Improving structural stability of water-dispersed MCM-41 silica nanoparticles through post-synthesis pH aging process

The colloidal and structural stabilities of MCM-41 mesoporous silica nanoparticles (MSNs) are of great importance in order to prepare optimal nanovectors. In this paper, MSNs (approximatively 160 nm in diameter) were synthesized using n-cetyltrimethylammonium bromide as a template and tetraethyl orthosilicate as a silica source under high N2 flow (MSN/N2) to obtain stable dispersions in water. The degradation of the porous nanoparticles was investigated by immersion in water. The morphology and the porous structure were studied by TEM, XRD, N2 sorption, and 29Si MAS NMR and were compared to that of MSNs prepared in ambient air (MSN/air). The volumetric properties of the MSN/N2 after 1 day in water were drastically more decreased than MSN/air (a pore volume decrease of 85 % for MSN/N2 and 59 % for MSN/air) and the 2D-hexagonal porous structure was totally lost. Furthermore, synthesizing MSNs under a high N2 flow leads to a decrease in the synthesis yield (45 % MSN/N2 and 75 % for MSN/air). The lower structural stability of the MSN/N2 is explained by the lower polycondensation degree of the MSN/N2 observed by 29Si MAS NMR (Q4/Q3 = 0.86 for MSN/N2 and 1.61 for MSN/air) and the lower silica molar ratio in the nanomaterials (SiO2/CTA = 3.9 for MSN/N2 7.1 for MSN/air). This allows for enhanced solubilization of silica in water. Four strategies were hence evaluated in order to reinforce the porous structure of the MSNs. Among them, the most efficient route was based on a pH adjustment of the colloidal suspension (pH 7.5) after 2 h of synthesis without any purification and while keeping a N2 static atmosphere (called MSN/N2/7.5). After 1 day in water, the volumetric and structural properties of MSN/N2/7.5 were similar to that obtained for MSN/air. The improvement of the stability arose as a result of the increase in the silica condensation (Q4/Q3 = 1.58) and silica molar ratio in the nanomaterials (SiO2/CTA = 6.8). After the post-treatment, the silica framework condensation is improved while keeping the colloidal stability, thus allowing further functionalization and/or drug loading. Cytotoxicity assays using SW480 cancer cells show a greater improvement in the cell viability.