Synthesis And Characterization Of Magnetic-Fluorescent（Fe₃O₄@SiO₂）/（PDDA/PSS）_n/（CdS/PAMAM） Nanoparticles

Magnetic-fluorescent nanoparticles have been widely used inthe biological field, in applications such as cell separation and image, drug labeling, and disease diagnosis,due to their superparamagnetic, fluorescent, and nano-scale properties. For long, researchers have been devoted in fabrication of such bifunctional systems with clear structure and both excellent magnetic and fluorescent properties without interfering each other. In preparation of ion-oxide nanopaiticles, the newly developed high-temperature decomposition method(ie.pyrolysis method) showadvantages above traditional co-precipitation method,in that NPs possess regular morphology, narrow size distribution, high crystallinity and strong saturation magnetization, thus drawing more and more attention. However, NPs abtained by this method are soluble in only organicsolvents, it is necessary to achieve phase transfer by certain modification before it can be used in biological applications. SiO2 is a kind of biocompatible material which happens to meet such modification desire, and its negatively charged surface can be easily further functioned by electrostatic adsorption. Layer-by-layer ssembling has been gradually recognized as a good way to fabricate magnetic-fluorescent bifunctional nanoparticles as it renders precise manipulation over partical morphology, size distribution and shell components. In this paper,Fe3O4 NPs were prepared throughpyrolysis method. The NPs were coated withSiO2 by a reverse micro emulsion method to achieve water solubility. Then magnetic-fluorescent bifunctional nanoparticles werefabricated via a LBL process, by adsorption of polyelectrolytes PDDA and PSSonto the negatively charged silica shell, quantum dots CdS/PAMAM with positive charge used as the outermost layer. A series of characterization were performed for NP preparation and silica coating and factors influencing their morphology were discussed;LBL process was verified, magnetic and fluorescent properties were studied as function of number of layers.The results showed as follows:1. In the pyrolysis method, Fe(acac)3 was used as precursor, dibenzyl ether as high boiling point organic solvent and oleylamine as both reductant and modifier. 5-7nm Fe3O4 nanopaticles were obtained by adjusting the volume ratio of oleylamine todibenzyl ether and were well dispersed in organic solvents such as cyclohexane. XRD results confirmed that inverse spinel Fe3O4 NPs were obtained and IR reults showed they were midified by oleylamine.Through TEM and VSM Characterization, the particles were found to be regular spherical with uniform size distribution and Super paramagnetic with a saturation magnetization of 30-40 emu/g. The relatively low value compared with literature was due to low heating rate confined by heating device.2. Fe3O4 nanoparticles were coated with a SiO2 layer via a reverse micro emulsion method by basic hydrolysis of TEOS, giving Fe3O4@SiO2 core-shell nanoparticles(CS NPs), in which Igepal Co-520 was used as surfactant, cyclohexane as continuous phase and ammonium hydroxide as disperse phase. The dosage of Fe3O4, ammonium hydroxide and TEOS were varied individually to discuss their influences to the final CS NPs. Through TEM observation,we found that: CS NPs with a single core could be obtained upon addition of a certain amount of Fe3O4, but pure SiO2 spheres or CS NPs with multi cores appeared when too little or too much Fe3O4 were added; The dosage of ammonium hydroxide affects the thickness ofSiO2 layer and they are positively correlated; The amount of TEOS added was also positively correlated with the thickness of SiO2 layer,but pure SiO2 spheres could be observed when too much TEOS was used. Upon coating with SiO2, the saturation magnetization of the particles decreased to 8 emu/g.3. Magnetic-fluorescent nanoparticlesCS NPs/(PDDA/PSS)n/( CdS/PAMAM) were fabricated through a LBL method, in which different layers of oppositely charged polyelectrolytes: PDDA and PSS were assembled to surface of the CS NPs, with CdS/PAMAM as the outermost layer. The LBL process was verified by Zeta potential and hydrodynamic diameter characterization, and clear images of the assembly could be seen in TEM observations. The saturation magnetization of the assembly decreased with increasing number of the assembly layers;fluorescent emission showed no difference in peak position in contranst with CdS/PAMAM QDs, and fluorescenceintensity gradually increased with increasing number of the assembly layers.