Stable Colloidal Suspension of Magnetic Nanoparticles for Applications in Life Sciences

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ScienceDirect Materials Today: Proceedings 2 (2015) 3813 – 3818

The Selected Papers of 10th International Conference on Physics of Advanced Materials, ICPAM10

Stable colloidal suspension of magnetic nanoparticles for applications in life sciences Emil Puscasua, Claudia Nadejdea, Dorina Creangaa*, Paul Fanninb, Cristian Pirghiec a Faculty of Physics, ”Alexandru Ioan Cuza” University, Bd. Carol I, no 11, 700506, Iasi, Romania Trinity College, Dept of Electronic and Electrical Engineering, The University of Dublin, College Green,Dublin 2, Ireland c Faculty of Mechanical Engineering, Mechatronics, Management,”Stefan cel Mare” University, Str. Universității 13, Suceava 720229 , Romania c

Abstract Various utilizations of magnetitc nanoparticles (MNPs), highly biocompatible, in biomedicine and environmental sciences require not only fine granulated powder but also stable dispersion in fluid media. MNPs were synthesized by chemical co-precipitation route. MNP surface modification was carried out with oleate ions as coating shell able to ensure colloidal MNP stability and also allowing further adding of secondary organic shell for conferring hydrophylicity or possibility of grafting various biomolecules. Granularity of colloidal MNPs was analyzed using microscopy techniques, crystalline properties by X-ray diffraction while superparamagnetic properties by magnetometry were assessed. The influences of magnetic and non-magnetic stirring, used alternatively during the preparation steps - on the microstructural and magnetic features are discussed. The MNP suspensions response to electromagnetic fields was studied by measuring the components of magnetic susceptibility. The investigation results suggested the usefulness of the yielding technology based on non-magnetic mixing during both ferrophase preparation and surface modification. © 2014Published Elsevier by Ltd. All rights © 2015 Elsevier Ltd. reserved. Selection andpeer-review peer-review under responsibility the conference committee of the 10th International onAdvanced Physics of Selection and under responsibility of theofconference committee of the 10th International ConferenceConference on Physics of Materials Materials. Advanced Keywords: iron oxide co-precipitation, microstructural investigation, magnetization curve, complex susceptibility;

* Corresponding author. Tel.: 040232201064; fax: 040232201150. E-mail address: [email protected]

2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of the conference committee of the 10th International Conference on Physics of Advanced Materials doi:10.1016/j.matpr.2015.08.008

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1. Introduction Magnetic nanoparticles with fine granularity in stable suspensions were studied extensively from viewpoint of their preparation and utilization in either technical or biomedical fields [1-3]. Chemical co-precipitation of magnetic metal oxides still remains one of the most available, versatile and less expensive technique for ferrophase particle preparation at relatively high temperature or even at room temperature [4-6]. Coating with oleate ions was one of the first methods that succeeded in yielding steric stabilization of single layer MNP core/organic shell systems in oily dispersion fluids or double layer similar systems in aqueous colloidal suspensions as current alternative option for MNP capping [710]. Less attention seems to be paid to the role of stirring method, i.e. mechanical or magnetic stirring which can influence ferrophase coating and thus dispersion efficacy. As summarized by Rosensweig [1] the stability of magnetic colloidal suspensions - against sedimentation in magnetic field gradient, in the gravitational field, and against magnetic dipole attraction - is strongly dependent on the solid phase granularity. Based on these theoretical considerations but also on practical observations it is generally accepted that for colloidal nanoparticles suspended within a carrier fluid with diameter less than 9-10 nm, stable magnetic suspension results. The role of the nonmagnetic surfactant shell is very important for ferrophase dispersion although the chemical or physical combination with the ferrophase increases the colloidal particle diameter by simultaneous diminution of the magnetic core. One of the most available and convenient ways of magnetite MNP synthesis is the alkaline hydrolysis of iron (II) and iron (III) salts. Since we are unaware of reporting of any research on the influence of the mixing method on the ferrophase granulation, the present study is focused on the comparison of magnetic versus non magnetic method during suspension finalization. 2. Experimental The basic procedure of Massart [2] was applied for the co-precipitation in alkali medium (NaOH 25%) of ferric and ferrous oxides from FeSO4u7H2O and FeCl3u6H2O in their stoichiometric ratio. All reagents were pure chemicals from Merck. During ferrophase precipitation the mechanical stirring (for 60 min) was carried out to favor chemical processes. Mechanical stirring device was based on neutral glass stick with four 2 cm branches rotating with 1200 rpm. Solid phase was separated by filtration and repeatedly washed with deionized water, acetone and ethanol. The MNPs were mixed with oleic acid (10% v/v) in hexane, under continuous stirring, for 40 min; kerosene was added after hexane vaporization at over 80 ºC. Oleic acid was chosen since it is known to develop strong interactions with iron ions; also oleate coating allows further surfacting with suitable organic molecules to confer hydrophylicity and thus dispersion in aqueous media for environmental and biomedical applications. During this second technological step magnetic and respectively mechanical stirring were applied for 60 min at about 80 ºC leading to MNP1 and MNP2 sample finalization. Ferrophase mechanical mixing with oleic acid was done by similar procedure applied for ferrophase yielding. Magnetic stirring was carried out with laboratory device based on permanent magnet controlled rotation –at 1200 rpm; 2 cm Teflon coated magnet bar was immersed into the reaction beaker placed on adjustable temperature hot plate. The microstructural characteristics of MNPs deposited on collodion sheet after dilution (10 4 in toluene) were imaged by Transmission Electron Microscopy (TEM), with Tesla device (resolution of 1.0 nm). AFM device worked in the tapping mode with standard silicon nitride cantilever (NSC21- force constant of 17.5 Nm-1, 210 kHz resonance frequency, tip radius of 10 and 20 nm). Diluted MNP suspensions were deposited on a mica substrate. Crystallinity with X-Ray Diffraction (XRD) device Shimadzu 6000 (Cu-Kα radiation) was analyzed. Magnetometry was carried out by Gouy’s method [11] at 25.0 ± 0.1 ºC; Walker Scientific MG 50D Gaussmeter with Hall probe was used to measure magnetic field. Complex magnetic susceptibility measurements (frequency from 100 Hz to 1MHz), were carried out by Toroidal Technique [12] in conjunction with a HP 4192A RF Bridge. 3. Results and discussions X-ray diffraction raw data (Table 1) evidenced typical spinel crystallites with characteristic peaks at practically standard positions [13]. The main qualitative difference is related to the lowest angle diffraction peak indicating probably the presence of maghemite/magnetite mixture in MNP2 sample. In Fig. 1 the TEM images show most of MNPs as relatively symmetrical (quasi-spherical) structures with polydispersity. Higher incidence of particle agglomeration appeared in MNP2 sample (Fig. 1 b). The box-plot

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technique [14], provided a suggestive comparison of the two MNP batches size distributions. From box edges Q1 and Q3, corresponding to 25% and 75% of cumulated data frequency, the box tails A1 and A3 (Table 2), were estimated as Q1-1.5(Q3-Q1A3>Q3 [4].

a

b

Fig. 1. (a) Ferrophase particles revealed by TEM investigation of MNP1 batch (bar size- 20 nm); (b) Ferrophase particles revealed by TEM investigation of MNP2 batch (bar size- 20 nm). Table 1. X-ray diffraction raw data. Peak

(111)

(220)

(311)

(400)

(422)

(511)

(440)

2θ (°) MNP1

-

30.2

35.5

43.3

53.7

57.3

62.8

2θ (°) MNP2

23.9

30.3

35.6

43.3

53.7

57.3

62.9

The comparative analysis of MNP diameter distribution (Fig. 2) revealed median values dTEM (corresponding to 50% of the cumulated frequency of diameter size incidence) of 13.6 (MNP1) and 14.9 nm (MNP2), respectively. Exceptional large diameters (symbol “R”) of up to 35.6 nm and 25.5 nm were found according to the inequalities Q1-3(Q3-Q1)< R R>A3 [4]: Table 2. Granularity of MNP batches given TEM and magnetometry investigation. Sample

dTEM (nm)

Q1

Q3

A1

A3

dM (nm)

MS (emu/g)

MNP1

14.9

7.5 nm

16.5 nm

3.5 nm

30.1 nm

12.8

53.1

MNP2

13.6

9.8 nm

17.1 nm

3.7 nm

22.5 nm

11.5

58.0

TEM data processing (Table 2, Fig. 2) shown narrower size distribution for the MNP2 sample where non-magnetic stirring was applied in both preparation steps; also the mean physical diameter dTEM and rare exceptionally large aggregates were smaller (Table 2) than those of the MNP1 sample. AFM 3-D investigations (Fig. 3) could visualize only largest particles or aggregates, with height reaching up to 40 nm (MNP1) and respectively up to 10 nm (MNP2). This could be the result of not only pure magnetite yielding, but also of the presence of maghemite with lower magnetization and consequently larger particle diameter that could lower the stability over time (Table 2). This fact could be associated with the relatively long durations of mixing steps and washings. Magnetization curves exhibited no hysteresis indicating superparamagnetic behavior with saturation magnetization (Fig. 4 a) of 53 and 58 emu/g respectively. The upper limit of the magnetic diameter, dM, of the ferrophase colloidal particles was assessed (Table 2) according to Langevin’s theory [15]:

d M3

§ 18k BT ·§ dM · ¨¨ ¸¸¨ ¸ SP M M © 0 b s ¹© dH ¹ H o 0

(1)

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where, kB is Boltzmann constant, μ0 is the free space magnetic permeability, H is magnetic field intensity, M is the magnetization, Ms and Mb are the saturation magnetizations of the sample and of the bulk magnetite [1] while (dM/dH) is the slope in the magnetization graph origin (where H is zeroed). The saturation magnetization (Table 2) was found to be higher for the MNP2 sample (58 emu/g compared to 53.1 emu/g for MNP1) while the magnetic core diameter (11.5 nm) was smaller than for MNP1 batch (12.8 nm) - with approximately equal width values of the coating shell of less than 2 nm for both samples. These values are also slighter higher than the theoretically estimated ones found for the cases of magnetic or non-magnetic stirring during ferrophase coating. The results of this experimental study are concordant with those reported by other authors [7, 16] that yielded single layer oleic acid magnetic nanoparticle systems with 10 nm and respectively 12-14 nm size.

Fig. 2. TEM diameter distributions of the two batches (MNP1 and MNP2) shown by the box-plot representation (M is dTEM)

a

b

Fig. 3. (a) AFM 3-D scanning (1.5u1.5 μm) of MNP1; (b) AFM 3-D scanning (1u1μm) for MNP2.

The MNP suspension response to electromagnetic field with frequency from 100 Hz to 1MHz (Fig. 4 b) was studied by measuring the real χ` and imaginary χ`` parts of its magnetic susceptibility, χ(ω), by means of the Toroidal technique [12], where ω is the pulsation:

F (Z )

F `(Z )  iF ``(Z )

(2)

There are two distinct mechanisms by which the magnetization of ferrophase suspension may relax after the applied field has been removed: either rotational Brownian motion [17] of the particle within the carrier liquid, with its magnetic moment locked in an axis of easy magnetization, or by rotation of the magnetic moment within the particle, Néel relaxation [18]. However, over the frequency range measured here Brownian relaxation is considered to be dominant. The time associated with the rotational diffusion is the Brownian relaxation time τB, is:

WB

3VHK k BT

(3)

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where VH is the MNP hydrodynamic volume, η is the dynamic viscosity of the dispersion fluid, fmax (i.e. 1/2SWB) is the frequency at which the χ``(ω) component reaches a maximum. For MNP2 sample (Fig. 4 b), the maximum of χ`` component occurs at a frequency of fmax = 2 kHz. a

b

Fig. 4. (a) Magnetization curves: magnetic moment per unit mass versus applied magnetic field; (b) complex magnetic susceptibility versus the frequency of applied electromagnetic field (normalized data)

Based on the above equations and using values of T=293 K and η for the carrier fluid (kerosene) equal to 2.5u10-6 cSt, the corresponding VH was estimated at about 37 nm; this is an indication of aggregation occurring during magnetic exposure to alternative fields. For MNP1 sample, susceptibility measurements failed in providing any meaningful data. This could have resulted from the reaction of the sample to the applied alternative field and also related to factors discussed above, which include the formation of multiple size aggregates, as revealed by TEM data box-plot representation where at least extreme large values could be identified for MNP2 (Fig. 3). This fact seems particularly important in choosing the preparation method for obtaining stable MNP suspensions with small width distribution of the physical diameter, where exposures to electromagnetic fields are necessary. We may say that the utilization of magnetic stirring during ferrophase dispersion into the carrier fluid, whilst being more vigorous than the mechanical mixing, has still favored small particle agglomeration around the stirring bar which has led to larger coated colloidal nanoparticles than in the case of the mechanical mixing; the most important inconvenient is consistent with the various large size aggregates that compromise the coherent behavior following the action of electromagnetic fields which could be unavoidable in some practical situations. Regarding the applications in life sciences of magnetic core/oleic acid systems dispersed in hydrocarbons, diluted suspensions were found useful in studying the influence on vegetal cells in early ontogenetic stages due to the convenient homogeneous mixing with agarized culture media that solidify at room temperature [19-20]. 4. Conclusion We conclude that MNPs can be better stabilized in kerosene suspension if non-magnetic stirring is applied during whole preparation technology since the resulting physical diameter is narrower than that found in the case of involving magnetic stirring procedure. Although saturation magnetization and magnetic core diameter are not much different for the two analyzed batches, the presence of larger aggregates seems to be the main reason for the magnetic instability risk during the exposure to electromagnetic fields, as evidenced for the MNP1 sample. Further optimization of the preparation protocol is planned by varying the stirring duration in order to improve the stability of oily MNP suspensions prepared by chemical co-precipitation and intended for use in various applications. Acknowledgements P.C.F acknowledges support from ESA.

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References [1] R.E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, Cambridge, 1985. [2] R. Massart, IEEE Trans. Magn. 17 (1981) 1247-1248. [3] R.M. Cornell, U. Schertmann, Iron oxides in the laboratory: preparation and characterization, VCH Publishers, Weinheim, Germany, 1991. [4] M. Nidhin, R. Indumathy, K.J. Sreeram, B.U. Nair, Bull. Mater. Sci. 31 (2008) 93–96. [5] C.C. Hua, S. Zakaria, R. Farahiyan, L.T. Khong, K. Nguyen, M. Abdullah, S. Ahmad, Sains. Malaysiana 37 (2008) 389–394. [6] D.K. Kim, Y. Zhang, W. Voit, K.V. Rao, M. Muhammed, J. Magn. Magn. Mater. 225 (2001) 30-36. [7] M. Bloemen, W Brullot, T.T. Luong, N. Geukens, A. Gils, T. Verbiest, J. Nanopart. Res. 14 (2012) 1-10. [8] M. Klokkenburg, J. Hilhorst, B.H. Erne, Vib. Spectrosc. 43 (2007) 243–248. [9] M. Mahdavi, M.B. Ahmad, M.J. Haron, F. Namvar, B. Nadi, M.Z.A. Rahman, J. Amin, Molecules 18 (2013) 7533-7548. [10] W.Wu, Q. He, C. Jiang, Nanoscale Res. Lett. 3 (2008) 397–415. [11] A. Saunderson, Phys. Educ. 3 (1968) 272–273. [12] P. C. Fannin, B.K.P. Scaife, S.W. Charles, J. Phys. E-Sci. Instr., 19 (1986) 238 – 239. [13] http://www.google.com/patents/WO2008131528A1?cl=en. [14] L.H. Koopmans, Introduction to contemporary statistical methods, Duxbury, Boston, 1987. [15] C. Kittel, Introduction to Solid State Physics, John Wiley & Sons, New York, 2005. [16] J. Liang, H. Li, J. Yan, W. Hou, Energy Fuels, 28 (2014) 6172–6178. [17] W.F. Brown, J. Appl. Phys. 34 (1963) 1319-1321. [18] L. Neel, Ann. Geophys. 5 (1949) 99-136. [19] A. Pavel, M. Trifan, I.I. Bara, D. Creanga, C. Cotae, J. Magn. Magn. Mater. 201 (1999) 443-445. [20] A. Manoliu, L. Antohe, D. Creanga, C. Cotae, J. Magn. Magn. Mater. 201 (1999) 446-449.