolution (z¼tol: toluene solution, z¼aq: aqueous solution). Thereaction scheme of the polyorganosiloxane (POS) nanoparticlessynthesis is shown in Fig. 1. The amount of monomers employedfor each step is given in Table 1.

2.4. Synthesis of magnetic iron oxide nanoparticles

Magnetic iron oxide nanoparticles (FexOy-I) were synthesizedby coprecipitation of Fe(III)- and Fe(II)-salts in NaOH solution asreported elsewhere [13]. Briefly, an aqueous solution ofFeCl3 �6H2O (0.1 M) and FeCl2 �4H2O (0.1 M) was added dropwiseto 50 mL of alkaline solution (1 M NaOH) under vigorous stirringfor 30 min at room temperature. The black precipitate waswashed three times with water and collected by centrifugation.After addition of HCl solution a stable aqueous dispersion wasobtained. After drying, 0.7–1.7 g of dark brown powder wasobtained.

2.5. Encapsulation of magnetic nanoparticles into

polyorganosiloxane system

The premixed monomers D, T and ClBz-T (Table 1) were addeddropwise to a solution of 125 g water, the surfactant/catalystdodecyl benzenesulfonic acid (DBS) (1.53 mmol) and the ironoxide nanoparticles through a syringe pump (15 mL/h) undercontinuous mechanical stirring at room temperature. The solutionwas stirred for 1 week before the monomer mixture for the shellwas added dropwise (10 mL/h). After additional stirring for threedays the dispersion was filtered before the end-capping wasperformed.

The bare polyorganosiloxane nanospheres were synthesized inan equivalent way and with the same amounts of monomerswithout addition of the iron oxide nanoparticles to the aqueousdispersion.

2.6. End capping process

To prevent particles from aggregation free SiOH groups wereend capped by addition of 2.5 g of ethoxytrimethylsilane (M) to12.5 g of the dispersion and stirred overnight. Afterwards, 1.25 gof M was added and stirring was continued for 5 h. The dispersionwas destabilized by precipitation in 150 mL methanol andcentrifuged. The precipitate was dissolved in 50 mL of tolueneand residual methanol and water were removed by rotaryevaporation at 40 1C in vacuum. 0.8 g of M was added to theremaining solution which was precipitated into 400 mL ofmethanol after stirring over night. The precipitate was collectedby centrifugation. This yielded 0.8–1.0 g of brown powder.

2.7. Magnetic separation

In a typical procedure, 1.0 g of nanospheres was dissolved intoluene and filtered through a commercially available magneticseparation column (Miltenyi Biotec: MACSs separation columns)under application of an external magnetic field (arrangement of

Fig. 1. Reaction scheme for the synthesis of magnetic core–shell nanospheres, T¼trimethoxymethylsilane, D¼diethoxydimethylsilane,

ClBz-T¼(chloromethylphenyl)trimethoxysilane.

Table 1Amount of monomer D, T and ClBz-T used in each step of the synthesis.

D/g T/g ClBz-T/g

Core 5.0 7.0 3.0

Shell 3.0 7.0

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–3526 3521

several neodymium ring magnets (NdFeB) with magnetizationsbetween 100 and 300 mT). Several washing steps were performedto rinse out the non-retained particles.

Fig. 2. TEM images of iron oxide nanoparticles deposited from aqueous dispersion

on a carbon coated copper grid, scale bar 100 nm.

Fig. 3. AF-FFF measurement of magnetic iron oxide nanoparticles in aqueous

dispersion.

3. Results and discussion

3.1. Magnetic iron oxide nanoparticles

In a previous publication, we reported the incorporation ofhydrophobically stabilized magnetic iron oxide nanoparticles synthe-sized via thermal decomposition of iron pentacarbonyl using oleicacid as capping agent into the polyorganosiloxane system [14]. Toprovide compatibility between the hydrophobic inorganic particlesand the siloxane system the magnetic particles had to be dispersed inoctadecyltrimethoxysilane (OD-T) before incorporation. Thereby, wewere able to incorporate the iron oxide nanoparticles in thepolyorganosiloxane core–shell system. Still the magnetic content ofthe polyorganosiloxane nanospheres was very low.

To obtain a higher magnetic content hydrophilic iron oxideparticles used for incorporation were synthesized via a simplecoprecipitation process. For this, a mixture of iron(II) chloride andiron(III) chloride was added to a basic solution. Immediately afterthe addition of the iron salt solution a black precipitate wasformed that was separated by centrifugation. By adjusting the pHto acidic values (pH¼2–3) a stable aqueous dispersion wasobtained, where the particles are stabilized by electrostaticinteractions between the charged particle surfaces. The particlespossess a positive zeta potential of 26.0 mV734%, but this valuecan only be treated as rough estimate, mainly due to the smallparticle size, which was additionally characterized with thetransmission electron microscopy (TEM) and the asymmetricalflow field-flow fractionation (AF-FFF). Fig. 2 shows a TEM image ofthe magnetic iron oxide nanoparticles after deposition from theaqueous dispersion.

The particles formed stable aqueous dispersions with apolydisperse size distribution that was determined from TEMimages to be 4.171.5 nm in radii i.e. a size variation of 35.4% (seesupporting information). The AF-FFF measurements of the aqu-eous dispersion shown in Fig. 3 revealed an averagehydrodynamic particle radius (Rh) of 3.2 nm, which correspondsreasonably well with the TEM data.

The magnetic properties were determined with the super-conducting quantum interference device (SQUID) measurementsof the dried sample. The magnetization curves we received byplotting the magnetization (M) against the magnetic field strength

(H) show no hysteresis loop meaning that neither remanence norcoercivity were found which is a characteristic behaviour ofsuperparamagnetic materials (Fig. 4). The determination of thesaturation magnetization (MS) resulted in 50.5 emu/g of samplefor measurements at 5 K and 39.7 emu/g of sample and at 300 K,respectively.

The saturation magnetizations of maghemite reported inliterature are 84.0 emu/g at 5 K and 78.0 emu/g at 300 K for the

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–35263522

bulk material [20]. The deviation between those values and theones we determined can be explained by the strong sizedependence of the saturation magnetization. To describe thiseffect Berkowitz et al. [15] developed the so-called core–shellmodel assuming that the magnetic particles are covered with athin diamagnetic oxide layer. The saturation magnetization of thenanoparticles relative to the bulk material can be calculatedaccording to the core-shell model using Eq. (1) (d: total particlediameter, h: shell thickness).

MSðdÞ

MSð1Þ¼ 1�6

h

dð1Þ

Based on the values reported in literature we considered anon-magnetic shell of approximately 0.5 nm for the calculation ofthe theoretical saturation magnetizations of maghemite nano-particles with a diameter of 6.4 nm which were determined to be

Fig. 4. SQUID measurements of iron oxide nanoparticles at 5 (solid) and 300 K

(dashed).

Table 2Saturation magnetizations of FexOy-I, g-Fe2O3 and Fe3O4.

T/K MS(FexOy-I)a/emu/g MS(g-Fe2O3)/emu/g MS(Fe3O4)/emu/g

Bulkb dc¼6.4 nm nm Bulkb dc

¼6.4 nm

5 50.5 84.0 45.8 98.0 53.4

300 39.7 78.0 42.5 92.0 50.2

a Determined by SQUID measurementsb Values reported in literature [20]c Calculated by using Eq. (1)

Fig. 5. Mossbauer spectroscopy measurements of FexOy-I at (a) room temperature (b) 80 K.

Fig. 7. AF-FFF measurements of bare and magnetically functionalized polyorga-

nosiloxane nanospheres in aqueous dispersion.

Fig. 6. TEM images of magnetically functionalized iron oxide nanoparticles

FexOy-I2.3%@POSaq deposited from aqueous dispersion, scale bar 100 nm.

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–3526 3523

45.8 emu/g at 5 K and 42.5 emu/g at 300 K and are therefore ingood agreement with the values we measured (Table 2) indicatingthat g-Fe2O3 is the dominate species present in the sample.

The superparamagnetic behaviour of the iron oxide nanopar-ticles was confirmed by Mossbauer spectroscopy at roomtemperature (Fig. 5). The isomeric shift was determined to be0.339 mm/s relative to a-Fe which fits well with the data ofmaghemite (g-Fe2O3) reported in literature [20]. Themeasurements at 80 K showed a central doublet component dueto the presence of superparamagnetic iron oxide nanoparticlesand several sextets with broad hyperfine field distributions whichis attributed to a wide range of particle sizes, also indicating thatmaghemite is the dominate species present in the sample (seeAppendix A).

3.2. Magnetically functionalized polyorganosiloxane nanoparticles

The iron oxide particles synthesized by the coprecipitationprocess were added to the aqueous solution used for thepolymerization reaction of the silane monomers. The monomerswere added dropwise to the solution to incorporate the iron oxideparticles into the polyorganosiloxane networks during the poly-condensation reaction. Afterwards, a second mixture of silanemonomers was added to build up core–shell structures.

Three different samples of magnetically functionalized poly-organosiloxane nanoparticles had been prepared wherein theamount of iron oxide added was varied from 1.2 up to 2.3 w%(mass (iron oxide)/mass (polymer+iron oxide), assuming 100%monomer conversion and 100% incorporation of iron oxide). Afurther increase of the amount of iron oxide did not result in anencapsulation due to instabilities of the dispersion. The TEMimages (Fig. 6) revealed that incorporation of the inorganic

Table 3Particle radii and polydispersities determined via DLS, AF-FFF and TEM measurements

Sample DLS AF-FFF

oRh4z/nm m2 oR14/nm oR24/n

POS 10.7 0.18 16.6 17.9

FexOy-I1.2%@POSaq 38.1 0.16 36.4 44.0

FexOy-I1.5%@POSaq 24.6 0.15 23.4 26.6

FexOy-I2.3%@POSaq 30.7 0.12 33.7 37.2

a Determined by AF-FFF for the peak maximum in aqueous dispersionb Determines from TEM images after deposition from aqueous dispersion

Fig. 8. SQUID measurements of magnetically functionalized polyorganosiloxane na

(b) per gram of iron oxide added.

particles was achieved; however, besides magneticallyfunctionalized polyorganosiloxane nanospheres multiple non-magnetic particles were present.

In Fig. 7, the aqueous AF-FFF data of the three magneticallyfunctionalized samples and bare polyorganosiloxane (POS)particles are compared. The bare polyorganosiloxane particlesshow a nearly Gaussian distribution whereas the magneticsamples show a polydisperse size distribution with a distincttailing of the peaks. The radii of the main fraction weredetermined to be 15.3 for POSaq, 23.1 for FexOy-I1.2%@POSaq, 16.4for FexOy-I1.5%@POSaq and 27.9 nm for FexOy-I2.3%@POSaq. Theaverage particle sizes of the magnetically functionalizedpolyorganosiloxane particles seem to be statistically distributedbut generally larger than those of the non-magnetic POS (seeTable 3).

A calculation of the first and second momentum of thedistribution as well as the PDI was performed using Eq. (2),neglecting the possible influence of band broadening andtherefore overestimating the size polydispersity [16]. The resultsare summarized in Table 3.

/R1S¼

RRnUVdRnR

UVdRn,/R2S¼

RR2

nUVdRnRRnUVdRn

, PDI¼/R2S/R1S

ð2Þ

Concerning the polydispersity a decrease with increasing ironoxide content is observed (see Table 3). Additionally, dynamic lightscattering measurements of the aqueous dispersions were used todetermine the particle radii confirming the general trend revealed viaAF-FFF measurements. However, when comparing the results of thetwo methods some deviations are obvious due to the differentaveraging of the data. The average particle radii and polydispersitiesdetermined by the DLS are also given in Table 3. In addition, the m2

values obtained at 901 scattering angle are also included and indicatethe polydispersity of the sample, which also decreases with increasing

.

TEM

m PDI Rh,maxa/nm Rnon-magnetic

b/nm Rnanocapsulesb/nm

1.1 15.3 8.6 -

1.2 23.1 14.3 15.0

1.1 16.4 12.9 20.1

1.1 27.9 17.8 26.0

nospheres at room temperature with magnetizations (a) per gram of sample

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–35263524

iron oxide content. To further characterize the magnetic capsules wemeasured the particle radii from the TEM images and obtained anincrease of the radii proportional to the amount of iron oxide added;in detail, the average particle radii of the dried magnetic capsuleswere determined to be 15.072.2 (FexOy-I1.2%@POSaq), 20.176.4(FexOy-I1.5%@POSaq) and 26.075.6 nm (FexOy-I2.3%@POSaq) for themagnetic capsules.

The SQUID measurements that are shown in Fig. 8 confirmedthe perpetuation of the superparamagnetic properties at roomtemperature. The determination of the saturation magnetizationsresulted in 0.44 emu/g of sample for FexOy-I1.2%@POStol, 0.55 emu/g of sample for FexOy-I1.5%@POStol and 0.85 emu/g of sample forFexOy-I2.3%@POStol. The increase of the saturation magnetizationwith the increasing amount of iron oxide added indicates that ahigher offer leads to a higher encapsulation of the magneticparticles. A comparison of the relative values for the saturationmagnetization and the amount of iron oxide added resulted ingood agreements, specifically: MS(FexOy-I1.2%@POStol):MS(FexOy-I1.5%@POStol)¼0.80, MS(FexOy-I1.5%@POStol):MS(FexOy-I2.3%@POStol)¼0.65 which fits well with the relative amounts of iron oxideadded (0.80 and 0.65, respectively) leading to the conclusion thatthe yield of magnetic capsules is proportional to the amount ofadded iron oxide. The loss in magnetic properties compared tothe pure iron oxides was also reported by other groups andderives from the spatial separation of the particles within thepolyorganosiloxane network [4,17]. Additionally, the non-magnetically functionalized polyorganosiloxane nanoparticlesare still present in the sample and therefore the values of MS

per gram of sample must be relatively low. Nevertheless, Fig. 8reveals that all samples show the same saturation magnetizationof approximately 37 emu/g when the values were normalized tothe theoretical amount of iron oxide assuming 100% conversion.

The complete data of the analysis are summarized in Table 4.

Table 4Saturation magnetization values evaluated by SQUID measurements.

Sample MSa/(emu/g sample) MS

a/(emu/g iron oxide)

POS - -

FexOy-I1.2%@POStol 0.44 37.1

FexOy-I1.5%@POStol 0.55 36.6

FexOy-I2.3%@POStol 0.85 37.8

a Determined by SQUID of the dried samples measurements at room

temperature

Fig. 9. TEM image of FexOy-I2.3%@POStol (a) before and (b) after magnetic

3.3. Magnetic separation

Commercially available magnetic separation columns fromMiltenyi Biotec (MACSs separation columns) in combination withNdFeB ring magnets were used for the magnetic separation. Theparticles were dissolved in toluene and filtered through themagnetic separation columns. A TEM image of the particles intoluene before (FexOy-I2.3%@POS)tol and after separation procedure(FexOy-I2.3%@POStol-sep) is shown in Fig. 9. Strikingly, all particlesseparated contain more than one magnetic iron oxidenanoparticle. Calculating the particle sizes that would benecessary for a magnetic separation using a cylindrical magnet(diameter 2, length 3 mm, remanescent magnetic inductionBr¼1.2 T) we found a minimum magnetic core radius ofR¼19.3 nm and a magnetic volume of V¼30 098 nm3 for acapsule with a diamagnetic shell of 14.5 nm thickness [14].Based on the TEM image shown in Fig. 9(b) we determinedthe average particle number present in the fractionatedpolyorganosiloxane particles to approximately 7. Consideringthis number and an average particle radius of 3.2 nm for theiron oxide nanoparticles this would result in a magnetic volumeof 961 nm3 which is obviously much lower than the calculatedvalue. This confirms the expected divergent field strength and theresulting inhomogenities inside the columns that resulted fromthe interaction between the iron spheres the columns are filledwith and the magnetic field of the external magnets.

The magnetic properties of the separated and the non-separated sample as determined by the SQUID are compared inFig. 10. Of course, the saturation magnetization of the separatedsample is increased (1.7 emu/g sample) compared to the non-separated material (0.85 emu/g sample) due to the absence of thebare polyorganosiloxane nanoparticles. Recalculatingthe magnetic content from the saturation magnetizations of thenon-separated sample resulted in an approximate value of 4.6 w%iron oxide. An absolute determination of the iron content from themagnetic measurements is difficult since the spatial separation ofthe encapsulated magnetic particles is generally decreasing themagnetic strength and causes thereby a lower saturationmagnetization compared to the same amount of pure iron oxideparticles.

57Fe-Mossbauer spectroscopy was performed for the separatedand the non-separated sample (Fig. 11). For both samples a singlepeak with an isomeric shift of 0.288 mm/s relative to a-Fe wasobserved by measurements at room temperature whichcorresponds to Fe(III) meaning that maghemite is the dominate

separation after deposition from toluene solution, scale bar 100 nm.

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–3526 3525

species present in the sample which is confirming the resultsobtained by the SQUID measurements [18]. The appearance of asinglet confirmed the superparamagnetic behaviour.

The absorption maxima were determined to be 0.45% forFexOy-I2.3%@POStol and 0.86% for FexOy-I2.3%@POStol-sep indicatingthat the iron oxide content in the separated sample is twice ashigh compared to the non-separated one. This result coincideswell with the values calculated from the saturation magnetiza-tions determined by the SQUID measurements shown in Fig. 10.

By measuring Mossbauer spectroscopy of FexOy-I2.3%@POStol-sepat 80 K it was possible to resolve the quadrupole splitting from thehyperfine splitting. The spectrum resembled the results of the pureiron oxide sample illustrated in Fig. 5 showing a composition ofsimilar sextet components with various hyperfine field distributionsrepresenting the polydispersity of the magnetic nanoparticles (seesupporting information). Fitting the corresponding data resulted inthe typical sextet structure (Fig. 12). Additionally, the decrease of thesecond and fifth peak indicates the hindered interaction betweenthe magnetic particles which is therefore a proof of successfulencapsulation [19].

4. Conclusion

The successful encapsulation of magnetic iron oxide nanopar-ticles into a polyorganosiloxane core–shell system as well as theirmagnetic separation was reported. First, magnetic iron oxidenanoparticles with an average particle radius of 3.2 nmwere synthesized by coprecipitation in basic solution. The

Fig. 10. SQUID measurements of magnetic polyorganosiloxane nanoparticles

before and after magnetic separation.

Fig. 11. Mossbauer measurements of the sample FexOy-I2.3%@POS

nanoparticles showed superparamagnetic behaviour at 5 K aswell as at room temperature with saturation magnetizations of50.5 and 39.7 emu/g. Afterwards, the iron oxide particles wereencapsulated into a polyorganosiloxane core–shell system viapolycondensation in aqueous dispersion. The AF-FFF measure-ments revealed a polydisperse size distribution wherein theaverage particle radii of the nanocapsules varied from 15.0 to26.0 nm and increased with increasing amount of iron oxideadded during synthesis. In all cases, SQUID measurementsshowed that the superparamagnetic properties of the iron oxidewere retained with saturation magnetizations of 37.0 emu/g ironoxide. The magnetic encapsulates could easily be separated by theuse of commercially available magnetic separation columns.The separated sample showed a saturation magnetization of1.7 emu/g of sample which is twice as high as the value of thenon-separated sample (FexOy-I2.3%@POStol) that was determinedto be 0.85 emu/g sample. Mossbauer spectroscopy reveals thatmaghemite is the dominant species in the sample. The possibilityof further functionalization like hydrophilic surface modificationand fluorescence labelling of the polyorganosiloxane core makesthe obtained particles promising candidates for biomedicalapplications [12]. In future investigations the magnetic core couldbe used for delivery, separation, heating (hyperthermia) orimaging (MRT) methods by external magnetic fields while theincorporation of dyes could be imagined for cell uptake control.

tol (a) before and (b) after separation at room temperature.

Fig. 12. Mossbauer measurements of the sample FexOy-I2.3%@POStol -sep at 80 K.

S. Utech et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3519–35263526

Finally, biologically active surface functionalities would providebiocompatibility and binding sites for specific targeting of cellsand biomolecules for drug delivery systems.

Acknowledgements

We thank Prof. M. Schmidt, University of Mainz, for thelaboratory environment and Christoph Bantz, University of Mainz,for the zeta potential measurements. For financial support wegrateful acknowledge Polymat Mainz and the DFG SPP1313.

Appendix A. Supplementary materials

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.jmmm.2010.06.056.

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