# Silica Mesoporous Structures: Effective Nanocarriers in Drug Delivery and Nanocatalysts
## 5 Application of Silica Mesopores in Drug Delivery
### Light-Responsive Drug Delivery  
Light irradiation is a convenient, remote-control method used in drug delivery systems. Light-sensitive molecules, such as azobenzene (AB), 0-nitrobenzyl ester, and thymine bases, are anchored to the surface of MSNs to render them photochemically susceptible to the light-controlled release [208,209]. [PERSON] et al. described a UV light-induced, reversible-release drug system [206]. They showed that the uptake, storage, and release of organic molecules in MCM-41 can be carried out through the photo-controlled, reversible intermolecular dimerization of coumarin derivatives attached to the surfaces of the pores. In 2009, [PERSON] et al. showed that cyclodextrin-covered mesoporous silica nanoparticles with photo-cleavable linkers exhibit photo-induced release characteristics [210]. [PERSON] et al. reported in vivo insertion of AB into the pores [204]. The drugs that were loaded included RB, polyimide (PI), and compotthecin. After the mesoporous silica carrier attacked the target tissue with light irradiation, the azo bond converted rapidly to its trans and cis isomers. Meanwhile, the drug was released from the cavities due to the repulsive force created by the very high mobility of the bulky phenyl-functional groups attached to the nitrogen. [PERSON] et al. proposed the use of the properties of a complex containing cyclodextrin, namely, its ability to create isomeric cis-and trans-azo, to alter an in vivo delivery time via light [205]. As shown in Figure 20, aromatic compounds were initially substituted for the MSN material. The bulk of the naphthalene then acts as an obstacle to the withdrawal of cyclodextrin. Furthermore, when the azo-isomeric bond is used, cyclodextrin acted as a washer and prevented the release of the drug. After a nanocarrier injection, light irradiation converted the cis-isomer into a trans-isomer, allowing the drug to reach the target site. [PERSON] et al. reported on a light-controlled drug delivery system for direct drug release that was based on coumarin cavity sizes [206]. When coumarin is exposed to UV light at wavelengths above 310 nm, it assumes a dimmer structure but returns to monomer form at a shorter wavelength (254 nm). The dimmer of coumarin, which is placed on the surface of the MCM-41 mesoporous material, reduces the size of the effective cavity and prevents the release of drugs loaded inside the channels. Hence, radiation with shorter wavelengths can open the channels and release the drug successfully. One of the main limitations of these devices is the low penetration capacity of UV-Vis light in living tissues. In order to overcome this problem, DDSs capable of responding to NIR radiation, which exhibits higher penetration capacity, have been reported. Au nanoparticles present plasmonic properties that allow them to transform NIR radiation into thermal energy. [PERSON] et al. have reported a system based on Au nanorods encapsulated within MSNs decorated with DNA double stands at gatekeepers [211]. Moreover, [PERSON] and coworkers designed a light-responsive, singlet-oxygen-triggered on demand drug release from photosensitized chlorin e6 doped MSNs nanorods, coated with bovine serum albumin modified with polyethylene glycol for cancer combination therapy [212].