Study of Preparation and Properties on Polymer-modified Magnetite Nanoparticles

In this paper, polyacrylamide (PAM)-modified magnetite (Fe3O4) nanoparticles were prepared by in situ polymerization in aqueous solution. The particle size, morphology, crystal phase and magnetic properties were measured utilizing scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. The size distribution and stability of the nanoparticles in aqueous solution were evaluated using the laser particle size analyzer and ultraviolet-visible spectroscopy (UV-vis). The influences of the dose of acrylamide (AM) and the pH value on the particle size and stability were also examined. The results showed that the Fe3O4 nanoparticles possessed superparamagnetic property and super dispersion stability in aqueous solution after PAM modification.


Introduction
2][3][4][5] Amongst various magnetic nanoparticles, Fe 3 O 4 nanoparticles have been recognized as a promising candidate for its good biocompatibility, strong superparamagnetism, low toxicity and easy preparation process. 6,7However, pure Fe 3 O 4 nanoparticles are likely to aggregate for their large specific surface area and strong magnetic dipole-dipole attractions between nanoparticles, resulting in aggregation in aqueous solution and the change of magnetic properties which should be avoided in applications. 8o avoid the aggregation of pure Fe 3 O 4 nanoparticles during the application processes as mentioned above, modification on the surface of Fe 3 O 4 nanoparticles with a surfactant or hydrophilic polymer is one of the most effective methods. 9,10Furthermore, another significant function of the modification on the nanoparitcles is that the polymer or surfactant provides chemical groups for further grafting to satisfy further applications. 11any researchers have discussed the modification of magnetic nanoparitcles.Yang et al. reported that the iron oxide nanoparticles were modified by various poly(amino acid)s for use as magnetic resonance probes. 12Yu et al. investigated the hydroxypropyl-bcyclodextrin/polyethylene glycol 400 modified on Fe 3 O 4 nanoparticles for congo red removal. 13Cui et al. studied perfluoropolyether carboxylic acid surfactant modified Fe 3 O 4 magnetic nanoparticles and the modified layer could withstand high temperature. 14Wang et al. presented modified magnetic nanoparticles using 3-aminopropyltriethoxy silane and subsequently activated by glutaraldehyde and then proteins were immobi-lized on the activated nanoparticles. 15Several other materials were also reported on the modification of magnetic nanoparticles.][18] Oleic acid was used to modify the surface of magnetite nanoparticles. 19,20PEG/PVA and Poly(4-MS-DVB-GMA) matrix grafted with poly(amidoamine) PAMAM dendrimer were also suggested to modify magnetic nanoparticles. 21,22owever, few studies on the modification of polyacrylamide (PAM) on the surface of magnetite nanoparticles appear in literature.PAM can be utilized to protect the particles' original properties and stabilize them in aqueous solution.In addition, this kind of magnetic composite can be moved effectively and simply by using applied magnetic field.4][25][26] The Fe 3 O 4 nanoparticles filler was encapsulated before and during the synthesis of the polymer. 27he particle size distribution and the morphology of PAMmodified Fe 3 O 4 nanoparticles were investigated.Other properties, such as crystal phase, magnetic properties and the mass lost value in TGA were also determined.Most importantly, the z-potential and stability of the PAM-modified nanoparticles in aqueous solution were illustrated.

Materials
The chemicals used in this work were all analytical reagents.Ferric chloride (FeCl

Preparation of Fe 3 O 4 Nanoparticles
3.25 g of FeCl 3 and 3.38 g of FeCl 2 •10H 2 O were successively dissolved in the 25 mL of deoxygenated water, which was obtained by bubbling nitrogen gas for 15 min, and then the solution was stirred and filtrated.The resulted solution was added dropwise into 200 mL of 1 M NaOH solution under vigorous stirring (600 r min -1 ) under N 2 atmosphere, followed by the generation of Fe 3 O 4 precipitate.
The total precipitate was isolated using a magnetic field, and the supernatant was removed by decantation.Then the precipitate was washed using purified deoxygenated water for several times with the same method, until no more Cl -was detected upon addition of Ag + .Finally, purified deoxygenated water was added to 1.7 g Fe 3 O 4 precipitate until the total volume was 100 mL to form the required solution.

Preparation of Polymer-modified Magnetite Nanoparticles
The polymer modified magnetite nanoparticles were formed by bottom-up approach. 27First, another 100 mL purified water was added to the Fe 3 O 4 suspension prepared previously, to give 200 ml solution.Subsequently, 0.02 mol % of K 2 S 2 O 8 was added as initiator.After 10 minutes of ultrasonic vibration, AM (34 % by weight) solution was added dropwise into the above 200 mL of mixed solution under stirring (600 r min -1 ) and ultrasonic vibration during the whole process.The solution was kept under N 2 atmosphere during the whole process.
Three kinds of products with different ratio of nanoparticles to AM were prepared, namely, PAM+10%Fe 3 O 4 , PAM+ 20%Fe 3 O 4 , PAM+50%Fe 3 O 4 .The pure Fe 3 O 4 nanoparticles were also prepared for contrast.After polymerization, the samples PAM+10%Fe 3 O 4 and PAM+20%Fe 3 O 4 were semi-liquid jelly and dried in electric blast drying oven.For sample PAM+50% Fe 3 O 4 , the product was liquid, it was lyophilized.All the dried samples could be dispersed into water to form a homogeneous, transparent colloid.The results are summarized in Table 1.

Characterization
The sample phases and particle sizes were determined by X-ray diffraction (XRD) (Rigaku-D/max-2500, Japan).The morphology of the Fe 3 O 4 Particles, before and after coated by PAM, were measured via micrographs obtained by scanning Electron Microscopy (SEM) (JSM-6700 JEOL, Japan) and transmission electron microscopy (TEM) (TECNAI-12, Philip Apparatus Co., USA).The size distribution and the stability of the PAM modified nanoparticles in aqueous solution were measured with a laser particle size analyzer (Horiba LB-550, Japan) and ultravioletvisible spectroscopy (UV-vis) (UV2102, UNICO USA), respectively.
The infrared measurements in the 4000-400 cm -1 range on powder specimens dispersed on a pressed KBr disk, using a Fourier-transform infrared spectroscopy (FTIR) (Tensor27, Bruker Germany).The magnetic properties were carried out using a vibrating sample magnetometer (VSM) (MPMSXL, Quantum Design USA).The composition particles were also determined with thermogravimetric analysis (TGA) (Pyris 1, PerkinElmer USA).

Electron Microscopy
Figure 1 shows SEM and TEM images of the Fe 3 O 4 particles before and after modification.The pure Fe 3 O 4 particles are nearly spherical and have uniform morphology with the diameter about 10 nm.After modification, the particles became larger and the size varies from 30 to 100 nm.Particularly, the TEM image (b-2) shows that the particle is separated from its neighbours and the boundary is very clear.

X-ray Diffraction
Diffraction patterns of the samples (pure Fe  4,28,29 The average particle size calculated from the Debye-Scherrer formula is 9 nm, which is consistent with SEM/TEM results.
Compared to the pure Fe 3 O 4 , the characteristic peaks of the PAM-modified Fe 3 O 4 particles are weak, but the position does not shift, which indicate that the PAM modified on the surface of the Fe 3 O 4 particles does not change the size and crystal phase of the Fe 3 O 4 .

IR Results
The IR spectrum of the four samples is shown in Fig. 3.The peak near 574 cm -1 of the pure Fe 3 O 4 curve belongs to the characteristic absorption band of the Fe-O bond, 4,7,30 and the absorption bands near 1632 cm -1 and 3418 cm -1 refers to the O-H stretching mode and H-O-H bending mode, indicating the presence of interstitial water in the samples. 7,28n the absorption curves of PAM-modified Fe 3 O 4 nanoparticles, the 3450 cm -1 peak belongs to H-O bond vibrations of water, 3,28 and 3139 cm -1 peak belongs to N-H stretching vibrations. 232783-2986 cm -1 can be assigned to the stretch vibrations of the C-H bonds which originates from the PAM modified on the surface of the particles. 3,28The absorption bands near 1655 cm -1 shows common characteristic of (NH 2 )C=O 23 and 1420 cm -1 represents the stretching vibration of the CN group. 30n addition, the characteristic absorption band of Fe-O (around 580 cm -1 ) still can be found in the PAM modified Fe 3 O 4 nanoparticles. 28,30Both of the absorption peaks from the PAM and Fe 3 O 4 indicates that the magnetite nanoparticles have been well modified.

Magnetic Measurement
In order to investigate the magnetic properties of the samples, VSM was employed.Figure 4  coercivity were not observed in magnetization curves, indicating that all the samples show typical superparamagnetic behaviour. 8he superparamagnetic behaviour also indicates that the particles' size range below 20 nm, 31 which is consistent with the morphological characteristics from the SEM/TEM in Fig. 1.The saturation magnetization was reached at a field of approximately 50 000 Oe. Figure 4 shows that the saturation measure-ments of the pure and PAM-modified Fe 3 O 4 samples are approximately 53.5, 20.7, 7.9, 4.3 emu g -1 , respectively.The saturation magnetization of the nanoparticles is significantly smaller than that of bulk magnetite which is 84 emu g -1 . 8It is believed that the decrease of measured saturation magnetization can be attributed to the reduction of size and the PAM coating.

TGA
Four dried samples were subjected to TGA in the 30-650 °C range under an N 2 atmosphere (shown in Fig. 5).The pure Fe 3 O 4 lost 13.3 % of its total weight, which is probably due to the absorbed water from the environment. 8The PAM-modified samples show that there are three stages of mass loss for the thermal degradation of PAM.The first stage involves a slight weight loss around 9 % at temperature below 250 °C.It can be attributed to the presence water absorbed in PAM-modified Fe 3 O 4 nanoparticles. 23The second stage appears in the range of 250-380 °C for the decomposition of pendant amide groups and the intactness of polymer main chains. 23The observed values of weight loss are around 24.5 %, 34.5 %, and 39.7 % for the three samples, respectively.The third stage appears in the range of 380-510 °C with 8 %, 10 %, and 11.5 % of weight loss for the different samples, respectively, due to the breakdown of the polymer backbones. 23

The Aggregation of PAM-modified Nanoparticles in Aqueous at Different pH Values
The aggregation variety of the nanoparticles dispersed in water under different pH values are shown in Fig. 6.For pure Fe 3 O 4 nanoparticles, at low pH value, the particles size is about 150 nm.As the pH rises, the aggregation becomes significant, for the size of the aggregate becomes larger and the maxi-mum is up to 550 nm and after which the aggregation decreases as the pH value continues to rise.For the polymer-modified Fe 3 O 4 nanoparticles, however, the slope of the aggregation curve is much more flat which means that polymer modified on the surface of Fe 3 O 4 nanoparticles can effectively decrease their sensitiveness to pH.In order determine the principles involved for this phenomenon, the z--potentials of samples at different pH values were studied.
Figure 7 shows the relationship between the pH and the z-potential of pure and PAM-modified Fe 3 O 4 nanoparticles.For pure Fe 3 O 4 nanoparticles, at low pH value, the surface charge of the particles is initially positive (more than 40 mV) at pH~4.As a result, the particles aggregate less (shown in Fig. 6) due to the repulsive Coulombic force.At an intermediate pH value near the isoelectric point (IEP), where the surface charge density of particles is very low, the aggregation of magnetite particles becomes significant (shown in Fig. 6) due to the attractiveness of Van der Waals force. 32,33At higher pH value far from the IEP, the overall charge is reversed and the repulsive Coulombic interactions among negative charged particles could again minimize the aggregation.So the z-potential of nanoparticle plays a vital role in aggregation of nanoparticles.But after modification of the particles with PAM, Fig. 7 shows that the nanoparticles' z-potential distribution is quite narrow which indicating that the z-potential of nanoparticles is not sensitive to the pH value.Aggregation of PAM-modified Fe 3 O 4 nanoparticles at different pH is not as significant as the case for the pure Fe 3 O 4 (shown in Fig. 6).

The Stability of the PAM-modified Nanoparticles in Water
The suspension stability in water of the pure and PAMmodified Fe 3 O 4 was also studied using UV-vis measurement, as the aggregation and deposition rates can be reflected by the intension of UV-vis absorbance. 31,34Figure 8 shows the absorbance intensity of the four samples, which were measured at different retention times.The absorbance of the pure Fe 3 O 4 nanoparticles decreased sharply in the first hour and nearly becomes zero after 72 hours, which indicated that the pure Fe 3 O 4 is not stable in the water and prone to aggregation and precipitation due high electromagnetic attractive forces between pure Fe 3 O 4 nanoparticles.But after modification by the PAM, the absorbance intensity decreased only slightly after 72 hours, which indicates that PAM largely improves the stability of the nanoparticles in water.It can be attributed to the water-soluble PAM molecular chains that surround the particles and form a hydration shell to prevent them from aggregation and precipitation.

Conclusions
In this paper, the PAM modified superparamagnetic Fe 3 O 4 nanoparticles were prepared.SEM shows that the single particle size is about 10-25 nm.XRD spectrum indicates that the produced nanoparticles are consistent with the standard pattern for Fe 3 O 4 .In addition, the FTIR spectrum demonstrated that PAM completely coated the surface of Fe 3 O 4 nanoparticles.The magnetic measurement proves that the nanoparticles show superparamagnetic property.The absorbance intensity of UV-vis implies that the suspension stability of the nanoparticles in aqueous solution can be remarkably improved after being modified by PAM.

Figure 2
Figure 2 XRD patterns of pure and PAM-modified Fe 3 O 4 nanoparticles.Figure 3 FTIR spectra of pure and PAM-modified Fe 3 O 4 nanoparticles.

Figure 3
Figure 2 XRD patterns of pure and PAM-modified Fe 3 O 4 nanoparticles.Figure 3 FTIR spectra of pure and PAM-modified Fe 3 O 4 nanoparticles.

Figure 5
Figure 5 TG curves of pure Fe 3 O 4 and PAM-modified Fe 3 O 4 nanoparticles.

Figure 6
Figure 6The influence of pH value on the size distribution of the nanaoparticles.

Figure 7
Figure 7The z-potentials of the suspension at different pHs.

Figure 4
Figure 4 Magnetization curves of pure and PAM-modified Fe 3 O 4 nanoparticles.