Synthesis and Characterization of Fe10BO3/Fe3O4/SiO2 and GdFeO3/Fe3O4/SiO2: Nanocomposites of Biofunctional Materials

Cancer is one of the leading causes of death worldwide; with
many different types, it kills thousands of people every day.
Various types of treatments have been developed to treat
cancer, and new approaches that are currently under investigation
include boron neutron capture therapy (BNCT)[1] and gadolinium
neutron capture therapy (GdNCT).[2] Neutron capture
therapy is primarily used to treat brain tumors, such as glioblastoma,
a particularly aggressive type of brain tumor that is
difficult to treat by conventional means such as radiation therapy.
BNCT and GdNCT involve a bimodal approach to treatment,
utilizing a cancer-specific drug and a neutron source
(neutron beam). The approach is based on the ability of
a boron isotope (10B) to absorb neutrons and emit localized
cell-killing particles. The main mechanism that takes place in
BNCT is the absorption of a neutron to convert 10B to 11B, with
the release of 4He2+, 7Li3+, and energy.[3] The energy that is released
can then destroy the tumor cell. Gadolinium also attracted
interest for its potential use in neutron capture therapy
because it is the element with the highest cross-sectional
value for thermal neutrons—2.55 105 b and 6.10 104 b for
157Gd and 155Gd, respectively.[4] In fact, the thermal neutron
value of 157Gd (2.55 105 b) is 65 times that of 10B, and it releases
Auger electrons, internal conversion electrons, g-ray and
X-ray after the capture of a single thermal neutron.[1, 5–7]

Targeted delivery of an anticancer drug is very desirable, as most of the commonly used agents have serious side effects associated with their use due to undesirable interactions with healthy cells. Moreover, targeted delivery can potentially enhance the therapeutic efficacy. [8] Research on nanomaterials has grown explosively in the last few years, including an increased emphasis on developing nanomaterials as drug delivery vehicles. [9,10] The size of such delivery vehicles (< 1000 nm) has attracted wide interest in the field of drug targeting. Nanomaterial-based drug systems provide the advantage of being able to penetrate cell membranes through minuscule capillaries in the cell wall of rapidly dividing tumor cells, while at the same time having low cytotoxicity toward normal cells. Nanomaterials have been found to have favorable interaction with the brain blood vessel endothelial cells of mice, and thus they might have the possibility of being transported to other brain tissues, making them potential neutron capture therapy agents. [11,12] In theory, in BNCT and GdNCT nanomaterials, a large number of boron and gadolinium atoms could be incorporated, thereby lowering the dose requirement for delivering critical amounts of 10 B and Gd to tumor cells. Accordingly, improvement of the drug storage capacity is very important. [7] Magnetic nanoparticles are being studied in terms of their highly promising applications in biology and medicine, including magnetic cell separation, magnetic resonance imaging (MRI) contrast enhancement, and magnetic targeted drug delivery for cancer magnetic hyperthermia. [10] MRI is a noninvasive technique for obtaining real-time three-dimensional images of the interior of solids (particularly cells), tissues, and organs. But magnetic nanoparticles tend to aggregate due to strong magnetic dipole-dipole attraction between particles brought together by van der Waals interparticle attractions and their inherently large surface energy. Therefore, coating agents, such as surfactants or capping ligands with some specific functional groups, have been used to modify these particles in order to prevent the sedimentation and to obtain better surface properties. [13] Silicates have attracted significant interest because of their rich structural chemistry, which makes the development of new structures and functionalities possible. Amorphous silica with a nontoxic nature, tunable diameter, and very high specific surface area with abundant SiÀOH bonds on the surface are promising candidates for use as carriers in drug delivery systems. Thus, nanocomposites of SiO 2 and magnetic particles have attracted considerable attention in targeted drug delivery because of the high surface area and magnetic separability. [14] Crystalline FeBO 3 material is known for its unique magnetic and acoustic resonance properties. [15] In contrast, GdFeO 3 shows promising relaxivity properties and has potential as an MRI contrast agent. [16] Fe 3 O 4 has been considered to be an ideal candidate for biological applications due to its special magnetic properties, lack of toxicity, and good biocompatibility. [17] The nanocomposites of these materials can carry an active agent (drug) and be guided to the target site inside the body, facilitating therapeutic efficiency and minimizing damage to normal tissue due to drug toxicity.
In recent years, several different routes have been used to synthesize biofunctional magnetic nanocomposites. [18] The gel combustion method has been developed and widely used to prepare phase-pure nanopowders. [19] The method has the advantages of using inexpensive precursors, requiring a simple experimental process, and resulting in an ultrafine, homogenous powder. Chavan and Tyagi used a combustion method to produce GdFeO 3 nanoparticles with sizes in the range of 40-65 nm. [20] However, there have been only a few reports on combinations of magnetic Fe 3 O 4 with FeBO 3 and GdFeO 3 nanoparticles. For this reason, we developed a route consisting of encapsulating preformed 10 B, Gd and Fe 3 O 4 nanoparticles into silica. The aim was to obtain core shell nanoparticles, denoted Fe 10 BO 3 /Fe 3 O 4 /SiO 2 , and GdBO 3 /Fe 3 O 4 /SiO 2 , which will 1) improve the drug storage capacity, 2) have sufficiently powerful magnetic properties, 3) form a stable dispersion at physiological pH, and 4) have facile surface chemistry to allow the use of coupling agents, such as commercially available alkoxysilane derivatives.
The reactions described herein were generally performed under air or argon, and Fe 10 BO 3 /Fe 3 O 4 /SiO 2 and GdFeO 3 /Fe 3 O 4 / SiO 2 nanocomposites were prepared via the route shown in Scheme 1. Powder X-ray diffraction (XRD) was used to investigate the variations in structure of the samples produced by the gel combustion method under different conditions (Figures 1). Fe 10 BO 3 and GdFeO 3 crystallized as a phase-pure material at calcination temperatures as low as 680 8C in 2 h (Figure 1 a,c). The diffraction peaks of the product can be readily indexed to the pure Fe 10 BO 3 and GdFeO 3 (Joint Committee on Powder Diffraction Standard (JCPDS) card no. 76-0701 and 78-0451, respectively). No additional peaks for other phases or impurities were found. These results demonstrate that well-crystallized Fe 10 BO 3 and GdFeO 3 can be obtained using the gel combustion technique. After coating with Fe 3 O 4 , the product is a nanocomposite of Fe 10 BO 3 or GdFeO 3 and Fe 3 O 4 (Figure 1 b,d), suggesting that a hybrid material, composed of Fe 10 BO 3 or GdFeO 3 and Fe 3 O 4 , had formed. No peaks of other phases were detected, indicating that no other reaction occurred between the core and the shell during the synthesis.
The typical microstructure of the sample was examined by transmission electron microscopy (TEM) analysis. Figure 2 shows the TEM images of the samples before and after coated with Fe 3 O 4 and SiO 2 . For pure Fe 10 BO 3 and GdFeO 3 , the TEM images indicate that the nanoparticles are spherical and the particle diameter is about 60 nm (Figure 2 a,d). When coated with Fe 3 O 4 , the particles tend to aggregate due to strong magnetic dipole-dipole attractions between them (Figure 2 b,e). After the particles were coated with SiO 2 , there was a thin layer of amorphous silica covering the surface (Figure 2 c,f).  In order to deduce the composition of the nanocomposites, energy-dispersive X-ray spectroscopy (EDS) analysis was carried out (Figure 3). Before coating with Fe 3 O 4 and SiO 2 , the EDS specta of the samples depict no other peaks except those for Fourier-transformed infrared (FT-IR) spectroscopy was used to identify the surface functional groups of the samples. Figure 4 shows the FT-IR spectra of Fe 10 BO 3 /Fe 3 O 4 /SiO 2 and GdFeO 3 /Fe 3 O 4 /SiO 2 nanocomposites in the region of 500 cm À1 to 4000 cm À1 . A broad band with a maximum at 3437.8 cm À1 is attributed to the OÀH stretching vibrations in both the SiÀOH groups and some physisorbed water, which is confirmed by the presence of an H 2 O deformation band (bending vibration of HÀOÀH) at 1633.6 cm À1 . For the Fe 10 BO 3 /Fe 3 O 4 /SiO 2 sample, bands at 1254.8 and 1965.6 cm À1 are due to vibrations of the BÀO bond and other bonds attached to the B or the O of the BÀO bond. [21][22][23] The band at 1050 cm À1 corresponds to u(SiÀOH); the bands at 1100 and~860 cm À1 correspond to u asym (SiÀOÀSi) and u sym (SiÀOÀSi) modes, respectively. The absorption at 767.7 cm À1 could be attributed to the OÀH stretching vibration on the surface of Fe 3 O 4 . An additional absorption at 667 cm À1 could be attributed to the FeÀOÀB bending vibration analogous to SiÀOÀ B. [23,24] A very small band shoulder at~560-580 cm À1 , observed in the IR spectra, can be assigned to u(FeÀO) and u(GdÀO) in FeÀOÀFe and GdÀOÀFe systems, respectively. [25][26][27] These results indicate that Fe 3 O 4 and SiO 2 are immobilized on the surfaces of the Fe 10 BO 3 and GdFeO 3 nanoparticles.
The magnetization curve measured at room temperature for the Fe 10 BO 3 /Fe 3 O 4 /SiO 2 and GdFeO 3 /Fe 3 O 4 /SiO 2 nanocomposites shows a small hysteresis loop suggesting that the nanocomposites have ferromagnetic behavior ( Figure 5). It has been reported that magnetic Fe 3 O 4 particles exhibit super-paramagnetic behavior when the particle size decreases to below a critical value, generally around 20 nm. [28] The Fe 3 O 4 particles are aggregated and connected to form larger particles, resulting from the ferromagnetic behavior. The magnetization saturation values for Fe 10    because they have sufficiently strong magnetization for efficient magnetic separation in the presence of an externally applied magnetic field. [29] A novel kind of magnetic sphere with 10 B or Gd and Fe 3 O 4 nanoparticles encapsulated in the cores of silica shells has been fabricated. The nanocomposite spheres, which combine the advantages of silica and magnetic carrier technology, are likely to be applied in targeted drug delivery. The main focus of this research was to synthesize novel neutron capture therapy materials that are both effective and relatively harmless to the patient. The next stage of this research involves the biological evaluation of the two nanocomposites reported here and is currently underway in our laboratories.
The combustion synthesis utilized Fe(NO 3 ) 3 ·9H 2 O (1.92 g, 0.0048 mol) and H 3 10 BO 3 (0.92 g, 0.015 mol) as the starting materials. Citric acid was used as the fuel, and chelation was in the ratio 1:1.25. The precursors and fuel were mixed in water (50 mL) to obtain a transparent aqueous solution, which on thermal dehydration resulted in a highly viscous liquid. On further heating to high temperature (190 8C), the viscous liquid swelled and dried. The resi-due was then ground to obtain a powder, which was then subjected to further heat treatment at 680 8C for 2 h under an argon atmosphere to isolate Fe 10  The phases of the final products were identified using an X-ray diffractometer (Rigaku D/max-2500 VPC) with Ni-filtered Cu Ka radiation at a scanning rate of 0.028 s À1 from 208 to 808. A Hitachi model H800 transmission electron microscope was used for determining the size and shape of the powder particles. Fourier-transform infrared (FT-IR) spectra were recorded using a MAGNA 550 FT-IR spectrometer on samples embedded in KBr pellets. Magnetization measurements were performed using an ACBH-100K B-H hysteresis loops measuring instrument. All measurements were performed at room temperature.