Materials Letters 89 (2012) 12–15

Contents lists available at SciVerse ScienceDirect

Materials Letters

0167-57

http://d

n Corr

E-m

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

Preparation of thermo-responsive hydrogel-coated magnetic nanoparticles

Aslam Khan n, Ahmed Mohamed El-Toni, Mansour Alhoshan

King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

a r t i c l e i n f o

Article history:

Received 19 June 2012

Accepted 15 August 2012Available online 27 August 2012

Keywords:

Magnetic materials

Nanocomposites

NIPAAm

Core–shell

7X/$ - see front matter & 2012 Elsevier B.V.

x.doi.org/10.1016/j.matlet.2012.08.064

esponding author. Tel.: þ966 1 4678369.

ail addresses: aslamkhan@ksu.edu.sa, aslamp

a b s t r a c t

We propose and demonstrate a facile synthetic method for obtaining magnetic nanoparticle cores with

a thermo-responsive polymeric hydrogel shell using the miniemulsion polymerization method. The

magnetic nanoparticles approximately 9 nm in diameter were first prepared by a thermal decomposi-

tion method, and then, the magnetic nanoparticles were used as hydrophobes during the reaction,

forming aggregated magnetic particles over-coated with a polymer layer. Transmission electron

microscopy shows that as-prepared particles possess core–shell structures of 40–100 nm of aggregated

magnetic-core nanospheres with an approximately 15 nm thick polymer shell over the iron oxide cores.

The nanocomposites were also characterized by X-ray diffraction, vibration sample magnetometer and

dynamic light scattering techniques.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic nanoparticles (MNPs) have attracted much interestdue to their wide applications in biomedical applications, espe-cially in biology and medicine. Superparamagnetic Fe3O4 MNPsare commonly desired because of their good biocompatibility [1].They have attracted significant attention as drug carriers inhyperthermia therapy [2] and as contrasting agents in magneticresonance imaging (MRI). They can also be used in cell separationprocedures [3]. MNPs generate heat via magnetic hysteresis loss,Neel-relaxation, and Brown-relaxation when exposed to a varyingmagnetic field [4], whereas thermo-responsive polymers cancollapse or expand upon heating [5]. Poly-N-isopropylacrylamide(PNIPAAm) is one of the most studied thermo-responsive poly-meric hydrogels [6], and in aqueous solution, it is well known toexhibit a sharp phase transition, called the lower critical solutiontemperature (LCST), at a temperature of approximately 32–33 1C,depending on the composition of the polymer chains. Below theLCST, the PNIPAAm random coil chains are hydrated, hydrophilic innature, and swollen. Above the LCST, the chains become hydro-phobic and dehydrated but weakly hydrogen-bonded with watermolecules and thus collapsed. Combining MNPs with thermo-responsive polymers to form core–shell structures may representa new class of composite responsive material with potentialapplications in biomedical fields. Faridi-Majidi et al. [7] success-fully prepared polymer-coated MNPs via emulsifier free miniemul-sion polymerzation. Frimpong and Hilt [8] have reported thesynthesis of core–shell PNIPAAm-based hydrogel coatings on MNPs

All rights reserved.

oly@gmail.com (A. Khan).

via the atomic transfer radical polymerization method. Althoughthe preparation of MNP composites with thermo-responsive poly-mers could be achieved [9,10], this synthesis route was notstraightforward for forming core–shell structures. Thus, therewas a need to both simplify the synthesis process and make itwidely applicable.

In this study, we report a simple method to synthesize core–shell structures of MNP-PNIPAAm composite particles by theminiemulsion polymerization method. The as-prepared compo-site particles were studied in detail by electron microscopy, X-raydiffraction, vibration sample magnetometer and dynamic lightscattering.

2. Experimental section

MNPs were synthesized [11] in 100 mL of benzyl ether in which12.92 g of 1,2 hexadecanediol and 3.22 g of Fe(acac)3 weredissolved until a clear solution was obtained. To this solution,8.5 g of oleic acid and 8.0 g of oleylamine were added. The solutionwas heated to 300 1C. After cooling, particles were precipitated byadding ethanol in a 3:1 ethanol:solution proportion and using apermanent magnet.

Miniemulsion polymerization was used to prepare MNP-PNIPAAm composite particles according to the following proce-dures. Ten milligrams of as-prepared MNPs was dispersed in 1 mLof acrylonitrile and 2.8 mL cyclohexane with the aid of ultrasoundto form the oil phase. To form the water phase, 0.08 g of SDS and0.91 g of NIPAAm monomer were dissolved in 25 mL of water. Theabove two phases were ultrasonicated in an ice-cooled bath for10 min to form a miniemulsion and transferred to a 50 mL three-necked flask equipped with a condenser, a nitrogen inlet and a

A. Khan et al. / Materials Letters 89 (2012) 12–15 13

stirrer. Potassium persulfate (0.5 mol%) dissolved in 0.5 mL ofwater was added to initiate the polymerization at 70 1C withcontinual stirring at 250 rpm. After 6 h, the MNP-PNIPAAmcomposite particles were obtained. To remove unreacted mono-mers, the dispersed solution was centrifuged and washed withdistilled water several times by repeated magnetic decantationfor further studies.

Fig. 1. (A) Transmission electron micrograph of MNPs only. (B) Transmission electron

structure of MNP-NIPAAm composite particles prepared using the miniemulsion polyme

the magnetic particles.

10 20 30 40

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50 60 70800

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nsity

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.)

2θ (degree)

440511

400

311

220

Fig. 2. (A) X-ray diffraction pattern. (B) Magnetic hysteresis loop of MNP-NIPAAm

(i) dispersed in water and (ii) magnetic recovery of the composite particles using an e

Transmission electron microscopy (TEM) images were obtainedusing a JEOL 2100 microscope operated at an acceleration voltageof 200 kV. Field emission scanning electron microscopy (FESEM)studies were performed on a JSM 7600 F microscope at 5 kV. X-raydiffraction (XRD) analysis was performed using an X’Pert PROPANalytical X-ray diffractometer (Cu Ka, 40 kV, 35 mA). Finally,the size of the composite particles dispersed in water (20 mg/mL)

micrograph and (C) field emission electron micrograph showing the core–shell

rization method. The insets show the corresponding histogram size distribution of

-20000 -15000 -10000 -5000 0 5000 10000 15000 20000

-10

-5

0

5

10

Applied Field (Oe)

Mag

netiz

atio

n (e

mu

g-1)

(ii)

composite particles. (C) Photographs of the MNP-NIPAAm composite particles

xternal neodymium magnet.

A. Khan et al. / Materials Letters 89 (2012) 12–1514

was determined by Malvern Nano ZS dynamic light scattering(DLS) from 20 to 50 1C.

Temp

Temp

20 25 30 35 40 45 50

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Temperature (°C)

NIPAAm NIPAAm-MNPs

Fig. 3. (A) Swelling curves of the PNIPAAm hydrogel particles with or without

MNPs (dashed and solid lines are provided to guide the eyes). (B) Sketch of the

temperature-dependent swelling behavior of NIPAAm hydrogel particles with

MNP cores. At low temperatures, water is a good solvent for the polymer made of

NIPAAm, and therefore, the particles are swollen. High temperatures cause a

collapse of the system. Water becomes a poor solvent for NIPAAm at temperatures

higher than 32 1C, and this leads to a decrease in size.

3. Results and discussion

Fig. 1A shows a TEM image of the MNPs synthesized by thethermal-decomposition method using oleic acid as a cappingagent. The size of each particle is approximately 9 nm, and theyhave a spherical shape. To observe the morphology of the MNP-PNIPAAm composite particles prepared by the miniemulsionpolymerization method, a drop of colloidal solution was drop-cast on a carbon-coated Cu grid, air dried and observed underFESEM (Fig. 1C), followed by TEM. Fig. 1C reveals that clusters ofMNPs are surrounded by a layer of polymer. The TEM image(Fig. 1B) of the same sample also clearly shows the formationof spherically shaped aggregated MNP cores (darker spots)surrounded by a grayish layer of polymer-shell�15 nm thick.From the above electron microscopy images, the results indicatethat the magnetic particles-polymer with core–shell structureswere successfully prepared via the miniemulsion polymerizationmethod.

Fig. 2A presents the XRD patterns of the prepared MNP-PNIPAAmcomposite sample. The diffraction peaks at 2y¼30.11, 35.41, 42.91,57.51 and 62.71 can be assigned to the (220), (311), (400), (511) and(440) planes of iron oxide, respectively. All the peak positions werebasically consistent with the standard data of the Fe3O4 structure(JCPDS 85-1436), and no other unexpected peaks were present. Theresults indicated that the MNP-polymer composite particles have ahighly crystalline cubic spinal structure.

The magnetic properties of the MNP-PNIPAAm compositeparticles were also investigated on a vibrating sample magnet-ometer (VSM) system at room temperature. The hysteresis loop ofthe MNP-PNIPAAm composite shown in Fig. 2B reveals ferromag-netic behavior with a saturation magnetization (Ms) value of ca8.5 emu g�1, which is much lower than that for the pure MNPs[12]. As shown in Fig. 2B, the magnetic hysteresis loop of thePNIPAAm-coated MNP is an S-type curve through the coordinateorigin. It exhibits negligible coercivity, which proved the super-paramagnetic properties of the particles. The superparamagneticbehavior may prevent particle aggregation and enable theirapplication in biomedical and bioengineering fields. Fig. 2C illu-strated the PNIPAAm-coated MNPs dispersed in water and theseparation process when an external magnetic field was applied.In the absence of an external magnetic field, the colloidal solutionwas saffron yellow in color and homogenous (Fig. 2C (i)).When the external magnetic field was applied, the compositeparticles were enriched, leading to transparence of the dispersion(Fig. 2C (ii)). This process was reversible. Moreover, it should bementioned here that the aqueous colloidal solution containingMNPs was stable over long periods of time without any sign ofprecipitation when stored at room temperature.

It is well known that thermo-responsive polymers can changetheir physical or chemical properties around the LCSTs. NIPAAm is awater-soluble monomer whose polymer exhibits many fascinatingproperties [6]. Fig. 3A shows a plot of swelling behaviors ofPNIPAAm hydrogel with or without MNPs versus temperature. Thedecrease In the hydrodynamic diameter occurred as the tempera-ture increased across its LCST value of ca 33.7 1C for the compositesample dispersed I water. Below the LCST, the polymer remainedhydrophilic in aqueous solution, and the NIPAAm polymer chainsextended and swelled. When the temperature was higher than theLCST, the polymer became hydrophobic, and the chains of thepolymer gel collapsed on the MNP cores. A schematic diagramrepresenting the swelling and collapse of PNIPAAm-containingmagnetic nanoparticles, below and above the LCST, is shown in

Fig. 3B. From the above-mentioned experimental results, it isconcluded that the as-prepared composite particles are responsiveto both magnetic and temperatures that may be useful in biomedi-cal applications.

4. Conclusions

In conclusion, we developed a novel, simple method for thesynthesis of well-defined core-shell structures of magnetic coreswith a thermo-responsive polymer shell using the miniemulsionpolymerization method. The TEM images show aggregatedmagnetic particles of 4100 nm cores with a polymer shell�15 nmthick coating the magnetic core to form a core–shell structure. Theas-prepared magnetic particles swell and shrink in water below andabove their phase transition temperature, respectively. The resultingcomposites were highly stable over long periods of time withoutsigns of precipitation when stored at room temperature. Thus, themethod developed in this article represent a novel way to synthesizethermo-responsive hydrogel-MNPs that may be useful materials forbiomedical applications.

Acknowledgments

This work was financially supported by KACST through KingSaud University under the National Plan for Science and Technology(NPST), Grant no. 10-NAN1008-02.

A. Khan et al. / Materials Letters 89 (2012) 12–15 15

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