Bi-Functional Silica Nanoparticles Doped with Iron Oxide and CdTe Prepared by a Facile Method
© to the authors 2009
Received: 15 December 2008
Accepted: 5 March 2009
Published: 20 March 2009
Cadmium telluride (CdTe) and iron oxide nanoparticles doped silica nanospheres were prepared by a multistep method. Iron oxide nanoparticles were first coated with silica and then modified with amino group. Thereafter, CdTe nanoparticles were assembled on the particle surfaces by their strong interaction with amino group. Finally, an outer silica shell was deposited. The final products were characterized by X-ray powder diffraction, transmission electron microscopy, vibration sample magnetometer, photoluminescence spectra, Fourier transform infrared spectra (FT-IR), and fluorescent microscopy. The characterization results showed that the final nanomaterial possessed a saturation magnetization of about 5.8 emu g−1and an emission peak at 588 nm when the excitation wavelength fixed at 380 nm.
Magnetic nanoparticles offer great potential in biomedical applications, including magnetic resonance imaging (MRI), targeted drug delivery, rapid biological separation, biosensors, and magnetic hyperthermia therapy. Among the magnetic nanomateials, iron oxide nanoparticles (including Fe3O4 and γ-Fe2O3) are by far the most commonly employed material because of their biocompatibility [1–4]. At the same time, cadmium telluride (CdTe) nanoparticles, as an important II–VI semiconductor material, have attracted considerable attention over the past decades. They have some unique physical and chemical properties, such as marvelous brightness, narrow and size-tunable emission, fairly high quantum yields, and good chemical and photo stability. These properties make CdTe nanocrystals suitable for biological applications, fabricating photoelectron devices and solar cells [5–11].
So the multifunctional nanomaterials which were composed of iron oxide nanoparticles and CdTe nanocrystals can enjoy both the advantages of iron oxide nanoparticles and CdTe QDs (quantum dots), they showed great potential in biological applications [12–14]. These multifunctional silica spheres are expected to serve as luminescent markers and are capable of being driven by an external magnetic field to a specific location . For example, in a targeting drug-delivery system, the magnetic fluorescent nanoparticles labeled drugs could be easily administered and transported to the terminal under the guidance of an external magnetic field, resulting in a safer and more effective tissue-specific delivery of drugs.
Tellurium powder was purchased from Shanghai Chemical Reagents Factory, China. NaBH4was commercially available from Guangming Reagent Company, China. Ferric chloride (FeCl3 · 6H2O) and Ferrous chloride (FeCl2 · 4H2O) were purchased from Tianjin Shuangchuan Chemical Reagent Factory, China. TEOS was purchased from Tianjin Chemical Reagent Company, China. 3-aminopropyltrimethoxysilane (APTMS) was commercially available from Fluka. Other routine chemicals were purchased from Shanghai Reagents Factory of China. Doubly deionized water was used throughout the experiment. All chemical reagents were of analytical grade and used as received without further purification.
Thioglycolic acid stabilized CdTe nanocrystals were prepared according to a reported procedure with some modification . Typically, 6 mL of 0.02 M CdCl2 and 100 mL of water was mixed together, and then the pH of the mixture was adjusted to 11.3 in the presence of 26 μL of thioglycolic acid. After stirring and bubbling with nitrogen for 40 min, NaHSe prepared by 6.0 mg Te and 6.0 mg NaBH4 was added. After 1 h stirring, it was refluxed for 3 h.
Silica coated iron oxide nanoparticles were prepared as described by our previous work with some modification . First, iron oxide nanoparticles were prepared from aqueous solution of Fe2+/Fe3+ by the addition of ammonia under magnetic stirring at room temperature. Then it was coated with silica. Typically, 2.5 mL of magnetic nanoparticles solution, 2.5 mL of distilled water, 30 mL of isopropyl alcohol, and 80 μL of TEOS were put together. The reaction was initiated by 0.8 mL of ammonia (25 wt%). After magnetic stirring for 10 h, the formed core–shell nanoparticles were collected by centrifugation and the silica coated iron oxide nanoparticles were denoted as FS nanoparticles.
The surfaces of FS nanoparticles were modified with amino group by treating with APTMS in refluxing toluene . Typically, 260 mg of FS nanoparticles was added to 200 mL of toluene, and then refluxed for 6 h after 2.0 mL of APTMS was added.
In order to prepare CdTe assembled FS nanoparticles, 5.0 mg of the prepared CdTe and amino group modified FS nanoparticles were dissolved in 2 mL of water, respectively. Then the two dispersions were mixed together and stirred for 12 h. The final product was isolated by centrifugation and it was denoted as FSC nanoparticles.
The prepared FSC nanoparticles were further coated with an outer silica layer. After being dispersed in 1 mL of H2O and 4 mL of ethanol, 4 μL of APTMS and 10 μL of TEOS were added to the FSC dispersion under magnetic stirring, the procedure was preceded for 10 h . Finally, the product (denoted as FSCS nanoparticles) was centrifuged and dispersed in water for further characterization.
Photoluminescence (PL) spectra were made with a RF-5301 PC spectrofluorophotometer at room temperature. Fluorescence microscope was used to show the optical images of the product. X-ray diffraction (XRD) pattern of the product was performed on an X’Pertpro Philips X-ray diffractometer with CuKαradiation (λ = 0.154056). The scan range (2θ) was from 10° to 90°. Transmission electron microscopy (TEM) images were used to show the morphology and size of the as-prepared samples. Nicolet AVATAR 360 Fourier transform infrared spectra (FT-IR) spectrometer was used to demonstrate the chemical nature of the products in KBr pellets. Magnetic hysteresis loops of these core–shell nanoparticles were carried out with a vibration sample magnetometer (VSM, Lakeshore 730, America) at room temperature.
Results and Discussions
The XRD spectrum of CdTe nanoparticles was depicted in Fig. 1b. The diffraction peaks were similar with the previous reports [26, 27]. It indicated the nanoparticles belonged to cubic structure. The average particle size of CdTe nanocrystallite was estimated to be about 3 nm by Scherrer formula.
Figure 1c showed the XRD pattern of FSCS nanoparticles. According to Fig. 1a, the diffraction peaks of iron oxide and silica were present in Fig. 1c. But the existence of CdTe in the particles could not be confirmed by the XRD spectrum because the peaks were ambiguous.
In summary, multifunctional silica spheres which were composed of CdTe and iron oxide nanoparticles was prepared by a convenient method. The TEM images proved the particle size was in nanometer range. But aggregation was occurred in some area. So improving their size distribution will be the further subject of our work. The PL spectra indicated the emission band of this material was located at 588 nm. The magnetization curves revealed the Ms value of the final particles was 5.8 emu g−1. The photograph indicated the color of the final particles was yellowish-brown in the daylight and it can be controlled by an external magnetic field. These results indicated the prepared multifunctional silica nanospheres, which were composed of iron oxide and quantum dots, possessed excellent magnetic and fluorescent properties.
This work 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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