Reversible pore-structure evolution in hollow silica nanocapsules: Large pores for siRNA delivery and nanoparticle collecting

The effective modulation of pore sizes for nanoporous silica nanoparticles still remains a great challenge not satisfactorily solved. In this paper, the pore sizes in the shell of hollow silica nanocapsules are well-tuned by a reversible Si-O bond breakage and reformation process under mildly alkaline conditions (e.g., Na ₂CO ₃ solution). The pores in nanosized hollow silica capsules can be modulated from 3.2 nm to larger than 10 nm by a novel, surfactant-directing alkaline-etching (SDAE) strategy. Interestingly, the pores can be fully filled through the regrowth of the dissoluted silicates by bonding to silanols (Si-OH) on the wall surface to generate the nonporous hollow silica nanocapsules. The large-sized pore hollow silica nanocapsules exhibit excellent siRNA-loading capabilities and intracellular transfection efficiencies in vitro. In addition, the large pores in the shell of hollow silica nanocapsules are explored as channels for collecting superparamagnetic, small-sized Fe ₃O ₄ nanoparticles as contrast agents for magnetic resonance imaging, initiating a special approach towards pore-size modulation and multifunctionalization of silica-based nanostructural materials for nanobiomedical applications. The pore structure in the shell of nanosized hollow silica capsules is reversibly modulated by a facile, surfactant- directing, alkaline etching strategy. The large-sized pore hollow silica nanocapsules exhibit high siRNA loading capabilities and intracellular transfection efficiencies. In addition, they are shown to successfully capture and collect nanoparticles.