Hollow metal oxide particles with macropore cavities (> 50 nm in diameter) and mesoporous shells (by definition, pores 2-50 nm wide) have tremendous potential utility as designed nanosystems for controlled adsorption, storage, and release of solutes. The hollow cavity provides a large volume for storage of solutes, while the shell can be designed for controlled dissolution (e.g. a multilamellar structure), controlled diffusion (dictated by pore size and orientation) and even gated diffusion of solutes. If controlled, these properties can lead to applications in targeted drug delivery, high-capacity fuel storage, enzyme encapsulation, and multifunctional catalysis. In this talk, we focus on tuning the pore size and accessibility of silica shells to make protein-accessible hollow particles. Previous studies in our group with latex/surfactant templating have allowed us to prepare hollow silica particles with ordered mesoporous shells [B. Tan; S.E. Rankin Langmuir 2005 21: 8180]. However, the mesopores in particles prepared with polystyrene latexes and cetyltrimethylammonium bromide (CTAB) have two disadvantages for protein applications: (1) they are too small to admit most proteins, and (2) the cylindrical channels in the shells almost always lie parallel to the particle surface, thus rendering them inaccessible. By switching to surfactants with pyridinium headgroups and treating the particles hydrothermally in aqueous ammonia, we will show that larger mesopores can be formed that are accessible from the outer particle surface. A detailed study of synthesis parameters suggests that the pyridinium headgroups decompose during hydrothermal treatment to cause extensive swelling, similar to the results reported by Yuan et al. [Z.Y. Yuan, J.L. Blin, B.L. Su, Chem. Comm. 2002: 504]. Traditional tools for analysis of mesoporous materials including x-ray diffraction, transmission microscopy, and nitrogen adsorption isotherm analysis are used to demonstrate the structural effects of the solution composition and hydrothermal treatment conditions. Swelling of the micelles during ammonia treatment results in a bimodal pore size distribution, and in extreme cases large mesopores form that provide a continuous pathway into the interior of the particle. The accessibility of the particles is demonstrated by fluorescence confocal microcopy with a sulforhodamine dye (that is insensitive to pore orientation) and green fluorescent protein (which is excluded from the hollow cores unless the pores are large and accessible).

Extended Abstract Status: Not Uploaded