Synthesis of thermosensitive microgels with a tunable magnetic core

Published: July 26, 2011

r 2011 American Chemical Society 10484 dx.doi.org/10.1021/la201723a | Langmuir 2011, 27, 10484–10491

ARTICLE

pubs.acs.org/Langmuir

Synthesis of ThermosensitiveMicrogels with a TunableMagnetic CoreMarco Laurenti,† Pablo Guardia,‡ Rafael Contreras-C�aceres,§ Jorge P�erez-Juste,||

Antonio Fernandez-Barbero,§ Enrique Lopez-Cabarcos,† and Jorge Rubio-Retama*,†

†Departamento de Química-Física II, Facultad de Farmacia, Universidad Complutense de Madrid, Spain‡Departamento de Física Fundamental e Instituto de Nanociencia y Nanotecnología, Universidad de Barcelona, Spain§Departamento de Física Aplicada, Universidad de Almería, Spain

)Departamento de Química-Física y Unidad Asociada CSIC, Universidad de Vigo, Spain

’ INTRODUCTION

Since Pelton and Chivante’s1 first work, the preparation andcharacterization of thermosensitive poly(N-isopropylacrylamide)(pNIPAM) microgels produced via surfactant-free emulsion po-lymerization (SFEP) have attracted extensive interest due to theirpotential applications in many fields, such as templates fornanoparticles,2 contrast agents,4 sensors,3 catalyst support,5,6 orbiomedical applications.7�9

In many of these works, the thermosensitive microgels arecombined with inorganic components such as quantum dots,10

silver,11 gold,12 or magnetic nanoparticles13 to yield nanostruc-tured and multifunctional hybrid material. The combinationbetween the organic and the inorganic components establishes asymbiotic relation in which the microgels give colloidal stability aswell as stimuli responsive features, while the inorganic counter-parts provide quantum properties such as photoluminescence,14,15

surface plasmon resonance,16 or magnetism.17

In some works, the localization of the inorganic material is onthe outer part of the microgels. This sort of decoration can beproduced either by exploiting charge interactions between thenanoparticles and themicrogels16,18 or by covalent bonds betweenthe microgels and the magnetic material.19 However, one of themajor drawbacks concomitant with this strategy is a reduction ofthe colloidal stability which leads to the system coagulation.

Another approach used to synthesized hybrid magneticnanoparticles is based on the preparation of magnetic nano-particles in a bulk solution, where microgels have beenpreviously dispersed. This approach usually yields nanoparti-cles homogeneously distributed within the microgels.20

Nevertheless, this sort of decoration presents two major disad-vantages: First, not all the inorganic nanoparticles can be done inthe presence of microgels, limiting this strategy only to synthesesperformed under mild conditions. Second, the high specificsurface of the nanoparticles can favor interactions between thenanoparticles and the microgel matrix, which normally affect thethermal response of the microgels being possible to block the lowcritical solubility temperature (LCST). This is the reason whyonly microgels with a limited amount of inorganic material havebeen produced using this method.21,22

To overcome the previous disadvantages, many efforts havebeen done to produce hierarchical microgels in which theinorganic nanoparticles are located in the core of thesystem.23�26 However, these methodologies have not beensuccessfully applied to Fe3O4 nanoparticles since its surfaceinhibits the polymerization reaction. This impediment has beenrelated to their capacity to mediate in redox processes27 and totransfer an electron to the radical, which subsequently stops thepolymerization. To overcome this problem, some authors haveincorporated pNIPAM on the surface of magnetic nanoparticlesusing a layer-by-layer deposition technique.28 However, thisprocedure requires many steps when one wants to grow a thickpolymer layer around the inorganic core.

Other authors cover the surface of the magnetic nanoparticleswith SiO2 to overcome the above problems.13,23,29 However,

Received: May 9, 2011Revised: June 29, 2011

ABSTRACT: In this work, we describe a new methodology for thepreparation of monodisperse and thermosensitive microgels withmagnetic core. In order to produce such a material, hydrophobicmagnetic Fe3O4 nanoparticles were prepared by twomethods: thermaldecomposition and coprecipitation. The surface of these nanoparticleswas modified by addition of 3-butenoic acid, and after that thesenanoparticles were dispersed in water and submitted to free radicalpolymerization at 70 �C in the presence of N-isopropylacrylamide(NIPAM) and bisacrylamide. The result of this reaction was mono-disperse microgels with a magnetic core. By varying the amount of3-butenoic acid, it was possible to obtain hybrid microgels withdifferent magnetic core sizes and different architectures.

10485 dx.doi.org/10.1021/la201723a |Langmuir 2011, 27, 10484–10491

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this method presents two mayor disadvantages: First, theprocess turns into a tedious multistep procedure, and secondthe incorporation of a second inorganic material diminishes themaximum magnetic moment of the final material. Theseproblems have been recently solved30 by covering the nano-particles with 3-butenoic acid (3-bt), which hinders the nano-particles' surface oxidation. With this molecule, it is possible tocreate a simple method that yields thermosensitive microgels,which present a magnetic core formed by a cluster of Fe3O4

nanoparticles. In this work, we perform a parametric studyabout the role of this molecule as well as the hydrophobic�hydrophilic character of the nanoparticles during the synthesisof the microgels and how they influence the architecture of thefinal hybrid material.

’MATERIALS AND METHODS

Chemicals.ThemonomerN-isopropylacrylamide, the cross-linkerN,N-methylenebisacrylamide, the initiator 2,20-azobis(2-methylpropionamidine),3-butenoic acid, cetyltrimethylammonium bromide (CTAB), tetramethy-lammonium hydroxide (TMAOH), iron(III) chloride hexahydrate, andiron(II) chloride tetrahydrate were purchased from Sigma-Aldrich. Iron(III)acetylacetonate (99%), 10-undecenoic acid (99%), and dibenzyl ether(99%) were purchased from Acros. All chemicals were used without furtherpurification. A commercial NdFeBmagnet with amagnetic field of 0.6 Twaspurchased (www.aimangz.com).Thermogravimetric Analysis. The weight composition of the

samples was determined using a Mettler TGA-DSC1 instrument.

The measurements were performed in a temperature range between 25and 700 �C under oxygen atmosphere to ensure the organic componentdecomposition.Transmission Electron Microscopy (TEM). The size and

morphology of the nanoparticles were measured with a JEOL JSM-2100 electronmicroscope working at 200 kV. Samples were prepared onTEM grids by a drop-casting technique.Dynamic Light Scattering (DLS). The DLS measurements were

recorded using a Zetasizer Nano ZS (Malvern Instruments, Worcester-shire, U.K.) instrument. Accumulation times for each sample weredetermined automatically, at selected temperatures. The correlationfunction was used to calculate the z-average (intensity mean) andhydrodynamic diameter (Dh) using the Einstein�Stokes equation.These results were calculated by selecting the multimodal analysismethod of the DTS software (5.0) provided by Malvern.Synthesis of theMagnetic Nanoparticles. Twomethods have

been used to producemagnetic nanoparticles: thermal decomposition ofiron(III) acetylacetonate and a coprecipitation of FeCl3 and FeCl2 in astoichiometric ratio 2:1.

Method I: Thermal Decomposition. In this case, the synthesis of themagnetic nanoparticles was a modification of a previously describedmethod.31,32 For the synthesis of 13 nm edge length particles, 0.353 g(1 mmol) of iron(III) acetylacetonate was mixed with 0.688 g (4 mmol)of 10-undecenoic acid in 25 mL of dibenzyl ether. After 1 h undervacuum in a Schlenk line, the solution was heated up to 200 �C with aconstant heating rate of 6�7 �C/min under an argon blanket flow andvigorous stirring. After 2 h at 200 �C, the solution was heated up to refluxwith a constant heating rate of 5.2 �C/min up to 300 �C and kept at this

Figure 1. TEM image of (A) 13( 2 nm edge length iron oxide nanoparticles synthesized by thermal decomposition and (B) 6( 1 nm edge length ironoxide nanoparticles synthesized by coprecipitation method.


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temperature for 1 h. After cooling to room temperature, a mixture oftoluene and acetone was added to the solution and then centrifuged toprecipitate. The precipitate was washed several times with a mixture oftoluene and acetone. Finally, the particles were stored in ethanol.Method II: Coprecipitation of Fe3+ and Fe2+. The particles are

synthesized using a modified coprecipitation recipe reported previously.33

Tetramethylammonium hydroxide (TMAOH) is used as alkaline baseinstead of ammonium hydroxide. A total of 25 mL of TMAOH 1 M and22 mL of H2O are mixed under mechanic stirring at 70 �C. After that, asolution containing 3mL of H2O, 650mg of FeCl3, and 250mg of FeCl2is added dropwise. The stirring stands for 30 min. After this time, thenanoparticles are recovered by magnetic decantation and washed severaltimes with milli-Q water.Surface Modification of the Magnetic Nanoparticles. The

modification of the magnetic nanoparticles was carried out as follows. Ina vial of 15mL, 350 μL ofmagnetic nanoparticles was diluted in 10mL ofwater and sonicated during 15 min to reduce the particle aggregation.Subsequently, the vial was immersed in a water bath at 70 �C, anddifferent amounts of 3-bt were added. This solution was stood for 1 h atthese conditions and then the nanoparticles were centrifuged during30 min to remove the 3-bt excess. With the aim of reducing theaggregation effects just before the centrifugation, 200 μL of 0.2 MCTAB was added before starting the centrifugation. After that, thesupernatant was discarded and the precipitate was diluted to 10 mL withwater and redispersed by sonication during 15 min.Synthesis of Thermoresponsive Microgels with Magnetic

Cores. The previous solution was put into a three-neck round-bottomflask and heated up to 70 �C, under mechanical stirring (350 rpm) andN2 atmosphere. Subsequently, 0.226 g of N-isopropylacrylamide and0.031 g of N,N0-methylenbisacrylamide were added to the magneticnanoparticles dispersion. The mixture was stirred during 15 min, thepolymerization was initiated by adding 100 μL of 0.1 M 2,20-azobis-(2-methylpropionamidine) dihydrochloride, the N2 flow was removed,and the polymerization was kept for 2 h. Finally, the milklike dispersionwas cooled down and the magnetic microgels were collected by placingthe reaction flask on a neodymiummagnet of 0.6 T during 24 h. A brownprecipitate appeared on the bottom of the flask, which was collected andredispersed in 10mL of water. The cleaning process was repeated at least10 times. The yield of hybrid microgels after this reaction was around

12% (w/w). After that a drop of 10 μL of this dispersion was placed on aTEM grid and observed by TEM.

’RESULTS AND DISCUSSION

Figure 1 shows a representative TEM image of the synthesizedmagnetic nanoparticles. From the TEM, images one can observethat the size of the nanoparticles obtained by thermal decom-position was 13 ( 2 nm while the size for those nanoparticlesproduced by coprecipitation was around 6 ( 1 nm. These twodifferent syntheses permitted us to obtain magnetic nanoparti-cles with different hydrophobic�hydrophilic character that wasexploited to control the magnetic core nature of the hybridmicrogels. The X-ray analysis showed that the crystalline struc-ture of the magnetic nanoparticles is in both cases Fe3O4.

The dispersion in water of the magnetic nanoparticles synthe-sized by thermal decomposition produced nanoparticle aggre-gates with a mean hydrodynamic diameter of 188 ( 27 nm.34

With the aim of getting magnetic clusters with smaller sizes, wetreated these magnetic nanoparticles with different amounts(μL) of 3-butenoic acid. In this way and after incubating thesenanoparticles with 3-bt during 1 h at 70 �C, it was possible toreduce the mean hydrodynamic diameter of the clusters (seeFigure 2).

Figure 2 depicts the hydrodynamic diameter measured byDLS of the clusters treated with different amounts of 3-butenoicacid. From this study, we can infer that 3-bt was able to disaggregatethemagnetic clusters and to reduce their sizes. The driving force ofthe size reduction would be the increment of the superficialcharge of the magnetic nanoparticles that occurs when 3-bt isadded. Scheme 1 summarizes the process that is involved in thenanoparticle disaggregation.

To substantiate that disaggregation is a consequence of theextra surface charge provided to the particles by the 3-bt, wehave measured the electrophoretic mobility and the hydrody-namic diameter of the cores after their treatment with 3-bt andprior to the CTAB addition. From these measurements,we observed an increment of the electrophoretic mobility from3.8 � 10�8 m2

3V�1

3 s�1 for untreated particles (sample S0I)

up to 5.3 � 10�8 m23V

�13 s�1 for sample S100I (see Table 1).

The reason why samples S40I and S100I present similarelectrophoretic mobility would be attributed to the fact thatelectrophoretical mobility is the result of two competitiveprocesses, charge density and friction coefficient. Sample S100Ihas higher surface charge than sample S40I, since it was preparedwith a higher concentration of 3-bt that would lead to a greatermobility. However, the increment of superficial charge wouldprovoke an increment of the water adsorbed on the clustersurface, as is described by L�opez-Leon et al.,41 increasing thefriction coefficient of the cluster and thus reducing its electro-phoretic mobility. It is the balance between terms, charge andfriction, what explains the result we observe.

In case of using 200 μL of 3-bt for treating the magneticnanoparticles synthesized by thermal decomposition, we observethat the sizes of the magnetic clusters were similar to thoseobtained after adding 100 μL. This effect would indicate thatabove 100 μL of 3-bt the charge density of the particles ismaximum and cluster segregation stops, being impossible toobtain single particles. In order to get the smallest magneticcluster, nanoparticles produced by coprecipitation were synthe-sized and modified with 3-bt. Prior to the treatment with 3-bt,they showed a hydrodynamic diameter of 20( 10 nm, which falls

Figure 2. Evolution of the cluster size distribution, after addition of3-bt: 0, 10, 20, 40, and 100 μL. The left-hand distribution curve showsthe size distribution for the magnetic nanoparticles obtained by methodII with 160 μL of 3-bt.


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down to 12 ( 7 nm after the addition of 160 μL of 3-bt (SeeTable 1).

When NIPAM is polymerized in the presence of the magneticclusters, we are able to obtain hybrid microgels. Figure 3 showsthe shape of the hybrid microgels obtained after using differentmagnetic clusters.

Themicrogel synthesis in the absence of 3-bt (sample S0I) leadsto large aggregates of the magnetic material, resulting in irregularclusters covered by pNIPAM. These hybrid particles show poorstability, provoking colloidal aggregation and strong sedimentation.The average particle size for pNIPAM@Fe3O4 in sample S0I isaround 500 nm, with the magnetic cores of these microgels beingclose to 400 nm, as determined by TEM. Such an amount ofinorganic counterpart decreases the colloidal stability of thesemicrogels, making it impossible to measure their size by DLS.

The use of magnetic clusters modified with 3-bt and polymer-ized with NIPAM leads to stable hybrid microgels with size,measured by DLS, between 320 and 370 nm (in the collapsedstate) and between 724 and 600 nm (in the swollen state) forsamples S10I�S100I (see Figure 4). In the TEM pictures ofFigure 3, we can observe that the pNIPAM reaction carried out inthe presence of magnetic clusters with sizes above 55 nm (samplesfrom S10I to S100I) produced hybridmicrogels with the inorganicmaterial located in the core, which maintains the magnetic clusterformed prior to the microgels synthesis. Furthermore, the shape ofthe clusters evolved from irregular to pseudospherical when theamount of 3-bt was increased. By contrast, when the smallestmagnetic clusters were used (S160II), hybrid particles with ameanhydrodynamic diameter of 1000 nm (in the collapsed state) and4800 nm (in the swollen state) were obtained. In this case, the

hybrid microgels present the magnetic nanoparticles randomlydistributed in the inner part of the microgels (Figure 3f)with multiple magnetic clusters per microgel subunit. Theseresults indicate that there is a strong influence of the aggregationstate of the magnetic nanoparticles on the final hybrid microgelarchitecture.

Despite the localization of the magnetic nanoparticles, all thesamples present volume phase transition; see panel 1 in Figure 4.The particles shrink continuously with increasing temperature asa consequence of the temperature-dependent Flory parameter.35

Furthermore, the shrinking�swelling cycles are reversible in allcases without any significant hysteresis. For microgels with a welldefinedmagnetic core, only the pNIPAM shell manifests swellingcapacity, while the inorganic core keeps constant. That has beenproved by comparing the relative shell swelling for pNIPAM@Fe3O4 with different core sizes. All curves overlap (Figure 4,panel 1, inset B), indicating that the swelling of the organic shell isnot influenced by the presence of the inorganic core. It is worthpointing out the big difference in size exhibited by the microgelsprepared with the smallest magnetic clusters, which have ahydrodynamic diameter of 4800 nm in the swollen state. Thisresult shows the tremendous influence that the cluster size has onthe microgel formation and their architecture.

As can be inferred from the previous results, the variation ofthe amount of 3-bt influences the aggregation state of themagnetic clusters used during the synthesis and permits control-ling the size of the magnetic clusters, allowing to entrap differentamounts of magnetic material.

Figure 5 shows the thermogravimetric analysis of the differenthybrid microgels. As can be observed, the sample S10I (magnetic

Scheme 1. (A) Representation of the Iron Nanoparticles' Surface Modification and the Effect on the Hydrodynamic Diameter;(B) TEM Micrograph of a Nanoparticle Cluster Obtained at the End of the Treatment with 3-bt.

Table 1. Amount of 3-bt Added for Surface Treatment, Electrophoretic Mobility (μ), Hydrodynamic Diameter (Dh) of MagneticCluster Treated with 3-bt but Without CTAB, Dh of Magnetic Clusters Treated with 3-bt and CTAB, Dh of pNIPAM@Fe3O4 andColloidal Stabilitya

sample

3-bt

(μL)

μ (10�8 m23V

�13 s�1)

(prior CTAB addition)

Dh (nm) magn. cluster

(prior CTAB addition)

Dh (nm) magn. cluster

(after CTAB addition)

Dh (nm)

PNIPAM@Fe3O4

colloidal

stability

S0I 0 3.8( 0.1 188( 21 188( 21 not measured <2 h

S10I 10 4.2 ( 0.1 120( 11 134( 12 600 ( 21 >2 weeks

S20I 20 4.4( 0.1 105( 16 110( 21 630 ( 23 >2 weeks

S40I 40 5.2( 0.1 67( 12 78( 15 674 ( 28 >2 weeks

S100I 100 5.3( 0.1 48( 9 55( 15 724 ( 32 >2 weeks

S160II 160 � 12( 7 12( 7 4800 ( 120 >2 weeksa Samples from S0I to S100I are synthesized by method I and S160II by method II.


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Figure 3. TEM pictures showing the core�shell structure of the pNIPAM@Fe3O4 microgels. The magnetic core reduces for increasing 3-bt: (a) 0 μL(S0I), (b) 10 μL (S10I), (c) 20 μL (S20I), (d) 40 μL (S40I), (e) 100 μL (S100I), and (f) 160 μL (S160II).

Figure 4. Panels 1: Insets (A) and (C) show the swelling curves against temperature for different pNIPAM@Fe3O4 particles. Inset (B) shows theswelling ratio for hybrid microgels with inorganic cores. Panel 2: Size distribution of the different hybrid microgels at 25 and 45 �C.


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core of 134 nm) has 37% organic content as determined bythermogravimetry, being colloidally stable.

The different particle decoration obtained from the synthesiscould be attributed to the different action mechanism involvedin the synthesis of the hybrid microgels. Several authors havedescribed the mechanism of the neat pNIPAM microgelproduction as a nucleation process of colloidally unstablepNIPAM polymer chains and nanogels that are formed duringthe first step of the polymerization reaction, which are colloid-ally unstable at the reaction temperature.1,37 Due to theircolloidal instability, they coagulate, forming a stable particle,which results in the final microgel. Using small-angle neutronscattering,36 these nanogels have been detected showing a sizearound 22 nm above the LCST.

Figure 6 shows pNIPAM microgels produced using thesame method described for the synthesis of the hybrid microgelsbut in the absence of magnetic clusters. Here we can observethe presence of nanogels that appear as dark dots within themicrogels.

However, when we introduce the functionalized magneticcores, the vinyl groups provided by 3-bt could form covalentbonds with the pNIPAM growing radicals. As result, the polym-erization process could create a polymer shell around the cores,which would grow thanks to the excess of pNIPAM oligomersand/or nanogels formed during the first stage of the polymeri-zation, in a process named “seed-feed” that has been previouslydescribed by Lyon and Neetu.42 This growing process stopswhen the microgels reach a size that provides enough colloidalstability, which is the same for hybrid and neat microgelsproduced under the same reaction conditions (370 nm at thecollapsed state, see Scheme 2A).

This scenario changes when the size of the magnetic clusters isreduced and the specific surface of the inorganic nanoparticle ismuch higher than in the previous case. Under this condition,more pNIPAM oligomers and nanogels would be required tocover the smallest magnetic clusters. This could provoke a rapidconsumption of the pNIPAM oligomers and nanogels, yieldingto small hybrid nanogels, which are unstable. In order to gaincolloidal stability, neighbor hybrid nanogels would aggregatewith each other, creating the final hybrid microgel. The bigdifference in size of S160II with respect to the other hybridmicrogels could arise from the smaller gain of colloidal stabilitythat the aggregation of hybrid nanogels produces in comparisonwith the aggregation of neat nanogels.

With the aim of studying the magnetic behavior of the hybridmicrogels, we have measured the magnetization moment of twodifferent microgel dispersions below and above the LCST as afunction of the magnetic field using a superconducting quantuminterference device. From these experiments, we can observe thatpNIPAM@Fe3O4 microgels show superparamagnetic behaviorwith very low hysteresis. In Figure 7A, one can observe that thesuperparamagnetic behavior is independent of the particle swel-ling for sample S100I since the magnetization curves are coin-cident above and below the LCST. That would indicate that thedistances between nanoparticles within the magnetic cluster are

Figure 5. Microgel polymer content (%) determined by thermogravi-metry as a function of the size of the magnetic cluster used in thesynthesis.

Figure 6. TEMmicrograph of neat pNIPAMmicrogels formed by smallnanogel subunits.

Scheme 2. Microgel Formation Mechanism (a) for MagneticCluster Bigger than the Nanogels and (b) for Magnet ClusterSmaller than the Nanogels


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very stable and the saturation magnetic moment remains con-stant independently of the polymer swelling. However, forsample S160II, in Figure 7B, the swelling determines the distancebetween the magnetic nanoparticles and influences the satura-tion magnetic moment of the hybrid microgels, which is stron-ger at high temperatures due to the close proximity, in the col-lapsed state, of the magnetic clusters that are spread within themicrogel.38�40

Superparamagnetism is an important property of single do-main magnetic nanoparticles originating from the fast flipping ofthe magnetic moments due to thermal energy. In the absence ofan external magnetic field, the magnetization of the nanoparticlesappears to be zero, but in the presence of a magnetic field theirmagnetic moments align along the field direction, leading to a netmagnetization. This property is very interesting since it ensuresthe disappearance of the nanoparticles' magnetization afterremoving themagnetic field, allowing in this way the redispersionof the colloidal magnetic material. However, in the presence of amagnetic field, the magnetic material is aligned and can becollected or concentrated.38,39

’CONCLUSION

In this paper, we report an easy and reproducible method toencapsulate a considerable amount of magnetic material withinmicrogels. The synthesis of hybrid particles is based on the use of3-bt to modify the magnetic cluster surface where pNIPAM cangrow. This strategy leads to controlled cluster segregation due tothe extra added charge and allows pNIPAM polymerization.3-Butenoic acid promotes terminal double-bond formation onthe nanoparticle surfaces where polymerization occurs. The final

hybrid particles show two different internal architectures as aconsequence of the polymer coagulative nucleation. The char-acteristics of these growing particles are determined by the ratiobetween the magnetic cluster size and the pNIPAM nanopre-cursors. Well-defined magnetic cores become apparent when themagnetic clusters are large enough. By contrast, when the size ofmagnetic clusters is small enough, hybrid microgels grow withthe magnetic material spread within the polymer lattice. Themagnetic response for the hybrid particles under external mag-netic fields is temperature independent for particles with well-defined cores, and it depends on temperature (at sufficiently highmagnetic fields) for the diffuse architecture for which thecompactness of the magnetic material depends on the polymerswelling.

’ACKNOWLEDGMENT

The authors acknowledge financial support from the SpanishScience and Innovation Ministry (Grant MAT2010-15349),from the UCM-BSCH program for Consolidation of ResearchGroups (Grant GR35/10-A-911033), and from the COSTAction D43. Marco Laurenti acknowledges the Comunidad deMadrid for a fellowship to perform this work. We want also tothank Agustin Fernandez from UCM-microscopy CAI for hishelp with TEM micrograph and the fruitful discussion.

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Figure 7. Magnetization for S100I (A) and S160II (B). Blue and red lines correspond to experiments performed at temperatures below and above theLCST, respectively. The insets show the hysteresis found for both temperatures.


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Synthesis of thermosensitive microgels with a tunable magnetic core

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