MICROSTRUCTURAL AND MAGNETIC ...

MAGNETOHYDRODYNAMICS Vol. 43 (2007), No. 4, pp. 11–18

MICROSTRUCTURAL AND MAGNETIC PROPERTIES OF MAGNETIC FLUID BASED ON MAGNETITE COATED WITH TARTARIC ACID M. R˜acuciu 1 , D.E. Creang˜a 2, A. Airinei 3 , V. Badescu 4, N. Apetroaie 2 1

“Lucian Blaga” University, Faculty of Science, 5-7 Dr. I.Ratiu Str., Sibiu, 550024, Romania 2 “Al. I. Cuza” University, Faculty of Physics, 11A Blvd.Copou, 700506, Iasi, Romania 3 “P. Poni” Institute of Macromolecular Chemistry, Iasi, Romania 4 National Institute of Research and Development for Technical Physics, 47 Blvd. D. Mangeron, Iasi, Romania

An aqueous magnetic fluid based on iron oxide particles as solid nano-magnetic phase was prepared by applying the chemical precipitation method. Tartaric acid (C4 H6 O6 ) was used to functionalize magnetic cores. Physical tests have been performed in order to reveal the microstructural and magnetic features, their optimization being needed for biomedical utilization. The particles’ size was investigated using transmission electron microscopy (TEM), atomic force microscopy (AFM) and magnetization measurements. Fourier transform infrared absorption spectra (FT-IR) have been recorded aiming to get some information on the solid phase structure.

1. Introduction. Recently, the synthesis of magnetic materials on the nano-scale has become a field of increased interest due to the mesoscopic properties shown by nanoparticles of quantum dimensions located in the transition region between atoms and bulk solids. Magnetic fluids (ferrofluids) are two-phase systems, consisting of small ferri- and ferromagnetic nanoparticles dispersed in a liquid [1]. Because the size of the magnetic particles lies in the nanometer range (often around 10 nm in diameter), those suspensions are referred to as magnetic nano-colloids. The colloidal nanoparticles are subject to random displacement due to their bombardment by impulsive forces from the solvent molecules, in which they are dispersed. Colloidal stability is assured by coating the nanoparticles with shells of a nonmagnetic molecular surfactant, which prevents close approach of the magnetic cores, thereby, reduces the possibility of aggregation via Van der Waals or dipolar attractions. When the dipolar interactions are much stronger than the thermal energies, particle chains start growing and forming more complex structures, depending on the particle volume fraction, size distribution, temperature or a magnetic field applied [2]. The colloidal stability of magnetic fluids is a complex issue related to the synthesis procedure followed, including the nature of surfactant(s) and carrier liquids used [3]. The magnetic fluid stability is the main characteristic that determines the possibility to exploit magnetic fluids in different industrial and biomedical applications. Magnetic fluids are said to be superparamagnetic, meaning that they are attracted by a magnetic field but retain no residual magnetism after the field is removed [1]. When the magnetic nanoparticles are dispersed in an ionic solution, the dissociation of ionogenic groups is followed by the differential adsorption on the particle surface generating a particle surface potential. Biophysicists pay more and more attention to the magnetic nanoparticles functionalized with a biocompatible nonmagnetic molecular surfac11

tant that prevents the irreversible aggregation of the magnetic cores and makes it suitable for biomedical applications. From a magnetic point of view, due to their small size, each particle represents a single magnetic domain with a magnetic moment proportional to its volume. Due to their convenient granulation comparative with cellular substructures as well as due to the superparamagnetic behaviour, the aqueous magnetic fluid has been designed for various biomedical applications [4, 5]. Separation, immunoassay, magnetic resonance imaging (MRI), drug delivery and hyperthermia can be optimized by the use of magnetic nanoparticles [6, 7]. Among the various methods for producing magnetic nanoparticles, chemical routes have the advantages of being relatively simple and providing good control over particles properties. Water-based magnetic fluids hold great potential for biological applications, considering their influence in plant growth as shown by Corneanu et al. (1998) [8], Pavel et al. (1999) [9], R˜ acuciu et al. (2007) [10, 11]. Tartrate-stabilized magnetic nanoparticles within magnetic fluids were yielded successfully by Neveu et al (2002) [12]; the biocompatibility of the magnetite coated with tartaric acid was discussed by Macaroff et al. (2004) [13], Lacava et al. (1999) [14] and others. We believe that the stabilization of magnetite with tartaric acid is the particular interest in plant biotechnology as this acid can be found naturally in some plants (particularly, grapes or bananas), consequently, we supposed that it could increase the biocompatibility already provided by magnetite. Nevertheless, the small size of the colloidal particles as well as their magnetic properties needs to be assured by adequate applications of the preparation method. 2. Experimental. The magnetite nanoparticles (though maghemite synthesis is not totally avoided) were prepared by co-precipitation of Fe3+ and Fe2+ oxides in an alkali medium (NH4 OH), following the method described in [15]. An aqueous solution of 4.3 g FeCl2 ·4H2 O and 11.75 g FeCl3 ·6H2 O in 200 ml deionized water was mixed at 80◦ C under vigorous and continuous stirring with 25 ml of 25% NH4 OH, dropwise added, as a precipitant agent. All necessary reagents were reactive grade (Merck). The ferrophase particles were decanted in a non-uniform magnetic field and further extracted by filtration. Repeated washings with deionized water (totally 800 ml) were carried out. 6 g of tartaric acid (C4 H6 O6 ) were added at 90◦ C under continuous mechanically stirring for sixty minutes. The tartrate ions bound to the iron oxides are supposed to generate the electrostatic repulsion among colloidal magnetic particles stabilizing in this way the magnetic fluid. A TESLA device, with a resolution of 1.0 nm, was utilized to investigate the ferrophase granularity in the sample prepared by 104 magnetic fluid dilution and deposition on a collodion sheet. Supplementary microstructural data from AFM scanning performed on the magnetic fluid sample 104 diluted and deposited on mica substrates were extracted. The AFM device assembled in our laboratory is working in the taping mode being provided with a commercial standard silicon nitride cantilever (NSC21) characterized by a force constant of 17.5 N/m, 210 kHz resonance frequency and tips with a radius ranging 5 to 10 nm. The AFM images cover a range of areas from 50 × 50 to 3 × 3. The image results from multiple scans, the surface being sampled with 256 × 256 pixels. Magnetization and magnetic susceptibility were measured revealing the suitability of the water-based magnetic fluid for magnetic carrier utilization. Magnetic susceptibility and magnetization measurements were carried out by the Gouy method, performed at the constant temperature (22.0 ± 0.1◦ C) using an air-tight non-magnetic cylindrical sample holder 3 mm in diameter and 25 cm long, placed perpendicularly to the magnetic field. Magnetic field intensity was measured by a 12

Fig. 1.

TEM image of magnetic nanoparticles deposited from the magnetic fluid sample aliquot.

Walker Scientific MG 50D Teslameter with a Hall probe. For sample weighting, an electronic balance ACULAB-200 with 10−4 g accuracy was used for measurements. The Fourier transform infrared absorption spectra (FT-IR) have been recorded aiming to get some information on the composition of colloidal nanoparticles. The FT-IR investigation was carried out using a Bruker Vertex 70 infrared spectrometer, with the magnetic nanoparticles being dispersed in KBr after previous thermal treatment at 100◦C up to constant weight. The suspension density (picnometric method), viscosity (capillary method using an Ubbelohde viscosimeter) and surface tension (stalagmometric method) were measured using standard methods. 3. Results and discussions. The magnetic fluid based on the ferrophase stabilized with tartaric acid was kept under observations for two months, during which insignificant changes in its homogeneity were noticed. The measurements carried out aiming to reveal the rheological properties of the magnetic fluid have resulted in the following values: 1025.35 ± 0.585 kg/m3 for density, (48.386 ± 0.895)×10−3 N/m for surface tension and (5.594±0.098)×10−3 kg/ms for dynamic viscosity. The TEM micrographs show that the size of the tartaric acid coated nanoparticles ranges 3 to 20 nm, exhibiting mostly a spherical shape. TEM image analysis was accomplished on about 1000 nanoparticles of the water-based magnetic fluid sample, using an interactive image analysis program. A TEM image of the tartaric acid coated magnetite nanoparticles is shown in Fig. 1.

Fig. 2. Physical diameter histogram of the ferrophase coated with tartaric acid.

particles number

200

150

100

50

0

4

6

8

10

12

14

16

18

20

diameter, nm

13

The statistic analysis of the TEM data led to the histogram of the nanoparticles’ physical diameter presented in Fig. 2. The mathematical approach with a normal distribution curve of the diameter histogram provided by an average diameter (8.945 nm) and standard deviation (2.389 nm) yields the following relation:
 1 1 2 exp − 2 [ln (d/d0 )] . (1) P (d) = √ 2s 2πsd Comparatively, other authors, who tested tartaric acid as a coating molecule for the ferrophase (Royer et al. (2004), in the case of CoFe2 O4 ) have succeeded to obtain an average size of about 6 nm in their magnetic fluids [16]. 3D-AFM scanning evidenced rare large particles and particle quasi-spherical aggregates 10–20 nm high (Fig. 3) as well as short particle chains. The interpretation of the aggregate frequency within the magnetic fluid on the base of the sample prepared by magnetic fluid deposition and drying on a specified support should not be taken as a proof of the presence of particle aggregates of the same frequency in the initial magnetic fluid, since the observed aggregates may be formed during the deposition process so that their frequency within the initial fluid might be much lower. The investigation on the basis of neutron diffraction would be an appropriate method, but this analysis belongs to a future research project. Magnetization and magnetic susceptibility curves for the water-based magnetic fluid measured at room temperature are presented in Fig. 4. Magnetization curves can be used to study both the particle interactions and the agglomerate

40 nm 10 nm −20 nm

0 µm

3 µm 1 µm 2 µm 2 µm 1 µm 3 µm

0 µm

Fig. 3. AFM analysis: 3-D record of magnetic nanoparticles deposited from a magnetic fluid sample by the AFM technique. 14

0.10

3000

0.08

2000

M (A/m)

χ (u.SI)

0.06

Susceptibility 0.04

Magnetization 1000

0.02

0

250000

500000

750000

0

H (A/m)

Fig. 4. Magnetization curves of magnetite-tartaric acid colloid. formation – the processes, which strongly influence the rheological behaviour of magnetic fluids. The saturation magnetization value was obtained from magnetization (M ) versus 1/H curves, by extrapolating to 1/H = 0. The initial susceptibility χi was defined from the susceptibility (χ) curve registered by extrapolating the initial linear curve to H → 0. For the magnetic fluid discussed here, the saturation magnetization value is 2425.9 A/m, being concordant with the solid nanoparticle volume fraction equal to 1.5% (from density measurement). The initial magnetic susceptibility χi value obtained for the analyzed magnetic nanoparticles functionalized with tartaric acid was of 0.1126. Using the magnetization data, the average size of the magnetic diameter (dM ) can be calculated from the Langevin equation. Assuming the spherical particle shape, particle magnetic diameters were calculated using Eq. 2, with the Mb value of bulk magnetite being (0.48 · 106 A/m) [1], so for the average magnetic diameter of nanoparticles, we obtained the value of 6.07 nm: d3M

18kB T = πµ0 Mb · Ms



dM dH

 H→0

,

(2)

where dM is the magnetic particle diameter, kB is the Boltzmann constant, T is the absolute temperature, Ms is the saturation magnetization of the sample, and µ0 is the vacuum magnetic permeability. Differences found between the particle diameter values from TEM measurements and those provided by the magnetization data can be assigned to the surfactant shell of the magnetite core. For our magnetic particles, we obtained dshell = dTEM − dM = 2.87 nm, an organic surface layer of 1.435 nm thickness. The surfactant layer may be considered as a magnetically dead coating, which can affect the uniformity or magnitude of magnetization due to quenching of surface moments [17]. As each magnetic particle in the magnetic fluid has a single magnetic domain, the number of particles n in the magnetic fluid sample was found 15

from Eq. 3 and the 4.08 · 1022 particles/m3 value was obtained Ms = n · m ,

(3)

with m being the dipole moment of the particle, which, when the particles are assumed spherical, is given by the following equation: m=

π 3 · d · Mb , 6 M

(4)

with dM being the magnetic particle diameter, Mb is the bulk magnetization value per unit volume, and µ0 is the magnetic permeability in vacuum. Further analysis was carried out using the FT-IR spectra (3500 cm−1 –500 cm−1 ), which are presented in Fig. 5. The FT-IR spectra of the nanoparticles coated with tartaric acid (HOOC(CH2 O)2 COOH) show an absorption band at 567 cm−1 associated with the stretching and torsional vibration modes of magnetite, concordant with Keiseret al. (1982) [18]. The tartrate ion consists of similar atom groups, each consisting of a carboxyl group, tetrahedral carbon and hydroxyl oxygen. The two atom groups are connected through the C–C bond. Therefore, stretching vibrations of hydroxyl and C–H groups are expected. Also, a free tartrate ion has two hydroxyl groups, which may give rise to two bands of stretching vibrations of the hydroxyl group in the same spectral domain as the C–H ones: 2800–3500 cm−1 ). As one can see in the IR spectrum from Fig. 5, one multiple band at 740 cm−1 , 819 cm−1 , 901 cm−1 , 930 cm−1 , 1007 cm−1 , 1043 cm−1 appears confirming the presence of iron tartrate in the solid phase of the analyzed water-based magnetic fluid. The large and intense band at 1542 cm−1 may be attributed to the C=O stretch of the carbonyl

Transmittance (%)

TA

TA-FF

3500

3000

2500

2000

1500

Wavenumber (cm

−1

1000

500

)

Fig. 5. FT-IR spectra for magnetite-tartaric acid colloid (TA-FF) and pure tartaric acid (TA), respectively. 16

group. The peak at 1407 cm−1 is assigned to C=O symmetric and δ (O–C=O) mode. The peaks of the multiple absorption band at 1243 cm−1 , 1277 cm−1 , 1300 cm−1 and, respectively, 1332 cm−1 are due to the C–O stretch and C–H bonds, while the peak at 1118 cm−1 is due to the C–O stretching [19]-[21]. The two bands of low intensity from 3448 cm−1 and 2958 cm−1 to the OH symmetric and asymmetric stretchings can be assigned; they practically vanish following the bonding of the COO− -anions from the tartrate ion structure to the iron ions from the magnetite core particles. As a consequence of this interaction, the shift of the C=O and C–O vibrations toward smaller wave numbers occurred also (from 1743 cm−1 to 1542 cm−1 and from 1447 to 1407 cm−1 ). The absorption band at 651 cm−1 appears confirming the presence of iron tartrate in the solid phase of the analyzed magnetic fluid. The narrow but intense absorption of tartrate from 668 cm−1 seems to be overlapped onto the weaker vibrations of the iron oxide skeleton situated in the same region. It seems that the tartaric acid efficiency binds to the iron oxide particles assuring their good dispersability in water. 4. Conclusions. Tartaric acid was used to stabilize magnetite nanoparticles within an aqueous magnetic fluid designed for further applications in the biological research. The physical diameter of the colloidal particles was assessed to less than 9 nm, while the magnetic diameter was found of about 6 nm. So, the small ferrophase particles are supposed to be able to penetrate a biological tissue thus recommending the magnetic fluid for further utilization as a putative biotechnological tool in plant growth. The compatibility with the vegetal structures results also from the vegetative origin of tartaric acid and from the magnetite biocompatibility. REFERENCES [1] R.E. Rosensweig. Ferrohydrodynamics. (Cambridge University Press, Cambridge, 1985). ´s. Magnetic nanofluids properties and some applications. Romanian [2] L. V´ eka Journal of Physics, vol. 49 (2004), no. 9–10 p. 707. [3] D. Bica. Preparation of magnetic fluids for various applications. Romanian Reports in Physics, vol. 47 (1995) no. 3–5, p. 265. [4] B.M. Berkovsky, V.F. Medvedev, M.S. Krakov. Magnetic Fluids: Engineering Applications. (Oxford University Press New York, 1993). [5] J. Roge, J.N. Pons, R. Massart, A. Halbreich, J.-C. Bacri. Eur. Phys. J. AP ., vol. 5 (1999), p. 321. [6] M.O. Aviles, A.D. Ebner, J.A. Ritter. Proc. 6th SCAMC ., vol. 5 (2006) p. 54. [7] L.M. Parkes, R. Hodgson, I. Robinson, D.g. Fernig, N.T.K. Thanh. Proc. 6th SCAMC ., vol. 5 (2006) p. 60. [8] G.C. Corneanu, M. Corneanu, C. Babeanu, D. Bica, L. Cojocaru. p. 447.

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