# Silica Mesoporous Structures: Effective Nanocarriers in Drug Delivery and Nanocatalysts
## 3 Factors in the Synthesis of a Mesoporous Material
### pH  
Typically, mesoporous materials are prepared in an acidic or alkaline environment. Neutral solutions are not suitable for these types of syntheses since, under neutral conditions, the rate of polymerization and transverse bonding is very high, resulting in the fabrication of a disordered silicate skeleton [85]. To overcome this problem, fluorine catalysts can be used in the syntheses of ordered solids under a neutral pH by adjusting the rate of hydrolysis and condensation of the silicate precursor [86; 87]. In an alkaline medium with the pH ranging from 9.5 to 12.5, the polymerization and creation of reversible networks of silicate species are possible. Hence, the above properties have led to the use of silicate precursors, such as colloidal solutions, Na\({}_{2}\)SiO\({}_{3}\), and TEOS [63]. During the synthesis, the pH changes. At the beginning of the reaction, due to the hydrolysis of silica, the pH decreases and then slightly increases as the components of the silicate species condensate [63]. A similar process occurs in strongly acidic environments since mesoporous design is less dependent on pH, reaction reversibility, and temperature [88]. As the pH decreases, the rate of synthesis of silica mesopores increases. High concentrations of acidic catalyst increase the rate of precipitation of the silica species. Strong acids, such as HCl, HNO\({}_{3}\), HBr, and H\({}_{2}\)SO\({}_{4}\), can be used as catalysts. In some cases, weak acids, such as H3 PO4 and CH3 COOH, are applied. When HCl is utilized as a catalyst, the pH of the system dips below one, whereas, as mentioned earlier, when there is a higher concentration of acid, the product does not reach the desired quality [88]. In general, the precipitation of mesoporous materials in the pH range 1-2 is strongly reduced, which can be related to the presence of an isoelectric point in the silicate source in this range. Since the common route for the synthesis of a mesoporous material is the sol-gel pathway, pH plays an important role.  
In the sol-gel method, hydrolysis and condensation processes are used to construct mesoporous nanostructures [89]. Organic compounds are integrated into the inorganic matrix of the mesoporous silica materials [90]. The presence of these organic compounds can then help in the formation of the final product through catalytic effects and improvements in the hydrolysis-condensation reactions. Depending on the nature and number of the organic groups that enter into the system, highly porous meso-structures can be achieved, even with heating [89]. The routes to the mesoporous nanostructure used by the sol-gel method under acidic and basic conditions are shown in Figure 2. In the first step, the acidic/basic silicate precursor is observed, which leads to the generation of a silanol group (creation of sol). In the next step, the silanols react together (condensation), and the gel structure is established by realizing the H\({}_{2}\)O or alcoholic by-products. A wet gel is obtained at this stage, which is ultimately converted into the original gel structure through drying and calcination process. In fact, under acidic conditions, the rate of nucleation determines the rate of the process. Due to the formation of a high concentration of the silanol groups, the final step is hydrolysis. Generally, hydrolysis in the presence of a base catalyst is faster than in an acid catalyst because the base prevents the agglomeration of the sol particles [91]. Therefore, hydrolysis and the condensation of silicate precursors can be considered as the key stage of the meso-structure's preparation method [91]. [PERSON] et al. showed that a low concentration of an acid catalyst, in addition to being a self-inducing stimulus, is required to begin the reaction [92].  
Figure 2: _Cont._