Ultrasonics Sonochemistry 23 (2015) 354–359
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Non-seeded synthesis and characterization of superparamagnetic iron oxide nanoparticles incorporated into silica nanoparticles via ultrasound Bashiru Kayode Sodipo ⇑, Azlan Abdul Aziz 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
a r t i c l e
i n f o
Article history: Received 27 August 2014 Received in revised form 24 September 2014 Accepted 25 September 2014 Available online 5 October 2014 Keywords: Acoustic cavitation Composite nanoparticles Inelastic collision Shockwave Silica SPION
a b s t r a c t A non-seeded method of incorporating superparamagnetic iron oxide nanoparticles (SPION) into silica nanoparticles is presented. Mixture of both SPION and silica nanoparticles was ultrasonically irradiated. The collapsed bubbles and shockwave generated from the ultrasonic irradiation produce tremendous force that caused inelastic collision and incorporation of SPION into the silica. Physicochemical analyses using transmission electron microscope (TEM), electronic spectroscopic imaging (ESI), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy demonstrated the formation of SPION/silica composite nanoparticles. The prepared composite nanoparticles exhibited superparamagnetic behaviour and nearly 70% of the initial saturation magnetization (Ms) of the SPION was retained. The presence and reactivity of the silica were demonstrated via assembling decanethiol monolayer on the composite nanoparticles. The silanol group of the silica provided the binding site for the alkyl group in the decanethiol molecules. Therefore, the thiol moiety became the terminal and functional group on the magnetic composite nanoparticles. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Superparamagnetic iron oxide nanoparticles (SPION) are material of interest for biomedical research and related applications such as magnetic resonance imaging (MRI) contrast agent, biocatalyst, biomarker, biosensor, drug delivery and hyperthermia therapy [1–4]. For these functions, stable SPION is required [5]. However, owing to energetic surface of SPION, the nanoparticles usually agglomerate in ionic medium [6]. Surface modification of SPION with biocompatible material is one of the methods used to control problems related to agglomeration of SPION [7]. Silica nanoparticle is among the preferred inorganic materials employed to modify the surface of SPION. Silica surfaces are chemically stable, biocompatible and can be easily functionalized for bioconjugation purpose [8]. Recently, many routes such as sol–gel, microemulsion, combination of microemulsion and sol–gel, reverse micelles, wet impregnated and hydrothermal method [9–14] were used to synthesize iron oxide/silica composite nanoparticles. As shown in Table 1,
⇑ Corresponding author at: Nano-Optoelectronics Research and Technology (NOR) Lab, School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia. E-mail address: [email protected] (B.K. Sodipo). http://dx.doi.org/10.1016/j.ultsonch.2014.09.011 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
the high saturation magnetization (Ms) value of the SPION is drastically reduced after their surface modification with silica. Unlike, most of the conventional methods which are based on seed growth mediated approach where SPION is used as a template for the growth of the silica; in this paper we report a non-seeded process of modifying the surface of SPION with silica. Colloidal suspension of both silica nanoparticles and SPION were synthesized separately. Subsequently, ultrasonic field was employed to incorporate SPION into the silica framework. The incorporation is due to intraparticle and inelastic collision between the silica and SPION, caused by turbulent flow from the shock waves and acoustic cavitation process generated by the ultrasonic irradiation. The structure and high saturation magnetization of the composite nanoparticles are demonstrated via physico-chemical analyses of the product. More importantly, the successful modification of the SPION’s surface with silica is validated through grafting of decanethiol monolayer on the composite nanoparticles. 2. Materials and method Ferric chloride hexahydrate (FeCl24H2O 99%), Ferrous chloride (98%), ammonium hydroxide (25 wt.%), sodium chloride salts, sodium hydroxide, 1-butanol (99%), triethoxyvinylsilane (TEVS 97%), and 1-Decanethiol (99%) were bought and used directly without any further purification from Sigma–Aldrich. The
B.K. Sodipo, A. Abdul Aziz / Ultrasonics Sonochemistry 23 (2015) 354–359
ultrasonic irradiation was carried out using 20 kHz Vibra-Cell ultrasonic with 13 mm diameter horn. 2.1. Synthesis of silica incorporated SPION The composite nanoparticles were synthesized in three steps: (1) synthesis of SPION, (2) synthesis of silica nanoparticles and (3) incorporation of SPION into the framework of silica nanoparticles via ultrasonic irradiation. The SPION was synthesized according to a co-precipitation as reported in our recent work [19]. Briefly, under degassed environment, room temperature and apparent pH of 10, Fe3+ and Fe2+ solution were co-precipitated at the ratio of 2 to 1 with 1 M sodium chloride solution. The system was mechanically stirred at 500 rpm and left to agitate for 2 h. The prepared particles were collected with magnets and washed using deionized water. The magnetic particles were divided into two portions. Part A and B are peptized with 3.5 M perchloric acid (HClO4) and 2.1% w/w of tetramethyl ammonium hydroxide (TMAOH), respectively. The colloidal silica nanoparticles were synthesized by a sol–gel method as reported [20]. Briefly, to 200 mL of distilled water, 200 lL of 10 M ammonia and 6 mL of 1-butanol were added and agitated at 4 rpm for few minutes. Then, 2 mL of triethoxyvinylsilane (TEVS) was added and agitated at 320 rpm and room temperature for 1 h. Unreacted chemicals were separated through dialysis using cellulose membrane for 4 days. At volume ratio of 1:1, colloidal suspension of SPION (part A) and silica nanoparticles were mixed. The pH was slightly adjusted to 3.5 and the mixture was sonicated for 30 min. For heat dissipation the ultrasonic irradiation were carried out in an iced bath environment. The product is named as sample H. The same procedure was repeated for part B with the pH adjusted to 10 and the product is named sample T. 2.2. Characterization The morphology and mapping of the various elements present in the composite nanoparticles were determined through electronic spectroscopy imaging (ESI) technique using energy filtering transmission electron microscopy (EFTEM) Zeiss Libra 120. The XRD of the prepared nanoparticles was characterized using D/ max-IIIC X-ray diffractometer (Shimadzu, Japan). Their peaks were compared to the JCPDS 5-0664 of International Centre for Diffraction Data to determine the crystalline structures. Fourier transforms infrared (FTIR) spectroscopy Perkin Elmer System 2000, was used to examine the binding of silica to SPION. The magnetization measurements were obtained at room temperature in magnetic field up to 10 kOe using DMS Vibrating Sample Magnetometer 8810 (10NRM). 3. Result and discussion It is well known that ultrasonic irradiation of liquid generates acoustic cavitation (formation, growth and collapse of bubbles in
Table 1 Comparing the saturation magnetization of silica coated SPION. Author
Yang et al. Sun et al. Zhang et al. Vogt et al. Gao et al.
Year
2004 2004 2008 2010 2011
Saturation magnetization Ms (emu/g) Uncoated SPION
Silica–SPION
– 46.3 – 81 –
3.2 13.9 15 17.6 12.7
Ref.
[15] [16] [17] [18] [8]
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the liquid). The cavitation creates a unique interaction between energy and matter, with temperature, pressure and cooling rate of approximately 5000 K, 1000 atm and 1010 Ks 1, respectively [21]. These extraordinary conditions permit access to wide range of chemical reactions and synthesis varieties of unusual nanostructured materials [22]. The dynamics of the growth and the collapse of cavity are dependent on local environment. The environment can either be homogenous liquid or heterogeneous solid–liquid interface. In homogenous liquid, spherical cavity is formed. Acoustic cavitation in homogenous liquid generates implosive bubble plus shock waves [23]. The shock waves produce high pressure with amplitudes exceeding 10 kbar [24]. Acoustic cavitation near solid–liquid interface (heterogeneous medium) is asymmetric and associated with high-speed microjets which impact the solid surface and cause mechanical damage [25–27]. The potential energy of the collapsed bubble is converted into kinetic energy of the microjet with velocities hundreds of meters per second. However from the report of Suslick et al. [28], at ultrasonic field of 20 kHz, solid particles with size less than the diameter of the collapsing bubble (150 lm) cannot cause microjet formation, instead a normal cavitation and emission of shock waves will occur [25,29]. Therefore, during the 20 kHz ultrasonic irradiation of our nanoparticles whose sizes are much less than the size of bubbles formed, normal acoustic cavitation were formed with shock waves production. Similar to what Suslick et al. [30] demonstrated with metallic nanoparticles, we observed fusion of silica nanoparticles into one another, when they were ultrasonically irradiated. As shown in Fig. 1, before ultrasonic irradiation of the colloidal suspension of silica, the particles were originally separated except few ones that overlap (Fig. 1A). After the sonication period, the extremely high speed from the turbulent flow and shock waves generated by the ultrasonic irradiation, induces effective melting at the point of impact [28]. As shown in Fig. 1B, fusion and neck formation between the silica particles were observed. The 30 min ultrasonic irradiation produce power (40 W), acoustic cavitation, shockwaves and rapid cooling that led to high velocity interparticle collision, melting and fusion of the silica nanoparticles. However, in an attempt to develop a non-seeded method of encapsulating SPION with silica, we investigated further with intraparticle collision between silica and SPION. Unlike the fusion and the neck formation of the silica nanoparticles, we observed the incorporation of SPION into the silica framework when their mixture was ultrasonically irradiated at apparent pH of 3.5. Fig. 2A is the micrograph of the SPION before mixing and irradiating ultrasonically with silica nanoparticles. The particles were well dispersed with sizes less than 10 nm. However, the ultrasonic irradiation of the mixture produced turbulent flow and shock waves that led to intraparticle collision between the SPION and the silica. As shown in Fig. 2B, after the sonication period, SPION was found embedded in the silica nanoparticles. Elemental mapping of the composite nanoparticles’ micrograph using ESI technique, demonstrated the formation of SPION/silica composite nanoparticles. The micro-analytical analysis (Fig. 3) reveals the various elements such as Fe, O and Si present in the product. Fig. 3A–C correspond to the maps of iron, oxygen and silicon. The maps demonstrate that each element present in the micrograph occupied same position. This can be related to the formation of composite nanoparticles. However, bright halo patches can be observed from the interior of the silicon maps. This can be correlated to the presence of pores in the silica nanoparticles. BET analysis of the prepared particles further confirmed the porosity of the composite nanoparticles with average pore diameter of 7.2 nm and total pore volume 0.0407 cm3/g. The nanocomposite particles are nearly spherical in shape with average size of 10 nm.
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Fig. 1. TEM micrograph with scale bar 100 nm of colloidal silica (A) before sonication (B) after ultrasonically irradiated showing fusion of the particles due to localized melting caused by the collisions.
Fig. 2. TEM micrograph with scale bar 100 nm of SPION (A) before collision with silica (B) after inelastic collision which led to the incorporation of SPION into the silica.
Fig. 3. Showing the maps of (A) iron, (B) Oxygen, and (C) silicon present in the composite site nanoparticles.
The final size of the composite nanoparticles reveals that after the non-seeded processes the silica with initial size of 50 nm might have encapsulated and contracted on the SPION. From the smaller size of the SPION compare to the size of the silica, the SPION must have gain much higher kinetic energy and collided with the silica during the ultrasonic irradiation period. The intraparticle collision between the SPION and the silica is totally inelastic. The total kinetic energy is not conserved; it is converted to other form of energy. However, the total momentum is conserved. Although, Doktycz and Suslick [28], determined interparticle collision velocity and transient temperature at the point of impact to be roughly 1800 km per hour and 3000 °C, respectively. The available data is not enough for us to determine the inelastic collision speed and the melting temperature of the silica
at point of interaction with the SPION. Further work needs to be done. More so, we hypothesize that the incorporation of the SPION into the silica framework under the influence of the ultrasonic field are due to 3 reasons: (1) total inelastic collision between the SPION and silica (2) structure and physical properties of the silica nanoparticles and (3) colloidal stability of the nanoparticles. A good colloidal stability is required by the particles to withstand the influence of the turbulent flow from the ultrasonic irradiation and shock waves. To maintain good colloidal stability of the system during the ultrasonic irradiation process, the pH of sample H and T were adjusted to 3.5 and 10, respectively. Otherwise, at a pH higher than 5 or less than 10, SPION were found unstable due to the point zero charge (PZC) effect [20].
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From the report of Costa et al.[31,32], which revealed silica nanoparticles synthesized through Stöber method to be highly porous, strongly swollen with water and ethanol, and can absorb molecules or ions. Therefore, we propose that the porosity of the amorphous silica increases the possibility of the SPION to be incorporated into the silica framework during the inelastic collision induced by the ultrasonic field. Physico-chemical analyses of both sample H and sample T demonstrate formation of the SPION/silica composite nanoparticles. The XRD spectra in Fig. 4 show the cubic inverse spinel crystal structure of both the prepared SPION and SPION/silica samples. The sharp peaks of the as-synthesised nanoparticles in Fig. 4A demonstrated the formation of iron oxide nanoparticles. The patterns are in consistent with either the standard data for magnetite or maghemite. However, based on this result alone is difficult to assign the spectrum to any of these iron oxide nanoparticles. The XRD pattern of amorphous silica nanoparticles can only be identified by a diffuse peak between 20° and 30° [33]. Therefore, the presence of amorphous silica nanoparticles in Fig. 4B and C were proven by the broad peak between 20° and 30°. These spectra indicate the formation of SPION/silica composite nanoparticles in both sample H and sample T. Conversely, the sharp crystalline peaks of the iron oxide nanoparticles drastically shrunk in Fig. 4B and C. This can be related to presence of the amorphous silica in the composite nanoparticles. Similarly, decrease in the sharp peaks of the SPION by the amorphous silica shell has been reported [34]. However, the effects of ultrasound on formation of the SPION/ silica composite nanoparticles as a function of irradiation time are demonstrated by the inset in Fig. 4. The XRD analysis of the samples for different sonication period between 10 and 40 min was evaluated. The results reveal that at all the sonication period the formation of the SPION/silica composite nanoparticles was observed. More importantly, the prominent 311 peak intensity of the SPION is more pronounced at the sonication period of 30 min. It is noteworthy to mention here that this is an ongoing research; more work is needed to be done to determine the optimum condition and mechanism of this incorporation of SPION into silica nanoparticles via ultrasonic irradiation. The FTIR spectra in Fig. 5 reveal that SPION is not only incorporated but also bind to the silica nanoparticles. The SPION (part A
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and part B) which were peptized with perchloric acid and tetramethyl ammonium hydroxide, contain hydroxyl and methyl functional groups, respectively. The various terminal groups provided the binding site for the silanol group of the silica. The presence of iron oxide nanoparticle in the naked SPION (Fig. 5A) is observed by two absorption bands corresponding to prominent peaks of Fe– O [35]. The 591 cm 1 and 637 cm 1 peaks result from the split of the first band ˆ1 which can be related to 570 cm 1 peak of Fe–O. The second band ˆ2 is observed by the 443 cm 1 peaks corresponding to either magnetite or maghemite. The appearance of the characteristic peaks of silica in 1059 cm 1, 1110 cm 1, 1160 cm 1 and 1062 cm 1, 1111 cm 1, 1163 cm 1 corresponding to siloxane (Si– O–Si) bond in Fig. 5B and C, respectively confirm the binding of the silica to the SPION. The slightly shift in peaks corresponding to Fe–O in Fig. 5B and C can be related to the binding of the silica to the SPION. The IR energy absorbed at 2926 and 2919 cm 1 corresponding to C–H bond in Fig. 5B and C is due to incomplete hydrolysis of the silica precursor. The stretching and bending bond that revealed the adsorption of hydroxyl (OH) group of water molecule on the SPION in Fig. 5A vibrated at 3420 and 1630 cm 1, respectively. Moreover, in Fig. 5B and C the adsorption of water molecule and presence of silanol (Si-OH) on the hybrid nanoparticles appeared in the same region with a slightly shift peaks. The shift in the composite nanoparticles spectra is due to binding of silica to the SPION [36]. In order to validate the incorporation and demonstrate the reactivity of the silica on the SPION, decanethiol monolayer is grafted on the composite nanoparticles as reported [37]. The sample demonstrated high decanethiol monolayer content on the composite nanoparticles. The FTIR spectrum shown in Fig. 6A demonstrated the binding of the decanethiol molecule to the silica encapsulated SPION. Similar to the Fig. 5 spectra, the presence of SPION in the Fig. 6A spectrum is shown by the 492, 592 and 665 cm 1 peaks corresponding to the Fe–O band. Unlike the IR spectra of the silica coated SPION in Fig. 5B and C, the prominent peaks absorbed due to the grafting of decanethiol on the composite nanoparticles are clearly seen in Fig. 6A. The C–H stretching bonds of the alkyl group which absorbed due to the binding of decanethiol monolayer to the silica shell are observed at 2954 cm 1, 2924 cm 1 and 2853 cm 1. The bending vibration of
Fig. 4. XRD spectrum of the (A) uncoated SPION (B) sample H and (C) sample T. The presence of the silica in the sample H and T are observed by the diffuse peaks of amorphous silica nanoparticles between 20° and 30°.
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Fig. 5. FTIR spectrum of (A) uncoated SPION (B) sample H (C) sample T. The siloxanes peaks that demonstrated the presence of silica in both sample H and sample T are delineated with the circle.
Fig. 6. Showing the (A) FTIR spectrum and (B) TEM micrograph of the self assembled decanethiol monolayer on the silica coated SPION.
the C–H group appeared at 1465 and 1383 cm 1 peaks. Unlike the three prominent peaks of siloxane bond that were observed in Fig. 5B and C, the peaks shrunk to only 1046 cm 1 peak. This can be related to the binding of decanthiol monolayer on the silica. However, owing to relatively low concentration of thiol molecules and poor sensitivity of IR to thiol group, the energy absorbed due thiol moiety is not clearly seen in the spectrum [38,39]. The binding of the decanethiol monolayer to the silica is possible due to the affinity of silanol functional group of the silica to the alkyl moiety of the decanethiol monolayer [40]. Furthermore, as shown in Fig. 6B, the surface concentration of the decanethiol molecule (3.2 nm thickness) on the composite nanoparticles delineated with red mark demonstrate and validate the presence of silanol group on the composite nanoparticles.
The saturation magnetization (Ms) values at room temperature for the samples are presented in Fig. 7. In agreement with the superparamagnetic nature, all the prepared nanoparticles show no hysteresis loop and zero coercivity. As expected, the unmodified SPION displays higher Ms 47.4 emu/g at room temperature. The difference in the Ms value between the composite nanoparticles and naked SPION indicates the silica might have caused surface spin disorder of the SPION [41]. The Ms of sample H and sample T are 30 emu/g and 23.6 emu/g, respectively. The sample H retains nearly 70% Ms value of its initial value of the unmodified SPION. The variation in the magnetization value of the composite nanoparticles can be due to the silica. Sample H might have experienced lesser surface spin disorder unlike the sample T during the nonseeded procedure.
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Fig. 7. Room-temperature magnetization curve of the unmodified SPION ( ), sample H ( ) and sample T ( ). Unlike sample T, the sample H might have experienced lesser magnetic spin disorder due to the surface modification with silica.
4. Conclusion We have demonstrated that through non-seeded method SPION/silica composite nanoparticles synthesized. The sonication of silica nanoparticles in the presence of SPION produced the integration of SPION into the framework of silica nanoparticle. Physicochemical analyses demonstrated the binding of the silica on the SPION. The presence of the silica is validated via functionalization of decanethiol monolayer on the composite nanoparticles. Acknowledgement This work is supported by Ministry of Education Malaysia (MoE) and Universiti Sains Malaysia (USM) through FRGS grant 203/PFIZIK/6711351. References [1] B. Chertok, B.A. Moffat, A.E. David, F. Yu, C. Bergemann, B.D. Ross, V.C. Yang, Biomaterials 29 (2008) 487–496. [2] F. Sonvico, S. Mornet, S. Vasseur, C. Dubernet, D. Jaillard, J. Degrouard, J. Hoebeke, E. Duguet, P. Colombo, P. Couvreur, Bioconjug. Chem. 16 (2005) 1181–1188. [3] C. Zhang, B. Wängler, B. Morgenstern, H. Zentgraf, M. Eisenhut, H. Untenecker, R. Krüger, R. Huss, C. Seliger, W. Semmler, Langmuir 23 (2007) 1427–1434. [4] F. Schulze, A. Dienelt, S. Geissler, P. Zaslansky, J. Schoon, K. Henzler, P. Guttmann, A. Gramoun, L.A. Crowe, L. Maurizi, J.-P. Vallée, H. Hofmann, G.N. Duda, A. Ode, Small (2014) (n/a–n/a). [5] F. Gao, Y. Cai, J. Zhou, X. Xie, W. Ouyang, Y. Zhang, X. Wang, X. Zhang, X. Wang, L. Zhao, Nano Res. 3 (2010) 23–31. [6] S. Mørup, M.F. Hansen, C. Frandsen, Beilstein J. Nanotechnol. 1 (2010) 182–190. [7] D. Ling, T. Hyeon, Small 9 (2013) 1450–1466. [8] M. Gao, W. Li, J. Dong, Z. Zhang, B. Yang, World J. Condens. Matter Phys. 1 (2011) 49. [9] J. Xu, S. Thompson, E. O’Keefe, C.C. Perry, Mater. Lett. 58 (2004) 1696–1700. [10] M.T.C. Fernandes, R.B.R. Garcia, C.A.P. Leite, E.Y. Kawachi, Colloids Surf., A 422 (2013) 136–142. [11] P. Tartaj, C.J. Serna, Chem. Mater. 14 (2002) 4396–4402. [12] M. Alcalá, C. Real, Solid State Ionics 177 (2006) 955–960. [13] Q. Yuan, N. Li, W. Geng, Y. Chi, J. Tu, X. Li, C. Shao, Sens. Actuators B Chem. 160 (2011) 334–340.
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Non-seeded synthesis and characterization of superparamagnetic iron oxide nanoparticles incorporated into silica nanoparticles via ultrasound Bashiru Kayode Sodipo ⇑, Azlan Abdul Aziz 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
a r t i c l e
i n f o
Article history: Received 27 August 2014 Received in revised form 24 September 2014 Accepted 25 September 2014 Available online 5 October 2014 Keywords: Acoustic cavitation Composite nanoparticles Inelastic collision Shockwave Silica SPION
a b s t r a c t A non-seeded method of incorporating superparamagnetic iron oxide nanoparticles (SPION) into silica nanoparticles is presented. Mixture of both SPION and silica nanoparticles was ultrasonically irradiated. The collapsed bubbles and shockwave generated from the ultrasonic irradiation produce tremendous force that caused inelastic collision and incorporation of SPION into the silica. Physicochemical analyses using transmission electron microscope (TEM), electronic spectroscopic imaging (ESI), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy demonstrated the formation of SPION/silica composite nanoparticles. The prepared composite nanoparticles exhibited superparamagnetic behaviour and nearly 70% of the initial saturation magnetization (Ms) of the SPION was retained. The presence and reactivity of the silica were demonstrated via assembling decanethiol monolayer on the composite nanoparticles. The silanol group of the silica provided the binding site for the alkyl group in the decanethiol molecules. Therefore, the thiol moiety became the terminal and functional group on the magnetic composite nanoparticles. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Superparamagnetic iron oxide nanoparticles (SPION) are material of interest for biomedical research and related applications such as magnetic resonance imaging (MRI) contrast agent, biocatalyst, biomarker, biosensor, drug delivery and hyperthermia therapy [1–4]. For these functions, stable SPION is required [5]. However, owing to energetic surface of SPION, the nanoparticles usually agglomerate in ionic medium [6]. Surface modification of SPION with biocompatible material is one of the methods used to control problems related to agglomeration of SPION [7]. Silica nanoparticle is among the preferred inorganic materials employed to modify the surface of SPION. Silica surfaces are chemically stable, biocompatible and can be easily functionalized for bioconjugation purpose [8]. Recently, many routes such as sol–gel, microemulsion, combination of microemulsion and sol–gel, reverse micelles, wet impregnated and hydrothermal method [9–14] were used to synthesize iron oxide/silica composite nanoparticles. As shown in Table 1,
⇑ Corresponding author at: Nano-Optoelectronics Research and Technology (NOR) Lab, School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia. E-mail address: [email protected] (B.K. Sodipo). http://dx.doi.org/10.1016/j.ultsonch.2014.09.011 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
the high saturation magnetization (Ms) value of the SPION is drastically reduced after their surface modification with silica. Unlike, most of the conventional methods which are based on seed growth mediated approach where SPION is used as a template for the growth of the silica; in this paper we report a non-seeded process of modifying the surface of SPION with silica. Colloidal suspension of both silica nanoparticles and SPION were synthesized separately. Subsequently, ultrasonic field was employed to incorporate SPION into the silica framework. The incorporation is due to intraparticle and inelastic collision between the silica and SPION, caused by turbulent flow from the shock waves and acoustic cavitation process generated by the ultrasonic irradiation. The structure and high saturation magnetization of the composite nanoparticles are demonstrated via physico-chemical analyses of the product. More importantly, the successful modification of the SPION’s surface with silica is validated through grafting of decanethiol monolayer on the composite nanoparticles. 2. Materials and method Ferric chloride hexahydrate (FeCl24H2O 99%), Ferrous chloride (98%), ammonium hydroxide (25 wt.%), sodium chloride salts, sodium hydroxide, 1-butanol (99%), triethoxyvinylsilane (TEVS 97%), and 1-Decanethiol (99%) were bought and used directly without any further purification from Sigma–Aldrich. The
B.K. Sodipo, A. Abdul Aziz / Ultrasonics Sonochemistry 23 (2015) 354–359
ultrasonic irradiation was carried out using 20 kHz Vibra-Cell ultrasonic with 13 mm diameter horn. 2.1. Synthesis of silica incorporated SPION The composite nanoparticles were synthesized in three steps: (1) synthesis of SPION, (2) synthesis of silica nanoparticles and (3) incorporation of SPION into the framework of silica nanoparticles via ultrasonic irradiation. The SPION was synthesized according to a co-precipitation as reported in our recent work [19]. Briefly, under degassed environment, room temperature and apparent pH of 10, Fe3+ and Fe2+ solution were co-precipitated at the ratio of 2 to 1 with 1 M sodium chloride solution. The system was mechanically stirred at 500 rpm and left to agitate for 2 h. The prepared particles were collected with magnets and washed using deionized water. The magnetic particles were divided into two portions. Part A and B are peptized with 3.5 M perchloric acid (HClO4) and 2.1% w/w of tetramethyl ammonium hydroxide (TMAOH), respectively. The colloidal silica nanoparticles were synthesized by a sol–gel method as reported [20]. Briefly, to 200 mL of distilled water, 200 lL of 10 M ammonia and 6 mL of 1-butanol were added and agitated at 4 rpm for few minutes. Then, 2 mL of triethoxyvinylsilane (TEVS) was added and agitated at 320 rpm and room temperature for 1 h. Unreacted chemicals were separated through dialysis using cellulose membrane for 4 days. At volume ratio of 1:1, colloidal suspension of SPION (part A) and silica nanoparticles were mixed. The pH was slightly adjusted to 3.5 and the mixture was sonicated for 30 min. For heat dissipation the ultrasonic irradiation were carried out in an iced bath environment. The product is named as sample H. The same procedure was repeated for part B with the pH adjusted to 10 and the product is named sample T. 2.2. Characterization The morphology and mapping of the various elements present in the composite nanoparticles were determined through electronic spectroscopy imaging (ESI) technique using energy filtering transmission electron microscopy (EFTEM) Zeiss Libra 120. The XRD of the prepared nanoparticles was characterized using D/ max-IIIC X-ray diffractometer (Shimadzu, Japan). Their peaks were compared to the JCPDS 5-0664 of International Centre for Diffraction Data to determine the crystalline structures. Fourier transforms infrared (FTIR) spectroscopy Perkin Elmer System 2000, was used to examine the binding of silica to SPION. The magnetization measurements were obtained at room temperature in magnetic field up to 10 kOe using DMS Vibrating Sample Magnetometer 8810 (10NRM). 3. Result and discussion It is well known that ultrasonic irradiation of liquid generates acoustic cavitation (formation, growth and collapse of bubbles in
Table 1 Comparing the saturation magnetization of silica coated SPION. Author
Yang et al. Sun et al. Zhang et al. Vogt et al. Gao et al.
Year
2004 2004 2008 2010 2011
Saturation magnetization Ms (emu/g) Uncoated SPION
Silica–SPION
– 46.3 – 81 –
3.2 13.9 15 17.6 12.7
Ref.
[15] [16] [17] [18] [8]
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the liquid). The cavitation creates a unique interaction between energy and matter, with temperature, pressure and cooling rate of approximately 5000 K, 1000 atm and 1010 Ks 1, respectively [21]. These extraordinary conditions permit access to wide range of chemical reactions and synthesis varieties of unusual nanostructured materials [22]. The dynamics of the growth and the collapse of cavity are dependent on local environment. The environment can either be homogenous liquid or heterogeneous solid–liquid interface. In homogenous liquid, spherical cavity is formed. Acoustic cavitation in homogenous liquid generates implosive bubble plus shock waves [23]. The shock waves produce high pressure with amplitudes exceeding 10 kbar [24]. Acoustic cavitation near solid–liquid interface (heterogeneous medium) is asymmetric and associated with high-speed microjets which impact the solid surface and cause mechanical damage [25–27]. The potential energy of the collapsed bubble is converted into kinetic energy of the microjet with velocities hundreds of meters per second. However from the report of Suslick et al. [28], at ultrasonic field of 20 kHz, solid particles with size less than the diameter of the collapsing bubble (150 lm) cannot cause microjet formation, instead a normal cavitation and emission of shock waves will occur [25,29]. Therefore, during the 20 kHz ultrasonic irradiation of our nanoparticles whose sizes are much less than the size of bubbles formed, normal acoustic cavitation were formed with shock waves production. Similar to what Suslick et al. [30] demonstrated with metallic nanoparticles, we observed fusion of silica nanoparticles into one another, when they were ultrasonically irradiated. As shown in Fig. 1, before ultrasonic irradiation of the colloidal suspension of silica, the particles were originally separated except few ones that overlap (Fig. 1A). After the sonication period, the extremely high speed from the turbulent flow and shock waves generated by the ultrasonic irradiation, induces effective melting at the point of impact [28]. As shown in Fig. 1B, fusion and neck formation between the silica particles were observed. The 30 min ultrasonic irradiation produce power (40 W), acoustic cavitation, shockwaves and rapid cooling that led to high velocity interparticle collision, melting and fusion of the silica nanoparticles. However, in an attempt to develop a non-seeded method of encapsulating SPION with silica, we investigated further with intraparticle collision between silica and SPION. Unlike the fusion and the neck formation of the silica nanoparticles, we observed the incorporation of SPION into the silica framework when their mixture was ultrasonically irradiated at apparent pH of 3.5. Fig. 2A is the micrograph of the SPION before mixing and irradiating ultrasonically with silica nanoparticles. The particles were well dispersed with sizes less than 10 nm. However, the ultrasonic irradiation of the mixture produced turbulent flow and shock waves that led to intraparticle collision between the SPION and the silica. As shown in Fig. 2B, after the sonication period, SPION was found embedded in the silica nanoparticles. Elemental mapping of the composite nanoparticles’ micrograph using ESI technique, demonstrated the formation of SPION/silica composite nanoparticles. The micro-analytical analysis (Fig. 3) reveals the various elements such as Fe, O and Si present in the product. Fig. 3A–C correspond to the maps of iron, oxygen and silicon. The maps demonstrate that each element present in the micrograph occupied same position. This can be related to the formation of composite nanoparticles. However, bright halo patches can be observed from the interior of the silicon maps. This can be correlated to the presence of pores in the silica nanoparticles. BET analysis of the prepared particles further confirmed the porosity of the composite nanoparticles with average pore diameter of 7.2 nm and total pore volume 0.0407 cm3/g. The nanocomposite particles are nearly spherical in shape with average size of 10 nm.
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Fig. 1. TEM micrograph with scale bar 100 nm of colloidal silica (A) before sonication (B) after ultrasonically irradiated showing fusion of the particles due to localized melting caused by the collisions.
Fig. 2. TEM micrograph with scale bar 100 nm of SPION (A) before collision with silica (B) after inelastic collision which led to the incorporation of SPION into the silica.
Fig. 3. Showing the maps of (A) iron, (B) Oxygen, and (C) silicon present in the composite site nanoparticles.
The final size of the composite nanoparticles reveals that after the non-seeded processes the silica with initial size of 50 nm might have encapsulated and contracted on the SPION. From the smaller size of the SPION compare to the size of the silica, the SPION must have gain much higher kinetic energy and collided with the silica during the ultrasonic irradiation period. The intraparticle collision between the SPION and the silica is totally inelastic. The total kinetic energy is not conserved; it is converted to other form of energy. However, the total momentum is conserved. Although, Doktycz and Suslick [28], determined interparticle collision velocity and transient temperature at the point of impact to be roughly 1800 km per hour and 3000 °C, respectively. The available data is not enough for us to determine the inelastic collision speed and the melting temperature of the silica
at point of interaction with the SPION. Further work needs to be done. More so, we hypothesize that the incorporation of the SPION into the silica framework under the influence of the ultrasonic field are due to 3 reasons: (1) total inelastic collision between the SPION and silica (2) structure and physical properties of the silica nanoparticles and (3) colloidal stability of the nanoparticles. A good colloidal stability is required by the particles to withstand the influence of the turbulent flow from the ultrasonic irradiation and shock waves. To maintain good colloidal stability of the system during the ultrasonic irradiation process, the pH of sample H and T were adjusted to 3.5 and 10, respectively. Otherwise, at a pH higher than 5 or less than 10, SPION were found unstable due to the point zero charge (PZC) effect [20].
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From the report of Costa et al.[31,32], which revealed silica nanoparticles synthesized through Stöber method to be highly porous, strongly swollen with water and ethanol, and can absorb molecules or ions. Therefore, we propose that the porosity of the amorphous silica increases the possibility of the SPION to be incorporated into the silica framework during the inelastic collision induced by the ultrasonic field. Physico-chemical analyses of both sample H and sample T demonstrate formation of the SPION/silica composite nanoparticles. The XRD spectra in Fig. 4 show the cubic inverse spinel crystal structure of both the prepared SPION and SPION/silica samples. The sharp peaks of the as-synthesised nanoparticles in Fig. 4A demonstrated the formation of iron oxide nanoparticles. The patterns are in consistent with either the standard data for magnetite or maghemite. However, based on this result alone is difficult to assign the spectrum to any of these iron oxide nanoparticles. The XRD pattern of amorphous silica nanoparticles can only be identified by a diffuse peak between 20° and 30° [33]. Therefore, the presence of amorphous silica nanoparticles in Fig. 4B and C were proven by the broad peak between 20° and 30°. These spectra indicate the formation of SPION/silica composite nanoparticles in both sample H and sample T. Conversely, the sharp crystalline peaks of the iron oxide nanoparticles drastically shrunk in Fig. 4B and C. This can be related to presence of the amorphous silica in the composite nanoparticles. Similarly, decrease in the sharp peaks of the SPION by the amorphous silica shell has been reported [34]. However, the effects of ultrasound on formation of the SPION/ silica composite nanoparticles as a function of irradiation time are demonstrated by the inset in Fig. 4. The XRD analysis of the samples for different sonication period between 10 and 40 min was evaluated. The results reveal that at all the sonication period the formation of the SPION/silica composite nanoparticles was observed. More importantly, the prominent 311 peak intensity of the SPION is more pronounced at the sonication period of 30 min. It is noteworthy to mention here that this is an ongoing research; more work is needed to be done to determine the optimum condition and mechanism of this incorporation of SPION into silica nanoparticles via ultrasonic irradiation. The FTIR spectra in Fig. 5 reveal that SPION is not only incorporated but also bind to the silica nanoparticles. The SPION (part A
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and part B) which were peptized with perchloric acid and tetramethyl ammonium hydroxide, contain hydroxyl and methyl functional groups, respectively. The various terminal groups provided the binding site for the silanol group of the silica. The presence of iron oxide nanoparticle in the naked SPION (Fig. 5A) is observed by two absorption bands corresponding to prominent peaks of Fe– O [35]. The 591 cm 1 and 637 cm 1 peaks result from the split of the first band ˆ1 which can be related to 570 cm 1 peak of Fe–O. The second band ˆ2 is observed by the 443 cm 1 peaks corresponding to either magnetite or maghemite. The appearance of the characteristic peaks of silica in 1059 cm 1, 1110 cm 1, 1160 cm 1 and 1062 cm 1, 1111 cm 1, 1163 cm 1 corresponding to siloxane (Si– O–Si) bond in Fig. 5B and C, respectively confirm the binding of the silica to the SPION. The slightly shift in peaks corresponding to Fe–O in Fig. 5B and C can be related to the binding of the silica to the SPION. The IR energy absorbed at 2926 and 2919 cm 1 corresponding to C–H bond in Fig. 5B and C is due to incomplete hydrolysis of the silica precursor. The stretching and bending bond that revealed the adsorption of hydroxyl (OH) group of water molecule on the SPION in Fig. 5A vibrated at 3420 and 1630 cm 1, respectively. Moreover, in Fig. 5B and C the adsorption of water molecule and presence of silanol (Si-OH) on the hybrid nanoparticles appeared in the same region with a slightly shift peaks. The shift in the composite nanoparticles spectra is due to binding of silica to the SPION [36]. In order to validate the incorporation and demonstrate the reactivity of the silica on the SPION, decanethiol monolayer is grafted on the composite nanoparticles as reported [37]. The sample demonstrated high decanethiol monolayer content on the composite nanoparticles. The FTIR spectrum shown in Fig. 6A demonstrated the binding of the decanethiol molecule to the silica encapsulated SPION. Similar to the Fig. 5 spectra, the presence of SPION in the Fig. 6A spectrum is shown by the 492, 592 and 665 cm 1 peaks corresponding to the Fe–O band. Unlike the IR spectra of the silica coated SPION in Fig. 5B and C, the prominent peaks absorbed due to the grafting of decanethiol on the composite nanoparticles are clearly seen in Fig. 6A. The C–H stretching bonds of the alkyl group which absorbed due to the binding of decanethiol monolayer to the silica shell are observed at 2954 cm 1, 2924 cm 1 and 2853 cm 1. The bending vibration of
Fig. 4. XRD spectrum of the (A) uncoated SPION (B) sample H and (C) sample T. The presence of the silica in the sample H and T are observed by the diffuse peaks of amorphous silica nanoparticles between 20° and 30°.
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Fig. 5. FTIR spectrum of (A) uncoated SPION (B) sample H (C) sample T. The siloxanes peaks that demonstrated the presence of silica in both sample H and sample T are delineated with the circle.
Fig. 6. Showing the (A) FTIR spectrum and (B) TEM micrograph of the self assembled decanethiol monolayer on the silica coated SPION.
the C–H group appeared at 1465 and 1383 cm 1 peaks. Unlike the three prominent peaks of siloxane bond that were observed in Fig. 5B and C, the peaks shrunk to only 1046 cm 1 peak. This can be related to the binding of decanthiol monolayer on the silica. However, owing to relatively low concentration of thiol molecules and poor sensitivity of IR to thiol group, the energy absorbed due thiol moiety is not clearly seen in the spectrum [38,39]. The binding of the decanethiol monolayer to the silica is possible due to the affinity of silanol functional group of the silica to the alkyl moiety of the decanethiol monolayer [40]. Furthermore, as shown in Fig. 6B, the surface concentration of the decanethiol molecule (3.2 nm thickness) on the composite nanoparticles delineated with red mark demonstrate and validate the presence of silanol group on the composite nanoparticles.
The saturation magnetization (Ms) values at room temperature for the samples are presented in Fig. 7. In agreement with the superparamagnetic nature, all the prepared nanoparticles show no hysteresis loop and zero coercivity. As expected, the unmodified SPION displays higher Ms 47.4 emu/g at room temperature. The difference in the Ms value between the composite nanoparticles and naked SPION indicates the silica might have caused surface spin disorder of the SPION [41]. The Ms of sample H and sample T are 30 emu/g and 23.6 emu/g, respectively. The sample H retains nearly 70% Ms value of its initial value of the unmodified SPION. The variation in the magnetization value of the composite nanoparticles can be due to the silica. Sample H might have experienced lesser surface spin disorder unlike the sample T during the nonseeded procedure.
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Fig. 7. Room-temperature magnetization curve of the unmodified SPION ( ), sample H ( ) and sample T ( ). Unlike sample T, the sample H might have experienced lesser magnetic spin disorder due to the surface modification with silica.
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