Synthesis of Organic Dye-Impregnated Silica Shell-Coated Iron Oxide Nanoparticles by a New Method
© to the authors 2008
Received: 26 August 2008
Accepted: 3 October 2008
Published: 23 October 2008
A new method for preparing magnetic iron oxide nanoparticles coated by organic dye-doped silica shell was developed in this article. Iron oxide nanoparticles were first coated with dye-impregnated silica shell by the hydrolysis of hexadecyltrimethoxysilane (HTMOS) which produced a hydrophobic core for the entrapment of organic dye molecules. Then, the particles were coated with a hydrophilic shell by the hydrolysis of tetraethylorthosilicate (TEOS), which enabled water dispersal of the resulting nanoparticles. The final product was characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, photoluminescence spectroscopy, and vibration sample magnetometer. All the characterization results proved the final samples possessed magnetic and fluorescent properties simultaneously. And this new multifunctional nanomaterial possessed high photostability and minimal dye leakage.
Recently, fluorescent-magnetic bifunctional nanomaterials which are composed of magnetic iron oxide nanoparticles and luminescent dye-doped silica matrix gained more and more attention [1–10]. On the one hand, superparamagnetic iron oxide nanoparticles including maghemite (γ-Fe2O3) and magnetite (Fe3O4) were widely investigated for in vivo and in vitro biomedical applications, such as magnetic resonance imaging (MRI), target drug delivery, and so on [11–14]. On the other hand, dye-doped silica nanoparticles were good candidate for bio-labeling and bio-imaging because they showed several advantages, including photostable, sensitive, water soluble, and easy surface modification [15–17]. So these bi-functional nanoparticles could provide fluorescent and magnetic properties simultaneously which make them useful in highly efficient human stem cell labeling, magnetic carrier for photodynamic therapy, and other biomedical applications [1–3][5–8].
Up to now, several methods have been developed for preparing such fluorescent-magnetic bi-functional nanomaterials [2–5]. Lee et al. have conjugated dye-doped silica with iron oxide nanoparticles by surface modification method . Alternatively, organic dye-incorporated silica shell-coated iron oxide nanoparticles can be prepared in a reverse micelle system [3, 4]. These strategies could produce high quality fluorescent-magnetic nanoparticles, but they either needed expensive reagents or complicated synthetic steps. Recently, Ma et al. have prepared inorganic dye-doped silica shell-coated iron oxide nanospheres by Stöber method which needed fewer organic solvents and the preparation procedure was convenient . But compared with inorganic dye, organic dye molecules seem to be better option for bio-labeling and bio-analysis because of their relatively high intrinsic quantum yield. However, organic dye molecules are not easily doped in a silica matrix . So, simple and economic method for preparing organic dye-impregnated silica shell-coated iron oxide nanoparticles is still needed to be developed.
Recent studies indicated that hydrophobic silane was a good candidate to entrap organic dye into the silica matrix [18, 19]. So we developed a new method for preparing organic dye-impregnated silica shell-coated iron oxide nanoparticles based on the hydrolysis of HTMOS and TEOS. Iron oxide nanoparticles were first coated with a dye-impregnated silica shell by the hydrolysis of HTMOS which produced a hydrophobic environment for entrapping organic dye molecules (Rhodamine 6G was used as model dye). Subsequently, the particles were coated with a hydrophilic shell by the hydrolysis of TEOS, which enabled the resulting nanoparticles to be dispersed in aqueous solution. Herein, the synthesis procedure and the characterizations of the final multifunctional nanomaterial were summarized in detail.
Rhodamine 6G was commercially available from Dongsheng chemical reagent company, China. Hexadecyltrimethoxysilane (HTMOS) was purchased from Fluka chemical company. Tetraethylorthosilicate (TEOS) was purchased from Tianjin chemical reagent company, China. NH3 · H2O was a product of Baiyin chemical reagent company, China. All chemicals were used as received without further purification. Distilled water was used through the experiment.
Iron oxide nanoparticles were prepared by adding ammonia to an aqueous solution of Fe2+/Fe3+ at a 1:2 molar ratio . The final product was denoted as S1.
Then the iron oxide nanoparticles were coated with Rhodamine 6G doped silica shell. Typically, 0.75 mL of S1, 1.5 mL of H2O, 0.6 mL of ammonia, and 10 mL of isopropyl alcohol were mixed together under magnetic stirring. Subsequently, 5 mL of Rhodamine 6G solution in isopropyl alcohol and appropriate volume of HTMOS was added into the mixture. After stirring for 3.0 h, 5 mL of isopropyl alcohol and 80 μL of TEOS were added into the reaction mixture. Two hours later, the formed product was centrifuged and washed with ethanol to remove the unreacted Rhodamine 6G and silane. The final particles were denoted as FS6 nanoparticles.
For comparison, Rhodamine 6G-doped silica shell-coated iron oxide nanoparticles were also prepared according to Ma’s report with some modification . Typically, 0.75 mL of S1, 1.75 mL of H2O, 0.4 mL of ammonia, 12.5 mL of isopropyl alcohol, and 10 μL TEOS were mixed together. Then it was stirred for 3 h. Subsequently, 5 mL of Rhodamine 6G solution in isopropyl alcohol and 20 μL of TEOS were added into the mixture. After stirring for 0.5 h, 5 mL of isopropyl alcohol, 2.5 mL of H2O, and 0.25 mL of ammonia was added dropwise into the reaction mixture simultaneously. The reaction mixture was further stirred for 24 h. The final product was denoted as FS62 nanoparticles.
X-ray diffraction (XRD) pattern of the synthesized products were measured on an X’ Pertpro Philips X-ray diffractometer from 10° to 90°. Transmission electron microscopy (TEM) was performed on a Hitachi-600 transmission electron microscope. A Nicolet Nexus 670 Fourier transform infrared spectra (FT-IR) spectrometer was employed to determine the chemical composition of the synthesized composites in the range of 4000–400 cm−1. Magnetic property of the final sample was measured at room temperature by a vibration sample magnetometer (VSM, Lakeshore 730, America). A RF-5301 PC fluorescence spectrophotometer was used to determine the photoluminescence (PL) spectra of this multifunctional nanomaterial.
Results and Discussion
In summary, a new method for preparing iron oxide nanoparticles coated with organic dye-doped silica shell was developed. The preparation procedure was carried out in a bulk aqueous/isopropyl alcohol system at room temperature, which make it environmental friendly and low cost. In addition, the preparation procedure was relatively facile. The characterization results by XRD, TEM, FT-IR, VSM, PL spectra, and Confocal fluorescence microscopy indicated that the final nanoparticles possessed magnetic and fluorescent properties simultaneously. So this new method was efficient in preparing organic dye-doped silica shell-coated iron oxide nanoparticles. In addition, we predict that this method can be applied to synthesis other fluorescent-magnetic nanoparticles.
The project was supported by the Open Subject Foundation of Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University. We also kindly acknowledge the National Science Foundation of China (No. 20875040) for supporting this work.
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