Synthesis and characterization of core-shell Fe3O4-gold-chitosan nanostructure
© Salehizadeh et al; licensee BioMed Central Ltd. 2012
Received: 6 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Fe3O4-gold-chitosan core-shell nanostructure can be used in biotechnological and biomedical applications such as magnetic bioseparation, water and wastewater treatment, biodetection and bioimaging, drug delivery, and cancer treatment.
Magnetite nanoparticles with an average size of 9.8 nm in diameter were synthesized using the chemical co-precipitation method. A gold-coated Fe3O4 monotonous core-shell nanostructure was produced with an average size of 15 nm in diameter by glucose reduction of Au3+ which is then stabilized with a chitosan cross linked by formaldehyde. The results of analyses with X-ray diffraction (XRD), Fourier Transformed Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM), and Atomic Force Microscopy (AFM) indicated that the nanoparticles were regularly shaped, and agglomerate-free, with a narrow size distribution.
A rapid, mild method for synthesizing Fe3O4-gold nanoparticles using chitosan was investigated. A magnetic core-shell-chitosan nanocomposite, including both the supermagnetic properties of iron oxide and the optical characteristics of colloidal gold nanoparticles, was synthesized.
Nanoparticles are nanostructures with at least one dimension being less than 100 nm. Gold-coated magnetic nanoparticles are a class of nanoparticles that have attracted much attention because of their advantageous characteristics, such as their inertness, non-toxicity, super magneticity, ease of detection in the human body, a magnetic core that is protected against oxidation, their facilitated bio-conjugating ability, catalytic surface, and their potential for a variety of biological applications [1, 2]. Gold-coated nanoparticles have great biocompatibility with the human body with the ability to interact with biomolecules such as polypeptides, DNA, and polysaccharides . Chitosan, poly-β-(1-4)-2-amino-2-deoxy-D-glucose, also has many favorable characteristics including: low toxicity and high biocompatibility. It has been widely used in many fields, such as water and wastewater treatment , biomedical applications as a drug carrier , therapy for repairing spinal damage  and for preserving nervous cell and mitochondrial membranes from harmful reactive oxygen species (ROS) . The production of core-shell Fe3O4-gold-biopolymer nanocomposites has attracted much attention over the past several years as they can be used in biotechnological and biomedical areas, including biotargeting for cancer treatment, drug delivery, biodetection, and downstream processing (i.e., the purification and bioseparation of biomolecules). Gold nanocomposites utilizing chitosan offer several potential benefits using the magnetic core for controllability, as well as the immobilization of biomolecules and other optical properties through their gold shell [8–10].
This paper describes a simple and rapid method for synthesizing controllable, agglomerate-free Fe3O4-gold-chitosan nanocomposites. Glucose was used as the reducing agent and chitosan as the protecting and stabilizing agent. Additionally, the spectral properties of core-shell Fe3O4-gold nanoparticles synthesized by this method have been evaluated by modern analytical techniques and the results discussed.
Materials and methods
Synthesis of Fe3O4 nanoparticles
Fe3O4 nanoparticles were synthesized according to Ahmed et al.  with several modifications resulting in substantial quality improvements. All of the chemicals used in this research were of analytical grade and obtained from commercial sources. FeCl2. 4H2O, FeCl3·6H2O, sodium hydroxide, sulphuric acid, nitric acid, hydrochloric acid, N-tetra methyl ammonia hydroxide, formaldehyde (37%), ammonium hydroxide, sodium phosphate monobasic, sodium phosphate dibasic and hydrogen tetrachloroaurate(III) (HAuCl4.4H2O, 99%) were obtained from Merck, Germany. Chitosan was prepared from Sigma-Aldrich, USA. Deionized water was obtained from Milli Q system and used throughout. The solutions of FeCl3·6H2O (4 ml, 2 M) and FeCl2·4H2O (2 ml, 2 M) were prepared in 250 ml flasks, added to a flat bottom beaker, and stirred at 30°C for 45 min. The Fe(III)/Fe(II) ratio was kept 2 throughout. Then, an aqueous ammonia solution (100 ml, 1 M) was added by droplet under the cover of N2 gas and the pH of the solution was carefully adjusted up to 10. The solution was stirred for about 1 h until stable, black Fe3O4 particles appeared. Next, the particles were filtered and then rinsed with distilled water and then methanol until the pH reached 7. They were then dried in a vacuum oven at room temperature for 24 h.
Synthesis of Fe3O4-gold nanoparticles
The synthesis of Fe3O4-gold nanoparticles was carried out according to Cui et al.  with some modifications. First, Fe3O4 nanoparticles were dispersed in a 0.1 M HAuCl4·4H2O solution in a flat bottom beaker for 20 minutes using sonication, and then slowly mixed in a shaking incubator at 38°C to allow the adsorption of Au3+ into the Fe3O4 surface. Glucose was then added to the system as a reducing agent and the mixture was incubated at room temperature in a shaking incubator (200 rpm). The core-shell nanoparticles that formed were then washed with pure water until the pH reached 7.
Synthesis of Fe3O4-gold-chitosan
Chitosan (200 mg) was added to 14 ml of acetic acid (1%, v/v) solution and stirred for 10 minutes at room temperature until it became a homogeneous viscous solution. Then, various concentrations of formaldehyde (2-10 ml, 5 M) were used to improve the gelation properties of the formed hydrogel. The prepared chitosan solution was simultanously added to the gold-coated magnetic nanoparticles being formed in the solution and incubated at room temperare with shaking in a shaking incubator (200 rpm) for 1.5 h leading to synthesis of the core-shell structure of Fe3O4-gold-Chitosan.
Fourier transformed infrared (FTIR) spectroscopy was carried out by a Bruker FTIR-6000 (Bruker, Germany) using KBr discs to investigate the interaction of functional groups in chitosan with the nanoparticles surface. The crystallographic characterization of nanoparticles was done by a powder X-ray diffraction (XRD) spectrometer (Bruker D8 Advance, Germany). Transmission electron microscopy (TEM) images to obtain the morphology and size of the nanoparticles were taken using a LEO920 TEM (Carl Zeiss, Germany). The topographic images of nanoparticles and their orientation in the chitosan texture were obtained by atomic force microscopy (AFM) (CSM-Bruker, Germany). The mean hydrodynamic diameter of nanoparticles was measured using Zetasizer (Malvern model, China),
Results and Discussion
Physical characteristics of Fe3O4 nanoparticles
Physical characteristics of Fe3O4-gold nanoparticles
Physical properties of Fe3O4-gold-chitosan hydrogel nanocomposite
Up until now, considerable effort has gone into the formation of gold-coated magnetite nanoparticles, but the use of them is still restricted due to some problems in the way it is synthesized [2, 12, 18, 19]. In most cases, hydroxylamine, citrate, and borohydride have been used as reducing agents in combination with the reverse micelle technique for reducing gold salt nanoparticles [13, 18, 20, 21]. Tamer et al.  reported a two-step synthetic method in which the magnetite nanoparticles were coated with gold using the borohydride reduction of HAuCl4 under sonication in order to achieve a better monodispersity and prevent aggregation problems. In this study, the use of the biopolymer chitosan as a template for the preparation of stable magnetite-gold core-shell monodisperse nanoparticles with a mean diameter of 15 nm was developed under mild temperature conditions.
In summary, a magnetic core-shell-chitosan nanocomposite was synthesized. A rapid, simple, agglomerate-free method was reported for the production of monodisperse gold-coated Fe3O4 nanoparticles using biopolymer chitosan as a stabilizing agent. Core-shell magnetic Fe3O4-gold-chitosan nanostructures show a great potential for biotechnological and biomedical applications in the near future, especially for biodetection and bioimaging, drug delivery, and magnetic bioseparation.
The authors gratefully acknowledge the staffs of University of Isfahan, central laboratory in Isfahan University of Technology and University of Ottawa for their assistance on this research. We are also thankful Dr. A. Zeini and Mr K. Hanif for their valuable helps.
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