Core–shell and multilayered magnetite nanoparticles—Structural and Mössbauer studies
Introduction
Magnetic nanoparticles with different core–shell structures have become very popular among many researches in last decades. A varied composition of a shell layer can add novel properties to magnetic nanoparticles. A metallic spacer layer influences magnetic properties of the system. When other magnetic material is used, such as a shell layer (like α-Fe2O3, γ-Fe2O3, Co, Ni), it influences very strongly all magnetic properties of the nanoparticle. On the other hand, using nonmagnetic, metallic elements (such as copper, silver, or gold), much less pronounced modification of magnetic properties can be obtained but, in addition, such modification causes lower toxicity, higher antibacterial properties, or effective antifungal resistivity, etc. [1], [2]. A surface layer of noble metals causes an increase on the palette of compounds possible to connect to magnetic nanoparticles. This effects in the protection of the core and surface of nanoparticles from external factors and, at the same time, isolates them from each other. Moreover, the shell composed of noble metals does not allow agglomeration processes of the cores and provides better biocompatibility and resistance to physiological conditions: for example, specific pH, enzymes interaction, etc. The shell of noble metals also guarantees high stability of nanoparticles in various solvents while keeping their core properties stable. All these arguments indicate a wide range of usage of core–shell nanoparticles in medicine, for example, in MRI contrast, in cancer treatment as hyperthermia agents [3], in chemistry in NMR spectroscopy. In case of a nonmetallic (e.g., silica, polymers) layer present in the core–shell, nanoparticles provide for its possible extensive applications as nucleators, reagents, carriers, chromatography fillers, etc. Therefore the shell layer becomes a key factor affecting the uniform dispersion of the particles, their further overall properties [4] and widespread application.
The preparation of nanoparticles can be done in many various ways, which can be, in general, divided to chemical and physical one. As a first example, a chemical synthesis consisting of thermal decomposition of acetylacetonate salts of iron and other metals in organic solutions can be presented [5], [6]. Core–shell nanoparticles can be also obtained in aqueous solutions by co-precipitation based on iron (II), iron (III) chlorides and ammonia solution [7], [8]. In this case, more intermixing of the elements in the structure can be expected in respect to the previous method. Both ways are quite well known and become a source of various modifications described in many research papers [9], [10], [11]. Those magnetite syntheses, however, are significantly different in preparation conditions, which causes other physical–chemical properties like magnetization, agglomeration, hydrophilicity, etc., of resultant particles.
Among scientists there are also widespread many physical methods of nanoparticles fabrication. A first example of the top-down approach is the reduction of size by long lasting grinding or ball milling. This method has two main disadvantages: firstly, it needs preparation of bulk material, which is time and energy consuming, secondly, the size and size distribution of prepared nanoparticles is poorly controlled [12]. Other popular methods are based on very sophisticated high vacuum systems where nanomaterials fabrication is obtained on the basis of the following phenomena: co-sputtering, laser vaporization, co-deposition of gas-phase, cluster beam deposition, etc. [13], [14], [15], [16], [17], [18]. This represents bottom-up approaches of nanomaterials production where physical methods are involved. High vacuum methods are ideal to study individual cluster/particle magnetism, collective dynamics or random anisotropy [19], [20]. On the contrary, chemical methods are important due to lower cost and much easier production procedures.
In this paper, the magnetite core of nanoparticles was obtained from thermal decomposition of Fe(acac)3 in organic solution. The shell layer of the nanoparticles was prepared from Cu, Ag and Au metal complexes. Furthermore, the obtained nanoparticles were functionalized with SH groups, and next immobilization of glucose oxidase was tested. The obtained core–shell nanoparticles and their respective biocomposites were measured by infrared spectroscopy (IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) imaging and differential scanning calorimetry (DSC). Magnetic properties were characterized by Mössbauer spectroscopy (MS).
Section snippets
Material and apparatus
To obtain core–shell nanoparticles with a various shell, the following chemicals were purchased from Aldrich: Fe(acac)3, Cu(acac)2, Gold(III) chloride 1,2-hexadecanediol, phenyl ether. AgNO3 and oleic acid were bought from POCH. In addition, 1-octadecanol and oleyl amine were obtained from Fluka. Cleaning and separation of nanoparticles were performed with the use of acetone, sonication bath and permanent magnet.
IR spectra were collected in range 400–4000 cm−1 by Nicolet 6700 infrared
Transmission electron microscopy studies
The samples selected from the series were characterized by transmission electron microscopy, which allows a reader to follow changes in the particles size and structure. TEM images of the prepared nanoparticles are depicted in Fig. 2. They are organized in the series according to the layered structure and type of Me (Ag, Au, Cu or Fe3O4).
It can be seen that the images of the fabricated particles show a good correlation between the postulated structure and particle size. The imaged particles
Conclusions
In our opinion, the presented series of the particles have well documented layered structure. It is approved by the characterization of the structures by TEM, XRD, IR, DSC and Mössbauer spectroscopy. Subsequent reduction of all used metals complexes suggests modification of the particles core in the layered manner. The proposed way of the modification by step-by-step growth of magnetic cores of the particles allows for the fabrication of every complex structure which helps in further
Acknowledgements
Mössbauer spectroscopy was done in close collaboration with the Department of Physics at the University of Bialystok. The work was partially financed by the EU funds via the project with contract number POPW.01.03.00-20-034/09-00 and via national grant (contract number UMO-2011/03/B/ST5/02691).
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