## 2 Synthesis of Magnetic-Silica Core/Shell Structures
In [PERSON]'s work, as mentioned above, Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) core/shell nanocomposites were synthesized through a sol-gel method and were considered promising tools in various biomedical applications such as targeted drug delivery, magnetic hyperthermia, MRI, and bio-separation [111; 112; 113]. Alternatively, the shape of the magnetite nanoparticles is an important aspect when it comes to evaluating their bio-applications; in this case, nanocubes are attractive options in terms of surface/volume ratio, which means an increase in biomolecules on the surface. The synthesis of Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\) core/shell nanocubes structures was performed in two steps [114; 115]. The first one consists of the synthesis of magnetite hydrophilic nanocubes, which is performed by a sonochemical method, and then the magnetite nanocubes obtained are taken from the precipitate using a magnet. Besides, the surface of the magnetite nanocube is functionalized through a modified sol-gel method. The obtained precipitate is collected and dried. If core/shell nanocubes with different thicknesses are desired to be developed, the amount of TEOS in the reaction must be controlled [45]. One of the syntheses of silica magnetite nanoparticles has been used oleic acid as a surfactant to stabilize the magnetite core on the silica shell [116]. Depending on the applications and conditions of synthesis, such as pH, temperature, stabilizing agents, and reaction time, the size, morphology, and even the magnetic properties of nanoparticles can be tuned [33]. To obtain core-shell magnetic nanoparticles for biomedical application, it is necessary to overcome some constraints on the Fe\({}_{3}\)O\({}_{4}\) effects on the blood as well as its limited stability in blood circulation. The above limitations can be resolved by functionalizing ionic oxides with stabilizers [117; 118]. The Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) preparation starts from the microemulsion water-oil, where nanocomposites are formed due to silica's ability to coat hydrophobic nanoparticles [5]. The solvothermal methods are based on the assistance of the temperature and pressure to form metal oxide nanoparticles. The solvothermal method is superior, concerning control of particle size and shape distributions, along with crystallinity, compared to the classical precipitation methods [6]. Using this synthesis method, monodisperse superparamagnetic single-crystal magnetite nanoparticles (MSSMN) were obtained [34]. These obtained materials were characterized by their morphology and crystalline structure, resulting in nanoparticles with a size of 70 nm. The most interesting aspect was that the magnetite nanoparticles had mesoporous structures, having a pore size of 5-10 nm. The energy-dispersive X-ray spectroscopy (EDS) and photoelectron spectroscopy (XPS) tests proved that the Fe\({}_{3}\)O\({}_{4}\) NPs are a pure phase. Besides, in the analysis of transmission electron microscopy (TEM), a thin shell of silica was observed to be coated on the core of Fe\({}_{3}\)O\({}_{4}\) NPs [34]. It has been shown that the change in the amount of sodium hydroxide (NaOH) controls the size of the mesoporous Fe\({}_{3}\)O\({}_{4}\) NPs, which shows that an excess of sodium hydroxide impacts the formation of irregular mesoporous Fe\({}_{3}\)O\({}_{4}\) NPs. This porous material was designed to incorporate drugs or genes into functionalized pores, which demonstrated that MSSMN has applications in drug delivery [34]. The titanium dioxide catalyst (TiO\({}_{2}\)) (including the photocatalyst) properties were used to develop nanomaterials with increased photoreactivity. Some researchers have coated the magnetite with silica (SiO\({}_{2}\)), followed by coating with a further TiO\({}_{2}\) shell through a facile and effective sol-gel method sustained by sonication. The photocatalytic assessment of the Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\)/TiO\({}_{2}\) core-shell/shell nanoparticles was conducted with or without UV irradiation.