Ultra-small mesoporous silica nanoparticles as efficient carriers for pH responsive releases of anti-cancer drugs.

Mesoporous silica has emerged as one of the most promising carriers for drug delivery systems. However, the synthesis of ultra-small mesoporous silica nanoparticles (UMSNs) and their application in drug delivery remains a significant challenge. Here, spherical UMSNs (∼25 nm) have been synthesized and tested as drug carriers. Anti-cancer drugs mitoxantrone (MX), doxorubicin (DOX) and methotrexate (MTX) have been utilized as model drugs. The pH-responsive drug delivery system can be constructed based on electrostatic interactions between carriers and drug molecules. The UMSNs could store drugs under physiological conditions and release them under acidic conditions. Different pH-responsive release profiles were obtained in phosphate buffer solutions (PBSs) at the designed pH values (from 4.0 to 7.4). MX and DOX can be used in the pH-responsive delivery system, while MTX cannot be used. Furthermore, we found that the physiological stabilities of these drug molecules in UMSNs are in a decreasing order MX > DOX > MTX, which follows the order of their isoelectric point (pI) values.