Inner and Outer Surface Functionalizations of Ultrasmall Fluorescent Silica Nanorings As Shown by High-Performance Liquid Chromatography

In the past two decades, ultrasmall fluorescent nanomaterials have garnered significant interest in the fields of bioimaging and nanomedicine. More recently, attention has shifted from purely spherical nanoparticles to objects with a variety of different shapes, such as high-aspect-ratio, hollow, and star-shaped nanomaterials. We have recently reported the synthesis and characterization of ultrasmall silica nanoparticles with complex shapes, including silica nanocages, silica nanorings, and single-pore silica nanoparticles. Here, we focus on fluorescent silica nanorings that are of particular interest for theranostic applications in nanomedicine. We present in-depth studies of the synthesis and orthogonal surface functionalization, successfully distinguishing the inside and outside of the silica nanorings, utilizing a combination of spectroscopic and analytical techniques including fluorescence correlation spectroscopy and reversed- phase high-performance liquid chromatography. Results suggest that despite the small silica ring diameter of around 10 nm and below, it is possible to effectively "hide" hydrophobic moieties on the inside of the rings but that their number must be carefully engineered. We expect the chemistry and methods developed here to be of interest to a range of differently shaped porous nanoparticles within the ultrasmall-size regime.