Morphological studies of randomized dispersion magnetite nanoclusters coated with silica
Introduction
Nanofabricated magnetite nanoparticles have been an intense focus of fundamental researchers since last decade because they have unique magnetic properties and ease to be engineered into many forms of composite materials, typically for biomedical applications. There are a number of synthetic methods being utilized to produce nanoscaled magnetite particles like, organo-metallic thermal decomposition [1], microemulsion [2], hydrothermal [3], solvothermal [4], and co-precipitation [5], [6]. The latter is the most commonly used method, mainly attributed to its simple and easy processing operation, high yield of products with superior crystallinity and magnetic behaviours and it involves only the utilization of non-organic based reactants. However, the major shortcoming of this method is the agglomeration of the particles due to their high magnetization properties.
To date, several findings have shown that the nanoparticles are favorably synthesized in the form of nanoclusters [7], [8]. Magnetic nanoclusters are a type of magnetic nanoscale materials containing a number of magnetic nanoparticles embedded or enwrapped by a polymeric shell such as thermoresponsive polymers [9], cyclodextrin [10], chitosan [11], and carbon nanocomposites [12]. These magnetic nanoclusters can easily be magnetized by an external magnetic field and loss of signal will greatly be avoided when it is in vivo transported together with the drugs to the targeted site. Previous studies have reported that the magnetic nanoparticles will tend to lose the magnetizability when the biopolymer coated nanoparticles are being carried into the body [13]. In the targeting drug delivery system, sufficient amount of drug is needed to be carried to the targeted organ by incorporating it with surface-functionalized magnetic nanoclusters under the manipulation of external magnetic field [14]. However, usually the nanoparticles coated insufficiently will have the difficulty to be magnetized by external magnetic field, resulting in lower efficiency of drug targeting treatment and to some extent the drug being transported may be erupted from the polymeric shell causing local cytotoxicity. Therefore, attention needs to be drawn towards the study of nanocluster formation due to their intriguing physical and chemical properties. However, formation of magnetic nanoclusters is a grand challenge bound to the magnetic properties of the particles, type of surface modifier whether it is dispersible in organic or non-organic solvent, biocompatibility of modifier and reproducibility. Therefore, particle dispersity is an issue that needs to be addressed earnestly in order to overcome the agglomeration of magnetic nanoparticles before the formation of magnetic nanoclusters. As a result, in this study, we adopted polyanionic citrate group to improve the dispersibility of the magnetite nanoparticles so that the nanoparticles can be homogeneously coated with the silica material. Previous study reported that the addition of anionic citrate to the surface of magnetite nanoparticles through chemical complexation process has greatly improved the particle's dispersibility [15].
Essentially, there are four types of nanocluster morphology including core–shell structure, reverse core–shell, inter-layered, and randomized dispersions [16], [17], [18], [19]. In this study, we are interested in the randomized dispersion nanoclusters since this particular morphology of nanoclusters offers larger cavity for the embedded seeds to be covered by the polymer layer. Fig. 1 depicts the typical type of nanoclusters in which the nanoseeds are randomly dispersed in the protective cavity resulting from the coating process.
Although many investigations have been done on the silica coated magnetite nanoparticles, however, to the best of our knowledge, the study of magnetic nanocluster formation has not been well developed. Silica is well-known for its biocompatibility, microporous properties, and excellent reproducibility [20], [21], [22]. However, most of the studies reported irregular shape of silica coating and showed severe agglomeration of the magnetic coated silica instead of well-confined in a silica shell [21], [23]. To have better understanding on the effect of silica coating on the magnetic nanoparticle surfaces, pre-treatment of magnetic nanoparticle surfaces is necessary in order to overcome the agglomeration of particles. Another issue that needs to be ascertained is the essential processing parameters that need to be adapted during the formation of silica material, in order to enhance the efficiency in the production of mono-dispersed silica spheres. Chou and Chen studied the critical condition related to the formation of new secondary silica particles during the seeded nucleation process [24]. They had varied the amount of TEOS to tune the size and shape of the amorphous silica particles.
In the present study, we have synthesized randomized dispersion magnetite nanoclusters using silica with minor modification from Stöber process. We have studied different reaction parameters including the different polarities of alcohol, ratio of alcohol–water as co-solvent, concentration of ammonia as catalyst, and concentration of TEOS monomer. The as-synthesized magnetite nanoparticles, citrate-treated magnetite nanoparticles, and RDMNS were then characterized using various instrumentations including morphology and size determination by transmission electron microscopy (TEM), degree of crystallinity before and after surface modification by X-ray diffraction (XRD), retainability of superparamagnetic properties of citrate and silica modified magnetite nanoparticles by vibrating sample magnetometer (VSM), the presence of citrate and silica layers on the surface of magnetite nanoparticles by Fourier transform infra red spectroscopy (FT-IR) and percentage of citrate and silica coated on the magnetite nanoparticles using thermogravimetric analysis (TGA). The mechanism of citrate-attachment to the magnetite nanoparticles surface will also be discussed.
Section snippets
Materials
Chemicals were used as received without further purification. Analytical-grade TEOS (tetraethyl orthosilicate, ≥98%), ethyl alcohol (>99%), NH3 (ammonia solution, 28 wt.%), and NaOH pellets (sodium hydroxide, assay ≥97%) were purchased from Merck & Co., Inc.; whilst ferrous dichloride heptahydrate (FeCl2·4H2O), ferric trichloride anhydrous FeCl3, trisodium citrate dihydrate (assay 99.3%), and methanol (ACS reagent, ≥99.8%) were procured from Sigma–Aldrich. Deionized water (DIW) was used
Morphology and size distribution of M1 and CM1
Fig. 3 shows typical TEM micrographs of both the unmodified magnetite nanoparticles (M1) produced via concentrated precipitation method (10 M of NaOH) and the citrate-modified magnetite nanoparticles (CM1). Referring to Fig. 3 (A), majority of the particles were in oval shape and some least number of cubic shaped magnetite nanoparticles also can be observed. The average crystallite size for the M1 nanoparticles is about 9.14 ± 3.14 nm. On the contrary, CM1 nanoparticles which are of better
Conclusion
In summary, herein we report a way to study the morphology of randomized dispersion magnetite nanoclusters coated with silica by controlling several reaction parameters during the silica sol polymerization process. The magnetite nanoparticles were firstly prepared using a well-known method i.e., co-precipitation, followed by surface-modification with polyanionic trisodium citrate dihydrate. The citrate-treated magnetite nanoparticles were then coated with silica using Stöber method with some
Acknowledgement
This project was supported by Universiti Kebangsaan Malaysia under the Research University Operation Grants (UKM-OUP-NBT-28-124/2010 & UKM-OUP-NBT-29-142/2010) and the Young Researcher Grant (UKM-GGPM-NBT-085-2010). Haw acknowledges the Ministry of Science, Technology and Innovation (MOSTI) for the disbursement of National Science Fellowship (NSF).
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