## 2 Synthesis of Magnetic-Silica Core/Shell Structures
The methods have similarities that showed changes in particle size, surface reactivity, and the composition of the core@shell, and optical and magnetic properties of the constituent parts confirmed the formation of the core@shell nanostructures [91; 92; 94]. The results also showed that the synthesis of core@shell nanostructures via the methods described presented magnetic and biocatalytic applications [91; 92]. The bottom-up approach is more useful than the top-down approach, as it is more proper for the preparation of materials at the nanoscale and generates uniform size, shape, and distribution. Top-down routes are not suitable for preparing uniform shapes and are difficult to design nanoparticles with, with the biggest problem being designing an imperfect surface structure. It has been reported that the grinding technique has been successfully used in the synthesis of magnetite nanoparticles by a top-down approach and that it significantly reduced the grinding time to 5 h compared to the previously reported studies [73; 97]. With the help of top-down nanofabrication techniques such as lithography, monodispersed particles with controlled shapes and dimensions can be generated. The top-down approach used to synthesize nanoparticles proves to be an alternative to overcome the disadvantages and obstacles of the bottom-up method [98; 99]. The approaches described above have multiple advantages and disadvantages; however, comparing the two techniques has shown that the bottom-up method is cost-effective and facilitates the manufacturing of 2D and 3D materials with multiple applications [78]. Table 1 describes various core@shell nanoparticles, their synthesis methods, and their application. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  
The table above presents examples with various core/shell nanoparticles, their synthesis methods, and their applications, and, next, will summarize some of the most used methods of synthesis for magnetite-silica core/shell nanostructures. The selection of the synthesis methods for core and shell are very important and require good judgment in choosing the proper material for a better outcome regarding the final applications of the core-shell nanoparticles. For the formation of core, nanoparticles have been used and have gained great attention in therapeutic applications. Besides their core, metal nanoparticles are being used for drug delivery applications. Using quantum dots as the core and shell can entrap diverse drugs within and can develop interesting core-shell nanoparticles for theranostic applications. Considering the type of precursors used to prepare the core, different preparation methods are needed [6]. The nanoprecipitation method has the advantage of being simple, fast, and reproducible; it has been used widely in the fabrication of nanoparticles as well as nanocapsules. In this method, a few factors affect the physicochemical properties of the nanoparticles, such as concentration, nature, and concentration of surfactant/dispersing agents. These parameters can significantly influence the particle size and entrapment, as well as loading efficiency, when it comes to deciding on the proper applications [6; 110]. The coprecipitation method is the most widely used method for the synthesis of metal oxide nanoparticles with controlled size and magnetic properties. In this method, aqueous salt solutions are used as precursors, with the precipitation of the desired nanoparticles being controlled by the addition of an alkaline solution. Different factors, such as ionic strength, pH, and temperature of the reaction system, as well as the nature of the precursors, highly influence the morphology and composition of the magnetic nanoparticles [6]. Therefore, [PERSON] et al. [4] describe an approach based on the preparation of electro-statically stabilized Fe\({}_{3}\)O\({}_{4}\) nanoparticles as seeds synthesized by the Stober method. They used stabilized trisodium citrate molecules to change surfaces and cover the magnetite nanoparticles with silica.