The Study On Bifunctional Magnetic-fluorescent Nanocomposite: Synthesis, Growth Mechanism And Application

Magnetic-fluorescent bifunctional nanocomposites could simultaneous exhibit the capabilities of magnetic-separation and fluorescent labeling for specific bimolecular and thus avoid the problem of further separation when using single-functional fluorescent probes. Moreover, Magnetic-fluorescent bifunctional nanocomposites reveal potentials in application of dual-mode imaging system based on magnetic resonance imaging （due to magnetic domain） and optical imaging （due to fluorescent domain）, which could greatly improve the imaging accuracy and sensitivity and therefore promote the study of early diagnosis and the pathology of certain diseases. Therefore, magnetic-fluorescent bifunctional nanocomposites have attracted intense attention in recent years and shown a promising future in applications of biomedical fields. However, there still remain lots of problems to solve. Among them, the synthesis of magnetic-fluorescent bifunctional in vivo probes and the better understanding of the growth mechanism of multicomponent nanocomposite in the system with large lattice mismatch are two main issues. Regarding that, we focus on the study of the synthesis and growth mechanism of magnetic-fluorescent nanocomposites.Herein, we first designed a hot-injection approach to synthesis Fe₃O₄-CdSe magnetic fluorescent nanocomposites based on the similar conditions of two classic synthetic approaches for Fe₃O₄ and CdSe nanocrystals, respectively, and therefore the synthesis of both Fe₃O₄ seeds and Fe₃O₄-CdSe magnetic fluorescent nanocomposites were preceded in the same noncoordinating octadecene （ODE） solution. By this method, heterogeneous nucleation of CdSe directly happened on a starting Fe₃O₄ seed by an injection of cadmium precursor, and the position of fluorescent emission peaks could be easily tuned by adjusting the reaction parameters, such as reaction temperature, time and OA dose, and in the range from 486 to 586 nm. Besides these, it was also found that the emission wavelength of Fe₃O₄-CdSe magnetic fluorescent nanocomposites was obviously blue-shifted in comparison with the pure CdSe quantum dots （QDs） synthesized under the same reaction conditions. Moreover, the increase of the concentration of Fe₃O₄ seed participated would lead to the enhancement of this blue-shift phenomenon. By evaluating the concentration of CdSe nuclei, the blue-shift was ascribed to the lower activation energy in heterogeneous nucleation and explained why CdSe QDs would be induced to deposit onto the surface of Fe₃O₄ seeds. This evaluation of this blue-shift phenomenon might be helpful for understanding the role of inorganic seed nanocrystals in the synthesis of multicomponent nanocomposites in some extent.Although Fe₃O₄-CdSe magnetic fluorescent nanocomposites were successfully obtained, it was still a great challenge to control the growth of CdSe domains on the Fe₃O₄ seed nanocrystals and the overall topology of nanocomposites owing to the significant lattice mismatch between Fe₃O₄ and CdSe nanocrystals. We lay emphasis on the surface status of Fe₃O₄ seed nanocrystals, since it is on the surface of seed nanocrystals that the nucleation and growth of the second component occur. By using Fe₃O₄ seed nanocrystals with different ligands as seeds, it was found that capping ligand affinity played an important role on the total number of heterojunctions on each Fe₃O₄ seed through influence on the addition process of second phase onto the surface of Fe₃O₄ seeds. Furthermore, the CdSe domains were preferred to grow on the corner of cubic Fe₃O₄ seeds, and cubic Fe₃O₄ nanocrystals are favored for the heterogeneous nucleation of CdSe as compared to spherical ones. Finally, with the help of theoretical calculations, the whole formation process of Fe₃O₄-CdSe magnetic-fluorescent nanocomposites was elucidated from the point of view of energy barrier, which might lead to the better understanding of heterogeneous nucleation and growth mechanism of multicomponent nanocomposites in the system with large lattice mismatch.However, pristine Fe₃O₄ nanocrystals and Fe₃O₄-CdSe magnetic fluorescent nanocomposites could not be directly used in biomedical applications. Suitable surface modification was required to obtain well biocompatibility and dispersity in aqueous solution, while the initial magnetic and fluorescent properties should be retained as much as possible. For this purpose, in this paper, surface modifications of Fe₃O₄ and Fe₃O₄-CdSe nanocrystals by polyethylene glycol （PEG） were successfully achieved via a physical grinding approach. The stability of resultant PEG-Fe₃O₄ and PEG-Fe₃O₄-CdSe aqueous solutions were investigated through an approach of measuring the UV-vis absorption and photoluminescence （PL） intensity. Additionally, the influence of PEG-coating on the magnetic and fluorescent properties of fluorescence of Fe₃O₄ and Fe₃O₄-CdSe were also studied. After that, cytotoxicity and uptake of PEG-Fe₃O₄ and PEG-Fe₃O₄-CdSe were carried out.NaYF₄: Er, Yb nanocrystals were known to have lower cytotoxicity and they could be excitated by near IR light, which dose less harm to living issues. Regarding that, in the final part of this paper, NaYF₄:Yb, Er nanocrystals were chosen to replace CdSe as fluorescent domains. We utilized the capping ligand with two functional groups to couple NaYF₄:Yb, Er with Fe₃O₄ nanocrystals to form nanocomposites. Through the observation of transmission electron microscopy （TEM）, Fe₃O₄ nanocrystals were proved to be successfully deposit on the surface of NaYF₄: Er, Yb seeds with morphologies of hexagonal plate, and the resultant nanocomposites revealed the capabilities of magnetic response as well as fluorescent emission. Next, through reverse microemulsion method, NaYF₄:Yb, Er-Fe₃O₄ nanocrystals were encapsulated by SiO₂ and achieved hydrophilic surface, which is essential for the further application. However, due to the larger lattice mismatch between NaYF₄ and Fe₃O₄, the existence of individual Fe₃O₄ nanocrystals could hardly be avoided with the formation of nanocomposites, which needed further studies.