## 3 Results and Discussion  
### Characterization  
The Fe\({}_{3}\)O\({}_{4}\) nanospheres were prepared through solvothermal reaction. The functionalized magnetic nanoparticles (Mag-PVP) were prepared with Fe\({}_{3}\)O\({}_{4}\) nanospheres and PVP. Its preparation route has been diagrammatically shown in Scheme 1. The TEM images of Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\), and Mag-PVP are presented in Figure 1a-d, respectively. The prepared magnetic nanoparticles exhibited a uniform distribution and the diameter of these nanospheres was approximately 150 nm. TEM images (Figure 1c,d) showed that the Fe\({}_{3}\)O\({}_{4}\) nanoparticles were coated by a gray polymer shell. Based on the TEM images of Mag-PVP, the thickness of the gray polymer shell was about 10 nm. The diameter of Mag-PVP was approximately 160 nm. Details about the structure of the gray polymer shell were investigated in the following characterizations.  
The FTIR spectra of Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\) and Mag-PVP are shown in Figure 2. The absorption bands around 1590, 1500 and 1490 cm\({}^{-1}\) of Mag-PVP were assigned to the characteristic vibration of pyridine rings, indicating the presence of PVP in the Mag-PVP [29]. The band at 747 cm\({}^{-1}\) was assigned to C-H stretching. The band at 584 cm\({}^{-1}\) was assigned to Fe-O stretching and bending modes [30], confirming the presence of Fe\({}_{3}\)O\({}_{4}\) in the Mag-PVP. All these indicated that the Mag-PVP composites were successfully coated with poly-divinylbenzene-2-vinylpyridine.  
As shown in Figure 3, the magnetic hysteresis curves of the Fe\({}_{3}\)O\({}_{4}\) and \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\) microspheres were 88.3 and 81.1 emu/g, respectively, which were far more than that of Mag-PVP microspheres (44.6 emu/g). The decrease in the saturation magnetization was caused by the coating layers of Fe\({}_{3}\)O\({}_{4}\), resulting in an increase of surface disorientation [31]. The results also strongly suggested that the remaining magnetic force in Mag-PVP could be effectively attracted by an external magnetic field. Mag-PVP was a feasible magnetic separation carrier with a good magnetic characteristic. When the external magnetic field was applied, Mag-PVP microspheres were quickly separated from the solution. Figure 4 showed the thermogravimetric analysis (TGA) curves of Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\) and Mag-PVP. In these curves, different reaction stages were observed. For Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\) and Mag-PVP, the exponential weight decay until 250 \({}^{\circ}\)C was attributed to absorbed humidity or surface hydroxyls. While the weight loss of Mag-PVP in the 380-480 \({}^{\circ}\)C region was thought to be due to degradation of the DVB chain, this loss did not occur in the Fe\({}_{3}\)O\({}_{4}\) and \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\) samples [32]. The 480-600 \({}^{\circ}\)C region was mainly attributed to thermo-oxidative degradation of the PVP polymer chain [33]. All of these results indicated that the thermo-stable Mag-PVP was a polymer based on magnetic nanoparticles.  
Figure 1: Transmission electron microscope (TEM) image of (a) Fe\({}_{3}\)O\({}_{4}\), (b) \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\), and (c,d) Mag-PVP.  
Figure 3: Magnetic hysteresis curves of Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\), and Mag-PVP.  
Figure 2: FTIR spectrum of Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-MPS-Fe\({}_{3}\)O\({}_{4}\), and Mag-PVP.