Mesoporous silica nanoparticles are promising materials for various applications, such as drug delivery and catalysis, but the functional roles of surfactants in the formation and preparation of mesostructured silica nanoparticles (MSN-as) remain to be seen. It was confirmed that the molar ratio of cationic surfactants to Si of alkoxysilanes (Surf/Si) can affect the degree of mesostructure formation (i.e., whether the mesochannels formed inside the nanoparticles actually pass through the outer surface of the particles), the particle diameter, and the dispersibility of MSN-as. Wormhole-like mesostructures formed with low Surf/Si ratios; however, the mesopores did not pass through the outer surface of the particles completely. At high Surf/Si ratios, the mesostructures extended. The particle diameter was 100 nm or larger at low Surf/Si ratios, and the primary particle diameter decreased as the Surf/Si ratio increased. This was because the surfactants enhanced the dispersity of the alkoxysilanes in water and the hydrolysis rate of the alkoxysilanes became faster, leading to an increased nucleation as compared to the particle growth. Moreover, primary particles aggregated at low Surf/Si ratios because of the hydrophobic interactions among the surfactants that were not involved in the mesostructure formation but were adsorbed onto the nanoparticles. At high Surf/Si ratios, the surfactant micelles were adsorbed on the surface of primary particles (admicelles), resulting in the dispersion of the particles due to electrostatic repulsion. In particular, molar ratios of 0.13 or higher were quite effective for the preparation of highly dispersed MSN-as. Surfactants played important roles in the mesostructure formation, decreasing the particle diameters, and the dispersibility of the particles. All of these factors were considerably affected by the Surf/Si ratio. The results suggested novel opportunities to control various colloidal mesostructured nanoparticles from the aspects of composition, structure, and morphology and will also be useful in the development of novel methods to prepare nanomaterials in various fields.

The particle sizes of mesoporouss ilica nanoparticles most greatly affect their properties, including cellular uptake, cytotoxicity,a nd dispersity.T he growth of colloidal mesostructured silica nanoparticles (CMSS) with particle sizes lesst han 100 nm was controlled by am odified seedgrowth method by using alkoxysilanes (Si(OR) 4 ,R:M e, Et, Pr, and Bu) with different hydrolysis rates as additives. It was confirmed that the approximate matching of the hydrolysis rates of the added silanes with the consumption rates, determined by the total outer surface area of the seeds, is most important.C MSSw ith narrow particle-size distributions (e.g.,6 0o r8 0nmi ns ize) were successfully prepared. Si(OPr) 4 was the most suitable additive, though Si(OEt) 4 was also useful for controlled growth. Si(OBu) 4 wasu sable but neededv aried conditions forc ontrolled growth. The mesostructures and the high dispersity of nanoparticles were retained, even after removal of the surfactants of the CMSS by dialysis.T hese findings should contribute to the more precise control of particle sizes of mesoporouss ilica nanoparticles and to the deeper understanding of their properties.