INTRODUCTION:

Nanoparticles are of great scientific interest because they represent a bridge between bulk materials and molecules and structures at an atomic level. With the rapid development of nanostructured materials and nanotechnology in the field of biotechnology, iron oxide (Fe₃O₄), in particular, has received considerable attention^1,4. Iron oxide nanoparticles are iron oxide particles with diameters between about 1 and 100 nanometres. They have attracted extensive interest due to their super paramagnetic properties and their potential applications in many fields.

At the nanoscale, particle-particle interactions are either dominated by weak van der Waals forces, stronger polar and electrostatic interactions or covalent interactions. Depending on the viscosity and polarisability of the fluid, particle aggregation is determined by the interparticle interaction. By the modification of the surface layer, the tendency of the colloid to coagulate can be enhanced or hindered.

Physical methods of synthesis of MNPs are usually elaborate and suffer from the inability to control the size of particles in the nanometer size range practically. Chemical methods are highly preferred because of its simplicity and convenient use for large scale production. Many chemical methods, such as co-precipitation, micro emulsion, thermal decomposition, and hydrothermal synthesis, have been applied and reviewed for the production of these magnetic nanoparticles. Among them, co-precipitation is a facile and convenient method in which the iron oxides from Fe2+/Fe3+ salt solutions in the presence of a base⁵. The nanoparticles are difficult to separate from the solution but using the magnetic property is a good solution to this problem⁹. Hence MNPs are easy to recover by a magnetic field.

Surface modification involves the functionalization of these magnetic nanoparticles by coating the surface with compounds such as aminosilanes, chitosan and polymer like silica. Every chemical modification process involves the reaction of one functional group with another, resulting in the formation of covalent bonds; APTES is one of the coupling agents that are used in the surface modification of magnetic nanoparticles.

Chitosan is an amine polysaccharide obtained from alkaline deacetylation of chitin. Amino groups that are found naturally in chitosan make it a cationic polyelectrolyte. Since the magnetic Fe₃O₄nanoparticles tend to aggregate in liquid media due to the strong magnetic dipole–dipole attractions between particles, some biocompatible and biodegradable polymers with speciﬁc functional groups have been used as stabilizer to modify and increase their stability⁶. Silica is the most prominent material for coating because it is chemically inert and, therefore, does not affect the redox reaction at the core surface.

MATERIALS AND METHODS:

Materials:

Ferric chloride hexa hydrate and ferrous chloride tetra hydrate, ammonia solution (29.6%), acetic acid, were purchased from Thomas Baker Chemicals Pvt. Ltd., Mumbai, India. APTES (3-aminopropyl triethoxy silane, 98%) and TEOS (tetra ethyl orthosilicate, 98%) were purchased from Alfa Aesar, Hyderabad, India. Chitosan (extrapure) was purchased from Sisco Research Laboratories Pvt. Ltd, Mumbai, India. Sodiumtripolyphosphate (TPP) was obtained from Merck Specialities Private Limited, Mumbai.

Methods:

Synthesis of magnetic Fe₃O₄ nanoparticles:

The magnetic Fe₃O₄ nanoparticles were prepared by co-precipitation method⁷. FeCl₂.4H₂O and FeCl₃.6H₂O were taken in the molar ratio 1:2 in 100 ml of deionized water and 75 ml of NH₄OH solution was added under vigorous stirring in the presence of nitrogen to maintain inert atmosphere at pH 10. The suspension was heated at 80 °C for 30 mins, black precipitate was formed and it was then cooled to room temperature and the precipitate was magnetically decanted. The particles thus obtained were washed with water and ethanol. Finally, the Fe₃O₄ nanoparticles were dried in vacuum oven.

Surface modification of Fe₃O₄ NPs by APTES:

The magnetic nanoparticles prepared (~2 g) were dispersed in ethanol and sonicated. 1.3 ml of APTES was then added and again sonicated for about 10mins for uniform dispersion. This is then followed by shaking the sample overnight at room temperature and the magnetic decantation yields the APTES-bound nanoparticles. The particles were then washed with ethanol several times.

Surface modification of Fe₃O₄ NPs by chitosan:

The Fe₃O₄ nanoparticles were coated with chitosan by the following method⁸. 100ml of Chitosan solution in 1% acetate solution (4 mg/ml, pH 4) was prepared. The initially prepared magnetic Fe₃O₄nanoparticles and 50ml of sodium tripolyphosphate solution (0.5 mg/ml) were added simultaneously to the chitosan solution and the above mixture was ultrasonicated for about 30mins. The Fe₃O₄-CS nanoparticles were then recovered by magnetic decantation and washed with deionized water, freeze-dried.

Surface modification of Fe₃O₄ NPs by Silica:

Silica was coated on iron oxide NPs by following method³. The magnetic NPs were added in a flask containing 80ml of anhydrous ethanol and 20ml of deionized water followed by sonication. Ammonium hydroxide was added to adjust the pH to 9. During sonication, 200 µl of TEOS was added and then the suspension was stirred for 4 hours in the presence of N₂ gas. Then the precipitate was washed with deionized water and ethanol thrice (using permanent magnet). It is then stored in 20ml of anhydrous ethanol and 2ml of this solution was taken and centrifuged. The pellet was washed with toluene thrice and then the particles were suspended in 12ml of DMF and 8ml of toluene. 1000µl of APTES was added drop by drop and stirred for 24 hours with N₂ gas flow. With the use of permanent magnet, the particles were washed with toluene for four times and stored in 10ml of toluene.

Characterization of Fe₃O₄ nanoparticles:

The size and morphology of the nanoparticles were determined by Transmission Electron Microscopy (TEM) using Technai 10, Philips in the magnification of 100 nm. XRD measurement was performed using Philips X'pert Pro Materials Research Diffractometer (MRD) in the receiving slit operation mode with a single Cu Kα radiation (λ=0.154 nm) and the XRD patterns were recorded in high angles (10-70 degree). The binding of Chitosan and Silica onto the Fe₃O₄ nanoparticles were confirmed by spectra of Fourier Transform Infra Red spectroscopy (Perkin elmer spectrum RX 1) using the KBr pellet method in the range of 400-2400 cm^-1_..

Fig.1. Transmission electron micrograph and its size distribution analysis for (a) Naked Fe₃O₄, (b) Fe₃O₄-CS and (c) Fe₃O₄-Silica nanoparticles

Fig.2. XRay Diffraction pattern for (a) Naked Fe₃O₄, (b) Fe₃O₄-CS and (c) Fe₃O₄-Silica nanoparticles

Fig.3.3. FTIR spectrum of (a) Naked Fe₃O₄, (b) Fe₃O₄-CS and (c) Fe₃O₄-Silica nanoparticles

RESULTS AND DISCUSSION:

TEM analysis:

The morphologies of the magnetic nanoparticles were determined using the Transmission Electron Microscopy (TEM). It is clear that the average particle size is 11.07± 2.00 nm. The naked Fe₃O₄ nanoparticles seem to be aggregated due to its dipole-dipole interactions. After coating CS onto the MNPs, there was good dispersibility of magnetite particles.

XRD analysis:

In the XRD pattern obtained there are series of characteristic peaks occurred at 2Ө for all the nanoparticles. For all the nanoparticles, the most intense characteristic peak was found to be at 35.6°. With the XRD pattern, the average size of the particles can be calculated using the Debye-Scherrer formula (D=Kλ/βcosӨ) where K is the Scherrer constant (0.9), λ is the X-ray wavelength (0.154 nm), β is the peak full width at half maximum (FWHM), and Ө is the Bragg diffraction angle⁹. From the most intense peak with the corresponding FWHM, the average size of the particles was calculated to be 10.33 nm, 11.99nm and 11.15nm for Fe₃O₄, Fe₃O₄- CS and Fe₃O₄-silica respectively.

FT-IR analysis:

The naked and coated Fe₃O₄ nanoparticles were characterized by FT-IR spectroscopy. The FTIR spectra of naked nanoparticles and surface modified nanoparticles are shown in fig.3.3.The FTIR spectrum of naked Fe₃O₄ shows the characteristic absorption peak at 585.9cm^-1 (Fe-O). The characteristic frequency at 1626.5cm^-1 in the naked Fe₃O₄ may be due to N-H stretching of the amine functional group.

In the FTIR spectra of Fe₃O₄-CS nanoparticles, the characteristic absorption peak for naked Fe₃O₄was observed at 585.9cm^-1 (Fe-O). The characteristic absorption band appeared at 1651.3cm^-1 for the chitosan, corresponding to the bending vibration of N-H. In the FT-IR spectra of Fe₃O₄-CS, the absorption peak shifted to 1614.4cm^-1 compared with the spectra of CS and the absorption peak was observed at 575.8cm^-1 (Fe-O). The shift of absorption peak from 585.9 cm^-1 to 575.8 cm^-1 in the Fe₃O₄-CS nanoparticles reveals the interaction between Fe₃O₄ and chitosan. The spectra show that CS was bound successfully to the Fe₃O₄ nanoparticles

FTIR spectrum of Fe₃O₄-Silica nanoparticles shows the presence of Fe-O bond in the magnetic nanoparticles by the characteristic absorption peak at 598cm^-1 and the characteristic absorption of SiO₂ can be confirmed by the presence of peak at 799.8 cm^-1 and 1081.3 cm^- (due to symmetric and un-symmetric linear vibration of Si-O-Si bonding) and 460.5 cm^-1 . This may be an evidence for the formation of silica shell.

CONCLUSIONS:

The magnetic nanoparticles were synthesized and the surface modification was performed with polymers (Chitosan and Silica). The average diameter of the nanoparticles was determined to be 11.15nm by XRD analysis. The surface modification of the magnetic nanoparticles by various coating showed good dispersability and reduced the agglomeration. FTIR analysis showed the binding of Chitosan and silica onto the nanoparticles.

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Received on 26.08.2013 Accepted on 01.09.2013

Research J. Engineering and Tech. 4(4): Oct.-Dec., 2013 page 268-271