## 2 Synthesis of Magnetic-Silica Core/Shell Structures
Based on the TEM evaluation, the Fe\({}_{3}\)O\({}_{4}\)/ SiO\({}_{2}\) core-shell nanoparticles have approximately 120 nm and the TiO\({}_{2}\) layer has 20-30 nm, meaning that the Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\)/TiO\({}_{2}\) core/shell/shell structures have \(\sim\)150 nm [119; 120]. The silica-coated magnetite nanoparticles can be used as T2-weighted MRI contrast agents for theranostic [33]. The usual method used for the manufacturing of magnetic nanoparticles was thermal decomposition followed by coating with silica shell by controlled hydrolysis [68]. Based on the TEM, it has been demonstrated that the particles have polydispersion and quasi-cubic morphology, with particles ranging from 40 to 60 nm, and with the silica shell being approximately 20 nm. The conclusion indicated that Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) nanoparticles are used as an effective tool in the T2-weighted MRI contrast agent within the tissue [68; 121]. Another study reported the core-shell magnetite-silica nanoparticle synthesis through two steps [108]. In the first phase, the synthesis of magnetite nanoparticles was performed by a coprecipitation method followed by the silica shell obtained with the addition of cetyltrimethylammonium bromide (CTAB), triethanolamine (TEA), and tetraethyl orthosilicate (TEOS) as surfactants, and a precursor for the silica shell formation via a sol-gel process. To use these magnetite-silica core-shell nanoparticles in specific applications, the pore characteristics from the silica shell should be controlled. The main aspects to be studied were the diameter of the pores, the volume of pores, and the surface area [108]. Through the analysis of the surface area of Brunauer-Emmett-Teller (BET), it was possible to determine the surface area of the pores of 177 m\({}^{2}\)/g; the pore size was 5.25 nm, and the model was the adsorption of gas. In the Barrett-Joyner-Halenda (BJH) pore size and volume analysis, the pore size distributions were estimated using the Kelvin model in the pore filling. One of the most important features that is also considered is the magnetic effect, evaluated using a vibrating sample magnetometer (VSM), measuring 60 emu/g. The images of the transmission electron microscopy (TEM) confirmed that the cores are uniformly surrounded by shells with an average particle size of 25 nm [108]. In their work, [PERSON] et al. [43] reported a modified Stober method for Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) core/shell nanoparticles using 20 nm hydrophilic Fe\({}_{3}\)O\({}_{4}\) NPs as seeds. This paper is relevant for synthesis, which interests us in manipulating the magnetic properties of silica/magnetic nanocomposite materials by controlling their surface properties and silica coating thickness [122]. The X-ray diffraction (XRD) results were confirmed by the TEM data that concluded that the magnetite core is of a single crystallite size. These Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) core/shell nanoparticles with controlled shell thickness are used in bioconjugation applications [43]. Another study reported the synthesis using a modified Stober method that was applied for the magnetite-silica nanostructures having 30-40 nm in diameter when the silica shell coats the magnetic nanoparticles. To use magnetite nanoparticles as a magnetic part in nanocomposites, they should be stabilized [43]. This paper supports the possible use of the magnetic properties of silica-magnetite composites, only stipulating that their properties and the thickness of the silica coating should be controlled. The study implies that the support of adequate encapsulation of magnetic nanoparticles is imperative in preserving adequate superparamagnetic properties, and the possibility of synthesizing large magnetic nanocomposites persists if the appropriate silica shell separates the individual magnetite nanoparticles [122]. The core/shell nanostructures have brought great attention to developing structures because the core and shell contribution is essential in tuning the performances of these structures.