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Synthesis and characterization of silica coated magnetic iron oxide nanoparticles

Vietnam Journal of Science and Technology 57 (3A) (2019) 160-166

Minh-Tri Nguyen-Le1, Dinh Tien Dung Nguyen1, Sophia Rich1, 3,
Ngoc Tram Nguyen1, 3, Cuu Khoa Nguyen1, Dai Hai Nguyen1, 2, *

Institute of Applied Materials Science, Vietnam Academy of Science and Technology,
1 Thanh Loc 29, District 12, Ho Chi Minh City


Graduate University of Science and Technology, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Cau Giay, Ha Noi


Tra Vinh University, No. 126, Nguyen Thien Thanh, Ward 5, Tra Vinh city, Tra Vinh province

Email: nguyendaihai0511@gmail.com

Received: 13 August 2019; Accepted for publication: 30 September 2019
Abstract. Advances in nanotechnology in recent years have led to a number of diverse
applications of nanomaterials. Magnetic iron oxide nanoparticles (Fe3O4 NPs), a representative
of magnetic nanomaterials, have gained much attention of many researchers all over the world
due to their unique properties such as superparamagnetism, biocompatibility and high magnetic
saturation. With such properties, Fe3O4 NPs can be exploited in many fields, particularly
biomedicine related fields such as cellular therapy, tissue repair, drug delivery, magnetic
resonance imaging, hyperthermia and magnetofection. However, owing to their self-aggregation
of Fe3O4 NPs, it is necessary to coat Fe3O4 NPs with a stable and biocompatible silica layer.
Therefore, in this report, Fe3O4 NPs were synthesized via a co-precipitation method using iron
(II)/ iron (III) chloride, ammonia and trisodium citrate. Then, the silica layer was coated onto
Fe3O4 NPs through the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in
ethanol. The as-synthesized samples were characterized with the infrared (IR) spectroscopy, Xray diffraction (XRD), thermogravimetric analysis (TGA), vibrating sample magnetometer
(VSM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The
results proved that silica was successfully coated on Fe3O4 NPs. The particle sizes measured by
TEM were found to be about 12 nm in diameter for Fe3O4 NPs and 45 nm in diameter for silica
coated Fe3O4 (SiO2@Fe3O4) NPs, while the dynamic diameters measured by DLS for Fe3O4 NPs
and SiO2@Fe3O4 NPs were 15.7 and 65.8 nm, respectively. Both Fe3O4 NPs and SiO2@Fe3O4
NPs were superparamagnetic materials in which Fe3O4 NPs have higher magnetic saturation
(45.8 emu/g) than the other (13.4 emu/g).
Keywords: Fe3O4 nanoparticles; silica; superparamagnetic.
Classification numbers: 2.2.1, 2.4.3, 2.10.2.

Synthesis and characterization of silica coated magnetic iron oxide nanoparticles

Recently, advances in nanotechnology have led to a number of diverse applications of
nanomaterials. Magnetic iron oxide nanoparticles (Fe3O4 NPs), a representative of magnetic
nanomaterials, have gained much attention of many researchers all over the world due to their
exceptional properties over the bulk materials such as superparamagnetism, biocompatibility and
high magnetic saturation. It is noteworthy that bulk magnetic materials are composed of
microscopic crystalline grains (also known as polycrystalline). Unlike bulk magnetic materials,
magnetic nanomaterials, with the size between 1 nm and 100 nm, are single-domain materials
which are uniformly magnetized to their saturation magnetization.
Several nanoparticles can be synthesized from oxides and alloys of Fe, Ni, Co, etc. In the
last decades, much research has been developed to the synthesis of Fe3O4 NPs due to their

biocompatibility, superparamagnetic behavior and chemical stability. Mahdavi et al. reported
synthesis of superparamagnetic Fe3O4 NPs with the average particle size of about 16.5 nm and
saturation magnetization of 80 emu.g−1[1]. For drug delivery, poly(ethyleneglycol)-modified
superparamagnetic Fe3O4 NPs with the average size of 40–50 nm and saturation magnetization
of 45-50 emu.g−1 were also reported by Gupta et al [2]. In addition to drug delivery, other
applications of Fe3O4 NPs such as cell labelling, hyperthermia and MRI contrast enhancement
were also demonstrated [3-5].
However, owing to large surface-to-volume ratio, Fe3O4 NPs possesses high surface energy
which results in aggregation of Fe3O4 NPs so as to minimize the surface energy. In order to
prevent them from loss of magnetism and dispersibility due to the aggregation and oxidation in
air, it is necessary to coat them with some protective layers such as organic molecules, including
small organic molecules or surfactants, polymers, and biomolecules, or coating with an
inorganic layer, such as silica, metal or nonmetal elementary substance, metal oxide or metal
sulfide [6]. Among them, surface coating of Fe3O4 NPs with silica not only stabilize the Fe3O4
NPs but also enhance surface functionalization through silanol groups, which is important for
biomedical applications [7].
Therefore, herein, we report one-pot synthesis of SiO2@Fe3O4 NPs through both coprecipitation of Fe (II) and Fe (III) ions, and hydrolysis and condensation of TEOS.
2.1. Materials and characterization
For synthesis of the NPs, FeCl2.4H2O (98 %) and FeCl3.6H2O (98 %) were purchased from
Merck. Tetraethyl orthosilicate (TEOS, 98 %) was purchased from Sigma-Aldrich. Ethanol
(99 %) was purchased from Scharlau. Ammonium hydroxide ( 30-33% NH3 in H2O) and
trisodium citrate (TSC, 99 %) were obtained from a China company. Deionized (DI) water with
resistance of 18 MΩ cm was used in all the experiments. All chemicals were used without
further purification.
For characterization, the structure morphology of synthesized Fe3O4 NPs was studied
through a transmission electron microscope (TEM, JEOL JEM-1400, 100 kV). The particle size
distribution of the NPs was recorded on a Zetasizer Nano ZS (ZEN 36000, Malvern Instruments,
UK) via dynamic light scattering (DLS). In order to study the crystalline structure, X-ray
diffraction (XRD) was conducted on a Rigaku X-ray diffractometer with Cu Kα radiation
(λ = 0.154056 nm). Moreover, the functional groups of the Fe3O4 NPs, SiO2@Fe3O4 NPs and
their intermediates generated during the synthesis were studied through FT-IR/NIR

Minh-Tri Nguyen-Le et al.

Spectroscopy (Frontier). Thermogravimetric analysis (TGA) was also performed on TGA-DSC
Mettler Toledo 3+. The magnetic properties of magnetic as-synthesized materials were
investigated using Vibrating Sample Magnetometer (DMS 880).
2.2. Synthesis of Fe3O4 NPs
Briefly, 0.796 g FeCl2.4H2O and 2.164 g FeCl3.6H2O were dissolved into 40 mL of DI
water under vigorous stirring and N2 purging for 30 minutes. Then, 13 mL of 10 % NH3 was
added into the mixture. The color of the obtained solution changed from orange to brown,
followed by black, indicating the formation of Fe3O4 NPs. The obtained Fe3O4 NPs were
separated from the solution by using a magnet, and then washed with DI water for several times.
Finally, the products were passed through freeze-drying in 1 hr.
2.3. Surface modification of Fe3O4 NPs with TSC
The surface functionalization of Fe3O4 NPs was carried out by mixing the obtained Fe3O4
NPs into 200 mL of 0.5 M TSC solution at 80 oC in 1 hr. The obtained solids (denoted as
TSC@Fe3O4 NPs) were separated from the solution by using a magnet, washed with DI water
for several times, and then dispersed into DI water for further use.
2.4. Surface coating of Fe3O4 NPs with silica
Firstly, ethanol and ammonium hydroxide were added into 0.5 mL of TSC@Fe3O4 NPs
containing solution. The mixture was then passed through sonication treatment in 30 mins. Next,
0.45 mL of TEOS was subsequently added to the mixture under vigorous stirring in 12 hrs in
order to obtain the silica coated Fe3O4 nanoparticles (denoted as SiO2@Fe3O4 NPs). Afterwards,
the separation and washing treatment of SiO2@Fe3O4 NPs were also carried out using the same
aforementioned procedure.
3.1. Effect of surface functionalization
In order to investigate functional groups of surface modified Fe3O4 NPs, FT-IR spectra of
Fe3O4 NPs, TSC@Fe3O4 NPs and SiO2@Fe3O4 NPs were recorded. As shown in Figure 1, three
characteristic peaks of Fe3O4 NPs were observed at 583 cm-1, 1616 cm-1 and 3360 cm-1, which
correspond to the stretching vibrations of Fe-O, bending and stretching vibrations of O-H,
respectively. After the treatment with TSC, other two new peaks were also recorded at 1402 cm-1
and 1610 cm-1 (Figure 1b) which are characteristic peaks of –COO– bonds due to successful
surface functionalization of Fe3O4 NPs with TSC through the carboxylic groups. Interestingly,
those aforementioned peaks were almost vanished or weaken in intensity after the surface
coating of Fe3O4 NPs with silica (Figure 1c), indicating strong interaction between silanol
groups of silica and surface hydroxyl and carboxylic groups of modified Fe3O4 NPs. The
presence of silica outer-coated Fe3O4 NPs layer can be evidenced by formation of an intensive
peak at 1091 cm-1 and other less intensive peaks at 803 cm-1 and 627 cm-1, which are indicative
of Si-O-Si and Si-C bonds, respectively. Additionally, a small peak of Si-OH bond was also
observed at 963 cm-1, probably due to incomplete condensation of TEOS substance.


Synthesis and characterization of silica coated magnetic iron oxide nanoparticles

Figure 1. FT-IR spectra of a) Fe3O4 NPs, b) TSC@Fe3O4 NPs, c) SiO2@Fe3O4 NPs.

3.2. Study of crystallinity
The crystalline structure of the as-synthesized NPs was investigated through XRD patterns,
as indicated in Figure 2. For Fe3O4 NPs, all diffraction peaks of the XRD patterns can be easily
indexed to a pure cubic phase of Fe3O4 (JCPDS No 65-3107) (Figure 2a). The characteristic
peaks were recorded at 2θ = 30.2°, 35.4°, 43.1°, 53.2°, 56.9°, and 62.5° which represent
corresponding indices (220), (311), (400), (422), (511), and (440) [8]. The average particle size
of Fe3O4 NPs calculated using the Debye–Scherrer formula from the refection peak of (311) is
about 13.9 nm [9]. It should be noted that the TSC functionalization did not lead to the phase
change. In contrast, the silica coating resulted in a decrease in the intensity of characteristic
peaks of Fe3O4, and the presence of the amorphous silica matrix observed at 2θ = 14.6 [10],
probably due to thick SiO2 coating.

Figure 2. XRD spectra of Fe3O4 NPs, TSC@Fe3O4 NPs and SiO2@Fe3O4 NPs.

3.3. Structural morphology
TEM images shown in Figure 3 revealed structural morphology of Fe3O4 NPs and
SiO2@Fe3O4 NPs. It is obvious that Fe3O4 NPs possess polydispersion with the average particle
size of 13.7 nm (Figure 3a,e) which is consistent with the XRD results. After modification with
TSC, there is not much change in particle size which was determined as 16.8 nm (Figure 3e).


Minh-Tri Nguyen-Le et al.

The particles became larger after the coating with silica. At this time, SiO2@Fe3O4 NPs revealed
a mosaic structure in which SiO2 acts as an outer layer of multi-core Fe3O4 NPs (Figure 3c).
3.4. Thermal stability
Thermal stability of Fe3O4 NPs, TSC@Fe3O4 NPs and SiO2@Fe3O4 NPs was explored by
TGA (Figure 4). Accordingly, all samples exhibited a weight loss of 5 % – 8 % below 200 oC
due to desorption of adsorbed water [11, 12]. At temperatures above 200 oC, Fe3O4 NPs are not
stable, probably due to oxidation of Fe3O4 into more thermal stable compound Fe2O3. This
statement is also true for TSC@Fe3O4. A significant weight loss of 6.7 % above 200 oC was
found for TSC@Fe3O4, which can be ascribed to the decomposition of surface functional groups
of modified Fe3O4, as well as transformation of Fe3O4 into Fe2O3. Interestingly, after surface
coating with silica, SiO2@Fe3O4 NPs revealed negligible weight loss above 200 oC, indicating
highly thermal stable state. The results demonstrated that SiO2 acts as a protective layer to
prevent Fe3O4 NPs from the oxidation.

Figure 3. TEM images and size distribution of Fe3O4 NPs (a, b) and SiO2@Fe3O4 NPs (c,d); DLS particle
size distribution (e) of Fe3O4 (the solid line), TSC@Fe3O4 NPs (the dotted line) and SiO2@Fe3O4 NPs (the
dashed line).

Figure 4. TGA of Fe3O4 NPs, TSC@Fe3O4 NPs and SiO2@Fe3O4 NPs.


Synthesis and characterization of silica coated magnetic iron oxide nanoparticles

3.5. Magnetic properties
The magnetization versus of Fe3O4 NPs, TSC@Fe3O4 and SiO2 coated Fe3O4 NPs samples
were measured in the field scanning from -15 to 15 kOe, and the curves are shown in Fig. 5.
From Figure 5, one can see that Fe3O4 NPs, TSC@Fe3O4 and SiO2 coated Fe3O4 NPs are all
superparamagnetic because no hysteresis loop can be observed [13-16]. The saturation
magnetization of Fe3O4 NPs is 45.8 emu/g, which is higher than that of TSC@Fe3O4 (~14.6
emu/g), SiO2 coated Fe3O4 NPs (~13.4 emu/g) due to the functionalization and the thick silica
shell layer (~ 15 nm), respectively.

Figure 5. Magnetization of superparamagnetic Fe3O4 NPs, TSC@ Fe3O4 NPs and SiO2@Fe3O4 NPs.

We demonstrated an effective method for preparation of SiO2 coated Fe3O4
superparamagnetic nanoparticles with saturation magnetization of 13.4 emu/g through a mild
synthetic conditions. The SiO2 coated Fe3O4 superparamagnetic nanoparticles revealed high
thermal stability and superparamagnetic property, which can be used in broad applications
including MRI, drug delivery and therapeutic fields.
Acknowledgements. This study was funded by the National Foundation for Science and Technology
Development (Grant No. 104.03-2018.46).


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