Kinetic analysis of pH influence on the fluorescein release from chitosan-coated mesoporous silica nanoparticles

The development of drug delivery systems that are able to respond to the different pH gradients found in the human body is a research field of vast scientific and technological interests. With such a responsive system, a specific pH condition can be used as a stimulus to trigger the local drug-releasing, increasing the efficiency of treatments in comparison to the traditional ones of systemic drug administration. In this study, a responsive drug nanocarrier to acid pH was produced by coating mesoporous silica nanoparticles with an outer chitosan layer that was covalently bonded to the nanoparticle. Fluorescein dye (FL) was loaded into the nanocarrier and used as a spectroscopic probe to evaluate the kinetics of release in buffered solutions with pH 6.0, 7.2 and 8.0. The kinetics analysis using mathematical models demonstrated higher rates of release in pH 6.0 than in pH 7.2 or 8.0. However, the experimental procedure used to load the dye into the nanocarrier had a huge influence on the kinetic mechanisms of delivering. When the dye was loaded into the nanocarrier before the chitosan coating had been made, the FL release was sustained and controlled through a Fickian's transport mechanism. When the dye was loaded after the nanocarrier coating a burst release effect was observed in every pH evaluated, which indicated that the dye was loaded only in the external coating of CS and did not inside of the silica mesopores.