# Therapeutic Potential of Polymer-Coated Mesoporous Silica Nanoparticles
## 2 Polymer Coated MSNs for Cancer  
Nanocarriers find enormous application potential in the diagnosis and treatment of cancer due to their capability of providing controlled release and targeted therapy. MSNs, due to their high surface area, tunable pore size, and flexible surface modification possibilities, along with good biocompatibility, have been prominently employed in cancer treatment research. In addition, the surface coating of MSNs offers the opportunity of achieving stimuli-responsive drug release at target sites, thus decreasing the systemic toxic effect of antineoplastic drugs [19]. Internal as well as external stimuli are widely explored for the selective release of drugs at tumor sites. Several polymers have been utilized to coat the MSNs in order to increase their potential in this respect and are discussed below. [PERSON] et al. prepared polydopamine (PDA) coated MSNs for dual stimuli-responsive drug delivery of doxorubicin (DOX). A simple stirring of bare MSNs and dopamine in Tris buffer allowed the coating of PDA over the MSN surface. The developed MSNs were responsive to ultrasound and pH. The accelerated release of DOX was observed at low pH owing to the high solubility of DOX and degradation of PDA under acidic conditions. In addition, a unique \"On-Off\" release of DOX was observed in the presence of HIFU (high-intensity focused ultrasound) irradiation due to the mechanical changes in the structure (Figure 4) [20]. Similarly, [PERSON] et al. developed gold modified PDA-coated DOX-loaded MSNs for synergistic chemo-photo therapy. High DOX release was observed at low pH from MSNs with an average diameter of \(60\pm 5\) nm due to the presence of PDA coating and a reduction in electrostatic interaction between MSN and DOX. On the other hand, less than 10% release was observed at physiological pH, advocating the advantage of PDA coating to avoid premature drug release. In addition, the irradiation of NIR light (808 nm) upsurged DOX release at low pH (pH--5.0) with the possibility of an on-off mechanism. It was proposed that NIR light increases the solution temperature due to the presence of gold particles and thus promote DOX diffusion from MSN [21]. Figure 3: Biological disorders that have been targeted using polymer coated mesoporous silica nanoparticles. In another example, PDA was used to coat MSN to facilitate pH sensitive drug release. Polyethylene glycol conjugated with epithelial cell adhesion molecule (EpCAM) aptamer was later attached to the MSNs to prepare a targeted drug delivery system for the treatment of colorectal cancer. DM-1, a cytotoxic agent, loaded MSNs demonstrate an increase in cytotoxicity to SW 480 cell lines by up to 50% after 48 h incubation compared to the free drug [22]. Recently, PDA coated MSNs were employed by us to further evaluate the effect of MSN shape on intracellular uptake and drug release rate. We demonstrated that rod-shaped MSNs exhibited delayed intracellular drug release with higher uptake compared to spherical MSNs [23] thus suggesting that polymer surface coating in combination with other design aspects can be utilized to fine-tune the drug delivery properties of the system [24]. This was a follow-up study to another one where we showed that on one hand, high drug loading (\(>\)30 wt% for water soluble compounds DOX and calcein) can be achieved owing to the \(\pi\)-\(\pi\) stacking interactions between the abundant aromatic rings of PDA and the aromatic backbones of drug compounds, and on the other hand catechol-metal-drug coordination systems can easily be constructed based on the coordination bonding between catechols in PDA and transition metal ions (Fe\({}^{3+}\), Zn\({}^{2+}\)) as well as that between metal ions and anthracycline drugs such as DOX, resulting in an acid-triggered drug release (Figure 5) [25]. PDA coated metal dichalcogenides (MoSe\({}_{2}\)) sheets were also employed to cap MSNs to develop a modified nanocarrier for chemo-photothermal therapy in cancer treatment (Figure 6).