IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011 1647

Comparative Study of Biomimetic Iron Oxides Synthesized UsingMicrowave Induced and Conventional Method

Soumya Bhattacharya, Dhriti Mallik, and Suprabha Nayar

National Metallurgical Laboratory, Jamshedpur 831007, India

Aqueous ferrofluids having high steric stability were prepared biomimetically by chemical co-precipitation of iron salts in poly (vinyl)alcohol. Both conventional and microwave heating modes were used for the synthesis of the fluids; the bottleneck of conventional heatingbeing low saturation magnetization. The uniqueness of this work lies in the fact that for the same initial constituents, microwave irra-diation enhances saturation magnetization without compensating stability. Superparamagnetic iron oxide nanoparticles with a narrowsize distribution were formed, and structural investigations of the dried fluid revealed that microwave irradiation increased the poly-dispersity and the average particle size of the nanocomposites which led to a loss of long-range ordering. X-ray diffraction patterns ofthe synthesized ferrofluids showed an increase in crystallinity for the microwave irradiated sample. All these structural rearrangementsaffected the saturation magnetization (��) which more than doubled from 12.97 to 27.07 kAm � with microwave irradiation.

Index Terms—Crosslinking, hydrogen bonding, magnetization, microwave irradiation, poly (vinyl) alcohol, polydispersity, superpara-magnetic iron oxide nanoparticles.

I. INTRODUCTION

A QUEOUS ferrofluids with high magnetic and fluidicproperties have a variety of applications in biomedical

sciences such as enzyme and protein immobilization, pharma-ceuticals, diagnostics, and controlled drug release [1]–[5]. Wehave synthesized aqueous magnetic nanofluids (nanocompos-ites) by chemical co-precipitation using poly (vinyl) alcohol(PVA) as the template, the advantage being, the control itexerts over the size and shape of the synthesized particles.This process mimics the natural “biomineralization” reactionand is, therefore, named a “biomimetic process.” PVA is awell-known synthetic polymeric material which is nontoxic,noncarcinogenic, and biocompatible. It has been used as a basicmaterial for a variety of biomedical applications includingcontact lenses, skin replacement material, artificial muscle, andvocal cord reconstruction [6]. On the other hand, the microwaveassisted organic synthesis has grown very rapidly since 1986,with the pioneering work by Gedye [7]. While heat or thermalenergy will increase molecular kinetics and hasten molecularreactions, the rapid movement of molecules directly inducedby the oscillating electromagnetic field will give rise to in-creased collision of molecules and, in turn, accelerate chemicalreactions. Although the energy generated by microwaves istoo small to alter covalent bonds, it does affect the integrityof noncovalent secondary bonding [8]. The main aim of thisstudy was to increase magnetization of the PVA-iron-oxidenanocomposite with the help of microwave irradiation. Thecombination of templated synthesis and microwave irradiationhas led to a dramatic increase in the saturation magnetizationof the microwave irradiated sample as compared to the con-ventional one. It is possible that microwave heating brings

Manuscript received May 12, 2010; revised August 17, 2010; acceptedDecember 06, 2010. Date of publication January 10, 2011; date of currentversion May 25, 2011. Corresponding author: S. Bhattacharya (e-mail:sbhatt@nmlindia.org).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2104418

about conformational changes of the polymer and probablyinduces a reduction in the complexity of the polymer matrix byrearrangement. A detailed structural comparative study revealsthe details.

II. EXPERIMENTAL SETUP

A. Materials

Iron (III) chloride (FeCl .H O) and iron (II) chloride (FeCl )was obtained from Rankem. 30% w/w liquor ammonia was fromMerck, USA. PVA (molecular ) was obtained fromAcros. Disodium hydrogen phosphate (Na HPO ), potassiumdihydrogen phosphate (KH PO ), potassium chloride (KCl),and sodium chloride (NaCl) was purchased from Merck. Allthe chemicals were of analytical grade with 99.99% purity andhence used without further purification.

B. Sample Preparation

Iron solution consisting of both ferrous and ferric ions in theratio 1M: 2M in phosphate buffer saline (PBS) containing PVAwas oxidized using liquor ammonia. Two sets of experimentswere performed with the same starting solution. In one (Mi-crowave induced, MI), the solution, before addition of liquorammonia was subjected to microwave irradiation in a domesticmicrowave oven (LG INTELLOWAVE MS—2342AE) oper-ating at a frequency of 490 MHz for 4 min followed by 3 mingap (7 min cycle); When the temperature of reactant mixturereached 40 C, liquor ammonia was added, stirred and subjectedto additional 15 such cycles (experiments showed that 15 cycleswere optimum to get the maximum iron oxide formation). Theother containing the same iron solution, designated as (Withoutmicrowave induction WMI), was stirred for 24 h after whichliquor ammonia was added at 40 C and further stirred for an-other 24 h. Both exhibited high steric stability with no phaseseparation post centrifugation (BL 165R) at 12000 rpm for 30min and dialysis for 24 h. The supernatant, in both cases, wasoven dried and used for further characterization.

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1648 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011

Fig. 1. TEM micrographs of (a) templated WMI with smaller particle size and(b) structurally rearranged MI with bigger particle size.

III. CHARACTERIZATION

X-ray diffraction (XRD) patterns were recorded withSiemens (D500) diffractometer using Cu K radiation(1.5418 ) source at a scan rate of one degree per minutewithin a range of 20 –80 . Fourier transform infrared spec-troscopy (FTIR) of the samples was done using NICOLET 759spectrometer. Scans were recorded in the range 4000 cm and400 cm using potassium bromide pellet method. Magneticmeasurements were done using the vibrating sample magne-tometer (VSM LAKESHORE 7040) in an applied magneticfield range of Tesla. Microstructures of the samplesevaluated using transmission electron microscope (JEOL-210).X-ray photo electron spectroscopy (XPS) measurements weredone in X-ray photoelectron spectrometer (M/s SPECS, Ger-many) with an Mg K X-ray source (Energy eV).The instrument was operated in the twin anode mode at a pres-sure of Torr in the analysis chamber. The pass energyfor both the samples was 80 eV. Full width at half maximum(FWHM) for both the samples was adjusted within the range1.675 eV–1.908 eV. The binding energies were calculated withreference to the maximum intensity of the C1s peak at 285 eV.

IV. RESULTS AND DISCUSSION

The same initial chemical composition of the reactantscan lead to different properties of the synthesized productdepending on the synthesis path followed. The followingexperimental analysis will compare microwave irradiationversus conventional heating of the oven dried PVA-iron-oxidenanofluids. In the beginning it is important to state that theoptical absorbance of pure PVA is at 202 nm and that of themicrowave-irradiated one at the same conditions is 211 nmindicating changes in the polymer conformation.

A. TEM

From the TEM micrographs, Fig. 1(a) and (b) we can clearlysee that the mean diameter of the nanocomposite MI (11.5 nm)

Fig. 2. XRD patterns of WMI and MI showing a slightly larger crystallite sizewith microwave irradiation.

Fig. 3. Comparative FTIR spectra of PVA, WMI, and MI showing substantialchanges in the polymer.

was larger than WMI (8.5 nm). The particle size distributionsreveal the formation of bigger particles in the microwave-irra-diated sample. The WMI has a more uniform morphology com-prising monodisperse iron oxide nanoparticles, in comparisonto MI possibly as a result of denaturation of PVA due to mi-crowave irradiation. The micrographs clearly show that expo-sure to microwave radiation resulted in the formation of polydis-perse nanostructures in MI, the distinctive feature that differen-tiates the two routes is the absence of long range ordering in MIsample compared to self-assembling long chains of nanoparti-cles in the WMI sample. The dark spots in both the micrographsmay be due to the formation of multi-core particles, a possibilityreported earlier by Fahlvik et al. and Qiu et al. [9], [10]. Increase

BHATTACHARYA et al.: COMPARATIVE STUDY OF BIOMIMETIC IRON OXIDES 1649

Fig. 4. XPS graphs of WMI showing the peaks of importance in the nanocomposite (a) Fe2p, (b) C1s, and (c) O1s peaks.

of core size increases particle aggregation due to enhanced mag-netic interactions. The TEM results can be co-related with theFT-IR results which indicate the effect of microwave irradiationon the polymer structure.

B. XRD

Fig. 2 shows the XRD patterns of the two samples. Braggreflections from certain characteristic planes prove that thenanocomposites primarily comprise of a mixture of magnetite(Fe O ) and maghemite ( -Fe O ) phases. The absence of cer-tain peaks corresponding to the crystal planes in the MI samplecompared to WMI indicated a preference for certain planes or atextured growth. Based on the quantitative estimation of phasesby X-ray diffraction [11], the percentage of magnetite andmaghemite phases formed in both the samples was calculated.It was seen that magnetite formation was higher in MI (59%)compared to WMI (51%). Prolonged heating by microwavesincreased the crystallinity of the sample considerably [12]–[14].The degree of crystallinity was approximately 28% in MI and21% in WMI. The crystallite size was calculated using FWHMin the Debye–Scherrer equation [14], with the shape factor K(0.9), considering the most intense 311 magnetite peak. The

calculated values of the mean crystallite size for magnetite wereequal to 9.43 nm in MI and 8.01 nm in WMI, respectively.

C. FTIR

A comparison between the FT-IR spectra of WMI, MI, andpure PVA highlight several differences Fig. 3. The intense peakat 475 cm in MI may be attributed to Fe-O stretching ofmagnetite and maghemite as a result of esterification. Absenceof such peaks in WMI confirms the formation of this peak tobe a result of microwave irradiation. The peaks at 499 cm ,588 cm in WMI and 675 cm in MI are the correspondingFe-O stretching modes of magnetite and maghemite [15], [16].The peaks at 844 cm and 847 cm observed in both sam-ples correspond to vinyl ethers, in phase C-0-C stretch. Pertur-bations in the O-H bending region around 1640 cm were ev-ident in both samples. The peak red shifted to 1630 cm inWMI as a result of iron oxide cross-linking, previous investiga-tions by M. N. Nadagouda et al. have showed that iron ions co-ordinate with PVA forming a polymer-metal crosslinked com-plex [17]. Temperature being an important factor during syn-thesis, we have observed that cross linking of chains in purePVA which normally occurs at elevated temperatures around

1650 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011

Fig. 5. XPS graphs of MI showing the peaks of importance in the nanocomposite (a) Fe2p, (b) C1s, and (c) O1s peaks.

150 C seems to occur at a much lower temperature of 60 Cdue to the presence of iron [18], [19]. The peak however, blueshifted to 1653 cm in MI due to an increase in the electron richgroups resulting in cage-like structures [20]. The peak at 1706cm is seen in MI due to esterification as a result of microwaveheating [21], inducing dehydration of polymer chains formingether bridges through C-O-C bond [22]. MI showed a narrowand sharp peak at 1093 cm corresponding to C-O-C stretchingvibration. WMI also exhibited similar behavior around the sameregion but the peaks at 1030 cm and 1110 cm were broadwith an intensity much lower compared to MI. With respect tothe absorption band of pure PVA at 1124 cm a red shift wasdetected in both cases, proving that cross linking of PVA chainshad occurred, the effect being more pronounced in MI. Absorp-tion bands at 1400 cm in WMI and 1409 cm in MI signi-fies CH bending vibrations while the peaks at 2921 cm and2930 cm arise due to asymmetric stretching of CH bonds alla combined result of iron oxide binding and microwave irradia-tion effects which results in structural rearrangement. The char-acteristic peak of OH stretching at 3040 cm and 3430 cm

in WMI also merged into a broad peak at 3396 cm in MIsample.

D. XPS

Since the samples contained in situ synthesized iron oxidesin PVA, we have focused on the regions showing iron (Fe2p),carbon (C1s), and oxygen (O1s) peaks in the XPS spectra inFigs. 4 and 5. The C1s peak of MI de-convoluted into four peaksat the binding energies 284.69 eV, 287.10 eV, 288.86 eV, and290.87 eV. The first peak though very weak may be associatedwith C-C and C-H groups. The remaining three peaks may beassigned to CH and C-OH groups [17], [23] the most promi-nent being 287.10 eV. Similarly the C1s peak of WMI con-sisted of five peaks at the binding energies 286.30 eV, 287.16eV, 288.09 eV, 289.46 eV, and 290.95 eV respectively, the mostprominent peak in this sample at 287.16 eV. The presence ofpeaks within the range 288.7 eV–287.15 eV indicates the pos-sibility of and COOR formation due to X-ray induceddegradation of C-OH groups in the polymer [23], [24]. Com-paring with the reference C1s peak at 285 eV peak shifts

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Fig. 6. Saturation magnetization curves of WMI and MI.

were detected in both the samples. Such a shift towards higherbinding energy might be due to the electron accepting ferrous/ferric ions present in the sample which decreases the electrondensity over the carbon atom [25]. Two weak but well definedpeaks were seen at 710.13 eV and 725.46 eV in the MI samplein agreement with the reported values of Fe 2p and Fe2p core level XPS peaks [14], [26], which in WMI appearedat 710.76 eV and 724.96 eV. In addition to these main peaksboth samples showed satellite peaks of lower binding energyabout 8 eV from the respective Fe 2p line and the peaksoccurred at 702.58 eV for WMI and 702.16 eV for MI, respec-tively. Both the samples showed O 1s peaks though the inten-sity of the WMI sample was much higher than the MI sampleindicating more secondary bond formations in MI. Deconvolu-tion of O1s peaks around the binding energy 530 eV showedmultiplet formation which distinguished between oxygen atomsbound to the iron and those bound to carbon. Prominent peaksat 532.76 eV for WMI and 532.72 eV for MI may be attributedto the O-H bond of H O absorbed by Fe O [27], [28].

E. VSM

When the concentration of the magnetic particles in the syn-thesized aqueous ferrofluid is not very high, the interparticlemagnetic interaction can be neglected. The magnetic particlescan then be treated as small thermally agitated permanent mag-nets in the carrier liquid. Ferrofluids differ from other para-magnetic salt solutions in that the magnetic moment is not ofa single molecule but of the whole system. From the magne-tization curves in Fig. 6, the initial susceptibility and sat-uration magnetization have been calculated to be 0.559,

Fig. 7. Magnetization versus 1/H (Gauss ) graphs of WMI and MI.

27.07 kAm and 0.260, 12.97 kAm for MI and WMI. Thecoefficient was obtained from Fig. 7(a) and (b) M versus 1/Hcurves with values (A/m) and (A/m)for WMI and MI, respectively. The retentivity and coercivityvalues of both the samples are low and the absence of hys-teresis proves that the samples are superparamagnetic in nature.The effect of microwaves on PVA is selective interaction withpolar groups; this provides effective heating of the polymer ascompared to conventional heating which induces intermolecularetherification of PVA, which increases its resistance to waterdue to formation of ether bridges. This could be leading to the

1652 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011

more controlled particle size and well shielded magnetic parti-cles in the conventional synthesis as compared to microwave in-duced one. Hence the steep rise in for the microwave heatedsample (probably due to less shielding), more than twice thevalue achieved by conventional heating, is an important findingfor all biomedical applications [29]. The XRD results also revealthat the crystallinity in MI was higher than WMI which can alsobe a possible cause for the high magnetic property of MI.

V. CONCLUSION

In our experiments, PVA has been used both as a template andas a surfactant forming possibly a mesh like or web like structurein which the magnetic nanoparticles are trapped or embeddedhindering their free mobility. All structural investigations con-clude that there is a structural rearrangement of PVA as a re-sult of microwave irradiation that induces stress and strain inthe polymer backbone as evidenced by FTIR peak shifts and in-tensity variation in XPS. This results in increased crystallite sizeand particle morphology and almost doubling of magnetizationof the composite without affecting its stability, something of im-mense importance for all biomedical applications.

ACKNOWLEDGMENT

The authors gratefully acknowledge the role of Councilof Scientific and Industrial Research (CSIR), Governmentof India, in providing financial assistance for this researchwork through the project “Nanostructured Advance Materials”NWP-0051 and Mr. A. Guha for editing of figures.

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