Preparation and Characterization of Silica-Coated Magnetic–Fluorescent Bifunctional Microspheres
© to the authors 2009
Received: 11 December 2008
Accepted: 24 May 2009
Published: 20 June 2009
Bifunctional magnetic–fluorescent composite nanoparticles (MPQDs) with Fe3O4MPs and Mn:ZnS/ZnS core–shell quantum dots (QDs) encapsulated in silica spheres were synthesized through reverse microemulsion method and characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, vibration sample magnetometer, and photoluminescence (PL) spectra. Our strategy could offer the following features: (1) the formation of Mn:ZnS/ZnS core/shell QDs resulted in enhancement of the PL intensity with respect to that of bare Mn:ZnS nanocrystals due to the effective elimination of the surface defects; (2) the magnetic nanoparticles were coated with silica, in order to reduce any detrimental effects on the QD PL by the magnetic cores; and (3) both Fe3O4MPs and Mn:ZnS/ZnS core–shell QDs were encapsulated in silica spheres, and the obtained MPQDs became water soluble. The experimental conditions for the silica coating on the surface of Fe3O4nanoparticles, such as the ratio of water to surfactant (R), the amount of ammonia, and the amount of tetraethoxysilane, on the photoluminescence properties of MPQDs were studied. It was found that the silica coating on the surface of Fe3O4could effectively suppress the interaction between the Fe3O4and the QDs under the most optimal parameters, and the emission intensity of MPQDs showed a maximum. The bifunctional MPQDs prepared under the most optimal parameters have a typical diameter of 35 nm and a saturation magnetization of 4.35 emu/g at room temperature and exhibit strong photoluminescence intensity.
KeywordsBifunctional microspheres Magnetic Fluorescent
Semiconductor quantum dots (QDs) have been widely explored as biomedical labeling agents [1–4]. However, the small-ensemble Stokes shift of intrinsic QDs can cause self-quenching. In addition, experimental results indicated that any leakage of cadmium from the QDs would be toxic and fatal to biological system , and cadmium-containing products can be environmentally problematic. Recently, Peng et al. [6–8] reported that doped QDs could not only replace cadmium in CdSe QDs with zinc, but also overcome a number of intrinsic disadvantages of undoped QDs emitters, i.e., strong self-quenching caused by their small-ensemble Stokes shift (energy difference between absorption spectrum and emission band)  and sensitivity to thermal, chemical, and photochemical disturbances . Mn2+-doped ZnS QDs have been extensively investigated for use in various applications other than biomedical labeling, such as displays, sensors, and lasers [11–13]. In addition, the luminescence lifetime of Mn2+-doped ZnS QDs is ~1 ms. Such a long lifetime makes the luminescence from the nanocrystal readily distinguishable from any background luminescence. Therefore, Mn2+-doped ZnS QDs could be potential candidates as fluorescent labeling agents, especially in biology . Magnetic nanoparticles of iron oxides (MPs) also show many advantages in biological applications. One unique feature of magnetic nanoparticles is to respond well to magnetic control, which has led to several successful applications, including biological separation, protein purification, bacteria detection, and drug delivery [15, 16]. Highly luminescent QDs could serve as luminescent markers, while magnetic nanoparticles could be easily manipulated under the external magnetic field. Therefore, combination of QDs and MPs to get fluorescent–magnetic bifunctional composite nanoparticles (MPQDs) has attracted intense attention in the past decade due to its appealing applications [17–25]. Surface modification of QDs and MPs with silica has led to improved stability, lower toxicity, and higher biocompatibility, and protection of the QDs against corrosion by the biological buffer. In addition, the rich and well-known surface chemistry of silica makes bioconjugation more convenient. However, it was still a challenge to obtain magnetic, multicolor barcoded nanospheres with controllable size and tunable readout.
All chemicals used were of analytical grade. Zn (CH3COO)2·2H2O, Mn(CH3COO)2·2H2O, Na2S·9H2O, FeCl2·4H2O, FeCl3·6H2O, Na2SiO3·9H2O, and thioglycolic acid (TGA) were obtained from Shanghai Chemical Reagents Company, tetraethoxysilane (TEOS), ammonia (NH4OH, 25–28 wt%), ethanol (95%),n-hexanol, cyclohexane, and acetone were obtained from Tianjin Hengxing Chemical Preparation Company; and TritonX-100 was obtained from Sinopharm Chemical Reagent Company. All chemicals were used as received. High-purity water with a resistivity of 18.2 MΩ/cm was used for preparation of all aqueous solutions.
Synthesis of Mn:ZnS/ZnS Core/Shell Quantum Dots
Mn:ZnS/ZnS core/shell QDs were synthesized according to our recent reports . Briefly, the stock solution was prepared by adding Zn(CH3COO)2·2H2O and Mn(CH3COO)2·2H2O into 100 mL 0.12 M TGA aqueous solution respectively. The Mn/Zn molar ratios in the four samples were fixed at 1%. Then the TGA–manganese solution reacted with Na2S aqueous solution at 80 °C for 20 min to form small-size MnS core. In order to obtain Mn:ZnS/ZnS core/shell QDs, the TGA–zinc complex aqueous solution was injected into the MnS core solution at two-step. At the first step, 75% of TGA–zinc solution was injected into the MnS core solution under vigorously stirring and heated at 80 °C for 10 h. The remaining TGA–zinc solution was then injected into the mixture and heated at 80 °C for another 2 h. The Mn:ZnS/ZnS core/shell QDs were obtained by adding excess ethanol to the solutions and then dried in vacuum.
Synthesis of Fe3O4Nanoparticles
Fe3O4 nanoparticles were synthesized as reported by Massart et al. . A mixture of 5.406 g of FeCl3·6H2O and 2.780 g of FeCl2·4H2O dissolved in 100 mL of high-purity water was placed in a 250-mL flask, following by the quick droplet-addition of 15 mL of 25% NH4OH. The mixture was irradiated with high-intensity ultrasound (600 W, 20 kHz) at room temperature in ambient air for 1 h. After irradiation, the precipitate was centrifuged and washed using distilled water and ethanol for several times. It was then freeze-dried at 223 K for 4 h in vacuum.
Synthesis of Core–Shell Fe3O4@SiO2Nanoparticles
The core–shell Fe3O4@SiO2nanoparticles were synthesized as follows: 1 g of Fe3O4nanoparticles were added to 100 mL of 2.84 wt% sodium silicate solution and ultrasonically dispersed for 30 min. Then, 2 wt% H2SO4was used to adjust pH value of the solution to 9. The mixture was irradiated with high-intensity ultrasound (600 W, 20 kHz) at room temperature in ambient air for 1 h. Twenty-five milliliters cyclohexane, 3.2 mLn-hexanol, 8 mL TritonX-100, 1 mL of the as-prepared magnetic sol, and 1.5 mL of TEOS were added in a flask in turn under vigorous magnetic stirring. Thirty minutes after the microemulsion was formed, 1 mL NH4OH (25 wt%) was added to initiate the polymerization process. The silica growth was completed after 10 h of stirring. The final product was denoted as FS.
Synthesis of Silica-coated Magnetic–luminescent Bifunctional Nanocomposites
One milliliter of Mn:ZnS/ZnS aqueous solution (10 g/L), 1 mL TEOS, and 1 mL NH4OH (25 wt%) were in turn added into the above-mentioned FS and allowed to stir at room temperature for 5 h. Acetone was used to terminate the reaction, and the resultant precipitates of MPQDs were washed with water and ethanol for three times, and then dried in vacuum.
The XRD patterns of the synthesized samples were obtained by a D/max-γA diffractometer using CuKα radiation (λ = 0.15418 nm). The size and morphology of the as-synthesized products were determined by a XL30 S-FEG SEM and a JEM-3010 high-resolution TEM. The PL spectra of the samples were recorded with a Fluorescence Spectrophotometer F-4500. The room temperature magnetization in the applied magnetic field was performed by model JDM-13 vibrating sample magnetometer.
Results and Discussions
Structural and Morphological Characterization
Agekyan  reported that the interaction between the Fe3O4 and the QDs would influence the PL properties of the nanocomposites. Hong et al.  reported that the PL properties of the magnetic–luminescent nanocomposites (Fe3O4/PE n /CdTe) were very sensitive to the distance between Fe3O4 nanoparticles and CdTe QDs separated by the polyelectrolyte multilayers. The interaction between the two particle types was suppressed only after having deposited 21 layers of polyelectrolyte between the magnetic and the luminescent nanoparticles. In this paper, a dense silica shell was deposited on Fe3O4 nanoparticles in order to prevent quenching of the QDs by the magnetic Fe3O4 nanoparticles. Hence, control of silica coating on the surface of Fe3O4 nanoparticles is an important consideration. With this in mind, we investigated several experimental parameters for silica formation with the aim of optimizing the resulting MPQDs fluorescence.
The Effects of the Ratio of Water to Surfactant
The Effect of the Amount of TEOS
The Effect of the Amount of NH4OH
Water-soluble bifunctional MPQDs with Fe3O4MPs and Mn:ZnS/ZnS core–shell QDs encapsulated in silica spheres were synthesized through reverse microemulsion method. The effects of the parameters for the silica coating on the surface of Fe3O4, such as the ratio of water to surfactant (R), the amount of NH4OH, and the amount of TEOS, on the PL properties of MPQDs were studied. It was found that the silica coating on the surface of Fe3O4could effectively suppress the interaction between the Fe3O4and the QDs under the most optimal parameters, and the emission intensity of MPQDs showed a maximum. The bifunctional MPQDs prepared under the most optimal parameters have a typical diameter of 35 nm and a saturation magnetization of 4.35 emu/g at room temperature, and exhibit strong photoluminescence intensity. In a few words, the PL, magnetic, and water-soluble properties of the MPQDs would allow them to find a large range of applications for biolabeling, bioseparation, immunoassay, and diagnostics.
This work was supported by the Provincial Excellent Ph.D. Thesis Research Program of Hunan (no. 2004-141) and the Graduate Educational Innovation Engineering of Central South University (no. LB08083).
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