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Iron oxide nanochains coated with silica: Synthesis, surface effects and magnetic properties
Applied Surface Science 476 (2019) 641–646
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Iron oxide nanochains coated with silica: Synthesis, surface effects and magnetic properties
T
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Marin Tadica, , Slavko Kraljb, Yoann Lalatonnec, Laurence Mottec a
Condensed Matter Physics Laboratory, Vinca Institute of Nuclear Science, University of Belgrade, POB 522, 11001 Belgrade, Serbia Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia c Inserm, U1148, Laboratory for Vascular Translational Science, UFR SMBH, Université Paris 13, Sorbonne Paris Cité, F-93017 Bobigny, France b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous silica shell Surface effects Superparamagnetism (SPION) Ferromagnetism Magnetic anisotropy Synthesis
Investigation and synthesis of anisotropic magnetic nanostructures, such as wires, rods, fibers, tubes and chains, is an important field of research due to the beneficial properties and great potential for practical applications ranging from magnetic data storage to biomedicine. Silica coated iron oxide nanochains of length up to 1 μm and diameter ∼80–100 nm have been synthesized by the simultaneous magnetic assembly of superparamagnetic iron oxide nanoparticle clusters (SNCs) as links (viz. maghemite, γ-Fe2O3) and the fixation of the assembled SNCs with an additional layer of deposited silica. We reveal that is possible to achieve either superparamagnetic or ferromagnetic behavior with the nanochains depending only on their physical orientation. The superparamagnetic behavior is observed for random orientation of nanochains whereas ferromagnetic properties (HC ≈ 100 Oe) come to the fore when the orientation is mainly parallel. These peculiar magnetic properties can be related to: (1) the specific size, which is ∼9 nm, of primary building blocks of the nanochains, i.e. of maghemite nanoparticles; (2) to the anisotropic chain-like shape of the particles; and (3) to inter-particle interactions. Large pore volume and pore size of silica shell as well as good colloidal stability and magnetic responsiveness of such nanochains enable applications in biomedicine.
1. Introduction Superparamagnetic and ferromagnetic properties of iron oxide nanoparticles have been widely discussed in both fundamental and applicative way [1–13]. The ferromagnetic property provides the iron oxide with potential applications in high-capacity magnetic storage, whereas superparamagnetism gives them potential applications as biomaterials [14–21]. These magnetic properties depend mainly on the size of nanoparticles: below size of ∼15 nm they are superparamagnetic whereas above ∼20 nm they are ferromagnetic. Moreover, enhanced magnetic properties have been observed in various nanoscale iron oxide morphologies (shape dependent properties), such as plates, cubes, rods, and tubes [22–26]. The assembling of these nanoparticle low-dimensional building blocks into 1D, 2D and 3D ordered nanostructures provides additional possibilities for tuning their physical properties and practical applications [27–34]. Therefore, the development and design of novel iron oxide nanostructures with superparamagnetic or ferromagnetic properties is an issue of consequence for technological advances and future nanodevices. The preparation of magnetic nanoparticles coated with non-
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magnetic material has been very interesting for fundamental studies and practical applications. Among different coated materials silica is advanced because of its non-toxic nature, high biocompatibility, prevention of agglomeration, temperature resistance, adsorbents, colloidal stability, chemical inertness and adjustable pore diameter [9,35–54]. Sol–gel method has been very useful for preparation of the nanoparticles coated or embedded in an amorphous silica [10,55–59]. In order to better understand the magnetic assembly of nanoparticles and figure out their potential applications, we have investigated magnetic properties of random and parallel-oriented anisotropic nanochains consisting of silica coated iron oxide (γ-Fe2O3) nanoparticle clusters (size ∼80 nm) which are composed of superparamagnetic nanoparticles (size ∼9 nm) as primary building blocks. These nanochains exhibit either superparamagnetism or ferromagnetism (HC ≈ 100 Oe) depending only on the physical orientation of the nanochains. 2. Experimental Iron oxide nanochains have been synthesized by colloidal chemistry
Corresponding author at: Condensed Matter Physics Laboratory, VincaInstitute, P.O. Box 522, 11001 Belgrade, Serbia. Tel./fax: +381-11-6308829. E-mail address: [email protected] (M. Tadic).
https://doi.org/10.1016/j.apsusc.2019.01.098 Received 10 October 2018; Received in revised form 27 December 2018; Accepted 12 January 2019 Available online 17 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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nanochain synthesis was completed within 3 h. Finally, the synthesized nanochains which had been fixated with an additional silica shell were magnetically separated from the suspension and washed first with ethanol and then 3-times with distilled water. The crystal structure was measured on a Rigaku Smartlab high-resolution diffractometer (RINTTTRIII) using Cu Kα radiation (λ = 1.5416 Å). M(T) and M(H) curves were measured on a vibrating sample magnetometer (VSM, Quantum Design, Versalab). A sample holder was filled with nanochains (NC) by slowly depositing a water dispersion of NCs (ferrofluid). The holder was then placed in the sample space of the VSM at 300 K and zero magnetic field. The magnetic field in VSM was zero for the sample NCrand and H = 1000 Oe for NC|| during decreasing temperature and freezing of the ferrofluid at 250 K. Zero-field-cooling (ZFC) data were collected using standard procedure below 250 K. Magnetic hysteresis measurements M(H) for nanochains were carried out at different temperatures under the applied magnetic field sweeping from −30 kOe to 30 kOe. For the TEM investigations, the magnetic nanochains were deposited by drying dispersion on a copper-grid supported, perforated, transparent carbon foil and analysed with TEM (Jeol, JEM, 2100) which was operated at 200 kV. For the NCrand sample the drop of the suspension of nanochains was only dried on the TEM grid without exposure to external magnetic field. On the other hand, the NC|| sample was prepared by depositing the drop of the nanochains on the TEM grid exposed to magnetic field of 1000 Oe. The TEM grid for the NC|| sample was exposed to that magnetic field during the entire process of drying. The zeta potential of the nanochains was measured by the Zeta sizer (NanoZS) from Malvern Instruments using its Dispersion Technology Software (DTS). Nitrogen adsorption-desorption measurement was carried out at −196 °C using a NOVA 2000e surface area and pore size analyser
Fig. 1. X-ray powder diffraction pattern of the iron oxide nanochains.
and magnetic field-induced self-assembly of nanoparticle clusters [35,36]. First, the aqueous suspension of the silica-coated clusters was transferred into the polyvinylpyrrolidone (PVP, MW 40 kDa) solution at pH 4.3. The sol-gel mixture was non-magnetically stirred at 250 rpm during the synthesis. A typical synthesis for the nanochain was obtained at a PVP concentration of 1.25 × 10−4 M, a clusters’ concentration of 1.6 × 10−8 M, a TEOS concentration of 60 mM, exposure to a magnetic field of 65 ± 15 mT for 85 min. The TEOS was added 10 min after the transfer of the clusters into the PVP solution. The pH value was set to 8.5, using 0.5% ammonia, after 80 min of the TEOS addition. The
Fig. 2. (a)–(c) TEM images with random orientation of the nanochains NCrand; (d)–(f) TEM images with aligned nanochains NC|| under applied magnetic field H = 1000 Oe; Inset of (f): magnified TEM image of the nanochains. 642
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Fig. 3. (a) EDS spectrum of the nanochains, (b) nitrogen adsorption-desorption isotherms measured at −196 °C, and (c) the corresponding pore size distributions of silica-coated nanochains.
(1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0), (5 3 3), (4 4 4), (6 4 2), (7 3 1) and (8 0 0) provide a clear evidence for the spinel structure of the iron oxide (maghemite, γ-Fe2O3) nanoparticles. This observation matches well with those of earlier reporters [35,36]. The crystallite size of the nanoparticles is calculated using Scherrer's formula and the determined value is D ∼ 8.4 nm. The particle size, structure and morphology of the sample were investigated by TEM microscopy (Fig. 2a–f). The cross-diameter of the nanochains is about 80–100 nm whereas their length is up to 1 μm (average about 600 nm) with aspect ratio ∼7 (Fig. 2). The observed samples are labeled as NCrand (Fig. 2a–c) and NC|| (Fig. 2d–f), which indicates random and parallel orientation of the nanochains, respectively. The experimental results demonstrated that applied magnetic field (H = 1000 Oe) is effective in aligning the anisotropic iron oxide nanochains (Fig. 2d–f). The procedure for aligning the nanochains on the TEM grid resulted in an average orientation, in which the long axis of the nanochains (magnetization easy axis) was parallel to the external magnetic field. Fig. 3a shows the EDS spectrum of the iron oxide/silica nanochains, where Fe, Si and O are the main components. The successful coating with silica is corroborated with nitrogen physical sorption (Fig. 3). Specific surface area of 27.1 m2/g, pore volume of 0.33 cm3/g and pore size of ∼3.6 nm confirmed the mesoporous type of silica. Large pore volume and pore size as well as high magnetic responsiveness of such nanochains in a magnetic field gradient enable its transformation into effective, magnetically-responsive drug delivery system in the future. The electrophoretic mobility of the nanochains was measured as a function of the operational pH (Fig. 4). The ζ-potential curve of the assynthesized maghemite nanoparticles shows relatively low absolute
Fig. 4. The curves of the ζ-potentials vs. pH for the uncoated as-synthesized maghemite nanoparticles and silica-coated nanochains.
(Quantachrome Instruments). Prior to the measurement, the sample was degassed at 160 °C for 6 h. Brunauer-Emmett-Teller (BET) specific area was calculated using desorption data. Pore size distribution was derived from adsorption branch of the isotherm using the BarrettJoyner-Halenda (BJH) method. Pore volume was determined from the amount of N2 adsorbed at the single point of P/P0 = 0.991. 3. Results and discussion The phase composition and structure of the investigated sample was characterized by measuring the X-ray powder diffraction (XRPD). The XRPD pattern of the synthesized sample is shown in Fig. 1. It can be seen that the XRPD pattern consists of wide and well-resolved peaks, which confirms the nanocrystalline and single-phase nature of the prepared material. The diffraction peaks corresponding to planes 643
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Fig. 5. ZFC-FC magnetization curves of the nanochains: (a) NCrand and (b) NC||.
value of ζ-potential along the whole pH range. The absolute values of ζpotential were below 20 mV along the whole pH range. The isoelectric point (IEP) is at pH 7.4 which reflects in relatively poor colloidal stability in bio-relevant pH range. However, the silica surface on nanochains shows a relatively acidic character, because its structure is terminated with negatively charged eOH surface groups at pH values above the IEP which is close to pH 2.8. The ζ-potential curve for the silica-coated nanochains reached negative values of above −20 mV in absolute value at pH above 7. The high absolute values of the zetapotential provide strong electrostatic repulsive forces between the nanochains, providing a good colloidal stability of the suspension in conditions relevant for possible application in biomedicine. To investigate the magnetic properties of nanochains magnetic measurements were performed using a VSM magnetometer (Figs. 5 and 6). We have presented ZFC-FC magnetization curves for NCrand and NC|| under magnetic field of H = 100 Oe (Fig. 5). It can be observed that the measured data displays the characteristic behavior expected from an iron oxide nanoparticle (Fig. 5a and b). The M(T) measurements exhibit wide maxima in ZFC curves and bifurcation of the ZFC-FC curves. Interestingly, the determined values for blocking temperatures of NCrand and NC|| are different, TBrand = 123.6 K and TB|| = 159.7 K, respectively. It should be noticed that the wider maximum in the ZFC curve is also observed for NC||. In the case of interacting magnetic nanoparticle systems the effect of stronger inter-particle interaction is observed as an increase of the blocking temperature TB and widening of ZFC maximum curve due to the enhanced distribution of energy barriers [31,35]. So, the experimental results in this work reveal stronger inter-particle interactions in NC|| sample. The field-dependence of magnetization M(H) of the NCrand (Fig. 6a) shows no hysteresis at 250 K, i.e. the coercivity and remanent
Fig. 6. Hysteresis loops of the nanochains: (a) NCrand, (b) NC|| and (c) HC vs T for nanochains with parallel (red circles) and random orientation (black squares). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
magnetization are zero that is typical for superparamagnetic state. This behavior has been expected for γ-Fe2O3 nanoparticles with diameters about 10 nm which is quite similar to this work [35,36]. At low temperatures (below TB), the M(H) curves display a non-zero coercivity indicating blocking state of the nanoparticles in NCrand sample. The HC 644
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towards higher values TB = 127.3 K (Fig. 7a) due to stronger interparticle interactions. 4. Conclusions Core-shell Iron oxide nanochains were obtained by the simultaneous magnetic assembly of superparamagnetic iron oxide nanoparticle clusters (SNCs) as links and the fixation of the assembled SNCs with an additional layer of deposited silica, produced by a sol-gel synthesis process. The silica-coating layer provides permanent structure and morphology of the nanochains. Moreover, large pore volume and pore size of silica shell as well as good colloidal stability and magnetic responsiveness of such nanochains enable applications in biomedicine. A comparative investigation of magnetic properties of anisotropic nanochains with parallel (NC||) and random orientation (NCrand) is also presented in this work. We found that the iron oxide (maghemite, γFe2O3) nanochains behave remarkably differently depending only on their physical alignment. When these nanochains are randomly distributed they manifest superparamagnetic behavior (HC = 0 Oe), whereas parallel alignment of nanochains leads to ferromagnetism (HC ≈ 100 Oe). The observed magnetic properties for NC|| (Tmax = 159.7 K and wider ZFC maxima) and NCrand (Tmax = 123.6 K and narrower ZFC maxima) reveal stronger inter-particle interactions in NC||. We conclude that shape anisotropy of nanochains, parallel orientation and inter-particle interactions render magnetic moments ordered along long axis for NC|| (stable state). The magnetic moments of the nanoparticles in NCrand which being in an unstable arrangement tend either to order along the long axis due to shape anisotropy or along the applied magnetic field producing some kind of their frustration (unstable state). Moreover, the investigated nanochains have permanent structure and morphology that provide a simple route toward tuning magnetic properties only by nanochain alignment. These findings bring about new possibilities for the design of nanodevices and further tailoring properties of 1D nanostructures with enhanced collective magnetic properties.
Fig. 7. ZFC-FC magnetization curves and M(H) at 250 K of the agglomerated nanochains NCagglom.
Acknowledgement
of NCrand monotonically increases with decreasing temperature (see Fig. 6c) as expected for iron oxide nanoparticles below TB [37]. Suprisingly, the M(H) curves of NC|| sample show ferromagnetic properties (HC = 98 Oe, Fig. 6b) which is in sharp contrast with the NCrand sample. The Hc values of NCs|| are much larger than those of NCsrand in all measured temperature range (Fig. 6c). Moreover, the HC of NC|| show almost constant values as the temperature increases from 50 K to 250 K (Fig. 6c), i.e. the effect of temperature on the coercivity is not noticable which is quite different in comparison with individual iron oxide nanoparticles [37]. Therefore, this property of NC is a consequence of collective magnetic properties of nanoparticles magnetically-assembled in anisotropic nanochains and is not an intrinsic property of the individual nanoparticle. The observed discrepancy in the magnetization curves derives from the magnetic anisotropy (anisotropic morphology) and physical orientation of the anisotropic nanochains. On the one hand, when the direction of the magnetic field is applied parallel to the nanochain long axis, the nanoparticle magnetic moments being in a stable arrangement, interacting strongly and form magnetic order structure along the long axis of the nanochains (TB = 159.7 K is higher and ZFC maximum wider in comparison with NCrand). On the other hand, when the magnetic measurements are performed on NCrand, TB value is lower 123.6 K and ZFC maximum narrower in comparison with NC||, due to weaker inter-particle interactions. Finally, we have prepared sample (NCagglom) with force agglomeration by adding NaCl to the dispersion of the nanochains in water to bring the particles into close contact and random orientation. We observed similar magnetic properties as in NCrand with superparamagnetic properties above blocking temperature (Fig. 7). The agglomeration of the nanochains slightly shifts the blocking temperature
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Full Length Article
Iron oxide nanochains coated with silica: Synthesis, surface effects and magnetic properties
T
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Marin Tadica, , Slavko Kraljb, Yoann Lalatonnec, Laurence Mottec a
Condensed Matter Physics Laboratory, Vinca Institute of Nuclear Science, University of Belgrade, POB 522, 11001 Belgrade, Serbia Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia c Inserm, U1148, Laboratory for Vascular Translational Science, UFR SMBH, Université Paris 13, Sorbonne Paris Cité, F-93017 Bobigny, France b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous silica shell Surface effects Superparamagnetism (SPION) Ferromagnetism Magnetic anisotropy Synthesis
Investigation and synthesis of anisotropic magnetic nanostructures, such as wires, rods, fibers, tubes and chains, is an important field of research due to the beneficial properties and great potential for practical applications ranging from magnetic data storage to biomedicine. Silica coated iron oxide nanochains of length up to 1 μm and diameter ∼80–100 nm have been synthesized by the simultaneous magnetic assembly of superparamagnetic iron oxide nanoparticle clusters (SNCs) as links (viz. maghemite, γ-Fe2O3) and the fixation of the assembled SNCs with an additional layer of deposited silica. We reveal that is possible to achieve either superparamagnetic or ferromagnetic behavior with the nanochains depending only on their physical orientation. The superparamagnetic behavior is observed for random orientation of nanochains whereas ferromagnetic properties (HC ≈ 100 Oe) come to the fore when the orientation is mainly parallel. These peculiar magnetic properties can be related to: (1) the specific size, which is ∼9 nm, of primary building blocks of the nanochains, i.e. of maghemite nanoparticles; (2) to the anisotropic chain-like shape of the particles; and (3) to inter-particle interactions. Large pore volume and pore size of silica shell as well as good colloidal stability and magnetic responsiveness of such nanochains enable applications in biomedicine.
1. Introduction Superparamagnetic and ferromagnetic properties of iron oxide nanoparticles have been widely discussed in both fundamental and applicative way [1–13]. The ferromagnetic property provides the iron oxide with potential applications in high-capacity magnetic storage, whereas superparamagnetism gives them potential applications as biomaterials [14–21]. These magnetic properties depend mainly on the size of nanoparticles: below size of ∼15 nm they are superparamagnetic whereas above ∼20 nm they are ferromagnetic. Moreover, enhanced magnetic properties have been observed in various nanoscale iron oxide morphologies (shape dependent properties), such as plates, cubes, rods, and tubes [22–26]. The assembling of these nanoparticle low-dimensional building blocks into 1D, 2D and 3D ordered nanostructures provides additional possibilities for tuning their physical properties and practical applications [27–34]. Therefore, the development and design of novel iron oxide nanostructures with superparamagnetic or ferromagnetic properties is an issue of consequence for technological advances and future nanodevices. The preparation of magnetic nanoparticles coated with non-
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magnetic material has been very interesting for fundamental studies and practical applications. Among different coated materials silica is advanced because of its non-toxic nature, high biocompatibility, prevention of agglomeration, temperature resistance, adsorbents, colloidal stability, chemical inertness and adjustable pore diameter [9,35–54]. Sol–gel method has been very useful for preparation of the nanoparticles coated or embedded in an amorphous silica [10,55–59]. In order to better understand the magnetic assembly of nanoparticles and figure out their potential applications, we have investigated magnetic properties of random and parallel-oriented anisotropic nanochains consisting of silica coated iron oxide (γ-Fe2O3) nanoparticle clusters (size ∼80 nm) which are composed of superparamagnetic nanoparticles (size ∼9 nm) as primary building blocks. These nanochains exhibit either superparamagnetism or ferromagnetism (HC ≈ 100 Oe) depending only on the physical orientation of the nanochains. 2. Experimental Iron oxide nanochains have been synthesized by colloidal chemistry
Corresponding author at: Condensed Matter Physics Laboratory, VincaInstitute, P.O. Box 522, 11001 Belgrade, Serbia. Tel./fax: +381-11-6308829. E-mail address: [email protected] (M. Tadic).
https://doi.org/10.1016/j.apsusc.2019.01.098 Received 10 October 2018; Received in revised form 27 December 2018; Accepted 12 January 2019 Available online 17 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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nanochain synthesis was completed within 3 h. Finally, the synthesized nanochains which had been fixated with an additional silica shell were magnetically separated from the suspension and washed first with ethanol and then 3-times with distilled water. The crystal structure was measured on a Rigaku Smartlab high-resolution diffractometer (RINTTTRIII) using Cu Kα radiation (λ = 1.5416 Å). M(T) and M(H) curves were measured on a vibrating sample magnetometer (VSM, Quantum Design, Versalab). A sample holder was filled with nanochains (NC) by slowly depositing a water dispersion of NCs (ferrofluid). The holder was then placed in the sample space of the VSM at 300 K and zero magnetic field. The magnetic field in VSM was zero for the sample NCrand and H = 1000 Oe for NC|| during decreasing temperature and freezing of the ferrofluid at 250 K. Zero-field-cooling (ZFC) data were collected using standard procedure below 250 K. Magnetic hysteresis measurements M(H) for nanochains were carried out at different temperatures under the applied magnetic field sweeping from −30 kOe to 30 kOe. For the TEM investigations, the magnetic nanochains were deposited by drying dispersion on a copper-grid supported, perforated, transparent carbon foil and analysed with TEM (Jeol, JEM, 2100) which was operated at 200 kV. For the NCrand sample the drop of the suspension of nanochains was only dried on the TEM grid without exposure to external magnetic field. On the other hand, the NC|| sample was prepared by depositing the drop of the nanochains on the TEM grid exposed to magnetic field of 1000 Oe. The TEM grid for the NC|| sample was exposed to that magnetic field during the entire process of drying. The zeta potential of the nanochains was measured by the Zeta sizer (NanoZS) from Malvern Instruments using its Dispersion Technology Software (DTS). Nitrogen adsorption-desorption measurement was carried out at −196 °C using a NOVA 2000e surface area and pore size analyser
Fig. 1. X-ray powder diffraction pattern of the iron oxide nanochains.
and magnetic field-induced self-assembly of nanoparticle clusters [35,36]. First, the aqueous suspension of the silica-coated clusters was transferred into the polyvinylpyrrolidone (PVP, MW 40 kDa) solution at pH 4.3. The sol-gel mixture was non-magnetically stirred at 250 rpm during the synthesis. A typical synthesis for the nanochain was obtained at a PVP concentration of 1.25 × 10−4 M, a clusters’ concentration of 1.6 × 10−8 M, a TEOS concentration of 60 mM, exposure to a magnetic field of 65 ± 15 mT for 85 min. The TEOS was added 10 min after the transfer of the clusters into the PVP solution. The pH value was set to 8.5, using 0.5% ammonia, after 80 min of the TEOS addition. The
Fig. 2. (a)–(c) TEM images with random orientation of the nanochains NCrand; (d)–(f) TEM images with aligned nanochains NC|| under applied magnetic field H = 1000 Oe; Inset of (f): magnified TEM image of the nanochains. 642
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Fig. 3. (a) EDS spectrum of the nanochains, (b) nitrogen adsorption-desorption isotherms measured at −196 °C, and (c) the corresponding pore size distributions of silica-coated nanochains.
(1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0), (5 3 3), (4 4 4), (6 4 2), (7 3 1) and (8 0 0) provide a clear evidence for the spinel structure of the iron oxide (maghemite, γ-Fe2O3) nanoparticles. This observation matches well with those of earlier reporters [35,36]. The crystallite size of the nanoparticles is calculated using Scherrer's formula and the determined value is D ∼ 8.4 nm. The particle size, structure and morphology of the sample were investigated by TEM microscopy (Fig. 2a–f). The cross-diameter of the nanochains is about 80–100 nm whereas their length is up to 1 μm (average about 600 nm) with aspect ratio ∼7 (Fig. 2). The observed samples are labeled as NCrand (Fig. 2a–c) and NC|| (Fig. 2d–f), which indicates random and parallel orientation of the nanochains, respectively. The experimental results demonstrated that applied magnetic field (H = 1000 Oe) is effective in aligning the anisotropic iron oxide nanochains (Fig. 2d–f). The procedure for aligning the nanochains on the TEM grid resulted in an average orientation, in which the long axis of the nanochains (magnetization easy axis) was parallel to the external magnetic field. Fig. 3a shows the EDS spectrum of the iron oxide/silica nanochains, where Fe, Si and O are the main components. The successful coating with silica is corroborated with nitrogen physical sorption (Fig. 3). Specific surface area of 27.1 m2/g, pore volume of 0.33 cm3/g and pore size of ∼3.6 nm confirmed the mesoporous type of silica. Large pore volume and pore size as well as high magnetic responsiveness of such nanochains in a magnetic field gradient enable its transformation into effective, magnetically-responsive drug delivery system in the future. The electrophoretic mobility of the nanochains was measured as a function of the operational pH (Fig. 4). The ζ-potential curve of the assynthesized maghemite nanoparticles shows relatively low absolute
Fig. 4. The curves of the ζ-potentials vs. pH for the uncoated as-synthesized maghemite nanoparticles and silica-coated nanochains.
(Quantachrome Instruments). Prior to the measurement, the sample was degassed at 160 °C for 6 h. Brunauer-Emmett-Teller (BET) specific area was calculated using desorption data. Pore size distribution was derived from adsorption branch of the isotherm using the BarrettJoyner-Halenda (BJH) method. Pore volume was determined from the amount of N2 adsorbed at the single point of P/P0 = 0.991. 3. Results and discussion The phase composition and structure of the investigated sample was characterized by measuring the X-ray powder diffraction (XRPD). The XRPD pattern of the synthesized sample is shown in Fig. 1. It can be seen that the XRPD pattern consists of wide and well-resolved peaks, which confirms the nanocrystalline and single-phase nature of the prepared material. The diffraction peaks corresponding to planes 643
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Fig. 5. ZFC-FC magnetization curves of the nanochains: (a) NCrand and (b) NC||.
value of ζ-potential along the whole pH range. The absolute values of ζpotential were below 20 mV along the whole pH range. The isoelectric point (IEP) is at pH 7.4 which reflects in relatively poor colloidal stability in bio-relevant pH range. However, the silica surface on nanochains shows a relatively acidic character, because its structure is terminated with negatively charged eOH surface groups at pH values above the IEP which is close to pH 2.8. The ζ-potential curve for the silica-coated nanochains reached negative values of above −20 mV in absolute value at pH above 7. The high absolute values of the zetapotential provide strong electrostatic repulsive forces between the nanochains, providing a good colloidal stability of the suspension in conditions relevant for possible application in biomedicine. To investigate the magnetic properties of nanochains magnetic measurements were performed using a VSM magnetometer (Figs. 5 and 6). We have presented ZFC-FC magnetization curves for NCrand and NC|| under magnetic field of H = 100 Oe (Fig. 5). It can be observed that the measured data displays the characteristic behavior expected from an iron oxide nanoparticle (Fig. 5a and b). The M(T) measurements exhibit wide maxima in ZFC curves and bifurcation of the ZFC-FC curves. Interestingly, the determined values for blocking temperatures of NCrand and NC|| are different, TBrand = 123.6 K and TB|| = 159.7 K, respectively. It should be noticed that the wider maximum in the ZFC curve is also observed for NC||. In the case of interacting magnetic nanoparticle systems the effect of stronger inter-particle interaction is observed as an increase of the blocking temperature TB and widening of ZFC maximum curve due to the enhanced distribution of energy barriers [31,35]. So, the experimental results in this work reveal stronger inter-particle interactions in NC|| sample. The field-dependence of magnetization M(H) of the NCrand (Fig. 6a) shows no hysteresis at 250 K, i.e. the coercivity and remanent
Fig. 6. Hysteresis loops of the nanochains: (a) NCrand, (b) NC|| and (c) HC vs T for nanochains with parallel (red circles) and random orientation (black squares). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
magnetization are zero that is typical for superparamagnetic state. This behavior has been expected for γ-Fe2O3 nanoparticles with diameters about 10 nm which is quite similar to this work [35,36]. At low temperatures (below TB), the M(H) curves display a non-zero coercivity indicating blocking state of the nanoparticles in NCrand sample. The HC 644
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towards higher values TB = 127.3 K (Fig. 7a) due to stronger interparticle interactions. 4. Conclusions Core-shell Iron oxide nanochains were obtained by the simultaneous magnetic assembly of superparamagnetic iron oxide nanoparticle clusters (SNCs) as links and the fixation of the assembled SNCs with an additional layer of deposited silica, produced by a sol-gel synthesis process. The silica-coating layer provides permanent structure and morphology of the nanochains. Moreover, large pore volume and pore size of silica shell as well as good colloidal stability and magnetic responsiveness of such nanochains enable applications in biomedicine. A comparative investigation of magnetic properties of anisotropic nanochains with parallel (NC||) and random orientation (NCrand) is also presented in this work. We found that the iron oxide (maghemite, γFe2O3) nanochains behave remarkably differently depending only on their physical alignment. When these nanochains are randomly distributed they manifest superparamagnetic behavior (HC = 0 Oe), whereas parallel alignment of nanochains leads to ferromagnetism (HC ≈ 100 Oe). The observed magnetic properties for NC|| (Tmax = 159.7 K and wider ZFC maxima) and NCrand (Tmax = 123.6 K and narrower ZFC maxima) reveal stronger inter-particle interactions in NC||. We conclude that shape anisotropy of nanochains, parallel orientation and inter-particle interactions render magnetic moments ordered along long axis for NC|| (stable state). The magnetic moments of the nanoparticles in NCrand which being in an unstable arrangement tend either to order along the long axis due to shape anisotropy or along the applied magnetic field producing some kind of their frustration (unstable state). Moreover, the investigated nanochains have permanent structure and morphology that provide a simple route toward tuning magnetic properties only by nanochain alignment. These findings bring about new possibilities for the design of nanodevices and further tailoring properties of 1D nanostructures with enhanced collective magnetic properties.
Fig. 7. ZFC-FC magnetization curves and M(H) at 250 K of the agglomerated nanochains NCagglom.
Acknowledgement
of NCrand monotonically increases with decreasing temperature (see Fig. 6c) as expected for iron oxide nanoparticles below TB [37]. Suprisingly, the M(H) curves of NC|| sample show ferromagnetic properties (HC = 98 Oe, Fig. 6b) which is in sharp contrast with the NCrand sample. The Hc values of NCs|| are much larger than those of NCsrand in all measured temperature range (Fig. 6c). Moreover, the HC of NC|| show almost constant values as the temperature increases from 50 K to 250 K (Fig. 6c), i.e. the effect of temperature on the coercivity is not noticable which is quite different in comparison with individual iron oxide nanoparticles [37]. Therefore, this property of NC is a consequence of collective magnetic properties of nanoparticles magnetically-assembled in anisotropic nanochains and is not an intrinsic property of the individual nanoparticle. The observed discrepancy in the magnetization curves derives from the magnetic anisotropy (anisotropic morphology) and physical orientation of the anisotropic nanochains. On the one hand, when the direction of the magnetic field is applied parallel to the nanochain long axis, the nanoparticle magnetic moments being in a stable arrangement, interacting strongly and form magnetic order structure along the long axis of the nanochains (TB = 159.7 K is higher and ZFC maximum wider in comparison with NCrand). On the other hand, when the magnetic measurements are performed on NCrand, TB value is lower 123.6 K and ZFC maximum narrower in comparison with NC||, due to weaker inter-particle interactions. Finally, we have prepared sample (NCagglom) with force agglomeration by adding NaCl to the dispersion of the nanochains in water to bring the particles into close contact and random orientation. We observed similar magnetic properties as in NCrand with superparamagnetic properties above blocking temperature (Fig. 7). The agglomeration of the nanochains slightly shifts the blocking temperature
The Ministry of Education and Science of the Republic of Serbia supported this work financially through Grant no. III 45015. M.T. thanks Dr. Veljko Dmitrašinović for a critical reading of the manuscript. The authors acknowledge the financial support from the Slovenian Research Agency (ARRS) for research core funding No. (P2-0089) and for the projects “Nanotheranostics based on magneto-responsive materials” (No. J1-7302) and “Tunnelling nanotubes for innovative urinary bladder cancer treatments” (No. J3-7494). We acknowledge the use of equipment at the Center of Excellence on Nanoscience and Nanotechnology from Institute Jozef Stefan. We thank the C Nano Platform from University Paris 13 for magnetic investigation. References [1] S.L. Jeon, M.K. Chae, E.J. Jang, C. Lee, Chem.-Eur. J. 19 (2013) 4217–4222. [2] L. Corbellini, C. Lacroix, D. Ménard, A. Pignolet, Scripta Mater. 140 (2017) 63–66. [3] J.T.-W. Wang, L. Cabana, M. Bourgognon, H. Kafa, A. Protti, K. Venner, A.M. Shah, et al., Adv. Funct. Mater. 24 (2014) 1880–1894. [4] S. Zhu, Y. Leng, M. Yan, X. Tuo, J. Yang, L. Almásy, Q. Tian, G. Sun, L. Zou, Q. Li, J. Courtois, Appl. Surf. Sci. 447 (2018) 381–387. [5] M. Fu, Meng, Xiangming Li, Rui Jiang, Zepeng Zhang, Appl. Surf. Sci. 441 (2018) 239–250. [6] K. Cendrowski, P. Sikora, B. Zielinska, E. Horszczaruk, E. Mijowska, Appl. Surf. Sci. 407 (2017) 391–397. [7] V.A. Svetlichnyi, A.V. Shabalina, I.N. Lapin, D.A. Goncharova, D.A. Velikanov, A.E. Sokolov, Appl. Surf. Sci. 462 (2018) 226–236. [8] S. Deka, V. Saxena, A. Hasan, P. Chandra, L.M. Pandey, Mat. Sci. Eng. C 92 (2018) 932–941. [9] S.S. Yakushkin, D.A. Balaev, A.A. Dubrovskiy, S.V. Semenov, K.A. Shaikhutdinov, M.A. Kazakova, G.A. Bukhtiyarova, O.N. Martyanov, O.A. Bayukov, J. Supercond. Nov. Magn. 31 (2018) 1209–1217. [10] S.S. Yakushkin, D.A. Balaev, A.A. Dubrovskiy, S.V. Semenov, Yu.V. Knyazev,
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