Multifunctional, multicompartment polyorganosiloxane magnetic nanoparticles for biomedical applications

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 1386–1388

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Multifunctional, multicompartment polyorganosiloxane magnetic nanoparticles for biomedical applications Stefanie Utech a,b, Christian Scherer b, Michael Maskos a,b, a b

¨ r Mikrotechnik Mainz GmbH (IMM), Carl-Zeiss-Str. 18-20, D-55129 Mainz, Germany Institut fu University Mainz, Institute of Physical Chemistry, Jakob-Welder Weg 11, D-55128 Mainz, Germany

a r t i c l e in f o

a b s t r a c t

Available online 21 February 2009

We present the synthesis and characterization of maghemite nanoparticles (average size 671.5 nm) and their incorporation into the core of polyorganosiloxane core–shell nanospheres (total average diameter 35710 nm). The nanoparticles are easily redispersable in organic solvents and can subsequently be modified by grafting of end-functionalized poly(ethylene oxide) to obtain water soluble nanospheres. The network structure of the nanospheres allows the diffusion of small molecules into the nanospheres, and consequently the nanospheres can be employed as nanocontainers and nanoreactors for potential biomedical applications. & 2009 Elsevier B.V. All rights reserved.

Keywords: Nanoparticle Maghemite Core–shell structure Polyorganosiloxane Nanosphere

Magnetic nanoparticles (magNPs) generate steadily growing interest for both research and potential biomedical applications including drug delivery, hyperthermia treatment, magnetic based cell separation, biosensors, and magnetic resonance contrast enhancement. Iron oxide magNPs are commonly and frequently used as magnetic carrier systems. On the other hand, increasing interest evolves in search for multifunctional nanoparticles, e.g. for multiplexing [1]. Some exciting examples include the combination of magNPs and quantum dots [2,3], or noble metal colloids such as gold [4]. To prevent undesired aggregation of the nanoparticles, for example in physiological environment, and thus loss of basic particle characteristics and properties, the magNPs need to be stabilized. A multitude of concepts have been reported in literature (e.g. [5–7]), including growth of silica shells. Besides the protective aspects, surface functionalization has been addressed and many interesting properties have been realized [8]. We report on the synthesis and characterization of magNPs and their incorporation in polyorganosiloxane core–shell nanospheres. Such nanospheres are nanonetworks where the interior can be accessed, for example by diffusion and, therefore, can be filled with appropriate small molecules [9]. For potential biomedical applications in aqueous solutions, the surface of the nanospheres can be modified by grafting end-functionalized poly(ethylene oxide), as shown before [10].

 Corresponding author at: Institut fu ¨ r Mikrotechnik Mainz GmbH (IMM), Carl-Zeiss-Str. 18-20, D-55129 Mainz, Germany. Tel.: +49 6131 990 130, +49 6131 39 24190; fax: +49 6131 990 205, +49 6131 39 22970. E-mail addresses: [email protected], [email protected] (M. Maskos).

0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.02.043

For the synthesis of the magNPs, we followed the route described by Hyeon et al. [11], leading to the formation of oleic acid-stabilized maghemite magNPs. Their average diameter as determined by TEM was 671.5 nm, excluding the surfactant layer. The mass fraction of maghemite was determined by thermogravimetric analysis (TGA, Pyris6 from Perkin) to be 15.2 wt%. The magNPs were subsequently incorporated into polyorganosiloxane core–shell nanospheres, as schematically shown in Fig. 1. The maghemite magNPs were further characterized by SQUID measurements showing the expected superparamagnetic properties including a saturation magnetization Ms ¼ 38 emu/g(gFe2O3). Electron diffraction and Mo¨ssbauer measurements confirmed the presence of g-Fe2O3. The incorporation of the stabilized maghemite magNPs into polyorganosiloxane nanoparticles to obtain magnetically responsive nanocontainers is a major challenge, because usually the long alkyl chains used for the stabilization will separate from the siloxane network. To make the magNPs compatible with the siloxanes, we first dispersed the magNPs in octadecyltrimethoxysilane (OD-T). Typically, 0.4 g of maghemite magNPs was dispersed in 1.8 g OD-T. The silane was capable of participation in the reactions of the siloxane network formation enabling the incorporation of the magNPs in the polyorganosiloxane nanospheres. Therefore, the OD-T modified magNPs were dispersed in a monomer mixture containing dimethyldimethoxysilane (D), methyltrimethoxysilane (T) and p-chloromethylphenyltrimethoxysilane (ClBz-T), which upon addition to the reaction mixture containing the surfactant dodecylbenzylsulfonic acid (typically, 0.2 g in 52 mL water) form the cores of growing nanospheres. Typically, 2.0 g D, 1.0 g T and 1.2 g ClBz-T were used. Subsequently, a monomer mixture containing D

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CH3 O

O

CH3

CH3 O

O O

+

D, T, ClBz-T,

Water,

γ-Fe2O3@OD-T

CI - OH

Endcapping

D, T

catalyst, surfactant

O Si O O

Fig. 1. Reaction scheme for the synthesis of magnetic polyorganosiloxane nanospheres. D ¼ diethoxydimethylsilane, T ¼ trimethoxymethylsilane, ClBz-T ¼ p-(chloromethylphenyl)trimethoxysilane, OD-T ¼ octadecyltrimethoxysilane.

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UV (254 nm) / a.u.

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Fig. 2. AF-FFF fractogram in water of magNP-loaded polyorganosiloxane nanospheres (solid line, bimodal) and non-loaded nanospheres (dashed) with the otherwise same composition (left); TEM pictures of the magNP-loaded sample showing the incorporated magNPs (right).

(1.2 g) and T (2.8 g) was added to form the shell of the particles. The size of the particles was determined by asymmetrical flow field-flow fractionation (AF-FFF) in aqueous dispersion and the obtained fractogram is shown in Fig. 2. The fractogram shows a bimodal distribution of particle sizes, which can be attributed to inherent segregation of cores into fractions containing magNPs (larger) and smaller spheres without magNPs, as confirmed by TEM, and which leads to the conclusion that two different nuclei start to grow. As known from previous work, this can be influenced by the amount of surfactant used to stabilize the growing particles [12,13]. TEM pictures also provided in Fig. 2 show the presence and incorporation of the magNPs in the fraction of larger polyorganosiloxane nanospheres. Depending on the magnetic properties, the magNP containing nanospheres could potentially be separated by application of magnetic fields. To obtain redispersable nanospheres, the still reactive surface of the particles needs to be passivated. This was achieved by first reacting with ethoxytrimethylsiloxane (M, 15.0 g) in aqueous solution, precipitation using methanol and subsequent transfer of the particles into organic solvents such as toluene (200 mL), followed by the repeated addition of M (3.2 g). Finally, the particles were precipitated using methanol and were fully redispersable, e.g. in toluene. Characterization included dynamic

light scattering (DLS) in toluene (average hydrodynamic diameter Dh ¼ 57.7 nm, m2(901) ¼ 0.164, where m240.1 indicates a polydisperse sample, which as in the present case can also originate from multimodality), TEM, shown in Fig. 2 (average diameter 35710 nm, determined by averaging 152 particles), thermogravimetric analysis, SQUID measurements and Mo¨ssbauer spectroscopy (Fig. 3). DLS yields an inverse z-average of the hydrodynamic size, whereas TEM gives a number averaged size, which explains the deviation due to the influence of the bimodality. The SQUID measurements confirmed the incorporation of the magNPs, also influenced by the diamagnetic contribution of the siloxane shell. The overall content of magNPs as determined by thermogravimetric analysis was approximately 1% of the total, non fractionated sample. This corresponds to an incorporation of approximately 85% of the administered magNPs into the polyorganosiloxane nanospheres. Also, the doublet structure observed in the Mo¨ssbauer spectrum confirms the presence of approximately 5 nm superparamagnetic magNPs, which fits very well with the size of the employed magNPs. The observed isomeric shift corresponds to Fe(III), relative to a-Fe [14]. In addition, the SQUID measurement shows the saturation magnetization of the employed magNPs, although the signal itself is disturbed due to the low overall content of g-Fe2O3 and the large diamagnetic part,

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80 60 40

M / emu g-1

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5.51 x 106

5.50 x 106

20 0 -20 -40 -60 -80

5.48 x 106 -4

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Fig. 3. Mo¨ssbauer spectrum of magNP-loaded polyorganosiloxane nanospheres (left); SQUID measurement of the same sample at T ¼ 5 K (right) in emu/g (g-Fe2O3).

which is seen by the scattering at high field strengths. Due to the small size of the magNPs and the incorporation of 1–3 magNPs per polyorganosiloxane nanospheres as determined from TEM, it has not yet been possible to magnetically fractionate the sample using standard neodymium magnets (NdFeB), which should be possible if larger magNPs are used. Simple text book based calculations [15] balancing the magnetic force (B(x) for a cylindrical magnet (diameter 2 mm, length 3 mm, remanescent magnetic induction Br ¼ 1.2 T) acting on a particle (magnetic core radius R and diamagnetic shell of 14.5 nm thickness) at a distance of 100 mm from the magnet yields a minimum particle radius R ¼ 19.3 nm when a viscosity of water is used for the media at a solution velocity of 100 mm/s. To conclude, the successful incorporation of magnetic g-Fe2O3 nanoparticles of 6 nm diameter into polyorganosiloxane nanospheres resulting in nanoparticles with 35 nm average diameter has been shown. The particles have been characterized by AF-FFF, TEM, SQUID and Mo¨ssbauer spectroscopy. As shown previously, the polyorganosiloxane nanospheres can be used as nanocontainers and nanoreactors [9,16], allowing potential incorporation of different functionalities in the future. In addition, the surface of the polyorganosiloxane can be modified by grafting with, for example, poly(ethylene oxide) to make the nanospheres water dispersable. This will also allow, for example, cell uptake experiments [16].

The authors would like to acknowledge additional financial support by the Deutsche Forschungsgemeinschaft DFG, Schwerpunktprogramm SPP1313 (BIONEERS), and the graduate schools POLYMAT and MAINZ. References [1] R. Kopelman, Y.-E. Lee Koo, M. Philbert, et al., J. Magn. Magn. Mater. 293 (2005) 404. [2] N. Insin, J.B. Tracy, H. Lee, et al., ACS Nano 2 (2008) 197. [3] D.K. Yi, S.T. Selvan, S.S. Lee, et al., J. Am. Chem. Soc. 127 (2005) 4990. [4] Y.L. Cui, L.Y. Zhang, J. Su, et al., Sci. Chin. B 49 (2006) 534. [5] U. Ha¨feli, W. Schu¨tt, J. Teller, M. Zborowski (Eds.), Scientific and Clinical Applications of Magnetic Carriers, Plenum, New York, 1997. [6] S.-Y. Yu, H.-J. Zhang, J.-B. Yu, et al., Langmuir 23 (2007) 7836. [7] Q. Zhang, M.S. Thompson, A.Y. Carmichael-Baranauskas, et al., Langmuir 23 (2007) 6927. [8] C.M. Niemeyer, Angew. Chem. Int. Ed. 40 (2001) 4128. [9] N. Jungmann, M. Schmidt, J. Ebenhoch, et al., Angew. Chem. Int. Ed. 42 (2003) 1713. [10] C. Diehl, S. Fluegel, K. Fischer, M. Maskos, Prog. Colloid Polym. Sci. 134 (2008) 128. [11] T. Hyeon, S.S. Lee, J. Park, et al., J. Am. Chem. Soc. 123 (2001) 12798. [12] N. Jungmann, M. Schmidt, M. Maskos, Macromolecules 34 (2001) 8347. [13] N. Jungmann, M. Schmidt, M. Maskos, et al., Macromolecules 35 (2002) 6851. [14] V.I. Goldanskii, R.H. Herber (Eds.), Chemical Applications of Mo¨ssbauer Spectroscopy, Academic Press, New York, 1968. [15] http://www.ibs-magnet.de. [16] N. Jungmann, M. Schmidt, M. Maskos, Macromolecules 36 (2003) 3974.