Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer

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RESEARCHARTICLE

Copyright copy 2009 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 9 5355ndash5361 2009

Magnetic Nanoparticles Encapsulated Within aThermoresponsive Polymer

A K Gaharwar12 J E Wong1lowast D Muumlller-Schulte3 D Bahadur2 and W Richtering1

1Institute of Physical Chemistry RWTH Aachen University Landoltweg 2 52056 Aachen Germany2Department of Metallurgical Engineering and Materials Science Indian Institute of TechnologyndashBombay Mumbai 400076 India

3Lionex GmbH Martelenberger Weg 8 52066 Aachen Germany

This study describes a facile two-step approach to modify the surface of nanoparticles therebyimparting a corendashshell structure to the system The core consists of magnetic nanoparticles and theshell is composed of thermoresponsive hydroxypropyl cellulose using a coupling agent to covalentlybind the core to the shell Hydroxypropyl cellulose is known for its biocompatibility and biodegradabil-ity and its thermoresponsive properties make it an excellent candidate for fabricating biocompatiblestimuli-responsive magnetic nanoparticles We report the synthesis of magnetic nanoparticles andthe successful binding of the polymer to them X-ray diffraction studies show that the surface mod-ification of the magnetic nanoparticles does not result in any phase change and the size of themagnetic core thus calculated (7 nm) reveals that such hybrid corendashshell system is superparamag-netic in nature as further confirmed by magnetization measurements The size obtained by X-raydiffraction is in good agreement with that obtained by transmission electron microscope Evidenceof binding is given by Fourier transform infrared spectroscopy and a quantitative analysis of thepolymeric content obtained by thermogravimetry analysis Dynamic light scattering as a function oftemperature reveals the thermoresponsive behavior of the particles with a lower critical solution tem-perature around 41 C which is also the temperature at which cellulose undergoes a coil-to-globuletransition

Keywords Magnetic Nanoparticles Superparamagnetic Hybrid CorendashShellThermoresponsive Hydroxypropyl Cellulose Biocompatible

1 INTRODUCTION

Magnetic nanoparticles (MNP) have unique size-dependentproperties and MNP based on iron oxides are attractivecandidates in the field of biomedical applications1ndash5 Forin vivo applications MNP should not form any agglom-erates hence to prevent this the idea is to modify theirsurfaces by either coating or encapsulating them in organicor inorganic materials One common approach is to embednanoparticles in silica6 because the latter can be read-ily functionalized to impart the protective shell Suchinorganicorganic nanocomposites78 have attracted muchattention lately in an attempt to exploit new hybrid proper-ties derived from the various components

Stimuli-responsive corendashshell systems can be achievedby choosing one of the components to be sensitiveto an external stimulus such as pH or temperature9

One particular class of stimuli-responsive materials that

lowastAuthor to whom correspondence should be addressed

has generated much attention is temperature-sensitivepolymers Thermoresponsive polymers possess a release-trigger mechanism when they undergo fast reversiblestructural changes from a swollen to a collapsed state byexpulsing the solvent and have recently been exploited asremote controlled drug delivery vehicles10 We previouslyreported surface modification via layer-by-layer techniqueof a thermoresponsive poly(N -isopropylacrylamide) PNI-PAM microgel core with polyelectrolyte multilayers andmagnetic nanoparticles while preserving the reversiblethermoresponsive behavior of the hybrid corendashshell11ndash15

Such corendashshell systems exhibit a lower critical solutiontemperature (LCST) of sim32 C when they undergo a tran-sition from a swollen hydrophilic to a collapsed hydropho-bic state However although PNIPAM is non-hazardousand soluble in water biocompatible polymers are usuallypreferred for any potential biomedical applications

Herein we report the synthesis of a biodegradablehybrid corendashshell system whereby the shell is thermore-sponsive and the core is the magnetic nanoparticle For this

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RESEARCHARTICLE

Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer Gaharwar et al

work the choice of the thermoresponsive shell is dictatedby the bio-compatibility aspect of the polymer with anLCST nearing (but slightly higher than) our body temper-ature Hydroxylpropyl cellulose16 (HPC) is a water-solublenatural cellulose with an LCST of sim41 C and with anexcellent bio-compatibility as well as bio-degradability asapproved by the United States Food and Drug Adminis-tration (FDA)17ndash19 The surface of the MNP is first mod-ified through a silanization reaction20 followed by HPCcoupling These new hybrid corendashshell sytems are char-acterized by X-ray diffraction (XRD) thermogravimet-ric analysis (TGA) Fourier Transform Infrared (FTIR)dynamic light scattering (DLS) and magnetic measure-ments to show the successful surface modification of MNP

2 EXPERIMENTAL DETAILS

21 Materials

Ferrous chloride tetrahydrate (FeCl2 middot4H2O) ferric chloridehexahydrate (FeCl3 middot 6H2O) 25 ammonium hydroxidesolution 37 hydrochloric acid (HCl) solution hydrox-ypropyl cellulose (HPC average Mwsim100 000 gmol)(3-aminopropyl)trimethoxysilane (APTMS) and sodiumperiodate (NaIO4) were purchased from Sigma-AldrichAll chemicals from commercial origin were used withoutpurification The water used in all experiments was double-distilled ultrapure water (Milli-Q-plus system Millipore)

22 Synthesis of MNP

The synthesis of MNP was carried out via a controlledchemical co-precipitation approach21 as described in detailin a recent paper1415 Briefly 081 g FeCl3 middot6H2O in 5 mlwater and 04963 g FeCl2 middot 4H2O in a mixture of 1 mlwater and 025 ml 37 HCl solution was prepared sepa-rately Both solutions were mixed in an ultrasonic bath for5 min to homogenize the mixture The mixture of Fe3+

and Fe2+ solutions was then added drop wise to 40 ml ofwater and 10 ml of 25 NH3 solution in a three-neckedround bottom flask under an inert N2 atmosphere and con-stant stirring for 30 min Aggregates were first separatedfrom the reaction mixture using a NdndashFendashB magnet andthen washed three times with 03 M NH3 Three cyclesof centrifugation at 5000 rpm for 10 min were carried outand the precipitate was dissolved in 20 ml of water toobtain a stable ferrofluid Exposure of this ferrofluid toa magnetic field revealed no phase separation confirmingthe complete redispersion of the MNP

23 Surface Modification of MNP

A solution of 10 l APTMS in 20 mL water was addeddrop wise under N2 atmosphere to a 2 mL 5 MNP solu-tion in 20 mL of water in an ultrasonic bath and mixingcarried out with a mechanical stirrer at 500 rpm for an

additional hour Ultrasonication is vital in reducing the par-ticle size and size distribution by promoting the mixing ofMNP and the APTMS22 Initial attempt of this step with-out sonification lead to aggregations The reaction mixturewas purified by two centrifugation cycles (20 and 15 min)with removal of the supernatant and redispersion in waterbetween each run The surface modified MNP hereafterdenoted as MNPAPTMS was finally collected and redis-persed in 20 ml water overnight

24 Surface Attachment of HPC to MNPAPTMS

A saturated NaIO4 solution was prepared by dissolving 2 g(an excess) of NaIO4 in 10 ml water using ultrasonicationkept in the dark for 1 hr and finally filtered to separate theexcess NaIO4 To partially oxidize HPC polymer chainsa 10 wt solution of HPC was added to the saturatedNaIO4 solution and mixed for 18 hr in the dark at roomtemperature To attach HPC onto APTMS modified MNPsurface 10 ml of the MNPAPTMS solution was added topartially oxidized HPC solution in an ultrasonic bath Likein the previous step ultrasonication proved to be impor-tant in reducing the particle size and size distribution bypromoting the mixing of MNPAPTMS and the HPC22

The solution was mixed for 12 hr Impurities and unre-acted polymers were removed from reaction mixture bythree centrifugation cycles at 40 000 rpm (25 15 10 min)The surface-modified MNPAPTMS hereafter denoted asMNPAPTMSHPC was finally collected and redispersedin 20 ml water

25 Characterization

Photon correlation spectroscopy (PCS) was used to deter-mine hydrodynamic size and size distribution of the par-ticles using a Zetasizer 3000HS (Malvern UK) at 25 Cwith a dilute dispersion of the particles in pure water Theparticle size of the themoresponsive MNP was measured asa function of the temperature by dynamic light scattering(DLS) on highly diluted samples with an ALV goniometer(ALV 5000E correlator) Temperature was varied from 20to 60 C in steps of 2 C Scattered light was detectedat 60 and particles size was calculated by cumulant fitsFourier transform infrared spectroscopy (FTIR) was car-ried out with a Nicolet spectrometer (Magna IR 550)within the wave range of 4000ndash400 cmminus1 A crystallo-graphic study of the particles was performed on a rotatinganode STOE STADI-P X-ray diffractometer system usingCoK radiation (g = 0179026 nm) X-ray diffraction(XRD) graphics were compared to the JCPDS standarddata in order to deduce the crystal structure of the prod-uct The size and morphology of particles were determinedby a Transmission Electron Microscope (TEM) (PhilipsCM 200) at 200 kV Magnetic properties were measuredby Vibrating Sample Magnetometer (VSM) (Lake Shore

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RESEARCHARTICLE

Gaharwar et al Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer

Model 7410) Thermogravimetric analysis (TGA) was car-ried out on dried samples from 20 to 600 C with aheating rate of 10 C minminus1 using a Setaram SETSYSthermogravimetric analyzer in a nitrogen atmosphere

3 RESULTS AND DISCUSSION

The surface modification of the MNP core with silane cou-pling agent (APTMS)2023 and a thermoresponsive poly-meric shell (HPC) is illustrated in Scheme 1 First MNPare prepared via co-precipitation technique followed bysurface modification by APTMS to introduce the desiredfunctional groups in this case amino groups onto thesurface2023 Partially oxidized HPC was then covalentlylinked to the amino groups present on the APTMS-surfacemodified MNP to get corendashshell particles with a magneticcore and a polymeric shell

31 Surface Modification of MNP An XRDEvaluation

Figure 1 shows the XRD patterns of MNP and MNPAPTMSHPC The XRD patterns of MNP and MNPAPTMSHPC show five characteristic peaks of magnetiteor maghemite marked by their indices ((220) (311) (400)(422) and (511))24 The peaks from MNPAPTMSHPCare slightly broader than those obtained for MNP due tothe amorphous polymeric (HPC) or amine (APTMS) coat-ing around the nanoparticles The volume average relativeparticle size of the crystal D was calculated using theScherrer formula giving a Dsim 7 nm for the magnetic coreindicating its superparamagnetic behavior No additionalpeak after surface modification with APTMS suggeststhat the coating does not result in any phase change

32 TEM

The TEM micrographs for the MNP MNPAPTMS andMNPAPTMSHPC are shown in Figure 2 together with an

HO H

O

HO

H O HO

H OHH O

HO

HOH

O

HO

HO

NH 2

NH2

NH2

NH2

NH

2H 2N

H 2N

H2 N

H2 N

NH

2

NH

2

H2N

HO

HO H

O

HO

HO

H OH O

HO

HOH

O

HO

HO

APTMS HPC

Scheme 1 Simplistic representation of surface modification of MNP first with APTMS and then with HPC

20 30 40 50 60 70 80 90

2 Theta (Co Kα)

MNPAPTMSHPC

MNP

511

422

400

220

311

Inte

nsity

au

Fig 1 XRD pattern of MNP and MNPAPTMSHPC Vertical dottedline is just a guide to the eye

electron diffraction pattern of the MNP Before modifica-tion MNP showed a relatively narrow size distribution witha mean diameter of sim7 nm After binding to APTMS andHPC respectively the magnetite cores were still monodis-perse with a similar size range Polymeric components(APTMS and HPC) possess a too low electron density tocontribute to the contrast in the image compared to MNPwhich have a high electron density The size obtained isan indication that the surface modification did not resultin agglomeration or a dramatic change in size of the MNPparticles This could be attributed to the fact that the reac-tion occurred only on the surface of the particles Theelectron diffraction pattern of MNP (Fig 2) consists ofconcentric rings consistent with a cubic inverse spinelstructure of magnetite Similar electron diffraction patternswere obtained for MNPAPTMS and MNPAPTMSHPCThe characteristic d spacing corresponds to the hkl valuesof (220) (311) (400) (440) and (533) which are in excel-lent agreement with the results obtained by XRD24

33 FT-IR

Further evidence of the successful surface modification ofMNP is provided by FTIR spectra as shown in Figure 3for MNP MNPAPTMS and MNPAPTMSHPC For as

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RESEARCHARTICLE

Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer Gaharwar et al

100 nm

MNP

MNPAPTMS

100 nm

MNPAPTMSHPC

100 nm

MNP

Fig 2 TEM images of MNP MNPAPTMS and MNPAPTMSHPCElectron diffraction pattern of MNP is also shown

prepared MNP a small absorption band between 3000 and3600 cmminus1 with peak at around 3427 cmminus1 and a bandat 1624 cmminus1 are associated with the fundamental valencestretching vibrations of the OndashH the band at 1380 cmminus1

is due to bending modes of OndashCndashH CndashCndashH and CndashOndashHof carbonate present on the surface of the MNP (as reac-tion was done in ambient conditions) Bands at 577 and637 cmminus1 are due to stretching vibration of FendashO at tetra-hedral site1 The attachment of APTMS onto the MNP wasmonitored by the following bands in the spectra(a) the broadening of the absorption band between 3000and 3600 cmminus1 with peak at around 3437 cmminus1 is dueto stretching vibrations of the NndashH group together withcontribution from OndashH bond

500 1000 1500 2000 2500 3000 3500 4000

3427

2923

2850

1380

1624

1120

1023

637

577

Tra

nsm

ittan

cea

u

Wavenumbercmndash1

MNPMNPAPTMSMNPAPTMSHPC

Fig 3 FTIR spectra of MNP MNPAPTMS and MNPAPTMSHPC

(b) absorption bands at 2854 and 2926 cmminus1 are due tostretching vibrations of the alkyl CndashH bond(c) transformation of band present at 1380 cmminus1 to a verynarrow and sharp band is due to the presence of strongamine group in APTMS and(d) bands at 1023 cmminus1 and at 1122 cmminus1 are assignedto SindashOndashSi bonds The decrease in intensity of absorptionband of FendashO (stretching vibration) at 638 and 585 cmminus1

indicates that APTMS is bound to the surface of the MNPMNPAPTMSHPC particles have almost similar bands asMNPAPTMS the major difference between the two are(a) a decrease in the intensity of the band at 1390 cmminus1

indicating that NndashH bonds have been used during the con-jugation of APTMS with HPC and (b) the appearance ofnew bands at 1754 and 1464 cmminus1 characteristics of thepresence of aldehyde groups in partially oxidized HPCThe band at 1754 cmminus1 is due to the aromatic CndashH out-of-plane bending vibrations while that at 1464 cmminus1 is dueto CndashH bending

34 TGA

TGA was carried out to confirm the shell formationand also to quantify the amount of APTMS and HPCadsorbed on the surface of the MNP Figure 4 shows TGAcurves of MNP MNPAPTMS and MNPAPTMSHPCAs expected the TGA curve for MNP shows almost nosignificant weight loss The slight weight loss is due to lossof residual water TGA results for MNPAPTMS revealeda two-step weight loss The initial weight loss is due tothe loss of residual water in the sample while the signif-icant weight loss (of about 7) between 250 and 500 Ccan be attributed to the degradation of APTMS There wasno significant weight change between 500 and 600 Cimplying the presence of only iron oxide within the tem-perature range The TGA curve for MNPAPTMSHPCshows a three-step weight loss The first step corresponds

100 200 300 400 500 60075

80

85

90

95

100

Wei

ght l

oss

Temperature TdegC

MNPMNPAPTMSMNPAPTMSHPC

Fig 4 Comparison between TGA curves of MNP MNPAPTMS andMNPAPTMSHPC

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Gaharwar et al Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer

ndash20000 ndash10000 0 10000 20000

Me

mu

g

HOe

MNPMNPAPTMSMNPAPTMSHPC

ndash60

ndash40

ndash20

0

20

40

60

Fig 5 Magnetization curves obtained at room temperature for MNPMNPAPTMS and MNPAPTMSHPC

MNP MNPAPTMS MNPAPTMSHPC

in

cla

ss

5 10 50 100 500 10005 10 50 100 500 1000 5 10 50 100 500 1000

Diameternm

(a)

20 30 40 50 60

100

150

200Agglomeration

Stable

LCS

T=

41

ordmC

Temperature T degC

Hyd

rody

nmiddotr

adiu

s R

hn

m

(b)

Fig 6 (a) Size distribution of MNP MNPAPTMS and MNPAPTMSHPC (b) DLS study of the thermoresponsive behavior of MNPAPTMSHPCcorendashshell particles Inset shows visual evidence (with the help of a NdndashFendashB magnet) of agglomerated particles when temperature is increased abovethe LCST of HPC

to the loss of residual water in the sample The pronouncedweight loss (of about 20) between 200 and 550 C is anoverlap of two degradation curves (one due to APTMS andanother due to HPC) The results confirm that APTMS andHPC were bound to the MNP and the more pronouncedweight loss observed for MNPAPTMSHPC shows thatmore non-magnetic polymeric materials are bound to itssurface than in the case of MNPAPTMS

35 Magnetic Measurements

Figure 5 shows the magnetization curves of MNP MNPAPTMS MNPAPTMSHPC nanoparticles No coerciv-ity and remanence are observed before and after surfacemodification indicating that the MNP retain their super-paramagnetic nature1415 The saturation magnetization ofMNP MNPAPTMS and MNPAPTMSHPC measured ina field of 2 T are 52 48 and 40 emug respectively The

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Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer Gaharwar et al

presence of non-magnetic materials on the surface of MNP(APTMS and HPC) leads to a decrease in the saturationmagnetization Successive surface modification leads to asim7 and sim20 decrease which is in very good agreementwith the amount of non-magnetic materials obtained inde-pendently by TGA The additional decrease in saturationmagnetization of MNPAPTMSHPC is a clear indicationof additional non-magnetic materials on the surface of theMNP (APTMS+HPC)23

36 DLS

Figure 6(a) shows the hydrodynamic size and size dis-tribution of particles as evaluated using PCS after eachmodification step The average hydrodynamic size of theparticles (scattering angle = 90) of MNP MNPAPTMSand MNPAPTMSHPC was 29 40 and 102 nm respec-tively Hydrodynamic size is always an overestimation ofthe real core size but comparing the values obtained byPCS with that obtained from TEM this difference is quitepronounced which could be an indication that in solutionwe have formation of some clustering of the nanopar-ticles However from the size distribution although theAPTMS modification step introduces some polydispersity(while the HPC modification step produces fairly monodis-perse particles) all the samples (MNP MNPAPTMS andMNPAPTMSHPC) are reasonably monodisperse

Figure 6(b) shows that the hydrodynamic size of theMNPAPTMSHPC as a function of temperature usingDLS (scattering angle = 60) The DLS curve also givesan indication of the stability of the particles in solution atdifferent temperatures At 20 C MNPAPTMSHPC arestable and do not show any agglomeration The stability isdue to the steric repulsion between neighboring particlesowing to the presence of the HPC network around the par-ticles The stability is further confirmed when no sedimen-tation is observed even after 6 hours exposure to a magnetas illustrated in the inset of Figure 6(b) As the tempera-ture is increased to around 40 C (simLCST of HPC) thesize of the corendashshell particles increases At temperatureshigher than the LCST there is a dramatic increase in sizecontrary to the usual thermoresponsive behavior of HPCgel1925 Such anomalous behavior was also reported byDou et al who observed an increase in the size but only attemperatures around and higher than the LCST (41 C)26

At temperatures greater than the LCST the hydrophilic-ity to hydrophobicity transition of HPC chains in watermay induce self-association leading to the formation ofmetastable nanoparticle aggregates192526 Gao et al alsoreported that HPC chains may form different metastableaggregates depending on how the system is brought to thattemperature19 A rapid increase of temperature from roomtemperature to the LCST resulted in fast self-associationof the HPC chains (and smaller nanoparticles) while incu-bating at intermediate temperatures gave HPC chains more

time to associate into larger particles The same author alsofound that increasing the temperature above 45 C wouldlead to even larger and denser particles19 The DLS curvein Figure 6(b) was obtained by increasing the temperaturefrom 20 C to 60 C in steps of 2 C with time intervalssufficiently long for the temperature to equilibrate Indeedstable and larger particles are observed Only on exposureto a magnet for about 6 hours do the particles at 60 C sed-iment Figure 6(b) inset revealing that larger and denseraggregates are formed Once larger particles were formedthe thermoresponsive behavior is no longer reversible

4 CONCLUSION

A facile two-step surface modification of magneticnanoparticles has been successfully carried out for the fab-rication of monodisperse corendashshell hybrid particles with abio-compatible and thermoresponsive hydroxylpropyl cel-lulose shell The modification steps involve silanization ofmagnetic nanoparticles followed by covalent binding ofthe cellulose through amino groups The physico-chemicalcharacterizations of the corendashshell system have been thor-oughly carried out and reveal that such hybrid systemsretain the individual properties of each component theyare made up of namely superparamagnetic behavior ofthe native magnetic nanoparticles and the thermorespon-sive behavior of the cellulose Such unique combinationof thermoresponsivity and magnetism could open up novelprospects in the field of nanomedical applications such asremote controlled drug carriers

Acknowledgment A K Gaharwar would like to thankthe Deutscher Akademischer Austauschdienst (DAAD)for a scholarship Support by the Fonds der Chemis-chen Industrie (Germany) and the Department of Scienceand Technology (Government of India) are gratefullyacknowledged

References and Notes

1 U Haumlfeli W Schuumltt J Teller and M Zborowski Scientific andClinical Applications of Magnetic Plenum New York (1997)

2 P Tartaj M P Morales S Veintemillas-Verdaguer T Gonzaacutelez-Carrentildeo and C J Serna J Phys D Appl Phys 36 R182 (2003)

3 Q A Pankhurst J Connolly S K Jones and J Dobson J PhysD Appl Phys 36 R167 (2003)

4 T Neuberger B Schopf H Hofman M Hofman and B vonRecherberg J Magn Magn Mater 293 483 (2005)

5 A K Gupta and M Gupta Biomaterials 26 395 (2005)6 I Pastoriza-Santos D Gomez J Perez-Juste L M Liz-Marzan

and P Mulvaney Phys Chem Chem Phys 6 5056 (2004)7 L M Liz-Marzan M Giersig and P Mulvaney Langmuir 12 4329

(1996)8 E Bourgeat-Lami J Nanosci Nanotechnol 2 1 (2002)9 A M Schmidt Colloid Polym Sci 285 953 (2007)10 D Muumlller-Schulte and T Schmitz-Rode J Magn Magn Mater 302

267 (2006)11 N Greinert and W Richtering Colloid Polym Sci 282 1146 (2004)

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Gaharwar et al Magnetic Nanoparticles Encapsulated Within a Thermoresponsive Polymer

12 J E Wong and W Richtering Prog Colloid Polym Sci 133 45(2006)

13 J E Wong C B Muumlller A Laschewsky and W Richtering J PhysChem B 111 8527 (2007)

14 J E Wong A K Gaharwar D Muumlller-Schulte D Bahadur andW Richtering J Magn Magn Mater 311 219 (2007)

15 J E Wong A K Gaharwar D Muumlller-Schulte D Bahadur andW Richtering J Nanosci Nanotechnol 8 4033 (2008)

16 E Doelker Hydrogels in Medicine and Pharmacy edited byN Peppas CRC Press Boca Raton (1987) Vol II p 115

17 B G Kabra S H Gehrke and R J Spontak Macromolecules 312166 (1998)

18 D C Harsh and S H Gehrke J Controlled Release 17 175(1991)

19 J Gao G Haidar X Lu and Z Hu Macromolecules 34 2242(2001)

20 M Yamaura R L Camilo L C Sampaio M A MacedoM Nakamura and H E Toma J Magn Magn Mater 279 210(2004)

21 R Massart IEEE Trans Magn 17 1247 (1981)22 J Chatterjee Y Haik and C J Chen Colloid Polym Sci 281 892

(2003)23 H Wakamatsu K Yamamoto A Nakao and T Aoyagi J Magn

Magn Mater 302 327 (2006)24 A Guinier X-ray Diffraction Freeman San Francisco CA (1963)

p 14225 X Lu Z Hu and J Gao Macromolecules 33 8698 (2000)26 H J Dou W H Yang and K Sun Chem Lett 35 1374 (2006)

Received 20 February 2008 Accepted 15 December 2008

J Nanosci Nanotechnol 9 5355ndash5361 2009 5361