Compartmentalized hollow silica nanospheres templated from nanoemulsions

Nanoemulsions with very high stability can be created by ultrasonication using a rich variety of surfactants, oils, and solution conditions. Multicompartments within a nanoemulsion droplet can also be created via a carefully chosen surfactant system. We will show in this paper that silica templating of a nanoemulsion system results in compartmentalized hollow silica nanospheres (HSN) of sub-100 nm size under neutral pH conditions. The system consists of water, cetyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate, n-hexadecane, n-octane, and n-hexanol. Two types of HSN can be obtained by manipulating the formulation; one is named single-compartment HSN (SC-HSN), where the HSN encapsulate a single water-in-oil droplet; the other is multiple-compartment HSN (MC-HSN), where the HSN encapsulate multiple smaller HSN. Using a high concentration of CTAB, we obtained a transparent solution of narrow size-distributed ultrasmall HSN (US-HSN) with a diameter of 12 nm. Parameters involved in the nanoemulsion have been examined and a possible mechanism is proposed. We show further that various new types of nested interior structures within HSN could be created by using other block copolymer type surfactants. Changing the oils to various food oils can also lead to biocompatible multicompartmentalized hollow silica nanospheres. A potential application of SC-HSN as a codelivery system of hydrophilic and hydrophobic drugs was demonstrated in simulated body fluid (SBF) using oil-soluble and water-soluble dyes as model compounds. Finally, we consider the mechanism responsible for the rich varieties of the nested structure in HSN and discuss factors promoting the stability of the nanoemulsion system for easy templating with ultrason-induced sol-gel silica chemistry.