Unexpected “Hammerlike Liquid” to Pulverize Silica Powders to Stable Sols and Its Application in the Preparation of Sub-10 nm SiO₂ Hybrid Nanoparticles with Chirality

Silane coupling agents are well-known as surface modifiers for various kinds of silica (SiO₂). However, in the present research, it has been found that they can also work as “hammerlike liquid” to pulverize different kinds of bulk amorphous SiO₂ in aqueous systems. This new function was typically clarified by using 3-aminopropyltrimethoxysilane (APS) and bundles of chiral SiO₂ nanofibers (with average diameter of ∼10 nm) as raw materials. By a simple reflux of the mixture of SiO₂ nanofibers and excessive APS in pure H₂O, the solid-containing mixture turned into a completely clear solution that contained sub-10 nm, amine-modified, and water-soluble hybrid SiO₂ sols (HS-sols). Moreover, this solution showed blue luminescence under ultraviolet irradiation. Furthermore, the circular dichroism and vibrational circular dichroism spectra revealed that the HS-sols are optically active even though the pristine chiral SiO₂ nanofibers were completely destroyed. It was considered that the chirality of SiO₂ nanofibers was due to the asymmetric arrangement of Si and O atoms in chiral domains (<10 nm) on the Si–O–Si network of SiO₂, and these domains are still preserved in chiral HS-sols. This green method has high potential for the recycling of rich SiO₂ sources to obtain functional SiO₂ nanomaterials with applications such as optical display, imaging, and chiral recognition. Also, it offers a tool for the analysis of the structural properties of SiO₂ on the molecular scale.