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Surfactant Effects on the Structural and on the Magnetic Properties of Iron Oxide Nanoparticles Maria Filippousi, Mavroeidis Angelakeris, Maria Katsikini, Eleni C. Paloura, Ilias Efthimiopoulos, Yuejian Wang, Demetris Zamboulis, and Gustaaf Van Tendeloo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5037266 • Publication Date (Web): 04 Jul 2014 Downloaded from http://pubs.acs.org on July 10, 2014

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The Journal of Physical Chemistry

Surfactant Effects on the Structural and on the Magnetic Properties of Iron Oxide Nanoparticles

Maria Filippousi,*1 Mavroeidis Angelakeris,2 Maria Katsikini,2 Eleni Paloura,2 Ilias Efthimiopoulos,3 Yuejian Wang,3 Demetris Zamboulis4 and Gustaaf Van Tendeloo 1

1

2

EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

Department of Solid State Physics, School of Physics, Aristotle University of Thessaloniki, GR54124, Thessaloniki, Greece 3

4

Department of Physics, Oakland University, Rochester, MI, 48309, USA

Laboratory of General and Inorganic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece

Author information * Corresponding author. Tel.:+32 3 265 35 29, fax: +32 3 265 33 18 E-mail address: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT Iron oxide nanoparticles were prepared with the simplest and most efficient chemical route, the co-precipitation, in the absence and the presence of three different and widely used surfactants. The purpose of this study, is to investigate the possible influence of the different surfactants on the structure and therefore on the magnetic properties of the iron oxide nanoparticles. Thus, different techniques were employed in order to elucidate the composition and structure of the magnetic iron oxide nanoparticles. By combining transmission electron microscopy with X-ray powder diffraction and X-ray absorption fine structure measurements, we were able to determine and confirm the crystal structure of the constituent iron oxides. The magnetic properties were investigated by measuring the hysteresis loops where the surfactant influence on their collective magnetic behavior and subsequent AC magnetic hyperthermia response is apparent. The results indicate that the produced iron oxide nanoparticles may be considered as good candidates for biomedical applications in hyperthermia treatments because of their high heating capacity exhibited under an alternating magnetic field, which is sufficient to provoke damage on the cancer cells.

Keywords: iron oxide nanoparticles, co-precipitation, hyperthermia, HRTEM, XRD, XAFS


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1. INTRODUCTION Nanoparticles, made of either inorganic or organic materials, possess many novel properties compared to their bulk counterparts 1. For example, a typical size effect in magnetic nanoparticles (MNPs) is superparamagnetism, i.e. the discrepancy between thermal and magnetic energy which affects the macroscopic magnetic behavior 2. The size-dependent magnetic features of MNPs can be employed in various biomedical applications, such as magnetic resonance imaging agents in diagnosis 3, heat mediators in hyperthermia treatments 4, and magnetic guides in drug delivery 5,6. These applications are based on the fact that MNPs have sizes comparable to that of the biological entities of interest (e.g. viruses and proteins) 7. Furthermore, the outermost surfaces of the MNPs can be easily modified and/or functionalized appropriately, without sparing their ability to respond to external magnetic fields. Iron oxide materials are widely utilized in biomedical techniques due to their biocompatibility 8. Several iron oxides that exist in nature and can be prepared in the laboratory; magnetite (Fe3O4) and maghemite (γ-Fe2O3) are mainly considered for biomedical applications, since these two compositions fulfill the prerequisites of (1) chemical stability under physiological conditions, (2) low toxicity, and (3) sufficiently high magnetic moments 9. Many reports have described efficient synthesis routes in order to produce shape-controlled, stable, biocompatible, and monodispersed iron oxide nanoparticles, whereas less effort was spent on invitro cytotoxicity

10

. The most common procedures for producing iron oxide nanoparticles

include the thermal decomposition, the micro-emulsion, and the co-precipitation 1. Regarding the latter, co-precipitation is a relatively simple, high-yield, and easily scalable pathway to produce magnetic nanoparticles for biomedical applications. A major disadvantage of this synthesis



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method is the yield of a wide particle size distribution, which can be partially regulated by a proper choice of surfactants 1,11. The aim of the present study is to investigate the surfactant effects on the morphological, structural, and magnetic properties of iron oxide nanoparticles produced by the co-precipitation route

12,13

. We have employed three diverse, yet widely-used surfactants, namely the cationic

Cetyl TrimethylAmmonium Bromide (CTAB) surfactant, the non-ionic Polyvinylpyrrolidone, K30 (PVP) surfactant, and the anionic Sodium Cholate (S. C.) surfactant. These surfactants differ in terms of molecule length, as well as polar headgroup and charge. The PVP and S.C. surfactants are water-soluble, non-toxic, and are used in various medical applications in order to control the particle size 14,15. CTAB is the surfactant that is most widely used for the synthesis of Au nanorods; even though it can control the particle size and shape, CTAB can be toxic to cells and tissues16–19. We have chosen the cationic CTAB, since it can interfere with the external crystal surface of the negatively charged iron oxide nanoparticles produced during coprecipitation within an alkaline pH. In addition, we have prepared a batch of iron oxide nanoparticles without any surfactant for direct comparison. All of the obtained iron oxide nanoparticle batches, which were prepared with and without the use of the different surfactants, were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) spectroscopy for elucidating their morphological, structural, and electronic properties, respectively. Finally, magnetic measurements performed on the various MNP batches have probed both their macroscopic magnetic features, as well as their hyperthermia response.


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2

EXPERIMENTAL METHODS

2.1

Materials

FeSO4·7H2O, Cetyl TrimethylAmmonium Bromide and NaOH (pellets pure) were purchased from Merck. FeCl3·6H2O and Polyvinylpyrrolidone, K30 were purchased from Sigma Aldrich. Sodium Cholate was obtained from Acros Organics. Doubly distilled water was used for all solutions. All other materials and solvents used in the analytical methods were of analytical grade.

2.2

Synthesis of iron oxide nanoparticles

Magnetic iron oxide nanoparticles were prepared following a simple co-precipitation method given in the literature. Essentially four different variations were implemented to synthesize tunable magnetic iron oxide nanoparticles. In the first one 0.656 g CTAB and 2.0 mmol FeCl3·6H2O were dissolved in 30 mL doubly distilled water in a 50 mL conical flask at room temperature. Then 1.0 mmol FeSO4 . 7H2O was added to the solution. 5 mL of a 5 N NaOH solution were added under stirring. A black precipitate appeared immediately. Exactly the same procedure was also followed for the other methods, but instead of CTAB as surfactant were used 0.2 g S.C. chosen as a biocompatible surfactant, 0.2 g Polyvinylpyrrolidone, K30 and finally, no surfactant at all. The iron oxide nanoparticles were prepared via the following reaction equation: Fe2+ +2Fe3+ +8OH−→ Fe3O4 ↓ +4H2O



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The resulting precipitate of iron oxide nanoparticles was stirred for another 20 min in the conical flask with a magnetic stirrer. The resulting black precipitates were collected by centrifugation, washed several times with double distilled water and finally dried in a freeze-dryer. 2.3

Characterization Samples suitable for transmission electron microscopy were prepared by drop casting the

aqueous solution containing the particles on holey, carbon-coated copper grids. Transmission electron microscopy, high resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were acquired using a Tecnai G2 electron microscope operated at 200 kV. The monochromatic angle-dispersive powder XRD measurements were performed at the 16BM-D beamline of the High Pressure Collaborative Access Team’s facility, at the Advanced Photon Source of Argonne National Laboratory. The X-ray beam wavelength was λ=0.4246 Å. The XRD patterns were collected with a MAR 345 CCD detector. The intensity vs. 2θ patterns were obtained using the FIT2D software

20

. Refinements of the measured XRD patterns were

performed using the GSAS+EXPGUI software packages 21,22. The Fe-K-edge XAFS measurements were conducted at the BESSY-II storage ring of the Helmholtz Zentrum Berlin. The spectra were recorded in the KMC-II beamline that is equipped with a double SiGe (111) graded-crystal monochromator, in the transmission mode. Fe3O4, FePO4 and FeS compounds were used for reference purposes. The EXAFS spectra were subjected to background subtraction and transformation from the energy to the k-space. The resulting χ(k) spectra were fitted using the FEFFIT program

23

paths used for the fitting, were constructed with the FEFF code

. The photoelectron scattering

24

using the magnetite structure

with unit cell parameter a= 8.36Å.


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The collective magnetic behavior of the iron oxide nanoparticles was recorded by room temperature hysteresis loops with an Oxford 1.2 H/CF/HT at maximum applied fields of 1 T. From each powder sample, an aqueous dispersion (of 2 mg Fe/mL) was prepared by the dissolution of nanoparticles into distilled water and sonication for 10 min. These dispersions were evaluated for their heating efficiency under an AC magnetic field (15, 20, 25 kA/m and 765 kHz). Measurements were performed using a 4.5 kW commercial inductive heater (Ultrahigh Frequency Induction Heating Machine SPG-10 of Shuangping Corporation). The experimental configuration included a glass vial, containing 1 mL of the sample, adapted in an insulating holder placed in the center of water-cooled induction coil. The temperature rise was recorded by an OpSens PicoM optic fiber thermometer. Apart from the precautions taken regarding insulation and accuracy of the temperature measuring point, the estimation of the Specific Loss Power (SLP) value involved a number of calculations required to avoid overestimation errors introduced by the heat transfer from the sample to the environment or from the coil surface to the sample, as previously

25,26

. Thus, by following the modified law of cooling we are able to

quantify more accurately the heating efficiency, i.e. calculate SLP values.

3.

RESULTS AND DISCUSSION

3.1

Morphology and structure In Figure 1, we summarize the TEM results of the various iron oxide nanoparticle

batches. Overall, the choice of surfactant does not appear to affect the structure and morphology of the obtained MNPs. The shape of the obtained iron oxide nanoparticles is polyhedral in all batches. Figures 1a-e clearly display the tendency of individual nanoparticles to form aggregates in all batches due to hydrophobic interaction, the inherent magnetism of MNPs

12

and van der



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Walls forces in order to reduce the surface energy. The SAED pattern reveals the highly crystalline nature of the iron oxide nanoparticles (shown as inset in Figure 1a). Analysis of the SAED rings shows that they correspond to the Bragg reflections of the {2 2 0}, {3 1 1}, {4 0 0}, {5 1 1}, and {4 4 0} crystal planes of either Fe3O4 or γ-Fe2O3. Furthermore, the single-crystalline nature of MNPs was gained through the detailed analysis of the respective HRTEM images (Figures 1 b-e). We have also estimated the average particle size for each iron oxide nanoparticle batch (Figure 1f). As it can be seen, the average particle size is smaller for the batch prepared without any surfactant, whereas the S.C. and PVP-based MNPs exhibit a slightly larger (but almost the same) particle size distribution. Finally, the CTAB-based batch displays the highest average particle size distribution values among the iron oxide samples. Hence, it appears that the choice of surfactant influences the iron oxide nanoparticle size distribution, i.e. the nanoparticle aggregation.


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Figure 1: (a) Representative TEM image of the iron oxide nanoparticles revealing that only a few surfactant effects on the morphology can be observed. As inset the SAED pattern of the iron oxide nanoparticles. The d-spacing of the SAED rings correspond to, from bottom to top, {2 2 0}, {3 1 1}, {4 0 0}, {5 1 1}, and {4 4 0} crystal planes of either magnetite or maghemite. Highresolution TEM images of the MNPs (b) in aqueous solution along a [111] zone axis, (c) in the presence of CTAB as surfactant along a [110] zone axis, (d) in the presence of PVP as surfactant along a [110] zone axis, (e) in the presence of S.C. as surfactant along a [110] zone axis. The



corresponding Fast Fourier Transforms are shown as insets. The scale bar in Figure a is 10 nm and for Figures b-e is 5 nm. (f) Size distribution of all the studied samples. More detailed structural information of the as-synthesized iron oxide nanoparticles was obtained from synchrotron-based XRD measurements. Figure 2 displays the refined XRD patterns obtained for the differently synthesized batches. In all cases, the main phase in the XRD spectra is indexed to the cubic Fe3O4 magnetite structure. In addition, small traces of the FeCl3 and FeSO4 reactants are present in all batches as impurities (Figure 2); the concentration of these foreign phases, however, lies below ~5% compared to the main phase, as estimated from their relative Bragg peak intensities. Fe3O4 Nanop. batch #

**

Intensity (arb. units)

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No Surf.

* *

CTAB

**

PVP (311) (440) (220)

Fe3O4 FeCl3 FeSO4

0

(511) (400)

**

5 10 15 20 Diffraction angle 2θ (degrees)

S.C.

25


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Figure 2: Refined XRD patterns for the different batches of iron oxide nanoparticles prepared with (top to bottom): an aqueous solution without surfactant, CTAB surfactant, PVP surfactant, and S.C. surfactant. The difference spectra between the measured (open circles) and the calculated (red lines) XRD patterns are also displayed (black lines). The asterisks and arrows mark the FeCl3 and FeSO4 Bragg peaks, respectively.

In Table 1 we present the main crystallographic parameters as obtained from the full Rietveld refinements of the XRD patterns (Figure 2). The different synthesis procedures yield similar results in terms of lattice parameters for the Fe3O4 nanoparticles; the lattice parameter values lie in-between those reported for bulk Fe3O4

27

and γ-Fe2O3 28, with the CTAB-based

batch exhibiting the smallest unit cell constant (Table 1). In addition, we have estimated the average crystallite size for all MNP batches by employing Scherrer’s formula (Table 1). The largest crystallite size is obtained when using the S.C. surfactant, whereas the smallest crystallites are produced in the case of PVP. The average crystallite sizes estimated from the XRD patterns exhibit lower values than the average particle sizes estimated from TEM, whereas they do not follow the same trend (Figure 1f). Even though the lower crystallite size values compared to the particle size may be attributed to the aggregation of the iron oxide nanoparticles (Figure 1), the crystallite sizes appear to depend on the choice of surfactant. Finally, we could not observe any clear correlation between the average crystallite sizes and the respective interatomic parameters, in agreement with Menard et al. 29. We should note, however, the presence of Fe vacancies for the CTAB-based and the S.C.-based iron oxide nanoparticle batches (Table 1). These Fe vacancies in the magnetite structure imply the oxidation of the respective nanoparticle batches, i.e. the transformation of Fe3O4 towards its oxidized



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analog γ-Fe2O3. Hence, the choice of surfactant seems also to regulate the defects within the crystal structure. TABLE 1: Structural Parameters for the Various Batches of Fe3O4 Nanoparticles. Structural parameters SG Fd-3m

Fe3O4 nanoparticle batch No Surf.

CTAB

PVP

S.C.

a (Å) V (Å3)

8.3698(1) 586.3

8.3552(1) 583.3

8.3663(1) 585.6

8.3677(1) 585.9

(nm) Fetet occ. Fetet Uiso (Å2) Feoct occ. Feoct Uiso (Å2) O-u position O occ. O Uiso(Å2) (Fe-O)tet × 4 (Fe-O)oct × 6 wRp (%)

7.3(1) 0.996(4) 0.0031(3) 1.021(4) 0.0031(3) 0.2602(2) 1 0.0068(6) 1.963(3) 2.011(2) 5.9

6.8(3) 0.917(3) 0.0039(3) 0.932(3) 0.0039(3) 0.2606(3) 1 0.0079(5) 1.962(4) 2.004(3) 5.12

6.5(1) 1.032(3) 0.0046(2) 1.032(3) 0.0046(2) 0.2539(2) 1 0.0099(7) 1.868(3) 2.059(2) 4.9

7.6(1) 1.006(4) 0.0043(4) 0.95(4) 0.0043(4) 0.2543(2) 1 0.0085(6) 1.874(3) 2.057(2) 4.78

χ2

0.374

0.448

0.309

0.33

Fe3O4 bulk 8.392 591.1

γ-Fe2O3 bulk 8.331 578.2

1

1 -

1 0.255 1 1.890 2.057 -

0.833 0.2512 1 1.821 2.073 -

Where occ. = occupancy and Uiso the isotropic atomic displacement parameter [the Debye– Waller factor is represented as exp(−8π2Usin2 θ/λ2)]. The atomic positions are Fetet: 8a (1/8, 1/8, 1/8), Feoct: 16d (0.5, 0.5, 0.5), and O: 32e (u, u, u).

3.2

XAFS measurements In order to get a more accurate micro-structural picture, we have additionally performed

EXAFS investigations on the different iron oxide nanoparticle batches. The normalized to the edge jump Fe-K-edge XANES spectra of the MNPs are shown in Figure 3a. The spectra of the II,III

reference compounds Fe3

O 4 , FeIIS and FeIIIPO4 are also depicted. Apart from a small blue

shift, the spectra of the iron oxide MNPs exhibit strong similarities with the spectrum of magnetite. The characteristic pre-edge peak was fitted using three Lorentzian functions as shown


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in the inset. The absorption edge was simulated using a sigmoidal function with amplitude equal to 1. According to the fitting results listed in Table 2, the different synthesis routes did not result in variations of the Fe oxidation state. In particular, the Fe valence state in all MNPs lies between 2.8-2.9. These values are slightly higher than that of magnetite (2.67), indicating the partial oxidation of all iron oxide nanoparticle batches. These results partly contradict the XRD measurements (Table 1), where only the CTAB- and the S.C.-based MNP batches are found to exhibit Fe vacancies or, equivalently, partial oxidation; this discrepancy can be attributed in part to the different sensitivity of the employed techniques, since XAFS serves as a more short-range and element-specific local probe, whereas XRD detects the average long-range structure of the material under study. On the other hand, the pre-edge peak characteristics do not exhibit strong variations, with their values being comparable to the corresponding values in magnetite (Table 2). This affinity of the pre-edge peak characteristics indicates that the MNPs adopt the spinel structure, consistent with both XRD and TEM studies.


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FeS

(a)

Fe3O4 PVP 2

No surf. S.C. CTAB FePO4

1

7108 7112 7116 7120

0 7100 Fe oxidation state

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorption coefficient (arb. units)


3.0

7125

7150 7175 Energy (eV)

7200

FePO4 No S.C. Surf. CTAB Fe3O4 PVP

(b)

2.5 FeS

2.0 7116

7118 7120 7122 7124 absorption edge position (eV)

Figure 3: (a) Fe-K-edge XANES spectra of the MNPs synthesized using different types of surfactants (CTAB, S.C., No Surfactant, PVP). The spectra of reference Fe3O4 (magnetite), FeIIIPO4, and FeIIS compounds are also included. The inset shows representative fitting of the pre-edge peak using three Lorentzian components (L1: 7113.3±0.1 eV, L2: 7114.6±0.1eV, L3: 7116.7±0.2 eV). (b) Determination of the Fe oxidation state in the MNPs using a linear interpolation of the position of the absorption edge of the FeS, Fe3O4 and FePO4 compounds. TABLE 2: Fitting Results of the Pre-Edge Peak of the XANES Spectra Edge position (eV) Oxidation state Area (arb. units) L1 L2 L3 Fe3O4 0.27 0.09 0.00 7121.6(2) 2.67(3) No Surfactant 0.22 0.16 0.10 7122.7(2) 2.80(3) CTAB 0.25 0.16 0.10 7123.1(2) 2.86(3) S.C. 0.25 0.14 0.10 7123.0(2) 2.84(3) PVP 0.19 0.14 0.08 7122.9(2) 2.84(3) Note: L1, L2 and L3 are the three Lorentzian functions with energy positions 7113.3±0.1, 7114.6±0.1 and 7116.7±0.2 eV, respectively, that were used to fit the pre-edge peak. sample


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The Fe-K-edge EXAFS spectra are shown in Figure 4. In the magnetite structure, Fe occupies tetrahedral and octahedral sites with a proportion of 1/3 and 2/3 respectively. Both contributions were taken into account using the parameter x as the fraction of Fe atoms that occupy octahedral sites, whereas the remaining (1-x) occupies tetrahedral sites. The fitting of the spectra in the k- and R- space is shown in Figure 4 (upper and lower panel, respectively). The fitting results are listed in Table 3. Overall, the obtained results indicate that the local bonding environment around Fe is similar in all MNPs irrespective of the type of surfactant that was used, whereas all iron oxide nanoparticle batches adopt the spinel structure. On the other hand, the fraction of Fe atoms that occupy octahedral sites is found slightly higher within the MNPs compared to magnetite; this deviation, however, lies within the error of the performed fitting (Table 3). Finally, the FeOh-O distance in the first nearest neighboring shell of all MNPs is found slightly smaller compared to the corresponding value in magnetite; such variation can be attributed to the partial oxidation of Fe in the MNPs 30.



0.3

Fe3O4

χ (k)

PVP No Surf. S.C. CTAB

0.0

4

6

8

10

12

14

-1

k (A ) 20

FT amplitude (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fe3O4

15

PVP

10

No Surf. 5

S.C. CTAB

0

0

1

2

3

4

5

6

7

8

o

R (A)

Figure 4: χ(k) (top) and Fourier Transform of the k3χ(k) (bottom) EXAFS spectra of the MNPs synthesized using different types of surfactants. The experimental and fitting curves are shown in thin and thick solid lines, respectively.


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TABLE 3: Fitting Results of the EXAFS Analysis model Fe3O4

(i)

sample

γ-Fe2O3

shell N1 FeOh-O N=6x

R1 ± 0.01

S.C.

No Surf.

PVP

0.76 ± 0.19 0.74 ± 0.13 0.77 ± 0.08 0.74 ± 0.09

4.0*

4.5

4.4

4.6

4.5

2.05

2.08

2.02

1.96

1.97

1.97

1.97

0.0117

0.0089

0.0079

0.0076

0.0073

N2

1.33

1.7

1.33*

1.0

1.0

1.0

1.0

R2 ± 0.01

1.89

1.80

1.88

1.88

1.88

1.88

1.88

0.0123

0.0093

0.0083

0.0080

0.0077

4

2.6

4.0*

4.5

4.4

4.6

4.5

2.97

2.94

2.98

3.01

3.00

3.00

3.00

0.0103

0.0126

0.0100

0.0098

0.0099

N3 (Fe) R3 ± 0.02

σ 32 ± 25% FeOh- FeTd FeTd- FeOh N4 (Fe) N=6x+12(1-x) R4 ± 0.02

σ

2 4 ±

8

6.9

8.0*

7.4

7.5

7.4

7.5

3.48

3.45

3.49

3.47

3.47

3.48

3.48

0.0088

0.0131

0.0101

0.0099

0.0101

8.0*

7.4

7.5

7.4

7.5

3.58

3.64

3.54

3.58

3.57

0.0280

0.0333

0.0294

0.0294

0.0296

25%

FeOh,Td-O N (O) N=6x+12(1-x) 5 R5 ± 0.05

4 4 3.49 3.66

5.1 3.4 3.45 3.60

σ 52 ± 40% FeTd- FeTd N=4x

CTAB

3.4

σ 22 ± 15% FeOh- FeOh N=6x

Fe3O4 x=0.67*

4

σ 12 ± 15% FeTd-O N=4(1-x)

(ii)

N6 (Fe)

1.33

1.7

1.33*

1.0

1.0

0.9

1.0

R6±0.05

3.63

3.60

3.64

3.62

3.62

3.62

3.62

2 6

σ ± 40%

0.0271 0.0322 0.0288 0.0287 0.0289 Note: N is the coordination number, x is the fraction of Fe atoms that occupy octahedral sites, R the nearest neighbor distance in Å and σ2 the Debye-Waller factor in Å2. The energy origin (3.8±0.3 eV) and the amplitude reduction factor (0.78±0.03) were commonly iterated for all the samples. The reported errors correspond to the fitting uncertainties. Values marked with an asterisk were not iterated. The first column indicates the type of the nearest neighbors and the octahedral (Oh) or tetrahedral (Td) site occupied by the absorbing atom. * 31 ; space group: Fd3m ; a= 8.3922Å; atom positions: FeTd (0.125, 0.125, 0.125), FeOh (0.5, 0.5, 0.5), O (0.255, 0.255, 0.255). 32 ** ; space group: P4132; a= 8.33Å; atom positions: FeTd (0.5, 0.5, 0.5), FeOh (0.875, 0.875, 0.875), FeOh (0.125, 0.875, 0.125), O (0.125, 0.125, 0.625), O (0.625, 0.625, 0.625).

3.3

Magnetic features



The magnetization versus magnetic field plots (M–H hysteresis loop) at room temperature for all the magnetic nanoparticles is shown in Figure 5. The magnetic characterization reveals the standard ferrimagnetic character exhibited by the spinel iron oxide samples. The magnetic properties for all the iron oxide nanoparticles are presented in Table 4. The saturation magnetization MS values for all the prepared samples are suppressed compared to that of ‘‘bulk’’ magnetite (90 Am2/kg)

33

, which may be attributed to: (a) The decrease of

particle size: smaller particles yield milder magnetic features while below a certain diameter particles undergo a magnetic transformation from a ferrimagnetic to a superparamagnetic collective behavior. If the majority of the MNPs reside within the superparamagnetic regime, MNPs eventually exhibit a smaller saturation magnetization compared to bulk values since a small fraction of particles only contributes to the overall behavior. (b) The relatively large particle size distributions originating from the co-precipitation synthesis route: since the blocking temperature depends on particle size, a wide size distribution will result in a wide range of blocking temperatures and, hence, non-optimum magnetic behavior due to polydispersity issues.1,13,34

60 2 Magnetization (A m /kg)

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Page 18 of 28

PVP No Surf. S.C.

30

CTAB

0

-30

-60 -1.0

-0.5

0.0

0.5

1.0

Field (T)


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Figure 5: Magnetic hysteresis loops for the four as-synthesized iron oxide nanoparticles recorded at room temperature and maximum field of 1 T. Details of the magnetization curves of the four as-synthesized iron oxide nanoparticles for field values in the range -0.25 to 0.25 T are shown in the inset.

The improved crystallinity of the nanoparticles, as deduced from the TEM and XRD measurements, facilitates to maintain the saturation magnetization at relatively high values. The MS values reported in Table 4 for all the synthesized MNPs are very close to iron oxide samples of similar sizes

35

. In addition, the saturation magnetization of the iron oxide prepared in the

presence of the surfactant CTAB is lower than that of the other three samples. This behavior can be ascribed to the presence of prominent Fe vacancies in the CTAB-based batch as discussed in previous section (Table 1) 36. Nevertheless, even smaller MS values (7–22 Am2/kg) are reported as being exploitable for bioapplications 37. TABLE 4: Magnetic Properties of the Iron Oxide Nanoparticles MS (Am2/kg)

Mr (Am2/kg)

Hc (T)

No surfactant

47.5

1.5

0.005

Fe3O4 surfactant CTAB

29.8

0.7

0.009

Fe3O4 surfactant S.C.

46.3

5.6

0.014

Fe3O4 surfactant PVP

50

4.2

0.010

Mr is defined as the magnetization at H＝0, Hc defined as the field magnitude necessary to obtain M＝0 Superparamagnetism is an essential feature of the magnetic nanoparticles, since after the removal of the external magnetic field, the magnetization disappears and, hence, the agglomeration and the possible embolization of the capillary vessels can be avoided

38,39

. From

Table 4 we see that the coercivity Hc exhibits a small value at room temperature, indicating a negligible (almost zero) ferrimagnetic character of the systems under study, which is a 19 ACS Paragon Plus Environment


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characteristic attribute for superparamagnetic materials

38

Page 20 of 28

. Furthermore the remanent

magnetization (Mr) is also close to zero for all samples. These magnetic features show that the measured MNPs can be easily manipulated under an external magnetic field, but they will not preserve any residual magnetism upon the removal of the external field. 3.4

Hyperthermia When magnetic nanoparticles are exposed in an alternating magnetic field, there are

different mechanisms that produce heat at the surrounding regions. In the case of superparamagnetic nanoparticles, the effect is attributed to Néel and/or Brown relaxation mechanisms while, the heat dissipation in the ferro(i)magnetic regime is mainly attributed to hysteresis losses 40. The specific loss power (SLP), defined as the energy conversion rate (W/g), is a gauge of the heating conversion efficiency and needs to be maximized. Since SLP strongly depends on the particle (size, composition, and magnetic profile) and field (intensity and frequency) parameters, the various approaches for enhancing SLP results in lower dosage level of nanoparticles and milder field conditions. The SLP values are proportional to the saturation magnetization MS of nanoparticles, show a maximum point for a certain particle size and magnetic anisotropy constant, whereas they are inversely proportional to the size distribution of the nanoparticles 41,42. The hyperthermia response of the four samples is comparatively illustrated in Figure 6. In all cases, the solution concentration and field frequency were kept constant at 2 mg/mL and 765 kHz respectively, while the magnitude of magnetic field was varying. It should be mentioned here that despite the use of a relatively high frequency (765 kHz), which leads in turn in a H·f product at least one order of magnitude above the estimated threshold for major


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discomfort (~ 5×108 Α m-1 s-1), analogous protocols are currently examined (also in-vitro) as alternatives to overcome the usual constraints of limited heating efficacy 43. Figure 6a displays the temperature variation for three different magnetic field magnitudes (15, 20 25 kA/m) for the four samples, the first indication of heating response. As expected, the magnitude of the field is proportional to the strength of the heating effect, showing the magnetic origin of this behavior. It can also be seen that the choice of surfactant strongly affects the collective heating response (Figure 6a), and that the effect may be further optimized upon the proper surfactant choice. As discussed in the previous section, the CTAB-based batch yields milder magnetic features (Figure 5 & Table 4) which are exhibited directly in hyperthermia measurements, since it is widely accepted that the higher the magnetization (along with coercivity, remanence, and magnetic anisotropy), the higher the AC heating response 44. It seems that the more prominent oxidation of the CTAB-based MNPs suppresses both the collective magnetic response, as well as the AC heating effect of this batch compared to the other MNPs. On the other hand, the batch synthesized without any surfactant maintains a relatively high saturation magnetization and reduces remanence and coercivity compared to the other two surfactant-covered samples (S.C., PVP), indicating the beneficial role of better crystallinity, size dispersion, and homogeneity in collective magnetic and AC heating response. On the contrary, the implementation of S.C. and PVP as surfactants leads to a much higher heating response. The latter is also depicted in Figure 6b, where the quantifiable measure of heating response, i.e. the SLP index against field magnitude is plotted for all samples. It seems that both S.C. and PVP surfactants manage to maintain colloidal and structural stability, as well as size homogeneity, though not significantly attenuating the collective magnetic features. The SLP values attained for these samples (≥ 200 W/g at 25 KA/m) render them as suitable hyperthermia agents

45

. Further



optimization of magneto-structural features via synthetic optimization is expected to establish stronger ferrimagnetic behavior, and enhance subsequently the heating response. 400

SLP (W/g)

PVP S.C. CTAB No Surf.

30 o Temperature ( C)

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Page 22 of 28

20

10

(a) 0

PVP S.C. CTAB No Surf.

300

200

100

15

20

25

Magnetic Field (kA/m)

(b)

0 15

20

25

Magnetic Field (kA/m)

Figure 6: (a) Temperature rise and (b) the corresponding SLP values for samples under study.

4.

CONCLUSIONS In summary, magnetic iron oxide nanoparticles have been synthesized through the widely

used co-precipitation method with (and without) the use of three common surfactants, namely CTAB, S.C., and PVP. All of the obtained MNP batches have been characterized in detail with a multi-experimental approach, namely with TEM, synchrotron-based XRD and XAFS, as well as with magnetic measurements. The TEM measurements on the various iron oxide nanoparticle batches have not revealed any surfactant effects on the morphological properties of the MNPs. On the other hand, the estimated average particle and crystallite size distribution appears to depend on the choice of surfactant. In addition, the presence of defects within the spinel structure, i.e. oxidation of the


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MNPs seems to be also influenced by the choice of surfactant. These structural deviations among the various iron oxide nanoparticle batches affect directly the magnetic properties of the MNPs. In particular, the CTAB-based batch yields the milder magnetic features compared to the rest of the samples due to its more prominent oxidation. Nevertheless, the remanent magnetization and coercivity are found to attenuate at room temperature due to finite-size effects for all iron oxide batches, indicating a superparamagnetic nature of the nanoparticles, a prerequisite in most biomedical applications. Furthermore, it is found that the iron oxide nanoparticles synthesized with the use of PVP and S.C. as surfactants, exhibit a higher heat capacity under an alternating magnetic field compared to the other ones. Thus, by proper choice of the surfactant, the nanoparticles maintain collective magnetic features along with an enhanced AC magnetic heating response, making them good candidates for hyperthermia modalities. Finally, we should point out that a direct comparison between the three surfactants employed in this work is not possible due to their significant physico-chemical differences. Nevertheless, it has been shown that the distinctive features of each surfactant (e.g. polar headgroup size and charge, hydrophobic chain length) may strongly influence the morphological (e.g. the nanoparticle aggregation), structural (stoichiometry), and magnetic properties of the produced MNPs

36

. These results can serve as a first step towards the understanding of the

correlation between the choice of surfactant and its effect on the magneto-structural properties of iron oxide nanoparticles synthesized by the co-precipitation method, and further enhance their application in magnetic particle hyperthermia.

Notes The authors declare no competing financial interest.



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Acknowledgements GVT and MF acknowledge funding from the European Research Council under the 7th Framework Program (FP7), ERC grant N°246791 – COUNTATOMS. This work is also performed within the framework of the IAP-PAI. The EXAFS characterization was financially supported from the European Community’s 7th Framework Program (FP7/2007-2013) under grant agreement No 226716. Part of this work was performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory (ANL). HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. We would like to thank Dr. D. Popov and Mr. T. Lochbiler for their assistance with the XRD measurements.

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