ARTICLE pubs.acs.org/Langmuir
Synthesis of Thermosensitive Microgels with a Tunable Magnetic Core Marco Laurenti,† Pablo Guardia,‡ Rafael Contreras-Caceres,§ Jorge Perez-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
)
‡
ABSTRACT: In this work, we describe a new methodology for the preparation of monodisperse and thermosensitive microgels with magnetic core. In order to produce such a material, hydrophobic magnetic Fe3O4 nanoparticles were prepared by two methods: thermal decomposition and coprecipitation. The surface of these nanoparticles was modified by addition of 3-butenoic acid, and after that these nanoparticles were dispersed in water and submitted to free radical polymerization at 70 °C in the presence of N-isopropylacrylamide (NIPAM) and bisacrylamide. The result of this reaction was monodisperse microgels with a magnetic core. By varying the amount of 3-butenoic acid, it was possible to obtain hybrid microgels with different magnetic core sizes and different architectures.
’ INTRODUCTION Since Pelton and Chivante’s1 first work, the preparation and characterization of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels produced via surfactant-free emulsion polymerization (SFEP) have attracted extensive interest due to their potential applications in many fields, such as templates for nanoparticles,2 contrast agents,4 sensors,3 catalyst support,5,6 or biomedical applications.79 In many of these works, the thermosensitive microgels are combined with inorganic components such as quantum dots,10 silver,11 gold,12 or magnetic nanoparticles13 to yield nanostructured and multifunctional hybrid material. The combination between the organic and the inorganic components establishes a symbiotic relation in which the microgels give colloidal stability as well as stimuli responsive features, while the inorganic counterparts 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 on the outer part of the microgels. This sort of decoration can be produced either by exploiting charge interactions between the nanoparticles and the microgels16,18 or by covalent bonds between the microgels and the magnetic material.19 However, one of the major drawbacks concomitant with this strategy is a reduction of the colloidal stability which leads to the system coagulation. Another approach used to synthesized hybrid magnetic nanoparticles is based on the preparation of magnetic nanoparticles in a bulk solution, where microgels have been previously dispersed. This approach usually yields nanoparticles homogeneously distributed within the microgels.20 r 2011 American Chemical Society
Nevertheless, this sort of decoration presents two major disadvantages: First, not all the inorganic nanoparticles can be done in the presence of microgels, limiting this strategy only to syntheses performed under mild conditions. Second, the high specific surface of the nanoparticles can favor interactions between the nanoparticles and the microgel matrix, which normally affect the thermal response of the microgels being possible to block the low critical solubility temperature (LCST). This is the reason why only microgels with a limited amount of inorganic material have been produced using this method.21,22 To overcome the previous disadvantages, many efforts have been done to produce hierarchical microgels in which the inorganic nanoparticles are located in the core of the system.2326 However, these methodologies have not been successfully applied to Fe3O4 nanoparticles since its surface inhibits the polymerization reaction. This impediment has been related to their capacity to mediate in redox processes27 and to transfer an electron to the radical, which subsequently stops the polymerization. To overcome this problem, some authors have incorporated pNIPAM on the surface of magnetic nanoparticles using a layer-by-layer deposition technique.28 However, this procedure requires many steps when one wants to grow a thick polymer layer around the inorganic core. Other authors cover the surface of the magnetic nanoparticles with SiO2 to overcome the above problems.13,23,29 However, Received: May 9, 2011 Revised: June 29, 2011 Published: July 26, 2011 10484
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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 iron oxide nanoparticles synthesized by coprecipitation method.
this method presents two mayor disadvantages: First, the process turns into a tedious multistep procedure, and second the incorporation of a second inorganic material diminishes the maximum magnetic moment of the final material. These problems have been recently solved30 by covering the nanoparticles with 3-butenoic acid (3-bt), which hinders the nanoparticles' surface oxidation. With this molecule, it is possible to create 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 study about the role of this molecule as well as the hydrophobic hydrophilic character of the nanoparticles during the synthesis of the microgels and how they influence the architecture of the final hybrid material.
’ MATERIALS AND METHODS Chemicals. The monomer N-isopropylacrylamide, the cross-linker N,Nmethylenebisacrylamide, the initiator 2,20 -azobis(2-methylpropionamidine), 3-butenoic acid, cetyltrimethylammonium bromide (CTAB), tetramethylammonium hydroxide (TMAOH), iron(III) chloride hexahydrate, and iron(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 further purification. A commercial NdFeB magnet with a magnetic field of 0.6 T was purchased (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 25 and 700 °C under oxygen atmosphere to ensure the organic component decomposition. Transmission Electron Microscopy (TEM). The size and morphology of the nanoparticles were measured with a JEOL JSM2100 electron microscope working at 200 kV. Samples were prepared on TEM grids by a drop-casting technique. Dynamic Light Scattering (DLS). The DLS measurements were recorded using a Zetasizer Nano ZS (Malvern Instruments, Worcestershire, U.K.) instrument. Accumulation times for each sample were determined automatically, at selected temperatures. The correlation function was used to calculate the z-average (intensity mean) and hydrodynamic diameter (Dh) using the EinsteinStokes equation. These results were calculated by selecting the multimodal analysis method of the DTS software (5.0) provided by Malvern. Synthesis of the Magnetic Nanoparticles. Two methods have been used to produce magnetic nanoparticles: thermal decomposition of iron(III) acetylacetonate and a coprecipitation of FeCl3 and FeCl2 in a stoichiometric ratio 2:1. Method I: Thermal Decomposition. In this case, the synthesis of the magnetic nanoparticles was a modification of a previously described method.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 under vacuum in a Schlenk line, the solution was heated up to 200 °C with a constant heating rate of 67 °C/min under an argon blanket flow and vigorous stirring. After 2 h at 200 °C, the solution was heated up to reflux with a constant heating rate of 5.2 °C/min up to 300 °C and kept at this 10485
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12% (w/w). After that a drop of 10 μL of this dispersion was placed on a TEM grid and observed by TEM.
Figure 2. Evolution of the cluster size distribution, after addition of 3-bt: 0, 10, 20, 40, and 100 μL. The left-hand distribution curve shows the size distribution for the magnetic nanoparticles obtained by method II with 160 μL of 3-bt. temperature for 1 h. After cooling to room temperature, a mixture of toluene and acetone was added to the solution and then centrifuged to precipitate. The precipitate was washed several times with a mixture of toluene 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 base instead of ammonium hydroxide. A total of 25 mL of TMAOH 1 M and 22 mL of H2O are mixed under mechanic stirring at 70 °C. After that, a solution containing 3 mL of H2O, 650 mg of FeCl3, and 250 mg of FeCl2 is added dropwise. The stirring stands for 30 min. After this time, the nanoparticles are recovered by magnetic decantation and washed several times with milli-Q water. Surface Modification of the Magnetic Nanoparticles. The modification of the magnetic nanoparticles was carried out as follows. In a vial of 15 mL, 350 μL of magnetic nanoparticles was diluted in 10 mL of water and sonicated during 15 min to reduce the particle aggregation. Subsequently, the vial was immersed in a water bath at 70 °C, and different amounts of 3-bt were added. This solution was stood for 1 h at these conditions and then the nanoparticles were centrifuged during 30 min to remove the 3-bt excess. With the aim of reducing the aggregation effects just before the centrifugation, 200 μL of 0.2 M CTAB was added before starting the centrifugation. After that, the supernatant was discarded and the precipitate was diluted to 10 mL with water and redispersed by sonication during 15 min.
Synthesis of Thermoresponsive Microgels with Magnetic Cores. The previous solution was put into a three-neck round-bottom
flask and heated up to 70 °C, under mechanical stirring (350 rpm) and N2 atmosphere. Subsequently, 0.226 g of N-isopropylacrylamide and 0.031 g of N,N0 -methylenbisacrylamide were added to the magnetic nanoparticles dispersion. The mixture was stirred during 15 min, the polymerization 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 dispersion was cooled down and the magnetic microgels were collected by placing the reaction flask on a neodymium magnet of 0.6 T during 24 h. A brown precipitate appeared on the bottom of the flask, which was collected and redispersed in 10 mL of water. The cleaning process was repeated at least 10 times. The yield of hybrid microgels after this reaction was around
’ RESULTS AND DISCUSSION Figure 1 shows a representative TEM image of the synthesized magnetic nanoparticles. From the TEM, images one can observe that the size of the nanoparticles obtained by thermal decomposition was 13 ( 2 nm while the size for those nanoparticles produced by coprecipitation was around 6 ( 1 nm. These two different syntheses permitted us to obtain magnetic nanoparticles with different hydrophobichydrophilic character that was exploited to control the magnetic core nature of the hybrid microgels. The X-ray analysis showed that the crystalline structure of the magnetic nanoparticles is in both cases Fe3O4. The dispersion in water of the magnetic nanoparticles synthesized by thermal decomposition produced nanoparticle aggregates with a mean hydrodynamic diameter of 188 ( 27 nm.34 With the aim of getting magnetic clusters with smaller sizes, we treated these magnetic nanoparticles with different amounts (μL) of 3-butenoic acid. In this way and after incubating these nanoparticles with 3-bt during 1 h at 70 °C, it was possible to reduce the mean hydrodynamic diameter of the clusters (see Figure 2). Figure 2 depicts the hydrodynamic diameter measured by DLS of the clusters treated with different amounts of 3-butenoic acid. From this study, we can infer that 3-bt was able to disaggregate the magnetic clusters and to reduce their sizes. The driving force of the size reduction would be the increment of the superficial charge of the magnetic nanoparticles that occurs when 3-bt is added. Scheme 1 summarizes the process that is involved in the nanoparticle disaggregation. To substantiate that disaggregation is a consequence of the extra surface charge provided to the particles by the 3-bt, we have measured the electrophoretic mobility and the hydrodynamic diameter of the cores after their treatment with 3-bt and prior to the CTAB addition. From these measurements, we observed an increment of the electrophoretic mobility from 3.8 108 m2 3 V1 3 s1 for untreated particles (sample S0I) up to 5.3 108 m2 3 V1 3 s1 for sample S100I (see Table 1). The reason why samples S40I and S100I present similar electrophoretic mobility would be attributed to the fact that electrophoretical mobility is the result of two competitive processes, charge density and friction coefficient. Sample S100I has higher surface charge than sample S40I, since it was prepared with a higher concentration of 3-bt that would lead to a greater mobility. However, the increment of superficial charge would provoke an increment of the water adsorbed on the cluster surface, as is described by Lopez-Leon et al.,41 increasing the friction coefficient of the cluster and thus reducing its electrophoretic mobility. It is the balance between terms, charge and friction, what explains the result we observe. In case of using 200 μL of 3-bt for treating the magnetic nanoparticles synthesized by thermal decomposition, we observe that the sizes of the magnetic clusters were similar to those obtained after adding 100 μL. This effect would indicate that above 100 μL of 3-bt the charge density of the particles is maximum and cluster segregation stops, being impossible to obtain single particles. In order to get the smallest magnetic cluster, nanoparticles produced by coprecipitation were synthesized and modified with 3-bt. Prior to the treatment with 3-bt, they showed a hydrodynamic diameter of 20 ( 10 nm, which falls 10486
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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 Magnetic Cluster Treated with 3-bt but Without CTAB, Dh of Magnetic Clusters Treated with 3-bt and CTAB, Dh of pNIPAM@Fe3O4 and Colloidal Stabilitya sample
μ (108 m2 3 V1 3 s1)
Dh (nm) magn. cluster
Dh (nm) magn. cluster
Dh (nm)
colloidal
(μL)
(prior CTAB addition)
(prior CTAB addition)
(after CTAB addition)
PNIPAM@Fe3O4
stability
S0I
0
3.8 ( 0.1
188 ( 21
188 ( 21
not measured
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
100 160
5.3 ( 0.1
48 ( 9 12 ( 7
55 ( 15 12 ( 7
724 ( 32 4800 ( 120
>2 weeks >2 weeks
S100I S160II a
3-bt
Samples from S0I to S100I are synthesized by method I and S160II by method II.
down to 12 ( 7 nm after the addition of 160 μL of 3-bt (See Table 1). When NIPAM is polymerized in the presence of the magnetic clusters, we are able to obtain hybrid microgels. Figure 3 shows the shape of the hybrid microgels obtained after using different magnetic clusters. The microgel synthesis in the absence of 3-bt (sample S0I) leads to large aggregates of the magnetic material, resulting in irregular clusters covered by pNIPAM. These hybrid particles show poor stability, provoking colloidal aggregation and strong sedimentation. The average particle size for pNIPAM@Fe3O4 in sample S0I is around 500 nm, with the magnetic cores of these microgels being close to 400 nm, as determined by TEM. Such an amount of inorganic counterpart decreases the colloidal stability of these microgels, making it impossible to measure their size by DLS. The use of magnetic clusters modified with 3-bt and polymerized with NIPAM leads to stable hybrid microgels with size, measured by DLS, between 320 and 370 nm (in the collapsed state) and between 724 and 600 nm (in the swollen state) for samples S10IS100I (see Figure 4). In the TEM pictures of Figure 3, we can observe that the pNIPAM reaction carried out in the presence of magnetic clusters with sizes above 55 nm (samples from S10I to S100I) produced hybrid microgels with the inorganic material located in the core, which maintains the magnetic cluster formed prior to the microgels synthesis. Furthermore, the shape of the clusters evolved from irregular to pseudospherical when the amount of 3-bt was increased. By contrast, when the smallest magnetic clusters were used (S160II), hybrid particles with a mean hydrodynamic diameter of 1000 nm (in the collapsed state) and 4800 nm (in the swollen state) were obtained. In this case, the
hybrid microgels present the magnetic nanoparticles randomly distributed in the inner part of the microgels (Figure 3f) with multiple magnetic clusters per microgel subunit. These results indicate that there is a strong influence of the aggregation state of the magnetic nanoparticles on the final hybrid microgel architecture. Despite the localization of the magnetic nanoparticles, all the samples present volume phase transition; see panel 1 in Figure 4. The particles shrink continuously with increasing temperature as a consequence of the temperature-dependent Flory parameter.35 Furthermore, the shrinkingswelling cycles are reversible in all cases without any significant hysteresis. For microgels with a well defined magnetic core, only the pNIPAM shell manifests swelling capacity, while the inorganic core keeps constant. That has been proved 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 is not influenced by the presence of the inorganic core. It is worth pointing out the big difference in size exhibited by the microgels prepared with the smallest magnetic clusters, which have a hydrodynamic diameter of 4800 nm in the swollen state. This result shows the tremendous influence that the cluster size has on the microgel formation and their architecture. As can be inferred from the previous results, the variation of the amount of 3-bt influences the aggregation state of the magnetic clusters used during the synthesis and permits controlling the size of the magnetic clusters, allowing to entrap different amounts of magnetic material. Figure 5 shows the thermogravimetric analysis of the different hybrid microgels. As can be observed, the sample S10I (magnetic 10487
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Figure 3. TEM pictures showing the coreshell 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 the swelling ratio for hybrid microgels with inorganic cores. Panel 2: Size distribution of the different hybrid microgels at 25 and 45 °C. 10488
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Scheme 2. Microgel Formation Mechanism (a) for Magnetic Cluster Bigger than the Nanogels and (b) for Magnet Cluster Smaller than the Nanogels
Figure 5. Microgel polymer content (%) determined by thermogravimetry as a function of the size of the magnetic cluster used in the synthesis.
Figure 6. TEM micrograph of neat pNIPAM microgels formed by small nanogel subunits.
core of 134 nm) has 37% organic content as determined by thermogravimetry, being colloidally stable. The different particle decoration obtained from the synthesis could be attributed to the different action mechanism involved in the synthesis of the hybrid microgels. Several authors have described the mechanism of the neat pNIPAM microgel production as a nucleation process of colloidally unstable pNIPAM polymer chains and nanogels that are formed during the first step of the polymerization reaction, which are colloidally unstable at the reaction temperature.1,37 Due to their colloidal instability, they coagulate, forming a stable particle, which results in the final microgel. Using small-angle neutron scattering,36 these nanogels have been detected showing a size around 22 nm above the LCST.
Figure 6 shows pNIPAM microgels produced using the same method described for the synthesis of the hybrid microgels but in the absence of magnetic clusters. Here we can observe the presence of nanogels that appear as dark dots within the microgels. However, when we introduce the functionalized magnetic cores, the vinyl groups provided by 3-bt could form covalent bonds with the pNIPAM growing radicals. As result, the polymerization process could create a polymer shell around the cores, which would grow thanks to the excess of pNIPAM oligomers and/or nanogels formed during the first stage of the polymerization, in a process named “seed-feed” that has been previously described by Lyon and Neetu.42 This growing process stops when the microgels reach a size that provides enough colloidal stability, which is the same for hybrid and neat microgels produced under the same reaction conditions (370 nm at the collapsed state, see Scheme 2A). This scenario changes when the size of the magnetic clusters is reduced and the specific surface of the inorganic nanoparticle is much higher than in the previous case. Under this condition, more pNIPAM oligomers and nanogels would be required to cover the smallest magnetic clusters. This could provoke a rapid consumption of the pNIPAM oligomers and nanogels, yielding to small hybrid nanogels, which are unstable. In order to gain colloidal stability, neighbor hybrid nanogels would aggregate with each other, creating the final hybrid microgel. The big difference in size of S160II with respect to the other hybrid microgels could arise from the smaller gain of colloidal stability that the aggregation of hybrid nanogels produces in comparison with the aggregation of neat nanogels. With the aim of studying the magnetic behavior of the hybrid microgels, we have measured the magnetization moment of two different microgel dispersions below and above the LCST as a function of the magnetic field using a superconducting quantum interference device. From these experiments, we can observe that pNIPAM@Fe3O4 microgels show superparamagnetic behavior with very low hysteresis. In Figure 7A, one can observe that the superparamagnetic behavior is independent of the particle swelling for sample S100I since the magnetization curves are coincident above and below the LCST. That would indicate that the distances between nanoparticles within the magnetic cluster are 10489
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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 the LCST, respectively. The insets show the hysteresis found for both temperatures.
very stable and the saturation magnetic moment remains constant independently of the polymer swelling. However, for sample S160II, in Figure 7B, the swelling determines the distance between the magnetic nanoparticles and influences the saturation magnetic moment of the hybrid microgels, which is stronger at high temperatures due to the close proximity, in the collapsed state, of the magnetic clusters that are spread within the microgel.3840 Superparamagnetism is an important property of single domain magnetic nanoparticles originating from the fast flipping of the magnetic moments due to thermal energy. In the absence of an external magnetic field, the magnetization of the nanoparticles appears to be zero, but in the presence of a magnetic field their magnetic moments align along the field direction, leading to a net magnetization. This property is very interesting since it ensures the disappearance of the nanoparticles' magnetization after removing the magnetic field, allowing in this way the redispersion of the colloidal magnetic material. However, in the presence of a magnetic field, the magnetic material is aligned and can be collected or concentrated.38,39
’ CONCLUSION In this paper, we report an easy and reproducible method to encapsulate a considerable amount of magnetic material within microgels. The synthesis of hybrid particles is based on the use of 3-bt to modify the magnetic cluster surface where pNIPAM can grow. This strategy leads to controlled cluster segregation due to the extra added charge and allows pNIPAM polymerization. 3-Butenoic acid promotes terminal double-bond formation on the nanoparticle surfaces where polymerization occurs. The final
hybrid particles show two different internal architectures as a consequence of the polymer coagulative nucleation. The characteristics of these growing particles are determined by the ratio between the magnetic cluster size and the pNIPAM nanoprecursors. Well-defined magnetic cores become apparent when the magnetic clusters are large enough. By contrast, when the size of magnetic clusters is small enough, hybrid microgels grow with the magnetic material spread within the polymer lattice. The magnetic response for the hybrid particles under external magnetic fields is temperature independent for particles with welldefined cores, and it depends on temperature (at sufficiently high magnetic fields) for the diffuse architecture for which the compactness of the magnetic material depends on the polymer swelling.
’ ACKNOWLEDGMENT The authors acknowledge financial support from the Spanish Science and Innovation Ministry (Grant MAT2010-15349), from the UCM-BSCH program for Consolidation of Research Groups (Grant GR35/10-A-911033), and from the COST Action D43. Marco Laurenti acknowledges the Comunidad de Madrid for a fellowship to perform this work. We want also to thank Agustin Fernandez from UCM-microscopy CAI for his help with TEM micrograph and the fruitful discussion. ’ REFERENCES (1) Pelton, R. H.; Chivante, P. Colloids Surf. 1986, 20, 247. (2) Schmidt, S.; Hellweg, T.; Von Klitzing, R. Langmuir 2008, 24, 12595. (3) Yin, J.; Li, C.; Wang, D.; Liu, S. J. Phys. Chem. B 2010, 114, 12213. 10490
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