## 2 Results and Discussion  
### Characterization of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO Nanoparticles  
The morphologies of Fe\({}_{3}\)O\({}_{4}\), silica coated Fe\({}_{3}\)O\({}_{4}\) nanoparticles (Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)) and Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO nanoparticles were characterized by TEM to visually observe the morphologies and the changes of nanoparticles during the preparation. TEM images of these nanoparticles are shown in Figure 1. Figure 1a shows the morphology of Fe\({}_{3}\)O\({}_{4}\) nanoparticles. It could be seen that the Fe\({}_{3}\)O\({}_{4}\) nanoparticles had a round shape and were well dispersed. The average diameter of Fe\({}_{3}\)O\({}_{4}\) nanoparticles was about 300 nm to 350 nm. Figure 1b shows the morphology of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles. It could be seen that a light grey coating layer existed on the surface of the Fe\({}_{3}\)O\({}_{4}\) nanoparticles and that the shell thickness of the silica layer was about 60 nm. After the amino functionalization and enzyme immobilization, the change in shape of the final Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO nanoparticles was small. The surface of the Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO nanoparticles showed some roughness, which might have been caused by the amino groups and the enzyme molecules. The structures of synthesized Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles were analyzed by XRD and the results are shown in Figure 2. For Fe\({}_{3}\)O\({}_{4}\) nanoparticles, the characteristic peaks were marked by corresponding indices (111), (220), (311), (400), (422), (511), (440) and (533) in accordance with the standard Fe\({}_{3}\)O\({}_{4}\) crystal XRD card. For Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles, the characteristic peaks were similar to that of Fe\({}_{3}\)O\({}_{4}\) nanoparticles except for the peak around 24\({}^{\circ}\). The peak around 24\({}^{\circ}\) could be attributed to the silica [22]. Based on these analyses, the coating of silica could be considered successful. To confirm the silica coating and amino groups on the surface of Fe\({}_{3}\)O\({}_{4}\) nanoparticles, FT-IR analysis was also performed. As shown in Figure 3a, the FT-IR spectrum of Fe\({}_{3}\)O\({}_{4}\) nanoparticles showed a characteristic Fe-O vibration band at 588 cm\({}^{-1}\)[23]. Figure 3b shows the FT-IR spectrum of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles. Compared with naked Fe\({}_{3}\)O\({}_{4}\) nanoparticles, the feature peak in the spectrum of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles at 796 cm\({}^{-1}\) and those around 1085 cm\({}^{-1}\) belonged to the Si-O-Si stretching vibration, the Si-O-H stretching vibration and the Si-O vibration, respectively [24]. After the amino functionalization, the FT-IR spectrum of amino Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles showed all of the peaks mentioned above. Moreover, the enhanced wide peak at 3400 cm\({}^{-1}\) could be attributed to the N-H stretching vibration [23]. These results indicated that the silica coating and the following amino functionalization on the surface of Fe\({}_{3}\)O\({}_{4}\) nanoparticles were successful. Figure 1: Transmission electron microscope (TEM) images of (**a**) Fe\({}_{3}\)O\({}_{4}\) nanoparticles; (**b**) silica coated Fe\({}_{3}\)O\({}_{4}\) nanoparticles (Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)) nanoparticles and (**c**) xanthine oxidase immobilized silica coated Fe\({}_{3}\)O\({}_{4}\) nanoparticles (Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO) nanoparticles. Figure 2: X-ray powder diffraction (XRD) patterns of (**a**) Fe\({}_{3}\)O\({}_{4}\) nanoparticles and (**b**) Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles. In order to evaluate the magnetic properties of prepared nanoparticles, the Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) and Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-XO nanoparticles were characterized by VSM.