# Silica Mesoporous Structures: Effective Nanocarriers in Drug Delivery and Nanocatalysts
## 5 Application of Silica Mesopores in Drug Delivery
### pH-Responsive Drug Delivery  
Since the pH readings in tumors or inflammatory sites (pH \(\sim\) 6.8), as well as endosomal or lysosomal compartments of cells (pH \(\sim\) 5-6), reveal acidic conditions, a number of nanocarriers, such as mesoporous silica nanoparticles (hereafter as MSNs), have been exploited for controlled drug delivery. [PERSON] et al., studied an ionically controlled nanoscopic molecular gate that was developed by exploiting functionalized mesoporous materials [176]. This study showed that control of mass transport at a nanometric scale can be achieved by using rigid solids and molecules that are active at physiological, pH-active molecules. In another study, [PERSON] et al. examined the performance of amin-functionalized mesoporous silica SBA-15 loaded with bovine serum albumin (BSA) encapsulated within a polyacrylic acid (PAA). The entrapped BSA was released from the PAA-encapsulated silica SBA-15 at a pH of 7.4 rather than at a lower pH value of 1.2 [177]. They found that this material has the potential for releasing protein drugs to sites with higher pH values, such as the small intestine or colon. [PERSON] et al. studied covalently immobilized PAA on MSN materials as a pH-sensitive drug delivery system. In this study, reversible addition-fragmentation chain transfer (RAFT) functionalities were attached to the exterior surface of the MSNs (Figure 16). Their results showed that the PAA-coated MSN could control the access of guest molecules by adjusting the pH value [178]. In 2013, [PERSON] et al. examined an MSN-based, pH-responsive drug release system with polyelectrolyte multilayers. The results showed that MSN under the acidic condition (pH \(\sim\) 5.2) had a higher drug release rate than it did under the neutral condition (pH \(\sim\) 7.4). [PERSON] et al. used cyclodextrin as a blocking agent for mesoporous channels and \(N\)-methylbenzamideimidazole (MBI) as a holder [178]. The channels were designed so that cyclodextrin formed a stable complex at a pH of 7.4 through interactions with MBI. However, in the acidic environment of the cancer cells, the cyclodextrin is released by the MBI. [PERSON] et al. developed mesoporous materials that due to pH = 0 values, could not release propidium iodide (PI) [179]. However, the drug could be released when the pH was low or high. In their work, cucurbit (CB) [180] and uril [180] were used as holders. [PERSON] et al. synthesized pH-responsive mesoporous materials, wherein polycation poly(dimethylethyldi-allylammonium chloride) (PDDA) interacted with anionic mesoporous material-based carboxylic acid at a pH = 0 [181]. In this case, the channel was closed,and the vancomycin drug was trapped within the mesoporous materials. The carboxylic acid was protonated in the acidic condition, eliminating the interactions between polycation and carboxylic acid; thus, polycation was removed from the surface, and the drug was released. [PERSON] et al. synthesized a type of pH-responsive gate keeper MSNs with self-immolative polymers blocking and the pore opening, which triggered drug release by acidic pH. The pH sensitivity of this nanosystem causes that cargo release took place in acidic conditions, while there was almost no release at physiological pH [182]. In such systems, controlled drug release at special time and space, on demand, could be achieved with a zero-release effect in blood circulation to prevent drug decomposition. Several controlled drug delivery nanos vesicles based on organic and inorganic platforms have been fabricated [183; 184; 185]. [PERSON] et al. prepared a tumor-adhesive pH-degradable PVA microgels decorated with tissue-adhesive dopamine moieties by a combination of microfluidics technology and photo-cross-linking chemistry with a considerable conceptualization efficiency of bevacizumab (Bev) and docetaxel (DTX) to enhance breast cancer chemotherapy. The results showed that MSN under the acidic condition (pH \(\sim\) 6.5) had an acceptable drug release rate e.g., ca. 73% [186].