IMMA. His current research interests include mesostructured materials, and several fields of inorganic materials chemistry. Some examples are intercalation chemistry, silicate chemistry, inorganic polymers, sol-gel chemistry, inorganic-organic hybrids, and self-organized materials. AbstractMesoporous silica nanoparticles (MSNs) with colloidal stability have attracted increasing interest because of their important properties, such as high transparency and cell uptake. Colloidal MSNs are comprehensively reviewed from the viewpoints of their preparation, characterization, and applications which include biomedical, catalytic, and optical materials. The following two points have been emphasized. The first point is that this review covers the widest range of introduction and discussion of the preparation and applications of both colloidal and noncolloidal MSNs. The second point is that this account contains a discussion from the viewpoints of both inorganic and colloid chemistries. After the introduction of the historical background of preparation of MSNs, preparative methods of colloidal MSNs and the major factors for the preparation of nanosized and colloidal MSNs are discussed. The methods to control the size, composition, morphology, and pore size of MSNs are also presented. Appropriate methods to obtain MSNs with high colloidal dispersibility are also discussed. Applications of colloidal MSNs are introduced with an emphasis on the colloidal stability. Issues to be addressed for further developments of colloidal MSNs are finally described.

Although mesoporous silica particles are useful building blocks for colloidal crystals, mesoporous silica nanoparticles smaller than 100 nm with sufficient monodispersity and colloidal stability to enable thermodynamic assemblies have not been reported. Here, we report that highly monodisperse colloidal mesoporous silica nanoparticles (CMS) can be prepared by combining the preparation of colloidal mesoporous silica nanoparticles with a shortened nucleation time. The nanoparticles exhibited a uniform shape and relatively smooth surface because an undesirable aggregate dispersion process was avoided. In addition, the diameter of the nanoparticles was controlled by seed-growth without spontaneous nucleation, which enabled the investigation of fundamental CMS properties. Using monodisperse CMS, the dependence of the ζ-potential of CMS on the diameter was revealed. Colloidal crystals composed of mesoporous silica nanoparticles were fabricated by drying the colloidal solution. This is the first report regarding the fabrication of colloidal crystals composed of mesoporous silica nanoparticles with a small particle size.

Both the pore size and particle diameter of aqueous colloidal mesostructured/mesoporous silica nanoparticles (CMSS/CMPS) derived from tetrapropoxysilane were effectively and easily controlled by the addition of trialkylbenzenes (TAB). Aqueous highly dispersed CMPS with large pores were successfully obtained through removal of surfactants and TAB by a dialysis process. The pore size (from 4 nm to 8 nm) and particle diameter (from 50 nm to 380 nm) were more effectively enlarged by the addition of 1,3,5-triisopropylbenzene (TIPB) than 1,3,5-trimethylbenzene (TMB), and the enlargement did not cause the variation of the mesostructure and particle morphology. The larger molecular size and higher hydrophobicity of TIPB than TMB induce the incorporation of TIPB into micelles without the structural change. When TMB was used as TAB, the pore size of CMSS was also enlarged while the mesostructure and particle morphology were varied. Interestingly, when tetramethoxysilane and TIPB were used, CMSS with a very small particle diameter (20 nm) with concave surfaces and large mesopores were obtained, which may strongly be related to the initial nucleation of CMSS. A judicious choice of TAB and Si sources is quite important to control the mesostructure, size of mesopores, particle diameter, and morphology.

Hollow siloxane-based nanoparticles (HSNs) have attracted significant attention because of their unique properties and applications. Recently, it was discovered that the simple covering of silica nanoparticles with an organosiloxane shell leads to the spontaneous formation of HSNs; however, the detailed mechanism of their formation has not yet been established. In this study, colloidal 30 nm HSNs were prepared by adding organically bridged alkoxysilane to an aqueous dispersion of mesostructured silica–surfactant composite nanoparticles, and the temporal changes of the morphology and chemical state of the nanoparticles were monitored to elucidate the formation mechanism. Core silica was dissolved after the formation of the core–shell structured nanoparticles, and almost all the dissolved silicate species were incorporated in the organosiloxane shell, changing the shell thickness. Two conditions were essential for silica dissolution induced by covering with organosiloxane: (i) presence of a sufficient amount of uncondensed Si–OH groups in the organosiloxane shell, and (ii) elevated temperature and pH for the promotion of the hydrolysis of silica. These findings will enable the fabrication of various HSNs through organosiloxane-induced silica dissolution and redeposition.

Colloidal crystals composed of mesoporous silica nanoparticles (MSNs) are expected to have various applications because of their unique hierarchical structures and tunable functions. The expansion of the mesopore size is important for introducing guest species which cannot be accommodated by using conventional colloidal crystals of MSNs; however, the preparation of MSNs with a controllable pore size, suitable for the fabrication of colloidal crystals, still remains a challenge. In this study, we fabricated colloidal crystals composed of pore-expanded MSNs using a sophisticated particle growth method to control the pore size of colloidal MSNs while retaining their monodispersity high enough to form colloidal crystals. By adding triisopropylbenzene (TIPB) only during the growth process with the stepwise addition of tetrapropoxysilane (TPOS), the particle size can be tuned from 60 nm to 100 nm, while the pore size can be tuned from 3 nm to ten plus several nm which is the largest size among the previous MSNs capable of forming colloidal crystals. These novel colloidal crystals should contribute to the expansion of nanomaterials science.