pH-Responsive Drug Release from Polymer-Coated Mesoporous Silica Spheres

A novel method was proposed to coat poly(methacrylic acid-co-vinyl triethoxylsilane) (PMV) on mesoporous silica spheres (MSSs) to obtain a core-shell pH-responsive drug-carrier (MSS/PMV). PMV was prepared by free radical polymerization of methacrylic acid (MAA) and vinyl triethoxylsilane (VTES), in which MAA acted as the pH-sensitive monomer and VTES acted as the siloxane-containing monomer to provide an anchoring effect with the MSS surface. The micrographs of MSSs before and after coating were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was found that the PMV shell with a thickness of 20 nm had been coated successfully on MSS particles. To confirm the pH-sensitivity of the PMV shell, we investigated the pH-response difference between MSS/PMV and MSS/PMV-H (acidified MSS/PMV) in NH4NO3/C2H5OH solution. It was found that the PMV shell of MSS/PMV was loose ("open" state) and allowed template molecules to penetrate it easily. By contrast, the PMV shell of MSS/PMV-H was compact ("closed" state) and confined template molecules effectively inside MSS pores. These results indicated that the PMV shell played the role of molecular switch that could control the transport of molecules via a pH-dependent "open-close" mechanism. In a test of in vitro drug release, MSS/PMV showed high response to the pH of a drug solution. At high pH (pH = 7.5), ibuprofen (IBU) that loaded in MSS/PMV released rapidly and completely (within 2 h); at low pH (pH = 4.0 or 5.0), only a small part of the IBU (15 wt %) was slowly released from the MSS/PMV, and most of the IBU was effectively confined in MSS pores.