A kinetic approach to the mechanism of formation of mesoporous silica nanoparticles

The synthesis of mesoporous silica nanoparticles (MSNs), widely investigated for application in nanomedicine, depends on many experimental factors and lacks reproducibility. Furthermore, its mechanism is not fully understood, in particular for the chemistry of the hydrolysis and condensation of tetraethoxysilane (TEOS). We coupled simple pH measurements with light diffusion monitoring directly within the reaction vessel in order to understand the early stages of the synthesis until the formation of nanoparticles. We found that two regimes of hydrolysis can be detected before the nanoparticles are formed. At the turning point, only 28% of the TEOS molecules are singly hydrolyzed and the condensation reaction only starts after the aggregation of the silicate-surfactant micelles. Experimental factors such as stirring strength or presence of carbonates of sodium hydroxide exert a strong influence on the kinetics of MSNs formation and on their size, and must be carefully controlled for reproducibility. Based on all our experimental evidence and on previous literature reports, a formation mechanism was proposed in order to highlight the importance of the interplay between chemical and physicochemical processes in such a complex system.