IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011 1647 Comparative Study of Biomimetic Iron Oxides Synthesized Using Microwave 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 heating being 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 narrow size 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 of the synthesized ferrofluids showed an increase in crystallinity for the microwave irradiated sample. All these structural rearrangements affected 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 fluidic properties have a variety of applications in biomedical sciences such as enzyme and protein immobilization, pharma- ceuticals, diagnostics, and controlled drug release [1]–[5]. We have synthesized aqueous magnetic nanofluids (nanocompos- ites) by chemical co-precipitation using poly (vinyl) alcohol (PVA) as the template, the advantage being, the control it exerts over the size and shape of the synthesized particles. This process mimics the natural “biomineralization” reaction and is, therefore, named a “biomimetic process.” PVA is a well-known synthetic polymeric material which is nontoxic, noncarcinogenic, and biocompatible. It has been used as a basic material for a variety of biomedical applications including contact lenses, skin replacement material, artificial muscle, and vocal cord reconstruction [6]. On the other hand, the microwave assisted organic synthesis has grown very rapidly since 1986, with the pioneering work by Gedye [7]. While heat or thermal energy will increase molecular kinetics and hasten molecular reactions, the rapid movement of molecules directly induced by the oscillating electromagnetic field will give rise to in- creased collision of molecules and, in turn, accelerate chemical reactions. Although the energy generated by microwaves is too small to alter covalent bonds, it does affect the integrity of noncovalent secondary bonding [8]. The main aim of this study was to increase magnetization of the PVA-iron-oxide nanocomposite with the help of microwave irradiation. The combination of templated synthesis and microwave irradiation has led to a dramatic increase in the saturation magnetization of 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; accepted December 06, 2010. Date of publication January 10, 2011; date of current version 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 online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2104418 about conformational changes of the polymer and probably induces a reduction in the complexity of the polymer matrix by rearrangement. A detailed structural comparative study reveals the 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 from Merck, USA. PVA (molecular ) was obtained from Acros. Disodium hydrogen phosphate (Na HPO ), potassium dihydrogen phosphate (KH PO ), potassium chloride (KCl), and sodium chloride (NaCl) was purchased from Merck. All the chemicals were of analytical grade with 99.99% purity and hence used without further purification. B. Sample Preparation Iron solution consisting of both ferrous and ferric ions in the ratio 1M: 2M in phosphate buffer saline (PBS) containing PVA was oxidized using liquor ammonia. Two sets of experiments were performed with the same starting solution. In one (Mi- crowave induced, MI), the solution, before addition of liquor ammonia was subjected to microwave irradiation in a domestic microwave oven (LG INTELLOWAVE MS—2342AE) oper- ating at a frequency of 490 MHz for 4 min followed by 3 min gap (7 min cycle); When the temperature of reactant mixture reached 40 C, liquor ammonia was added, stirred and subjected to additional 15 such cycles (experiments showed that 15 cycles were optimum to get the maximum iron oxide formation). The other containing the same iron solution, designated as (Without microwave induction WMI), was stirred for 24 h after which liquor ammonia was added at 40 C and further stirred for an- other 24 h. Both exhibited high steric stability with no phase separation post centrifugation (BL 165R) at 12000 rpm for 30 min and dialysis for 24 h. The supernatant, in both cases, was oven dried and used for further characterization. 0018-9464/$26.00 © 2011 IEEE