Author's Accepted Manuscript

Structural and Magnetic Investigation ofStyrene-Divinylbenzene Encapsulated IronOxide Nanoparticles

Anselmo Rodriguez, Fernando S.E.D.V. Faria,Renildo M. Cunha, Jose Coaquira, Judes G.Santos, Ricardo B. Azevedo, Marco Morales,Hory Mohammadpour, Fabricio M. Almeida,Eryvaldo S.T. Egito, Paulo Cesar Morais

PII: S0167-577X(14)00662-4DOI: http://dx.doi.org/10.1016/j.matlet.2014.04.085Reference: MLBLUE16823

To appear in: Materials Letters

Received date: 3 February 2014Accepted date: 17 April 2014

Cite this article as: Anselmo Rodriguez, Fernando S.E.D.V. Faria, Renildo M.Cunha, Jose Coaquira, Judes G. Santos, Ricardo B. Azevedo, Marco Morales,Hory Mohammadpour, Fabricio M. Almeida, Eryvaldo S.T. Egito, Paulo CesarMorais, Structural and Magnetic Investigation of Styrene-DivinylbenzeneEncapsulated Iron Oxide Nanoparticles, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2014.04.085

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Structural and Magnetic Investigation of Styrene-Divinylbenzene Encapsulated Iron Oxide Nanoparticles Anselmo Rodriguez1,*, Fernando S.E.D.V. Faria1, Renildo M. Cunha1, Jose Coaquira2, Judes G. Santos5, Ricardo B. Azevedo2, Marco Morales3, Hory Mohammadpour4, Fabricio M. Almeida5, Eryvaldo S.T. Egito6, and Paulo Cesar Morais2 1Centro de Ciências Biológicas e da Natureza, Universidade Federal do Acre, Rio Branco, 69915-900, Brazil 2Universidade de Brasília, Instituto de Física, Núcleo de Física Aplicada, Brasília DF 70910-900, Brazil 3Universidade Federal do Rio Grande do Norte, DFTE, Natal, RN, 59078-970, Brazil 4Department of Physics, Science and Research Branch, Islamic Azad University, Khouzestan, Iran 5Fundação Universidade Federal de Rondônia, Porto Velho, RO 76801-059, Brazil 6Universidade Federal do Rio Grande do Norte, Faculdade Farmácia, Natal, RN 59010-180, Brazil

Abstract

The magnetite nanoparticles were prepared in the mesoporous styrene-divinylbenzene template

by in situ chemical cyclic route. Precipitation of magnetite ocurred after adding NaOH into the

Fe aqueous solution. Each nanoparticle was prepared in a layer-by-layer manner, N chemical

synthesis cycles was applied. The crystalline quality and crystal phase were examined by X-ray

diffraction (XRD) measurements. The size distribution was obtained from transmission electron

microscopy (TEM). The particle size increased with the number of chemical cycles. Dynamic

properties assessed by AC susceptibility revealed a slowing of the magnetic relaxation of the

smaller particles due to dipolar interaction between nanoparticles. The AC data was well

described by the Vogel-Fulcher relationship. The magnetic parameters obtained from

magnetization hysteresis loops showed a dependence with N.

a) Author to whom correspondence should be addressed. E-mail: ruiz@ufac.br. �������

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2

I. INTRODUCTION Nanotechnology demands have fostered efforts towards the design and production of nanoscaled

components for incorporation into devices. Magnetite nanoparticles have attracted a great deal of

attention because of their unique physicochemical properties and great potential use in various

biomedical applications, such as contrast agents in magnetic resonance imaging (MRI), carriers

for targeted drug delivery, magnetic separation in microbiology and biochemical sensing.

There is a considerable interest in preparation of particles which can be manipulated in different

systems by external stimuli such as thermal, electric or magnetic field. Polymeric particles can be

easily prepared and their size, morphology and surface chemistry can be tuned. However,

conventional polymers cannot provide magnetic response properties. Therefore, incorporation of

magnetic nanoparticles in polymeric particles can be an interesting route for preparation of

hybrid particles which can provide this interesting feature.

Magnetic nanocomposites are an important class of functional materials, possessing unique

magnetic properties due to the reduced size ( D < 30 nm) of the magnetic component, with

potential for use in devices with reduced dimensions1-7. However, as far as the nanosize and

dispersivity control are concerned, there is a tendency of preventing isolated magnetic

nanostructures from aggregation into bigger clusters during in situ synthesis process4. The

aggregation is driven by particle-particle interaction and/or by reduction of the energy associated

to the high surface-to-volume ratio.

The magnetic polymer-based spheres have been considered as an important material for the

biotechnology industry, as for instance in magnetic resonance imaging4, DNA and RNA

isolation5 and gene and drug delivery6. In particular, mesoporous polymeric templates can be

produced as micron-sized spheres, thus allowing in situ chemical synthesis of nanosized

3

magnetic particles with tunable magnetic properties and mass density1. The fine-tuning of

composite physical parameters can be obtained by performing several chemical synthesis cycles.

In pharmaceutics, styrene-divinylbenzene (Sty-DVB) has diversified applications, the primary

among which is as carriers of cationic or anionic drugs6, for stem cell labeling and tracking7, as a

carrier for capturing of viruses8, among others. Aiming to develop a polymeric-magnetic drug

carrier system, we report on the microscopic, structural and magnetic investigation of magnetite

nanoparticles precipitated within mesoporous Sty-DVB microspheres.

II. EXPERIMENTAL The Sty-DVB copolymer used in this study was synthesized by suspension polymerization.9

Sulfonation of the Sty-DVB microspheres (200 �m average diameter) was performed using

concentrated sulfuric acid. The nanocomposite preparation uses a four step procedure. First, the

mixture containing ferrous aqueous solution and the porous micron-sized polymeric spheres was

stirred at room temperature. Second, the spheres are separated by filtration and washed with

water. Third, the precipitation of iron oxide nanoparticles in the template-incorporated ferrous

ions is performed in alkaline medium. Fourth, the obtained black composite was filtered, washed

with water until the pH of the eluent was neutral. The above-described procedure can be

performed N times to obtain the N-cycle composite samples with increasing amount of magnetic

material.

In this study, we will discuss results of the composite samples prepared by treating the

polymeric template with a 10 mmol/L ferrous solution, while using up to six chemical synthesis

cycles. From here on the samples will be labeled as 10MNC, where N=1, 2, 3, 4, 5, 6. The

structural characterization was carried out using a powder diffractometer from Rigaku that uses

a Cu-K radiation and TEM instrument from JEOL 100 CX-II system. Magnetic properties were

4

investigated using AC and DC magnetometers (Quantum Design PPMS 6000). The AC

susceptibility was carried out at frequencies from 10 Hz to 10 kHz, under an applied field of 2

Oe.

III. RESULTS AND DISCUSSION

Figure 1a presents the XRD patterns of the composite for different chemical cycles. The

patterns show reflections whose positions correspond to magnetite. The lattice parameter

obtained from the Rietveld refinements has a value of 8.365 Å, this value is close to the one

expected for magnetite (8.39 Å). The smaller value obtained in our samples could be related to

some Fe2+ oxidation. Sample 10M1C shows broad peak reflections while the peak broadening

decreases in samples with larger N. The tendency observed in the XRD peak’s width suggests

that the particle size increases as N increases3.

Fig. 1- (a) XRD patterns of all samples. Numbers close to peaks are the miller indices. (b) TEM micrograph of the sample with N=2. (c) The particle size histogram and the fit to the log-normal function.

5

The particle diameter, DXRD, was calculated by using the Scherrer relationship10 by using the

reflection peak (311) and without considering the possible contribution of crystal stress. The

particle size values are present in Table 1. The TEM image presented in Figure 1b reveals a

particle size distribution (see Figure 1c) which was well-represented by a log-normal function

(Equation 1):

,2

)/(lnexp2

1)( 20

2

���

����

��

�DD

DDP (1)

Where D0 is the median diameter and is the diameter-dispersity. For sample N=2 we found

D0= (21.8 � 0.7) nm and = (0.23 � 0.03) nm, revealing a reasonable agreement with the value

found in the XRD analysis.

To study the dynamics of the magnetic nanoparticles, AC susceptibility measurements were

carried out. Figure 2 presents the temperature dependence of the ’ and ” components,

recorded at different frequencies ( f ) for sample 10M5C. The in-phase component shows two

main features: First, a steady increase of the susceptibility signal is observed in the whole range

of temperature. Second, a broad shoulder is observed at around 40K which shifts towards higher

temperatures when the frequency is increased. Similar features were observed for all samples.

The out-of-phase component exhibits two main features: first, a sharp peak at 30 K and its

position shifts towards higher temperatures when the frequency increases (Figure 2b). Second,

the susceptibility signal increases in the whole range of temperatures.

6

Fig. 2. (a) The in-phase component as a function of the temperature (T). (b) Short range temperature of the out-of-phase component vs. T.

To characterize the relaxation process associated to the sharp peak at 30 K in the ” signal, we

used the frequency dependence of the ” peak temperature for selected samples. Attempts to

describe the ” peak data using the Arrhenius plot resulted in unphysical �o values of the order

of 10-13 s which is smaller than the characteristic relaxation time of �o = 10-9 s. Therefore, based

on the assumption of a cooperative particle interaction system11, the analysis of the ” peak

temperature was carried out using the Vogel-Fulcher relaxation process, which is described by

equation 2:

0 0 0ln( / ) ln( / )eff BT K V k T� � � � � (2)

Where T0 is a temperature–correction related to the interaction between nanoparticles, � =1/f,

KeffV is the magnetocrystalline energy barrier, Keff is the magnetic anisotropy constant which

ranges from 1.4 x 104 to 5.0 x 104 J/m3 for magnetite12,13 and T is the peak temperature. Table 1

shows the parameters obtained from the fits, the values of T0 indicates that nanoparticles in

samples with N > 3 interact more strongly that the sample with N = 3.

7

On the other hand, the increasing signal in ”and ’ susceptibility could be accounted for via

the coexistence of two distinct sets of nanoparticles: small particles, responsible for the peak at

~ 30 - 40 K, and large particles (D > 20nm) responsible for the steady increase in the signal up

to 300 K. From the TEM analysis, (Figure 1b), particles with sizes smaller than 10 nm is

observed and there are larger particles with D > 20 nm which would be magnetically blocked at

300 K. A particle size of 25 nm is requiered for superparamagnetic behavior11 at temperatures

above ~ 300 K. From the term KeffV/kB and Keff = 2.0 x 104 J/m3 the size of the smaller particles

(d=(6V/�)1/3) was calculated and it ranged from 3.9 to 4.2 nm. The T0 parameter shows that the

small particles are interacting with the larger ones.

As described above, each cycle uses a constant concentration of iron ions. In the first cycle,

particles of around 14.5 nm were formed. At any cycle, for N > 1, a large amount of iron ions is

used to increase the size of the preformed particles, and the exceeding fraction is used to form

smaller particles. The small intensity of the AC susceptibility signal associated to the smaller

particles is an evidence of their small magnetic moment and reduced number.

Figure 3a shows the M vs. H curves obtained at 300 K and figure 3b shows the dependence of

the coercive field (Hc) and particle size on N. For all samples the saturation magnetization

increases with N. The increment of twice in the saturation magnetization (Ms) for samples

10M5C and 10M6C is attributed to the passage to the magnetically blocked regime for sample

10M6C.

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Fig. 3. (a) Magnetization hysteresis loops of all samples. (b) The dependence of the coercive field and particle size on N.

A remarkable feature, the HC increases with the number of cycles. This may be the result of

magnetic moments pinning at the interfaces due to the layer-by-layer growth process of each

nanoparticles and due to stress anisotropy13. Table 1 summarizes the values of HC and the

remanence-to-saturation magnetization ratio (MR/MS). The MR/MS values ranged from 0.036 to

0.176. MR/MS = 0.5 is expected for non-interacting and randomly oriented particles14. This

result confirms the magnetic interaction between nanoparticles14.

Table 1. Particle size and magnetic properties of the iron oxide nanoparticles.

Sample DXRD

(nm)

HC

(Oe) MR/MS

KeffV/KB(K)

d

(nm)

T0 (K)

10M1C

10M2C

10M3C

10M4C

10M5C

10M6C

14.5 ± 0.5

19.5 ± 0.3

21.5 ± 0.3

22.5 ± 0.3

24.0 ± 0.3

27.2 ± 0.3

0,0

29.3

47.0

67.3

73.9

78.6

0,0

0.036

0.143

0.158

0.171

0.176

---

---

119 � 9

130 � 6

136 � 7

133 � 5

---

---

3.9

4.1

4.2

4.1

---

---

14.5 � 0.7

20.1 � 0.4

19.8 � 0.5

19.9 � 0.4

9

IV. Conclusions

Iron-oxide nanoparticles were prepared in the mesoporous network of Sty-DVB by a cyclic

chemical synthesis process. The AC magnetic susceptibility and XRD measurements showed

that there are two sets of particles: superparamagnetic-like with blocking temperature (TB) of ~

35 K and bigger particles with TB � 300 K. The first ones have diameters of � 4.0 nm and the

second ones of � 20.0 - 25.0 nm. The dynamical properties of the smaller particles was well

described by the Vogel-Fulcher equation, the T0 value revealed a magnetic interaction between

the nanoparticles. The MS value increased at each cycle, having a large step when the sample’s

particle size became larger than the critical size for superparamagnetism in magnetite. The

variation of MR, HC and MS with N shows that a fine tunning of the magnetic properties can be

reached when adopted an in situ chemical cyclic process for preparing maganetite in a porous

system.

VII. ACKNOWLEDGMENTS

Authors thank the financial support of the Brazilian agencies FINATEC, CTPETRO/FINEP,

MCT/CNPq and INCT in Nanobiotechnology.

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Highlights

� Magnetite nanoparticles were prepared in the porous of styrene-divinylbenzene beads

� The chemical synthesis was performed N times to obtain the N-cycle composite

� Saturation magnetization and the coercive field increases with N

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