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Superparamagnetic iron oxide nanoparticles incorporated into silica nanoparticles by inelastic collision via ultrasonic field: Role of colloidal stability Bashiru Kayode Sodipo and Abdul Aziz Azlan Citation: AIP Conference Proceedings 1657, 100002 (2015); doi: 10.1063/1.4915209 View online: http://dx.doi.org/10.1063/1.4915209 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1657?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of sonication on the colloidal stability of iron oxide nanoparticles AIP Conf. Proc. 1657, 040006 (2015); 10.1063/1.4915167 Maneuvering the chain agglomerates of colloidal superparamagnetic nanoparticles by tunable magnetic fields Appl. Phys. Lett. 105, 183108 (2014); 10.1063/1.4901320 Controlled synthesis of superparamagnetic iron-oxide nanoparticles by phase transformation J. Appl. Phys. 111, 07B520 (2012); 10.1063/1.3676232 Energy absorption of superparamagnetic iron oxide nanoparticles by microwave irradiation J. Appl. Phys. 97, 10J510 (2005); 10.1063/1.1859212 Rheological study of the stabilization of magnetizable colloidal suspensions by addition of silica nanoparticles J. Rheol. 47, 1093 (2003); 10.1122/1.1595094

Superparamagnetic Iron Oxide Nanoparticles Incorporated into Silica Nanoparticles by Inelastic Collision via Ultrasonic Field: Role of Colloidal Stability Bashiru Kayode Sodipo and Abdul Aziz Azlan Nano-Optoelectronics Research and Technology (NOR) Lab, School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia Nano-Biotechnology Research and Innovation (NanoBRI), Institute for Research in Molecular Medicine (INFORMM), Universiti Sains Malaysia, 11800, Pulau Pinang, Malaysia Abstract. Superparamagnetic iron oxide nanoparticles (SPION)/Silica composite nanoparticles were prepared by ultrasonically irradiating colloidal suspension of silica and SPION mixture. Both silica and SPION were synthesized independently via co-precipitation and sol-gel method, respectively. Their mixtures were sonicated at different pH between 3 and 5. Electrophoresis measurement and other physicochemical analyses of the products demonstrate that at lower pH SPION was found incorporated into the silica. However, at pH greater than 4, SPION was unstable and unable to withstand the turbulence flow and shock wave from the ultrasonic field. Results suggest that the formation of the SPION/silica composite nanoparticles is strongly related to the inelastic collision induced by ultrasonic irradiation. More so, the formation the composite nanoparticles via the ultrasonic field are dependent on the zeta potential and colloidal stability of the particles. Keywords: Acoustic cavitation, Composite nanoparticles, Colloidal stability, SPION, Silica, zeta potential PACS: 81

INTRODUCTION Recently, research on the synthesis iron oxide/silica composite nanoparticles is of intense interest due to their numerous physical, chemical and biomedical applications [1-3]. Several synthetic routes such as sol-gel, reverse microemulsion, combination of microemulsion and sol–gel, wet impregnated and gas phase or thermal method [4-8] have been deployed to synthesized the composite nanoparticles. Both sol-gel and the microemulsion routes are seed mediated growth approach that involved the use of SPION as a template for the growth of silica. On the other hand, the gas phase involves thermal decomposition of iron oxide and silica precursor mixture at high temperature to form iron oxide/silica composite nanoparticles [9]. The wet impregnation method is based on the soak of a trivalent iron salt over a calcined silica and subsequent reduction at high temperatures under a reducing atmosphere [10]. However, literatures have reported the seed mediated approach (sol-gel) as the most cited and adopted method of synthesising iron oxide silica composite nanoparticles. These could be due to its advantages over the other methods such as relatively mild reaction condition, low cost and surfactant-free [11]. However, recently we demonstrated that via a non-seeded approach SPION/silica composite nanoparticles can be synthesized [12]. A sizecontrolled colloidal suspension of silica and superparamagnetic iron oxide nanoparticles (SPION) were synthesized separately. Subsequently, the unique conditions such as shock wave plus turbulence flow generated along with high temperature and pressure from acoustic cavitation process were employed to ultrasonically irradiate mixture of both SPION and silica nanoparticles. Therefore, the SPION were found incorporated into the silica framework. In this paper, we systematically study the role of colloidal stability of the system and the formation of the iron oxide/silica composite nanoparticles via ultrasound assisted non-seeded process.

MATERIAL AND METHOD Ferric chloride hexahydrate (FeCl3.6H2O >99%), Ferrous chloride tetrahydrate (FeCl2.4H2O >99%), Sodium chloride salts, Sodium hydroxide, Triethoxyvinylsilane (TEVS), 1-butanol, Ammonia and perchloric acid (HClO4) were purchased from Sigma Aldrich and used directly without any further purification. SPION with sizes less than 10 nm were synthesized as reported in our previous work [13]. Briefly, both Fe3+ and Fe2+ were co-precipitated at the ratio of 2 to 1 with sodium chloride solution in an inert environment of nitrogen gas. The sizes of the National Physics Conference 2014 (PERFIK 2014) AIP Conf. Proc. 1657, 100002-1–100002-5; doi: 10.1063/1.4915209 © 2015 AIP Publishing LLC 978-0-7354-1299-6/$30.00

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nanoparticles were controlled by maintaining the temperature and pH of the process at room temperature and 9, respectively. The silica nanoparticles were synthesized by a modified sol-gel method [14]. Briefly, 200 µL of 10 M ammonia and 6 mL of 1-butanol were added to 200 mL of distilled water, and agitated at 4 rpm for few minutes. At room temperature 2 mL triethoxyvinylsilane (TEVS) was added to the solution and agitated at 320 rpm for 1 hour. Unreacted chemicals were separated for 4 days via dialysis using cellulose membrane. The SPION/silica composite nanoparticles were synthesized by ultrasonically irradiating SPION and silica nanoparticles mixtures as reported in our previous publication [12]. SPION and silica nanoparticles were mixed at volume ratio of 1:1, pH of the system was adjusted with NaOH to 3.5, 4.25 and 5, and then ultrasonically irradiated for 30 min. The products are denoted as IS3, IS4 and IS5, respectively. For proper heat dissipation the sonochemical reaction were carried out in an iced bath environment.

RESULT AND DISCUSSION It is well-known that ultrasonic irradiation of liquid medium generates acoustic cavitation process (formation, growth and collapse of bubbles) with huge temperatures and pressure of 5000 K and 1000 atm, respectively. Alongside with this cavitation process shockwaves or microjets are often generated [15]. The production of the either shock waves or microjets depends on the environmental medium. In homogenous medium, spherical (symmetrical) cavitation plus shock waves are formed while in heterogeneous environment asymmetrical cavitation and microjets are produced at the solid/liquid interface. However, the microjets formation depends on the size of the solid surface in the medium. The size of the material should be greater than the size of the collapsing bubbles which is ~150 µm [16, 17]. Otherwise, spherical cavitation and shock waves is formed. In the ultrasonic irradiation of our nanomaterials whose various size distributions were much less than 150 µm (Figure 1), symmetric cavitation and shock waves were produced. As demonstrated in Figure 1A and 1B, the size of the SPION and the silica nanoparticles were ~10 nm and ~50 nm, respectively. The ultrasonic irradiation of the SPION and silica nanoparticles mixture produces shock waves travel with huge velocity and pressure. Consequently, it causes turbulence flow and stirring of the liquid content. Therefore, the nanoparticles were in continuous random motion and collision with one another. Due to the intraparticle collision between the iron oxide and silica nanoparticles, after the sonication period the SPION were found embedded into the silica nanoparticles (Figure 1C). The size distribution in Figure 1C reveals that silica is an elastic material. Following the incorporation process, the silica contracted on the SPION. The formation of SPION/silica composite nanoparticles is demonstrated via electronic spectroscopy imaging (ESI) mapping of Figure 1C. The maps reveal the presence of Fe, O and Si which correspond to Figure 1D, E and F. More so, all the elements occupied same position in the micrograph. This shows SPION successfully embedded into the silica nanoparticles. The FTIR spectra shown in Figure 2a and 2b are the as-synthesized and composite nanoparticles respectively. The presence of prominent peaks such as 443, 583 and 634 cm-1 which can be assigned to Fe-O in both IR spectra confirmed the as-synthesised particles to be iron oxide and composite nanoparticles contain iron oxide, respectively. Unlike in Figure 2a, the occurrence of asymmetric stretching vibrations peaks of siloxane (Si-O-Si) bond at 1110 and 1050 cm-1 in Figure 2b confirmed binding of the silica nanoparticles to the SPION. Furthermore, the energy absorbed at 874 cm−1 in Figure 2b is a characteristic of Si-O stretching vibration. The band at 3420 cm-1 and 1630 cm-1 in both spectra are due to the stretching and bending vibration of H2O molecules, respectively. However, the 3787 cm-1 shoulder in Figure 2b can be related to the presence of silanol vibration. The band that appeared at 2926 cm-1 in Figure 2b can be assigned to the C-H stretching vibration of the ethyl group. The presence of this C-H band in the IR of the composite nanoparticles can be due to incomplete hydrolysis of the silica precursor.

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FIGURE 1: TEM micrograph of (A) SPION, (B) Silica (C) SPION/Silica Composites Nanoparticles. The incorporation of the SPION into the silica nanoparticles is demonstrated through the ESI mapping of the composite nanoparticles, where the various maps show (D) Fe, (E) O and (F) Si.

FIGURE 2: FTIR spectra showing (a) iron oxide nanoparticles (b) iron oxide/silica composite nanoparticles. The binding silica to the Iron oxide nanoparticles is confirmed with the prominent siloxane peaks at 1110 and 1050cm-1.

The XRD spectrum shown in Figure 3, further confirmed the formation of iron oxide/silica composite nanoparticles. Like Figure 3a and 3b, the prominent peaks of IONP such as magnetite (Fe3O4) or maghemite (γFe2O3) pattern were observed. It is difficult to match these spectra with either Fe3O4 or γ-Fe2O3 based on only XRD analysis. However, as shown in the inset, XRD pattern of amorphous Silica is identified with a diffusive peak that

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emerges at about 20o-30o [18, 19]. Consequently, in Figure 3b, the presence of amorphous silica in the composite nanoparticles was observed by the hump that appeared between 20o and 30o. In addition, as observed in Figure 3b the amorphous silica caused reduction in the initial sharpness of the iron oxide peaks.

FIGURE 3: XRD pattern showing (a) naked SPION with the prominent pattern of magnetite nanoparticles (b) SPION/silica composite nanoparticles, the presence of silica shell is observed with the diffuse peak that appeared between 20o and 30o

However, electrophoresis measurement shows a good colloidal stability of the particles is required for the successful incorporation of the SPION into the silica nanoparticles. Colloidal stability of a suspension is often measured via its zeta potential value (ζ). Moreover, suspension with |ζ| values between ±40 mV and ±60 mV are considered stable [20]. As illustrated in Table 1, prior to the ultrasonic irradiation of the suspensions, all the samples display good colloidal stability. On the other hand, at a pH less than or equal to 3, the system might be too acidic. In addition, above pH of 5 the suspension was found unstable. This is due to the point zero charge (PZC) or isoelectric point (IET) of iron oxide nanoparticles which was reported to be around pH of 7 [14]. After the ultrasonic irradiation of the various samples, only IS3 was found to retain it colloidal stability with nearly 90% of its initial ζ. More importantly, the incorporation of the SPION into the silica nanoparticles was only obtained in the sample IS3. However, in samples IS4 and IS5 the particles were found settled at the bottom of the container. This implies that the incorporation of SPION into the silica through inelastic collision under the influence of ultrasonic irradiation depend mainly on the colloidal stability of the suspensions. TABLE (1). Illustrating the influence of ultrasonic irradiation on the colloidal stability of the iron oxide nanoparticles. Zeta potential (mV) Samples pH Before sonication After sonication IS3 3.50 45 41 IS4 4.25 40 10 IS5 5.00 35 0

CONCLUSION We have demonstrated that ultrasonic irradiation of SPION and silica nanoparticle mixture leads to formation of SPION/silica composite nanoparticle. The physico-chemical analyses reveal that the SPION was embedded into the silica. More importantly, ultrasonic irradiation of the mixture at different pH and zeta potential values shows that good colloidal stability is required for successful incorporation of SPION into the silica nanoparticles.

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ACKNOWLEDGMENTS This work is supported by the Universiti Sains Malaysia through FRGS grant 203/PFIZIK/6711351.

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