# Silica Mesoporous Structures: Effective Nanocarriers in Drug Delivery and Nanocatalysts
## 1 Introduction  
Porous nanostructures are a special group of materials that have cavities/porosities at the nanoscale and exhibit different physicochemical features [1]. These properties are determined by their shape [2], size [3], and composition. Compared to uniform particles of the same size, porous nanomaterials have unique properties due to their vacant spaces, including low densities [4; 5; 6; 7], large active surfaces [8; 9; 10],low refractive coefficients [11; 12; 13; 14], desirable permeabilities [15; 16; 17], good selectivity [18; 19; 20], and thermal and acoustic resistances [21; 22; 23]. The above-mentioned characteristics of porous nanostructures have resulted in the presentation of diverse nanoporous materials, which have attracted the attention of scientists in a variety of fields. The ratio of free-space pores to the total volume of a material is defined as porosity [24; 25], which in this category, a pore connected to the free surface of substance is called an open cell [26], and materials with open cells are suitable for use in, e.g., filtration [27], membranes [28], separation [28], and chemical operations, playing roles as catalysts [28] and in chromatography [29]. A pore that is far away from the free surface of a composition is called a closed-cell [30], and these closed cells, while they increase the thermal and acoustic resistance of these materials, as well as reduce their weights, do not contribute to any chemical applications [21; 22; 23; 31]. Pores come in a variety of shapes and can be spherical [30], cylindrical [32], grooves [33], and hexagonal [34]. A wide variety of porous nanomaterials are available with very diverse properties, structures, and applications. The most useful method for the classification of these materials is to use the diameters of the cavities. Many of the important exclusivities in adsorption and permeation technology relate to these diameters. According to the definition provided by the IUPAC, porous solids are sorted into three basic types based on diameters of their pores: Microporous (<2 nm) [35], silica mesoporous (2-50 nm) [36], and macroporous (>50 nm) [37]. Among these porous solids, mesopores are more efficient than the others due to their large surface areas [38], high porosities [39], tuneable pores [40], excellent thermal properties [41], and mechanical stability [42]. They are used in many different applications, including drug delivery systems (DDSs) [43], catalysts [44], sensors [45], adsorption [46], and separation [47], and are divided into the SBA (Santa Barbara amorphous) [48], MCM (Mobil Composition of Matter) [49], FSM (folding shield mechanism) [50], TUD (Delft University of Technology) [51], and HMS (hollow mesoporous silica) types [52]. In the preparation of silica nanoparticles, different types of precursors can be used depending on whether hardness or flexibility is desired, whether a lipophilic or hydrophilic material is required, and what the density of catalytic active centres on the surface is to be. Since the early 21 st century, attempts have been made to make all kinds of silica nanoparticles. By using organic fluids, the structural elements of the structures, and solutions of water and alcohol as solvents, new inorganic substrates with regular structures in nanometre dimensions were prepared. In the late 1990s, various studies were conducted to identify the silica structures and the factors affecting the morphological features, such as temperature and pH, of compositions and solvents. In addition, other compounds, including salts, were used as additives in the production of various types of these materials. Interestingly, these nanostructures have internal (cylindrical pores) [53] and exterior [54] surfaces that allow selective functionalization for catalytic and drug delivery applications. The main purpose of this review is providing a general knowledge over the recent applications of mesoporous silica as nanocatalyst and carrier for drug delivery.