Semi-Biosynthesis of Magnetite-Gold Composite Nanoparticles Using an Ethanol Extract of Eucalyptus camaldulensis and Study of the Surface Chemistry

Abstract

Green synthesis of metal nanoparticles, such as silver or gold nanoparticles, has been attracting increasing attention in recent years. Functionalized magnetite nanoparticles have many uses in various applications, including nanoelectronic devices, molecular recognition, biomedical applications, drug delivery targeting, and optical devices. In this investigation, magnetic cores (Fe₃O₄) were synthesized using a fabrication method involving coprecipitation of Fe²⁺ and Fe³⁺. In the next step, magnetite-gold composite nanoparticles were synthesized with size ranging from 6–20 nm, using an ethanol extract of Eucalyptus camaldulensis as a natural reducing agent. Transmission electron microscopy, energy-dispersive spectroscopy, X-ray diffraction spectroscopy, and visible absorption spectroscopy confirmed the fabrication of magnetite-gold composite nanoparticles. In the UV spectra diagram, a red-shift of the surface plasmon of the Au was evidence that contact between gold and Fe₃O₄ had occurred. The surface chemistry of the as-prepared magnetite-gold nanoparticles was studied using infrared spectroscopy. The presence of organic compounds with a carboxyl moiety was confirmed on the surface of the magnetite-gold nanoparticles fabricated by this combined chemical and biological reducing process, which we have designated as a semi-biosynthesis method.

1. Introduction

Composite nanoparticles that contain two or more different nanoscale materials are of increasing interest in present day research because of their unique properties. These nanoparticles, have many applications in a number of fields, including use in nanoelectronic devices, molecular recognition, biomedical applications, and optical devices [1]. Recently, different nanocomposite materials, such as SiO₂-coated Fe nanocapsules, cobalt ferrite core/shell nanoparticles, silver/polystyrene nanospheres, Au-Cu₂O core-shell nanocubes, and Al₂O₃-Co nanocapsules have been synthesized by different fabrication methods [2–5]. One of the prevalent nanostructures that has many uses in biomedicine is the magnetic nanoparticle. Magnetic nanoparticles, for instance those composed of magnetite (Fe₃O₄) or maghemite (𝛾-Fe₂O₃), have unique thermal, chemical, and magnetic properties that make them particularly well suited for medical applications [6].

However, materials that are used in medical diagnoses or as therapeutic agents require specific characteristics, such as colloidal stability [7], biocompatibility [8], resistance against environmental degradation [2], and nontoxicity [6]. For these applications, nanoparticles must be passivated with an external inert shell in order to protect the magnetic core against oxidation. Moreover, the surfaces of these particles should be functionalized with organic molecules in order to allow them to chemically bind with biomolecules like DNA, proteins, amino acids, and so forth [9].

At present, a number of different methods have been reported for coating magnetic nanoparticles. Protective layers include polymeric molecules such as polyethylene glycol and dextran or inorganic compounds like silica [8]. Although some of these methods have been recently commercialized, most have substantial deficiencies. For example, hydrophobic surfaces arising from polymeric chains can result in improper protein bonding [10]. A nonmetallic coating also potentially decreases the magnetic properties of iron-oxide nanoparticles, thus limiting the applications of these particles [11].

Among the different techniques available for surface modifications, gold-coated magnetic nanoparticles have lately attracted significant attention. Since gold is an inert element from the viewpoint of chemical reactivity, it is very useful as a coating for protecting magnetic nanoparticles. Kinoshita et al. synthesized a gold-iron nanocomposite powder by a reverse micelle method [12]. Similar structures of Fe/Au nanoparticles have been successfully synthesized and the particles used as a magnetic resonance (MR) contrast agent [13]. According to Yu et al. and Au-Fe₃O₄ nanoparticles with dumbbell-like shapes could be prepared via decomposition of iron pentacarbonyl, FeCO₅, onto the surface of gold nanoparticles [14]. In other work, gold and magnetite composite nanoparticles have been prepared by a laser process in water [15]. Gold-coated ferric oxide nanoparticles have been obtained using sodium citrate as the reducing agent, yielding particles with an average size of 25 nm, as reported by Pham et al [6]. Amino acids have also been adsorbed onto the Au/𝛾-Fe₂O₃composite nanoparticles synthesized by gamma-ray irradiation [9].

A number of these methods for synthesizing gold-iron oxide nanocomposites may not be suitable for biological application because they use harmful surfactants or organic solvents. In addition, maintaining the magnetic nanoparticles in liquid media and preventing aggregation are difficult in practice. In the current study, Au-Fe₃O₄composite nanoparticles were prepared with a combined chemical and biological reducing process, designated here as a semi-biosynthesis method. Magnetic cores (Fe₃O₄) were primarily synthesized using a fabrication method consisting of coprecipitation of Fe²⁺ and Fe³⁺. An ethanol extract of Eucalyptus camaldulensis [16] was used for the reduction of Au⁺³ on the surface of the Fe₃O₄magnetic nanoparticles and for the functionalization of the Au-Fe₃O₄ nanocomposite particles.

2. Experimental

Magnetite (Fe₃O₄) nanoparticles were prepared at room temperature by a previously reported method [17]. Briefly, deionized water (200 ml) was deoxygenated by bubbling nitrogen gas for 1.5 hour, then 50 ml of NH₄OH (1 M) was added, and the mixture was stirred with mechanical agitation at 1000 rpm. Next, 6.76 g FeCl₃.6H₂O (2.5 mmol) and 4.97 g FeCl₂.4H₂O (2.5 mmol) were separately dissolved in 50 ml distilled water to make 0.5 M solutions. Subsequently, 10 ml of the ferrous chloride and 20 ml of the ferric chloride solutions were added to the mentioned ammonium hydroxide solution and the reaction mixture was stirred for 2.5 minutes at 1000 rpm. At this stage, a black precipitate formed and it was washed four times with deionized water. The prepared magnetite (Fe₃O₄) nanoparticles were separated from the solution with a magnet.

The reduction of the Au³⁺ onto the surface of magnetite (Fe₃O₄) nanoparticles was carried out using an ethanol extract prepared from E.camaldulensis whole plant [16]. Chloroauric acid was purchased from Merck, Germany. Magnetic nanoparticles (200 mg) were dispersed in 100 mL of aqueous HAuCl₄ (chloroauric acid) solution (1 mM) and mixed with 10 mL of an ethanol extract of E.camaldulensis (10 mg/mL). The resulting mixture was allowed to stand for 15 minutes at room temperature, then the prepared magnetite-gold nanocomposites were separated from the colloid by a magnet. Finally, the nanoparticles were washed three times with distilled water for use in further experiments.

The UV–visible spectra of the magnetite nanoparticles and magnetite-gold nanocomposites samples, before and after separation with the magnet, were measured on a Labomed Model UVD-2950 UV-VIS Double Beam PC Scanning spectrophotometer, operated at a resolution of 2 nm. All samples were characterized by transmission electron microscopy (model EM 208 Philips), energy-dispersive spectroscopy (EDS), and X-ray diffraction spectroscopy. The surface chemistry of the separated magnetite-gold nanocomposites was further studied with Fourier transform infrared spectroscopy (Nicolet Magna 550).

3. Results and Discussion

This study reports a new method for preparation of functionalized magnetite-gold nanocomposites that used a combined chemical and biological procedure. In the first stage, Fe₃O₄ magnetite nanoparticles were chemically synthesized and their shape and size were studied by transmission electron microscopy. Figures 1(a) and 1(b) show different representative TEM images recorded from the drop-coated film of the magnetite nanoparticles synthesized by this method [17]. The particle size histogram of the synthesized Fe₃O₄ nanoparticles shows that the particles ranged in size from 6 to 20 nm, with an average size of 10 nm (Figure 1(c)). As can be seen in Figures 1(a) and 1(b), the broad surface area of the nanoparticles and the magnetic forces between them could cause a considerable agglomeration between the fabricated Fe₃O₄magnetite nanoparticles [18]. The histogram of magnetite nanoparticle size dispersion obtained from TEM images showed diameter ranges between 6–20 nm with an average of 10 nm (Figure 1(c)).

(a)

(b)

(c)

(d)

Figure 1 

TEM images of magnetic nanoparticles with low (a) and high (b) magnifications, (c) Particle size distribution histogram of Fe3O4nanoparticles, (d) EDS spectrum of magnetite particles.

Analysis of the Fe₃O₄nanoparticles by energy dispersive spectroscopy (EDS) confirmed the presence of Fe and O element signals (Figure 1(d)). The peaks shown in Figure 1(d) were attributed to Fe and O elements, which confirmed the existence of iron-oxide nanoparticles (copper and carbon peaks existed due to the grid used for TEM imaging). In the second stage, the magnetite-gold nanocomposite was fabricated in the presence of Au³⁺ by adding an ethanol extract of E. camaldulensis as a reducing agent. The TEM images of the magnetite-gold nanocomposite illustrated in Figure 2 show gold nanoparticles as dark spots. In contrast, Fe₃O₄magnetite nanoparticles are observed as a gray color because elemental gold (Au) has a higher electron density than Fe₃O₄and subsequently more electrons are transmitted in bright field imaging [14].

(a)

(b)

Figure 2 

TEM images of magnetic-gold composite nanoparticles with low (a) and high (b) magnifications. Dark particles represent gold, and gray particles are magnetite.

The EDS pattern of the separated Fe₃O₄magnetite-gold nanocomposite indicated the presence of gold, iron, and oxygen, as illustrated in Figure 3. The EDS experiment was repeated for different nanoparticles and the obtained EDS spectra confirmed that the formed nanoparticles were comprised of both elemental gold and Fe₃O₄magnetite ingredients. The UV–Visible spectra were examined before and after separation of the magnetic nanoparticles from the gold nanoparticles (Figure 4). Plasmon resonance peaks detected after removing the magnetic nanoparticles from the reaction mixture established the formation of magnetite-gold nanocomposites. It should be noted that pure iron-oxide did not show a significant plasmon resonance peak in the absence of gold nanoparticles. As seen in Figure 4, the highest absorbance was observed for gold nanoparticles located at 530 nm (solid line curve), whereas Au-Fe₃O₄composite nanoparticles showed a maximum peak at 608 nm (dotted line) which agreed with other previous reports [2, 9, 14, 19]. This wavelength shift in the gold surface plasmon, known as red-shift, is a probable occurrence and has been reported previously [9, 12, 14, 19].

Figure 3 

EDS pattern of magnetite-gold composite nanoparticles.

Figure 4 

Absorbance spectra of gold nanoparticles (solid line) and magnetite-gold composite nanoparticles (dotted line).

The XRD method was further used to determine the chemical composition of the nanocomposites (Figure 5). The Fe₃O₄and Au peaks were observed and again verified the connection of magnetite nanoparticles with elemental gold. The pattern of gold nanoparticles was face-centered cubic, and the dominant crystal plane for these particles was (111). The surface chemistry of the prepared magnetite-gold nanocomposites was studied using infrared (IR) spectroscopy and the IR peaks centered at 3426 cm⁻¹ and 1675 cm⁻¹ confirmed the presence of a carboxyl group on the surface of the semi-biosynthesized magnetite-gold nanocomposites (spectrum not shown).

Figure 5 

X-ray diffraction pattern of Au-Fe3O4 composite nanoparticles.

4. Conclusion

In this study, the green synthesis and functionalization of a Au-Fe₃O₄nanocomposite (from Fe₃O₄magnetic nanoparticles) were performed using an ethanol extract of E. camaldulensis. As one advantage, a carboxyl group was detected on the surface of the prepared Au-Fe₃O₄nanocomposite, which would make these particles suitable for rapid separation and purification of biomolecules by magnetic field and drug delivery. It should be noted that the importance of the external inert shell in protecting the magnetic core against oxidation is well established but however, in this study we prepared the Au-Fe₃O₄nanocomposites that may have contained some portions of Au-Fe₃O₄ core-shell nanoparticles. Further treatments such as acid washing can be used for separation of the Au-Fe₃O₄ core-shell fraction and merit further investigation. Moreover, since surfactant has not been used for functionalization of these nanocomposites, this makes these nanoparticles also potentially useful for biomedical applications.

Acknowledgment

This wok has been supported by Medicinal Plants Research Center, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran.