In Situ Mineralization of Magnetite Nanoparticles in Chitosan Hydrogel
© to the authors 2009
Received: 26 February 2009
Accepted: 17 May 2009
Published: 30 May 2009
Based on chelation effect between iron ions and amino groups of chitosan, in situ mineralization of magnetite nanoparticles in chitosan hydrogel under ambient conditions was proposed. The chelation effect between iron ions and amino groups in CS–Fe complex, which led to that chitosan hydrogel exerted a crucial control on the magnetite mineralization, was proved by X-ray photoelectron spectrum. The composition, morphology and size of the mineralized magnetite nanoparticles were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and thermal gravity. The mineralized nanoparticles were nonstoichiometric magnetite with a unit formula of Fe2.85O4and coated by a thin layer of chitosan. The mineralized magnetite nanoparticles with mean diameter of 13 nm dispersed in chitosan hydrogel uniformly. Magnetization measurement indicated that superparamagnetism behavior was exhibited. These magnetite nanoparticles mineralized in chitosan hydrogel have potential applications in the field of biotechnology. Moreover, this method can also be used to synthesize other kinds of inorganic nanoparticles, such as ZnO, Fe2O3and hydroxyapatite.
Mineralization, leading to the formation of minerals in the presence of organic molecules, is a widespread phenomenon in biological system [1, 2]. In the process of mineralization, a small amount of organic component not only reinforces mechanical properties of the resulting materials but also controls the mineralization process, which endows materials with interesting properties such as special crystal morphology and superb mechanical properties . Therefore, mineralization is becoming a valuable approach for novel materials synthesis.
One of the most intriguing examples for mineralization is magnetic bacteria [4, 5]. Each magnetic bacteria acts as a small reaction vessel for mineralization, and the bacterial cell wall can control the iron ions diffusion. Consequently, the magnetite nanoparticles mineralized in magnetic bacteria have perfect shape and size, and the magnetite nanoparticles are assembled into a highly ordered chain structure. Furthermore, the mineralized magnetite nanoparticles in magnetic bacteria are water soluble and biocompatible, which makes it suitable for being used in the fields of bioscience and biomedicine, such as separation for purification and immunoassay , drug target delivery [7, 8], magnetic resonance imaging (MRI) [9, 10] and hyperthermia . However, the yield of mineralization of magnetite nanoparticles in magnetic bacteria is too low to make it practical for industrial applications.
Enlightened by the phenomenon of mineralization in magnetic bacteria, a large number of organic molecules have been investigated to realize controllable magnetite mineralization in laboratory. These organic molecules usually contain carboxylic groups , sulfate or hydroxyl groups as functional groups [13, 14], which may bind iron ions or control crystal nucleation and growth by lowering the interfacial energy between the crystal and organic molecules. However, most of these studies focus on the mineralization in solution state that is quite different from the gel state in case of magnetic bacteria. Therefore, researches on mineralization of magnetite in organic hydrogel have great scientific and practical significance.
Inspired by magnetic bacteria, we propose in situ mineralization of magnetite nanoparticles in chitosan hydrogel under ambient conditions. CS–Fe complex was used as a precursor for the mineralization, and the chelation effect of CS–Fe complex can control magnetite mineralization. The mineralized magnetite nanoparticles were well investigated, and the mineralization principle was discussed.
Materials and Methods
Biomedical grade chitosan (viscosity–average molecular weight 3.4 × 105) was supplied by Qingdao Haihui Bioengineering Co., Ltd. with 91.4% degree of the deacetylation. All chemicals were analytical grade reagents and used without further purification.
Preparation of chitosan hydrogel was performed as follows. Three grams of chitosan powder was dissolved in 100 mL of 2% (v/v) acetic acid solution to get 3% chitosan solution. 0.3 mL glutaraldehyde solution (50%) was added to the 100 mL chitosan solution under vigorous stirring to obtain homogeneous solution, in which the molar ratio of aldehyde/amino groups was equal to 1:5. The solution was held until chitosan hydrogel formed completely due to cross-linking effect of glutaraldehyde.
In situ mineralization of magnetite nanoparticles in chitosan hydrogel was carried out as follows. First, the chitosan hydrogel was soaked in 0.15 mol/L FeCl3solution for 30 min. Then, the chitosan hydrogel with iron ions was washed with deionized water, and soaked in 0.075 mol/L FeCl2solution for another 30 min. After that, the chitosan hydrogel containing iron ions was subsequently washed with deionized water. This cycle was repeated for 3 times, and the CS–Fe complex was formed. The pH value of the CS–Fe complex was approximately 1.0. Finally, the CS–Fe complex was soaked in NaOH solution (1.25 mol/L) for 12 h, and the magnetite/chitosan composite was achieved. The amount of NaOH was extremely excessive for magnetite mineralization, which induced the concentration of NaOH approximately 1.25 mol/L during the reaction process. Magnetite nanoparticles were obtained after the magnetite/chitosan composite was degraded by H2O2, in which the molar ratio of amino/H2O2was equal to 1:2.
X-ray photoelectron spectroscopy (K-Alpha, Thermo Fisher Company) was employed to study interactions between iron ions and chitosan. Crystal structure of mineralized magnetite nanoparticles was investigated by an X-ray diffractometer (D/max-2550, Rigaku) using Cu Kα radiation and a graphite monochromator. The Raman spectra (HORIBA T64000) were excited by 514.5 nm radiation from an argon ion laser. The laser power reaching the sample surface was 20 mW, and the typical acquisition time was 60 s. Transmission electron microscopy (H-7650, Hitachi, Japan) was used to observe the morphology of the magnetite nanoparticles. The mineralized magnetite nanoparticles were also investigated by thermal gravity (STA 449C, Netzsch Company, Germany) to obtain the amount of chitosan layer on the mineralized magnetite nanoparticles. Magnetic properties were determined by Physical Property Measurement System (PPMS-9, Quantum Design, America).
Results and Discussion
Crystal Structure of the Mineralized Magnetite Nanoparticles
The calculated unit formulas of magnetite/chitosan composite and mineralized magnetite nanoparticles
Unit formula (Fe3−δO4)
Mineralized magnetite nanoparticles
Morphology of the Mineralized Magnetite Nanoparticles
As can be seen in Fig. 4b, there was a blurred layer coating on the Fe3O4nanoparticles. It is believed that the blurred layer could be assigned to chitosan layer on mineralized magnetite nanoparticles.
Chitosan Layer on the Mineralized Magnetite Nanoparticles
As can be seen in Fig. 5a, in the interval of 200–800 °C, there was no weight loss for pure magnetite. However, the mineralized magnetite nanoparticles experienced a 19.1% weight loss that was assigned to the decomposition of acetylated and deacetylated units of chitosan layer coating on mineralized magnetite nanoparticles (Fig. 5b). The existence of chitosan layer changes the properties of magnetite nanoparticles and makes it water soluble and biocompatible, which makes it has potential applications in the field of biotechnology as magnetic resonance imaging contrast agents and drug carrier.
Magnetic Properties of the Mineralized Magnetite Nanoparticles
Principle of In Situ Mineralization of Magnetite in Chitosan Hydrogel
In situ mineralization of magnetite nanoparticles in chitosan hydrogel under ambient conditions was proposed. The chelation effect between iron ions and amino groups in CS–Fe complex was proved by XPS. The mineralized magnetite nanoparticles, which were coated by chitosan layer, have a narrow size distribution and small diameter. XRD analysis and Raman spectra indicated that the mineralized nanoparticles were nonstoichiometric magnetite and the unit formula was Fe2.85O4. The mineralized magnetite nanoparticles with a mean diameter of 13 nm dispersed in chitosan hydrogel uniformly. Magnetization measurement indicated that superparamagnetism behavior was shown and the coercitivity and the remanence were 16.5 Oe and 0.9 emu/g respectively. The principle of magnetite mineralization in chitosan hydrogel can be expatiated as follows. First, iron ions were chelated by the amino groups of chitosan, and the CS–Fe complex was fabricated. When the CS–Fe complex encountered OH−, the iron ions chelated by the amino groups, which providing nucleation site for magnetite crystals. The iron ions diffusion was restricted by chelation effect, and abnormal crystal growth of magnetite was avoided; thus, magnetite nanoparticles with small diameter and narrow size distribution were formed.
The authors thank the financial support from National Science Foundation of China (50702017), the Innovation Foundation of Harbin Institute of Technology (HIT. NSRIF. 2008.51) the Post-Doctor Foundation (20060390786), and the program of excellent team in Harbin Institute of Technology.
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